Environmental Engineering Reference
In-Depth Information
rather than food crops (Murphy and Power, 2009). What is referred to as
grass is typically a mix of grass varieties and clover from grasslands. Other
ensiled crops relevant for biogas production include other cereals such as
sorghum and barley, as well as sugar beet tops, although a much wider
range of materials can be stored in this way and used for biogas production,
for example hemp (Pakarinen et al., 2011), pineapple processing waste (Rani
and Nand, 2004), mango peel (Madhukara et al., 1993) and green pea shells
(Madhukara et al., 1997).
There have been several studies evaluating ensiling as a storage method
for biogas substrates, particularly grass. These show that well-preserved
silage has a relatively constant methane yield, even after months of storage
(e.g. grass (Pakarinen et al., 2008; Seppa¨ la¨ et al., 2008) and cereals
(Herrmann et al., 2011)). Ensiling of maize or hemp has been shown to
increase the available sugars and the amount of biogas produced (Pakarinen
et al., 2011; Amon et al., 2007), and this is particularly true when acid was
used as an additive (Pakarinen et al., 2011). This is presumably because acid
addition means fewer WSCs are used up by LAB and because the acidic
conditions break down hemicelluloses. It also has been shown that some
biological additives increase methane yield in maize silage (Vervaeren et al.,
2010), but also that some have no significant effect on methane yield from
grass silage (Pakarinen et al., 2008).
4.3
Pre-treatment technologies for biogas production
Anaerobic digestion is a well-established process for energy production. The
fermentation takes place in four steps associated with different microbial
populations: hydrolysis, acidogenesis, acetogenesis and methanogenesis.
The time needed for the degradation of biomass to biogas, or macro-
molecules to mainly methane and carbon dioxide, varies depending on the
nature of the chemical bonding of the carbohydrate in the biomass (Noike
et al., 1985). The microorganisms in anaerobic digestion convert simple
molecules, including sugars such as glucose, into biogas (see Chapter 5 for a
more detailed description). Starch and cellulose are both chains of glucose
units, but while starch is used by the plants as an energy store and is
therefore easy to break down, cellulose is used to maintain the structure of
the plant and is, by necessity, difficult to break down. The breakdown of
cellulose is further complicated by the bonds between different cellulose
chains, and between cellulose and hemicelluloses and lignin (see Figure 4.1).
Converting this lignocellulose complex to sugar is the key to biofuel
production, whether that is biogas or bioethanol.
Different pre-treatment technologies have been developed in recent years
to increase the availability of carbon, particularly in lignocellulolytic
material, for anaerobic digestion. Many of these technologies come from the
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