Technical Note GB32: Agricultural crops for biogas production

The New Zealand agricultural sector produces rotation break crops, agricultural residues and horticultural residues which can be a feedstock to produce biogas via anaerobic digestion. This has been widely put into practice, including at large scale in some EU countries, with more than 7000 on-farm digesters in Germany alone. This has proven very useful at a time when outside natural gas supply to the EU has been lost.

NZ actually has a better opportunity than the EU to produce domestic energy, with 3.4 times more agricultural land in use per head of population than the more populous EU countries. The residues from crops for exported food (in excess of those used to sustain soil life and fertility) are abundant and often wasted or underutilised. The option of land use change of lower value arable land and a small part of under utilised pastoral grazing land to biomass crops is also more feasible and justified than in the EU and USA. Such practices in those countries are rightly criticised for achieving energy supply at the cost of food supply. An objective look at the NZ situation gives a different conclusion: diversification by pastoral or arable land users by sourcing feedstocks only from residues or supplementary crops can be beneficial to land management at the same time that it increases food production and greatly reduces carbon emissions. Long term contracts between growers of biomass and a new sector of processors into biofuels and heat energy would increase food income stability for growers.

The research to identify for NZ the best non-woody plant species and characterise their dry mass yield potential, crop production requirements, suitability to various cropping regions, how best to store biomass, and how much energy can be produced per ha of crop has all already been achieved—a decade ago! The primary research reports which have been published relevant to the production of biogas in New Zealand from agricultural crop residues and break crops are set out in this Technical Note, with links to specific research reports.