Environmental Engineering Reference
In-Depth Information
scale-up technologies. FT diesel production can be combined with electricity
production to increase energy efficiency and reduce production costs. FT diesel
made from biomass is free from sulfur and nitrogen and contains no (or very few)
aromatic compounds, which makes it more environmentally friendly than diesel
produced from fossil resources.
7.4.3
Pyrolysis
By definition, the pyrolysis of carbon-containing compounds entails their incom-
plete thermal degradation in the absence of oxygen at moderate temperatures
(350-700
o
C) under pressure. It produces various organic liquids and gases along
with char. The liquid product of biomass pyrolysis is known as bio-oil or pyrolysis
oil (and by several other names, including biofuel-oil, wood liquid, and wood oil).
Bio-oils are a mixture of different molecules (alcohols, aldehydes, ketones, esters,
and phenolic compounds) derived from the fragmentation of lignin, cellulose,
hemicellulose, and extractives. It is much easier to handle and to transport bio-oil
than solid biofuel. However, the composition of bio-oil can change during storage
if the products in the condensate (bio-oil) did not reach thermodynamic equilib-
rium during pyrolysis. The moisture content of bio-oil is typically about 15-30 wt%
of that in the original feedstock used in the pyrolysis process [44].
Biomass pyrolysis processes can be divided into three categories: slow pyroly-
sis (carbonization); fast pyrolysis; and flash pyrolysis. Slow pyrolysis is a conven-
tional method involving a slow heating rate of 0.1-1°C s
-1
. This gives a high yield
of char relative to liquid and gaseous products and has been utilized for thousands
of years in the production of charcoal. Slow pyrolysis (or carbonization) requires
low temperatures and very long residence times, with typical vapour residence
times of 5-30 min.
Fast pyrolysis uses much higher heating rates (10-200°C s
-1
) and high tem-
peratures (500-700°C) for short periods of time, and is considered to yield bet-
ter results than slow pyrolysis. For example, the pyrolysis of pine wood samples
at 550°C releases large quantities of aldehydes, ketones, and methoxylated phe-
nols. The most important products derived from pine by pyrolysis are turpentine
and pine oil (sometimes called tall oil). Biomass should normally be dried to a
moisture content of around 10% before fast pyrolysis [45]. Recently, biomass
has been converted to bio-oil and then into hydrogen by catalytic steam reform-
ing. However, the yield of this process is relatively low. The main aim of such
processes is to achieve a high yield of liquid or gaseous biofuels and other
chemicals [46].
Flash pyrolysis of biomass usually occurs at 500-700°C and involves very high
heating rates (e.g. 300°C min
-1
) over very short periods of only a few seconds. It
therefore requires feedstocks with very small particle sizes (105-250 µm) [47].
The gaseous products of fast pyrolysis require rapid cooling or quenching to
minimize secondary reactions of the intermediate products (radical components).
