Page 282 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

258 Biomass Gasification, Pyrolysis and Torrefaction

Throated downdraft gasifiers are not suitable for scale-up to larger sizes
because they do not allow for uniform distribution of flow and temperature
in the constricted area. To above 1 MW th capacity, an annular constriction
can be employed, but this has not been the practice to date.

8.2.3 Crossdraft Gasifiers

A crossdraft gasifier is a cocurrent moving-bed reactor, in which the fuel is
fed from the top and air is injected through a nozzle from the side
(Figure 8.7) of the gasifier. It is primarily used for gasification of charcoal
with very low ash content. Unlike the downdraft and updraft types, it
releases the product from its sidewall opposite to the entry point of the air
for gasification. Because of this configuration, the design is also referred to
as sidedraft. High-velocity air enters the gasifier through a nozzle set at a
certain height above the grate. Excess oxygen in front of the nozzles facili-
tates combustion (oxidation) of part of the char, creating a very-high-

temperature (.1500 C) zone. The remaining char is then gasified to CO
downstream in the next zone (Figure 8.7). The product gas exits from the
opposite side of the gasifier. Heat from the combustion zone is conducted
around the pyrolysis zone, so the fresh biomass is pyrolyzed while passing
through it.
This type of gasifier is generally used in small-scale biomass units. One
of its important features is a relatively small reaction zone with low thermal
capacity, which gives a faster response time than that of any other moving-
bed type. Moreover, start-up time (5 10 min) is much shorter than in down-
draft and updraft units. These features allow a sidedraft gasifier to respond
well to load changes when used directly to run an engine. Because its tar
3
production is low (0.01 0.1 g/nm ), a crossdraft gasifier requires a relatively
simple gas-cleaning system.

Biomass

Drying
Gasification
Pyrolysis
Combustion
Air Gas

Ash
FIGURE 8.7 Schematic of a crossdraft gasifier.