Bioenergy has the potential to be a significant energy resource in Australia with an estimated 371PL p.a. available. Bioenergy is a form of renewable energy derived from biomass (organic materials) to generate electricity and heat, and liquid fuels for transport.
Biofuels are liquid fuels, produced by chemical conversion processes that result in the production of ethanol and biodiesel. Biofuels can be broadly grouped according to the conversion processes. The fuel type (the heating value and moisture) and the conversion technology will influence the energy conversion efficiency. For example, the energy conversion efficiency for wood waste in a direct combustion facility is about 35%, compared to between 70-85% efficiency in a combined heat and power facility. Ethanol is produced by and not limited sugar by-products, waste starch from grain, and biodiesel is produced from used cooking oils, tallow from abattoirs and oilseeds.
Typically, the resources used for bioenergy are dominated by forestry and agriculture residues from the sugar cane, grain and vegetable oil crops, and organic waste streams, which is typically used to produce biogas. Biogas is produced from anaerobic digestion using waste effluents such as wastewater, sewage sludge and municipal solid waste. Anaerobic bacteria digest organic material in the absence of oxygen and produce biogas. Anaerobic processes can be managed in a digester or airtight tank or covered lagoon. Currently Australia’s use of bioenergy for electricity generation is limited to bagasse (sugar cane), wood waste, and gas from landfill and sewage facilities. There is increasing use of this technology on both a small and large scale.
Bioenergy offers the potential for considerable environmental benefits. Biomass releases Carbon dioxide (CO2) and other trace amounts of greenhouse gases when converted into another form of energy. However, CO2 is absorbed during vegetative growth through the photosynthesis process and Carbon assimilation. biogas is composed principally of methane and CO2 produced by anaerobic digestion of biomass. Although bioenergy offers a potential renewable form of energy good management of resources is needed, minimising chemical and fertiliser use, land degradation, energy, and water consumption.
The biomass available for potential bioenergy is dependent on a range of factors such as feedstock prices, seasonal availability, and the relative value of biomass to produce other commodities. Things to consider for each bioenergy resource include moisture content, resource location and distribution, and type of conversion process. There is also a range of potential impacts on the resources including drought, flood, fire, climate change and energy prices.
Commercialisation of advanced technologies will likely increase the range of resources, such as the non-edible (woody) parts of plants, Agaves and Algae. There is potential to expand Australia’s bioenergy sector utilising more of these organic and waste products. The right economic conditions may encourage growers to diversify towards bioenergy production or, upgrade infrastructure to produce energy onsite and offset consumption.
The sugarcane industry is one of few industries self-sufficient in energy, through the combustion of bagasse in cogeneration plants. The sugar mill directly consumes the heat and electricity generated and any surplus steam is used to generate electricity and feed into the power grid. The total annual sugarcane crop is about 35.5 million tonnes (Mt), of which 14 per cent is cane fibre, resulting in a total available energy of above 90 PJ.
The three main biomass combustion conversion technologies are grate boilers, fluidised bed combustion (gasification) and co-firing in utility boilers. In the most efficient electricity generation plant, around 30 per cent of the energy in the biomass is converted into electricity; the rest is lost into the air and water. Cogeneration or combined heat and power plants have greater conversion efficiencies because they produce both electricity and process heat. Trigeneration technology provides cooling in addition to heat and electricity generation. The process waste heat can be usefully applied for heating in winter and, via an absorption chiller or refrigeration, for cooling in summer. The use of gasification is more efficient for energy recovery in terms of electricity generation than traditional combustion. In gasification, solid biomass is heated to high temperatures (800–1000°C) in a gasifier and converted to a syngas primarily composed of Hydrogen, Carbon monoxide, Carbon dioxide, water vapour and Methane. One benefit is that there are lower amounts of Sodium oxide, Nitrous oxide, and dioxin emissions than in a traditional combustion process.