2.6.1 Gaseous Products
The products from gasification may be of low, medium, or high heat-content (high-Btu) content as dictated by the process as well as by the ultimate use for the gas (Speight, 2008, 2011a, 2013). However, the products of gasification are varied insofar as the gas composition varies with the system employed (Speight, 2013). It is emphasized that the gas product must be first freed from any pollutants such as particulate matter and sulfur compounds before further use, particularly when the intended use is a water gas shift or methanation (Cusumano et al., 1978; Probstein and Hicks, 1990).
Product gases from fixed-bed versus fluidized-bed gasifier configurations vary significantly. Fixed-bed gasifiers are relatively easy to design and operate and are best suited for small to medium-scale applications with thermal requirements of up to several thermal megawatts (megawatts thermal, MWt). For large-scale applications, fixed-bed gasifiers may encounter problems with bridging of the feedstock (especially in the case of biomass feedstocks) and non-uniform bed temperatures. Bridging leads to uneven gas flow, while non-uniform bed temperature may lead to hot spots, ash formation, and slagging. Large-scale applications are also susceptible to temperature variations throughout the gasifier because of poor mixing in the reaction zone.
Pressurized gasification systems lend themselves to economical synthesis gas production and can also be more flexible in production turndown depending on the reactor design. Typically this is the case for both a pressurized bubbling reactor and a circulating fluidized-bed reactor, while the flexibility of an atmospheric fluidized-bed reactor is typically limited to narrower pressure and production ranges. Both designs are well suited for pressurized synthesis gas production. Pressurized designs require more costly reactors, but the downstream equipment (such as gas cleanup equipment, heat exchangers, synthesis gas reactors) will consist of fewer and less expensive components (Worley and Yale, 2012).
In the process, the feedstock undergoes three processes during the conversation to synthesis gas – the first two processes, pyrolysis and combustion, occur very rapidly. In pyrolysis, char is produced as the feedstock heats up and volatile products are released. In the combustion process, the volatile products and some of the char reacts with oxygen to produce secondary products (primarily carbon dioxide and carbon monoxide) and the heat required for subsequent gasification reactions. Finally, the char reacts with steam to produce hydrogen (H2) and carbon monoxide (CO).
Combustion:
Gasification:
At the gasifier temperature, the ash and other feedstock mineral matter liquefies and exits at the bottom of the gasifier as slag, a sand-like inert material that can be sold as a co-product to other industries (e.g., road building). The synthesis gas exits the gasifier at pressure and high temperature and must be cooled prior to the cleaning stage. Full-quench cooling, by which the synthesis gas is cooled by the direct injection of water, is more appropriate for hydrogen production. The procedure provides the necessary steam to facilitate the water gas shift reaction, in which carbon monoxide is converted to hydrogen and carbon dioxide in the presence of a catalyst:
Water Gas Shift Reaction:
This reaction maximizes the hydrogen content of the synthesis gas, which consists primarily of hydrogen and carbon dioxide at this stage. The synthesis gas is then scrubbed of particulate matter and sulfur is removed via physical absorption (Speight, 2008; Chadeesingh, 2011; Speight, 2013). The carbon dioxide is captured by physical absorption or a membrane and either vented or sequestered.
2.6.1.1 Low Btu Gas
During the production of gas by oxidation of the feedstock with air, the oxygen is not separated from the air and, as a result, the gas product invariably has a low Btu value (low heat-content) on the order of 150 to 300 Btu/ft3. Low Btu gas is also the usual product of in situ gasification of coal (Speight, 2013) which is essentially used as a method for obtaining energy from coal without the necessity of mining the coal, especially if the coal cannot be mined or of mining is uneconomical.
Several important chemical reactions, and a host of side reactions, are involved in the manufacture of low heat-content gas under the high-temperature conditions employed (2011; Speight, 2013). Low heat-content gas contains several components, four of which are always major components present at levels of at least several percent; a fifth component, methane, is marginally a major component.
The nitrogen content of low heat-content gas ranges from somewhat less than 33% v/v to slightly more than 50% v/v and cannot be removed by any reasonable means; the presence of nitrogen at these levels makes the product gas low heat-content by definition. The nitrogen also strongly limits the applicability of the gas to chemical synthesis. Two other noncombustible components (water, H2O, and carbon dioxide, CO) further lower the heating value of the gas; water can be removed by condensation and carbon dioxide by relatively straightforward chemical means.
The two major combustible components are hydrogen and carbon monoxide; the H2/ CO ratio varies from approximately 2:3 to approximately 3:2. Methane may also make an appreciable contribution to the heat content of the gas. Of the minor components hydrogen sulfide is the most significant and the amount produced is, in fact, proportional to the sulfur content of the feed coal. Any hydrogen sulfide present must be removed by one, or more, of several procedures (Mokhatab et al., 2006; Speight, 2019).
Producer gas is a low Btu gas typically obtained from a coal gasifier (fixed-bed) upon introduction of air instead of oxygen into the fuel bed. The composition of the producer gas is approximately 28% v/v carbon monoxide, 55% v/v nitrogen, 12% v/v hydrogen, and 5% v/v methane with some carbon dioxide. Water gas is a medium Btu gas which is produced by the introduction of steam into the hot fuel bed of the gasifier. The composition of the gas is approximately 50% v/v hydrogen and 40% v/v carbon monoxide with small amounts of nitrogen and carbon dioxide. Also, low heat-content gas is of interest to industry as a fuel gas or even, on occasion, as a raw material from which ammonia, methanol, and other compounds may be synthesized.
2.6.1.2 Medium Btu Gas
Medium Btu gas (medium heat-content gas) has a heating value in the range 300 to 550 Btu/ ft3) and the composition is much like that of low heat-content gas, except that there is virtually no nitrogen. The primary combustible gases in medium heat-content gas are hydrogen and carbon monoxide (Kasem, 1979). Medium heat-content gas is considerably more versatile than low heat-content gas; like low heat-content gas, medium heat-content gas may be used directly as a fuel to raise steam, or used through a combined power cycle to drive a gas turbine, with the hot exhaust gases employed to raise steam, but medium heat-content gas is especially amenable to synthesize methane (by methanation), higher hydrocarbon derivatives (by Fischer-Tropsch synthesis), methanol, and a variety of synthetic chemicals.
The reactions used to produce medium heat-content gas are the same as those employed for low heat-content gas synthesis, the major difference being the application of a nitrogen barrier (such as the use of pure oxygen) to keep diluent nitrogen out of the system.
In medium heat-content gas, the H2/CO ratio varies from 2:3 C to 3:1 and the increased heating value