Mild Gasification

A slurry of coal-derived fuel and upgraded char has the potential of being a very versatile fuel that can be burned in both coal- and oil-fired boilers. If the char is upgraded to a high degree, even feedstock coal with a high sulfur content can be used without alternating heat rates or capacity factors. 

Two primary methods exist for converting coal into liquid fuels (1) direct liquefaction (Chapters 18 and 19) and (2) indirect liquefaction using the Fischer-Tropsch process (Chapters 20 and 21). 

Direct liquefaction is the conversion of coal directly to liquid products. In general chemical terms, coal liquefaction involves addition of hydrogen to the coal by various techniques so that the ratio of hydrogen to carbon in the product is increased to a level comparable to petroleum-based fuels. Indirect liquefaction is coal gasification followed by conversion of the synthesis gas (a carbon monoxide, CO, hydrogen, mixture) to liquid fuels. 

Coprocessing (Chapters 18 and 19), a recent development for coal liquefaction technology, involves the production of liquids from a mixture of coal and heavy petroleum residue, with the residual oil providing all or most of the hydrogen needed for the conversion process. Once produced, the coal-derived liquid can be refined by sulfur mineral matter (ash) ranoval before use. 

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A newer concept has been developed that is given the name mild gasification (33). It is not a gasification process in the tme sense of the word. The process temperature is some several hundred degrees lower, hence the term mild, than the usual gasification process temperature and the objective is not to produce a gaseous fuel but to produce a high value char (carbon) and Hquid products. Gas is produced, but to a lesser extent. 

Mild gasification is actually a pyrolysis-based process, closely related to coking. It is designed to produce the most profitable slate of alternative fuels and chemicals by decomposing coal at relatively mild conditions of temperature and pressure.20 21 Mild gasification differs from... 

Khan, M. R., and Kurata, T., in The Feasibility of Mild Gasification of Coal Research Needs, DOE/METC-85/4019, NTRS/DE85013625, 1985. 

The treatment with a flow containing SO2+H2O+O2 gives an amount of 400 mg of sulphuric acid. Heating up this sample, sulphuric acid is removed from the surface by reduction that leads to carbon consumption. This mild gasification can produce either an opening of the microporosity to mesoporosity and/or the creation of new microporosity. This can be followed by the increase of pore volume calculated by Horvath-Kawazoe (HK, for micropores) and Barret-Joyner-Halenda (BJH, for mesopores) methods. 

US Department of Energy. 1994. Toxicity studies of mild gasification products. Morgantown, WV Office of Fossil Energy, U.S. Department of Energy. DOE/MC/26018-3730. DE94004113. 

The mild gasification of chars with steam or carbon dioxide at red temperatures (thermal or physical activation). 

A more recent concept termed mild gasification has also been investigated in detail (Cha et al, 1988 Merriam et al, 1992). The purpose of the process is to produce added-value products (such as high heat content char), which will facilitate removal of impurities and low value materials (such as water), thereby making the economics of shipment more favorable. The process conditions are similar to the low-to-medium temperature carbonization processes and do offer an attractive option for coal use. 

The current emphasis on the use of coal is the production of added-value products using a variety of processes by which the total coal can be used with very little wastage of the products. Such is the character of the mild gasification process (Chapter 22) and a variety of other clean coal processes (United States Department of Energy, 1993) (Chapter 22), which have brought about renewed interest in briquetting and pelletization of coal. 

Merriam, N.W. and Jha, M.C. 1991. Development of an advanced continuous mild gasification process for the prodnction of co-products. Final Report Report No. WRI-91-R068 (DE-AC21-87MC24268). Western Research Institute, Laramie, WY. 

Texaco gasification is based on a combination of two process steps, a liquefaction step and an entrained bed gasifier. In the liquefaction step the plastic waste is cracked under relatively mild thermal conditions. This depolymerisation results in a synthetic heavy oil and a gas fraction, which in part is condensable. The noncondensable fraction is used as a fuel in the process. The process is very comparable to the cracking of vacuum residues that originate from oil recycling processes. 

Coal Refineries, DOE, July, 1991 ENCOAL Mild Coal Gasification Project, A DOE Assessment, NETL, March 2002... 

What, then, does the future hold This author believes that the catalytic hydrocarbonization/gasification concept will ultimately achieve commercial success for the production of liquid and gaseous fuels from coal. In selected applications, the mild hydrocarbonization of western coal to produce liquid and gaseous fuels with power generation from the low-sulfur char may also be commercially attractive. Finally, further development of the flash hydropyrolysis technology, as exemplified by the Rocketdyne project, may eventually lead to a technically and economically attractive liquefaction process. But the most important questions still remain unanswered. Does private industry have sufficient interest to pursue the possibilities Where is the interest focused Will a private consortium build a hydrocarbonization/ cogeneration complex using western coal Will the phoenix arise from the ashes ... 

The major techniques and methods for removal acid gases, including H2S, COS, and C02, from the raw coal gasification gas include (a) the absorption by chemical and/or physical solvents, (b) sorption by metals or metal oxides, and (c) membrane separation. The techniques and methods for sulfur removal from the raw coal gasification gas and other streams by solvent absorption at low temperature and by sorption at high or mild temperature were reviewed in detail by Mahin Rameshni,188 Korens et al.,189 Newby et al.,190 and Uhde,186 respectively. 

This hypothesis is supported by additional experimental data. Therefore, when sample SI300 (obtained at 1300°C) is mildly gasified up to 1% burn-off the benzene/cyclohexane ratio clearly decreases (142.6). Moreover, when the gasification is increased up to a medium burn- off (6%) the molecular sieve effect disappears (benzene/cyclohexane ratio, 0.93), and Vs for 2,2 DMB is far from the gas hold-up time and can be measured (0.75 cm /m ). This means that this treatment produced carbon removal which enlarged the pore constrictions. This fact is consistent with Vs values of linear hydrocarbons rising with increasing burn-off... 

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