Fossil fuels, although originally derived from organic matter, have been created over many millennia through biological and geological processes and are essentially non-renewable.
Organic matter used to produce bioenergy are called biomass or bioenergy feedstocks. Bioenergy can be traced back to energy from sunlight, produced via photosynthesis, making it a major renewable energy source. As a storage house of bioenergy, biomass can be considered to be natures ‘solar batteries’.
The energy biomass produces can be converted into electricity, heat or biofuels. Bioenergy can be as simple as a log fire or as complex as an advanced second generation liquid biofuel.
Bioenergy is the most widely used renewable energy in the world, providing around 10% of the world’s primary energy supplies, mostly as thermal energy for heating and cooking.
Types of biomass
Sources of biomass include agricultural crops, animal and plant wastes, algae, wood and organic residential/ industrial waste. The type of biomass will determine the type and amount of bioenergy that can be produced and the technology that can be used to produce it.
For example, agricultural crops, like corn and canola, can be used to produce liquid biofuels such as ethanol and biodiesel. Alternatively, wet wastes like manure are well suited to produce biogas through anaerobic digestion, which can be combusted to generate electricity and heat or upgraded into a transport fuel, biomethane.
How is bioenergy produced?
There are many ways to produce bioenergy. Choosing the best pathway and technology depends on the biomass material and the type of bioenergy you want to produce. Some processes can be relatively simple, like growing, harvesting and
Other complex processes, like algae production for transport fuels, require a controlled growing environment using specific microalgae species. The algae are then processed to separate the oils which are refined into biofuels.
|Photo 1: Feeding briquettes into biomass boiler. Photo taken by Edith Paarhammer, Paarhammer Joinery.|
A variety of conversion pathways can be used to convert biomass into bioenergy in the form of heat, electricity, or transportation fuels. Biomass conversion pathways include thermal, biochemical or mechanical either alone or in combination. Biomass can be converted to energy via a range of technologies from simple solid wood combustion heaters, to boilers and biodigesters which in
turn produce gas for process steam for heat or for powering engines and turbines.
Biorefineries are facilities used to convert biomass into a number of fuel types and other bio-products; similar to conventional oil refineries. Fuel cells
can convert hydrogen, produced from biomass, into electricity for stationary or transport energy.
Biomass conversion technologies
Biomass conversion technologies for stationary electricity and heat generation
There is a wide range of feedstocks that can be used, and technologies and processes for extracting energy from biomass and converting it into stationary bioenergy for heat and/or electricity. The more commonly used being conventional combustion, gasification, pyrolysis and anaerobic digestion. Each is discussed further below.
Direct combustion is the simplest and most widely used bioenergy technology for converting biomass to heat which can then be used for space heating or cooling, to heat water, for use in industrial processes, or to produce electricity via a steam engine or turbine. Combustion typically has an electrical efficiency of only
|Photo 2: Woodlands biomass power plant, Woodlands, California.|
- Fixed bed combustion – a common technology which involves burning materials on a fixed or moving grate with air passing through it.
- Fluidised bed combustion – 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 or LPG. Co-firing can be a cost-effective way for fossil fuel power generators to reduce greenhouse gas (GHG) emissions.
Various fuels can be combusted with coal with minimal processing beyond chipping or shredding and drying. Most fuels are easier to handle, transport and store if they are processed and compacted into pellets or briquettes, which are denser and more consistent in quality and moisture content.
Co-generation, also known as Combined Heat and Power (CHP) has greater energy conversion efficiencies because it captures ‘waste’ heat from electricity generation, which can be used for space and water heating, or 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 and produce steam which can then be used to drive a steam turbine to generate more power. The principle is that the exhaust of one gas engine is used as the heat source for another, thus extracting more useful energy from the heat, increasing the system’s overall efficiency.
Tri-generation technology adds cooling to the co- generation process. Waste heat can be used in cooling via:
- Absorption chilling/refrigeration – where heat drives a cooling system using a closed cycle of evaporating, dissolving and separating out two liquids at different pressures, or,
- 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 is a thermo-chemical process that involves heating 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 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 existing 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.
The need to scrub gases and dispose of tars can be an
issue if the syngas is to be run through a gas engine to generate power.
Plasma gasification technology uses electricity in combination with an arc gasifier to create high temperatures which breaks down biomass and inorganic matter into syngas that can be used to generate electrical energy. At present, Plasma gasification is mainly deployed as a waste treatment technology focussing on inorganic waste streams including household rubbish.
Pyrolysis is similar to gasification, in that it involves thermal degradation of biomass heated in the absence of air – or with very limited air or oxygen. It produces solid, liquid and/or gaseous products at ratios dependent on the speed and temperature of the pyrolysis process. The gases and compounds in the liquids 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, (bio-oil) and syngas.
|Photo 3: Un-cleaned mustard seed.|
Fast ‘flash’ pyrolysis is done at much higher temperatures, and can yield up to 80% bio-oil which can then be used in other stationary energy and biofuel production systems. Fast pyrolysis produces small quantities of syngas and biochar.
Biochar is a stable form of charcoal that can be produced in many different ways and thus have many varied chemical and structural properties. It may be used later to produce heat or it may have other commercial value as a soil improvement and carbon sequestration product.
Anaerobic digestion is the biological breakdown of biomass in oxygen-free conditions. Anaerobic digestion occurs naturally e.g. in oxygen starved peat swamps and in man-made environments, including landfills, effluent lagoons and purpose-built biodigesters.
Many types of biomass can be anaerobically digested in biodigesters, but the process is particularly suited to wet feedstocks that do not contain lignin. Sewage, manure, wet agricultural residues, straw and effluents can be anaerobically digested to produce biogas – a mixture of mostly methane and carbon dioxide. Biogas can then be combusted to produce heat and/or power in a gas turbine. It can also be upgraded to natural gas
Biodigesters are sealed systems that intentionally promote the controlled build-up of biogas. They can be very simple in design and are commonly built and used by households in developing countries. There are also many larger systems used by farmers and food processors in countries such as Germany.
| Photo 4: Dairy cows grazing pasture.
Photo 5: Biogas engines. Photo courtesy of Murray Goulburn Co-operative.
The undigested sludge residue from biodigesters can be dehydrated and burnt to produce more bioenergy, or used as an organic fertiliser or compost.
Biogases can be captured in landfills and sewage treatment ponds through collection pipes and flared (burnt off) or used to generate bioenergy. Most of the larger landfills and sewage treatment plants in Victoria capture biogas and use it to generate heat and/or electricity for use on-site or sell it for a premium price as ‘green power’.