Page 218 - Advances in bioenergy (2016)

limited fraction of biomass converted and the larger capital intensity of the process, as
compared with first generation bioethanol. 4

Various types of thermochemical processes, such as liquefaction, pyrolysis, and gasification,
5-7
can be applied to produce gaseous and liquid products. Liquefaction and pyrolysis take
place in the absence of air and produce mainly liquid products, accompanied by solid and
6
gaseous fractions. Liquefaction (50–200 atm and 250–325 °C) and pyrolysis (1–5 atm and
375–525 °C) proceed via thermal depolymerization of the lignocellulosic matter, producing
1
bio-oils. A wide range of feedstock can be used, including wood, black liquor, agricultural
wastes, and forest wastes. This bio-oil production path is advantageous as it requires only a
single reactor and a large fraction of the biomass energy (50–90%) can be preserved into the
liquid. Slow pyrolysis produces large amounts of coke, whereas fast pyrolysis produces bio-
oils in high yields, of up to 80 wt% of dry feed.

Bio-oils are complex mixtures of more than 400 highly oxygenated compounds, including
acids, alcohols, aldehydes, esters, ketones, and aromatic compounds, and can contain up to
1,8
50 wt% water. A representative distribution of components from a fast pyrolysis reactor,
operated to maximize bio-oil liquid, is: 65 wt% organics, 10 wt% water, 12 wt% char, and
1
13 wt% gas. However, the relative ratios of these fractions are highly dependent on reaction
conditions, reactor design, and biomass alkali content. Liquefaction oils have higher heating
content, lower oxygen content, and lower moisture content than fast pyrolysis oils.

Biomass can be decomposed via the microbiological action of bacteria under anaerobic
conditions. This decomposition occurs naturally in landfills, whereas the commercially proven
technology of anaerobic digestion is widely used for treating high moisture content organic
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wastes. The main advantage of this approach is the fact that the digester's feedstock can be any
biodegradable raw material, originating from sewage sludge, municipal solid wastes, animal

manure, agroindustrial wastes, energy crops, and by-products from biofuels’ production. 5,7,9-11
In many cases, the raw material is not only of negative cost, but it also results in a considerable
reduction of disposable wastes, having thus both financial and environmental benefits. The
main product is biogas, a gas mixture consisting mainly of methane and carbon dioxide in
various ratios, depending on the type of feedstock, the technical design of the digester and
operating conditions. 9,11 CH content in biogas derived from sewage is approximately 70%,
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that originating from waste of food industry can be as high as 85%, whereas CH content of
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landfill gas can be as low as 30%. 12