Download Greenhouse gas abatement through AD

Bioenergy
Greenhouse gas abatement through AD
Study undertaken by Barry Caslin, Bioenergy Specialist, Teagasc, Oak Park, Carlow
Ireland is unusual in that more than one third of its human induced greenhouse gas emissions
originate in agriculture. It is estimated that agriculture produces 35 percent and was the single
largest producer of GHGs (Department of the Environment). This is primarily due to the structure
of Irish agriculture, where the livestock sectors typically account for over 80 percent of agricultural
output value. Between cattle and sheep, Ireland has more than 14 million ruminant animals. These
animals represent the main source of methane, a gas with a relatively high global warming
potential. Consequently, one of the proposed strategies in the National Climate Change Strategy,
is to target agriculture as a source of greenhouse gases. The objective is to achieve a reduction in
agricultural emissions of 2.2 million tonnes (Mt) carbon dioxide (CO2) equivalents by the end of the
commitment period 2008-2010 from a `business as usual' projected level of 18.7 Mt CO2
equivalents. National Climate Change Strategy identifies agriculture as the sector with the highest
emissions in Ireland and sets a reduction target of 1.2Mtpa from the National herd, 0.06Mtpa from
changes in manure management and 0.9Mtpa from reduced fertiliser use
Biogas offers the greatest potential in reduction of Green House Gasses because the AD process is
a physical treatment process accelerating a natural process which occurs in manure and organic
waste streams to maximise methane capture in controlled conditions. Methane (CH4) as a
Greenhouse Gas is 20 Times more Potent than Carbon Dioxide, Methane is released from manure
naturally in storage particularly when stored for long periods of time in traditional open storage
tanks. For efficient capture of Methane in the Digestion Process it is important that manure is
transferred to the digester within 2 – 4 weeks of production by livestock. Approximately 25% of
the Methane will emit from the manure naturally in storage. This can be seen as bubbles in
manure. The biogas produced by the AD process will displace fossil fuel energy, resulting in a
further reduction in greenhouse and acidifying gases, which Ireland has international
commitments to reduce.
The sources of carbon reduction are:
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Methane emissions from stored manure and sludge
Nitrous oxide emissions reduction achieved by mineralisation of the Nitrogen during the
digestion process
The carbon dioxide emissions avoided by replacing fossil fuel to generate the net output
of electricity and heat
Allowing for emissions of unburned methane (1% of fuel) in the CHP exhaust
NPK fertiliser substitution
The Sustainable Energy Authority of Ireland is partly financed by Ireland’s EU Structural Funds Programme co-funded by the Irish Government and the European Union.
Bioenergy
The CO2 reduction due to NPK substitution the following upstream specific energy and CO2
contents have been assumed: (38MJ/kg pure N) 9.36kgCO2/kg pure N, (17MJ/kg pure P)
2.67kgCO2/kg pure P, and (6MJ/kg pure K) 0.80kgCO2/kg pure K
The recently published PROBIOGAS “Promotion of Biogas for Electricity and Heat Production in
EU- Countries, Economic and Environmental Benefits of Biogas from Centralised Co-digestion”.
Project co-ordinator: University of Southern Denmark, Department of Bioenergy, Esbjerg, Denmark
concluded that calculated emissions of between 65 kg and 150kg of CO2 equivalent are saved per
ton of biomass treated in AD Process. The total CO2 saving from a typical AD Plant of say 0.4MW
Generating capacity using 22,000 tonnes of Biomass would be between 1400 tonnes and 3300
tonnes of CO2. If the equivalent value of the CO2 saving at €20/tonne paid to the developer as a
rate per kW/h the rate would equate to between 7c/kWh and 16.5c/kWh.
The Sustainable Energy Authority of Ireland is partly financed by Ireland’s EU Structural Funds Programme co-funded by the Irish Government and the European Union.