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 ------- Baghouse and exhaust stack controlling secondary aluminum smelter (80,000 ft3/mm) ------- 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. ------- Table 1. Characteristics of Uncontrolled Emissions from Acid-Emitting Industries ------- 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 ------- 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. ------- 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. ------- Figure 3. Aluminum Reduction Cell Figure 4. Secondary Aluminum Magnesium Reduction Process ------- Charging of secondary aluminum furnace ------- Settling chamber upstream of baghouse to capture sparks and large debris ------- 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 ------- 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 ------- 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 ------- Figure 7. Comparison of Uncontrolled and Controlled Opacities Using Sorbent Systems ------- 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) ------- Table 2. Cost of Air Pollution Controls for 300-ton/d Primary Aluminum Plant (Pot Vents Only) ------- 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. ------- 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. ------- Figure 8. Glass Manufacturing Process ------- 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 ------- |