Methane gasification

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,... 

The initial biogas recovered is an MHV gas and is often upgraded to high heat value (HHV) gas when used for blending with natural gas suppHes. The aimual production of HHV gas ia 1987, produced by 11 HHV gasification facihties, was 116 x 10 m of pipehne-quaUty gas, ie, 0.004 EJ (121). This is an iacrease from the 1980 production of 11.3 X 10 m . Another 38 landfill gas recovery plants produced an estimated 218 x 10 m of MHV gas, ie, 0.005 EJ. Additions to production can be expected because of landfill recovery sites that have been identified as suitable for methane recovery. In 1988, there were 51 sites ia preliminary evaluation and 42 sites were proposed as potential sites (121). 

The processes that have been developed for the production of synthetic natural gas are often configured to produce as much methane in the gasification step as possible thereby minimizing the need for a methanation step. In addition, methane formation is highly exothermic which contributes to process efficiency by the production of heat in the gasifier, where the heat can be used for the endothermic steam—carbon reaction to produce carbon monoxide and hydrogen. 

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... 

Gasification technologies for the production of high heat-value gas do not all depend entirely on catalytic methanation, that is, the direct addition of hydrogen to coal under pressure to form methane. 

Thermodynamically, the formation of methane is favored at low temperatures. The equilibrium constant is 10 at 300 K and is 10 ° at 1000 K (113). High temperatures and catalysts ate needed to achieve appreciable rates of carbon gasification, however. This reaction was studied in the range 820—1020 K, and it was found that nickel catalysts speed the reaction by three to four orders of magnitude (114). The Hterature for the carbon-hydrogen reaction has been surveyed (115). 

Euture large gasification plants, intended to produce ca 7 x 10 m standard (250 million SCE) of methane per day, are expected to be sited near a coal field having an adequate water supply. It is cheaper to transport energy in the form of gas through a pipeline than coal by either rail or pipeline. The process chosen is expected to utilize available coal in the most economical manner. 

Characteristics of moving-bed gasifiers are low gasification temperatures, relatively low oxygen requirements, relatively high methane content in... 

Synthetic Natural Gas. Another potentially very large appHcation of coal gasification is the production of synthetic natural gas (SNG). The syngas produced from coal gasification is shifted to produce a H2-to-CO ratio of approximately 3 to 1. The carbon dioxide produced during shifting is removed, and CO and H2 react to produce methane (CH, or SNG, and water in a methanation reactor. 

Hydrogen and carbon monoxide are produced by the gasification reaction, and they react with each other and with carbon. The reaction of hydrogen with carbon as shown in reaction (27-15) is exothermic and can contribute heat energy. Similarly, the methanation reaction (27-19) can contribute heat energy to the gasification. These equations are interrelated by the water-gas-shift reaction (27-18), the equilibrium of which controls the extent of reactions (27-16) and (27-17). 

It is not possible, however, to calculate accurately actual gas composition by using the relationships of reactions (27-14) to (27-19) in Table 27-12. Since the gasification of coal always takes place at elevated temperatures, thermal decomposition (pyrolysis) takes place as coal enters the gasification reactor. Reaction (27-15) treats coal as a compound of carbon and hydrogen and postulates its thermal disintegration to produce carbon (coke) ana methane. Reaction (27-21) assumes the stoichiometiy of hydrogasifying part of the carbon to produce methane and carbon. 

In Lurgi coal gasification, an example of extremely important treating is in the sulfur removal step ahead of methanation where the catalyst is poisoned by even small traces of any sulfur compound. The sulfur removal step is a relatively high capital and operating cost item. 

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