Page 101 - Materials Chemistry, Second Edition
P. 101
Life-Cycle Assessment of Biomethane 87
40
e MJ -1 energy replaced 35 Direct GHG emissions
30
Indirect GHG emissions
25
Total GHG emissions
20
15
g CO 2 10
5
0
Crop production Herbicide volatilization Lime dissolution N2O emissions Transportation Anaerobic digestion plant Upgrading Total processing emission Biogas losses
Agriculture Transportation Biomethane production process
Fig. 4 The direct, indirect, and total GHG emissions from grass biomethane production (Korres
et al. 2010, 2011)
the problem of high water contents, low biogas production rates, and high econ-
omies are barriers in AD of manure (Gerin et al. 2008). To overcome this problem,
manure can be codigested with other lignocellulosic feedstocks. This results in
higher biomethane production (Jagadabhi et al. 2008) with improved digester
microbiology (Nizami and Murphy 2010).
3.3 Digestate: A Source of Fertilizer and Bioproducts
AD results in a residual digestate. This digestate can be a great source of com-
mercial fertilizer. This additional environmental benefit included in the biofuel
process chain lowers the production costs and loss to the environment and
increases the process efficiency (Cherubini et al. 2009). The use of maize and grass
silage as AD feedstocks and their digestate used as fertilizer have been studied by
Gerin et al. (2008). Matsunaka et al. (2006) studied the Timothy grass for digestate
purposes. They observed the benefit of nitrogen uptake by the grass digestate,
especially during the spring. Liquid and fiber components are obtained from
digestate (Salter and Banks 2009) and some of the liquid can be re-used to enhance
the digestion process (Berglund and Börjesson 2006). The rest is processed into
liquid biofertilizer or can be used for many practical purposes (Fig. 5). The solid
digestate can be processed into either soil conditioner or high value insulating
materials (Grass2004; Salter and Bank 2009). The concept of using biomethane as
a biofuel and digestate for value-added products evolves into the concept of
