Manure can be gasified under anaerobic conditions and the biogas can be converted into heat and power in combustion plants or engines.
The present data refer to heat and power production from pig manure at "farm scale plants" (where pig farmers have their own production units) and "joint biogas plants" (where several farmers share one large production unit).
The main production processes are following: 1) pig manure is transported from the pig stable to a pre-storage tank 2) the manure is heated to about 37ºC and transferred to an anaerobic reactor where it is mixed with organic matter from e.g. slaughter houses and fish industry, 3) biogas produced in the reactor from gasification of manure and organic matter from industry is transferred to a gas storage tank 4) biogas is transferred to a stationary engine or a gas combustion unit which is producing heat and electricity or just heat, 5) degassed manure and organic matter from industry is transferred to a manure storage tank and distributed on agricultural fields as fertilizer 6) electricity and heat is used locally and/or transferred to the national electric grid respectively the local district heating system.
Average manure transportation distances in step 1 and 4 range between 1.5 km and 7.5 km (Seadi, 2000) at joint scale plants. At farm scale plants, transportation of pig manure is small.
About 80% of manure gasified in Denmark is gasified in joint scale units and about 20% is gasified in farm scale units (Biogasbranchen, 2004)
Biogas is to a large extent combusted in stationary engines at both farm and joint scale plants. However, gas furnaces are also used occasionally e.g. in cases of excess gas production and in cases of extraordinary heat demand (personal communication with various actors in business).
All electricity produced at both farm scale and joint scale plants is utilized and most of heat produced at joint scale plants is utilized (personal communication with various actors in business). The degree of heat utilization at farm scale plants is unknown.
Data for heat and power production from manure have been derived from biogas yield factors, energy yield factors and air emission factors related to gas engines, estimates of internal energy consumption at biogas plants and avoided emissions of CH4 and N2O as a result of changed manure handling praxis.
1) The range is around 34 to 40%, 2) the range is around 20 to 32%.
emissions of CH4 and N20
The avoided emissions have therefore been estimated to 1100 g CH4 /ton manure and 14 g N2O/ton manure when gasification is applied as an alternative to no gasification with present biogas technology.
Heat and electricity used e.g. for handling of manure during all processes from the first to the last storage tank are included (see process description). Main emissions to air from biogas combustion are included but gas emissions from eventual leaks are not. Avoided emissions as a result of gasification of manure as an alternative to no gasification is included. Gas generated from organic matter from industry is excluded. Small synergetic effects from combining industrial waste and manure in gasification process have been ignored. Transport of manure from pig stable to the first storage tank of the biogas plant and from the last storage tank at the biogas plant to the field is not included. Non-fossil CO2 from biogas combustion have been ignored.
Danish production of heat and electricity from manure is heterogeneous and the present data seek to represent averages for the two main types of plants (farm scale plants and joint scale plants) by applying general estimates of inputs and outputs related to manure gasification. The most important assumptions on biogas yield, heat and electricity yield and emission factors are considered pretty good whereas many other assumptions are considered more uncertain. In total, the data set provides a reasonable representation of energy production from manure in biogas plants in Denmark although realized values at specific plants may vary from the average.
Data have been validated by comparing and combining information from different sources.
Inputs and outputs associated with heat and power production from manure (about 5% (weight) dry matter) at farm and joint scale plants Data are provided per ton of manure entered into the gasification tank. Intermediate CH4 production is 22 m3).
Biogasbranchen (2004): www.biogasbranchen.dk
Dansk Bioenergi, BioPress (www.biopress.dk) each issue.
Energistatistik (2002): Energistyrelsen.
NERI (2002): Emission factors, stationary combustion for year 2002: National Environmental Research Institute, Denmark (www.dmu.dk).
Poulsen HD, Børsting CF, Rom HB and Sommer SG (2001): Kvælstof, fosfor og kalium i husdyrgødning – normtal 2000.DJF rapport nr. 36 Husdyrbrug (p. 9).
Seadi T (2000): Danish Centralised Biogas
Plants - Plant Descriptions.Publisher:
Sommer SG, Møller HB and Petersen SO (2001): Reduktion af drivhusgasemission fra gylle og organisk affald ved biogasbehandling. DJF Rapport no. 31. (In Danish, Summary in English). http://www.agrsci.org/
J de (2004): Personal communication with Jan de Wit, Danish
Data URL: http://www.lcafood.dk/processes/energyconversion/heatandpowerfrommanure.htm