 The
Biogas Anaerobic Digestion Process
Delivery of Feedstocks
Animal slurry is to be pumped via an underground main to the anaerobic
digestion (AD) plant. As slurry is 95% water this is the most economical
way of transporting it, thus removing the need to haul a large
volume of material using farm machinery and lorries on the road.
All other feedstocks will be hauled to the site either when seasonally
produced or on a daily basis depending on feed stock type.
Storage on Site
Grass and maize silage will be hauled to the site and stored outside
in normal sheeted agricultural forage clamps. Any run off from
the clamps will be stored in underground tanks and then used within
the digestion process.
Low dry matter materials e.g. cattle slurry, will be stored in
sealed silos or underground storage tanks. Because this material
will remain in sealed tanks and pipes until mixing with other feed
stocks in the mixing hall, it will produce little or no smell.
Other odorous materials e.g. poultry manure and abattoir waste
will be discharged into the reception hall. This hall will be a
sealed controlled environment, only being opened for access for
delivery lorries. Discharging will take place within the sealed
hall which will have a negative air pressure environment ensuring
that all odours are contained within the plant. Air is fed from
this hall to the engines in the CHP plant and put through a thermal
odorising unit to remove smells.
Feedstock Mixing
Forages will be conveyed into the hall where it will be mixed
with other materials such as poultry manure, food waste etc to
create the optimum carbon to nitrogen mix. This will then be blended
with liquid feeds, e.g. cattle slurry, to produce the optimum dry
matter mix for digesting.
Before entering the digestion tanks the mixed feedstock will be
pasteurised at 70oC for one hour to destroy any harmful pathogens.
This ensures that the fertiliser residue remaining at the end of
the digestion process is free from any potential contamination
risk and is therefore safe to be applied back to the land.
 Anaerobic Digestion
The mixed and pasteurised feedstock
is pumped into the sealed digestion tanks where it remains for
a period 21 to 30 day. Bacteria are encouraged to grow in the
warm environment and through the natural 4 stage process of hydrolysis,
acidification, acetogenesis and methanogenesis biogas is given
off. This contains mainly Methane and Carbon Dioxide with some
trace gases e.g. Hydrogen, Hydrogen Sulphide and Nitrogen. The
Biogas is collected, the trace gases are removed and is then
used for powering the electricity generating engines.
Combined Heat and Power
Initial Generation from the Energy Centre will be in the region
of 2MW but has potential for up to 3MW depending on local waste
management.
T he Combined Heat and Power building will initially house 4 large
0.5MW generating engines. Although large, these engines are contained
in a sound proof and insulated building where the heat is captured
using a combined heat and power process (CHP). This heat is then
used within the biogas plant for the pasteurisation process and
to heat the feedstock which improves the efficiency of the digestion
and gives greater gas production.
The heat capturing process is so efficient in the CHP unit that
it can be used in the other integrated energy processes. This surplus
heat will be used firstly for the production of bioethanol, where
it will be supplemented with extra heat from methane powered boilers,
and secondly within the Biodiesel production process. The final
use of the heat will be back within the anaerobic digester where
it will be used to warm the digestate and speed up the biogas process.
Electricity generated from the CHP plant will be exported onto
the national electricity grid. Because it will have been generated
from renewable energy sources it will have added value. Currently
there is a large demand for electricity generated from renewable
sources with large industry, retailers and the service industry
as well as to the private consumer.
It is now well understood that renewable energy generated electricity
replaces generation from coal and oil, both of which deplete natural
reserves, release greenhouse gases and are therefore harmful to
the environment.
Bio Fertiliser
The rich organic fertiliser that remains after the biogas production
will be split into a dry and a concentrated wet fraction. This
virtually odourless fertiliser will be stored and spread onto agricultural
crops when they require it. The process of AD not only destroys
the pathogens and eliminates most of the smell, it also makes the
nutrients more available to the growing plants and dramatically
reduces the potential for contaminating water courses. Weed seed
populations normally present in animal manure are also destroyed
by the AD process and thus takes away the need for usual chemical
pesticide control.
By reducing the need for artificial fertilisers which rely on
fossil fuels for their production it also has a huge beneficial
effect on the environment. |
