17.3.1 Solid Fuel

The CV of a biomass is a crucial property needed to be considered for its conversion into fuels. The importance of oxygen:carbon (O:C) and hydrogen:carbon (H:C) ratios on the CV of solid fuels can be illustrated using a Van Krevelen diagram. High content of oxygen and H2 reduces the energy value of the fuel, since energy contained in carbon-oxygen (C-O) and carbon-hydrogen (C-H) bonds are lower than energy in carbon-carbon (C-C) bonds. Materials with low O:C and H:C ratios are usually preferred as fuels for gasification and combustion since they contain more energy. Composition of lignocellulose component of wood is seen to be closely related to coal since wood is the natural precursor for the formation of coal through diverse coal liquefaction processes [4]. The content of alkali metal such as Na, K, Mg, P, and Ca, in biomass is crucial for its thermo-chemical conversion into fuels. Reaction between alkali metals and silica present in ash produces sticky, mobile liquid phase

which can lead to blockages of airways in furnace. Total silica content may increase significantly due to contamination with soil during harvesting.

In a work performed by Abdullah and Yusup [29], the characterization of eleven samples ofMalaysian biomass, listed inTable 17.5, has been performed andevaluated to determine their potential utilization as feedstock for biomass gasification into hydrogen.

In this work, the biomass is physically pre-treated via grinding and sieving. Anal­yses are carried out on sieved biomass to ensure homogeneous samples. Details on the types and working principle of the analysers used in characterizing the biomass are given in Table 17.6. Table 17.7 shows lists of characteristics of eleven types of biomass properties. The elemental analysis results of this biomass are given in Table 17.8. The composition of the hydrocarbon fuel is expressed in terms of its basic elements except for its moisture (M) and inorganic constituent. This is represented by the ultimate analysis. In comparison, the composition of biomass in terms of its volatile matter, FC, moisture, and ash content is represented by proximate analysis. A typical ultimate and proximate analysis is presented by Eqs. 17.4 and 17.5 [35];

Ultimate analysis: C + H + O + N + S + ASH + M = 100 % (17.4)

Proximate analysis: VM + FC + M + ASH = 100 % (17.5)

Table 17.6 Biomass characterization

Analysis Working principle

The C, H, O, N, and S are expressed as the weight percentages of carbon, hydrogen, oxygen, nitrogen, and sulphur, respectively in the fuel.

Biomass moisture content can either be expressed in dry or wet basis as presented in Eqs. 17.6 and 17.7 [35];

The dry-basis moisture is: Mdry = (Wwet — Wdry)/Wdry (17.6)

The wet-basis moisture is: Mwet = (Wwet — Wdry)/Wwet (17.7)

The wet basis (Mwet) and dry basis (Mdry) are related as in Eq. 17.8 [35];

Mdry = Mwet/1 — Mwet (17.8)

The FC content is calculated based on Eq. 17.9 [35];

FC = 1 — M — VM — ASH (17.9)

The suitability of biomass as a gasification feedstock is screened using an aggregate matrix as shown in Table 17.9 based on its CV, O:C and H:C ratios and moisture, ash, volatiles, and FC contents [29]. A weighting factor is assigned to each characteristic according its significance for gasification process. The biomass is ranked, based on the scoring performed using the weighting factor. It was found that palm kernel shell (PKS), after being subjected to physical pre-treatment (grinding and sieving) and thermal pre-treatment (oven-drying), is the most preferred biomass among the eleven biomass as the gasification feedstock as shown in Fig. 17.7.