used in the chemical and crude oil industries to increase reaction rates, sometimes making certain previously unachievable products possible (Speight, 2002; Speight, 2014a; Hsu and Robinson, 2017; Speight, 2017). Acids, through donated protons (H+), are common reaction catalysts, especially in the organic chemical industries. This it is not surprising that catalysts can be used to enhance the reactions involved in gasification and use of appropriate catalysts not only reduces reaction temperature but also improves the gasification rates.
In addition, thermodynamic constraints of the gasification process limit the thermal efficiency are not inherent but the result of design decisions based on available technology, as well as the kinetic properties of available catalysts. The latter limits the yield of methane to that obtainable at global equilibrium over carbon in the presence of carbon monoxide and hydrogen. The equilibrium composition is shown to be independent of the thermodynamic properties of the char or feedstock. These limitations give non-isothermal two-stage processes significant thermodynamic advantages. The results of the analysis suggest directions for modifying present processes to obtain higher thermal efficiencies, and a two-stage process scheme that would have significant advantages over present technologies and should be applicable to a wide range of catalytic and non-catalytic processes (Shinnar et al., 1982; McKee, 1981).
Alkali metal salts of weak acids, such as potassium carbonate (K2CO3), sodium carbonate (Na2CO3), potassium sulfide (K2S), and sodium sulfide (Na2S) can catalyze the carbon-steam gasification reaction. Catalyst amounts on the order of 10 to 20% w/w K2CO3 will lower the temperature required for gasification of bituminous coal from approximately 925°C (1695oF) to 700°C (1090oF) and that the catalyst can be introduced to the gasifier impregnated on coal or char.
Disadvantages of catalytic gasification include increased materials costs for the catalyst itself (often rare metals), as well as diminishing catalyst performance over time. Catalysts can be recycled, but their performance tends to diminish with age or by poisoning. The relative difficulty in reclaiming and recycling the catalyst can also be a disadvantage. For example, the potassium carbonate catalyst can be recovered from spent char with a simple water wash, but some catalysts may not be so accommodating. In addition to age, catalysts can also be diminished by poisoning. On the other hand, many catalysts are sensitive to particular chemical species which bond with the catalyst or alter it in such a way that it no longer functions. Sulfur, for example, can poison several types of catalysts including palladium and platinum.
2.5.6 Physical Effects
Depending on the type of feedstock being processed and the analysis of the gas product desired, pressure also plays a role in product definition. In fact, some (or all) of the following processing steps will be required: (i) pretreatment of the feedstock, (ii) primary gasification of the feedstock, (iii) secondary gasification of the carbonaceous residue from the primary gasifier, (iv) removal of carbon dioxide, hydrogen sulfide, and other acid gases, (v) shift conversion for adjustment of the carbon monoxide/hydrogen mole ratio to the desired ratio, and (vi) catalytic methanation of the carbon monoxide/hydrogen mixture to form methane.
Most notable effects in the physical chemistry of the gasification process are those effects due to the chemical character of the feedstock as well as the physical composition of the feedstock (Speight, 2011a, 2013). In more general terms of the character of the feedstock, gasification technologies generally require some initial processing of the feedstock with the type and degree of pretreatment a function of the process and/or the type of feedstock. For example, the Lurgi process will accept lump feedstock (1 in., 25 mm, to 28 mesh), but it must be non-caking with the fines removed – feedstock that show caking or agglomerating tendencies form a plastic mass in the bottom of a gasifier and subsequently plug up the system thereby markedly reducing process efficiency. Thus, some attempt to reduce caking tendencies is necessary and can involve preliminary partial oxidation of the feedstock thereby destroying the caking properties.
Another factor, often presented as very general rule of thumb, is that optimum gas yields and gas quality are obtained at operating temperatures of approximately 595 to 650oC (1100 to 1200oF). A gaseous product with a higher heat content (BTU/ft.3) can be obtained at lower system temperatures but the overall yield of gas (determined as the fuel-to-gas ratio) is reduced by the unburned char fraction.
The major difference between combustion and gasification from the point of view of the chemistry involved is that combustion takes place under oxidizing conditions, while gasification occurs under reducing conditions. In the gasification process, the feedstock (in the presence of steam and oxygen at high temperature and moderate pressure) is converted to a mixture of product gases. In the initial stages of gasification, the rising temperature of the feedstock initiates devolatilization of the feedstock and the breaking of weaker chemical bonds to yield tar, oil, volatile species, and hydrocarbon gases. These products generally react further to form hydrogen, carbon monoxide, and carbon dioxide. The fixed carbon that remains after devolatilization reacts with oxygen, steam, carbon dioxide, and hydrogen.
Depending on the gasifier technology employed and the operating conditions, significant quantities of water, carbon dioxide, and methane can be present in the product gas, as well as a number of minor and trace components. Under the reducing conditions in the gasifier, most of the sulfur in the fuel sulfur is converted to hydrogen sulfide (H2S) as well as to smaller yields of carbonyl sulfide (COS). Organically bound nitrogen in the feedstock is generally (but not always) converted to gaseous nitrogen (N2) – some ammonia (NH3) and a small amount of hydrogen cyanide (HCN) are also formed. Any chlorine in the feedstock (such as coal) is converted to hydrogen chloride (HCl) with some chlorine present in the particulate matter (fly ash). Trace elements, such as mercury and arsenic, are released during gasification and partition among the different phases, such as fly ash, bottom ash, slag, and product gas.
Fuels for gasification reactors differ significantly in chemical properties, physical properties, and morphological properties and, hence, require different reactor design and operation. It is for this reason that, during more than a century of gasification experience, a large number of different gasifiers has been developed – each reactor designed to accommodate the specific properties of a typical fuel or range of fuels. In short, the gasification reactor that is designed to accommodate all (or most) types of fuels does not exist.
However, before choosing a gasifier for any individual fuel it is important to ensure that the fuel meets the requirements of the gasifier or that it can be treated to meet these requirements. Practical tests are needed if the fuel has not previously been successfully gasified. In other words the fuel must match the gasifier and the gasifier must match the fuel.
2.6 Products
The original concept of the gasification process was to produce a fuel gas for use in homes (including street lighting) and industrial operations. Thus, the gasification of carbonaceous residues is generally aimed to feedstock conversion to gaseous products. In fact, gasification offers one of the most versatile methods (with a reduced environmental impact with respect to combustion) to convert carbonaceous feedstocks into electricity, hydrogen, and other valuable energy products. Depending on the previously described type of gasifier (e.g. air-blown, enriched oxygen-blown) and the operating conditions, gasification can be used to produce a fuel gas that is suitable for several applications.
Gasification agents are typically air, oxygen-enriched air or oxygen and the products of the combustion or gasification oxidation reaction change significantly as the oxygen-to-fuel ratio changes from combustion to gasification conditions, which are dependent upon gasifier design and operation (Luque and Speight, 2015). The mixture under gasifying conditions is fuel-rich and there is not enough oxygen to effect complete conversion of the feedstock, in terms of gas quality. As a result, the feedstock carbon reacts to produce carbon instead of carbon dioxide
