Methane from animal waste management systems
Authors: Bruce Trangmar, Dave Stwart
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Executive summary
This report describes research conducted as part of MAF’s Climate Change “Plan of Action” Research Programme 2007/8 on the sub-category topic of “Methane from Animal Waste Management Systems”. Greenhouse gas (GHG) emissions from animal wastes are described and the sources of the specific greenhouse gases carbon dioxide, methane and nitrous oxide in the context of animal wastes management are outlined. It is important in this research to set down in some detail the current animal waste management methodologies on New Zealand farms for dairy, pig and poultry wastes because these methods, and possible future enhancements to them, have much relevance to on-farm methane generation and an accompanying controlled approach to collecting and utilising these emissions via anaerobic lagoons or biogas digesters. Accordingly, Section 3 of this report describes these current collection and management methods for dairy, pig and poultry wastes.
To obtain relevant and robust input data for the economic models developed in this project the expected volumes of wastes from dairy cows, pigs and poultry have been calculated and the associated expected annual volumes of biogas (methane) have thus been derived. These estimates have been extended to calculate the national methane emissions from dairy cows, pigs and poultry and thus the total quantity of carbon dioxide equivalents from methane emissions from animal wastes in New Zealand on an annual basis. The geographic potential and associated variation for on-farm biogas production based on climate differences in New Zealand has also been established.
There are potential modifications and enhancements to the collection and management of animal wastes and to the generation, collection and management of associated methane emissions which are being or could be made to ultimately reduce greenhouse gas emissions from animal wastes in New Zealand. Section 5 of this report outlines these improvements and enhancements.
The role of on-farm biogas plants is discussed in some detail. The various potential benefits from an environmental stand-point in achieving regulatory compliance and, ultimately, in economic terms are considered in this report. The historical context of biogas plants in New Zealand is outlined and the reasons for the fall-off in biogas generation and utilisation in recent years are examined. The concepts underpinning a typical biogas plant are illustrated, along with various examples. The utilisation of biogas as a fuel is also discussed, with the various options of process heating, electricity generation, and use as a vehicle fuel being examined. The further option of flaring the gas to produce carbon dioxide as an emission, rather than simply releasing methane, has major implications in GHG reduction terms and economically via the concept of carbon credits, which is also discussed.
Manure and biogas generation was calculated for seven different farm types that could be reasonably representative of New Zealand dairy, pig and poultry farms. Biogas volumes and net emission reductions as a result of biogas use were calculated for each farm type based on typical biogas systems (covered existing anaerobic lagoon, new lined covered anaerobic lagoon, and tank digestion).
The results of the study indicate that capture and management of methane from collection and management of animal waste using biogas systems have potential to reduce greenhouse gas emissions from livestock farms. For example, potential net emission reductions of about 3,868 t CO2e could be achieved from a 10,000 head pig farm by capturing methane from animal wastes through use of anaerobic digestion (either covered lagoons or tank digestion) and conversion to CO2 by combustion.
Introduction of a biogas system to a farm operation for reduction in methane emissions from animal waste may require changes in animal management to maximize waste collection. This is especially the case for dairy cows, where currently only about 10% of total manure produced is able to be collected because typically the only time cows spend on hard surfaces is in the dairy shed during milking. If manure collection for dairy cows could be increased significantly (e.g. through feeding on hard standing pads, or animal housing for longer periods) greater potential exists for methane capture and management in a biogas system. For example, potential annual net emission reductions from biogas digestion of wastes for a 900 head dairy herd is about 217t CO2e based on 10% manure collection compared to about 1,305 t CO2e based on 60% manure collection. While there is potential for biogas systems to reduce on-farm methane emissions, the scenarios analysed under this study indicate that such systems are generally not economically viable at present (poultry and some pig farm scenarios excepted). Analysis for farm scenarios indicates that use of biogas for on-farm electricity generation and C credits is non-economic for most dairy and some pig farm scenarios at current prices and costs under the assumptions made in the study. However, economic viability does vary according to a wide range of factors, including livestock type and number of head per farm, manure management systems used, biogas technology used, electricity price (where electricity generated from biogas is used to substitute for grid supplied electricity), C credit price (if methane emissions reductions from biogas can be eligible under the Emissions Trading Scheme), and capital and operating and maintenance costs. Due to these many variables affecting biogas viability in New Zealand, it is recommended that detailed analysis (with steps similar to those used in Section 8 of this study) should be conducted by all farms considering investment in biogas because viability will be farm specific.
Biogas technology is still relatively new in New Zealand, with few systems currently operating despite a number of large biogas investments in the 1980s and 1990s. The lessons learned from these earlier investments and also current biogas investments should be collated so that new entrants to biogas in New Zealand have access to the full range of knowledge generated in this area. Given the changing energy situation and potential ETS in New Zealand, it is timely for MAF Policy to consider drawing this experience together for the benefit of rural sector investors considering biogas development in the future.
This work has developed and presented a detailed model, encompassing a series of variables, and with a significant degree of associated necessary complexity, to investigate possible scenarios for methane generation from animal wastes on dairy, pig and poultry farms. For optimum utility it will be necessary to produce a simplified version of the model, probably with an associated “User Guide”, to lead farmers through the practical application of the model to their particular animal waste management circumstances.
It is therefore recommended that the results of this work and, in particular, the mechanics of application of the economic model, be simplified and consolidated into a user-friendly package that farmers can adapt to the circumstances of their individual operations. This would enable them to assess the physical and economic viability of collecting wastes and carrying out anaerobic digestion to produce biogas, with that biogas either utilised for electricity generation (and possibly waste heat usage) or simply flared, in each case with associated carbon credits.
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