Back­ground

Already today, almost 70 % of renew­able ener­gies in Germany are gen­er­at­ed from biomass. However, at around 300 g CO2 / kWh, elec­tric­i­ty from biogas from con­ven­tion­al corn mono­cul­tures is only slight­ly better than the con­ver­sion of fossil natural gas into elec­tric­i­ty. However, the balance can be opti­mised by various mea­sures such as the car­bon­i­sa­tion of the fer­men­ta­tion residue into biochar.

Neg­a­tive climate balance

For the entire process of biogas pro­duc­tion by maize mono­cul­tures, the author deter­mined a climate balance of around ‑4 t CO2 / ha. This results from the fol­low­ing assump­tions / values:

  • On 1 hectare (ha), an average of 12 t dry maize mass is pro­duced for the pro­duc­tion of Si layer. On average, 200 kg nitro­gen (N), 100 kg phos­phate (PO), 150 kg potas­si­um (K) are fer­til­ized for this purpose. The pro­duc­tion, storage, appli­ca­tion and out­gassing of these fer­til­iz­er quan­ti­ties cause green­house gases of around 2 t CO22 /ha.
  • Cul­ti­va­tion, tillage, her­bi­cides, har­vest­ing, chop­ping, trans­port to the biogas plant and silage cause green­house gases of around 0.4 t CO22 /ha.
  • On average, 2% of the biogas pro­duced escapes into the atmos­phere. This causes green­house gases of around 2.2 t CO2 /ha.2 /ha.
  • 18 % of the meth­a­nized biomass is pro­duced as fer­men­ta­tion residue, via which CO2 is released back into the atmos­phere. This causes green­house gases of around 12 t CO2/ha.2/ha.
  • If the approx­i­mate­ly 4.5 t of methane pro­duced from the harvest of 12 t of corn is burned, green­house gases of approx­i­mate­ly 12.4 t CO2/ha are pro­duced.

This pro­duces 23.8 t CO2 in the entire biogas cycle. However, only 19.7 tons of CO2 are absorbed and bound by the maize itself during growth. If the methane is con­vert­ed into elec­tric­i­ty, 14,000 kWh of this can be gen­er­at­ed, which results in a climate balance (without waste heat util­i­sa­tion) of around 300 g CO2/kWh of elec­tric­i­ty.

Opti­mi­sa­tion pos­si­bil­i­ties

Most of the approach­es pre­sent­ed by the author improve both the climate balance of biogas and the effi­cien­cy and thus prof­itabil­i­ty of the pro­duc­tion process.

 

  • Increas­ing plant diver­si­ty: In con­trast to 1‑year-old maize, the advan­tage of a species-mul­ti­ply­ing seed mixture is that it only needs to be reseed­ed every 3–5 years. Biomass yields are higher, and the bio­log­i­cal activ­i­ty of the soil, carbon seques­tra­tion in the soil and plant health are also improved.
  • Car­bon­a­tion of the fer­men­ta­tion residue: When methane is pro­duced from maize silage, only around 60% of the carbon it con­tains is con­vert­ed into methane gas. As a rule, 95% of the remain­ing carbon escapes into the atmos­phere when the fer­men­ta­tion residue is spread as fer­til­iz­er. By car­bon­i­sa­tion (pyrol­y­sis) the fer­men­ta­tion residue can be refined to biochar or the fer­men­ta­tion residue car­bon­ate can be used as fuel (calorif­ic value of about 20 MJ /kg). This improves the climate balance by 5.3 t CO2 /ha.2 /ha.
  • Reduc­tion of nutri­ent losses due to biochar: The fer­tilis­ers absorbed by the maize are still almost com­plete­ly present in the fer­men­ta­tion residue after fer­men­ta­tion. However, about 20 % of the nitro­gen (as ammonia) is lost by spread­ing the fer­men­ta­tion residues and a further 15 % by soil leach­ing. This can be reduced by adding biochar, as its high spe­cif­ic surface area binds nutri­ents to it for longer. If the fer­men­ta­tion residue is also fer­ment­ed togeth­er with lactic acid bac­te­ria, the climate-dam­ag­ing methane gas losses are reduced. This improves the climate balance by 1.1 t CO22 per ha und year.
  • More effi­cient use of land: If biogas from maize silage is only con­vert­ed into elec­tric­i­ty, this is an extra­or­di­nar­i­ly inef­fi­cient use of space in terms of energy. After all, 14 MWh of elec­tric­i­ty (pro­duced from 1 ha of maize per year) can also be pro­duced on only 1 % of the hectare by using solar panels – and without the use of fer­til­iz­ers.

Orig­i­nal article (German): Klima­bi­lanz für Biogas aus Mais­monokul­turen (1) & Biogas durch Kli­ma­farm­ing nach­haltig pro­duzieren (2) (Climate balance for biogas from maize mono­cul­tures (1) & sus­tain­able biogas pro­duc­tion through climate farming (2))

Author: Hans-Peter Schmidt
Pub­lished in: Ithaka Journal 1/2012, p. 57–60 (1); Ithaka Journal 1/2012, p. 61–66 (2)