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Energy & Fuels 2001, 15, 794-800
Control of Trace Elements in Gasification: Distribution to the Output Streams of a Pilot Scale Gasifier G. P. Reed,* D. R. Dugwell, and R. Kandiyoti Department of Chemical Engineering and Chemical Technology, Imperial College, University of London, Prince Consort Road, London SW7 2BY, U.K. Received July 12, 2000. Revised Manuscript Received March 26, 2001
The distribution of trace elements to the output streams in a coal and waste gasification scheme for clean energy production have been investigated on a pilot scale gasifier. A 2 MWt air-blown fluidized bed gasifier operated at nominal bed conditions of 960 °C and 13 bar has been used for these investigations. The effects of feedstock variations (coal/sewage sludge/sorbent) and hot gas filter temperatures of 580 to 450 °C were studied. Feedstock sampling and deposition within the plant were found to be significant contributions to mass balance closures outside the expected range. The total trace element emission in the fuel gas from co-gasification of sewage sludge with coal was not significantly increased, and remained within potential legislative limits. No Zn was present in the fuel gas, despite the high levels of this element in the sewage sludge, but there was evidence of an increase in Sn emissions under certain conditions. Hg and Se were only detected in the fuel gas, while Cd and Pb were concentrated in the fine dust removed by the hot gas filter. A reduction in hot gas filter temperature to 450 °C was shown to substantially reduce Pb emissions in the fuel gas. B was retained in the solid streams when sulfur sorbents were used.
Introduction Gasification. Gaseous primary fuels such as natural gas have an amenity value which generally favors their application in power generation systems over alternative fossil fuels such as coal, even when the cost of energy from the latter may be lower. Solid fuels may be made more amenable by gasification to make a fuel gas; after purification, the fuel gas may be used instead of natural gas to fire a gas turbine for power generation,1,2 or to provide process heat. The economics of power generation systems employing gasification may be greatly improved by substituting a waste with a significant heating value and low or negative costs, such as sewage sludge, for part of the solid fuel. Gasification schemes for such applications are usually modest in scale (ca. 10 MWt), because of the limited amount of waste available in a single location. Air-blown gasification is often selected, because of the lower infrastructure requirements and advantages for fuel with a high or refractory ash content and moderate sulfur content.3 Hot Gas Cleanup. The fuel gas from the gasifier must be cleaned of contaminants that might be harmful to the gas turbine, before it is combusted. Studies have shown the advantages of hot gas cleanup over conven* Corresponding author. E-mail:
[email protected]. (1) Takematsu, T.; Maude, C. Coal Gasification for IGCC power generation; IEA Coal Research: London, 1991(March); IEACR/37, ISBN 92-9029-190-7. (2) EPRI Gasfication Technologies Conference; San Francisco, CA, 1997(October). (3) Wheeldon, J. A. Review of PFBC Power Plant Designs. In Proceedings of the 13th International Pittsburgh Coal Conference; Pittsburgh, PA, 1996; pp 247-260.
tional low-temperature wet scrubbing; the Air-Blown Gasification Cycle (ABGC)4 and other similar systems gain much of their efficiency advantage through the use of hot gas cleaning.5 These advantages are greatest if a mechanical particle separator such as a hot gas filter is sufficient to protect the gas turbine. A critical issue in the performance of hot gas cleaning systems is the vapor concentration of metal species in the fuel gas. These can pass through the cleaning systems as a vapor and deposit either in the ducts, or cause potentially more problematic deposition and corrosion damage in the gas turbine. Minor elements in the coal such as Na and K are the common concern, and the hot gas cleanup temperature is usually restricted to below 600 °C to overcome this problem. However, other metals such as Pb and Zn are found in sewage sludge in significant amounts; their volatility and corrosiveness6 may require a further reduction in hot gas cleanup temperature. Trace Elements. Elements present in coal at a level of up to about 1000 ppm(wt) in dry coal may be described as “trace elements”.7 Although these elements are present at low levels, the annual consumption of coal is large, making the potential for their dispersal into the environment a cause for concern.8 (4) Welford, G. B. Gasification and Mitsui Babcock Energy Ltd. In Proceedings of the International IChemE Conference on Gasification; Dresden, Germany, 1998(September). (5) McMullan, J. T.; Williams, B. C.; Sloan, E. P. Clean Coal Technologies; Proc. Instn. Mech. Engrs: 1997; Vol. 211, Part A, pp 95107. (6) ASTM Standard Specifications for Gas Turbine Fuel Oils; ASTM Standard Designation: 1990(November), D 2880-90a. (7) Swaine, D. J. Trace Elements in Coal; Butterworths: Markham, ON, Canada, 1990. (8) Bowen, H. J. M. Environmental Chemistry of the Elements; Academic Press: London, 1979.
10.1021/ef000156k CCC: $20.00 © 2001 American Chemical Society Published on Web 06/06/2001
Control of Trace Elements in Gasification
The literature on the behavior of trace elements in combustion systems is quite extensive, in line with their current industrial importance. The International Energy Agency (IEA) has produced several reports targeted on trace elements and their behavior; Smith9 has investigated coal combustion as one of the sources of trace elements to the environment, and Clarke10 the sources of trace elements in the atmosphere, their occurrence in coal and their partitioning through coal-fired combustion and gasification processes, and in downstream pollution control equipment. The literature cited shows a classification of the elements broadly in line with their relative volatility. A more recent review by Davidson and Clarke11 focused upon the analytical and sampling problems which can cause gross uncertainties in experimental measurements. The mechanisms of nucleation, condensation, and coagulation by which trace element vapors may be removed in a particle-laden combustor gas path have been reviewed by Biswas and Wu;12 an essential feature in any model of this process is the chemical speciation of each element. Theoretical studies13-15 have been made of the trace element speciation for the reducing conditions that prevail in a gasifier, but their experimental validation is impractical.. Experimental data from gasification systems is limited16-19 and even more so for systems using hot20,21 rather than cold gas cleanup. In most cases measurements of trace element levels in the fuel gas were either not available,21 not validated by mass balance,20 or not published for commercial reasons. In the work reported here a pilot scale 2 MWt airblown fluidized bed gasifier with a hot gas filter has been operated over a range of feedstock combinations including coal, sewage sludge pellets, and various Ca(9) Smith, I. M. Trace elements from coal combustion: emissions; IEACR/01; IEA Coal Research: London, 1987(June). (10) Clarke, L. B.; Sloss, L. L. Trace elements-emissions from coal combustion and gasification; IEACR/49; IEA Coal Research: London, 1992(July). (11) Davidson, R. M.; Clarke, L. B. Trace Elements in Coal; IEAPER/ 21; IEA Coal Research: London, 1996(January). (12) Biswas, P; Wu, C.Y. Control of Toxic Metal Emissions from Combustors Using Sorbents: A Review. J. Air Waste Manage. Assoc. 1998, 48, 113-127. (13) Frandsen, F.; Dam-Johansen, K.; Rasmussen, P. Trace Elements from Combustion and Gasification of CoalsAn Equilibrium Approach. Prog. Energy Combust. Sci. 1994, 20, 115-138. (14) Helbe, J. J.; Mojtahedi, W.; Lyyranen, J.; Jokiniemi, J.; Kauppinen, E. Trace element partitioning during coal gasification. Fuel 1996, 75 (8), 931-939. (15) Mojtahedi, W. Trace Metals Volatilisation in Fluidised-Bed Combustion and Gasification of Coal. Combust. Sci. Technol. 1989, 63, 209-227. (16) Forney, A. J.; Haynes, W. P.; Gasior, S. J.; Johnson, G. E.; Strakey, J. P. Analyses of tars, chars, gases and water found in effluents from the Synthane process. Bureau of Mines Application of Improved Technology to Provide Clean Energy Program, Technical Progress Report 1974, 76. (17) Beishon, D. S.; Hood, J.; Vierrath, H. E. The fate of trace elements in the BGL gasifier. In Sixth International Pittsburgh Coal Conference; AIChemE: Pittsburgh, PA, 1989(September). (18) Baker, D.C. Projected emissions of hazardous air pollutants from a Shell coal gasification process-combined cycle power plant. Fuel 1994, 73 (7), 1082-1086. (19) Williams, W. A.; Behrens, G. P. A Study of Trace Substance Emissions from a Coal-Fired Gasification Plant. In EPRI/DOE International Conference on Managing Hazardous and Particulate Air Pollutants; EPRI: Toronto, ON, Canada, 1995(August). (20) Mojtahedi, W.; Salo, K. Fate of a Few Selected Trace Elements in Pressurised Fluidised-Bed Gasification and Hot Gas Cleanup. In Thirteenth Annual Pittsburgh Coal Conference; AIChemE: Pittsburgh, PA, 1996. (21) Bushell, A.; Williamson, J. The Fate of Trace Elements in Coal during Gasification. In Proceedings of the 8th International Conference on Coal Science; Oviedo, Spain, 1995(September).
Energy & Fuels, Vol. 15, No. 4, 2001 795
Figure 1. Schematic of the gasifier pilot plant.
based sulfur sorbents, and with a range of hot gas filter temperatures. Eighteen trace elements (As, B, Ba, Be, Cd, Co, Cr, Cu, Hg, Mn, Mo, Ni, Pb, Sb, Se, Sn, V, Zn) were measured in the fuel gas after the hot gas filter and in the solid streams, to enable trace element balances to be determined. The effect of the gasifier feedstocks and hot gas filter temperature on the fuel gas levels and distribution of these elements to the gasifier output streams is presented. Experimental The Gasifier Pilot Plant. The work described in this section was carried out on the 2 MWt pilot plant gasifier at British Coal, Coal Technology Development Division (CTDD), Stoke Orchard. The plant, described in outline in an earlier paper by Reed et al.,22 is shown schematically in Figure 1. A mixture of air and steam was used to gasify the fuel, which here was crushed coal and sewage sludge pellets (top size 3 mm). A sulfur sorbent such as limestone, with a top size of 250 µm, was injected to retain the sulfur. In the reactor the fuels were gasified producing fuel gas, char, and ash. Some of the solid material (95% of an element such as B which would remain in the gas phase until about 80 °C above the impinger conditions, and >98% of an element that should condense at higher temperatures, such as Sn.23 Solid stream samples were taken by methods adapted from BS1017 to the practical limitations imposed by the quantity of these materials, their nature, and the need to store them. All of the samples were analyzed by inductively coupled plasma/mass spectrometry or inductively coupled plasma/atomic emission spectrometry; a validation of the analytical data by comparison has been given elsewhere.24 (23) Reed, G. P. Control of Trace Elements in Gasification. Ph.D. Thesis, Imperial College, University of London (March 2000).
Figure 2. Typical distribution of trace elements in combined input to gasifier.
Results and Discussion Feedstocks. The trace element compositions of the feedstocks are summarized in Table 2; the estimated uncertainty of analysis was (20%, so only the first significant figure may be regarded with confidence. Pb, Sn, and Zn levels in the sewage sludge pellets were higher than in the coal or any of the sorbents. Zn was especially high in the sewage sludge pellets, and quite variable in the range of 2500 to 12000 mg kg-1, due to feedstock heterogeneity. Input Distribution. The feedstocks were not fed to the gasifier in equal amounts; in those tests where they were used, the sewage sludge pellets were 25 wt % of the total fuel and the sorbent was about 10 wt % of the coal. The typical distribution of the trace elements in the combined feedstock is given in Figure 2. It can be seen that the sewage sludge pellets contributed most of the Cr, Cu, Ni, Pb, Sb, Sn, and Zn in the tests where it was fed, while As, B, Mo, and V were always predominantly derived from the coal. The sorbent made a major contribution to the input of Co, and also contributed a significant proportion of elements such as Mn, Pb, and Zn. Fuel Gases. The mean values of the trace element concentrations in the fuel gas, after deduction of the appropriate sample system blank values, are given in Table 3. Measurements below the analytical limit of (24) Richaud, R.; Lachas, H.; Healey, A. E.; Reed, G. P.; Haines, J.; Jarvis, K. E.; Herod, A. A.; Dugwell, D. R.; Kandiyoti, R. Trace Element Analysis of Gasification Plant Samples by ICP-MS: Validation by Comparison of Results from Two Laboratories. Fuel 2000, 79, 10771087.
Control of Trace Elements in Gasification
Energy & Fuels, Vol. 15, No. 4, 2001 797
Table 2. Trace Element Analyses of Feedstocks trace element concentration, mg kg-1 as receivedc,d test
As
B
Ba
Be
Cd
Co
Cr
Cu
Hg
Mn
Mo
Ni
Pb
Sb
Se
Sn
V
Zn
LODb DMa SSPa CDa CSDLa GMLa
0.05 8.1 2.9 0.8 tr tr
4 tr tr 40 tr nd
2 140 400 nd 370 nd
0.1 tr tr nd nd nd
1 nd tr nd nd nd
0.1 4.1 7.9 30 53 44
2 tr 55 tr tr tr
3 tr 310 nd nd nd
3 tr tr tr nd tr
1 200 280 520 110 39
1 10 12 nd nd tr
4 tr 40 nd nd nd
0.7 11 140 7 47 tr
0.2 tr 6.9 tr tr tr
10 tr tr tr nd nd
0.6 tr 81 tr tr nd
0.7 61 45 39 29 31
4 tr 7700 69 83 tr
a
See Table 1 for key to feedstocks. b LOD, limit of detection. c nd, not detected.
d
tr, trace.
Table 3. Summary of Fuel Gas Trace Element Measurements trace element concentration as measured, µg.mn-3 (correcteda mean)c,d test
As
B
Ba
Be
Cd
Co
Cr
Cu
Hg
Mn
Mo
Ni
Pb
Sb
Se
Sn
V
Zn
LODb 4 7 9 10 11#1 11#2 11#3 12
0.01 0.23 0.38 tr tr 0.45 tr 0.26 0.24
8.0 tr tr tr 218 281 223 255 tr
1.6 tr tr 23 54 tr 38 tr 140
0.12 nd nd nd nd nd nd nd nd
0.08 2.7 4.3 nd tr 1.4 tr tr 14
3.9 nd nd nd nd nd nd nd nd
2.4 95 188 88 133 329 270 451 181
3.2 nd nd nd nd nd nd nd nd
0.39 tr nd nd nd 6.4 11 nd 11
0.79 62 75 60 102 77 64 99 40
0.10 1.8 16 5.0 2.1 5.2 tr 14 11
7.9 175 114 169 tr 149 80 154 118
0.04 53 62 7.7 68 72 37 0.91 176
0.03 tr tr tr tr 0.73 nd 1.3 tr
3.9 tr tr tr 61 73 79 87 42
0.02 nd 0.19 nd 4.8 27 70 84 36
3.2 nd nd nd nd nd nd nd nd
2.0 tr tr nd tr nd nd nd tr
a
Blanks deducted. b LOD, limit of detection. c nd, not detected.
detection are shown as “not detected”, while those below the limit of quantification (10 × limit of detection) are given as a “trace”. The estimated error in the measurements was (70%,23 so only the first significant figure should be considered. It can be seen that Be, Co, Cu, and V were never detected in the fuel gas. High levels of Cr, Mn, and Ni were seen in all of the measurements; this is attributed to corrosion products from the pilot plant components. The measurable levels of Mo are also thought to arise from contamination by MoS2 anti-seize lubricants used in the pilot plant. Pb was quantifiable in all of the tests, Sn in the tests with sewage sludge pellets, and B in the tests without sulfur sorbent. As, Ba, Cd, Hg, Sb, Se, and Zn were measured in a few tests. The Hg measurements have been shown to be an underestimate,25 and later work has demonstrated the benefits of dry sampling for this element.26 The presence of Pb in the fuel gas can be expected to cause gas turbine corrosion. The level of Pb seen during most of the tests is close to the equivalent of the standard specification for gas turbine fuel oils.6 The absence of Zn in the fuel gas for the tests with sewage sludge pellets, despite the high levels in this waste fuel, is a significant and very positive outcome with respect to the commercial application of the ABGC gasifier to sewage sludge gasification. Element Balances. A total mass balance of 100 ( 3% was normally considered to be satisfactory for the gasifier pilot plant, and was achieved for all of the tests. The total mass balance was dominated by the flowrate of the gas streams. These were determined by measuring the input mass flows of air and steam, and the composition of the output fuel gas; the mass flowrate of nitrogen was assumed to be constant, allowing the output mass flowrate of fuel gas to be calculated. The (25) Richaud, R.; Lachas, H.; Collot, A-G.; Mannerings, A. G.; Herod, A. A.; Dugwell, D. R.; Kandiyoti, R. Trace Mercury Concentrations in Coals and Coal-derived Material Determined by Atomic Absorption Spectrophotometry. Fuel 1998, 77 (5), 359-368. (26) Reed, G. P.; Ergu¨denler, A.; Grace, J. R.; Watkinson, A. P.; Herod, A. A.; Dugwell, D.; Kandiyoti, R. Control of Gasifier Mercury Emissions in a Hot Gas Filter: The Effect of Temperature. Fuel 2001, 80, 179-194.
d
tr, trace.
major components of the gas composition (i.e., N2, CO2, CO, H2, CH4) can be readily measured by continuous gas analysis, without any significant sampling errors. However, balance closures for elements that are mainly in the solid streams are always worse than the total mass balance, as the former depend on solid stream mass flow rates, sampling, and analysis. An example of this is provided by the balances for Al; this is an abundant nonvolatile element in coal ash, for which the closures in these tests ranged from 89 to 112%.27,28 The input and output mass flowrate data were used together with the analyses to calculate element balances. In work of the type presented here, it is normally found that one or more of the process streams may contain a level of a trace element that is below the limit of quantification, and therefore not quantifiable to the same level of uncertainty as other measurements. The balance closure may be substantially affected if this stream contributes a major part of the total input or output. Assumptions are sometimes made that the level in such a stream is a certain multiple of the limit of detection, but although this may be sufficient for assessing stream toxicity, balances prepared on this basis cannot be regarded as sufficiently reliable for evaluating process behavior. To provide a better basis, the protocol was applied that at least one input stream and 90% of the total input and output of any element must exceed the limit of quantification. Element balances prepared on this basis are given in Table 4, with all of the tests that fail to meet these requirements eliminated. The extent to which the remaining closures may be regarded as acceptable depends on the uncertainty of the measurements; on the basis of the analytical uncertainty alone, this has been estimated to be 100 ( 30%.23 An acceptable closure range of 100 ( 30% has also been used by other workers to assess trace element balances.29,30 The means of the closure values for As, (27) Paterson, N.; Watkins, C. J.; Williams, J. D. Gasification Tests with Northumbrian Water Sewage Sludge Pellets. APG Report 302, British Coal CTDD Stoke Orchard (November 1996). (28) Paterson, N.; Watkins, C. J.; Williams, J. D. Process Performance and Operability Report for Run PG 13. APG Report 306, British Coal CTDD Stoke Orchard (January 1997).
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Reed et al.
Table 4. Trace Element Balance Closures balance closure (mean, %) test
As
B
Ba
Be
Cd
Co
Cr
Cu
Hg
Mn
Mo
Ni
Pb
Sb
Se
Sn
V
Zn
4 7 9 10 11#1 11#2 11#3 12 mean
164 213 E 151 64 89 87 36 114
E E E E E E E E E
129 143 100 125 93 128 132 142 124
E E E E E E E E E
E E E E E E E E E
71 75 64 124 81 120 129 84 94
E E E E E E E E E
E E E E 104 126 134 110 118
E E E E E E E E E
83 82 65 105 80 95 100 86 87
E E 35 E E 54 49 E 46
E E E E E E E E E
99 56 44 77 66 73 68 75 70
E E E E 43 66 66 103 70
E E E E E E E E E
E E E E 37 53 55 108 63
36 44 25 40 32 46 42 37 38
E E E E 55 86 101 66 85
B, Ba, Co, Cu, Mn, Pb, Sb, and Zn fall within the range of 100 ( 30%. Examination of the closure values for individual tests shows that of the 66 valid mass balance closures, 35 (53%) were within the range of 100 ( 30%. No closure values can be reported for B, Be, Cd, Cr, Hg, Ni, or Se. The balances for Mo and V are seen to be consistently below 70%. The sampling of the solid streams collected by the primary cyclone and the hot gas filter has been investigated and shown not to be a significant source of additional uncertainty, as these streams were found to be remarkably homogeneous.23 The most important causes of balance closures outside of the acceptable range are believed to be feedstock sampling and deposition within the plant. Fluidized beds operate with a much coarser fuel particle size (
