Environmental Engineering Reference
In-Depth Information
6.6 Matching feedstocks and digester type
6.6.1 Effect of pre-treatments on energy conversion
Pre-treatments are discussed in Chapter 4 but, as with selection of a suitable
temperature range, from the viewpoint of process optimisation it is essential
to consider both inputs and outputs. When dealing with energy crops or
other feedstocks that have a production cost associated with them, many
processes that apparently improve the specific methane yield per kilo-
gramme VS may also reduce the overall mass of VS, thus giving little or no
net gain in terms of yield in MJ tonne
1
or MJ ha
1
year
1
: examples include
loss of VS during ensilage, heat treatment or pre-aeration. Conversely, for a
given digester capacity, increasing the rate of substrate conversion may
improve volumetric gas production as the digester can process a larger
volume of substrate in the same period even if the specific methane yield of
the material is not increased.
6.6.2 Feedstock selection
Some feedstocks are difficult or unsuitable for mono-digestion because of
their unfavourable C/N ratios or high lipid content: examples include
abattoir wastes, fats and oils, and paper. In these circumstances, co-
digestion is the best approach to resolve any imbalance and improve
volumetric methane productivity: examples include the co-digestion of
slaughterhouse wastes with animal slurries or municipal solid wastes and the
addition of whey or glycerol (biodiesel by-product) to the digestion of
livestock manures (Alvarez and Lide´ n, 2008; Kavacik and Topaloglu, 2010;
Astals et al., 2011). Feedstock selection or blending should also be
considered as a means of optimising other aspects of performance. The
physical characteristics of the digestate and in particular its dewaterability
may have a significant effect on the overall process energy balance: wastes
such as sugar beet pulp are often extremely difficult to dewater without
chemical addition and centrifugation (Brooks et al., 2008), giving the
operator a choice between the energy cost of transporting large volumes of
digestate versus that of processing the material. In some cases the chemical
composition of feedstocks can result in precipitation of struvite (an
insoluble salt of magnesium phosphate and ammonia) in the digestion
plant, leading to blockages, physical damage and heat transfer losses; dense
inert materials such as grit and stones in the original feedstock can have a
similar effect and effective upstream pre-treatment is required to protect the
system. The issue of final digestate quality is also a major factor determining
feedstock selection. This is discussed in more detail in Chapter 12 but, in
general, high-quality source-segregated materials should not be downgraded
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