United States
Environmental Protection
Agency
Industrial Environmental Research
Laboratory
Cincinnati OH 45268
v°/EPA
Research and Development
EPA-600/S2-81-026> Apr. 1981
Project Summary
Emission of Volatile Organic
Compounds from Drum-Mix
Asphalt Plants
Thomas W Beggs
This research program was under-
taken in order to develop a quantitative
estimate of the emission of volatile
organic compounds (VOCs) from
drum-mix asphalt plants.
The study was carried out by field
sampling of five drum-mix plants
under a variety of operating conditions.
Included in these plants was a plant
that processed a mixture of recycled
pavement and virgin aggregate, and a
plant that employed a wet scrubber,
which was tested both at the stack and
also upstream of the scrubber to
determine if wet scrubbing provides
any significant VOC removal. The
sampling method used was EPA Pro-
posed Method 25, modified to filter
out particulate emissions which would
interfere with the laboratory determi-
nation of VOC concentration in the
collected samples. In most cases,
three simultaneous samples were
taken for each set of operating condi-
tions in order to calculate a mean and
standard deviation fora statistical
comparison of VOC emissions under
different conditions.
Results are that VOC emission fac-
tors for drum-mix plants are on the
order of 0.1 to 0.4 pounds of VOC (as
carbon) per ton of asphalt concrete
produced. VOC emissions appear to
be independent of operating param-
eters, over the normal range of plant
operation and within the limited scope
of the statistical testing employed. It
appears that a wet scrubber reduces
VOC emissions somewhat but the
reduction is difficult to quantify be-
cause of variation in the results.
The nationwide emission of VOCs
from all drum-mix asphalt plants is
estimated to be about 20,600 tons per
year.
This Project Summary was devel-
oped by EPA's Industrial Environmen-
tal Research Laboratory, Cincinnati,
OH, to announce key findings of the
research project that is fully docu-
mented in a separate report of the
same title (see Project Report ordering
information at back).
Introduction
Although most existing asphalt con-
crete plants are still the batch type, the
majority of new plants sold in the last
few years have been the drum-mix type.
The drum-mix process represents the
state-of-the-art in the production of
asphalt concrete. The process is a vast
improvement over the asphalt batching
process as it involves fewer items of
equipment, is simpler, and more porta-
ble. Because the product mix is controlled
at the feed end of the drum rather than
at the discharge end, the process is
more versatile than the conventional
batching process, allowing rapid changes
in production rate, mix type, and mix
temperature.
The simplicity of the drum-mix proc-
ess, as compared to the conventional
process, can be seen in Figure 1. The
aggregate and asphalt are mixed in the
same vessel in which the aggregate is
dried and heated, namely the dryer
-------
Aggregate
Paniculate
Laden Gas
Burner
Burner Fuel
Figure 1. Typical drum-mix process.
drum, which obviates the need for the
pugmill that is found in the conventional
batch process. Also, the hot screens and
weigh hoppers required in the batch
process are not required in the drum-
mix process due to careful control of the
incoming aggregate blend by variable-
speed conveyors.
In a drum-mix recycling process,
salvaged asphalt pavement (or base
material) that has been crushed and
screened is introduced into the dryer
drum at a point somewhere downstream
of the virgin aggregate inlet. Current
blends generally range from about 20
percent to a maximum of 50 percent
recycled material.
Emissions of organic compounds
from drum-mix and drum-mix recycle
processes can be either liquid (blue
smoke) or gaseous (VOC). The former
are actually paniculate emissions; in an
EPA Method 5 test, the liquid aerosol
particles are trapped by the filter. There-
fore, these emissions must be removed
from the VOC sampling train so as not to
be counted as VOC emissions.
Little is known about the amount of
gaseous organic (VOC) emissions gen-
erated by these processes; much less
about how to control them if they are
significant. (It has been suggested that a
wet scrubber may reduce these emis-
sions.) Intuitively, it can be expected
that VOC emissions from the dryer in a
drum-mix or the drum-mix recycle
process are greater than VOC emissions
from the dryer in a conventional asphalt
batch process, due to the heating of a
petroleum-based product (liquid asphalt)
in a closed space
This study was undertaken with the
objectives of gathering data to support
or refute this speculation, attempting to
develop a quantitative estimate of these
emissions, and determining the effi-
ciency of a wet scrubber in VOC removal.
Sampling and Analysis
Methodology
Five drum-mix plants were selected
for testing, on the basis of location and
the owners willingness to participate.
The test method chosen for measuring
VOC emissions from the drum-mix
asphalt process was EPA Proposed
Method 25, "Determination of Total
Gaseous Nonmethane Organic Emis-
sions as Carbon: Manual Sampling and
Analysis Procedures" (sometimes re-
ferred to as the "TGNMO" procedure).
The principle of this procedure is to
anisokinetically draw a sample of stack
gas through a stainless steel probe and
condensate trap and into an evacuated
cylinder. Heavy VOCs condense in the
trap, which is packed in dry ice; light
VOCs remain gaseous and are collected
in the tank. Both trap and tank are
subsequently analyzed for total carbon
by a laboratory procedure in which all
nonmethane organic compounds are
oxidized to carbon dioxide, reduced to
methane, and then measured by a flame
ionization detector (FID).
One plant was tested simultaneously
for paniculate and VOC emissions. For
this purpose a combined train was used
in which the VOC sampling train begins
as a slip-stream, taken off the Method 5
train downstream of the heated filter.
This combination method required iso-
kmetic sampling at preselected traverse
points representing equal areas of stack
cross-section, as in sampling by Method
5 alone. The addition of the VOC tram
does not appreciably affect the isokinetic
factor of the Method 5 test since the
sampling rate of the VOC train is two
orders of magnitude less than the
sampling rate normally employed in
Method 5.
To perform VOC sampling only, a
combined train was not necessary. A
modified filter assembly was used
instead, consisting of a filter and holder
enclosed in a box which was maintained
(along with the probe) at the desired
temperature by means of an electrical-
resistance heater such as that used in
Method 5. After solid and liquid particu-
lates were removed by the filter, the gas
passed into the VOC sampling train.
For each test run, sampling was
begun after steady-state operation had
been achieved at the specified condi-
tions. Parameters varied for purposes of
testing included production rate, mix
temperature, fuel type, and percentage
of recycled material. When sampling
included paniculate measurement, the
VOC train was connected as a slip-
stream after the particulate filter and
the probe were traversed in the stack
according to EPA Method 5. For straight
VOC testing, the probe to the heated
filter box was placed in the stack near
the center, and sampling was performed
anisokinetically according to EPA Meth-
od 25. When testing under conditions of
high particulate loading (upstream of a
scrubber in one plant), a pre-impactor
was also employed along with other
devices to keep particulates out of the
VOC train. In most cases, three simulta-
neous VOC samples were obtained for t
-------
each test run; each test run also included
a velocity traverse and a Fyrite gas
analysis for COa and 02 in the stack gas.
Plant process parameters were moni-
tored and recorded during each run. At
the completion of each test run, the
sample tanks were packed away and the
traps were sealed and packed in dry ice
until analysis. Subsequent laboratory
analysis of samples was done and the
results were reported in terms of total
carbon for the tank, the trap, and the
total.
All VOC values were converted to
emission factors of pounds carbon per
ton of asphalt produced by employing
the measured stack gas velocity and
recorded process parameters. For each
test run the multiple VOC emission
factors were averaged and the standard
deviation was calculated. Table 1 pre-
sents these results. Statistical tech-
niques were then used to compare
results for parameter changes to deter-
mine if apparent differences were sta-
tistically significant.
Results
As a result of this study, VOC emission
factors for the drum-mix asphalt process
are established to be in the range of 0.1
to 0.4 pounds carbon per ton. Within the
limits of the procedures used and the
narrow ranges of process parameters
found in most plants, no real dependence
of VOC emission factors could be de-
tected for parameters such as mix
temperature, percentage of recycled
material, production rate, and type of
fuel. A high-energy wet scrubber (ven-
turi) is capable of reducing VOC emis-
sions, but the reductions achievable
varied widely so that a separate emission
factor range could not be established for
plants with wet scrubbers.
The nationwide impact of VOC emis-
sions from drum-mix asphalt plants is
estimated to be approximately 20,600
tons per year.
The procedure of EPA Proposed Meth-
od 25, as modified to filter out particu-
lates performed well in the field. How-
ever, additional modifications were
necessary to sample under the high
paniculate loading experienced upstream
of a particulate control device.
Table 1. Summary of VOC Test Results by Plant
VOC Emission Factors, Ib/ton*
Plant
A
B
C
D
E
Production
tons/hr
350-375
100-200
240-320
200-275
175-200
Burner Fuel
Diesel oil
Natural gas
Propane
No. 4 fuel oil
Natural gas
No. 2 fuel oil
Control
Device
Baghouse
Baghouse
Baghouse
Baghouse
None**
Venturi
Scrubber
No. of
Samples
8
14
12
15
7
10
Mean
0.21
0.062
0.13
0.41
0.36
0.19
Standard
Deviation
0.07
0.034
0.05
0.17
0.22
0.03
* As carbon.
** Upstream of venturi scrubber.
Thomas W. Beggs is with JACA Corporation, Fort Washington, PA 19034.
Mark J. Stutsman is the EPA Project Officer (see below).
The complete report, entitled "Emission of Volatile Organic Compounds from
Drum-Mix Asphalt Plants," (Order No. PB 81 -157 943; Cost: $9 50, subject to
change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Industrial Environmental Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
> US GOVERNMENT PRINTING OFFICE 1961 757-013/7059
-------
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Fees Paid
Environmental
Protection
Agency
EPA 335
Official Business
Penalty for Private Use $300
RETURN POSTAGE GUARANTEED
Third-Class
Bulk Rate
EPA
------- |