Page 263 - Synthetic Fuels Handbook

FUELS FROM BIOMASS 249

deforestation (though this is negligible compared to deliberate destruction to clear land for
agricultural use) but more importantly it means that more work has to be put into gathering
fuel, thus the quality of cooking stoves has a direct influence on the viability of biofuels.

8.4.4 Biofuels from Synthesis Gas

Alternatively, biomass can be converted into fuels and chemicals indirectly (by gasification
to syngas followed by catalytic conversion to liquid fuels) or directly to a liquid product by
thermochemical means. The process yields synthesis gas (syngas) composed primarily of
hydrogen and carbon monoxide (Chap. 7), also called biosyngas (Cobb, 2007)
The production of high-quality syngas from biomass, which is later used as a feedstock
for biomass-to-liquids (BTL) production, requires particular attention. This is due to the
fact that the production of synthesis gas from biomass is indeed the novel component in the
gas-to-liquids concept—obtaining syngas from fossil raw materials (natural gas and coal)
is a relatively mature technology.
Gasification (qv) is actually thermal degradation of the feedstock in the presence of
an externally supplied oxidizing (oxygen-containing) agent, for example, air, steam, and
oxygen. Various gasification concepts have been developed over the years, mainly for the
purposes of power generation. However, efficient biomass-to-liquids production imposes
completely different requirements for the composition of the gas. The reason is that in
power generation the gas is used as a fuel, while in biomass-to-liquids processing it is used
as a chemical feedstock to obtain other products. This difference has implications with
respect to the purity and composition of the gas.
In contrast, for biomass-to-liquids production only the amount of carbon monoxide
and hydrogen is important (the larger the amount, the better), while the calorific value is
irrelevant. The presence of other hydrocarbons and inert components should be avoided or
at least kept as low as possible. This can be achieved via the following ways:
1. The amount of components other than CO and H (primarily hydrocarbons) can be
2
reduced via their further transformation into CO and H . This is however rather energy
2
intensive and costly (two processes—gasification and transformation). As a result, the
overall energy efficiency of syngas production and of biomass-to-liquids processing is
also reduced, leading to higher production costs.
2. The amount of various components can be minimized via a more complete decomposi-
tion of biomass, thereby preventing the formation of undesirable components at the gas-
ification step. This approach seems to be more appropriate from energy efficiency and
cost point of view. The minimization of the content of various hydrocarbons is achieved
by increasing temperatures in the gasifier, along with shortening the residence time of
feedstocks inside the reactor. Because of this short residence time, the particle size of
feedstocks should be small enough (in any case—smaller than in gasification for power
generation) in order that complete and efficient gasification can occur.
3. In gasification for power generation, typically air is employed as oxidizing agent, as it
is indeed the cheapest amongst all possible oxidizing agents. However, the application
of air results in large amounts of nitrogen in the product gas, since nitrogen is the main
constituent of air. The presence of such large quantities of nitrogen in the product gas
does not hamper (very much) power generation, but it does hamper biomass-to-liquids
production. Removing this nitrogen via liquefaction under cryogenic temperatures is
extremely energy intensive, reduces substantially the overall biomass-to-liquids energy
efficiency, and increases costs. Amongst other potential options (steam, CO , O ), from
2
2
a technical and economic point of view oxygen appears to be the most suitable oxidizing
agent for biomass-to-liquids manufacturing. It is true that the oxygen-blown gasification