Environmental Engineering Reference
In-Depth Information
Amount and water content of feedstock
An important piece of information about a feedstock is the amount
produced or accumulated per year (or season). This information can be used
to estimate if sufficient feedstock is available for treatment in a centralised
plant. The water content of a feedstock (liquid, paste-like/semi-solid or
solid) is also important first-hand information.
Seasonal variation of feedstock accumulation and composition
In general, a biogas plant is operated all year round. If the feedstocks are
only seasonally available, they must be storable. Some feedstocks like crops
can be ensiled for storage. However, in industrial processes that work in a
campaign (e.g. sugar beet factories), the energy will also be needed during
the campaign. In this case, a biogas process would have to be adapted to
operate during the campaign and to stop for the rest of the year. Although
this is not a state-of-the-art operation mode of biogas plants, it is a viable
possibility.
The composition of other waste fractions (municipal organic waste for
example) can also vary due to seasonal changes. One example is organic
waste produced in suburbs - its composition can depend on the seasonal
change in gardening activities. For example, during the summer the waste
may contain grass cuttings, which are replaced by hedge prunings in the
winter. This has an obvious effect on the carbon to nitrogen (C:N) ratio.
Feedstock temperature
The temperature at which a feedstock will enter the biogas plant can be
important information (Braun, 1982). This is especially the case if the biogas
process is integrated into an existing process rather than being transported
long distances or stored for a long time. In addition, the local climate is of
relevance since the heating demand of the digester depends on the outside
temperature. Very low temperatures of the feedstock combined with high
water content cause a high heating demand. As a consequence, the net
energy output of a biogas process can decrease substantially. Figure 3.2,
shows the relationship between VS content, fermentation temperature and
net energy for a substrate temperature of 5
8
C. According to this figure, a
slurry of 1% VS at 5
C.
Heat exchangers can be applied to improve the energy balance, but this
increases investment costs. Apart from low temperatures, if the feedstock
temperature is too high, this can have a negative effect on the microbes in
the digester. Therefore, feedstocks at very high temperatures occasionally
will have to be cooled down before entering a biogas plant.
8
C would not yield any energy when fermented at 40
8
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