Biomass properties of importance for gasification

The important properties of biomass of significance for gasification can roughly be divided as

follows:

• Physical and thermodynamic properties: The physical properties of biomass depend markedly on the type of feedstock. For instance, density may vary from 100kg/m3, for balsa, bagasse and straw, to 1200 kg/m3, for lignum vitae (Di Blasi, 1997). Permeability to gas flow and thermal conductivity not only vary with the biomass species, but also along, across and tan­gentially to the wood fiber-chains. Furthermore, the effective physical properties of char probably reflect those of the virgin biomass and thus show significant variation with the feedstock. All cereal straws resist compaction, which make them difficult to compress for economical transport and storage. Processed biofuels do generally have a higher calorific value than the raw material itself, caused by the disposal of the moisture content and air in the structures of the biomass during processing. Since gasification is a thermochemi­cal conversion process the thermodynamic properties of a biomass plays a significant role in the gasification process. Biomass, as such, is highly anisotropic with different thermal conductivity along the fiber compared to across the fiber bundles. The conductivity also depends on factors such as moisture content, porosity and the present temperature. An exam­ple of the thermal conductivity dependency on the dry density along and across the grain is shown in Figure 6.4. Other important properties are the specific heat, depending on the moisture content and the actual temperature, and the ignition temperature. The ignition tem­perature is the temperature where the thermochemical process becomes self-sustainable and is generally lower for biomass fuels with a higher volatile matter content. However, it also depends on the surrounding conditions such as oxygen partial pressure, particle size and heating rate.

• Chemical composition and energy content: All organic compounds in lignocelluloses contain oxygen, but the degree of oxidation varies, so that the polysaccharides contain more oxygen than lignin and the extractives. The quantities of cellulose, hemicellulose and lignin vary between different types of biomass plants. Each of these components has their own pyrolysis chemistry in thermo-chemical conversion, and therefore the composition of the volatilized intermediary compounds can vary substantially depending on biomass used and (Mohan, 2006) and hence also influence the composition after gasification. The energy content varies between different biomass fuels as exemplified by straw, refused derived fuel (RDF) and municipal solid waste (MSW), algae and non-organic residue with low heating values (LHV) of 18.0, 22.9, 23.1 and 33.0MJ/kg, respectively (Phyllis, 2012).

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Figure 6.4. Thermal conductivity dependency on the dry density along and across the grain. Straight line is along the fibers and the curved is across the fibers (Basu, 2010).

Table 6.1. Typical gas compositions using air or oxygen/steam as gasifying agents.

Product gas composition

Oxygen/steam (Nossin, 2009) Pressure = 22 bar Wood chips (Vol.%)

Air (Knoef, 2005) Atmospheric RDF1) (Vol.%)

H2

14.8

8.6

CO2

8.4

15.6

CO

20.3

8.8

CH4

8.9

6.5

H2O

47

9.5

n2

45.8

CxHy

na

4.9

na = not analyzed. 1)Refuse-derived-fuel.