Page 300 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

276 Biomass Gasification, Pyrolysis and Torrefaction

of the gasifying medium (i.e., steam/oxygen ratio). Performance parameters
of a gasifier include carbon conversion and cold-gas efficiency.
A typical process design starts with a mass balance followed by an
energy balance. The following subsections describe the calculation proce-
dures for these.

8.6.2 Mass Balance
Basic mass and energy balance is common to all types of gasifiers. It
involves calculations for product gas flow and fuel feed rate.
8.6.2.1 Product Gas Flow-Rate
The gasifier’s required power output, Q (MW th ), is an important input
parameter specified by the client. Based on this, the designer makes a pre-
liminary estimation of the amount of fuel to be fed into the gasifier and the
amount of gasifying medium. The volume flow-rate of the product gas, V g
3
3
(N m /s). For a desired LHV g (MJ/N m ) is found by:
Q
3
V g 5 Nm =s (8.5)
LHV g
The net heating value or LHV of producer gas (LHV g ) can be calculated
from its composition. The composition may be predicted by the equilibrium
calculations, described later, or by more sophisticated kinetic modeling of
the gasifier, as discussed in Chapter 7. In the absence of these, a reasonable
guess can be made either from published data on similar fuels in similar gas-
ification conditions or from the designer’s experience.
For example, for air-blown fluidized-bed biomass gasifiers, the LHV g is
3
in the range 3.5 6 MJ/N m (Enden and Lora, 2004). For oxygen gasifica-
3
tion, it is in the range 10 15 MJ/N m (Ciferno and Marano, 2002). So, for
3
an air-blown gasifier, we start with a value of 5 MJ/N m as a reasonable
guess (Quaak et al., 1999).
8.6.2.2 Fuel Feed Rate
To find the biomass feed rate, M f , the required power output is divided by
the LHV of the biomass (LHV bm ) and by the gasifier efficiency, η gef .
Q
M f 5 (8.6)
LHV bm η gef
The LHV may be related to the higher heating value (HHV) and its
hydrogen and moisture contents (Quaak et al., 1999) as:

LHV bm 5 HHV daf 2 20; 300 3 H daf 2 2260 3 M daf (8.7)
Here, H daf is the hydrogen mass fraction in the fuel, M daf is the moisture
mass fraction, and HHV daf is the HHV in kJ/kg on a dry on moisture-ash-free