APPENDIX A
Anaerobic Digestion Plant Overview
Colwick, Nottingham
Prepared by: Gary Burgess BSc (hons) DIS
Enerlux Limited
22-36 Kettering Road
Abington Square
Northampton NN1 4AH
Dated:
Page 1 of 10
July 2013
PROPOSED USE
The proposal is for the installation of an Anaerobic Digestion (AD) plant to take the waste
streams from food waste and other organic waste materials, and by treating and digesting
these wastes to produce biogas that can then be converted into energy.
A portion of the electricity and heat generated will be used in the AD plant, the remainder
being available for export or use by the local industrial estate. By a considered approach to
the recycling proposals for waste management it is possible to deliver a significant power
resource that reduces the carbon footprint whilst generating sustainable electricity.
ANAEROBIC DIGESTER
Anaerobic Digestion is the fermentation and digestion of organic matter in the absence of
oxygen and relies on creating the right environment for bacteria to thrive that produce
methane and other gases, together known as biogas, by the digestion of the organic
material. The type of bacteria in waste streams is selected by the temperature range in
which they are able to live and reproduce. Generally there are two temperature ranges that
are commonly used in AD plants to create the right biology, one that is mid-range around
37oC known as mesophilic and the other is a higher temperature thermophilic above 50 oC.
The biology in both ranges produce efficient biogas yields, the higher range typically allows
faster fermentation and digestion of torpid feed-stocks depending on the type of materials
available. It is proposed to operate initially within the mesophilic range.
ANAEROBIC DIGESTER CAPACITY
The AD plant capacity is proportionate to the availability of the waste-streams available and
is proposed to be of 2,000kWe electrical and 3,000kWt heat capacity determined by the
onsite generation from a Combined Heat & Power (CHP) unit incorporated within the AD
plant.
The volume of waste food and other vegetable waste, which is relatively fresh and therefore
has yet to decay, that is to be handled per annum is approximately 50,000m3, this
represents up to 140m3 per day of organic material available that lends itself to gas
production. This translates into an average of 5-7 vehicle movements and loads per day.
In addition to the production of electricity and heat the facility will also produce
approximately 30,000m3 of digestate which will act as a bio-fertiliser and a further
15,000m3 of bio-liquid, a quick release crop fertiliser. This can be supplied to local farmers
for use on the land by spreading as a substitute for chemical fertilisers and will also add
organic matter to the land soils. Additional organic crops, such as break crops of maize or
beet, can be sourced from these farmers to supplement the input to the AD process if
required.
Page 2 of 10
THE PROPOSED STRUCTURES
The AD plant functional diagram is show below; the following description of each element is
explained. In addition it is proposed to add a hydroponic facility to produce additional
organic feed-stocks for the AD plant grown using the digestate and bio-fertilisers from the
AD output.
FOOD & ORGANIC
WASTE BIO-PLANT
DEPACKER
HOLDING CLAMP
FOOD WASTE
HOLDING CLAMP
ORGANIC WASTE
MACERATOR
PASTEURISER
BIO-GAS
CONDITIONER
BIO-GAS
C
H
P
BIO-LIQUID
Secondary
Digester
HEAT OUTPUT
BIO-FERTILISER
DIGESTATE
SEPARATOR
Primary
Digester
ELECTRCITY
SEMI-DRY
BIO SOLIDS
HYDROPONIC
BIO-PLANT
BIO-FERTILISER
TO AD PLANT
Hydroponic bed
Page 3 of 10
PAS110
MACERATOR
The waste-streams available are first divided into packed - typically date expired food waste
from retail and hospitality businesses, and unpacked – typically organic waste from food
processes such as vegetable residues or kitchen waste, and temporarily held in their
respective holding clamps.
The packed food is then processed through a de-packing line that strips away any nonorganic materials and then the resultant de-packed output is introduced into the macerator.
The additional organic material, such as food waste and vegetable residues, are then added
into the system through the macerator. The purpose of the macerator is to mill the material
into a paste; this paste then passes through a chopper pump device in order to macerate
further the material to a finer paste prior to entry into the pasteurising tank. By reducing
the size of suspended particles this exposes more surface area of the organic mix for
bacterial digestion.
Page 4 of 10
PASTEURISER TANK
The pasteuriser tanks contain pump mixers and heating pipes that pre-heat the material.
This tank pasteurises the material by heating the contents up to 75 oC for 30-60 minutes and
then allows it to cool down to below 40oC. Pasteurisation will enable any potentially
harmful bacteria such as E.coli and Streptococcus to be eradicated. This material is then
sample screened to analyse the biology prior to introduction of the porridge into the
primary digester. By pasteurising and screening, any contamination of the digester culture
is prevented.
The pasteurisation will also allow the output digestate to be considered as enhanced which
allows additional uses as a valuable fertiliser. The aim is to become certified and comply
under the PAS110 specifications by continuous screening and monitoring. The process cycle
from origin of waste streams, the timing of samples and outcomes, through to the removal
of digestate from site and destination of product will be electronically recorded to ensure
full compliance and traceability.
Typical pasteuriser tanks insitu
Page 5 of 10
DIGESTER TANKS
The anaerobic digestion stages are carried out by the use of two digester tanks, a primary
digester in which the majority of the potential gas yield from the organic substrate is
collected, and a secondary digester for additional gas production and digestate storage. To
produce sufficient gas output to drive the intended 2MWe generation there are to be four
tanks in total, two as primary digesters and two as secondary digesters and storage. The
digester tanks are equipped with circulating pumps and heaters to keep the material fully
mixed to allow for maximum gas production. The conditions for digestion are maintained
separately in each of these tanks. If other materials are introduced then one tank at a time
can be dosed in order to minimise any possibility for contamination or biology mismatch
caused by the new material. The tanks are typically 30m diameter and hold approximately
3,360m3 of organic substrate. Each of the tanks are topped with a gas cap in which the
biogas is held, these caps are double skinned with a rubber sealing ring where it meets the
tank lip in order to create an air-tight seal and to prevent any gas leakage. The plant would
have automatic monitors for leak detection in the digesters and the liquid digestate tanks.
Typical layout of proposed AD plant
The digester forms a block of four insulated tanks residing on the site. As the AD process
advances and biogas is created from the digestion of the input materials, the gas is being
collected in the domes above the digester tanks, and at the end of process the digestate
(which through the process has removed the odour creating elements) is moved into a
three-sided clamp for temporary storage and cooling prior to removal.
Page 6 of 10
ELECTRCITY & HEAT GENERATION
The biogas is cleaned to minimise particles and other contaminates and then used as the
fuel for the CHP unit to produce electricity and heat. A proportion of the electricity
generated is used to provide the AD plant with power to run the pumps and systems, the
remaining power generated is proposed to be exported to the grid as sustainable electricity
or potentially used by the local industry to reduce its carbon footprint by the use of this
renewable energy.
Typical CHP installation,
with sound insulation
cover removed.
Heat is also produced by the CHP engine by a heat exchanger operating from the engine
cooling fluid water jacket, some of this heat is used to feedback into the AD plant to provide
the input for the tank heating and to keep the digester material at the required
temperature. The remaining heat can then be used for other heating requirements, such as
heating for the hydroponics or for use by the local industry where appropriate or possible.
The heat being generated from the biogas produced by the AD plant is considered to be
carbon neutral to reduce the site carbon footprint by offsetting.
The efficiency of the CHP engine is up to 90%, in
addition to the CHP the plant will be installed with
an Organic Rankine Cycle (ORC) turbine engine.
The ORC uses the high temperature from exhaust
flue gases, from 300oC to 1,000oC, as its input to
generate electricity and to provide heat exchanger
output of cooler temperatures for process heat
use. The ORC provides a further 10% of electrical
power generation increasing the efficiency of the
plant. The lower temperature heat, typically now
less than 85oC, can be used for other heat uses on
the site.
Page 7 of 10
Example ORC engine without covers
DIGESTATE
The digestate is initially separated into liquids and semi-dry solids, some of the liquid
element is fed back into the front end of the plant to mix with the macerator output
material, the remaining liquid is held in small storage vessels to top-up the liquid
requirements within the digesters or for later tankering off-site for use as a bio-fertiliser.
The separated semi-dry matter is a bio-fertiliser containing the same nutrients as the
original material less the methane and carbon-dioxide that would have been released had
the material naturally decayed. The AD process essentially accelerates the release of the
methane and carbon-dioxide for capture and subsequent energy conversion, making use of
the power rather than simply releasing into the atmosphere. The digestate can be used
directly by spreading onto fields for crop growth, particularly for energy crops that can be
reintroduced into the whole AD process. The digestate is a valuable alternative to chemical
derived fertiliser products, saving not only cost but also the potential downsides of nitrogen
and phosphate granular fertilisers.
Page 8 of 10
PLANT LAYOUT – NOISE AND ODOUR ABATEMENT
The incoming waste holding clamps, de-packing line, macerator, pasteurisation treatment,
process control management and analysis lab are proposed to be housed in an industrial
building. This will enhance the visual impact by providing a common structure and also to
further reduce any noise output that may come from the de-packing or macerator plant.
Also by housing the feedstock holding clamps and macerator internally odours from organic
materials can be contained. In addition automatic closing doors and an odour control
system are to be installed within the building to eradicate odour escape.
Page 9 of 10
By its nature anaerobic means in the absence of oxygen, and as the AD process is a sealed
system, meaning that air cannot get in, it then follows that odours cannot get out. Further
the AD process removes the majority of the odour creating gases from the digestate - biogas
is made up of methane CH4, carbon dioxide CO2, nitrogen N2, water vapour H2O, and other
trace gases such as hydrogen sulphide H2S - of which the methane is burnt to produce
energy and the other gases are removed in the process. The resultant semi-dry digestate is
low odour, similar to garden centre compost.
The CHP’s are to be housed in sound insulated containers, typically the noise specification
for a 500kW-2,000kW generator is 70dBA at 7m. The nearest residential building is
approximately 700m away from the generator, and for each multiple factor of distance the
noise reduces by 20dB, so the noise profile at the residential building would be around
30dBA. This is the equivalent of the likely noise heard in a library so it is unexpected that
the generators will have any impact on any surrounding residential areas.
The layout of the plant, including tanks, is shown below.
Page 10 of 10