Environmental Engineering Reference
In-Depth Information
and model calculations confirm that transport is not an issue in terms of
energy demand, but is in terms of cost.
9.2.2 Substrate pre-treatment
Substrate pre-treatments are now becoming more common and in many
cases are relatively energy intensive. Mechanical and thermal pre-treatment
methods such as milling, grinding and heating require large amounts of
energy. Some pre-treatment steps cannot be avoided (e.g. separation and
crushing of wastes and sanitisation or even sterilisation); however, the
energy demand can still be optimised. Typically, about 20% of the energy
from the biogas may be required (mainly as electricity, with less required as
heat) for the whole pre-treatment chain of source-separated organic
municipal solid waste prior to fermentation.
When the purpose of the pre-treatment is an increased biogas yield (as
promised by a large number of technology providers), extremely thorough
verification tests are required. This type of technology often promises an
increase of 5-20% in biogas yields; however, the energy demand for the pre-
treatment can increase by the same amount. The effects achieved are heavily
dependent on the substrates used and the fermentation technology adopted.
Positive effects (such as shorter retention times in the fermenter or less
mixing energy) can be obtained, but a detailed comparison between the
energy demand and the expected yield increase must be undertaken. It is
also advisable to investigate the practical experiences of other biogas plant
operators who have used this technology.
9.2.3 Substrate supply to the fermenter and fermentation:
electricity demand
The fermentation process itself is often the main source of energy demand,
with pumps and stirrers representing the main consumers of electricity.
Different fermentation technologies display significant differences in their
parasitic electricity demand. For example, CSTRs and continuously mixed
plug flow fermenters have relatively high electrical energy demands, while
dry batch fermenters (such as vertical garage door fermenters) have very low
electrical energy demand. The vertical garage door fermenters have a high
fuel demand because of the wheeled machinery used for transportation in
and out of the fermenters and for mixing outside the fermenters. However,
batch systems offer lower biogas yields than CSTR systems (FNR, 2011).
The design of the plant as a whole has an important effect on the energy
demand. For example, one-stage fermentation can have a lower energy
demand, but is also likely to have a lower biogas yield than multi-stage
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