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234 8 Pre-combustion Air Emission Control

For conventional pyrolysis, where the heating rate is below 10 °C/s and the
residence time is long, the primary products are tar and char. In order to maximize
the production of oil, flash pyrolysis with rapid heating of about 100–10,000 °C/sec
is employed. External heating is also applied for the process to allow pyrolysis in
the absence of combustion. The char and tar can be burned with air for heating.

8.3.2 Gasification and Syngas Cleaning

8.3.2.1 Gasification Chemistry

Gasification is a thermal chemical conversion process in which carbonaceous
materials are converted to gases by incomplete oxidation. Gasification is actually
fuel rich combustion operating at 25–40 % of the oxygen that would be needed to
convert the hydrocarbon fraction of the fuel to CO 2 and H 2 O. In this process part of
the fuel is combusted to provide the heat needed to gasify the rest.
Major reactions involved in the gasification process are as follows [36].

1
C þ O 2 ! CO ðCombustion reaction) ð8:4Þ
2
C þ O 2 ! CO 2 ð Combustion reactionÞ ð8:5Þ
C þ CO 2 ! 2CO ð Boudouard reactionÞ ð8:6Þ

C þ H 2 O ! CO þ H 2 ð Gasification with steamÞ ð8:7Þ
Steam is produced by oxidation of hydrogen atoms in the fuel. There are also
other minor chemical reactions including gasification with hydrogen, water gas shift
reaction, and methanation.

C þ 2H 2 ! CH 4 ðGasification with H 2 Þ ð8:8Þ

CO þ H 2 O ! H 2 þ CO 2 ð Water gas shift reactionÞ ð8:9Þ
CO þ 3H 2 ! CH 4 þ H 2 O ðMethanation) ð8:10Þ

A more complicated way to present the gasification of solid or liquid feedstock
involves devolatilization to produce volatile hydrocarbons and charts. Then both
the chars and volatile hydrocarbon are further gasified to produce syngas and other
compounds.

1 1
C n H m þ nO 2 ! mH 2 þ nCO ðVolatile gasification) ð8:11Þ
2 2