United States Environmental Protection Agency Air and Energy Engineering Research Laboratory Research Triangle Park NC 277t 1 Research and Development EPA/6QO/S7-90/011 June 1990 &EPA Project Summary An Evaluation of the E-SO Process on the EPA Pilot Electrostatic Precipitator X Louis S. Hovis The E-SOX Process makes use of an electrostatic precipitator (ESP) for combined sulfur dioxide (SO2) removal and particulate collection. The concept of spray drying is introduced to the inlet and/or first section of the ESP in which electrical components are removed. Because of the many ESPs at coal-fired power plants, the process is well suited to retrofitting. The work described in this report was a small pilot-scale evaluation of the process to obtain the information needed to undertake a planned 5 MWe field pilot demonstration. The results from this evaluation indicate that a 50 - 60% removal of SO2 at a calcium to sulfur ratio of 1.2 - 1.4 can be obtained. Furthermore, this reduction in SO2 can be achieved without degrading the particulate emissions even though the process requires a reduction in the collecting surface of the ESP. The utilization of a temperature-controlled electrode precharger to compensate for loss of collecting surface is also described. This Project Summary was developed by EPA's Air and Energy Engineering Research Laboratory, Research Triangle Park, NC, to announce key findings of the research project that is fully documented in a separate report of the same title (see Project Report ordering information at back). Introduction E-SOX is a retrofit process for coal-fired boilers, which combines in a single unit electrostatic precipitator (ESP) technology for collecting particles and spray dryer technology .'for sulfur dioxide (SO2) removal. The process uses a modified existing ESP equipped with an auxiliary system for preparation and injection of a lime slurry into the ESP. The front end of an existing ESP is converted to a spray ..chamber where contact is made between gaseous SO2 and lime slurry droplets. Water also evaporates in this converted section of the ESP so that the reaction product, excess lime, and fly ash that enter the remaining portion of the ESP are sufficiently dry for efficient ESP operation. A dry solid waste product containing reaction products, unreacted lime, and fly ash is collected. That portion of the ESP not converted to a spray chamber is left with electrical components intact to operate as a particle collector. However, to provide the contacting chamber sufficient space for a finite drying time of 1s or greater, the ESP will lose 25 to 30% of its collector plate surface. At the same time, the particulate load will increase by a factor of 3 or more. This particulate, a large fraction of which is calcium based, has an extremely high electrical resistivity at normal ESP operating temperatures. The sorbent reaction with SO2 will eliminate the effects of sulfur trioxide (SO3) that would lower this resistivity. On the other hand, lowering the gas temperature by the water spray will more than compensate for resistivity increases due to ash composition. Advanced ESP technology to aid in maintaining good particulate collection performance is available for E-SOX retrofitting if required. Cooled pipe precharging, for example, can be introduced in retrofit ESPs to ------- compensate for less collector surface or changes in resistivity characteristics of the participate. The concept of E-SOX appears to offer an attractive option for acid rain mitigation. An economic evaluation of E- SOX based on reasonable rates of SO2 removal and lime utilization has indicated that it can be cost effective. A large pilot- plant evaluation is underway at Ohio Edison's Burger Station sponsored by the U.S. Environmental Protection Agency (EPA) and the Ohio Coal Development Office. Preparation for that pilot plant evaluation started in 1987 and the actual testing is being carried out in 1989. This report covers work that was performed in- house at EPA to verify the original results and to define the parameters that control SOg removal. The work- reported was completed and the technology transferred for use in starting up the field site evaluation. The E-SOX concept raises two fundamental questions which can be answered only by experiment. The first question concerns the feasibility of removing substantial SO2 by contacting the rapidly moving gas with slurry droplets and drying the droplets within the space of one ESP section. The second question has to do with maintaining an acceptable level of ESP performance under a reduced collector area and an increased particulate loading. The results of experiments to partially answer these questions are reported here. Test Facility All the experiments were conducted in the ESP pilot-plant located at EPA's Air and Energy Engineering Research Laboratory (AEERL). The pilot-plant consists of a four-section, single-lane ESP operating at a flue gas capacity equivalent to 0.47 m3/s".Outside air is heated to 149°C by a natural gas heater. Gaseous SO2 is injected into the heated air to the desired concentration (usually 1,500 to 2,500 ppm) to simulate burning of moderately high sulfur coal. The ESP is operated under about 0.5 kPa negative pressure so that no SO2 is released to the room. Fly ash is aspirated counter currently into the simulated flue gas stream just before the cocurrent injection of the lime slurry. Lime slurry containing 10 - 20% solids is pumped through a spray nozzle designed to provide an oval "Readers mom familiar with nonmetric units may uso the conversion factors at the back. spray pattern in the 1.27 x 0.381 m contact chamber. The atomized slurry droplets have a 1.5 - 2 s residence time in the chamber to evaporate most of the water. The evaporation causes a flue gas temperature drop and results in a relatively dry, powder-like product which contains the unused lime, the absorbed and reacted SO2, and fly ash. The spray chamber consists of the entrance section and the first of four ESP sections with all the electrical internals (i.e., discharge electrodes) removed. The electrical configuration of the ESP is flexible but, for most experiments reported, two cold pipe prechargers were used: one in the connecting space between sections 1 and 2 and one between sections 3 and 4. Conventional wire-plate electrodes were assembled in sections 2 and 4. Summary of Results The primary objectives of the E-SOX experiments carried out at EPA using the 0.47 m3/s modified pilot ESP were to verify E-SOX as a competitive retrofit process for S02 removal and to determine the critical parameters which influence the degree of SO2 removal. Once the critical factors were determined, they could be adjusted within limitations of the process to give the best conditions for SO2 removal and sorbent utilization. To meet these objectives, experiments were planned to investigate impacts of indirect variables as well as those directly influencing the lime slurry/SO2 reaction. SO2 Removal Dependence on Critical Factors A number of tests were performed in which only concentration and rate of injection of slaked lime were varied. In essence, this permitted an examination of the effect of the two most critical parameters on SO2-removal; the temperature of approach to saturation (ATAS) and the stoichiometric ratio of calcium to sulfur (Ca/S) in the slurry/gas mixing. When other parameters are held constant, including spray chamber geometry, gas flow rate, S02 concen- tration in the gas, and the inlet gas temperature, these injection parameters can be manipulated to give a ATAS and Ca/S combination. There is a lower limit on ATAS for adequate droplet drying and an upper limit on the solids concentration for consistent spraying. The fixed conditions are listed in Table 1. The removal of SO2 as a function of approach temperature for various stoichiometric ratios is shown in Figure 1. This correlation between approach to Table 1. E-SOX Fixed Conditions for SO2 Removal Studies Air Flow Inlet temperature SO2 concentration Fly ash concentration Nozzle configuration Inlet chamber 28 m3/min 149°C 2000 ppm 1.9 g/l Single two-fluid CasterJet oval spray pattern 1.5 m spray chamber plus 1.2 m ESP section "saturation"and SO2' removal shows that good removal is possible at very low stoichiometric ratios, but only at very close approach temperatures. At these close approaches the droplets are not completely evaporated and excess moisture will pass into the ESP. The lower practical ATAS is believed to be between 16 and 17°C. Stoichiometric ratios of 1.3 to 1.4 produced a removal of 50% or better at a 17°C approach to saturation. Results indicate a marginal improvement in SO2 capture at a ratio of 1.4. The leveling off of removal rate with increasing Ca/S is also reflected in the plot of percent removal as a function of Ca/S in Figure 2. Particulate Removal The second fundamental question about E-SOX as a retrofit concerns maintenance of an acceptable level of particulate removal. In conjunction with the SO2 removal testing, the ESP electrical configuration was"varied to determine effects of the increased load and change in characteristics of the particulate matter collected by the ESP. With sections 2, 3, and 4 energized and containing 0.32 cm wires, the ESP has an 18.8 s/m (96 SCA). With only two of the sections energized, the ESP was reduced to 12.5 s/m (64 SCA). Cold pipe prechargers, between sections 1 and 2 and between 3 and 4, could be activated, but the SCA would remain the same. The data in Table 2 "show that high mass efficiencies were obtained during a 4-fold increase in particulate loading and a 50% reduction in the SCA. There appears to be no significant change in efficiency with the amount of particulate as long as some moisture is present. The ESP ------- 60 50 1" o I DC w o CO 40 30 6 O S02 2000 ppm Ca/S • 1.5 O 1-4 A 1.3 A 1.2 ...a.i.1 n 13.9 16.7 19.4 19.4 Approach to saturation, °C Figure 1. Effect of approach temperature on S02 removal at several stoichiometric ratios. 55 50 as 75 (D o: t. * -..40 35 7.0 J.T 7.2 7.3 Ca/S 7.4 7.5 emission rates listed in Table 2 also indicate that under E-SOX conditions the ESP can meet or exceed the NSPS standard of 43 ng/J. For fly ash only, the efficiency was reduced severely without moisture addition. In this case, the 38 kV could be maintained on the cold pipe precharger, but not on the wires because of back corona. Evaporation of water during the drying step lowers the temperature which accounts for the low resistivity of the lime sorbent/fly ash mixture in the E-SOX process. As a consequence of the low resistivity, back corona is not a problem. Future Work K The primary immediate E-SOX follow-up will occur at the Burger Plant of Ohio Edison where the EPA, Ohio Coal Development Office (OCDO), Babcock & Wilcox Research Division and Southern Research Institute are evaluating the process. The evaluation is being carried out in a 5 MWe pilot ESP which is connected by a slipstream off of the main ducts between the boilers and the plant ESP. The work plan at this site has been designed to verify the SO2 removal results and the ESP efficiencies that have been obtained in the in-house process and reported here. The work, having been done on a larger unit, should provide experience in design that will be more meaningful for a full- scale demonstration. The pilot evaluation is slated for completion in late 1989 with results to be reported in the spring of 1990. Figure 2. Effect of stoichiometric ratio on SO2 removal at two approaches to saturation. ------- Table 2. E-SO, Particulate Removal Efficiencies for Various ESP Electrical Configurations Sections Energized 38 kV 2,3,4 2,3,4 2,3,4 2,3 2,3 2,3,4 2,3,4 2.3.4 Cold Pipe 38 kV Yes No No No No No Yes Yes Approach Temperature (°C) 17 17 17 17 19 56 56 89* Particulate E-SOX & fly ash Fly ash E-SOX & fly ash E-SOX & fly ash Fly ash Fly ash Fly ash Fly ash ESP Efficiency (%) 99.5 97.9 98.5 98.6 95.6 97.6 97.8 89.0 Emission Rate (ngtJ) 7.319 13.776 12.915 37.884 24.969 12.915 16.359 55.104 *No moisture injection NONMETRIC EQUIVALENTS Readers more familiar with nonmetric units may use the following conversion factors: Metric °C "C (app to sat.) cm g/i kPa m MWe nglJ Multiplied by 9/5 x"C + 32 9/5 x °C 0.394 0.526 4.00 3.28 2128 3000 0.0023 Yields nonmetric "F "F (app to sat.) in. in. H2O ft cfm acfm lb/106 Btu The EPA author, Louis S. How's, also the EPA Project Officer (see below), is with Air and Energy Engineering Research Laboratory, Research Triangle Park, NC 27711. The complete report, entitled "An Evaluation of the E-SOX Process on the EPA Pilot Electrostatic Precipitator," (Order No. PB90-216 4411 AS; Cost: $17.00, 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: Air and Energy Engineering Research Laboratory U.S. Environmental Protection Agency Research Triangle Park, NC 27711 United States Environmental Protection Agency Center for Environmental Research Information Cincinnati OH 45268 Official Business Penalty for Private Use $300 EPA/600/S7-90/011 ------- |