Environmental Engineering Reference
In-Depth Information
Fig. 8.8
Biomass gasification and pyrolysis routes to synthetic biofuels
Source: Sustainable biofuels: prospects and challenges, RS Policy document 01/08, ISBN 978 0
85403 662 2. Reproduced with permission of the Royal Society.
(Fig. 8.8) include bioalcohols (methanol, ethanol and linear higher chain alcohol
mixtures) and synthetic biofuels.
At this point is worth mentioning that although bio-SNG could be classified as
synthetic biofuel,
it comes first since all the reported biofuels from gasification are
obtained from it and thus the technologies (up to the preparation of the syngas) are
very similar.
Bio-SNG
Bio-SNG can be produced by a conventional gasification process (methanation) at
high temperatures (800-1000
◦
C) aiming at producing large quantities of methane.
The current technology employed allows the use of a wide range of biomass feed-
stocks including wood chips and waste wood [84-86]. The conventional gasification
process involves various steps (Fig. 8.9). Firstly, the biomass undergoes endothermal
steam gasification (reaction 1) to give a mixture of CO and H
2
, which is subse-
quently converted into methane, CO
2
and hydrogen (reactions 2 and 3). The net
overall reaction from biomass to methane and CO
2
(reaction 4) is slightly exother-
mic. However, the main drawback of the conventional gasification technology is the
formation of tars and char [87].
Interestingly, the gasification of biomass can be performed at lower tempera-
tures (250-400
◦
C) in supercritical water. It has currently been reported at lab scale,
employing different Ni and Co based catalysts [86]. In this process, the biomass
