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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.
