Anaerobic Digestion
A case study - McDonnell Farms Biogas
Limited, Shanagolden, Co. Limerick
Source: Biogas plant McDonnell Farms Biogas Limited
Anaerobic Digestion - a case study McDonnell Farms Biogas Limited, Shanagolden, Co. Limerick
The biogas produced can be upgraded to natural gas (fossil)
quality and injected into the gas grid or used as a vehicle fuel
but is normally used on site to generate heat and electricity in a
Combined Heat and Power–unit (CHP).
Source: FNR
Source: Containerised CHP-unit with gas flare and reception building
The biogas yield depends on the composition of the feedstock
and on the ambient conditions in the digester (e.g. temperature,
retention time). It is possible that the same feedstock could have
different gas yields. Approximately 2 kWh electricity and 2 kWh
heat can be produced from 1 cubic metre biogas depending on
the CHP unit and gas composition (e.g. 55% CH4 content in the
biogas, 20 MJ/m3, 38% electrical and thermal efficiency CHP unit).
Figure 1: On farm digester model
Anaerobic Digestion (AD) is the process whereby organic matter
is broken down by bacteria and enzymes in an oxygen-free
environment. The organic matter is released as biogas; this is a
mixture of the combustible gas methane (50-75%), carbon dioxide
(25-45%), small amounts of water (2-7%) and trace gases. This
process occurs in bogs, landfill and in the stomachs of animals.
Component
Methane
Chemical symbol
CH4
Concentration
50 – 75%-vol.
Carbon dioxide
CO2
25 – 45%-vol.
Water vapour
H2O
2 - 7%-vol.
Oxygen
O2
< 2 %-vol.
Nitrogen
N2
< 2 %-vol.
Ammonia
NH3
< 1 %-vol.
Hydrogen
H2
< 1%-vol.
Hydrogen sulphide H2S
20–20.000 ppm
ppm: Parts per million; %-vol.: Volumetric percentage
The type of feedstock used by anaerobic digesters varies; it can
include pig or cattle slurry, food waste, energy crops (grass silage,
maize silage, grain), municipal solid waste from households and
organic solid waste from industry. Materials with high lignin
content, e.g. any kind of wood, are not suitable for biogas production.
Feedstock is pumped into a closed vessel (digester) which has
been inoculated with suitable bacteria. Anaerobic (0% oxygen)
conditions are then maintained in the vessel and the temperature
is held at a constant value (typically 40°C).
The residue or digestate of the AD process can be separated into a
liquid and fibrous fraction. The liquid can be returned to the land as
a high value fertiliser and the solid fibre used as a soil conditioner.
Fermentation improves the quality of manure as nutrients are
more available for plants and pathogens and weed seeds are killed.
Furthermore, as odours are broken down and neutralised during
the fermentation process, the development of odours during liquid
manure storage and spreading is greatly reduced.
Organic products from industry which are used to produce biogas
provide interesting agricultural opportunities. By using organic
residues such as distiller’s pulp, grease or food wastes, the natural
material cycles (carbon and nitrogen) is closed and provides a
recirculation of the nutrients into agriculture.
Biogas technologies contribute to environmental protection
as they release no carbon dioxide (CO2). Energy from biogas
is largely CO2 neutral because the CO2 released from burning
biogas was already removed from the atmosphere through
photosynthesis. The fermentation of manure also reduces
emissions from methane, a gas that would have far more
devastating effects on the climate than CO2 if it escaped
uncontrolled from raw liquid manure.
Developing an AD project on your farm - a personal
perspective David McDonnell, McDonnell Farms Biogas
Limited, Shanagolden, Co. Limerick
Where to start?
There are a number of essential items which must be attended
to at the very early stages; these include planning consent,
application for waste permit/licence, Animal By-Product (ABP)
approval from Department of Agriculture, Fisheries & Food
(DAFF), grid connection and grant aid eligibility. Power purchase
agreements and Renewable Energy Feed In Tariff (REFIT)
applications need to be lodged. Planning for an AD plant has
the added complication of involving ‘waste’ e.g. disinfection
according to the ABP-regulation.
Plant size
The first issue is the scale of the plant. A farm plant is relatively
small in scale, usually processing all its own feedstock and possibly
taking in some outside local bio-degradable material like dairy
sludge. This is a very different type of plant to a large scale centralised plant in an industrial environment which will process all its
material outside the plant environs. The planning process for these
two types of development is very different. It is more of a challenge
commercially to make a small plant viable but the environmental
benefits to the local community are significant and it integrates
well with farming practices and infrastructure.
Pre-planning
It is essential to have a pre-planning meeting with the County
Council in the early stages of the project development. This will
give a clear indication as to what areas will need to be dealt with in
detail in a planning application and the timeframe involved.
Issues that need to be clarified at this point include what
the proposed location of the CHP unit will be and what type
of building or container it will be in. This is an important
consideration for noise reduction, safety equipment and
procedures for example an emergency gas flare. You will
also need to be mindful of the distance of the plant to any
neighbouring dwellings which are not owned by the developer.
A waste permit from the local authority or a waste licence from
Anaerobic Digestion - a case study McDonnell Farms Biogas Limited, Shanagolden, Co. Limerick
Source: Biogas plant McDonnell Farms Limited, Primary digester and first covered storage digester
the EPA will also be required depending on the scale of the
plant. These take several months to process and therefore the
application should be submitted early.
The Animal By Product section of DAFF also need to be informed
of the proposed development and an application made to them
for their approval, which is guided by conditions they have set out
for the design, layout and operation of all AD plants. This process
is done in three stages, before construction, before operation and
finally validation of operation when the plant is commissioned.
Once a draft plant layout and design is available it is best to submit
the first stage application for approval in case changes need to be
made. This may save having to re-submit a change in planning
later on.
Grid connection
Revised guidelines for the grid connection process for smaller
scale generators is now evolving with the recent publication by
the Commission for Energy Regulation (CER). A grid connection
application for an AD plant of under 5 MW can be processed
without the need for grid interaction studies, and the overall
time lines have decreased from years to months. It is, however,
very important to have an application submitted by an electrical
consultant early in the process; this process is quite technical and
the costs associated with grid connection can be considerable. The
sooner you receive an offer the better for your business plan. The
CER also provides licensing for the construction and operation of a
generator, but units under 1 MW are automatically licensed by order.
Selling to the grid
Finally, once all the above come together and a realistic timeline
is in place, discussions for the sale of the electrical output can
commence. It is also time to apply for the Renewable Energy Feed
in Tariff (REFIT), which for AD-CHP applications is up to 15 ct/
kWh. This can only be done after planning is granted and a grid
connection offer has been issued.
Generally it takes a minimum of two year to get the project from
inception to completion.
Case study AD Project - David McDonnell Farms Biogas Ltd, Co.
Limerick
During 2009 and 2010, Limerick farmer David McDonnell installed
one of the most advanced anaerobic digestion (AD) on-farm
plants in the country. David started looking at alternative energy
several years ago and visited numerous AD plants in Germany and
Denmark to see what was being done in these countries.
David milks 300 dairy cows and runs a medium-sized free range
poultry farm with his brother Richard. As well as the digester they
have recently installed two 2.3 MW wind turbines which were
connected to the grid towards the end of 2010.
Having seen what was being done in Germany and Denmark,
he knew there was no reason he could not create a similar plant
to carry out AD in Ireland. He engaged a German engineering
company to design his plant. They have built over 150 AD plants
worldwide. David’s plant is now constructed and is undergoing the
commissioning process.
When it is up and running it will have a validation inspection.
He predicts that when the plant is working most of the input
product will be manure from the family farm. Due to the large
number of cattle and poultry he holds, David realised there was
a significant benefit from AD. Gases from slurry could be burned
to produce electricity, while he would also end up with a higher
quality fertiliser for use on the land, not to mention the numerous
environmental benefits of the AD process e.g. better availability of
nutrients, less odour from the slurry.
Having seen what was being done in Germany and Denmark,
he knew there was no reason he could not create a similar plant
to carry out AD in Ireland. He engaged a German engineering
company to design his plant. They have built over 150 AD plants
worldwide. David’s plant is now constructed and is undergoing the
commissioning process.
When it is up and running it will have a validation inspection. He
predicts that when the plant is working most of the input product
will be manure from the family farm. Due to the large number of
cattle and poultry he holds, David realised there was a significant
benefit from AD. Gases from slurry could be burned to produce
electricity, while he would also end up with a higher quality fertiliser for use on the land, not to mention the numerous environmental benefits of the AD process e.g. better availability of nutrients,
less odour from the slurry.
Some feedstock will need to be imported and can range from
glycerine from biodiesel plants to food waste. Once operational
the biogas plant will produce electricity and heat constantly for
8,000 hours per year, which means in the region of 2,000,000 kWh
electricity and 2,100,000 kWh heat. He will receive 15 ct/kWh
electricity being exported. The heat is going to be used in the plant
itself, for the pasteurisation and on farm for heating the poultry
sheds to replace fossil fuels.
Anaerobic Digestion - a case study McDonnell Farms Biogas Limited, Shanagolden, Co. Limerick
Construction commenced:
September 2009
Output:
Commissioning:
Autumn 2010
Availability CHP unit: ~91%; 8,000 full load hours/a
Biogas production: ~950,000 m3/a (55% CH4 content, 20 MJ/m3)
~2,000,000 kWh/a, mostly exported to the national grid
Feedstock:
Cattle slurry:
5,800 t/a
(10% dry matter content)
Food waste:
2,800 t/a (22% dm)
Electricity production:
Poultry litter: 900 t/a (40% dm)
Heat production:
~2,100,000 kWh/a, heat exported Dairy sludge: 900 t/a (22% dm)
Glycerine:
360 t/a (90% dm)
Sum: 10,760 t/a
for heating the plant, for pasteurisation and for heating the poultry sheds
Primary energy savings:
~1,200 MWh/a
Technique and plant components:
CO2 savings: ~1,500 t CO2/a
Total capital cost:
~ €1.5m
SEAI CHP grant: €108.000
Payback time: approx. 10 years
• Primary-digester and first covered storage tank 980 m3net (17 x 6m) insulated and covered with double layer foil
REFIT:
15 ct/kWh exported electricity indexed linked for 15 years
•
Integrated gas- and mechanical mixing systems (agitators)
•
Integrated progressive cavity pump
•
External heat exchanger to heat feedstock
The Sustainable Energy Authority of Ireland (SEAI) recognized
the importance of this project as a demonstration of AD-CHP
in Ireland and supported this project (CHP-unit) with a grant of
€108,000.
•
Covered storage tank 2,500 m3net (26 x 6m)
•
Separator to separate digestate in liquid and fibre
•
Biological gas cleaning, gas cooling, gas analyser
•
CHP unit: MAN, 250 kWel., 263 kWth.
•
eception hall for liquids and semi solid waste including
R
disinfection unit, feeding and storage tank 200 m3
• Reception area for on farm animal by-products
(slurry, poultry litter) and sludges
•
•
Approximate efficiency rates: 38% electrical, 41% thermal (integrated heat exchanger)
Abbreviation:
Full automation of the plant
Operating parameters:
Operating temperature: 40°C (mesophilic), heated by hot water from CHP-unit
Organic loading rate: ~4.0 kg odm/m3d (primary digester), 2.0 kg odm/m3d primary digesters and first storage tank
Retention time: ~40 days primary digester, 80 days primary digester and first storage tank
€
Euro
H2S
%
Percent
kWel. Kilowatt electric
%-vol. Volumetric percentage
Hydrogen Sulphide
kWh Kilowatt hours
~
Approximately
kWth.
°C
Degree Celsius
m
Meter
a
Annum, year
m
Cubic meter
Anaerobic digestion
MJ
AD
CH4 Methane
CHP Combined Heat and Power
ct
Cent
3
Kilowatt thermal
Mega Joule
MWh Megawatt hours
N2
Nitrogen
NH3
Ammonia
dm
Dry matter
odm Organic dry matter
H2
Hydrogen
ppm Parts per million
H2O Water
t
Tonnes
Sustainable Energy Authority of Ireland
Wilton Park House
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Ireland
t +353 1 808 2100
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e [email protected]
w www.seai.ie
Sustainable Energy Authority of Ireland
Renewable Energy Information Office
Unit A, West Cork Technology Park
Clonakilty
Co Cork
t +353 23 8842193
f +353 23 8863398
e [email protected]
w www.seai.ie
The Sustainable Energy Authority of Ireland is financed
by Ireland’s EU Structural Funds Programme co-funded
by the Irish Government and the European Union