Page 129 - Advances in bioenergy (2016)

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Integrated Biomass Hydropyrolysis and Hydrotreating:

A Brief Review

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Martin Linck, Larry Felix, Terry Marker and Michael Roberts
Department of Energy Conversion, Gas Technology Institute, Des Plaines, IL, USA

PYROLYSIS

Pyrolysis involves heating biomass in the absence of air or oxygen to drive off water and
vapors evolved by the thermal breakdown of biopolymers—cellulose, hemicellulose, and
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lignin. Biomass tends to thermally decompose above approximately 230C, with pyrolysis
occurring at temperatures nearer 500C. The products formed during pyrolysis depend on
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temperature, heating rate, and product residence time at elevated temperatures. Three types of
products are usually obtained: species that remain gaseous under standard conditions; species
that condense to form liquids under standard conditions; and solid char, which contains mostly
carbon and ash. It has been observed that very rapid heating of biomass, combined with
immediate quenching of the vapors obtained, produces a large proportion of liquid products.
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This approach is referred to as ‘fast’ pyrolysis. Although a range of results is reported in the
literature, in the case of fast pyrolysis, approximately 70% of the biomass can be recovered as
liquids, 15% as solid char, and another 15% is evolved as noncondensable gases (such as
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CO ). If fast pyrolysis is carried out at very high temperatures, (above 650C), the fraction of
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char produced can be driven well below 10%. 5

However, in this case, a shift from condensable liquid products toward gaseous products is
observed (i.e., CO, H , CO , and CH ). The liquid product of fast pyrolysis is known as bio-
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oil, bio-crude, or pyrolysis oil and has been the focus of great interest because it has a much
higher volumetric energy density than biomass. It has been suggested that pyrolysis oil can be
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upgraded to replace crude oil in certain applications. Finally, another, more time-consuming
biomass conversion approach is referred to as ‘slow’ pyrolysis, and produces more gas and
more solid product, with relatively little liquid being produced. Traditional charcoal burning
is an example of slow pyrolysis. 1

Unfortunately, pyrolysis oil has a number of undesirable properties if the desired end product

is a deoxygenated liquid hydrocarbon fuel. The oil consists of a great variety of chemical
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species, many of which remain unidentified, and many of which contain oxygen. These
oxygenated species are generally chemically reactive. The compounds are also hydrophilic,
and they mix readily with water. Indeed, pyrolysis oil is usually approximately 25% water; this
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water reduces the viscosity of the oil and considerably reduces its energy density. The oil is
inherently acidic, which means that systems designed to handle pyrolysis oil must be made of