Electricity generation from waste biomass has the advantage that it can provide continuous base or peak load energy. This compares favourably with wind and solar energy systems which vary in electricity-producing capability with the weather and usually involve the use of expensive battery systems to achieve continuous supply. Also, smaller, distributed generators located near resources and the communities they serve offer better energy security than large fossil-fuel based generation plants.
Australia’s globally competitive agricultural output means that waste biomass is abundant. Sources include grain, rice, cotton crop residues, waste from tree plantations, native forest thinning and waste from wood and paper processing facilities.
These feedstocks can be converted to electricity through a number of processes including slow pyrolysis which is the thermo-chemical decomposition of organic material at high temperatures and in the absence of oxygen. This produces charcoal and syngas – a gaseous fuel composed of hydrogen, carbon monoxide, carbon dioxide, nitrogen, methane and other hydrocarbons.
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Although not widely used in Australia, the engineering involved is proven technology overseas and represents a more efficient energy production system than simply burning residues for heat as is sometimes done at sugar cane mills.
Crop residues can be expensive
However, using waste biomass as a feedstock for bioenergy production does have disadvantages. Production can be seasonal and adversely affected by drought. Crop residues are already used to maintain soil health and as animal feed or bedding material. The material can also be expensive to collect and transport.
This trade-off between energy potential and logistics led to the development of the Bioenergy Atlas of Australia several years ago. The atlas is an online tool that provides access to maps indicating potential biomass energy resources, based on agricultural census data, as well as existing industry and transport infrastructure.
Dr Ian Nuberg of the University of Adelaide, one of the developers of the atlas, says that although it was a ‘blue sky’ idea at the time, it is a valuable tool for regional planners looking to develop a bioenergy industry. Potential sources of biomass can be explored within different radii of infrastructure such as towns or railways. He believes it is these sorts of logistics, rather than processing technologies for the biomass, that can determine the viability of bioenergy projects.
The expense of gathering waste biomass such as that left behind after logging operations may be prohibitive and could lead to a negative net efficiency for the fuel produced.
However, woody crops offer some advantages, says Nuberg. A woody crop planted extensively in an area, with a processing plant or distribution hub in the centre, offers multiple revenue streams and may minimise the costs associated with dispersed sources of biomass.
BEST Energies solves the problem
Nuberg says there is a problem with using agricultural residues, especially from grain crops. “People see all of that straw being left behind, but if you take that away you are also taking away a lot of the carbon which should go back into the soil for organic matter. It has a lot of implications for soil health so while that biomass may be available, it may be better kept in the ground,” he said.
The amount of organic carbon in the soil determines soil structure and is linked to nutrient cycling and availability brought about by soil microbes. It is a problem that BEST Energies Australia believes it has solved. BEST is an R&D engineering company that has a one tenth commercial scale slow pyrolysis bioreactor at Somersby, north of Sydney. The reactor turns waste biomass into syngas and charcoal that can be used as a carbon filtration media, pelletised fuel or as bio-char for soil enrichment and carbon sequestration.
“People are saying that biofuels are just mining the soils of nutrients and are unsustainable in that regard. The nutrients such as phosphorus and potassium that are in the wheat stalks, for example, remain in the bio-char so they can get recycled back on to the field,” said Adriana Downie, technical manager at BEST. “We are not only sequestering carbon but we are doing it in a beneficial way. Once you put the carbon in a soil it has all of the flow-on effects of actually being a soil improver.”
Soil organic carbon is a very important component of the global carbon cycle. It accounts for about as much carbon as is in the atmosphere and in living vegetation combined. If more carbon is stored in the soil as organic carbon, it will reduce the amount present in the atmosphere, and therefore help to alleviate global warming and climate change.
Syngas combusted to dry feedstock
However, soil carbon resulting from the decomposition of crop residues may have different properties to the charcoal that is produced from burning wood in pyrolysis kilns and BEST is currently conducting extensive field trials to quantify the benefits of returning the bio-char to agricultural soils.
Feedstocks for the BEST bioreactor include poultry litter, dairy manure, green waste, nut shells, paper sludge, straw, wood waste, woody weeds, distillers grain, cotton trash, rice hulls and switch grass.
The feedstock is dried and fed through a stirred and heated kiln. Approximately 35 percent by weight of the dried material is converted to bio-char. The syngas that is generated can be combusted to dry the feedstock and to provide a heat source for the pyrolysis kiln. Excess syngas can be used as engine fuel, fuel for industrial boilers or as a feedstock for the production of liquid fuel. Processing waste this way can minimise the release of volatile gases and odours that are given off during decomposition.
The bioreactor at Somersby is set up for continuous processing and has a small footprint for stand alone or integrated heat and power applications. Processing can be optimised to maximise either syngas or bio-char production – lower temperatures favour the production of bio-char.
Fast pyrolysis is a process involving moderate temperatures and a shorter residence time in the kiln that can produce bio-oil from waste biomass feedstock but Downie says they have steered away from that because much more research is needed before bio-oil can be used as a transport fuel. In contrast, syngas can replace LPG or natural gas in industrial applications or it can be used for electricity generation.
Technology uptake
Although the BEST technology is being taken up in some boutique applications where the cost of disposing of waste biomass provides incentive for installing a bioreactor, Downie says the relatively cheap cost of electricity in Australia, compared to world standards, is a barrier to uptake of the technology. Industry is waiting for a price on carbon to be established before taking up the technology more widely.
Dr Stephen Schuck, manager of Bioenergy Australia, believes that Australia is currently at a political crossroads in the development of biomass energy and needs a stable, long-term policy framework with incentives such as carbon trading and mandatory renewable energy targets. Bioenergy Australia is a government-industry forum that fosters the development of biomass for energy and Schuck has played a key role in the development this year of the Australian Bioenergy Roadmap produced by the Clean Energy Council.
He believes that all levels of government need to recognise the potential of bio-electricity.
The Australian Bioenergy Roadmap predicts that, despite the potential, issues such as the high cost of logistics for such a dispersed resource, means that waste biomass is not likely to make a sizeable contribution to bioenergy before 2020.
“There is lots of biomass around. There is lots of technology. It is just really the integration of policy and incentives that is needed to get the industry to progress,” said Schuck.