Coal, gasification

Coal gasification is essentially the transfer of the exergy from solid coal to an excellent chemical gaseous fuel and base chemical. Suppose gasification is shown by 

Schematic of coal gasification. (http //www.fossil.energy.gov/programs/powersystems/gasification/howgasificationworks.html http //en.wikipedia. org/wiki/Integrated gasification combined cycle). 

Components that cannot gasify, such as mineral components in the fuel, leave the gasifier either as an inert glass-like slag or in a form useful to marketable solid products. A small fraction of the mineral matter is blown out of the gasifier as fly ash and requires removal downstream. 

Sulfur impurities in the feedstock are converted to hydrogen sulfide and carbonyl sulfide, from which sulfur can be easily extracted, typically as elemental sulfur or sulfuric acid, both valuable by-products. Nitrogen oxides, potential pollutants, are not formed in the oxygen-deficient (reducing) environment of the gasifier instead, ammonia is created by nitrogen-hydrogen reactions. The ammonia can be easily washed out of the gas stream. 

All or part of the syngas can also be used in other ways  

Coal gasification is an endothermic process requiring high temperature for the reachon to proceed. In this process, solid coal reacts with oxygen and steam to produce a syngas mixture (CO/H2) according to the following reachon  

Typical Coal Gas Composition for Selected Oxygen-Blown Gasifiers 

Manufacturer Lurgi Winkler Destec Koppers- Totzek Texaco Shell 

Cola Illinois No. 6 Texas Lignite Appalachian Bit. Illinois No. 6 Illinois No. 6 Illinois No. 6 

A combination of pyrolysis and gasification is applied to produce hydrogen from solid fuels. In the past, a variety of methods has been used to gasify solid fuels, to 

6 The process-related details of the various types of gasifier are not discussed here in depth. For more information, refer to the relevant literature (see, e.g., Trevino (2002) Chiesa et al. (2005) IE/IPTS (2005) BMELV (2005)). 

In IGCC plants used only to generate power, in which the synthesis gas is directly converted into electricity in the gas turbine and thus neither a shift reaction nor C02 separation are necessary, the specific investment for CCS is 30% to 40% higher 

IGCC (H2) H2 production, min. H2 production, average H2 production, max. (reference) 

The volatile matter increases from less than 8% in antracite to more than 27 wt% in lignite. In addition, the content of water may vary from less than 5 wt% in antracite to about 60% in German brown coal. Nitrogen (0.5-2%) will be converted into ammonia. The sulphur content may typically vary from 0.5-5 wt%. Sulphur will be converted to COS and H2S. Sulphur will poison downstream synthesis catalysts and must be removed. Chlorine is normally below 1 wt%. Chlorine may cause corrosion problems in downstream equipment. Chlorine will react with ammonia from the nitrogen and deposition of ammonia chloride may foul waste heat boilers and limit their operating temperature [230]. 

The ash may contain a number of components which are volatile at high temperatures such as As, Hg. The ash content may t3 ically be around 10% in anthracite coal, but up to 40% in some coals considered for gasification [230]. The properties of the ash (melting point, etc.) are important parameters. 

The C/H ratio in coal varies around 14 wt/wt in lignite to 25 wt/wt in antracite as shown in Table 1.13. 

Coke made from coal consists mainly of fixed carbon plus the ash. It is of high value for use in blast furnaces because of its strength. 

Petroleiun coke, petcoke and liquid refinery resid feedstocks are used for gasification. Petcoke is made from heavy residues in refineries by the coking process (delayed coking). Resids and petcoke are characterised by a low C/H ratio of 7-10 wt/wt and high content of sulphur 1-7 wt%. 

The Methanex Vancouver web site lays emphasis on methanol production and hopes for the direct methanol fuel cell (DMFC). Fiydrogen is mentioned, but not the need for a hydrogen mine for the nearby Ballard Power Inc. 

The hydrogen industry clearly needs to refresh its sense of proportion and begin to tackle its major new development problems. Eikewise the fuel cell industry has much to reappraise and many new problems to tackle. 

And those behind cried forward, and those in front cried back  

At such a juncture as the one covered here, the best way forward is not easy to establish. 

A large question, looked at below, is the future of gasified coal, supplying complete fuel cells, integrated with gas turbines and with greatly reduced carbon dioxide output, relative to present-day plants based only on heat cycles. 

Arsenic impurities may be removed from synthetic gas with various sorbents, such as zinc ferrite (ZnFe204) or, under carefully controlled temperatures, mixtures of copper(II) oxide and carbon (Quinn et al., 2006 Diaz-Somoano, L6pez-Ant6n and Martfnez-Tarazona, 2004). Specifically, zinc ferrite may capture AS4 vapors through the following reaction (Diaz-Somoano, Lopez-Anton and Martfnez-Tarazona, 2004)  

4ZnFe204 + As4 + 8H2S + 8H2 - 4FeAs + 4ZnS + 4FeS + 16H20 (5.2) 

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Synthesis gas is obtained either from methane reforming or from coal gasification (see Coal conversion processes). Telescoping the methanol carbonylation into an esterification scheme furnishes methyl acetate directly. Thermal decomposition of methyl acetate yields carbon and acetic anhydride,... 

Although the rapid cost increases and shortages of petroleum-based feedstocks forecast a decade ago have yet to materialize, shift to natural gas or coal may become necessary in the new century. Under such conditions, it is possible that acrylate manufacture via acetylene, as described above, could again become attractive. It appears that condensation of formaldehyde with acetic acid might be preferred. A coal gasification complex readily provides all of the necessary intermediates for manufacture of acrylates (92). 

Fig. 3. Overall block flow diagram for Eastman s coal gasification—acetic anhydride complex (35).

Ammonia from coal gasification has been used for fertilizer production at Sasol since the beginning of operations in 1955. In 1964 a dedicated coal-based ammonia synthesis plant was brought on stream. This plant has now been deactivated, and is being replaced with a new faciUty with three times the production capacity. Nitric acid is produced by oxidation and is converted with additional ammonia into ammonium nitrate fertilizers. The products are marketed either as a Hquid or in a soHd form known as Limestone Ammonium Nitrate. Also, two types of explosives are produced from ammonium nitrate. The first is a mixture of fuel oil and porous ammonium nitrate granules. The second type is produced by emulsifying small droplets of ammonium nitrate solution in oil. 

In 1984, the Ube Ammonia Industry Co. began operating the largest Texaco coal gasification complex to date. This faciUty is located in Ube City, Japan, and has a rated gasification capacity of 1500 t/day of coal, and production capacity of 1000 t/day of ammonia. The plant has successfully gasified coals from Canada, AustraUa, South Africa, and China. At the present time the plant uses a mixture of petroleum coke and coal (43). 

When completed in 1996, the Weihe plant will gasify 1500 t/day of coal to produce 300,000 t/yr of ammonia, which will be used to manufacture 520,000 t/yr of urea fertilizer. This project is the eighth Texaco oil or coal gasification plant Hcensed by Chinese industry. 

Suface Coal Gasification Program Fiscal Year 1991, Summary Program Plan, pubhcation DOE/FE-0235P, U.S. Dept, of Energy, Washington, D.C.,... 

Alternative feedstocks for petrochemicals have been the subject of much research and study over the past several decades, but have not yet become economically attractive. Chemical producers are expected to continue to use fossil fuels for energy and feedstock needs for the next 75 years. The most promising sources which have received the most attention include coal, tar sands, oil shale, and biomass. Near-term advances ia coal-gasification technology offer the greatest potential to replace oil- and gas-based feedstocks ia selected appHcations (10) (see Feedstocks, coal chemicals). 

The importance of coal gasification as a means of produciag fuel gas(es) for iadustrial use caimot be underplayed. But coal gasification systems also have undesirable features. A range of undesirable products are also produced which must be removed before the products are used to provide fuel and/or to generate electric power (see Power generation) (22,41). 

Chemistry. Coal gasification iavolves the thermal decomposition of coal and the reaction of the carbon ia the coal, and other pyrolysis products with oxygen, water, and hydrogen to produce fuel gases such as methane by internal hydrogen shifts... 

Indirect Hquefaction of coal and conversion of natural gas to synthetic Hquid fuels is defined by technology that involves an intermediate step to generate synthesis gas, CO +. The main reactions involved in the generation of synthesis gas are the coal gasification m2LC ions Combustion... 

Coal gasification technology dates to the early nineteenth century but has been largely replaced by natural gas and oil. A more hydrogen-rich synthesis gas is produced at a lower capital investment. Steam reforming of natural gas is appHed widely on an iadustrial scale (9,10) and ia particular for the production of hydrogen (qv). 

Methanol, a clean burning fuel relative to conventional industrial fuels other than natural gas, can be used advantageously in stationary turbines and boilers because of its low flame luminosity and combustion temperature. Low NO emissions and virtually no sulfur or particulate emissions have been observed (83). Methanol is also considered for dual fuel (methanol plus oil or natural gas) combustion power boilers (84) as well as to fuel gas turbines in combined methanol / electric power production plants using coal gasification (85) (see Power generation). 

B. K. Schmid and D. M. Jackson, "The SRC-11 Process," paper presented at Third Annual International Conference on Coal Gasification and Eiquefaction. University of Pittsburgh, Aug. 3—5, 1976 D. M. Jackson and B. K. Schmid, "Production of Distillate Fuels by SRC-11," paper presented at ACS Div. of Ind. and Eng. Chem. Symposium, Colorado Spriags, Col., Feb. 12,1979. 

E. L. Huffman, Proceedings of the Third Annual International Conference on Coal Gasification andEiquefaction, Pittsburgh, Pa., 1976. 

New furnace concepts in evolutionary stages include fluidized-bed furnaces (25), coal gasification furnaces (26), and MHD furnaces (27,28). Of these technologies, fluidized-bed combustion has reached commercial-scale operations (Fig. 11). 

J. T. Stewart and T. D. Pay, "Coal Gasification Processes and Equipment Available for Small Industrial AppHcations," paper presented at the Fifth Annual International Conference on Coal Gasification Eiquefaction and Conversion to Electricity, University of Pittsburgh, Pa., Aug. 1—3,1978. 

Fig. 4. Coal gasification process. PSA = pressure-swing adsorption.

Partial oxidation of heavy Hquid hydrocarbons requires somewhat simpler environmental controls. The principal source of particulates is carbon, or soot, formed by the high temperature of the oxidation step. The soot is scmbbed from the raw synthesis gas and either recycled back to the gasifier, or recovered as soHd peUetized fuel. Sulfur and condensate treatment is similar in principle to that required for coal gasification, although the amounts of potential poUutants generated are usually less. 

Coal gasification technical data, Texaco Development Corp., Jan. 1978. 

This process is one of the three commercially practiced processes for the production of acetic anhydride. The other two are the oxidation of acetaldehyde [75-07-0] and the carbonylation of methyl acetate [79-20-9] in the presence of a rhodium catalyst (coal gasification technology, Halcon process) (77). The latter process was put into operation by Tennessee Eastman in 1983. In the United States the total acetic anhydride production has been reported to be in the order of 1000 metric tons. 

The high cost of coal handling and preparation and treatment of effluents, compounded by continuing low prices for cmde oil and natural gas, has precluded significant exploitation of coal as a feedstock for methanol. A small amount of methanol is made from coal in South Africa for local strategic reasons. Tennessee Eastman operates a 195,000-t/yr methanol plant in Tennessee based on the Texaco coal gasification process to make the methyl acetate intermediate for acetic anhydride production (15). 

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