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Life Cycle Assessment of Slurry Management Technologies

Annex D. Fibre Pellets combusted in Energy Plant – Life Cycle Inventory data

• D.1 System description
• D.2 In-house storage of slurry
• D.3 Storage of slurry in pre-tank
• D.4 Energy plant
• D.5 Outdoor storage of the liquid fraction
• D.6 Transport of the liquid fraction to field
• D.7 Field processes (Liquid fraction)
• D.8 Avoided production of mineral fertilisers
• D.9 Storage of fibre pellets
• D.10 Avoided heat production
• D.11 Storage of ash
• D.12 Transport of Ash to Field
• D.13 Field processes (ash)

D.1 System description

This annex contains Life Cycle Inventory data for fibre pellet production in a Samson Bimatech Plant MaNergy 225 producing heat from the fibre pellets.

The fibre pellets are produced in a number of steps, which include mechanical separation of pig slurry, drying of the fibre fraction and pressing the dried fibres into pellets. The pellets can be used for heat production at the farm in a Samson Bimatech Energy Plant. The heat production demands that the farmer needs the heat for heating, for example his private house or the housing units for farrowing sows with piglets. The phosphorus is left in the ash from the combusted fibre pellets might be used on the fields as fertiliser. The drying process of the wet fibres requires heat consuming app. 40 % of the energy from the fibre pellets. The Energy plant is described in section D.4.

Alternatively, the fibre pellets could be applied directly to the field as fertiliser utilizing the N, P and K content. This possibility is covered in Annex E.

The fibre pellets might also be transported to a central biogas plant and used for combined heat and power production. This possibility is covered in a following study.

The fibre pellets might also be used as fuel in central power plants. However, this opportunity is not covered in this study.

Samson Bimatech also produces a “stand-alone” mechanical separation plant, which is described in Annex C.

The scenario containing the Energy Plant producing energy based on fibre pellets is shown in figure D.1. The process numbers refer to the heading of the section in this Annex D.

Figure D.1. Flow diagram for the scenario with the Samson Bimatech Energy Plant (Annex D).

Click here to see Figure D.1

D.2 In-house storage of slurry

This process is identical to process C.2 in Annex C.

D.3 Storage of slurry in pre-tank

This process is identical to process C.3 in Annex C.

D.4 Energy plant

The processes in the energy plant are:

• Separation of the slurry into a fibre fraction and liquid fraction
• Drying of fibres in a tumble dryer
• Pellet production
• Combustion in gasifier and furnace and heat exchange in a boiler

The fibre pellet production starts by mechanical separation of the pig or cattle slurry. The fibre fraction is dried in a tumble dryer, and the dried, warm fibre fraction is then pressed into pellets. The pellets are cooled before storage for avoiding condensation in the outdoor silo.

In this Annex it is assumed that all the pellets are combusted in the Energy Plant, giving energy that can be used for heating the farmer’s private house. The consequence of the heat delivered is that the farmer saves energy for heating. Accordingly, this avoided heating is subtracted from the system. The nitrogen is transformed to N₂ and the phosphorus is left in the ash and might be used on the fields as fertiliser.

The flow diagram for energy plant is shown in figure D.2. The processes in the combustion plant are shown in figure D.3.

The composition of the slurry leaving the pre-tank is the same as the “ex housing” composition in the reference scenario, as it has been assumed that there are not loss or emissions during the storage in the pre-tank. As discussed in Annex C, this is a rough estimate as there is significant biological activity in the pre-tank and in the slurry system below the stables.

As the Samson Bimatech Energy Plant is based on the same mechanical separation as in Annex C, the mass balances for the separation are established in Annex C. In the Energy Plant, fibre pellets are produced from the fibre fraction from the mechanical separation.

Figure D.2. Flow diagram for Samson Bimatech energy plant.

Figure D.3. Principles for the Samson Bimatech energy plant

The electricity consumption for the plant corresponds to 19 kWh per 1000 kg pig slurry. The electricity is used for pumps, the screw separator, pellet machine (biggest motor in the plant) and other electric devices. This consumption is exclusive of stirring. The energy for drying the fibres comes from the heat production from the plant, corresponding to 120 MJ per 1000 kg slurry. The energy needed for heating and evaporating the water in the fibre fraction corresponds to 77 MJ per 51.98 kg fibre fraction (which corresponds to 1000 kg slurry) ¹. Accordingly, the energy for drying the fibres seems as a reasonable conservative estimate (when including the fact that a considerable amount of the heat is wasted to the surroundings.).

The amount of surplus heat produced is 128 MJ per 1000 kg slurry. The pellets works a “energy storage” – at winter time, they are all combusted for producing heat for the farmers private house, at summer time, the plant only produce the heat needed for drying the fibres, producing pellets that can be stored for next winter. All data are based on information from Samson Bimatech (personal communication with J Mertz, 2009).

It is assumed that all the heat is used at the farm. Calculation example: A farm with a production of 1400 fattening pigs per year (farm type 20 from Dalgaard et al., 2008, paper 3), with a production of 0.47 tons slurry per fattening pig the surplus heat production is in the magnitude of 84000 MJ per year ².

The heat consumption of a Danish household varies a lot with the age of the house, the insulation of the house and the number of people in the household. An estimate, based on data from Dong Energy (2009) shows that the heat consumption of an average Danish household is in the order of 46400-102000 MJ per year, assuming a house of 140 m² and a family of 4 persons ³. Accordingly, the slurry production from 1400 fattening pigs should be able to produce the heat needed for a family of 4 in a 140 m² house (build before 1962 and with some insulation) or for a larger house (if it is newer or the insulation is better) or for a family with 5-6 persons. The rough estimates show that the heat produced by the pig slurry probably is at the same magnitude as the need for heat by the farmer and his family.

The uncertainty on the measurements is considered to be rather high, as the emissions and energy production to a great degree depend on the actual slurry composition, and as it has not been possible to construct a “Norm Data Slurry” for testing in real life.

The above data corresponds to an Energy Plant where all the fibre pellets are used for combustion within the plant.

According to personal communication with J Mertz (2009), measurements of the emissions from the Energy Plant correspond to:

• 45 kg carbon dioxide (CO₂) per 1000 kg pig slurry
• 0.071 kg carbon monoxide (CO) per 1000 kg pig slurry
• 0.104 kg nitrogen oxides (NO_X) per 1000 kg pig slurry
• 0.0295 kg particulates (dust)

However, when using the mass balances, the combustion of the “reference pig slurry” based on the Norm Data pig slurry in this study, the CO2 emission is 36.13 kg per 1000 kg slurry ⁴, which is used in this study. The measurements by Samson Bimatech are made for at different slurry composition than the reference slurry in this study.

During drying of the fibres, ammonia (NH₃) volatiles. The air from the “drying tumbler” is sent to the combustion in the furnace, where it is combusted to nitrogen (N₂). Some of the ammonia (NH₃) is also absorbed by the liquid slurry fraction. The fraction that goes to the liquid fraction vs. the fraction that is combusted to N₂ in the furnace is not known. However, NH₃ is not emitted to the environment in significant amounts. There will probably be small amounts of NH₃ emissions (as no plant is 100% tight), however, the amounts are assumed to be insignificant compared to the emissions from the storage of slurry due. The assumption is supported to the fact that when being in the Energy Plant or outside the Energy Plant, the NH₃ odour is not strong – and by no means as strong as from the pre-tank or in the housing units.

The N₂ emissions are based on mass balances. According to Hjort-Gregersen and Christensen (2005) all nitrogen is emitted when the solid fraction is incinerated.

As decribed in the calculation for table C.3 in Annex C, the separation of the “Norm Data fattening pig slurry” will lead to 51.98 kg fibre fraction. The fibre fraction is heated and dried. It is assumed that the content of DM is not reduced during this process.

The fibre pellets has a varying content of DM and water, depending on the original raw slurry. In this study, data is based on measurements from “real life fibre pellets” produced on slurry from piglets. No data has been available on slurry from fattening pigs. The fibre pellets contain 88.93% DM (measurements performed by OK Laboratorium for jordbrug, 03-03-2009).J

Accordingly, 1000 kg of “Norm data” pig slurry “ex animal” gives 51.98 kg fibre fraction, leading to 23.199 kg fibre pellets ⁵. This corresponds to 20.62 kg DM.

The ash content of the fibre pellets correspond to 0.209 kg ash per kg DM. Accordingly, combustion of the fibre pellets from 1000 kg “Norm Data” pig slurry gives 4.3 kg ash (= 20.62 kg DM * 0.209 kg ash per kg DM).

Data on the loss of N during heating and pressing the fibre fraction into fibre pellets has not been available. Instead the theoretical loss of N has been calculated as the difference between the N in the fibre fraction (Norm data) and the fibre pellets (measurements). This is a rough approximation. The fibre fraction contains 0.37 kg N (per 1000 kg slurry) (see table C.3 in Annex C and table D.1 below). The measured fibre pellets contain 11.59 kg N per 1000 kg fibre pellets, i.e. 0.27 kg N per 1000 kg slurry ⁶. Accordingly, 0.1 kg N seems to be lost during the fibre pellet production. This loss of N is at the same magnitude as when calculation the loss from “measured fibre fraction” to “measured fibre pellets”. It is assumed that the loss of N is lost as NH₃ but as the air from the heating and fiber pellet pressing is gathered within the plant and used as combustion air for the combustion plant, the amounts of NH₃ is converted to NO_X and N₂ during the combustion.

When the fibre pellets are combusted, all N in the fibre pellets is emitted as NO_X or N₂. The ash do not contain N (at least not in significant amounts).

The emissions of N₂ is calculated as the difference between total loss of N and the NO_X emissions to 0.355 kg N₂ ⁷.

Table D.1. Mass balances for mechanical separation of pig slurry and fibre pellet production from fattening pigs. Per 1000 kg of slurry “ex housing”.

^a Loss of N, see text.

^b See text above

c The values in the column “Amount in fibre pellets after drying” divided by the mass, i.e. 1000 kg / 23.199 kg.

For a range of the emissions, data has not been available. It is assumed that the methane (CH₄) emissions are insignificant, as they are mainly caused by biological activity.

It is likely that some of the easy degradable fatty acids evaporate during the drying process, reducing the content of VS and DM in the fibre fraction. It has not been possible to find data on this.

The life cycle data for the Energy Plant are shown in table D.2

Table D.2. Life cycle data for treatment of slurry in the Samson Bimatech energy. Data per 1000 kg slurry treated.

The theoretical composition of the ash after combustion of the fibre pellets in the energy plant is shown in table D.3. As mentioned above, all nitrogen is emitted when the solid fraction is incinerated (Hjort-Gregersen and Christensen, 2005). As can be seen, it has not been possible to estimate the DM and the total mass of the ash. However, as the ash is just added to the liquid fraction and applied to the field, it is only important to know the amount of P, K and Cu and Zn in order to estimate the amount of these added to the field.

Table D.3. Mass balances for calculating the composition of the ash. Per 1000 kg of slurry “ex housing”.

A list of the materials used in for the construction of the Samson Bimatech Energy Plant, container and silo for storing the pellets are shown in table D.4. The consumption is based on rough estimates.

Table D.4 Material consumption and for the energy plant.

The density of slurry roughly 1000 kg per m³ used for these estimates (as it is rough estimates anyway).

Note 1): Estimated life time: 30 years. 10000 m³ slurry per year = 300000 m³ slurry in a life time.

Note 2): Life time: 1 year. 10000 m³ slurry per year = 10000 m³ slurry in a life time

Note 3): Life time: 0.5 year. 10000 m³ slurry per year = 5000 m³ slurry in a life time

D.5 Outdoor storage of the liquid fraction

This process is identical to process C.5 in Annex C.

D.6 Transport of the liquid fraction to field

This process is identical to process C.6 in Annex C.

D.7 Field processes (Liquid fraction)

This process is identical to process C.7 in Annex C.

D.8 Avoided production of mineral fertilisers

The “fertiliser replacement value” for N is calculated for the total system is based on Danish Law combined with “common practice”.

First, the “fertiliser replacement value” for the reference system is stated (see Annex A for further details):

• Mineral N fertiliser: 5.00 kg N per 1000 kg slurry ex storage [the value given by the Danish Norm Data, as explained in Annex A] * 1086 kg slurry ex storage per 1000 kg slurry ex animal * 75% [the replacement value according to (Gødskningsbekendtgørelsen, 2008] = 4.073 kg mineral N fertiliser
• Mineral P fertiliser: 1.04 kg P per 1000 kg slurry ex storage * 1086 kg slurry ex storage per 1000 kg slurry ex animal = 1.13 kg P
• Mineral K fertiliser: 2.60 kg K per 1000 kg slurry ex storage * 1086 kg slurry ex storage per 1000 kg slurry ex animal = 2.82 kg K

The “N fertiliser replacement value” for the “Energy Plant scenario” in this Annex is calculated as follows:

The amount of N in the liquid fraction of the slurry replace mineral N fertiliser by 85% according to Danish Law (Gødskningsbekendtgørelsen (2008), BEK 789, § 21 ⁸) when the fibre fraction is combusted. This 85% is different from the 75% used for the reference system above. Accordingly, 4.71 kg N [per kg liquid fraction after storage] * 948.02 kg liquid fraction after separation per 1000 kg slurry ex animal * 1086 kg liquid fraction ex storage per 1000 kg liquid fraction after separation * 85% = 4.122 kg N per kg slurry ex animal (i.e. for the total system) is replaced when the fibre fraction is combusted (see table C.8). This amount is almost identical to the mineral fertiliser value of the reference system (only 1% higher than for the reference system, 4.073 kg mineral N fertiliser).

The system does not affect the total amount of P and K applied to field, as these are in the ash from the combusted fibre pellets, and as the ash is added to the liquid fraction at the farm before application (i.e. at the same farm).

D.9 Storage of fibre pellets

The fibre pellets are typically stored in 4-6 months, shorter during the winter period.

The fibre pellets are stored in a silo. During storage, the fibre pellets are protected against rain water, however, they might absorb moisture from the air.

Data for the emissions during storage of the fibre pellets has not been immediate available. It has been considered to initiate measurements of the emissions, however, due to the fact that the pellets are relatively dry (10-17% water), it is estimated that the biological decomposition is relatively low. According to E Fløjgaard Kristensen (2009), there is no decomposition in straw when the water content is lower than 15%.

As denitrification is restricted to environments without oxygen, denitrification will not take place.

As mentioned in Annex C, Hansen et al. (2006) measured the emissions from covered and uncovered heaps of separated fibre fraction (which has a higher water content). As can be seen in table C.10, the emissions from covered heaps are relatively low, only a few % of the initial values. As the emissions from the fibre pellets are likely to be lower due to the low water content, it is assumed that the emissions from storage of the fibre pellets are insignificant.

D.10 Avoided heat production

The produced heat is assumed to be used for heating the housing systems for the private housing units, and the consequence of this is that heat production is avoided. The avoided heat will vary depending on the other possibilities the farmer has in mind. Accordingly, two alternatives for the avoided heat production are calculated, one for a fuel oil boiler and another based on wood chips. The SimaPro processes chosen for the avoided heat is:

• Heat, light fuel oil, at boiler 10kW condensing, non-modulating
• Heat, wood pellets, at furnace 15kW

D.11 Storage of ash

The ash is stored in a covered / closed container. It is assumed that the emissions from storage of ash are insignificant. If handled careless or if it is not covered it will produce a lot of dust.

D.12 Transport of Ash to Field

Often, the ash is mixed with the liquid fraction of the slurry and applied to the local fields. However, the high content of P in the ash gives a possibility of transporting P from areas with excess P to areas with P deficiency. It is assumed that it is not being done right now as ash is not regarded as a valuable saleable fertiliser product in Denmark right now.

D.13 Field processes (ash)

According to Birkmose and Zinck (2008), P and K in the ash are less soluble in water than superphosphate. However, tests shows that the plant uptake of P from ash is at the same level as in mineral fertilisers. Birkmose and Zinck (2008) refers to a test where the conclusion where that P and K in ash from combusted chicken manure has a plant availability of 90-100% compared to mineral fertiliser. Accordingly, P and K will be calculated as replacing K and P mineral fertilisers 1:1.

The assumption has been questioned by members of the steering group of the project. It is beyond the scope and budget of the project to perform further analysis of the availability of the P in the ash. Further scientific research is needed in the area.

Sensitivity analysis has been performed for that the fertiliser value of the ash is 0.

As the ash does not contain N and C there will be no contributions to emissions of CO₂, CH₄ og N-compounds.

Table D.5. Life cycle data for application of ash and field processes. All data per 1000 kg of slurry ex storage

[1] As can be seen below, 1000 kg of slurry corresponds to 51.98 kg fibre fraction. The fibre fraction has a DM content of 39.69%, i.e. water content of 60.31%. 51.98 kg * 60.31% = 31.35 kg water per 1000 kg slurry.
As can be seen below, 1000 kg of slurry corresponds to 23.19 kg fibre pellets with 88.93% DM, i.e. 11.07% water, i.e. a water content of 23.19 kg * 11.05% = 2.57 kg water per 1000 kg slurry.
The water that needs to be evaporated is 31.35 kg – 2.57 kg = 28.78 kg
All the water is assumed to be heated from 10°C to 100°C.
Energy for heating water:
31.35 kg * 4.187 kJ/kg°C * (100°C-10°C) / 1000 MJ/kJ= 11.8 MJ
Energy for evaporating water: 28.78 kg * 2.26 MJ/kg = 65 MJ
Total energy consumption: 65 MJ + 11.8 MJ = 76.8 MJ

[2] 1400 fattening pigs * 470 kg slurry per year * 128 MJ / 1000 kg slurry = 84000 MJ per year.

[3] According to Dong Energy (2009) http://www.dongenergy.dk/privat/energiforum/tjekditforbrug/typisk%20varmeforbrug/Pages/fjernvarme.aspx the heat consumption is in the order of:
200 kWh per m² per year for houses build before 1962 – and no insulation
170 kWh per m² per year for houses build before 1977 – and no insulation
110 kWh per m² per year for houses build after 1977
90 kWh per m² per year for houses build after 1998
The heat consumption for hot water (showers and hygiene) is in the order of 75 kWh per person per year.
Assuming 4 persons in the household, 140 m² house, old house, no insulation:
4 persons * 75 kWh/person + 140 m² * 200 kWh per m² per year = 28300 kWh = 101880 MJ per year.
Assuming 4 persons in the household, 140 m² house, new house:
4 persons * 75 kWh/person + 140 m² * 90 kWh per m² per year = 12900 kWh = 46440 MJ per year.

[4] As can be seen from table C.3 in Annex C, the fibre fraction from the separation contains 9.86 kg C per 1000 kg slurry. If all this is combusted to CO₂ the maximum CO₂ emission is: 9.86 kg * 44.01 g/mol / 12.01 g/mol = 36.13 kg CO₂. An insignificant part of this becomes CO.

[5] 1000 kg slurry gives 51.98 kg fiber fraction * 39.69% DM = 20.62 kg DM
The amount of fibre pellets can then be calculated to: 20.62 kg * 1000 kg / 889.3 kg DM per 1000 kg = 23.199 kg fiber pellets per 1000 kg slurry.

[6] 11.59 kg N per 1000 kg fibre pellets * 23.187 kg fibre pellets / 1000 kg slurry = 0.27 kg N per 1000 kg slurry.

[7] The amount of N in the fibre fraction corresponds to 0.37 kg N per 1000 kg slurry, see table D.1. Of these, 0.015 kg N is emitted as NO_X, see text above. The rest: 0.37 kg N – 0.015 kg N = 0.355 kg N is assumed to be lost as N₂.

[8] BEK 786, § 21: Ved beregning af forbruget af kvælstof i husdyrgødning skal følgende andele af det totale indhold af kvælstof i gødningen anvendes:
væskefraktion efter forarbejdning, hvor fiberfraktionen afbrændes: 85 pct

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Version 1.0 July 2009, © Danish Environmental Protection Agency