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Cutting the carbon tax bill
Achieving the upmost resource recovery is part of the way A.J. Bush & Sons, from Beaudesert in Australia's southern Queensland, says it stays competitive. As a part of a long-standing meat rendering industry the company maximises the value of the by-products of processing. Optimising bioenergy is part of its overall business approach to sustainability, and an added impetus to its drive for efficiency is an annual carbon tax bill each year of A$2 million ($2.1 million), caused mostly by having coal-fired boilers that heat process water.
One way of cutting this liability is through the production of electric power from biogas derived from the meat waste.
It was money from the federal government in 2005 that set the company on its journey into biogas production. Some A$715,000 came from the Food Innovation Grant programme, which added to an earlier, smaller grant from the Queensland State Government to allow the company to evaluate the feasibility of biogas capture.
A.J. Bush developed a project at its Beaudesert site in co-operation with Quantum Power that sees the meat renderer take responsibility for the gas treatment system and everything downstream, including a covered anaerobic lagoon (CAL) and gas capture pipework, While, Quantum owns the project's gensets, and is responsible for their operation and maintenance. Quantum also sells all the electricity generated to A.J.Bush at a discounted price as part of a long-term power purchase agreement. Historically the meat renderer imported its total electricity requirements from the grid.
Today coal-fired boilers at the plant still generate steam for the company, which it uses to process the fairly constant supply of non-meat by-products it receives from poultry, pork and beef processors in southern Queensland and northern New South Wales. The company renders about 4000 tonnes of biomaterial over a five day working week, and the plant operates 24 hours a day, 52 weeks per year, with three shifts, from Monday morning until midday Saturday.
Fat, bone and slaughter floor materials yield tallow and meat & bone meal, while poultry by-products produce poultry oil and poultry meal. A.J.Bush hydrolyses poultry feathers and dries them to produce hydrolysed feather meal. Some 65 per cent of the weight of raw material received is water and the remainder yields tallow and protein meal.
Australia retains its status as a country free of bovine disease BSE, so feedstocks from within the country require no extra sterilisation or pasteurisation other than for control of normal bacterial pathogens, and no limitations exist on which material can be used for biogas production. The plant produces around 65,000 tonnes of rendered tallow, oil and protein meals each year.
All of this activity means the plant consumes about 1000 MWh of electricity per month and has a peak demand of about 2.2 MW.
The biogas project started in April 2005 with a trial version of a CAL, which held 2 million litres of wastewater. It had a 1.5 mm thick high-density polythene cover, which was operated with a ten-day residence time. Its success led to the commissioning of a CAL with a capacity of 26 millon litres in 2007, which is 6 metres deep and can hold 28 days' production of wastewater. The new CAL's covers are secured around the edge of the pond with concrete trenches and rise and fall depending on gas generation and usage rates.
Gas continues to accumulate over Saturday afternoon and Sunday, when the main processing plant is idle, and it falls gradually over the week as the gas volume and pond cover lowers. The pond cover includes safety vents, which allow biogas to escape if the pond cover rises to a certain height above the water level. This involves the use of 0.5-metre risers attached to the underside of the vents.
The pH control of the influent is critical to biogas production rates, so maintaining the pH at between 6.6 and 7.6 through the addition of lime has been required on occasion.
The wastewater leaves the CAL by gravity flow (an inflow-outflow balancing system) and is then further treated in other on-site ponds before being recycled for irrigation of crops and pastures.
A manual pumping system removes rainwater that accumulates on the CAL cover during the wet season, and the area has an anti-personnel fence to prevent unauthorised access.
Power generation
Generation of gas occurs at a rate of about 220 m3/h. Fans draw gas from both ponds through ports attached to the top of the cover into pipework that transports it uphill to the gas treatment skid. The uphill location of the skid and the elevation of the pipework from the pond to the skid allows condensate to drain back to the pond. The skid includes a heat exchanger that lowers the temperature of the gas to 3-9°C, which removes most of the condensable materials from it. A flare unit forms part of the skid and burns excess gas whenever a generator is taken off line for maintenance.
Biogas travels from the skid to two 0.5 MW Shengdong engines, the first of which was commissioned in July 2010 and the second in March 2011.
Electric power generation from the gensets has amounted to about 200,000 kWh per month, or about 20 per cent of the total site electricity consumption, with an electrical efficiency of about 33 per cent. However, availability has been much lower than hoped because of operational and quality issues associated with the engines.
As the cost of the engines was less than a biogas treatment unit to remove hydrogen sulphide (H2S), the decision was made not to include a stage for the removal of the acid gas, but instead to sacrifice the engines. The residual H2S level in the gas after treatment is generally less than 1200 ppm.
Major modifications had to be made to the engines to ensure they complied with Australian standards for electrical and gas safety, and could run without being continuously attended to, but there are still operations that must be performed manually, such as inspections, and checks on the coolant.
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