Page 57 - Materials Chemistry, Second Edition

The Application of Life Cycle Assessment on Agricultural 43

Fig. 2 Flowchart of biogas/
biomethane production chain

Anaerobic digestion (the second activity within ‘‘process’’ phase as shown in
Fig. 3) is a versatile biochemical process by which organic matter is converted to
biogas under anaerobic conditions (Korres et al. 2011). This is achieved as a result
of the consecutive biochemical breakdown of polymers to methane and carbon
dioxide in an environment in which various microorganisms harmoniously grow
and produce/reduced end products (McCarty 1982).
Four successive biological processes are involved in the anaerobic degradation
of organic matter, namely hydrolysis, acidogenesis, acetogenesis, and methano-
genesis (Fig. 3). Complex polymers, as stated above, are converted into monomers
by extra-cellular enzymes during hydrolysis while these monomers are trans-
formed mainly into volatile fatty acids (acetic, propionic, and butyric acids) during
acidogenesis. Acetate, carbon dioxide (CO 2 ), and hydrogen (H 2 ) are produced
from volatile fatty acids during acetogenesis which is finally converted into
methane (CH 4 ) during methanogenesis (Bernet and Beline 2009).
The biogas produced during anaerobic digestion is composed of CH 4 (55–75%),
CO 2 (25–45%), and trace elements such as hydrogen sulfide (H 2 S which range
from 0 to 2,000 ppm) and ammonia (NH 3 within the ranger of 0–590 ppm) (Rasi
et al. 2007). In addition, trace amounts of hydrogen (H 2 ), nitrogen (N 2 ), carbon
monoxide (CO), saturated or halogenated hydrocarbons, and oxygen (O 2 ) are
occasionally present. The biogas is usually saturated with water vapor (H 2 O) (Rasi
et al. 2007). If the biogas is to be used as a transport fuel or to be injected in the
natural gas grid for other use, it has to be upgraded or scrubbed (i.e., removal of
corrosive components, particles, water and increase heating value to approxi-
mately 50 MJ/kg or methane content of 97%) to gain natural gas standards. The
upgraded and pressurized biogas is then ready to be used (Nilsson 2001).