12. Heat Capacity of Combustible Gas [17].

Process	Gasification agent	Heat capacity of combustible gas kJ/m3N
Direct gasification	Air	4000-7000
	Oxygen	10000-12000
Indirect gasification	Steam	15000-20000

Indirect gasification process generates combustible gas with higher heat capacity at low production rate, and has low cost of energy recovery and gas cleanup and high investment costs.

Direct gasification process with oxygen has the same advantages as indirect process but the cost of oxygen production is estimated at over 20% of the generated electricity cost using combustible gas.

A gasification system consists of the following main components:

• gasifier;

• gas cleanup subsystem;

• energy recovery subsystem.

The gasifiers (reactors in which the conversion of biomass into combustible gas takes place) can be divided as follows:

• direct gasifier (Fig. 18):

  • fixed bed;

  • fluidized bed;

• indirect gasifier (Fig. 19).

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Figure 18. Direct gasifiers.

Figure 18. Indirect gasifiers [17].

A good gasifier should produce a combustible gas with low tar content. The up-draught gasifiers generate a combustible gas with high content of tar because the pyrolysis tar is carried upward by the flowing hot gas. The down-draught gasifiers generate combustible gas with low content of tar because the pyrolysis vapours offer conditions for thermal cracking of tar. Fluidised bed is suitable for gasification of biomass with high ash content. Gasifiers with fluidized are five times more effective than fixed bed [17]. The indirect gasifiers produce combustible gas with higher quality but they have higher investment and maintenance costs compared to direct gasifiers.

The combustible gas produced by gasification has to be pre-treated before the use in energy recovery system in order to remove the contaminants and dangerous components and to increase heat capacity and hydrogen content. The treated combustible gas is called syngas and can be used in boilers, internal combustion engines, gas turbines and molten carbonate fuel cells and in methanol production. The contaminants present in combustible gas with problems they cause and their cleanup methods are presented in Table XIII. The combustible gas treatment methods depend on the used energy recovery technology.

13. Contaminants in Combustible Gas: Problems and Cleanup Methods [17].

Contaminant	Range (g/Nm3)	Problems	Cleanup method
Particulate (ash, char, inert material)	3-70	Erosion, pollutant emissions	Filtration, scrubbing
alkali metals (sodium and potassium compounds)	-	Hot corrosion	Condensation and filtration
Fuel nitrogen (NH3, HCN)	1.5-3.0	NOx formation	Scrubbing, SCR
Tars	10-100	Clog filters, deposit	Tar cracking, scrubbing
Sulphur, chlorine (H2S, HCl)	2.5-3.5	Corrosion, pollutant emissions	Lime scrubbing

In Table XIV the required quality parameters of syngas are shown.