United States Energy
Research and Development
Administration
Office of Fossil Energy
Washington, D.Ci 20545
United States Environmental Industrial Environmental Res ;arch
Protection Agency Laboratory
Office of Research and Development Research Triangle Park, N.C. 27711
EPA-600/7-76-019
October 1976
A DEVELOPMENT PROGRAM
ON PRESSURIZED
FLUIDIZED-BED COMBUSTION
Interagency
Energy-Environment
j
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
I ' 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-600/7-76-019
October 1976
A DEVELOPMENT PROGRAM
ON PRESSURIZED
FLUIDIZED-BED COMBUSTION
by
G.J. Vogel, I. Johnson, P.Cunningham, B. Hubble,
S. Lee, J. Lenc, J. Montagna, F. Nunes, S. Siegel,
G. Smith, R. Snyder, S. Saxena, W. Swift, G. Teats,
I. Wilson, and A. Jonke
Argonne National Laboratory
9700 South Cass Avenue
Argonne, Illinois 60439
EPA No. IAG-D5-E681
ERDA No. 14-32-0001-1780
Program Element No. EHE623A
Project Officers:
Walter B. Steen John F. Geffken
EPA/Indus trial Environmental Research Lab ERDA/Office of Fossil Energy
Research Triangle Park, NC 27711 Washington, DC 20545
Prepared for
U.S. Energy Research
U.S. Environmental Protection Agency and Devel ent Administration
Office of Research and Development ^^ Qf Fosgil Energy
Washington, DC 20460 Washington, DC 20546
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TABLE OF CONTENTS
Page
Abstract 1
Summary 1
Introduction • 13
Regeneration of S02-Accepting Additive,
Bench-Scale Studies 13
Materials 14
Equipment 14
Regeneration of Sulfated Tymochtee Dolomite by the
Incomplete Combustion of Methane 16
Regeneration of Sulfated Greer Limestone by the
Incomplete Combustion of Coal 32
Regeneration of Sulfated Tymochtee Dolomite by the
Incomplete Combustion of Coal 45
Carbon Utilization and Carbon Balance Calculations 48
Effect of Coal Ash on the Agglomeration of Sulfated
Additive During Regeneration 48
Mass and Energy Constrained Model for the
Regeneration Process 57
Cyclic Combustion/Regeneration Experiments 68
Preliminary Experiment. Sulfation of Regenerated
Tymochtee Dolomite 69
Combustion Step, Cycles 1, 2, and 3 72
Regeneration Step, Cycles 1 and 2 79
Coal Ash Buildup During Sulfation and Regeneration
Steps 80
Pressurized, Fluidized-Bed Combustion: Bench-Scale Studies ... 84
Materials 85
Bench-Scale Equipment. . 85
Experimental Procedure 88
Replicate of VAR-Series Experiment 88
Effect of Coal and Sorbent Particle Size on Sulfur
Retention, Nitrogen Oxide Level in the Flue Gas,
and Combustion Efficiency 89
Combustion of Lignite in a Fluidized Bed of Alumina 94
Effect of Fluidizing-Gas Velocity on Decrepitation Rate. . . 95
Binary Salt of Magnesium and Calcium Sulfate ...-..,.. 95
Synthetic S02 Sorbents , , . , . 99
Introduction 99
Preparation of Synthetic Sorbents ...,,.... 99
Sulfation Studies , . 100
111
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TABLE OF CONTENTS (Contd'.i)
Metal Oxides in a-A!203 Ill
Regeneration Studies ............ 114
Cyclic Sulfation-Regeneration Studies. . , . . , 121
Methods of Preparing Supports with Optimum
Physical Properties 124
Sorbent Attrition Studies 131
Introduction 131
Attrition of Dolomite. 131
Attrition of Supports and Synthetic Sorbents 133
Combustion-Regeneration Chemistry , . . , 134
Half-Calcination Reaction 134
Regeneration by the CaSO^-CaS Reaction .... 139
Coal Combustion Reactions 145
Determination of Inorganic Constituents in the Effluent
Gas from Coal Combustion « 145
Systematic Study of the Volatility of Trace Elements
in Coal 148
Equipment Changes 156
Miscellaneous Studies ....... 158
Preparation and Testing of Supported Additives
(Dow Subcontract) 158
Limestone and Dolomite for the Fluidized-Bed
Combustion of Coal: Procurement and Disposal 159
The Properties of a Dolomite Bed of a Range of Particle
Sizes and Shapes at Minimum Fluidization 162
Mathematical Modeling: Noncatalytic Gas-Solid Reaction
with Changing Particle Size, Unsteady State Heat
Transfer . . . 164
Acknowledgments 172
References 172
Appendix A. Characteristics of Raw Materials Used in Fluidized-
Bed Combustion Experiments 177
Appendix B. Plots of Operating Data and Experimental Results of
Combustion Experiments
IV
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LIST OF FIGURES
No. Title Page
1. Schematic Diagram of Present Regeneration System 15
2. Particle Size Distribution of the Fines Leaving the
Cyclone in FAC-5 19
3. The Effect of Fluidizing-Gas Velocity on Sulfur
Regeneration for Sulfated Tymochtee Dolomite 20
4. Effect of Temperature on Sulfur Regeneration for
Sulfated Tymochtee Dolomite 21
5. Pore Distributions of Dolomite Samples from Different
Process Stages 23
6. Sulfation Reaction Data Obtained with a Thermogravimetric
Analyzer at 900°C, 0.3% S02, and 5% 02 24
7. Electron Microprobe Analyses of Sulfated Tymochtee
Dolomite Particles 26
8. Electron Microprobe Analyses of Regenerated Tymochtee
Dolomite Particles from Experiment FAC-1R2 27
9. Electron Microprobe Analyses of Regenerated Tymochtee
Dolomite Particles from Experiment FAC-4 28
10. FAC-1, Fractional Feed and Product Particle Size
Distributions 30
11. Bed Temperature and Gas Concentrations in Off-Gas,
Experiment LCS-2 35
12. Bed Temperature and Gas Concentrations in Off^Gas,
Experiment LCS-3 «... 37
13. Bed Temperature and Gas Concentrations in Off-Gas,
Experiment LCS-4D 40
14. Geometry of Oxidizing and Reducing Zones in Relation
to Position of Coal Injection Line . 41
15. Bed Temperature and Gas Concentrations in Off^-Gas,
Experiment LCS-7 42
16. Regeneration of CaO and S02 Concentration in the Dry
Flue Gas for Sulfated Limestone and Tymochtee Dolomite
as a Function of Solids Residence Time , . . , 44
17. Regeneration of Calcium Oxide as a Function of Solids
Residence Time ............. 46
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LIST OF FIGURES (Contd.)
No. Title Page
18, Predicted and Experimental Sulfur Dioxide Concentrations
at Three Regeneration Temperatures . 43
19. Schematic of DTA Apparatus , 53
20. Flow Diagram for the One-Step Regeneration Process . , . . , 58
21. Experimental Solids Regeneration and Predicted Increase
in Gas Volume During Regeneration Versus Solids Residence
Time ,..,..,... 61
22. Predicted Required Coal Feed Rate and Oxygen Concentration
in the Feed Gas as Functions of Solids Residence Time. .... 62
23. Predicted Off-Gas Constituent Concentrations as Functions
of Solids Residence Time ..,..,..,.., . 63
24. Predicted Individual Heat Requirements as a Function of
Solids Residence Time 64
25. Predicted Fuel Cost for Regeneration per Electric Power
Unit Produced when Burning 3% Sulfur Coal as a Function
of Solids Residence Time 65
26. Bed Temperature and Flue-Gas Composition, Segment of
Experiment RC-1A . . , , . 70
27. Bed Temperature and Flue-Gas Composition, Segment of
Experiment REC-1 (REC-1K and -1L).............. 74
28. Bed Temperature and Flue-Gas Composition, Segment of
Experiment REC-2 75
29. Bed Temperature and Flue-Gas Composition, Segments of
Experiment REC-3 (REC-3A, REC-3B) 76
30. Bed Temperature and Gas Concentrations in Off-Gas for
Experiment CCS-1, the Regeneration Step of the First
Tymochtee Dolomite Utilization Cycle . 82
31. Bed Temperature and Gas Concentrations in Off-Gas
for Experiment CCS-2 . ,.....,.,.... 83
32. Simplified Equipment Flowsheet of Bench^Scale Fluidized-^Bed
Combustor and Associated Equipment . . ............ 86
33. Detail Drawing of 6-in.-dia, Pressurized, Fluidized-Bed
Combustor 87
VI
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LIST OF FIGURES (Contd.)
No. Title Page
34. CaO Concentration in a-A!203 as a Function of Calcium
Nitrate Concentration in Reactant Solution from which
Prepared ................ • ...... ... 100
35. Comparison of Calculated (Eq. 1) and Experimental Sulfation
Rates at 900°C in 5% 02-N2 of 6.6% CaO in a-A!203 (Heat-
Treated at 800°C) as a Function of S02 Concentration .... 102
36. Rate of Sulfation at 900°C of 1100°C Heat-Treated 6.6%
CaO in a-A!203 as a Function of S02 Concentration ...... 103
37. Effect of Heat-Treatment Temperature on Rate of Sulfation
of 6.6% CaO in a-Al203 at 900°C ............... 103
38. Effect of Oxygen Concentration on the Rate of Sulfation
of 6.6% CaO in a-A!203 at 900°C ...... , ..... . • • 104
39. Calculated and Experimental Rates of Sulfation of 6.6%
CaO in a-Al2Os (800°C H.T.) with 0.3% S02-5% 02<-N2 as a
Function of Sulfation Temperature. ....,..,.•••• 1°5
40. Calculated and Experimental Rates of Sulfation of 6.6%
CaO in «-Al203 (1100°C H.T.) with 0.3% S02-5% 02-N2 as a
Function of Sulfation Temperature ....... . ..... > 106
41. Sulfation of Sorbents Having Various CaO Loadings. ,,..., 109
42. Sorbent Weight Gain at 900°C as a Function of Calcium
Loading in Sorbent .............. • •, .....
43. Comparison of the Rates of Sulfation of Tymochtee Dolomite
and 6.6% CaO in a-A!203 ...... ...... ........ 112
44. Comparison of Sulfation Rates of Various Metal Oxides
at 900°C ................... ....... 113
45. Regeneration of Sulfated 6.6% CaO-a-Al203 Pellets, Using
Hydrogen at 1100°C ........... ,..,..,... 114
46. Regeneration of Sulfated 6.6% CaO-ci-Al203 Pellets, Using
Carbon Monoxide at 1100°C ..................
47. Effect of C02 Concentration in the Reducing Gas on the
Rate of , Regeneration of Sulfated 6.6% CaO in a~A!203 at
1100°C ....... .......... ...» ...... 117
48. Regeneration of Sulfated 6.6% CaO in a-Al203 Sorbent,
Using 1% H2-N2 ............. • ..... «... H8
49. Comparison of the Rate of Regeneration of CaSOit in a-r-Al203
and Sulfated Dolomite using 3% H2 or 3% CO at 1100°C . . , . . 120
vii
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LIST OF FIGURES (Contd.)
No. Title Page
50. Regeneration Rate of Various Metal Sulfates in a-Al203
at 1100°C using 3% H2 122
51. Cyclic Sulfation of 1100°C Heat-Treated Pellets (6.6%
CaO in ct-A!203) Using 3% S02-5% 02-N2 at 900°C 123
52. Cyclic Sulfation of 11.1% CaO in 1500°C H.T. a-A!203
Granular Sorbent Using 0.3% S02-5% 02-N2 at 900°C. ...... 125
53. Cyclic Sulfation of 12.5% CaO in 1350°C H.T. a-A!203
Granular Sorbent Using 0.3% S02-5% 02-N2 at 900°C 126
54. Relationship of Cumulative Pore Volume to Pore Diameter. . . 127
55. Sulfation Rate at 900°C of CaO in Granular Supports 127
56. Relation of Pore Volume to Pore Diameter for the Original
a-A!203 Support, the CaO in a-A!203 Sorbent, and the
Sulfated Sorbent 129
57. Relation of Pore Volume to Pore Diameter as a Function
of Indicated CaO Concentrations in the Support 130
58. Percent Conversion VS Time for Half-Calcination Reaction
under 100% C02 Environment 136
59. Percent Conversion vs Time for Half-Calcination Reaction
under 40% C02-60% He Environment 137
60. CaO Content as a Function of Reaction Time ......... 144
61. Schematic Diagram of Batch Fixed-Bed Combustor System. . . . 147
62. Effect of Heat-Treatment Conditions on Weight Loss of
340°C Ash , . . , 154
63. Overall View of New Regeneration Facility. . . . , 157
64. Schematic of TGA Apparatus 160
65. Gas-Mixing Apparatus for TGA Sulfation Experiments 161
66. Qualitative Dependence of the Pressure Drop Across the
Bed, AP, on the Fluidizing-Gas Velocity , . 163
67. Gas-Solid Reaction of a Growing Particle: The Concen-
tration and Temperature Profiles • • 166
Vlll
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LIST OF FIGURES (Contd.)
No. Title Page
B-l Bed Temperature and Flue-Gas Composition,
Experiment PSI-1R ............... ...... 184
B-2 Bed Temperature and Flue-Gas Composition,
Experiment PSI-2 ..... .......... . .....
B-3 Bed Temperature and Flue-Gas Composition,
Experiment PSI-3 . . ...... ............. 186
B-4 Bed Temperature and Flue-Gas Composition,
Experiment PSI-4 ......... . ........... 187
B-5 Bed Temperature and Flue-Gas Composition,
Experiment LIG-2D ..................... I88
B-6 Bed Temperature and Flue-Gas Composition,
Experiment LIG-2R ..................... 189
ix
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LIST OF TABLES
No. Title Page
1. Design Experimental Conditions and Extent of Regeneration
Results for the FAC-Series 17
2. Chemical Analysis of Regenerated Product, Calculated
Material Balances, and Regeneration Results from the
FAC-Series 18
3. Qualitative Chemical Compositions of Regenerated and
Unregenerated Samples of Additive 31
4. Qualitative Chemical Composition (Obtained by X-ray
Diffraction Analysis) of Reacted Samples from DTA
Experiments 33
5. Experimental Conditions and Results for Regeneration
Experiments in which Arkwright and Triangle Coals were
Incompletely Combusted in a Fluidized Bed 34
6. Experimental Conditions and Results for Regeneration
Experiments Designed to Test the Effectiveness of the
Two-Zone Reaction in Minimizing CaS Buildup 38
7. Experimental Conditions and Results for Regeneration
Experiments with Combustion of Arkwright and Triangle
Coal in a Fluidized Bed 43
8. Experimental Conditions and Results for Regeneration
Experiments with Sulfated Tymochtee Dolomite Using
Triangle Coal 47
9. Experimental Conditions, Carbon Balances, and Carbon
Utilizations for Regeneration Experiments 49
10. Fusion Temperatures, Under Reducing Conditions, of Ash
from Arkwright No. 2 coal, Sewickley Coal, and Triangle
Coal 50
11. Compositions of Greer Limestone and Sewickley Coal Ash ... 51
12. Coal Ash Level in Sulfated Greer Limestone 52
13. Differential Thermal Analysis Results and X-ray Diffraction
Analysis of the Reaction Products of Unsulfated Tymochtee
Dolomite 54
14. Differential Thermal Analysis Results and X-ray Diffraction
Analysis of the Reaction Products of PER Sulfated Greer
Limestone and Residual Coal Ash 55
15. Regeneration of Tymochtee Dolomite 67
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LIST OF TABLES (Contd.)
No. Title Page
16. Operating Conditions and Flue-Gas Analysis for Segment
of Combustion Experiment RC-1A 69
17. Utilization of Calcium in Overflow, Primary Cyclone,
and Secondary Cyclone Samples from Experiment RC-1A 71
18. Screen Analysis Results for Combustion Experiment RC-1A ... 72
19. Operating Conditions and Flue-Gas Composition for Cyclic
Combustion Experiments 73
20. Steady-State Flow of Calcium Through the ANL, 6-in.-
dia Combustor during Combustion Experiment REG-IK 78
21. Experimental Conditions and Results for Regeneration Segments
of the Regeneration Step of the First Two Utilization Cycles. 81
22. Calculated Ash Buildup During Sulfation and Regeneration
of Tymochtee Dolomite, Based on Silicon, Iron, and
Aluminum Concentration Changes 84
23. Concentrations of Ash Constituents in Sulfated Dolomite
and Sulfated Greer Limestone 84
24. Operating Conditions and Flue-Gas Compositions for VAR-6
Replicate Experiments 89
25. Sieve Analyses of Arkwright Coal Size Fractions Used in
PSI-Series Combustion Experiments 90
26. Sieve Analyses of Tymochtee Dolomite Size Fractions Used
In PSI-Series Combustion Experiments 90
27. Operating Conditions and Flue-Gas Analyses for PSI-Series
of Combustion Experiments 92
28. Sulfur Retention, Combustion Efficiency, and Sorbent
Utilization for PSI-Series of Experiments , . . 93
29. Operating Conditions and Flue-Gas Analyses for Combustion
Experiments LIG-2D and LIG-2-R. . 96
30. Operating Conditions and Flue-Gas Compositions for Experiments
REG-IK and VEL-1 to Test the Effect of Gas Velocity on
Decrepitation 97
31. Analysis of Decrepitation in Experiment VEL-1 , 98
32. Calcium Content and Percent Sulfation of 6.6% CaO in
a-A!203 Sorbent Sulfated at 900°C 107
XI
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LIST OF TABLES (Contd.)
No. Title Page
33. Sorbent Weight Gain during Sulfation for Various Calcium
Oxide Concentrations. . 110
34. Most Promising S02 Sorbents Based on Thermodynamic
Screening Results 112
35. Regeneration of CaS03 to CaO, Calculated from Weight Loss . . 116
36. Carbon Content of Pellets after Regeneration with Methane . . 117
37. Product of the Regeneration Reaction at Various Temperatures. 118
38. Calcium Utilization and Regeneration during Sulfation-
Regeneration Cyclic Experiments, Using 1100°C H.T. Pellets. . 124
39. Sulfation of Supported Sorbents ..... 128
40. Pore Volume for Various Synthetic Sorbents 130
41. Fluidized-Bed Attrition Experiments 132
42. Half-Calcination Experiments on 1337 Dolomite 138
43. Summary of TGA and X-Ray Diffraction Results for Solid-
Solid Experiments at 950°C. . 141
44. Results of CaS04-CaS Reaction Kinetic Measurements at 945°C . 143
45. Elemental Concentration of High-Temperature Ash Corrected
for Weight Losses at the Stated Temperatures 150
46. Elemental Concentrations in High-Temperature Ash Calculated
on the Original 340°C-Ash Basis 151
47. Elemental Concentration in High-Temperature Ash as a
Function of Oxygen Concentration in Gas Flow 152
48. Effect of Heating on Weight Loss of 340°C Ash 153
49. Elemental Concentrations in High-Temperature Ash Calculated
on the Original 340°C-Ash Basis 155
A-l Particle-Size Distribution and Chemical and Physical
Characteristics of Arkwright Coal 178
A-2 Particle-Size Distribution and Chemical and Physical
Characteristics of Triangle Coal 179
A-3 Particle-Size Distribution and Chemical Characteristics
of Glenharold Lignite ....... 180
xii
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LIST OF TABLES (Contd.)
No. Title Page
A-4 Particle-Size Distribution and Chemical Characteristics
of Tymochtee Dolomite. . 181
A-5 Particle-Size Distribution and Chemical Characteristics
of Type 38 Alundum Grain Obtained from the Norton Company. 182
xiii
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A DEVELOPMENT PROGRAM ON
PRESSURIZED FLUIDIZED-BED COMBUSTION
Annual Report
July 1975—June 1976
by
G. J. Vogel, I. Johnson, P. T. Cunningham, B. R. Hubble,
S. H. Lee, J. F. Lenc, J. Montagna, F. F. Nunes, S. Siegel,
G. W. Smith, R. B. Snyder, S. Saxena, W. M. Swift,
G. F. Teats, C. B. Turner, W. I. Wilson, and A. A. Jonke
ABSTRACT
Information is presented on a continuing research and
development program in which the concept of fluidized-bed
combustion for ultimate use in power stations and steam-
raising applications is being investigated. Laboratory-
scale equipment and process development units are being used
in investigating the effect of combustion operating variables
on release of pollutants to the atmosphere, on process
efficiency, and on release of compounds injurious to turbine
materials of construction. Methods are being investigated
for regenerating CaSOit, a product of reaction of CaO additive
with sulfur compounds in the combustion process. Significant
progress has been made in demonstrating that (1) reductive
decomposition of CaSOi* is a viable process for recovering CaO
for reuse in the combustion step, (2) cyclic use of limestone
in the combustion/regeneration processes appears feasible,
and (3) synthetic additives can be prepared that have good
sulfur retention and sulfur release properties.
SUMMARY
Ongoing studies are in progress in support of the national program on
fluidized-bed combustion for electric power and industrial/commercial
applications. The concept involves burning coal in a fluid bed of calcium-
containing stone such as limestone. The sulfated stone is removed from
the combustor and can be regenerated for resse in the combustor. Both
atmospheric pressure and pressurized concepts are under investigation. In
the pressurized combustion concept, the hot flue gas from the combustor
would be expanded through a turbine to generate electricity.
Current studies are aimed at (1) demonstrating a regeneration process,
(2) determining the effect of cyclic (combustion-regeneration) operation on
limestone quality, (3) continuing the study of the effect of combustion
operating variables on the release of pollutants and corrosive compounds, (4)
developing and testing a synthetic stone, and (5) elucidating the chemistry
of reactions in the combustion and regeneration processes.
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Regeneration of SO?- Accepting Additive, Bench-Scale Studies
The feasibility of reductive decomposition of sulfated S02-accepting
additive in a fluidized bed is being investigated. The reduction of the
CaSQi+ constituent of the sulfated stones to CaO is favored by high temper-
atures (>1040°C) and mildly reducing conditions. The solid-gas reactions
by which regeneration occurs can be summarized as follows :
CaS04 + CO -> CaO + C02 + S02 (1)
H2 -> CaO + H20 + S02 (2)
The present experimental system consists of a 10.8-cm-ID refractory-
lined reactor, an off-gas cleanup system, and a continuous gas sampling
and analyzing system. The coal and sulfated sorbent are fed separately
to a common transport feed line.
Sulfated Tymochtee dolomite and Greer limestone have been regenerated.
Methane and coal (in separate studies) have been combusted to provide the
required heat and reductants for the reactions. The effects of fluidizing-
gas velocity, temperature, and solids residence time have been evaluated.
Regeneration of Sulfated Tymochtee Dolomite by the Incomplete
Combustion of Methane. Additional results are presented on an earlier
reported FAC-series of regeneration experiments in which sulfated dolomite
with 10.2 wt % S was regenerated by the incomplete combustion of methane
in a 7.6-cm-dia fluidized bed. Total CaO regeneration values calculated
from chemical analyses of the regenerated samples were within analytical
agreement of fehe values calculated from flue-gas analyses. Most of the
sulfur and calcium balances ranged from 90 to 110%, which is an acceptable
variation. When the regeneration temperature was increased from 1010 to
1095°C, the extent of CaO regeneration increased from 21 to 89% and the
S02 concentration in the wet effluent gas increased from 0.7 to 5.3%.
Dolomite that had been regenerated at a higher temperature (1095°C
as compared with 1040°C) contained a larger amount of large pores (>0.4 urn).
These large pores have been shown by others to be beneficial to sulfation.
However, when the reactivity of these samples as S02 acceptors was
evaluated in TGA experiments, the sample that had been regenerated at the
lower temperature (1040°C) was found to be more reactive. Although higher
regeneration rates are obtained at high regeneration temperatures, the
effect of regeneration temperature on the reactivity of the additive in
subsequent cycles also has to be considered.
Electron microprobe analyses of sulfated and regenerated Tymochtee
dolomite particles showed no sulfur concentration irregularities that
might lead to poor utilization "in subsequent sulfation cycles.
At steady state, sulfide (S ) levels were £0.1% in the product
obtained in experiments performed at low reducing gas concentrations (3%
reducing gas in the effluent), and sulfide levels of 0.3 and 0.7% were
obtained in experiments performed at high reducing gas concentrations
-------
(15% reducing gas in the effluent). At these reducing conditions, it was
predicted that only CaS should exist at equilibrium. The very low CaS
concentrations found were attributed to the formation in the regeneration
reactor of two zones (oxidizing zone at the bottom of the fluidized bed
and reducing zone at the top).
The extent of decrepitation ranged from 5 to 15% of the total feed
(calcium basis). These particles, which were elutriated, were basically
smaller than the smallest sulfated particles fed to the regenerator. For
all experiments considered, a decrease in the number of larger particles
(>800 ym) and an increase in the number of medium-size particles (>300
and <800 ym) were found in the bed.
Agglomeration of sulfated additive occurred during some regeneration
experiments. X-ray diffraction analyses showed that compounds of calcium-
magnesium silicates had formed during the agglomeration (melting) process.
In DTA (differential thermal analysis) experiments, sulfated dolomite
melted at 1200°C in atmospheres of nitrogen and air. The X-ray diffraction
analysis of the DTA samples revealed the possible formation of a calcium
silicate compound, but no calcium-magnesium silicate trace was found.
Regeneration of Sulfated Greer Limestone by the Incomplete Combustion
of Coal. In a preliminary experiment, sulfated Greer limestone from Pope,
Evans and Robbins was regenerated. The results were comparable with those
obtained when using sulfated Tymochtee dolomite.
The next experiments made with Greer limestone explored the effect
of relative lengths of the oxidizing and reducing zones in the bed. At
relatively high reducing gas concentrations (^4% in the effluent gas),
increasing the length of the oxidizing zone relative to the length of
the reducing zone resulted in a drastically reduced CaS concentration in
the regenerated product and an increase in the extent of regeneration.
All subsequent experiments were made with the larger oxidizing zone.
In the main experimental studies, the Greer limestone was regenerated
at two temperatures, 1050 and 1100°C. In experiments at ^1050°C, decreasing
the solids residence time (which is inversely proportional to the sulfated
limestone feed rate) from 31 min to 12 min decreased the extent of CaO
regeneration from 89% to 70%. At 1050°C, increasing the fluidizing-gas
velocity from 1.2 to 1.37 m/sec had no significant effect on the extent
of regeneration. In the earlier regeneration study (FAC-series), it had
been found that increasing the fluidizing-gas velocity adversely affected
the extent of regeneration of sulfated additive.
In one of the experiments performed at 1100°C, the solids residence
time in the regenerator was low, 7.5 min. The S02 concentration in the
dry off-gas of 7.6% obtained at this solids residence time was the highest
in this series. Although the extent of regeneration of CaO decreased with
decreasing solids residence time, it remained quite high (61%), even at a
relatively low solids residence time of 7.5 min.
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Regeneration of Sulfated Tymochtee Dolomite by the Incomplete Combustion
of Coal. Sulfated Tymochtee dolomite was regenerated at bed temperatures
of 1000, 1050, and 1100°C. At 1000°C, the S02 concentration in the effluent
gas was not affected by reducing the solids residence time, but the extent
of regeneration decreased. At 1050°C, when the solids residence time was
decreased from 34 min to 12 min, the S02 concentration in the dry effluent
increased from 3.0% to 4.8% and the extent of regeneration dropped from
90% to 56%.
At 1100°C, the solids residence time was decreased from 13 to 9.4 min
and the SC-2 concentration in the dry effluent gas increased from 6.4% to
7.8%. The extent of regeneration decreased from 85% to 77%. Since the
extent of regeneration remained high at 1100°C at the above solids residence
times, it is expected that higher SC>2 off-gas concentrations.can be achieved
by further decreasing the solids residence time in the reactor (i.e., by
increasing the sulfated reactant throughput rate).
Carbon Utilization and Carbon Balance Calculations. Carbon balances
calculated for three of the earlier regeneration experiments were 97% and 99%,
and an unaccountably low 55%. Carbon utilizations were ^80%.
Effect of Coal Ash on the Agglomeration of Sulfated Additive during
Regeneration. During sulfation of the sorbent (combustion step), some coal
ash is retained in the fluidized bed and is removed with the sulfated sor-
bent. Tymochtee dolomite has been sulfated during the combustion of Arkwright
coal at ANL and Greer limestone during combustion of Sewickley coal at Pope,
Evans and Robbins. The ash fusion temperatures (initial deformation) under
reducing conditions for both of these coals are very close to the regeneration
temperature. The presence of these coal ashes could contribute to initial
coalescing of particles in the fluidized bed and subsequent reactions with
the sulfated acceptor. The coal ash level was found to be ^5% in once-sul-
fated Tymochtee dolomite and was M.0% in sulfated Greer limestone.
Reactions in two different materials have been investigated in the
Differential Thermal Analyzer (DTA) to date: (1) unsulfated Tymochtee
dolomite and (2) sulfated Greer limestone which contained ^10% coal ash.
The decomposition peak temperatures of MgC03 and CaC03 in the Tymochtee
dolomite samples were found to be 819 and 933°C, respectively, in good
agreement with literature values. In the study of sulfated Greer
limestone, dehydration of Ca(OH)2 was observed at ^530°C, and decomposition
of the residual CaC03 occurred at 790-840°C. The formation of 2Ca2Si1300°C), the samples fused and complex compounds of
calcium-aluminum silicates formed. Since the fluidized-bed temperature
was ^1100°C, the formation of calcium silicates may be the first stage
in the agglomeration process, followed by loss of fluidity in the fluidized
bed and greater localized temperature increases.
Mass and Energy Constrained Model for the One-Step Regeneration
Process. A mass and energy constrained model for the one-step regeneration
process is being developed which can be used to perform sensitivity analyses
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for key variables such as regeneration temperature and pressure, fluidizing-
gas velocity, reactor size, solids residence time, and feed gas composition
and temperature. The early objectives of this work are to predict the
effects of experimental variations and to use these results to guide further
experimental efforts in the investigation of the feasibility of the one-
step regeneration process. The effects of some of these variables on
dependent variables such as off-gas composition (SC>2, C02, H^O), fuel and
oxygen requirements, and energy cost of regeneration have been evaluated.
Cyclic Combustion/Regeneration Experiments
Preliminary Experiment. A preliminary experiment was performed to
evaluate the effects of sorbent recycling on sorbent reactivity and decrepi-
tation. Tymochtee dolomite which had been sulfated and regenerated under
various operating conditions was used as the sorbent. Arkwright coal was
combusted at a bed temperature of 840°C, 810 kPa pressure, and ^17% excess
combustion air. The Ca/S mole ratio, which was based on the unreacted
calcium in the regenerated dolomite, was 1.6. Based on the flue-gas
analysis for SC>2 (250 ppm), sulfur retention was ^90%, indicating that
the sorbent retained its activity for sulfur retention into the second
combustion cycle. Data showed that about a third of the dolomite fed to
the combustor was elutriated and recovered in the cyclones and filters.
Screen analyses of the feed, overflow, and cyclone materials indicated that
entrainment was not entirely the result of decrepitation of the dolomite
in the combustor.
Combustion Step, Cycles 1, 2, and 3. A ten-cycle series of combustion/
regeneration experiments was initiated using Arkwright coal and Tymochtee
dolomite. Nominal operating conditions selected for the combustion portions
of each cycle were a 900°C bed temperature, 810 kPa system pressure, 1.5
Ca/S mole ratio, ^17% excess combustion air, 0.9 m/sec fluidizing-gas
velocity, and a 1.07-m bed height.
The first 2 1/2 cycles (three combustion experiments) have been
completed. The sulfur dioxide level in the flue gas increased from 290
ppm (86% retention) in the first combustion cycle to 400 and 490 ppm (81
and 77% retention) in cycles two and three. Sulfur retention in cycle
three was sufficient to meet the EPA environmental emission standard for
sulfur dioxide.
As the sorbent passed through the combustor in the first cycle, ^20%
of the +30 mesh additive feed was reduced in size to -30 mesh. Based on
an approximate 5-hr solids residence time in the combustor, the decrepitation
rate can be expressed as ^4 wt %/hr. For the same experiment, ^25% of
the additive entering the combustor was entrained with the flue gas and
the remaining ^75% was removed in the product overflow. This corresponds
to an entrainment rate of ^5 wt %/hr.
Quantitative evaluations of decrepitation and elutriation of sorbent
during the second and third cycles have not been completed, but estimated
losses are 2-3 wt %/hr for cycle two and 1-2 wt %/hr for cycle three,
indicating a significant reduction in the sorbent loss rate in succeeding
cycles.
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Regeneration Step, Cycles 1 and 2. The sulfated dolomite from the
combustion step was regenerated at a bed temperature of 1100°C, a pressure
of 153 kPa, and a solids residence time of ^7 min, with relatively good
results. In the first cycle, the SC>2 concentration in the dry effluent
gas was 6.5% and the extent of CaO regeneration was 71%. Solids losses
from the bed due to elutriation and/or attrition were found to be lower
than expected, 2%. In the second cycle, CaO regeneration was 67%. The
S02 concentration in the dry off-gas was 8.6%. The solids losses due to
elutriation and/or attrition were negligible. The low sorbent losses due
to attrition can be attributed to the short solids residence time in the
regenerator.
Coal Ash Buildup during Sulfation and Regeneration Steps. In the
sulfation (coal combustion) step, Arkwright coal is combusted under oxidizing
conditions. In the regeneration step, Triangle coal is partially combusted
under reducing conditions. A coal-ash level of 4-5% in the bed was
estimated for the combustion-sulfation step of the first cycle. In the
regeneration step, no additional ash buildup was observed.
Pressurized, Fluidized-Bed Combustion; Bench-Scale Studies
The objectives of the experiments were to evaluate the following: (1)
the effects of coal and additive particle size on sulfur retention, nitrogen
oxide flue-gas levels, and combustion efficiency; (2) the sulfur retention
capability of lignite ash, which has a high calcium content; and (3) the
relative effects of fluidizing-gas velocity and residence time on the
decrepitation of sorbent.
Combustion experiments were performed, using a highly caking, high-
volatile bituminous, Pittsburgh seam coal from the Consolidation Coal
Company Arkwright mine and a lignite coal from the Consolidation Coal
Company Glenharold mine. Tymochtee dolomite obtained from C. E. Duff and
Sons, Huntsville, Ohio, was air-dried and screened before being fed to the
combustor.
The equipment, designed for operation at pressures up to 10 atm,
consists of a 6-in.-dia fluidized-bed combustor, a compressor for supplying
fluidizing-combustion air, a preheater for the fluidizing-combustion air,
coal and additive feeders, and an off-gas system (cyclones, filters, gas-
sampling equipment, and pressure let-down valve). The system is thoroughly
instrumented and is equipped with an automatic data logging system.
Replicate of VAR-Series Experiments. A replicate of a previously
performed VAR-series experiment was made after the completion of equipment
modifications (separation of the combustion and regeneration systems).
In this experiment, the operation of the combustor and analytical instru-
mentation was checked and the validity of comparing current experiments
and past experiments was verified. Except for a slight discrepancy in
the level of NO in the flue gas, the results of the experiment agreed very
well with the results of the previously performed experiments.
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Effect of Coal and Sorbent Particle Size on Sulfur Retention,
Nitrogen Oxide Level in the Flue Gas, and Combustion Efficiency. Four
combustion experiments (PSI-series) were completed to measure the effects
of coal and sorbent particle size on sulfur retention, NOX emission, and
combustion efficiency. Arkwright coal with mass-mean particle size levels
of ^150 ym and ^640 ym and Tymochtee dolomite with mass-mean particle
size levels of ^370 ym and ^740 ym were used in the experiments (22 factorial
design). The experiments were performed at 843°C (1550°F) , 8 atm, and
excess combustion air.
The level of S02 in the effluent gas ranged from a low 160 ppm (^93%
sulfur retention) in experiment PSI-3 (fine coal, fine sorbent) to 240 ppm
0^89% sulfur retention) in experiment PSI-4 (fine coal, coarse sorbent).
The results indicate a slight increase in sulfur retention with decreasing
dolomite particle size. No significant effect of coal particle size on
sulfur retention was observed.
There was no noticeable effect of coal or dolomite particle size on
NO levels in the flue gas (120 to 150 ppm). N02 levels were ^40 to ^60
ppm, which are considerably higher than the anticipated N02 levels of 5
to 10 ppm.
Combustion efficiencies ranging from 89 to 93% were determined, but
no consistent effect of particle size of coal or sorbent on combustion
efficiency was indicated. Utilization of sorbent material ranged from
58 to 71% and increased with decreasing dolomite particle size.
Combustion of Lignite in a Fluidized Bed of Alumina. Glenharold
lignite was combusted in a fluidized bed of alumina in two replicate experi-
ments to test the sulfur-retention capability of the high calcium lignite
coal. Sulfur retentions were 86 and 89%, which compared favorably with
a sulfur retention of 85% when lignite was combusted in a bed of dolomite
under similar operating conditions. Hence for this lignite, sulfur
emissions can be adequately controlled without the addition of limestone.
Effect of Fluid iz ing-Gas Velocity on Decrepitation Rate. An experi~
ment was performed to duplicate the conditions of the first-cycle combustion
experiment discussed above, except that the velocity was reduced to 0.6
m/s from 0.9 m/sec. At the lower gas velocity, the coal and thus the
additive feed rate is less, resulting in a higher solids mean residence
time. The data show that the rate of decrepitation was essentially
unaffected (it increased slightly) by the decrease in fluidiz ing-gas
velocity. Although decrepitation increased slightly due to the increased
residence time of the additive in the combustor, the decreased velocity
was apparently effective in retaining a greater percentage of the additive
feed in the product overflow stream. Thus, the entrainment was actually
less than the decrepitation rate.
Synthetic SO? Sorbents
Calcium oxide impregnated in a-A!203 is being investigated as an
alternative to dolomite and limestone as an an additive for lowering the
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8
sulfur dioxide level in the off-gas from fluidized-bed coal combustors.
In this program, the capability of the supported calcium oxide to react
with sulfur dioxide and of the resulting calcium sulfate to be regenerated
is being studied experimentally, using thermogravimetric analysis.
Alpha-alumina pellets containing 6.6% calcium oxide by weight were
sulfated at 900°C, using concentrations of sulfur dioxide in the gas stream
ranging from 0.05 to 3%. Calcium utilization ranged from 67 to 98%, and
the reaction went to completion in 4 to 10 hr.
The oxygen concentration in the gas phase had only a small effect on
the sulfation rate when oxygen was present in stoichiometric excess. However,
when sulfur dioxide was in excess, the rate was first order in oxygen
concentration.
Water concentration in the synthetic combustion gas had no effect on
the sulfation rate.
The sulfation rate of the pellets increased with sulfation temperature
up to 900°C, where it became independent of temperature. Above 900°C, the
reaction is diffusion controlled.
Synthetic sorbents containing 2-16.5% CaO in a-A!203 were studied for
their ability to capture S02. The sulfation rate decreased with increasing
CaO concentration when measured as a fraction of the maximum possible
sulfation; however, the amount of S02 captured for a given residence time
was usually higher for higher CaO concentrations.
Tymochtee dolomite and calcium oxide in a-A!203 pellets were sulfated
under similar experimental conditions to allow comparison. The calcium
oxide in a-A!203 pellets was 95% sulfated in 6 hr at 900°C, using 0.3%
sulfur dioxide and excess oxygen; the dolomite was only 60% sulfated in
19 hr. However, the dolomite contained four times as much calcium as the
sorbent did.
In addition to CaO and Li20, the metal oxides, Na20, K20, SrO, and
BaO, have been impregnated into alumina and tested as S02 sorbents. Lithium
sulfate was found to be unstable at sulfation conditions (900°C). Sodium
oxide and potassium oxide sorbents have a higher rate of reactivity with
S02 than does CaO sorbent; however, their sulfates would have appreciable
volatility at regeneration conditions (1100°C). Strontium oxide and barium
oxide sorbents have essentially the same reactivity as does calcium oxide.
Sulfated sorbents were regenerated using various reducing gases (CO,
H2, and CH^) . In each case, the reaction is 0.8 order in reducing gas
concentration. The rate was the same for hydrogen and methane and the rate
for carbon monoxide was one-third the former rate, requiring only 4 min
when using 2_&% reducing gas. The addition of carbon dioxide (15% or more)
to the reducing gas lowered the regeneration rate since carbon dioxide
reacted with hydrogen to form carbon monoxide.
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The reaction product for the CaO in a-Al203 sorbent was dependent
upon the regeneration temperature; above 1050°C, the product was CaO;
below 900°C, it was CaS. Between 900 and 1050°C, the product was a mixture
of CaO and CaS, with CaO concentration increasing with temperature.
Other data showed that dolomite was regenerated at a slightly lower
rate than was the supported additive; moreover, the product of the reaction
when dolomite was regenerated in the TGA apparatus contained 50% CaS and
not 100% CaO as with the supported additive.
A cyclic sulfation-regeneration experiment (five cycles) was performed
on 6.6% CaO in a-Al203 additive that had been heat-treated at 1100°C.
The rates of sulfation and regeneration were the same in each cycle.
Porosity measurements have been made on granular supports that had
been heat treated at 1100, 1200, and 1500°C. The higher heat-treatment
temperatures produced supports containing larger pores. The supports that
contained larger pores produced sorbents having a higher reactivity with
S02 and a greater calcium utilization.
Sorbent Attrition Studies
Preliminary attrition studies on dolomite have shown that sulfated
dolomite is more attrition-resistant than is fresh half-calcined dolomite.
The attrition rate of regenerated dolomite is high; however, due to the
short residence times in regenerators, the material loss per cycle is
low. Supports for synthetic sorbents have approximately the same attrition
rate as does sulfated dolomite.
*
Combustion-Regeneration Chemistry
The fundamental aspects of the chemical reactions associated with
the cyclic use of BCR 1337 dolomite, chosen as a model sorbent system, to
control sulfur emissions are being investigated. Kinetic studies are based
on the use of a thermogravimentric analysis (TGA) technique, and structural
studies are based on X-ray diffraction and microscopy techniques.
The half calcination reaction
[CaC03'MgC03] ->- [CaC03-MgO] + C02
has been studied at 1 atm over the temperature range 640-800°C in two
environments: (a) 100% C02 and (b) 40% C02-60% He. The reaction rate
increases rapidly with temperature for both 100% CO and 40% C02 environ-
ments, particularly above a temperature near 700°C. The effect of C02
concentration is subtle; in general, the rate is higher in the 40% C02
environment. X-ray diffraction studies show that size and degree of
preferred orientation of the CaC03 (calcite) pseudo crystals formed by
Sponsored in part by ERDA, Division of Physical Research.
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10
this reaction are dependent on the kinetics (temperature) of the reaction,
i. e. j larger sizes and greater degrees of preferred orientation result
when the reaction proceeds slowly (at lower temperatures).
Optical microscopic examination of the half-calcined dolomite sample
shows that dolomite crystals are transformed to calcite along grain
boundaries, as well as within dolomite crystals and that the half-calcined
grain structure formed as a result of the half-calcination reaction is
completely destroyed by heat treatment.
Experiments have been performed to establish whether the solid-solid
reaction between CaSOi^ and CaS would be practical as a regeneration scheme.
The yield of the reaction is shown to depend on the percentages of CaS
and CaSOij in the dolomite starting material for the reaction. In addition,
the kinetics of the reaction are shown to be such that at 950°C, the
reaction reaches 80% completion in less than 6 hr.
Coal Combustion Reactions
Determination of Inorganic Constituents in the Effluent Gas from
Coal Combustion. Some chemical elements carried by the combustion gas
from coal combustion are known to cause severe metal corrosion, for example,
to turbine blades. A study is under way to determine quantitatively which
elements are present in the hot combustion gas of coal, in either volatile
or particulate form, and to differentiate between volatile and particulate
species. Of interest are (1) identification of the compound forms and
amounts of particulate species and (2) determination of the amount and
form of condensable species.
The detailed design of the laboratory-scale batch fixed-bed combustor
for this study has been completed, the combustor has been fabricated in
the central shop of ANL, and the combustor system is now being assembled.
A schematic flow diagram of the combustor system is shown and described.
Prior to fabrication of the combustor, detailed engineering drawings,
specifications for fabrication, and stress calculations to support the
safety of the design were reviewed and approved by a design/preliminary
safety review committee. Modifications to the design are described in
this report. The combustor is now considered to have a safe design.
Systematic Study of the Volatility of Trace Elements in Coal. The
effluent gases from coal combustion are known to contain trace elements,
some of which cause severe metal corrosion in coal-fired boilers and gas
turbines. Knowledge of the temperature at which trace elements in coal
start to volatilize and of the rate of volatilization is important for
the utilization of coal. The objective of this study was to obtain data
on the volatility of these trace elements under practical coal combustion
conditions. The experimental phase of this study has been completed.
In all experiments, ash samples prepared from Illinois Herrin No. 6
Montgomery County coal by ashing at 340°C were heat-treated in a tubular
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11
furnace to various temperatures in an air flow of 0.6 scfh for 20 hr or
more, and the ash residue was analyzed for trace elements of interest.
From the experimental results, it is concluded that under dry oxidizing
conditions, most of the chlorine in the coal evolves at temperatures below
640°C, although trace amounts of chlorine still remain in the high-temperature
ashes. However, the metallic elements, Na, K, Fe, Al, Mg, Ca, Ti, Zn, Mn,
Ni, Co, Cu, Cr, and Li are generally retained in the fused ash up to 1250°C
under both dry and wet oxidizing environments, and the retention of these
metallic elements in the fused ash is not affected by the oxygen concen-
tration in dry flowing gas. Water vapor in the flowing gas did not affect
the evolution of these metallic elements; however, it caused a greater weight
loss of the ash, indicating that water vapor in the flowing gas affects
the evolution of certain substances from the 340°C ash during heat treatment.
Equipment Changes
As originally installed, the pressurized, fluidized-bed combustor and
the regenerator utilized several common components and therefore could not
be operated simultaneously. Additional equipment and instruments were
installed to permit concurrent investigations. Equipment is also being
installed so that solids can be transported between the combustion and
regenerator reactors.
Miscellaneous Studies
Preparation and Testing of Supported Additives (Dow Subcontract). A
thermogravimetric analysis apparatus to be used in evaluating various
candidate S02 sorbents has been constructed as part of a research program
subcontracted to Dow Chemical Company. Three synthetic sorbents (CaO,
BaO, and SrO) have been impregnated into low-surface-area and intermediate-
surf ace-area Al2C>3 support material. The work plan includes screening
six kinds of metal oxide-support combinations in both the sulfation and
regeneration modes.
Limestone and Dolomite for the Fluidized-Bed Combustion of Coal:
Procurement and Disposal. The potential demand, supply, and cost as well
as the disposal aspects associated with the use of limestone or dolomite
as sulfur-accepting additives in the fluidized-bed combustion (FBC) of
coal are assessed. Also, the market for the regeneration by-products
(sulfur and sulfuric acid) is discussed.
The Properties of a Dolomite Bed of a Range of Particle Sizes and
Shapes at Minimum Fluidization. Experiments have been performed to determine
minimum fluidization velocity as a function of temperature and pressure
for a dolomite bed having a wide size range of nonspherical particles.
An improved correlation has been developed in the form of Ergun's correlation.
Experiments revealed that partial segregation of the bed occurred if the
ratio of the diameter of the largest particle to the diameter of the
smallest particle exceeded at least 16. As a result, the prediction of
minimum fluidization velocity for the entire bed becomes complicated, making
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12
precise prediction uncertain. A procedure is suggested for the prediction
of minimum fluidization velocity of the entire bed.
Mathematical Modeling; Noncatalytic Gas-Solid Reaction with Changing
Particle Size, Unsteady State Heat Transfer. A mathematical expression
for the unsteady state heat balance is derived for a nonisothermal, non-
catalytic, first order gas-solid reaction. The formulation is based on
a shrinking core model and takes into account the changing size of the
spherical particle during reaction. Thus, the model has two moving
boundaries viz.3 the reaction front and external particle diameter which
grows or shrinks due to reaction. This formulation constitutes the first
basic step in the modeling of combustion efficiency of coal in a fluidized
bed.
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13
INTRODUCTION
In a national program, the feasibility of an environmentally
acceptable fluidized-bed combustion process for use in power and steam
plant applications is being studied. The concept involves burning fuels
such as coal in a fluidized bed of particulate lime solids which react
with the sulfur oxides released during combustion. The sulfated lime
solids will be either discarded or regenerated, the choice depending on
environmental and economic considerations. One concept is pressurized
combustion, in which the hot flue gas would be expanded through a gas
turbine.
The current program for demonstration of regeneration process
feasibility consists of studying: (1) the reductive decomposition of
CaSOtf at VL100°C, using coal as a source of reducing agent, (2) the
conditions at which sintering of particles in the bed can occur, (3)
the modeling of the process, and (4) other possible methods of regener-
ation.
To demonstrate that sulfur retention by the lime solids during
combustion and sulfur release during regeneration are possible without
excessive breakup of the solids during cycling, batch cyclic studies
(alternate combustion and regeneration of the lime solids) have been
started. Combustion studies have been performed to determine (1) the
effects of stone and coal particle sizes on sulfur retention, NO level
in flue gas, and combustion efficiency, (2) whether the concentration of
calcium in lignite is sufficient that enough sulfur is retained in the
bed and the EPA specification for S02 in the flue gas is met, (3) the
effect of fluidzing-gas velocity on decrepitation rate, and (4) the fates
of the biologically hazardous trace elements and compounds which can
cause hot corrosion of turbine metal blades. Miscellaneous peripheral
studies include (1) a survey on procurement and disposal of sulfated
limestone, (2) properties at minimum fluidization of a bed having a range
of particle sizes and shapes, and (3) a mathematical modeling of a
particle undergoing a noncatalytic reaction and changing size; unsteady
state heat transfer effects are taken into consideration.
REGENERATION OF S02-ACCEPTING ADDITIVE, BENCH-SCALE STUDIES
[J. Montagna (Principal Investigator),. R. Mowry, C. Sehoffstoll, G. Smith,
G. Teats]
The technical feasibility of a process is being investigated for
the one-step reductive regeneration of additive in a fluidized bed in
which the required heat and reductants are provided by partial combustion
of a fuel in the regeneration reactor. The reduction of CaSO^ to CaO is
favored by high temperatures (>1040°C/1900°F) and mildly reducing
conditions.
+ CO -+ CaO + S02 + C02 (1)
+ H2 -* CaO + S02 + H20 (2)
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14
Other reductants such as CH4 can play the same role as CO and H2. At
lower temperatures (<1000°C) and more highly reducing conditions the
formation of CaS is favored:
CaSOi4 + 4CO -> CaS + 4C02 (3)
CaS04 + 4H2 -> CaS + 4H20 (4)
Since these are competing reactions, the reaction conditions must be
chosen to minimize the buildup of CaS, maximize the conversion of CaSOi+
to CaO, and maximize the S02 concentration in the off-gas from the
reactor for economical reprocessing of the off-gas from the regeneration
process.
Sulfated dolomite (Tymochtee) and limestone (Greer) have been regen-
erated. Methane and coal (in separate studies) have been partially
combusted during the regeneration experiments. The effects of temperature,
fluidizing-gas velocity, and solids residence time on the regeneration
of CaO from CaSO^ have been evaluated at different experimental conditions.
Materials
Tymochtee dolomite obtained from C. E. Duff and Sons, Huntsville,
Ohio, which had been sulfated during coal combustion experiments (using
Arkwright coal) at ^900°C and 810 kPa, was regenerated. Also regenerated
was Greer limestone which had been sulfated by Pope, Evans and Robbins
at 840-900°C and atmospheric pressure during the combustion of Sewickley
coal.
Initially high-volatile Arkwright bituminous coal and later Triangle coal
(ash fusion temperatures of VL100°C and 1380°C, respectively).were combusted
during these regenerations. Chemical and physical characteristics of these
coals are presented in Appendix A, Table A-l.
Equipment
The bench-scale experimental system used for regeneration experiments
(in which methane was combusted) has been previously described.1 .Recently,
the experimental regeneration system was separated from the combustion
system and the regenerator was rebuilt and modified. Figure 1 is a
schematic diagram of the new regeneration system. The inside diameter
of the reactor vessel has been enlarged from 7.62 cm (3.0 in.) to 10.8
cm (4.25 in.), and the interior metal overflow pipe formerly used to
control the fluidized-bed height has been replaced with an overflow pipe
that is external to the fluidized bed. The coal and the sulfated sorbent
are fed separately (for independent control) to a common pneumatic transport
line which enters the bottom of the reactor. In addition to the regeneration
reactor, the experimental system consists of an L-valve for metering
sulfated additive, a peripherally sealed rotary feeder for metering coal,
and the flue-gas solids-cleanup system. Another component is an electrically
heated pipe heat exchanger for preheating some of the fluidizing gas and
preheating air (used in startup only) to ^400°C.
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TO GAS
ANALYZERS
COAL
ROTARY VALVE
AIR
SAMPLE
GAS CONDITIONER
SULFATED-ADDITIVE
HOPPER
PULSED
AIR
Fl LTER
•EXHAUST
FILTER
PRESSURE
CONTROL
VALVE
REGENERATOR
(10.8-cmID)
LINE PREHEATER
SURGE
TANK
PRODUCT
COLLECTOR
-C
AIR
N2
Fig. 1. Schematic Diagram of Present Regeneration System.
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16
The 10.8-cm-dia pressurized, fluidlzed-bed reactor is lined with a
^4.8-cm-thick Plibrico castable refractory encased in an 8-in. (20.3-cm)
dia Schedule 40 pipe (Type 316 SS), approximately 2.29 m (7 1/2 ft) long,
with its entire length contained within a 12-in.-dia Schedule 20 carbon
steel pipe. Differential thermal expansion of the inner and outer pipes
is accommodated by the use of packing glands on lines entering the bottom
flange of the unit.
The solids transport air constitutes ^40% of the total fluidizing
gas added to the bed in the reactor. The remaining fluidizing gas is a
mixture of pure % and G£. The gases are added separately to permit
better temperature control in the fluidized bed (if there should be a
mild temperature excursion, combustion could be stopped, without de-
fluidizing the bed, by shutting off the oxygen). Also, the concentration
of oxygen in the entering gas can be higher than that in air, allowing
more coal to be combusted without significantly changing the fluidization
gas velocity. A large amount of heat is required to compensate for (1)
the heat losses in the comparatively small experimental system and (2)
the heat load imposed by feeding cold sulfated additive to the system.
Regeneration of Sulfated Tymochtee Dolomite by the Incomplete Combustion
of Methane
The FAC-series of regeneration experiments was performed in the 7.6-
cm-ID regenerator using the -in situ combustion of methane to evaluate
the effects of temperature, residence time, height of fluidized bed,
and total reducing gas conditions on the regeneration of CaO in sulfated
Tymochtee dolomite. Earlier results, which were based only on flue-gas
analyses, have been reported and discussed.1 To aid in the discussion
of additional results obtained by chemical analysis of particulate
product samples, the experimental conditions and the results based on
flue-gas analyses from the FAC-series are re-presented in Table 1.
Sampling was initiated when over 90% of the fluidized bed had been
replaced while the regenerator was at the design experimental conditions.
Steady-state samples, each composed of the overflow product taken over a
30-min period, were analyzed for total sulfur, sulfide, and calcium (Table
2). The accuracy of the analytical techniques ranged from 5 to 10%.
Regeneration of CaO. Regeneration of CaO was calculated from chemical
analyses of the steady-state products. It was based on the sulfur to
calcium ratios in (1) the sulfated-dolomite feed and (2) the steady-state
product after regeneration. These calculated regeneration ratios are
compared in Table 2 with ratios that are based on off-gas analyses. The
latter values are the ratios of the total sulfur released into the off-gas
stream to the total sulfur contained in the sulfated dolomite feed. Since
the off-gas flow rate was not measured, it had been assumed to be equal
to the feed gas flow rate. This has caused a discrepancy in the two calcul-
ated regeneration results. As discussed in the "Mass and Energy Constrained
Model for the Regeneration Process" section of this report, the gas flow rate
increases during regeneration due to the coal combustion and the decomposition
reactions that occur. The predicted gas volumetric increase was included in
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Table 1. Design Experimental Conditions and Extent of Regeneration Results
for the FAC-Series.
Reactor Diameter, in.: 3
Pressure, psig: 12
Additive: Sulfated Tymochtee dolomite (10.2% S)
Design Conditions
Exp.
FAC-1
FAC-1R1
FAC-1R2
FAC-2
FAC-3
FAC-4
FAC-5
FAC-9A
FAC-7
FAC-8
FAC-9
Temp
(°C)
1040
1040
1040
1095
1040
1095
1040
1040
1040
1010
1040
Additive
Feed Rate
(Ib/hr)
6
6
6
6
10
10
10
6
10
6
6
Residence
Time
(min)
30
30
30
30
18
18
30
30
30
30
30
Bed
Height
(ft)
1.5
1.5
1.5
1.5
1.5
1.5
2.5
1.5
2.5
1.5
1.5
Total
Reducing Gas
in Effluent3
(vol %)
3
3
3
15
15
3
15
3
3
3
3
Fluidizing- Measured S02,
Gas Velocity in Effluent
(ft/sec) (%
2.2
2.4
2.5
Experiment
2.6
2.5
2.7
3.0
2.5
2.3
2.5
wet)/(% dry)
3.0 / 4.0
2.6 / 3.6
2.3 / 2.9
could not be
4.1 / 5.7
5.3 / 7.3
3.5 / 4.4
1.3 / 1.5
2.7 / 3.3
0.65/ 0.78
1.3 / 1.7
CaO
Regeneration
(%)
70
65
60
completed
70
83
61
40
43
15
58
The actual reducing gas concentrations were within 15% of the design levels.
^Determined by infrared 862 analysis of the dry flue gas.
"Based on flue gas analysis.
-------
Table 2. Chemical Analysis of Regenerated Product, Calculated Material
Balances, and Regeneration Results from the FAC-Series.
Chemical Analysis of Regenerated
Samples Taken at Steady State
Temp Total Sulfur
Exp.
FAC-1
FAC-1R1
FAC-1R2
FAC-2
FAC-3
FAC-4
FAC-5
FAC-9A
FAC-7
FAC-8
FAC-9
( c)
1040
1040
1040
1095
1040
1095
1040
1040
1040
1010
1040
(%)
3.30
3.73
5.08
2.89
1.53
2.79
7.34
7.41
8.38
2.74
Sulfide
(%)
0.1
<0.1
<0.1
5.2
0.3
<0.1
0.7
0.1
<0.1
<0.1
<0.1
Calcium
(%)
29.8
28.7
26.2
30.2
30.0
29.0
26.8
25.3
23.1
29.1
Elutriated3
Material Balances Calcium x 100
CaO
Sulfur Calcium Feed Calcium Regeneration
(%) (%)
94 109
99 109
106 113
This experiment
91 102
88 83
Production
— -
109 110
71 72
95 91
(%) 0
5
15
13
could not be completed
12
9
rate was not measured
-
15
14
11
0 /(%)
70/76
65/72
60/58
70/79
83/89
61/79
40/40
43/36
15/21
58/55
00
Only particles larger than 15 ym were monitored.
Based on flue gas analysis.
GBased on chemical analysis of particulate dolomite samples.
-------
19
the recalculation of regeneration based on off-gas analysis. The sulfur
regeneration values obtained by chemical analysis of the regenerated products
are generally within analytical agreement of the values calculated from off-
gas analyses.
Material Balances. Material balances for sulfur and calcium were
not performed for the entire experiments, as was the case for balances
in other experiments that are reported later. Instead, the balances were
each made over a 30-min steady-state period.
Because the experimental system did not contain a final filtration
system, particles smaller than -vL5 ym escaped from the system unmonitored.
The total mass rate of these fines was measured only in FAC-5, in which
the flue gas exiting from the last cyclone was passed through a sintered-
metal filter. A particle pad ^0.3 cm thick was built up on the filter,
ensuring good particle collection efficiency for the fine particles. A
mass collection rate of 4.6 g/min was obtained in this sintered-metal
filter, in comparison with a 8.3 g/min captured in the other particle
collection devices. The particle size distribution for the FAC-5 sample
of fines was obtained with a Coulter counter particle analyzer and is
given in Fig. 2. None of the calculated mass balances contained the
contribution from these fines and thus they are biased low.
99.9
O.I
10
DIAMETER,
Fig. 2. Particle Size Distribution of the Fines
Leaving the Cyclone in FAC-5.
-------
20.
The calcium balances ranges from 72% in FAC-8 to 113% in FAC-1R2.
These balances were based on the mass rates and chemical analyses of the
feed, product, and elutriated solids. The calcium analyses are reliable
within +5%. Errors greater than ^5% are due mainly to unreliability of
the measurements of the mass rates.
The sulfur balances were based on the flue-gas flow rates, the con-
centrations of sulfur (as S02 only) in the flue gas, and the flow rates
and chemical analyses of the solid streams. The sulfur balances ranged
from 71% in FAC-8 to 109% in FAC-7. Acceptable material balances should
range from 90 to 110%; most sulfur balances were acceptable.
The lowest sulfur and calcium balances obtained (FAC-8) were probably
caused by incomplete removal of the 30-min steady-state sample from the
receiver. Thus the measured mass rate of regenerated solid was probably
low. (The pre-steady-state removal rate was ^50% larger than the calculated
steady-state rate.) Generally, sulfur and calcium balances agreed within
10%.
Effect of Fluidizing-Gas Velocity. The effect of fluidizing-gas
velocity on extent of regeneration was evaluated and reported earlier.1
Regeneration data were calculated using S02 concentrations in the off-gas.
Now these data with additional CaO regeneration values based on chemical
analyses of the regenerated product are plotted in Fig. 3. As the gas
velocity was increased from 0.67 m/sec to 0.91 m/sec, CaO regeneration
decreased from 76 to 40% and the SC>2 concentration in the wet effluent
gas decreased from 3% to 1.3%. In a study by Martin et a£. 2 on the
reductive decomposition of gypsum in a 25.4-cm-dia (lO-in.-dia) fluidized
bed, the optimum fluidizing-gas velocity (for decomposition) was found
to be about 0.6 m/sec. However, the effect of fluidizing-gas velocity
was reported to be small.
a BASED ON SOLID
ANALYSIS
o BASED ON FLUE
GAS ANALYSIS"
0.60 075 0.90
FLUIDIZING-GAS VELOCITY, m/sec
Fig. 3. The Effect of Fluidizing-Gas Velocity on Sulfur
Regeneration for Sulfated Tymochtee Dolomite.
-------
21
In this study, fluidizing-gas velocity had a meaningful effect in the
velocity range investigated. The relatively small internal diameter (7.62 cm)
of the fluidized-bed reactor probably enhanced the detrimental effect of
gas velocity, lowering fluidization quality.
Effect of Temperature on Regeneration and Resulfation. The effect
of regeneration temperature (ranging from 1010 to 1095°C) on the regener-
ation of dolomite was reported earlier.1 The extent of regeneration was
based on the S02 content of the off-gas. Now, regeneration based on
solid product analyses are given in Pig. 4 with the earlier reported data.
(Regeneration temperature was the only variable in all experiments,
except that in the 1095°C experiment the solids residence time was 18 min
instead of 30 min.) As the temperature was increased from 1010°C to 1095°C
and the solids residence time remained constant or decreased, CaO regen-
eration (based on analysis of the regenerated product) increased from 21%
to 89%. With increasing regeneration temperature, the SC>2 concentration
in the wet off-gas increased from 0.7% to 5.3% (from 0.8% to 7.3% in the
dry off-gas).'
The temperature dependence of the regeneration of sulfated Tymochtee
dolomite in these experiments was compared (see Fig. 4) with similar
results obtained by Martin et al. 2 for the decomposition of gypsum in a
25.4-cm-dia fluidized bed with a solids residence time of ^90 min. In
the latter investigation, carbon was the reductant. The extent of regen-
eration was based on chemical anlaysis of the gypsum before and after
decomposition.
100
o CaO REGENERATION
(Based on flue gas analysis)
A CaO REGENERATION
(Based on product analysis!
a S02 CONCENTRATION
-"•--DECOMPOSITION OF GYPSUM,
CaO REGENERATION (MARTIN et al.)
(Based on product analysis)
s?
to"
-------
22
To evaluate the effect of regeneration temperature on the quality of
the regenerated dolomite as a S02~acceptor, the pore size distribution of
samples (-25 +35 mesh particles) of calcined, sulfated and regenerated
dolomite were obtained (Fig. 5). Porosity was measured as the extent of
penetration of mercury as a function of pressure. The cumulative pore
volume (mercury penetration) per given mass is plotted (Fig. 5) as a
function of pore diameter. It was reported by Hartman and Coughlin3 that
most sulfation takes place in the larger pores (>_0.4 ym). Pores smaller
than MD.4 ym are relatively small and easy to plug and do not contribute
much to the extent of sulfation. The pores shrink during sulfation of
CaO as a result of changes in molecular volume.
Curve A represents the pore size distribution of virgin dolomite
that had been completely calcined at 900°C for 2 hr in a furnace (preheated
to 900°C) with a 20% C02-80% air environment. The pore volume for pores
0.4 ym and larger was found to be 0.1 cm3/0.5 g. This pore distribution
was compared with that of sulfated and regenerated dolomite because
dolomite is fully calcined during regeneration.
Curve B represents the pore structure of dolomite that had been
sulfated (10.2 wt % S) in coal combustion experiments at 900°C and was
used as the feed for the FAC regeneration experiments. The pore volume
for pores 0.4 ym and larger was found to be 0.045 cm3/Q.5 g. In contrast
to the pore volume distribution of the calcined material, the pore structure
of the sulfated material was very much plugged by sulfation.
Curves C and D illustrate the pore structures of the sulfated dolomite
that had been regenerated at 1040°C and 1095°C. At the higher regeneration
temperature, the volume of the larger pores (>0.4 ym) increased. On the
basis of this result, the sulfation reactivity of dolomite regenerated
at the higher temperature was expected to be greater.
Sulfation experiments were performed in a thermogravimetric analyzer
with the regenerated samples and the calcined dolomite sample. The data
show (see Fig. 6) that the higher regeneration temperature did not improve
the reactivity of the dolomite with S02- The dolomite that had been
regenerated at 1095°C had a lower reactivity than did the dolomite regen-
erated at 1040°C and also the dolomite precalcined at 900°C.
Although high regeneration reaction rates are obtained at higher
temperatures, the potential detrimental effect of high temperatures on the
reactivity of the dolomite in subsequent sulfation cycles requires further
study. A decrease in the reactivity of regenerated additive during cyclic
exposures to high temperature was reported by Skopp e~k at. and the reason
was presumed to be loss in porosity. In the FAC experiments, the porosity
increased after one and a half cycles.
Electron Microprobe Analysis of Sulfated and Regenerated Particles.
Regenerated Tymochtee dolomite particles from FAC-1R2 and FAC-4, and
unregenerated, sulfated Tymochtee dolomite particles were analyzed with
an electron microprobe to determine the effect of regeneration on the
-------
A= CALCINED TYMOCHTEE
DOLOMITE 900°C
B = SULFATED TYMOCHTEE
DOLOMITE 900°C
C=REGENERATED TYMOCHTEE
DOLOMITE I040°C
D=REGENERATED TYMOCHTEE
DOLOMITE I095°C
Q_
0
10 1.0 O.I
D, PORE DIAMETER,
N>
OJ
Fig. 5. Pore Distributions of Dolomite Samples from Different Process Stages.
-------
24
sulfur concentration profiles. The radial distributions of calcium,
magnesium, and sulfur were measured. To prepare the sample for analysis
a random sample of particles was screened, and approximately twenty
-20 +25 mesh (707-841 ym) particles were mounted in epoxy and machined to
remove the equivalent of one-half the diameter of a typical mounted
particle (^390 ym). A thin gold layer (^50 angstroms) was sputtered on
the machined surface to enhance the conductivity of the mounts. Apatite
(39.9 wt % Ca), FeS2 (53.4 wt % S), and MgO (60.3 wt % Mg) were used
as standards to calibrate the probe and to obtain a rough quantitative
estimate of the local component concentrations. One-half of the particles
were analyzed.
n VIRGIN DOLOMITE PRECALCINED AT
900°C FOR 2 hr IN 20% C02
o REGENERATED AT 1040°C
REGENERATED AT 1095 °C
2 3
SULFATION TIME, hr
Fig. 6. Sulfation Reaction Data Obtained with a
Thermogravimetric Analyzer at 900°C, 0.3%
S02, and 5% 02.
-------
25
Microprobe analyses of sulfated Tymochtee dolomite particles were
compared with chemical anlayses that indicated average sulfur and calcium
concentrations of 10.2 wt % and 22.2 wt %, respectively. The component
radial concentration distributions for calcium and sulfur for three
typically sulfated particles are given in Fig. 7 (magnesium concentration
profiles are not presented because they are similar to those for calcium).
The extent of sulfation in these particles varied. These particles had
been in the continuously fed, fluidized-bed coal combustor for different
periods of time (solids are backmixed). The extent of sulfation was least
for the particle represented at the top of the figure and greatest for
the bottom particle. As sulfation progressed, the edge of the sulfated
shell moved towards the center of the particle. The local sulfur and
calcium concentrations obtained were lower than the true local concen-
trations because irregularities on the surfaces cause scattering of the
characteristic emitted by X-ray (the sample surfaces could not be machined
as smoothly as the calibration sample surfaces).
Regenerated particles from experiment FAC-1R2 (performed at 1040°C)
were chemically analyzed and found to contain 5.1 wt % S and 26.2 wt %
Ca. The electron probe analyses of two sample particles from this experi-
ment are given in Fig. 8. Regeneration appears to take place in two
stages. In the particle whose analysis is given at the top of Fig. 8,
regeneration apparently is in an early stage, with the primary regener-
ation-reaction zone moving radially into the uniformly sulfated particle.
Some residual sulfur (sulfur that is less available, probably due to
diffusion limitation) is left behind the advancing primary reaction zone.
The bulk of the sulfur is removed in the first stage of regeneration.
The particle represented by the other electron probe analysis is in a
later (slower) stage of regeneration, and the residual concentration of
sulfur in the entire particle is relatively low (note difference in scales
for the two diagrams). In an industrial process in which the solids
residence time would be low (high solids throughput rate), probably only
the first (rapid) stage of regeneration would occur. Hence, most but not
all of the sulfur would be removed in an industrial process.
Electron microprobe analyses were performed on regenerated particles
from experiment FAC-4, which was conducted at the higher temperature of
1095°C. The calcium and sulfur concentrations in regenerated Tymochtee
dolomite from this experiment were 30 wt % and 1.5 wt %, respectively.
The electron probe analyses for calcium and sulfur in three particles
from this experiment are shown in Fig. 9. The particle whose analysis is
given at the top of Fig. 9 appears to be near the end of the first regen-
eration stage; the primary desulfurization reaction zone has almost reached
the center of the particle. Regeneration of the other two particles is
more nearly complete (note difference in scale of sulfur concentrations).
A greater extent of regeneration was observed in particles from FAC-4
than in particles from FAC-1R2 which were regenerated at a lower temperature.
Generally, the regenerated particles showed no sulfur concentration
irregularities that might lead to poor utilization in subsequent sulfation
cycles.
-------
26.
30
10
" 0
I" 12
c •>" 8
PARTICLE, 1060
-------
27
CO
PARTICLE, 1170/im
PARTICLE, 1500 /an
Fig. 8. Electron Microprobe Analyses of Regenerated
Tymochtee Dolomite Particles from Experiment
FAC-1R2.
with 'v.S vol % total reducing gas in the effluent). With vL5% reducing
gas in the effluent, the formation of CaS was enhanced—for example, in
two experiments, sulfide concentrations in the products were 0.3 and 0.7%,
respectively.
The experimental sulfide concentrations obtained are much lower
than those predicted for the equilibrium equation (below) at the experi-
mental off-gas conditions.
1/4
+ CO •* 1/4 CaS + C02, - =
PCO
PC02
(3a)
At 1040°C, the maximum partial pressure ratio of CO to C02(?CO/PC02) for
which CaS and CaSO^ coexist is ^0.017. At higher partial pressure ratios,
only CaS should exist at equilibrium.5 Since the experimental values of
partial pressure ratios of CO to C02 in the effluent for the FAC-experi-
ments were all above 0.02, higher sulfide concentrations would be expected.
-------
28
s 30
-8s 20
-
o
CO
0
10
5
0
JV/v
PARTICLE, 1200
^ ou
§5? 20
«*'8
o 0
of 1.0
=>s«
i£0.5
<" n
A^/-^*^-*^^. /^-^xx-v A\
-
•PARTICLE, MOOfim-
s 30
Ess 20
^'°0
00
PARTICLE, 900/im
Fig. 9. Electron Microprobe Analyses of Regenerated
Tymochtee Dolomite Particles from Experiment
FAC-4.
The experimental results may be explained on the basis that (1) the
reaction in the f luidized-bed reactor is not at equilibrium and (2) the
conditions in the fluidized bed are not reducing throughout. Rather,
solid reactants and products are ciculated between reducing and oxidizing
zones , allowing CaS to be oxidized to
Elutriation and Decrepitation of Additive. The extent of decrepitation
of additive is of major concern in the development of one-step reductive
regeneration. The physical integrity of additive particles may be affected
by thermal stresses at the high process temperature (^1100°C) and by
molecular rearrangements within the particles as a results of regeneration
reactions.
-------
29
Losses of regenerated product due to decrepitation (including decrepi-
tation during pneumatic transport) at the various experimental conditions
were measured. Since the sulfated feed dolomite was nominally -14 +45 mesh
and the elutriated particle samples in the flue gas were all nominally
-45 mesh (<350 ym), it was assumed that the elutriated particles were
decrepitation fragments of the feed Tymochtee dolomite. Table 2 shows the
fraction of additive lost during regeneration due to decrepitation and
subsequent elutriation expressed as a percentage of feed calcium. The
loss ranged from 5 to 15%.
In a more direct approach to the evaluation of decrepitation, the
fractional distributions of the sulfated feed and regenerated product for
experiment FAC-1 were plotted (Fig. 10, upper plot). The fractional
distribution.curves suggest a particle population shift to smaller diameters.
This was confirmed by plotting the mass ratios (i.e., the calcium mass
content of the product to the calcium mass content of the feed additive)
as a function of particle diameter (Fig. 10, lower plot). Generally, all
plots of product-to-feed calcium ratio as a function of fractional particle
diameter showed that during regeneration, the number of larger particles
(>800 ym) decreased and the number of medium-size particles (>300 and <800 ym)
increased due to the decrepitation of the larger particles.
Composition and Agglomeration of Sulfated Additive. Partial agglom-
eration of the sulfated dolomite bed occurred in experiment FAC-2 which
was attempted using a bed temperature of 1095°C and a 15% reducing gas
concentration in the off-gas. It is believed that upsets in the fluidization
of the additive bed caused local temperatures to rise higher than the
measured operating temperature which, in turn, caused partial melting of
the additive followed by agglomeration. (It has been previously reported
by Wheelock and Boylan5 that the surface of gypsum becomes glassy during
reductive decomposition at 1250°C.)
Fresh, sulfated, regenerated (FAC-4), and agglomerated (FAC-2)
Tymochtee dolomite were analyzed by X-ray diffraction (Table 3). In unsul-
fated dolomite, traces of a-quartz were found as expected, since the dolomite
contains ^2% Si02. Sulfated dolomite contained the expected components,
including traces of a-quartz and weak traces of other phases, possibly
merwinite, Ca3Mg(SiOit)2. The regenerated dolomite from FAC-4 which was not
agglomerated contained mostly CaO and MgO with traces of a-quartz, CaSO^
and weak traces of other phases (possibly merwinite). During a regeneration
experiment (FAC-2), partial agglomeration of the sulfated additive occurred.
The loosely bound particles around the agglomerate were found to contain
approximately the same major constitutents and the same minor constituents
of CaSO^ and a-quartz as were found in the regenerated sample from FAC-4.
In addition, traces of CaMg(Si03)2 were found. In the cores of the
agglomerates, the additive particles were fused together and contained
Ca3Mg(SiOt).)2 as a medium constituent and CaO as a medium rather than a
major constituent.
Samples of Pope, Evans and Robbins (PER) sulfated Greer limestone were
also analyzed by X-ray diffraction. An agglomerate taken from a regeneration
-------
30
100
8s ,,-
% FRACTIONAL WEIGHT,
r>o 01 -
O CJl O CJ
1 1 r
o = SULFATED S02
°=REGENERATED
W"
^
i
ACCEPTOR
S02 ACCEPTOR
/
(
I \
1
)
s
\
\
\
= 200"
<
cc.
in 150"
CO
<£
^E i /\/\
PRODUCT/FEED
cji C
-° P . ^
•^
00
/
/
^x
•^
\
s
N
\
\
1000
DIAMETER.pn
Fig. 10. FAC-1, Fractional Feed and Product Particle Size
Distributions. Decrepitation is characterized
by the fractional product to feed mass ratios
at different particle diameters.
experiment (Coal Test 3)7 was analyzed at two locations in the agglomerate.
The loosely bound limestone particles contained Ca3Mg(SiOit)2 as a minor
constitutent. The fused limestone particles contained CaaMg (810^)2 as a
major constituent. Again, as the additive softened, the formation of the
calcium-magnesium silicate compound progressed.
On the basis of the X-ray diffraction analysis described, it appears
that as the particles soften, calcium-magnesium silicate compounds form.
The melting point of this class of materials is %1250°C which indicates
that the localized temperature at which agglomeration occurred was much
higher than the design experimental temperature
Preliminary differential thermal analyses (DTA) were performed on
unsulfated and sulfated Tymcohtee dolomite in several gas environments.
-------
Table 3. Qualitative Chemical Compositions of Regenerated and Unregenerated
Samples of Additive.
Analysis Method: X-ray diffraction
Material
o
Regenerated
Pertinent Constituents
Source
fused particles
Major
MgO
Medium
CaO and possible
Minor
CaSOi+
Tymochtee dolomite
from FAC-2
Q
Regenerated
Tymochtee dolomite
from FAC-2
f\
Regenerated
Tymochtee dolomite
from FAC-4
Sulfated Tymochtee
dolomite
Virgin
Tymochtee dolomite
Regenerated
PERC limestone
from Coal Test
No. 3
Regenerated
PER limestone
from Coal Test
No. 3
from regenerator
bed
Loosely bound
particles from
regenerator bed
Unbound regen-
erated additive
from regenerator
bed
Combustor product
Ca3Mg(S10^)
Fused particles
from regenerator
bed
Loosely bound
particles from
regenerator bed
MgO, CaO
CaO, MgO
, CaC03
CaMg(C03)2
MgO and
probable
Ca3Mg(Si04)2
CaS04, MgO
Possible
a-quartz, and
CaMg(Si03)2
a-quartz, possible
CaS04, CaMg(C03)2,,
Ca(OH)2, and other
phases
CaO, possible
a-quartz, and other
phases
a-quartz and possible
Possible CaS04,
MgAl2Oit; very minor:
possible CaO
Possible Ca3Mg(SiOtt)2
Regeneration temperature was
Regeneration temperature was 1040°C.
Pope, Evans and Robbins.
-------
32
The X-ray diffraction analyses of the reacted samples are given in Table 4.
Only one reaction was observed in unsulfated dolomite between ambient temper-
ature and 1300°C in either air or nitrogen. The calcination reaction occurred
at 550-850°C. The sulfated dolomite (which contained a small amount of
carbonate) calcined at 600-750°C, and melted at 1200°C in both air and
nitrogen. This is lower than the melting point of any of the major
consitutents in sulfated dolomite. In addition to the weight loss attributed
to calcination of these sulfated samples, a weight loss was noted as their
temperature was raised above 1100°C. This might have been caused by decom-
position of CaSO^. During cooling of the sample from 1300°C, an exothermic
reaction (solidification) occurred at 1150°C. X-ray diffraction analysis
showed that CaSO^ and MgO were the major constituents and CaO the minor
constituent. CaSOij was present as larger crystals (indicative of post-
melting) instead of the small crystals normally present in sulfated dolomite.
Also, in both of these samples, calcium silicate was present as a possible
minor constituent, instead of the calcium-magnesium silicate compounds
found in agglomerated dolomite samples.
The last DTA experiment in this series was performed with sulfated
dolomite in a reducing atmosphere (100% H2). A calcination weight loss
occurred at 600-700°C, followed by a further weight loss which continued
as the temperature rose to ^1000°C. The latter weight loss was probably
due to reduction of CaSO^. to CaS which, with MgO, was found to be the
major constituents in the final product. No melting was observed up
to 1300°C.
The samples, which were small, ^30 mg , may not have been representative
of nonhomogeneous Tymochtee dolomite. Larger samples are being used in
experiments currently being performed in another DTA apparatus.
Regeneration of Sulfated Greer Limestone by the Incomplete Combustion
of Coal
Exploratory Experiments. Sulfated Greer limestone from Pope, Evans
and Robbins (Test 620) has been regenerated, with the reducing gas and
the required heat generated by incomplete combustion in the bed by either
Arkwright or Triangle coal.
For experiment LCS-1 the experimental conditions and results are
given in Table 5. A S02 concentration of 2.5% in the wet effluent gas and
a CaO regeneration of 88% were obtained. The extents of regeneration
based on chemical analysis of the regenerated products from these experi-
ments were almost identical. The steady-state concentrations of COS and
CS2 in the flue gas, obtained by mass spectrometric analysis, were 0.02%
and £0.01%, respectively.
Experiment LCS-2 was performed at a higher bed temperature, 1100°C,
than LCS-1. The operating temperature and the concentrations of gases
in the effluent gas are given in Fig. 11, and the results are given in
Table 5. The feed rate in this experiment was 5.4 kg/hr, in comparison
to 4.9 kg/hr in LCS-1. The S02 concentration in the wet off-gas was 3,1%
-------
Table 4. Qualitative Chemical Composition (Obtained by X-Ray
Diffraction Analysis) of Reacted Samples from DTA
Experiments.a
Reaction
Atmosphere
during DTA
Material Experiment
Tymochtee N2
dolomite
Tymochtee Air
dolomite
Sulfated N2
Tymochtee
dolomite
Sulfated Air
Tymochtee
dolomite
Sulfated H2
Tymochtee
dolomite
Pertinent Constituents
Major Medium
MgO CaO
CaO, MgO
MgO,' CaSOiLj
(very crystalline)
MgO, CaSOi^
(very crystalline)
CaS, MgO
Minor
Ca(OH)2, possible
CaC03 and other
weak phases
very minor :
possible CaAl2Ott
CaO, possible
calcium silicate
CaO, possible
calcium silicate
Final DTA furnace temperature was 1300°C in each experiment.
-------
Table 5.
Experimental Conditions and Results for Regeneration Experiments in
which Arkwright and Triangle Coals were Incompletely Combusted in a
Fluidized Bed.
Nominal Particle Residence Time: 15-30 min
Nominal Fluidized-Bed Height: 46 cm
Reactor ID: 10.8 cm
Coal: Arkwright coal (2.82 wt % S)
reduction conditions, 1105°C (LCS-1, -2)
Triangle coal (0.98 wt % S), Ash fusion temperature under
reduction conditions, 1390°C (LCS-3)
Additive: Sulfated Greer limestone
(LCS-1, -2, -3).
Ash fusion temperature under
-2)
iion temperature under
(initial deformation)
(9.48 wt % S) -10 +50 mesh
Exp. Pressure
No. (kPa)
LCS-1
LCS-2
LCS-3°
Based
Based
184
153
153
Bed
Temperature
1040
1100
1060
on flue gas analysis,
on chemical analysis
Feed
02 Cone.
18.9
18.6
23.6
Fluidiz ing-
Gas
Velocity
(m/sec)
0.91
1.1
1.0
Additive
Feed
Rate
(kg/hr)
4.9
5.4
8.6
Reducing Gas
Concentration
in Effluent (%)
1.3
1.3
1.6
Measured S02/H2S
in Wet
Effluent Gas
2.5/96
3.1/235
3.5/76
CaO
Regeneration
88/88
94/94
72/84
•
of dolomite and limestone samples.
u>
-P-
^Triangle coal was used; in the other two experiments, Arkwright coal was used.
-------
35
^1200
E
QJ
O)
CD
1000
*Srv*
.AW-C7
2000
E
o.
o.
o"
.2 c^
O)
o
cz
o
o gS
% £
CD o
GO
I
OJ
| 5«
-------
36
in LCS-2, and the CaO regeneration based on off-gas analysis was 94%. In
comparison with LCS-1, the extent of regeneration and the S02 content of
the effluent gas were improved by increasing the temperature in LCS-2.
The concentrations of COS and CS2 in the flue gas for LCS-2 were 0.02%
and 0.01%, respectively.
An additional experiment was performed with sulfated Greer limestone,
using Triangle coal instead of. Arkwright coal (a bituminous coal with
a higher ash fusion temperature than Arkwright coal). The experimental
conditions for this regeneration experiment (LCS-3) were an additive feed
rate of 8.6 kg/hr (which is higher than in earlier experiments at a bed
temperature of 1060°C), a fluidizing-gas velocity of 1.0 m/sec, and a
reducing gas concentration of 1.6% in the effluent gas. The operating
temperature and the concentrations of gases in the effluent gas are given
in Fig. 12; the experimental results are given in Table 5. From the
average SC>2 concentration of 3.5% in the wet effluent gas, a sulfur (or
CaO) regeneration of 72% was calculated (based on off-gas analysis). A
greater sulfated additive feed rate (i.e., reduced solids residence time)
increased the S02 concentration in the ^effluent gas without greatly
decreasing the extent of regeneration.
Effect of the Relative Lengths of the Oxidizing and Reducing Zones
during Regeneration of Greer Limestone. When fuel (coal) is combusted
under reducing conditions in the fluidized bed, both an oxidizing zone
and a reducing zone are present. An oxidizing zone is established at
the bottom of the fluidized bed (mainly below the fuel injection point)
where combustion air enters; a reducing zone is established in the upper,
fuel-rich portion of the fluidized bed. Calcium sulfide is formed in
the reducing zone by the following typical reaction,
CaS04 + 4 CO -»- CaS + 4 C02 (3)
and its formation is favored by higher reducing gas concentrations. The
presence of large amounts of CaS is detrimental to the regeneration of
CaSOif to CaO since CaO does not form directly from CaS at the conditions
used. However, since the additive is continually circulated in the bed,
the CaS can be reoxidized in the oxidizing zone of the bed by the
reaction:
CaS + 2 02 -> CaS04 (5)
Thus an effective oxidizing zone is important for preventing the buildup
of CaS.
To test the effect of the relative lengths of the oxidizing and
reducing zones on the steady-state level of CaS in the bed, two experiments
have been performed using sulfated Greer limestone feed and sufficient
coal to give a reducing gas concentration of ^4% in the effluent gas
(approximately twice as high a concentration as in other regeneration
experiments with coal). The results for experiments LCS-4D and LCS-7, in
which the ratios of the nominal heights of the reducing zone to the oxidizing
zone were 5 and VL.3, respectively, a're given in Table 6.
-------
H-
OQ
O H
0) (D
i-!
(0
t-1 O
O (B
U) O
o
(D
3
rt
H
rt
H-
O
Above Bed Gas Concentrations
S02, % CH4, % CO, % 02, % C02, % NO, ppm Bed Temp, °C
0 50 10 20 10 30 0 100 1000 1200
o I • - ' o I to ' ' '—i—| I—*--—i—•—| 1—•—'—•—)i—( I ' * * x-
t-
oo
-------
Table 6.
Experimental Conditions and Results for Regeneration Experiments Designed
to Test the Effectiveness of the Two-Zone Reactor in Minimizing CaS Buildup.
15-30
Temperature, °C: 1060
Pressure, kPa: 153
Nominal Particle Residence Time, min:
Nominal Fluidized-Bed Height, cm: 46
Reactor ID, cm: 10.8
Coal: Triangle coal (0.98 wt % S). Ash fusion temp, under red.
conditions: 1390°C (initial deformation)
Additive: Sulfated Greer limestone (9.48 wt % S) -10 +30 mesh
Sulfur/Sulfide Measured S02/
Exp.
No.
LCS-4D
LCS-7
Red. Zone Length
Oxid. Zone Length
5
^1.3
Fluidizing
Gas Velocity
(m/sec)
0.97
0.95
Feed
Rate
(kg/hr)
5.3
5.9
Reducing Gas in Regenerated H2S in Wet
Concentration
in Effluent (%)
3.8
4.1
Additive
(%)/(%)
7.5/4.9
2.3/1.1
Effluent Gas
(%)/(ppm)
0.9/625
1.9/1900
CaO
Regeneration
(%)a/(%)b
20/41
67/72
U)
oo
a
Based on flue-gas analysis.
Based on chemical analysis of limestone samples before and after regeneration.
-------
39
In the first experiment, LCS-4D, regeneration (conversion of CaSO^ to
CaO) was very poor, 41%; sulfide (S ) concentration was high, 4.9%. The
intermittent off-gas analysis for this experiment is given in Fig. 13.
The total fluidized-bed height was 46 cm (illustrated in Fig. 14).
In experiment LCS-7 (and in all subsequent regeneration experiments)
the coal injection line was 13 cm longer than that formerly used (Fig. 14),
decreasing the ratio of the heights of the reducing and oxidizing zones.
The intermittent flue-gas analyses for LCS-7 are given in Fig. 15. Solids
regeneration was 71%, and the regenerated additive contained 1.1% sulfide.
By increasing the length of the oxidizing zone relative to the length of
the reducing zone, the buildup of CaS was reduced and greater regeneration
was obtained.
Since 40% of the total fluidizing gas is used to inject the coal and
additive into the fluidized bed, a fluidizing-gas velocity discontinuity
would have existed in LCS-7 between the portion of the fluidized bed below
and above the injection line. To compensate for this discontinuity, a
20-cm-long ceramic insert with an ID of 8.9 cm (3.5 in.) was installed
above the gas distributor, as illustrated in Fig. 14. This insert was
retained in all future experiments.
Effect of Solids Residence Time and Temperature on Regeneration.
Additional regeneration experiments were performed with sulfated limestone
from Pope, Evans and Robbins (PER) to study the effects of solids residence
time and bed temperature on regeneration. .In five of these experiments
(LCS-8 through LCS-12), Greer limestone which had been sulfated 0\J.l wt
% S) by PER in Test 621 was regenerated. In the remaining two experiments
(LCS-16, -17), sulfated limestone (^9.3 wt % S) from PER which had an
uncertain experimental origin, but was probably a mixture of Greer and
Germany Valley limestones, was regenerated.
The first three experiments, LCS-8, -9, and -10, were performed at
the same fluidizing-gas velocity (^1.2 m/sec) and at nearly the same bed
temperature (^1050°C). The results and experimental conditions for these
and the other regeneration experiments are given in Table 7. The sulfated
Greer limestone feed rate, was increased from 5.9 kg/hr to 15.4 kg/hr in
this series of runs, and as a result, the extent of regeneration (based on
chemical analysis of solids) decreased from 89% to 70%. This decrease
was expected since increasing the solids feed rate decreased the particle
residence time in the regenerator from 31 min to 12 min. Nevertheless, the
extent of regeneration for LCS-10 (70%) was higher than that obtained for
a similar experiment using sulfated Tymochtee dolomite (9.4 wt % S) in
which 58% of the sulfur was regenerated (LCS-10, discussed below). The
reason for the higher extent of regeneration in these experiments was
probably the lower sulfur content (7.1 wt % S) of the Greer limestone.
Experiment LCS-11 was performed at the same temperature and sulfated
additive feed rate as was LCS-10. It was found that increasing the
fluidizing-gas velocity from 1.2 to 1.4 m/sec caused no significant change
in the extent of regeneration. In contrast, in the FAC-series of regeneration
-------
U>
O W
Ml (D
l-h CU
O H
PJ (D
•d rt
ft) C
i-i i-i
H- n>
3
(t> fu
3 0
rt CL
f Q
O S3
CO CO
-P- O
o
(D
0
s
CO
S02, %
0 10 0
o4e-- — • — i — i
Above Bed Gas Concentrations ----------
% CO, % 02, % C02, % NO, ppm Bed Temp, °C
20 40 20 30 0 200 1000 1200
8
§
i-
-------
41
FLUIDIZED
BED
COAL AND
ADDITIVE
INJECTION
LINE
GAS
DISTRIBL
-*•
IA A A
ITOR-^
i- DISTANCE ABOVE GAS DISTRIBUTOR, cm
o —
co — ro<-M.c>encr>->joou3o
1 O OOOOOOOOOOO
FLUIDIZEO
BED
COAL AND
ADDITIVE
INJECTION
LINE
al »»i V
) 10 20 30
(YGEN CONC.,%
4D
"100
90
80
70
60
50
40
30
20
0
5 "0 10 20 30
OXYGEN CONC.,%
LCS-7
Fig. 14. Geometry of Oxidizing and Reducing Zones in
Relation to Position of Coal Injection Line.
experiments (discussed above), the extent of regeneration decreased at
higher fluidizing-gas velocities. However, the FAC-experiments were per-
formed in a smaller reactor (7»6-cm ID), at lower fluidizing-gas
velocities (0.6-0.9 m/sec), and with CH^ instead of coal. Additional
experiments will be performed to test the effects of fluidizing-gas
velocity on regeneration.
The other experiments listed in Table 7 were performed at 1100°C.
LCS-16 was performed with the same solids feed rate as was used in LCS-12.
The S02 content of the dry flue gas was 4.8% in LCS-16, in comparison with
3.4% in LCS-12. The higher S02 content in the flue gas in LCS-16 was due
mainly to the lower fluidizing gas velocity (less dilution) used in LCS-16.
Experiment LCS-17 was performed with a much higher sulfated-additive
feed rate, 25.4 kg/hr. This corresponds to a solids residence time in the
regenerator of 7.5 min. The SC-2 concentration in the dry flue gas was
7.6%, and the extent of CaO regeneration was 61%.
-------
CW
O W
l-h fD
O H
(u n>
X pi
"rj ri*
n> e
H- (D
(D P
rt CL
O
O
o
§
3
s
CO
Above Bed Gas Concentrations
S02, % CH4, % CO, % 02, % C02, % NO, ppm Bed Temp, °C
0 10 020 40 20 300 290 1000 1200
\
1
:
^
S3
-------
Table 7.
Experimental Conditions and Results for Regeneration Experiments with
Combustion of Arkwright and Triangle Coal in a Fluidized Bed.
Pressure, kPa: 153
Nominal fluidized-bed height: 46 cm
Reactor ID: 10.8 cm
Coal: Triangle coal (0.98 wt % S)
Ash fusion temp under reducing conditions, 1390°C (initial
deformation)
Additive: Sulfated Greer limestone
a. -10 +30 mesh, 7.12 wt % S (LCS-8 through -12)
b. -14 +30 mesh, 9.26 wt % S (LCS-16 through -17)
Bed
Exp. Temperature
No. (°C)
LCS-8
LCS-9
LCS-10
LCS-11
LCS-12
LCS-16
LCS-17
1043
1065
1050
1050
1100
1100
1100
Fluidizing- Feed
Gas Velocity Rate
(m/sec) (kg/hr)
1.22
1.21
1.20
1.37
1.48
1.14
1.20
5.9
10.4
15.4
15.0
14.3
15.4
24.5
Particle
Residence
Time (min)
31
18
12
12
13
12
7.5
Reducing Gas Measured S02/H2S
Concentration in Dry Effluent
in Effluent (%) Gas (%)/(ppm)
2.1
1.9
2.2
^2.0
^2.0
2.4
2.5
2.3/300
3.4/0
3.2/400
3.1/400
3.4/300
4.8/1100
7.6/400
CaO
Regeneration
(%)a/(%)b
97/89
81/80
52/70
59/72
70/75
57/76
59/61
Based on flue-gas analysis.
Based on chemical analyses of limestone samples before and after regeneration.
-------
44
The experimental results obtained with sulfated PER limestone at
1100°C, including experiment LCS-2 (Table 5), have been plotted in Fig. 16
as a function of solids residence time. The SC>2 concentration obtained in
LCS-12 was adjusted to compensate for dilution due to the higher fluidizing-
gas velocity used in that experiment. The extent of regeneration of CaO
from sulfated Tymochtee dolomite at 1100°C (described in the next section
of this report) is also included on Fig. 16. The S02 concentration of the
dry off-gas from the sulfated Tymochtee dolomite experiments was higher
than that for the sulfated limestone experiments. At low solids residence
time, the 862 concentrations in the off-gas were almost equal because the
sulfur content of the sulfated stones no longer affected the rate of SC>2
evolved. The extent of regeneration was vLO% lower for limestone than for
dolomite.
100
I—I—I
TYMOCHTEE
DOLOMITE
EXTENT OF LIMESTONE
REGENERATION
O SOLID ANALYSIS
D GAS ANALYSIS
LIMESTONE
A S02 CONCENTRATION
A ADJUSTED SO, CONCENTRATION FOR LC5-I2
10 15 20 25 30
SOLIDS RESIDENCE TIME, min
Fig. 16. Regeneration of CaO and S02 Concentration in the
Dry Flue Gas for Sulfated Limestone and Tymochtee
Dolomite as a Function of Solids Residence Time.
T=1100°C, Fluidizing Gas Velocity =1.0-1.5 m/sec.
-------
45
Regeneration of Sulfated Tymochtee Dolomite by the Incomplete Combustion of
Coal
Effect of Solids Residence Time and Temperature on Regeneration. A
series of regeneration experiments with sulfated Tymochtee dolomite has
been initiated to further investigate the effects of regeneration temper-
ature and solids residence time on the extent of regeneration and the
concentration of S02 in the dry effluent gas. The experimental conditions
and results are given in Table 8. The extents of regeneration as a
function of solids residence time, based on solids analysis, are plotted
in Fig. 17 for the three bed temperature levels 1000, 1050, and 1100°C.
At 1000°C, when the solids residence time was decreased from 37 min to 18
min, the extent of regeneration decreased drastically from 77% to 30%
(Table 8), but the S02 concentration in the dry effluent gas, 2.5%, did
not change.
At 1050°C, as the solids residence time was decreased from 34 min to
12 min, the extent of regeneration decreased from 90% to 56% and the S02
concentration in the dry effluent gas increased from 3.0% to 4.8%. At the
highest temperature level, 1100°C, it was found that as the solids residence
time was decreased from 13 min to 9.4 min, the extent of regeneration
decreased from 85% to 77% and the S02 concentration in the dry effluent gas
increased from 6.4% to 7.8%. Relatively high S02 concentrations in regen-
erator off-gases have also been reported by Pope, Evans and Robbins.8
Since the extent of regeneration remained quite high at 1100°C when
the solids residence time was lowered to 9.4 min, it is expected that S02
concentrations in excess of 10% in the effluent gas can be obtained by
further increasing the solids rate through the reactor.
Effect of Solids Residence Time and Temperature on S02 Concentrations
in the Off-Gas. The curves representing regeneration as a function of
solids residence time in Fig. 17 were extrapolated through the experimentally
obtained solids regeneration values. By use of the regeneration curves
and the predicted gas volume increases during regeneration (see section on
Mass and Energy Constrained Model for the Regeneration Process), the S02
concentrations in the regenerator off-gas were predicted at the same three
temperature levels, a pressure of 153 kPa, and a fluidizing-gas velocity
of 1.06 m/sec. The predicted S02 concentrations in the effluent gas are
given in Fig. 18. At 1000°C, the effluent gas S02 concentration cannot be
increased by decreasing the solids residence time (increasing the solids
throughput rate) because the regeneration rate would be too low.
At 1100°C, higher S02 concentrations can be obtained by decreasing the
solids residence time. Additional experiments are being performed to further
quantify these results. The regeneration results with limestone and dolomite
show that as the solids residence time is decreased (-i.e., at a higher solids
throughput rate), the extent of regeneration is sacrificed but the S02 con-
centration in the dry off-gas is enriched.
High temperatures improve both the extent of regeneration and the
S02 concentration in the off-gas from the regeneration process. However,
-------
REGENERATION OF CaO, %
H-
era
era
n>
H
cu
rt
8
o
n
cu
O
X
co
P
§
o
s
en
o
H-
CL
CO
CO
H-
CU
o
ro
-------
Table 8. Experimental Conditions and Results for Regeneration Experiments
with Sulfated Tymochtee Dolomite Using Triangle Coal.
Nominal Fluidized-Bed Height: 46 cm Pressure: 153 kPa
Reactor ID: 10.8 cm
Coal: Triangle coal (0.98 wt % S) Ash fusion temperature
under reducine conditions. 1390°C (initial deformation)
Additive: (1) -14 +50 mesh, 9.0 wt % S (CS-6, -7, -8)
(2) -14 +50 mesh, 9.4 wt % S (CS-10, -11, -12)
Bed
Temperature
Exp. (°C)
CS-6
CS-8
CS-7
CS-10
CS-11
CS-12
1000
1000
1050
1050
1100
1100
Fluidizing- Feed
Gas Velocity Rate
(m/sec) (kg/hr)
0.98
0.92
0.92
0.98
1.07
1.16
5.0
10.0
5.4
15.0
14.3
19.5
Solids
Residence
Time (min)
37
18
34
12
13
9.4
Reducing Gas Measured SO in CaO
Concentration Dry Effluent Regeneration
in Effluent (%) Gas (%) (%)a/(%)5
1.4
2.2
1.9
2.5
2.9
2.9
2.5
2.5
3.0
4.8
6.4
7.8
83/77
39/30
84/90
53/56
80/85
79/77
Based on flue-gas analysis.
Based on chemical analysis of dolomite samples.
-------
48
PREDICTED S02 CONCENTRATION
i I I I I I 1
10 15 20 25 30 35 40
SOLIDS RESIDENCE TIME , min
Fig. 18. Predicted and Experimental Sulfur Dioxide
Concentrations at Three Regeneration Temperatures.
Pressure: 153 kPa, fluidizing-gas velocity:
1.06 m/sec.
at regeneration temperatures above 1100°C, agglomeration of the sulfated
additive has been found to be aggravated.
Carbon Utilization and Carbon Balance Calculations
In regeneration of additive, coal is combusted under reducing
conditions to provide the heat and the reductants required for reaction.
Under such reducing conditions, sufficient oxygen is not available to
ensure complete combustion of the coal. Thus, relatively low carbon
utilization may be expected.
To determine the utilization of the coal, carbon material balances
and carbon utilizations were calculated for three earlier reported
experiments listed in Table 9. The carbon balances were good for
experiments CS-5 (97%) and LCS-2 (99%) and low for LCS-1 (55%). The
carbon utilizations were found to be ^80%. (The carbon was assumed to
have been utilized if it was oxidized to CO or C02.) Additional data
are being obtained.
Effect of Coal Ash on the Agglomeration of Sulfated Additive during
Regeneration
Ash Buildup in Sulfated Additive. Partial agglomeration of sulfated
-------
Table 9. Experimental Conditions, Carbon Balances, and Carbon
Utilizations for Regeneration Experiments.
Nominal Particle Residence Time: 30 min
Nominal Fluidized-Bed Height: 46 cm
Reactor ID: 10.8 cm
Coal: Arkwright
Additive: (a) Sulfated Tymochtee dolomite
-14 +50 mesh (CS-5)
(b) Sulfated Greer limestone
-10 +50 mesh (LCS-1, -2)
Exp.
No.
CS-5
LCS-1
LCS-2
Pressure
(kPa)
184
184
153
Bed
Temperature
(°C)
1040
1040
1100
Fluid iz ing-
Gas Velocity
(m/sec)
0.93
0.91
1.1
Additive
Feed Rate
(kg/hr)
5.0
4.9
5.4
Coal
Feed Rate
(kg/hr)
1.59
1.47
1.47
Carbon
Balance
(%)
97
55
99
Carbon
Utilization
(%)
80
72
82
-e-
VO
Carbon was assumed to have utilized when oxidized to C02 or CO.
-------
50
additive (specifically, sulfated Greer limestone from Pope, Evans and
Robbins) has occurred, even during mild temperature upsets near 1100°C.
Observations of agglomerated interior surfaces with an optical microscope
revealed ash cenospheres, which appeared as tiny glass-like beads. These
beads are formed as tiny bubbles as gases evolve from the ash. X-ray
diffraction analysis showed them to be amorphous.
The agglomerating tendency of molten ash (the Godel phenomenon) is
utilized as a means of removing the ash from a fluidized bed of bituminous
coke in Ignifluid gasification boilers,9 which are operated at high
fluidizing-gas velocities (10-15 m/sec) and at temperatures of 1200-1400°C.
A suggested mechanism for one type of agglomeration in the presence
of coal ash is that at temperatures where these spheres and/or the ash
are molten, they could serve as coalescing agents between adjacent additive
particles in the regenerator. Although it has been reported that molten
ash has a high surface tension,10 its interfacial tension with sulfated
additive at high temperature might be low enough to wet the additive,
causing the additive particles to become sticky and agglomerate. This
could occur at temperatures above the ash fusion point and below the fusion
point of the additive. Other types of agglomeration initiated by the
fusion of sulfated additive or by the formation of coal-ash additive com-
pounds are also possible.
The sulfated additive feed to the regenerator contains coal ash because
during the preceding sulfation (combustion) ;;step, some coal ash is retained in
the fluidized bed. Tymochtee dolomite has been sulfated during the combustion
of Arkwright No. 2 coal at ANL, and Greer limestone has been sulfated
during the combustion of Sewickley coal. The ash fusion temperatures
(determined by ATSM D-1857-74) for these coals and for Triangle coal are
given in Table 10. The ash fusion temperature under reducing conditions
for both Arkwright and Sewickley coals are very close to the regeneration
temperatures used. The presence of ash in the additive might have con-
tributed to the above coalescing mechanism and disrupted the fluidization
in the regenerator. Once the fluidity of the bed was disrupted, localized
temperature excursions provide the atmosphere for chemical reactions of
the coal ash with additive.
Table 10. Fusion Temperatures, Under Reducing Conditions,
of Ash from Arkwright No. 2 Coal, Sewickley
Coal, and Triangle Coal.
Arkwright Ash Sewickley Ash Triangle Ash
Initial Deformation 1104 1100 1383
Softening (H = W) 1177 1188 1444
Softening (H = 1/2 W) 1193 1215 1485
Fluid 1232 1288 1510
-------
51
During some regeneration experiments, agglomeration of additive and
coal ash occurred even during mild temperature fluctuations caused by
upsets in the regenerating environment at ^1100°C. Molten ash is believed
to be responsible for initiating some agglomeration at these temperatures;
analyses of sulfated additive have been performed in an attempt to further
understand this postulated mechanism.
From a chemical analysis of once-sulfated Greter limestone (-8 mesh)
provided by Pope, Evans and Robbins (Test 620), its composition was cal-
culated and compared (Table 11) with the composition of unreacted Greer
limestone.11 The concentrations of Fe203, Si02, and A1203 increased
markedly during sulfation. These compounds are the major constituents of
the ash of Sewickley coal (Table 11) used in Test 620. The temperature
of initial deformation of the ash from this coal (Table 10) under reducing
conditions is
Calcium concentration was used as a basis (ash contains only a small
amount of calcium) for estimating the amount of ash in the sulfated
limestone. Increases in silicon, iron, and aluminum were calculated from
the analyses. The corresponding calculated ash contents of the sulfated
limestone are given in Table 12. Since SiC>2 is the most abundant ash
.constituent, about 50% of the ash, its chemical analysis is expected
to given the most reliable result. The ash content of the sulfated lime-
stone based on the silicon analyses was 10%; This high level of ash
may have been responsible for the formation of agglomerates during
relatively mild temperature upsets.
Table 11. Compositions of Greer Limestone and Sewickley Coal Ash.
Unsulfated Greer
Limestone
CaO
CaSO^
MgO
Fe203
Si02
A1203
S
Others
C02 Loss
Total
(wt %)
44.80
1.90
0.80
10.50
3.60
0.17
0.71
37.52
100
Sulfated Greer
Limestone3
(wt %)
36.3
30.4
1.94
2.53
16.63
5.75
7.16b
—
3.54
100
Sewickley
Coal Ash
(wt %)
5.0
1.0
20.3
49.8
19.5
4.4
—
100
Sewickley coal was combusted in the Pope, Evans and Robbins experiment
in which this limestone was sulfated.
Also included in CaSOij value.
-------
52
Table 12. Coal Ash Level in Sulfated Greer Limestone.
Mass basis: 100 g unsulfated limestone
Equivalent
Ash Content
Ca
Fe
Si
Al
a.
0
8.1
10.4
9.1
3.
It was assumed that the ash contributed no calcium
(Sewickley coal ash contains only 3.6 wt % calcium).
The effect of the presence of these ash constituents on the fusion
temperature of the limestone is being investigated with a differential
thermal analyzer combined with X-ray diffraction analysis.
Differential Thermal Analysis Study of Additive and Coal Ash
Reactions. A differential thermal analyzer (DTA) has been constructed to
investigate the problem of agglomeration of sulfated additive with residual
coal ash at elevated temperatures (>1100°C) during the regeneration of
sulfated additive. In future and ongoing work with the DTA apparatus, the
objectives are to study the following:
1. Chemical reactions and their temperatures, as well as fusion
temperatures, involved in the agglomeration process.
2. The effects of key ash constituents (Si02, A1203, Fe203).
3. The effects of residual coal ash on chemical reactions and
physical changes.
The DTA experimental apparatus (Fig. 19) consists of a platinum-
wound tube furnace which contains two crucibles, one for holding the sample
material and the other containing a reference material, alumina powder.
Thermocouples imbedded in the crucibles are connected to temperature
recorders or a potentiometer. The sample temperature and the temperature
difference between the sample and the reference material are recorded as
a function of time. The latter plot is of particular interest since it
permits observation of pertinent heat changes with a rise in sample
temperature (a heating rate of 15°C per minute is employed). Heat of
reaction, heat of fusion, and other energy changes can be observed. They
appear as peaks (deviations from the baseline) on the plot of temperature
difference (AT) versus time. The gas environment in the furnace is air.
The experimental approach consists of first heating (at 15°C/min) a
sample to a high temperature—1400°C, for example. All detectable chemical
and physical changes which occur up to this temperature are surveyed. In
the next phase, fresh samples of the same material are heated just beyond
each of the reaction peaks noted in the original AT versus time plot. The
composition of a sample from each stage of the reaction is determined by
-------
53
AMPLIFIER
SAMPLE
TEMPERATURE
RECORDER
SAMPLE
TEMPERATURE
POTENTIOMETER
FURNACE
AMBIENT
TEMPERATURE
RECORDER
AT RECORDER
PROGRAMED
TEMPERATURE
CONTROLLER
0 °C REFERENCE JUNCTION
FURNACE CHAMBER
"EXPERIMENTAL SAMPLE
REFERENCE SAMPLE'
Fig. 19. Schematic of DTA Apparatus.
X-ray diffraction. Relatively large ground samples (>JL g) are used, to
compensate for the nonhomogeneity of the materials.
The reactions in two different materials have been investigated to
date: (1) unsulfated Tymochtee dolomite and (2) sulfated Greer limestone
from Pope, Evans and Robbins, Test 620. The reaction temperatures and
product analyses from these two series of DTA experiments are given in
Tables 13 and 14. In each of these tables, the column headed "maximum
temperature" is the temperature at which heating of the sample was
terminated; "peak temperatures" are the observed (AT) peak temperatures;
"product condition" represents visual evaluations of the post-reaction
samples at room temperature; and the remaining columns list the product
constituents in decreasing order of importance.
To assist in the calibration of the newly constructed differential
thermaly analyzer, the decomposition temperatures of unsulfated Tymochtee
dolomite [CaMg(C03)2] were determined. Literature values for the main
peak temperatures for tite two dolomite decomposition peaks are 790°C
(for the decomposition of MgCOs to MgO) and 940°C (for the decomposition
of CaC03 to CaO).12 The temperatures obtained of 819°C and 933°G (see
-------
Table 13. Differential Thermal Analysis Results and X-Ray Diffraction
Analysis of the Reaction Products of Unsulfated Tymochtee Dolomite.
Maximum
Temperature
Sample No. (°C)
DOL-3A
DOL-3B
836
990
Peak
Temperatures Product
(°C) Condition
819 Unchanged
819, 933 Unchanged
Product Constituents
Major Medium
CaC03 MgO
CaO, MgO
Minor Very Minor
CaO a-quartz
possible
Ca2SiOi+,
a-quartz
Ln
-------
Table 14. Differential Thermal Analysis Results and X-Ray Diffraction Analysis of the
Reaction Products of PER Sulfated Greer Limestone and Residual Coal Ash.a
Maximum
Temperature
Sample No. (°C)
PER-620
PER-620-1
PER-620-2
PER- 62 0-3
PER-620-4
PER- 62 0-5
PER-620-6B
PER-620-6A
PER-620-6A°
not
heated
625
790
960
1180
1218
1297
1409
Peak
Temperatures
not
heated
516
510, 770
523, 787,
842
523, 785,
843,1093
506, 789,
841,1098
1208
520, 786,
846,1086,
1212,1277
534, 790,
843,1101,
1208,1264
_ , Product Constituents
Product
Condition Major Medium
-
unchanged CaSOif , CaO
unchanged CaO CaSOi^
CaSOtf CaO
unchanged
b
sintered, 2Ca2SiOtt'CaSOtt CaSOif
very brittle
b
fused, 2Ca2Si04'CaSOit CaSO^
slightly
brittle
fused, very 2CaO-Al203-Si02,
slightly CaSOi^.
brittle
completely possible Ca^AlgO^SO^
fused, hard, or sulfo-aluminous
dark-colored clinker, CaSOi^
Cae.iMg^sia.eO!^
Minor Very Minor
CaO, Ca(OH)2, a-quartz
CaC03
CaC03 a-quartz,
Ca(OH)2
a-quartz
a-quartz
CaO, possible
Ca2Al2SiOy ,
CaO, possible
$-Ca2S±Oi+ possible
possible CaSOi^.,
2CaO-Al203-Si02,
High ash content (^22 weight percent).
Composition reported to be uncertain.
Sample taken from product surface, instead of from the core (other PER-620-6A sample).
Ln
Ui
-------
56
Table 13) are in good agreement with these literature values. The X-ray
diffraction analysis confirmed that the expected decomposition products
were obtained. In a thesis by Beck,13 it was sugggested that the first
dolomite peak (decomposition of MgC03) occurs at a higher temperature than
does that for magnesite alone because energy must be supplied to break
down the crystal structure of dolomite, as well as to decompose the magnesium
carbonate.
The sulfated Greer limestone from Pope, Evans and Robbins (PER-620)
contained 36.3% CaO, 2.53% Fe203, 16.6% Si02, 5.75% A1203, and 3.5% C02.
Based on this analysis, it was estimated that the sulfated additive con-
tained 10% coal ash. The X-ray diffraction analysis of a sample also
revealed the presence of Ca(OH)2 (Table 14).
In this series of DTA experiments, experiment PER-620-6A was performed
first (a sample was heated to 1409°C) to survey all detectable chemical
and physical changes. Next, individual samples were heated past each of
the temperature peaks observed in the first experiment. The maximum heating
temperatures, peak temperatures, and product conditions for this series of
experiments are given in Table 14.
Comparison of the X-ray diffraction analysis of the product sample
from experiment PER-620-1 (a sample that was heated to 625°C) with that
of the unheated material (PER-620) indicates that Ca(OH)2 dehydrated to
form CaO. This dehydration occurred at temperatures ranging from 506 to
534°C where an endothermic peak was observed. The handbook value for
the dehydration temperature of pure Ca(OH)2 is 580°C. Our samples are
not pure, however, and the possible effects of other constituents on the
dehydration peak temperature of Ca(OH)2 must be considered. There is
evidence14 of decreasing peak temperatures of Ca(OH)2 with increasing
dilution of the original sample. In other words, pure Ca(OH)2 yielded
the highest dehydration peak temperature, while the most dilute samples
yielded the lowest values.
The results from the next two experiments, PER-620-2 and -3 (in which
endothermic peak temperatures of ^790°C and ^840°C that represented
areaetionshwere observed)-were compared with results for magnesian limestone
that have been discussed by MacKenzie.15 MacKenzie observed three peaks
for a magnesian limestone. The first peak at 780°C was attributed to
the decomposition of MgC03. The last two peaks were attributed to the
decomposition of the dolomitic CaC03 (at 880°C) and the free calcium
carbonate (at 940°C). It is unlikely that any MgC03 is present in the
PER sulfated limestone samples since the sulfation temperature of the
material was ^900°C. It was concluded that the peaks at 790°C and 840°C
are CaCOs calcination peaks. X-ray diffraction analysis confirmed the
disappearance of CaC03. The decomposition peak temperatures obtained
(Table 14) are about 100°C below the referenced values. This may have the
same explanation as the discrepancy for the Ca(OH)2 peak, i.e., the dilute
nature of the sample may tend to decrease the peak temperatures of individual
components.
In experiment PER-620-4, the sample was sintered and the peak temper-
ature ranged from 1086-110l"C (the definition of the exothermic peak was
-------
57
not clear) . This is near the temperature at which agglomeration of PER-620
material has occurred in the regenerator. X-ray diffraction analysis of the
product from PER-620-4 indicates that 2Ca2SiCVCaSOit is the major constituent.
The high silica content of the sulfated limestone contributes to the for-
mation of this compound. Since the CaO level was reduced from a medium
constituent to a minor constituent in the PER-620-4 experiment when the
sample was heated to 1180°C, it is suspected that the principal reaction
occurring may have been:
2CaO + Si02 -»• Ca^SiOit (6)
Sample PER-620-5 (heated to 1218°C) showed no change in the analysis
from that for the sample heated to 1180°C although an endothermic peak
was observed at 1208 °C. This peak may represent the transition of g-CaSOtt
to a-CaS04. Previous DTA work16 by West and Sutton on pure CaS04 showed
that this transition occurred at 1225 °C and that on cooling, an exothermic
peak appeared 40 to 50 degrees lower than in the heating thermogram. This
agrees with the results in Table 14 and with unreported cooling peak
temperature results.
Analyses of samples taken from the products of experiments PER-620-6B
and PER-620-6A, showed that complex molecules of calcium-aluminum silicates
formed in the sample interior. Gag . jMgj . iS±3. 6°14. 4 was found at the
surface of the sample from PER-620-6A (which had been heated to 1409°C).
West and Sutton16 had similar results, demonstrating that significant
reaction between anhydrite (a-CaSO^) and Si02 and Fe20s occurs above the
3-a transition temperature (1225°C) for anhydrite. The results obtained
indicate that ash contributes further to the agglomeration problem since
aluminum (as well as silicon) is a constituent of the complex molecules.
Agglomerates from regeneration experiments (at ^1100°C) with sulfated
Greer limestone (PER-620) have also been analyzed by X-ray diffraction;
y- and 3-Ca2SiOi+ and Ca2Al2SiC>7 have been found in different locations in
the agglomerates. The agglomerates consists of large particles of sulfated
limestone and residual coal ash (-10 +30 mesh) . Since the one-step regen-
eration process is operated at VL100°C, the formation of calcium silicates
at this temperature may be the first stage in agglomeration of the fluidized-
bed material, which may be followed by a loss of fluidity and greater
localized temperature increases. The objectives are to find the tolerable
limits of silicates and other compounds for the regeneration process.
Mass and Energy Constrained Model for the Regeneration Process
In the investigation of the feasibility of the one-step regeneration
process for sulfated S02-acceptors, one objective is to optimize the process
conditions from technical and economic points of view. The one-step
reductive decomposition is carried out in a fluidized bed at elevated
temperature (M.100°C). The heat and the reducing gases are provided by
the combustion of coal (or other fuels) in the fluidized bed under reducing
conditions.
-------
58
A mass and energy constrained model for the one-step regeneration
process is being developed which is being used to predict the effects of
experimental variables on:
1. Off-gas composition (SC>2, C02, H2<), etc.)
2. Volumetric gas changes
3. Fuel and oxygen requirements
4. Individual energy terms
5. Energy cost of regeneration
Sensitivity analyses can be performed for key variables such as:
1. Regeneration temperature, T
2. Regeneration pressure (near atmospheric), P
3. Fluidizing-gas velocity, V
4. Reactor size
5. Feed solids and gas temperatures and compositions
The first phase of this work is concentrated only on what occurs
in the regeneration reactor, as illustrated in the flow diagram (Fig. 20).
The early objectives are to predict the effects of experimental variations
and to use these results to guide future experimental efforts.
In the one-step reductive regeneration process, the assumed combustion
reactions for coal (CH + nH^O) are as follows:
oxidizing conditions:
(CH + nH20) (s) + (y + 1)02 + CQ2
n)H20
(7)
OFF -GAS
ELUTRIATED COAL PARTICLES
SULFATED
ADDITIVE
t
TEMPERATURE,
PRESSURE,
FLUIDIZING-
GAS VELOCITY
i r
HEAT LOSS
REGENERATED
ADDITIVE
COAL °2'N2
Fig. 20. Flow Diagram for the One-Step Regeneration Process,
-------
59
reducing condition:
(CHm + nH20) (s) + 1/2 02 -> CO + y H2 + nH20 (8)
The main decomposition reactions that the S02-accepting additive
(natural stones) undergo are the calcination reaction:
CaC03(s) -> CaO(s) + C02 (9)
and the reductive decomposition reactions:
) + CO ->- CaO(s) + S02 + C02 (1)
) + H2 -> CaO(s) + S02 + H20 (2)
All of these decomposition reactions are endothermic, and their thermal
requirements are provided by the coal combustion reactions. From the
above combustion and decomposition reactions, it is apparent that gas
volume throughput increases as a result of reactions in the regeneration
reactor.
The total internal heat requirement (Qc) in the regeneration reactor
is:
Q = Q, +Qj +Q-,+QJJ+Q +Q (10)
sc xlost des cal add gas ec v '
where Q-. = heat losses from the reactor
xlost
Q, = heat of desulfation
Q ., = heat of calcination
cal
Q = sensible heat difference between regenerated and sulfated
additive
Q = sensible heat difference between effluent and input gas
gas
Q = sensible heat for the unburned elutriated coal
ec
It is assumed that this internal heat requirement will be satisfied by
the combustion coal.
Q = Q . + Q (11)
xc xci xcc
where
Q . = heat liberated during the required incomplete combustion of
coal
If Q - < Q ,
ci c
Q = additional heat requirement to be supplied by the complete
cc
combustion of coal
-------
60
if Qci > Qc,
Q = heat which must be removed from the process
In the model, no kinetic predictions are made; instead, experimental
results for the extent of solid (CaO) regeneration that have been correlated
as a function of solids residence time are used. The above mass and energy
relations are interlocked and are solved at different reactor solids resi-
dence times (the solids residence time is equal to the equivalent mass of
the fluidized bed devided by the mass feed rate of sulfated additive). The
major assumptions and properties of the model are summarized below.
1. Solids and the off-gas exit at the reactor temperature.
2. The additive is fully calcined.
3. The extent of regeneration is a function of solids residence
time and temperature (the assumption is based on experimental
data).
4. The chemical composition of the additive is a function of the
extent of regeneration.
5. The heat capacities of solids and gas constituents are
temperature-dependent.
6. Oxygen and nitrogen can be fed separately.
Examples of plotted calculated output are given in Fig. 21 through
25. The input regeneration conditions were:
Temperature: 1094°C (2000°F)
Fluidizing-gas velocity: 1.07 m/sec (3.5 ft/sec)
Pressure: 153 kPa (22.5 psia)
Reactor ID: 10.8 cm (4.25 in.)
Nominal fluidized-bed height: 46 cm (19 in.)
Feed gas temp: 344°C (650°F)
Feed solids temp: 25°C (77°F)
Solids residence time: 2-200 min (mass feed rates:
90-0.9 kg/hr)
Sulfated Tymochtee dolomite: 9.5% sulfur and 9.5 CO^
Effects of Solids Residence Time on Volumentric Gas Change. The input
functional dependence of extent of solids regeneration on solids residence
time (shown in Fig. 21) was obtained from experimental results. This
graph also shows the calculated increase in gas volume during regeneration
as a function of solids residence time (SRT). At SRTs of less than 10 min,
large gas volumentric increases are predicted, caused by increased coal
combustion due to increased heat requirements (sensible heat differences
of solids and gases, and heat of decomposition reactions) and increased
solids decomposition for the process, equations 1, 2, 7, 8, 9. The volu-
metric increase dilutes the S02 ifl the effluent gas.
Relation of Coal Feed Rate and Oxygen Concentration to Solids Residence
Time. In Fig. 22, the predicted coal feed rate and the predicted required
oxygen concentration are given as functions of SRT. The experimental
-------
61
o-
V
EXTENT OF REGENERATION , %
0 50 60 70 80 90 100 UO
i i i i i i i
\
\
\
\
\
\
/
\
'
\
1
\
;
\
/
\
'
i i i i i 1 1
o = EXTENT OF REGENERATION . %
o = INCREASE IN GAS VOLUME , %
\
/
EXPERIMENTAL CONDITIOf^
T=2000°F, V= 3.5 FT/SE
P = 22.5 PSIA
IS
^
FEED GAS 650 °F
FEED SOLID
\
\
'D
^
^
77 °F
•
»^
•fe
•^-,
•~~-
^^-^^
-&
-8
D 10 20 30 40 50 60 70
INCREASE IN GAS VOLUME , %
RESIDENCE TIME . MIN
100
Fig. 21. Experimental Solids Regeneration and Predicted Increase
in Gas Volume During Regeneration Versus Solids Residence
Time.
-------
62
8
K
Q
W
O -
*H
P
o
8
£
n
S
•-4
\
\
\
\
\
\
\
\
\
\
s
H
\
\
0
0
a
\
LI
L
\
\
D
L
>
III 1
a = COAL FEED RATE
o = OXYGEN IN FEED GAS ,
SYMBOLS : EXPERIMENTAL VALUES
OW SYMBOLS : CALCULATED VALUES
CS-12
\
•\cs-
CS-12
\ CS-
\
-II
\
-II
\
\
tJ
--^
•v
--«,
3,
>-
-•
-—
•& — &—&-fif
•8
s
•8
x
o
•a
10
RESIDENCE TIME , MIN
100
Fig. 22. Predicted Required Coal Feed Rate and Oxygen
Concentration in the Feed Gas as Functions of
Solids Residence Time.
-------
63
i—i—r
a = S02
A = C02
rm
DRY BASIS
WET BASIS
SOLID SYMBOLS: EXPERIMENTAL VALUES
HOLLOW SYMBOLS:CALCULATED VALUES
8
CS-12
C/2
O
w
CS--II
o
£s
w
u
K
W
(X,
\
CS-12
CS
-•II
10
100
RESIDENCE TIME , MIN
Fig. 23. Predicted Off-Gas Constituent Concentrations as
Functions of Solids Residence Time.
-------
64
I I I I I I I I I
D = HEAT OF COMBUSTION
o = SENSIBLE HEAT OF SOLID
A = SENSIBLE HEAT OF GAS
+ = HEAT OF DESULFATION
x = HEAT OF CALCINATION
o = HEAT LOST
100
RESIDENCE TIME . MIN
Fig. 24. Predicted Individual Heat Requirements as a Function
of Solids Residence Time.
-------
65
(O
CO
CM
CO
co
N
o
t— 1
E-i
CM
53 «
o -
o
W
E«
7
10
100
RESIDENCE TIME . MIN
Fig. 25. Predicted Fuel Cost for Regeneration per Electric Power
Unit Produced when Burning 3% Sulfur Coal as a Function
of Solids Residence Time. (These costs are not repre-
sentative of those in a commercial plant since conditions
have not been optimized.)
-------
66
conditions and results for the measured variables in experiments CS-11
and-12 are also plotted for comparison with the predictions. Agreement
of predicted with experimentally obtained values was good.
Relation of Off-Gas Composition to Solids Residence Time. The predicted
off-gas constituent concentrations as functions of solids residence time
are given in Fig. 23. The concentrations of all pertinent constituents
increase with decreasing SRT (increasing solids feed rate). At a SRT of
5 min, a S02 concentration of 9.4% in the dry off-gas is predicted for
the experimental conditions described above.
Relation of Heat Requirements to Solids Residence Time. The predicted
individual heat requirements for the one-step regeneration process are
plotted in Fig. 24 as functions of solids residence time. At low SRT, the
sensible heat requirements of the solids and gas constitute most of the
required heat.
Relation of Regeneration Fuel Cost to Solids Residence Time. The fuel
cost for regeneration per electric power unit produced when burning 3%
sulfur coal has been predicted as a function of SRT and is shown in Fig. 25.
The experimental conditions for the regeneration are given above.
The costs plotted are relative costs obtained to estimate the effect of
operating conditions such as SRT and feed solid and gas temperatures. In
an industrial process, these costs would be reduced by recovering the
sensible heat from the effluent regenerator streams. (A coal cost of $26/ton
was used in predicting the regeneration fuel costs.) It has been found that
for the experimental conditions outlined above, the energy cost of regen-
eration is optimum at a SRT of ^9 min. At lower solids residence times,
the extent of regeneration decreases rapidly and the sensible heat require-
ments for the solids and gas increase rapidly.
Predicted Effects of Input Conditions on Regeneration Results. The
effects of solids and gas feed temperatures, pressure, carbonate concentration
in the additive, and reactor diameter on: S02 concentration in the dry
effluent gas, oxygen requirement, increase in gas volume, process heat
requirement, and energy cost of regeneration were predicted. The results
at a SRT of 5 min are given in Table 15. Case 1 represents the design
experimental condition capabilities of the existing bench-scale experimental
regeneration system at ANL. In cases 2, 7, 8, 9, and 10, the effects of
preheating the feed solids and gas to temperatures as high as 1094°C (2000°F)
were predicted for a SRT of 5 min. The SC>2 concentration in the wet effluent
gas (not shown) was predicted to increase from 7.8% in case 1 (gas containing
17% water) to 8.3% in case 10 (gas containing 2.9% water). However, it is
predicted that if the temperatures of the feed solids and gas are increased
as in case 2 and dases 7-10, the S02 concentration in the dry effluent gas
would decrease because the water content decreases. The required oxygen
concentration in the feed gas decreased from 53% in case 1 to 4.0% in case
10. These data are useful in selecting operating conditions for a realistic
plant situation in which air rather than oxygen-enriched gas would be utilized.
-------
Table 15. Regeneration of Tymochtee Dolomite.
Regeneration Temperature: 1094°C
Solids Residence Time: 5 min
Fluidizing Gas Velocity: 1.07 m/sec
Input Conditions
Case
1
2
3
4
5
6
7
8
9
10
Feed
Solids
Pressure Temp
(kPa) (°C)
153
153
153
153
153
102
153
153
153
153
25
650
650
650
'650
650
870
1094
870
1094
Feed
Gas
Temp
345
650
650
650
650
650
870
870
1094
1094
$
9.5
9.5
9.5
9.5
0
9.5
9.5
9.5
9.5
9.5
Reactor
Dia
(cm)
10.8
10.8
21.6
43.2
10.8
10.8
10.8
10.8
10.8
10.8
S02 in Dry
Effluent
Gas (%)
9.4
8.9
8.8
8.8
9.7
12.0
8.7
8.6
8.7
8.6
Req . 02
in Feed
52.9
22.8
19.1
17.8
17.6
30.9
10.8
4.0
9.8
4.0
Predicted Results
Gas
Volume
Increase
48.5
41.7
40.9
40.6
28.1
60.9
39.0
37.7
38.8
37.7
Process Heat
Requirement
(Btu/hr)
+92 K
+34 K
+109K
+395K
+24 K
+30 K
+11 K
-9 K
+9 K
-11 K
Fuel Cost for
Regeneration3
(raill/kWh)
0.60
0.30
0.27
0.25
0.25
0.28
0.18
0.10
0.17
0.10
Tons of Coal for
Regeneration/Ton of
Coal for Combustion
.069
.035
.031
.029
.029
.032
.021
.012
.020
.012
aThese costs are not representative of those in a commercial plant since conditions have not been optimized.
Coal cost, $26/ton.
A 3 wt % S coal was assumed to be burned during the combustion step.
-------
68
The process heat requirement (Qcc) is the heat that must be added
to or removed from the regeneration process in addition to the heat liberated
by incomplete combustion of coal. In ciases 8 and 10 in which temperatures
of feed solids and gas are high, heat must be removed from the reactor to
maintain the design experimental conditions.
The energy cost of regeneration per unit of electric power generated
with the combustion of 3% sulfur coal decreased from 0.6 (case 1) to 0.1
mill/kWh (case 10). It was assumed in these calculations that no energy
was recovered from the hot (1095°C) regenerated additive. In terms of
coal usage, VL-3% additional coal to that combusted in the power generating
system would be required for regeneration of the sulfated sorbent.
In cases 2, 3, and 4, the effects of increasing the reactor ID from
10.8 cm to 43.2 cm were predicted. The effects were not dramatic. For
larger reactors, the percent heat losses are reduced and thus the energy
cost of regeneration is reduced. The SC-2 concentration in the dry
effluent gas is predicted to remain almost the same.
The effect of carbonate content of the sulfated additive was predicted
by comparing cases 2 and 5. By decreasing the concentration of CO^ from
9.5% to 0, the S02 concentration in the dry effluent gas was predicted
to be increased from 8.9% to 9.7%. The energy cost of regeneration would
be decreased from 0.3 to 0.25 mill/kWh.
The parameter that was most effective for increasing the S02 concen-
tration in the dry effluent gas was found to be regeneration pressure. It
was predicted that by decreasing the pressure from 153 kPa (22.5 psig) in
case 2 to 102 kPa (15 psia) in case 6, the S02 concentration would increase
from 8.9% (case 2) to 12% (case 6) because the total gas feed rate would
decrease by 50%.
These predicted effects of experimental variations on dependent
variables (such as 50% concentration in the effluent gas) will be used
to guide future experimental efforts.
CYCLIC COMBUSTION/REGENERATION EXPERIMENTS
J. Montagna and W. Swift (Principal Investigators), H. Lautermilch,
R. Mowry, F. Nunes, C. Schoffstoll, G. Smith, S. Smith, J. Stockbar, G. Teats
In the utilization of regeneration technology, recycling of the
sorbent a sufficient number of times without loss of its reactivity for
either sulfation or regeneration and without severe decrepitation is import-
ant. Unless both of these requirements are met, the sorbent makeup rate will
be so high that regeneration will not be justified. An experimental effort
is being made, therefore, to evaluate the effects of cyclic operation on
the resistance to decrepitation and the reactivity of Tymochtee dolomite in
ten combustion/regeneration cycles, 2 1/2 of which have been completed.
Prior to the cyclic experiments, Tymochtee dolomite which had been sulfated
and regenerated was again sulfated in the combustor to obtain preliminary
operating data.
-------
69
Preliminary Experiment. Sulfation of Regenerated Tymochtee Dolomite
Before the 10-cycle series of experiments was begun, experiment RC-1A
was performed as the initial experiment* in the planned evaluation of the
effects of sorbent recycling on sorbent reactivity and decrepitation. The
sorbent used was Tymochtee dolomite (^50% CaCO$, ^40% MgC03. as received)
which had been sulfated and regenerated. Sulfation was done in the
bench-scale combustor in the PSI-series experiments (combustion with
Arkwright coal.; see Pressurized, Fluidized-Bed Combustion Studies). After
sulfation, it was found that utilization of the additive had been ^62% and
that the sorbent contained VLO-11% sulfur by weight. The sulfated dolomite
was then regenerated in the 4 l/4-in.-dia regenerator under various operating
conditions; it was then thoroughly mixed and resulfated in experiment RC-1A.
It contained 4.3 wt % sulfur, which corresponded to an additive utilization
of 18%.
Experiment RC-1A was performed over a period of three days; 0;54.5 kg
of regenerated dolomite was processed. Operating conditions and flue-gas
analysis results are given in Table 16 for the steady-state conditions
existing during the longest (^6 1/2-hr), most stable operating period of
the experiment. Bed temperature and flue-gas composition data for that
period of operation are plotted in Fig. 26.
Based on the flue-gas analysis for S02 (250 ppm), sulfur retention
(based on sulfur in the coal and not including the sulfur in the original
sorbent) was determined to be ^90%. This S02 level agrees very well with
the expected level in the flue gas of 240 ppm S02 calculated from correlations
of previous experiments.17 Thus, the results of the preliminary recycle
experiment were very encouraging regarding the capability of the sorbent to
retain sulfur in the second combustion cycle.
Table 16. Operating Conditions and Flue-Gas Analysis for Segment
of Combustion Experiment RC-1A.
Combustor: ANL, 6-in.
Bed Temp: 840 °C
Pressure: 810 kPa
Fluidizing-
Gas
Feed Rate (kg/hr) Ca/S Velocity
Coal Sorbent Ratioa (m/sec)
14.1 3.2 1.6 1.0
-dia Excess Air: 17%
Coal: Arkwright
Sorbent : Regenerated
Tymochtee Dolomite
Flue-Gas Analysis
S02 NO NOX CO C02 02
(ppm) (ppm) (ppm) (ppm) (%) (%)
250 120 150 40 17 3.1
Based on unreacted calcium (CaO) in regenerated dolomite and sulfur in coal.
Equipment, materials, and procedures used in the combustion steps of
the cyclic experiments are described in a following section, Pressurized,
Fluidized-Bed Combustion; Bench-Scale Studies.
-------
Gas Concentrations
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-------
71
Calcium utilizations were calculated from steady-state samples of the
overflow, the primary cyclone, and the secondary cyclone (Table 17). Utili-
zations based on the sulfur and calcium contents of the respective samples
ranged from 56% in overflow to 67% in secondary cyclone samples. Also given
in Table 17 are utilization values adjusted to reflect the 18% calcium
utilization of the dolomite feed to the combustor. The adjusted utilizations
ranged from 47% in-overflow to 60% in secondary cyclone samples. These
utilization values provide evidence that finer dolomite particles are more
active in S02 retention.
A calcium material balance for experiment RC-1A showed that of the
sorbent fed to the combustor, ^62% was removed in the product overflow,
v30% was recovered in the primary cyclone, ^2% was recovered in the
secondary cyclone, and ^6% was unaccounted for.
Screen analyses of samples of regenerated dolomite.feed, overflow,
primary cyclone, and secondary cyclone materials are presented in Table 18.
Although conclusive evidence in the form of chemical determinations is
lacking, it appears from the screen analyses in Table 18 that decrepitation
of dolomite particles in the fluidized bed was not the sole source of ash
products recovered in the cyclone. Comparison of the steady-state overflow
with the regenerated dolomite feed shows a larger loss by entrainment (in
percent) for the smaller particle sizes. Examination of the primary
cyclone screen analysis shows a significant amount of +80 mesh material,
which is the size of the smallest particles in the regenerated dolomite
feed. This indicated that at least some of the sorbent carryover into the
cyclones may have occurred without decrepitation of the sorbent in the
fluidized bed. Based on these results, more rigorous determinations of
decrepitation during selected cycles of the ten-cycle series of combustion/
regeneration experiments are planned.
Table 17. Utilization of Calcium in Overflow, Primary Cyclone,
and Secondary Cyclone Samples from Experiment RC-1A.
Calcium Utilization (mol S/mol Ca) x 100
Based on Sulfur and Adjusted for Feed
Calcium Content of Having 18% Calcium
Sample
Overflow
Primary Cyclone
Secondary Cyclone
Sample
56%
62%
67%
Utilization
47%
54%
60%
Weighted Average
(based on product
distribution of
calcium)
-------
72
Table 18. Screen Analysis Results for Combustion Experiment RC-1A.
Wt Fraction in Size Range
U.S.
Sieve No.
+14
-14 +25
-25 +35
-35 +45
-45 +80
-80 +170
-170
Regenerated
Dolomite Feed
0.00
0.17
0.18
0.43
0.22
0.00
0.00
Steady-State
Overflow
0.00
0.36
0.28
0.30
0.05
0.00
0.00
U.S.
Sieve No.
+45
-45 +80
-80 +100
-100+170
-170+230
-230+325
-325
Wt Fraction
Primary
Cyclone
0.30
0.25
0.04
0.10
0.06
0.08
0.16
in Size Range
Secondary
Cyclone
0.20
0.12
0.08
0.10
0.13
0.14
0.24
Total
1.00
0.99
0.99
1.01
Combustion Step, Cycles 1, 2, and 3
After the initial recycle experiment (RC-1A), a ten-cycle series of
combustion/regeneration experiments was initiated using Arkwright coal and
Tymochtee dolomite. Nominal operating conditions selected for the combustion
portions of each cycle are a 900°C bed temperature, 810 kPa pressure, 1.5
Ca/S mole ratio, ^17% excess combustion air, 0.91 m/sec fluidizing-gas
velocity, and a 1.07 m bed height. Chemical and physical characteristics
of the coal and dolomite are presented in Tables A-l and A-4, respectively.
The coal was fed to the combustor as received; the dolomite was prescreened
to -14 +30 mesh.
Three combustion steps (2 1/2 cycles) have been completed. During the
first combustion test, which required VL70 hr of operating time, 620 kg
of dolomite (^124 kg on a calcium basis) was sulfated. The second and third
combustion tests were made with 270 kg and 250 kg of sorbent (^80 and 75 kg
on a calcium basis) and required approximately 100 and 95 hr of operating
time, respectively. As experience is gained regarding sorbent losses which
can reasonably be expected per cycle, the quantity of sorbent processed
will be reduced to shorten the operating time required to complete the full
ten-cycle test. Operating conditions and flue-gas composition data for
representative steady-state segments of the first three combustion cycle
experiments are presented in Table 19. Bed temperature and flue-gas data
for representative segments of the cyclic combustion experiments are plotted
in Fig. 27-29.
Sulfur Retention. The sulfur dioxide level in the flue gas increased
from 290 ppm in the first combustion cycle to 400 ppm and 490 ppm in
combustion cycles two and three. Sulfur retention, based on flue-gas analysis
has correspondingly decreased from 86% in cycle one to 81% and 77% in cycles
two and three. The sulfur retention in cycle three, which corresponds to an
emission of 0.95 Ib S02/106 Btu, meets the EPA environmental emission
standard of 1.2 Ib S02/106 Btu.
-------
Table 19.
Operating Conditions and Flue-Gas Composition for
Cyclic Combustion Experiments.
Combustor: ANL, 6-in.-dia
Coal: Arkwright, -14 mesh, 2.8 wt % S
Sorbent: Cycle 1, Tymochtee dolomite,
-14 +30 mesh
Cycles 2 and 3, Regenerated
Tymochtee dolomite
900°C
Temperature:
Pressure: 810 kPa
Excess Air: M.7%
Additive
Feed
Analysis Feed Rate
(wt %) (kg/hr)
Cycle
REC-1
REC-2
REC-3
Ca S Coal
20 - 14
29.7 4.7 13
29.7 4.6 13
.6
.3
.5
Sorbent
4.1
2.8
2.9
Fluidizing-
Ca/S Gas
Mole Velocity
Ratio3- (m/sec)
1.6
1.4
1.5
0.94
0.91
0.91
Bed
Height
(m)
1.1
0.9
0.9
Flue-Gas Analysis (avg values)
S02
(ppm)
290
400
490
NO
(ppm)
200
120
130
(ppm)
32
30
N.D.
CO C02 02 I
(PPm) (%) (%)
90 16.0 3.4
40 15.5 3.1
° 20 16.0 3.2
Sulfur
letention^
86
81
77
kRatio of unsulfated calcium in dolomite feed to sulfur in coal.
Based on flue-gas analysis.
No data.
-------
74
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1200
1440
1680
Eig. 27. Bed Temperature and Flue-Gas Composition, Segment
of Experiment REC-1 (REG-IK and -1L).
-------
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-------
GAS CONCENTRATIONS
PH BED
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-------
77
The sulfur retention capability of the dolomite would be expected to
decrease during the first few cycles after less than 100% regeneration of
the sorbent has been achieved. For the first combustion cycle, the Ca/S
mole ratio of 1.6 is based on the total calcium content of the sorbent.
If a shrinking-core sulfation model is assumed, the sorbent from the first
combustion cycle consists of inner unreacted calcium, a layer of unregen-
erated calcium sulfate, and a surface layer of regenerated calcium oxide.
For subsequent combustion cycles, the Ca/S mole ratio of 1.5 is selectively
based on regenerated calcium oxide at the surface and less available
calcium (due to diffusional mass transport limitations) at the core of
the particle. If this were the only factor affecting sulfur retention
during cyclic operation, however, a balance between sulfation and regen-
eration would be expected, after which there would be no further decrease
in the reactivity of the dolomite.
Decrepitation Rate. Analysis of samples of solids from a representative
segment of the first combustion cycle experiment, REG-IK, has been completed,
permitting evaluation of the decrepitation of Tymochtee dolomite during the
experiment. (The analysis does not distinguish decrepitation in the feeding
operation, which might be appreciable, from decrepitation which occurs
during dolomite residence in the combustor.) The approach taken in the
analysis was to determine the degree to which +30 mesh dolomite (^99.2% of
feed) was reduced to -30 mesh additive, elutriated from the bed, and collected
in the cyclones. The primary cyclone material, a mixture of ash and dolomite,
was separated into +30 mesh and -30 mesh fractions which were analyzed for
both calcium and magnesium to determine the dolomitic calcium in each size
fraction.
The results of the calculations are presented in Table 20. The overall
calcium material balance is 103%. Decrepitation of the Tymochtee dolomite,
defined as the amount of +30 mesh additive reduced to -30 mesh, was ^20%
for the segment REG-IK of the first combustion cycle or ^4%/hr (solids mean
residence time in the combustor was ^5 hr). Entrainment, defined as the
percentage of calcium entering the combustor which subsequently leaves the
combustor in the flue gas, was ^25% or ^5%/hr.
The high level of decrepitation measured during the first combustion
cycle does not agree with measurements of decrepitation of Tymochtee dolomite
reported previously.17 In the earlier study, an inventory was made of +45
mesh Tymochtee dolomite during eleven combustion experiments with an average
loss for all experiments of only 4% (solids mean residence times were
between 2 and 16 hr). The +45 mesh fraction accounted for approximately
87 wt % of the -14 +100 mesh additive feed used in the previous investigation,
as compared with experiment REC-lK in which the +30 mesh additive accounted
for ^99% of the -14 +30 additive feed. The previous decrepitation measure-
ments were based on balances made around the entire experiments; the current
determination is based on a steady-state material balance. However, neither
of these differences in the determination of decrepitation seems sufficient
to account for the large difference in decrepitation values reported.
-------
78
Stream
Table 20. Steady-State Flow of Calcium Through the
ANL, o-in.-dia Combustor during combustion
Experiment REG-IK
Composition Dolomite Calcium
Rate
(kg/hr)
Ca Mg +30 mesh -30 mesh Coal Calcium
(wt %) (wt %) (g/hr) (g/hr) (g/hr)
IN
Additive
Feed
Coal
Feed
4.08 20.0
14.6
OUT
Product
Overflow
Primary
Cyc, +30 mesh
Primary
Cyc , -30 mesh
Secondary
Cyclone, all
-30 mesh
2.4
0.4
2.0
0.15
N.D.
19.3
8.'5
11.3
810
6.7
0.27 negl.
TOTAL
39
810 6.7 39
GRAND TOTAL ^860 g/hr
N.D. 600
32
10.6
4.19
72
150
6.1 2.22 - 5.9
TOTAL 670 190
GRAND TOTAL
negl.
2.1
22
3.2
27
^890 g/hr
No data.
-------
79
The most likely explanation for the higher decrepitation rate in exper-
iment REG-IK than those in previous combustion experiments at Argonne is
that experiment REC-1 was performed using a different batch of dolomite
from the supplier, C. E. Duff and Sons, Huntsville, Ohio. Exxon has
reported similar increases in attrition of Tymochtee dolomite received in
later shipments from the same quarry. Exxon tested (in a batch combustion
experiment) a sample of Tymochtee dolomite used in the earlier Argonne
experiments and reported a significant reduction in the attrition rate
for the sample.
There is a discrepancy in the method of reporting attrition data by
the various contractors investigating FBC which could make the comparison
of results difficult. In results reported by Argonne, a distinction has
been made between the amount of sorbent entrained in the flue gas and the
amount of decrepitation (i.e.3 the quantity of additive particles reduced
below a certain size). Such a distinction is important since it has been
found that some sorbent particles that are larger than the smallest particle
size in the sorbent feed can be carried over into the cyclones. Other
investigators of fluidized-bed combustion have reported as attrition the
amount of additive which leaves in the fluidized-bed overhead and is
recovered from the flue gas.
Entrainment and decrepitation for an experiment can differ greatly.
For example, in earlier work at Argonne in which the average attrition of
Tymochtee dolomite was reportedly ^4% of the dolomite fed to the combustor*,
carryover of sorbent material into the cyclone was reported to be as high
as ^80% at fluidizing-gas velocities of 1.5 m/sec.
Exxon has reported attrition rates of 20-25 wt %/hr (expressed as
percent of the bed weight lost per hour) for Tymochtee dolomite in batch
combustion tests.18 If this basis were used for reporting, the 25%
entrainment reported here for experiment REG-IK would correspond to an
"attrition" rate of ^5 wt %/hr. However, on the basis of the solids mean
residence time of 5 hr for experiment REG-IK, the rate at which +30 mesh
additive feed material was reduced to -30 mesh material in the combustor
was ^4 wt %/hr.
Quantitative evaluations are being made of decrepitation and
elutriation of sorbent from the combustor during the second and third
combustion cycles. Preliminary indications are that the sorbent losses
are significantly less than the 4 to 5%/hr (based on the weight of the
bed) measured in the first combustion cycle. Losses were ^2-3%/hr during
the second combustion cycle and VL-2% during the third combustion cycle.
Regeneration Step, Cycles 1 and 2
In the regeneration steps, Triangle coal (for which the ash fusion
temperature is high) is being partially combusted at a system pressure of
153 kPa and a bed temperature of 1100°C. Chemical and physical character-
istics of Triangle coal are presented in Appendix A. The results, up to a
third regeneration step of the ten-cycle experiment, are reported.
-------
80
Extent of Regeneration of CaO. The experimental conditions and results
for the regeneration steps (CCS-1 and CCS-2) of the first two cycles are
presented in Table 21, and the intermittent off-gas analyses are given in
Figs. 30 and 31. In the regeneration step of the first sorbent utilization
cycle, the S02 concentration in the dry flue gas was 6.5% and the extent
of CaO regeneration was 71% (based on analyses of solid products).
The extent of CaO regeneration in cycle two (67%) was slightly lower than
that in the first cycle (71%). This difference was probably due to the higher
sulfur content of the feed sulfated dolomite (10.7 wt % as compared with 9.1
wt % in cycle 1). The S02 concentration in the dry off-gas increased from
6.5% in the first cycle to 8.6% in the second cycle. There were two reasons
for the increase: the fluidizing-gas velocity was lower in the second cycle
and hence the extent of S02 dilution was lower. Secondly, the sulfur content
of the sulfated additive fed was higher in the second cycle and hence more
S02 was evolved per unit time during regeneration.
Attrition Rate. A first estimate of the attrition rate was obtained
by calculating the difference between the calcium in the feed and the
product streams per unit time at steady-state conditions. The calcium
feed rate for CCS-1 was 7.11 kg/hr, and calcium was removed in the regen-
erated product at a rate of 6.94 kg/hr. On the basis of these rates,
0.17 kg/hr or approximately 2% of the sulfated feed dolomite was attrited
and elutriated. The attrition rate during the second cycle has been found
(within analytical accuracy) to be negligible based on the calcium feed rate
(5.50 kg/hr) and the calcium removal rate (5.52 kg/hr) in the regenerated
sorbent. The low sorbent losses from attrition and elutriation can be
attributed to the short solids residence time in the regenerator reactor.
Coal Ash Buildup during Sulfation and Regeneration Steps
Of concern in developing a regenerative fluidized-bed coal combustion
process is the extent of coal ash buildup in the fluidized bed and the
effect of ash on (1) the S02-accepting capability of the additive bed and
(2) ash-additive reactions during regeneration. The ash buildups during
the first-cycle sulfation and regeneration steps have been calculated
from wet-chemical analyses of the samples and are given in Table 22. As
a basis for calculation, 100 g of unsulfated Tymochtee dolomite was used.
Some of the analytical methods used for unsulfated Tymochtee dolomite were
obtained from the literature.19 (Analysis of the virgin dolomite is
presently being performed at ANL, and ash builup will be recalculated on
the basis of these results.) An ash buildup of 4-5% during sulfation was
calculated, based on the enrichment of Si, Fe, and Al.
The concentrations of ash constituents in sulfated dolomite are
compared to the concentrations in Greer limestone in Table 23. As expected,
the extent of ash buildup was smaller in the experiments with Tymochtee
dolomite (in which a relatively low ash Arkwright coal was used).
-------
Table 21. Experimental Conditions and Results for Regenerative Segments
of the Regeneration Step of the First Two Utilization Cycles.
Nominal Fluidized-Bed Height: 46 cm
Reactor ID: 10.8 cm
Pressure: 153 kPa
Temperature: 1100°C
Coal: Triangle (0.98 wt %• S), Ash fusion temperature under
reducing conditions: 1390°C (initital deformation)
Additive: -14 +30 mesh, sulfated Tymochtee dolomite
Fluidizing- Solids
Cycle Exp.
No.
1 CCS-1
2 CCS-2
Gas
Velocity
(m/sec)
1.43
1.26
Residence
Time
(min)
7
7.5
Reducing Gas
Concentration
in Off-Gas
(
2
3
y\
i°)
.8
.0
Sulfur in
Feed
Sulfated
Sorbent
(%)
9.1
10.7
CaO
Regeneration
(%)a/(%)
73/71
67/67
Major
S02
6.5
8.6
in
0
0
Sulfur Compounds
Dry
H2S
.04
.02
Off-Gas
COS
0.06
0.1
(%)
CS2
0.04
0.1
00
Based on off-gas analysis.
Based on chemical analysis of dolomite samples.
-------
82
|