Case Story Australia

Profitable on-farm biogas in the Australian pork sector

February 2018

Summary

Methane emissions from uncovered anaerobic ponds contribute about 60–80% of the total on-farm greenhouse gas emissions for Australian pork production. Capturing and using this methane as a source of heat and electricity on-farm improves environmental and business performance of the piggery. Covered anaerobic ponds are the most common digester technology used to capture manure methane at Australian piggeries. These on-farm biogas production systems rely on offsetting on-farm energy costs (primarily electricity, LPG and diesel used for electricity generation) and selling surplus power to the grid for their basic financial viability. This ensures that the capital investment is met within the first decade of operation, regardless of possible changes in Government policy relating to carbon credit systems.

Technical overview

It is common practice for Australia’s pig producers to collect and treat effluent (primarily manure and waste feed) flushed from the sheds housing the pigs in anaerobic ponds. To capture methane, the ponds are usually covered with a high density polyethylene (HDPE) cover, which is trenched in around the crest of the pond embankment to form a gas-tight seal, (Figure 1). There are fewer mixed and heated digesters at piggeries in Australia. In contrast to heated/mixed digesters, the volume of biogas produced by covered ponds varies with seasonal temperature changes, but Australia has a reasonably temperate climate, so that in colder months the biogas produced is about 20% less than the average yearly flow, and during warmer months about 20% more than the average yearly flow.

A blower (Figure 2) is generally used to convey the biogas through sealed pipework (underground HDPE, PVC; aboveground stainless steel) to a generator engine or boiler. Australia has relevant standards that direct the zoning for electrical equipment in the vicinity of a biogas system. Importantly, biogas equipment described here usually operates at low pressure (<100kPa or 14.5Psi), including the covered pond where the pressure rarely exceeds 50 pascals (0.007Psi) even when visibly inflated.

Figure 3 summarises the equipment commonly used on Australian piggeries. Coarse solids in flushed piggery effluent, such as barley husks and pig hair, may be removed from the effluent stream with a solids screening step; this reduces accumulation of solids as a floating crust layer under the cover.

To protect engines or boilers, corrosive ingredients from the biogas are often removed before use. This is typically done by contacting the biogas with a solid cleaning-medium (Figure 4) containing iron oxide, which strips corrosive hydrogen sulphide (H2S) from the biogas. Usually these cleaning systems use solid media that become spent with use and require replacement, contributing to the operating costs. More recently, biological oxidation in an external vessel has become more popular with piggery biogas installations, due to low operating cost and effectiveness in removing H2S.

Where biogas is conveyed in long sections of underground pipework, cooling of the biogas to the soil temperature has also been used to condense moisture with the pipeline; this is laid at a gradient sufficient to allow drainage of the condensate to a collection point. Biogas is also chilled at some piggeries above-ground using chilled water before being sent underground (Figure 5).