Process G.28: Avoided production and application of mineral fertilizers and yield changes

Life Cycle Assessment of Biogas from Separated slurry

G.28 Avoided production and application of mineral fertilizers

G.28 Avoided production and application of mineral fertilizers

G.28.1 General description

In this scenario, nitrogen is spread to the field at 3 points: Via the liquid fraction (section G.7), via the degassed fibre fraction (section G.23) and via the degassed liquid fraction (section G.27).

Before continuing this section, it is very important to clarify the difference between “The fertiliser value” and “The replaced amount of mineral fertiliser”:

The agronomic fertiliser value regards the nutritional value for the plants. It is estimated on the basis of the N amount, origin (pig, cattle) and content of organic matter in the slurry. This is used for calculating the yield increase. An increase in the crop production occurs if the agronomic fertiliser value of organic fertilisers applied in scenario G (all together for the total system) is higher than the fertiliser value for the reference scenario A, and vice versa. The calculations regarding the agronomic fertiliser value aim at representing the behaviour of the biophysical system.
The replaced amount of mineral fertiliser is the amount of mineral fertiliser that the farmer is not allowed to bring out to the field, due to spreading the slurry (i.e. the substituted amount of mineral fertiliser). These calculations are based on Danish laws as well as on what the farmers actually do in practice. It has not a one-to-one relation to the net mineralisation in the growing season caused by the animal slurry, so it may differ from “real plant availability”.

The agronomic fertiliser value and the replaced amount of mineral fertiliser are hence two different things, and in consequence they may also differ numerically.

The calculations of the replaced amount of mineral fertiliser (based on Danish law) are explained in section G.28.2. The agronomic fertiliser value and the yield changes are explained in section G.28.3.

G.28.2 Calculation of the replaced amount of mineral fertiliser

As explained in Annex F, the starting point for calculating the replaced amount of mineral fertiliser is the Danish law and the guidelines for this (Gødskningsloven (2006), Gødskningsbekendtgørelsen (2008), and Plantedirektoratet (2008b)).

The foundation for the law is that there is a “quota” of nitrogen for each field, depending on the crop and soil type ^[22]. In addition to this, there is an upper limit for how much of the “nitrogen quota” that can be applied as animal slurry.

When applying cattle slurry, the N in the slurry replace 70% mineral fertiliser, which means that if applying 100 kg N in slurry, the farmer has to apply 70 kg mineral N fertiliser less (Gødskningsbekendtgørelsen (2008), paragraph 21). For example, if the farmer has a field with winter barley, and the soil type is JB3, the farmer has a “Nitrogen quota” for that field at 149 kg N per ha (Plantedirektoratet, 2008). If the farmer applies 100 kg N per ha as dairy cow slurry, this accounts for 70 kg N per ha, which means that the farmer is allowed to apply the remaining 149 kg N per ha – 70 kg N per ha = 79 kg N per ha as mineral N fertiliser.

However, for separated slurry and for degassed slurry, the rules are not as straightforward.

For separated slurry, the “mineral fertiliser replacement values” of the separated fractions is set by the producer (i.e. the farmer or the biogas plant that separate the slurry). However, they have to follow the rule of conservation:

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^[23].

For degassed biomass from biogas plants, there are three rules that can be applied, and the biogas plant can choose which one to apply^[24]:

The “mineral fertiliser replacement value” of the outgoing biomass is calculated in accordance with the ingoing biomass (“rule of conservation”).
The producer of the degassed biomass (i.e. the biogas plant staff) sets the “mineral fertiliser replacement value” for the degassed biomass based on representative measurement of samples of the degassed biomass.
Or, the “mineral fertiliser replacement value” for the degassed biomass can be set to 75% as for pig slurry.

In the following, calculations have been performed for some of the rules mentioned above.

When following rule a) + b) strictly, the “mineral fertiliser replacement value” is calculated as follows:

The replaced amount of mineral N fertiliser for Annex F is based on 4 steps:

Step 1: Use a substitution value of 50% for the fibre fraction of slurry.
Step 2: Acknowledging the above, make the weighted sum of the substitution values (liquid and fibre), i.e. 70 % for cattle and 75 % for pig.
Step 3: Make a weighed sum of the substitution values for the materials entering the biogas plant – this is the substitution value for the end product before separation.
Step 4: Use a substitution value of 50% for the fibre fraction of the degassed material from the biogas plant, and put “the rest” upon the liquid fraction (much like step 1 and 2).

The calculations for scenario F are shown in table G.36.

Table G.36. Replaced amount of mineral N fertiliser in Annex G. All calculations per 1000 kg slurry ex-animal

Calculations

Step 1: Substitution value for fibre fraction to biogas plant
Amount of fibre fraction: 133.995 kg (see figure G.1). N in fibre fraction: 10.96 kg per 1000 kg fibre fraction (see table G.6). Substitution value: 50% of 10.96 kg per 1000 kg fibre fraction * 133.995 kg fibre fraction / 1000 kg = 0.73429 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 cattle slurry, the substitution value is 70 %.
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 dairy cow 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 6.02 kg N per kg slurry ex storage (see also table A.6 and A.2). The Danish Norm Data is what the farmer use for the accounts^[1]: 6.02 kg N per 1000 kg slurry ex storage (table A. 2). However, there is only 463.347 kg slurry being separated (see figure G.1).

For the system, the mineral fertiliser substitution value is then: 6.02 kg N per 1000 kg slurry ex storage * 1044 kg slurry ex storage / 1000 kg slurry ex animal * 70% = 4.3994 kg N per 1000 kg slurry ex-animal.

However, there is only 463.347 kg slurry being separated (see figure F.1), i.e. 4.3994 kg/1000 kg * 463.347 kg = 2.03846 kg N.

Of this 2.03846 kg N, 0.73429 kg N belongs to the fibre fraction (as calculated in step 1).
The difference i.e.: 2.03846 kg N – 0.73429 kg N = 1.30417 kg N belongs to the liquid fraction.

Mineral fertiliser replacement value for the liquid fraction (at the farm): 1.30417 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 4.3994 kg N per 1000 kg slurry (as described under step 2 above – 70% of 6.02 kg N ex storage). The amount of this raw slurry is 536.653 kg (see figure G.1). Its mineral fertiliser replacement value is: 4.3994 kg N per 1000 kg slurry * 536.653 kg slurry/1000 kg = 2.36095 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: 0.73429 kg N (fibre fraction, step 1) + 2.36095 kg N (raw slurry, see above) = 3.09524 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 value is used for the further calculations.

Step 4a: Use a substitution value of 50% for the fibre fraction of the degassed material from the biogas plant (like step 1)
Amount of degassed fibre fraction: 156.443 kg (see figure G.1). N in fibre fraction: 6.60 kg per 1000 kg fibre fraction (see table G.27). Substitution value: 50% of 6.60 kg per 1000 kg fibre fraction * 156.443 kg fibre fraction / 1000 kg = 0.51626 kg N

Mineral fertiliser replacement value the degassed fibre fraction: 0.51626 kg N

Step 4b: Calculation of the substitution value for the liquid fraction as “the rest”.
Here, rule (a) applies again: “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”.

Total substitution value out of biogas plant = total substitution value in biogas plant, as calculated in step 3: 3.09524 kg N.
Substitution value for the liquid fraction = total from biogas plant – fibre fraction (from step 4a) = 3.09524 kg N - 0.51626 kg N = 2.57898 kg N

Mineral fertiliser replacement value for the degassed liquid fraction (after the biogas plant: 2.57898 kg N

Total amount of substituted mineral N fertiliser in the system
1.30417 kg N + 0.51626 kg N + 2.57898 kg N = 4.3994 kg N

[1] It should be noted, that it might be more logical to use “ex housing data” for separation, but the farmers do not have information from the Norm Data on these. Furthermore, it can be argued that the loss of N during the outdoor storage is relatively low (2% according to the Norm Data), accordingly, it does not make a big difference whether the calculations are based on “ex housing” data or “ex storage” data. Accordingly, the N substitution value of the untreated slurry (before separation) is based on the Danish Norm Data (DJF, 2008).

Note that this 4.3994 kg N is identical to 70% of the initial 6.02 kg N per 1000 kg slurry ex storage * 1044 kg slurry ex storage per 1000 kg slurry ex animal. This is logical, as this is the amount that is “divided” into the different fractions when applying rule (a) and rule (b) which both conserve the masses.

It should also be noted, that this amount is identical to the amount of substituted mineral N fertiliser for the reference system in Annex A.

As this study is a comparison, the calculations of the replaced amount of mineral N fertiliser are shown in table G.37, based on the explanations in Annex A, section A.6.1.

Table G.37. Replaced amount of mineral N fertiliser in scenario A

Fraction	Calculations	Replaced amount of mineral N fertiliser [kg N per 1000 kg slurry ex animal]
Slurry	Calculations for Annex A, see explanations in section A.6.1: 70% of 6.02 kg N (per 1000 kg slurry ex storage) * 1044 kg slurry ex storage / 1000 kg slurry = 4.3994 kg	4.3994 kg N

G.28.3 Yield changes

The yield changes are calculated as a function of the agronomic fertiliser value in order to determine the “extra amount of N” available for crop uptake. This is translated to a response in extra wheat, as in Annex B of Wesnæs et al. (2009). This means that the production of this extra wheat does not have to be produced somewhere else in Denmark and can consequently be deduced from the system.

The yield change calculations are explained in Annex F, section F.28.3. Utilising the same methods as in section F.28.3^[25], the overall N difference between Scenario A and Scenario G is (in kg mineral N equivalent):

0.4663 kg N per 1000 kg slurry ex-animal for soil JB3;
0.4432 kg N per 1000 kg slurry ex-animal for soil JB6.

Accordingly, the extra corresponding wheat is:

For soil JB3: 0.4663 kg N surplus * 9.0 kg extra wheat/kg N surplus = 4.20 kg extra wheat (per 1000 kg slurry ex-animal).

For soil JB6: 0.4432 kg N surplus * 8.1 kg extra wheat/kg N surplus = 3.59 kg extra wheat (per 1000 kg slurry ex-animal).

This same procedure was also applied with the 100 years values for both soil types.

G.28.4 Avoided P and K mineral fertilisers

In this scenario, it is assumed that the degassed fibre fraction is transported to fields with lack of phosphorous. This is in fact the whole purpose of separating the degassed biomass after the biogas plant: To collect the main part of the phosphorous in order to increase the possibilities for using this as fertiliser where P is need (at fields with P deficiency) instead of at the fields close to the pig farm areas where there is surplus phosphorus in the soil (mainly in Jutland).

Accordingly, as the degassed fibre fraction (which contains the main part of the phosphorous) is transported to fields with phosphorous deficiency, it is assumed that 100 % of the phosphorous in this fraction replace mineral P fertiliser.

It is assumed that the same, i.e. 100 % replacement, applies for potassium (K). The actual amount of K substituted may in fact be less than 100 % if the K applied is greater than the crops needs. However, as previous modelisations (e.g. Wesnæs et al., 2009) showed that the avoided K fertilisers have a rather insignificant effect on the overall environmental impacts of slurry management, it is believe that the amount of K avoided (100 % or less) is not likely to affect the results.

The avoided emissions per kg of inorganic N, P and K avoided are modelled as in Annex A, Table A.18.

[22] Gødskningsbekendtgørelsen (2008), chapter 3, paragraph 3 and 4:

”Stk. 3. En marks kvælstofkvote opgøres på grundlag af den eller de afgrøder, der dyrkes på arealet, dog på grundlag af den senest etablerede afgrøde, hvis arealet er sået om, fordi afgrøden er slået fejl.”

[23] Gødskningsbekendtgørelsen (2008), paragraph 20:

§ 20. For det enkelte forarbejdningsanlæg gælder, at den totale mængde kvælstof i den forarbejdede husdyrgødning skal svare til den indgående totale mængde kvælstof. Ligeledes skal den andel, der skal udnyttes, af den totale mængde kvælstof i forarbejdet husdyrgødning mindst svare til andelen, der skal udnyttes, af den indgående totale mængde kvælstof […].

Stk. 2. Producenter af forarbejdet husdyrgødning fastsætter ved salg eller afgivelse til en virksomhed registreret efter lovens § 2 det totale antal kg kvælstof i gødningen og den andel af det totale antal kg kvælstof, der skal udnyttes.

[24] Gødskningsbekendtgørelsen (2008), paragraph 19: § 19. Indholdet af kvælstof i afgasset biomasse skal beregnes på grundlag af oplysninger om den mængde kvælstof i husdyrgødning, der er tilført biogasanlægget samt oplysninger om den mængde kvælstof i anden organisk gødning, der er tilført biogasanlægget, jf. § 22, stk. 6. Alternativt kan biogasanlæg, der leverer afgasset biomasse til virksomheder omfattet af lovens § 2 eller til andre virksomheder med henblik på endelig brug i virksomheder omfattet af lovens § 2, få indholdet af kvælstof i afgasset biomasse bestemt ved analyse af repræsentative prøver foretaget mindst en gang inden for perioden 1. august til 31. juli i den planperiode, gødningen skal anvendes, jf. stk. 2. Biogasanlægget skal opgøre den leverede mængde afgasset biomasse, som analysen gælder for. Stk. 2. Analyse af indhold af kvælstof i gødning skal foretages af et laboratorium, der er autoriseret hertil af Plantedirektoratet […].

Plantedirektoratet (2008b): Udnyttelsesprocenten beregner producenten (ud fra indgangsmaterialet eller analyse af repræsentative prøver). For afgasset gylle kan udnyttelsesprocenten i stedet sættes som andelen for svinegylle, der i 2007/08 er 75 pct.

[25] The values needed to apply the methodology presented in Annex F can be found in the sections for N leaching, namely G.7.6, G.23.7 and G.27.6.