Design of Biomass Gasifiers - Biomass Gasification, Pyrolysis and Torrefaction - page 268

TABLE 8.4 Effect of Medium, Temperature, and Residence Time on the

Calcination of CaCO 3 in a Lime-Based Chemical Looping Gasifier

Medium CO 2 H 2 O

Temperature ( C) Conversion (%) Time (min) Conversion (%) Time (min)

600 8.78 30

700 73.22 30

800 7.58 30 96.94 30

900 20 30 100 25

950 72.89 30 100 19.16

1000 92.95 30 100 10

Source: Data taken from Acharya, et al. (2012).

( Table 8.4 ). If it is, on the other hand steam, a very high conversion is
achieved even at lower temperature.

8.4 ENTRAINED-FLOW GASIFIERS
It is the most successful and widely used type of gasifier for large-scale gasi-
fication of coal, petroleum coke, and refinery residues. Entrained-flow gas-
ifier is ideally suited to most types of coal except low-rank coal, which, like
lignite and biomass, is not attractive because of its large moisture content.
High-ash coal is also less suitable because cold-gas efficiency decreases with
increasing ash content. For slurry-fed coal, the economic limit is 20% ash;
for dry feed it is 40% (Higman and Burgt, 2008, p. 122).
The suitability of entrained-flow gasification for biomass is questionable
for a number of reasons. Owing to a short residence time (a few seconds) in
entrained-flow reactors, the fuel needs to be very fine, but grinding fibrous
biomass into such fine particles is difficult. Entrained-flow gasifiers need ash
to be molten. For biomass with high CaO and low alkali metals (Na, K), the
ash-melting point is high (Mettanant et al., 2009), and therefore to provide for
such high combustion temperature, a higher amount of oxygen is required. On
the other hand, for biomass with high alkali content, the ash-melting point is
much lower. This reduces the oxygen required to raise the temperature of the
ash above its melting points. However, molten biomass ash is highly aggres-
sive, which greatly shortens the life of the gasifier's refractory lining.
For these reasons, entrained-flow reactors are not preferred for biomass
gasification. Still, they have the advantage of easily destroying tar, which is
very high in biomass and is a major problem in biomass gasification.