Page 95 - Materials Chemistry, Second Edition

Life-Cycle Assessment of Biomethane 81

1.3 Anaerobic Digestion: A Source of Biofuel

AD is a process where organic waste and lignocellulosic biomass are converted
into biogas and digestate for value-added products. The organic wastes include
slaughterhouse waste, agricultural slurries and residues, and OFMSW. According
to Prasad et al. (2007), among the entire biomass available in the world ligno-
cellulosic biomass consisting of industrial, agricultural, and forest residues is the
mainstream feedstock for biogas production. Different potential feedstocks for
biogas production are listed in Table 1. The biogas produced can be used for the
production of electricity or heating purposes at combined heat and power (CHP)
units. Biogas can be further purified and upgraded to enriched biomethane
(*97 % CH 4 , *3%CO 2 and some minor constituents) and can be injected into
the gas grid or used as gaseous biofuel for transport and heating purposes. The
enriched biomethane used as a transport fuel has recently started to gain consid-
eration in many European countries, such as in Sweden, Austria, France, and
Switzerland (Korres et al. 2011). All across Europe, the biomethane yield from
3
various lignocellulosic biomass ranges from 10 to 1,150 m h -1 (Dena et al.
2009). The methane (CH 4 ) yield of various feedstocks is exemplified in Table 2.
AD brings a promising perspective for stakeholders in the discussion of carbon
trading and carbon neutral production chains, when doing an LCA study.
The aim of this chapter is to evaluate the LCA studies of biomethane produced
from lignocellulosic biomass as a biofuel and its release into the environment in
comparison with other bioenergy systems. A case study of grass biomethane,
produced by AD of grass silage and used as a transport fuel, is described.

Table 1 Different feedstocks for biogas production
Agricultural residues Municipal waste and residues
• Livestock manure • Sewage sludge
• Animal mortalities • Municipal solid waste
• Citrus waste • Food residuals
• Green waste • Organic fraction of municipal
• Agricultural slurries solid waste
• Sugarcane bagasse
Energy crops Industrial origin
• Energy maize • Wastewater
• Grass • Industrial sludges
• Miscanthus • Industrial byproducts
• Oilseed rape • Slaughterhouse waste
• Sugar beet • Animal fats
• Sweet sorghum • Biosolids
• Switchgrass • Spent beverages
• Willow