Design of Biomass Gasifiers - Biomass Gasification, Pyrolysis and Torrefaction - page 286

Energy out
in char

Energy out in
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
through gasification
medium

Energy input
through feed

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 external heat

Energy output: Enthalpy of product gas at gasifier temperature

1 heating value of biomass 1

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
or otherwise) needed to gasify F kg of fuel to produce 1 N m 3 of product