s>EPA
United States
Environmental Protection
Agency
Office of Air Quality
Planning and Standards
Research Triangle Park NC 27711
EPA-450/3-79-014
June 1979
Air
A Review of Standards
of Performance for New
Stationary Sources -
Asphalt Concrete Plants
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Federal Register / Vol. 44. No. 171 / Friday. August 31. 1979 / Rules and Regulation.
51225
ENVIRONMENTAL PROTECTION
AGENCY
40 CFR Part 60
[FRL 1231-3]
Standards of Performance for New
Stationary Sources: Asphalt Concrete;
Review of Standards
AGENCY: Environmental Protection
Agency (EPA)
ACTION: Review of Standards.
SUMMARY: EPA has reviewed the
standard of performance for asphalt
concrete plants (40 CFR 60.9. Subpart I).-
The review is required under the Clean
Air Act. as amended August 1977. The
purpose of this notice is to announce
EPA's intent not to undertake revision of
the standards at this time.
DATES: Comments must be received by
October 29,1979.
ADDRESS: Comments should be
submitted to the Central Docket Section
(A-130), U.S. Environmental Protection
Agency. 401 M Street. S.W.,
Washington, D.C. 20460, Attention:
Docket No. A-79-O4.
FOR FURTHER INFORMATION CONTACT:
Mr. Robert Ajax, telephone: (919) 541-
5271. The document "A Review of
Standards of Performance for New
Stationary Sources—Asphalt Concrete"
(EPA-450/3-79-014) is available upon
request from Mr. Robert Ajax (MD-13),
Emission Standards and Engineering
Division. U.S. Environmental-Protection
Agency, Research Triangle Park, North
Carolina 27711.
SUPPLEMENTARY INFORMATION:
Background
'In June 1973. EPA proposed a
standard under Section 111 of the Clean
Air Act to control particulate matter
emissions from asphalt concrete plants.
The standard, promulgated on March 8,
1974. limits the discharge of particulate
matter into the atmosphere to a
maximum of 90 mg/dscm from any
affected facility. The standard also
limits the opacity of emissions to 20
percent. The standard is applicable to
asphalt concrete plants which
commenced construction or
modification-after June 11.1973.
The Clean Air Act Amendments of
1977 require that the Administrator of
the EPA review and. if appropriate.
revise established standards of
performance for new stationary sources
at least every 4 years [Section
lll(b)(l)(B)J. Following adoption of the
Amendments, EPA contracted with the
MITRE Corporation tojundertake a
review of the asphalt concrete industry
and the current standard. The MITRE
review was completed in January 1979.
Preliminary findings were presented to
- and reviewed by the National Air
Pollution Control Techniques Advisory
Committee at its meeting in Alexandria,
Virginia, on January 10.1979. This notice
announces EPA's decision regarding the
need for revision of the standard.
Comments on the results of this review
and on EPA's decision are invited.'
Findings
Overview of the Asphalt Concrete
Industry
The dsphalt concrete industry consists
of about 4,500 plants, widely dispersed
throughout the Nation. Plants are
stationary (60 percent), mobile (20
percent), or transportable (20 percent),
i.e., easily taken down, moved and
reassembled. Types of plants include
batch-mix (91 percent), continuous mix
(6.5 percent), or dryer-drum mix (2.5
percent). The dryer-drum plants, which
are becoming increasingly popular,
differ from the others in that drying of
the aggregate and mixing with the liquid
asphalt both take place in the same
rotary dryer. It is estimated that within
the next few years, dryer-drum plants
will represent up to 85 percent of all
plants under construction.
Current national production is about
263 to 272 million metric tons (MG)/
year, with a continued rise expected in
the future. It is estimated that
approximately 100 new and 50 modified
plants become subject to the standard
each year. Operation is seasonal, with
plants reportedly averaging 666 hours/
year although many operate more
extensively.
Particulate Matter Emissions and
Control Technology
The largest source of particulate
emissions is the rotary dryer. Both dry
•(fabric filters) and wet (scrubbers)
collectors are used for control and are
both capable of achieving compliance
with the standard. However, all systems
of these types have not automatically
achieved control at or below the level of
the standard.
Based on data from a total of 72
compliance tests, it was found that 53 or
about three-fourths of the tests for
particulate emissions showed
compliance. Thirty-three of the 53
produced results between 45 and 90
Mgs/dscm (.02 and .04 gr/dscf). Of the 47
tests of fabric filters or venturi scrubber
controlled sources over 80 percent
showed compliance. The available data
do not provide details on equipment
design and an analysis of the cause of
failures has not been performed.
However, EPA is not aware of any
instances in which a properly designed
and installed fabric filter system or high-
efficiency scrubber has failed to achieve
compliance with the standard. The fact
that certian facilities controlled by
fabric filters and high-efficiency
scrubbers have failed to comply is
attributed to faulty design, installation.
and/or operation. This conclusion and
these data are consistent with data and
findings considered in the development
of the present standard.
On the basis of these findings, EPA
concludes that the present standard for
particulate matter is appropriate and
that no revision is needed.
Much less test data are available for
opacity than for particulates. Of the 26
tesjs for which opacity levels are
reported, only 5 failed to show
compliance with the opacity standard.
However, none of these 5 met the
standard for particulate matter. Of the
21 plants reported as meeting the
current standard for opacity, 19 met the
particulate standard. On the basis of
these data. EPA concludes that the
opacity standard is appropriate and
should not be revised. While the data do
indicate that a tighter standard may be
possible, the rationale and basis used to
establish the present standard are
considered to remain valid.
Enforcement of the Standard
Because the cost of performance tests
which are required to demonstrate
compliance with the standard are
essentially fixed and are independent of
plant size, this cost is disproportionately
high for small plants. Due to this, the
issue was raised as to whether formal
testing could be waived and lower cost,
alternative means be established for
determining compliance at small plants.
Support for such a waiver can be found
in the fact that emission rates are
generally lower at these plants and
eiror,s in compliance determinations
would not be large in terms of absolute
emissions. However, testing costs at all
sizes of plants are small in relation to
the cost of asphalt concrete production
over an extended period and these costs
can be viewed as a legitimate expense
to be considered by an owner at the
time a decision to construct is made. A
number of State agencies presently.
require, under SIP regulations, initial
and in some cases annual testing of
asphalt concrete plants. Moreover,
available compliance test data show
that performance of control devices is
variable and even with installation of
accepted best available control
technology the standard can be
exceeded by a significant degree if the
control system is not properly designed.
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51226 Federal Register / Vol. 44. No. 171 /Friday. August 31. 1979 / Rules and Regulations
operated, and maintained. Relaxing the
requirement for formal testing thus
could lead to a proliferation of low
quality or marginal control equipment
which would require costly repair or
retrofit at a later timev
A further performance testing problem
indentifled in the review of the standard
concerns operation at less than full
production capacity during a compliance
test. When this occurs, ERA normally
accepts the test result as •
demonstration of compliance at the
tested production rate, plus 23 Mg (25
tons)/hr. To operate at a higher
production rate, an owner or operator
must demonstrate compliance by testing
at that higher rate. Industry
representatives view this limitation as
an unfair production penalty. It is noted
in particular that reduced production is
sometimes an unavoidable consequence
associated with use of high moisture
content aggregate. Furthermore, it is
argued that facilities which show
compliance at the maximum production
rate associated with a given moisture
level can be assumed to comply at
higher production rates when moisture
is lower. However, this argument
assumes that the uncontrolled emission
rate from the facility does not increase
as production rate increases and EPA is
not aware of data to support this
assumption.
As a general policy it is EPA's intent
to minimize administrative costs
imposed on owners and operators by a
standard, to the maximum extent that
this can be done without sacrificing the
Agency's responsibility for assuring
compliance. Specifically, in the cases
cited above. EPA does not intend to
impose costly testing requirements on
small facilities or any facilities if
compliance with the standard can be
determined .through less costly means.
However. EPA at this time is not aware
of a procedure which could be employed
at a significantly lower cost to
determine compliance with an
acceptable degree of accuracy. Although
opacity correlators with grain loading
and serves as a valid means for
identifying excess emissions, due to
dependence on stack diameter and other
factors opacity alone is not adequate to
accurately assess compliance with the
mass rate standard. Similarly, the
purchase and installation of a baghouse
or venturi scrubber does not in itself
necessarily imply compliance. EPA is
concerned that approval of such
equipment without compliance lest data
or a detailed assessment of design and
operating factors would provide an
incentive for Installation of low cost,
under-designed equipment. This would
place vendors of more costly systems
which are well designed and properly
constructed and operated at a
competitive disadvantage: in the long
term this would not only increase
emissions but would be to-ihe detriment
of the industry.
EPA has. however, concluded that a
study program to investigate alternative
compliance test and administrative
approaches for asphalt plants is needed.
An EPA contractor working for the
Office of Enforcement has initiated a
study designed to assess several
administrative aspects of the standard.
Including possible low cost alternative
lest methods; administrative
mechanisms to deal with the problem of
process variability during testing; and
physical constraints affecting the ability
to perform tests. If the results of this
program, which is scheduled to be
completed laler in 1979. show that the
regulations or enforcement policies can
be revised to lower costs, such revisions
will be adopted.
Hydrocarbon Emissions
While the principal pollutant
associated with asphalt concrete
production is particulate matter, the
trend nqted previously toward dryer-
drum mix plants has raised question as
to the significance of hydrocarbon .
emissions from these facilities. In the
dryer-drum mix plant, drying of the
aggregate as well as mixing with asphalt
and additional Tines takes place within a
rotary drum. Because the drying takes
place within the same container as the
mixing, emissions are partly screened by
the curtain of asphalt added so that the
uncontrolled particulate emissions from
the dryer are lower than from
conventional plants. In contrast, it has
been reported that the rate of
hydrocarbon emissions may be
substantially higher than from
conventional plants. However, data
Decently reported from one test in a
plant equipped with fabric filters
showed only traces of hydrocarbons in
dust and condensate and did not *
support this suggestion. Thus, while
these data do not indicate a need to
revise the standard, more definitive data
are needed on hydrocarbon emission
rates and related process variables. This
has been identified as an area for
further research by EPA.
An additional source of hydrocarbon
emissions in the asphalt*industry is the
use of cutback asphalts. Although not
directly associated with asphalt
concrete plants, this represents a
significant source of hydrocarbon
emissions. As such, the need for
possible standards of performance
pertaining to use of cutback asphalt was
rasied in this review. The term cutback
asphalt refers to liquified asphalt
products which are diluted or cutback
by kerosene or other petroleum
distillates for use as a surfacing
material. Cutback asphalt emits
significant quantities of hydrocarbons—
at a high rate immediately after
application and continuing at a
diminishing rate over a period of years.
It is estimated that over 2 percent of
national hydrocarbon emissions result
from use of cutback asphalt.
The substitution oT emulsified
asphalts, which consist of asphalt
suspended in water containing an
emulsifying agent, for cutback asphalt
nearly eliminates the releasejof volatile
hydrocarbons'from paving operations.
This substitute for petroleum distillate is
approximately 98 percent water and 2
percent emulsifiers. The water in
emulsified asphalt evaporates during
curing while the non-volatile emulsifier
is retained in the asphalt
Because cutback asphalt emissions
result from the use of a product rather
'than from a conventional stationary
source, the feasibility of a standard of
performance, is unclear and the Agency
has no current plans to develop such a
standard. However. EPA has issued a
control techniques guideline document.
Control of Volatile Organic Compounds
from Use of Cutback Asphalt (EPA-4SO/
2-77-037) and is actively pursuing
control through the State
Implementation Plan process in areas
where control is needed to attain-
oxidant standards. Because of area-to-
area differences in experience with
emulsified asphalt, availability of ~
suppliers, and ambient temperatures, the
Agency believes that control can be
implemented effectively by the States.
Asphalt Recycling Plants
A process for recycling asphalt paving
by crushing up old road beds for
reprocessing through direct-fired asphalt
concrete plants has been recently '
implemented on an experimental basis.
Plants using this process, which uses
approximately 20 to 30 percent virgin
material mixed with the recycled
asphalt, are subject to the standard and
at least two have demonstrated
compliance. However, preliminary
indications are that the process may
have difficulty in routinely attaining the
allowable level of perticulate emissions
and/or that the cost of control may be
higher than a conventional process. The
partial combustion of the recycled
asphalt cement reportedly produces a
blue smoke more difficult to control than
the mineral dusts of plants using virgin
material.
It is EPA's conclusion that there is no
need at this time to revise the standard
as it affects recycling, due to its limited
practice and due to the data showing
that compliance can be achieved at
facilities which recycle asphalt.
However, this-matter is being studies
further under the previously noted study
by an EPA contractor.
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Federal Register / Vol. 44. No. 171 / Friday, August 31, 1979 / Rules and Regulations
Educational Program for Owners and
Operator*
The asphalt industry consists of a
large number of facilities which in many
cases are owned and operated by small
businessmen who are not trained or
experienced in the operation, design, or
maintenance of air pollution control
equipment. Because of this, the need to
comply with emission regulations, and
the changing technology in the industry
(i.e., the introduction of dryer-drum
plants, recycling, the possible move
toward coal as a fuel, and the use of
emulsions), the need for a training and
educational program for owners and
operators in the operation and
maintenance of air pollution control
equipment has been voiced by industry.
This offers the potential for cost and '
energy savings along with reduced
pollution.
To meet this need. EPA's Office of
Enforcement, in cooperation with the
National Asphalt Paving Association.
conducted a series of workshops in 1978
for asphalt plant owners and operators.
Only limited future workshops are
currently planned. However. EPA will
consider expansion of the programs if •
continued need exists.
Adminiitrotor
F* Dm. rfr^mn nw •-*•->« MI ••)
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EPA-450/3-79-014
A Review of Standards
of Performance for New
Stationary Sources -
Asphalt Concrete Plants
by
Kathyrn J. Brooks, Edwin L. Keitz, and John W. Watson
Metrek Division of the MITRE Corporation
1820 Dolley Madison Boulevard
McLean, Virginia 22102
Contract No. 68-02-2526
EPA Project Officer: Thomas Bibb
Emission Standards and Engineering Division
Prepared for
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Air, Noise, and Radiation
Office of Air Quality Planning and Standards
Research Triangle Park, North Carolina 27711
June 1979
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This report has been reviewed by the Emission Standards and Engineering
Division, Office of Air Quality Planning and Standards, Office of Air, Noise
and Radiation, Environmental Protection Agency, and approved for publica-
tion . Mention of company or product names does not constitute endorsement
by EPA. Copies are available free of charge to Federal employees, current
contractors and grantees, and non-profit organizations - as supplies permit
from the Library Services Office, MD-35, Environmental Protection Agency,
Research Triangle Park, NC 27711; or may be obtained, for a fee, from the
National Technical Information Service, 5285 Port Royal Road, Springfield,
VA 22161.
Publication No. EPA-450/3-79-014
11
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ABSTRACT
This report reviews the current Standards of Performance for New
Stationary Sources: Subpart I - Asphalt Concrete Plants. Emphasis
is given to the state of control technology, extent to which plants
have been able to meet current standards, experience of representatives
of industry and of EPA officials involved with testing and compliance,
economic costs, environmental and energy considerations, and trends
in the asphalt industry. Information used in this report are based
upon data available as of June 1978. Recommendations are made for
possible modifications and additions to the standard, including future
studies needed of unresolved issues.
iii
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ACKNOWLEDGMENT
The authors wish to acknowledge Ms. Sally Price who contributed
so much to the preparation of this document. Her time and patience
in editing and overseeing the preparation of this report is greatly
appreciated.
iv
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TABLE OF CONTENTS
LIST OF ILLUSTRATIONS ix
LIST OF TABLES x
1.0 EXECUTIVE SUMMARY 1-1
1.1 Overview of Asphalt Concrete Industry 1-1
1.2 Control Technology Available 1-2
1.3 Test Results 1-3
1.4 Possible Changes: Analysis, Conclusions and
Recommendations 1-5
1.4.1 Industry Trends 1-5
1.4.2 NSPS for Particulates 1-5
1.4.3 NSPS for Opacity 1-5
1.4.4 Fugitive Emissions 1-6
1.4.5 Monitoring 1-6
1.4.6 Formal Particulate Testing 1-6
1.4.7 Other Pollutants 1-7
1.4.8 Future Work 1-8
2.0 INTRODUCTION 2-1
3.0 CURRENT STANDARDS FOR ASPHALT CONCRETE PLANTS 3-1
3.1 Facilities Affected 3-1
3.2 Controlled Pollutants and Emission Levels 3-2
3.3 Compliance Testing 3-3
3.4 Terms Applicable to Asphalt Concrete Plants 3-3
3.5 Regulatory Basis for Waivers 3-4
4.0 STATUS OF CONTROL TECHNOLOGY 4-1
4.1 Scope of Industrial Operations 4-1
4.1.1 Nature of Present and Projected Plant
Operations 4-1
4.1.2 Geographic Distribution of Asphalt Plants 4-3
4.1.3 Plant Size Capacity 4-6
4.1.4 Summary 4-12
4.2 Control Methods to Meet NSPS 4-14
4.2.1 Overview 4-14
4.2.2 Types Available 4-18
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TABLE OF CONTENTS (Continued)
Page
4.2.3 Efficiencies Achieved 4-25
4.2.4 Operation of Controls in Asphalt Plants 4-26
4.2.5 Control System Costs 4-33
4.3 Comparison of Achievable Levels with NSPS 4-38
4.3.1 Best Available Control Technology 4-38
4.3.2 Effect of Different Control Levels 4-40
4.4 Energy Needs and Environmental Effects 4-41
4.4.1 Energy Requirements 4-41
4.4.2 Environmental Effects 4-46
5.0 INDICATIONS FROM TEST RESULTS 5-1
5.1 Test Coverage in Regions 5-1
5.2 Analysis of Test Results 5-4
5.2.1 Particulates 5-4
5.2.2 Opacity 5-12
6.0 ANALYSIS OF POSSIBLE REVISIONS TO NSPS 6-1
6.1 Source and Nature of Revisions 6-1
6.2 Industry Development and Trends 6-1
6.2.1 Control Devices 6-2
6.2.2 Dryer-Drum Mix Plants 6-3
6.2.3 Asphalt Recycling Plants 6-7
6.2.4 Hot Water Emulsions 6-8
6.3 Levels for Particulate Emissions 6-9
6.3.1 Variables Affecting Compliance 6-9
6.3.2 Environmental Considerations 6-17
6.3.3 Effects on the Asphalt Industry 6-21
6.4 Levels for Opacity .6-23
6.5 Fugitive Emission Control 6-24
6.6 Changes in Tests and Procedures 6-27
6.6.1 Monitoring Requirements 6-27
6.6.2 Production Penalty 6-29
vi
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TA5LE OK CONTENTS U'ont
Page
6.6.3 Exemptions for Small Plants 6-31
6.6.4 Waiving of Particulate Tests 6-35
6.7 Control of Other Pollutants 6-36
6.8 Use of Liquified Asphalt Cutbacks 6-38
7.0 CONCLUSIONS 7-1
7.1 NSPS for Particulate Emissions 7-1
7.1.1 Retention of Present Level 7-1
7.1.2 Justification for Retention 7-1
7.1.3 Clarification of Items 7-3
7.2 NSPS for Opacity 7-4
7.2.1 Justification for Retention 7-4
7.2.2 Actual Correlation Between Opacity and
Particulate Emissions 7-5
7.3 Testing Procedures 7-7
7.3.1 Waiving of Formal Particulate Testing 7-7
7.3.2 Production Penalty 7-8
7.4 Control of Other Pollutants 7-9
7.4.1 Pollutants Involved 7-9
7.4.2 HC Rates from Drum Mix Plants 7-9
7.4.3 HC Emissions from Cutbacks 7-9
7.4.4 Emissions from Recycling Plants 7-10
7.4.5 Emissions from Hot Water Emulsion Mixes 7-10
8.0 RECOMMENDATIONS 8-1
8.1 Specific Changes in Regulations 8-1
8.1.1 Current Levels of Pollutants 8-1
8.1.2 NSPS Applied to Emission of Other Pollutants 8-1
8.1.3 Enforcement Policy 8-2
8.2 Areas of Further Investigation 8-2
vii
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TABLE OF CONTENTS (Concluded)
8.2.1 Percent of Opacity and Level of Particulates
8.2.2 Determination of Uncontrolled HC Emissions
from Drum Mix Plants
8.2.3 Technology for Development and Use of
Improved Control Devices 8-3
8.2.4 Control of Particulates from Recycling Plants 8-4
8.2.5 Control of Emissions from Hot-Water Emulsions 8-4
8.2.6 Standards for Cutbacks 8-5
9.0 REFERENCES 9-1
viii
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LIST OF ILLUSTRATIONS
Figure Number Page
4-1 Asphalt Hot Mix Production, 1965-1985 4-4
4-2 Geographic Distribution of Existing
Asphalt Concrete Plants 4-7
4-3 1976 Vehicle Registration in Each Region 4-8
4-4 Population of United States and Canada 4-9
4-5 Regional Percentages of Existing Asphalt
Concrete Plants Subject to NSPS 4-10
4-6 Regional Average Operating Capacity of
Asphalt Concrete Plants Subject to NSPS 4-11
4-7 Ratio of Mobile Asphalt Concrete Plants
to Total Number in Each Region 4-13
4-8 Asphalt Hot Mix Industry 4-15
4-9 Dryer Dust Loading as Fuction of Percent
of Fines Input and Drum Gas Velocity 4-17
4-10 Venturi Scrubber Fractional Efficiencies
for Various Pressure Drops 4-24
4-11 Materials Flow for Representative Asphalt
Plant (Batch or Continuous Mix) 4-28
4-12 Typical Flow in a Dryer-Drum Mix Asphalt
Plant 4-34
5-1 Results of Opacity Tests 5-13
6-1 Relation Between ACF and DSCF 6-12
6-2 Tons Per Hour Capacity at Different
Moisture Content (For Specific Dryer
Operating at Constant Temperature) 6-14
6-3 Increase in DSCF/Ton at Different
Moisture Content (With Use of No. 2
Fuel Oil, Dryer Exhaust at 350°F) 6-15
ix
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LIST OF TABLES
Table Number Page
4-1 Integration of Company Operations 4-2
4-2 Asphalt Concrete Plants Subject to NSPS 4-5
4-3 Collection Efficiencies as Function of
Particle Size (-200 Mesh; 4-27
4-4 Estimated Costs for Control Systems for
Representative Plant Sizes 4-36
4-5 Additional Energy Requirements for Plants
Using Venturi Scrubbers 4-43
4-6 Estimated Reduction in Particulate Emissions
from NSPS (New and Modified Asphalt Plants) 4-48
4-7 Estimated Additional Emission of Pollutants
from Increased Energy (Horsepower) Require-
ments for Venturi Scrubbers 4-50
5-1 MITRE/Metrek Survey of NSPS Test Data 5-2
5-2 Distribution of Particulate Test Results
(Averages) by Control System 5-5
5-3 Emission Rates for Dryer-Drum Mix Plants 5-9
6-1 Asphalt Concrete Uncontrolled Emission
Factors, (Kilograms of Particulates/Metric
Ton Asphalt Product) 6-5
6-2 Contribution of Asphalt Hot Mix Industry
to National Emissions of Other Pollutants 6-37
x
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1.0 EXECUTIVE SUMMARY
The objective of this report is to review the New Source Perfor-
mance Standard (NSPS) for asphalt concrete plants in terms of develop-
ments in control technology, economics and new issues that have
evolved since the original standard was promulgated on March 8, 1974.
Possible revisions to the standard are analyzed in the light of com-
pliance test data available for plants built since the promulgation of
the NSPS. The NSPS review includes the particulate standard (current-
ly 90 milligrams/dry standard cubic meter (mg/dscm) or 0.04 grains/dry
standard cubic foot (gr/dscf) and the opacity standard (currently less
than 20 percent). The following paragraphs summarize the results and
conclusions of the analysis as well as the recommendations for future
action.
1.1 Overview of Asphalt Concrete Industry
The asphalt concrete industry, which currently consists of about
4500 plants, is widely dispersed throughout the nation. Plant loca-
tions correlate well with populations and numbers of motor vehicles.
Plants are stationary (60 percent), mobile (20 percent) or trans-
portable (20 percent), i.e., easily taken down, moved and reassembled.
Types of plants include batch-mix (91 percent), continuous mix 6.5
percent) or dryer-drum mix (2.5 percent). The dryer-drum plants,
which are becoming increasingly popular, differ from the others in
that drying of the aggregate and mixing with the liquid asphalt both
1-1
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take place in the same rotary dryer. It is estimated that within the
next few years, dryer-drum (drum-mix) plants will represent up to 85
percent of all plants under construction.
Current national production is about 263 to 272 million metric
tons (Mg)/year (290 to 300 million tons), with a continued rise in the
future. EPA estimates 100 new and 50 modified plants become subject
to NSPS each year.
Most plants have a mixer capacity of under 218 Mg (240 tons)/
hour with the average being 160 Mg (176 tons)/hour. Operation is
seasonal, with plants averaging only 666 hours/year although many
operate more extensively.
1.2 Control Technology Available
The largest source of particulate emissions is the rotary dryer.
The exit gas carries small particles of the mineral aggregate that
makes up over 90 percent of the asphalt concrete product. Emissions
also occur from screens, elevators and weigh hoppers. Both particu-
lates and opacity in the exit gas reflect the presence of fine par-
ticles. Both dry (fabric filters) and wet (scrubbers) collectors are
used for particulate control. Although many plants use primary col-
lectors for large particles and more efficient secondary collectors
for fines, recent experience supports the use of a single collector
that may be either a baghouse (used in 40 percent of the plants) or a
high-efficiency scrubber (used in 24 percent of the plants).
1-2
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A principle underlying NSPS is the establishment of the best
technological system (BTS) of continuous emission reduction, taking
into consideration costs, and non-air quality health and environmental
impacts. For particulate control from asphalt concrete plants, two
control systems qualify as BTS. These are the fabric filter system
and the high-energy scrubber of the variable-throat venturi-type oper-
ated at a sufficient level of energy to provide efficient dust removal
equivalent to that of a fabric filter. It is important to note that
not all systems of these types automatically achieve control at or be-
low the NSPS level. The systems selected must be properly designed,
installed, operated and maintained in order to ensure NSPS compliance.
The reduction in particulate emissions is estimated to be 7700 Mg
(8500 tons) each year from plants that have become subject to the
standard in that year. Thus, in 1978 a reduction of approximately
30,800 Mg (3400 tons) is estimated to have occurred. This reduction
has been achieved at a cost (capital plus operating) to owners of
about 15 to 24 cents/Mg of product (14 to 22 cents/ton), depending
principally upon the control system employed, the extent to which
fines are recycled, and the plant size.
1.3 Test Results
Quantitative data from 72 tests conducted for compliance with
NSPS were made available by EPA regional personnel with the aid of the
Compliance Data System (CDS). About three-fourths of the tests for
particulate emissions showed rates less than NSPS. Of the 26 tests
1-3
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for which detailed results on opacity were reported, 21 showed a
percentage less than the 20 percent NSPS level.
Specific equipment design, operating status and condition of con-
trol systems were not available. In addition, for 13 of the 72 tests
even the general type of control system was not identified. Of the 47
that identified as either a baghouse or venturi scrubber, over 80
percent achieved compliance. Fourteen of the tests (four of which had
unspecified control systems) showed emissions of less than 90 mg/dscm
(0.04 gr/dscf) but higher than 68 mg/dscm (0.03 gr/dscf), which is the
standard in a few states.
The test results comprise a sample large enough to support valid
statistical inference as to the state of control systems as installed
and operated. It may be estimated on this basis that the average
percentage of baghouse and venturi scrubber systems which actually
achieve compliance under the broad range of conditions represented in
the tests is between about 69 and 91 pecent. The fraction could be
less than two-thirds, at the 99 percent confidence level. No data are
available to support analysis of the conditions under which some
plants with baghouses or venturi scrubbers achieved significantly
lower emission rates than others.
The small sample of data and the broad tolerances within which
opacity readings were reported do not permit detailed analysis of the
results. However, the consensus of EPA regional officials is that the
particulate requirement dominates that for opacity.
1-4
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1.4 Possible Changes: Analysis, Conclusions and Recommendations
1.4.1 Industry Trends
The most important development in the ashphalt concrete industry
regarding emissions is the increased usage of dryer-drum mix plants,
which is predicted to account for approximately 85 percent of the new
plants in the next few years. These plants provide an uncontrolled
emission rate for particulates that appears to be lower by one or more
orders of magnitude than either the batch or the continuous-mix
plants. However, because of the nature of the process, the hydro-
carbon emission rate from the dryer-drum plant may be higher than the
rate from conventional plants.
1.4.2 NSPS for Particulates
The current NSPS of 90 mg/dscm (0.04 gr/dscf) for particulate
emissions is being satisfactorily met. No basis exists for relaxing
the standard. However, it is also concluded that no change should be
made in current NSPS at the present time for the following reasons:
1. Test results show that although many plants are meeting NSPS
with currently available BTS, the margin of compliance is too
small to justify tighter standards.
2. The possible environmental gain would be slight.
1.4.3 NSPS for Opacity
Stricter standards for opacity are feasible but unwarranted. The
opacity standard is, by intent, set at a level which will be achieved
by any source which does not exceed the particulate mass standard.
Thus, meeting the NSPS particulate level implies that opacity will be
1-5
-------
less than the current maximum of 20 percent and changing the NSPS for
opacity would, therefore, not in itself reduce particulate emission.
Hence, any environmental gain would be minor and not worth the ad-
ditional increased administrative and procedural efforts.
1.4.4 Fugitive Emissions
A major source of fugitive emissions in batch and continuous
process plants is now controlled by venting to the control system
emissions from screens, elevators, weighing and handling, and the
dryer. No NSPS for additional control of fugitive emissions is
considered to be warranted at present.
1.4.5 Monitoring
The intermittent nature of the asphalt concrete industry makes it
a difficult process to monitor. Periodic monitoring would be techno-
logically useful but practical constraints dominate the situation. The
purchase, installation, operating and maintenance costs of monitors is
relatively prohibitive. In addition, skilled technical operators are
not available at the plants. Additional regulations to require moni-
toring are not warranted at this time.
1.4.6 Formal Particulate Testing
A significant impetus has been generated by some EPA regional per-
sonnel and asphalt concrete plant owners to consider the elimination
of formal particulate testing for small plants (plants of less than
150 tons/hr), which have well designed operational control systems of
the types that are known to be capable of meeting NSPS. The result
1-6
-------
would be substantial savings to the industry and minimal environmental
risk. There are minimal risks associated with elimination of certi-
fication testing for small plants. However, there are considerations
militating against a policy that does not require testing for plants
which have proven control systems. The most important of these is the
fact that test data show that the mere presence of a fabric filter or
venturi scruber system does not guarantee compliance with NSPS. The
particulate test is a way of ensuring that the control system vendor
has provided a well designed system that complies with the NSPS under
actual operating conditions.
1.4.7 Other Pollutants
Other pollutants (NOX, SC>2, HC and CO) are emitted in very
small amounts when compared with:
• Total national emissions
• Rates achieved by controlled industries
• Rates for particulate emissions, even under current NSPS.
No apparent need exists at this time to consider NSPS for emis-
sions of NOX, S02, or CO from any plant or for HC emissions from
batch or continuous plants. However, the unknown rate of HC emissions
from dryer-drum plants should be determined, since it may be higher
than that from other processes.
The largest and most significant source of HC emissions from the
asphalt industry is in application of asphalt diluted with volatile HC
1-7
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fluids, i.e., "cutback." Emissions from this source continue for more
than 3 years after the asphalt is placed as pavement. The use of
water-based emulsified asphalt wherever feasible would reduce HC
emissions and achieve energy savings. While a cutback standard would
not apply to asphalt plant emissions per se, EPA should investigate
regulatory approaches to controlling emissions from this source.
1.4.8 Future Work
Certain technical areas relating to NSPS for asphalt concrete
plants should be investigated. It is recommended specifically that
further development activities address the rate of uncontrolled HC and
particulate emissions from dryer-drum plants, and investigate the
possiblity of less costly control devices to achieve NSPS for
dryer-drum plants.
Investigation of these areas would be useful in any future con-
sideration of possible new or modified NSPS for asphalt concrete
plants.
1-8
-------
2.0 INTRODUCTION
The Clean Air Act of 1977 requires that the NSPS for control of
emissions from designated facilities be reviewed every 4 years. Such
review may lead to revision of the NSPS or of the regulations govern-
ing them as presented by the U.S. Environmental Protection Agency.
This report analyzes current NSPS for asphalt concrete plants with re-
spect to the adequacy of current standards, the need for their revi-
sion, and the probable effects of the standards on the industry and
on the emissions generated. Finally, recommendations are developed
for EPA.
The levels of performance achievable under the best technologi-
cal system (BTS) of continuous emission reduction are compared with
existing NSPS in Section 4.3. Estimated energy needs, environmental
effects produced by emission controls, and potential effects on in-
dustrial operations are also considered. Results of testing emis-
sions from asphalt plants under NSPS are analyzed based on detailed
information obtained from some 70 tests, primarily for particulate
emissions, which were monitored by EPA regions and/or state agencies.
Possible revisions to the standards are analyzed with attention
given to the recommendations submitted by personnel in the 10 EPA
regions. Factors examined are changes in acceptable emission levels,
additions to the list of pollutants controlled, process facilities
from which emissions are measured and controlled, and regulations
governing testing and monitoring procedures.
2-1
-------
The probable effects of changes in standards and/or associated
regulations with respect to industrial trends and possible research
and development needs created by process or control changes are pre-
sented as conclusions. Specific recommendations are made regarding
whether standards and/or regulations should be changed or retained,
as well as unresolved issues to be addressed.
2-2
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3.0 CURRENT STANDARDS FOR ASPHALT CONCRETE PLANTS
3.1 Facilities Affected
An asphalt concrete plant may be stationary, transportable, or
mobile. The stationary plants are permanent fixtures that can not be
moved. All asphalt concrete plants built before 1925 are stationary
plants. The term stationary has been expanded in the current termin-
ology to include transportable plants that have been built within the
past 50 years (NAPA, 1978). Transportable plants are modular units
that permit easy disassembly, relocation and reassembly. The trans-
portable plant is not to be confused with the mobile unit (often
referred to as portable) which is actually constructed on wheels
(NAPA, 1978).
Each asphalt concrete plant planned for, under construction, or
under modification as of June 11, 1973, is subject to the NSPS listed
in 40 CFR 60. Plant facilities controlled include dryers; systems
for screening, handling, storing, and weighing hot aggregate; systems
for loading, transferring, and storing mineral filler; systems for
mixing asphalt concrete; and/or systems for loading, transferring,
and storing that can be associated with emission control systems.
An asphalt concrete plant is defined as any facility that is used
to manufacture asphalt concrete by heating and drying aggregate and
mixing the aggregate with asphalt cements (40 CFR 60.91). Plants
planned or constructed prior to the proposal of the standards are
exempt from the regulations unless a physical change to the plant
3-1
-------
causes an increase in the amount of air pollutants emitted, or unless
the plant qualifies as a reconstruction. Routine maintenance, repair
and replacements; relocation of a transportable plant or of a mobile
plant; change of aggregate; and transfer of ownership are not consid-
ered to be modifications that require an existing plant to comply
with the standard (40 CFR 60.14).
3.2 Controlled Pollutants and Emission Levels
The pollutants to be controlled by asphalt concrete plants are
particulate emissions. The standards for asphalt concrete plants
were first proposed to be 68 mg/dscm which is equivalent to 0.03
gr/dscf for particulate emissions and 10 percent for opacity
(40 CFR 60.1). After proposal and evaluation of comments presented
by the asphalt industry and others, the standards were made slightly
less stringent:
On or after the date on which the (required)
performance test... is completed, no owner or
operator...shall discharge or cause the dis-
charge into the atmosphere from any affected
facility any gases which:
(1) Contain particulate matter in excess of
90 mg/dscm (0.04 gr/dscf).
(2) Exhibit 20 percent opacity or greater
(40 CFR 60.92).
The opacity standards help an operator of an asphalt concrete
plant to determine whether his particulate emission control equip-
ment is operating and maintained properly. An observed opacity of
more than 20 percent is an indication that the particulate emissions
standard of 90 mg/dscm may be violated (39 FR 9308, March 8, 1974).
3-2
-------
3.3 Compliance Testing
Performance tests to verify compliance with particulate and
opacity standards for asphalt concrete plants must be conducted
within 60 days after the plant has reached its full capacity produc-
tion rate, but not later than 180 days after the initial startup
of the facility. Unless exceptions are approved by EPA, each per-
formance test consists of three hour-long runs with a sampling rate
of at least 0.9 dscm/hr (0.53 dscf/min). The standard applies to
the arithmetic mean of the three runs (40 CFR 60.8).
No continuous monitoring requirement currently exists for
particulate NSPS for asphalt concrete plants.
3.4 Terms Applicable to Asphalt Concrete Plants
Several terms that apply to asphalt concrete plants are de-
fined by 40 CFR 60 and are listed below.
• Affected facility - with reference to a stationary source,
any apparatus to which a standard is applicable.
• Commenced - an owner or operator has undertaken a
continuous program of construction or modification or
an owner or operator has entered into a contractual
obligation to undertake and complete, within a reasonable
time, a continuous program of construction or modification.
• Modification - any physical change in, or change in the
method of operation of, an existing facility which increases
the amount of any air pollutant (to which a standard applies)
emitted into the atmosphere by that facility or which results
in the emission of any air pollutant (to which a standard
applies) into the atmosphere not previously emitted.
• Opacity - the degree to which emissions reduce the
transmission of light and obscure the view of an object
in the background.
3-3
-------
• Particulate matter - any finely divided solid or liquid
material, other than uncombined water, as measured by
Method 5 of Appendix A to this part or an equivalent
or alternative method.
• Reconstruction - the replacement of components of an
existing facility to such an extent that:
(1) The fixed capital cost of the new components exceeds
50 percent of the fixed capital cost that would be
required to construct a comparable entirely new
facility, and
(2) It is technologically and economically feasible to
meet the applicable standards set forth in this part.
• Run - the net period of time during which an emission
sample is collected. A run may be either intermittent
or continuous.
• Shutdown - the cessation of operation of an affected
facility for any purpose.
• Startup - the setting in operation of an affected facility
for any purpose.
3.5 Regulatory Basis for Waivers
Operations during periods of startup, shutdown, and malfunction
shall not constitute representative conditions of performance tests.
Such operations are thus exempt from the standard. In addition,
when systems of emission reduction which are meeting the mass
standard do not meet the opacity limits, the source is exempt
from the opacity standard at that time (39 FR 9309, March 8, 1974)
and an ad hoc opacity standard will be established for that plant.
3-4
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4.0 STATUS OF CONTROL TECHNOLOGY
4.1 Scope of Industrial Operations
4.1.1 Nature of Present and Projected Plant Operations
For the past 50 years most asphalt concrete plants have
been modularly constructed so that they can be transported from
one location to another. These transportable plants can be dis-
assembled for movement. Mobile plants are constructed on wheels.
Sixty percent of all asphalt concrete plants are transportable,
20 percent are mobile, and the remaining 20 percent are stationary
units (NAPA, 1977).
In 1976, 64 percent of the transportable and stationary plants
ranged from 109 Mg (120 tons)/hr capacity production rate to 218 Mg
(240 tons)/hr. Twenty-nine percent of the mobile plants fell in this
size range, while 30 percent were continuous mixer units. Sixty-one
percent of hot mix asphalt concrete plants had hot storage (surge)
facilities, of which 54 percent had a production capacity of under
181 Mg (200 tons) and 46 percent a capacity of over 181 Mg (200 tons)
(NAPA, 1977). The surge facility makes it easier for a plant to
operate continuously throughout the duration of a test, with no
problems encountered in the three separate test runs required. Most
asphalt concrete companies place (lay) their own hot mix (Table 4-1),
and approximately 16 percent operate gravel pits or quarries.
Approximately 16 percent produce Portland cement concrete in addition
to asphalt concrete.
4-1
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TABLE 4-1
INTEGRATION OF COMPANY OPERATIONS
Operation
Produces hot mix asphalt
Places (lays) hot mix asphalt
produced by your company
Owns pit or quarry
Produces Portland cement concrete
Company is contractor for:
Road construction
Other types of construction
Distributes asphalt emulsion
Distributes liquid asphalt
Number of
1975
293
264
152
48
254
159
44
44
Companies
1976
299
269
153
46
260
160
45
46
Source: NAPA, 1977.
As of April 1977 there were an estimated 4539 transportable,
stationary and mobile asphalt concrete plants operating in the U.S.
(JACA Corp., 1977). During a comprehensive study, 3579 of these were
formally identified by JACA Corporation. The EPA Compliance Data
System (CDS)* has formally identified 1751 plants. Thus, a consider-
able discrepancy between the JACA list and the CDS is evident. Only
486 (13.2 percent) of those found by JACA Corporation were considered
subject to NSPS. This figure compares with an informal estimate by
* CDS is a computerized management information system operated by
EPA for tracking compliance and enforcement information pertaining
to all facilities subject to NSPS, National Emission Standards for
Hazardous Air Pollutants (NESHAPS) and/or State Implementation
Plans (SIPs).
4-2
-------
NAPA of 15 percent subject to NSPS (NAPA, 1978). As of February 8,
1977, 15 asphalt concrete plants (Table 4-2) were identified in the
CDS as being either planned or under construction. This does not
mean that no others are under construction or in the planning stage.
The CDS files are known to be incomplete in regard to future plants,
reflecting the fact that regional information often becomes available
on a piecemeal basis. In some cases, information is not received up
until the time of plant operation (MITRE Corp., 1978). The 15 future
plants specifically identified through CDS represent only 10 percent
of the 150 plants estimated to come under NSPS regulations annually
(100 new plants plus 50 modifications per year). This estimate (EPA,
1974) correlates reasonably well with the nearly 500 plants identi-
fied as new in the JACA survey, and with the informal NAPA estimate
(1978) that about 15 percent of the 4500 plants in the U.S. are
subject to NSPS.
Production of asphalt concrete declined in both 1975 and 1976
(NAPA, 1977), but is projected to increase steadily to 1985 (Figure
4-1).
4.1.2 Geographic Distribution of Asphalt Plants
Unlike some industries which tend to be concentrated geographi-
cally, asphalt concrete plants are dispersed throughout the 50 states
Because of the principal uses of asphalt for paving highways, roads,
parking surfaces and the like, the distribution of plants by state
4-3
-------
en
C
o
4-1
a
•H
H
O
^>
a
o
&
Pn
400
300
200
160
1965
1970
1975
YEAR
1980
1985
SOURCE: Khan and Hughes, 1977.
FIGURE 4-1
ASPHALT HOT MIX PRODUCTION, 1965-1985
4-4
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TABLE 4-2
ASPHALT CONCRETE PLANTS SUBJECT TO NSPS
Region
As a Percentage
of all Existing
Plants in the Region3
Plants Planned or Under Construction
As a Percentage of
all Plants in the
Region Known to be
Numberb Subject to NSPS c
I
II
III
IV
V
VI
VII
VIII
IX
X
10.1
11.3
9.3
9.3
10.0
24.3
6.7
21.0
40.7
14.0
2
1
2
0
2
5
2
0
0
1
10.0
2.8
3.3
0
2.5
7.4
16.6
0
0
4.0
Based on JACA Corp., 1977. Not all plants are subject to NSPS.
3MITRE Corp., 1978.
•»
"The ratio of known new plants to plants that actually do come on
line (usually considerably more in number than the number of known
new plants) is about the same for all regions.
correlates reasonably well with highway miles, vehicle miles traveled,
and population. While the effects of emissions from asphalt plants
and from any changes in NSPS or governing regulations may be felt
more in industrial areas and regions of high population density,
these effects will occur nationwide rather than in the few localities
containing most of the plants.
4-5
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Figure 4-2 shows the distribution of existing asphalt plants
within the states in each of these 10 EPA regions. This distribution
can be compared with vehicle registration and population density as
shown in Figures 4-3 and 4-4, respectively. Most asphalt concrete
plants are located in the Northeast, along the Ohio and Mississippi
River basins, and on the West Coast (Figure 4-2). Population densi-
ties and urban areas appear to follow roughly the same pattern
(Figure 4-4). Figure 4-5 presents estimates of regional percentages
of total plants that are subject to NSPS as reflected in the JACA
Corporation survey (1977).
4.1.3 Plant Size Capacity
The operating capacities of asphalt concrete plants range from
36 to 544 Mg (40 to 600 tons)/hr. The most prevalent plant size is
less than 218 Mg (240 tons)/hr (NAPA, 1977). This overall average
has not changed significantly for newer plants due to the large
number of smaller mobile units which are currently operational.
The overall average productivity rate is about 160 Mg (176 tons)/hr
(Khan and Hughes, 1977). Figure 4-6 shows the average operating
capacity of new asphalt concrete plants by region according to CDS.
Plants in Regions I and VIII, for which average operating size was
not available, are estimated to fall within the 145 to 181 Mg (160
to 200 tons) and 255 to 290 Mg (281 to 320 tons)/hr categories,
respectively, based on numbers of plants. The pattern of distribu-
tion shown in the Figures 4-1 through 4-5 suggests that the eastern
4-6
-------
A dot represents five asphalt
concrete plants
aAs identified by JACA Corp., 1977.
FIGURE 4-2
GEOGRAPHIC DISTRIBUTION OF
EXISTING ASPHALT CONCRETE PLANTS8
-------
i
CO
LEGEND
Number of Registered Vehicles3
Less than 6,000,000
6,000,000 to 12,000,000
12,000,000 to 18,000,000
18,000,000 to 24,000,000
24,000,000 to 30,000,000
Based on The World Almanac and Book of Facts 1978.
FIGURE 4-3
1976 VEHICLE REGISTRATION IN EACH REGION
-------
hopleths are drawn on the hnii of county and minor civil division
boundaries All population in urban area* of more than 20,000 hai
been excluded in computing rural densities. Data from United States
Census of 1960 and Census of Canada. 1960.
CANADA 1961
POPULATION OF URBAN CENTERS
* Urban Centers
DENSITY OF POPULATION
INHABITANTS PER UNIT AREA
Source: ESPENSHADE. 1970.
FIGURE 4-4
POPULATION OF UNITED STATES AND CANADA
4-9
-------
As of April 1977. Based on Jaca Corp., 1977.
FIGURE 4-5
REGIONAL PERCENTAGES OF EXISTING3
ASPHALT CONCRETE PLANTS SUBJECT TO NSPS
-------
/ \
LEGEND
| | 145 - 181 metric tons/hr
182 - 218 metric tons/hr
219 - 254 metric tons/hr
255 - 290 metric tons/hrC
291 - 308 metric tons/hr
•a
Region I - size unavailable - size based on previous pattern tendencies.
Region VIII - size unavailable - size based on previous pattern tendencies.
SOURCE: Mitre/Metrek Survey
FIGURE 4-6
REGIONAL AVERAGE OPERATING CAPACITY
OF ASPHALT CONCRETE PLANTS SUBJECT TO NSPS
-------
half of the U.S. (Regions I, II, III, IV, V) has older, smaller,
transportable or stationary plants built before the promulgation
of the NSPS. This pattern also appears in Region IX. These plants
do not operate as efficiently as the never, larger, transportable
units (NERC, 1973), a fact which may explain the need for more
construction of plants in the six regions. Regions VI, VII, and
VIII have relatively large average plant operating capacities, with
less than 25 percent of the existing sources subject to NSPS.
4.1.4 Summary
In summary, most of the newer plants subject to NSPS are
located in Regions IV, V and IX; while a higher percentage of plants
in Regions I, II, VII and X were generally built before 1973 and are
not subject to NSPS.
Mobile asphalt concrete plants are more prevalent in the remote
and less populated areas of the country where vast expanses of land
separate urban areas. On the other hand, "stationary [and trans-
portable] plants are located in urban areas where there is a con-
tinuing market for paving and resurfacing work. Mobile plants are
usually involved in highway projects since they can be ... [easily]
located..." (Khan and Hughes, 1977). Figure 4-7 shows mobile plants
as a percent of the total number of asphalt concrete plants in each
region. When comparing Figures 4-5 and 4-7, it appears that Region
IX has newer and more mobile sources than any other region, with the
exception of Region VIII. Region VI, which contains an average
4-12
-------
I
h-"
OJ
aAs identified by NAPA, 1977.
FIGURE 4-7
RATIO OF MOBILE ASPHALT CONCRETE PLANTS
TO TOTAL NUMBER IN EACH REGION3
-------
proportion of asphalt concrete plants subject to NSPS, has a similar
percentage of mobile plants. Regions X and VII contain few plants
subject to NSPS and have the highest percentage of mobile plants.
Regions I, II, III and IV, which contain the more populated areas,
have fewer mobile asphalt plants; whereas Regions VI, VII, VIII, IX
and X contain a larger percentage of highway systems and nonurban
areas and, therefore, more mobile asphalt units*
It is difficult to be totally'accurate in determining plant
distribution, since both transportable and mobile plants can and
sometimes do cross state and regional boundaries.*
4.2 Control Methods to Meet NSPS
4.2.1 Overview
In asphalt hot-mix production a combination of aggregates,
ranging from small stones to fine particles such as sand, is mixed
with liquid asphalt. There are three major types of processes:
batch, continuous mix, and dryer-drum (drum-mix) (Figure 4-8). In
all three processes, cold-feed aggregate is heated in a rotary dryer
and most of the moisture is carried out by an exhaust fan. After
this operation, hot liquid asphalt is blended with the mineral aggre-
gate to produce the desired product. The batch process now accounts
for over 90 percent of all asphalt production. The mixing takes
* A complete and accurate account of the number of mobile units needs
to be determined. CDS does not differentiate transportable and
stationary from mobile consistently, due to inaccurate data and
inconsistent use by regions.
4-14
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INDUSTRY
PROCESS TYPF
PLANT MOBILITY
MOBILE (1.3%)
DRYER DRUM PROCESS (2.6%)
ASPHALT HOT
MIX PLANTS
PERMANENT3(1.3%)
CONTINUOUS PROCESS (6.6%)
MOBILE (4.3%)
PERMANENT (2.3%)
MOBILE (14.3%)
BATCH PROCESS (90.8%)
FUEL TYPE EMISSION CONTROL TYPE
(0%)
GAS
WET COLLECTOR (0%)
OIL (1.3%)
GAS (0.9%)
-JWET COLLECTOR (1.3%)
BAGHOUSE (0.1%)
OIL (0.4%)
GAS (1.2%)
WET COLLECTOR (0.8%)
BAGHOUSE (0.1%)
-JWET COLLECTOR (0.3%)"
BAGHOUSE (0.7%)
WET COLLECTOR (0.5%)
OIL (3.1%)
BAGHOUSE (1.7%)
~WET~ COLLECTOR (1.4%)
GAS (1.3%)
(0.6%)
COLLECTOR (0.7%
OIL (1.0%)
BAGHOUSE (0.4%)
GAS (0.9%)
-I WET COLLECTOR (0.6%)
BAGHOUSE (0.3%)
-fWET COLLECTOR (0.6%)
OIL (13.4%),B-AGHOUJ?AA-2%> -
1 WET COLLECTOR (9.2%)
BAGHOUSE ""(12.5%)
GAS
PERMANENT (76.5%)
WET COLLECTOR (17.3%)
OIL (46.7%)
BAGHOUSE (19.6%)
WET COLLECTOR (27.1%)
Numbers in parentheses represent
% of total industry
0% indicates no industry response
PLANT MOBILITY
SUMMARY
FUEL TYPE EMISSION CONTROL TYPE
PERMANENT PLANTS (80%) GAS (34%) BAGHOUSE (40%)
MOBILE PLANTS (20%) OIL (66%) • WET COLLECTOR (60%)
SOURCE: Khan and Hughes, 1977.
aPermanent includes stationary and transportable
FIGURE 4-8
ASPHALT HOT MIX INDUSTRY
4-15
-------
place in batches and each batch requires roughly 1 minute. Thus, a
plant with a mixer size of 2.7 Mg (3 tons) has a rated capacity of
163 Mg (180 tons)/hr. In the continuous mix process, aggregate and
liquid asphalt are metered through separate control systems in the
desired proportions on a continuous basis. In both processes, drying
and mixing occur as distinct operations in separated enclosed com-
ponents of the plant. The dryer-drum or drum mix process differs
from these two processes in that drying of the aggregate and mixing
with the liquid asphalt occur in different compartments of a drum
dryer (Khan and Hughes, 1977; NAPA, 1978).
Exhaust gases from the dryer comprise about 80 to 90 percent
of the total gas flow in the system. Most of the dust loading of
particles from the process is contained in the gas from the dryer.
Exit velocities typically range from 2.3 to 4.6 m/sec (450 to 900
ft/min). It has been found that a 50-percent increase in exit gas
velocity will lead to an increase of 125 to 150 percent in dust
carry-out from the dryer (Crim et al., 1971; Barber-Greene, 1976)
as shown in Figure 4-9.
Opacity and particulate loadings both reflect particles in the
emissions from an asphalt concrete plant. The control of particu-
lates basically controls opacity and may be achieved with the use of
one or more devices for trapping or removing the particles.
4-16
-------
9O
M
•vl
LU
CD
£cc
C/) «
QJ LJ
80
70
60
zee
UJ CC
0<
ceo
LU
Q.
50
40
UPPER BOUNDARY
RANGE OF
VALUES
LOWER BOUNDARY
1
2.8 (550)
SOURCE: Robert, J. et al., 1975.
3.0 (600) 3.3 (650) 3.6 (700)
DRUM GAS VELOCITY, m/sec (ft/min)
3.8 (750) 4.1 (800)
FIGURE 4-9
DRYER DUST LOADING AS FUNCTION OF PERCENT
OF FINES INPUT AND DRUM GAS VELOCITY
-------
4.2.2 Types Available
Control devices used in asphalt concrete plants may be classi-
fied as either wet or dry. Dry devices range in complexity from the
simple settling chamber or knockout box, through cyclones utilizing
centrifugal force, to a baghouse using fabric filters. Wet devices
are washers (also commonly called scrubbers) that range in complexity
from the low-energy spray chamber, through centrifugal or cyclonic
wet washers at low to medium energy levels, to the high-energy
venturi scrubbers.* Distribution of control systems among the types
of asphalt plants is shown in Figure 4-8.
4.2.2.1 Dry Collectors. In the settling box the velocity of
the carrier gas is reduced to a point such that gravity causes some
of the particles to fall out of the air stream. This device is
effective only for particles greater than about 40 microns. Cen-
trifugal or cyclone collectors use changes in direction and speed
of the air stream as it passes through an enclosed area to settle
out progressively smaller sized particles. Small diameter cyclone
units operate more efficiently. In dealing with an air volume too
great for a small unit, several small cyclones can be placed in a
parallel operation as a multiple cyclone collector (sometimes termed
a "multi-clone"). The load is, thus, divided among numerous small
cyclones mounted in a common housing. The fine particles recovered
* A discussion of venturi scrubbers and energy levels associated
with high efficiency is given in Section 4.3.1.
4-18
-------
by a dry collector such as a settling box or cyclone unit are
valuable as fines for recycling into the asphalt product. Hence,
such a device has for many years been used in nearly all asphalt
concrete plants as an essential part of production. Although the
efficiency of dry collectors is typically too low to meet current
NSPS particulate levels, they may still be employed as primary
collectors ahead of a more efficient control system termed a second-
ary collector (NAPA, 1975; Danielson, 1973; Barber-Greene, 1976).
As a dry system for meeting NSPS, a bag collector (more pre-
cisely, a "fabric filter" dust collector) is widely used. An esti-
mated 40 percent of all asphalt plants are now fitted with such
devices as shown in Figure 4-8 (Khan and Hughes, 1977). In a bag
collector, the dust-laden exhaust gases from the dryer, as well as
those from so-called ventlines carrying "scavenger air" with dust
particles from other components of the process, are drawn through a
filtering fabric, the fibers of which capture the dust particles.
The filter cloth is most commonly arranged in the form of a cylin-
drical bag to handle the large volume of exhaust gas. The bag is
commonly fitted over cylindrical wire forms called "cages" to
support the bag in operating condition and to give maximum cloth
exposure within minimum space. Effective operation in asphalt con-
crete plants reportedly results from a filter of 14-ounce Nomex,
needled, scrimback felt at an air-to-cloth ratio of 6:1, although
ratios ranging from 9:1 to 4:1 and even lower may be employed.
4-19
-------
A number of bags assembled into a single airtight unit make up
what is termed a baghouse. The dust is trapped on the dirty side
of the bag so that clean air passes out by means of the exhaust fan.
Dust cake deposited on the dirty side of the bags actually aids in
trapping the smaller particles in the exhaust gas, up to a point.
However, removal of the dust cake at regular intervals is required
for effective performance so as to maintain the design exhaust capac-
ity of the fabric filter system. Uninterrupted operations can be
maintained by cleaning only a portion of the baghouse at a time.
Fabric filters or baghouses are generally regarded as the most effi-
cient control system available for asphalt concrete plants under
current technology for removal of particulates (NAPA, 1975, 1978;
Soderberg, 1974; Danielson, 1973; Barber-Greene, 1976).
4.2.2.2 Wet Collectors. Wet collectors or scrubbers introduce
water into the gas stream to condition the fine particles so as to
increase their effective size for easier removal and/or to trap
the particles in a liquid film that washes them away. Wet collector
efficiency is a function of several variables, including resistance
to air flow measured as pressure drop, or the amount of pressure
lost due to friction and condensation between two points, such as
the inlet and outlet of the collector. In general, the higher the
pressure drop the more efficient the wet collector.
4-20
-------
Venturi scrubbers as typically used by asphalt concrete plants
have a pressure drop of about 51 centimeters (cm) (20 in.). A
venturi scrubber consists of a convergent section and a divergent
section. As dust-laden gas enters the convergent section, the con-
striction increases both gas stream velocity and velocity of the
particles relative to the droplets of water interjected at a typical
ratio of about 30 liters (8 gal/min) to each 28.3 m3 (1000 ft3)/
min of gas flow. The high velocity gas stream atomizes the liquid
into a fine mist. Dust is entrapped in the water and the droplets
agglomerate to a relatively large size. In the divergent section,
the dust-laden gas is slowed down. Changes of direction in the gas
flow result in further impaction and agglomeration. In the separator
the liquid is thrown to the walls by centrifugal forces and then
through gravity drains to the bottom. Clean gas passes out through
the upper portion of the separator, while the liquid typically drains
into a settling pond (Kahn and Hughes, 1977; NAPA, 1975).
Venturi scrubbers are often categorized by their operating
characteristics and capabilities. The terms "high gas velocity,"
"medium energy," and "high efficiency" are frequently applied,
but are not defined quantitatively in the available literature.
Although some indication of their range of application is implied
by the following discussion the description is not clear. In the
4-21
-------
background document for NSPS, EPA (1973) stated that "In order to
reduce emissions by about 99.7 percent as required by the proposed
standard, fabric filters or medium energy venturi scrubbers, normally
preceded by a cyclone or multiple cyclone, are used to collect dust
from the dryer." Reported test results include those in which plants
controlled by venturi scrubbers with a pressure drop in the range of
25 to 48 cm (10 to 19 in.) water gauge (WG) emitted particulates at
a rate less than the proposed standard of 90 mg/dscm (0.04 gr/dscf).
Venturi scrubbers with a pressure drop up to 51 cm (20 in.) WG are
common in the asphalt concrete industry. The Scrubber Handbook
(Calvert et al., 1972) cites test data for asphalt concrete plants
in which venturi scrubbers with pressure drops in the range of 35
to 50 cm (about 14 to 20 in.) provided the control system.
Pressure drops in the range of 25 to 51 cm (10 to 20 in.) are
considered to provide an efficiency of about 97 percent for parti-
cles of at least 1 micron (Robert et al., 1975). Indications are
that in the range of 102 cm (40 in.) WG pressure drop, efficiencies
exceeding 99 percent may be attained in removing submicron particles
(Calvert et al., 1972; Robert et al., 1975; Soderberg, 1974; American
Air Filter Co., 1978).
The distinction between "medium" and "high energy" scrubbers
appears to occur with a pressure drop of 51 to 76 cm (20 to 30 in.)
WG. Some sources, however, consider the venturi scrubber to .repre-
sent a "high energy" scrubber, the centrifugal and cyclonic as low
4-22
-------
energy scrubbers, and the orifice as a medium energy wet collector
(Robert et al., 1975). Figure 4-10 shows the efficiencies of
venturi-type scrubbers with specified pressure drops as a function
of particle size and indicates the range of "medium" and "high
energy" scrubbers.
Gas velocities used with venturi scrubbers may range from
61 m/sec (200 ft/sec) to 152 m/sec (500 ft/sec) or as much as
213 m/sec (700 ft/sec). No indication has been found in the liter-
ature of cutoff points for "high velocity" as contrasted with "low"
or "medium velocity" within this range (Calvert et al., 1972;
Robert et al., 1975).
4.2.2.3 Aggregate Size Distribution. Aggregate comprises
more than 90 percent of asphalt hot mix product (Khan and Hughes,
1977). The size distribution of the aggregate entering the dryer
is an important factor in determining what the inlet loading to the
control system will be. At a given velocity of the gas stream only
some of the particles will become airborne, depending on their size,
weight and shape. Because smaller particles become airborne more
easily with the dryer gases than the larger ones, the inlet loading
to the collector is strongly influenced by the amount of mineral
dust in the aggregate (Khan and Hughes, 1977; Robert et al., 1975).
Baghouses are much less sensitive to this variable than venturi
scrubbers.
4-23
-------
**
to
M
O
I
Q
99.9
99.5
99
98
97
95
90
80
70
60
50
LOW
ENERGY
RANGE
HIGH
ENERGY
RANGE
MEDIUM
ENERGY
RANGE
0.1
0.2
0.3 0.4 0.5 0.6 0.8 1
PARTICLE SIZE, microns
Sources: Robert, et al., 1975;
Soderberg, 1974.
8 10
FIGURE 4-10
VENTURI SCRUBBER FRACTIONAL EFFICIENCIES
FOR VARIOUS PRESSURE DROPS
-------
4.2.3 Efficiencies Achieved
Efficiency is expressed throughout this report as
output loading
a percentage - 100 x input loading
The efficiency achievable for any collection device (assuming
proper maintenance and operation) may vary under different conditions.
Wet collector efficiency is affected by the amount of power supplied
in forcing the gas stream through the collector—a function of the
pressure drop, or amount of pressure lost, due to friction and con-
densation between inlet and outlet. An increase in pressure drop
by a factor f times the original value is reflected as an increase
in power required of f2 i.e., increasing the pressure from 41
to 51 cm (16 to 20 in.) or 1.25 times the original value requires
25/16 as much power or an increase of 9/16).
Overall efficiencies may be as low as 60 percent for large-
diameter dry cyclones, as much as 95 percent for small diameter
cyclones and multiclones, and as high as 94 percent for spray type
wet scrubbers (Grim et al«, 1971). Only scrubbers such as the
venturi and the baghouse can generally be relied on to achieve
efficiencies of well over 99 percent in particulate removal. The
higher efficiencies (in the range greater than 99.5 percent) are
reportedly easier to achieve with a baghouse system. The range of
efficiencies for various control devices as a function of particle
4-25
-------
size is given in Table 4-3. Data are limited for the smallest par-
ticles (< 7 microns). None of the references cited in Table 4-3
reported results specifically for asphalt concrete plants. Collec-
tion efficiencies of baghouses for submicron particles reflect a wide
range based on test data from utility and industrial boilers, lime
recovery at a pulp mill, and laboratory studies of fabric performance*
Engineering experience reports increasing reliance on either the bag-
house or the venturi scrubber as a single collection device; however,
dry primary collectors are still used as precleaners and have par-
ticular application in one or both of the following:
• Providing a cost-effective means to filter out particulates
to be recycled as fines for use in the aggregate.
• Reducing the dust-loading on the final collection device by
removing particles of the size and nature which could impede
its operation (e.g., larger particles which form too porous
a cake in a baghouse filter) (NAPA, 1975).
4.2.4 Operation of Controls in Asphalt Plants
A simplified flow diagram with materials balance applicable to
a batch process or continuous mix operation in an asphalt plant is
shown in Figure 4-11. A plant of representative size has been
assumed at 159 Mg (175 tons)/hr of product output. The use of
both primary and secondary collectors for control of emissions is
illustrated. Differences in the dryer drum plant are also explained.
4-26
-------
TABLE 4-3
COLLECTION EFFICIENCIES AS FUNCTION OF PARTICLE SIZE
(-200 Mesh)
Particle Size
(Microns)
>74
>30
>10
>5
>1
>0.5
>0.3
Low-Rests tance
Cyclones
(Percent)
99b
80-90b
50-80b
<20b
Multicones
(Percent)
>99°
95-99°
80-9 5b
40-50b
Wet
Collectors
(Percent)
299 C
299°
97-99°
90-9 6b
50-60b
>50f
Cyclone Scrubbers ,
Wet Fans
(Percent)
>99°
£99°
299°
98-99
95-98b
>50f
High Pressure
Venturis
(Percent)
>99.9°'d
>99.9°'d
99-99. 9°'d
99.99.98
98-99. 7g
95-9S8
90-958
Baghouse
(Percent)
>99.9°'e
>99.9b'e
99-99. 9b>i
97-99. 91*11'1
97-99. 91'1
85_99J >k
70-99 J >k'1
to
-J
e.g. - Gravity Spray Tower.
bNAPA, 1975.
°Khan and Hughes, 1977.
HPatankar and Foster, 1978.
Standard Havens, 1978.
fDanielson, 1973.
Robert et al., 1975, Soderberg, 1974.
Pressure drop of 38 to 51 cm (15 to 20 in.) WG.
Slarmon, 1977.
^Lanib et al., 1978.
SlcKenna, 1974.
^radway and Cass, 1975; 1976.
-------
Discard
or
Recycle
141.3 kg/hr
(311.5 Ib/hr)
ho
00
AGGREGATE
STORAGE
(Coarse +
Fine)
151 Mg/hr
(166 tons/hr)
Primary
Collector
(Cyclone)
(96% Efficiency)
2858 kg/hr
(6300 Ib/hr)
ROTARY
DRYER
0 143 kg/hr
315 Ib/hr)
(7A) 714 kg/hr
(1575 Ib/hr)
3.43
Mg/hr
Ventline
to
r
i
(11A) 1.43
kg/hr
O.15
Ib/hr)
Secondary Collector
(Fabric Filter)
(99% Efficiency)
Secondary Collector
(Venturi Scrubber)
(98% Efficiency)
2.9
kg/hr
6.3
Ib/hr
140.0 kg/hr
308.7 lb/hr)(dry weight)
J Primary or Secondary
D.78 I Collector j
tons/hr) J I
151.16
Mg/hr
Water and Mud
Disposal as
via Settling Pond
147.73
Mg/hr
(162.85
tons/hr)
(166.63
tons/hr)
Vibrating
Screens, Bins,
Weigh Hopper
and Mixer
6.89 Mg/hr
(7.58 tons/
159
Mg/hr
(175 tonsThrT
SOURCE: Khan and Hughes, 1977; NAPA, 1975; Danielson, 1973..
FIGURE 4-11
MATERIALS FLOW FOR REPRESENTATIVE ASPHALT PLANT
(BATCH OR CONTINUOUS MIX)
-------
Aggregate of appropriate mix is fed (see Figure 4-11) into the
rotary dryer (Stream 1) at a controlled rate. The aggregate, which
is generally composed of locally available material, will contain
both coarse-sized crushed rock and fines. Fines typically comprise
less than 10 percent of the total weight (Crim et al., 1971).
Moisture content of the cold aggregate is usually 3 to 5 percent by
weight; however, ranges well above 10 percent are encountered.* The
rotary dryer is an inclined rotating cylinder (usually employing oil
or gas as fuel) into which the aggregate is fed at the raised end and
discharged at the lower end. A dryer exhaust temperature of between
90° and 100°C (-200° and 250°F) is often considered to be optimum,
although temperatures up to 175°C (~350°F) are encountered (Khan and
Hughes, 1977, 1977; NAPA, 1975; Foster, 1977).
The rotary dryer is the principal source of particulate
emissions in a hot-mix asphalt plant (Stream 2). Based on the EPA
emission factor for uncontrolled particulate emissions of 22.5 kg/Mg
(45 Ib/ton) of product (EPA, 1973a) and on the assumption of 80
percent emission contribution by the dryer (Khan and Hughes, 1977;
Danielson, 1973), a 159 Mg (175 ton)/hr plant is estimated to emit
2858 kg (6300 Ib) of particulates per hour from the dryer. In
*The National Asphalt Paving Association (NAPA) provides tables
showing balance between air flow and available heat under various
conditions for ranges of aggregate moisture content between 4 and
15 percent (NAPA, 1975).
4-29
-------
Figure 4-11 these emissions are shown entering the primary collector,
for which a dry cyclone is assumed to be the representative type
operating at a 96 percent efficiency (Khan and Hughes, 1977). In
many plants no primary collector is provided, and all emissions go
to the secondary collector.
The vibrating screens, bins, weigh hopper and mixer are also
sources of particulate emissions which be controlled. These areas
are normally enclosed. The dust emitted is carried by ventline to
the control system (Stream 7A or 7B). The materials balance depicted
in Figure 4-11 is based on the assumption that the ventline emissions
will move through the dry cyclone (Stream 7A) along with emissions
from the dryer. However, in some plants these emissions bypass the
primary collector and go directly to the secondary collector (Stream
7B) (Khan and Hughes, 1977; Danielson, 1973).
Two output streams from the primary collector are also shown
in the figure. The primary collector removes an estimated 3.43 Mg
(3.78 tons)/hr of particulates shown as being recycled (Stream 4)
by a service conveyor back to the process. Here it is combined with
the hot aggregate from the dryer (Stream 3) and hauled (Stream 5) by
a bucket elevator to the vibrating screens. These screens sort the
aggregate to predetermined uniform grades and drop it into an appro-
priate storage bin. Aggregate to be used is weighed and fed into a
mixer. After a few seconds of dry mixing, asphalt is added and the
blended material is discharged (Stream 10) into trucks for delivery.
4-30
-------
The particulate emissions not captured by the primary collector
pass (Stream 6) to a secondary collector. These emissions consist
largely of very fine particles (less than 20 to 30 microns), for which
the primary collector has a relatively low efficiency. This type of
cyclone alone will not suffice to meet current NSPS, but may be used
to facilitate recycling of larger particles (in the range of 74
microns, of which it may remove up to 100 percent) and to improve
performance of the secondary control system. In Figure 4-11 dust-
laden air from the primary collector is shown moving by exhaust fan
to the secondary collector, although performance of some wet collec-
tors may be improved by placing them ahead of the fan (Khan and
Hughes, 1977; NAPA, 1975; Danielson, 1973).
A control device of the type shown in Figure 4-11 is usually
crucial to meeting current NSPS for particulates. In a plant using
a single collector, the device is likely to be either a baghouse
(fabric filter system) or a wet scrubber of the venturi design.
If ventline emissions bypass the primary collector, they enter the
secondary collector directly (Stream 7B) along with the output of
the cyclone.
Figure 4-11 shows typical results with the use of either a
fabric filter (from which output emerges as Streams HA or 12A) or
a venturi scrubber (outputs as Streams 11B and 12B). Efficiencies
hypothesized in the figure are slightly lower than those used as
4-31
-------
typical ratings (Khan and Hughes, 1977) since the emissions entering
the secondary collector contain a high percentage (by weight) of
particles below 30 microns and a significant percentage of particles
in the 5-micron range. Efficiencies of all collectors (baghouses
and Venturis included) diminish when collecting the smaller range
particles as shown in Table 4-3 (NAPA, 1975).
The fines filtered out by a baghouse (fabric filter) collector
(Stream HA) can be recycled along with particles from a primary
collector or they can be discarded as solid waste. A survey of
asphalt plants indicated that 53 percent recycled this material
(Khan and Hughes, 1977). Particles removed by a wet scrubber must
be disposed of as solid waste, typically through use of a settling
pond.
Small changes in the materials balance (Figure 4-11) would be
required under the assumption of only a single collector (venturi
scrubber or fabric filter). Since the 714 kg (1575 lb)/hr of par-
ticulate emissions from the ventline would not be recycled via the
primary collector, a small increase in the amount of material from
storage would be required to offset the difference. Slightly higher
atmospheric emissions from the single collector would be expected,
although a very small increase in efficiencies on an overall weight
basis would be likely as the larger particles would not already
have been removed.
4-32
-------
The outputs postulated for the hypothetical plant in Figure
4-11 from either secondary collector would be expected to meet
current NSPS. A loading of 2.86 kg (6.3 lb)/hour (approximately
735 gr/min) at a flow rate of 520 dscm/min (18,375 dscf/min) would
yield a grain loading of 90 mg/dscm (0.04 gr/dscf). A flow rate of
520 dscm/min (18,375 dscf/min) is not particularly high for a 159 Mg
(175-ton)/hour plant (NAPA, 1975).
The dryer-drum mix plant differs from the unit shown in Figure
4-11 in that the aggregate, mineral fines and asphalt all go directly
from storage into a dryer drum where mixing takes place. A block
diagram of the flow in such a plant is shown in Figure 4-12.
The distribution of types of control systems among asphalt
plants is shown in Figure 4-8.
4.2.5 Control System Costs
Purchase, installation, and operation and maintenance costs of
the various types of control systems increase with the approximate
efficiency of the given system. In general, dry collectors are the
least expensive, although baghouse systems are initially more expen-
sive than wet collectors.
The increase in cost by control system type is not linear
with the increase in range of overall efficiency. The incremental
increase in efficiency provided by the baghouse system is likely
to be relatively expensive as an initial investment.
4-33
-------
t
ATMOSPHERIC
EMISSIONS
SECONDARY
COLLECTOR
RECYCLE
OR
, DISCARD
RECYCLE OR DISCARD
PRIMARY
COLLECTOR
HOT MIX
STORAGE
FACILITIES
AGGREGATE STORAGE
(COARSE + FINE)
DRYER DRUM
TO ASPHALT TRUCK
(PRODUCT)
MINERAL FINES
STORAGE
ASPHALT
STORAGE
FIGURE 4-12
TYPICAL FLOW IN A DRYER-DRUM
MIX ASPHALT PLANT
4-34
-------
However, cost in cents per ton of asphalt product for the
baghouse system can be offset at least partly by the recycling of
valuable fines recovered. These fines would be disposed as solid
waste at the owner/operator's expense when a scrubber is used.
In addition, as energy costs increase, the economics of baghouses
become increasingly attractive.
Theoretical calculations of expected costs to asphalt plants by
EPA (1974) and estimates of expected costs by Grim (1971) as given
in Table 4-4 are partly borne out by limited experimental data. The
costs do not match on the basis of plant-size and dollar per actual
cubic meter per minute (acmm) because the ratio of actual cubic meter
per minute to product output in the observed operating situations was
much higher than the ratio used in EPA's theoretical calculations.
(For a plant of given size, the observed actual cubic meter per
minute was on the order of 1.5 times the EPA estimate.)
One plant operating at 132 Mg (146 tons)/hr was reported as
having a baghouse and fan installed for a total price of $115,000 at
a cost of $91.83/acmm or $2.60 per actual cubic foot per minute
(acfm). The EPA estimate for the same kind of installation in a 136
Mg (150 ton)/hr plant was $79,500 at an estimated cost of $97.13 to
$112.32/acmm ($2.75 to $3.18/acfm). Another plant with a 272 Mg
(300 ton)/hr capacity was reported as using a baghouse installed
4-35
-------
TABLE 4-4
ESTIMATED COSTS FOR CONTROL SYSTEMS FOR REPRESENTATIVE PLANT SIZES
PLANT CAPACITY
Control
Device
Inlet Gas f ACMM
Volume "[ ACFM
Control Efficiency (%)
Equipment Cost
Installation Cost
Total Installed Cost
Investment fACMM
Cost j^ACFM
Comparative Es- /ACMM
timate of cost* 1 ACFM
b
Total Annual Cost
Cost in C/unit fag
Product iTon
136 Metric Tons (150 Tons) /Hour
Fabric Filters
Without Dust
Recovery
708
25,000
99.8
$47,600
20.400
68.000
$96.07
$2.72
$88.30-141.28
$2.5-$4.00
$24,700°
22. Oc
24. 3C
With Dust
Recovery
708
25,000
99.8
$57,300
22,200
79,500
$112.32
$3.18
$88. 30-141. 2«
$2.5-$4.00
d
$22,500
20. Oc
22. OC
Venturi
Scrubber
708
25,000
99.8
$27,700
29,600
57.300
$80.88
$2.29
Not Given
$24,300°
21. 6c
23. 8c
Multi-Centrifugal
Scrubber
708
25,000
96.9
$21,400
26.300
47,700
$67.46
$1.91
$44.15-70.64
$1.25-$2.00
$19,700°
17.50
19. 3C
272 Metric Tons (300 Tons) /Hour
Fabric Filters
Without Dust
Recovery
1416
50,000
99.8
$69,500
29.200
98,700
$69.58
$1.97
$88.30-141.28
$2.5-$4.00
$39.400°
17. 5c
19. 3C
With Dust
Recovery
1416
50,000
99.8
$79,600
31,100
110,700
$78.06
$2.21
$88.30-141.28
$2.5-$4.00
$32,600d
14. 5c
16. OC
Ventur i
Scrubber
1416
50,000
99.8
$48,500
47,600
96,100
$67.81
$1.92
Not Given
$43,200°
19. 2c
21. 3C
Multi— Centrifugal
Scrubber
1416
50,000
98.3
$35,200
41,400
76,600
$54.04
$1.53
$44.15-70.64
$1.25-$2.00
$34,600°
15. 4c
17. DC
SOURCE (Except where otherwise stated): U.S. Environmental Protection Agency, 1974.
aCrim, J. A., et al., 1971.
EPA figures include labor, materials, utilities, depreciation, interest and property taxes.
Q
Includes cost of dust disposal.
value of recovered fines subtracted from annual cost.
-------
at a total cost of $106,000. This cost was $4000 less than the EPA
estimate of $110,000 and much lower in $/acmm ($51.57) or $/acfm
($1.46) than the EPA estimate of $70.06/acmm ($2.21/acfm) (New York
State, 1976; 1977).
Engineering experience indicates that the actual initial costs
of a baghouse system can be up to three times higher than EPA's
estimate of the actual initial costs of a venturi scrubber. Venturi
scrubbers range from $40K and up for initial costs; whereas baghouses
may run well over $100K (NAPA, 1978).
Engineering experience indicates that the cost of a baghouse
system may represent one-fourth to one-third of the plant invest-
ment. This approximation is consistent with EPA predications that
a model plant of 272 Mg (300 tons)/hr with a capital investment
of $354,000 without control equipment would cost an additional
$99,000 to $111,000 for a baghouse system, with the higher price
being for a system that provided dust recovery.
The cost of controls to an asphalt plant must be expanded to
include the cost for formal testing for particulates. Under the
present regulations, this cost represents a one-time charge somewhat
less than 1 percent of the total plant investment. Estimates vary as
to test costs, but range from $2000 to $5000 with an overall average
of about $2500 (NAPA, 1978). This figure is substantially lower
4-37
-------
than the upper level estimate of $10,000 per test, indicated by EPA
(1974). Replies from representative testing firms indicated a range
of $1500 to $2500 for a one-time Method 5 particulate test which can
be concluded in 1 day. This cost estimate does not include retesting
or any indirect expenses incurred by the plant in preparing for and
supporting the test (Valentine et al., 1978; Entropy Environmental,
1978; Snowden, 1978).
4.3 Comparison of Achievable Levels with NSPS
4.3.1 Best Available Control Technology
An important purpose and role of NSPS is the establishment
of a level of efficiency achievable by BTS of continuous emission
reduction (taking into consideration costs, and nonair quality
health and environmental impact). It is generally anticipated that
all plants subject to NSPS will need to be equipped with collector
systems representing BTS. For removal of particulates from asphalt
concrete plants, the NSPS have been set at a level reflecting effi-
ciencies which BTS can achieve.
In the background document discussing the proposed standard,
EPA (1973) stated (as part of the analysis of costs for new plants
of typical size) "Either the fabric filter or the venturi scrubber
will enable a new plant to comply with the proposed standards...."
Particulate and opacity levels for asphalt concrete plants specified
by current NSPS can indeed be met and even exceeded by BTS as repre-
sented in the use of these systems.
4-38
-------
However, the mere fact that a control system is of the fabric
filter or venturi scrubber type does not necessarily mean that it
will represent BTS. Indeed, in reporting test results considered in
formulating the NSPS for particulates, EPA reported (1974) that tests
of two plants equipped with baghouses were not conisdered representa-
tive of good operation and maintenance.
Many authorities consider the fabric filter the ultimate in
particulate control (Soderberg, 1974; Danielson, 1973). Baghouses
are particularly effective in removing the finer particles through
building up a dust cake which then "collects basically all dust
particles irregardless of size" (Soderberg, 1974). The efficiency
of venturi scrubbers against submicron particles is highly dependent
upon the amount of energy supplied (as measured by pressure drop).
Letters on file with EPA have indicated, however, the capability of
manufacturers of both venturi scrubbers and baghouses to provide
equipment meeting the NSPS particulate level (EPA, 1974).
The capability of fabric filters and high-energy venturi scrub-
bers to achieve the efficiency required by NSPS for particulates
in asphalt concrete plants is illustrated in Figure 4-11. The
theoretical calculations, based on typical ratings for the control
systems, are supported by successful implementation of even more
rigid standards in a few states and by reports of the test studies
conducted. Efficiencies of control systems for particulates of
various sizes (fractional efficiencies) are given in Table 4-3.
4-39
-------
4.3.2 Effect of Different Control Levels
To the extent that BTS can regularly provide efficiencies
exceeding those required by current NSPS, a change in levels presents
no problems. However, the cost of installing and operating a control
system is by no means a simple function of the efficiency required of
the device and, hence, of emission levels achievable. For a given
type of aggregate in a specific plant, a decrease in the grain loading
permitted (i.e., a tightening of the standards) may be translated
into higher costs for the industry if scrubbers are used.
The need for a control system that is more costly to install
and/or to operate than one that would otherwise be required (as shown
in the EPA comparisons of a baghouse and venturi scrubber with a less
expensive and less efficient multiple centrifuge). This is of
primary concern when comparing cost of control under NSPS vs no NSPS.
However, it may also play a role in changing the NSPS.
Efficiency achieved by a venturi scrubber is a function of
pressure drop through the device. The attempt to raise efficiency by
increasing water flow rate, or the ratio of water in liters (gallons)
to gas flow in cubic meters (cubic feet/minute), would result in a
nominal increase in water consumption. The extent of the increase
would be limited by the cutoff in efficiency gain above a rate of
1341/m3/min (10 gal/10 ft3/min) (Figure 4-10).
An increase in the amount of particulate removal in a venturi
scrubber is roughly linear with an increase in pressure drop (as the
4-40
-------
gas to be cleansed is forced through the orifice at a faster rate,
for example, by narrowing the throat); whereas the power require-
ments increase as the square of the pressure drop in centimeters
(inches). Thus, an increase in the pressure drop of from 41 to 51 cm
(16 to 20 in.) would raise power requirements to 1.56 times the
original.
4.4 Energy Needs and Environmental Effects
4.4.1 Energy Requirements
The energy requirements for a baghouse (fabric filter) system
represent no appreciable increase over those needed for a centrifuge
or cyclonic system. Under this option, essentially no additional
energy is expended to meet NSPS. However, the energy requirements
for a venturi scrubber, based on estimates by EPA (1974), are about
67 percent higher than for a multicentrifugal scrubber in smaller
plants and about 60 percent higher in larger asphalt concrete plants.
As shown in Table 4-5 about 24 percent of 150 new and modified
plants a year would be using venturi scrubbers. These plants are
assumed to be distributed by size so that 76 percent would have a
capacity no greater than 218 Mg (240 tons/hr) and 24 percent would
be larger (based on a survey of plants by NAPA, 1977).
The additional kilowatt-hours per year for these estimated 36
plants have been calculated to be approximately 2 x 10^ (Table 4-5),
4-41
-------
based on NAPA estimates (Khan and Hughes, 1977; NAPA 1978) of an
average of 666 hours/year of actual operation per plant or and
additional requirement of 1.98 x 106 J/Mg (0.67 hp-hr/ton) for a
136 Mg (150 ton)/hr plant and of 1.48 x 106 J/Mg (0.5 hp-hr/ton)
for a 272 Mg (300 ton)/hr plant.
The energy input to generate the electricity required can be
estimated by using the factor of 10? J (10^ Btu) as an approxi-
mate guide for 1 kWh of electricity generation at central power
plants (based on 33 percent conversion efficiency). However, higher
factors are applicable for small generators typical of those used by
mobile asphalt plants. These may be greater than 1.58 x 10^ J
(1.5 x 10^ Btu) per kWh. Using this later figure would yield a
requirement for 3.19 x 1013 J (3.02 x 1010 Btu) per year for the
additional energy used by the 36 venturi scrubbers.
The additional requirement corresponds to approximately 680 Mg
(5000 barrels) per year of oil, based on an estimate of 4.47 x 10*"
J/Mg (5.8 x 106 Btu/barrel) for distillate oil and 4.84 x 1010
J/Mg (6.3 x 106 Btu/barrel) for residual oil. This is a very small
amount of oil when compared with the average rate of oil consumption
in the U.S. For the year 1976 an average 2.2 x 106 Mg (1.7 x
10^ barrels) of oil per day were consumed in the U.S. (Inter-
national Petroleum Encyclopedia, 1977).
4-42
-------
TABLE 4-5
ADDITIONAL ENERGY REQUIREMENTS FOR PLANTS
USING VENTURI SCRUBBERS
Parameter
Percent Plants3
Number of Plants
Number Using Scrubbers
Percent Using Scrubbers'3
Average Number of Hours
Plant
<218 Mg/Hour
(<240 Tons/Hour)
76
114
27
24
666
Capacity
>218 Mg/Hour
(>240 Tons/Hour)
24
36
9
24
666
Operating per Yearc
Horsepower Requirements^
(Additional)
HP Hr/Yr for Plant
Total Additional
Energy Requirements
(106 kWh/yr)
100
66,600
1.34
150
99,900
0.67
aNAPA, 1977.
"Khan and Hughes, 1977. Adapted from total percent of plants
now using EPA-recommended control devices. Consistent with
percent of venturi scrubbers observed in test results (Section 5).
cKhan and Hughes, 1977.
dEPA, 1974.
4-43
-------
The figures for energy (fuel) usage to operate venturi scrubbers
may be compared with overall fuel usage at asphalt plants as given by
NAPA (1977a). Under stoichiometric conditions, which can in fact be
approximately achieved, the energy required to heat and dry 1 Mg of
typical aggregate is 2.81 x 108 J (241,600 Btu/ton). NAPA estimates
the efficiency of the process to be 84.9 percent. Therefore the
energy input required would be 3.31 x 108 J/Mg (284,600 Btu/ton) of
aggregate. If a typical hot mix of 95 percent aggregate and 5
percent asphalt is prepared, the energy input would be 3.14 x 108
J/Mg (270,400 Btu/ton) of hot mix.
This energy usage can be compared with the energy required to
provide the additional electricity used in the venturi scrubbers at
stationary plants. Using the figures shown above for additional
requirements for venturi scrubbers, combined with an electricity
generation efficiency of 33 percent, yields the following energy
input requirements at the power plant:
• For the 136 Mg (150 ton)/hour plant
5.93 x 106 J/Mg of hot mix
(5.10 x 103 Btu/ton of hot mix)
• For the 272 Mg (300 ton)/hour plant
4.43 x 106 J/Mg of hot mix
(3.81 x 103 Btu/ton of hot mix)
This is an increase of 1.9 percent in the energy required by the
smaller plant and 1.4 percent in the energy required by the larger
plant.
4-44
-------
A similar analysis can be made for the case where it is assumed
that all the plants are mobile and/or use portable generators fueled
with distillate oil. In this case an electricity generation
efficiency of 22 percent would yield the following energy input
requirements.
• For the 136 Mg (150 ton)/hour plant
8.16 x 106 J/Mg of hot mix
(7.73 x 103 Btu/ton of hot mix)
• For the 272 Mg (300 ton)/hour plant
6.09 x 106 J/Mg of hot mix
(5.77 x 103 Btu/ton of hot mix)
This amounts to an increase of 2.9 percent in the energy required by
the smaller plant and 2.1 percent in the energy required by the
larger plant*
The increased cost per unit mass of product from stationary
plants can be estimated by assuming that the power plant burns a
typical fuel such as No. 6 residual oil with less than 1 percent
content. The October 1978 market price for such oil in Chicago
(typical) was $14.00 per barrel (Oil and Gas Journal, 1978). The
cost of using such fuel to provide the additional energy required to
operate the venturi scrubber becomes 1.25 cents/Mg of hot mix (1.13
cents/ton) for the 136 Mg (150 ton)/hr plant and 0.94 cents/Mg
(0.85 cents/ton) for the 272 Mg (300 ton) per hour plant.
4-45
-------
In a similar way the increased costs for mobile plants can be
estimated by assuming that distillate oil is burned. The October 1978
market price for such oil in Chicago was $15.54/barrel (Oil and Gas
Journal, 1978). The cost of using the fuel to provide the additional
energy required to operate the venturi scrubbier becomes 2.28 cents/Mg
of hot mix (2.07 cents/ton) for the smaller plants and 1.71 cents/Mg
(1.55 cents/ton) for the larger plants.
In the final analysis, these increases in fuel consumption and
costs cannot be considered an inevitable result of the particulate
emissions standards set for asphalt concrete plants, since they
would have been avoided by use of fabric filter control systems. The
higher energy requirements result from choice of venturi scrubbers
from the two representative control technologies. As operating
data for plants subject to NSPS are not available, it is not known
to what extent energy penalties (as well as loss of fines) associated
with choice of venturi scrubbers actually offset higher capitaliza-
tion costs for baghouses. However, as already noted, EPA (1973) has
estimated baghouses as the more cost effective of the two options.
4.4.2 Environmental Effects
In addition to the increased consumption of fuel which occurs
with scrubbers, possible significant environmental effects from NSPS
for asphalt concrete plants represent principally the following:
• Reduction in particulate emissions to the atmosphere from
asphalt plants.
4-46
-------
• Increased solid waste disposal requirements from solid
pollutants.
• Increased emission of atmospheric pollutants from additional
horsepower generation resulting from choice of venturi
scrubbers by plant owners.
4.4.2.1 Reduction in Particulate Emissions. Without the NSPS,
particulate emissions from the 150 new and modified asphalt plants
per year can be estimated to have been on the average 0.86 kg/Mg
(1.7 Ib/ton) of product, using the EPA emission factor for high-
efficiency cyclones. With the NSPS, the emissions are conservatively
estimated to have been on the average 0.015 kg/Mg (0.03 Ib/ton)
taking the average of emission rates for venturi scrubbers and bag-
houses (EPA, 1978c).
The total reduction in particulate emissions from new plants
each year from 1974 through 1977 is estimated to be between 6985
and 8527 Mg (7700 and 9400 tons) (as calculated in Table 4-6). This
represents about 12 percent of the annual emissions from all asphalt
plants for the year 1975 as estimated by Khan and Hughes (1977). By
the year 1977, a total of about 600 plants would have been operating
for 4 years, their cumulative emission reduction amounts to over
76,200 Mg (84,000 tons) of particulates, or an amount greater than
the 1975 annual total from all plants.
4.4.2.2 Increased Solid Waste Disposal. Reduction in atmos-
pheric pollutants represents a partial trade-off with increased solid
4-47
-------
TABLE 4-6
ESTIMATED REDUCTION IN PARTICIPATE EMISSIONS FROM NSPS
(NEW AND MODIFIED ASPHALT PLANTS)
I
-P*
oo
Year
1974
1975
1976
1977
TOTAL
Asphalt
Production
106Mg (106 tons)
Total
319(352b)
267(294b)
264(291b)
263(290C)
New Plants
10.64(11.73)
8.89(9.80)
8.79(9.69)
8.77(9.67)
Estimated Annual
Emission of Particulates
Thousand Mg (thousand tons)
0.85 kg/Mg of Product
(1.7 Ib/ton of Product)3
9.04 (9.97)
8.01 (8.33)
7.48 (8.24)
7.46 (8.22)
31.99 (34.76)
0.05 kg/Mg of Product
(0.1 Ib/ton of Product)3
0.16 (0.18)
0.13 (0.15)
0.13 (0.15)
0.13 (0.15)
0.55 (0.63)
Estimated Reduction
Thousand Mg (thousand tons)
Annual
8.88(9.79)
7.88(8.18)
7.35(8.09)
7.33(8.07)
31.44(34.16)
Cumulative
8.88(9.79)
16.76(17.98)
24.11(26.08)
31.44(34.16)
81.19(88.01)
3 EPA, 1978c.
b NAPA, 1977.
c Estimated.
-------
waste. Particulates removed by wet scrubbing and approximately one-
half of those recovered by fabric filters (the remainder being assumed
to be recycled) must be so disposed. On this basis, 24 percent of
about 7711 Mg (8500 tons) or about 1814 Mg (2000 tons) (on a dry-
weight basis) must be disposed in settling ponds; and one-half of
the remainder, or about 2903 Mg (3200 tons), must be disposed from
fabric filters, making a total of about 4717 additional Mg (5200
tons) each year from plants that become subject to NSPS.
4.4.2.3 Emissions Due to Increased Fuel Usage. Based on promul-
gated NSPS for residual-oil-burning and natural-gas-burning plants,
pollutants per year estimated to result additionally over the preced-
ing year from increased energy requirements by venturi scrubbers are
shown in Table 4-7. These additional emissions reflect the choice of
a particular control system which is allowable but not necessary as
a means of complying with NSPS.
4-49
-------
TABLE 4-7
ESTIMATED ADDITIONAL EMISSION OF POLLUTANTS
FROM INCREASED ENERGY (HORSEPOWER) REQUIREMENTS
FOR VENTURI SCRUBBERS
Specific Emission Factor3
Pollutant gm/106J(gm/hp-hr)
Carbon Monoxide
Exhaust Hydrocarbons
NOX
Aldehydes
sox
Particulates
73.6(199)
2.47(6.68)
1.91(5.16)
0.08(0.22)
0.099(0.268)
0.121(0.327)
Annual (gm/yr)^
(106)
537
18
14
0.59
0.72
0.88
Emissions
(tons/yr)c
593
20
15
0.66
0.80
0.98
aEPA, 1973, Part A, Section 3.3.3.
bBased on an additional 2.01 x 106 kWh (2.7 x 106 hp-hr) per year
as calculated in Table 4-5.
C453.59 gm/lb.
4-50
-------
5.0 INDICATIONS FROM TEST RESULTS
5.1 Test Coverage in Regions
A survey conducted by MITRE/Metrek obtained information on a
total of 72 tests from CDS (Table 5-1). These tests cover the period
since the promulgation of the standards in 1974. The sample repre-
sents approximately 14.9 percent of all asphalt concrete plants
subject to NSPS as estimated by the JACA Corporation (1977). Of the
10 EPA regions, Region V was the only one that did not submit test
data.
Tests for particulates and opacity are listed under the pollutant
column in Table 5-1. At least one kind of pollutant compliance was
determined in each test. A total of 36.5 percent tested for both
particulates and opacity compliance, and 3.8 percent did not test
for particulates. Of the 72 plants tested, 22.2 percent were not
in compliance. Plant status is an identification of new sources as
opposed to modified sources. Only four plants (5.5 percent) were
successfully identified; and all were new sources.
The pollution control technology of most of the plants tested
consisted of scrubbers (25 percent), baghouses (25 percent), or
a combination of baghouses and cyclones (16.6 percent). Another
4 percent used cyclones, and 11.1 percent reported the use of
other methods (or none) for particulates and opacity control.
Approximately 19 percent did not report the technology used.
5-1
-------
TABLE 5-1
MITRE/METREK SURVEY OF MSPS TEST DATA
Test
Region No.
I: 1
II: 1
2
3
4
5
6
7
8
9
10
11
12
III: 1
2
3
4
IV: 1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
Compliance
With NSPSa
X
X
X
X
X
X
-
X
X
X
X
X
X
X
X
X
X
X
X
X
X
-
X
-
-
X
-
X
X
X
X
X
X
-
X
X
X
-
—
Process
Equipment
Unknown
Drum Dryer
Drum Mix
Dryer
Unknown
Unknown
Dryer
Unknown
Stansteel Model
(RM 120 A)
Unknown
Unknown
Rotary Dryer
Unknown
Dryer Mix
Unknown
Rotary Dryer
Unknown
Unknown
Unknown
Unknown
Unknown
Unknown
Unknown
Unknown
Unknown
Unknown
Unknown
Unknown
Unknown
Drum Mix
Unknown
Unknown
Unknown
Unknown
Drum Mix
Unknown
Unknown
Unknown
Unknown
Pollutant Plant
(m^Sic
3.4
34.2
66.6
6.4
77.6
76.7
198.0
6.8
48.4
61.6
7.8
13.5
22.8
73.0
80.1
-
33.5
20.5
73.0
63.9
63.9
155.1
68.4
205.3
111.8
21.7
180.2
9.8
55.4
64.3
89.0
52.5
13.7
84.4
66.9
79.9
57.0
105.0
95.8
Opac. Status13
- Unknown
0 Unknown
Unknown
Unknown
- Unknown
Unknown
- Unknown
- Unknown
Unknown
- Unknown
- Unknown
Unknown
0 Unknown
0 Unknown
Unknown
0 to 20 Unknown
- Unknown
Unknown
<20 Unknown
< 5 Unknown
<10 Unknown
5 to 10 Unknown
10 Unknown
- Unknown
- Unknown
Unknown
Unknown
Unknown
Unknown
Unknown
0 Unknown
0 to 15 Unknown
New
Source
10 to 40Unknown
Unknown
Unknown
Unknown
Unknown
- Unknown
Control Technology
Cyclone and aeropulse fabric filter
Barber-Greene Model CF Baghouse
Venturi Scrubber
McCarter size 1410 single cyclone; McCarter 540-D fabric filter
with double bags of 540 Nomex 16 oz. bnp.houso
Unknown
Baghouse system
Research Cottrell Flex Kleen Model 512 pulse jcl type baghou?e
Dustex NOIS (Louver Collector) Dustex No. 2120 34 (Fabric Collector)
Baghouse
Cyclone-Stansteel Model 9836
Baghouse-Stansteel Model S 0205
Stansteel reverse air baghouse — 672 bags
Aeropulse, Inc. 612-10; Modern Model 100 Fan and Chicago blower
15LS-SQI Baghouse
Cyclone collector and aeropulse bag collector model '/756-10T
Baghouse
Unknown
Unknown
Cyclone separator, baghouse with Nomex bags
Barber-Greene Cyclone; Barber-Greene Baghouse
Unknown
Venturi Scrubber
Baghouse
Baghouse
Baghouse
Baghouse
Washer and wet fan
Scrubber - wet fan/air wash
Dry Cyclone Collector and Baghouse
Wet washer (Scrubber ?)
Baghouse (Nomex)
Primary Cyclone and Astec Baghouse
Dual Venturi Scrubber
Baghouse
Venturi Scrubber
Baghouse
Unknown
Wet Scrubber
Baghouse
Baghouse
Baghouse
Venturi Scrubber
-------
TABLE 5-1 (Concluded)
u>
Pollutant
Region
V:
Vi:
Test
No.
Compliance
With NSPSa
Process
Equipment
Part.
(mg/dscm)
Opac.
(%)
Plant
Status
Control Technology
No tests submitted
1
2
3
X
X
X
Subpart I
Dryer
Dryer, Screening
73.0
68.0
55.2
_
-
-
Unknown
Unknown
Not Indicated
Venturi Scrubber
Scrubber
Baghouse
Tower, Roll crusher
VII:
VIII:
IK
X:
4
5
6
7
8
1
2
3
4
5
6
7
8
1
2
3
4
5
6
1
2
1
2
3
4
5
6
7
8
9
-
-
X
X
-
_
_
X
X
X
X
X
X
_
X
-
X
X
X
X
X
X
X
-
X
X
X
X
X
X
Mineral filler
Dryer
Unknown
Dryer
Unknown
Unknown
Unknown
Bar ber-Gr eene
(DM-60)
Unknown
Dryer Mixer
Rotary Dryer
Drum Dryer
Unknown
Kiln Stack
Rotary Dryer
Drum Dryer,
Rotary Mixer
Dryer
Unknown
Unknown
Unknown
Unknown
Unknown
Drum Mixer
Veneer Dryer
Dryer
Unknown
Mobile Drum
Mix Dryer
Continuous Mixer
Drum Mixer
Drum Mixer
Thermodrum
silo
155.1
2,236.0
37.6
45.9
—
93.5
1,357.6
26.9
18.0
52.5
23.3
71.6
93.1
16,778.9
70.7
6,778.6
51.6
93.1
68.4
68.4
22.8
70.7
18.3
319.4
57.0
73.0
61.6
80.0
70.7
41.1
-
-
-
-
64
0.
25
Unknown
Unknown
Unknown
Unknown
Unknown
57 Unkown
to Unknown
Unknown
Unknown
Wet Scrubber
Unknown
Unknown
Unknown
Venturi Scrubber
30
1.
_
5 to
1.
_
6.
-
-
-
-
-
-
<5
-
<5
<1
<20
-
0 to
7
17.
10
<5
6 Unknown
Unknown
25 Unknown
6 Unknown
Unknown
8 New Source
Unknown
Unknown
Unknown
Unknown
Unknown
Unknown
Unknown
Unknown
Unknown
Unknown
Unknown
Not Indicated
5 New Source
New Source
5 Unknown
Unknown
Unknown
Cyclone
Cyclone, baghouse, hood
Cyclone, Flex-Clean Corp. Baghouse & Hood
Single Cyclone dry dust collector and impinger
wet dust collector
Boeing Venturi Scrubber
Demister or Venturi Scrubber
None
Unknown
None
Unknown
Baffle/Spray nozzle
Cyclone, Scrubber tower
Baghouse-Cedaropids
Cyclones and Baghouse
Stansteel Wet Scrubber Venturi
Venturi Scrubber
Unknown
Unknown
Orifice Scrubber
Stansteel Model D Scrubber
Venturi Scrubber
Venturi Wet Scrubber
Barber-Greene Venturi Wet Scrubber
.90 mg/dscm (0.04 gr/dscf) and'20% opacity.
Indicates if plant is a new source or a modified source.
-------
5.2 Analysis of Test Results
5.2.1 Particulates
A test for particulates typically requires measurement of the
grain-loading observed in three individual sample runs. In the
discussion which follows, a test result represents the reported
average from the runs. The test data made available to MITRE/Metrek
by the EPA regions consists of results from 72 such tests. Generally,
there was one test per plant, however, two plants required retesting
to achieve compliance. Hence, the 72 tests represent a total of 70
different plants (Table 5-2).
The results show 53 of the tests yielded an average grain
loading of less than or equal to 90 mg/dscm (0.04 dscf), whereas
19 tests (including first runs from the plants that required
retesting) failed to meet NSPS. Accordingly, 73.61 percent of the
tests showed control systems efficient enough to meet current
standards.
Results showed a narrow range. Most tests showed less than
228 mg/dscm (0.1 gr/dscf). Only five tests or 6.94 percent yielded
a grain loading greater than 228 mg/dscm (0.1 gr/dscf). The five
plants included two with no controls for which the grain loadings
were very high. Control systems used with the other three plants
were not indicated on the records provided.
5-4
-------
TABLE 5-2
DISTRIBUTION OF PARTICULATE TEST RESULTS (AVERAGES)
BY CONTROL SYSTEM
Interval
Range mg(gr)
<22.5 (<.01)
<45 (<«02)
<67.5 (<.03)
<90 (<.04)
TOTAL ^90 (<-04)
< 113 (<.05)
< 183 (<.08)
<228 (<.10)
<2282 (<1.00)
>2282 (>1.00)
TOTAL > . 04
TOTAL -90 mg/dscm
Percent (^.04 gr/dscf
i
Control System Type
Cyclone
Scrubber
1
1
2
0
2
) 100.0
Baffle/
Spray
Nozzle
0
1
1
1
0.0
Fabric
Filter
(Baghouse)
4
2
7 .
3a,b
16
2a
1
1
0
0
4
20
80.0
Fabric
Filter
and
Cyclone
6
1
3a
0
10
la
0
0
0
0
1
11
90.9
Venturi
Wet
Scrubber
1
1
3
6
11
1
2
0
0
0
3
14
78.6
Wet
Scrubber
0
1
3
0
4
1
1
1
0
0
3
7
57.1
Wet
Scrubber
and
Cyclone
0
1
0
0
1
0
0
0
0
0
0
1
100.0
Orifice
Scrubber
0
0
0
1
1
0
0
0
0
0
0
1
100.0
None
0
0
0
0
0
0
0
0
0
2
2
2
0.0
Unknown
1
0
3
4
8
1
1
0
2
1
5
13
61.5
Cn
a2 tests required for one plant
bParticulate loading = 0.039 gr/dscf for one plant,
SOURCE:
Data made available through CDS file.
-------
The data sample is large enough to support valid statistical
inferences regarding the effectiveness of control systems installed
in asphalt concrete plants subject to NSPS. The basic problem is
that the data are not sufficiently detailed to allow a determination
of whether the control systems, as installed, represented the BTS.
As discussed in Section 4.3.1, a collector system does not neces-
sarily represent BTS just because it is of the fabric filter or
venturi scrubber type. Details are not available in the test data
as to the condition of collector systems, the adequacy of instal-
lation, or the design and operating parameters (e.g., air-to-cloth
ratio for fabric filters, pressure drop for venturi scrubbers).
Thus, it is not clear how many of the venturi scrubbers tested could
be termed "high-energy" with pressure drops in the range above 76 cm
(30 in.) WG designed to be effective particularly against submicron
particles as noted in Section 4.3.1. It is known that verturi
scrubbers with pressure drop in the range of 51 cm (20 in.) WG and
less are not uncommon in asphalt concrete plants.
While not specifically applicable to BTS, the test results do
clearly reveal the success achievable with fabric filter devices and
venturi scrubbers. Table 5-2 presents a breakdown of test results by
type of control system employed. Controls used in 13 plants were not
identified.
For baghouse systems (with or without a cyclone as a primary
collector), 26 tests (83.9 percent) out of 31 met current standards.
5-6
-------
Venturi scrubbers also achieved good results. Of the 14 tests
for plants explicitly identified as using such devices, 11 (or 79
percent) met current NSPS.
Results recording the use of only scrubbers and/or wet washers
are questionable. It is not certain whether any of the systems
identified were venturi scrubbers. Of 10 tests reported as using
scrubbers or washer and wet fan, seven met current NSPS, all at
loadings less than or equal to 68 mg/dscm (0.03 gr/dscf).
A total of 45 plants were reported to be using either fabric
filter (with or without a cyclone as primary collector) or a venturi
scrubber. Of these 37 (82.2 percent) met current NSPS. This sample
may be deemed large enough to use the normal approximation to the
binomial distribution. On this basis the expected percent of such
devices as installed and operated which will meet current NSPS lies
between 64 and 92 at the 99 percent confidence level and between 69
and 91 at the 95 percent confidence level.
Of the 53 tests meeting NSPS, 14 gave results (averaged over the
multiple runs) of between 68 and 90 mg/dscm (0.03 and 0.04 gr/dscf)
(the total of 14 includes one unsuccessful test of a plant using
fabric filters). All of the plants with known control methodologies
for which results fall in this range were identified as using either
baghouses or venturi scrubbers. The test samples show that of the
control devices necessary to achieve stricter standards, over 25
percent of the devices that met present NSPS could not have met
5-7
-------
the stricter threshold of 68 rag (0.03 gr) prevailing in some states.
These results do not imply that the systems represented BTS or that
they could not have been designed to meet a more restrictive stan-
dard.
Results of these tests may be compared with other recent surveys
reported. Based on available information there is no way to verify
the extent of possible overlap among tests included in any of the
samples.
Patankar and Foster (1978) report data on a sample of 63 dryer-
drum mix plants. None of the plants used baghouses. Only 50 percent
of the 18 systems using venturi scrubbers reduced particulate emis-
sions to the level of 90 mg/dscm (0.04 gr/dscf) required by current
NSPS. Only five plants (28 percent) showed emission levels less
than or equal to 68 mg/dscm (0.03 gr/dscf). Of 24 low-energy wet
scrubbers, seven (29 percent) met current NSPS and only two (8
percent) were tested at levels less than or equal to 68 mg/dscm (0.03
gr/dscf). No other type of device in the sample tests reduced
particulate emissions to the level of current NSPS (although one
cyclone or multicyclone showed marginal results at about 90 mg/dscm
or 0.04 gr/dscf.
Khan and Hughes (1977) report a survey of 16 dryer-drum mix
plants. The particulate emissions (Table 5-3) are reported on the
basis of grams per second. Without information on the flow rate
of exit gas in standard volume per minute, there is no way to
5-8
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TABLE 5-3
EMISSION RATES FOR DRYER-DRUM MIX PLANTS
Plant
a
i
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
Average
Production
Rate
CMg/hr)
454
200
159
272
272
454
251
272
181
227
363
91
136
109
259
318
Uncontrolled
Emissions Rate
(gm/sec)
12.5
5.5
4.4
7.5
7.5
12.5
6.7
7.5
5.0
6.2
10.0
2.5
3.7
3.0
10.0
8.7
(gm/Mg)
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
Primary Collector
Type
cyclone
none
none
cyclone
none
settling
chamber
settling
chamber
none
cyclone
none
cyclone
none
multi-
cyclone
cyclone
none
none
Emission Rate
(gm/sec)
(gm/Mg)
0.47 ; 3.75
— ; —
— | —
0.28
—
4.16
2.29
__
0.19
—
0.38
—
0.03
0.11
—
—
3.75
—
33.0
32.5
_
3.75
—
3.75
—
0.5
3.75
—
—
Efficiency
(%)
96.2
—
96.2
—
66.67
66.67
—
96.2
—
96.2
—
99.3
96.2
—
—
Secondary Collector
Type
baghouse
cyclone
scrubber
cyclone
scrubber
none
none
Venturi
scrubber
cyclone
scrubber
Venturi
scrubber
none
baghouse
none
baghouse
Venturi
scrubber
gravity spray
tower
Venturi
scrubber
Venturi
scrubber
Emission Rate
(gm/sec)
1 x 10~4
0.04
0.03
—
—
0.004
0.02
0.01
—
0.001
—
0.0005
3 x 10~5
0.001
0.008
0.08
(gm/Mg)
8 x 10~4
0.65
0.65
—
—
2.9 x 10~3
0.25
0.05
—
0.02
—
0.02
0.001
0.003
1.0
0.85
Efficiency
(%)
99.98
99.3
99.3
—
—
99.9
99.3
99.9
—
99.98
—
99.98
99.9
99.1
99.9
99.9
Overall j
Efficiency
(%)
>99.99
99.3
99.3
96.2
Not applicable
99.97
99.33
99.9
96.2
99.98
96.2
99.98
99.9
99.1
99.9
99.9
SOURCE: Khan and Hughes, 1977.
-------
convert these results to mass loading. However, the uncontrolled
emission rates are given, enabling the percent of efficiency to be
calculated and shown for each control system tested. It should be
noted that a uniform value for uncontrolled emissions of 100 gm/Mg
(0.2 Ib/ton) is given for all plants in Table 5-3, which implies
that this value did not result from observations made in tests, but
rather assumptions. The average loadings as the ratio of reported
mass of particulates to mass of product have also been calculated on
the basis of reported yearly production rate and annual hours of
operation.
The methodology of calculation used in deriving the data in
Table 5-3 is not known. It may be noted that as reported the data
yield essentially constant factors for loadings in grams/metric ton
for efficiencies achieved by each type of control device. All of the
control devices reported, with the exception of the settling
chambers, would have met NSPS (less than or equal to 90 mg/dscm or
0.04 gr/dscf at any reasonable flow rate of stack gas that might be
postulated. No secondary device would have been required for the
cyclones (at a reported emission rate of 1.25 gm/Mg or 0.0025 Ib/ton)
or the multicyclones (at 0.5 gm/Mg or 0.001 Ib/ton), given a rate of
no higher than 41 scm (1312.5 scf) of gas per metric ton (ton) of
product—a very low ratio indeed. This ratio corresponds to a flow
rate for exit gas of less than 311.4 scm/mln. (11,000 scfm) for a 454
Mg (500 ton)/hour plant (and correspondingly lower rates for smaller
plants).
5-10
-------
The emission figures in Table 5-3 may be compared with those
expected from conventional plants in which the drying and mixing
take place in separate components (i.e., batch and continuous mix
plants). Using the EPA (1973a) estimate of 22.5 kg (45 Ib) of
particulates per metric ton (ton) of asphalt product on an uncon-
trolled basis, a plant with a control system providing 99.9 percent
efficiency would have a controlled emission rate of 22.5 gm/Mg (0.045
Ib/ton). A control system efficiency of 99.99 percent would lower
the controlled emission rate to 2.25 gm/Mg (0.0045 Ib/ton). By con-
trast, the emission rate in grams/metric tons (pounds/ton) for drum
mix plants with baghouses and venturi scrubbers ranged from 1.0 gm/Mg
(0.002 Ib/ton) to as low as 8 x 10~4 gm/Mg (16 x 10~7 Ib/ton)
(Table 5-3).
These differences are significant and somewhat at variance with
both the results obtained in the actual tests as furnished by the EPA
region shown In Table 5-1 and with the data reported by Patankar and
Foster (1978). It is true that all of the 11 plants specifically
Identified as dryer drum mix passed the tests, in contrast with 19
failures in tests of plants not so identified. All except two of the
plants specifically identified as drum mix used baghouse or venturi
scrubber, with the exception of one control system not identified and
one reported as an orifice scrubber. The large number of plants not
identified in the test data (Table 5-1) as to process type precludes
5-11
-------
a detailed comparison of results for dryer drum mix vs. conventional
plants.
Nothing in the EPA regional test data specifically supports or
denies the assumption that these are substantially lower controlled
emission rates for drum-mix than for conventional plants, as suggest-
ed by Table 5-3.
5.2.2 Opacity
Much less test data are available from the regional sources
for opacity than for particulates. Of the 26 tests for which opacity
is reported as a percent, only five (or less than 20 percent) failed
to meet the NSPS of opacity less than or equal to 20 percent. None
of these five met current NSPS for particulates.* Results are
shown in Figure 5-1.
It is difficult to assess the correlation between opacity and
particulate emissions from the test data available. Many of the
results were reported only as not exceeding a specified high thres-
hold (such as 20 percent), and it is not clear how far the readings
were below the stated upper limit. Of the 21 plants reported as
meeting the current NSPS for opacity, only two (less than 10 percent)
also failed to meet particulate standards. Both of these were observ-
ed between 5 and 10 percent. The five plants for which readings
One of the tests reported opacity results in a manner that pre-
vented determination of the particulate grain loading associated
with the opacity level (measured at over 60 percent).
5-12
-------
o
en
a)
tH
rt
00
•S
T3
0)
:¥»
^w$S$::x$:?i:::::Sx^::^
10 15 20 25 30 35 40 45 50 55 60
65
Percentage of Opacity
FIGURE 5-1
RESULTS OF OPACITY TESTS
5-13
-------
were reported only as less than or equal to 15 or 20 percent (all
of which met particulate standards) might have had much lower
percentages of opacity determined by more detailed reporting of
results.
The small data base available supports the policy underlying
asphalt NSPS that an opacity reading of greater than 20 percent
will be associated with control equipment not functioning at the
level required to meet particulate standards.
5-14
-------
6.0 ANALYSIS OF POSSIBLE REVISIONS TO NSPS
6.1 Source and Nature of Revisions
The revisions considered in this section are based on sugges-
tions from EPA officials, particularly those at the 10 regions (MITRE
Corp., 1978); from representatives of the asphalt concrete industry
and private concerns involved with control equipment and plant
testing; from analysis of published literature; and from analyses of
available data.
These potential revisions fall under the following headings:
• New levels for pollutants now controlled by NSPS.
• Fugitive emissions.
• Changes in tests and procedures, including monitoring
requirements.
• Control of other pollutants emitted by asphalt concrete
plants.
The analysis of possible revisions considered potential changes
from the following points of view:
• Near-future developments and trends in the industry.
• Impacts that changes might have on the environment and
on industry.
• Administrative procedures involved in compliance.
6.2 Industry Development and Trends
Among the significant industrial developments that have affected
or are likely to affect NSPS for asphalt concrete plants is the
6-1
-------
trend toward dryer-drum mix plants in the asphalt industry. Other
important developments include the recycling of asphalt pavement and
the use of hot-water emulsion mixes.
6.2.1 Control Devices
The use of fabric filters or baghouses and high-efficiency wet
scrubbers has been particularly effective in achieving compliance
with the standard. EPA (1973; 1974) and Kinsey (1976) predicted that
either baghouses or venturi scrubbers would be required to reduce
emissions to a level less than or equal to 90 mg/dscm (0.04 gr/dscf).
As shown by the sample test results discussed in Section 5, some 80
percent of plants tested which used fabric filters or orifice or
venturi scrubbers achieved compliance—a slightly higher ratio than
for plants in general. Estimates of the percent of industry usage of
baghouses and venturi scrubbers range from 56 to 70 (Khan and Hughes,
1977; NAPA, 1978). It is not known how many of these control
collector systems actually represent BTS.
Baghouses account for approximately 40 percent of all control
systems used in asphalt concrete plants, and the trend is toward an
"all dry" control system using fabric filter devices as the single
or secondary, and critical, collector (Khan and Hughes, 1977; NAPA,
1978). Use of the fabric filter collector has grown significantly
within the last 4 years. At present, particularly good results are
obtained by use of 397 gm (14-oz) Nomex filter bags employing a
cloth-to-air ration of about 1:6.
6-2
-------
Venturi wet scrubbers have also proved effective. A pressure
drop of from 36 to 51 cm (14 to 20 inches) is typical (NAPA, 1978).
Venturi scrubbers are employed by about 16 percent of the industry as
secondary collectors and orifice scrubbers by about 8 percent (Khan
and Hughes, 1977). In the past few years they have been particularly
favored by dryer-drum mix plants; about 37 percent of a sample of 49
drum mix plants with various control devices used venturi scrubbers
(Patankar and Foster, 1978).
6.2.2 Dryer-Drum Mix Plants
The dryer-drum mix plant represents what has been termed a
"recently revitalized process for manufacturing asphalt hot mix"
(Kahn and Hughes, 1977). Drying of the aggregate as well as mixing
with asphalt and additional fines takes place within a rotary drum,
so that the whole process is simplified. Equipment requirements
are reduced and important gains in operational efficiency result,
including reduced manpower requirements. The capital cost of a
dryer-drum mix plant is estimated to be only 75 to 85 percent that
of a conventional plant (Robert et al., 1975). It is not surpris-
ing that the drum mix plant is proving increasingly popular in the
industry. At present about 2.6 percent of all U.S. plants are
estimated to use this process. It is estimated, however, that
30 percent of those put in operation during the last 3 years are
of the dryer-drum mix type. It has also been estimated that use
6-3
-------
of such plants will grow at an accelerated rate, and in the next
few years up to 85 percent of all new plants will be dryer-drum mix
(Khan and Hughes, 1977; Patankar and Foster, 1978; Moe, 1978).
Because the drying takes place within the same container as
the mixing, emissions are partly screened by the curtain of asphalt
added so that the particulate loading from the dryer is much lower
than that from conventional plants. Further, the emissions from
the hot elevators, screens, bins, weigh hopper and mixer, which
in conventional plants are conveyed by the scavenger ductwork to
the collector, are not present in the drum-mix plant. In the latter
type of plant, these elements are replaced by proportioning feed
controls that provide all components as input directly to the drum
where both drying and mixing take place. The overall inlet loading
to the collector of particulates is much lower than the rate from
conventional plants, perhaps by one or more orders of magnitude
(see Table 6-1).
A possible drawback to the dryer-drum mix plant from an envi-
ronmental point of view is that the rate of HC emissions may be
substantially higher than from conventional plants (Robert et al.,
1975). However, one test recently reported in a plant equipped with
6-4
-------
TABLE 6-1
ASPHALT CONCRETE
UNCONTROLLED EMISSION FACTORS,
(kilograms of particulates/metric ton asphalt product)
Source
EPA-AP 42
Conventional
Dryer-Drum
Air Pollution
Engineering
Manual
Test C426
Test C537
kg/Mg
22.5
4.9
23.9
15.8
Calculated from Known Loading
Value Known
(Ib/ton) Stated Parameters Parameters
(45) x
(9.8) x
(47.8) x
(31.6) x
Kinsey
(Plant A)
(Dryer-Drum)
Kinsey
(Plant B)
(Dryer-Drum)
Khan & Hughes
(Average)
Standard-Havens
(Dryer-Drum)
Khan and Hughes
(Dryer-Drum)
22.8
2.2
2.79
6-8
0.1
(45.6)
(4.4)
(5.57)
(12-16)
(0.20)
6-5
-------
baghouses showed only traces of HC in dust and condensate (Forsten,
1978). The HC emission rate from dryer-drum plants has not been
determined experimentally.
Because the dust particles from the dryer-mixer drum are
coated with sticky asphalt, it was formerly considered that the use
of fabric filter controls would not be feasible with the drum mix
plant (Robert et al., 1975; Kinsey, 1976). However, it has now
been found that baghouses can be used with this process. Some 8
percent of dryer-drum mix plants are estimated to use fabric filter
collectors (Khan and Foster, 1977). No concern over the feasibility
of baghouses for these plants was expressed by the National Asphalt
Pavement Association in recent discussions of the asphalt industry
(NAPA, 1978).
A new development in dryer-drum mix plants has been reported by
one plant manufacturer. The process uses natural draft flow of air
with no exhaust fan. Less equipment is required than with conven-
tional drum mix plants. According to the manufacturer, the process
also consumes less horsepower and less fuel and no control equipment
external to the drum is needed. Although some plants have been
installed and a few first test results reported, the significance
of this process as a new development in dryer-drum mix technology
has not yet been established. An apparent problem is the difference
in output particulate loadings expressed as grams/metric ton (pounds/
6-6
-------
ton) of product and as milligrams/dry standard cubic meter (grains/
dry standard cubic foot). Figures supplied by the manufacturer show
that in both cases, output loadings increase with production rate.
Because the volumetric flow (as well as the velocity) of exit gas
from the drum is much less than from conventional plants, an output
of particulates which is relatively low when measured in grams
(pounds) per hour or per metric ton (ton) of product may exceed
the allowable level when measured as milligrams/dry standard cubic
meter (grains/dry standard cubic foot) as required by current NSPS.
Manufacturer-supplied data reflecting test results indicate that at a
production rate of 82 Mg (90 tons)/hr or less, the emission rate is
within current NSPS; whereas, the output of particulate emissions
tends to exceed 90 mg/dscm (0.04 gr/dscf) at production rates from
109 to 272 mg (120 to 300 tons)/hr. The resulting rates for all of
these production levels when shown in mass of particulates per hour
were less, according to the data, than results reported from EPA
tests of plants using scrubber or baghouse control systems (Nelson,
1978).
6.2.3 Asphalt Recycling Plants
A process for recycling asphalt paving by crushing up old road
beds for direct firing has been recently implemented on an experi-
mental basis. It is estimated that 40 plants using this process have
made production runs to up to 12 weeks. These plants operate
particularly in the Midwest relaying asphalt to cover potholes.
6-7
-------
Although EPA has ruled the plants subject to NSPS and at least two
have demonstrated compliance, preliminary indications are that the
process may not meet the allowable level of particulate emissions*
Partial combustion of the recycled asphalt cement produces a blue
smoke reportedly more difficult to control than the mineral dusts of
plants using virgin material. Tests have been reported of plants
with opacity less than 20 percent, but particulate emissions exceed
90 mg/dscm (0.04 gr/dscf). The plants use approximately 20 to 30
percent virgin material mixed with the recycled asphalt. While
considered effective in conservation of energy and reclamation of
solid waste, this process involves some pollution penalties (EPA,
1978b; Patankar, 1978).
6.2.4 Hot Water Emulsions
Recently EPA has encouraged the use of hot-water emulsion mixes
rather than cutback asphalts. The emission levels that occur during
production of hot water emulsions are undetermined. Some potential
pollution problems exist in the reported practice of bypassing con-
trol equipment partly for safety purposes when a sudden surge of
steam carrying particulate emissions occurs as the water is dumped
into the pugmill. These emissions are vented directly to the
atmosphere. Changes in operating procedures and modifications of
control equipment (e.g., to avoid damage of the fabric filters from
the surge of steam) may be indicated (EPA, 1978b).
6-8
-------
6.3 Levels for Particulate Emissions
6.3.1 Variables Affecting Compliance
Current NSPS are now being met to a considerable degree and, in
some situations, compliance is achieved with the more rigid standard
of 68 mg/dscm (0.03 gr/dscf) prevailing in a few states. A natural
question is, therefore, whether BTS supports a stricter level for
particulate emissions. In analyzing such a possible revision, it is
useful to consider some aspects of asphalt concrete plant operations
that affect the emission rate achieved.
Current NSPS for particulate emissions from asphalt concrete
plants are prescribed in terms of milligrams/dry standard cubic meter
(grains/dry standard cubic foot) of exit gas from the stack. Con-
sequently, the efficiency demanded of a control system in order to
meet current NSPS depends upon two variables: the quantity of
uncontrolled emissions with which the collector must deal (sometimes
referred to as the inlet loading to the control system) and the flow
rate in dry standard cubic meters/minute (dry standard cubic feet/
minute)• Neither of these variables is completely subject to opera-
tor control.
Asphalt concrete plants are designed to operate at a specific
capacity in metric tons (tons) per hour. This capacity is related to
the capability of the dryer to handle a given quantity of aggregate
at a time. Since approximately 1 minute is required for drying, the
hourly output of the plant is rated at 60 times the dryer capacity.
6-9
-------
A plant with a 2722 kg (6000-lb) mixer has a capacity of 163 Mg (180
tons)/hr. As the plant works most efficiently at this capacity, it
is advantageous to operate at full production level. The exhaust fan
is designed to handle a specified volumetric flow of exist gas, which
contains not only the combustion products from the heat used in
drying but also the moisture removed from the aggregate and any
excess air supplied to ensure complete combustion (NAPA, 1977; 1978).
Particulates in the aggregate are carried out in the exit gas
from the dryer and are the principal source of particulate emissions
from an asphalt concrete plant. Small additions are, of course,
provided by the ventline from the screens, weigh hopper, storage,
etc. The quantity of particles that become airborne in the dryer
and, hence, the inlet loading to the collector vary with the
velocity of the exit gas. It has been shown that the increase in
particulate loading with gas velocity is nonlinear; for example, a
50 percent increase in exit gas velocity from 3 to 4.5 m/sec (600 to
900 ft/min) will increase the quantity of particulates by from 2 1/4
to about 2 1/2 times (Grim et al., 1971; Robert et al., 1975).
6.3.1.1 Volumetric Flow Rate. The exit gas velocity, sometimes
called the velocity index, represents the volumetric flow rate
provided by the exhaust fan divided by the cross-sectional area of
the dryer. For example, in a dryer 2.44 m (8 ft) in diameter with a
flow rate of 1133 acmm (40,000 acfm), the velocity of the exit gas
is nearly 4.06 m/sec (800 ft/min). For a different size dryer, the
6-10
-------
velocity for the same volumetric flow rate would vary as the square
of the radius (half the diameter) of the dryer. Since the volumetric
flow rate is essentially fixed, so is the exit velocity, or velocity
index. It is not normal practice for an asphalt plant to vary the
velocity of exit gas. If for any reason a smaller quantity of
asphalt is to be produced than the full capacity allows, a damper is
applied to reduce the total volumetric flow. The gas exits at
essentially the same velocity. Many asphalt plants are provided with
automatic dampers to adjust the volumetric flow. Thus, an increase
in inlet loading of particulates as a result of change in velocity of
the exit gas is not to be expected (NAPA, 1977; 1978).
The translation of actual volume per minute to dry standard
volume represents an adjustment for the presence of moisture
in the gas and for temperature, since the volume of a gas varies
directly with temperature. The volume of exit gas can be computed
by the formula
294.27 100 - % moisture
dscm - acm x 273.16 + T(°C) x 100
460 + 70 100 - % moisture
dscf - acf x 460 + T(°F) x 100
where T represents temperature of the exit gas in °C (°F), and 273.16
(460) is applied in converting the temperature to the absolute
(Kelvin) scale. An example of this conversion is shown in Figure 6-1.
6-11
-------
N)
45
40
35
30 —
THOUSANDS OF DRY
STANDARD CUBIC i
FEET PER MINUTE 20
(at stack exit
temperature
of 275° F)
25 —
15 —
10 —
5 —
THOUSANDS OF ACTUAL CUBIC FEET PER MINUTE
FIGURE 6-1
RELATION BETWEEN ACF AND DSCF
-------
Thus, the flow rate In dry standard volume varies inversely with the
percent of moisture In the exit gas; the principal source of moisture
is that removed from the aggregate in drying. The percent of moisture
by weight in the aggregate translates into a much larger percent of
moisture (by volume) in the exit gas, depending particularly upon
temperature, type of fuel used, and amount of excess air provided.
It is common for the moisture in the aggregate to range from about
4 to 10 percent; whereas the percent of moisture in the exit gas may
vary from about 15 to as much as 50 percent. It has been found from
experience that the effect of atmospheric humidity is not signifi-
cant (NAPA, 1977; 1978).
Because the volumetric flow rate is essentially fixed and the
moisture in the aggregate may vary, the effect of a high percentage
of aggregate moisture is to reduce the capacity at which the plant
may operate as shown in Figure 6-2. The result is to require a
higher ratio of actual volume per minute to tons of asphalt produced.
Simultaneously, the ratio of dry standard volume to actual volume in
the volumetric flow will decrease. The increase in both actual
%
volume and dry standard volume required per mass of product as the
percent of moisture removed from the aggregate is nearly linear (for
a given type of fuel and dryer exhaust gas temperature) and can be
approximated quite well by a straight line as shown in Figure 6-3.
As a result of these variables, the decrease in production is greater
than the decrease in dry standard volume per minute associated with
6-13
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250
200
150
TONS PER
HOUR
100
50
0
25.3% decrease
2%
moisture
increase
I
1 L J
I
0
5678
MOISTURE REMOVED (%)
10
11
12
SOURCE: Barber-Greene Co., 1976.
FIGURE 6-2
TONS PER HOUR CAPACITY AT DIFFERENT MOISTURE CONTENT
(FOR SPECIFIC DRYER OPERATING AT CONSTANT TEMPERATURE)
6-14
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8.5
8.0
7.5
7.0
6.5
£ 6.0
CJ
5.5
5.0
4.5
>* 55 4.0
Ptf O
Q H
o w 3.5
P-I
CO
CO
5
3.0
2.0
1.0
0
32.3% Increase
— 2%
Moisture
Increase
I
I
I
I
0
2345678
PERCENT MOISTURE REMOVED
9 10
SOURCE: Barber-Green Co., 1976.
FIGURE 6-3
INCREASE IN DSCF/TON AT DIFFERENT MOISTURE CONTENT
(WITH USE OF NO. 2 FUEL OIL, DRYER EXHAUST AT 350°F)
6-15
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a given flow rate in actual volume per minute. Thus, as higher
moisture in the aggregate reduces the production of asphalt and,
hence, the drying of aggregate, the ratio of exit gas in dry standard
volume to mass of product tends to increase.
6.3.1.2 Uncontrolled Emission Rate. The inlet loading to the
collector, representing the rate at which airborne particles are
emitted in the uncontrolled situation, depends on several variables
in a way not precisely defined in the literature. The U.S. Environ-
mental Protection Agency (1973a) has published a list of pollutant
emission factors in which the uncontrolled emission rate of particu-
lates for asphalt concrete plants is given as 22.5 kg/mg (45 Ib/ton)
of product. This figure was based on information in the literature
for conventional (i.e., other than drum-mix) plants. In fact, the
uncontrolled emission rate is reported to vary under unspecified
conditions by at least one order of magnitude, as shown in Table 6-1.
Regrettably, available data do not permit a determination of flow
rate as a function of independent variables.
The nature of the aggregate used in asphalt production affects
the inlet loading of particulates to the collector. Available Infor-
mation in the literature indicates that at a given exit gas velocity,
essentially a constant percentage of the total weight of the aggre-
gate fines will be airborne from the dryer (Robert et al., 1975;
Danielson, 1973). This result would be expected from the physical
principles involved, since the tendency of a particle to become air-
6-16
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borne depends upon its size and weight in relation to the velocity
of the air stream. At typical velocity indices, from about one-half
to about three-fourths of the total aggregate fines (particles of
-200 mesh size, i.e., those which pass a 200 mesh screen and are
less than 74 microns) will be carried out in the exit gas from a
dryer* Such particles can be expected to total about three-fourths
of the particulate weight in the inlet loading. Thus, an unusually
high percentage of fines in the aggregate will result in much
heavier inlet particulate loadings than usual. In addition to the
percent of fines, the distribution of particles of -200 mesh size
can also vary.
6.3.2 Environmental Considerations
6.3.2.1 Particulate Emission Rate in Pounds per Ton. In addres-
sing the environmental effects of current NSPS for particulates from
asphalt concrete plants (and the effects of possible changes), it
would be useful to have reliable data on the allowable emission
rate per unit of product.
To convert from emission in mass per dry standard volume to total
mass of particulate output or to output in mass per mass of product
from a given plant, it is necessary to know the flow rate of stack
gas from the plant in dry standard volume, either overall or as a
ratio to production* At various percentages of moisture in the
aggregate (by weight) from 2 to 10 and stack gas exit temperature in
the range from 135° to 177°C (275° to 350°F), variations in
6-17
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the flow rate range from about 109.24 dscm/Mg (3500 dscf/ton) to
about 265.28 dscm/Mg (8500 dscf/ton) (Foster, 1977; NAPA, 1975). On
this basis, the particulate emission rate in kilograms/metric ton
(pounds/ton) of asphalt corresponding to 90 mg/dscm (0.04 gr/dscf)
can be tabulated as follows:
Stack Gas Flow Rate, Kilograms of Particulate per
dscm/mg (dscf/ton) Metric Ton of Product (Ib/ton)
109.24 (3500) 0.010 (0.020)
124.84 (4000) 0.012 (0.023)
156.05 (5000) 0.015 (0.029)
187.26 (6000) 0.017 (0.034)
218.47 (7000) 0.020 (0.040)
249.68 (8000) 0.023 (0.046)
265.29 (8500) 0.025 (0.049)
These figures are generally lower by an order of magnitude
than most of the factors estimated by EPA (1976) for the output of
various control devices. However, the EPA estimates do include a
factor of 0.02 kg/mg (0.04 Ib/ton) for an orifice-type scrubber and
indicate that emission rates an order of magnitude lower can be
achieved by baghouses. They are substantially higher than the rates
reported for 16 dryer-drum mix plants by Khan and Hughes (1977).
They can serve as a reasonable range within which the current NSPS
for particulates may be converted to mass per mass of product.
6.3.2.2 Emissions from New Plants. Using the values calcu-
lated above, a ceiling on particulate emissions from the 150 new and
modified plants estimated to come under NSPS each year can readily
be derived. Although the average production of asphalt plants overall
6-18
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was estimated to be 160 Mg (176 tons)/hr (Khan and Hughes, 1977), the
trend is clearly towards larger plants (NAPA, 1978; EPA, 1974) so
that an average asphalt production rate of 181 to 200 Mg (200 to 220
ton)/hr from plants subject to NSPS is reasonable. On this basis,
the output per plant per hour would range from 1.8 kg (4 Ib) to
nearly 5.0 kg (11 Ib). Taking a midpoint in the average rate of
0.015 kg/Mg (0.03 lb/ton)* in order to have a single figure and a
production rate of 191 Mg (210 ton) Air on the average results in a
figure of 2.86 kg (6.3 lb)/hr per plant or just under 191 Mg/yr (2.1
tons/yr). The estimated 150 plants becoming subject to NSPS each
would then produce a gross total of about 286 Mg (315 tons) of
particulate. In one sense, this figure represents a near maximum,
because it does not take into account plants with control systems
that reduce particulate emissions to levels less than NSPS. On the
other hand, inherent in the method of calculating the emission factor
is a possible error of as much as + 25 percent.
In summary, narrowing the standards to 69 mg/dscm (0.03 gr/dscf)
would reduce the ceiling on possible particulate emissions by 25
percent or about 73 Mg (80 tons). Seventy-three Mg (80 tons) is
about 0.11 percent of the estimated annual emissions from asphalt
plants in 1975 (Khan and Hughes, 1977).
*Such a rate would be quite representative, as it corresponds to a
flow rate at about 4 to 5 percent aggregate moisture and a stack gas
exit temperature in the 135° to 177°C (275° to 350 °F) range.
6-19
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6.3.2.3 Other Possible Impacts. While all environmental
effects possible from a change in the NSPS for particulates would be
very slight, they would not necessarily all be beneficial. As noted
in Section 4, if the option of using a venturi scrubber is exercised,
the efficiency of the scrubber can be increased by providing more
energy (through higher horsepower) so as to increase the pressure
drop. The higher efficiency necessary to remove an additional 25
percent of the particulate load would result in a corresponding
increase in pressure drop but an increase of about 50 percent
(actually 9/16) in energy and, hence, in fuel requirements. When
this increase is applied to the estimates of fuel usage and resul-
tant emissions, the effects (which could be avoided by choice of
fabric filter as control system) are seen to be as insignificant
as the possible gains from reducing particulate emissions. Neverthe-
less, these effects are negative.
Similarly, an increase in solid waste disposal would be antici-
pated from stricter particulate emission standards since 25 percent
of the particulates not now being captured would be removed. This
very small incremental increase in mass would presumably be disposed
of as waste. It would not be reasonable to assume any recycling of
this additional quantity, when not all fines presently removed are
being reused. A change of one quarter in the standard would result
in the additional collection of 25 percent of the 286 Mg (315
6-20
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tons) which would be emitted under current standards. This annual
increase in solid waste would be about 72 Mg (79 tons)*
6.3.3 Effects on the Asphalt Industry
Any change in the particulate standards would clearly affect
the asphalt industry in some adverse fashion, the extent of which
would be extremely difficult to quantify. New plants gear up to
meet a level of 90 mg/dscm (0.04 gr/dscf) (except in states with
stricter standards such as Maryland, New Jersey and New York) through
installation of control systems with rated efficiencies that are
adequate. Many of the devices installed up to now are clearly able
to meet stricter standards, but as shown in Section 5, about 25
percent of those that can meet a level of 90 mg/dscm (0.04 gr/dscf)
do not meet a level of 68 (0.03). Detailed data are not available
for analyzing why some plants have achieved lower mass-loadings.
Factors involved are assumed to include type of control system and
design parameters, details of installation, and operating and
maintenance practice. It would be interesting to know how lower
emissions rates correlate with the price of installed equipment and
with systems actually representative of BTS.
As a minimum, very strong opposition from the industry to tight-
ening of NSPS at this time would have to be anticipated. Plants
would be faced with a higher failure rate (already 20 percent) with
the use of control systems considered representative of BTS based on
a sample of sufficient size to support valid statistical inference.
6-21
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However, as already noted in Section 5, test data are not sufficiently
detailed to determine how many of the control systems of plants not
meeting NSPS actually represent BTS in terms of the conditions under
which they were designed, installed or operated. No information has
come to the attention of EPA (1978) that indicates any plants equipped
with baghouses have had to close down.
The probability of retesting would be increased. The estimate
of about 40 percent under present conditions (NAPA, 1978) seems high
in contrast with the test sample which showed two retests out of 72.
No quantitative information is available on the fate of plants that
require retesting, i.e., how many achieve compliance after a given
number of tests and how many face indefinite injunction against
operating. The average figure of $1848 for each standard Method 5
test supplied by one firm (Snowden, 1978) could be expected to apply
to retests. This amount is less than other more general estimates.
While this amount represents less than 1 percent of gross annual
income to an asphalt plant, retesting can mount up costs and erode
the margin out of which annual profits must come. Of course, it is
not necessarily true that cost of testing must come out of the
profits of a plant for a single year. Such costs logically repre-
sent part of the total plant investment. It is also possible that
cost of retesting where a control system had failed to meet speci-
fications might be borne partially by the vendor. Adequate data
6-22
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are not available, however, to assess any possible inflationary
effect on the price of asphalt products.
6.4 Levels for Opacity
EPA regional personnel have noted that most tests of asphalt
concrete plants subject to NSPS result in opacity readings within
the 20 percent limit and often well below this maximum allowable
level. These comments are consistent with the limited amount of
detailed data from tests for opacity, as discussed in Section 5.
Of the plants in the small test sample, over 80 percent were cer-
tified at opacity less than 20 percent. It is possible that in
addition to the approximately 25 percent tested at opacity less
than 5 percent, other plants may have exhibited a percentage this
low if more detailed reporting had been supplied. Under existing
regulations, a measurement of simply "less than 20 percent" is
adequate.
Opacity is clearly related to the amount of particulate loading
in the stack gas. The opacity standard was designed to provide an
easy visual means of determining that a plant is operating satis-
factorily as evidenced by the transparency of its stack exhaust.
Heavy emission of particulates leads to a dense plume, which
exhibits a higher percentage of opacity than one relatively free
of particulates.
A number of regional personnel have verified that opacity is
dominated by the particulate standard. That is, meeting the NSPS
6-23
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level of 90 mg/dscm (0.04 gr/dscf) for particulates implies a very
low opacity reading—much lower than the current standard of 20
percent. This judgment is supported by engineering experience. In
short, the consensus is that an opacity Beading above 5 percent is
generally associated with particulate emissions exceeding the NSPS.
As noted in Section 5, although the test sample is too small to
support definitive inferences, test results are consistent with this
view. Opacity readings in the upper range of 0 to 20 percent seem to
be associated with excess particulate emissions. Conversely, low
emission levels of particulates tend to imply very low opacity
percentages. Readings as low as 0 percent opacity were reported in
test results from plants for which the particulate mass loading was
well below the NSPS level.
Available data for defining the relationship between particu-
late levels and opacity percentages are not exact. Other factors
appear to influence the relationship, including size and color of
the particles emitted in the plume and path length of emissions.
(Cooper and Rossano, 1971; EPA, 1978).
6.5 Fugitive Emission Control
The types of emissions encompassed by the term "fugitive1' are
variously classified in the literature. In the interests of compre-
hensive consideration, three types are noted as to nature and source,
even though some authorities exclude the first from the category of
fugitive emission. The three types discussed are:
6-24
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• Scavenger or fugitive dust emissions (controlled under
NSPS)
*
• "Open source" emissions
• Miscellaneous emissions.
Scavenger emissions or "fugitive dust emissions" are those that
go through the ventline carrying "scavenger air" to the collector
from enclosed components of a continuous mix or batch plant. Since
the components involved are not represented in a dryer-drum mix
plant, scavenger emissions do not occur in a dryer-drum plant. The
scavenger air contains dust and gaseous emissions from the following
principal sources (Khan and Hughes, 1977; Patankar and Foster,
1978; NAPA, 1975; Robert et al., 1975):
• Hot aggregate elevator
• Vibrating screens
• Hot aggregate storage bins
• Weigh hopper
• Mixer.
Scavenger air may range from about 7 to over 25 percent of the
total system gas volume, depending on moisture content in the dryer
air (NAPA, 1973). The proportion of emissions in the scavenger air
may range around 15 to 25 percent of the total, as shown by relative
particulate loadings reported in representative tests (Danielson,
1973). One study estimated the particulate concentration from the
mixer alone to be about 2 percent of that from the dryer (Khan and
6-25
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Hughes, 1977). These scavenger emissions flow through the control
system and exit in the stack gas, so that they are controlled under
current NSPS. Hence, they are of no further concern in the present
context.
"Open source" emissions is a term sometimes applied to those
that emanate from stockpiles, cold plant towers, reject chutes, feed
bins, loading operations, and truck traffic around the plant. These
emissions are not now controlled (Khan and Hughes, 1977; Patankar
and Foster, 1978). One estimate is that they may constitute about
10 percent of the total dust output of an asphalt concrete plant
(Robert et al., 1975). However, as this estimate also includes
emissions from the hot material elevator which are usually vented
as part of the scavenger air, the estimate should be reduced.
No quantitative data are available on which to confirm or refine
this single estimate. Measuring such fugitive emissions is regarded
as difficult if not impossible to achieve. The rate is inevitably
affected by weather. It has been estimated that on a dry, windy
day, open source emissions may greatly exceed all others from an
asphalt concrete storage plant.
Control of open-source emissions appears to be inherent in
effective maintenance or good housekeeping practices (Khan and
Hughes, 1977). Wetting aggregate stockpiles, as recommended in
some reports, may not be in the best interests of an asphalt manu-
facturer because high moisture content decreases production and
6-26
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degrades efficiency. However, the National Asphalt Pavement Asso-
ciation urges its members to enclose the stockpiles and cold feed
bins to prevent dust blowing while simultaneously protecting the
aggregate from moisture (NAPA, 1978). Oiling traffic surfaces can
reduce particulate emissions but gives rise to small quantities of
hydrocarbon emissions.
Miscellaneous emissions occur during finished product discharge
to the trucks from the mixer. Particulates from this coated product
are extremely low—about 2 percent of the stack concentration.
However, gaseous emissions, particularly hydrocarbons (HC), occur at
an unknown rate. One study estimates the concentration of HC from
the mixer to be less than 3.5 ppm, or about 8 percent of that from
the stack. Polyeyelie organic material is estimated to be about
0.36 mg/m^, less than 1 percent of the rate in stack emissions
(Khan and Hughes, 1977). Because these emissions occur only during
the fraction of operational time when the mixer is engaged in dis-
charging its product, emissions on an hourly rate are very low.
Other minor sources of miscellaneous emissions include those from
handling and storage of raw liquid asphalt and from disposal of
mineral fines.
6.6 Changes in Tests and Procedures
6.6.1 Monitoring Requirements
As a means of ensuring that NSPS are maintained, monitoring re-
quirements are sometimes specified. However, they are not specified
6-27
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in the Code of Federal Regulations for asphalt concrete plants (40
CFR 60).
By its very nature, the asphalt concrete industry appears to be
a poor candidate for continuous monitoring. Because of the intermit-
tent operations involved in virtually all plants, monitoring cannot
be continuous, but would consist of a sequence of start-and-stop
measurement operations maintained over varying periods of time. As
previously noted, an asphalt concrete plant operates on the average
of 666 hr/yr. Reinstallation, calibration, and all the fine
adjustments necessary for accurate monitoring would be necessary.
Several processes have been developed for continuous monitoring
of particulate emissions, including photometric detection, use of
tape detectors, chemical determination, and beta-ray attenuation
(Cooper and Rossano, 1971). None of these was developed for asphalt
concrete manufacture. The cost and complexity of these processes
makes them ill-suited to an industry where no gain in process con-
trol can be expected; thus, they are an additional expense.
Periodic testing for opacity percentage by visual means is a
relatively cheap and readily performed operation. It is provided
for in current regulations and can be used to indicate whether a
soundly designed control system, once installed and determined to
meet NSPS requirements, is maintained and operated at the proper
level of efficiency. In most instances, even admission to the plant
area itself is not required.
6-28
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6.6.2 Production Penalty
The change considered in this subsection relates to a procedure
applied in administering compliance. It does not concern emission
levels established by NSPS or regulations for monitoring.
It is a common practice in certifying plants as in compliance
to limit the production capacity when testing occurs at substantially
less than the full production rate. In some regions, the procedure
is reportedly to certify the plant at the tested level plus 23 Mg (25
tons)/hr (MITRE Corp., 1978). The plant can then be cited for
violating regulations if it attempts to produce at capacity without
retesting. This procedure appears to be pursuant to the requirement
in 40 CFR 60 that testing be conducted under "representative condi-
tions." If a plant must operate a greater number of hours to produce
a given quantity of asphalt, the production cost per unit mass will
be raised, since these costs increase directly with the hours of
plant operation. Moreover, asphalt plants are designed to operate
at a specific production rate that is optimum in terms of efficiency.
If required to operate significantly below this rate, reduction
in this maximum possible efficiency results in further indirect
costs for each unit mass of asphalt produced. Individual plants
thus affected are placed at a competitive disadvantage.
Regional EPA officials and others who certify compliance are not
unreasonable in attaching a production ceiling. Indeed, the practice
6-29
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can be justified on technical grounds which are complex and somewhat
controversial.
Production at less than full capacity is required when the per-
centage of moisture removed from the aggregate raises the ratio of
actual volume per minute per mass of product to a level that would
exceed the capacity of the exhaust fan.
Assuming that the efficiency of the collector system and the
inlet mass loading per unit mass of aggregate remain somewhat constant
for a given type of aggregate, then higher emission rates can be
expected if the flow rate of stack gas decreases. Thus, when the
plant operates at substantially higher production levels, the same
output in mass of particulate per mass of product should result. But
in this situation the rate measured will be higher because the
increase in production will exceed the increase in flow rate. For
example, a plant with a control system operating at 99.9 percent
efficiency, which will just meet current NSPS particulate levels at
about 187.3 dscm/Mg (6000 dscf/ton) at a given inlet mass loading,
will exceed 103 mg/dscm (0.045 gr/dscf) as the flow rate decreases by
12.5 percent to 163.9 dscm/Mg (5250 dscf/ton). Certifying officials
have good reason for somewhat limited views of how far compliance has
been demonstrated for higher production rates.
Some situations may result in a hardship to the plant operator
and achieve no environmental gains. Test measurements may imply
6-30
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that if the plant had been operating at full capacity, all other
conditions remaining the same, it would have been in compliance.
This situation may occur when a plant is being operated at a produc-
tion rate which is maximum for the percentage of moisture in the
aggregate and is very different from one in which a marginal control
system is maneuvered into meeting the NSPS level by adjusting produc-
tion. By extrapolating the observed results in controlled mass
loading and the flow rate to operation at full capacity it may be
possible to estimate whether the plant would have met NSPS if
operating at a higher rate.
6.6.3 Exemptions for Small Plants
Concern has been expressed by EPA regional personnel that formal
testing for particulates places a burden on small plants. The sug-
gestion has been made that these plants be exempted from this
requirement (MITRE Corp., 1978).
The direct costs of testing for rate of particulate emissions,
which vary from less than $2000 to as much as $5000, may be compared
with published figures on gross income for asphalt companies. As
noted in Section 6.3, testing costs would not necessarily come out
of income for a single year. A survey of member companies by the
National Asphalt Pavement Association reports results from about 850
plants (not distinguished as to NSPS status) producing 59 x 10*> Mg
(65 million tons) (about 22 percent of total national production).
6-31
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On the basis of this survey, plants producing less than 90,700 Mg
(100,000 tons)/year (less than 136 Mg (150 tons)/hour have produced
about 9 percent of the total asphalt tonnage; the annual gross income
per plant was about $465,000. This figure contrasts with an average
of all plants of from about $700,000 to $800,000 per plant. The
higher figure represents results based on the survey, whereas the
lower figure is derived by dividing the estimated national total of
4500 asphalt plants into the total value of hot mix asphalt (NAPA,
1977).
Thus, it is clear that the burden can weigh inequitably on small
plants simply because of their smaller income. In terms of gross
income, the costs remain a small fraction for plants of all sizes.
A price range of about $2000 to $2500 for test costs represents no
more than about 136 Mg 0.5 percent of the gross income for plants in
the group producing less than 136 Mg (150 tons)/hour. The distribu-
tion of plants by size below 136 Mg (150 tons)/hour is not known;
however, it is known that plants of 109 Mg (120 ton)/hr capacity or
less now represent about 12 percent of the total. Detailed data on
cost margins of asphalt plants are unavailable.
It is not reasonable to consider exempting small plants entirely
from NSPS. It can be shown that in the extreme the resulting parti-
culates from all such plants becoming subject to current standards in
any one year could exceed the total emissions calculated in Section
6-32
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6.3 for all new plants, if they meet current NSPS level. Extreme
results are derived from the use of the emission factor (EPA, 1973a)
of 0.85 kg/Mg (1.7 Ib/ton) with use of a cyclone as control device,
and from estimates of total production from the plants at issue as
slightly under 907,000 Mg (1 million tons) in the first year of
operation. The latter production estimate is obtained by the assump-
tion that the fraction of 9 percent of the total mass of asphalt
produced by plants with capacities less than 136 Mg (150 tons)/hr
applies also to the ratio of new and modified plants (0.0333) to all
asphalt plants.
Merely exempting small plants from formal particulate testing,
as originally proposed by EPA regional personnel, would have a very
small environmental impact. Plants still subject to NSPS would have
to be certified on a basis other than formal tests (as provided for
in 40 CFR 60). They might be required to satisfy the certifying
officials that they had installed a suitable control system, such as
a baghouse or wet venturi scrubber, and meet a satisfactory opacity
level as determined by visual observation (i.e., equal to or sub-
stantially less than the NSPS level of 20 percent). If such plants
provide about 9 percent of the annual asphalt production from plants
newly subject to NSPS, then at a level of 90 mg/dscm (0.04 gr/dscf)
they would be expected to emit about 25.4 Mg (28 tons) of particulates
per year. An increase of 10 percent would not exceed 2.7 Mg (3 tons)
6-33
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annually. At a level of success in predicting control system per-
formance that approaches 100 percent, no appreciable increase in
particulate emissions would be expected.
It is not certain what the rate of success would be in predict-
ing that a control system will perform satisfactorily and meet the
standard for particulates. The failure rate of approximately 20 per-
cent observed in the sample of test results examined in this analysis
appears high as an estimate of the frequency of error in certifying a
control system as adequate. It must be assumed that assurance would
be required on engineering details in design and installation of the
system, which would sharply reduce the likelihood of failure from
that reflected in the essentially random sample of baghouse and
venturi scrubber system in the available test data. On this basis,
20 percent of the mass produced annually by the plants involved would
result in somewhat higher emission rates of particulates. If these
rates are approximately 1.5 times the rate achieved under full
control, then the overall increase of about 10 percent in particulate
emissions would be expected.
Comparing the estimates of 10 percent increase in particulate
emissions with the estimate of 286 Mg (315 tons) emitted per year by
plants newly subject to NSPS indicates that the amount would be
approximately 29 Mg (32 tons) annually.
In contrast to these factors favoring exemption of small plants
from the particulate test are other offsetting factors. Since only
6-34
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new or modified plants are tested, these tend to be large units.
Thus, the number of small units exempt from the test would be
relatively small. In addition, the particulate test is the only
way to determine independently that the installed control system is
operating as designed. In an economic sense the cost of the test can
be considered to be one of the necessary costs of plant construction
and treated accordingly during planning.
6.6.4 Waiving of Particulate Tests
It has also been suggested by some EPA regional personnel that
wherever a plant has properly installed a well designed particulate
control system, formal testing could be waived and compliance granted.
This idea is appealing, both because of the savings to the asphalt
industry and because of the procedural simplification for officials.
These estimates could be considerably refined given comprehen-
sive and detailed data on the performance of venturi scrubbers and
baghouses under various conditions and on the emission rates of
plants so equipped.
It can be seen, however, that the order of magnitude of the
possible increase is extremely small. Even an error of 100 percent
in the estimate of 29 Mg (32 tons) per year would still give a figure
less than 0.1 percent of the annual estimate of particulates (63,500
Mg/70,000 tons) from the asphalt industry as a whole (Khan and
Hughes, 1977).
6-35
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6.7 Control of Other Pollutants
Other pollutants emitted by asphalt concrete plants are nitrogen
oxides (NOX), sulfur dioxide (S02>, hydrocarbons (HC) and carbon
monoxide (CO). Estimates of the amount of each emitted annually by
all asphalt concrete plants and by the estimated 150 new and modified
plants coming under NSPS each year are given in Table 6-2. The amount
of these pollutants emitted is very small and does not exceed 454 mg
(500 tons) per year for any pollutant. The contribution of the
asphalt industry to national emissions in these categories is minute;
no pollutant emission reaches as much as 1/10 of 1 percent of national
emissions from stationary sources. The fractional percentage of
total emissions in the U.S. for any category is even lower—by as
much as an order of magnitude.
It may be noted that the fraction of total national emissions
resulting from asphalt concrete plants is smaller for the above
pollutants—even though they are not expressly controlled—than
is true for particulate emissions from asphalt plants, which are
controlled. The total of about 63,500 Mg (70,000 tons) of particu-
lates emitted from asphalt plants is about 0.35 percent of all
particulate emissions nationwide (about 17.9 million Mg or 19.7
million tons). This percentage is approximately one order of
magnitude higher than the percentage of other emissions in the
respective national totals (Khan and Hughes, 1977).
6-36
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TABLE 6-2
CONTRIBUTION OF ASPHALT HOT MIX
INDUSTRY TO NATIONAL EMISSIONS OF OTHER POLLUTANTS
Pollutant
Sulfar Oxides
Nitrogen Oxides
Hydrocarbons
Carbon Monoxide
National
Emissions3
(106 Mg/yr)
29.96
22.25
25.06
96.89
Total Emissions
from Stationary
Sources
(106 Mg/yr)
29.36
13.35
8.68
22.28
Emissions from Hot-Mix Asphalt
106 Mg/Yr
All Plants3
0.014
0.007
0.005C
0.008
New Plants
Each Yeard
0.00045
0.00024
0.00018
0.00027
Percent Asphalt Plant Emissions
Total
Sources
0.05
0.033
0.022
0.008
Stationary
Sources
0.05
0.06
0.06
0.04
aKhan and Hughes, 1977.
^Estimated from percent of stationary to total, 1972 and 1973 (Khan and Hughes, 1977;
EPA, 1976).
°As methane equivalent.
Based on EPA estimate of 150 new plants each year or 3.33 percent of total industry.
-------
The stack emission rates of NOX and SC>2 estimated for asphalt
concrete plants are far lower than those set by NSPS for these
pollutants from acid plants. The rate of less than 0.05 kg/Mg (0.1
Ib/ton) for SC>2 compares with the 2 kg/Mg (4 Ib/ton) set for
sulfuric acid plants. Similarly, the rate of 0.25 kg/rag (0.5 Ib/ton)
for NOX is a small fraction of the NSPS emission rate of 1.5 kg/rag
(3 Ib/ton) set for nitric acid plants (Khan and Hughes, 1977; 40 CFR
60). The previously mentioned pollutants—NOX, S02, HC and CO—
are controlled to some degree by the use of scrubbers or fabric
filters which wash out or trap impurities. S02 is also reduced by
the use of limestone or dolomite which is estimated to make up 85
percent of all aggregate in asphalt concrete mixes. These substances
are widely used catalytically in scrubbers for S02 reduction.
The emission rate of HC for drum-mix or dryer-drum plants has
not been determined experimentally but is believed to be greater than
that of conventional (batch or continuous mix) processes (Robert et
al., 1975). Therefore, the drum-mix plant should be considered in a
somewhat special category as meriting study specifically to determine
whether its HC emissions are environmentally significant. This
issue is especially important in view of the anticipated growth of
the drum-mix process.
6.8 Use of Liquefied Asphalt Cutbacks
The suggestion has been raised by EPA regional personnel that
the use of "cutbacks" in application of liquefied asphalt be elim-
6-38
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inated or sharply reduced (MITRE Corp., 1978). A similar interest
has also been expressed by the asphalt industry (NAPA, 1978).
The issue is somewhat tangential to the present review because it
is concerned with the application of liquefied asphalt in surfacing
operations, rather than with production of asphalt concrete, as
specified under current NSPS. The term cutback refers to liquefied
products in which the asphalt is cut back or diluted by kerosene or
other volatile HC fluids for use as a surfacing material. However,
because the issue was raised in the context of the present study and
because restriction of cutbacks provides the real opportunity for
reducing HC emissions from asphalt products, it is briefly discussed
here.
Recent studies by or under contract to EPA have confirmed the
significance of cutbacks in surfacing operations as a source of HC
emissions (Kirwan and Maday, 1977; Midwest Research Institute, 1978).
It is estimated that well over 2 percent of national HC emissions
result from cutbacks used in pavements and other surfaces. Cutbacks
were found in laboratory tests to emit HC at a peak rate within the
first minute of exposure. It was also found both in the laboratory
and from field samples that such emissions continue at a diminished
rate for long periods. Some samples taken from highways in the
Midwest were emitting HC more than 3 years after paving operations
6-39
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were completed. It Is in the interests of the asphalt industry to
substitute other forms of liquefied asphalt for cutbacks. For
example, significant reductions in energy requirements and savings
of fuel can result from substituting water-based emulsions. Such
alternative products can be generally used, although cutbacks may
continue to be required in surfacing operations at temperatures
below about 10°C (50° F).
6-40
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7.0 CONCLUSIONS
7.1 NSPS for Particulate Emissions
7.1.1 Retention of Present Level
Current NSPS of 90 mg/dscm (0.04 gr/dscf) for particulate emis-
sions are being satisfactorily met. No basis exists for relaxing the
standards, since more than three-fourths of the plants tested met
current NSPS, and there have been no reports of any excessive
numbers of failures. The fact that many plants are able to
achieve even lower particulate emission rates is evident both from
test results and from the fact that standards of 68 mg/dscm (0.03 gr/
dscf) have been successfully implemented in a few states. Thus, it
would clearly be possible to tighten the standards.
However, it is concluded that no change should be made in
current NSPS at the present time for the following reasons.
7.1.2 Justification for Retention
• The current standards are sufficiently stringent.
Current standards are being met, sometimes at levels
notably less than 90 mg/dscm (O.OA gr/dscf); however, a
significant number of failures have occurred. Nearly
one-fifth of the plants equipped with one of the control
systems considered representative did not achieve compli-
ance. Two of the plants using fabric filters achieved
compliance only after a second test. Of the tests invol-
ving plants known to be using either a baghouse or a scrub-
ber of the venturi type, about 25 percent of those achieving
compliance would not have achieved the level of 68 mg/dscm
(0.03 gr/dscf). This indicates that even the devices
counted on to achieve greater particulate control, while
succeeding most of the time and sometimes even surpassing
present requirements, cannot always be relied upon to meet
7-1
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stricter standards in all situations because of the possi-
bility of faulty equipment design or of inadequate equip-
ment maintenance programs.
• Achieving the standards is subject to variability in the
aggregate.
Variations in the distribution of particles within the
fines can result in higher emission rates than usual.
The likelihood of test failure from this cause is in-
creased as the standards become more restrictive.
Variability in aggregates used has been reported and
could be involved in the variability of test results
observed. Aggregates used in some parts of the country
are known to be particularly high in fines passing 200
mesh screen and in the finer particles much below 74
microns. For example, the sand used in the southeastern
U.S. and other types of aggregate are clayey or contain
very fine silt (Barber-Greene, 1976; NAPA, 1978). Fac-
tors that are not known include the distribution of
particle sizes below 20, 10 and 5 microns in particular
types of aggregate and the inlet mass loading as a func-
tion of aggregate characteristics. The occasional labo-
ratory tests that have been conducted are inconclusive.
In some such tests, baghouses have met current standards
with little difference regardless of the aggregate
used. In other tests, however, variations have occurred
in both the inlet loading and the output emission rates
(EPA, 1978; University of Texas, 1973).
• Efficiencies required of control systems may already be at
the limits of technological capability.
As noted in Sections 4 and 6, the efficiencies demanded
of control systems by current NSPS for particulates are
already quite high, both in relation to the rated effi-
ciencies and the theoretical maximum. This fact applies
particularly to collectors used with aggregates having
a high distribution of small particles, for which rated
efficiencies of control devices may be relatively low.
Certainly the efficiencies demanded are extremely high if
based on the EPA average factor for uncontrolled emissions
of 22.5 kg/Mg (45 Ib/ton) of product (EPA, 1973a). Actual
efficiencies achieved in compliance testing are largely
unknown because inlet loadings are seldom measured. To do
so would be both difficult and expensive. Test data avail-
able during present analysis indicate a practice of scoring
7-2
-------
efficiencies by comparison of test results with the
EPA estimate of 22.5 kg/Mg (45 Ib/ton) (using known
parameters of production and of flow rate in dry
standard volume) (New York State, 1976).
Test results in the sample analyzed indicate that
stricter particulate levels could probably be met
in most instances through a baghouse combined with a
cyclone as primary collector.
• The possible environmental gain would be slight.
The maximum reduction of about 73 Mg (80 tons) annual-
ly for each 23 mg/dscm (0.01 gr/dscf) by which the
standard is tightened is a small fraction (about 0.11
percent) of the total annual particulate emissions of
asphalt plants at present and is infinitesimal com-
pared with the national level from all sources. The
cost and other administrative burden to EPA in promul-
gating new standards may not presently be justified by
environmental benefits.
7.1.3 Clarification of Items
An important question is the role of dryer-drum mix plants in
the asphalt industry and the performance of control systems installed
in such plants. As already noted, it was formerly believed that
baghouses could not be used because of the tendency of the sticky,
asphalt-coated particles to clog the fabric filters. Although experi-
ence in some plants has demonstrated that baghouses can be effective,
further information is needed. The remarkable results reported by
Khan and Hughes (1977) raise a valid question as to whether different
particulate emission levels are required for drum-mix plants in order
to ensure that future plants subject to NSPS install BTS. These
results are somewhat at variance both with those reported by Patankar
and Foster (1978) and with those in the test data supplied by EPA
7-3
-------
regional personnel. However, the discrepancies imply the need for a
study to define what the controlled rate actually is and what levels
of particulate emissions would be expected from properly designed and
operated venturi scrubbers and baghouses. If the trend toward dryer-
drum mix plants approaches the predicted level of 85 percent, it will
be particularly important for consideration of future changes in NSPS
to know how these plants and their control systems operate in regard
to particulate levels.
Also related to a tighter standard is the education of owners to
the need for equipment that is well engineered, maintained, and oper-
ated. Owners need better guidance on the performance and cost bene-
fits of baghouses vs. scrubbers.
7.2 NSPS for Opacity
7.2.1 Justification for Retention
Although the results of the relatively small body of data avail-
able on opacity tests indicate that it would be feasible to tighten
the current standard of less than 20 percent opacity, no significant
environmental gain would be achieved. The cost and administrative
burden to EPA and other officials both inside and outside of the
Federal Government would be unwarranted.
In asphalt concrete plants the opacity standard is essentially
dominated by the NSPS for particulate emissions. When a particulate
level less than or equal to 90 mg/dscm (0.04 gr/dscf) is met, the
opacity is much lower than 20 percent. Tests results are consistent
7-4
-------
with experienced engineering judgment than an opacity reading of
higher than 5 percent will be associated with a plant in which the
particulate emissions exceed NSPS. Tightening the opacity standards
to require a lower percentage will not in itself reduce pollutants or
otherwise aid in protection of the environment. Indeed, a lower
percentage for opacity will normally be achieved as the automatic
result of an emission rate for particulates of less than or equal to
90 mg/dscm (0.04 gr/dscf).
7.2.2 Actual Correlation Between Opacity and Particulate
Emissions
The exact correlation between opacity readings and rate of par-
ticulate emissions for asphalt concrete plants is unresolved.
However, some inference can be drawn from related studies. One such
study involved a survey of member companies of the Industrial Gas
Cleaning Institute (Stastny, 1973). The member companies were asked
to express an opinion as to what emission level would generally
produce clear or near clear stacks for 42 industrial applications
Unfortunately, asphalt concrete plants were not included in the 42.
However, in the rock products category data were given for seven
operations which included:
• Dry cement kilns
• Wet cement kilns
• Gypsum
• Alumina
7-5
-------
• Alumina
• Lime
• Bauxite
• Magnesium oxide.
The average mass loading for these industries, which would
yield no visible emissions (except condensed water vapor), was 84 rag/
dscm (0.037 gr/dscf) with actual values ranging from 55 to 110 mg
(0.024 to 0.048 gr). If it is assumed that there are significant
similarities between these industries and the drying of asphalt
aggregates, then the NSPS level of 90 mg/dscm (0.04 gr/dscf) should
produce near clear stacks. The overall average for the 42 industries
was 76 mg/dscm (0.034 gr/dscf).
It is unlikely that opacity readings alone could ever provide
a legal basis on which to certify a plant as in compliance with
particulate NSPS. Opacity readings reflect a number of variables
in addition to particulate loading (Stastny, 1973). Among these
are path length, angle of incidence of the light, moisture content
of effluent, weather conditions and process changes. The opacity
standard should be set at a level such that the specific features
of all plants with BTS fully meeting the NSPS for particulates
will be in compliance. On this basis, the opacity level as now
set appears to satisfactorily reflect the numerous considerations
involved. Existing regulations provide for subsequent opacity
readings to be taken on plants where it is suspected that improperly
7-6
-------
functioning equipment may be causing excess particulate emissions.
(EPA 1978a). These considerations reinforce the conclusion
already stated that no change in opacity levels is warranted now.
7.3 Testing Procedure
7.3.1 Waiving of Formal Particulate Testing
7.3.1.1 Small Plants. The cost of formal particulate testing
places a disproportionate burden on small plants of less than about
36 Mg /hr (150 tons/hr). These costs could be eliminated at very
little expense to the environment; increase in particulate emissions
would be from near 0 to about 2.7 Mg (3 tons) per year for plants
newly subject to NSPS if these plants were exempt from formal testing
but required to be certified on the basis of control system.
On the other hand, there are considerations militating against
such a policy. The actual cost of testing is small in relation
to net income over the life of the plant. The precedent implied by
granting a blanket exemption may be undesirable. There are also
procedures providing for the use of alternative methods in certain cases,
It is concluded that class exemption of plants of any size is presently
unwarranted.
7.3.1.2 Other Plants. All asphalt concrete plants might be
certified on the basis of optimal control systems and opacity readings
of less than about 5 percent. The result would be substantial
savings to the industry and minimal environmental risk. However,
7-7
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there are considerations militating against such a policy, in addi-
tion to the difficulty of correlating opacity with particulate emis-
sion level. One of these is the difficulty of providing accurate
opacity readings from wet plumes such as those that occur with ven-
turi scrubbers. Perhaps more important is the uncertainty of pre-
dicting efficiency of even those control systems of the types.
considered representative of BTS. It is therefore concluded that no
effort should be made to implement a certification policy on parti-
culate testing based solely on the presence of one optional control
system and an opacity reading of less than 5 percent*
7.3.2 Production Penalty
Attaching a production penalty (i.e., celling on maximum produc-
tion authorized), as now practiced in certifying plants tested at
less than capacity, may result in a hardship to some plants. Where
moisture is the factor limiting production, any environmental gains
expected from this practice are minimal. Therefore, explicit guide-
lines should be considered to eliminate the possible hardship which
may be Imposed upon individual operators in the asphalt industry.
The crucial question is whether the rate of uncontrolled emissions
(i.e., the inlet loading to the control system) remains the same per
ton of product under varying degrees of moisture in the aggregate.
Presently, there is a lack of experiential data to answer this
question. The matter should be thoroughly investigated.
7-8
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7.4 Control of Other Pollutants
7.4.1 Pollutants Involved
The other pollutants (NOX, SO2, HC and CO) are emitted by
asphalt concrete plants in amounts that are very small when compared
with:
1. Total national emissions
2. The rates achieved by controlled Industries
3. The rates for particulate emissions even under current
NSPS.
No apparent need exists at this time to consider NSPS for emissions
of NOX, S02 or CO from asphalt plants generally or for HC emis-
sions from conventional plants (i.e., batch and continuous mix).
7.4.2 HC Rates from Drum Mix Plants
The rates for HC reflect a state of the industry in which
dryer-drum mix plants represent less than 3 percent of the total.
It is not known to what extent the expected growth up to 85 percent
of the total of these plants will have on overall HC emissions, since
the rate for such plants is not established. A study to determine
the HC emission rates from dryer-drum mix plants is warranted.
7.4.3 HC Emissions from Cutbacks
The big source of HC emissions from asphalt is in liquefied
asphalt cutbacks. Although this issue is somewhat tangential to
the present study, it does represent the most effective way to
reduce HC emissions from industrial use of asphalt. Therefore, it
7-9
-------
is concluded, that work now in progress under EPA aegis should
continue toward development of regulations restricting the use of
liquefied asphalt cutbacks and promoting the use of emulsions.
7.4.A Emissions from Recycling Plants
It is concluded that determination of particulate emission
rates is needed from plants that recycle asphalt concrete. The
effectiveness of baghouses and venturi scrubbers under various
operating conditions as defined by process parameters should also
be determined.
7.4.5 Emissions from Hot Water Emulsion Mixes
It is concluded that particulate emissions that occur from use
of hot water emulsion mixes in asphalt concrete production should be
determined as well as of suitable means to control those with avail-
able equ ipmen t.
7-10
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8.0 RECOMMENDATIONS
Recommendations to EPA regarding NSPS for asphalt concrete
plants fall into two categories: specific changes in the regulations
and unresolved issues or areas warranting further investigation.
Specific changes involve the development of an enforcement policy
covering testing and certification regulations. Further study is
needed in regard to the unresolved issues of percent of opacity and
level of particulates, R&D of uncontrolled particulate and hydro-
carbon emissions from dryer-drum mix plants, standards for cutbacks,
and the technology for development and use of improved control
devices.
8.1 Specific Changes in Regulations
8.1.1 Current Levels of Pollutants
As of this review, no changes in the current levels of standards
for pollutants (particulates and opacity) from asphalt concrete
plants are recommended. Both standards — 90 mg/dscm (0.04 gr/dscf)
and less than or equal to 20 percent opacity — should be retained
for the present.
8.1.2 NSPS Applied to Emission of Other Pollutants
There is no need for NSPS to be applied to the emission of any
other pollutants or to be extended to any other sources from hot-mix
asphalt concrete plants; therefore, none should be promulgated at
this time.
8-1
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8.1.3 Enforcement Policy
No change is recommended in the requirement to test asphalt
plants of all sizes. Development of an enforcement policy regarding
the testing and certification of plants as in compliance should be
considered. Research and development to define inlet loadings from
different degrees of moisture in the aggregate should be carried out
to determine whether a change is warranted.
The production penalty (a ceiling on production at some small
increment above the production at which tested) should be removed so
that all plants can be certified up to production capacity based on
the following:
1. Particulate testing at a level less than 90 mg/dscm
(0.04 gr/dscf) when operating at a production level
that represents full capacity for the percentage of
moisture in the aggregate used which can be determined
from mathematical tables to correspond to a rate no
higher than 90 mg/dscm (0.04 gr/dscf) at the nominal
capacity of the plant.
2. Installation of a soundly designed fabric filter or wet
scrubber system of the orifice or venturi type.
8.2 Areas of Further Investigation
8.2.1 Percent of Opacity and Level of Particulates
It is unlikely that a precise correlation between opacity and
particulate emissions exists which is precise enough to ever serve
as a basis for certifying plants as in compliance based on percent-
age of opacity alone. However, a more definitive relationship
between these two measures in which the effect of other variables
8-2
-------
(e.g., path length, process changes) are taken into account could
improve the use of opacity reading as a surveillance tool to ensure
continuing compliance with NSPS for particulates. It is therefore
recommended that further study be undertaken by the appropriate
organizational units in EPA to meet this objective.
8.2.2 Determination of Uncontrolled EC Emissions From
Drum Mix Plants
Further development activities are needed to determine the rate
of uncontrolled HC emissions from dryer-drum mix asphalt plants as
a function of significant variables, such as production rate and
exit-gas velocity, in dry standard volume per minute. The basis for
promulgating NSPS for HC emissions should be: (1) a finding that the
HC emission rate is on the average greater than 4.54 kg (10 lb)/hr/
plant (which is 1.25 times the maximum rate permitted under Los
Angeles Rule 66 as federally modified) and (2) a growth rate that
indicates that dryer-drum mix plants will exceed 50 percent of all
new plants by 1982.
8.2.3 Technology for Development and Use of Improved Control
Devices
Further development activities are needed to develop reliable
projections on inlet loading to control devices (i.e., uncontrolled
emission rates) for each type of asphalt plant (continuous mix,
batch, and dryer-drum mix) as a function of aggregate input. Pro-
jections for exit-gas flow rate and projections of distribution of
particle size in the uncontrolled emission should also be determined.
8-3
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These could be used in evaluating expected results of control systems
under operating conditions and, hence, become a basis for possible
future modification of the particulate emission standards.
In addition, it is recommended that an educational program be
considered, either sponsored and organized by EPA or the industry,
for the purpose of providing better guidance to owners on need for
well engineered, maintained and operated control devices. This pro-
gram should include detailed information on the performance and cost
benefits associated with baghouses and scrubbers.
8.2.4 Control of Particulates from Recycling Plants
Determination should be made of the effectiveness of BTS in
controlling particulate emissions from plants which recycle asphalt
pavement. If, as some evidence indicates, emissions from these
plants exceed NSPS even when equipped with collector systems that
adequately control emissions from plants using virgin material, a
study should be made of the extent to which recycling conserves
energy and alleviates the solid waste disposal problem. Findings in
these areas of investigation should be used in considering whether
plants recycling asphalt pavement warrant a separate and less strin-
gent standard for particulate emissions.
8.2.5 Control of Emissions from Hot-Water Emulsions
A study should be made of particulate emissions vented to the
atmosphere from asphalt concrete plants using hot-water emulsions.
This investigation should include particularly the effectiveness
8-4
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of existing equipment to control such emissions during the sudden
surge of steam when hot water is added directly to the pugmill.
An objective should be to determine what, if any, modifications
to operating practice and control technology are necessary if
significant emissions are occurring as a result of bypassing the
control system*
8.2.6 Standards for Cutbacks
Continued study should be made to develop standards on the use
of cutbacks in application of liquefied asphalt.
8-5
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9.0 REFERENCES
American Air Filter Co., 1978. Dust Control Bulletin. DC-1-294D.
Barber-Greene Company, 1976. Bituminous Construction Handbook.
Aurora, 111. Fifth Edition.
Bradway, R.M. and Cass, R.W., 1975. Fractional Efficiency of a
Utility Boiler Baghouse: NUCLA Generating Plant, EPA-600/2-75-013a.
U.S. Environmental Protection Agency, Research Triangle Park, N.C.
Calvert, S.J., J. Goldshmid, D. Leith and 0. Mehta, 1972. Wet
Scrubber System Study, Vol. II. Wet Scrubber Handbook. Prepared by
Ambient Purification Technology, Inc. for U.S. Environmental Protec-
tion Agency. NTIS PB 213 016.
Cass, R.W. and R.M. Bradway, 1976. Fractional Efficiency of a
Utility Boiler Baghouse: Sunbury Steam - Electric Station. EPA-600/
2-76-077a. U.S. Environmental Protection Agency, Research Triangle
Park, N.C.
Code of Federal Regulations, 1977. Standards of Performance for
Asphalt Concrete Plants. 40 CFR 60 Subpart I. Office of the
Federal Register, U.S. Government Printing Office, Washington, D.C.
Cooper, H.B.H. and A.T. Rossano, Jr., 1971. Source Testing for
Air Pollution Control. Environmental Science Services. 24 Danbury
Rd., Wilton, Conn.
Grim, J.A., et al., 1971. Asphaltic Concrete Plants. Atmospheric
Emissions Study. Valentine, Fisher and Torn!inson. APTD 093.
EPA-68-02-0076. Seattle, Wash.
Danielson, J.A., ed., 1973. Air Pollution Engineering Manual.
County of Los Angeles, Air Pollution Control District. National
Center for Air Pollution Control. Cincinnati, Ohio. PHS Pub 999
AP 40.
Entropy Environmental, 1978. Personal communication with Mr. Walter
Smith, August 24.
Espenshade, E.B., Jr., ed., 1970. Goode's World Atlas. Thirteenth
Edition. Rand McNally and Company. Chicago, 111.
9-1
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Federal Register, Vol. 39, Pages 9308-9309 - Friday, March 8, 1974.
Forsten, N.H., 1978. Applications of Fabric Filters to Asphalt
Plants. Presented at 71st Annual Meeting of the Air Pollution Control
Association, Houston, Tex. June 25-30.
Foster, C.R., 1977. Theoretical Computarions of the Fuel
Used and the Exhaust produced in Drying Aggregates. National
Asphalt Pavement Association. Information Series 61. Riverdale, Md.
Harmon, D.L., 1977. Field Tests with a Mobile Fabric Filter.
Published in proceedings of Third Symposium on Fabric Filters for
Particulate Collection. U.S. Environmental Protection Agency, EPA-600/
7-78-087.
International Petroleum Encyclopedia, 1977. John C. McCaslin (ed.).
The Petroleum Publishing Company, Tulsa, Okla.
JACA Corp., 1977. Identification of Asphalt Concrete Plants.
Prepared for U.S. Environmental Protection Agency. Fort Washington,
Pa.
Khan, Z.S. and T.W. Hughes, 1977. Hydrocarbon Emissions from Asphalt
Plants. Prepared for seminar on Asphalt Industry Environmental
Solutions, Cincinnati, Ohio. March 1-2, 1978. Monsanto Research
Corporation. Dayton, Ohio.
Kinsey, J.S., 1976. An Evaluation of Control Systems and Mass
Emission Rates from Dryer-Drum Hot Asphalt Plants. Journal of the
Air Pollution Control Association. 26(12):1163-1165.
Kirwan, F.M., and C. Maday, 1977. Some Air Quality and Energy Conser-
vation Considerations for the Use of Emulsions to Replace Asphalt
Cutbacks in Certain Paving Operations. U.S. Environmental Protection
Agency. Strategies and Air Standards Division, Office of Air Quality
Planning and Standards. Research Triangle Park, N.C. March.
Lamb, G.E., P.A. Costanza and D. O'Meara 1978. Contributing Role of
Single Fiber Properties to Nonwoven Fabric Performance. Published in
proceedings of Third Symposium on Fabric Filters for Particulate
Collection. U.S. Environmental Protection Agency. EPA-600/7-78-087.
McKenna, D., 1974. Applying Fabric Filtration to Coal Fired Indus-
trial Boilers. Office of Research and Development, U.S. Environmental
Protection Agency. EPA-650/2-74-058.
9-2
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Midwest Research Institute, 1978. Nonmethane Organic Emissions from
Asphalt Pavement. Draft Final Report. Prepared under Contract to
EPA. Kansas City, Mo.
MITRE Corporation, 1978. Regional Views on NSPS for Selected
Categories. MITRE Technical Report-7772. Mitre Corporation/Metrek
Division. McLean, Va.
Moe, T., 1978. Dryer-Drum Plants. Paper presented at the Seminar
on Asphalt Industry Environmental Solutions; March 1-2, Cincinnati,
Ohio. Barker-Greene Co. Aurora, 111.
National Asphalt Pavement Association (NAPA), 1975. The Maintenance
and Operation of Exhaust Systems in the Hot Mix Plant. Information
Series 52 (2nd edition) and 52A (Combined Volumes). Riverdale, Md.
National Asphalt Pavement Association (NAPA), 1977. Hot Mix Asphalt
Plant and Production Facts 1975-76. Information Series 64. River-
dale, Md.
National Asphalt Pavement Association (NAPA), 1977a. Theoretical
Computations of the Fuel Oil Used and the Exhaust Produced in
Drying Aggregates. Information Series 61. Riverdale, Md.
National Asphalt Pavement Association (NAPA), 1978. Informal Meeting
with Fred Kloiber of NAPA and L. Duncan, J. Watson, and R. Brooks
of the MITRE Corporation, March 27, 1978, and subsequent informal
discussions. Riverdale, Md.
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Emissions from the Asphalt Industry. PB-277 372. December.
Nelson, J.E., 1978. Personal communications, May 1978. E.D.
Etnyre & Co. 200 E. Jefferson St., Oregon, 111.
New York State, 1976; 1977. Department of Environmental Conservation.
Test results from asphalt plants.
Oil and Gas Journal, 1978. Industry Statistics, October 10, 1978.
(76)42:126. Petroleum Publishing Company, Tulsa, Okla.
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Patankar, U. and K.E. Foster, 1978. Control Technology Evaluation
of the Drum-Mix Process for Asphalt Concrete Manufacturing. Prepared
for U.S. Environmental Protection Agency. Environmental Research
Information Center. Cincinnati, Ohio. Presented at the Seminar on
Asphalt Industry Environmental Solutions. Cincinnati, Ohio. March
1-2.
Patankar, U., 1978. Personal Communication. Consultant to
Division of Stationary Source Enforcement, EPA. JACA Corporation,
Fort Washington, Pa.
Robert, J. et al., 1975. Air Pollution Emissions and Control
Technology, Asphalt Paving Industry. Combustion Sources Division.
Air Pollution Control Directorate. Report EPS 3-AP-74-2. Canada.
Snowden, W.D., 1978. Sampling Costs on Asphaltic Concrete Plants.
Personal Communication. March 15. Alsid, Snowden and Associates.
Bellevue, Wash.
Soderberg, H.E., 1974. Environmental, Energy and Economic Considera-
tions in Particulate Control. Mining Congress Journal, Vol. 16.
No. 12.
Standard Havens, Inc., 1978. Case Study of a Baghouse on a Drum-
Mixer. Prepared for: Asphalt Industry Enviromental Solutions
Seminar. Cincinnati, Ohio. March 1-2. Standard Havens, Inc.
Kansas City, Mo.
Stastny, E.P., 1973. Industrial Gas Cleaning Institute Reports
Consensus on Industrial Emission Levels Producing "Clear" or
"Near Clear" Stacks. Proceedings, Baltimore Meeting, South
Atlantic States Section of APCA, May.
The World Almanac and Book of Facts - 1978, 1977. Published for
the Washington Star. Edited by George E. Pehory. Newspaper Enter-
prise Association, Inc., New York, N.Y.
U.S. Environmental Protection Agency, 1973. Background Information
for Proposed Now Source Performance Standards. Vol. si, Main Text.
APTD-1352a. PB-221 736. Vol. II APTD 1352b. Office of Air and
Water Programs. Office of Air Quality Planning and Standards.
Research Triangle Park, N.C.
9-4
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U.S. Environmental Protection Agency, 1974. Background Information
for Proposed New Source Performance Standards, Asphalt Concrete
Plants, etc. Vol. III. EPA 450/2-74-003. Office of Air and Water
Programs. Office of Air Quality Planning and Standards. Research
Triangle Park, N.C.
U.S. Environmental Protection Agency, 1976. 1973 National Emissions
Report. Office of Air and Waste Management. Office of Air Quality
Planning and Standards. EPA-450/2-76-007. Research Triangle Park,
N.C.
U.S. Environmental Protection Agency, 1976a. Supplement No. 6 for
Compilation of Air Polutant Emission Factors. AP-42. Second Edition.
Research Triangle Park, N.C.
U.S. Environmental Protection Agency, 1978. Personal Discussions.
Emission Standards and Engineering Division. Office of Air Quality
Planning and Standards. Research Triangle Park, N.C. April.
U.S. Environmental Protection Agency, 1978a. Personal Communication.
Division of Stationary Source Enforcement, Research Triangle Park,
N.C.
U.S. Environmental Protection Agency, 1978b. Memorandum from Kirk
Foster to Bob Ajax. Division of Stationary Source Enforcement,
Research Triangle Park, N.C. •
U.S. Environmental Protection Agency, 1978c. Supplement No. 8 for
Compilation of Air Pollutant Emission Factors. AP-42. Third Edition.
Research Triangle Park, N.C.
University of Texas, 1973. Bench Model Filter Tests. Sponsored by
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Valentine, Fisher and Torn!inson, 1978. Personal Communication,
March 14.
9-5
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
REPORT NO.
EPA-450/3-79-014
3. RECIPIENT'S ACCESSION NO.
4. TITLE AND SUBTITLE
A Review of Standards of Performance for New
Stationary Sources - Asphalt Concrete Plants
5. REPORT DATE
June 1979
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
8. PERFORMING ORGANIZATION REPORT NO.
Kathryn J. Brooks, Edwin L. Keitz, and John Watson
MTR-7826
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Metrek Division of the MITRE Corporation
1820 Dolley Madison Boulevard
Me Lean, VA 22102
10. PROGRAM ELLMENT NO.
11. CONTRACT/GHANT NO.
68-02-2526
12. SPONSORING AGENCY NAME AND ADDRESS
13. TYPE OF REPORT AND PERIOD COVERED
DAA for Air Quality Planning and Standards
Office of Air, Noise, and Radiation
U. S. Environmental Protection Agency
Research Triangle Park, NC 27711
14. SPONSORING AGENCY CODE
EPA 200/04
15. SUPPLEMENTARY NOTES
16. ABSTRACT
This report reviews the current Standards of Performance for New Stationary
Sources: Subpart I - Asphalt Concrete Plants. Emphasis is given to the
state of control technology, extent to which plants have been able to meet
current standards, experience of representatives of industry and of EPA
officials involved with testing'and compliance, economic costs, environmental
and energy considerations, and .trends in the asphalt industry. Information
used in this report is based upon data available as of June 1978. Recommenda-
tions are made for possible modifications and additions to the standard,
including future studies needed of unresolved issues.
17.
KEY WORDS AND DOCUMENT ANALYSIS
a.
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS C. COSATI Field/Group
13B
18. DISTRIBUTION STATEMENT
Release Unlimited
19. SECURITY CLASS (ThisReport)
Unclassified
21. NO. OF PAGES
150
20. SECURITY CLASS (Thispage)
Unclassified
22. PRICE
EPA Form 2220-1 (Rev. 4-77) PREVIOUS EDITION is OBSOLETE
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