Australia’s advanced brown coal-fired power generation technology

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Advanced Brown Coal Combustion Technology in Australia by D. Brockway 1 Overview Coal is the main fuel used for power generation in Australia, with coal-fired power accounting for 80% of the country’s total electricity production. Two-thirds of the coal used for power generation in Australia is high-ranking coal (bituminous coal), while one-third is lignite (a low-ranking coal with a moisture content of 30% to 70%). According to the Electricity Supply Association of Australia (ESAA), Australia’s electricity demand is expected to increase by 37% over the next 15 years. Part of this additional electricity demand will be met by the excess power generation capacity in New South Wales, while new power plants will also need to be built. A recent research report prepared by Sinclair Knight Merz (SKM) for the Cooperative Research Centre on New Technologies for Low-Rank Coal Power Generation (CRC) suggests that Victoria and South Australia will need to build 9 to 11 new coal-fired power plants with a capacity of 1,000 MW each over the next 40 years, in order to meet a minimum electricity demand growth rate of 2%. 　　In the subsequent decades, the growth rate of renewable and bioenergy sources is likely to be high, but their contribution to Australia’s total energy supply will remain relatively small. Coal will remain the main fuel for power generation in Australia in the future. Developing efficient, inexpensive, and clean brown coal power generation technologies holds significant economic strategic importance for Australia, particularly for Victoria and South Australia. 2 Cooperative Research Center for New Technologies in Low-Rank Coal Power Generation (CRC) This center was established in July 1993, and its main task is to research and develop new technologies and processes for power generation, in order to address the issues of high combustion costs associated with low-rank coal as well as the severe environmental pollution it causes. New power generation technologies are key to reducing power generation costs and environmental pollution. Companies and institutions working in collaboration with the CRC include: Loy Yang Power Corporation, Yallourn Energy Corporation, Hazelwood Power Corporation, Flinders Power Corporation, the Minerals Division of the Commonwealth Scientific and Industrial Research Organization (CSIRO), Transfield Technical Services, Lurgi Australia, Monash University, University of Adelaide, Swinburne University of Technology, and the Industrial Strategy Research Foundation. 　　Private or jointly-owned power companies in Victoria and South Australia are strong supporters of research into efficient power generation technologies. These companies are very optimistic about the long-term lignite power generation market in Victoria and South Australia, and are particularly interested in reducing power generation costs and minimizing environmental pollution. 　　Currently, the Research Center for New Technologies in Low-Rank Coal Power Generation is carrying out 40 research and development projects on new technologies for lignite power generation. With the active involvement of its partners (including the industry, CSIRO, and academic institutions), the center has a strong research capacity. The researchers involved in the research projects at this center include over 110 scientists, engineers, and technical managers, among whom 35 are postgraduate students. 3 Opportunities and Challenges Facing Lignite-Based Power Generation Technology Undoubtedly, coal-fired power plants around the world will continue to rely heavily on higher-grade coal. However, countries with abundant lignite resources **will actively increase the use of lignite as fuel. These countries include Germany, Russia, the United States, Poland, the Czech Republic, Greece, Turkey, Australia, China, Romania, Canada, Bulgaria, India, Thailand, Hungary, Spain, and Indonesia. It is worth noting that many Asia-Pacific regions facing power shortages **have large reserves of lignite**. 　　Lignite contains a large amount of moisture, and it generally needs to be processed before being transported or exported. Lignite is mainly used in pithead power stations. It is estimated that the world’s proven lignite reserves are sufficient to supply 2,100 power plants with a capacity of 1,000 MW for 30 years. At Australia’s current rate of lignite consumption, the economically viable reserves of lignite would last the country for 1,000 years. 3.1 Strategic advantages of lignite power generation The strategic advantage of lignite power generation, particularly for the Australian economy, lies in the very low cost of this fuel. The direct electricity generation costs for lignite in Victoria, Australian hard coal, and natural gas are 3 AUD/MWh, 11–15 AUD/MWh, and >20 AUD/MWh, respectively. The reason for the very low electricity generation costs at the lignite surface power plants in Victoria is the large thickness of the coal seams, ranging from 60 to 100 meters ； The overlying strata of the coal seam are thin, generally 5–10 meters ； A bucket wheel excavator is used, enabling large-scale production. 　　Fuel cost is just one factor in power generation costs; other major costs include equipment expenses, as well as operational and maintenance costs for the power plant. However, fuel costs alone have already demonstrated the strategic advantage of lignite power generation in Australia. 　　By using current coal injection boilers and the profits generated from reasonable investment, the ultimate cost of brown coal power generation in Victoria is 40 Australian dollars/MWh. 3.2 Strategic Challenges in Lignite Power Generation The high moisture content of lignite, combined with the low thermal efficiency of power plants that use conventional coal injection technologies, poses strategic challenges to lignite-based power generation, namely the issue of greenhouse gas (GHG) emissions. This issue is currently being addressed by developing efficient power generation technologies. 4 Australian lignite-fired power plants at the mine site The main use of lignite in Australia is for power generation. However, the physical properties of lignite pose special challenges to its use. The physical properties of lignite include high moisture content, low ash fusion point, and tendency to form slag. 　　Due to its high moisture content and tendency to catch fire easily during drying, the lignite in Victoria is used in power plants located near open-pit mines. Lignite is first transported by conveyor to a temporary coal bin; after a short period of storage (usually less than 24 hours), it is sent to pulverized coal injection boilers for combustion to generate electricity. Lignite is in a high-moisture state, and all of that moisture passes through the boiler; therefore, large and expensive boilers are required. Since the lignite produced in South Australia comes from remote and barren mining areas that lack the cooling water required for power generation, the lignite produced there must be transported by rail to power plants in Port Augusta, 260 km away. The power companies in Victoria and South Australia produce 45 TWh of electricity per year, consuming 60 million tons of lignite. 5 Advanced combined cycle power generation technologies for burning lignite To improve the thermal efficiency of coal-fired power plants and reduce emissions of CO2, NOx, and SOx, many advanced power generation technologies are in various stages of development. These technologies include: Circulating Fluidized Bed Combustion technology (CFBC), Supercritical Pulverized Coal Combustion boiler technology (SCPC), Pressurized Fluidized Bed Combustion technology (PFBC), Integrated Gasification Combined Cycle power generation technology (IGCC), and Advanced Pressurized Fluidized Bed Cycle power generation technology (APFBC). The main focus of research and development in these advanced technologies is on the use of higher-grade coal. 　　The high moisture content of lignite **affects the power generation efficiency of traditional pulverized coal injection combustion boilers. The modern boilers at the Loy Yang Power Plant burn lignite with a moisture content of 62%, achieving a combustion efficiency of around 29% under high heat value (HHV) conditions. In contrast, the combustion efficiency of high-rank hard coal in conventional boilers is 37%. 6 Efficiency of Advanced Cyclic Power Generation Using Lignite The overall efficiency of advanced combined cycle power generation technologies using higher-grade coals has been assessed by many international organizations, among which the assessments conducted by the International Energy Agency (IEA) are likely to be the most comprehensive. The CRC Center also conducted a similar assessment of the utilization process of lignite in advanced power generation technologies. The assessment took into account the necessity of coal drying processes (using steam fluidized bed drying technology (SFBD)), and identified the characteristics of lignite relevant to its utilization. Figure 1 Comparison of power generation efficiency between low-rank coal and high-rank coal (see page 57 of the third issue of \"China Coal\"). Figure 1 shows the net combustion efficiency of lignite (with a moisture content of 62%) and high-rank coal under high calorific value conditions. The combustion efficiency of higher-grade coal is taken from references (including IEA data) and includes many different values. When calculating lignite data, it is assumed that the carbon conversion rate is 100%. Since a complete carbon conversion process in a fluidized-bed gasifier is not possible, an 91% carbon conversion rate was experimented with and proven suitable for IGCC technology (in which carbon is burned in an air-pressure boiler). 　　The APFBC cycle power generation technology achieves the highest combustion efficiency; when generating electricity from lignite with a moisture content of 62%, the combustion efficiency is 44% (under high calorific value conditions). Figure 2 shows the significant reduction in CO2 emissions from various lignite-fired power generation technologies. Figure 2: Comparison of CO2 emissions from various lignite-fired power generation technologies (100% carbon conversion rate) (for details, see page 57 of the third issue of ‘China Coal’) 7 Advanced hydraulic fluidized bed combustion technology: The APFBC technology (see Figure 3) involves a partial gasification (carbonization) process as well as a pressure combustion process using the resulting carbon. During the cycle, fuel gas (from part of the gasifier) and hot flue gas (from the carbon burner) are generated. The fuel gas and flue gas are mixed together and burned together with the supplementary air in the burner at the top; the gases generated by this combustion expand and then pass through the gas turbine. The temperature of the generated gas must be controlled at the allowable turbine inlet temperature to ensure high combustion efficiency. Electric power is generated both by the gas turbines in the combined cycle and by the steam turbines in the high-pressure steam cycle (where the steam comes from hydraulic fluidized bed combustion). 　　Compared with IGCC technology, the main advantage of APFBC technology is its high combustion efficiency under poor conditions: at a temperature of 1000°C for higher-grade coals, and even lower temperatures for lignite, with a carbon conversion rate of 100%. The lower gas temperature in APFBC technology reduces the need to cool the gas before it is exhausted, which allows combined-cycle gas turbines to produce more electricity than high-pressure steam cycle processes with lower combustion efficiency. At the same time, since the APFBC technology does not require air separation equipment, auxiliary power consumption is also reduced. Figure 3: Flow diagram of the advanced pressurized fluidized bed combustion technology (for details, see page 57 of the third issue of “China Coal”). ① Coal feeder ② Coal ③ Dryer ④ Pressurized carbonizer ⑤ Gas filter ⑥ Gas burner ⑦ Gas turbine ⑧ Air ⑨ Coke ⑩ Hot fuel gas ⑪ To the chimney ⑫ Heat recovery boiler ⑬ Steam ⑭ Air ⑮ Pressurized fluidized bed ⑯ Water supply pipe ⑰ Steam turbine. Compared with other combustion technologies, APFBC technology has not received as much development effort to date, but it is recognized as the most efficient combustion technology (including both coal gasification and combustion processes). Foster Wheeler has established a large hard coal combustion demonstration plant in Wilsonville, Alabama, United States. The U.S. Department of Energy (USDOE) has approved funding for the construction of a 240Mwe commercial hard coal-fired demonstration plant in Lakeland, Florida; construction of this plant is set to begin at the beginning of the next century. 8 APFBC technology for burning lignite developed by the CRC Center Based on the basic principles of the APFBC cycle technology, the CRC Center has developed a unique combustion process in which the combustion efficiency of high-moisture lignite is higher than that of various combustion technologies shown in Figure 2. The improved APFBC technology combustion process developed by the CRC Center has the following main features: (1) A drying process is employed ； (2) Remove the alkali vapor from the steam before it enters the gas turbine ； (3) High inlet temperature of the gas turbine ； (4) The carbon was completely transformed. 9 Cost of generating electricity from lignite 　For the advanced coal-fired cycle power generation technology used, the adjusted electricity price over the plant’s operational life should be more competitive compared to the prices of other power generation technologies. The costs of conventional power plants are well known, and due to market pressures, these costs tend to decline; the costs of new power generation technologies, however, are still unclear. Due to standardized design, as well as good technical risk management and redundancy, the most advanced power generation equipment is more expensive than conventional ones. 　　Although many sources analyze power generation costs, the assumptions and ranges of variation used in electricity price calculations are often unclear. The Australian Energy Research and Development Organization and the Australian Electricity Supply Association conducted a comparative cost study in 1992, and later the International Energy Agency carried out a similar study in 1993. For lignite power generation, the CRC Center commissioned SKM Engineering Consultants to conduct a cost comparison study on a series of advanced combustion technologies using the same principles. Recently, the results of a study on power generation costs conducted by the U.S. Department of Energy were published; this study compared the power generation costs of APFBC technology, various IGCC power generation approaches, PFBC technology, and conventional coal injection technology. The conclusions of these power generation cost assessments can be summarized as follows: (1) The basic cost of various lignite-based power generation technologies used in Australia is approximately 1,350–1,800 Australian dollars/kW ； (2) The adjusted generation cost for lignite-fired power plants that meet environmental requirements is estimated to be 35–43 Australian dollars/MWh. (3) The generation cost of power plants using the mature APFBC technology to burn lignite in Victoria is estimated to be 35 Australian dollars per MWh. (5) The generation cost of power plants using APFBC technology to burn lignite in Australia is expected to be more competitive than that of advanced power generation technologies that use higher-grade coal and natural gas. (Translated by Li Xiaoshang)