Gasification fuels
1.3.2 Need for selection of the right gasifier for each fuel
Biomass fuels available for gasification include charcoal, wood and wood waste (branches, twigs, roots, bark, woodshavings and sawdust) as well as a multitude of agricultural residues (maize cobs, coconut shells, coconut husks, cereal straws, rice husks, etc.) and peat.
Because those fuels differ greatly in their chemical, physical and morphological properties, they make different demands on the method of gasification and consequently require different reactor designs or even gasification technologies. It is for this reason that, during a century of gasification experience, a large number of different gasifiers has been developed and marketed, all types geared towards handling the specific properties of a typical fuel or range of fuels.
Thus it follows that the "universal" gasifier, able to handle all or most fuels or fuel types, does not exist, and in all probability will not exist in the foreseeable future.
The range of designs includes updraught, downdraught, crossdraught, fluidized bed as well as other biomass gasification systems of less importance (see section 2.3). All systems show relative advantages and disadvantages with respect to fuel type, application and simplicity of operation, and for this reason each will have its own technical and/or economic advantages in a particular set of circumstances.
Each type of gasifier will operate satisfactorily with respect to stability, gas quality, efficiency and pressure losses only within certain ranges of the fuel properties of which the most important are:
— energy content
— moisture content
— volatile matter
— ash content and ash chemical composition
— reactivity
— size and size distribution
— bulk density
— charring properties
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 the next sections the most important fuel properties will be discussed and fuels of current interest will be reviewed.