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A suite of technologies makes up Australia’s existing electricity generation portfolio. New technologies are currently being developed to add to the mix and enhance the ability of Australia to reach its long term emissions reduction target. Least-cost delivery of greenhouse gas emissions reduction requires the broadest range of technologies available including some technologies which are not yet commercially available in Australia (including carbon capture and storage). Currently the lowest greenhouse gas intensity technology that is commercially able to provide base load power is gas CCGT, ie combined cycle gas turbine. The Federal Government has ruled out nuclear power for Australia. Fossil fuel technologies Combined Cycle Gas Turbine (CCGT)
The integrated combustion of gas and steam turbines is called a combined cycle gas turbine (CCGT). CCGT plants achieve high thermal efficiencies by utilising most of the waste heat from the gas turbine exhaust. While used mainly to supply intermediate or peak load in Australia, they are increasingly becoming an economic option for base load generation where natural gas prices are competitive. Supercritical (SC) and ultra-supercritical (USC) pulverised fuel (coal)
Conventional (sub-critical) pulverised fuel (PF) power generation is the main generation technology used in Australia. It involves burning finely milled coal in air in a boiler to generate steam to drive turbo-alternators. Over the last 60 years significant improvements have lead to supercritical (SC) and ultra supercritical (USC) technology being developed. SC and USC plants are characterised by progressively higher boiler steam pressures and temperatures, whereby thermal efficiencies can be increased. Dry-cooling techniques are increasingly used in conditions where a lack of adequate water resources is a problem. The water savings associated with dry cooling need to be balanced with the efficiency losses that generally result. Integrated Gasification Combined Cycle (IGCC) – black coal
Internationally, IGCC is a technically proven and near-commercial technology. In the IGCC process, coal is converted into a synthetic gas (syngas), which is cooled and cleaned to remove particulates and sulphur compounds, passed through a shift reactor (to separate carbon dioxide) and then burned in a CCGT unit. The main components are thus a coal gasification facility, an air separation unit (oxygen instead of air is typically used in the gasification process), a gas cleaning facility, carbon dioxide shift reactor and a CCGT power plant. As concentrated carbon dioxide is one of the by-products of the gasification process this technology is highly suited to carbon capture and storage (CCS). IGCC systems using air instead of oxygen in the gasification process have also been developed. These avoid the upfront capital and energy cost of the air separation unit, but are not as ideally suited to CCS. Integrated Gasification Combined Cycle (IGCC) - brown coal
Research has been underway for some time to develop IGCC technology to utilise brown coal, which requires a technology to be developed to dry the coal before or as part of the gasification process. So-called Integrated Drying Gasification Combined Cycle is one such process that has been developed in Victoria. Brown coal drying and dewatering
The efficiency of brown coal-fired pulverised fuel (PF) power stations can be substantially increased (with corresponding greenhouse gas emissions reductions) if the high moisture content of the coal is reduced prior to combustion. Steam fluidised bed drying (SFBD): a technique that has been under development in Germany for over 20 years whereby finely divided coal is dried in a fluidised bed with an internal heat exchanger to provide heat for drying. Mechanical thermal expression (MTE): a process originally developed in Germany which is undergoing significant development in Victoria. Coal is heated to 150 to 200 degrees Celsius at saturation pressure to prevent evaporation, and then squeezed by applying mechanical pressure.
Oxy-firing
Oxy-firing (also referred to as oxy-fuel combustion) is currently experimental but relatively simple reconfiguration of existing pulverised fuel (PF) technology to burn pulverised coal in a mixture of oxygen and recirculated flue gas instead of air. There are a number of possible variants of the process, but in simple terms the main technology modifications involve an air separation unit and the flue gas recycling process (and if equipped for CCS, the carbon dioxide compression facility). Oxy-firing reduces the net volume of flue gases and substantially increases the concentration of carbon dioxide in the flue gases compared with the normal air-fired process. These features make oxy-firing highly suited to CCS and provide for a potentially lower cost option for achieving near-zero emissions coal-fired electricity generation compared with IGCC and USC with post combustion capture. Ultra clean coal (UCC)
UCC is coal that has had virtually all of the mineral contamination removed from it using a chemical leaching process similar, in part, to the process used to refine bauxite into alumina. The cleaned coal can then be used as an alternative to natural gas in a combined cycle turbine. The technology is most attractive to countries with adequate coal supplies but limited access to competitively priced natural gas resources. Zero emissions technologies incorporating carbon capture and storage Carbon capture and storage (CCS) technologies could be combined with any of the generation options outlined above and can in-principle be retrofitted to existing plants. While many of the techniques and technologies are well-established in other industrial applications, their adaptation to power generation systems is currently at the experimental or demonstration stage. The type of fuel and generation technology that is used will determine the type of capture technology that is most suitable. The three main approaches that could be used to capture carbon dioxide are: Post combustion capture (PCC) or flue gas capture: after combusting either pulverised coal or natural gas, carbon dioxide can be separated and captured from the flue gas using contact with chemical solvents, physical absorption, cryogenic separation or membrane separation. Oxygen separation: carbon dioxide concentrations in the flue gas can be increased significantly by increasing the level of oxygen and reducing the nitrogen content in the combustion air (as in the oxy-firing process described above). Hydrogen (or syngas) approach: a pre-combustion capture approach suited to the IGCC process described above. The fuel is first reacted with oxygen, air, or in some cases, steam to produce a gas consisting mainly of carbon monoxide and hydrogen.
The carbon monoxide is then reacted with steam in a catalytic shift converter to produce carbon dioxide and more hydrogen. The carbon dioxide is then separated from the hydrogen, typically using chemical absorption methods. Carbon dioxide can then be transported using high pressure pipelines – a technique that has already been proven for use in enhanced oil recovery (EOR) projects since the 1980s (although on a much smaller scale than what would be required for transporting power generation emissions). It could also be transported in tankers similar to those used to transport LPG. Captured carbon dioxide has the potential to be stored in a variety of geological or ocean sites including active and depleted oil and gas reserves, deep and unminable coal seams and saline aquifers. Renewables Hydro
From an elevated barrier, water can be diverted through a tunnel or tube into a turbine coupled to a generator that converts the kinetic energy of the falling water into electricity. Biomass
Biomass generators produce electricity from organic matter such as energy crops, plant derivatives (bagasse), industrial and animal waste (landfill, faeces, carcasses). There are various technologies with quite different features (fuel availability, costs etc) including: Direct combustion: the most widely-used technology (for example, burning wood waste in a boiler to produce steam). Co-firing of coal (PF) with biomass: the combustion of biomass in an existing coal fired power plant furnace. Typically the biomass (for example, wood chip) is added to the feed coal making up five per cent of the total fuel combusted to produce steam. Liquid fuels: methanol, ethanol and bio-fuel can be produced from biomass. These can be used for both electricity generation and for transport energy uses. Gasification: a process of decomposition of organic matter at elevated temperatures (called pyrolysis) to produce a combustible gas. The resulting gas is a more versatile fuel than the original biomass and can be used in more efficient combined cycle power generation systems. Anaerobic digestion: a biological process whereby organic wastes are converted to biogas usually comprised of a mixture of methane and carbon dioxide. The biogas can then be used as a fuel source for generation. The process is based on the breakdown of the organic molecules by naturally occurring bacteria (for example as in landfill gas).
Wind power
Wind turbines use the kinetic energy of the wind to spin the rotor blades of a turbine, driving a generator that produces electricity. Wind farms increasingly consist of a large number of individual turbines. Wind farms can be located either onshore or offshore but costs have so far prohibited offshore wind farm development in Australia. The availability of sites with an ideal wind regime is a naturally limiting factor since the economics of a wind farm are heavily impacted by wind speed. Problems with intermittency may be increasingly addressed through emerging technological solutions (electricity storage) and advanced forecasting techniques. Solar photovoltaic
Photovoltaic (PV) technology transforms the energy of solar photons into direct electric current (DC) using semiconductor materials and then into alternating current (AC) using an inverter. The basic unit is a solar or PV cell. When photons enter the cell, electrons in the semiconductor material are freed, generating electric current. PV technology has a wide range of applications; those directly associated with electricity production include: Stand alone off-grid systems: most likely in developing countries or remote rural areas. Such systems can be cost effective in isolated areas compared with the cost of delivering electricity from the grid. Grid-connected systems: these can be attached to buildings or other infrastructure and can be used to sell electricity into the grid when not needed. Utility-scale (or concentrated) PV systems: power plants made up of many PV arrays installed together (capacity of up to several megawatts). These are likely to achieve sent out costs lower than the other applications.
Solar hot water
Whilst not an electricity generation option as such, increased uptake of solar hot water systems could avoid the need for additional centralised generation and could thus deliver greenhouse benefits. Using established commercial technology, solar energy can be collected with either flat-plate or evacuated tube collectors to heat water for residential and industrial applications. Generally, more efficient (and more expensive) systems are required to generate the higher temperatures required for industrial use. Solar hot water systems generally require auxiliary back up systems run on off-peak electricity or gas. Heat pump systems use a refrigerant to extract heat from the atmosphere and also require electricity to operate. Solar thermal energy (STE)
Solar Thermal, also called Concentrating Solar Power, includes three main types of systems - parabolic trough, parabolic dish and power tower. All of these technologies rely on a process whereby a concentrator captures and concentrates direct solar radiation, which is then delivered to the receiver. The receiver absorbs the concentrated sunlight, transferring its heat energy to a conventional power conversion system such as a steam turbine. Geothermal
Geothermal energy is derived from heat originating within the earth. Processes using the natural steam generated by geothermal energy have been commercially applied in other parts of the world since the 1950s to generate electricity. A relatively new and unproven application of this concept known as ‘hot dry rock’ is currently being examined in a number of locations in Australia, including the Cooper Basin (South Australia). Heat producing granites, located three kilometres or more below the earth’s surface, are trapped by overlying rocks which act as an insulating blanket. The heat is extracted from these granites by circulating water through them in an engineered, artificial reservoir or underground heat exchanger and is converted into electricity in a turbine. Wave power and tidal power
Wave and tidal power technologies generate electricity using the kinetic energy of moving water. There are a number of experimental technologies that are currently under development, including in Australia. A number of significant barriers are likely to be faced; for example, the best sites are likely to be located far from the electricity grid – for wave power this is in the deep ocean and for tidal power off the remote north-west coast of Western Australia. Other electricity generation technologies Cogeneration
Cogeneration is the simultaneous production of electricity and useful heat from the same energy source (usually combustion of a fossil or biomass fuel). Cogeneration is generally used where there is a combined need for electricity and process heat, the latter normally being supplied as steam, though not necessarily so. Higher overall efficiencies are achieved by using the heat energy that is otherwise wasted. Fuel cells
Fuel cells convert hydrogen directly into electricity via a thermochemical reaction. The fuel cell reacts the hydrogen with oxygen from the air to produce electricity and water. A power conditioner is required to convert the DC produced by the fuel cell into AC for use in conventional electrical systems. The reaction of fuel produces very little oxides of nitrogen compared with normal combustion. However, most fuel cells operating today use natural gas as the fuel for hydrogen production and hence still produce greenhouse emissions.
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