The properties of biomass and coal related to the gasification process are useful for understanding and predicting the gasification process. In this section, the relevant coal and biomass properties are presented and discussed.
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2.2.1 Proximate and Ultimate Analysis
Proximate and ultimate analyses are normally the first steps in evaluating the feedstock solid fuels. Proximate analysis gives the fuel characteristics in terms of mass percentage of moisture, volatile matters, fixed carbon and ash content in the solid fuel. It is performed by heating the raw material to a set temperature, and in the case of coal or biomass, the solid fuel decomposition takes place at this temperature to generate volatile gas substances. The moisture content is the water molecules that physio-chemically bond to the solid fuel material; however, for coal or biomass, the moisture content can be removed by heating without any chemical reactions occurring. The volatile matters that are released from coal or biomass decomposition reactions contain a series of gaseous molecules of hydrogen, carbon monoxide, carbon dioxide and other hydrocarbons. The decomposition rate and released gas composition are affected by temperature and heating rate. The decomposition reactions are also termed as pyrolysis or devolatilization. The remaining solid from devolatilization of the solid fuel (biomass or coal) is called char, which consists of fixed carbon and ash. The ash content is defined as the mass percentage (or weight percentage, wt %) of the remaining solid to the chars after char complete combustion. The proximate analysis results for selected biomass and coals are listed in the table below:
Table 2.1: typical proximate analysis data of selected biomass and coals (wet base).
Fuel type Moisture
(wt %) Volatile matter (wt %) Fixed carbon (wt %) Ash (wt %) Pine (dried)(1) 12 71.5 16 0.5 Eucalyptus (dried)(2) 10.6 74.8 13.9 0.7 Lignite(3) 31.03 34.82 11.86 22.28 Sub-bituminous coal(4) 29.2 30.8 34.4 5.5
(1) & (2)(Franco et al. 2003b) (3) (Haykiri-Acma and Yaman 2007) (4) (Gordillo et al. 2009)
From Table 2.1, it can be seen that in general biomass contains more volatile matters while coals contain more fixed carbon and more ash; therefore, the char yield from the biomass is much lower than that of coals. The moisture content in the biomass is lower because the coal moisture
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content is measured as that received whereas the biomass is air-dried from an initial moisture content normally above 100%.
Ultimate analysis gives the elemental constitution of a particular fuel in mass fraction or weight percentage (wt %) in a dry ash-free basis (daf). Ultimate analysis is performed by complete combustion of the fuel normally on the oven-dried material. Composition of the combustion final products are analysed and the main elements of the solid fuel are determined. Ultimate analysis results of the selected biomass and coals are listed in Table 2.2 below.
Table 2.2: Ultimate analysis data of selected biomass and coals (dry ash free basis, daf).
Fuel type C (wt%) H (wt%) O (wt%) N (wt%) S (wt%) H/C O/C Pine (1) 51.6 4.9 42.6 0.9 (-) 0.095 0.826 Eucalyptus (2) 52.8 6.4 40.4 0.4 (-) 0.121 0.765 Lignite (3) 56.6 5.6 35.1 1.5 1.2 0.129 0.424 Sub-bituminous(4) 81.2 5.7 8.8 1.0 3.3 0.072 0.239
(1)&(2) (Franco et al. 2003b); (3) (de Souza-Santos 2004); (4) (Skodras and Sakellaropoulos 2002)
From Table 2.2, it can be seen that the biomass contains more oxygen and less carbon compared with the coal. The high oxygen content in the biomass is due to the acid alcoholic groups in the cellulose, hemicelluloses and lignin. However, the coals contain higher contents of sulphur and nitrogen as well as other metal elements. The metal elements are not given in the ultimate analysis but can be determined from ash analysis. It is also observed in Table 2.2 that the ratio of O/C for the biomass is much higher than that of coals while the H/C ratios for the two types of solid fuels (biomass and coal) are similar.
2.2.2 Microstructure and chemical composition
In order to better understand and describe the co-gasification process of biomass and coal, examination and analysis of the microstructure and chemical composition of each fuel as well as their blend is necessary. The chemical composition of the coal and biomass is highly dependent
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on the material species and origin. Coal is a combustible, brown to black sedimentary rocky fossil fuel degraded from slow carbonization or a deterioration process of ancient forest body and other plants, which has lain under the earth’s surface layer for millions of years. It consists principally of organic substances, namely vitrinite, liptinite, intertinite maceral groups, inorganic
minerals, and moisture (Kural 1994). Elementally, coal is composed of mainly carbon and a
small proportion of hydrogen and oxygen, and its structure is highly complicated and random; there is no exact structural formula for coal. The chemical formula of coal is generally expressed
in the empirical form of CHxOy, based on the molar fraction of each element of the coal being
analysed. Based on the formation age of the carbonization process, the coal can be classified into
different types (de Souza-Santos 2004). In general, the carbon content in the coal increases with
the age of carbonization, thus coal with a high carbon fraction is classified as high quality coal that has a higher heating value, such as anthracite and bitumen. On the other hand, coal with a higher oxygen fraction is classified as low quality coal that has a lower heating value, such as lignite.
Oven-dry woody biomass contains three main components: cellulose, hemicellulose and lignin (McKendry 2002). Cellulose and hemicellulose are polysaccharides, which have chemical
formulae of (C6H10O5)m and (C5H8O4)n with the degree of polymerization m and n being less than
10000 and 50-300 respectively. Lignin consists of aromatic, phenolic and various hydro-carbon groups, and the structure has a high degree of randomness. The fraction of these three main
components of biomass varies among the species and origin (Couhert et al. 2009), therefore,
biomass composition is normally expressed in empirical form CHxOy based on the molar fraction
of the elements.