Anaerobic Digestion (AD) converts 30-60% of the digestible solids in
the feedstock
into biogas by bacterial
fermentation. The biogas is then burned to generate renewable energy. When used for heat
only, the biogas is burned in a modified gas boiler to provide heat energy to heat the digester and
for export. Alternatively, the biogas can be used to fuel engines of vehicles and other machinery.
When biogas is used as a fuel for a combined heat and power (CHP) unit, electricity and heat are
produced. The heat from the CHP unit can be used to maintain digester temperature and supply heat
energy for export.
The products of AD are:
- Biogas
- mixture
of methane (55-80%), carbon dioxide (20-40%), hydrogen sulphide (0-3%) plus ammonia and nitrogen (0-5%).
Calorific value is 17-25 MJ/m3 (21.4 MJ/m3
at 60% methane content). The biogas needs “cleaning”
to remove hydrogen sulphide and water prior to use as a vehicle fuel.
- Digestate
–
The
pollution potential (BOD) of digestate is considerably less than the feedstock (60 to >80% reduction),
though it is still very high. Most of the nitrogen (N), phosphorus (P), and potassium (K) present in
the feedstock remain in
the digestate.
Digestate can be applied to land as an organic
fertiliser, or can be mechanically separated to
produce:
- Fibre – high in organic matter and plant
nutrients. Fibre can be used
as a nutrient-rich soil conditioner or further processed to produce granular organic fertiliser or a
combustible fuel.
- Separated liquid – contains plant nutrients
and can be applied
to
land as an organic fertiliser.
The quantity
of biogas produced depends
on type of feedstock, temperature of the digester and the hydraulic retention time (HRT) of feedstock
in the digester. In temperate climates heating of the digester is normally required.
There
are two main types of AD system:
- Mesophilic:
The digester
is heated to 25-35oC. HRT
of feedstock is typically 15-30 days. Most on-farm biogas plants are mesophilic.
- Thermophilic:
The digester is heated to 49-60oC. HRT of feedstock is typically 12-14 days.
Compared
with mesophilic digestion, thermophilic digestion produces more biogas per unit time, requires smaller
digestion tanks for a given volume of feedstock and gives better pathogen ‘kill’. However, thermophilic
digestion is less robust and less tolerant than mesophilic digestion, requires more expensive technology,
greater energy input and a higher degree of operation and monitoring.
