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MINERAL MATTER in COAL and WOOD - REPLICATIONS for SOLID FUELED GAS TURBINES by Kenneth W

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MINERAL MATTER in COAL and WOOD - REPLICATIONS for SOLID FUELED GAS TURBINES by Kenneth W MINERAL MATTER IN COAL AND WOOD - REPLICATIONS FOR SOLID FUELED GAS TURBINES by Kenneth W. Ragland University of Wisconsin, Madison, WI and Andrew J. Baker U.S. Forest Products Laboratory, Madison, WI INTRODUCTION Advanced industrial and utility power systems typically use direct fired gas turbine engines. Using coal or wood to directly power a gas turbine has yet to be accomplished commercially, primarily because the ash causes erosion of the blades and deposition on the blades. If the combustion products contain a significant fraction of molten ash particles, deposition on the turbine blades occurs which blocks the flow path and degrades performance. If the ash particles are solid, erosion of the blades occurs which also degrades per- formance. In addition, mineral matter can cause corrosion of the blades. The size distribution, concentration and composition of the ash, as well as the turbine design, determine the lifetime of the turbine blades (1,2). Ash particulates are formed from mineral matter due to three different mechanisms. Part of the mineral matter (in coal but not wood) is found in layers or bands separate from the organic matter. This is called adventitious mineral matter, and it can be partially separated from the coal after crushing and fine grinding. The adventitious mineral matter is transformed directly to ash in the combustor, and the shape is semi-rounded. Secondly, mineral matter in coal and wood is contained within the organic matrix in the form of chemically bound molecules and submicron crystals. Grinding does not liberate this intrinsic mineral matter. During char combustion the intrinsic mineral matter forms ash nodules on the char surface which then coalesce to form particle sizes in the 1 to 10 micron range. Thirdly, some of the mineral matter is vaporized during combustion and condenses on cooler surfaces such as turbine blades. The ash loading and size distribution depend on the type of solid fuel, the extent of grinding and fuel cleaning, and the combustion time-temperature history. Regardless of the type of combustion system, much of the ash is in the 1 to 20 micron size range. Particles down to about 10 microns can be efficiently removed by hot cyclone collectors. Particles in the 1 to 10 micron range can be removed by other advanced methods, however this tends to cause additional pressure drop and heat loss. Currently there are no commercially available fine particle control devices which can operate at high temperature for long time periods. MINERAL MATTER IN COAL Mineral matter includes all elements in coal except C, H, O, N and S. Mineral matter varies widely, and is present as discrete bands In: Combustion fundamentals and applications: 1987 spring technical meeting of the central 117 states section of The Combustion Institute; 1987 May 11-12; Argonne, IL. Argonne, IL: Argonne National Laboratory; 1987: 117-122. and crystals, and as elements bound to the organic matrix. The major constituents (say greater than 0.5% of the coal) are of primary interest with regard to deposition and erosion. Mineral matter primarily includes clays, shales, pyrite, quartz, and calcite. Sometimes overburden rock material mixes in with coal during mining, but this can be removed by cleaning techniques. According to Harvey and Ruth (3), over 125 different minerals have been reported in coal. Frequently occurring minerals are listed in Table 1. Minerals occur as discrete grains, flakes or aggregates in one of five physical modes: (1) as microscopically disseminated inclusions within macerals (distinct organic designations such as vitrinite, liptinite and inertinite), (2) as layers of partings wherein fine-grained clay minerals usually predominate, (3) as nodules including lenticular and spherical concretions, (4) as fissures including cleat and other fracture or void fillings, and (5) as rock fragments found within the coal bed as a result of faulting, slumping or related disturbances. Mineral matter is considered to have been formed by three different mechansims - detrital deposition, syngenesis, or epigenesis. Detrital grains were introduced into a coal forming basin (such as swamp land) by rivers, tidal waves and wind. Syngenetic minerals were formed during the peat stage of coal formation and include minerals formed by crystallization of inorganic elements in plants (intrinsic mineral matter). Epigenitic minerals are those found as filling of fissures and voids after the peat was formed. Palmer and Filby (4) determined the size distribution of major minerals in Powhatan, Ohio coal. pyrite was concentrated in the 5-20 and >20 micron size. Clays were in the 0.2-2 and 2-5 micron range while Quartz was in the 0.2-2.0 micron range. Straszheim et al (5) recently reported data on mineral analysis versus mineral particle size for Illinois No. 6 coal and Pittsburgh No. 8 coal. For the Illinois coal (Table 2) more than 90% of the mineral matter consisted of pyrite, kaolinite, illite or quartz whichwere more or less uniformity distributed among the particle sizes. Removal of mineral matter by sink-float at 1.3 sg ranged from 75% for less than 4 microns to 100% for particles greater than 36 microns (Table 3). For Pittsburgh coal the pyrite was more coarse, while the other minerals were more fine-sized, and the fine grained minerals were relatively untouched by cleaning. For Illinois coal the quartz dropped from 2.51% of the dry coal to 0.63% by cleaning, whereas for the Pittsburgh coal quartz decreased from 0.39% to 0.34%. ASH FROM COAL In the reducing and oxidizing environment of a combustion chamber, mineral matter undergoes a variety of transformations (6). Clays are transformed to aluminosilicates and mullite (Al6Si2O 13), calcite goes to calcuim oxide and quartz may remain unchanged. 118 119 Mixtures of Al2O3-SiO2-FeO-CaO-K 2O occur which are partially molten at 900 c. Illite is the first phase to be partially converted to molten form (6). Representative analysis of ash from bituminous and lignite coal is shown in Table 4. Of course, Table 4 does not represent the actual molecular composition. Adventitious mineral matter is transformed directly to ash in the combustion zone. Depending on the temperature-time history, the ash particles will be spherical or semi-rounded (7). The size distribution of this ash depends on the size distribution of the adventitious mineral matter. Intrinsic mineral matter forms tiny ash nodules in the pores of the char, and as char burnout proceeds, the ash nodules coalesce on the surface of the char. Frequently, cenospheres (hollow glass-like spheres) are formed. The size distribution of this ash depends on the temperature-time history in the combustor. In PFBC tests three hot cyclones yield an ash size distribution of 98% less than 10 microns and 80% less than 4 microns (2). MINERAL MATTER IN WOOD The composition of mineral matter in wood depend somewhat on the soil conditions under which the tree grew, and the location of the sample within the tree. A number of mineral constituents are necessary for plant growth. These and other minerals are transported from the soil thru the roots. The minerals are comprised mainly of salts of calcium, potassium and magnesuim, with other salts in lesser amounts. The acid radicals are carbonates, phosphates, silicates, sulfates and oxalates. In some species, sub micron, crystals of calcium oxalate (CaC2O 4) have been observed (8). For bark, in addition to the minerals transported from the soil, there are wind blown minerals and minerals picked up during harvesting. Relatively little mineral matter is extractable from wood with water. 120 Wood is a desirable fuel because it has low sulfur content compared to coal. In spite of the high oxygen and moisture content compared to coal, wood has an adequate heating value. The heating value of dry wood is roughly 60% that of dry coal. The ash content of wood grown in the temperate zones is 0.1-1.0%, whereas wood grown in the tropical and subtropical zones contains up to 5% ash (9). The higher ash is mainly due to crystals of silica in the wood structure (which tend to dull saw blades). The ash content of bark is typically 3-8%. Bark tends to be available more for combustion than wood at many sites because it remains after the wood is utilized. A whole tree contains 15 to 20% bark. On a equivalent heat input basis, bark generates nearly as much ash as some coals. ASH FROM WOOD Table 4 presents representative data on the mineral content of wood and bark ash. Wood ash often contains 40-70% calcium oxide, 10- 30% potassium oxide, and 5-10% magnesium oxide, as well as oxides of sodium, iron, silicon and, phosphorous. Wood and bark have significant levels of sodium and potassium which tend to promote fouling because they volatilize and recondense on particles and surfaces making them sticky. Bark has 10-20 times the ash content of wood with the greatest increase due to calcium oxide. Hardwood species tend to have more potassium than softwood. Wood and bark as it is used for fuel often somewhat higher ash content due to extraneous mineral matter inadvertently picked up during handling. Regarding the size distribution of wood ash, the State of Oregon has found that for wood and bark burning stoker spreader boilers, 70% of the particulate emissions are less than 10 microns in size (13). Experiments on our novel gravel bed combustor for a gas turbine which uses wood chips and operates in a downdraft mode with high excess air have shown that 90% of the particulate are less than 10 microns and 70% less than 5 microns. CONCLUSIONS Wood is a good turbine fuel because the mineral matter is disseminated in sizes less than 1 micron.
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