Process I.26: Avoided production and application of mineral fertilizers and yield changes

Life Cycle Assessment of Biogas from Separated slurry

I.26 Avoided production and application of mineral fertilizers

I.26.1 Calculation of the replaced amount of mineral fertiliser

This is calculated using the same method as in Annex F – however, at it involves incineration/ combustion of some of the fibre pellets, the replacement value for this part is accounted as 85% as in Annex D, see section D.8. This is in accordance with a special rule that only applies when slurry is separated and the fibre fraction is combusted. In that case, the fertiliser replacement value should be calculated as 85% of the liquid fraction (Gødskningsbekendtgørelsen (2008), paragraph 21^[13])

Table I.8. Replaced amount of mineral N fertiliser in Annex I. All calculations per 1000 kg slurry ex-animal

Calculations
Step 1: Substitution value for fibre fraction to biogas plant Amount of fibre pellets: 12.388 kg (see figure I.1) N in fibre fraction: 11.75 kg N per 1000 kg fibre fraction (see table I.1 in Annex I) Substitution value: 50% of 11.75 kg per 1000 kg fibre fraction * 12.388 kg fibre fraction / 1000 kg = 0.07278 kg N per 1000 kg slurry ex-animal. This is the substitution value that “belongs” to the fibre fraction that is sent to the biogas plant. This is “input” to the biogas plant.
Step 2: Acknowledging the above, make the weighted sum of the substitution values (liquid and fibre). For raw pig slurry, the substitution value is 75 %. Here rule (a) applies: “The sum of the “mineral fertiliser replacement value” of the outgoing fractions shall be the same as the “mineral fertiliser replacement value” of the ingoing slurry before separation”. The mineral fertiliser replacement value of untreated, raw pig slurry is calculated based on the Danish Norm Data (DJF, 2008), which was also done in Annex A (section A.6.1). From the Danish Norm Data tables, the farmer knows the value of 5.00 kg N per kg slurry ex storage (see also table A.5 and A.1). See further explanation in Annex F, section F.28. For the system, the mineral fertiliser substitution value is then: 5.00 kg N per 1000 kg slurry ex storage * 1086 kg slurry ex storage / 1000 kg slurry ex animal * 75% = 4.0725 kg N per 1000 kg slurry ex-animal. However, only 890.002 kg slurry is being separated. Furthermore, this is a bit more complicated than in Annex H, as 40% of the fibre pellets are combusted internally in the Energy Plant in order to provide heat for the drying of the fibre pellets, see section E.4 in Annex E. It means that when 12.388 kg fibre pellets are sent to the biogas plant (in Step 1 above), actually 12.388 kg * 100/60 = 20.65 kg fibre was produced from the separated fibre fraction. It also means, that 60% of the 890 kg separated slurry should be calculated in accordance with the “separation rules” (as in Annex H), and 40% of the 890 kg should be calculated in accordance with the “combustion rules” (as in Annex E). Calculation for the 60% in accordance with the “separation rules”: The “replacement value” of 60% of the 890 kg slurry BEFORE the separation is: 4.0725 kg N per 1000 kg slurry ex animal * 60% * 890 kg/1000 kg = 2.174715 kg N per 1000 kg slurry ex animal. From this, the replacement value from the fibre fraction in step 1 is subtracted: 2.174715 kg N - 0.07278 kg N = 2.101935 kg N. This is the “replacement value” for the liquid fraction corresponding to the amount of fibre pellets sent to the biogas plant. Calculation for the 40% in accordance with the “combustion rules”: The “replacement value” for the 40% of the 890 kg slurry is calculated as 85% of the N in the liquid fraction. It is calculated in relation to the “ex storage” values. The “ex storage” N for the liquid fraction is 4.712 kg N per 1000 kg liquid fraction ex storage (see table H.4 in Annex H). It is assumed that this value is *measured* on samples of the “ex storage” liquid fraction in correspondence with the Norm Data discussion in Annex F regarding “ex separation” or “ex storage” data. Accordingly, it is assumed that the farmer actually measures 4.712 kg N per 1000 kg liquid fraction ex storage at this point. The amount of liquid fraction ex storage is 916.301 kg. Accordingly, the “replacement value” for this is: 85% of 4.712 kg N per 1000 kg liquid fraction ex storage * (40% of 916.301 kg / 1000 kg) = 1.4680 kg N The total “replacement value” for the liquid fraction is then: 2.101935 kg N + 1.4680 kg N = 3.56994 kg N Mineral fertiliser replacement value for the liquid fraction (at the farm): 3.56994 kg N
Step 3: Make a weighed sum of the substitution values for the materials entering the biogas plant. Rule (b): “Mass balance in and out of Biogas Plant – i.e. the “mineral fertiliser replacement value” of the outgoing biomass is calculated in accordance with the ingoing biomass”. · The raw slurry going directly to biogas plant (without separation) has a mineral fertiliser replacement value of 4.0725 kg N per 1000 kg slurry (as described under step 2 above – 75% of 5.00 kg N ex storage). The amount of this raw slurry is 110 kg (see figure I.1. Its mineral fertiliser replacement value is: 4.0725 kg N per 1000 kg slurry * 110 kg slurry/1000 kg = 0.447975 kg N per 1000 kg slurry ex-animal. This is the substitution value for the raw slurry into the biogas plant. · At the plant, a biomass mixture is made from this raw slurry and the fibre fraction from step 1, so the substitution value for this input mixture is: 0.07278 kg N (fibre fraction, step 1) + 0.447975 kg N (raw slurry, see above) = 0.520755 kg N. This is the substitution value for the input biomass mixture going into the biogas plant, and accordingly also the substitution value for the degassed biomass mixture coming out of the biogas plant – i.e. the degassed biomass before separation. This is the substitution value for the end product before separation, used for the further calculations. Mineral fertiliser replacement value for the degassed biomass: 0.520755 kg N
Total amount of substituted mineral N fertiliser in the system 3.56994 kg N + 0.520755 kg N = 4.0907 kg N

I.26.2 Yield changes

Using the same methods as in section F.28.3^[14], the overall N difference between Scenario A and Scenario I is (in kg mineral N equivalent):

0.2307 kg N per 1000 kg slurry ex-animal for soil JB3;
0.2368 kg N per 1000 kg slurry ex-animal for soil JB6.

Accordingly, the extra corresponding wheat is:

For soil JB3: 0.2307 kg N surplus * 9.0 kg extra wheat/kg N surplus = 2.08 kg extra wheat (per 1000 kg slurry ex-animal).

For soil JB6: 0.2368 kg N surplus * 8.1 kg extra wheat/kg N surplus = 1.92 kg extra wheat (per 1000 kg slurry ex-animal).

I.26.3 Avoided P and K mineral fertilisers

As the amount of P and K in the slurry is the same as in Annex A, as there are no loss of P and K in the system, and as it is assumed that all the slurry fractions (the liquid fraction at the farm, the ash from the combusted fibre pellets and the degassed biomass) ends in the same area, the amount of replaced mineral fertilisers are the same as in Annex A, see section A.6.

[13] Gødskningsbekendtgørelsen (2008), paragraph 21: ”Ved beregning af forbruget af kvælstof i husdyrgødning skal følgende andele af det totale indhold af kvælstof i gødningen anvendes: 9) væskefraktion efter forarbejdning hvor fiberfraktionen afbrændes: 85%.”

[14] The values needed to apply the methodology presented in Annex F can be found in the sections for N leaching, namely I.7 (referring to H.7.6) and I.22.7.