Processes I.15 to I.18: Biogas production, co-generation of heat and power and avoided heat and electricity production

Life Cycle Assessment of Biogas from Separated slurry

I.15 Biogas production
I.16 Co-generation of heat and power from biogas
I.17 Avoided electricity production
I.18 Avoided heat production

I.15 Biogas production

I.15.1 Biogas principles

The principles for the biogas production in this annex is identical to the principles for the biogas production described in Annex F, see section F.15.1.

However, the composition of the biomass entering the biogas plants is changed. This is described in the following.

I.15.2 Biomass mixture entering the biogas plant

The biomass mixture input in the anaerobic digester is constituted of raw slurry (which composition is identical to the ex-housing pig slurry from table I.1) and fibre pellets (which composition is shown in table D.1 of Annex D). According to the composition and the degradability of both fractions, the amount of both fractions in the mixture is determined in order to obtain a biomass mixture that has a DM of approximately 10% during the digestion in the reactor, in order to obtain realistic production conditions (Jensen, 2009).

According to calculations provided by Xergi (Jensen, 2009), the 1000 kg mixture of the biomass entering the biogas plant consists of:

898.78 kg raw slurry (ex pre-tank)
101.22 kg fibre pellets

The mixture composition and mass balances is shown in table I.1 below.

Table I.1. Mass balances for the biomass entering the biogas plant, i.e. a combination of fibre pellets and raw pig slurry (slurry from fattening pigs).

			Mass balances
	Composition of the raw slurry ^a)	Composition of fibre pellets ^b)	Amount in untreated slurry	Amount in fibre pellets	Sum of mass	Composition of biomass entering the biogas plant ^c)
	[kg per 1000 kg slurry]	[kg per 1000 kg fibre pellets]	[kg]	[kg]	[kg]	[kg per 1000 kg biomass]
Total mass	1000 kg	1000 kg	898.78 kg	101.22 kg	1000 kg	1000 kg
Dry matter (DM)	69.7 kg	889.3 kg	898.78 /1000 * 69.7 kg = 62.645 kg	101.22 /1000 * 889.3 kg =90.015 kg	152.66 kg	152.66 kg
Total-N	5.48 kg	11.75 kg	898.78 /1000 * 5.48 kg = 4.925 kg	101.22 /1000 * 11.75 kg = 1.189 kg	6.115 kg	6.115 kg
Total-P	1.13 kg	4.433 kg	898.78 /1000 * 1.13 kg = 1.016 kg	101.22 /1000 * 4.433 kg = 0.4487 kg	1.464 kg	1.464 kg
Potassium (K)	2.85 kg	3.563 kg	898.78 /1000 * 2.85 kg = 2.562 kg	101.22 /1000 * 3.563 kg = 0.361 kg	2.922 kg	2.922 kg
Carbon (C)	33.3 kg	424.88 kg	898.78 /1000 * 33.3 kg = 29.929 kg	101.22 /1000 * 424.88 kg = 43.006 kg	72.936 kg	72.936 kg
Copper (Cu)	0.03 kg	0.0595 kg	898.78 /1000 * 0.03 kg = 0.02696 kg	101.22 /1000 * 0.0595 kg =0.006023 kg	0.03299 kg	0.03299 kg
Zinc (Zn)	0.0894 kg	0.2428 kg	898.78 /1000 * 0.0894 kg = 0.08035 kg	101.22 /1000 * 0.2428 kg = 0.02458 kg	0.1049 kg	0.1049 kg

a) Same as in table A.1 (which is from ex-housing slurry in Annex A)

b) Same as in table D.1 from Annex D.

c) Composition of biomass mixture of slurry and fibre pellets entering the biogas plant, i.e. the biomass input into the digester

In this project, the functional unit is “Management of 1000 kg slurry ex-animal”. The biogas production therefore has to be related to the functional unit by the use of mass balances, i.e. the values expressed per 1000 kg of biomass mixture must be converted in order to be expressed per 1000 kg of slurry ex-animal. To do this, the amount of biomass mixture (898.78 kg raw slurry plus 101.22 kg fibre pellets) used per 1000 kg of slurry ex-animal must be calculated. This calculation can be done in 6 steps:

Step 1: Defining the total amount of “ex-animal” slurry involved – contribution from the raw slurry input
The 898.78 kg raw slurry entering the biogas plant is “ex pre-tank ”, which corresponds to the same amount of “ex-animal” slurry, since it is assumed that no water was added during the storage in the pre-tank. Therefore, the amount of raw slurry ex-animal from this input is 898.78 kg.
Step 2: Defining the total amount of “ex-animal” slurry involved – contribution from the input of fibre pellets
The 101.22 kg of fibre pellets origins from 7272.07 kg slurry ex-housing (as 1000 kg raw slurry gives 13.919 kg fibre pellets, see section E.4 in Annex E) ^[2].
Step 3: Defining the total amount of “ex-animal” slurry involved – sum of the two biomasses input
It means that a biomass mixture of 898.78 kg raw slurry + 101.22 kg fibre fraction origins from: 898.78 kg + 7272.07 kg = 8170.85 kg pig slurry ex-animal.
Step 4: Relating the 898.78 kg of raw slurry input to the functional unit (1000 kg slurry ex-animal)
As the functional unit in this study is 1000 kg slurry ex-animal, the amount of “raw slurry for biogas mixture” is: 898.78 kg *1000 kg / 8170.85 kg = 109.998 kg raw slurry (ex pre-tank) per functional unit (1000 kg slurry ex-animal) (and 109.998 kg raw slurry ex pre-tank corresponds to 109.998 kg slurry ex-animal, as there is no water addition during the in-house storage).
Step 5: Relating the 101.22 kg of fibre pellets input to the functional unit (1000 kg slurry ex-animal)
The amount of fibre fraction needed for the biogas mixture is: 101.22 kg *1000 kg / 8170.85 kg = 12.3879 kg fibre pellets per 1000 kg slurry ex-animal (and 12.388 kg fibre pellets corresponds to 890.002 kg pig slurry ex-animal ^[3]).
Step 6: Total biomass input needed per functional unit
The biomass needed for the process is then 109.998 kg pig slurry (ex pre-tank) + 12.3879 kg fibre pellets = 122.386 kg “biomass mixture” entering the biogas plant per 1000 kg of slurry “ex-animal”.

The mass flows in figure I.1 are based on the mass flows calculated above.

I.15.3 Energy consumption during biogas production and heat value of the biogas produced

The energy parameters for the biogas production are calculated using the same principles and calculation methods as in Annex F. However, there is one important difference: The specific methane yields for the fibre pellets is 187 Nm³ per ton (170 Nm³ per ton from primary digester + 10 % extra from secondary step), i.e. the same as for the fibre fraction in Annex H (the fibre fraction from the Samson Bimatech separation). These data are based on Møller (2007) (the same reference as used for the corresponding data in Annex F). The fibre fraction data used are those referred to as “solids from separation by mechanical equipment (solid 2, 3)” by Møller (2007).

Accordingly, the data for the calculations are:

The amount of VS corresponds to 80 % of DM.
The specific methane yields for the untreated pig slurry is 319 Nm³ per ton (290 Nm³ per ton from primary digester + 10 % extra from secondary step)
The specific methane yields for the fibre fraction is 187 Nm³ per ton (170 Nm³ per ton from primary digester + 10 % extra from secondary step).
The biogas is constituted of 65 % CH₄ and 35 % CO₂ (table F.17 in Annex F).

The calculation principles are explained in Annex F and will not be repeated here. The results of the calculations are:

A total of 45.31 Nm³ biogas is produced per 1000 kg of “biomass mixture” ^[4].
The biogas density being 1.158 kg/Nm³, a mass of 52.47 kg of biogas per 1000 kg of “biomass mixture” is therefore produced.
The heat value of the biogas corresponds to 1053.8 MJ per 1000 kg biomass mixture”^[5].
During the process, both heat and electricity are consumed. See further description in section F.15.3. The electricity therefore consumed for producing the biogas corresponds to 5.85 kWh per 1000 kg “biomass mixture”^[6].
The heat consumption for the process is 116.49 MJ per 1000 kg “biomass mixture” ^[7].

I.15.4 Emissions of CH₄ and CO₂

As the biogas plant is constructed tight in order to reduce losses of biogas, the emissions to air during the digestion are assumed to be rather small. As described in Annex F, section F.15.4, the emission of CH₄ from the biogas plant is estimated as 1% of the produced methane.

For the emissions of CO₂, Jungbluth et al. (2007) used an emission of 1 % of the produced carbon dioxide in the biogas. In this project, the calculated ratio between emissions of CO₂ and CH₄ in anaerobic conditions will be used, i.e. 1.42 kg CO₂ per kg CH₄ (see section F.5.5 in Annex F). This, in the present case, corresponds to 0.96 % of the CO₂ produced, which is in the same magnitude as proposed by Jungbluth et al. (2007).^[8] .

I.15.5 Emissions of NH₃ and N₂O

As described in Annex F, section F.15.5, the emissions of NH₃ and N₂O from the biogas plant are assumed to be insignificant.

I.15.6 Life cycle data and mass balances for anaerobic digestion process

In this scenario, the biogas is not upgraded (which is necessary if it is going to be used as fuel for transport). The biogas is used for co-production of electricity and heat. Table I.2 presents the life cycle data for the anaerobic digestion process.

Table I.2. Life cycle data for the anaerobic digestion process. Data per 1000 kg biomass mixture into the biogas plant.

	Biomass mixture	Comments
Input
Biomass mixture	1000 kg	All emissions are calculated relatively to 1000 kg “biomass mixture” (i.e. 89.88% raw slurry and 10.12% fibre pellets)
Output
Biogas	52.47 kg i.e. 45.31 Nm³	Density 1.158 kg/Nm³, see text.
Degassed slurry	947.53 kg	Gas output is dried. No water loss. Therefore, the only loss is the mass of the biogas : 1000 kg – 52.47 kg= 947.53 kg
Energy consumption
Electricity	5.85 kWh	Estimated own consumption of electricity: 5 % of net production, engine efficiency of 40 %, see text. Electricity from the grid.
Heat	116.49 MJ	Heating the biomass from 8°C to 37°C, see text. Heat from the co-generation unit.
Emissions to air
Carbon dioxide (CO₂)	0.314 kg	1.42 kg CO₂ per kg CH₄. 0.211 kg CH₄ * 1.42 kg CO₂ per kg CH₄.= 0.314 kg CO₂
Methane (CH₄)	0.211 kg	1% of the methane content of the biogas is assumed to be emitted to the environment. 45.31 Nm³ biogas * 65% CH₄ * 0.717 kg/Nm³ * 1% = 0.211 kg CH₄.
Ammonia (NH₃-N)		Assumed to be insignificant, see text
Nitrous oxide (N₂O-N)		Assumed to be insignificant, see text
Nitrogen oxides (NO_X)		Assumed to be insignificant, see text
Nitrogen monoxide(NO)		Assumed to be insignificant, see text
Nitrogen(N₂)		Assumed to be insignificant, see text
Hydrogen sulphide (H₂S)		Assumed to be insignificant compared to the emissions from the following co-production of electricity and heat.
Odour		No data
Emissions to water
		No emissions to water
Emissions to soil
		No emissions to soil

The composition of the degassed slurry after biogas production is shown in table I.3. It is based on mass balances from data presented in table I.2 for the total mass, the DM content and the total N.

Table I.3. Mass balances for the biogas mixture before and after the biogas plant

	Composition of Mixture of slurry and fibre pellets entering the biogas plant	Mass balance: Change during biogas production	Mass balance: Amount after biogas production	Composition of Degassed biomass after biogas production ^a)
	[kg per 1000 kg biomass mixture]	[kg]	[kg]	[kg per 1000 kg degassed biomass]
Total mass	1000 kg	- 52.47 kg^b)	947.53 kg	1000 kg
Dry matter (DM)	152.66 kg	- 52.47 kg ^c)	100.19 kg	105.74 kg
Total-N	6.115 kg	No change	6.115 kg	6.4533 kg
Total-P	1.464 kg	No change	1.464 kg	1.5454 kg
Potassium (K)	2.922 kg	No change	2.922 kg	3.0840 kg
Carbon (C)	72.936 kg	- 24.61 kg ^d)	48.322 kg	50.9975 kg
Copper (Cu)	0.03299 kg	No change	0.03299 kg	0.0348 kg
Zinc (Zn)	0.1049 kg	No change	0.1049 kg	0.1107 kg

a) All the data are the same as in the precedent column, but adjusted to be expressed per 1000 kg of degassed mixture, instead of per 947.53 kg of degassed mixture.

b) This loss corresponds to the biogas produced, expressed in mass terms.

c) No water loss and therefore change in dry matter is equal to change in total mass.

d) This corresponds to the losses in the biogas itself and the losses that occurred during the digestion process:
Losses in the biogas are calculated as the sum of CH₄-C and CO₂-C: (45.31 Nm³ biogas * 65 % CH₄ * 0.717 kg CH₄/Nm³) * (12.011 g/mol /16.04 g/mol) + (45.31 Nm³ biogas * 35 % CO₂ * 1.977 kg CO₂/Nm³) * (12.011 g/mol /44.01 g/mol) = 24.4 kg C
Losses from the digestion process are the aggregated losses as CO₂-C + CH₄-C: 0.313 kg CO₂ * (12.011 g/mol /44.01 g/mol) + 0.211 kg CH₄ * (12.011 g/mol /16.04 g/mol) = 0.24 kg C
Total C loss : 23.2 kg C + 0.23 kg C = 24.6 kg C.

I.15.7 Material consumption for the anaerobic digestion plant

The materials for the anaerobic digestion plant are identical to the material consumption for the anaerobic digester in Annex F, section F.15.7, see this.

I.16 Co-generation of heat and power from biogas

Also in this annex it is assumed that the biogas produced is used for the production of electricity and heat. The technology and basic methods for calculations are the same as in Annex F, however, the biogas production per 1000 kg slurry “ex-animal” is somewhat different.

As detailed in section I.15.3, the system produces 45.31 Nm³ biogas per 1000 kg of biomass mixture. As there are 122.386 kg biomass mixture per 1000 kg slurry ex-animal (see detailed calculation in section I.15.2), this corresponds to a production of 5.545 Nm³ biogas per 1000 kg slurry ex-animal^[9].

The net energy production after the co-generation unit is therefore 59.3 MJ heat plus 14.33 kWh electricity (51.59 MJ) per 1000 kg slurry ex-animal^[10].

As also detailed in section I.15.3, some of the produced heat is used to fulfil the heat demand of the biogas production. The amount of heat needed for this purpose is 116.49 MJ per 1000 kg mixture input, which corresponds to 14.257 MJ per 1000 kg slurry ex-animal^[11]. The heat consumption by the biogas plant thus corresponds to 14.257 MJ/ 59.329 MJ = 24.0 % of the heat produced. The surplus heat for the system is 59.329 MJ – 14.257 MJ = 45.072 MJ for the total system.

As described in Annex F (section F.16), it is considered that only 60 % of the surplus heat produced at the biogas plant is used, the remaining 40 % being wasted. Therefore, out of the 45.072 MJ per 1000 kg slurry ex-animal of net surplus heat, only 27.04 MJ (i.e. 45.072 MJ * 60%) are used to fulfil the heat demand. The wasted heat thus corresponds to 18.0 MJ.

The energy produced from the biogas can be summarized as:

14.33 kWh electricity (51.59 MJ) per 1000 kg slurry ex-animal, all used through the national electricity grid, low voltage electricity.
59.3 MJ heat per 1000 kg slurry ex-animal, of which:
- 14.257 MJ per 1000 kg slurry ex-animal is used for fulfilling the heat demand of the biogas process itself;
- 27.04 MJ per 1000 kg slurry ex-animal is used to fulfil national heat demand;
- 18.0 MJ per 1000 kg slurry ex-animal is wasted.

As for Annex F, the emissions from the biogas engine were estimated from recent data from the Danish National Environmental Research Institute (DMU, 2009) (plants in agriculture, combustion of biogas from stationary engines).

Table I.4 presents the life cycle data related to the co-generation of heat and power from the biogas engine.

Table I.4. Life cycle data for the co-generation of heat and power from biogas. Data per 1 MJ energy input.

	Per MJ input	Comments
Input
Biogas	0.043 Nm³ (1 MJ)	Amount of biogas corresponding to an energy content of 1 MJ input.[1 MJ/23.26 MJ/Nm³] = 0.043 Nm³.
Co-generation unit	5.0 E-9 p	Engine, generator, electric parts etc. divided by lifetime (Data from Jungbluth et al., 2007, table 13.20 of page 259)
Lubricating oil	3.0 E-5 kg	Production and disposal of used mineral oil included (Data from Jungbluth et al., 2007, table 13.20 of page 259)
Output
Heat Of this: Used for biogas plant Surplus heat	0.46 MJ 0.110 MJ 0.35 MJ	The efficiency of the heat production is 46% (see table I.17) The heat consumption used by the biogas plant is 24.0 % : 0.46 MJ * 24.0 % = 0.110 MJ. Surplus heat: 0.46 MJ – 0.110 MJ = 0.35 MJ
Electricity	0.40 MJ	The electricity efficiency is 40% (see Annex F, table I.19)
Emissions to air
Carbon dioxide (CO₂)	8.36 E-2 kg	DMU (2009)
Carbon monoxide (CO)	2.73 E-4 kg	DMU (2009)
Methane (CH₄)	3.23 E-4 kg	DMU (2009)
Non-methane volatile organic compounds (NMVOC)	1.40 E-5 kg	DMU (2009)
Ammonia (NH₃-N)		No data
Nitrous oxide (N₂O)	1.59 E-7 kg	DMU (2009)
Nitrogen oxides (NO_X)	5.40 E-4 kg	DMU (2009)
Nitrogen monoxide(NO)		No data
Nitrogen(N₂)		No data
Particulates PM₁₀ PM_2.5	4.51 E-7 kg 2.06 E-7 kg	DMU (2009)
Hydrogen sulphide (H₂S)		No data
Sulphur dioxide (SO₂)	1.92 E-5 kg	DMU (2009)
NMVOC	1.40 E-5 kg	DMU (2009)
Odour		No data
Emissions to water
		No emissions to water
Emissions to soil
		No emissions to soil

I.17 Avoided electricity production

The electricity that is replaced is the marginal electricity as described in Annex A, following the same principles as in Annex F, see section F.17.

However, the amounts of replaced electricity are different than in Annex F.

I.18 Avoided heat production

The avoided heat production is described in Annex F, section F.18. However, the amounts of replaced heat are different than in Annex F.

[2] According to the footnotes for table C.3 in Annex C, the separation of 1000 kg pig slurry gives 51.98 kg fibre fraction. From Annex D (table D.1) and the text for this, it can be seen that when 1000 kg of pig slurry undergoing mechanical separation and pellet production, 23.199 kg fibre pellets are produced. As described in section E.4, Annex E, 40% of the fibre pellets are used for drying the fibre pellets, i.e. the amount left is 23.199 kg fibre pellets * 60% = 13.919 kg fibre pellets.

Accordingly, in order to produce 101.22 kg fibre pellets for the biogas plant, 1000 kg pig slurry * 101.22 kg/13.919 kg = 7272.07 kg raw slurry is needed.

[3] 12.388 kg fibre pellets * (1000 kg slurry ex-animal / 13.919 kg fibre pellets) = 890.002 kg pig slurry ex-animal.

[4] From pig slurry: 898.78 kg slurry* 69.7 kg DM/ 1000 kg slurry * 0.8 kg VS per kg DM * 319 Nm³ CH₄ per ton VS / 0.65 Nm³ CH₄ per Nm³ biogas * ton/1000 kg = 24.60 Nm³ biogas.

From fibre pellets: 101.22 kg fibre pellets * 889.3 kg DM/1000 kg fibre fraction * 0.8 kg VS per kg DM * 187 Nm³ CH₄ per ton VS / 0.65 Nm³ CH₄ per Nm³ biogas * ton/1000 kg = 20.72 Nm³ biogas.

Total biogas produced per 1000 kg of “biomass mixture”: 45.31 Nm³ biogas (24.60 Nm³ from slurry + 20.72 Nm³ from fibre fraction).

[5] This is calculated using the heat value and the total biogas produced: 6.46 kWh/Nm³ biogas (see table F.19) * 45.31 Nm³ biogas/1000 kg “biomass mixture” * 3.6 MJ/kWh = 1053.8 MJ/1000 kg “biomass mixture”.

[6] Estimated internal consumption of electricity in kWh per 1000 kg biomass mixture : 45.31 Nm³ biogas/1000 kg biomass mixture x 6.46 kWh/Nm³ biogas x 40 % engine power efficiency x 5 % internal consumption = 5.85 kWh per 1000 kg biomass mixture.

[7] It is assumed that the average temperature for the biomass is 8 °C when entering the process and that it is heated to 37°C (the process temperature). Specific heat is calculated based on the content of DM and water (calculated as 1-DM), assuming that the specific heat for DM corresponds to 3.00 kJ/kg°C and to 4.20 kJ/kg°C for water. As the DM for biomass mixture is 152.66 kg/1000 kg biomass mixture (table I.6), it involves that the water content is 1000kg – 152.66 kg = 847.34 kg/1000 kg biomass mixture. The heat consumption for heating the biomass mixture from 8°C to 37°C is thus:

For DM: 152.66 kg DM/1000 kg biomass mixture * 3.00 kJ/kg DM*°C * (37-8) °C = 13281.4 kJ/1000 kg biomass mixture;

For water : 847.34 kg water/1000 kg biomass mixture * 4.20 kJ/kg DM*°C * (37-8) °C = 103206 kJ/1000 kg biomass mixture;

Total: (13281.4 kJ + 103206 kJ) kJ/1000 kg biomass mixture * MJ/1000 kJ = 116.49 MJ/1000 kg biomass mixture.

[8] When calculating in accordance with the biogas composition, which is defined as 65% CH₄ and 35% CO₂ (see table F.19 in Annex F), then the ratio is 1.477 kg CO₂ per kg CH₄: 0.65 mol CH₄-C corresponds to 0.35 mol CO₂-C i.e.

1 mol CH₄-C gives 0.538 mol CO₂-C (= 0.35/0.65)

Accordingly: 16.04276 g CH₄/mol = 0.538 * 44.0098 g CO₂/mol

i.e. 1 g CH₄ = 1.477 g CO₂

1.42/1.477 = 96%

[9] 122.386 kg biomass mixture (per 1000 kg slurry ex-animal) * 45.31 Nm³ / 1000 kg biomass mixture = 5.545 Nm³ biogas per 1000 kg slurry ex-animal.

[10] Heat produced: 5.545 Nm³ biogas (per 1000 kg slurry ex-animal) * 23.26 MJ/ Nm³ biogas (heat value of the biogas, see table F.19 in Annex F) * 0.46 (engine efficiency for heat) = 59.329 MJ heat per 1000 kg slurry ex-animal.

Electricity produced: 5.545 Nm³ biogas (per 1000 kg slurry ex-animal) * 23.26 MJ/ Nm³ biogas (heat value) * 0.40 (engine efficiency for electricity) = 51.59 MJ electricity per 1000 kg slurry ex-animal. This corresponds to 51.59 MJ * MJ/3.6 kWh = 14.33 kWh electricity per 1000 kg slurry ex-animal.

[11] There is 122.386 kg biomass mixture per 1000 kg slurry ex-animal, see section I.15.2. The heat required for the process is 116.49 MJ per 1000 kg mixture (section I.15.3). The heat needed per functional unit corresponds to: 122.386 kg biomass mixture / 1000 kg slurry ex-animal * 116.49 MJ / 1000 kg biomass mixture = 14.257 MJ per 1000 kg slurry ex-animal.