Author: Liz Hamilton
Bioenergy is energy produced from organic matter derived from plants or animals. It is living, or recently harvested matter, as opposed to fossil fuel. Fossil fuels, although originally derived from organic matter, have been created over long periods through biological and geological processes and are essentially non-renewable.
Specific types of organic matter used to produce bioenergy are called biomass or bioenergy feedstocks. The energy biomass produces can be converted into electricity, heat or biofuels. Bioenergy products can range from a wooden log to refined transport fuel.
Most bioenergy can be traced back to energy from sunlight, making it a major renewable energy source.
Bioenergy is the most widely used renewable source of energy in the world, providing around 10% of the world’s primary energy supplies and mostly used for heating and cooking.
Types of biomass
Sources of bioenergy include agricultural crops, animal and plant wastes, algae, wood and organic residential/industrial waste. The type of biomass will determine how much energy it will produce, and how. For example some agricultural crops, such as canola, can be used to produce liquid biofuels such as ethanol and biodiesel for transport. Or waste streams, such as manure, can be used to produce biogas, which can then be used to generate electricity and heat - and even be upgraded to use as a transport fuel.
How is bioenergy produced?
There are many ways to produce bioenergy. It depends on the biomass material and the type of bioenergy you want to produce. Some processes can be relatively simple, like growing, harvesting and burning wood for electricity and/or heat generation. Others can be more complicated, like algae production for transport fuels, for example, which requires a controlled system with measured amounts of special algae species, water, nutrients and carbon dioxide. This is followed by separating the algae and oils, which need to be refined further to make biofuels.
A variety of conversion pathways can be used to convert biomass into bioenergy to provide heat, electricity, or transportation. Biomass conversion pathways are usually thermal, biochemical or physical, either alone or in or a combination. Biomass can be converted to energy via a range of technologies including engines, boilers, refineries, turbines, fuel cells, and others.
Biomass conversion technologies for stationary electricity and heat generation
There's a wide range of feedstocks, technologies and processes for extracting energy from biomass and converting it into stationary bioenergy for heat and/or electricity. The following are the most common:
Conventional combustion
Direct combustion is the simplest and most widely used bioenergy technology for converting biomass to heat. This heat can then be used for space heating or cooling, to heat water, for use in manufacturing processes or to produce electricity using a steam engine or turbine. Combustion typically has an electrical efficiency of only 20-35%, but co-generation techniques (such as CHP - combined heat and power production) can reach energy efficiencies of over 85%. Modern thermal-only bioenergy systems typically operate at around 85% efficiency and are much cheaper than CHP systems. Worldwide, thermal systems are far more common than biomass CHP systems.
The two main combustion technologies are:
- Fixed bed combustion - an old technology which involves burning materials on a fixed or moving grate with air passing through it.
- Fluidised bed combustion - here, biomass is mixed with sand which then acts more like a fluid, burning more evenly and leading to increased efficiencies. It also allows a wider range of fuel types and higher moisture contents. Generally, fluidised bed boilers produce lower emissions than fixed bed boilers.
Co-firing is where biomass fuels, such as sawdust, biomass pellets, or biogas are combined and burnt with another base fuel, such as coal. Co-firing can be a relatively cost-effective way for fossil fuel power generators to reduce greenhouse gas emissions.
Various fuels can be combusted with minimal processing beyond chipping or shredding and drying. However, most fuels are easier to handle, transport and store if they are processed and compacted into pellets or briquettes, which are denser and of a more consistent quality.
Co-generation, also known as CHP, plants have greater energy conversion efficiencies because they capture 'waste' heat from electricity generation, which can be used for space and water heating, or even cooling via absorption chillers. Co-generation is well suited in situations where heating or cooling requirements are constant and where electricity can be used on site. Co-generation has traditionally been used to capture waste heat from conventional steam turbines.
Combined cycle electricity generation describes gas turbine systems that capture exhaust gases to heat water. This water can then be used to drive a steam turbine to generate more power.
Tri-generation technology adds cooling to the co-generation process. Waste heat can be turned into cooling by:
- Absorption chilling/refrigeration - using heat to drive a cooling system that works on a closed cycle of evaporating, dissolving and separating out two liquids at different pressures.
- Desiccant cooling - using waste heat in a closed system to dry chemicals that are, in turn, used to extract moisture from the air before it is cooled in an evaporative cooler.
Gasification
Gasification is a thermo-chemical process that involves heating a solid biomass to temperatures of around 800-1000°C in a gasifier with a limited supply of oxygen. Under these conditions, fuel is only partly burnt and is largely converted to ‘syngas’ which contains a mixture of methane, hydrogen, carbon monoxide, carbon dioxide and nitrogen. Small amounts (about 1%) of char are produced through gasification.
Syngas can be used directly for heat or power applications, for example to run gas engines, gas turbines or combined cycle power systems. It can also be upgraded for biofuel production via a number of new and emerging technologies.
Gasification is generally more efficient than combustion-based routes in terms of electricity generation. However, it is more demanding in terms of biomass specifications like moisture content and particle size, and there can be issues with tars and pollution control.
Pyrolysis
Pyrolysis is similar to gasification, in that it involves thermal degradation of biomass heated in the absence of air - or very limited air or oxygen. It produces solid, liquid and/or gaseous products at ratios dependent on the speed and temperature of the process. The energy in the liquids and gases can be used to generate bioenergy.
Slow pyrolysis involves heating biomass to temperatures of around 500°C and results in roughly equal proportions of biochar, liquid and gas.
Fast 'flash' pyrolysis is done at much higher temperatures, and can yield up to 80% bio-oil which can then be used in other energy production systems. Also, it produces less gases and biochar.
Anaerobic digestion
Anaerobic digestion is the biological breakdown of biomass in oxygen-free conditions. Anaerobic digestion occurs in many situations, including landfills, effluent lagoons and purpose-built digesters.
Many types of biomass can be anaerobically digested, but the process is particularly suited to wet feedstocks. Sewage, manures, wet agricultural residues and effluents can be anaerobically digested to produce biogas - usually a mixture of methane and carbon dioxide. Biogas can then be burnt to produce heat and/or generate power for onsite use. It can also be upgraded to natural gas standards (biomethane) and exported to the gas grid, or used in gas-engine vehicles.
The undigested sludge residue from anaerobic digesters can be dehydrated and burnt to produce more energy, or used as a soil improver.
Anaerobic digestion also occurs naturally in landfills. Gases can be captured through collection pipes and burnt off, or used to generate heat and/or power. Most larger landfills in Victoria capture landfill biogas and use it to generate electricity.
Biofuels for transportation
Biofuels can be used as a substitute for, or mixed with diesel, petrol and LPG. Many biofuels can be produced from biomass, and there are many biofuel technologies being used at various stages of commercialisation. The main bioenergy transport fuels are:
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Biodiesel, which can be produced from waste fats and oils, and oilseed energy crops through various processes. Biodiesel can be used with diesel, or used unblended in many modern diesel engines.
- Biomethane, which is biogas upgraded to natural gas standard and exported to the natural gas grid for use in gas-powered vehicles.
- First-generation bioethanol is produced through fermentation of sugars and starches extracted from crops such as sugar cane, sugar beet and corn. Bioethanol is typically blended with petrol at rates of 5-10%. E85 engines have been in use for some decades now and are specifically designed to run on 85% bioethanol, 15% petroleum. Bioethanol is produced in many countries around the world, with Brazil producing around 80% of the world’s bioethanol from sugar cane.
- Second-generation bioethanol can be produced via different technologies that are in varying stages of development and commercialisation, using a wide range of feedstocks such as wood or straw. Second generation ethanol production processes from lignocellulosics, (woody biomass), is considerably more complex than 1st generation production as it involves the extra steps of pre-treatment to separate out the biomass into lignin, cellulose and hemicelluloses. This process is then typically followed by either thermochemical hydrolysis of cellulose, followed by fermentation or, via more recently developed biochemical pathways, using enzymes and micro-organisms to hydrolyse sugars prior to fermentation to produce ethanol, butanol and other fuels. Another second generation technology being developed is via thermochemical processes using pyrolysis or gasification technologies. Fast pyrolysis processes can produce bio-oil, syngas and/or biochar. Bio-oil can be further refined to produce petrol, biodiesel and other high value chemicals. A number of industries and researchers are investigating the upgrading of pyrolysis oil into transport fuels. Gasification methods produce syngas which, when purified, can be used in vehicles as a substitute for LPG, or further processed using Fischer-Tropsch synthesis or other established catalyst based conversion processes to produce syndiesel and aviation biofuels. The process was originally developed in Germany in the 1920’s and used to produce transport fuels from coal. Alternatively the syngas from a gasification process can be converted by a biological process to produce alcohol based biofuels such as ethanol. Greater efficiencies in some of the processes are needed to make second generation biofuels cost-competitive with other first-generation biofuel and fossil fuels.
- Third-generation bioethanol technologies, including hydrogen production from biomass, which can be used in fuel cells or special internal combustion engines, and algae to biofuels.
- Algae to biofuel systems, which can be separated into two categories:
- Macroalgae - for example seaweed, which can potentially be used for heat and power generation, such as in a biodigester to produce biomethane, or fermented to produce ethanol. Macroalgae is still at an early stage of development.
- Microalgae - microscopic, photosynthetic organisms that produce substances, such as lipids, that can be harvested and converted to a range of products, including biodiesel.
Algae production systems need to overcome many technical challenges before they become commercially viable. Other potentially high-value bi-products can be extracted from certain types of algae, which may help bring production costs down.
What are the benefits of bioenergy?
Bioenergy is renewable, and can generate many additional benefits. Its success depends on a combination of factors, including the technologies used, the types of feedstocks, (biomass) used - and how they are produced, transported and processed.
Renewable energy
Bioenergy is a renewable alternative for fossil fuels.
Reducing greenhouse gas (GHG) emissions
Bioenergy is potentially carbon-neutral. The carbon dioxide that is released from the production of bioenergy is from carbon that has entered the atmosphere through the process of photosynthesis, therefore is does not contribute extra carbon dioxide to the atmosphere like fossil fuels.
Bioenergy's GHG reduction benefits are potentially greater than those of other renewables. The extent of GHG emissions reduction varies widely and depends on many factors including the type of feedstocks used, how they are grown/produced, harvested, transported and converted to energy. Generally, GHG emissions reduction from bioenergy systems is greatest where waste streams are converted to bioenergy in combined heat and power plants on site where they are generated.
Considerable research is underway around the world to quantify the total lifecycle impacts of various bioenergy and other renewable energy systems. For example, through the IEA Bioenergy Task 38 project 'Greenhouse Gas Balances of Biomass and Bioenergy Systems'.
Better air quality
Biomass residues that would otherwise be burnt in the field or forest, such as stubble or forest slash, are removed and burnt in an emissions controlled environment.
Biofuels are biodegradable
Petroleum-based fuels and chemicals are harmful to the environment and a major surface and ground water-pollutant. Biofuels, including ethanol and biodiesel, are much less toxic, and are biodegradable.
Regional and rural economic development opportunities
Bioenergy helps stimulate regional economic development and employment by providing new, decentralised and diversified income streams from bioenergy and biomass production. This gives landholders have more options with new and diversified uses of agricultural and tree crops.
Supporting agricultural and food-processing industries
Using biomass can help build resilience in agricultural and food-processing industries. Biomass provides a use for their waste streams, can help them reduce their energy costs, and potentially add a new revenue stream if they can sell biomass-derived heat and/or export electricity to the grid.
Cost savings
Using the right bioenergy technology in the right situation can help achieve greater cost savings than using fossil fuels, especially in remote and rural areas where electricity often comes from centralised electricity grid networks, with higher transmission costs.
Less landfill
Using waste streams to generate bioenergy saves the environmental and economic costs of disposal in landfills and reduces contamination risks.
Energy reliability and security
Rural and regional energy reliability and security can be enhanced by providing a domestic energy source that can run continuously, or at peak times required by the electricity market, as with many coal-fired plants.
Generating heat and electricity
Unlike most other renewable energy sources, biomass can generate both heat and electricity in a combined heat power (CHP) plant. This can then be used or a range of heating and cooling applications in industry, or for small communities.
A growing range of technologies and applications
There's a growing range of proven, adaptable technologies available for converting biomass into heat, electricity and biofuels.
Bioenergy production can link with the development of other bioproducts and biotechnologies. For example, producing useful chemicals as part of an integrated biorefinery system.
Alternatives to prescribed forest-burning
Bioenergy production is an alternative to prescribed burning of forests. Mechanical thinning and biomass removal for bioenergy can reduce use of hazardous fuels, especially in areas where the cost and risks associated with prescribed burning are high. These techniques are widely used in forests and woodlands in the USA .
Environmental benefits from growing certain bioenergy crops
Bioenergy crops can be grown in areas that benefit from the additional vegetation cover. For example, trees can be grown and harvested for their woody biomass on farms in configurations that help improve farm shelter, shade, salinity control and biodiversity.
Production of biochar
Biochar production also yields bioenergy in the form of syngas, which can be used to produce heat and/or power.
How can I produce bioenergy?
To determine what type of bioenergy you can produce, you need to assess the type and amount of biomass you can grow or source. If you wish to grow a bioenergy feedstock, such as a biofuel oil crop, you also need to determine who'll process it or buy it from you - and the economics of producing it. Like any agricultural crop, it's about assessing your farm's capacity, commodoty prices and the market’s demand.
More information
https://www.energy.vic.gov.au/renewable-energy/bioenergy