Environmental Engineering Reference
In-Depth Information
The green biorefinery utilizes untreated-wet-green biomass, including grasses, green crops,
and immature cereals (Kamm and Kamm, 2004). The first step consists of pressing the green
material to produce a nutrient-rich juice and fiber-rich press cake (Fernando et al., 2006).
The two-platform biorefinery feedstocks are treated and separated into a sugar and a ther-
mochemical line. Sugars are then used to produce fuel (e.g., ethanol) or chemicals (e.g., lactic
acid) via fermentation (biochemical route); and the remaining material sent to the thermo-
chemical conversion line where is transformed via gasification into syngas—a mixture of
carbon monoxide, hydrogen, and other gases (Clark and Deswarte, 2008).
In terms of processing, there are two routes: biochemical and thermochemical.
Biochemical route
When sugars are not readily available for fermentation, feedstocks need a pretreatment to
hydrolyze them and release fermentable sugars. Typical treatments include hydrolysis with
acids, enzymes, or a combination of both. Hydrolyzates rich in glucose can be transformed
into ethanol via fermentation with
Saccharomyces cerevisiae
. When hydrolyzates contains
xylose, the primary sugar contained in hemicelluloses, the fermentation needs a different type
of microorganism, for instance the recombinant bacterium
Zymomonas mobilis
(ZM4(pZB5) )
(Panesar et al., 2006).
Thermochemical route
The thermochemical route has two alternatives: gasification and fast pyrolysis. Gasification
consists of heating biomass to temperatures between 650 and 1,400°C by direct or indirect
heating. In direct heating, the biomass is combusted in an oxygen-limited atmosphere yield-
ing a gas that is rich in carbon monoxide and hydrogen. If air is used instead of oxygen, then
nitrogen is part of the end product and the gas in this case is called “producer gas.” In indirect
heating systems, biomass is heated using a heat transferring system (Ebert, 2008).
An alternative to gasification is pyrolysis, which is a thermochemical conversion that takes
place at lower temperatures than gasification. Biomass is heated in the absence of oxygen in
an indirect heating reactor to temperatures between 400 and 800°C (Ebert, 2008). The main
final products are liquid bio-oil, pyro-gas, and char (Table 14.1). Bio-oil (also known as
“pyrolysis oils, pyrolysis liquids, bio-crude oil, wood liquids, wood oil, liquid smoke, wood
distillates, pyroligneous acid, and liquid wood” (Mohan et al., 2006) ) is a polar liquid
containing a mixture of alcohols, furans, ketones, carboxylic acids, pyrolytic lignin, carbohy-
drates, and other minor compounds (Piskorz et al., 1988). Bio-oil compositions depend on the
feedstocks including species, portion of the plant, and age (Mohan et al., 2006).
Table 14.1
Pyrolysis end products of three wood species in weight percentage.
Brockville Poplar
White Spruce
Red Maple
Temperature (°C)
497
500
508
Bio-oil
62.9
66.5
67.9
Char
14.4
12.2
13.7
Pyro-gas
12.0
7.8
9.8
Water
10.3
11.6
9.8
From Piskorz et al., 1988.
