Biomass 04: Mimosa Pigra - Turning a weed into electricity
Renewable Energy Commercialisation in Australia, Australian Greenhouse Office, 2003
NOTE: The status of these projects will have changed since the time of publication, and project contacts may also have changed.
Converting mimosa pigra weed into electricity will contribute to greenhouse gas abatement and provide distributed generation support to the electricity network.
In 1999 the Northern Territory Power and Water Corporation (PAWC) and Biomass Energy Services and Technology (BEST) conducted research under a project supported by the Renewable Energy Industry Program into the possible use of Mimosa pigra (mimosa) as an energy source. The Northern Territory Department of Primary Industry and Fisheries assisted the research into the extent of the resource and some of the harvesting issues (with strong emphasis on not spreading the seeds).
The current project, assisted by a grant of $1 million under the Renewable Energy Commercialisation Program, aims to optimise the mimosa harvesting, briquetting and gasification processes and the integrating of the gas engine technology. The planned output of the pilot scheme is 350kW. This would be enough to light about 200 homes in Darwin and would reduce greenhouse gas emissions by over 2,400 tonnes of CO2annually.
The Adelaide River floodplain location has been selected as a demonstration site because it is close to a large, accessible infestation of mimosa. In addition, it is on the Arnhem Highway which will expose thousands of Kakadu National Park visitors every year to the technology, thereby helping to promote sustainable energy principles to the wider community.
The conversion process involves harvesting the mimosa and compressing the mimosa chips into briquettes on site to store for the wet season and to prevent spreading of the seeds. Then the mimosa briquettes are gasified to produce fuel gas, which is burnt in an optimised spark ignition gas engine to produce electricity and ash.
The gas that results from gasifying the mimosa is very high in hydrogen. This makes it unsuitable for gas turbines and difficult for the majority of spark ignition reciprocating gas engines. However, it could be very suitable for fuel cells when they become commercially available.
In the 1999 research, BEST used a Holden motor to run on the gas and to produce 30kW. PAWC has been conducting market research on available and suitable engines in the 500kW range.
PAWC modelling suggests that although the pilot scheme may have costs in the range of 25 to 35 cents per kWh, upscaling the project to 3MW in future may produce costs competitive with grid power.
PAWC coordinated research on harvesting in conjunction with the Australian Cooperative Research Centre for Renewable Energy in 2000. Harvesters that have been developed for the oil mallee projects in Western Australia may be suitable. Other harvesters in Scandinavia for clearing up forest residue may also be adaptable. For the pilot scheme it is envisaged that the landholder will bulldoze (or chain) the mimosa and feed it into a chipper. The harvesting would need to be done in about four months during the dry season and produce enough fuel for a full year's operation of the power station.
The mimosa resource has 'renewable status' as the mimosa regenerates and is back to its original density after 3 to 5 years. The pilot scheme will require only a few hectares to be cleared each year. PAWC does not expect to make much impact on mimosa initially but, in conjunction with other biological control and eradication methods, it will serve as a fuel resource for a 7MW plant producing renewable energy for at least 20 years.
For more information please contact
Trevor Horman
Power and Water Corporation
18-20 Cavenagh Street
Darwin NT 0812
Tel (08) 8924 7093
Fax (08) 8924 7222
Email trevor.horman@powerwater.com.au
Internet www.pawa.com.au
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