Environmental Engineering Reference
In-Depth Information
2.3.5 Feedstock as methane yield boosters
The group of methane yield boosters includes compounds belonging to
various feedstock categories, all having the common features of very high
methane potential (Table 2.1). Methane boosters are supplied to biogas
plants for economic reasons and are added in rather small amounts,
precisely dosed, to the feedstock mixture, with the purpose of increasing the
methane yield per volume of digested biomass. As with any biogas
feedstock, it is important that methane boosters are of high quality, as
their addition to the feedstock mixture will affect both the quantity and the
quality of biogas production. Frequently used methane boosters are fatty
materials such as fish oil from fish processing industries, soya bean oil and
margarine from the food industry and various alcohol residues from
brewing industries. Along with these, residues from the sugar and beverage
industries are also used to enhance methane yields of the substrate mixture.
More recently, glycerin from biodiesel production was introduced as a
biogas feedstock and methane booster to the group of so-called oleo-
chemicals. Use of glycerin as a feedstock for biogas is part of the concept of
integrated biorefinery in which by-products from biofuels production (such
as glycerin and press cake from the production of biodiesel) are further
processed through AD with the extraction of methane. The press cakes
remaining after pressing juice or oil from grapes, olives, apples and other
fruits are also a good and homogeneous biogas feedstock. These press cakes
contain the skins, pulp, seeds and stems of the pressed fruits. Compared
with other feedstock types, the industrial wastes used as methane boosters
are highly homogenous materials. They can be added to the digester in
precise small amounts and their effect on the microbiology inside the
digester and on the methane yield is easily predictable. In general, certain
upper limits for the addition of methane boosters to the feedstock mixture
inside the digester are recommended as excessive amounts negatively
influence the AD process.
2.3.6 Feedstock influence on plant operation
The characteristics and composition of the feedstock influence the
configuration, design and operational parameters of an anaerobic digester.
The feedstock also determines the quality and quantity of biogas and
digestate produced, and therefore has a direct impact on the overall
economy of the biogas plant. The composition of the feedstock that is to be
digested is one of the most important elements in determining the size of the
digester and thus the investment cost of the plant, as longer retention times
require a larger digester volume. The feedstock supplied determines to a
large extent the main objective of the AD treatment. The digestion of animal
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