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.
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))