Fluid Mechanics Applications/B35:Study of MANOMETER

Introduction 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 [1]. 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 [2]. 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 [3]. 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. Contaminants present in syngas: Contaminants removed from syngas generally include particulate matter, condensable hydrocarbons (i.e. tars), sulfur compounds, nitrogen compounds, alkali metals (primarily potassium and sodium), and hydrogen chloride (HCl). Carbon dioxide (CO2) is also removed in various industrial applications concerned with acid gases or carbon sequestration, but it is not considered in this review. Contaminant levels vary greatly and are heavily influenced by the feedstock impurities and the syngas generation methods (see Table 1). The level of cleaning that is required may also vary substantially depending on the end-use technology and/or emission standards (see Table 2).

Tars Tars are composed of condensable organic compounds. They vary from primary oxygenated products to heavier deoxygenated hydrocarbons and polycyclic aromatic hydrocarbons (PAHs) [23]. Thermochemical conversion processes create hundreds or even thousands of different tar species in response to the operating parameters. Particularly important are feedstock composition and processing conditions, especially temperature, pressure, type and amount of oxidant, and feedstock residence time [24,25]. For instance, gasification of wood shows higher tar concentrations with greater amounts of stable aromatics in comparison to coal or peat [26]. An updraft gasifier operates very differently from a downdraft gasifier and may yield 10%e20% tar composition, while the latter may yield less than 1% tar (unless otherwise stated these discussions are also provided on a mass basis) [27]. Regardless of the amount or type, tar is a universal challenge of gasification because of its potential to foul filters, lines, and engines, as well as deactivate catalysts in cleanup systems or downstream processes [24]. The complex chemical nature of tar creates difficulties in collecting, analyzing, and even defining what constitutes tar [28]. A recent intergovernmental effort has produced an explicit definition of “tar” as “all hydrocarbons with molecular weights greater than that of benzene.” [29] In addition to this definition, a widely recognized “tar standard” was created which now provides technical specifications for sampling and analysis of tars [8,30]. This guideline was designed to provide a consistent basis of tar measurement among researchers. Essential to measuring and controlling this contaminant is a fundamental understanding of the nature and formation of tar compounds.

The formation of tar is commonly understood to be a progression from highly oxygenated compounds of moderate molecular weight to heavy, highly reduced compounds. Longer reaction times and higher temperatures (referred to as increased reaction severity) reduce tar yields but result in more heavy hydrocarbons, which are very refractory to further reaction. These compounds are conveniently grouped into primary, secondary, and tertiary tars (see Fig. 2). Primary tars are organic compounds, such as levoglucosan and furfurals, which are released from devolatilizing feedstock (coal or biomass) [23]. Higher temperatures and longer residence times result in secondary tars, including phenolics and olefins. Further increases in temperature and reaction time encourage the formation of tertiary tars, such as PAHs [24]. Overall, the severe conditions of thermochemical processes produce an array of tarry compounds with diverse properties that can be differentiated by structure as shown in Table 3. Tars in classes 1, 4 and 5 can readily condense even at high temperature, making them responsible for severe fouling and clogging in gasification-based fuel and power systems [31]. Class 2 and 3 tars, including heterocyclic aromatics and benzene/toluene/xylene (BTX) compounds, are problematic in catalytic upgrading because they compete for active sites on the catalysts. These tars are also water soluble and create issues with waste water remediation in water based cleanup processes. In general, removal or decomposition of all organic compounds is encouraged as they represent impurities in the synthesized product [13]. Although eliminating all tar is desirable, a more practical strategy is to simply remove sufficient tar for its dew point to be less than the minimum temperature experienced by the gas stream. The Energy Research Center of the Netherlands (ECN) has developed an extensive database with information on more than 50 common tar compounds, as well as calculation procedures for estimating the tar dew point [32]. An analyzer has also been developed that is capable of online tar dew point measurements, which are critical to preventing tar problems in biomass gasification systems [33,34].