Environmental Engineering Reference
In-Depth Information
between electricity, fuel and heat demand; furthermore, these are also
dependent on factors such as substrate parameters and climatic variations
over the year, meaning that a single static average energy demand will never
reflect the true energy demand behaviour of the plant.
This chapter aims to provide an overview of the data required for energy
balance analysis, both in terms of carrying out the analysis itself and in
interpreting the results. The chapter's focus is principally on medium- and
large-scale agro-industrial type biogas plants with automated feeding, active
heat control, active agitation (or leachate recirculation) and technical
application of the biogas (electricity, heat or upgraded biogas).
9.2
Energy demand of biogas plants
A broad range of different technologies for biogas plants are available on
the market; the selection of a certain type of technology has a significant
effect on the energy demand of the plant . For example, continuously stirred
tank reactors (CSTRs) differ substantially from 'vertical garage door' dry
batch digesters in terms of their electricity consumption (which is greater for
the stirrers in the CSTR system) and their fuel consumption (which is much
higher for a vertical garage door system mixed and fed by a wheel loader).
The actual parasitic energy demand depends on the substrates and their pre-
treatment, as well as on the treatment of the biogas and the liquid or solid
residues. The energy demands associated with the different process steps in a
biogas plant are discussed below. Precise standardisation is not possible, but
all energy demand data will be referenced to the amount of biogas (typically
with 55-60% methane content) in the unit kWh/m
3
raw biogas.
9.2.1 Transport and storage
The transport and storage of agricultural substrates typically require
relatively low energy inputs. The main energy consumption in this step is
associated with fuel consumption by tractors, trucks and wheel loaders. This
might be different in the case of centralised biogas plants or waste treatment
plants where the transport of fresh and digested material might be
significant (up to hundreds of kilometres).
In a few cases, the transportation of liquid substrates such as effluents and
manure is carried out in pipeline systems, which require electricity for the
pumps. During substrate transport and storage, heat demand is minimal
and is only associated with substrates that enter a solid phase at low
temperatures (e.g. fats) or to avoid freezing at low temperatures in cold
climates.
As a proportion of the gross energy production associated with biogas,
the energy demand for storage is extremely low. Both practical experience
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