U.S. Environmental Protection Agency Industrial Environmental Research CDA fiOH/T 7fi
Office of Research and Development Laboratory
Cincinnati. Ohio 45268 December 1976
ENVIRONMENTAL
CONSIDERATIONS OF
SELECTED ENERGY
CONSERVING MANUFACTURING
PROCESS OPTIONS:
Vol. X. Cement Industry
Report
Interagency
Energy-Environment
Research and Development
Program Report
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S.
Environmental Protection Agency, have been grouped into seven series.
These seven broad categories were established to facilitate further
development and application of environmental technology. Elimination
of traditional grouping was consciously planned to foster technology
transfer and a maximum interface in related fields. The seven series
are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
6. Scientific and Technical Assessment Reports (STAR)
7. Interagency Energy-Environment Research and Development
This report has been assigned to the INTERAGENCY ENERGY-ENVIRONMENT
RESEARCH AND DEVELOPMENT series. Reports in this series result from
the effort funded under the 17-agency Federal Energy/Environment
Research and Development Program. These studies relate to EPA's
mission to protect the public health and welfare from adverse effects
of pollutants associated with energy systems. The goal of the Program
is to assure the rapid development of domestic energy supplies in an
environmentally—compatible manner by providing the necessary
environmental data and control technology. Investigations include
analyses of the transport of energy-related pollutants and their health
and ecological effects; assessments of, and development of, control
technologies for energy systems; and integrated assessments of a wide
range of energy-related environmental issues.
This document is available to the public through the National Technical
Information Service, Springfield, Virginia 22161.
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EPA-600/7-76-034j
December 1976
ENVIRONMENTAL CONSIDERATIONS OF SELECTED
ENERGY CONSERVING MANUFACTURING PROCESS OPTIONS
Volume X
CEMENT INDUSTRY REPORT
EPA Contract No. 68-03-2198
Project Officer
Herbert S. Skovronek
Industrial Pollution Control Division
Industrial Environmental Research Laboratory - Cincinnati
Edison, New Jersey 08817
INDUSTRIAL ENVIRONMENTAL RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OHIO 45268
For sale by the Superintendent of Documents, U.S. Government Printing Office, WMhburton. D.O. XMOB
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DISCLAIMER
This report has been reviewed by the Industrial Environmental Research
Laboratory, U.S. Environmental Protection Agency, and approved for publica-
tion. Approval does not signify that the contents necessarily reflect the
views and policies of the U.S. Environmental Protection Agency, nor does
mention of trade names or commercial products constitute endorsement or
recommendation for use.
ii
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FOREWORD
When energy and material resources are extracted, processed, converted,
and used, the related pollutional impacts on our environment and even on our
health often require that new and increasingly more efficient pollution con-
trol methods be used. The Industrial Environmental Research Laboratory -
Cincinnati (lERL-Ci) assists in developing and demonstrating new and im-
proved methodologies that will meet these needs both efficiently and
economically.
This study, consisting of 15 reports, identifies promising industrial
processes and practices in 13 energy-intensive industries which, if imple-
mented over the coming 10 to 15 years, could result in more effective uti-
lization of energy resources. The study was carried out to assess the po-
tential environmental/energy impacts of such changes and the adequacy of
existing control technology in order to identify potential conflicts with
environmental regulations and to alert the Agency to areas where its activi-
ties and policies could influence the future choice of alternatives. The
results will be used by the EPA's Office of Research and Development to de-
fine those areas where existing pollution control technology suffices, where
current and anticipated programs adequately address the areas identified by
the contractor, and where selected program reorientation seems necessary.
Specific data will also be of considerable value to individual researchers
as industry background and in decision-making concerning project selection
and direction. The Power Technology and Conservation Branch of the Energy
Systems-Environmental Control Division should be contacted for additional
information on the program.
David G. Stephan
Director
Industrial Environmental Research Laboratory
Cincinnati
111
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EXECUTIVE SUMMARY
The manufacture of cement in the United States required 0.52 x lO Btu
in 1971. This ranked cement as the eighth most energy intensive industry at
that time. The production of cement in 1972 totaled 84.6 million tons, with
Portland cement constituting 96% of this amount, and the balance being natural,
masonry, and pozzolan cements. This cement had a value of about $3.5 billion,
and was produced by 50 cement companies, which operated 150 plants.
Approximately 80% of the total energy required for cement manufacture is
fuel, which is required for the high-temperature reaction step of clinker pro-
duction. The balance is electrical energy primarily used in grinding the raw
materials and the finished cement. Presently, coal, oil, and natural gas are
all used for cement production. During the past several decades, coal has
been declining in use, as it was progressively displaced by oil and natural
gas. In the 1970's, this trend has been reversed.
Coal is an acceptable fuel for cement manufacture, both technologically
and environmentally. Coal can replace all of the oil and gas now being used
by the cement industry. Most of the industry's rotary kilns (in which most of
the fuel is burned), can be converted to coal firing. Almost all of the fuel's
sulfur leaves the cement process chemically bound up as part of the cement pro-
duct and the waste kiln dust.
This study of possible process modifications or the use of alternative
fuel forms in the cement industry focused on the unit process of clinker pro-
duction, since it requires about 80% of the total energy for cement manufacture.
The process modifications analyzed in this study were the suspension preheater,
flash calciner, and fluidized-bed cement process. The use of coal instead of
oil or gas was also considered .
All of the process options investigated will require less fixed capital
investment and use less total energy than the long rotary kiln base case. The
pollution control costs are also expected to be lower for these process options.
It appears that the amount and nature of effluents from these process options
will be the same or less than from the long rotary kiln.
The conversion from oil or gas fuel to coal will require additional cap-
ital for the coal storage and handling. Fugitive emissions and runoff from
coal storage and handling are expected to increase pollution control costs.
This report was submitted in partial fulfillment of contract 68-03-2198
by Arthur D. Little, Inc. under sponsorship of the U.S. Environmental Protec-
tion Agency. This report covers a period from June 9, 1975 to February 9, 1976.
iv
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TABLE OF CONTENTS
FOREWORD iii
EXECUTIVE SUMMARY iv
List of Figures viii
List of Tables x
Acknowledgments xiii
Conversion Table xv
I. INTRODUCTION 1
A. BACKGROUND 1
B. CRITERIA FOR INDUSTRY SELECTION 1
C. CRITERIA FOR PROCESS SELECTION 3
D. SELECTION OF CEMENT INDUSTRY PROCESS OPTIONS 3
II. FINDINGS, CONCLUSIONS, AND RECOMMENDATIONS 6
A. APPROACH 6
B,. POTENTIAL CONFLICT WITH ENVIRONMENTAL REGULATIONS 8
1. Suspension Preheater 8
2. Flash Calciner 9
3. Fluidized-Bed Cement Process 10
4. Conversion to Coal from Natural Gas and Oil 11
C. ADDITIONAL RESEARCH 11
i
III. INDUSTRY OVERVIEW 13
IV. ALTERNATIVE PROCESSES 16
A. SUSPENSION PREHEATER 16
1. Process Description 16
2. Definition 19
3. United States Situation 22
4. Current Applications 22
5. Development 25
6. Economic Factors 26
7. Environmental Factors 31
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TABLE OF CONTENTS (Cont.)
B. FLASH CALCINER 34
1. Process Description 34
2. Current Status 37
3. Energy, Economics, and Environment 39
C. FLUIDIZED-BED CEMENT PROCESS 40
1. Process Description 40
2. Reactor 44
3. Mechanical Advantages 44
4. Energy Use 46
5. Economic Factors 52
6. Environmental Factors 54
D. CONVERSION TO COAL FUEL FROM OIL AND NATURAL GAS 60
1. Background 60
2. Coal-Firing Factors 61
3. Conversion to Coal Firing 61
4. Physical Facilities Required 66
5. Economic Aspects 67
6. Environmental Aspects 68
V. IMPLICATIONS OF POTENTIAL INDUSTRY/PROCESS CHANGES 79
A. SUSPENSION PREHEATER AND FLASH CALCINER 79
1. Environmental and Energy Impact 79
2. Systems Implications 79
3. Probability of Change 79
B. FLUIDIZED-BED CEMENT PROCESS 80
1. Environmental and Energy Impact 80
2. Systems Implications 81
3. Probability of Change 81
C. CONVERSION TO COAL FUEL FROM OIL AND NATURAL GAS 82
1. Environmental and Energy Impact 82
2. Systems Implications 82
3. Probability of Change 82
APPENDIX A - BASE LINE CEMENT TECHNOLOGY 84
APPENDIX B - BASE LINE PROFILE OF ENERGY USE IN THE CEMENT INDUSTRY 90
vi
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TABLE OF CONTENTS (Cont.)
Page
APPENDIX C - CURRENT POLLUTION PROBLEMS AND EFFECTIVENESS OF
AVAILABLE POLLUTION CONTROL TECHNOLOGY 99
APPENDIX D - FLASH CALCINING SYSTEMS 110
vil
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LIST OF FIGURES
Number Page
IV-1 Schematic Diagram of the Cement Clinker Burning Process 16
IV-2 Schematic Diagram of a Typical Four-Stage Suspension
Preheater 20
IV-3 Representative Four-Stage Suspension Preheater Systems
Presently Being Offered by the Machinery Industry 20
IV-4 Alkali and Chloride Cycles in Four-Stage Suspension
Preheater Kiln 23
IV-5 Total Capital Costs of Various Clinkering Sections 27
IV-6 Flash Calcining System with Combustion Air for Precalciner
Drawn up through Kiln 36
IV-7 Detail of Kiln for Scientific Design Fluid-Bed Cement
Process 41
IV-8 Material Balance Around the Reactor 43
IV-9 Scientific Design Fluid Bed Cement Process 49
IV-10 Total Capital Costs of Various Clinkering Sections,
April, 1975 52
IV-11 Comparison of NOX Emissions from Fluidized-Bed Reactor
and Rotary Kiln 58
IV-12 Basic Elements in the Systems Installed to Convert
to Coal Firing 65
B-l Types of Energy Used by the U.S. Portland Cement
Industry, 1974 90
B-2 Trends in Types of Energy Used, 1950-1974 92
B-3 Trends in Fuel and Electricity Use, 1950-1974 93
viii
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LIST OF FIGURES (Cont.)
Number Page
B-4 Trends in Energy Consumption by Process Step, 1950-1974 95
B-5 Trends in Unit Energy Use For Wet and Dry Processing,
1950-1970 96
B-6 Distribution of Unit Energy Consumption by Number of
Plants, 1974 96
B-7 Percent Distribution of Unit Energy Consumption,
1972 and 1974 97
ix
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LIST OF TABLES
Number Page
1-1 Summary of 1971 Energy Purchased in Selected Industry Sectors 2
II-l Summary of Costs/Energy/Environmental Aspects of Process
Options in the Portland Cement Industry . 7
II-2 Summary of Results of Process Options in the Portland
Cement Industry 8
IV-1 Chemical Analyses of Raw Meal, Ash, Clinker and Dust for
Long Wet-Process Kiln 17
IV-2 History of U.S. Sales of Four-Stage Suspension Preheaters,
1953-1973 23
IV-3 World and U.S. Data on Suspension Preheater Kilns 24
IV-4 Portland Cement Production Cost: Suspension
Preheater/Flash Calciner Kiln 28
IV-5 Portland Cement Production Cost: Long Rotary
Kiln (Oil-Fired) 29
IV-6 Comparison of Typical Energy Requirements for Suspension
Preheater and Long Kiln 30
IV-7 Operating Costs for Air Pollution Control System: Long
Rotary Kiln System (Dry-Process/No Insulation) 32
IV-8 Operating Costs for Air Pollution Control System: Long
Rotary Kiln System (Dry-Process/Insulated Lining) 32
IV-9 Operating Costs for Air Pollution Control System: Four-
Stage Preheater Kiln System 33
i
IV-10 Basis for Operating Cost Estimates for Air Pollution Control
in Cement Manufacturing 33
IV-11 Wastewater Treatment Costs: Suspension Preheater/Flash
Calciner 35
IV-12 Optimum Particle Size Distribution for Bed of Clinker
Particles 42
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LIST OF TABLES (Cont.)
Number Page
IV-13 Comparison of Typical Energy Requirements for Fluidized-
Bed Process and Long Kiln 46
IV-14 Portland Cement Production Cost: Fluidized-Bed Cement
Process 53
IV-15 Operating Costs for Air Pollution Control System:
Fluidized-Bed Cement Process 55
IV-16 Wastewater Treatment Costs: Fluidized-Bed 59
IV-17 Required Pulverized-Fuel Fineness at Maximum Rating 61
IV-18 Maximum Sulfur Specifications for Cement in Selected Countries 64
IV-19 Plant Characterization Data for Three Amcord Plants
Converted to Coal Fuel 67
IV-20 Portland Cement Production Cost: Long Rotary Kiln
(Coal-Fired) 69
IV-21 Typical Composition of Dried Kiln Dust 71
IV-22 Composition of West Virginia Coal Ash 73
IV-23 Particle Size' Analysis and Distribution of Alkalies in a
Specimen Kiln Dust from an Electrostatic Precipitator 76
IV-24 Wastewater Treatment Costs: Coal Firing 78
A-l Types and Quantities of Raw Materials used in Producing
Portland Cement in the United States, 1972-1973 85
A-2 Types of Portland Cement Shipped in the United States 1974 89
B-l Clinker Produced in the U.S. by Kind of Fuel, 1974 91
B-2 Electrical Energy used in Portland Cement' Manufacture, 1974 93
B-3 Energy Use by Process Step 94
B-4 Energy Efficiency 98
B-5 Energy Consumption by Type of Kiln 98
xi
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LIST OF TABLES (Cont.)
Number Page
C-l Sulfur Dioxide Emission Factors for Cement Kilns 102
C-2 Reported Cooling Water Usage in Cement Plants 104
C-3 Water Usage for the Cement Industry 105
C-4 Comparison of Waste Loadings for Leaching and Nonleaching
Subcategories 106
C-5 Wastewater Treatment Costs: Base Case Cement Plant 109
xii
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ACKNOWLEDGMENTS
This study could not have been accomplished without the support of a
great number of people in government agencies, industry, trade associations
and universities. Although it would be impossible to mention each individual
by name, we would like to take this opportunity to acknowledge the particular
support of a few such people.
Dr. Herbert S. Skovronek, Project Officer, was a valuable resource to us
throughout the study. He not only supplied us with information on work
presently being done in other branches of EPA and other government agencies,
but served as an indefatigable guide and critic as the study progressed. His
advisors within EPA, FEA, DOC, and NBS also provided us with insights and
perspectives valuable for the shaping of the study.
During the course of the study we also had occasion to contact many
individuals within industry and trade associations. Where appropriate we
have made reference to these contacts within the various reports. Frequently,
however, because of the study's emphasis on future developments with compara-
tive assessments of new technology, information given to us was of a confiden-
tial nature or was supplied to us with the understanding that it was not to be
credited. Therefore, we extend a general thanks to all those whose comments
were valuable to us for their interest in and contribution to this study.
Finally, because of the broad range of industries covered in this study,
we are indebted to many people within Arthur D. Little, Inc. for their parti-
cipation. Responsible for the guidance and completion of the overall study were
Mr. Henry E. Haley, Project Manager; Dr. Charles L. Kusik, Technical Director;
Mr. James I. Stevens, Environmental Coordinator; and Ms. Anne B. Littlefield,
Administrative Coordinator.
i
Members of the environmental team were Dr. Indrakumar L. Jashnani,
Mr. Edmund H. Dohnert and Dr. Richard Stephens (consultant).
Within the individual industry studies we would like to acknowledge the
contributions of the following people'.
Iron and Steel; Dr. Michel R. Mounier, Principal Investigator
Dr. Krishna Parameswaran
Petroleum Refining; Mr. R. Peter Stickles, Principal Investigator
Mr. Edward Interess
Mr. Stephen A. Reber
Dr. James Kittrell (consultant)
Dr. Leigh Short (consultant)
xi'ii
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Pulp and Paper;
Olefins:
Ammonia:
Aluminum:
Textiles:
Cement:
Glass:
Chlor-Alkali:
Phosphorus/
Phosphoric Acid;
Primary Copper;
Fertilizers:
Mr. Fred D. lannazzi, Principal Investigator
Mr. Donald B. Sparrow
Mr. Edward Myskowski .(consultant)
Mr. Karl P. Pagans
Mr. G. E. Wong
Mr. Stanley E. Dale, Principal Investigator
Mr. R. Peter Stickles
Mr. J. Kevin O'Neill
Mr. George B. Hegeman
Mr. John L. Sherff, Principal Investigator
Ms. Nancy J. Cunningham
Mr. Harry W. Lambe
Mr. Richard W. Hyde, Principal Investigator
Ms. Anne B. Littlefield
Dr. Charles L. Kusik
Mr* Edward L. Pepper
Mr. Edwin L, Field
Mr* John W. Rafferty
Dr. Douglas Shooter, Principal Investigator
Mr* Robert M. Green (consultant)
Mr* Edward S* Shanley
Dr* John Willard (consultant)
Dr.. Richard F* Heitmiller
Dr, Paul A. Huska, Principal Investigator
Ms. Anne B. Littlefield
Mr., J., Kevin O'Neill
Dr. D, William Lee, Principal Investigator
Mr* Michael Rossetti
Mr
Mr
Dr* Ravindra M, Nadkarni
Mr. Roger E. Shamel, Principal Investigator
Mr, Harry W. Lambe
Mr*. Richard P. Schneider
Mr. William V. Keary, Principal Investigator
Mr. Harry W. Lambe
Mr. George C. Sweeney
Dr., Krishna Parameswaran
Dr. Ravindra M. Nadkarni, Principal Investigator
Dr* Michel R. Mounier
Dr. Krishna Parameswaran
Mr* John L. Sherff, Principal Investigator
Mr. Roger Shamel
Dr. Indrakumar L. Jashnani
* R* Peter Stickles
* Edward Interess
xiv
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ENGLISH-METRIC (SI) CONVERSION FACTORS
To Convert From
To
Metre2
Pascal
3
Metre
t Joule
Pascal-second
Degree Celsius
Degree Kelvin
Metre
3
Metre /sec
3
Metre
Metre2
Metre/sec
2
Metre /sec
I) Metre3
Ibf/sec) Watt
.c) Watt
Watt
Metre
Joule
Metre3
Metre
Metre
i Metre
Pascal-second
Newton
Kilogram
Kilogram
Kilogram
Kilogram
Kilogram
Kilogram
Multiply By
4,046
101,325
0.1589
1,055
0.001
t^ - (tj -32]
0.3048
0.0004719
0.02831
0.09290
0.3048
0.00002580
0.003785
745.7
746.0
735.5
0.02540
3.60 x 106
1.000 x 10"3
1.000 x 10"6
0.00002540
1,609
0.1000
4.448
0.4536
0.02916
1,016
1,000
907.1
1,000
Acre
Atmosphere (normal)
Barrel (42 gal)
British Thermal Unit
Centipoise
Degree Fahrenheit
Degree Rankine
Foot
3
Foot /minute
Foot
2
Foot
Foot/sec
2
Foot /hr
Gallon (U.S. liquid)
Horsepower (550 ft-1
Horsepower (electric)
Horsepower (metric)
Inch
Kilowatt-hour
Litre
Micron
Mil
Mile (U.S. statute)
Poise
Pound force (avdp)
Pound mass (avdp)
Ton (assay)
Ton (long)
Ton (metric)
Ton (short)
Tonne
Source: American National Standards Institute, "Standard-Metric Practice
Guide," March 15, 1973. (ANS72101-1973) (ASTM Designation E380-72)
xv
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I. INTRODUCTION
A. BACKGROUND
Industry in the United States purchases about 27 quads* annually, approxi-
mately 40% of total national energy usage.** This energy is used for chemical*
processing, raising steam, drying, space cooling and heating, process stream
heating, and miscellaneous other purposes.
In many industrial sectors energy consumption can be reduced significantly
by better "housekeeping" (i.e., shutting off standby furnaces, better thermo-
stat control, elimination of steam and heat leaks, etc.) and greater emphasis
on optimization of energy usage. In addition, however, industry can be expected
to introduce new industrial practices or processes either to conserve energy
or to take advantage of a more readily available or less costly fuel. Such
changes in industrial practices may result in changes in air, water or solid
waste discharges. The EPA is interested in identifying the pollution loads of
such new energy-conserving industrial practices or processes and in determin-
ing where additional research, development, or demonstration is needed to
characterize and control the effluent streams.
B. CRITERIA FOR INDUSTRY SELECTION
In the first phase of this study we identified industry sectors that have
a potential for change, emphasizing those changes which have an environmental/
energy impact.
Industries were eliminated from further consideration within this assign-
ment if the only changes that could be envisioned were:
• energy conservation as a (result of better policing or "housekeeping,"
• better waste heat utilization,
• fuel switching in steam raising, or
• power generation.
*1 quad = 1015 Btu
**Purchased electricity valued at an approximate fossil fuel equivalence of
10,500 Btu/kWh.
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After discussions with the EPA Project Officer and his advisors, industry
sectors were selected for further consideration and ranked using:
• Quantitative criteria based on the gross amount of energy (fossil
fuel and electric) purchased by industry sector as found in U.S.
Census figures and from information provided from industry sources.
The cement industry purchased 0.52 quads out of the 12.14 quads pur-
chased in 1971 by the 13 industries selected for study, or 2% of the
27 quads purchased by all industry (see Table 1-1).
• Qualitative criteria relating to probability and potential for proc-
ess change, and the energy and effluent consequences of such changes.
In order to allow for as broad a coverage of technologies as possible, we
then reviewed the ranking, eliminating some industries in which the process
changes to be studied were similar to those in another industry planned for
study. We believe the final ranking resulting from these considerations identi-
fies those industry sectors which show the 'greatest possibility of energy con-
servation via process change. Further details on this selection process can be
found in the Industry Priority Report prepared under this contract (Volume II).
On the basis of this ranking method, the cement industry appeared in eighth place
among the 13 industrial sectors listed.
TABLE 1-1
SUMMARY OF 1971 ENERGY PURCHASED IN SELECTED INDUSTRY SECTORS
SIC Code
•15 In Which
Industry Sector 10 Btu/Yr Industry Found
1. Blast furnaces and steel mills 3.49(1) 3312
2. Petroleum refining 2.96*2' 2911
3. Paper and allied products 1.59 26'
4. Olefins 0.984<3) 2818
/£.)
5. Ammonia 0.63V ' 287
6. Aluminum 0.59 3334
7. Textiles 0.54 22
8. Cement 0.52 3241
9. Glass 0.31 3211, 3221, 3229
10. Alkalies and chlorine 0.24 2812
11. Phosphorus and phosphoric ,g,
acid production 0.12V 2819
12. Primary copper 0.081 3331
13. Fertilizers (excluding ammonia) 0.078 287
Estimate for 1967 reported by FEA Project Independence Blueprint,
p. 6-2, USGPO, November 1974.
Includes captive consumption of energy from process byproducts
(FEA Project Independence Blueprint)
Olefins only, includes energy of feedstocks:- ADL estimates
(4)
Amonia feedstock energy included: ADL estimates
ADL estimates
Source: 1972 Census of Manufactures, FEA Project Independence Blueprint,
USGPO, November 1974, and ADL estimates.
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C. CRITERIA FOR PROCESS SELECTION
Within each of the 13 industry sectors, there are a variety of potential
changes in industrial practice. In this study we have focused on identifying
changes in the primary production processes which have clearly defined pollu-
tion consequences. In selecting those to be included in this study, we have
considered the needs and limitations of the EPA as discussed more completely
in the Industry Priority Report mentioned above. Specifically, energy conser-
vation has been defined broadly to include, in addition to process changes,
conservation of energy or energy form (gas, oil,.coal) by a process or feed-
stock change. Natural gas has been considered as having the highest energy
form value followed in descending order by oil, electric power, and coal. Thus,
a switch from gas to electric power would be considered energy conservation
because electric power could be generated from coal, existing in abundant
reserves in the United States when compared to natural gas. Moreover, pollu-
tion control methods resulting in energy conservation have been included within
the scope of this study. Finally, emphasis has been placed on process changes
with near-term rather than long-term potential within the 15-year span of time
of this study.
In addition to excluding from consideration better waste heat utilization,
"housekeeping," power generation, and fuel switching, as mentioned above, cer-
tain options have been excluded to avoid duplicating work being funded under
other contracts and to focus this study more strictly on "process changes."
Consequently, the following have also not been considered to be within the
scope of work:
• Carbon monoxide boilers (however, unique process vent streams yielding
recoverable energy could be mentioned);
• Fuel substitution in fired process heaters;
• Mining and milling, agriculture, and animal husbandry;
• Substitution of scrap (such as iron, aluminum, glass, reclaimed tex-
tiles, and paper) for virgin materials;
• Production of synthetic fuels from coal (low- and high-Btu gas, syn-
thetic crude, synthetic fuel oil, etc.); and
• All aspects of industry-related transportation (such as transportation
of raw material).
D. SELECTION OF CEMENT INDUSTRY PROCESS OPTIONS
Within each industry, the magnitude of energy use was an important crite-
rion in judging where the most significant energy savings might be realized,
since reduction in energy use reduces the amount of pollution generated in the
energy production step. Guided by this consideration, candidate options for
in-depth analysis were identified from the major energy consuming process steps
with known or potential environmental problems.
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After developing a list of candidate process options, we assessed
subjectively
• pollution or environmental consequences of the process- change,
• probability or potential for the change, and
• energy conservation consequences of the change.
Even though all of the candidate process options were large energy users,
there was wide variation in energy use and estimated pollution loads between
options at the top and bottom of the list. A modest process change in a major
energy consuming process step could have more dramatic energy consequences than
a more technically significant process change in a process step whose energy
consumption is rather modest. For the lesser energy-using process steps
process options were selected for in-depth analysis only•if a high probability
for process change and pollution consequences was perceived.
Because of the time and scope limitations for this study, we have not
attempted to prepare a comprehensive list of process options or to consider all
economic,, technological,, institutional, legal or other factors affecting imple-
mentation of these changes. Instead we have relied on our own background exper-
ience, industry contacts, and the guidance of the Project Officer and EPA
advisors to choose eight promising process options (with an emphasis on near-
term potential) for study in the cement industry:
• Suspension preheater
• Flash calciner
• Fluidized-bed cement process
• Conversion to coal fuel from oil and natural gas
• Roller mill for raw material grinding
• Oxygen enrichment of kiln combustion air
• New cement process which uses no pyroprocessing step
• Use of slag and other pozzolanic additives to portland cement.
After discussion with the EPA Project Officer, his advisors, and industry
representatives, the first four of these options were chosen for in-depth
analysis because:
I
• They represent technology that can be implemented in the near term,
• The promise of energy savings is significant, and therefore could
'motivate the industry to implement the technology, and
• There is a recognized or expected effect upon effluent streams with
attendant environmental impact.
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In this study, the cement industry description is based on 1974, the
latest representative year for the industry for which we had good statistical
information. Recognizing that capital investments and energy costs have esca-
lated rapidly in the past few years and have greatly distorted the traditional
basis for making cost comparisons, we developed costs representative of the
first half of 1975, using constant 1975 dollars for our comparative analysis of
new and current processes.
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II. FINDINGS, CONCLUSIONS, RECOMMENDATIONS
A. APPROACH
The changes in portland cement technology and cement industry practices
examined in this study have an effect on only one of the major cement-making
process steps, the clinkering, or cement burning, step. The major new develop-
ments in cement technology and plant practice are focused on this single proc-
ess step because it uses 70-80% of the total energy required for cement
manufacture.
Although some major departures from conventional cement making by pyro-
processing are currently under investigation, the only changes with a reasonably
high probability of implementation during the next 15 years are those which
exist today and are the product of a considerable number of years of active
development. Further, these new technologies or alternatives to present state-
of-the-art in clinker production, with the exception of the production of
cement clinker in a fluidized-bed reactor, are being implemented in commercial-
scale facilities around the world.
Since the four process or practice alternatives considered in this study
(suspension preheater, flash calciner, fluidized-bed reactor, and conversion to
coal fuel) affect only the cement clinker production step, the design, layout,
fixed capital investment, operating costs, effluent streams, and environmental
aspects of the other processing steps will be essentially unchanged for cement
plants operating today, and modified or new plants which employ these process
or practice alternatives. Therefore, in this study we compare only the affected
clinker production step represented by the current long rotary kiln (base line)
and the alternative processes or practices: suspension preheater, flash cal-
ciner, fluidized-bed reactor, and conversion to coal fuel.
This study and analysis have shown that the quantities and compositions
of the various effluent and process streams associated with these alternative
processes and practices are essentially the same as those associated with the
long rotary kiln (Tables II-l and II-2). In all cases, a hydrocarbon fuel is
burned with air to generate the heat required for cement clinker production.
These combustion gases carry dust and volatilized elements from the reacpor,
(i.e., rotary kiln or fluidized bed). The percent excess air, the chemical;
composition and the particle distribution of the particulates will change, but
it appears that no new species of pollutants and no new effluent streams are
created.
-------
TABLE II-l
SUMMARY OF COSTS/ENERGY/ENVIRONMENTAL ASPECTS OF
PROCESS OPTIONS IN THE PORTLAND CEMENT INDUSTRY
Base Line Process: New Cement Plant, Dry Process, Long Rotary Kiln
PROCESS OPTIONS
COSTS
SUSPENSION PREHEATER
Lower capital cost. Lower
operating cost. Lower
pollution control costs.
FLASH CALCINER
Lower capital cost; about
the same as suspension pre-
heater. Lower operating
cost; lower pollution con-
trol costs.
FLUIDIZED BED
Lower capital cost; lowest
of these options. Lower
operating cost. Pollution
control costs about the
same as base line process.
COAL FUEL
Higher capital cost due to
coal storage & handling.
Slightly lower operating cost
due to lower fuel cost. Higher
pollution control costs due to
coal storage & handling.
ENERGY Lower process energy re-
quirements, primarily due
to significantly lower
fuel energy. About 20-
25% overall energy
saving.
Lower process energy re-
quirements, primarily due
to significantly lower
fuel energy. About 20-
25% overall energy
saving.
Lower process energy due
to generation of total
electrical energy require-
ments from reactor exit
gases.
Conservation due to use of coal
instead of natural gas and oil
for heat energy.
ENVIRONMENT No change, except that
waste dust recycled if
alkali specifications
in cement product can be
met.
Lower N0x> Other aspects
.are about the same as with
suspension preheater. All
waste dusts recycled if
alkali specifications can
be met.
Significantly less waste
dust. Process has poten-
tial for converting waste
rotary kiln dust into cement,
which can reduce pollution
control costs at existing
plants. Lower NO . Waste
dust almost pure alkali
salts, with potential by-
product value.
Fugitive emissions and rainwater
runoff from coal handling and
storage cause additional air and
Water pollution control costs.
Coal ash combines with cement raw
materials reducing environmental
problems.
-------
TABLE I1-2
SUMMARY OF RESULTS OF PROCESS OPTIONS IN THE PORTLAND CEMENT INDUSTRY
(Basis: 1350 Tons Cement Per Day)
Base Line: Long Kiln (Oil) - Energy Consumption from 3.4 to 6x10^ Btu/Ton
PRODUCTION FACILITY
Fixed Capital Investment ($106)
Production Cost ($/ton)
Energy Requirements (106 Btu/ton)
ENVIRONMENTAL CONTROL
FACILITIES
Fixed Capital Investment ($106)
Operating Cost ($/ton)
Energy Requirements (106 Btu/ton)
PRODUCTION PLUS ENVIRONMENTAL
CONTROL FACILITIES
Fixed Capital Investment ($106)
Operating Cost ($/ton)
Energy Requirements (10 Btu/ton)
Long Kiln Suspension Preheater FluidJ.zed Long Kiln
(Oil) & Flash Calciner Bed (Coal)
42 40 38 45
47.81 43.71 44.30 45.56
5.6 4.2 5.0- 5.6
1.6 1.2 1.9 2.0
1.97 1.40 2.10 2.27
.069 .047 0.102 .069
43.6 41.2 39.9 47.0
49.78 45.11 46.40 47.83
5.7 4.2 5.1 5.7
The available literature presents insufficient data to permit us to compare
the dust or particulate emissions from the clinkering step as a function of the
chemical composition 'and nature of the raw materials and fuel inputs (especially
the composition of the coal ash, or the mineral impurities in the coal burned
as fuel) for each of the various alternative technologies. Also, no data are
available to indicate the composition of the gases emitted to the atmosphere
after passage through a suitable dust collector, such as a glass fabric filter
or an electrostatic precipitator. Therefore, this final but important aspect
of specific elemental or component material balancing cannot be accomplished.
(See Section II-C for recommended research and development areas.)
B. POTENTIAL CONFLICT WITH ENVIRONMENTAL REGULATIONS
The changes in both cement industry practice and process technology which
we have studied will not result in any potential-conflict with the environmental
regulations (see Appendix C). Significant environmental aspects of the indus-
trial practice and process changes studied are summarized in the following
subsections.
1. Suspension Preheater
Outside the United States, the suspension preheater-equipped rotary kiln
is a well-developed, established cement clinker production step. Although it
gained rapid acceptance in the United States in the 1950*s, this clinkering
-------
alternative fell into total disfavor with the U.S. cement industry because of
problems with the operation and the quality of the cement. However, the present
high fuel costs combined with continued and apparently successful development
and operation of the suspension preheater has led to its recent reacceptance.
Due to extensive experience with actual commercial-scale operation in a large
number of plants throughout the world, the environmental aspects of the suspen-
sion preheater-equipped rotary cement kiln are quite well known.
Suspension preheater-equipped cement plants are dry process plants, and
therefore, have no process water discharge, typically, suspension preheater-
equipped plants operate with a total dust return to the clinkering step, and
therefore have no problem with disposal of waste kiln dust. Occasionally, to
meet alkali specifications in the finished cement, preheater kilns are operated
with a bypass of some of the kiln exit gases. The dust collected from this
bypass is discarded, since it is high in alkali content, and thereby provides
an alkali purge stream from the process. The quantities of particulates and S02
.from a suspension preheater kiln are well known and present no more problems in
either magnitude or nature than those with which the cement industry is already
familiar.
If waste kiln dust from a suspension preheater bypass system is discarded,
its physico-chemical nature should lie within the range of characteristics of
kiln dust from cement plants now operating in the United States. Therefore,
the rain water run-off and leaching problems associated with the disposal of
waste kiln dust from such a system should also be no different than those asso-
ciated with the disposal of kiln dust from plants now operating.
2. Flash Calciner
This is a significant new variation of the suspension-preheater rotary
kiln which has gained wide acceptance in Japan and Europe. The first commer-
cial installation in the United States is nearing completion. Since the flash
calciner is a dry process, the same observations and comments we made on the
suspension preheater are applicable. Approximately 50% of the total fuel
required for the clinker production step is burned at a relatively low tempera-
ture, with a low percent excess combustion air and quite uniform combustion gas
composition throughout the combustion chamber. It has been reported that these
characteristics are responsible for the NOX produced by a flash-calciner-equipped
kiln being significantly lower thaSn for either the suspension preheater or long
rotary kiln.
The particulates and SC>2 emissions from the flash-calciner-equipped rotary
kiln are expected to be approximately the same as those from a suspension pre-
heater, except when part or all of the rotary kiln combustion gas bypasses the
flash-calciner and suspension-preheater vessels in order to produce low alkali
cement. Although no data are available on the efficiency of the collection
within the rotary kiln of S02 by its chemical reaction with the calcined, cement-
making raw materials fed from a flash calciner to form calcium, potassium, and
sodium sulfates, the efficiency is expected to be quite high. Therefore, there
is a possibility that the S(>2 emissions from a partial or total bypass system
may be in conflict with air pollution regulations.
-------
3. Fluidized-Bed Cement Process
The fluidized-bed cement process utilizes a fluidized-bed reactor, rather
than a rotary kiln, for the production of portland cement clinker. Although
no commercial plant has yet been built using the fluidized-bed clinkering reac-
tor, a semi-commercial-scale plant of 100-ton-per-day (tpd) capacity was built
and operated successfully for a period of several years.
The reported data indicate that the combustion gases leaving the fluidized-
bed reactor are as low in S02 as those of a rotary kiln and are significantly
lower in particulates and NOX. In fact, the particulates consist almost entirely
of water-soluble potassium and sodium sulfates. This suggests that these par-
ticulates, when collected, could prove to be a valuable byproduct, or inter-
mediate product. The fluidized-bed cement process is a dry process, and there-
fore has none of the process water effluent which is common to the conventional
wet process plant.
This process, offered by two U.S. firms to the cement industry, employs
the generation of steam as one mode of process heat recovery and is reported to
be equivalent in overall thermal efficiency to the suspension preheater-equipped
rotary kiln, which exhibits the highest thermal efficiency (and consequently
the lowest Btu consumption per ton of cement produced) of any of the available
rotary kiln-type cement clinkering process alternatives.
All other things, such as the chemical and physical characteristics of the
raw material, being constant, the fluidized-bed clinkering reactor will produce
cement clinker of significantly lower alkali concentration than any of the
rotary-kiln-type clinkering processes. This results from the significantly
higher alkali volatilization in the fluidized-bed reactor and the indirect means
of heat recuperation from the hot combustion gases exiting the reactor compared
with the direct heat recuperation by raw material particles in the rotary-kiln
types of clinkering process alternatives.
Therefore, most of the particulates contained in the combustion gases
leaving the fluidized-bed reactor are quite different from those from any of
the rotary kil.n-type processes. Approximately 97% are water-soluble potassium
and sodium sulfate, and the remaining 3% are finished clinker particles. Also,
since the extent of alkali volatilization in the fluidized-bed process is sig-
nificantly greater than in the rotary kiln-type clinkering process, the quantity
of alkali sulfate emitted in the effluent combustion gas stream will be signifi-
cantly higher than in a comparable rotary kiln-type clinkering process, maybe
two or three times higher. Since most of these particulates are alkali sulfates
which have been volatilized from the clinkering raw materials, they are expected
to be extremely fine and are more appropriately defined as a fume. Although no
specific data are reported concerning the operation of such a collection device,
glass cloth filters should suitably collect these particulates. The total
pounds of particulates emitted per ton of cement clinker produced is expected
to be considerably less than from any of the rotary kiln-type clinkering proc-
esses. This should be a significant benefit of this process in the discarding
of alternative disposal of the particulates, especially if these alkali sul-
fates have a value as a chemical raw material or plant nutrient, for example.
10
-------
Actual data obtained from the operation of a pilot-scale fluidized-bed
cement reactor show that the NOX concentration in the combustion gases is sig-
nificantly less than from an equivalent rotary kiln process. The reasons for
this are that the fluidized-bed reactor operates at a lower temperature and
the fuel in the fluidized-bed reactor can be burned with only a very small
quantity of excess air. Also, the high heat and mass transfer rates which are
exhibited by fluidized beds reduce oxygen concentration gradients within the
gas phase to very low levels.
4. Conversion to Coal from Natural Gas and Oil
Although this is not a process change per se, as a change to a lower value
fuel, it is within the scope of our study. Until recently, 45% of the cement
produced in the United States came from cement plants using natural gas as fuel
and 15% from plants using oil. Approximately 40% of cement was produced using
coal fuel. Pulverized coal can be successfully burned as the fuel in any of the
current rotary kiln cement installations in the United States. It appears
that industry is presently converting its kiln-firing systems to coal. The two
main environmental consequences of switching from natural gas or oil to coal
are:
• Fugitive particulate emissions and rainwater run-off which come from
the storage and handling of coal and
• The presence of coal fly ash in kiln dust which is wasted or discarded.
Coal-fired steam electric generating facilities handle and store large
quantities of coal. The equipment and handling techniques used by these utili-
ties should prove equally satisfactory for the control of fugitive emissions
which will attend the use of coal in cement plants. The presence of coal fly
ash in the kiln dust will increase the number of elements, and possibly their
concentration in the dust. However, this does not appear to be in potential
conflict with any environmental regulations.
C. ADDITIONAL RESEARCH
During this study, we identified several areas in which additional data or
information would have been helpful. This forms the basis of our recommenda-
tions for additional research into current or future processes and industry
practices in the United States port.land cement industry, i.e.:
(1) Develop and implement a program to sample and analyze dust from var-
ious kiln systems, especially those burning coal, in order to corre-
late the trace elements, especially the heavy metals in the dust
wasted, with the presence of those elements or constituents in the
raw materials and coal burned.
(2) Develop and implement a test program at a number of cement plants
with clinkering facilities employing long-rotary-kiln, suspension
preheater, or flash-calciner processes burning coal as the fuel.
11
-------
Coal of various sulfur levels should be tested to determine the effect
on operation of the level and nature of sulfur in gas, dust, and
clinker. The benefits which derive from the physical and/or chemical
cleaning of coal to reduce pyritic sulfur levels in coal for cement
manufacturing could also be quantified. '
(3) Develop and implement a program to analyze, and study ways of using
waste kiln dust (for example, as a soil conditioner or plant nutrient,
or as the primary or major raw material feed component to the
fluidized-bed cement process).
(4) Develop and implement a commercial-scale test program on one or more
flash-calciner-equipped rotary-kiln cement-making facilities to
characterize the gaseous and particulate emissions. Of particular
interest would be the emissions from operating with a bypass of a
considerable amount of the combustion gases to eliminate alkalies.
12
-------
III. INDUSTRY OVERVIEW*
In 1974, 53 companies in 41 states and Puerto Rico produced more than 79
million short tons of cement, which brought about $2.1 billion in net sales.
Of all the hydraulic cement products shipped, more than 90% was portland cement;
the remainder was masonry, natural, or pozzolanic cement.
In general, cement companies market locally, where they may compete with
as many as 15 to 20 pther companies. Currently, all but 10 states have one or
more cement plants. Some companies have as many as 14 plants. Since cement
has a high weight-to-value ratio, it is generally transported on land by rail
or truck over a radius of 200-300 miles -surrounding the cement plant. For com-
panies with access to water transportation, market areas are extended consider-
ably beyond this radius. Distribution terminals are a vital part of cement
marketing and transportation. Corresponding with the increase in excess capa-
city that started about 1959, the number of distribution terminals increased
rapidly. By 1964, 164 new terminals., which accounted for more than 20% of all
direct shipments to customers, had been built. This change reflected the inten-
sified efforts of cement producers to hold or increase sales by being able to
provide faster service.
One method of lowering distribution costs for cement distribution is to
increase the use of water transportation. Three major cement-consuming areas
where water transportation is possible are: (1) the East Coast and the Hudson
River, (2) along the Great Lakes, and (3) along the Mississippi River. The
plants in these areas are among the largest in operation.
Plant age is difficult to define since a single plant often has major
processing equipment with different ages. In the cement industry, 168 plants
operate 434 kilns, the major piece of equipment. Almost half (47%) the kilns
now operating have been built since j!955. They provide 68% of the total cement-
producing capacity.
A major factor in determining plant size is the cost of distribution from
a plant location. This cost depends on the demand centers which the plant
might supply, the distances involved, and the types of transportation to which
the plant would have access. Larger and fewer kilns per plant offer several
cost-saving opportunities, relating to fuel economies, labor economies, quality
control, and ease of automation.
*See Appendix A for supporting data.
13
-------
Among world producers of hydraulic cement, the United States ranks third.
In 1973, the United States produced 11% of the total world production. While
world production of hydraulic cement has grown at 7.9% per year, U.S. produc-
tion has grown at 2.0% per year.
Cement is manufactured via two processes. In the dry process the raw
materials are dried before being ground and blended; in the wet process, water
is added to the raw materials, which are then ground wet. Although more than
half of all cement was produced by the wet process in 1974, this fraction has
been declining since 1968. The dry process production has grown to 32.8 mil-
lion short tons of cement clinker in 1974. This growth was largely a result of
increased fuel prices, since the dry process consumes less energy. Despite its
position as one of the world's largest cement producers, the United States has been
importing cement and clinker at an increasing rate during the last ten years.
Imports in 1974 totaled 5.7 million short tons. The major source of imports is
Canada, which contributes 39% of all imports, followed by the Bahamas with 14%,
Norway with 12%, and the United Kingdom with 11%.
Imports in 1974 declined 14% from the peak level of 6.6 million short tons
in 1973. Various factors, such as decreasing domestic demand and increased
prices of imports, affected the decline. Bulk clinker is contributing an
increasing percentage to the total cement imports.
Cement is sold primarily to ready-mix concrete producers who subsequently
sell concrete to various contractors. In 1974, ready-mix concrete producers
consumed 66% of the total cement shipped by domestic producers. The next
largest consumer, concrete product manufacturers, used 14% of the total shipped
to make concrete block and pipe and precast, prestressed concrete, among other
products.
Because of the relatively high level of fixed costs associated with cement
production, the industry's rate of capacity utilization correlates closely with
profitability. The 1950's were profitable years for the cement industry. When
the rate of utilization peaked at 94% in 1955, the highest'rate of return,
18.6%, was achieved. This profit rate was 25% above the profit rate of all
manufacturing companies for that year.
Attracted by the high profits of the 1950's, established firms expanded
capacity, but capacity expanded far more rapidly than demand. Between 1950
and 1968, production rose 74% from 43 to almost 75 million short tons,
while capacity rose 100% to its peak level of almost 96 million short tons
in 1968.
From 1970 to 1972, the cement industry operated at nearly 90% of its
capacity—the spread between supply and demand was narrowed to the point where
a d'efinite shortage existed. By 1974, demand declined due to depressed housing
construction activity, increased inflation, and an uncertain national economy.
Faced with the prospect of continued low returns, a growing number of what
were once predominantly cement firms began to diversify. Vertical integration
with cement's leading market, ready-mixed concrete producers, is relatively new.
Before 1956, only 2 cement companies operated ready-mixed concrete facilities;
by March 1966, the number had grown to 19.
14
-------
In addition to integrating vertically, cement companies have: (1) inte-
grated horizontally with other cement producers in the same market area, (2)
merged with firms to extend market areas, and (3) merged with non-cement com-
panies to extend product lines. Most of the acquisitions were market-extension
mergers.
Largely due to consolidations and acquisitions, the number of cement com-
panies has been declining steadily, going from 94 in 1923 to 51 in 1974. No
single company accounts for more than 7.5% of the total cement production.
While the four largest firms account for nearly 24% of the total capacity, they
are contributing smaller percentages of the total- capacity than they did in
either 1950 or 1964.
Though no cement company serves the entire United States, the largest
firms cover major portions of the country by operating numerous plants. For
example, the four leading firms operate an average of 11 plants.
15
-------
IV. ALTERNATIVE PROCESSES
A. SUSPENSION PREHEATER
1. Process Description
A portland cement rotary kiln consists of essentially three separate
zones (Figure IV-1). The three zones, based on the temperature range, and
the nature of the physical and/or chemical changes or reactions which occur
within them, are:
• preheating
• calcining
• sintering (or clinkering)
Comparing the three zones for the conventional long rotary kiln versus those for
a rotary kiln using a suspension preheater and a flash calciner (Figure IV-1),
we see that there is no sharp demarcation between adjacent zones in the rotary
cement kiln. However, the temperature profile and the chemical composition
of the raw materials in the kiln show approximately where the zones are
(Table IV-1).
60—800°C
750-"950°C
CLINKER PHASE FORMATION
(950~1.460°C)
LONG ROTARY KILN: RAW MEAL.
SUSPENSION PREHEATER:
FLASH CALCINER:
CALCINING
CEMENT CLINKER
40-46% (DEGREE OF CALINATION)
SUSPENSION PREHEATER _l ROTARY KILN
SUSPENSION
PREHEATER
80-90% (DEGREE OF CALCINATION)
"FLASH FURNACE"
ROTARY KILN
Source Seki, M. m al, (1974) (IEEE, 1974 Cement Industry Technical Conference, Mexico City)
Figure IV-1. Schematic Diagram of the Cement Clinker Burning Process
16
-------
TABLE IV-1
CHEMICAL ANALYSES OF RAW MEAL, ASH, CLINKER AND DUST FOR LONG WET-PROCESS KILN
No.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
constituents
insoluble residue
Si02
A12°3
Ti02
P2°3
Fe2°3
Mn2°3
CaO
MgO
SO- total
S
K20
Na20
loss on ignition
sum 1-14
non-volatile oxidic
components^
sum 1-9
C
CaO free
co2
H20 (<100°C)
H20 (>100°C)
raw meal
a
_
12.26
4.30
-
-
1.45
-
43.93
0.61
0.13
-
0.67
0.091
36.78
100.22
62.55
0.07
-
33.64
39.2
3.14
ignited
b
_
19.39
6.80
-
-
2.29
-
69.49
0.97
0.20
-
1.06
0.14
-
100.33
-
0.17
-
-
-
ash
c
_
37.43
20.41
-
traces
28.96
-
4.14
1.75
3.21
-
3.40
0.18
-
99.48
92.69
-
-
-
-
-
clinker
d
0.08
20.19
6.63
0.20
-
3.75
0.20
64.50
0.98
1.89
0.00
1.07
0.087
0.16
99.74
96.53
-
1.02
-
-
-
ignited
e
0.08
20.22
6.64
0.20
-
3.76
0.20
64.60
0.98
1.89
0.00
1.07
0.087
-
99.73
-
-
1.02
-
-
-
dust in clean gas precipitated dust
f
_.
16.90
5.09
0.21
-
13.28
-
16.58
0.72
15.54
-
9.60
1.70
19.75
99.37
52.78
-
-
-
-
-
ignited
g
21.06
6.34
0.26
-
16.55
-
20.66
0.90
19.37
-
11.96
2.12
-
99.22
-
-
-
-
-
-
h
_
16.10
6.33
0.22
-
5.24
-
35.87
0.57
7.86
-
6.70
0.35
20.20
99.44
98.60
-
0.93
-
-
-
ignited
i
_
20.17
7.93
0.28
-
6.57
-
44.95
0.71
9.85
-
8.39
0.44
99.29
-
-
1.16
-
-
-
Source: Weber, P., "Heat Transfer in Rotary Kilns", Zement-Kalk-Gips, Special Edition, 1973
-------
a. Preheating Zone
Essentially no chemical reactions occur in this zone. The raw material
is completely dried of moisture and its temperature progressively increases
as heat is transferred to the feed material from the hot combustion gases
passing countercurrent to the raw material flow as they exit the cold feed
end o'f the rotary kiln. This heat is transferred by conduction and con-
vection between the gas and the raw material, and also between the hot
refractory brick or castable refractory lining (of the inner surface of the
kiln) in the raw material. The refractory lining of the kiln in the pre-
heating zone is heated by the hot combustion gases; since the rotary kiln
is turning on its longitudinal axis, the hot refractories turn under the bed
of raw material, and thereby transfer heat into the bottom of, the bed by
conduction. During its passage through this preheated zone, the cold raw
material feed is heated to approximately 1400°F.
b. Calcining Zone
The interface between the preheating and the calcining zone is not a
physical one within the rotary kiln proper, but is marked by the onset of
significant thermal decomposition of the calcium carbonate in the raw
material, which constitutes approximately 75% of the raw feed. This thermal
decomposition, or calcination, with consequent liberation of carbon dioxide,
is the first major chemical reaction which occurs, and is the precursor of
a complex series of solid-solid and solid-liquid reactions which are respon-
sible for the ultimate production of the four main portland cement compounds.
c. Clinker ing Zone
After the calcium carbonate has essentially finished decomposing to
calcium oxide and carbon dioxide (the latter carried out of the kiln by the
combustion gases), a series of reactions -between the calcium oxide and the
other components of the raw-material ultimately results in the formation of
the four major portland cement compounds:
• tricalcium silicate (C3S)*
• dicalcium silicate (C2S)
• tricalcium aluminate (C~A)
• tetracalcium alumino-ferrite (C.AF)
*C = CaO, A = A1203, S = S.^, F =
18
-------
The formation of these major cement compounds is an exothermic reaction
which liberates a sizeable quantity of heat in the sintering (clinkering)
zone. This process generates a sufficient liquid phase for the reacting
materials to consolidate to clinker in the form of dense solid modules
which range in size from approximately one-half inch to three inches.
2. Definition
A suspension preheater is a modification to, or an addition to, a cement
rotary kiln. It is attached to the raw feed inlet end of the kiln, totally
replacing the preheating zone of the ro,tary kiln. The preheater is an
assemblage of refractory-lined steel ducts and vessels in which the hot gases
leaving the calcining zone of the rotary kiln contact the incoming cold raw
feed. This is accomplished by mixing the raw feed into the hot combustion
gases flowing at high velocity through the ducts and vessels. The raw
material particles are entrained by the hot gases, resulting in a cloud of
raw material particles carried by the hot gases. This cloud consists of a
uniform dispersion of raw feed particles in intimate contact with the hot
combustion gases. Suspension preheating achieves heat transfer characteristics
both rate and amount - from the hot combustion gases which greatly exceed
those of the simple preheating zone of the conventional cement rotary kiln.
Since the suspension preheater actually replaces the preheating zone of
a cement rotary kiln, it can result in a significant shortening of the rotary
kiln. If an operating, dry-process rotary cement kiln is converted to a sus-
pension preheater kiln, approximately one-half of two-thirds of the original
rotary kiln can be discarded.
In addition to preheating raw material, the suspension preheater also
accomplishes a considerable amount of raw material calcination. Typical
suspension preheaters heat cold raw feed to approximately 1400°F, and accom-
plish 30-40% of the total calcination, or thermal decomposition of the calcium
carbonate, the main component of the raw feed. Consequently, the rotary kiln
receives hot and partially calcined raw material.
There are several variations of the suspension preheater (Figure IV-3);
however, the process concept can be illustrated by the four-stage Humbolt
suspension preheater (Figures IV-2 and IV-3). The key element of this pre-
heater is the combination of a vertical section of ducting with a relatively
small cross-sectional area. The hot combustion gases from the rotary kiln
flow up through this duct, which is refractory-lined to protect the steel
from the high-temperature gases and from abrasion of the solids carried by
those gases. Raw feed is dispersed in the hot gases near the bottom of the
duct. This results in a cloud of fine particles within the high-velocity,
upward-moving stream of hot combustion gases.
Further details on preheaters as well as other process options discussed
can be obtained from the references accompanying the Figures.
19
-------
EXIT GASES
6SO-680°F
DUST: 6-8%
1500°F
2100°F|
1900°F
Source: Norbom, H.R. I.E.E.E. Cement Technical Conference,
Miami. Florida, May 1973.
Figure IV-2. Schematic Diagram of a Typical Four-Stage
Suspension Preheater
FEEDPOINT
STAGE 1
HUMBOIDT
FEEDPOINTS
STAGE 4
FEEDPOINT
FEEDPOINT
MIGG
KRUPP
Source: Garrett, H.M. and J.A. Murray - Rock Products, p. 58, August 1974.
Figure IV-3. Representative Four-Stage Suspension Preheater Systems
Presently Being Offered by the Machinery Industry
20
-------
Because of the very small size of the raw feed particles and the excel-
lent gas/solid contact, the temperature difference between the solids and
the gas is equilibrated within a fraction of a second. The final feed pre-
heat temperature is primarily a function of the ratio of the mass flow rate
of solids and combustion gases.
After the solid particles have extracted the useable heat from the com-
bustion gas stream, the hot feed particles are recovered from that gas stream.
The Humboldt suspension preheater uses a cyclone to accomplish this. The
vertical duct section carrying dust-laden gases makes a 90° bend and tangen-
tially enters a cyclone. The steel cyclone is refractory-lined for temperature-
and abrasion-resistance. The solids discharge from the bottom of the cyclone,
and the gas stream exits through the top center of the cyclone.
What has been described thus far is a single-stage suspension preheater,
which is a commercial variation of the more common four-stage suspension
preheater, and some of these have recently been installed in North America.
The advantage of such a single-stage suspension preheater over the more com-
mon four-stage form is that the single-stage unit recuperates a significant
amount of heat from the hot combustion gases for a minimum of fixed capital
investment, and has less operating and maintenance costs than the four-stage
units.
A four-stage suspension preheater consists of four of these duct/cyclone
unit elements assembled in series. This then provides four separate counter-
current heat transfer stages with greater thermal efficiency than the single-
stage preheater.
When the duct/cyclone unit representing a single-stage suspension pre-
heater is expanded into a series of four separate stages, the resulting
preheater looks like those shown in Figure IV-3.
Stage one, which is located at the top of the unit, consists of two
cyclones in parallel. This provides higher velocities within these cyclones
which generate higher dust collection efficiency and minimize the amount
of raw feed carried to the subsequent dust collector. The raw feed enters
the main vertical duct, which makes a 90° bend, and is divided into two
streams, each of which passes into one of the two first-stage cyclones. The
combustion gas temperature at this point is quite low. -The partially pre-
heated raw feed solids collected by both of the cycloiies in the first-stage
exit through the bottom of these vessels and drop into the gas stream leaving
the third-stage cyclone. This duct is the gas inlet to the second-stage
cyclone. This process is repeated four times, with the final preheated raw
material, partially calcined, and at about 1400°F, passing down into the
feed end of the rotary kiln.
21
-------
3. United States Situation
In the last few years, there has been' a significant renewal of interest
in the suspension preheater in the United States. The design and operation
of the suspension preheater for the United States has evolved along lines
which permit the manufacture of lower-alkali cement clinker and a reduction
in the operating problems due to sticking or clogging of the preheater system,
which is due in part to the presence of alkalies.
One of the key developments specifically for the reduction of the alkali
content in clinker, which also tends to diminish the problem of raw material
sticking in the lower stages of the suspension preheater, is the incorpora-
tion of a bypass between the rotary kiln and the suspension preheater. This
permits the direct removal of some of the combustion gases leaving the rotary
kiln, bypassing some of the combustion gases from the kiln around the pre-
heater. This produces an effective outlet for alkalies, sulfur and any
chlorine present, since the combustion gases at this point in the system
usually contain the highest concentration of these recirculating materials.
The heat efficiency is diminished somewhat through the use of such a bypass.
Compared to the long wet-process and the dry-process rotary kiln systems, the
thermal efficiency of a four-stage suspension preheater with sufficient gas
bypass for use in the United States is still quite attractive because of the
reduction in fixed capital investment and in fuel required.
Figure IV-4 shows the alkali, chloride, and sulfur cycles in a four-stage
Humboldt-type suspension preheater, both with and without a bypass. For the
case with bypass, 15% of the combustion gas is bypassed. The diagrams show
the actual flow of the alkalies and chlorine. The width of each line is
proportional to the flow rate. For each case, 100 parts of the species being
considered enter with the raw material feed, and the numbers on the figures
indicate the quantity of these species in their recirculation through various
parts of the preheater kiln system.
4. Current Applications
There is a strong trend in the U.S. cement industry toward the application
of the suspension preheater (Tables IV-2 and IV-3). Several of these instal-
lations are new facilities, such as Gifford Hill & Company's Harleyville, S.C.,
facility. Recently, there have been some modifications of older wet-process
cement plants to dry process through the conversion of an existing long kiln
to a suspension preheater. It appears that a logical series of conversions
and additions at an old wet-process cement plant with several relatively small
kilns would be as follows:
• Convert the old wet-process rotary kiln to a four-stage suspension
preheater. This would be done by cutting the kiln approximate!^ '
in half and removing the feed end. The suspension preheater tower
would be constructed adjacent to the longitudinal axis of the rotary
kiln, allowing for the construction of a second preheater tower
adjacent to the first, and on the other side of the kiln axis,
thereby providing symmetry in plan view. The purpose of this offset
22
-------
NA20 CYCLES
LiN
| KILN | CYCLONES j COLLECTOR | j KILN j CYCLONES j COLLECTOR j
! ~j P | CLINKER! | ' |
| KILN CYCLONES (COLLECTOR j KILN CYCLONES ICOLLECTOB
Source Nofbom. H.R . I.E E.E. Ceieul TecFinical Confeienw,
Mum. FtoiKjj. Mn '9'3-
Figure IV-b. Alkali and Chloride Cycles in Four-Stage
Suspension Preheater Kiln
TABLE IV-2
HISTORY OF U.S. SALES OF FOUR-STAGE SUSPENSION PREHEATERS, 1953-1973
No
1
3
1
2
13
1
1
1
3
1
1
1
1
1
1
1
2
1
1
1
1
1
1
1
35
6 Hfg
F
F
F
F
F
F
F
F
F
Subtotal
F
FLS
K
F
AC
AC
F
F
F
F
F
F
AC
F
FLS
F
F
F
Total
Year
Sold
1953
1954
1954
1954
1955
1955
1955
1955
1955
1959
1963
1970
1970
1971
1971
1971
1972
1972
1972
1972
1972
1973
1973
1973
1973
1973
1973
1973
Company
National Gypsum
Alpha
Lehigh
Marquetce
National Gypsum
Coplay
Heduaa
Whitehall
Ideal
Alpha
Whitehall
American
Loneatar
Monarch
California Portland
Fllntkote
Centex
Southweatem
Glfford-Hlll
Meduia
Unlveraal Atlaa
Monarch
Fllntkote
Whitehall
Hlaaourl Portland
Capital
unlveraal Atlas
Killer
Location
Evanavllle, Pa.
Cementon, N.Y.
Fogelavllle. Pa.
Hagerstown, Md.
Alpena, Mich.
Nazareth, Pa.
Dixon, 111. i
Cenenton, Pa.
Boettcher, Col.
Cementon, N.Y.
Oahu, Hawaii
Maryneal, Tex.
Humbolt, Kan.
Rilllto, Aril.
Ciena Falls, N.Y.
La Ssllo, 111.
Falrborn, Ohio
Hartevllle, S.C.
Cllnchfleld, Ga.
Bufflngton, Ind.
Humboldt, Kan.
Koamoedale, Ky.
Cementon, Pa.
Joppa. Ho.
San Antonio, Tex.
Leeda, Ala.
San Antonio, Tex.
Heaark
Shutdown approx 1966 and replaced with 2 long dry
kilns. Reatarted 1973.
Shutdown approx 1964 and replaced with 1 wet kiln.
Shutdown plant In 1970.
Shutdown approx 1970 & replaced with one long dry kiln.
Shutdown approx 1969 S removed approx 1972.
In operation.
In operation.
In operation.
In operation with modified feed to reduce combuatlblea.
Shutdown approx 1964 & replaced with 1 long kiln.
In opera Ion.
In opera lon-Hodlflcatlon of existing rotary kilns.
In opera Ion.
In opera ion.
In opera Ion.
Under construction.
Under conatruction.
Under construction.
Under construction.
Under construction.
In design.
In design.
In design.
In design.
In design.
In design.
In design.
Note: Excludes five 2-stage SP unite by F.L. Smldth in 1959-60 for American Cement at Oahu. Hawaii (1);
Oro Grande, California (2); Clarksdale, Arizona (2).
* Legend: F Fuller Company
FLS F.L. Smldth
AC Alll> Oulmera
K Krupp
Source: Garrece, H.M. and J.A. Murray, Rock Product.. p. 58. Auguat 1974
23
-------
TABLE IV-3
WORLD AND U.S. DATA ON SUSPENSION PREHEATER KILNS
Developer &
Manufacturer
Humboldt1
Germany
Wedag
Germany
F.L. Smidth
Denmark
Polysius
Germany
Krupp3
Germany
MIAG
Germany
TOTAL
United States Year World Sales to
Representative Developed 1966 1971
Fuller
F.L. Smidth
Polysius
Krupp
Allis-Chalmers
1950
1962
1955
1958
196A
1968
180
15
24
55
11
0
285
267
Incl.-
Above
75
132
26
5
505
United States
Sales Through
1971
16
0
3
_2
22
1) Humboldt purchased Wedag about 1969
2) Excludes 2-stage SP systems sold in United States
3) Krupp purchased Polysius about 1970
Source: Garrett, H.M. and J.A. Murray, 1974
preheater tower is for the addition of a. second preheater at a
future stage of capacity expansion. The major impact of this first
step conversion to a four-stage suspension preheater kiln is the
significant reduction-in fuel energy. Where the older and rela-
tively small wet-process kiln may have been operating with a fuel
consumption on the order of 6 x 10^ Btu/ton, the new four-stage
preheater kiln should have a fuel requirement of approximately
3 x 10^ Btu/ton; this reduction of fuel consumption by 50% provides
high motivation for such a conversion. In addition, there will be
a modest increase, 20-30%, in the production capacity of the kiln.
Add a flash-calcining vessel. In this second step, the flash calciner
is added between the rotary kiln and the four-stage suspension pre-
heater. The design of the suspension preheater should allow room
for this flash-calcining vessel. The flash calciner would increase
the production capacity of the total facility by about 25%, and
should again slightly decrease the quantity of fuel required for '
clinker production. This increase in capacity results from the com-
bustion of part of the required fuel in the flash calcining vessel,,
and not within the rotary kiln; this provides almost totally calcined
feed to the rotary kiln. The rotary kiln would probably be operated
at a higher speed to maintain proper bed depth and residence time.
24
-------
• Add a second calciner to the kiln. This third step employs the
construction of the second suspension preheater and flash calciner
in the space initially provided for this tower. This flash calciner
and suspension preheater would be identical with the existing one,
and would serve the same rotary kiln. The combustion gas leaving
the rotary kiln would be divided into two streams and would be fed
to the two flash calciner units operating in parallel. Half of the
total raw feed to the rotary kiln will go to each flash calciner.
The major effect of this third step would be essentially a doubling
in kiln clinkering capacity, or a 100% increase in the capacity of
the kiln with only a suspension preheater installed.
5. Development
The suspension preheater was invented in Czechoslovakia in 1933. However,
this idea was not aotmnercialized for 17 years, until Humboldt built its first
commercial installation in 1950. Following Humboldt's initial commercializa-
tion in 1950, three other suspension preheaters were installed, all in
Germany. Shortly thereafter, in 1953,-the first commercial suspension
preheater unit was built in the United States by the Fuller Company, operating
at that time as the licensor of the Humboldt suspension preheater design.
The suspension preheater was quickly adopted by the portland cement
community in the United States (Table IV-2) and in the other major cement-
producing countries of the world, such as Germany and Japan (Table IV-3).
In rapid succession, twelve more Humboldt suspension preheater units were sold
in the United States by the end of 1955. All 13 plants came onstream during
the 1955-1958 period. After 1955, there was a significant hiatus in the U.S.
sales of Humboldt suspension preheaters because of considerable operating
difficulties in the early units due to alkalies and the presence of combustible
materials in the raw feed. One of the main problems, the alkalies — i.e.,
potassium and sodium values — are widespread in clays, shales, and other argil-
laceous materials, the second most important raw feed material for making cement.
During the sintering or clinkering reactions in the high-temperature zone ot
the cement rotary kiln, the original crystal lattice, which binds the potassium
and sodium atoms, is disrupted and reforms into the portland cement compounds.
During this disruption, potassium and sodium appear to volatilize as the
sulfates. These vapor species form in the vicinity of the flame produced by
the burning fuel. i
If coal is burned, the coal ash also contributes some potassium and
sodium. Coal and, to some extent, oil contribute sulfur, which goes into the
formation of the potassium and sodium sulfate vapor. Any chlorine present
forms some potassium and sodium chloride in chemical equilibrium with the
potassium and sodium sulfates. This mixture of vapor species condenses to
form a fume of very fine particle size. A fraction of these alkali compounds
also condenses out on the surface of the raw material dust entrained by the
combustion gases. These fine particles of highly concentrated alkali sulfates
and/or chlorides leave the kiln and are collected by the fourth and third
stages of the suspension preheater and are returned to the kiln with the
preheated raw feed. This process sets up a large recirculation of alkali
compounds within the kiln/suspension preheater system.
25.
-------
Due to the relatively low melting point of the mixtures of potassium,
sodium and calcium sulfate, this material can become sticky and can adhere
to the inner refractory-lined surfaces of the ducting, cyclones, and trans-
fer pipes of the preheater. As these deposits accumulate, they add their
own measure of insulation to the refractory lining of the preheater com-
ponents, thereby permitting progressively higher temperatures, which accel-
erate the continuing deposition of these materials. Such a process progresses
at an increasing rate until sections of the preheater actually become choked
with solid material, thereby requiring shutdown and cleaning of the unit.
This can become costly due to loss of cement production. It can also be
dangerous because of the high temperature of the material being cleaned,
since the unit is not permitted to cool even for such cleaning.
The other main problem experienced by the early suspension preheater
units in the United States is caused by the presence of fuel, or combustible
values, in the raw feed. In certain cement plants in the United States, the
argillaceous component of the raw feed is a kerogen-containing shale, and
kerogen is a combustible hydrocarbon material. These hydrocarbons are ignited
when such raw feed is preheated to the 1400°F temperature found in the fourth
stage of a suspension preheater. Sufficient oxygen for combustion is usually
present in the combustion gases because of the excess air required to maintain
proper combustion conditions in the firing end of the kiln, and air in-leakage
at the rotary kiln seals. The heat liberated from this kerogen combustion
within the preheater causes severe local overheating. This is accompanied by
excessive calcination and reaction of the raw material components within the
preheater, leading to sticky raw feed and the accumulation of hard solid
deposits within the preheater and, again, requires shutdown for cleaning.
The first Humboldt suspension preheater built in the United States was at
the Evangville, Pennsylvania, plant of Allentown Portland Cement Company, in
1953. Apparently this unit functioned well and there were no excessive alkali
problems, nor was kerogen-containing shale a component of the cement raw
material. Because acceptance of the suspension preheater in the United States
was so rapid and widespread, a large number of units were built before many
were tried. When the alkali and kerogen-containing shale problems were then
encountered, the U.S. portland cement industry concluded that the suspension
preheater was unacceptable for application to U.S. cement raw materials and
this process fell into disfavor.
6. Economic Factors
a. Fixed Capital Investment
The fixed capital investment for the clinkering section of a short'.kiln
with a four-stage suspension preheater is lower than that of a long kilA /'
(Figure IV-5), simply because the large, heavy, refractory-lined rotary kiln
is more expensive than the simpler stationary suspension preheater.
26
-------
24
20
18
CAPITAL
COST 14
(SI 06)
12
10
200
400 600 800
ANNUAL CEMENT CAPACITY (000 TONS)
1000
1200
'NO COST ESCALATION CONTINGENCIES INCLUDED
Source: Margiloff, I.B. and R.F. Cascone, Rock Products Great
Industry Seminar, Chicago, Dec. 8,1975.
b.
Figure IV-5. Total Capital Costs of Various Clinkering Sections
Operating Cost
The most significant difference in operating cost between the suspension
preheater kiln (Table IV-4) and the conventional rotary kiln (Table IV-5) is
in the unit fuel cost. (The dry-process kiln was selected for comparison
because the preheater system is dry.)
In comparing the energy use between a suspension preheater system and a
long conventional rotary kiln note that the electrical energy used to drive
the induced draft fan that draws the combustion gases through all of the
ducting and cyclone vessels of the preheater is not needed in a conventional
long rotary kiln.
Furthermore, one must consider the specific raw material moisture content,
grindability, type of clinker cooler used, etc. For example, raw materials
containing 10% moisture may be appropriate for a wet-process cement plant but
may be too moist for crushing and grinding in a dry-process plant and thus
require an expensive drying step. However, the use of a suspension preheater
kiln system provides a significant quantity of high-temperature gases (from
the clinker cooler), not needed for combustion because of the high thermal
efficiency which can be incorporated into a closed-circuit raw material
grinding and drying system to handle such raw materials.
27
-------
TABLE IV-4
PORTLAND CEMENT PRODUCTION, COST:
SUSPENSION PREHEATER/FLASH CALCINER KILN
Product: Type I Portland Cement
Byproducts: None
Annual Capacity: 445,500 tons
Annual Production: 445,500 tons .
Four-Stage Suspension Preheater Working Capital: $2.8 x 10
Process: with or without precalcining vessel
Stream Days/Yr: 330
Fixed Investment; $40 x 10
Location: U.S. East Coast
VARIABLE COSTS
Raw Materials: Limestone
Argillaceous
Components
Gypsum & Minor
Additives
Purchased Energy: Fuel (oil)
Electric Power
Water: Cooling
Operating Labor
Labor Overhead
Operating, Main. & Repair Supplies
FIXED COSTS
Plant Overhead
Taxes & Insurance -
Depreciation
TOTAL PRODUCTION COST
Pre-Tax Return on Investment
TOTAL
Units
inclt
incli
106 Btu
kWh
103 gal
man-hr
30%
2%
70%
Quantity /Ton
jded in other co
ided in other co
2.8
130
0.45
0.6
of Operating La
of Plant Cost
Unit Cost
sts
sts
2.00
0.02
0.03
6.00
>or
of Operating Labor
2% of Plant Cost
20 j
ear, straight 1
20%
ine
$/Ton
1.00
5.60
2.60
0.01
3.60
1.08
1.80
2.52
1.80
4.49
24.50
19.21
43.71
Source: Arthur D. Little, Inc. estimate.
28-
-------
TABLE IV-5
PORTLAND CEMENT PRODUCTION COST:
LONG ROTOARY KILN (OIL-FIRED)
Product: Type I Portland Cement
Byproducts: None
Long Rotary Kiln
Process: Dry Process
Annual Capacity: 445,500 tons
Annual Production: 445,500 tons
Fixed Investment: $42 x 10
Working Capital; $2.8 x 10
Stream Days/Yr ; 330
Location: U.S. East Coast
VARIABLE COSTS
Raw Materials: Limestone
Argillaceous
Components
Gypsum & Minor
Additives
Purchased Energy: Fuel (oil)
Electric Power
Water: Cooling
Operating Labor
Labor Overhead
Operating, Main. & Repair Supplies
FIXED COSTS
Plant Overhead
Taxes & Insurance
Depreciation
TOTAL PRODUCTION COST
Pre-Tax Return on Investment
TOTAL
Units
inc:
inc]
106 Btu
kWh
103 gal
man-hr
30%
2%
70%
Quantity /Ton
.uded in other c
.uded in other c
4.2
130
0.45
0.6
Unit Cost
osts
osts
2.00
0.02
0.03
6.00
of Operating Labor
of Plant Cost
of Operating Labor
2% of Plant Cost
20 :
rear, straight ]
20%
ine
$/Ton
1.00
8.40
2.60
0.01
3.60
1.08
1.89
2.52
1.89
4.71
27.70
20.11
47.81
Source: Arthur D. Little, Inc. estimate.
29
-------
The electrical-energy-saving roller mill operates to its best advantage
in the raw material grinding circuit when supplied with gases which are even
hotter than a conventional ball mill can accept. Therefore, the roller mill
is particularly well suited to inclusion in a suspension preheater circuit,
thereby increasing the suspension preheater facility's capacity for handling
wet raw materials. Such a roller-mill-equipped, suspension-preheater facility
can accept raw materials with moisture contents as high as 15-20%. In addi-
tion, the roller mill produces cement raw feed with 15% less consumption of
electrical energy than comparably sized ball mills. This adds another
dimension of total energy savings to the suspension preheater system but one
which does not derive directly from the suspension preheater/kiln clinkering
system only.
c. Energy Requirements
Table IV-6 compares the energy requirements for a suspension-preheater-
equipped rotary kiln and a long dry-process rotary kiln in new facilities.
TABLE IV-6
COMPARISON OF TYPICAL ENERGY REQUIREMENTS FOR SUSPENSION
PREHEATER AND LONG KILN
(Btu/ton cement)
Available
Feed Preparation Clinkering Finishing Energy Recovery
(quarry, crush_, (burn, cool) (grind, pack) (steam/power gen-
dry, mix feed) eration, dryer
fuel savings)
Net Energy
Required After
Energy
Recovery
Preheater, Short Kiln
Electrical
Fuel
Total
534,000
336.000
870,000
374,000
3,200,000
3,574,000
760,000
760,000
(346,000)
(300.000)
(646,000)
1,322,000
3.236,000
4,558,000
Dry, Long Kiln
Electrical
Fuel
Total
534,000
336,000
870,000
315,000
4,600,000
4,915,000
(320.000)
(320,000)
1,609,000
4,616,000
6,225,000
Source: Margiloff, I.E. and R.F. Cascone, Rock Products Cement Industry Seminar,
Chicago, December 8, 1975
30
-------
7. Environmental Factors
a. Air Pollution
(1) Particulates
One of the environmentally advantageous aspects of the suspension pre-
heater is its propensity to trap the alkalies and sulfur values within the
lower- and higher- temperature stages of the preheater. These alkalies remain
with the cement clinker. Therefore, a four-stage suspension preheater, oper-
ating with no bypass, would send a relatively cool combustion gas stream con-
taining solid particulate material which is physically and chemically similar
to cement raw feed to the dust collecting system.
Because the dust removed from the combusion gases leaving the suspension
preheater system is -essentially the same as cement raw feed, all of it is
returned to the preheater system. Therefore, the adoption of the suspension
preheater presents no new dimensions to the collection or disposal of solid
particulates.
(2) SO
v ' _ x
Raw material which has been partially calcined is highly reactive with
the sulfur dioxide, forming calcium sulfate. Any SOX which might form in the
combustion gases in a rotary kiln using extremely high-sulfur coal as fuel
contacts the raw material so intimately that the use of a suspension preheater
system should not present any sulfur dioxide emission problems.
(3) N0
The concentration of nitrogen oxides (NOX) in the combustion gases from
both the long rotary kiln and the preheater system will probably be equivalent
because fuel is burned in the same way in both systems. However, the absolute
quantity of nitrogen oxides generated per ton of cement clinker produced by
the suspension preheater kiln will be less than that produced by the long kiln,
since the thermal efficiency of the suspension preheater kiln is so much better
than either the long wet- or dry-process kiln.
The number of pounds of nitrogen oxides generated per ton of cement
clinker produced by a suspension preheater facility operating at 3 x 10° Btu
per ton should be exactly half of the quantity of nitrogen oxides produced by
a long kiln which is operating at 6 x 106 Btu per ton; however, the concentra-
tion of nitrogen oxides in the combustion gases leaving both of these systems
should be about the same.
(4) Costs
The fixed capital and operating costs for the air pollution control sys-
tems required by two different long conventional kilns and a four-stage pre-
heater kiln system are shown in Tables IV-7 through IV-9. The basis for these
estimates is presented in Table IV-10.
31
-------
TABLE IV-7
OPERATING COSTS FOR AIR POLLUTION CONTROL SYSTEM:
LONG ROTARY KILN SYSTEM (DRY-PROCESS/NO INSULATION)
i
Production, ton/yr 470,000
Fuel Required, Btu/ton 4.2 x 10
Capital Investment, $ 1,085,000
Control Device (Kiln, Dryer, Cooler) Glass Bag Filter
Annual Operating Costs:
Electricity (2.70 x 106 kHh/yr), $/yr 54,000
Direct tabor (14,100 hr/yr), $/yr 84,600
Maintenance Labor (7,000 hr/yr), $/yr 49,000
Plant Overhead, $/yr 133,600
Materials, $/yr 98,000
Depreciation, $/yr 54,250
Taxes and Insurance, $/yr 21,700
Return on Investment, $/yr 217,000
Total Operating Cost, $/yr 712,150
Total Operating Cost, $/ton 1.52
Energy Consumption, 109 Btu/yr 28.35
Energy Consumption, Btu/ton 60,400
Source: Arthur D. Little, Inc. estimate
TABLE IV-8
OPERATING COSTS FOR AIR POLLUTION CONTROL_ SYSTEM:
LONG ROTARY KILN SYSTEM (DRY-PROCESS/INSULATED LINING)
Production, ton/yr 470,000
Fuel Required, Btu/ton 3.4 x IQ&
Capital Investment, $ 933 QOO
Control Device (Kiln, Dryer, Cooler) Glass Bag Filter
Annual Operating Costs:
Electricity (2.33 x 106 kWh/yr), $/yr 46,600
Direct Labor (12,200 hr/yr), $/yr 73,200
Maintenance Labor (6,100 hr/yr), $/yr 42,700
Plant Overhead, $/yr 115,900
Materials, $/yr 85,400
Depreciation, $/yr 49,150
Taxes and Insurance, $/yr 19,650
Return on Investment, S/yr 196,600
Total Operating Cost, $/yr 629,200
Total Operating Cost, $/ton 1.34
Energy Consumption, 10^ Btu/yr 24.47
Energy Consumption, Btu/ton 52,100
Source: Arthur D. Little, Inc. estimate
32
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TABLE IV-9
OPERATING COSTS FOR AIR POLLUTION CONTROL SYSTEM:
. FOUR-STAGE PREHEATER KILN SYSTEM
Production, ton/yr
Fuel Required, Btu/ton
Capital Investment, $
Control Device (Kiln, Dryer, Cooler)
Annual Operating Costs:
Electricity (1.74 A 106 kWh/yr), $/yr
Direct Labor (9,150 hr/yr), $/yr
Maintenance Labor (4,100 hr/yr)
Plant Overhead, $/yr
Materials, $/yr
Depreciation, $/yr
Taxes and Insurance, $/yr
Return on Investment, $/yr
Total Operating Cost, $/yr
Total Operating Cost, $/ton
Energy Consumption, 10^ Btu/yr
Energy Consumption, Btu/ton
470,000
2.8 x 106
798,000
Glass Bag Filter
Source: Arthur D. Little, Inc., estimate
TABLE IV-10
BASIS FOR OPERATING COST ESTIMATES FOR
AIR POLLUTION CONTROL IN CEMENT MANUFACTURING
Power Costs, $/kWh
Operating Labor (Incl. Supervision), $/hr
Maintenance Labor (Incl. Supervision), $/hr
Depreciation, years
Method of Depreciation
Taxes and Insurance
Return on Investment
Annual Operating Hours
Plant Capacity, ton/yr
Btu/kWh
0.02
6.00
7.00
20
Straight Line
2% of Capital Investment
20%
7,200
470,000
10,500
33
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b. Water Pollution
The wastewater characteristics, necessary wastewater treatment measures,
and treatment costs for cement plants employing the suspension preheater/
flash calciner process are very much similar to those associated with the base
case cement plant. (See Appendix C.)
It is estimated that the suspension preheater/flash calciner alternative
will generate the same quantity of"non-contact cooling water as the base case
plant, i.e., 648,000 gallons per day (gpd) for a 1350-tpd plant.
As in the case of the base case cement plant, the suspension preheater/
flash calciner alternative will produce a waste dust, which probably will be
stored in large piles or holding ponds. The quantity of dust generated is
expected to be substantially less than that of the base case, 60 tpd versus
140 tpd. The'dust is expected to contain a slightly higher soluble fraction
than that generated by the base case cement plant. Since the quantity of dust
generated is less than that of the base case, for a given storage pile depth
the amount of exposed area (and thus the run-off flow rate) will be propor-
tionally decreased. It is estimated that a 1350-tpd cement plant employing
the suspension preheater/flash calciner process will require a 10-year dust
storage area of 4.5 acres (20-ft depth) versus 10.6 acres for the base -case.
As with the base case cement plant, the storage area will have to be diked and
will have to have provisions for collecting run-off water and subjecting it to
clarification and neutralization prior to discharge.
Due to the lower volume of run-off water to be treated, the suspension
preheater/flash calciner alternative has a slightly lower wastewater treatment
cost compared to the base case - $0.39/ton of cement versus $0.46/ton of
cement (Table IV-11).
B. FLASH CALCINER
1. Process Description
Although the design of flash calcining systems varies (Appendix D), the
main feature which characterizes the flash calciner rotary kiln is the flash
calcining vessel added between a rotary kiln and a suspension preheater
(Figure IV-6).
The combustion gases leaving the rotary kiln pass through the flash
calcining vessel. The hot raw material leaving the bottom of Stage 3 of the
suspension preheater discharges into the flash calcining vessel. Fuel is
burned in the flash calcining vessel to further calcine and preheat this raw
material stream. The combustion gases combined with kiln gases carry the; raw
material from the flash calcining vessel into Stage 4 of the suspensiojb pre-
heater, from which the hot and almost completely calcined raw material dis-
charges into the rotary kiln.
The flash calciner arrangement in Figure IV-6 requires that a considerable
amount of excess combustion air be used in burning the fuel in the rotary kiln
so that enough air is present in the combustion gases leaving the kiln to per-
mit combustion of the fuel in the flash calciner vessel.
34
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TABLE IV-11
WASTEWATER TREATMENT COSTS: SUSPENSION PREHEATER/FLASH CALCINER
Basis
1350 tpd Cement Production
330 Operating Days Per Year
CAPITAL INVESTMENT - $427.000
VARIABLE COSTS
Operating Labor
(including overhead)
Maintenance
(inc. Ibr & mtls)
Chemicals
Sulfuric acid
Electrical Power
TOTAL VARIABLE COST
FIXED COST
(Depreciation @ 5%)
(Taxes & Insurance @ 2%)
TOTAL FIXED COST
Annual
Quantity
Cost Per
Unit
Quantity
2630 man-hr $12/hr
6 tons
323,1007
kWh
$100/ton
$0.02/
kWh
Quantity
Per Ton of
Production
0.0059
1.35x10
0.73
-5
Unit Cost
($ Per Ton
of Product)
0.0709
0.0384
0.0110
0.0146
0.1349
0.0480
0.0191
0.0671
TOTAL ANNUAL COST
RETURN ON INVESTMENT @ 20%
0.2020
0.1917
TOTAL
0.39/ton
Notes:
1)
2)
3)
Capital investment adjusted to 1975 level (ENR Construction
Cost Index = 2126)
Wastewater treatment includes:
a) Non-contact cooling water thermal pollution control
via spray pond
b) Dust pile runoff containment, collection, clarification
and neutralization
Estimates are for the specific example of a dry-process, non-
leaching cement plant and are in no way intended to represent
industry-wide wastewater treatment costs.
Source: Arthur D. Little, Inc. estimates
35
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STAGE 1
STAGE 3
FLASH CALCINING
VESSEL
CLINKER
COOLER
FAN
RAW FEED INTO SYSTEM
STAGE 2
STAGE 4
KILN
Figure IV-6. Flash Calcining System with Combustion Air
for Precalciner Drawn up through Kiln
The advantages of the flash-calciner-equipped rotary cement kiln are:
• Significantly increased rotary kiln capacity. This permits very
high clinker production capacities from average size rotary kilns.
• Improved kiln availability. This results from the use of conven-
tional sized rotary kilns which exhibit refractory life considerably
in -excess of the large rotary kilns required for equivalent pro-
duction capacities without the flash calciner.
• Reduced fuel consumption. The heat losses through the rotary kiln
shell are less than those of a conventional rotary kiln. The cement
produced per square foot of kiln shell area is very high.
• Reduced fixed capital investment. The flash calciner represents a
slightly lower fixed capital investment than that required for the
incremental amount of rotary kiln which it replaces.
• Alkali removal with less heat energy penalty than incurred by the
>• use of a bypass in a suspension preheater kiln. Since only 40-50%
of the total fuel is burned in the rotary kiln, and since the
clinkering zone of the rotary kiln is the only place where the alkali
compounds are volatilized, the alkali compounds are reported to be
more highly concentrated in a smaller quantity of gas; therefore,
bypassing less of this gas is reported to eliminate more alkalies.
36
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• Reduced nitrogen oxide emissions. Since 50-60% of the total fuel
burned in this system is burned in the flash calciner, and since
the temperature of the flash calciner is maintained at only about
1500°F, the nitrogen oxides formed in this vessel are reported to
be considerably less than formed in the high-temperature, free-
standing flame which is burned in the rotary kiln.
• More predictable, constant and easily controlled operation. The
tendency for uncontrolled periodic "rushes" within the kiln is
eliminated and the function of the kiln is simplified to an extent
where the prospect for truly automatic control of the whole
clinkering process is much more probable with almost totally
calcined raw feed than with any other clinkering system.
One of the important advantages of the flash calcining system is the very
rapid calcination which takes place in the suspension flash furnace or fluid-
ized bed vessel. By monitoring temperature, it is possible to maintain close
control over the extent of calcination of the raw material, thereby pro-
viding a preheated and precalcined raw material of very uniform chemical com-
position to the rotary kiln. Any variations in the extent of calcination are
only short-term, and the residence time and the mixing of the raw material
being clinkered in the rotary kiln evens out those short-term variations. The
consequence of this is a very uniform and steady rotary kiln operation and
clinker product of high uniformity. This not only minimizes downtime, with
consequent increase in kiln availability, but also significantly increases
the prospects for the fully automated control of a rotary portland cement
kiln - perhaps very soon.
Therefore, by burning a large fraction of the total fuel outside the
rotary kiln, the capacity of the rotary kiln can be increased significantly,
with simultaneous accrual of a host of other potential benefits: e.g., better
technical performance, lower fuel energy consumption, higher-quality cement,
lower costs, and reduced nitrogen oxide emissions.
2. Current Status
Several major equipment manufacturers have developed their own particular
versions of the flash calciner, differing mainly in gas flow and precalcining
vessel location. The following systems are now in commercial operation:
(a) Japanese
• Ishikawajima Harima Heavy Industries (IHI)
• Onoda Cement-Kawasaki Reinforced Suspension Preheater System
• Mitsubishi Fluidized Calcinator
37
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(b) European
• Polysius System
• F. L. Smidth System
Some of these systems have been designed to use alternative fuels such as coal.
(See Appendix D.) The high production capacity and the small geographic area
of Japan combine to make extremely large cement plants the most economical there.
The trend in Japan toward large single kilns of extremely high output has been
unparalleled in the other cement-producing countries. Because the flash
calciner concept has made it practical and technically feasible to operate
(with high kiln availability) extremely large single-kiln facilities, the
flash calciner has been adopted by the Japanese cement industry at a very
rapid rate.
IHI started its research program in 1963 for the development of its ver-
sion of a flash calcining system. Initial research and development work was
done on a scale ranging from 2 to 20 tons per day. From there, a full-scale
commercial facility with a capacity of 2200 tons per day was designed and
built.
This development program solved the following problems:
• Sticking of a calcined raw material coating to the inner wall of
the flash calcining furnace;
• Clogging in the preheater cyclone;
• Misfiring of the furnace-burner; and
• Coating or clogging in the rotary kiln.
At present, there are 33 flash calciner cement facilities either oper-
ating or being built in Japan. Because of the large number of installations
being constructed, the Japanese equipment manufacturers who are developing
and offering the flash calcining system .have had an opportunity to develop
their designs faster than equipment manufacturers in other countries. The IHI
process became so popular that 19 plants were already sold through 1974, and
12 are now in operation in Japan. Japan is presently making 40% of its
cement production with flash calcining systems, almost 60% of them using the
IHI process.
The largest reported flash calciner system is at Chichibu, with a capacity
of 8500 tons per day of cement clinker, and a heat consumption of less than '
2.6 x 106 Btu per ton. This appears to be a record' for not only the largest
daily productive capacity from a single kiln facility, but also the lowest
reported heat consumption.
38
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3. Energy, Economics, and Environment
The published data on flash-calciner-equipped rotary cement kiln energy
requirement, fixed capital investment, operating cost and environmental fac-
tors are reported by major types of units or for specific plants. Because of
the nature of the data, we present them in this combined form for each of the
five major flash calciner systems. The available data in these categories
enable us to make the following general characterizations of these flash
calcining systems.
a. Energy Requirements
The energy required is essentially the same or slightly less than that
for a suspension preheater system. One of the reasons for this is that the
capacity of the rotary kiln operating with a flash calciner system is twice
that of a suspension preheater kiln. Therefore, with capacity held constant,
the flash-calciner-equipped rotary kiln is significantly smaller, thereby
presenting less shell area through which heat can be lost to the environment.
Also, the fuel for the flash calcining vessel is burned at a low temperature,
and the calcining capacity of the precalcining vessel, expressed in terms
such as tons of raw feed precalcined per cubic foot (and therefore per square
foot of external surface of the precalciner), is significantly greater than
the precalcining or calcining zone of the rotary kiln.
b. Economics
The fixed capital investment should be slightly less than that for a
suspension preheater, since a section of rather expensive rotary kiln is
replaced by a stationary and smaller precalcining vessel. Except for the
Mitsubishi fluid-bed precalcining system, which is the only atypical one,
no data are yet available in the literature.
Operating costs are probably slightly lower than a suspension preheater
only to the extent of a smaller kiln and better refractory life and possibly
more stream days per year.
c. Environmental Factors
(1) Air Pollution
(a) Particulates
The quantity and dust loading of the combustion gas stream leaving a
flash calciner should be essentially the same as for a comparable suspension
preheater. The difference would be in the gases leaving through a bypass.
However, there are no data in the available literature to clarify this.
39
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(b) SO
x
By the time the combustion gases exit to the atmosphere, essentially all
of the SOX should1 be absorbed and reacted with the raw feed. • However, any
gases which might be bypassed could be different in SO content from sus-
pension preheater bypass gases because the raw feed entering the flash cal-
ciner kiln are almost completely calcined, and also have a significantly lower
kiln residence than in a suspension preheater.
(c) NO
v x
NOX is reported to be lower than from a suspension preheater or long kiln
because half of the fuel being burned in the precalciner is at a low and uni-
form temperature and oxygen content.
(d) Costs
The fixed capital and operating costs for the air pollution control systems
required by a flash calciner are essentially the same as for a suspension pre-
heater. (See Table IV-9.)
(2) Water Pollution
The wastewater characteristics, necessary wastewater treatment measures,
and treatment costs for cement plants employing the flash-calciner-equipped
rotary kiln are essentially identical with those associated with the suspension
preheater. (See Section A.)
C. FLUIDIZED-BED CEMENT PROCESS
1. Process Description
The difference between the fluidized-bed cement making process and the
conventional processes is in the high-temperature clinkering step. All of the
other steps are essentially identical.
In a fluidized-bed reactor (Figure IV-7), the raw material is introduced
at the bottom of the bed of fluidized cement clinker particles which is main-
tained at a temperature high enough to permit the clinkering reactions to
occur. The extremely large heat transfer coefficients of the fluidized bed
quickly heat the incoming raw material particles up to clinkering temperature.
As these raw materials are heated and begin to chemically react, an intermedi-
ate liquid composition is reached. This permits the partially reacted liquid
reactants to adhere to the surface of the individual particles of clinker,,
rather than be carried out of the bed by the fluidizing gases. Upon com-
pletion of the clinkering reactions, this thin liquid, or semi-solid layer,
quickly solidifies.
40
-------
HOT KILN GASES
SIZED RECYCLE
CLINKER
REFRACTORY
LINING
CARBON STEEL
SHELL
PREHEATED
AIR
CLINKER OVERFLOW
Source. Mjrgilolt, I B , wid R.F C«cone, Hock
Pioctucit, Current Indutiry Seminar. Chicago
lltmon. OK 8. 1976
Figure IV-7. Detail of Kiln for Scientific Design Fluid-Bed Cement Process
The new surface"is itself refractory and solid at the reactor tempera-
ture. The clinker particles thus produced are spherical, and increase in
diameter as successive increments of clinkering raw materials are applied
to their surface. This continues as long as the clinker particles remain in
the fluidized bed. The individual particles do not stick together to form
larger agglomerates, but remain discrete spheres.
The bed of clinker particles is fluidized by hot combustion gases pro-
duced by the introduction of preheated combustion air through an air dis-
tribution grid which forms the flooV of the reactor vessel. Fuel is metered
into the bed, and burns in the continuous gas phase present between the indi-
vidual particles of cement clinker. Any hydrocarbon fuel such as natural gas,
oil, or coal, can be used. When operated properly, it is reported that there
are no visible flames in the free space above the upper surface of the fluid-
ized bed, and all combustion takes place in the interstices of the fluidized
bed proper. The bed operates at a temperature of 2400°F.
The bed depth is apparently determined by combustion requirements. Too
shallow a bed can permit fuel combustion above the surface of the bed, where
the heat would not be adequate for the clinkering reactions and would also
probably be hazardous to heat transfer surfaces located downstream of the
reactor.
41
-------
The optimum particle size distribution of the bed material (Table IV-12)
is maintained in the bed by the continuous removal of bed material and the
reintroduction of crushed and screened clinker product to act as nuclei for
continuing growth of new particles. This extraction of bed material and
recycling of the finest fraction (combined with a suitable quantity of
crushed and screened fine "seed" material) permits the steady-state, contin-
uous operation of the fluid bed reactor. This appears to be one of the key
operational aspects of this process.
Figure IV-8 illustrates the recycle of both the fine fraction of
extracted bed material and the finer product obtained by crushing coarse
particles removed from the bed to act as new "seed" particles. The values
shown on this figure illustrate a simple material balance around the reactor.
This material balance demonstrates the approximate flow rat§ of the major
streams of solid reactants, products, and recycle streams. The stream of
raw feed material entering the bottom of the fluidized bed is shown to be
6000 Ib. After calcination and clinkering, this material produces 4000 Ib
of finished portland cement clinker. Essentially all of the 2000-Ib differ-
ence, or weight loss, is accounted for by the mass of carbon dioxide liberated
from the raw feed material. During calcination, calcium carbonate, the main
chemical constituent of portland cement raw feed material, is thermally
decomposed to yield calcium oxide and carbon dioxide gas, which is carried
from the reactor along with the combustion gases generated within the bed of
fluidized clinker.
TABLE IV-12
OPTIMUM PARTICLE SIZE DISTRIBUTION
FOR 'BED OF CLINKER PARTICLES
-0.3" + 4 mesh 10%
-4 mesh + 6 mesh 20%
-6 mesh + 8 mesh 35%
-8 mesh + 10 mesh 25%
-10 mesh + 20 mesh 10%
Source: Sadler, A.M., Paper presented at A.I.Ch.E., New York
Meeting, Nov. 30, 1967.
42
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100 LB
ALKALI COMPOUNDS
400 LB
1500LB
CRUSHER
1100LB
SCREEN
5500 LB
4000 LB
CLINJKER
PRODUCT
Source: Sadler, A.M., A.I. Ch. E. New York meeting, Nov. 30, 1967.
6000 LB
RAW FEED
(4000 LB
CLINKER EQUIV.)
Figure IV-8. Material Balance Around the Reactor
The 4000 Ib of equivalent clinker generated from this raw feed stream
are deposited on the surface of the particles of clinker which constitute the
fluidized bed. Accordingly, the bed increases not only in mass by that amount,
but also in total volume, raising the upper surface of the bed. As the sur-
face rises, there is a consequent increase in the rate at which clinker par-
ticles spill through the overflow outlets from the reactor. Due to the well-
mixed nature of the fluidized bed, the clinker particles which leave the
reactor over a period of time have a particle size distribution which is the
same as the average particle size distribution of the entire bed of fluidized
particles.
i
Only the largest (diameter) particles are considered finished product.
They are separated from the overflow stream of reactor material by a screen-
ing step. To remove the 4000 Ib of clinker product, a total overflow of
reactor contents equal to 5500 Ib is screened. The 1500 Ib of finest mate-
rial are then returned to the reactor for further growth.
If a simple screening and recycle process were carried out, the particle
size distribution of the fluidized bed would be impossible to maintain, and
would progressively shift toward the larger end of the size spectrum. To
maintain a constant aid predetermined particle size distribution, part of the
1500 Ib of recycled particles of clinker are crushed to a smaller size, to
provide the nuclei necessary to maintain a steady-state operation. Of the
1500 Ib of recycled material, 1100 Ib are recycled directly to the reactor,
while 400 Ib are crushed to a finer size before being returned to the reactor.
43
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2. Reactor
The fluidized-bed reactor is a vertical cylinder fabricated of carbon
steel and lined with high-temperature refractories. The inner refractory
lining would probably consist of high-temperature refractory brick and an insu-
lating layer of refractory material would be located between that brick lining
and the outer carbon steel shell. The bottom of the vessel is a cast refrac-
tory grid. This grid would contain a large number of vertical holes through
which the fluidizing and combustion air would pass upward into the vessel.
Also, appropriately sized pipelines for conveying cement raw feed and fuel
would pass up through this cast grid.
The depth of the fluidized bed within the reactor is determined by the
position of the clinker overflow pipe. This water-jacketed pipe extends
through the carbon steel shell and refractory lining of the vessel and forms
the outlet for the clinker particles moving around within the fluidized bed.
The fluidization of relatively large-diameter particles with hot combustion
gases usually produces a type of fluidization which is similar to a rapidly
boiling and agitated liquid. Therefore, as the upper surface of the fluidized
bed moves in the form of waves, and as bubbles burst through the surface, the
undulating and probably splashing effect of these phenomena would cause the
bed to wash up over the clinker overflow, thereby discharging clinker particles
with each cycle of this kind of wave motion.
3. Mechanical Advantages
During the 1950's and 1960's, the cement industry installed progressively
larger rotary kilns. This trend was motivated by the increased profitability
of larger cement plants which derived from the economies of scale for large
plants based on only one or two large rotary kilns rather than many smaller
ones. In the late 1950fs and early 1960's, kilns being installed ranged up
to 27 ft- in diameter and 700 ft long. These are probably the largest and
heaviest rotating pieces of processing equipment used by any industry.
A rotary kiln consists of a cylindrical welded-steel plate shell lined
with refractory brick and encircled along its length with steel rings, called
tires. Each tire rests on a pair of steel support rollers or bearings.
Because of the great weight of such an., assemblage, some shell deformation
is unavoidable. When viewed from the end, a large rotary kiln is oval shaped,
as the weight of the kiln tends to flatten it slightly. This distortion
produces a regular cycle of major stresses as the kiln rotates. These
stresses are not a serious problem in the elastic steel kiln shell, but are
a problem in the refractory brick. The rigidity of refractory brick, combined
with its low tensile strength, is responsible for increasing brick damage
(e.g., spalling) in the larger rotary kilns. 'This results in a significant
shortening of the refractory life, and increased refractory cost in the larger
rotary kilns.
By contrast, the fluidized-bed reactor is a vertical cylinder of rather
low height and large diameter. The reactor does not rotate or move in any
other way. Although it consists of a steel shell lined with refractory brick,
none of the refractory problems of the rotary kiln apply to the fluidized-bed
reactor. Therefore, refractory life in the fluidized-bed process is expected
to be high.
44
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In the rotary kiln, because of the high flame temperatures which result
from the suspension combustion of fuel, it is necessary to protect the refrac-
tory brick in the clinkering zone with a coating of sticky or semi-plastic
clinkering raw material, deliberately induced by its composition. Even more
important is the protection of the refractory from the highly abrasive action
of the clinker particles which roll along the inner surface of the rotary kiln.
Without such a coating, refractory bricks are quickly worn too thin to protect
the steel shell.
The down-time required for cooling the kiln, removing the worn brick,
rebricking, and startup is in itself a significant interruption leading to
loss of production. In addition the cost of refractories is quite high. The
sum of these two items can amount to a significant manufacturing cost item.
Refractory life in the fluidized-bed cement reactor, however, is extremely
high because the walls are vertical, the clinker particles are small and
spherical, and the impact of fluidized particles on the vertical inner wall
of the fluidized-bed reactor is cushioned by the suspension of the particles
in air and the flow of air around the particles. The production and mainte-
nance of a coating of clinkering raw material is not necessary on the inner
surface of t-he brick lining the fluidized-bed cement reactor.
The fluidized-bed cement reactor is also a more compact piece of equip-
ment than the rotary kiln. For example, a fluidized-bed reactor with a
production capacity of about 250,000 tpy of cement clinker would have an
outside diameter of approximately 23 ft and be 50 ft tall. By contrast, a
dry-process rotary kiln of similar capacity would be 11 ft in diameter and
375 ft long (wet process - 12 ft diameter by 450 ft long).
A rotary kiln requires a complex and expensive drive mechanism and speed
control, which the fluidized-bed cement reactor does not need. Air in-leakage
with its attendant energy losses is common to rotary kilns, but is not a
problem for the fluidized-bed cement process.
However, all of the combustion air supplied to the fluidized-bed reactor
must be provided at a pressure high enough to overcome the resistance to flow,
or pressure drop, it experiences as it passes through the air preheating sys-
tem, through the air distribution grid which forms the floor of the reactor
vessel, and finally through the fluidized bed of particles. A considerable
expenditure of energy is required to drive the blowers for fluidizing air
compression.
Another advantage of the fluidized-bed clinkering process is the ease
with which clinker production can be changed from one type to another, pri-
marily because of the absence of a coating (of either clinker or calcined
raw material) on the inner surface of the refractory brick which lines the
reactor. Also, the total inventory of clinker within the reactor is rela-
tively small, and it can be displaced by the formation of new clinker in a
relatively short time.
45
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4. Energy Use
a. Comparison of Fluidized Bed with Rotary 'Kiln
i
Table IV-13 show's the comparative energy use of the conventional rotary
kiln versus the fluidized bed. In the conventional rotary kiln, the hot com-
bustion gases and the incoming cold raw materials pass continuously in a
countercurrent manner. The raw materials are gradually heated to the final
clinkering temperature, in excess of 2400°F. The combustion gases and the
reactants, or raw materials, are at their highest temperature at the firing
end of the kiln, where the fuel is burned in suspension within the kiln itself.
The combustion gases pass thrqugh the kiln simultaneously giving up heat to
the raw materials in the kiln and are at their lowest temperature at the gas
discharge end, which is also the raw material feed inlet end, of the kiln.
The temperature of the combustion gases exiting from a long dry process
rotary cement kiln which has a relatively high fuel consumption (e.g.,
4.0-4.4 x 106 Btu/ton), is 1300-1400°F. A long dry-process rotary cement kiln
of approximately the same capacity (1300-1400 tpd), with chains hung in the
preheating section and exhibiting low fuel consumption (e.g., 3.4-3.6 x
10° Btu/ton) has an exit gas temperature of 800-900°F. The combustion gas
has.been cooled from near flame temperature to this relatively low tempera-
ture by countereurrent heat exchange with the incoming raw material feed
stream. By contrast, the combustion gases which exit from the fluidized-bed
cement reactor leave at the bed temperature, which is approximately 2400°F.
TABLE IV-13
COMPARISON OF TYPICAL ENERGY REQUIREMENTS
FOR FLUIDIZED-BED PROCESS AND LONG KILN
(Btu/ton Cement)
S.D. Fluidized-Bed Kiln
Electrical
Fuel
Total
Feed Preparation
(quarry, crush,
dry, mix feed)
Clinkering
Finishing
Available
Energy Recovery
(burn, cool) (grind, pack) (steam/power gen-
eration, dryer
fuel savings)
Net Energy
Required After
Energy
Recovery
490,000
310.000
800,000
5,000,000
5,000,000
760,000
760,000
(1,400,000)
(180,000)
(1,580,000)
(150,000)
5,130,000
4,980,000
Dry. Long Kiln
Electrical
Fuel
Total
534,000
336.OOP
870,000
315,000
4.600.000
4,915,000
760,000
760,000
(320.000)
(320,000)
1,6091,000!
4.616.000
6,225,000
Source: Margiloff, I.B. and R.F. Cascone, Rock Products .Cement Industry Seminar, Chicago, December 8, 1975.
46
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In the conventional rotary kiln, the raw material feed is preheated by
the exiting combustion gases to temperatures where the rate of thermal
decomposition of the calcium carbonate constituent of the raw feed becomes
relatively high. Carbon dioxide is then liberated from the calcining raw
material. This calcination occurs over a temperature range of approximately
1200-1600°F. Therefore all of the carbon dioxide liberated from the raw
feed in the calcination zone has only been heated to the calcination
temperature.
In the fluidized-bed clinkering reactor, however, all of the carbon
dioxide contained in the original calcium carbonate reactant is heated to
2400°F, since the entire reactor content of raw material and finished clinker
is at a uniform clinkering temperature of 2400°F. Since about 35% of cement
raw feed is liberated as carbon dioxide during the calcination process the
fluidized-bed reactor subjects almost 50% more solid reacting materials to a
temperature range from approximately 1400°F to 2400°F. The heat required to
increase the temperature of all of the carbon dioxide from 1400°F to 2400°F
represents an additional heat load in the fluidized-bed cement reactor which
the conventional rotary kiln does not require.
Another difference in thermal or fuel energy use between the fluidized-
bed reactor and the conventional rotary kiln relates to the recycle of the
fine fraction of clinker particles. The clinker product which leaves the
fluidized-bed represents the average particle size distribution within the
bed of fluidized clinker particles. Therefore, there is a fine fraction of
clinker particles leaving the fluidized-bed reactor which must be returned
-for further growth until it becomes larger than approximately 8 mesh.
The present process concept employs a cooler to reduce the temperature
of the clinker from 2400°F to almost ambient temperature, for easy handling
of the clinker, and to recuperate heat from the hot clinker particles. The
cooled clinker is screened, the +8-mesh fraction being sent on to clinker
product storage. The -8-mesh fraction is recycled to the reactor (with a
part of it first being crushed, as described earlier).
Since the recycled clinker is essentially at ambient temperature, it
must be preheated again to 2400°F. This is accomplished through the extrac-
tion of heat from the hot particles within the reactor. This extra step
requires burning of additional fuel and is not part of the operation of a
conventional cement rotary kiln.
b. Specific Requirements
Any form of carbonaceous fuel can be used in the fluidized-bed reactor.
Semi-commercial scale demonstrations have been successfully accomplished using
the following fuel forms:
• natural gas
• fuel oil (No. 4 and No. 6)
47
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• bituminous coal
• petroleum coke
• oil shale (where the shale was one of the raw material constituents).
The fluidized-bed cement process compares favorably with the most energy-
efficient current cement plant design.
(1) Raw Material Preparation
The total energy used for the raw material preparation steps would be the
same for conventional rotary kiln plants and the fluidized-bed cement process.
Dry grinding operations all consume about the same amount of electrical power
per ton of raw mix. However, wet grinding consumes less power, and since the
fluidized-bed cement process is a dry one, this difference would indeed exist.
Most of the new cement plants are dry process plants, indicating the beginning
of a major trend in the United States away from the wet process cement plant.
(See Appendices A,B, and C.)
In a rotary kiln, the typical loss of potential clinker through the
wasting of kiln dust is about 8% of the raw material, whereas in the fluidized-
bed reactor, only about 3% of the clinker equivalence of the raw material is
lost. Therefore, less raw material is required to produce a ton of cement by
the fluidized-bed process then by the conventional rotary kiln process. This
higher yield of product per ton of raw material with the fluidized-bed process
has, of course, an attendant savings in total energy required for raw material
preparation.
(2) Clinker Production
The process design studies which have been conducted by Scientific Design
Company indicate that the fluidized-bed clinkering reactor requires as much
purchased fuel as the long dry rotary kiln equipped with a chain preheating
section but less purchased fuel than the typical wet-process rotary kiln.
Since the steam from the hot combustion gases leaving the fluidized-bed
reactor generates power which is projected to be significantly in excess of
that required to drive the fluidization air blowers, Table IV-13 shows the
typical energy requirement in Btu/ton of cement, and neither credits nor debits
this amount of power. However, the power in excess of this amount, from the
steam generation system, which can be used in other areas of the cement plant,
is shown on this table.
Figure IV-9 is a schematic process flow diagram which shows the main components
and important material flow streams for the fluidized-bed clinkering sections
of a cement plant. The reactor is as described in the preceding section.
48
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Source: Margiloff, I.e. and R.F. Cascone, Rock Products,
Cement Industry Seminar, Dec 8, 1975.*
Figure IV-9. Scientific Design Fluid-Bed Cement Process
A pressurized feeder conveys a dense fluidized stream of cement-making
raw materials at a suitably high pressure and at a controlled feed rate into
the fluidized bed of hot portland cement clinker particles through one or more
transfer lines. The overflow of hot cement clinker from the fluidized-bed
reactor discharges from the overflow pipe by gravity into a clinker cooler.
A horizontal reciprocating grate cooler of the type designed and built by
Fuller Company probably would suitably handle and cool such a spherical product.
The cooled clinker is screened to separate the clinker product from the
recycled material.
Fuel is introduced through fuel transport lines in a manner analogous to
the raw material feed. A gaseous or liquid fuel would be pumped directly into
the bed, probably through the cast refractory grid. A crushed solid fuel,
such as coal or petroleum coke, would probably be introduced through the grid.
The raw material and the fuels would be introduced at the bottom of the
fluidized bed to maximize their residence time. The hot combustion gases
leaving the reactor at 2400°F would pass through a heat exchanger, which would
transfer heat from these hot gases to incoming cold fluidizing and combustion
air, as well as to water for the generation of steam. Probably the heat
exchanger would be divided into two sections, which would exist in series. The
first section would receive the hottest gas leaving the reactor and would be
designed to exchange radiant heat energy from the hot gas to the fluidizing
and combustion air contained in alloy tubes, for the generation of steam, or
49
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for both of these options operating together. After the combustion gases
leaving the reactor have been cooled to a temperature sufficiently low so
that the radiant component of the gases' heat transfer capability are suf-
ficiently diminished, the cooler gases would probably pass through a con-
ventional preheating unit. This unit would do the initial heating of the
fluidizing and combustion air and/or heating of water condensed from a steam
system. The steam generated in such a heat exchanger would be used either to
directly drive the air blowers required to supply the pressurized fluidizing
and combustion air to the reactor or to generate electrical energy through a
turbine drive which could in turn power an electric motor drive connected to
the air blower system. The steam generated by such a system would be in
excess of that required simply for providing the pressurized fluidizing and
combustion air for the fluidized-bed reactor. In fact, sufficient steam is
available (after all of the heat that can be utilized in preheating the
fluidizing and combustion air has been extracted from the combustion gases
leaving the reactor) to provide electrical energy not only for grinding all
of the raw material required by the fluid-bed reactor but also to drive the
finish cement grinding mill.
Therefore, in addition to fuel, the only other uilities which are
apparently required for the fluidized-bed cement process are cooling water
(or air coolers) for condensation of turbine exhaust steam and a small amount
of boiler feedwater makeup for the steam system.
This makes the fluidized-bed cement process competitive on a total energy
basis with the most energy efficient cement-making processes presently avail-
able to the portlant cement industry, namely the suspension-preheater-
equipped rotary kiln, and the flash-calciner-equipped suspension preheater
rotary kiln facility. In addition to the recuperation of heat from the hot
combustion gases leaving the fluidized-bed reactor, the hot air leaving the
clinker cooler provides another source of high-temperature gases which can
be utilized to reduce the overall energy required by this process. Either
the highest temperature cooling air exiting from the hot clinker inlet end
of the cooler can be sent to the convection section of the main heat exchanger
for the initial preheating of the fluidizing and combustion air and/or the
initial heating of the feedwater to the steam generator, or else that hot
air can be utilized for the preheating of, the cement raw feed.
An additional advantage of the fluid-bed process is that the raw feed
particles would be kept isolated from the hot combustion gases leaving the
fluidized-bed reactor, and therefore would not become contaminated by the
volatilized alkali elements leaving the clinkering raw feed in the fluidized
bed. This alkali volatilization characteristic of the fluidized-bed clinkering
process is a significant advantage and results in large part from the totally
indirect heat transfer between the hot combustion gases carrying the alkali/
values from the bed of clinkering materials and any of the air or raw material
inlet streams to that reactor.
50
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(3) Power Recovery
The fourth column in Table IV-13 is the excess heat from clinkering which
is recoverable as steam suitable for power generation. This includes steam
generated in excess of that required for driving the blowers for the pres-
surized fluidizing air, exit gas fan, cooler air fan, etc., all of which are
associated with the fluidized-bed clinkering reactor system. This also
includes heat recoverable from the hot air leaving the clinker cooler. The
fuel credits shown in this column assume that available heat (down to 300°F)
in the flue gases or heated air is limited only to the heat that can be used
for feed drying in that particular train of the production system, although
more heat may actually be available in those particular streams.
The electrical energy required is based on a 30% fuel efficiency on
delivered power, and a 26% efficiency of captively generated power to convert
electrical power requirements to the equivalent and actually required fuel
energy for that electrical power generation. The fifth column of Table IV-13,
shows the net energy required, including energy recovery. In the case of
the fluid-bed cement process, all of the steam which can be generated in
excess of that required to supply the motive power for the fluidized-bed
clinkering system has been considered available for use in the other areas of
the cement plant, such as for driving the ball mills for raw material grinding
and finished cement grinding.
On a total energy basis, the process design studies recently conducted
by Scientific Design Company indicate that the cement process employing the
fluidized-bed cement reactor, with proper heat recovery, requires signifi-
cantly less total energy than either the conventional wet or dry long rotary
kiln, and actually is close to the preheater-equipped short rotary kiln, which
represents the most energy-efficient cement clinkering process available to
the cement industry today.
Further savings in purchased fuel can be achieved through the use of raw
materials containing fuel values such as the kerogen-contained in oil shale.
Also, when rotary kiln waste dust is used as the raw feed or forms a portion
of the raw feed to the fluidized-bed clinkering reactor, significant savings
in purchased fuel can be achieved since kiln dust has already been subjected
to some degree of calcination. For example, cement clinker can be produced
in a fluidized-bed reactor using 100% kiln dust (with a suitable correction of
the calcium, silicon, iron, and aluminum values to make the proper balance of
Portland cement compounds) with a savings of up to 2 x 10^ Btu/ton of clinker
produced. This would reduce the total energy required to approximately
3 x 106 Btu/ton of cement, including the fuel energy required to produce all
of the electrical power for operating the entire cement plant built around
the fluidized-bed clinkering reactor.
51
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5. Economic Factors
a. Fixed Capital Investment
' ' ' ' i
Figure IV-10 shows the total fixed capital cost associated with the
clinkering section only for the Scientific Design fluid-bed kiln and the long
kiln with chain section. This figure shows the fixed capital investment, in
1975 dollars, as a function of the annual installed cement production capacity.
Fixed capital investment includes the purchase cost of all major items of
equipment, and other materials such as instrumentation, insulation, founda-
tions, etc. Also included are the construction labor and construction super-
vision required to build the facility, and all design, engineering, procurement,
and expediting costs.
These fixed capital investment figures are only for the clinkering sector,
and do not include the front end of the cement plant which begins in the
quarry, and proceeds through the raw material grinding send blending system,
or the back end of the cement plant which begins with portland cement clinker
and proceeds through crushing, finish grinding, and storage. However, since
the front and back end of the cement plant will be essentially the same for
both processes shown, and the only difference in design and fixed capital costs
is-in the clinkering section, this figure clearly compares the standard or
present state-of-the-art clinkering process used by the cement industry with
the fluidized-bed clinkering system.
LONG
KILN
FLUID
BED
400 600 800
ANNUAL CEMENT CAPACITY (000 TONSI
•NO COST ESCALATION CONTINGENCIES INCLUDED
Souict: Mirgllofl, I.B.. wd R.F. Ciiccne, Rode Producn,
C«ram Induitrv Semlnir, Chicago, Dec. 8, 1976.
Figure IV-10.
Total Capital Costs of Various
Clinkering Sections, April, 1975
52
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TABLE IV-14
PORTLAND CEMENT PRODUCTION COST: FLUIDIZED-BED
CEMENT PROCESS
Product: Type I Portland Cement
Byproducts: None
Annual Capacity: 445,500 tons
Annual Production: 445.500 tons
Fluidized-licd Clinker
Process: Reactor
Fixed Investment: $38 x 10°
Working Captial: $2.8 x 10
Stream Days/Yr.; 330
Location: U.S. East Coast
VARIABLE COSTS
Raw Materials: Limestone
Argillaceous
Components
Gypsum
Purchased Energy: .Fuel (Oil)
Water: Cooling
Operating Labor
Labor Overhead
Operating, Main. & Repair Supplies
FIXED COSTS
Plant Overhead
Taxes & Insurance
Depreciation
TOTAL PRODUCTION COST
Pre-Tax Return on Investment
TOTAL
Units
Quantity /Ton
Unit Cost
included in other costs
included in other costs
106 Btu
103 gal
Man-hour
5.0
3.45
0.06
2.0
0.03
6.00
30% of Operating Labor
2* of Plant Cost
70%
of Operating Lai
or
2% of Plant Cost
20 y
ear, straight li
20%
1
nc
$/Ton
1.00
10.00
0.10
3.60
1.08
1.71
2.52
1.71
4.26
25.98
18.32
44.30
Source: Arthur D. Little, Inc. estimate.
53
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It is interesting to note the significantly lower fixed capital invest-
ment required for the fluidized-bed process as envisioned by Scientific
Design Company compared to that of the long kiln. Scientific Design Company
indicated that the fixed capital investments for the various modern rotary
kiln systems were independently estimated by cement specialists. If waste
kiln dust is used as the primary or as a major raw material component, the
feed grinding department capital investment will be reduced.
b. Operating Cost
Table IV-14 shows our estimate of cement manufacturing cost for a fluid-
bed cement process plant of 1350 tpd, or 445,500 tpy capacity. The objective
of this estimate is to compare the fluidized-bed process costs with the con-
ventional long rotary kiln cement process plant manufacturing costs in order
to identify the significant differences.
6. Environmental Factors
a. Air Pollution
The main effluent stream from the fluidized-bed cement process is the
combustion gas stream leaving the reactor. This stream is analogous to the
hot combustion gas stream leaving the conventional rotary cement kiln. With
regard to its main gaseous and vapor constituents, such as nitrogen, carbon
dioxide, etc., the two principal differences between the effluent gas streams
from the conventional rotary kiln and the fluidized-bed cement reactor are
the quantity and composition of the solid particulates carried by that gas
stream and the concentration of NO .
x
Table IV-15 shows our estimated operating costs for the air pollution
control system required by the fluid-bed cement process.
(1) Particulates
The composition of the solid particulates carried from the fluidized-bed
cement reactor is very different from that of rotary kiln dust. Rotary kiln
dust consists of partially calcined cement raw feed and potassium and sodium
sulfates in the range of 5-10% total alkalies, expressed as the stoichiometric
equivalent of sodium oxide. In contrast, it is reported that the solid par-
ticulates carried by the hot combustion gases exiting the fluidized-bed cement
reactor consist of 97% water-soluble potassium and sodium sulfates, and 3%
cement clinker. Therefore, since the dust from the fluidized-bed process is
essentially pure potassium and sodium sulfate, the quantity of dust collected
per ton of cement clinker produced is very small compared with the dust col-
lected from the conventional rotary kiln process.
The volume of dust which must be disposed of per ton of cement produced by
the fluidized-bed cement process is consequently only a small fraction of the
volume which must be discarded from the conventional rotary kiln process. In
addition, since the dust from the fluidized-bed cement process is essentially
pure potassium and sodium sulfate, it is readily water-soluble, and contains
almost no hydraulically cementitious materials, it should be technically
54
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TABLE IV-15
OPERATING COSTS FOR AIR POLLUTION CONTROL SYSTEM:
FLUIDIZED-BED CEMENT PROCESS
Production, ton/yr 470,000
Fuel Required, Btu/ton 5.0 x 106
Capital Investment, $ 939,000
Control Device (FLuidlzed Bed, Cooler) Glass Bag Filter
Annual Operating Costs:
Electricity (2.17 x 106 kWh/yr), $/yr 43,300
Direct Labor (11,340 hr/yr), $/yr 68,040
Maintenance Labor (5,700 hr/yr), $/yr 39,900
Plant Overhead, $/yr 107,940
Materials, $/yr 79,800
Depreciation, $/yr 46,950
Taxes and Insurance, $/yr 18,800
Return on Investment, $/yr 187,800
Total Operating Cost, $/yr 592,530
Total Operating Cost, $/ton \.2f>
Energy Consumption, 109 Btu/yr 22.79
Energy Consumption, Btu/ton 48,500
Source: Arthur D. Little, Inc. estimate
feasible, and perhaps even economically attractive to further process this
dust to extract potassium sulfate, which could have commercial value, for
instance as a plant nutrient.
(2) Alkali Volatilization (Particulates Source)
An important advantage of the fluidized-bed cement reactor over the con-'
ventional rotary kiln is the very high degree of alkali volatilization from the
raw materials during their conversion to cement clinker. The direct contact
between the hot combustion gases and the raw feed in the rotary kiln is
responsible for the recirculation of volatilized alkalies between the high-
temperature clinkering zone (where the alkali values are liberated from the
raw materials and become a vapor species) and the cool end of the rotary kiln
where the combustion gases leave the kiln after giving up much of their heat
by direct contact with the incoming cold raw material.
By contrast, the hot combustion gases leaving the fluidized-bed reactor
give up their heat indirectly, through steel heat transfer surfaces to the
incoming fluidization and combustion air, to water and steam in the steam
generating portion of the heat exchange system. Therefore, as the potassium
and sodium sulfate vapor condenses into a fume in the hot combustion gases
passing through the heat exchange system, these alkali particulates are kept
from contact with the incoming cement raw feed. Since there is no route by
which the volatilized alkali compounds can be returned to the bed of clinker
55
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in the fluidized-bed reactor system the alkalies, once volatilized, will
leave the clinkering system and be removed from the cooled combustion gases
by appropriate dust removal equipment, such as glass cloth filters.
In addition to the indirect heat transfer between the hot combustion
gases carrying the alkali sulfates and any of the incoming material streams
to the reactor, other factors which are probably responsible for the high
degree of alkali volatilization demonstrated by the fluidized-bed cement
reactor are as follows:
• Fluidized beds characteristically demonstrate extremely high heat
transfer and mass transfer rates. Therefore, the temperature and
combustion gas composition is expected to be quite constant through-
out the continuous fluidizing gas medium phase in the "fluidized
bed, which should tend to maximize the volatilization efficiency.
• The long residence time of the cement clinker particles in the high
temperature (2400°F) in the fluidized-bed reactor compared with the
relatively short period of time that the reactants are in the high-
temperature clinkering zone of a rotarykiln. The extent of
volatilization should increase in proportion to residence time.
• The raw feed particles (containing the highest quantity of alkali
materials present in the ^fluidized-bed system) are deposited on the
surfaceof the individual fluidized-bed particles. Since the total
surface area of these small particles is large, and the thickness of
the new incremental layer of clinker deposited on each particle at
any time is probably very small, the distance for diffusion of the
alkali components is short. Clinker particles in a rotary kiln are
quite large in diameter, compared with the average diameter of the
clinker spheres in the fluidized-bed cement process.
(3) Nitrogen Oxides
Combustion conditions in the rotary cement kiln favor NOX formation, due
to the high peak flame temperatures associated with the combustion of fuel in
suspension, as well as the existence of regions of high oxygen concentration
due to the absence of good fuel/air mixing, further enhanced by in-leakage of
ambient air through the rotary kiln seal.
By contrast, the fluidized-bed cement reactor operates at a constant and
uniform temperature of 2400°F, which is considerably below the peak flame tem-
peratures in a rotary kiln. This temperature is constant and uniformly dis-
tributed throughout the entire volume of the fluidized bed, due to the excel-
lent heat and mass transfer characteristics of fluidized beds. Finally, I
because of the excellent mass transfer exhibited by fluidized beds, the oxygen
concentration in the gas phase within the bed is quite uniform at any elevation,
thereby preventing regions of high oxygen concentration.
56
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Figure IV-11 compares the emission of nitrogen oxides, expressed as NOX,
from a fluidized-bed reactor and from a rotary kiln, both being operated to
produce solid products at 2400°F. The fuel used during these tests was oil,
and percent stoichiometric air was the independent variable. The fluidized
bed clearly generates significantly less NOX than the rotary kiln.
b. Water Pollution
The wastewater generated by a cement plant employing the fluidized-bed
process will be similar in composition but different in flow rate from that
of the base cement plant. (See Appendix C.)
The incorporation of steam generation facilities within the fluidized-bed
process configuration greatly increases the amount of non-contact cooling
water generated. The amount of cooling water generated is estimated as follows
Plant Cooling Water (same as base case) 648,000
Steam Generation Condenser Waste 4,320,000
Total Non-Contact Cooling Water 4,968,000
Due to restrictions on wastewater temperature rise imposed upon the cement
industry (maximum permissible temperature rise above inlet water = 3°C) , it
will be necessary to cool the exit cooling water prior to discharge. As in
the base case cement plant, it is anticipated that a spray pond would be the
most practical means of cooling. Since the total non-contact cooling water
flow rate is much greater than that of the base case (4,968,000 gpd vs
648,000 gpd), the spray pond will be larger and more costly.
The steam generation facilities will also produce wastewater streams con-
sisting of boiler blowdown and boiler feedwater treatment regeneration brines,
both of which are small in volume and largely contain inorganic salts; they
are not considered in the cost comparison.
Cement plants employing the fluidized-bed process will produce a waste
dust, which for a 1350-tpd cement plant is estimated to be generated at a rate
of 40 tpd. Unlike the base case cement plant, dust generated by the fluidized-
bed process will consist of relatively high-grade potassium and sodium sulfate,
both of which are highly soluble. There are possibilities for selling this
material as a byproduct. However, if the material cannot be sold as a byproduct,
it will have to be stored on-site in a manner similar to that described for
the base case. The high solubility of the material imposes even a stronger
need to dike the storage area and to collect and treat runoff water.
The run-off would be treated primarily to remove suspended solids and
soluble heavy metals. Dissolved species, such as potassium, sodium and sul-
fate, would be much more difficult to remove, aiid probably would not need to
be removed.
57
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BASIS: TEMPERATURE = 2400°
FUEL = OIL
F LUIDIZED-BEDREACTO P
100 110 120 130 140
STOICHIOMETRIC COMBUSTION AIR (%>
Source: Lawall. T.R. and Cohen, S.M., Paper presented at
A.I.Ch.E., Cincinnati meeting. May 1971.
Figure IV-11.
Comparison of NOX Emissions from Fluidized-Bed
Reactor and Rotary Kiln
Due to the smaller quantity of dust generated, the required storage area
will be smaller than that of the base case (3.0 acres for a 10-year storage
area 20 feet deep vs 10.6 acres for the base case cement plant).
Due to the greatly increased cooling water flow rate, the unit treatment
cost is substantially greater than that of the base case - $0.84/ton vs
$0.45/ton for the base case. (See Table IV-16.)
c. Solid Wastes
The main solid waste is the collected particulate material. When firing
a rotary kiln with coal, some of the coal ash is carried out of the kiln with
the combustion gases, and forms part of the collected kiln dust. Any of the
heavy metals commonly present in this ash can be leached out of such kiln
dust storage piles by rainwater.
Essentially all of the coal ash generated within a coal-fired, fluid-bed
cement reactor will form part of the clinker. Therefore, the dust from the
coal-fired fluid bed should have a much lower coal ash content.
58
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TABLE IV-16
WASTEWATER TREATMENT COSTS: FLUIDIZED BED
Basis
1350 tpd Cement Product
330 Operating Days Per Year
CAPITAL INVESTMENT - $937,000
Cost Per Quantity
Annual Unit Per Ton of
Quantity Quantity Production
VARIABLE COSTS
Operating Labor 3125 man-hr $12/hr 0.007
(including overhead)
Maintenance
(inc. Ibr & mtls)
Chemicals 4 tons $100/ton 9.0 x 10~
Sulfuric acid
Electrical Power 2,283,200/ $0.02/kWh 5.13
kWh
TOTAL VARIABLE COST
FIXED COST
(Depreciation @ 5%)
(Taxes & Insurance @ 2%)
TOTAL FIXED COST
TOTAL ANNUAL COST
RETURN ON INVESTMENT @ 20%
Unit Cost
($ Per Ton
of Product)
0.0842
0.0842
0.0009
0.1025
0.2718
0.1053
0.0420
0.1473
0.4191
0.4207
TOTAL
0.84/ton
Notes:
1) Capital investment adjusted to 1975 level (ENR Construction
Cost Index = 2126)
2) Wastewater treatment includes:
a) Non-contact cooling water (plant cooling water plus steam generation
condenser cooling water) thermal pollution control via spray pond
b) Dust pile runoff containment, collection, clarification
and neutralization
3) Estimates are for the specific example of a dry-process, non-
leaching cement plant and are in no way intended to represent
industry-wide wastewater treatment costs.
Source: Arthur D. Little, Inc. estimates
59
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D. CONVERSION TO COAL FUEL FROM OIL AND NATURAL GAS
1. Background
Portland cement is manufactured in most of the countries of the world
using all commercially available carbonaceous fuels and, in some cases,
unusual fuels which are not commercially used by other industries. The
majority of cement plants use a rotary kiln for the clinkering reactor. Most
of the fuel used in the cement-making process is burned in the rotary kiln,
where it is burned in suspension and forms a free-standing flame in one end
of the kiln.
The specific fuels which are used, or have been used, for manufacturing
Portland cement in a rotary kiln are:
• natural gas
• crude oil
• fuel oil (primarily No. 6)
• anthracite coal
• bituminous coal
• petroleum coke»
• various waste materials such as peanut shells
• combinations of the above, such as:
- anthracite coal/bituminous coal
anthracite coal/No. 6 fuel oil
The specific fuel or combination of fuels used to produce portland
cement clinker in a rotary kiln has an effect upon:
• burner design
• ratio of combustion air to fuel
•• ratio of primary to secondary combustion air.
With a rather wide latitude in the interchangeability of fuels burned in
any specific rotary kiln installation, essentially any rotary kiln now being
fired by gas or oil can be converted to coal.
60
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2. Coal-Firing Factors
The air necessary to sustain combustion of the fuel comprises primary air,
which enters the kiln along with the fuel, and secondary air, normally hot air
obtained from the clinker cooler. When firing with pulverized coal, primary
air may be as much as 25% of the total quantity of combustion air supplied,
depending upon the kind of coal employed and the draft provided within the
kiln by the induced draft fan. When firing with oil or gas, primary air is
only about 3% of the total combustion air, since these fuels do not require
air as a carrier medium for the fuel. An advantage of firing pulverized coal
is that the primary air can be preheated to significantly higher temperatures
than with oil or gas. The fineness to which coal must be pulverized or ground
to burn properly in a rotary kiln depends to a large degree upon its ash con-
tent and also its rank (percent volatile matter). As the coal ash content
increases, or as the volatile matter decreases, the fineness to which the coal
must be ground increases (Table IV-17).
3. Conversion to Coal Firing
In converting a rotary kiln now burning gas or oil to coal, the following
areas of manufacturing process technology, plant operation, and plant equip-
ment must be considered.
TABLE IV-17
REQUIRED PULVERIZED-FUEL FINENESS AT MAXIMUM RATING
(Percent through 200 U.S.S. Sieve*)
ASTM CLASSIFICATION OF COALS BY RANK
Fixed Carbon (%) Fixed Carbon below 69%
97.9-86
Btu
Petroleum
Type of
Furnace
coke
85.
9-78
77
.9-69
above
13
,000
Btu
12
11
,900-
,000
Btu
below
11,000
Marine boiler furnace - 85 80 80 75
Water-cooled furnace 80 75 70 70 65** 60**
Cement kiln 90 85 80 80 80
Metallurgical (As determined by process, generally from 80 to 90%)
*The 200-mesh screen (sieve) has 200 openings per linear inch, or 40,000 openings per square
inch. From U.S. and ASTM sieve series, the nominal aperture for 200 mesh is 0.0029 inch, or
0.074 mm. The ASTM designation for 200 mesh is 74 microns.
**Extremely high-ash-content coals will require higher fineness than indicated.
Source: Schwarzkopf, F., Ro.ck Products, July 1974
61
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a. Raw Feed Chemistry
Coal is a unique fuel for the manufacture of portland cement, since it
contributes a substantial amount of ash to the'interior of the rotary kiln.
Chemically, coal ash consists primarily of silicon, iron, aluminum, sodium,
and a host of other elements in decreasing percentages. Because of their
form in the original minerals, and also because of the high temperature of the
oxidizing atmosphere within the combustion zone, coal ash consists primarily of
glassy or amorphous silica, iron oxide, aluminum oxide, sodium sulfate, and
other elemental oxides, many of which have combined into various complex
solid combinations.
Typical bituminous coal contains approximately 10% ash by weight.
Between 50% and 100% of all of the coal ash produced by the combustion of
coal in a rotary cement kiln contacts and chemically combines with the
clinkering raw materials, thereby losing its identity as coal ash and
becoming portland cement clinker. Since the coal ash has a chemical com-
position which is not at all appropriate for the production of portland
cement clinker, the raw feed chemical composition must be adjusted to
incorporate the quantity and composition of coal ash from the coal combus-
tion in the rotary kiln. In this way - regularly done in the portland cement
industry - the combination of raw feed and coal ash meets the specific and
stringent chemical composition requirements for portland cement clinker.
There is no adverse affect on the quality of the cement clinker if the raw
feed has been adjusted to incorporate the coal ash, and if variations in the
quantity and chemical composition of the coal ash accompanying the coal being
burned can be predicted and suitable corrections made in the raw feed to the
kiln.
b. Sulfur
The major potential problem associated with the use of coal as a fuel in
manufacturing cement is the sulfur content of the coal'. Portland cement
typically contains between 1.5% and 2.5% sulfur, expressed as 803. Most of
this sulfur comes from the deliberate addition of gypsum to portland cement
clinker. This is done before final grinding to the finished, fine-powdered
product, in order to increase the setting time of the concrete and allow
sufficient time for mixing and placement. Without the addition of gypsum,
most portland cement clinkers will produce a concrete with an unpredictable
and extremely short setting time.
In addition to the sulfur contributed by'the gypsum, the portland cement
clinker itself contributes an amount of sulfur, usually in the form of potas-
sium and sodium sulfate. This is rather uniformly distributed throughout each
individual particle of ground clinker and has only a minor effect on setting
time. The sulfur in the clinker originates from both the raw material 'com-
ponents and the fuel used for clinkering. With the raw materials and fuels
62 _
-------
typically used by the industry, there is at least sufficient sulfur present
in the kiln to stoichiometrically react with the alkali elements in the raw
materials to produce potassium and sodium sulfate. Any additional sulfur
usually exists in a dynamic equilibrium between sulfur oxides in the high-
temperature burning end of the kiln, and calcium sulfate in the cooler, raw
material feed end.
There are two main aspects to the effect of sulfur in the clinkering
process. The first is the effect of sulfur upon the quality of the cement
clinker, and upon the finished portland cement made from that clinker. The
second is the effect upon the operation of the rotary kiln. The rotary kiln
with a suspension preheater or a flash calciner is most affected by high
levels of sulfur.
Operating experience has shown that a suspension-preheater-equipped
rotary kiln (or the suspension preheater part of a flash calciner) exper-
iences progressively more serious,problems with sticking materials, clogging
of transfer lines, and bridging of the bottom solids outlet portion of cyclone
suspension vessels as the sulfur level in the rotary kiln/suspension pre-
heater system increases. In particular, as the stoichiometric ratio between
sulfur and the alkalies (potassium and sodium) increases above one, these
problems become more pronounced. It appears that as long as the alkalies and
sulfur are in balance, most of the sulfur forms potassium and sodium sulfate.
However, as the sulfur is allowed to increase beyond this point, other low
melting point phases, such as calcium sulfate, or combinations of alkali and
calcium sulfate, or even other sulfates, form and concentrate between the
rotary kiln and Stage 4 of the suspension preheater.
Although this additional sulfur does not significantly increase the level
of S02 in the suspension preheater or flash calciner exit gases, the opera-
tional problems experienced within the suspension preheater or flash calciner
can become so severe as to preclude the use of coal containing excessively
high levels of sulfur. Unfortunately, it is not possible to generalize and
cite some level of sulfur in coal which forms the threshold for severe opera-
tional problems, since the composition of the raw material, the content of
alkalies in the raw material, and the specific clinkering and preheating sys-
tem, all contribute importantly to the quality of clinkering process equipment
performance or operation.
Excessive sulfur in portland cement can delay some of the hydration reac-
tions too long beyond the final setting of the concrete. This results in
considerable expansion within the concrete mass and consequent cracking of the
finished structure. Because of this, all major cement-producing countries of
the world have set maximum sulfur specifications on cement (Table IV-18).
It is, therefore, one of the operating goals of most of the world's cement
plants to produce a clinker with as low a sulfur content as possible. This
permits the greatest latitude in adding gypsum to control setting time. If the
clinker sulfur content is too high, then it may not be possible to add the
amount of gypsum actually required for set control and still have a cement
which meets the specified maximum sulfur content. This would result in a pro-
duct with poor or even unacceptable physical properties.
63
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TABLE IV-18
MAXIMUM SULFUR SPECIFICATIONS FOR CEMENT
IN SELECTED COUNTRIES
Type of Cement
Maximum Sulfur
Content
(Wt Z SOa)
UNITED STATES
JAPAN
VEST GERMANY
ITALY
FIANCE
UNITED KINGDOM
SPAIN
3CaO-Al203 i82
ASTM TYPE I 2.5
ASTH TYPE II 2.5
ASTM TYPE III 3.0
ASTM TYPE IV 2.3
ASTM TYPE V 2.3
3CaO-Al203 >8Z
ASTM TYPE I 3.0
ASTM TYPE III 4.0
Ordinary Portland Cement 2.5
Rapid Hardening Portland Cement 2.8
Medium Low Heat Portland Cement 2.5
Portland Blast Furnace Cement (3 types) 3.0
Pozzolanic Cement (3 types) 2.5
Portlandzement (6 types)* 3.5
Portlandzement (6 types)** 4.5
Eisenportlandzecient (4 types)* f 3.5
Elsenportlandzement (A types)** 4.5
Hochofenzement (5 types)* 4.0
Hochofenzenent (5 types)** 4.5
Hochofenzement (5 types) [Containing more than 70Z slag] 4.5
Trasszement (3 types)* 3.5
Trasszement (3 types)** 4.5
Cemento Portland Normale 3.0
Cemento Portland Ad Alta Resistenza 3.0
Cemento d'Alto Forno 3.0
Cemento d'Alto Forno Ad Alta Resistenza 3.0
Cemento Fozzolanico 2.5
Cemento Pozzolanlco Ad Alta Resistenza 2.5
Clment Portland (19 types) 3.5
Ciment Portland de Fer (2 types) 3.5
Clment MStallurgique Mixte (2 types) 3.5
Clment de Haut Forneau (2 types) 3.5
Clment de Laitler au Clinker (2 types) 5.0
Ordinary Portland Cement* 2.5
Ordinary Portland Cement** 3.0
Rapid Hardening Portland Cement* 2.5
Rapid Hardening Portland Cement** 3.0
Low Heat Portland Cement* 2.5
Low Heat Portland Cement** 3.0
Sulfate-Resisting Portland Cement 2.5
Portland Blast Furnace Cement 6.75++*
Cemento Portland (3 types) 4.0
Cemento Portland Resistente'a Las Aguas Selenitosas (6 types) 4.0
Cemento Portland Siderurgico (2 types) 4.0
Cemento Portland de Horno Alto (2 types) 4.0
Cemento Puzolanico 4.0
Footnotes:
•Specific Surface <4,000 co2/g
"Specific Surface >4,000 cmz/g
+3CaO-Al203 37Z
++3CaO'Al203 >7Z
++*Max S03 - 3.0Z
Max S - 1.5Z
Source: "Cement Standards of the World", published by Cembureau, Paris, 1968.
64
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c. Equipment
To convert a cement plant from gas or oil to coal firing, it is neces-
sary to design and install the basic system for handling, processing, and
burnzng that coal. The main elements of such a system for coal conversion
are (Figure IV-12):
• coal unloading facilities
• storage
• secondary reclaiming
• primary crushing
• storage bunkers
• coal milling
• feed to the kilns plus related conveyor systems
• appropriate instrumentation and controls.
RAIL CARS
STORAGE
BIN
COAL
MILL
Source: Pit and Quarry, June, 1975.
Figure IV-12.
Basic Elements in the Systems Installed
to Convert to Coal Firing
65
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4. Physical Facilities Required
a. Receiving
The typical portland cement plant uses fuel at such a high rate that (if
it is burning coal) it justifies the receipt of coal by unit train. This will
require the installation of enough track to receive and store all the cars of
a unit train its unloading. A typical unit train might consist of 30-35 cars,
carrying a minimum of 2000 tons of coal. Sufficient track for storing these
cars, plus car handling equipment must be installed. Usually, demurrage
charges begin 24 hours after arrival of a unit train. Therefore, the car
handling and unloading system should be designed to permit unloading and rail
car turnaround during that period.
b. Storage
Coal storage will be either open or covered. Covered storage is desirable,
since it avoids coal pile run-off containment and treatment. However, the
larger the cement plant, and consequently the quantity of coal to be stored,
the more fixed capital investment is necessary for covered storage. At some
cement plant size, open storage probably becomes economically justified. Open
storage piles must be properly compacted to prevent fires and contamination
and to minimize the buildup of moisture. Cement plants located in geographic
areas of sufficiently severe winter weather must consider the use of de-icing
compounds, such as calcium chloride, to permit recovery of coal as needed.
The use of calcium chloride or other de-icing compounds adds to the coal pile
run-off treatment necessary.
c. Reclaiming
Reclaiming is done either manually or automatically. Manual reclaiming
requires the use of a piece of mobile equipment, such as a bulldozer, to move
the coal from a stockpile into a reclaiming hopper from which the coal is then
automatically fed to a conveyor for transfer to the rest of the system. Auto-
matic reclaiming is done by locating feeders and a conveying system under the
coal storage pile. A large outdoor coal storage area may require a bulldozer
for maintaining the proper shape and compaction of the coal pile, whether
automatic or manual reclaiming is used.
d. Pulverizing
Coal is usually pulverized in a ball mill or a roller mill. The roller
mill is also referred to as a bowl, ring-roller, or ball-race mill. Typically,
both types of mill are used in a direct-dash firing system, where the flow rate
of coal to the mill is controlled or metered and the pulverized coal, as soon
as it is sufficiently pulverized, is blown directly into the burner in the
rotary kiln. In this system, there is no storage of fine pulverized coal. The
indirect firing system uses an intermediate storage of fine pulverized coal,
but the fire and explosion potential that accompanies such storage has dis-
couraged its use.
66
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5. Economic Aspects
a. Fixed Capital
The costs required to convert a cement plant from oil or gas to coal
firing are highly site-specific. Therefore, the following example of a
recent fuel conversion program is cited to provide a set of specific cost
data.
Amcord, Inc., ranked seventh in cement production in the U.S., with an
annual cement production capacity in excess of 4 x 106 tons, recently con-
verted its western cement plants to coal from gas and oil. Three separate
plants were converted, at a cost of $9 million. A total of 11 kilns are
operated at three separate cement plant location (Table IV-19).
The Clarksdale, Arizona plant of Phoenix Cement was converted first.
Actual construction on this 3-kiln plant began early in 1974. The 6-kiln
conversion at Riverside Cement's Oro Grande, California plant followed.
The third and final conversion, involving two kilns, was at the Crestmore
plant at Riverside, California.
The annual production capacity of these three plants is 2.5 x 10 tons
per year. The fixed capital investment required for this conversion to coal
fuel, therefore, was $3.60 per annual ton of cement production capacity.
TABLE IV-19
PLANT CHARACTERIZATION DATA FOR THREE AMCORD PLANTS
CONVERTED TO COAL FUEL
Plant Location
Clarksdale,
Arizona
Riverside,
California
Oro Grande,
California
Process
Number of Kilns
Capacity (000 rpy)
Number of Preheaters
Kiln Data
Year
Number
Capacity (000 tpy)
Dry
3'
620
2
1959
2
189
1961
216
Dry
2
733
0
1964
2
432
Dry
6
1,147
1
1948
3
162
1951
2
162
1959
1
180
Source: Portland Cement Association U.S. Portland Cement Industry:
Plant Information Summary, December 31, 1974
67
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b. Operating Costs
Table IV-20 shows the estimated cement manufacturing cost at a 1350-tpd
cement plant using coal fuel. For purposes of direct comparison, the basis
for this estimate is the same as for the base case (Section IV-A).
6. Environmental Aspects
Two major environmental aspects attend the use of coal fuel for the
manufacture of portland cement:
• Fugitive coal dust emissions from receiving, storing, and handling
the coal; and
• Emissions to air and water of the products from burning coal, par-
ticularly the coal ash contained in the cement kiln dust.
a. Fugitive Dust Emissions
Airborne coal particulates will be generated by the receiving and handling
of coal. The primary sources of this dust will be during railcar unloading.
A water-spray system should satisfactorily eliminate this source. The con-
tainment and treatment of rainwater run-off from open coal storage will require
suitable grading, diking, and treatment facilities. The treatment of run-off
from coal piles where de-icing compounds such as calcium chloride are used will
require special attention.
The main points of fugitive coal dust emission and methods for their con-
trol are as follows:
• coal transport to and from plant - Rail cars and conveyors probably
will have to be covered.
• coal storage piles - Wet pile storage would probably have to be
used, or else silos and wind breakers will be employed.
• stacker/reclaimer - The conveyor would be covered and a suitable
hood built to enclose the reclaim wheel.
• coal conveyors - Transfers would be fitted with suitable hoods, and
conveyors would be covered.
• crushing and screening building - Transfer points would be hooded,
building vents would be enclosed and treated.
68
-------
TABLE IV-20
PORTLAND CEMENT PRODUCTION COST:
LONG ROTARY KILN (COAL FIRED)
Product: Type I Portland Cement
Byproducts;
Process: Long Rotary Kiln Dry Process Location: U.S. East Coast
Annual Capacity; 445.500 tons
Annual Productioni 445,500 tons
Fixed Investment: $45 x 10
Stream Days/Yr.:
330
Working Capital; $2-8 x 10
VARIABLE COSTS
Raw Materials: Limestone
Argillaceous
Compounds
Gypsum & Minor
Additives
Purchased Energy: Fuel (Coal)
Electric Power
Water: Cooling
Operating Labor
Labor Overhead
Operating, Main. & Repair Supplies
FIXED COSTS
Plant Overhead
Taxes & Insurance
Depreciation
TOTAL PRODUCTION COST
Pre-Tax Return on Investment
TOTAL
Units
incl
incl
106 Btu
kWh
103 gal
Man-Hour
Quantity/Ton
uded in other c<
uded in other c<
4.2
130
0.45
0.6
30% of Operating L
23
70?
of Plant Cost
of Operating L
Unit Cost
>sts
)StS
$1.00
0.02
0.03
6.00
abor
abor
2% of Plant Cost
20 year, straight line
t
20%
$/Ton
1.00
4.20
2.60
0.01
3.60
1.08
2.02
2.52
2.02
5.05
24.10
21.46
45.56
69
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b. Coal Combustion Products
Products of environmental concern from coal combustion are SOX and coal
ash. The sulfur oxides appear to react readily with the alkalies (potassium and
sodium) in cement raw materials, as well as the calcium oxide in the raw
material in the calcining zone of the rotary kiln, or within the suspension
and preheater to form calcium sulfates, and to react with other basic con-
stituents of cement clinker. The operating problems which result from the
presence of too much sulfur dioxide in rotary kiln combustion gas has been
discussed in detail in section IV-A. However, it appears that the amount of
sulfur dioxide in combustion gases emitted from rotary kilns burning coal
with a high sulfur content is still extremely low. This has received con-
siderable attention in the past.
To a large extent, the coal ash from rotary combustion contacts and
chemically combines with the clinkering raw material within the kiln. How-
ever, some of the coal ash escapes from the rotary kiln along with partially
calcined raw feed. This combined dust and fine coal ash is removed from the
gas by either electrostatic precipitators or glass cloth filters before the
combustion gas is emitted to the atmosphere. Depending upon the kind of
cement being produced, the nature of the raw material being used, and the
design and operation of the clinkering system (i.e., long rotary kiln, sus-
pension preheater, etc.), the collected kiln dust is returned to the kiln,
discarded, or both. Rainwater run-off from the typical uncovered storage pile
of waste kiln dust contains a high concentration of soluble potassium and
sodium sulfate and calcium hydroxide. In addition, iron, aluminum, and mag-
nesium are typical major chemical constituents of coal ash. Because of the
thermal history of the coal ash, it is probably present in a glassy or
amorphous state; therefore, it is quite highly chemically reactive, especi-
ally in the highly alkaline aqueous solution formed by the percolation of
rainwater through the dust pile. Any of the many elements present in the coal
ash associated with the discarded kiln dust which are soluble in high pH
aqueous solutions can and probably will be present in such run-off. The actual
elements present in coal ash will depend upon the specifc coal being burned.
The concentration of minor constitutents in the individual raw materials
used for making portland cement is expected to vary considerably from plant
to plant. The literature contains few exhaustive chemical analyses of waste
kiln dust. In one of these (Table IV-21) the typical elements, potassium,
sodium, calcium, and sulfur, predominated, as expected. Also, a high concen-
tration of carbonate coming from the limestone which was not calcined is also
present. Other elements which form compounds, such as sulfates or oxides with
high vapor pressures, were concentrated to a significant extent: e.g., rubidium,
zinc, and lead.
70
-------
TABLE IV-21
TYPICAL COMPOSITION OF DRIED KILN DUST
Clay (HC1 insoluble, fired at 800°c)
Organic substance
Cations
Weight %
4.61
2.06
Lithium
Sodium
Potassium
Rubidium
Cesium
Magnesium
Calcium
Strontium
Na
K +
Rb+
Cs+
Mg4"1"
Sum of Cations
Anions
Fluoride
Chloride
Bromide
Iodide
Carbonate
Sulfate
Sulfide
Borate
Phosphate
Br~
i ~
C03~
$04"
s -
B03-
P04-
0.0064
12.25
24.50
0.475
0.0074
Trace
9.26
0.015
0.46
1.43
0.040
0.0552
29.59
9.06
Trace
0.152
Not detectable
Sum of Anions
Heavy Metals (Weight %)
Heavy Metal Oxides (Weight %)
Chromium
Manganese
jron
Zinc
Lead
Cr
Mn
Fe
Zn
Pb
0.011
0.013
0.84
1.62
0.562
Sum of all determinations
Oxygen (from CaO not bound in carbonate)
Sum of all constituents
Cr2°3
Mn02
ZnO
PbO
0.016
0.021
1.19
2.02
0.607
97.825
2.98
100.805
Source: Davis, T.A. and D.B. Hooks, "Disposal and Utilization of Waste Kiln
Dust from Cement Industry", EPA-670/2-75-043, May 1975
71
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c. Coal Impurities
The composition of the coal impurities is important since they form part
of the waste dust from a coal-fired cement kiln. The major mineral impurities
found in coal, ranked in decreasing order of the amount present, are as
follows: :x
(1) Shale group
• Muscovite
• Illite
• Montmorillonite
(These are principally sodium, potassium, calcium, aluminum,
magnesium and/or iron silicates.)
(2) Kaolin group
• Kaolinite (aluminum silicate)
(3) Sulfide group
• Pyrite
• Marcasite
(4) Carbonate group
• Calcite
• Ankerite
(5) Cloride group
• Sylvite
• Halite
The minor minerals that have been identified in coal, Roughly in order of
decreasing abundance, are as follows:
(1) Quartz (6) Apatite (11) Prochlorite (16) Staurolite
(2) Feldspar (7) Zircon (12) Diaspore (17) Topaz
(3) Garnet (8) Epidote (13) Lepidocrocite (18) Tourmaline
(4) Hornblend (9) Biotite (14) Magnetite (19) Hematite
(5) Gypsum (10) Augite (15) Kyanite (20) Pennitite
Of these 20 minerals, 13 are silicates.
72
-------
The typical limits of the oxides present in the ash of bituminous coals
from the United States are as follows:
Constituent
Percent
Silica (Si02) 20-60
Aluminum oxide (Al20o) 10-35
Ferric oxide (Fe203) 5-35
Calcium oxide (CaO) 1-20
Magnesium oxide (MgO) 0.3-4
Titanium oxide (Ti02) 0.5-2.5
Alkalies (Na20+K20) 1-4
Sulfur trioxide (S03) 0.1-12
The West Virginia Geological Survey analyzed 596 spot samples for 38 ele-
ments from 16 coal beds representing major coal-producing areas of that state
(Table IV-22).
TABLE IV-22
COMPOSITION* OF WEST VIRGINIA COAL ASH
Oxide
Li20
Na20
K20
Rb20
CaO
SrO
BaO
MgO
A120'3
Si02
Fe2°3
Ti02
Ag20
AS2°3
B203
BeO
B1203
Average,
Percent
0.075
1.78
1.60
.030
2.76
.38
.22
.98
29.9
43.9
15.9
1.52
.0010
<.07
.12
.008
<.004
.010
Oxide
CoO
Cr203
CuO
GaO
Ge02
HgO
La203
MnO
NiO
P2°5
PbO
Sb2°3
Sn02
v2o5
wo3
ZnO
Zr02
Average,
Percent
.010
.023
.061
.022
.011
.011
.030
.046
.016
.01,7
.35
.048
<-005
.020
.050
•e.Ol
.053
.029
* Spectrographically determined
Source: Leonard, J.W. and D.R. Mitchell, Editors, "Coal
Preparation", 3rd Edition, AIME, 1968
73
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d. Kiln Dust
(1) General
The most significant impacts on the environment of the various effluent
streams from a cement plant are associated with, or result indirectly from,
the solid particulate matter carried out of the rotary kiln by the exiting
combustion gases. The available literature contains little data concerning
the quantity and composition of this kiln dust because:
• only during approximately the last 15 years has kiln dust been dis-
carded as a waste material by any significant number'of cement
plants. Prior to that time, kiln dust was considered a valuable
material, representing a considerable amount of processing cost,
and was returned to the kiln for conversion into cement clinker.
• The quantity and chemical composition of kiln dust is very variable,
and is quite sensitive to the operating conditions of the rotary
kiln, and to the nature of the raw material components, as well as
to the chemical composition of the raw feed.
The motivation for the disposal.of kiln dust was the trend of continually
increasing cement compressive strength and steadily declining cement alkali
content. The specification for low alkali cement (0.6% total alkalies ex-
pressed as Na2<3) is not unusual for major projects such as municipal water
facilities, to avoid the destructive alkali-aggregate reaction. These two
cement quality characteristics have tended to become widespread among cement
users, resulting in cement plants discarding progressively more kiln dust
to diminish the alkali content of the finished cement.
(2) Dust Quantity
The quantity of dust carried from a portland cement-rotary kiln usually
varies from 3% to 40% of the clinker production. Usually, the amount increases
directly in accordance with kiln production rate because ate the latter
increases, the fuel consumption rate must also increase to provide sufficient
heat for clinkering. This increased fuel consumption commensurately increases
the production of hot combustion gases, consequently resulting in an increase
in the kiln gas velocity within the rotary kiln itself. This increased kiln
gas velocity, therefore, carries a higher quantity of dust particles from the
kiln. As a rotary kiln is operated over its design production capacity,
usually the production of kiln dust drastically increases. This interrela-
tionship of processing parameters results in the description of maximum pro-
duction capacity from any cement rotary kiln, since the collection and return
of a rate of dust generation equivalent to more than 40 or 50% of the clinker
production rate usually makes operation technically and probably economically
undesirable or infeasible.
74
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Therefore, the quantity of dust generated per ton of cement clinker pro-
duced by a rotary kiln system, which is being operated at its design capacity,
can be set within some reasonable limits.
Technically, kiln dust can be returned to a cement rotary kiln in any one
of several ways. The two most common are: (1) addition to the raw material
feed going to the rotary kiln feed end; and (2) insufflation. In the latter
case, the dust is pneumatically conveyed through a pipe adjacent to and
aligned in parallel with the fuel burner pipe in the firing end of the kiln.
The incoming cloud of suspended dust particles is rapidly heated and tends to
adhere to the coating adhering to the refractory lining of the rotary kiln, as
well as to the nodules of clinker.
The recently published results of a survey among 101 cement plants con-
cerning the disposal and utilization of waste kiln dust (Davis, T.A., &
D.B. Hooks, "Disposal and Utilization of Waste Kiln Dust from Cement Industry"
EPA-670/2-75-043, May 1975) showed that 57 discard some and 16 discard all of
the -dust which is collected. The most common current method of dust disposal
is to simply pile the dust on cement plant property. Between one-third and
two-thirds of the total alkalies present in kiln dust are water-soluble, and
are continually leached from the dust pile by rainwater. The leachate from
such piles typically has a pH in the range of 12-13. It has also been reported
that this high pH does not appear to diminish rapidly, since an old pile of
kiln dust had rainwater run-off of 12-13 pH even after five years with no new
dust additions.
The high pH characteristic of rainwater run-off from waste kiln dust piles
is probably in large part due to the calcium hydroxide produced by the hydra-
tion of calcium oxide in the dust. Typically, the pH is lowered by either the
addition of waste acid to this leachate or by bubbling carbon dioxide through
a reservoir of this leachate. The carbon dioxide is conveniently obtained at
a cement plant by taking some of the stack gases, which contain a high con-
centration of carbon dioxide, and sparging them into the supernatent liquor on
the surface of a waste kiln dust disposal pond.
(3) Reuse of Kiln Dust
At some cement plants, kiln dust which is discarded because it contains
too high a concentration of alkalies is reused after the water-soluble alkali
values have been leached by water treatment. Typically, the leaching occurs
in a waste kiln dust pond, where the water can be recirculated to the hydraulic
conveying system for transporting dust to the pond, until it becomes saturated
in alkali salts. Periodically, the leached solids are dredged and reintroduced
as an additive to the raw feed, with suitable chemical correction of the raw
feed.
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(4) Chemical Composition of Dust
The chemical composition of kiln dust varies so widely that it is very
difficult to characterize this waste material, or the recycled material.
This is one of the reasons that the literature contains very few analyses of
kiln dust. In one chemical analysis of the potassium and sodium content of a
sample of kiln dust from an electrostatic precipitator, as a function of par-
ticle size, the collected sample contained approximately 0.4% Na20, and 9%
K20 (Table IV-23). But a complete chemical analysis, including minor and trace
elements, is typically not done on kiln dust. A sample of kiln dust which was
collected in an electrostatic precipitator at a cement plant in Blaubeuren,
West Germany, analyzed by chemical methods and X-ray fluorescence spectro-
scopy, showed an extremely interesting high concentration of rubidium, zinc,
and lead. The high concentration of zinc and lead are probably due to the
relatively high vapor pressures of the oxides of these materials.
Depending upon the specific compounds formed in the high-temperature
clinkering zone, the rotary kiln gases will contain species of varying vapor
pressures (or volatilities) and concentrations. As a result, certain elements
will be volatilized to a greater extent than others, and will tend to concen-
trate in the kiln dust, rather than in the clinker. For example, under the
oxidizing conditions present in the high-temperature zone of the rotary kiln,
zinc will probably oxidize, and due to the high vapor pressure of zinc oxide,
will tend to volatilize, and reform as a fume in the cooler regions of the
kiln. Due to the very small particle size of this fume, it will probably be
concentrated in the dust collected by the last compartments of the electro-
static precipitator. Also, the alkali family of elements, potassium, sodium,
rubidium, etc., will tend to be concentrated in the kiln dust rather than
in the clinker due to the formation of sulfates and chlorides of these elements;
and the high vapor pressures of these compounds will tend to concentrate these
elements in the kiln dust.
TABLE IV-23
PARTICLE SIZE ANALYSIS AND DISTRIBUTION OF ALKALIES IN A SPECIMEN
KILN DUST FROM AN ELECTROSTATIC PRECIPITATOR
Total Alkalies Water Soluble Water
Particle Size Weight £%) Alkalies (%) Insoluble
Range (Microns) Percent Na20 K20 Na20 K20 K20 (%)
+68 0
-68+48 0.3
-48+34 0.4
-34+24 0.7 0.35 4.51 0.094 1.927 2.58
-24+17 1.8 0.38 5.08 0.117 2.560 2.52
-17+12 5.1 0.40 5.15 0.134 3.072 2.08
-12+6 27.3 0.33 5.35 0.134 3.252 2.10
-6 64.4 0.42 10.72 0.242 8.191 2.53
* Insufficient sample for analysis
Source: Davis, T.A. and D.B. Hooks, "Disposal and Utilization of Waste
Kiln Dust from Cement Industry"; EPA-670/2-75-043, May 1975
76
0.30
0.31
0.35
0.38
0.40
0.33
0.42
3.62
3.46
4.51
5.08
5.15
5.35
10.72
*
*
0.094
0.117
0.134
0.134
0.242
*
*
1.927
2.560
3.072
3.252
8.191
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e. Water Pollution
A coal-fired cement plant of the same capacity (1350 tpd) and general
process configuration as the base case will produce a cooling water wastewater
stream and a dust storage pile run-off stream of exactly the same size and com-
position as that of the base case cement plant. (See Appendix C.)
However, an additional wastewater stream must be dealt with: i.e., run-off
water from the coal storage area. The composition of coal pile run-off water
can vary greatly, but generally the water tends to be slightly acidic and may
contain a variety of sulfur compounds and leached heavy metals. The generally
recommended form of treatment consists of clarification with lime, which is
intended to neutralize the water, remove suspended solids, and precipitate
heavy metals. The coal pile storage area will be about 33,000 ft2. Unlike
discarded dust, which is often left in a pond partially filled with water, the
coal pile must be maintained in a relatively dry condition. Thus, all run-off
from the area must be collected and treated; it cannot be allowed to accumulate.
The coal pile run-off wastewater collection and treatment system must therefore
be sized for the total annual precipitation, not just precipitation in excess
of evaporation. For the purpose of cost estimates, we used a rainfall rate of
30 inches per year, which amounts to an average flow rate of 17,000 gpd. To
contain surges from heavy storms a substantial surge capacity must also be
provided.
Because of the need to treat coal pile run-off, the treatment cost
(Table IV-24) is substantially higher than that of the base case cement plant:
$0.75/ton vs $0.45/ton.
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TABLE IV-24
WASTEWATER TREATMENT COSTS: COAL FIRING
Basis
1350 tpd Cement Production
330 Operating Days Per Years
CAPITAL INVESTMENT - $931.000
Annual
Quantity
Cost Per
Unit
Quantity
VARIABLE COSTS
Operating Labor
(including overhead)
Maintenance
(including Labor & Mtls)
Chemicals
Sulfuric acid
Lime
Electrical Power
TOTAL VARIABLE COST
FIXED COST
(Depreciation @ 5%)
(Taxes & Insurance @ 2%)
TOTAL FIXED COST
3075 man-hr $12/hr
Quantity
Per Ton of
Production
0.007
Unit Cost
($ Per Ton
of Product)
0.0828
0.0836
14 tons
1 ton
365,0007
kWh
$100/ton
$100 /ton
$0.02/
kWh
3.1xlO-5 1
2.2x10-6]
0.0034
0.0164
0.1862
0.1045
0.0418
0.1463
TOTAL ANNUAL COST
RETURN ON INVESTMENT @ 20%
0.3325
0.4180
TOTAL
0.75/ton
Notes;
1) Capital investment adjusted to 1975 level (ENR Construction
Cost Index = 2126)
2) Wastewater treatment includes:
a) Non-contact cooling water thermal pollution control via spray pond
b) Dust pile runoff containment, collection, clarification, and
neutralization
c) Coal storage pile runoff containment, collection, lime precipita-
tion and clarification
3) Estimates are for the specific example of a dry-process, non-leaphing
cement plant, and are in no way intended to represent industry-wide
wastewater treatment costs.
Source: Arthur D. Little, Inc. estimates
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V. IMPLICATIONS OF POTENTIAL INDUSTRY/PROCESS CHANGES
A. SUSPENSION PREHEATER AND FLASH CALCINER
1. Environmental and Energy Impact
The suspension-preheater-equipped rotary kiln requires less heat energy,
of fuel, per ton of cement clinker produced than any other commercially avail-
able clinkering step. Although the flash calciner may have a slightly lower
energy requirement, both of these process alternatives to the long rotary
kiln are reported to be very similar in thermal energy requirements. It also
appears that the suspension preheater or flash calciner represents the low-
est total energy (i.e., combined electrical and"fuel energy) required for pro-
ducing portland cement of any of the commercially available process alternatives.
Neither the suspension preheater nor flash calciner appear to present any
new dimension to the environmental aspects of the manufacture of portland
cement. In fact, when these processes are operated on suitably low alkali
raw materials, and with suitably low sulfur fuels the present state-of-the-
art employs total dust recycle, which eliminates the environmental problems
associated with disposal of waste kiln dust. In addition, the flash calciner
appears to produce combustion gases with a. significantly lower NOX content
than either the suspension preheater or the long rotary kiln.
2. Systems Implications
An important overall (systems) implication of the use of the flash cal-
ciner is that significant quantities of high-temperature gases are available
for drying raw materials. The temperature of these gases is typically higher
than can be used with a conventional closed-circuit ball mill raw material
grinding system. However, the roller mill is gaining rapid and wide accep-
tance for raw material grinding. This new mill can be operated with these
high-temperature gases and can utilize their additional drying capacity to
help grind raw materials of significantly higher moisture content. The roller
mill is reputed to grind raw materials for cement making with a 25-35% raw
grinding energy savings.
3. Probability of Change
The acceptance of the suspension preheater process alternative is wide-
spread throughout the world, and has recently risen to a high level in the
United States. It appears that the U.S. cement industry will continue to
move from the wet to the dry process, and toward the suspension preheater in
the latter.
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Although the flash calciner has been totally accepted in Japan and in
much of Europe, the U.S. cement industry appears to be waiting for a demonstra-
tion of satisfactory performance by its first flash calciner, which is presently
being completed in the United States.
One of the major advantages of the flash calciner is the extremely high
output of a moderate-sized rotary kiln, which permits the construction and
successful operation of extremely large cement plants—possibly as high as
10,000 metric tons per day capacity—with rotary kiln sizes which are small
enough to insure good performance. One of the reasons for its wide acceptance
in Japan is that the Japanese cement market is geographically distributed to
favor such very large single cement plants.
However, in the United States, cement plants of 2,000- to 3,000-tpd capac-
ity are typical. Therefore, the advantages which derive from the use of the
flash-calciner-equipped rotary kiln may not be sufficiently great in the
United States to cause it to be favored this strongly over the suspension
preheater. The roller mill for raw material grinding effectively combines
with the flash calciner. Other key aspects of the flash calciner are its
significantly lower NOX emissions, better refractory life, and apparently
slightly lower fixed capital investment (and probably slightly lower operating
costs) as well. These aspects along with the ability of the flash calciner to
operate on raw materials containing fuel values such as oil shale will prob-
ably motivate other cement manufacturers to install this newest process alter-
native to the long rotary kiln.
B. FLUIDIZED-BED CEMENT PROCESS
1. Environmental a_nd Energy Impact
t_,
No Commercial installations of the fluidized-bed cement process are oper-
ating, or have ever been constructed. A 100-tpd semi-commercial fluidized-bed
facility was successfully operated for several years. This process is pres-
ently offered by two U.S. firms. It appears that the total energy required
for the manufacture of cement by this process is about 10% more than that
required for cement manufactured by the four-stage suspension-preheater-
equipped Notary kiln, and is therefore potentially directly competitive with
the suspension preheater system which is rapidly assuming the preeminent posi-
tion of standard process technology in the international portland cement
community.
The fluidized-bed cement process also appears to emit extremely low levels
of particulates compared with a rotary kiln, and these particulates are essen-
tially all water-soluble alkali sulfates. This holds the promise of eliminat-
ing large landfill or storage pond areas for the satisfactory and environmen-
tally acceptable disposal of waste kiln dust. In- fact, these alkali sulfates
could be valuable byproducts from cement manufacture, and could possibly/be/
sold for their potash value.
Due to the exceptionally high alkali volatilization characteristics of
the fluidized-bed process, waste kiln dust from conventional rotary kiln cement
manufacturing operations (high in alkali) can form either a main raw material
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component or the total raw material (suitably adjusted in chemistry to make
cement clinker) for the fluidized-bed cement process. Therefore, an exist-
ing cement plant, which must discard waste kiln dust in order to produce sat-
isfactory cement quality, could operate a fluidized-bed cement process to
convert the waste kiln dust from the rotary kiln operations into acceptable
cement, thereby dtastically reducing or totally eliminating the environmental
aspects associated with the disposal of waste kiln dust.
2. Systems Implications
The fluidized-bed cement process is a total departure from any of the
other clinker-producing cement process alternatives. Its economic viability
rests on heat recuperation from the hot combustion gases leaving the reactor,
at least in part, in the generation of steam. This steam could be used
directly through turbine drives, or indirectly through the generation of elec-
trical power, to supply all of the electrical energy requirements for the
rest of this cement plant (e.g., raw material and finish cement grinding, and
supply of fluidizing and combustion air at suitable pressure).
Several decades ago, the utilizing of waste heat from rotary kiln plants
was accomplished through the generation of steam for electrical power production.
This was ultimately abandoned due to the high operating and maintenance costs
associated with cleaning the resulting raw material and alkali sulfate deposits
which developed rapidly on the tubes, as well as the problems associated with
maintaining and operating electrical generating equipment at plant sites with
high particulate concentrations in the ambient air. The generation of steam
associated with the fluidized-bed cement process is totally different, in that
the high-temperature gases leaving the fluidized-bed reactor could generate
high-pressure steam, rather than the low-pressure steam associated with true
waste heat boiler operation of old cement plants. In addition, it is reported
that the concentration of particulates in this high-temperature gas is very
low compared with rotary kiln waste gas, which permits the development of non-
fouling boiler tube and steam generating designs. In fact, it is reported that
the fluidized-bed cement process could generate more electrical energy than is
required by a cement plant, and such a cement plant could be a supplier of
electrical energy to the power grid, rather than a consumer of electrical
energy. The widespread acceptance of ,the fluidized-bed cement process would
therefore have a major impact upon energy generation and transmission patterns
and demand upon power generating stations.
3. Probability of Change
The first successful production of portland cement in a rotary kiln in the
United States was achieved in 1890. Since then, the rotary kiln has grown in
diameter, length, and production capacity up to a giant kiln 27 feet in diameter
and 700 feet long which has a production capacity of 1.2 x 10$ tpy.
For decades, the rotary kiln was the only technically sound way of con-
tinuously producing portland cement clinker on a relatively large scale and with
good quality control. Because of this century-long history of development,
the cement rotary kiln has been-firmly entrenched as the primary clinkering
alternative technology.
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The suspension preheater process alternative to the long rotary kiln was
commercially accepted approximately 18 years after it was developed. Inter-
estingly, the fluidized-bed cement process was developed about 17 or 18 years
ago. It does appear that the U.S. cement industry is seriously considering
the fluidized-bed cement process, at least initially as a process alternative
for converting waste kiln dust from rotary kilns into marketable cement. We
believe that the first commercial-scale, fluidized-bed cement process will be
commissioned in the very near future, probably within 5 years.
Another major resistance to the adoption .of this process alternative to
the rotary kiln, is that the main technology which constitutes this process
alternative is foreign to the cement industry, even though the chemical proc-
ess industry at large has employed large, high-temperature fluidized-bed i
reactors for decades. We believe that the recent engineering design studies
and commercial offering of the fluidized-bed cement process by Scientific
Design within the past year are a significant new motivating factor in the
consideration and acceptance of this new process technology by the U.S. cement
industry.
C, CONVERSION TO COAL FUEL FROM OIL AND NATURAL GAS
1. Environmental and Energy Impact
The energy conservation potential of the use of coal fuel is primarily
one of form rather than quantity of energy. The energy required to transport,
handle, and pulverize coal for suitability of combustion in cement manufacture
may be different from the energy associated with the transportation and use
of oil or natural gas, but we believe the difference will be insignificant.
The environmental effects of the switch to coal, however, will be pri-
marily the fugitive dust emissions from the handling and storage of coal,
which will have to be suppressed to comply with air pollution regulations.
In addition, the rainwater run-off from outdoor coal storage will contain some
particulates and also soluble de-icing compounds used in northern latitudes
to prevent" freezing of coal storage piles. This will have to be collected
and treated.
However, the coal ash from the combustion of coal and the manufacture of
cement is an additional raw material component and chemically and physically
combines with the clinkering raw materials to form cement. Therefore, this
market for fly ash converted to the form of cement will tend to offset any
increase in energy required by the use of coal.
2. Systems Implications
There appear to be no noteworthy systems implications of the conversi'on
of fuel form to coal.
3. Probability of Change
The probability of conversion to coal fuel is extremely high, as evidenced
by the actual conversion from natural gas and oil to coal fuel by cement plants
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in the United States. The major constraints upon the rate at which this con-
version will occur are as follows:
• availability of sufficient coal of appropriate quality;
• sulfur content of available coal - although cement raw materials
absorb all of the SC>2 from the combustion of high sulfur coal, if
the sulfur level of the coal becomes too high, severe operating
problems occur with suspension preheaters and flash calciners, as
well as unacceptable chemical and physical characteristics of the
finished cement;
• availability of suitable coal pulverizing and handling systems and
equipment.
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APPENDIX A
BASE LINE CEMENT TECHNOLOGY
1. FEEDSTOCKS
Hydraulic cement is a powder made by burning lime, silica, alumina, iron,
and magnesia together in a kiln and then pulverizing the product. It reacts
with water to bond rock or sand and gravel into concrete. During '1973, 139 x
106 tons of raw materials were used to manufacture 85 x 1C)6 tons of cement;
i.e., 1.6 tons of raw materials are needed to produce 1 ton of cement. Weight
is lost during calcination in the kiln when moisture, carbon dioxide, and ,
other gases are driven off.
In making cement, more limestone is used than any other raw material
(Table A-l) since it provides one of the key materials in cement manufacture—
lime (CaO). Other sources of lime include shells. Areas bordering the
Gulf of Mexico and San Francisco Bay provide oyster shells, while in Florida
coquina shells are used. Recently, oolitic aragonite sands from the Bahamas
added another source of lime. Occasionally, slag and other industrial wastes
rich with lime have been used in cement manufacture. Natural argillaceous
limestones known as cement rock are found in the Lehigh Valley in Pennsylvania.
These deposits contain the correct amount, of all cement making raw materials
so that no other material needs to be mixed in. Marls are accumulations of
calcareous material secreted by plants or animals in lakes and marshes; they
are important in Michigan and Ohio.
When alumina and silica are not present in the limestone in sufficient
amounts, secondary raw materials are needed to supply the balance. The ratio
of silica and alumina has to be controlled closely. Natural sources of silica
include sediments, i.e., sand, silt, clay and loess, or their corresponding
rocks, i.e., sandstone, siltstone, shale, or mudstone. Alumina sources include
mud, clay, loess and related rocks and wastes, such as fly ash, slag, red muds
from bauxite processing and wash plant or mill tailings. Coal fly ash can
contribute significant amounts of raw materials.
Iron is sometimes added in small amounts to adjust the composition of the
cement mix. Commonly used sources are iron ores, mill scale, and certain /
metallurgical process waste slags. In recovering these mineral raw materials,
the tonnage of overburden handled each year may equal the amounts of raw ,
materials used.
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TABLE A-l
TYPES AND QUANTITIES OF RAW MATERIALS USED IN PRODUCING
PORTLAND CEMENT IN THE UNITED STATES1, 1972-1973
(Thousand tons)
Raw Materials
Limestone (includes aragonite)
Cement rock (Includes marl)
Oyster Shell
Argillaceous:
Clay
Shale
Other (includes staurolite, bauxite,
aluminum dross, pumice, and
volcanic material)
Siliceous:
Sand
Sandstone and quartz
Ferrous:
Iron ore, pyrites, millscale, and
other iron-bearing material
Gypsum and anhydrite
Blast furnace slag
Fly Ash
Other
Total
811,922 86,699
25,879 26,067
5,081 5,144
7,931
4,099
2,053
748
8,062
4,096
1,993
781
839
4,094 4,253
759 682
271 299
33 5
136,920
139,188
Includes Puerto Rico
Source: U.S. Bureau of Mines, Minerals Yearbook
2. PROCESSING
Processing of raw materials into finished cement follows four steps:
• Crushing
• Grinding
• Clinkering
• Finish grinding
a. Crushing
The first step is simply size reduction. Depending on the raw material
and the design of the raw grinding system, the crushing system can vary con-
siderably. In the usual case with limestone, crushing produces material at a
given maximum size, which typically varies from 3/8 to 2-1/2 inch in diameter.
Primary, secondary, and often tertiary crushing stages produce the product for
the mills.
The types of crushers used vary according to the hardness, size, and type
of the rock. Primary crushers include gyratory crushers, which consist of a
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steel cone moving eccentrically inside a cone-shaped housing; jaw crushers, in
which the rock is broken between substantially vertical moving breaker plates;
and roll crushers, where the reduction takes place between two rolls, which
often have case-hardened steel teeth, or between one roll and a breaker plate.
Typically, primary crushers reduce the rock from shovel size to 5 inch in
diameter, while secondary crushers then reduce the rock to 3/4 to 2-1/2 inch
in diameter. Secondary crushers include hammer mills in which the rock is
fractured by heavy hammers swung by centrifugal force from a horizontal shaft.
The crushed stone is then transported by elevator and belt conveyors for stor-
age in separate compartments by raw material type (limestone, shale, etc.).
b. Raw Grinding
Through milling, sizes and mixtures of raw materials are prepared. Not
all raw materials can feed directly into grinding mills, especially wet raw
materials like clays and chalks which first require disintegration in wash
mills where a slurry containing about 50% water is formed. This wash mill
product, known as a clay slip, is then fed into wet mills with the roughly
proportioned amounts of limestone and other raw materials. The first wet mill
stages are commonly ball mills, which get their name from the thousands of
large steel balls inside -them which are carried on ribs up one side of the
rotating mill and cascaded down onto the materials being ground. The second
stage of grinding occurs in tube mills, which are similar to ball mills but
are longer, of smaller diameter, and charged with smaller balls. Often ball
and tube mills are combined into a single machine which has two or three com-
partments separated by perforated steel diaphragms and charged with differently-
sized grinding balls.
Wet milling produces a slurry of the ground kiln feed in which the water
content is kept as close as possible to the minimum that can be pumped success^
fully, i'.e., 30-45%. Coarse fractions are returned to be ground again while
the finished fractions are pumped to storage tanks or basins for blending.
Sedimentation in the tanks is prevented by constant agitation, either mechani-
cally or with air.
The dry milling is very much like the wet except that no water is added
and the material is ground dry, usually at 1% moisture content or less. When
necessary, dryers are used, supplied with either their own heat sources or
recuperated kiln heat. The variety of mills is greater in dry process plants,
which in addition to ball and tube mills, use vertical, roller and ball-race
mills. Air separators classify the milled product and return the coarse frac-
tion to the milling system. The finished fractions are then blended and homog-
enized before going to the kiln. It is common practice to combine drying and
raw grinding in a single, closed-circuit ball mill system.
c. Clinkering
In the clinkering step, the accurately controlled mixture of raw materials
reacts chemically at high temperatures in the kiln to produce clinker, which
is subsequently ground into cement. The kiln is the heart of the cement plant;
thus, any plant capacity changes reflect changes in kiln capacity.
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Briefly described, rotary kilns are nearly horizontal steel cylinders
which range up to 25 ft in diameter and 750 ft in length. Inside, the kiln
is lined with refractories to protect the steel shell and conserve heat. The
inclination of the kiln together with its rotation at rates near one revolu-
tion per minute causes the kiln feed to move gradually down the kiln toward
the lower, discharge end in several hours.
The burner is at the discharge end. Thus, the flame is pointed in a
direction opposite to the feed move direction. As the feed travels toward the
firing end, it gets progressively hotter. At first, the heat causes water to
evaporate; then it causes carbon dioxide to be driven off during calcination
of the carbonates. As the feed approaches the discharge end, it enters the
hottest zone, with temperatures about 2,800°F, where the main chemical reac-
tions of hot lime with silica, alumina, and iron begin causing clinker to
form. While the size of clinker ranges greatly, a typical range is "buck-
shot" to "golf ball" size.
Many kilns use chains to improve the heat exchange between the hot gases
and feed as they move towards the burning zone. The combustion gases pass
through the kiln countercurrent to the material and leave the kiln through
its feed end at temperatures between 600° and 1,600°F, depending on the kiln
length and the process used.
Formerly, exit gas temperatures from dry process kilns were so high that
waste heat boilers were used to generate all the electric power used in the
cement plant. But the cost of such a system increased more rapidly than the
.cost of purchased power. Besides, most new kilns have low exit gas tempera-
tures, thus making purchased power more attractive.
Typically, wet process kilns are designed to be longer than dry kilns
since part of the kiln is used to evaporate the raw feed slurry water. Such
kilns are equipped with elaborate arrangements of chains which serve as heat
exchangers between the gas stream and slurry.
After leaving the kiln, the clinker enters coolers which reduce its tem-
perature before storing or grinding and recover its heat for reuse inside the
kiln. There are numerous ways for cooling clinker, ranging from primitive
pits to highly sophisticated forced air-cooled reciprocating grate units. These
grate units permit a blast of coolin'g air to pass through a slowly moving bed
of hot clinker. When air quenching is used, often the clinker quality is
improved because the magnesia freezes into the glass phase in the clinker.
Slow cooling could permit the magnesia to crystallize, producing delayed expan-
sion and cracking in the final concrete. Other types of coolers include rotary
coolers, which are separate cylinders located under the kilns, and planetary
coolers, which consist of smaller cylinders built around, and thus rotating
with, the kiln.
d. Finish Grinding
Beyond the cooler, clinker generally is moved by cranes or conveyors into
storage, where it is segregated, tested, blended, and moved into bins for feeding
to the finish grinding mills. The resulting quality of the cement product varies
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with the type of storage and time of grinding. Typically, 3-6% gypsum is
interground with clinker to .control the setting time of the final concrete.
The mills used in finish grinding are essentially the same as those used
in raw grinding. Rod, ball, roller, race, and tube mills are found in various
finish-grinding installations. Most finish-grinding systems are closed-circuit
systems in which air separators provide classification. Fine finished products
are sent to storage while coarser fractions are returned for further grinding.
3. PRODUCTS
Superficially, cement seems to be a one-product industry, but the prod-
ucts are more complex. Different cement types are distinguished by (1) their
proportions of lime, silica, alumina and iron and (2) the specifications
which they meet.
Of all the hydraulic cements, more portland cement is produced than any
other cement. Five types are recognized in the United States:
Type I; For use in general concrete construction;
Type II; For use in general concrete construction exposed to moderate
sulfate action, or where moderate heat of hydration is required;
Type III; For use when high early strength is required;
Type IV; For use when a low heat of hydration is required; and
Type V; For use when high sulfate resistance is required.
These five types can be modified or combined with other materials to
qualify for different uses. To act as buffers against freeze-thaw deteriora-
tion, air-entraining agents can be interground with the clinker to produce
the "A" varieties of cement (mainly, IA, IIA, and IIIA).
A number of cements sold under specifications are known by names which
describe their use or composition, including (Table A-2):
• Masonry Cement, tvhich is used in mortars for masonry work;
• Oil-Well Cement, which is designed for use under high temperature
and pressure;
• White Cement, which is ordinary portland cement with a low propor-
tion of iron oxide so its color is white instead of grey;
• Water-Proof Cement, which is designed for stucco work and to improve
water impermeability;
• Portland-Pozzolan Cement, which is produced by grinding together
portland cement clinker and a. pozzolana (a material capable of react-
ing with lime in the presence of water at ordinary temperature to
produce cementitious compounds);
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TABLE A-2
TYPES OF PORTLAND CEMENT SHIPPED IN THE UNITED STATES 1974*
Quantity Value Average Value
103 ton ($) ($/ton)
General use and moderate heat
(Types I and II) 73,474 1,927,557 26.23
High-early-strength (Type III) 2,596 71,423 27.51
Sulfate-resisting (Type V) 323 8,653 26.79
Oil-well 989 27,667 27.97
White 474 26,697 56.32
Portland-slag and portland pozzolan 672 16,843 25.06
Expansive 132 4,681 35.46
Miscellaneous** 822 24.385 29.67
Total or .average 79,482 2,107,906 26.52
^Includes Puerto Rico
**Includes waterproof cement
Source: U.S. Bureau of Mines, Minerals Yearbook Preprint 1974
• Portland Blast Furnace Slag Cement, which is produced by grinding
together a portland cement and granulated blast furnace slag.
Portland blast furnace slag cement usually contains 35 to 45%
by weight of granulated slag. This slag is produced by rapid quench-
ing in water and air of hot slag near 2,500°F as it comes from the
furnace. Because of the rapid cooling, a glass forms that chemically
resembles a low lime clinker, yet it is cheaper.
89
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APPENDIX B
BASE LINE PROFILE OF ENERGY USE IN THE CEMENT INDUSTRY*
1. TOTAL ENERGY
In 1974, the U.S. hydraulic cement industry consumed more than 490 x
Btu of fossil fuels and about 11 x 109 kWh of electricity, which is about 1%
of all energy used in the United States. To produce one ton of cement, the
average cement plant used 6.3 x 106 Btu of fuel and 134 kWh of electricity.
The cost of this energy represented about 40% of the production cost.
The cement industry uses all forms of energy: coal, fuel oils (distil-
lates and residual oils), natural gas, and electricity. The consumption of
coal and natural gas is about equal (Figure B-l).
Source: Arthur D. Little and U.S. Bureau of Mines
Figure B-l. Types of Energy Used by the U.S. Portland Cement Industry, 1974
The U.S. Bureau of Mines is cited throughout this Appendix as a source. In all
cases our information has come either from their Annual Mineral Yearbook or
Monthly Cement Industry Survey.
90
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a. Fuel
Almost 25% of all clinker Is produced in 42 coal-fired plants (Table B-l).
Nearly 22% more of the total clinker production is produced in plants fired
solely by natural gas or oil. The remaining clinker is produced in 89 cement
plants, fired by a combination of fuels: coal and oil, coal and natural gas,
oil and natural gas, and coal, oil, and natural gas.
From 1950 to 1960, the use of coal declined rapidly while the use of
natural gas simultaneously increased (Figure B-2). Between 1960 and 1969, the
use of coal, oil, and gas leveled off. In 1970, the use of natural gas took
the lead, oil use increased dramatically, and coal use fell off. In 1971,
natural gas use peaked and then started its present decline. Meanwhile oil
use increased until 1973, when price and availability caused coal use to
increase,.1
TABLE B-l
CLINKER PRODUCED ,IN THE .U.S. BY KIND OF FUEL, 1974
Number of
Fuel
Single Fuel
Coal
Natural Gas
Oil
Multiple Fuels
Oil and Natural Gas
Coal and Natural Gas
Coal and Oil
Coal, Oil, Natural Gas
TOTAL
Clinker Produced
(percent
Fuel Consumed
Plants (103 ton) of total) (103 ton) (106 cf) (bbl)
42
27
10
19,298
10,980
5,801
24.8
14.1
7.4
4,724
70,246
5,465
31
33
16
9
15,313
12,950
8,465
5.170
77,977
19.6
16.6
10.9
6.6
100.0
-
1,516
1,367
487
8,094
74,843
47,331
-
15,962
208,382
1,902
-
2,604
339
10,310
Note: Includes Puerto Rico
Source: U.S. Bureau of Mines
b. Electric Energy
In 1974, of the total energy consumed in cement manufacture, 18% was electri
city of which 94% was purchased and 6% was generated by cement plants (Table B-2)
The wet process produced 57.5% of the cement but consumed only 54.6% of the
total electricity. Since 1950, total electricity used to produce one ton of
cement has increased, since cement is being ground finer (Figure B-3).
91
-------
50
40
Q
LU
2
CO
O 30
o
>
a
cc
UJ
Z
UJ
_J
J5 20
1-
10
ELECTRICITY
I
1950
52 54
55
58 60
62
YEAR
64 66 68 70 72 74
Source: U.S. Bureau of Mines and Arthur D. Little, Inc.
Figure B-2. Trends in Types of Energy Used, 1950-1974
92
-------
TABLE B-2
ELECTRICAL ENERGY USED IN PORTLAND CEMENT MANUFACTURE, 1974
(106 kWh)
Wet
Dry
Both
Generated
135
499
11
Purchased
5,700
3,938
401
Total
5,835
4,437
412
Electricity
Use/Ton
Cement
127.6
142.9
151.6
TOTAL 645 10,039 10,684
*Equivalent to 1^411,000 Btu/ton (based on 10,500 Btu/kWh)
Note: Includes Puerto Rico
Source: U.S. Bureau of Mines
134.4*
2 =
U
§
TOTAL ENERGY
TOTAL ELECTRICITY
_J I I I
1950 52 54 56 58 60 62 64
YEAR
Source: U.S. Bureau of Mines
70 72 74
Figure B-3. Trends in Fuel and Electricity Use, 1950-1974
93
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2. ENERGY USE BY PROCESS
Of all processes, the one using a pfeheater consumes the least energy.
As shown in Table B-3, the preheater dry or wet process consumes 4.3 x 10°
Btu/ton cement versus 5.7 x 10° and 6 x 10" Btu/ton for the dry and wet processes
respectively. For all processes the clinkering step consumes the most energy,
with an average of 80% of the total energy used, most of which is fossil fuel.
Most electricity, on the other hand, is used in grinding.
a. Grinding
Whether or not the grinding is raw or finish, less energy is consumed in
the wet process than either the dry or preheater process. In the wet process,
98 x 103 Btu/ton of cement are consumed in raw grinding and 197 x 103 Btu/ton
of cement for finish grinding compared with 15 x 103 Btu/ton and 215 x 103
Btu/ton for the dry and preheater processes. The Portland Cement Association
has reported a range or raw grinding, in Btu equivalents of purchased electric
energy, of 9.7-226 Btu/ton clinker and 86-240 Btu/ton clinker for finish
grinding.
TABLE B-3
ENERGY USE BY PROCESS STEP
(103 Btu/ton Cement)
Process Step
Quarrying-Electric
-Fuel
Crushing and Drying
-EjLectric
-Fuel
Raw Grinding-Electric
Mixing Feed-Electric
Clinkering and Cooling
-Electric
-Fuel
Finishing Grinding-Electric
Pack Handling-Electric
TOTAL
Wet
8
16
98
7
86
5,560
197
15
5,996
Dry
6
16
15
600
126
14
Preheater
6
16
15
600
126
14
Source: Garrett, H.M. and J.A. Murray, p. 76, Rock Products, May, 1974
94
-------
b. Clinkering
Energy consumption in clinkering varies according to type of kiln. In
1974, the wet process consumed 61.3% of the total energy consumed in cement
manufacture, yet it contributed only 57.9% of the total clinker produced. The
dry process, on the other hand, consumed 38.7% of the total energy to produce
42.1% of the total clinker (Figure B-4). Since 1950, the wet process has
been consuming a greater percentage of the total energy. Starting in 1950,
the wet process consumed 56% of the total, peaking at a high percentage of
almost 65% in 1969. This difference was also reflected in the dry process,
which dropped in energy use. After 1969, energy use in the wet process
leveled out at around 62.1% (still above the 1950 level) as the dry process
simultaneously leveled out at around 37.9%.
The trends in unit energy use for the wet and dry processes are more dra-
matic (Figure B-5). In 1950, unit energy consumption for the two processes
was within 1%, that is 9.29 x 10" Btu/ton cement produced for the wet process
versus 9.18 x 10^ Btu/ton cement for the dry. By 1974, the spread in values
increased to 13% with the wet process at 8.21 x 106 Btu/ton and the dry at
7.13 x 106 Btu/ton
Figure B-6 shows the distribution for wet and dry plants. Although not
indicated on the figure, there were three wet and dry plants in 1974, one which
consumed less than 6 x 106 Btu/ton, one which consumed 8.1-9.0 x 106 Btu/ton,
and one which consumed 9.1-10.0 x 106 Btu/ton.
70
60
O
cc
IU
z
LU
< 50
O
u.
O
a?
40
WET
DRY
30
1950 52 54 56
58 60 62 64
YEAR
66
68
70
72
74
Source: U.S. Bureau of Mines
Figure B-4. Trends in Energy Consumption by Process Step, 1950-1974
95
-------
9.5 i-
Sourcv: U S. Bureau of Mines
Figure B-5. Trends in Unit Energy Use For Wet and Dry Processing, 1950-1970
45
40
35
to 30
I 25
U.
o
DC
S 20
s-
* 15
10
5
0
TOTAL
<6.0 6.1-7.0 7.1-8.0 8.1-9.0
10* BTU/TON*
9.1-10.0
>10.0
•TONS OF EQUIVALENT PRODUCTION (92% CLINKER AND 8% CEMENT)
Figure B-6. Distribution of Unit Energy Consumption by Number of Plants, 1974
96
-------
Compared with 1972, the percent unit energy consumption shifted by 1974,
when only 17% of the plants used over 9 x 1()6 Btu/ton, down from 21% in 1972
(Figure B-7).
c. Energy Efficiency
Energy efficiency varies according to plant age and size (Table B-4).
On the whole, older plants consumed 21% more energy than newer plants. Now,
only 13% of the cement plant capacity is 40 years old or older. By 1984 this
figure will rise to 30% or more, assuming the same rate of replacement.
With plant size, the larger the plant the more energy-efficient it is.
For all plants the difference in energy efficiency for small and large plants
is 14%, but for wet plants the difference can be as much as 17% compared with
7% for dry plants. Ideally, the larger the plant the better, but there are
practical limitations on plant size.
Table B-5 shows the range of unit energy consumption for the major types
of kiln systems.
CO
cc
g
Q
30
20
10
1972
1974
1
J
<6.0 6.1-7.0 7.1-8.0 8.1-9.0
106 BTU/TON*
9.1-10.0
>10.0
"TONS OF EQUIVALENT PRODUCTION ARE USED
Source: Portland Cement Association and Arthur D. Little
Figure B-7. Percent Distribution of Unit Energy Consumption, 1972 and 1974
97
-------
TABLE B-4
ENERGY EFFICIENCY
Btu/ton finished portland cement)
I. Plant Age
<10 years
10-40 years
>40 years
II. Plant Size
<300 x 103 tons
300-600 x 103 tons
>600 x 103 tons
All Plants
1972
1974
7.57 7.04
7.54 7.56
8.79 8.62
8.32 8.08
7.83 7.77
7.08 6.96
Wet Plants
1972 1974
7.4e
9.1C
8.69 8.67
8.41 8.41
7.14 7.28
Dry Plants
1972 1974
6.8*
7.9
7.57 7.18
6.93 6.87
6.98 6.69
*Based on 10 plants 40 years old or older and 25 plants less than 10 years old.
Source: Portland Cement Association, May 1974 and June 1974
TABLE B-5
ENERGY CONSUMPTION BY TYPE OF KILN
Long Wet Kiln with Chains
Long Dry Kiln
Long Dry Kiln with Chains
Long Dry Kiln with Waste-Heat Boiler
Short Dry Kiln with Grate Preheater
Short Dry Kiln with Suspension Preheater
10 Btu/ton
Clinker
4.7-9.45
5.0-7.8
3.9-6.1
4.95-6.1
3.55-3.85
2.85-4.45
Source: lammartino, N.R., Chemical Engineering
98
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APPENDIX C
CURRENT POLLUTION PROBLEMS AND EFFECTIVENESS OF AVAILABLE
POLLUTION CONTROL TECHNOLOGY
1. AIR POLLUTION
a. Emissions from Cement Manufacturing Plants
The major source of particulate emissions in cement plants is the kiln.
Dust is generated in kiln operations by the hot combustion gases entraining
feed particles. Also involved is the tumbling action within the kiln, the
liberation of gases during calcination, which tends to expel particles into
the gas stream, and the condensation of material that is volatilized at the
firing end of the kiln. Volatilization and condensation generally produce
smaller particles than the mechanical processes, thereby increasing the dif-
ficulty of the air pollution cleanup problems.
In the wet process a dryer is not used. However, in some dry process
plants the raw materials are first dried. The concentration of dust in the
dryer exit gases is related to the velocity of the gases, the quantity and
size of the fine particles, and their degree of dispersion in the gas stream.
The volume of the flue gas from the dryer depends on the moisture content of
the feed material.
As the clinker is discharged from the lower end of the kiln, it is passed
through a clinker cooler that reduces the temperature of the clinker. The
clinker cooler represents another source of airborne pollutants in the cement
plants.
Emissions from the crusher area depend on the type and moisture content
of the raw material and the characteristics and type of crusher. If the mate-
rial has a high moisture content, it may not be necessary to provide dust con-
trol, due to very little emissions.
b. Air Pollution Control Laws
The Environmental Protection Agency has established new sources standards
in some industries. The cement industry is one of these. These standards
are applicable to kiln and clinker coolers and facilities; they are also appli-
cable to modified equipment.
99
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(1) Kiln
• The maximum 2-hour average discharge of particulates shall be 0.3
Ib/ton of feed to the kiln.
• The maximum discharge of particulates shall be limited to 20%
opacity, except where the presence of uncombined water is the only
reason for failure to meet the requirements.
i2) Clinker Cooler
• The maximum 2-hour average discharge of particulates shall be
limited to 0.1 Ib/ton of feed to the kiln.
• The maximum discharge shall be limited to 10% opacity.
(3) Other Facilities
The maximum discharge shall be limited to 10% opacity. Cement plants
scheduled for construction after August 17, 1971, were assumed to be subject
to national standards of performance.
While existing cement plants (starting construction before August 17,
1971) are subject to state air pollution regulations, the regulations vary
from state to state. In the cement industry, the federal standards for new
sources may be used as an average of state standards for existing plants. The
only difference is that the electrostatic precipitator has an acceptable col-
lection efficiency in existing plants, whereas it will not be acceptable
according to the new source performance standards. ^
c. Control Practices and Equipment for Cement Plants
(1) Control Practices
Particulate emissions can be adequately controlled in the cement industry
by proper selection of dust control equipment. Particulate emissions as low
as 0.03 to 0.05 gr/scf have been obtained in newly-designed, well-controlled
plants.
The hot kiln gases are the main source of emissions and they present a
major problem because gas volumes are large; they contain acid gases such as
H2S and S02, varying amounts of 1^0, and are in a temperature range usually
above 500 or 600°F. A kiln producing 20 tons of cement clinker per hour will
produce about 240,000 pounds of exit gases per hour, or about 92,000 acfm.
(2) Control Equipment
(a) Multicyclones
Although a number of types of dust collectors are used in the cement indus-
try, only the high-efficiency collectors, such as the electrostatic precipita-
tor and fabric filter sometimes used in -series with inertial collectors,
100
-------
effectively collect fine dust. The multicyclones alone are not an acceptable
means of reducing dust emission from the kiln to the atmosphere, since they
can only be expected to remove about 70% or all the coarser particles.
(b) Electrostatic Precipitators
In a wet process plant, the performance of an electrostatic precipitator
is greatly enhanced by the extra water vapor in the exhaust gases from the
slurry. Dry process kilns do not have this water in the feed, so it often is
necessary to add water as an aid to precipitator operation. In the past, the
operation of electrostatic precipitators has not been entirely satisfactory
because of decreasing efficiency over extended periods due to the effects of
the cement dust on the high-voltage components. Also, when kilns have been
shut down and then restarted, it has been necessary to bypass the electrostatic
precipitator for periods up to 24 hours because of the danger of explosion
from combustible gas or coal dust.
(c) Fabric Filters
Fiberglass baghouse filters have had much success in controlling kiln
emissions. Bag life averages 18 months or more. A big plus in baghouse instal-
lations is the fact that duct designs are simple and uncomplicated, requiring
little study for the flow of gases when compared with the frequently complicated
model studies necessary for good gas flow patterns in the electrostatic type
dust collector.
Moisture condensation in glass fabric filters can present problems. However,
dew point temperatures are normally avoided by proper application of insula-
tion to ducting, etc., and by proper operation to avoid condensation.
Investment and capital cost estimates were based on the following:
• For dry-process rotary kilns, assumed that glass fabric filters would
be used.
• For existing wet-process rotary kilns, we assumed that electrostatic
precipitators would be used. Investment costs for wet process kilns
were estimated by assuming a migration velocity of 0.35 ft/sec and
a gas volume as predicted by,a linear regression equation.
• We assumed glass fabric filter controls for raw material dryers and
clinker coolers. For the clinker we further assumed that only the
secondary section would be vented to a control system, while air
from the primary section would be returned to the kiln.
The cost data represent March 1975 dollars; we used the Engineering and
News Record Index (ENR Index) to update the cost data obtained from various
sources. The capital cost is extrapolated by using the six-tenths rule. Depre-
ciation was assumed to be straight line over 20 years. Return on investment
(ROI) was arbitrarily estimated to be 20% of the capital investment.
101
-------
d. Effect of Fuel Used in the Kiln on Emissions
Gaseous emissions from the combustion of fuel in the kiln are usually not
sufficient to create significant air pollution problems. Most of the sulfur
dioxide formed from the sulfur in the fuel is recovered as it combines with
the alkalies and also with the lime when the alkali fume is low. Tests of the
kiln exit gases from one portland cement plant burning 2.8% sulfur coal showed
a concentration of sulfur dioxide ranging from 6 to 39 ppm. Nitrogen oxides
can form at kiln temperatures of 2,600-3,000°F and may be of some concern in
areas that experience photochemical-type air pollution, but no federal restric-
tions exist or are anticipated.
TABLE C-l
SULFUR DIOXIDE EMISSION FACTORS FOR CEMENT KILNS*
Dry-Process Kiln Wet-Process Kiln
Gas Combustion
kg/103 ton Negligible Negligible
Oil Combustion
kg/103 ton 2.IS 2.IS
Coal Combustion
kg/103 ton 3.AS 3.4S
Mineral Source
kg/103 ton 5.1 5.1
The sulfur dioxide factors presented take into account the
reactions with the alkaline dusts when no baghouses are used.
With baghouses, approximately 50% more S02 is removed because
of reactions with alkaline particulate filter cake. The total
S02 from the kiln is determined by summing emission contribu-
tions from the mineral source and the appropriate fuel.
2. WATER POLLUTION
In discussing wastewater characteristics, regulatory constraints, and
wastewater treatment technology/economics, it is necessary to distinguish
between wet and dry process plants and between leaching and non-leaching plants,
for each has its own set of wastewater effluent problems, guidelines, and
recommended treatments. Dry process plants outnumber wet process plants, and
non-leaching plants outnumber leaching'plants. The anticipated energy-saving
process changes within the cement industry apply to the dry process, non-
leaching type cement plant; and for this reason, water pollution considera-
tions of this report are almost entirely focused on the dry process, non-
leaching plant.
102
-------
a. Sources of Wastewater
Cement plants, in general, have 2 major sources of wastewater:
• Noncontact cooling water;
• Wastewater previously in contact with raw material, final product,
or discarded cement dust.
Depending on the type of plant, wastewater may come in contact with raw
material, product, or discarded dust, either directly as an intended part of
the manufacturing process, or indirectly, either as plant cleaning water or
as surface run-off from accumulated piles of discarded cement dust.
(1) Wet Process, Leaching Plants
Wet process plants feed raw material to the kiln in the form of a slurry.
The slurry water is subsequently evaporated in the kiln and therefore should
not constitute a discharge.
The relatively constant volume of water in the preparation of slurry
averages 260 gal/ton.
At a few plants, excess water containing a high concentration of suspended
solids is discharged from the slurry thickeners. This constitutes a nonessen-
tial discharge and is easily avoided by recycling the water to make the slurry.
Other losses of slurry may occur due to poor maintenance of pumps, which
become worn and develop leaky seals. If not controlled, the resulting spill-
age may result in a waste discharge with high solids.
In "leaching" plants, soluble alkalies from collected kiln dust are
removed by leaching so that portions of the dust can be returned to the kiln
as recovered raw material. In all plants employing leaching, the overflow
(leachate) from this operation is discharged. For all plants that employ
leaching, the wastewaters from the leaching step are very similar, varying to
some extent in concentration of individual constituents because of differences
in raw materials. Wastewaters from leaching operations are high in pH and
alkalinity, and contain appreciable amounts of suspended solids and dissolved
solids (calcium, potassium, chlorides,, and sulfates). Of all the wastewater
streams associated with the manufacture of cement, the leachate overflow is
environmentally the most objectionable.
(2) Dry Process, Non-leaching Plants
In the dry process, non-leaching plant, there are but two major wastewater
streams: noncontact cooling water, and overflow/run-off water from discarded
dust storage piles.
In terms of volume, the largest wastewater stream is usually the non-
contact cooling water. This water is used to cool bearings on the kiln and
grinding equipment, air compressors, burner pipes and the finished cement
103
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prior to storage or shipment (Table C-2). While cooling water is mostly
noncontact, it can sometimes become polluted as a result of poor water manage-
ment practices. This pollution may include oil and grease, suspended solids,
and even some dissolved solids. If cooling towers are used, blowdown dis-
charges may contain residual algicides.
In non-leaching plants discarded cement dust is not recovered; instead it
is usually allowed to accumulate in large storage ponds or piles. (In many
cases, particularly in older plants, discarded dust is returned to the quarry
from where it originated.) Often, the discarded dust is transported to the
pond in the form of a slurry. If the plant is located in a region of net pre-
cipitation, periodic overflows from the storage areas or ponds can occur.
Discharges can also occur from use of excessive slurry water and other poor
water management procedures. The characteristics of dust storage area over-
flow and run-off are not unlike those of leachate overflow from leaching plants,
in that the wastewater will be high in pH, alkalinity, and dissolved solids.
The volume of dust storage area overflow or run-off is very much a function of
site-specific conditions.
TABLE C-2
REPORTED COOLING WATER USAGE IN CEMENT PLANTS
Average Flow Number of Range
Use . (gal/ton of product) Plants Minimum Maximum
Bearing cooling 284 39 1.0 1,530
Cement cooling 200 22 0.5 985
Clinker cooling 23 12 0.6 64
Kiln-gas cooling 85 4 24 203
Burner-pipe cooling 70 2 68 72
Source: "Development Document for Proposed Effluent Limitations Guidelines and
New Source Performance Standards for the Cement Manufacturing Point
Source Category", U.S. Environmental Protection Agency, EPA 440/1-73/
005
(3) Miscellaneous
All cement plants have some accumulation of settled dust on the plant
property, and this dust may show up in the wastewater in a number of ways.
Many plants spray water on the roads to prevent the dust from becoming air-
borne by truck traffic. Most plants also routinely wash accumulated dust off
104
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the trucks. At some plants, certain parts of the plant areas are also washed
down to remove accumulated dust. The amount of water used for these purposes
varies widely, ranging from 250 to 2,500 gpd. Some of this water undoubtedly
evaporates, but depending on the topography of the plants, some of this water
may drain into storm sewers or natural waterways.
Water from process area surface run-off after rain may also be laden with
the dust that accumulates on the plant site. Run-off from dust piles, coal
piles, and raw material piles may also become contaminated. Plants with boilers,
cooling towers, and intake water-treatment facilities have blowdown and back-
wash discharges associated with these operations. Relatively few of the plants
employing wet scrubbers for air pollution control have a wastewater stream
consisting of spent scrubber water.
A summary of reported water usage within the cement industry is presented
in Table C-3.
TABLE C-3
WATER USAGE FOR THE CEMENT INDUSTRY
Use
Cooling
Number of
Plants
117
Reported Flow
Average
450
Minimum
Maximum
21,000
Units
gal/ton of
Product
Raw Material
Washing and
Beneficiation
Process
Dust Control
78
13
29
29
250
0.6
0.7
72
118 gal/ton of
Raw Material
108 gal/ton of
Product
510 gal/ton
Dust Leaching
Dust Disposal
Wet Scrubber
11
703
55
8,100
627
2.3
1,200
773
140
12,300
gal/ton of
Dust
gal/ton of
Product
gal/ton of
Product
Source: "Development Document for Proposed Effluent Limitations Guidelines and
New Source Performance Standards for the Cement Manufacturing Point
Source Category", U.S. Environmental Protection Agency, EPA 440/1-
73/005
105
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b. Wastewater Characteristics
While a wide variety of inorganic chemical constituents are present in
both leaching and non-leaching plants (Table C-4), it was EPA's decision
that none are of sufficient concern to warrant the setting of specific
limitations for those species. Pollution control measures for this industry
are designed primarily to remove suspended solids, control pH, and to limit
the temperature rise of water going through the plant.
As Table C-4 shows, the overall waste loading from non-leaching plants is
much lower than that of leaching plants.
With the exception of lead and chromium, significant loadings of heavy
metals have not been generally detected in the wastewaters from cement plants.
The plants that do have lead and chromium present appear to be isolated cases.
c. Regulatory Constraints
Briefly, the Effluent Guidelines for the_ Cement Industry* pertaining to
dry process, non-leaching plants are solely intended to limit the quantity of
suspended solids discharged, reduce thermal pollution from discharged cooling
water, and to maintain the pH within acceptable limits.
TABLE .C-4
COMPARISON OF WASTE LOADINGS FOR LEACHING AND
NON-LEACHING SUBCATEGORIES
Parapeter
Units
Alkalinity
BOD, 5 day
COD
Total Solids
Total Dissolved Solids
Total Suspended Solids
Total Volatile Solids
Ammonia
rjeldahl Nitrogen
Nitrate Nitrogen
Phosphorus
Oil and Grcare
Chloride
Sulfarc
Sulfidc
Sulflte
Fhmols
Chroalum
Acidity
Total Organic Carbon
Total Kardness
Fluoride
Aluminum
Calcium
Copper
Iron
lead
Magnesium
Hercury
Nickel
Potassium
Sodium
Zinc
Ib/ton
Ib/ton
lt>/ton
Ib/ton
IWton
Ib/ton
Ib/ton
Ib/ton
Ib/ton
Ib/ton
Ib/ton
Ib/ton
Ib/con
Ib/ton
Ib/ton
Ib/ton
.001 Ib/ton
.001 Ib/ton
Ib/ton
Ib/ton
Ib/ton
Ib/ton
.001 Ib/ton
Ib/ton
.001 Ib/ton
.001 Ib/ton
.001 Ib/ton
Ib/ton
.001 Ib/ton
.001 Ib/ton
Ib/ton
Ib/ton
.001 Ib/ton
2.76
0
0.06
14.99
13.24
1.81
1.65
0
0
0
0
0
2.40
7.33
0
0
0.16
_
4.41
0
1.28
1.93'
9.53
1.98
0.03
_
6.60
0.74
0
10
9
9
10
10
10
8
8
£
B
8
4
6
6
4
0
4
6
0
0
4
1
3
4
0
3
2
4
0
0
4
4
2
0.17
0
0
0.63
0.54
0
0
0
o
0
0
0
0
Q
o
0
0
o
o
0
1.73
0
0.02
0.19
0
0.31
0
0.31
0
o
0.15
0.48
0
Mean Value
Mean Value for Non-
tar Leaching Nuober leaching Number
Subcategory of Planes Subeategory of Plants
61
57
53
61
60
58
57
53
52
53
55
47
56
56
41
5
47
51
6
4
21
5
10
18
5
15
3
15
3
4
11
12
9
Source: " "fluent Guidelines Development Document—Cement Industry -;.
U.S. Environmental Protection Agency. EPA 440/1-73/005
*"Effluent Guidelines and Standards - Cement Manufacturing," 40 CFR 411,
Federal Register, February 20, 1974.
106
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Effluent guidelines are divided into two parts, those pertaining to
wastewater discharged from the plant itself (defined as "Subpart A-Non-leaching
Subcategory) and those pertaining to materials storage pile run-off (Subcate-
gory C). For both these subcategories the BPTCA level (Best Practicable Con-
trol Technology Currently Available 1977) and the BATEA level (Best Available
Technology Economically Achievable) are the same, and are listed below:
Subpart A - Non-leaching Subcategory
Effluent
Characteristics
Total Suspended Solids
Temperature (heat)
pH
BPCTCA (1977) and BATEA (1983)
Effluent Limitations
(max, for any one day)
(lb/1000 Ib of product)
0.005
Not to exceed 3°C rise above
inlet temperature
Within the range from 6.0 to 9.0
Subpart C - Materials Storage Piles Run-off Subcategory
BPCTCA (1977) and BATEA (1983)
a. Effluent
Characteristics-
Total Suspended Solids
pH
Effluent
Limitation
Not to exceed 50 mg/liter
Within the range from 6.0 to 9.0
b. Any untreated overflow from facilities designed, constructed, and
operated to treat the volume of run-off from materials storage
piles which is associated with a 10-year, 24-hour rainfall event
shall not be subject to the pH and TSS limitations stipulated above.
d. Treatment Technology and Costs
In order for the dry process, non-leaching cement plant to achieve the
recommended effluent discharge levels, it is generally necessary for the fol-
lowing measures to be implemented:
• Isolation of cooling water circuits from possible sources of pollu-
tion and reduction of discharged cooling water temperature by means
of either cooling towers or spray ponds; and
• Diking, collection, clarifications, and neutralization of all waste-
water from discarded dust and material storage areas.
107
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On the basis of these requirements, we have prepared wastewater treatment
cost estimates for the specific 1350-tpd base case dry process, non-leaching
cement plant (Table C-5) in accordance with the following design basis:
• Noncontact cooling water flowrate = 648,000 gallons
• Spray pond area = 1 acre
• Dust generation @ 7% of raw material feed, or 140 tpd. Run-off con-
trol measures are based on a 10-year accumulation, which at a 20-ft
depth amounts to 10.6 acres. Of the total dust, 15% is expected to
be soluble.
• Discarded dust storage area is diked to contain and collect the over-
flow caused by precipitation run-off. The amount of run.-off is basically
the amount of precipitation in excess of evaporation, which of course
varies from location to location. For the purpose of these estimates,
a run-off rate resulting from precipitation in excess of evapora-
tion of 4 inches per year was used. This results in a flowrate of
3100 gpd average.
• Overflow wastewater treatment system consists of a 500,000-gallon
holding basin to contain heavy stormwater surges plus a clarifier
with acid feed equipment (both capable of treating a flowrate of
20,000 gpd).
Our cost estimates are for the specific example of a dry process, non-leaching
cement, and are in no way intended to represent industry-wide wastewater treat-
ment costs. Wet process leaching plants can incur substantially higher
costs due to greater wastewater volumes arid waste loads.
108
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TABLE C-5
WASTEWATER TREATMENT COSTS: BASE CASE CEMENT PLANT
Basis
1350 tpd Cement Production
330 Operating Days Per Year
CAPITAL INVESTMENT - $519.000
Cost Per Quantity
Annual Unit Per Ton of
Quantity Quantity Production
VARIABLE COSTS ,
Operating Labor 2630 man-hr $12/hr 0.0059
(including overhead)
Maintenance
(including Labor & Mtls)
Chemicals
Sulfuric acid 14 tons $100 /ton 3.1xlO~5
Electrical Power 360, OOO/ $0.02/ 0.81
kWh kWh
TOTAL VARIABLE COST
FIXED COST
(Depreciation @ 5%)
(Taxes & Insurance @ 2%)
TOTAL FIXED COST
TOTAL ANNUAL COST
RETURN ON INVESTMENT @ 20%
Unit Cost
($ Per Ton
of Product)
0.0709
0.0467
0.0031
0.0162
0.1369
0.0584
0.0233
0.0817
0.2186
0.2330
TOTAL
0.45/ton
Notes:
1)
2)
3)
Capital investment adjusted to 1975 level (ENR Construction
Cost Index = 2126)
Wastewater treatment includes:
a) Non-contact cooling water thermal pollution control via
spray pond
b) Dust pile runoff containment, collection, clarification,
and neutralization
Estimates are for the specific example of a dry-process, non-
leaching cement plant and are in no way intended to represent
industry-wide wastewater treatment costs.
Source: Arthur D. Little, Inc. estimates
109
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APPENDIX D
FLASH CALCINING SYSTEMS
1. JAPANESE
a. Ishikawajima Harima Heavy Industries
The IHI suspension flash preheater system was developed by IHI and
Chichibu Cement. The suspension preheater portion of this system receives
raw feed in the conventional manner and the raw feed progresses down through
the first three stages of the preheater in a conventional manner. The prod-
uct from the third stage, instead of discharging into the fourth and final
stage, is discharged into the flash furnace, which contains several burners
mounted in the furnace roof. The flash furnace is a cyclonic suspension fur-
nace with gas inlet at the bottom and tangential gas and calcined raw feed
outlet at the top.
One of the important features of the IHI suspension flash preheater is
the addition of high-temperature preheated air, taken from the midsection of
the grate-type clinker cooler, to the combustion gas leaving the rotary kiln
to form the atmosphere in which the flash furnace fuel is burned. This per-
mits the fuel in the rotary kiln to be burned with the appropriate minimum
quantity of excess air, thus optimizing the burning conditions in the kiln.
A main disadvantage of providing all of the combustion' air to the flash
calcining vessel or furnace by using sufficient excess combustion air within
the rotary kiln is the high volumetric flow., rate of gas and its attendant high
spatial velocity within the rotary kiln. Consequently, it is desirable, if
not necessary, to provide the combustion air for the flash furnace from a
source other than the excess air contained in the combustion gases leaving
the rotary kiln. A second disadvantage is that the high volumes of combus-
tion gas and excess air passing through the rotary kiln do not concentrate
the volatilized alkalies in that gas stream, thereby negating the removal of
alkalies by use of a bypass.
The design of the IHI suspension flash preheater avoids these difficulties
by taking hot air from the center of the clinker cooler and conveying it '
through a separate refractory-lined duct, located parallel to the kiln, and'
mixing this hot air with the combustion gases leaving the rotary kiln. This
mixture of combustion gas and preheated air from the clinker cooler is then
introduced into the flash furnace. The flow rate of combustion gases through
the rotary kiln and the preheated air through the secondary air duct between
110
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the clinker cooler and the flash furnace is provided by the suction from the
induced draft fan, which is the prime air mover through the entire kiln/flash
calciner/preheater system. The proper balance of air flow to the kiln and the
flash furnace is controlled by:
• a constricted portion of the kiln extract duct, which functions as
an orifice, to achieve a fixed gas flow resistance on the outlet of
the kiln combustion gases; and
• an adjustable damper in the secondary air duct, which can control
the pressure drop through this duct, and.thereby balance the air
flow system.
This system permits operation without a secondary air fan, which generally
provides hot secondary combustion air from the clinker cooler to the rotary
kiln. In the conventional preheater kiln or long non-preheater kiln system.
the temperature limitation of this fan prevents the use of secondary air tem-
peratures as high as they actually could be. Because no secondary air fan is
required in this system, the hottest possible secondary air from the cooler
can be used for the rotary kiln, thereby further reducing the overall heat
consumption.
This system for controlling and balancing the flow of hot secondary air
and combustion gases has been demonstrated at over 12 suspension flash pre-
heater facilities which IHI has built, and which are now in operation. The
IHI design also eliminates the need for a fan to move the hot air from the
clinker cooler through the secondary air duct. This results in the use of the
hottest available air from the mid-section of the clinker cooler to be sent
to the flash furnace for the raw feed precalcining, which further maximizes
the heat recuperation from the clinker cooler.
At a typical IHI suspension flash preheater system, the temperature of
the hot combustion gases leaving the rotary kiln is higher than the tempera-
ture of the air in the secondary air duct, at the point where these two hot
gas streams are mixed, just prior to their entry into the bottom of the flash
furnace. The temperature of the hot combustion gases leaving the rotary kiln
is about 2050°F, while the temperature of the hot air in the secondary duct,
at the point of mixing, is approximately 1380°F. The mixture of these two
streams results in a cooling of the hot combustion gases from the rotary kiln.
This sudden temperature reduction, especially in this temperature range,
would tend to cause the solidification of alkali-coated raw material and dust
particles, which would tend to build up on the walls of ducts and the internal
surfaces of the flash furnace. Therefore, one of the design features of the
IHI system is that the hot secondary air and the kiln gases are sufficiently
well-mixed in a short enough time and in a way which prevents the build up of
solid alkali-rich materials.
Hot secondary air and hotter kiln combustion gases mix as they tangen-
tially enter the bottom of the flash furnace. The gases spiral upward through
the flash furnace and exit, also tangentially. The raw material inlet is
located in the roof of the flash furnace at a point which maximizes the reten-
tion time of the raw material inside the flash furnace vessel and produces the
111
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most uniform dispersion of the calcining raw material within the hot combus-
tion gases generated by the burning fuel. All of the plants thus far have
used oil in the flash furnace but Fuller and IHI are presently conducting
tests for the use of coal.
b. Onoda Cement-Kawasaki Reinforced Suspension Preheater System
The Onoda/Kawasaki flash calciner is called the "reinforced suspension
preheater system" (RSP). The main features of the RSP system are similar to
the suspension flash preheater system developed by IHI. The principal dif-
ference between the IHI and the Onoda/Kawasaki systems is that the kiln exit
gases do not pass through the flash calcining vessel; instead, they mix with
the precalcined raw material and combustion gases coming from the flash cal-
cining vessel on their way to the Stage 4 cyclone. The only gas going to the
flash calcining vessel is the hot preheated secondary combustion air coming
from the clinker cooler. Since the air required for the combustion of the
fuel introduced into the flash calcining vessel is provided directly from the
clinker cooler without mixing with kiln gases, it is reported that the higher
concentration of oxygen present in that vessel provides more stable and posi-
tive combustion than the IHI system. However, this higher concentration of
oxygen is probably responsible for a higher concentration of NOX formed within
the flash calcining vessel. This would tend to negate one of the main envi-
ronmental advantages ascribed to the flash calcining system.
There are two main parts to the flash caicining vessel used by the RSP
and the secondary hot air stream from the clinker cooler is divided into two
parallel streams. One of these secondary air streams goes to the swirl burner,
in which an ignition burner operates. The other stream of hot secondary air
goes to the swirl calciner, in which the single main firing burner is operated.
The final remaining major difference between the IHI and the RSP systems
is that the latter requires an induced draft fan to provide the hot secondary
air from the clinker cooler at a sufficiently high pressure for its introduc-
tion into the swirl burner and swirl calciner. The operating temperature
limitations of a fan in this secondary air stream limit the temperature of
the hot air taken from the clinker cooler to a level which is below the maxi-
mum which could be taken from the cooler. This would tend to reduce the heat
recuperated from the clinker cooler, and, consequently, increase the overall
fuel energy required to make cement clinker by this process.
c. Mitsubishi Fluidized Calcinator
Although the basic characteristics and the process conceptual goal of the
Mitsubishi fluidized calcinator (MFC) are equivalent to both the IHI and the
Onoda/Kawasaki flash calciner processes, this third alternative has some sigr
nificant differences which put it in a quite different category.
The primary feature which sets this concept apart from the preceding two
is that the preheated raw material is calcined in a fluidized bed instead of
in a vortex-type suspension vessel. Secondly, only a fraction of the preheated
raw material is sent to the precalcining or "flash-calcining, fluidized-bed
112
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vessel, whereas the preceding two processes are designed to operate with 100%
of the preheated raw material fed to the flash calcining vessel. Presently,
only 20% of the total raw feed is diverted to the fluidized-bed calcining
vessel. Although this material is precalcined to an extent of 90% when it is
recombined with the remaining 80% of the preheated raw feed, the precalcined
mixture which is fed to the rotary kiln has been calcined to an extent of only
55%. However, Mitsubishi is currently working toward increasing the bypass
percentage to 50% or possibly higher. But at present this system remains a
hybrid between the flash calcining system and the suspension preheater system.
The available literature provides no basis for a prediction of how much of the
raw feed can be passed through the fluidized-bed, flash calcining step.
When the fluidized bed is fired with oil about one-third of the air
required for complete combustion of the oil is introduced along with the oil
through burners which are submerged below the fluidized-bed surface. The
remainder of the two-thirds of the required combustion air is introduced above
the bed. Therefore, a significant amount of combustion takes place above the
fluidized bed. This is probably necessary because of the small particle size
of the raw feed, and the consequently low fluidizing velocity.
One of the present advantages of the system is that Mitsubishi has reported
the use of coal as the sole fuel in the fluidized-calcining vessel. Although
Fuller Company and IHI are presently conducting development programs aimed at
the use of coal in their flash-calcining vessels, we understand that Mitsubishi
is the only company which has successfully used coal as the only fuel in the
flash-calcining-vessel section of their system.
Fluidization of the preheated raw material in the fluidized calcinator
is done with hot air from the clinker cooler which has first passed through a
cyclone-type dust collector to remove the fine clinker dust.
An interesting benefit of the MFC process is that materials containing
fuel value but not normally used for fuel can be successfully burned in the
fluidized calcinator. For example, waste material from coal dressing opera-
tions has been effectively burned. Such refuse with a heating value of between
2500 and 5000 Btu/lb has been successfully burned with the ash forming part
of the raw material and being converted to clinker. Such coal dressing
refuse, in addition to being unsuitable as the sole fuel for firing a con-
ventional rotary kiln system, is also unsuitable as a supplemental fuel
additive, blended with coal or another fuel, because the refuse will cause
a decrease in burning temperature, has a tendency for kiln ring formation
and the lack of suitable mixing between the ash from the burned coal dress-
ing refuse and the clinkering raw material in the firing zone of the rotary
kiln produces a variation in clinker quality which is unacceptable. Actual
commercial operation of this system began in December, 1971, with a signif-
icant increase in kiln capacity, and in system availability.
113
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2. EUROPEAN
a. Polysius System
Polysius has modified their Dopol suspension preheater system in which
all of the air required for combustion in'the precalciner is contained in the
rotary kiln exit gas as excess air. The kiln exit gases enter the bottom of
the calcining shaft where a number of burners supply up to 50% of the total
process heat.
This Polysius system was developed in conjunction with Portland-Zementwerk
Dotternhausen, in Southern Germany, expressly for the purpose of utilizing an
oil shale raw material component successfully in a suspension preheater with-
out the sticking, clogging, and other solid material buildups which create
severe operating problems in suspension preheaters when the raw material feed
contains fuel values. Also, this cement company wished to use the fuel values
of the oil shale in its raw material feed to effectively reduce the amount of
purchased fuel used in the rotary kiln. Both of these objectives were reached
by the successful development of the Polysius version of the flash calciner.
The salient features of the Polysius system, as embodied in the full-
scale commercial operation at Dotternhausen, are:
• Coal can be utilized in the flash calciner.
• No kiln bypass duct is used.
• Planetary coolers can be used since all of the combustion gas is
conducted through the rotary kiln.
• Raw material containing fuel values, such as oil shale, can be
successfully used.
• Six years of actual plant experience have been obtained.
b. F.L. Smidth System
In Denmark, F.L. Smidth & Company has been developing a flash calcining
system. Although it is claimed that a fluidized bed of raw material and fuel
exists in the bottom of this precalciner, it appears that most or even all of
the combustion of fuel, and calcination of the raw material, occurs in the
toroidal recirculation zone which exists in most of this vessel, where the raw
feed particles, after being preheated, are calcined in suspension.
The kiln exit gas passes up through a conventional four-stage suspension
preheater and the preheated air from the clinker cooler passes through the
secondary air duct which runs parallel to the kiln and then enters a second
and separate suspension preheater which is equipped with a flash calciner.
The preheated raw material from the discharge of stage four of the suspension
preheater system is passed through into the flash calciner and is precalcined
along with the preheated raw material entering the flash calciner from stage
114
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three of the flash calciner-equipped suspension preheater. In this way, the
two high temperature gas streams are kept separate until they have passed
the two separate induced draft fans and the distribution of the combustion
air from the kiln and the clinker cooler secondary air can be regulated by
means of the two separate fans.
115
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
EPA-600/7-76-034J
2.
3. RECIPIENT'S ACCESSIOI*NO.
4. TITLE AND SUBTITLE
ENVIRONMENTAL CONSIDERATIONS OF
SELECTED ENERGY CONSERVING MANUFACTURING PROCESS
OPTIONS. Vol. X. Cement Industry Report
5. REPORT DATE
6. PERFORMING
"^Q*y<5 j_5 III-IX,. EPA-600/7-?6-034c. through EPA-600/7-76-034i, and XI-
XV, EPA-600/7-76-034k through EPA-600/7-76-034o, refer to studies of other industries
as noted below: Vol. I. EPA—600/7—76—G34a is the Industrv Sutranarv Ket>ort and Vol. IX.
is. ABSTRACT EPA-600/7-76-034b is the Industry Priority Report.
' This study assesses the likelihood of new process technology and new practices being
! introduced by energy intensive industries and explores the environmental impacts
of such changes.
Specifically, Vol. X deals with the cement industry and examines four options:
(1) suspension preheater, (2) flash calciner, (3) fluid-bed cement process, and
(4) conversion to coal fuel from oil and natural gas, all in terms of process economics
and environmental/energy consequences. Vol. III-IX and Vol. XI-XV deal with the fol-
lowing industries: iron and steel, petroleum refining, pulp and paper, olefins,
ammonia, aluminum, textiles, glass, chlor-alkali, phosphorus and phosphoric acid,
copper, and fertilizers. Vol. I presents the overall summation and identification
of research needs and areas of highest overall priority. Vol. II, prepared early in
.the study, presents and describes the overview of the industries considered and
presents the methodology used to select industries.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS C. COSATI Field/Group
Energy; Pollution; industrial Wastes;
Cement
Manufacturing Processes;
Energy Conservation;
'Kiln; Flash Calciner;
Suspension Preheater;
Alkali Dusts
13B
18. DISTRIBUTION STATEMENT
Release to public
19. SECURITY CLASS (ThisReport}'
unclassified
21. NO. OF PAGES
132
20. SECURITY CLASS (Thispage)
unclassified
22. PRICE
EPA Form 2220-1 (9-73)
116
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