Page 57 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

36 Biomass Gasification, Pyrolysis and Torrefaction

in excess of what the plant is designed for. In such cases, biomass needs to
be dried using appropriate drying equipment, which is of course, an integral
part of a torrefaction plant.
Depending upon the feedstock specification of the reactor, the biomass is
to be screened, crushed, or milled. For example, a fixed bed gasifier can
accept few tens of millimeter-sized particles, while an entrained flow gasifier
can accept only micron-sized biomass. Thus, both capital and operating cost
of biomass preparation for entrained flow gasifier would be higher than that
in other types of gasifiers.
The pretreatment cost comprises the cost associated with cleaning, dry-
ing, screening, and sizing the biomass. Bridgwater (1995) gave an estimate
of feed pretreatment cost in terms of electricity produced and showed that
larger plants have lower specific costs. For example, for a 20 MWe plant the
cost of preprocessing of feed is $600/kWe, while it is $300/kWe for a
100 MWe plant.

2.3.3 Gasifier Cost
The cost of gasification depends, to some extent, on the technology applied
for gasification of the biomass. Technological options include:
1. Downdraft gasifier
2. Updraft gasifier
3. Side draft gasifier
4. Bubbling fluidized bed gasifier
5. Circulating fluidized bed gasifier
6. Entrained flow gasifier (atmospheric or pressurized)
7. Supercritical water gasifier.
The capital cost depends on the capacity or fuel throughput into the plant.
Figure 2.1 plots the installed capital cost of a gasification plant as a function
of fuel throughput into the gasifier. The total plant cost (TPC) is correlated
with the fuel input for both pressurized and atmospheric pressure plants.
These figures are given on the year 1994 basis. Table 2.3 presents data on
three types of gasifiers in 2001 dollars. It clearly shows that entrained flow
gasifier is more expensive than bubbling bed units on dry ton basis, but on
heat output basis it is most economic. The comparison is not quite uniform
as cost for fluidized bed gasifiers are for biomass while that for entrained
flow gasifier is for coal as the feedstock.
There is a considerable difference between pressurized and atmospheric
pressure gasifier plants. Even after discounting for smaller size of the gasifier
reactor, high-pressure gasifiers could cost between two and four times more
than atmospheric pressure plants would (Bridgwater, 1995). The feed system
in a pressurized plant also adds to the cost.