Releases Archive 2011

Anaerobic digestion (AD) of organic material creates biogas, an excellent source of renewable energy and digestate, an extremely useful biofertiliser.  Northern Ireland produces almost 12 million tonnes of organic material each year, which if fed through anaerobic digesters could generate about 8% of Northern Ireland’s electricity requirement.  Over 80% of the organic material available is manure from housed livestock, with cattle manures accounting for almost 90% of this.  Agriculture has therefore tremendous potential to generate renewable energy through anaerobic digestion.  Because of this, AFBI installed a 660 m3 anaerobic digester at Hillsborough in 2008 in order to research the potential of farm based feedstocks for AD in Northern Ireland.  Capital funding was provided by the Secretary of State’s Environment and Renewable Energy Fund.  The first phase of the research programme funded through DARD has been to benchmark AD of dairy cow slurry.

Table 1 summarises inputs, outputs and performance of the Hillsborough digester over its first two years of operation.  During this time the digester was maintained at an average of 37.50C, had an average retention time for slurry of 27 days, was fed with 14,560 tonnes of mainly dairy cow slurry and produced 216,944 cubic meters (m³) biogas.  The gross energy value of the biogas was 1,208 megawatt hours (MWh*), equivalent to 113 thousand litres of heating oil.

Table 1: Inputs, outputs and performance of AFBI anaerobic digester over 2 years (24 January 2009 – 21 January 2011).

On average over the 2 years, 1 m3 (1 tonne) of slurry at 6.8% dry matter produced 0.989 m3 (0.989 tonne) of digestate, plus 14.9 m3 of biogas with a gross energy value of 83 kWh.  Maintaining digester temperature required 32 kWh of heat per tonne of slurry fed (equivalent to 38% of the total biogas energy produced).  Electricity required to run the plant averaged 5.4 kWh of electricity per tonne of slurry input.  Biogas was utilised through a 23 kW electrical output combined heat and power unit (CHP) to produce electricity and heat for use on the AFBI site.  The performance of the CHP over almost 9 thousand hours of operation from August 2009 (date of commissioning) to January 2011 is summarised in Table 2.

Table 2: AFBI Combined Heat and Power outputs over 8,670 hours between August 2009*** and January 2011

***CHP was commissioned in August 2009

The overall energy efficiency was 78% for the CHP (27% electrical efficiency).  The average net renewable energy gain as electricity plus heat from the CHP was 28 kWh per t of slurry input, with zero use of fossil fuel energy**.  Higher biogas yields per tonne of fresh input would be obtained from using higher dry matter slurry or from co-digestion with, for example, grass silage.  Grass silage has enormous potential for AD and could produce in the region of 1kW continuous of electricity per hectare ensiled (8,760 kWh).  Work to determine the biogas yield from dairy cow slurry co-digested with grass silage will be the next phase of the AD research programme (see photo).

When compared with the volume of slurry fed, the volume of digestate recovered was almost the same.  The total quantities of nutrients (N, P₂O₅ and K₂O) in the digestate were almost identical as those in the input raw slurry.  However, digestion

increased the plant available N (ammonia-N) content of the digestate by 17%

(Table 1).  Field trials will be undertaken by AFBI to determine the effect of this enhanced available nitrogen content on crop yields and nitrogen use efficiency.

Further information on the performance of the digester can be found on the AFBI web site (www.afbini.gov.uk).

Following intensive monitoring of the on-farm anaerobic slurry digester at AFBI, Hillsborough during 24 months of operation with dairy cow slurry as the main input, AFBI has observed, through work funded by DARD, that on average:-