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Life Cycle Assessment of Biogas from Separated slurry

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

• F.28 Avoided production and application of mineral fertilizers and yield change
  • F.28.1 General description
  • F.28.2 Calculation of the replaced amount of mineral fertiliser
  • F.28.3 Yield changes
  • F.28.4 Avoided P and K mineral fertilisers

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F.28 Avoided production and application of mineral fertilizers and yield change

F.28.1 General description

In this scenario, nitrogen is spread to the field at 3 points: Via the liquid fraction (section F.7), via the degassed fibre fraction (section F.23) and via the degassed liquid fraction (section F.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 F (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 F.28.2. The agronomic fertiliser value and the yield changes are explained in section F.28.3.

F.28.2 Calculation of the replaced amount of mineral fertiliser

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 ^[21]. In addition to this, there is an upper limit for how much of the “nitrogen quota” that can be applied as animal slurry, where a maximum of 1.4 “DE per ha” is allowed for pig farmers (1.4 DE per ha corresponds to 1.4 animal unit per hectare and 1 DE equals 100 kg N – or, that is to say, did originally correspond to 100 kg N, however, this varies slightly with the Norm Data for each animal category each year).

When applying pig slurry, the N in the slurry replace 75% mineral fertiliser, which means that if applying 100 kg N in slurry, the farmer has to apply 75 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 pig slurry, this accounts for 75 kg N per ha, which means that the farmer is allowed to apply the remaining 149 kg N per ha – 75 kg N per ha = 74 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:

1. 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^[22].

For degassed biomass from biogas plants, there are three rules that can be applied, and the biogas plant can choose which one to apply^[23]:

2. The “mineral fertiliser replacement value” of the outgoing biomass is calculated in accordance with the ingoing biomass (“rule of conservation”).
3. 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.
4. 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 F.35.

Table F.35. Replaced amount of mineral N fertiliser in Annex F. All calculations per 1000 kg slurry ex-animal

[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).

This 4.0725 kg N (per 1000 kg slurry ex-animal) is identical to 75% of the initial 5.00 kg N per 1000 kg slurry ex storage * 1086 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 F.36, based on the explanations in Annex A, section A.6.1.

Table F.36. Replaced amount of mineral N fertiliser in scenario A

F.28.3 Yield changes

The yield changes reflect the difference in the “extra” amount of N available for “extra” crop uptake in Scenario F as compared to Scenario A. For a given scenario, this delta N can be expressed as:

ΔN = Harvested N - N received from slurry according to the substitution rule.

The N received from slurry according to the substitution rule is in fact the avoided inorganic N. Because this N is expressed in terms of inorganic N, the harvested N must be translated in terms of inorganic N as well.

For Scenario A, this delta N is referred to as ΔN_A and for Scenario F, as ΔN_F.

The overall difference in N is then expressed as the difference between ΔN_A and ΔN_F.

This difference is afterwards 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. It is acknowledged that this may be a simplistic approach to reflect the impact of a higher yield. In fact, the actual consequence of a higher yield of a given crop consists of the market response to the additional amount of that crop suddenly provided on the market. This response is however not straightforward and requires a comprehensive analysis of trade and market mechanisms (e.g. Kløverpris, 2008), which is out of the scope of the present project.

The calculation of the harvested N is made separately for each organic fertiliser type (liquid fraction, degassed fibre fraction and degassed liquid fraction):

1. Liquid fraction
    
   • Step 1: Modelling the N from liquid fraction as a given proportion of slurry N + a given amount of mineral N
     These proportions are as described and explained in section F.7.6, i.e.: 0.21 from slurry and 0.79 from mineral N. This means that a fraction of 0.21 is assumed to affect the field as slurry and a fraction of 0.79 is assumed to affect the field as mineral N.
     
     After ammonia volatilisation of the liquid fraction there is 3.61-0.02-0.19= 3.4 kg N left per 1000 kg liquid fraction (table F.16).
     
     Giving a calculation example for JB3, this means that:

     0.21*3.4 kg N per 1000 kg liquid fraction = 0.71 kg N (per 1000 kg liquid fraction) is assumed to take the pathway of pig slurry;
     
     0.79*3.4 kg N per 1000 kg liquid fraction = 2.69 kg N (per 1000 kg liquid fraction) is assumed to take the pathway of mineral N;

   • Step 2: Amount of N harvested – portion modelled as slurry
     Table A.15 of Annex A presents the proportion of the different fate of N following pig slurry application. Based on this table, for soil JB3, a fraction of 0.36 of the 0.71 kg N (per 1000 kg liquid fraction) is harvested after ammonia losses ^[24], which corresponds to:

     0.71kg N (slurry) *0.36 = 0.26 kg N harvested (per 1000 kg liquid fraction).

   • Step 3: Amount of N harvested – portion modelled as mineral
     Similarly as the procedure of step 2, after ammonia losses, a fraction of 0.44 of applied mineral N is assumed to go to harvest (see table A.14, and applying the same principle as in footnote 24), which corresponds to:

     2.69 kg N (mineral N)*0.44 = 1.19 kg N harvested (per 1000 kg liquid fraction).

   • Step 4: Total N harvested
     From the 3.61 kg N applied, the harvested N is:

     0.26 kg N (step 2) + 1.19 kg N (step 3) = 1.44 kg N harvested (per 1000 kg liquid fraction).

     For JB6, the total N harvested corresponds to 1.65 kg N (per 1000 kg liquid fraction).
      
2. Degassed liquid fraction
   The calculation of harvested N for degassed liquid fraction follows the exact same 4 steps described for the liquid fraction.
   
   The proportion of N modeled as slurry and as mineral N is however based on section F.27.7 and are as follow: 0.49 from slurry and 0.51 from mineral N.
   
   As a result, the N harvested is 2.87 kg N on JB3 and 3.29 kg N on JB6 (per 1000 kg degassed liquid fraction).
    
3. Degassed fibre fraction
   For degassed fibre fraction, there is, for soils JB3 and JB6 respectively, 2.81 and 2.44 kg N left for harvest and leaching after all gaseous losses, according to the calculations of section F.23.7. Knowing the amount of N leaching for each soil types (table F.31), the N left for harvest can be calculated by a simple difference.
   
   This gives a harvest of 1.2752 kg N for soil JB3 (per 1000 kg degassed fibre fraction) and 1.3434 kg N for soil JB6 (per 1000 kg degassed fibre fraction).

Aggregating the amount of N harvested from each of these 3 organic fertilizer and taking into account the amount of each that is actually applied (figure F.1), the total harvested N for soil JB3 is:

[1.43 kg N per 1000 kg liquid fraction * 708 kg liquid fraction/1000 kg slurry ex-animal] + [1.2752 kg N per 1000 kg degassed fibre fraction *77.3 kg degassed fibre fraction /1000 kg slurry ex-animal] + [2.87 kg N kg degassed liquid fraction *263.4 kg degassed liquid fraction /1000 kg slurry ex-animal] = 1.873 kg N harvested on soil JB3 per 1000 kg slurry ex-animal.

On soil JB6 the amount of harvested N is 2.139 kg N per 1000 kg slurry ex-animal.

As this harvested N will be used in an equation where the N is expressed in term of mineral N, it must be translated in terms of mineral N.

To do so, the amount of mineral N needed to obtained these harvest N (1.873 kg N on soil JB3 and 2.139 kg N on soil JB6) must be determined. This can be done through the partitioning factors presented in table A.14 of Wesnæs et al. (2009). This harvested N equivalent therefore corresponds to:

1.873 kg N harvested/0.431(see table A.14) =4.3467 kg N harvest equivalent for soil JB3 (per 1000 kg slurry ex-animal).

2.139 kg N harvested/0.494(see table A.14) =4.3308 kg N harvest equivalent for soil JB6 (per 1000 kg slurry ex-animal).

Therefore, ΔN_F corresponds to :

Soil JB3: 4.3467 kg N – 4.0725 kg N (table F.35) = 0.2742 kg N per 1000 kg slurry ex-animal

Soil JB6: 4.3308 kg N – 4.0725 kg N (table F.35) = 0.2583 kg N per 1000 kg slurry ex-animal

For Scenario A, the calculation of harvested N is more straightforward, as it is simply the amount of N in the slurry ex-storage minus the ammonia losses, on which the partitioning ratios of table A.15 of Wesnæs et al. (2009) are applied to determine the N harvested. Then the equivalent in mineral N can be determined as above, using the values presented on table A.14 of Wesnæs et al. (2009). This gives:

Soil JB3: (4.8-0.02-0.48)*36.05% kg N per 1000 kg slurry ex-storage *(1086 kg slurry ex-storage/1000 kg slurry ex-animal) *(1/0.431) = 3.9096 kg N harvest equivalent for soil JB3 (per 1000 kg slurry ex-animal).

Soil JB6: (4.8-0.02-0.48)*41.4% kg N per 1000 kg slurry ex-storage *(1086 kg slurry ex-storage/1000 kg slurry ex-animal) *(1/0.494) = 3.9096 kg N harvest equivalent for soil JB6 (per 1000 kg slurry ex-animal).

Therefore, ΔN_A corresponds to :

Soil JB3: 3.9096 kg N – 4.0725 kg N (table F.36) = -0.1629 kg N per 1000 kg slurry ex-animal

Soil JB6: 3.9096 kg N – 4.0725 kg N (table F.36) = -0.1629 kg N per 1000 kg slurry ex-animal.

The overall N difference between both scenarios corresponds to:

ΔN_F - ΔN_A = 0.2742 kg N – (-0.1629 kg N) = 0.4371 kg N surplus for JB3 (per 1000 kg slurry ex-animal)

ΔN_F - ΔN_A = 0.2583 kg N – (-0.1629 kg N) = 0.4212 kg N surplus for JB6 (per 1000 kg slurry ex-animal)

According to the yield N responses (see section A.5.5, Annex A and section B.10, Annex B), one kg extra mineral N yields:

• Soil JB3: 9.0 kg more wheat grain;
   
• Soil JB6: 8.1 kg more wheat grain;

The yield increase is therefore:

For soil JB3: 0.4371 kg N surplus * 9.0 kg extra wheat/kg N surplus = 3.93 kg extra wheat (per 1000 kg slurry ex-animal).

For soil JB6: 0.4212 kg N surplus * 8.1 kg extra wheat/kg N surplus = 3.41 kg extra wheat (per 1000 kg slurry ex-animal).

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

F.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 modelling (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.

[21] 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.”

[22] 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.

[23] 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.

[24] In table A.15, the partitioning value for ammonia volatilisation is 10.4 %. Without the ammonia losses, the sum of the values presented in table A.15 is 89.6 % (i.e. 100% - 10.4%). Based on this, the harvest partitioning value of 32.3 % (10 years) becomes: (32.3*100%)/89.6 = 36 %.

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Version 1.0 August 2010, © Danish Environmental Protection Agency