How grass-clover ley and cereals can be used more efficiently as biogas substrates by optimizing harvesting dates and chopping length

Grass-clover ley as well as rye and wheat harvested as whole-crop silage (WCS) are used as biogas substrate in Sweden today. Earlier studies have shown that the specific methane potential, the energy yield and consequently also the substrate costs differ depending on when crops are harvested and how finely the material is chopped. The goal of this project was therefore to investigate how grass-clover ley and cereals can be used more efficiently as biogas substrates by optimizing harvesting dates and chopping length. Other research questions were how biomass yield and chopping length settings affect diesel consumption and harvest capacity.
Diesel consumption and harvest capacity for different cutting lengths, harvest dates and choices of crop were computed using dynamic harvest- and transport logistic models employing data for diesel consumption and machinery hours derived from a machinery harvest experiment. The results show that increasing cutting length from 3,5 to 9 mm reduced diesel consumption and machinery hours by 17 and 33 %, respectively.

Cutting length settings affected also the specific methane potential which was assessed in lab test. However, the relation is more complex than expected earlier. Our studies show that did not always pay off to chop finely – on the contrary, a too small cutting length setting can reduce energy yield and increase costs. For ley and wheat a reduced cutting length (4 instead of 12 mm) lead to a reduction in specific methane potential with 9 and 13 %, respectively, while for rye the specific methane potential increased by 12 %. Additional studies are needed to investigate the mechanisms behind these results as well as the role of crop maturity and dry matter content as an influencing factors.

Potential energy yields and substrate costs were computed following life cycle methodology. Energy yields of rye and wheat were high, 36 and 33 MWh/ha, respectively. This represents an energy factor of 5 times the energy used in the production. Energy yield for ley was 24 % lower than for rye, 28 MWh/ha. These results are based on biomass yields for wheat, rye and ley crops of 12,2, 12,1 and 12,1 tonnes dry matter per hectare and year, respectively, as measured in the field experiments. Values have been adjusted for losses during harvest, transport and storage according to project results.

The energy yield for rye and wheat could be further increased, since other studies in cooperation between SB3 and SLU Alnarp have presented up 25-30 % larger biomass yields. The biomass yield of ley crops are not expected to increase, on the contrary, as other studies have shown that second year biomass yields of ley crops can be up to 30 % lower than these of the first year.

All potential biogas substrates had lower production costs than the ability to pay of a potential biogas plant. Under the best circumstances, rye and wheat had the lowest total costs (514 and 521 SEK/MWh, respectively) for production of vehicle gas. Vehicle gas from ley crops harvested 3 times per year resulted in a ca 10 % higher cost, 570 SEK/MWh.

The function of ley crops as break crop or its positive properties concerning soil carbon add to the sequestration of carbon dioxide in the soil and potentially increases soil fertility. According to a current suggestion for EU rules, vehicle fuel from ley crops will be counted as a second generation advanced biofuel, which entitles to tax reductions and allows double accounting in national statistics on production of renewable energy.

The project is funded by The Swedish Farmers' Foundation for Agricultural Research (SLF)

Duration of the project: 2012-2015

Project Leader: Thomas Prade
Other contributors: Sven-Erik Svensson, Emma Kreuger, Torsten Hörndahl, Jan Erik Mattsson

Publications:
Factsheet (Swedish): Vall ger mycket biogas! link: http://pub.epsilon.slu.se/12552/
LTV-rapport (Swedish with English summary): Vall och helsäd som biogassubstrat link: http://pub.epsilon.slu.se/12402/