Bulk density of the fuel
Bulk density is defined as the weight per unit volume of loosely tipped fuel. Fuels with high bulk density are advantageous because they represent a high energy-for-volume value. Consequently these fuels need less bunker space for a given refuelling time. Low bulk density fuels sometimes give rise to insufficient flow under gravity, resulting in low gas heating values and ultimately in burning of the char in the reduction zone. Average bulk densities of wood, charcoal and peat are given in Table 2.6. Inadequate bulk densities can be improved by briquetting or pelletizing.
V The bulk density varies significantly with moisture content and particle size of the fuel.
2.4.2 Charring properties of the fuel
The occurrence of physical and morphological difficulties with charcoal produced in the oxidation zone has been reported. Some feedstocks (especially softwoods) produce char that shows a tendency to disintegrate. In extreme cases this may lead to inadmissible pressure drop.
A number of tropical hardwoods (notably teak) are reported (38) to call for long residence times in the pyrolysis zone, leading to bunker flow problems, low gas quality and tar entrainment.