Page 297 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

Chapter | 8 Design of Biomass Gasifiers 273

Pyrolysis gas + tar Oxygen Ash + char
Biomass
Combustion
Gasifier (stage 2)
(stage 3)
Air
Cooler
Low-temperature
gasifier (stage 1) Pulverizer
Product gas
Deduster
Ash
FIGURE 8.18 Choren process. The biomass is gasified in an entrained-flow gasifier, facilitated
by a rotary-type partial gasifier (stage 1).

low-temperature reactor for pregasification at 400 500 C in a limited supply

of air. This produces solid char and a tar-rich volatile product. The latter
flows into the second chamber (stage 2), an entrained-flow combustor, where
oxygen and the product gas from the first stage are injected downward into

the reactor. Combustion raises the temperature to 1300 1500 C and
completely cracks the tar. The hot combustion product flows into the third
chamber (stage 3), where the char is gasified.
The solid char received from the first stage is pulverized and fed into the
third stage of the Choren process. It is gasified in the hot gasification
medium produced in the second stage. Endothermic gasification reactions
reduce the temperature to about 800 C. Char and ash from the product gas

are separated and recycled into the second-stage combustor. The ash melts
at the high temperature in the combustor and is drained from the bottom.
Now the molten ash solidifies, forming a layer on the membrane wall that
protects the wall against the corrosive action of fresh molten biomass ash.
The product gas is processed downstream for Fischer Tropsch synthesis or
other applications.

8.5 PLASMA GASIFICATION
In plasma gasification, high-temperature plasma helps gasify biomass hydro-
carbons. It is especially suitable for MSW and other waste products. This
process may also be called “plasma pyrolysis” because it essentially involves
thermal disintegration of carbonaceous material into fragments of compounds
in an oxygen-starved environment. The heart of the process is a plasma gun,
where an intense electric arc is created between two electrodes spaced apart
in a closed vessel through which an inert gas is passed (Figure 8.19).

Though the temperature of the arc is extremely high (B13,000 C), the
temperature downstream, where waste products are brought in contact with