ENVIRONMENTAL HEALTH SERIES
Air Pollution
Air Pollution in the
Roasting Industry
U.S. DEPARTMENT OF HEALTH,
EDUCATION, AND WELFARE
Public Health Service
-------
AIR POLLUTION
IN THE
COFFEE ROASTING INDUSTRY
Frank Partee
Technical Assistance Branch
Division of Air Pollution
U.S. DEPARTMENT OF HEALTH, EDUCATION; AND WELFARE
Public Health Service
Division of Air Pollution
Cincinnati, Ohio
September 1964
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The ENVIRONMENTAL HEALTH SERIES of reports was established
to report the results of scientific and engineering studies of man's en-
vironment: The community, whether urban, suburban, or rural, whe re
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Public Health Service Publication No, 999-AP-9
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CONTENTS
FOREWORD . . ., . iv
ABSTRACT v
INTRODUCTION .... . .... 1
SUMMARY ... . . 1
PROCESSING COFFEE . . 2
Preparing the coffee cherry. . .... 2
Regular coffee . 2
Cleaning the green beans . . 2
Blending ... 4
Roasting. ... . 4
Cooling 5
Stoning ... . . 6
Instant coffee .... 6
Unit operation . .... 6
Disposal of spent grounds 7
Decaffeinated coffee . 7
NATURE AND EXTENT OF AIR POLLUTION 9
Chemistry of coffee 9
Coffee roasting . . 9
Stack emissions 10
Instant coffee 12
Decaffeination . . . 12
METHODS OF CONTROLLING EMISSIONS 12
Particulate and smoke emissions . ... 12
Soluble coffee emissions . .... 13
REFERENCES . . 15
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FOREWORD
This pamphlet is one of a series of studies undertaken by the Public
Health Service to provide information concerning air pollution encounter-
ed in specific industries. It is intended as a compilation of facts regard-
ing the nature and extent of air pollution resulting from the processing
of coffee and current methods available for control of atmospheric
emissions from typical installations.
Many people have contributed their time and knowledge toward
making the publication of this pamphlet possible. Gratitude is extended
to all; in particular to Messrs. Frank A. Bell, Jr. and "Walter Smith,
Public Health Service; The National Coffee Association, for guidance
and photographs of plant operations; Jabez Burns -- Gump Division of
Blaw-Knox Company, for technical information on modern processing
and control equipment; members of the industry for their counsel; and
all the state and local health officials who generously relayed their
knowledge and experience in this area.
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ABSTRACT
This review provides a guide for the inventorying and control of
emissions arising from coffee processing. Information -was collected
from published literature and other sources. Emission factors -were
established for the various processes involved, i.e. roasting, stoning,
and cooling. The air pollution aspects of the production of regular
grades, instant, and decaffeinated coffee are discussed. Also discussed
are the types and operating characteristics of control equipment used.
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AIR POLLUTION
IN THE
COFFEE ROASTING INDUSTRY
INTRODUCTION
Coffee is roasted from coast to coast and from the northern tier
of states to the Rio Grande. The extent of possible air pollution from
this industry is of general interest in the United States, although the
bulk of the coffee is processed in and around New York, Los Angeles,
Chicago, and New Orleans. During November and December 1961,
city air pollution control and health agencies were queried in regard
to their experiences with air pollution caused by the processing of
coffee. The survey indicated that of 37 cities responding 18 cities have,
or have had, problems with that source of air pollution. Some indicated
the problem was of relatively minor importance to them. Officials in
15 cities reported receiving no complaints. Four of the responses
gave no information about the complaints.
Coffee roasters are finding that although they may have been located
on the outskirts of communities at one time, they now are being engulfed
by burgeoning residential growth. This has created pollution "receptors"
where formerly there were none and a concomitant demand for control.
Coffee processing produces four types of emissions: dust, chaff,
odor, and smoke. Dust is generated in the handling of green beans,
which are bagged in cloth. Chaff consists of the outer covering, or
skin, that bursts when the bean swells during roasting. The odor and
smoke are combinations of organic constituents volatilized at roasting
temperatures and steam produced when the roast is quenched with water.
Further processing to produce instant coffee causes an additional emis-
sion in the form of powdered coffee, which escapes during the drying
process. During decaffeination, odors can be produced by trichloro^
ethylene, the solvent used in extracting caffeine from the green coffee
beans.
SUMMARY
Emission inventory factors are presented in Table 1. These fac-
tors can be used to approximate the gross amount of particulate matter
emitted from a typical roasting plant with or without control equipment.
Particulate emissions are held well within generally prescribed
limits when cyclone collectors are employed. Odor-laden smoke pre-
sents a more difficult problem, however. Smoke density often exceeds
No. 2 Ringelmann (black smoke) or 40 percent opacity (other than black
smoke), the limit usually established in smoke regulation codes. Smoke
emissions have been lessened by the development of a roaster that re-
circulates effluent gases through the flame of the roaster burner. Cur-
rently, the best method of smoke elimination involves use of a separate
1
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afterburner. Powdered coffee particulate, being highly soluble, can be
effectively controlled by means of a simple water scrubber.
Table 1. EMISSION INVENTORY FACTORS FOR COFFEE
PROCESSING
Solid emissions, lb/1000 Ib green beans
Process • — • "
With no control With usual control
Roaster
Direct fired 3. 8 1. la
Indirect fired 2. 1 0. 6a
Stoner and cooler 0. 7 .0. 2a
Instant coffee spray Control always 0.7
dryer employed
a Vi
Cyclone. Cyclone and wet scrubber.
In summary, the most effective means of control now available are
centrifugal collectors for dust and chaff, afterburners for smoke and
oior, and water scrubbers for instant coffee particulate,
PROCESSING COFFEE
PREPARING THE COFFEE CHERRY
Before arrival at a roasting plant in the United States, the coffee
cherry (the fruit) is prepared by one of two methods. The older, "dry"
method consists of spreading the cherries on flat, drying grounds called
"barbecues, " which often have brick or cement surfaces. The cherries
are raked until they are dried thoroughly by the wind and sun. Artifi-
cial drying has supplanted natural sun drying on many plantations.
The modern, "wet" method utilizes a fermentation process.
Cherries are depulped in water, and the mixture of pulp and liberated
seeds is allowed to ferment for periods lasting from hours to days.
This loosens the tough, parchment shell of the beans (See Figure 1).
Then, the beans are dried and the shell and silver skin removed by
mechanical rubbing.
REGULAR COFFEE
A flow diagram for a typical coffee roasting operation is shown in
Figure .2.
Cleaning Green Beans
Although the beans are cleaned before they are exported to this
country, dirt enters the bags during transit. Other trash accumulates
from repeated handling. Dust,- lint, and strings become mixed with the
beans when the bags are opened. This trash must be removed at the
plant not only to ensure a quality product, but to eliminate hazards in
9 AIR POLLUTION IN THE
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the roasting process. Bits of combustible material can cause serious
fires in the roasters, clog up conveyors and piping, and otherwise re-
duce production efficiency. Small stones and pieces of metal, if not
eliminated, could ruin the grinders.
Figure I. Cross section of a coffee cherry showing outer skin, mucilaginous matter, or
pulp, (dotted); parchment cover (white); silver skin (shaded); and coffee bean
(black). (Reference I).
POINTS OF EMISSION
USUAL OR POSSIBLE CC
* ALTERNATE CONTROL
Figure 2. Flow diagram of coffee roasting plant.
Much dust is generated in dumping the green coffee from coarse,
cloth bags. Dust from the dumping pit can be controlled by means of
a hood and exhaust system vented to a cyclone collector.
Cleaning was formerly carried out by screening methods dependent
on the difference in the size of the green beans and attendant trash.
These inefficient methods were unsatisfactory. Presently, excellent
separation is achieved by utilizing differences in specific gravity of
the beans and that of foreign materials. Before roasting, only those
materials lighter than the bean, i.e., dust, string, etc., are removed.
An air stream, at a carefully regulated pressure, is drawn up through a
rectangular duct. The beans are introduced into this air stream through
a slit in the duct. As the beans fall through the duct to a. bin below,
material lighter than the beans is carried up the duct and discharged
from the process. '
COFFEE ROASTING INDUSTRY 3
-------
During roasting, the coffee beans lose weight while gaining about
50 percent in volume, thus the specific gravity of the bean is decreased
by about one-half. Those pieces of material that closely matched the
specific gravity of the green bean and, therefore, escaped the first
cleaning step would, after roasting, be about twice as dense as the bean.
This new disparity in the density of the "stones" and the roasted beans
is employed to remove these materials during a "stoning" operation,
which is described later.
Blending
After the green beans have been cleaned, they are generally lifted
by bucket conveyor to the highest point in the plant to permit gravity
feed to succeeding operations. The blender consists of a hollow cylin-
der, mounted horizontally. ' The proper weight of each grade of
coffee for the blend is charged and the cylinder is rotated. The batch
is mixed in a few minutes by helical flanges inside the blender. Any
dust or chaff loosened by the mixing is removed by an exhaust system.
The blended coffee then goes to holding bins or to feed hoppers over
the roasters.
Roasting
Roasting is the most important step in coffee making because it
develops the flavor. For the average roast, about 370 Btu of heat
energy is required per pound of green coffee. More than 95 percent
of modern roasters are gas-fired; the remainder are oil-fired. '
A roaster consists of a perforated, horizontal cylinder with internal
helical flanges enclosed in a metal jacket. Of the three types of roasters
in use, two are batch-fed. They roast up to 1, 500 pounds per hour in
500-pound batches that require 15 to 20 minutes of roasting time each.
The older, direct-fired roasters utilize a gas jet inside the cylinder,
which operates at an air temperature of about 2, 000°F (Figure 3), and
FEED BIN
EXHAUST
GASES
Figure 3. Simplified drawing of direct fired roaster.
AIR POLLUTION IN THE
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heats the roaster by radiation. In the newer, indirect-fired roasters,
the gas burner is located in a separate chamber behind the cylinder.
Hot combustion gases are recirculated, thus effecting more efficient
heat transfer. This reduces the operating temperature range to 850° to
900°F and produces a more uniform, higher quality roast (Figure 4).
EXHAUST GASES
FEED BIN
FUEL AND AIR
RECIRCULATED GASES
Figure 4. Simplified drawing of indirect fired roaster.
The third type, and the most modern, is a continuous roaster with a
rated capacity of up to 10, 000 pounds per hour and a roasting time of
only 5 minutes; it is operated at 450° to 500°F. 3
During the first 10 minutes of batch-fed roasting, the charge heats
at a fairly uniform rate and moisture is driven off. In the last 5 to 10
minutes, the temperature rises rapidly and the chemical degradation,
which produces the familiar odor and flavor, occurs. The beans swell
and turn brown. While the beans are roasting, the operator constantly
compares samples from the roaster with beans of a standard prescribed
by cup testers as having the desired flavor. At the moment the color
of the roasting beans matches that of the standard beans, the operator
applies a water quench to stop the roasting action. Because of the high
temperature in the roaster, the water flashes off and passes out of the
stack as steam. Roasters that perform the roasting operation auto-
matically, i. e. , that heat to a predetermined temperature and then
quench the batch, are also used extensively. The changes that occur
in the chemical composition of the coffee beans during roasting are
listed in Table 2.
When the roasting process has been completed, the batch is dumped
into a cooler where the temperature of the brown beans drops until no
further chemical changes occur. The cooler usually consists of a bin
wherein the beans can be agitated while a draft of cool air is drawn
through them. Chaff and residual smoke are picked up by the air
stream and discharged from the cooler.
COFFEE ROASTING INDUSTRY
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Table 2. CHANGES IN COFFEE COMPOSITION ON ROASTING3-
Solids on basis of dry weight of green coffee, %
Before After
Water
Sugar
Crude fiber
Ether extract
Water extract
Ash
Caffeine
Chlorogenic acid
Trigonelline
Total nitrogen
Total sulfur
10.
8.
24.
11.
30.
3.
1.
7.
1.
2.
0.
73
62
00
08
35
00
22
8
02
34
10
2.
0.
13.
13.
12.
4.
1.
4.
0.
2.
0.
16
75
03
75
62
03
31
5
73
45
11
aReferences 3 and 4.
Stoning
This step would more properly be called "de-stoning" because
heavy materials, which were not removed in the green bean cleaner,
are removed during the stoning operation. The principle is the same
as that used to remove light-weight debris. The actual operation is
different in that the air flow is regulated to lift the roasted beans and
leave the bits of rock and metal behind. The "stones" fall into a recep-
tacle and the beans are "airveyed" up the duct to bins above the grinders.
The air is then discharged from the process.
INSTANT COFFEE
Unit Operation
Production of instant coffee is an extension of the roasting process.
The operation basically is an extraction. Apparently, all instant coffee
installations are custom designed; however, enough similarities exist
to allow general treatment.
About 3. 5 pounds of green coffee are required to make 1 pound
of instant coffee. The roasted beans are ground very coarsely. The
moisture is allowed to remain much higher than that of beans used to
make regular grades of coffee; this promotes immediate dissolution of
the coffee solubles.
Stainless-steel cylinders, some as large as 2 feet in diameter, with
a capacity of 10, 000 pounds of coffee, are filled with the coarse grounds.
(Figure 5). Zeolite-softened water at 300°F is introduced under pres-
sure, usually. 10 to 12 atmospheres, at the top of the cylinder. The
water travels through the bed, extracting the solubles. Leaving the
bottom of the first cylinder, the stream of liquor enters the top of the
6 AIR POLLUTION IN THE
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cylinder. The "water travels through the bed, extracting the solubles.
Leaving the bottom of the first cylinder, the stream of liquor enters the
top of the second cylinder. The liquor stream always enters a cylinder
of fresher grounds. This provides a maximum concentration gradient
at all points in the system. When a cylinder is exhausted, after about
6 hours of extraction, it is bypassed in the line and the spent grounds
are blown out with steam.
GROUND
COFFEE
HOT _
WATER
JL
SPENT
GROUNDS
E
| FILTER ]
XT R ACTION
TOWER
S
"x
[PA
1 |
SPRAY
DRYER
^
CKAGI
HOT AIR
^
TO SP
NG~|
INSTANT
COFFEE
" © POINTS OF EMISSION
Figure 5. A typical instant coffee process.
Concentrated liquor is drawn off in 500- to 600-pound batches.
At this point, the extract is a brownish fluid containing 30 to 35 percent
solids. It is filtered to remove residual grounds, and sometimes further
purified by use of a centrifugal clarifier.
In the final step, the coffee concentrate is spray-dried and becomes
a powder recognizable as the familiar instant coffee. The liquor is
sprayed through an atomizer into a huge metal chamber. The evapor-
ator chamber may be as tall as 80 feet and have a diameter as great as
20 feet. Figure 6 is a schematic diagram showing such a drying process.
As the droplets enter the chamber, they meet a. blast of hot air and are
dried as they fall. Inlet and outlet temperatures of the drying air,
which range respectively from 500° to 700°F and 200° to 400°F, deter-
mine the color, taste, and final moisture content of the product. To
prevent the moistened, hot air from rewetting the dried powder, cold
air is introduced near the bottom of the evaporator. This forms a
"dam" that diverts the hot air and creates a calm zone where the par-
ticles settle out. To recover fines, the exiting air stream is passed
through a cyclone collector and a high-efficiency multiple-cyclone in
series. Effluent from the secondary collector is usually exhausted into
the atmosphere. The powdered coffee is screened to remove lumps,
and stored under controlled humidity (35 to 50 percent at 75°F) until
it is packaged.
6,7
Disposal of Spent Grounds
Disposal of spent grounds has been a major problem to the industry.
The most prevalent method is simple elimination into a sewer. A less
economical alternative is transferring the waste to a sanitary landfill.
Another method, under study recently, is an incineration process.
DECAFFEINATED COFFEE
Decaffeinated coffee represents only a small fraction of the in-
dustry's total production and is limited to only a few major plant sites,
COFFEE ROASTING INDUSTRY
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COFFEE
CONCENTRATE
HOT AIR STREAM
SPRAY NOZZLE
COOL AIR "DAM"
POWDERED COFFEE
EFFLUENT TO MULTIPLE CYCLONES
DEHUMIDIFIED COOL AIR
— STREAM
J77T
TO STORAGE
Figure 6. Schematic diagram of instant coffee spray-dryer.
and therefore will be discussed briefly. Figure 1 shows the process
units used in extracting the caffeine. The procedure consists of five
main steps:
SOLVENT
WASTE
WATER
CONDENSER
CONDENSER
CAFFEINE TO
REFINING
EVAPORATOR
WATER
SEPARATOR
SOLVENT
COFFEE TO
ROASTERS
Figure 7. Decaffeinated coffee process (W, water; S, steam}.
AIR POLLUTION IN THE
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1. Raising the moisture content of the green bean from 10 percent
to about 18 percent.
2. Extraction with trichloroethylene at 160°F.
3. Removal of the solvent by steaming the coffee for several
hours.
4. Drying the extracted beans.
5. The usual roasting process.
Approximately 97 percent of the caffeine is removed and is re-
covered from the solvent as a valuable byproduct.
NATURE AND EXTENT OF AIR POLLUTION
CHEMISTRY OF COFFEE
The characteristic odor and flavor of coffee result from the forma-
tion of esters of chlorogenic acid, furfuryl derivatives, and other aro-
matic substances. Coffee contains 1 to 2 percent caffeine (trimethyl-
xanthin). The caffeine seems to occur in combination with tannic and
caffetannic acids rather than alone. Five crystalline derivatives of
coffee that are important in its use as a beverage are caffeine, potas-
sium chlorogenate, caffalic acid, valeric acid, and trigonellin. There
are also some pyridine-like bodies, furfural, furfuralcohol, and cer-
tain others described generally as "caffeol, " which may be a pure sub-
stance or, more likely, a mixture of volatile alcohols and derivatives.
As shown in Table 2, many changes occur in coffee during roasting.
There is a great loss of water; the sugar is caramelized and largely
disappears; and there is a considerable loss of crude fiber, caused by
destructive distillation. About 90 percent of the loss in weight results
from elimination of wa-ter vapor and crude fiber. Also, during roast-
ing, there is a slight gain in the percentage composition of nitrogenous
substances, sulfur, and caffeine because they are not broken down or
driven off at roasting temperature.
Oils and fatty substances are present in considerable quantity.
These fats, mostly olein with small amounts of palmitin and stearin,
are of both the saturated and unsaturated varieties. Of the saturated
fats, which make up about 40 percent of the total fatty matter, the
compounds of palmitic acid comprise 25 to 28 percent of the total;
compounds of carnaubic acid, 20 percent; capric acid, 5 percent; and
daturic acid, 1 percent. Of the unsaturated, fatty acids, compounds
of oleic and linoleic acids comprise the greater portion, linoleic and
its compounds making up 50 percent of the total fatty material. ' '• 1(-)
The ash of coffee is alkaline, consisting largely of phosphate and
carbonate of potash; in Mocha coffee, salts of magnesium and calcium
also are found.
COFFEE ROASTING
Coffee chaff is similar to peanut skin and is dark brown. The
particles are large in relation to most airborne particulates, attaining
a maximum size of about 0. 5 inch. Probably 80 to 90 percent of the
COFFEE ROASTING INDUSTRY 9
-------
lacroparticles are in the order of 0. 125 inch in diameter. Smoke and
odor are also produced in the roaster. These are emitted intermittently
because of the cyclic nature of the roasting process and are composed
of minute droplets of condensed, organic volatiles. A weight loss of
about 16 percent occurs during roasting, of which 2 to 5 percent is dry
weight loss. The amount of emission is a function of the grades of
coffee, the method of curing, and the degree and type of roasting.
Stack Emissions
Data available concerning emission rates from coffee roasters are
limited. Emissions from different sources in several plants are com-
pared in Table 3.
Table 3. PARTICULATE EMISSIONS FROM THE COFFEE ROASTING PROCESS
Processes
Direct-fired roaster
Indirect-fired roaster
Cooler
Stone r
Stoner and cooler
combined
Run
1
I
Avg
1
L
3
Avg
1
1
1
2.
Avg
Solid emissions
gr/scf
0.213
0. 173
0. 193
0. 137
0.091
0. 153
0. 127
0. 036
0. 097
0. 026
0. 008
0.017
lb/100'0 Ib
of
green beans
1.26
0. 97
1. 12
0. 44
0. 29
1. 00
0. 58
0. 29
0. 37
0. 22
0. 10
0. 16
Condensed
tar, %
75
87
8J
36
36
-
16
38
27
Control
equipment
Cyclone
Cyclone
Cyclone
Cyclone
Cyclone
Cyc lone
Cyclone
None
None
Cyclone
Cyclone
Cyclone
Reference
number
10
10
12
12
13
10
10
10
13
As shown in Table 3, the use of low-draft-loss cyclone collector
to treat the effluents of the cooler and stoner reduces total emissions
by about 70 percent, as compared with corresponding effluents dis-
charged directly into the atmosphere.
Smoke emission from plants, with and without cyclone collectors,
has been studied. Typical smoke desities are illustrated in Figure 8.
In general, during each cycle, which averages 15 minutes, the opacity
is 0-1 Ringelmann at the beginning of each roast; this opacity increases
10
AIR POLLUTION IN THE
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gradually for about 10 minutes, then rapidly for 1 to 3 minutes. At the
end of the cycle, characterized by a large cloud of steam from the
•water quench, the smoke density returns to 0-1 Ringelmann. In con-
trast with indirect-fired units (Figure 8), the 1- to 3-minute peak smoke
emission from direct-fired roasters usually does not exceed No. 2
Ringelmann.
of 4
a. I
I I 1
Direct-fired roaster
10 15 20 25 30 35 40
TIME, minutes
10 15 20 25 30 35 40
Figure 8. Variation of smoke with time batch roasters (References 10 and 12).
Kuratsune analyzed soot deposits taken from roaster stacks for
various polycyclic, aromatac hydrocarbons; he discovered the presence
of benzopyrene (about 0. 2 ppm by weight), a substance known to produce
cancer in experimental animals.
COFFEE ROASTING INDUSTRY
11
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INSTANT COFFEE
The effluent stream from the spray dryer used in the produc-
tion of instant coffee contains fine, particulate matter. The amount
emitted is dependent on the efficiency of the multicyclone collection
system through which the effluent stream passes and the rate of plant
production. Naturally, this emission is held to a minimum, as any loss
is loss of salable product. This resolves to a consideration of the cost
of a highly efficient collection system versus economic return. For a
plant producing 1 ton of soluble coffee per hour, the particulate loading
of exit streams would be about 0. 02 grains per cubic foot at 60°F and
1 atmosphere, or 0.7 pounds of particulate per 1,000 pounds of green
beans.
In operations where spent coffee grounds are disposed of by in-
cineration, smoke and odor problems are likely to occur.
DECAFFEINATION
Loss of solvent is the only emission from the decaffeination pro-
cess. This solvent, trichloroethylene, is a colorless, nonflammable
liquid, has an odor like chloroform, and is a narcotic and anesthetic.
In a closed plant operation, this would be of more concern to the indus-
trial hygienist than to air pollution control personnel. It should be re-
membered that in both decaf feination and production of instants the
roasting process is also employed and that its attendant emissions may
be expected to be present in these operations.
METHODS OF CONTROLLING EMISSIONS
PARTICULATE AND SMOKE EMISSIONS
Because of the size and nature of the dust and chaff particulate,
cyclone collectors provide a simple, economic means of emissions con-
trol. A simple cyclone will handle, with great efficiency, emissions
of a size greater than 20 to 40 microns. It can tolerate temperatures
to 750°F and accommodate flow rates in excess of 25, 000 cfm. 14 Care
must be exercised in removing the collected solids so that another air
pollution problem is not generated. In some locations, the chaff is
burned, with the inherent possibility of creating more smoke and odor.
Even where cyclones are employed, the submicron particles in
the smoke and the odor leaving the roaster are not controlled. There
is, however, the so-called "smokeless roaster." Its manufacturers
claim that this roaster eliminates smoke and odor completely; however,
all do not accept this claim as entirely true. A damper system recircu-
lates the combustion gases that are ordinarily vented directly into the
atmosphere through the gas flame of the roaster. The additional heat
required increases fuel consumption about 40 percent. This modifi-
cation is shown in Figure 4. Better smoke control is experienced with
an afterburner in the roaster stack, but fuel requirements are increased
100 to 150 percent over that for a conventional roaster.
12 AIR POLLUTION IN THE
-------
Other methods of emission control, such as catalytic oxidation,
scrubbers, and ultrasonic agglomeration of submicron particles to the
point that they be handled by conventional collection equipment, have
been attempted. They have failed for various reasons, usually poor
economics or poor performance.
In modern coffee plants, cyclones are included as an integral part
of the roaster design. Particularly in installations of small capacity,
however, the effluent air streams from the cleaning, cooling, and
stoning processes may have no such individual collection system. In
general, these can be connected in manifold to a common exhaust stack
and serviced by a single cyclone collector. Efficiency ratings as high
as 97 percent may be expected from such devices. In some communi-
ties, however, the remaining 3 percent may constitute a nuisance prob-
lem. In such situations, water scrubbers have been used to eliminate
the remaining emission.
SOLUBLE COFFEE EMISSIONS
Emissions from the production of soluble coffee consist of the fine,
spray-dried particles that escape the multicyclone system. This col-
lecting device has an expected efficiency of 95 percent for particles with
a median diameter greater than 10 to 30 microns. ^ Thus, the particu-
lates emitted would be expected to be smaller than 10 microns. Mea-
surements have shown that these particles range from 2 to 10 microns.
The powder is very hygroscopic, which precludes the use of a bag filter
installation since the porosity of any cloth filter would be seriously im-
paired by the sticky particles covering its surface. Cleaning the filters
would be impractical by conventional methods. The control method
currently used is water scrubbing. This takes the form of a simple,
gravity scrubber in which the particle-laden effluent rises countercur-
rently to water droplets falling through an empty tower. Such a scrub-
ber is usually constructed of corrosion-resistant material, such as
stainless steel, because the dissolved coffee is slightly acidic (pH 4. 8
to 5. 2) and can cause corrosion.
The efficiency of the gravity scrubber, which is dependent on -water
droplet size and contact time, decreases rapidly for particles smaller
than 5 microns. More effective treatment is produced by a Venturi
scrubber, which costs more initially but consumes only half as much
water as the gravity scrubber and controls the smaller particulates
with a higher efficiency than a gravity scrubber.
COFFEE ROASTING INDUSTRY 13
GPO 814—835—3
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REFERENCES
1. MacKaye, Payson. The Coffee Man's Manual. Spice Mill Publish-
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