Synthesis Gas. James G. Speight
* The term low-rank coal refers to the lowest heating value and higher moisture content lignite and sub-bituminous coals.
The production of gas from coal has been a vastly expanding area of coal technology and, as a result, the characteristics of rank, mineral matter, particle size, and reaction conditions are all recognized as having a bearing on the outcome of the process; not only in terms of gas yields but also on gas properties (Massey, 1974; van Heek and Muhlen, 1991). The products from the gasification of coal 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 (Figure 2.1) (Fryer and Speight, 1976; Mahajan and Walker, 1978; Anderson and Tillman, 1979; Cavagnaro, 1980; Bodle and Huebler; Argonne, 1990; Baker and Rodriguez, 1990; Probstein and Hicks, 1990; Lahaye and Ehrburger, 1991; Matsukata et al., 1992; Speight, 2013).
Coal is a fossil fuel formed in swamp ecosystems where plant remains were saved from oxidation and biodegradation by being covered with water and mud. Coal is a combustible organic sedimentary rock (composed primarily of carbon, hydrogen and oxygen as well as other minor elements including sulfur) formed from ancient vegetation and consolidated between other rock strata to form coal seams. The harder forms can be regarded as organic metamorphic rocks (e.g., anthracite coal) because of a higher degree of maturation.
Coal remains in adequate supply and at current rates of recovery and consumption, the world global coal reserves have been variously estimated to have a reserves/production ratio of at least 155 years. However, as with all estimates of resource longevity, coal longevity is subject to the assumed rate of consumption remaining at the current rate of consumption and, moreover, to technological developments that dictate the rate at which the coal can be mined. But most importantly, coal is a fossil fuel and an unclean energy source that will only add to global warming. In fact, the next time electricity is advertised as a clean energy source, just consider the means by which the majority of electricity is produced – almost 50% of the electricity generated in the United States derives from coal (EIA, 2007; Speight, 2013).
Coal occurs in different forms or types (Speight, 2013). Variations in the nature of the source material and local or regional variations in the coalification processes cause the vegetal matter to evolve differently. Various classification systems thus exist to define the different types of coal. Chemically, coal is a hydrogen-deficient hydrocarbon with an atomic hydrogen-to-carbon ratio near 0.8, as compared to crude oil hydrocarbon derivatives, which have an atomic hydrogen-to-carbon ratio approximately equal to 2, and methane (CH4) that has an atomic carbon-to-hydrogen ratio equal to 4. For this reason, any process used to convert coal to alternative fuels must add hydrogen or redistribute the hydrogen in the original coal to generate hydrogen-rich products and coke (Speight, 2013).
The chemical composition of the coal is defined in terms of its proximate and ultimate (elemental) analyses (Speight, 2013). The parameters of proximate analysis are (i) moisture, (ii) volatile matter, (iii) mineral matter, which is determined as combustion ash, and (iv) fixed carbon. Elemental or ultimate analysis encompasses the quantitative determination of carbon, hydrogen, nitrogen, oxygen, and sulfur within the coal. Additionally, specific physical and mechanical properties of coal and particular carbonization properties are also determined.
Carbon monoxide and hydrogen are produced by the gasification of coal in which a mixture of gases is produced. In addition to carbon monoxide and hydrogen, methane and other hydrocarbon derivatives are also produced depending on conditions. Gasification may be accomplished either in situ or in processing plants. In situ gasification is accomplished by controlled, incomplete burning of a coal bed underground while adding air and steam. The gases are withdrawn and may be burned to produce heat, generate electricity or are utilized as synthesis gas in indirect liquefaction as well as for the production of chemicals.
Producing diesel and other fuels from coal can be performed through the conversion of coal to synthesis gas, a combination of carbon monoxide, hydrogen, carbon dioxide, and methane. Synthesis gas is subsequently reacted through Fischer-Tropsch Synthesis processes to produce hydrocarbon derivatives that can be refined into liquid fuels. By increasing the quantity of high-quality fuels from coal (while reducing costs), research into this process could help in mitigating the dependence on ever-increasingly expensive and depleting stocks of crude oil.
While coal is an abundant natural resource, its combustion or gasification produces both toxic pollutants and greenhouse gases. By developing adsorbents to capture the pollutants (mercury, sulfur, arsenic, and other harmful gases), it is possible not only to reduce the quantity of emitted gases but also to maximize the thermal efficiency of the cleanup. Thus, gasification offers one of the most clean and versatile ways to convert the energy contained in coal into electricity, hydrogen, and other sources of power. Turning coal into synthesis gas isn’t a new concept; in fact the basic technology dates back to World War II.
2.3.5 Biomass
As the last two to three decades have evolved, biomass has been considered as any renewable feedstock which is in principle carbon neutral (while the plant is growing, it uses the energy of the sun to absorb the same amount of carbon from the atmosphere as it releases into the atmosphere).
Biomass is a broad term used to describe any organic material or resource which is derived from plant or animal matter, and primarily used as fuel. This includes wood, food crops, grass and woody plants, agriculture and forestry residues, and organic components of municipal and industrial wastes. Biomass excludes organic material which has been transformed via geological processes to fossil fuels such as natural gas, crude oil, and coal. The chemical composition of biomass varies substantially, because of the broad range of materials involved, but the main components are moisture, carbohydrates or sugars, lignin, and mineral matter which becomes ash upon combustion or gasification.
There are two methods for converting biomass into high-value products: (i) biochemical conversion and (ii) thermochemical conversion. Biochemical conversion involves the use of biological processes to convert biomass into biofuels, chemicals and electrical power. In the case of ethanol production, enzymes and/or chemical processes are used to extract sugars from the biomass, which can then be converted to ethanol via fermentation. Thermochemical conversion, either gasification using less than stoichiometric oxygen or pyrolysis (the gasification of biomass in the absence of oxygen), uses heat and pressure to convert biomass to liquid fuels, chemicals and electrical power. Combustion is an option for conversion of biomass to electrical power; however, the synthesis gas produced by gasification is much easier and economical to clean than are the exhaust gases produced by combustion. This results in gasification providing better environmental performance, including a cheaper method of capturing carbon dioxide. In addition, synthesis gas produced by gasification can also be processed into a variety of marketable products, where combustion is limited to electrical production via the steam cycle.
The two main advantages that gasification has over biochemical conversion processes are (i) the speed with which the end product is produced (minutes for gasification compared to days for biochemical conversion) and (ii) the ability of the gasification process to extract the energy held in lignin, the harder structural part of the biomass. Fermentation methods currently are unable to extract the energy stored in the lignin; however, this does present the possibility of using gasification as a waste treatment method for materials that cannot be fermented at a biochemical conversion facility.
Through gasification, biomass is converted into a