Environmental Engineering Reference
In-Depth Information
Biogas
Biogas is the product of anaerobic digestion of organic matter by microorganisms. Traditional
feedstocks for production of biogas include manure, sewage, and municipal waste. Also
energy crops and crop residues are good candidates that can be added to the mix (Krakat et al.,
2010; Weiland, 2010).
The main components of biogas are variable amounts of methane and carbon dioxide;
proportions depend on the feedstocks and gasification method. The term
biogas
is reserved for
the gas resulting from microbial fermentation of organic matter and should not be confounded
with wood gas, which is basically producer gas (syngas produced by direct gasification using
air as source of oxygen) containing carbon monoxide, hydrogen, carbon dioxide, and nitrogen
(see Chapters 9 and 10).
FEEDSTOCKS FOR FUELS AND CHEMICALS
The International Energy Agency (IEA) estimates that the world satisfies 11 percent of its
energy needs from biomass sources (IEA, 2005). This basically takes place in Africa, Asia,
and China where wood and dung are used as heating fuels (Sims et al., 2006). However,
the total amount of biomass used to produce electricity in the world is merely 1.1 percent
(IEA, 2005). Therefore, besides the limited harvest of biomass from crop residues
(e.g., straw and corn stover), forest residues, and municipal solid waste (see Table 14.4),
dedicated energy/chemical crops will be needed to satisfy demand of a biobased economy.
From this point on, crops for production of energy or chemicals will be referred generically
as energy crops.
Energy crops are plants and algae grown exclusively to produce biomass. Energy crops
capture energy from the sun via photosynthesis and store that energy in the chemical bonds
of sugars, which can remain free in the plant or transformed into starch and cellulose. Biomass
is then harvested and the contained energy recovered in the form of heat (by direct burning)
or transformed in a biorefinery into energy carrier fuels and electricity or feedstocks for
chemicals.
Energy crops, if grown without the input of fossil fuels, cycle carbon over and over again
and do not add new carbon dioxide to the atmosphere. Because energy crops can be replanted
after harvest, they are a truly renewable energy source.
Table 14.4
Heating value of biomass byproducts from selected crops.
Biomass
Residue yield (metric tons/hectare)
Heating value (MJ/kg, dry)
Wheat straw
2.97
17.9
Rice straw
4.52
16.8
Almond branches
6.21
18.4
Olive kernels
64.0
18.9
Ptolemais lignite
—
16.9
Forest residue
—
19.5
Hazelnut shell
—
15.43
Safflower seeds
—
23.86
Rapeseed
—
26.7
Cotton seed residue
—
16.9
Reproduced from Saxena et al., (2009), with permission from Elsevier.
