Technology Transfer                          EPA 625/2-79-020
Capsule  Report


Control of
Acidic Air Pollutants by
Coated Baghouses
January 1979
       Lir
       -EDISOH.
This report was developed by the
Industrial Environmental Research Laboratory
Cincinnati, OH 45268

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Baghouse and exhaust stack controlling secondary aluminum smelter (80,000 ft3/mm)

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1.    Significance
Emissions from the aluminum, glass,
phosphate fertilizer, and sulfunc acid
industries and from waste incineration
have exhaust gas emission character-
istics unique to their sources because
of process variations.  However, they
also share common problems: com-
bined particulate, corrosive acid vapor,
and acid mist emissions. The uncon-
trolled  pollution characteristics of
these sources are shown in Table  1.
Unless controlled, these pollutants
form a  dense plume that can damage
property and animals and may be harm-
ful to human health.

In sufficient quantities, fluorides from
aluminum and phosphate emissions
can cause severe bone malformations
(called fluorosis)  in grazing animals.
Acids can etch glass,  damage paint,
and corrode metal.  Particulate matter
and acid mist reduce visibility and can
adversely affect human health.

These combinations of pollutants also
cause problems in operation of the
more common air pollution control
systems. Dry electrostatic precipita-
tors, cyclones, and  ordinary fabric  fil-
ters (baghouses) do not remove gases
and are subject to rapid deterioration
from the acids. Mist eliminators are
effective only on  liquids; acid gases
pass through unaffected. Scrubbers
and wet  precipitators  introduce a
wastewater source that often requires
treatment
To avoid potential water pollution
problems, a dry control system is de-
sirable. To avoid corrosion problems,
neutralization of the acid is desired.
For environmental reasons, efficient
removal of particulate matter, acid
mist, and acid gases is necessary. This
capsule report presents an approach
that accomplishes all these require-
ments  through the use of a dry scrub-
bing agent to neutralize and capture
the acids, followed by removal of par-
ticulates  and captured acids in a bag-
house  filter.

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Table 1.



Characteristics of Uncontrolled Emissions from Acid-Emitting Industries

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2.   The  Process
The key element in the dry scrubbing
process is the intimate contact between
the exhaust stream constituents and
the dry sorbent. This contact ensures
both capture of particulates and neu-
tralization of acid gases by the sorbent.
There are currently three techniques
for achieving contact:

•   Fluidized bed system
•   Injection system
•   Batch-charge system
Afluidized bed contact system is illus-
trated in Figure 1. Sorbent is fed from
the crude sorbent tank to a screened
plate under the baghouse. Process
gases pass up through the screened
plate and fluidize the sorbent granules,
providing good contact. The gases
then pass through the baghouse before
being released. Abraded fines from the
process are caught in the baghouse,
where they assist in capturing the acid
gases.
                                     Limestone feed supply tower

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Figure 1.
Fluidized Bed System for Emissions from Primary Aluminum Production
Figure 2 is a diagram of an injected
sorbent contact system. In this proc-
ess a powdered sorbent is blown into
the gas stream before it enters the
baghouse. The sorbent can then react
with acids both in the duct and in the
baghouse. A quench chamber may be
placed upstream of the injection point,
as shown in the figure, to reduce
temperature and raise humidity to im-
prove sorption.

The third method, the batch-charge
process, is  actually a variation  of the
injection process. In the batch-charge
process, sorbent is blown directly into
the bags and retained until its ability to
remove acids is near exhaustion; it  is
then replaced.
Details of the design of these control
systems vary somewhat with the in-
dustry; however, all rely on some form
of dry sorbent to coat the inside of the
baghouse filter and scrub exhaust
gases.

Sorbents commonly used include
alumina, limestone, and nepheline
syenite (a silicate). Industries using dry
sorbents often  demonstrate a prefer-
ence for a particular system for reasons
that will  be discussed. It is perhaps
best,  then, that each system be dis-
cussed in the  context of the industry
that uses it.
Primary Aluminum

Fluidized bed contact systems have
been used in the primary aluminum
industry for several years; typically
these systems use alumina as the
sorbent.

Primary aluminum is  produced almost
solely from alumina (AI203) in shallow,
rectangular cells (Figure 3) called pots,
by use of carbon electrodes. These
electrodes may be prebaked or may be
baked  m situ (Soderberg process).
Cryolite (Na3AIF6) serves as both a
solvent and an electrolytic material for
alumina. During the  production proc-
ess, solid and gaseous fluorides are
evolved.

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Figure 2.

Injected Sorbent System

Both the fluidized bed and injected
sorbenttechniques have been used for
prebake potlmes and Soderberg pot-
lines. These processes appeal to the
primary aluminum industry because of
the lack of heavy oils in the exhaust,
the use of alumina as a feed for the
process, and the usability of the cap-
tured fluorine to replace makeup fluo-
rine in the  process. The amount of
alumina used in production and the
amount of fluorine in the  exhaust are
such that a large quantity of alumina
can be used without economic penalty.

Unfortunately, because other indus-
tries use sorbent inefficiently in the
fluidized bed  process and cannot use
the spent sorbent, this system is appli-
cable only to the primary  aluminum
industry. It is also more difficult to use
the system for Soderberg potlines
because of the presence of high molec-
ular weight organics from the baking of
the anode. Moreover, the accumula-
tion of unwanted acid gases in the cell
gases may necessitate  supplemental
treatment of some  of the gas stream.


Secondary Aluminum

The secondary aluminum industry
recycles aluminum  scrap of varying
quality from a multitude of sources
(Figure 4). Troublesome impurities
found in scrap include magnesium and
oils. Fluorine and chlorine are used to
react with and remove the magnesium
in a  process known as  demaggmg.
Sparks and fluorine or chlorine, as well
as oils and solids, are emitted from the
process. The oils blind uncoated bag-
houses, and sparks tend to cause burn-
through of the filters. Batch-charge
seems to be the favorite sorbent con-
tact  method for this industry because
the relatively low acid gas concentra-
tions allow a reasonable cycle time (3
to 10 days). Before the exhaust enters
the baghouse it  is usually quenched
with water to cool it and to extinguish
sparks from the process.

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Figure 3.
Aluminum Reduction Cell
Figure 4.
Secondary Aluminum Magnesium Reduction Process

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Charging of secondary aluminum furnace

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Settling chamber upstream of baghouse to capture sparks and large debris

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3.    Performance
Emissions on which dry sorbent sys-
tems have been demonstrated to be
effective include acid gases such as HF
and HCI, sulfur oxides (S03 and, to a
lesser extent, S02), particulates (both
carbonaceous  and inorganic), and re-
sulting visible  stack plumes.

In the three dry sorbent systems the
acid gas outlet concentrations depend
only on the gaseous stream and the
sorbent type, provided that sufficient
contact between the two occurs and
that the sorbent is not depleted. The
control system, therefore, will exhibit a
constant outlet concentration inde-
pendent of inlet loading; a conven-
tional system,  on  the other hand, will
exhibit constant efficiency irrespective
of the inlet concentration. This point is
illustrated in Figure 5, where acid gas
concentrations for the aluminum in-
dustry are shown  along  with outlet
concentrations of acid gases for a dry
sorbent system. For example, inlet
concentrations of  1,000 ppm would be
reduced by over 99.9 percent while an
inlet concentration of 2  ppm would
only be reduced by about 50 percent.
The particulate collection efficiency of
dry sorbent systems depends primarily
on particle size.  Collection efficiency
will be almost 100 percent for large
particles; very small particles will pass
through the baghouse at significantly
less efficiency. A given process stream
from a primary aluminum plant will
have a particle size distribution very
similar to that of a process stream from
a secondary aluminum plant. This simi-
larity makes it possible to compare
particulate outlet concentrations. Fig-
ure 6 shows the inlet and outlet partic-
ulate concentrations and the effect of
dry sorption control. Coated baghouses
will increase efficiency in capturing
very small particles as compared to
conventional, uncoated baghouses.
                                     Emergency cooling damper linkage in baghouse

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        300
        250
        200
         150
     Q
     O
         100
          50
                         Inlet,
                         primary
                         aluminum
Inlet,
secondary
aluminum
Outlet,
all
gases
                                 Dry sorption baghouse systems are,
                                 therefore, particularly effective in re-
                                 ducing the opacity of the stack plume.
                                 The opacity is determined by a com-
                                 plex relationship of particle concentra-
                                 tion, total particle surface area, particle
                                 size, shape and distribution, optical
                                 properties, acid gas concentrations,
                                 and climatic conditions. The dry sor-
                                 bent system is capable of eliminating
                                 the visible plume from the stack. Figure
                                 7 shows controlled  and uncontrolled
                                 opacities from the aluminum industry
                                 using dry sorbents.
Figure 5.

Inlet Gas Concentrations Compared to Outlet Concentrations from
Sorbent System

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                                                                           Dry sorbents also influence baghouse
                                                                           operation.  Earlier in the discussion,
                                                                           major problems in baghouse control
                                                                           were identified as the corrosiveness of
                                                                           the gaseous stream, the oiliness of the
                                                                           paniculate, and the presence of sparks.
                                                                           Thus, there is often a high rate of
                                                                           failure owing to bag corrosion, blind-
                                                                           ing, and perforation.

                                                                           Dry sorption systems, however, act as
                                                                           a precoat to protect the filter bags from
                                                                           both blinding and burning, while they
                                                                           absorb and neutralize acid gases.
                                                                           Once applied, the coating is effective
                                                                           for several (3 to 7) days. In this manner,
                                                                           the dry sorbent increases the life of the
                                                                           bags.
Figure 6.

Inlet and Outlet Particulate Concentration and the Effect of Dry
Sorbent Control

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Figure 7.
Comparison of Uncontrolled and Controlled Opacities Using
Sorbent Systems

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4.    Economics
Costs for a primary aluminum plant
producing 300 tons per day of alumi-
num, and for a secondary aluminum
plant producing 200 tons per day of
aluminum, are given in Tables 2 and 3,
respectively. These costs are intended
to represent typical  retrofit situations
for each industry, and assumptions are
identified to enable conversion to dif-
ferent values. Operating costs are m
proportion to volume of gas handled
rather than to amount of pollutant
removed.
Costs for these systems will, of course,
vary widely with the exact ducting
configuration, with the size of the flow
volume, and with differing values of
the cost components covered in the
tables.
                                     View of baghouse and limestone tower (foreground)

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Table 2.



Cost of Air Pollution Controls for 300-ton/d Primary Aluminum Plant (Pot Vents Only)

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Table 3.

Cost of Air Pollution Controls for 200-ton/d Secondary Aluminum Smelter3
                                  Item
Injected or
  batch
Tesisorb*
 Capital cost-
    Equipment, total installed cost at SB/actual ft3/min	    $540,000
    Working capital at 20 percent of equipment cost	     108,000
    Royalty	      45,000

      Total capital cost	       693,000

 Annual operating cost.
    Operating laborb	         $3,800
    Maintenance	       2,200
    Depreciation, administrative overhead, property tax,  and insurance at 1 5
      percent of capital cost and interest at 8 percent of  capital cost	      159,000
    Utilities:
      Electricity at $0.03/kWh  	       4,300
      Sorbent	       4,800
    Disposal for recovered material at $10/ton   	       9,000

      Total annual cost	     183,100

 Cost per pound of product	         0.0028

 a60,000 ft3/mm average air flow at 200° F.

 b750 h/yr at $8/h.

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5.    Areas of Application
In light of the information presented in
Table 1, the glass industry and indus-
trial or municipal incineration can
appropriately be considered potential
areas for the use of the described dry
sorption systems. Only a few glass
plants now use dry sorbent in bag-
houses. At least one municipal incin-
erator plans to use  a dry sorption
system.

Figure 8 illustrates the glass manufac-
turing process.  Glass is  formed by
melting and mixing sand, soda ash,
cullet, and limestone or lime, along
with numerous  other materials added
to give the glass its special  character-
istics.

Typical emissions from glass manufac-
ture  include very fine particulates,
NOX, and SOX (especially S03), which
are difficult to remove by scrubbers,
wet precipitators, or conventional
baghouses.

The  preferred scheme for using dry
sorption in the glass industry is injec-
tion  of the sorbent some time after
quenching.  In the case of fiberglass
and opal glass, lime maybe used in the
quench to reduce the consumption of
sorbent and to  preclude formation of
extremely fine boron fumes that are
very difficult to remove. In some cases,
the recovered materials can be reused
in the glass process. IMephelme sye-
nite  is the only sorbent known to have
been used, to date, for the glass in-
dustry.
Industrial or municipal incineration is
another possible area of application.
Waste incinerators can emit large
quantities of particulates  and varying
amounts of acid gases, depending on
the material incinerated. Acid gases
may result from the incineration of
materials containing sulfur, chlorine,
or fluorine.

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Figure 8.



Glass Manufacturing Process

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                                             This report was prepared for the U.S. Environmental Protection Agency by the
                                             Centec Corporation. Mr. James A. McCarthy and Mr. Philip A. Militello are the
                                             principal contributors. Mr.  Ben Smith is the EPA Project Officer. The R J.R.
                                             Archer Company, Winston Salem NC, and the Toronto Metals & Alloys Company
                                             Limited, Toronto,  Ontario,  provided the photographs. Comments or questions
                                             regarding this  report should be addressed to:

                                             Metals and Inorganics Branch
                                             Industrial Environmental Research Laboratory
                                             U.S. Environmental  Protection Agency
                                             Cincinnati OH  45268
                                             This report has been reviewed by the Industrial Environmental Research
                                             Laboratory, U.S. Environmental Protection Agency, Cincinnati OH, and approved
                                             for publication. Approval does not signify that the contents necessarily reflect
                                             the views and policies of the  U.S. Environmental Protection Agency,  nor does
                                             mention of trade names or commercial products constitute endorsement or
                                             recommendation for use.
US GOVERNMENT PRINTING OFFICE 1979 658-636

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