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The role of Circulating Fluidised Bed (CFB) technology in future coal power generation

Posted: 1 November 2017 By: Qian Zhu

Update from the IFSA2017 - 6th Conference on Industrial Fluidization South Africa  

The 6th Conference on Industrial Fluidization South Africa (IFSA2017) was held on 3-5 October 2017. It is a triannual event which took place this year at the Glenburn Lodge situated in the tranquillity of the Zwartkops Mountains, on the banks of the Blaauwbank and Crocodile Rivers in South Africa. Around 50 delegates from a number of countries with a mix of academic, industrial and organisational backgrounds, attended the conference. The moderate number of attendees meant that delegates could easily get to know each other and participate in interesting conversations.

October in South Africa is spring season so the sun was shining, and it was pleasantly warm, which was reflected in the atmosphere of the lecture hall. Dr Rebecca Maserumule, Chief Director of Hydrogen and Energy, Department of Science and Technology, SA, gave the opening speech, stressing the importance of coal in South Africa as an energy source supplying affordable electricity and providing employment. Coal has played a key role in the South African economy for decades and will continue to do so for the foreseeable future. Therefore, it is vital to find ways to use coal in a much cleaner way.

Fluidisation is employed in a wide range of industries across the world. Some or all of the properties of a fluidised bed such as excellent gas-solids mixing and high heat transfer rates, have led to its application in a broad range of processes including combustion/power generation, gasification, minerals treatment, drying, coating/encapsulation, and more recently, chemical looping. CFB technology for power generation from coal has been advancing steadily and significant development has been made within the last decade. This technology has continued to evolve as demonstrated by the presentations from delegates from universities, research institutes and technology providers at IFSA2017.

The conference venue, Glenburn Lodge situated in a famous UNESCO World Heritage Site, next to the Blaauwbank and Crocodile rivers.

After the opening speech by Dr. Maserumule, two lectures were given by invited speakers. The first guest speaker was Prof. Anthony from Cranfield University (UK). He described oxyfuel combustion as a promising technology for carbon capture and storage (CCS), and circulating fluidised bed combustion (CFB) as a power generation technology which can utilise the oxyfuel concept. After a brief review of the R&D that has been carried out worldwide, Prof. Anthony discussed the technical challenges and potential solutions in the design and scale-up of the oxyfuel CFB process. He also examined some novel approaches such as 100% oxygen firing in the calciner of a Calcium looping unit and pressurised oxy-fired fluidised bed combustion units. Oxyfuel combustion is one of the main options for CO2 capture from coal combustion plants. Several universities/research institutes, technology providers, such as Alstom and Foster Wheeler, and utility companies are exploring the oxyfuel CFB concept using pilot-scale tests. One of the major advantages of oxyfuel CFB technology is that circulation of hot solids in the primary reaction loop, in conjunction with the recycle of flue gas, can potentially provide an effective means to control combustion and extract heat during the combustion process, thus allowing either a significant reduction of the amount of recycled flue gas, or alternatively permitting the use of a much higher oxygen concentration in the combustor. This can lead to significantly improved economics of oxy-fired CFB over oxyfuel pulverised coal (PC) combustion by reducing the size of the CFB boiler island by as much as 50%, making it highly competitive to PC combustion power plants with CCS.

The second guest speaker was Prof. Franz Winter, Head of Research Group of Reaction Engineering & Combustion, Vienna University of Technology in Austria. Prof. Winter declared CFB to be a key technology for the efficient utilisation of resources such as waste, biomass and coal. He presented the recent R&D from the Vienna University of Technology, in developing FBC technology for various processes. His work showed the versatility of CFB technology as it can be applied to combustion, gasification, chemical looping combustion (CLC), CO2 capture, synthetic biofuels and fluid catalytic cracking.

The CFB process for power generation was developed to burn low grade coal or coals that are difficult to burn in a pulverised coal boiler. The low combustion temperature and in-bed desulphurisation means that a CFB boiler has low SOx and NOx emissions that meet emission standards in most countries without the need for pollutant removal systems such as FGD/SCR. Since the first CFB unit went into operation around 40 years ago, this technology has been applied to power generation using low-grade and waste coal worldwide. The technology has evolved over the years with improved reliability and availability, and the unit size increased from 25 MWe to 320 MWe early this century but the operating conditions remained subcritical. In 2009, Foster Wheeler successfully commissioned the world’s first supercritical (SC) CFB at Lagisza power plant in Poland. It marked a milestone in the development of CFB technologies in the scaling-up of the unit size and the adoption of SC steam parameters. Since then, a number of SC CFB coal power generating units have been put into service, or are under construction, in countries including China, Russia and South Korea. 

Foster Wheeler claims that now it can supply 800 MWe SC/USC CFB boilers. Rapid development and deployment in CFB technology have also taken place in China. CFB technology plays an important role in power generation using waste and low rank coal in China. Significant progress in research, development, demonstration and deployment of SC CFB technology has been made recently and the technology is continuing to evolve rapidly in China. By the end of 2016, more than 4,000 CFB power generating units were in operation or under construction. In 2013, China successfully demonstrated the domestically designed and built 600 MWe Baima SC CFB power plant. The Baima CFB boiler was designed to burn high ash, high sulphur content and low calorific value lean coal. After a year of commercial operation, a boiler performance test was conducted. The results showed that the boiler performance and emission levels continue to meet or exceed the design values at 192, 112 and 9 mg/m3 for emissions of SO2, NOx and particulates (PM), respectively.

Based on the knowledge and experience gained from operating the Baima CFB power plant, designs for 350 MWe class SC CFB units have been developed. The smaller SC CFB boilers are more popular because they are more flexible, and have been widely used in China. By July 2016, nine 350 MWe SC CFB power generating units were in operation and around 70 more were being installed. In addition, two 660 and one 500 MWe SC CFB units were under construction and a 600 MWe SC CFB boiler was manufactured for a foreign client.

Recently, China launched a plant upgrading programme to improve efficiency and reduce emissions of existing coal power plants to the levels comparable to gas-fired power plants. R&D has been carried out to identify the optimum technical routes for CFB boilers to achieve the ultralow emissions. There have been a number of reports that new as well as the upgraded existing CFB coal power plants achieved emissions of 30, 50 and 10 mg/m3 for SO2, NOx and PM, respectively. Approaches, such as air-cooling, ash utilisation and solar-coal hybrid, have also been taken to improve the plants’ environmental performance. In October 2016, China launched the ‘USC CFB technology RD&D (research, development and demonstration) programme’. The objective of the programme is to develop and build a 660 MWe high efficiency, ultralow emission and energy-saving USC CFB demonstration power plant. It will cost around 139 million Chinese yuan and is scheduled to start plant operation and technical demonstration by 2020.

A major advantage of a CFB power generation system is its ability to consume all types of coal, coal wastes and a wide variety of other fuels such as biomass and waste derived fuels, either individually or cofired. When economic and policy constraints dictate use of difficult to burn indigenous fuels or co-firing with biomass or agricultural waste, CFB is the technology of choice. In particular, CFB technology can benefit countries such as South Africa. According to the Ministry of Energy (SA), approximately 60 million tonnes of discard coal are generated each year, and the accumulated waste coal left over from mining activities is estimated to be over 1 billion tonnes in South Africa. CFB power generating units fired with waste coal can serve to clean up the waste piles, turning waste coal into valuable electricity.

The recent technology advances and engineering design optimisations have resulted in new and existing CFB coal power plants with improved energy efficiencies and ultralow emissions. With the net energy efficiency reaching 45% and unit size of up to 800 MWe, CFB technology is becoming very competitive to PC boiler, in particular for power generation from waste and low rank coal. With the emergence of low-emission, energy-saving, highly efficient USC CFB power generating systems and the development of oxyfuel CFB process, it can be anticipated that CFB technology will play an increasingly important role in future coal power generation.

 

 
About the Author

Dr Qian Zhu is an author and analyst at the IEA Clean Coal Centre. Her focus is on a range of coal combustion related topics covering air pollutant emission control technologies, clean coal technologies and low-rank coal combustion.

 
 

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