Heat and power production from pig manure

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).

Process description:

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 collection and treatment

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.

Methane yield:
Methane yield from manure vary for different growth stages of pigs and has been assumed to be 22 m3 per ton pig manure on average (Volatile Solid (VS) content of pig manure is about 5% (about 5% of manure is dry matter (Poulsen et al., 2001) and about 98% of dry matter is VS (IPCC 1996)), theoretic yield is about 520 l CH4 per kg VS (Møller et al., 2004), efficiency of plants (farm scale as well as joint scale) has been estimated to 85%). See also Birkmose 2000.

Energy yield
Electricity and heat yield per input of methane in gas engines vary from time to time and from plant to plant depending on actual conditions (see records in every issue of Dansk Bioenergi). Energy content of biogas is about 23 MJ/m3 (Energistatistik, 2002) and the following efficiencies specific for gas motors applied at joint scale plants and farm scale plants have been assumed in dialogue with Laugesen (2004) and de Wit (2004).
Type of energy Unit Joint scale plants Farm scale plants
Efficiency
% yield kWh/m3 Efficiency
% yield kWh/m3
Electricity kWh 371) 2.4 262) 1.7
Heat kWh 48 3.1 52 3.3

1) The range is around 34 to 40%, 2) the range is around 20 to 32%.

Air emissions factors:
Average air emissions factors for biogas applied in combustion plants and stationary engines are derived from NERI (2002), see table below. Emissions provided in “g per GJ” have been converted into g per m3 biogas using a heat value of 23 MJ/m3 (Energistatistik, 2002).
Substance Unit Combustion plants Stationary engines
SO2 g per GJ 11 19.2
NOx g per GJ 31 540
NMVOC g per GJ 4 14
CH4 g per GJ 4 323
CO g per GJ 36 273
CO2 kg per GJ 83.6 83.6
N2O g per GJ 2 0.5
PM2.5 g per GJ 1.5 0.206
PM10 g per GJ 1.5 0.451

Internal energy use:
Internal energy use at biogas plants (electricity use (for pumps, light etc.) and heat (manure heating and room heating etc.) varies from plant to plant depending production facilities and conditions. Many plants use their own gas for heat and electricity supply and it has been roughly estimated that both farm scale and joint scale plants use 0.09 kWh electricity per m3 biogas produced and 0.6 kWh heat per m3 biogas produced. Estimates of internal energy use is based on data from two joint scale biogas plants.

Avoided emissions of CH4 and N20
Fermentation of pig manure in biogas plants reduces the content of volatile solid in manure which drives the microbial processes that lead to CH4 and N2O during storage in storage tanks and manure channels and from the fields when the manure is applied as fertilizer. Quantities of emissions are determined by a wealth of factors in pig production, crop production and biogas production and estimates by Sommer et al. (2001) have been applied (assuming that CH4 is solely generated in manure canals, storage tanks etc. and that N2O is solely generated at the field), see Table below.
Substance No gasification of manure, g per ton manure (5% VS) Gasification with existing biogas technology,
g per ton manure (5% VS)
CH4 2700 1600
N2O 34 20

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.

Technical scope

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.

Representativity

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.

Validation

Data have been validated by comparing and combining information from different sources.

Inputs and outputs

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).

Unit

Quantity
Joint scale plants Farm scale plants
Inputs

Manure

ton
1.0 1.0
Electricity kWh 2 2
Heat kWh 13 13

Outputs

Products

Electricity

kWh
53 37
heat kWh 68 73
Degassed manure

ton
~1.0 ~1.0

Air emissions
SO2 g 9.7 9.7
NOx g 270 270
CO g 138 138
CH4 g 160 160
NMVOC g 7.1 7.1
N2O g 0.25 0.25
PM10 g 0.23 0.23
PM2.5 g 0.10 0.10

Avoided emissions
CH4 g 1100 1100
N2O g 14 14

Location in database: Waste treatment/Others/Energy prod. from pig manure…

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