United States Environmental Protection Agency Air and Energy Engineering Research Laboratory Research Triangle Park NC 27711 Research and Development EPA/600/S7-89/004 Nov. 1989 Project Summary Experimental Study of High Levels of SO2 Removal in Atmospheric-Pressure Fluidized Bed Combustors D. D. Kinzler and K. R. Drake Tests were conducted in an atmos- pheric-pressure fluidized bed com- bustor (FBC) having a cross-section of 1 x 1.6 m, for the purpose of demonstrating high levels of SO2 removal when burning a high-sulfur coal and feeding limestone sorbent for SO2 removal. The goal was to achieve SO2 removals of 90-plus % with reasonable sorbent feed rates, through suitable reductions in sor- bent particle size (to improve reac- tion kinetics) and increases in gas residence time (to increase gas/sor- bent contact time), in a manner predicted by an existing mathemat- ical model. At particle sizes averaging from 800 and 1300 pm (mass mean), and with gas residence times of 0.5 to 1.5 sec, the measured SO2 retention levels ranged from 88 and 98% when sor- bent was fed at Ca/S molar ratios be- tween 2 and 3. This result supports model predictions. Reducing sorbent particle size and Increasing gas resi- dence time results in modest in- creases in SO2 removal over the range of conditions tested here. In- creases in flue gas O2 content also increased removals. Only one of the three sorbents considered for this project had the attrition resistance necessary to permit use in this test- ing, indicating that some sorbents will not be suitable for use in dense- phase FBCs. Emissions of NOX ranged from 130 to 236 ng/J during these tests. Partic- ulate emissions following the cyclone but upstream of the baghouse ranged from 9 to 35 g/m3; after the baghouse, at the stack, the particle loading ranged from 0.4 to 22 ng/J. This Project Summary was devel- oped by EPA's Air and Energy En- gineering Research Laboratory, Re- search Triangle Park, NC, to announce key findings of this research project that is fully documented In a separate report of the same title (see Project Report ordering information at back). Introduction For FBCs to be competitive with conventional coal-fired boilers, the FBCs will have to be able to provide reductions in S02 emissions comparable to those possible with conventional boilers using scrubbers. These comparable reductions must be achieved at a competitive cost. Some New Source Performance Standards (NSPS), which have been considered or promulgated for various coal-fired boilers, have envisioned S02 reductions up to 90% with high-sulfur coals. However, early experimental test- ing of FBCs had generally focussed on the earlier NSPS for large steam generators which had been promulgated in 1971 (520 ng S02/J). This earlier standard corresponds to a percentage S02 reduction of only about 80 to 85% with a high-sulfur coal. Little experimental work had been conducted with sorbent feed rates necessary to achieve reductions > 90%. Accordingly, there ------- Table 1. Text Matrix for Tests i3-28» (Effects of Sorbent Particle Size and Gas Residence Time) Mass mean sorbent particle size (ftm) Gas velocity (m/sec) Bad depth (m) Residence time (sec) Ca/S 0.9 0.9 1.0 2.0b, 2.75" 800 1.4 1.5 2.0, 2.75 1.4 0.9 0.7 2.0, 275 1.4 1.4 1.0 2.0, 2.75 1300 1.8 0.9 0.5 2.0, 2.75» 1.4 0.75 2.0, 2.75b Conditions for all 16 tests: Illinois No. 6 coal (3.5% sulfur); Greer limestone; bed temperature 844 °C; excess O2 5% (30% excess air); two coallsorbent feed ports; and no carryover recycle. bThese tests were replicated, to yield a total of 16 tests. was not a substantive data base in large pilot fluidized beds (FBs) to confirm how a requirement for 90-plus % reduction might impact the design of FBCs, and their capital and operating costs. Earlier EPA-sponsored research at Westinghouse Research and Develop- ment Center had involved the develop- ment of a mathematical model predicting SO2 removal in a FBC, based on sor- bent/S02 reaction kinetics and on FB de- sign and operating parameters. Using this model, it had been predicted that FBCs should generally be able to achieve high levels of SO2 removal economically, if sorbent reactivity is sufficiently great (e.g., through decreases in sorbent particle size), and if the gas residence time in the bed (i.e., the gas/solids contact time) is sufficiently great, through a suitably increased bed depth and/or decreased superficial gas velocity. The reduced operating costs resulting from reduced sorbent feed requirements would more than compensate for increased capital costs associated with the larger, deeper combustors that would be needed. The purpose of the current study is to demonstrate that high levels of SO2 removal (>90%) can in fact be routinely achieved in FBCs with reasonable sorbent feed rates, if sorbent particle size and gas residence time in the bed are appropriately adjusted. This objective is to be met through a statistically designed test program on a reasonably large experimental FBC which has the flexi- bility to operate over the range of gas velocities and bed depths needed for this evaluation. Experimental Equipment The experimental FBC consists of a carbon steel shell lined with castable refractory to inside dimensions of 1 x 1.6 m. The unit can burn from 55 to 250 kg coal per hr. In-bed temperature is controlled by an air-cooled tube bundle. Crushed coal and sorbent are premixed and fed near the bottom of the bed. Flue gas leaving the combustor first passes through an overbad heat exchanger to reduce temperature, then through a cy- clone and a baghouse to remove panic- ulate. The baghouse is a reverse-jet pulse type containing 93 m2 of Nomex cloth. Test Program The test program consisted of two segments. In the first segment (Tests 1 through 12), testing was carried out with one vs. two coal/sorbent feed ports, and with and without carryover recycle. The purpose was to determine how these parameters should be set for the remain- der of the testing. The second segment (Tests 13 through 28) was designed to investigate the effects of sorbent particle size and gas residence time (i.e., the relationship of bed depth and gas velocity). The test matrix for this second segment is shown in Table 1, covering: two sorbent particle size distributions (mass means of 800 and 1300 urn); three superficial gas velocities (0.9, 1.4, and 1.8 m/sec); two bed depths (0.9 and 1.4 m); and two sorbent feed rates (Ca/S ratios of 2.0 and 2.75), expected to provide reductions in the vicinity of 90- plus % at these test conditions. The gas velocities/bed depths were selected to give nominal gas residence times in the bed ranging from 0.5 to 1.5 sec. Usually, 6 hours of steady state operation was maintained for each test condition. During that time, SO2, O2, C02, and CO were monitored con- tinuously in the flue gas, and grab samples for NOX were taken. We chemistry of SO2 and NOX (EPA Method 6 and 7) was measured once each run t confirm the results from the instrument; A cascade impactor was used t determine the particle size distributio upstream of the baghouse, and EP< Method 5 was used to determine particl mass loading in the duct downstream c the baghouse. Coal, limestone, fly asf and bed material were sampled fc chemical analysis and determination c size distribution, as appropriate. The tests were all conducted burnin Illinois No. 6 coal (3.5% sulfur) and usin Greer limestone. Of the three sorbenl considered for this project, Greer was th only one having sufficient resistance t attrition/elutriation. Bed temperature wa held at 844°C, and excess air wa generally held at 30% (5% exces oxygen), although there were som limited, unavoidable variations. The tesl in the second segment (Tests 13-2J were conducted with two feed ports an without carryover recycle. The results c Tests 1-12 showed no significant benef either to sulfur retention or to combustio efficiency, of operating with one vs. tw feed ports, or with or without recycle; th selected options gave the best centre over freeboard temperature. Results The S02 retentions observed durin the 28 tests ranged from 88 to 98%. A expected, within the range of condition tested here, the highest retention level were generally achieved with the greate; gas residence times (i.e., with deep bed and low gas velocities), the highes sorbent feed rates, the smaller sorber particle size, and the highest levels ( excess air. Results of a multiple lines regression analysis show that the SO ------- retention increased: by about 2% as gas residence time was increased over the tested range; by about 2% as sorbent particle size was decreased; and by 3% as excess oxygen increased from 5.0 to 6.1% excess. As expected, the sorbent feed rate had the dominant effect (increasing SC>2 retention by 6% as the Ca/S increased from 2 to 3). The addition of fly ash recycle and increasing the number of feed ports from one to two, each resulted in a 2% increase in retention. The fact that Greer limestone was the only sorbent of three candidates which had sufficient attrition resistance for these tests illustrates that some sorbents will not be suitable for use in dense-phase FBCs. NOX emissions during the 28 tests ranged from 200 to 300 ppm, or 130 to 236 ng/J. The highest NOX levels were measured at the greatest excess air values. NOX also tended to be higher when the S02 concentrations were lowest. Particulate mass loadings upstream of the baghouse (after the cyclone) ranged from 9 to 35 g/m3, with 15 to 25% of the particulate smaller than 10 pm. Particulate mass loadings at the stack (downstream of the baghouse) generally ranged from 0.4 to 22 ng/J. Particulate emissions below the NSPS of 13 ng/J were generally achieved for baghouse air-to-cloth ratios less than about 1.2 m3/min/m2. U.S. GOVERNMENT PRINTING OFFICE: 1989/748-012/07186 ------- D. D. Kinzler and K. R. Drake are with FluiDyne Engineering Corp., Minneapolis, MN 55422. D. Bruce Henschel is the EPA Project Officer (see below). The complete report, entitled "Experimental Study of High Levels of SO2 Removal in Atmospheric-Pressure Fluidized-Bed Combustors," (Order No. PB 89-194 1871 AS; Cost: $21.95, 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 UNOFFICIAL MAIL United States Environmental Protection Agency Center for Environmental Research Information Cincinnati OH 45268 Official Business Penalty for Private Use $300 EPA/600/S7-89/004 000085833 PS 0 S EMVia PROTECTION AGENCY BEGICS 5 LIBBABT C]\/ 230 S DEABBOEM STBEEI Vv CHICAGO 1L 60604 ------- |