Introduction - Biomass Gasification, Pyrolysis and Torrefaction - page 10

enzymatic hydrolysis technology though the latter could have a significant
cost advantage.

1.2.2 Thermochemical Conversion
In thermochemical conversion, the entire biomass is converted into gases, which
are then synthesized into the desired chemicals or used directly ( Figure 1.6 ).
The Fischer

Tropsch synthesis of syngas into liquid transport fuels is an exam-
ple of thermochemical conversion. Production of thermal energy is the main
driver for this conversion route that has five broad pathways:

1. Combustion
2. Carbonization/torrefaction
3. Pyrolysis
4. Gasification
5. Liquefaction.

Table 1.2 compares the above five thermochemical paths for biomass
conversion. It also gives the typical range of their reaction temperatures.
Combustion involves high-temperature exothermic oxidation (in oxygen-
rich ambience) to hot flue gas. Carbonization covers a broad range of processes
by which the carbon content of organic materials is increased through thermo-
chemical decomposition. In a more restrictive sense for biomass, it is a process
for production of charcoal from biomass by slowly heating it to the carboniza-
tion temperature (500

900 C) in an oxygen-starved atmosphere. Torrefaction
is a related process where biomass is heated instead to a lower temperature
range of 200

300 C without or little contact with oxygen.
Unlike combustion, gasification involves chemical reactions in an
oxygen-deficient environment producing product gases with heating values.

Biomass Feedstock Steam for heat

and power
Product gas clean-
up and heat recovery

Gas

Electricity

Gasifier

Syngas

Syngas (CO+H 2 )

Ash

Ethanol

Syngas
catalyst or
fermentation

Dilute
ethanol

FIGURE 1.6 Thermochemical route for production of energy, gas and ethanol.