Fluid Mechanics Applications/B35: gasification study
Syngas is a mixture of hydrogen (H2) and carbon monoxide (CO) produced from the gasification of carbonaceous feed-stocks. Since its first commercial use by the London Gas, Light, and Coke Company in 1812, syngas and its coal based antecedents (town gas, producer gas, coal gas) have been influential in the development of human society . They have illuminated cities, provided heat and power, and fuelled vehicles through both direct use and conversion to liquid fuels. As global energy demand rises by nearly 44% from 2006 to a projected 715 EJ in 2030, syngas will become increasingly important for process heat, electric power generation, and liquid fuels . There is renewed emphasis on coal gasification for enhancing national security, while mounting environmental sustainability issues have increased interest in biomass gasification. Raw product gas generated from gasification contains contaminants that must be mitigated to meet process requirements and pollution control regulations. This paper provides a comprehensive overview of the technologies used to remove these contaminants. The term ‘syngas’ is widely used as an industry shorthand to refer to the product gas from all types of gasification pro-cesses. However, syngas is technically a vapor stream composed of only H2 and CO derived from a steam and oxygen gasification process. While not entirely accurate, this industry shorthand will be used in this paper with appropriate adjectives to maintain clarity and simplicity of discussion with regard to the industry and published literature . Syngas has many uses which range from heat or power applications such as IGCC to a variety of synthetic fuels as shown below (Fig. 1). With such applications, each contaminant creates specific downstream hazards. These include minor process inefficiencies such as corrosion and pipe blockages as well as catastrophic failures such as rapid and permanent deactivation of catalysts. A multitude of technologies exist to purify the raw synthesis gas stream that is produced by gasification. Some methods are capable of removing several contaminants in a single process, such as wet scrubbing, while others focus on the removal of only one contaminant. Techniques are available that minimize the syngas contamination by reducing the contaminants emitted from within the gasifier; an approach typically termed ‘primary’ or ‘in-situ’ cleanup. Also available are a variety of secondary techniques that clean the syngas downstream of the reaction vessel in order to meet the stringent requirements of today’s applications. Gas cleanup technologies are conveniently classified according to the process temperature range: • hot gas cleanup (HGC) • cold gas cleanup (CGC) • warm gas cleanup (WGC).
There is considerable ambiguity in these definitions with no accepted guidelines to distinguish among them. Cold gas cleanup generally describes processes that occur near ambient conditions, while hot gas cleanup has been used to describe applications at a broad range of conditions from as low as 400 C to higher than 1300 C. Before reviewing these different kinds of gas cleaning, the nature of the contaminants to be removed from the gas stream is described.
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