Page 310 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

286 Biomass Gasification, Pyrolysis and Torrefaction

Energy out Energy out in
in char product gas
Energy given
to gasification
Gasification
Energy loss
from reactor
Energy
out from
combustion
Combustion
Energy given
to pyrolysis

Pyrolysis

Energy given
to drying
Drying
External
energy
Gasifier boundary
Energy input Energy input
through feed through gasification
medium

FIGURE 8.22 Energy flow in and out of a gasifier.

Figure 8.22 shows the energy flow in and out of a gasifier. Biomass
enters with its chemical energy and sensible heat. The gasifying agents enter
with sensible heat at the reference temperature. External heat is added for
heating the feeds to the gasification temperature for meeting any shortfall in
the reaction heat requirement and for wall losses from the reactor. The prod-
uct gas, with its chemical energy, leaves at the gasifier temperature. Unburnt
char leaves with a potential energy in it. The unutilized steam and other
gases also leave at the gasification temperature.
The overall energy balance may be written as:

Energy input: Enthalpy of (biomass 1 steam 1 oxygen) at reference tem-
perature 1 heating value of biomass 1 external heat
Energy output: Enthalpy of product gas at gasifier temperature 1 heating
value of product gas 1 heat in unconverted char 1 heat loss from the
reactor.
If A is the amount of air needed and W is the total steam (from moisture
3
or otherwise) needed to gasify F kg of fuel to produce 1 N m of product