By Li Xing
Dr. Cen Kefa is one of the world’s foremost experts in the field of thermophysics engineering. Born in January 1935, Dr. Cen graduated from the Department of Power Engineering, Huazhong Engineering College (now the Huazhong University of Science and Technology) in 1956 and later received his doctorate from the Department of Power Engineering of Bauman Moscow State Technical University in 1962. He then accepted a professorship in the Department of Mechanical and Energy Engineering at Zhejiang University. He was appointed as an Academician of the Chinese Academy of Engineering in 1995. Currently Dr. Cen is President of the Institute for Thermal Power Engineering at Zhejiang University.
Academician Cen has overseen groundbreaking achievements in a range of fields, including high-efficiency, clean energy technologies; resource utilization, including utilization of fossil fuels and waste-to-energy; development and utilization of renewable energy; utilization of biomass energy, hydrogen production, clean coal combustion and gasification technologies; coal-water slurry combustion; fluidized bed combustion for power generation; technologies for simultaneous multi-pollutant removal during the course of energy utilization; engineering gas-solid multiphase flow technologies; power plant boiler computer-aided testing technologies; and advanced laser diagnostic technologies.
Cornerstone sat down with Dr. Cen to discuss his perspective on how the environmental impact of coal utilization in China can be most effectively reduced.
Q: Coal is the principal energy source in China and has thus supported rapid economic growth over the past few decades. However, coal is considered one of the important contributors to air quality issues. In the report of the 18th National Congress of the Communist Party of China, it was proposed that an “energy production and consumption revolution” must be carried out. How do you think a coal utilization revolution can and should be realized?
A: In 2014, coal consumption in China was approximately 3.5 billion tonnes. Coal-fired power is the largest coal-consuming industry, accounting for 58% of China’s coal consumption. Presently, the main high-efficiency, low-emissions coal-based power generation technologies include ultra-supercritical steam cycles, ultra-low emission technologies, and integrated gasification combined-cycle (IGCC) technologies. Due to the severe shortage of oil and gas in China, there has been a focus on the production of clean coal-based fuels or chemicals using high-efficiency, low-emissions conversion technologies, the core of which is the complete gasification of coal. Such technologies include production of coal-derived oil and synthetic natural gas, coal-to-olefins, coal-to-ethylene glycol, coal-to-aromatics, and methanol-to-gasoline. To revolutionize coal utilization, the dual nature of coal, as an energy source and as a raw material, should be considered—leading to a change in the current industrial model in China and realizing energy conversion and emission reductions simultaneously.
Staged coal conversion, a clean power generation technology, is one of the revolutionary approaches to coal utilization that has been proposed based on China’s specific conditions. This technology is based on the nature and conversion properties of various components found in coal. This approach utilizes coal as a raw material and an energy source simultaneously by organically combining coal pyrolysis, combustion, and other processes—thus realizing staged conversion and the stepwise utilization of coal.
In this process, coal is first pyrolyzed in a pyrolysis furnace, and the gases evolved are extracted to generate a gas mixture (i.e., fuel gas or syngas) and tar. The leftover char is then sent to a boiler for combustion. Steam, a gas mixture, and flue gas are generated during the process. Emission controls, including the removal of carbon, can be employed to achieve ultra-low emissions. The gas mixture generated can be used to produce natural gas and other fuels. The steam can be used for electricity generation and to supply heat. Based on the chemical speciation, value-added components in the ash can be extracted. Ash can also be used to produce cement and other construction materials.
The staged conversion of coal is closely linked to China’s demands for industrial restructuring, a circular economy, energy conservation, and emission reductions. These needs are spawning new industrial models and driving the conversion and upgrading of the coal-based power industry. The basis of the technology is the innovative concept of simultaneously controlling emissions of centralized and distributed energy sources. To explain, coal can be converted and ultimately used by distributed consumers. The first step of staged coal conversion is to process the coal at centralized facilities that generate low-emissions electricity, natural gas, and steam. These products are cogenerated without the need for high-pressure, pure (or enriched) oxygen so that the conversion costs are greatly reduced and valuable water resources are conserved compared to standalone coal conversion facilities. At the same time ultra-low emissions are realized in the production of power. The cogenerated electricity, natural gas, and/or steam provide surrounding or dispersed industrial kilns and private boilers with sources of energy to realize the low-emissions operation of these facilities as well.
Q: Starting this year, China proposed stricter emission control requirements for newly built coal-fired power plants. In your opinion, how big a role will achieving ultra-low emissions from coal-fired power plants play in reducing energy-related environmental impacts? Also, how should one evaluate the economic and environmental benefits of ultra-low emission controls?
A: In 2012, China’s total SO2 emissions were 21.176 million tonnes. Out of this total, the SO2 emissions from the electricity industry were 8.83 million tonnes, accounting for 41.7% of the country’s SO2 emissions. Similarly, the total national NOx emissions were 23.378 million tonnes, of which the NOx emissions from the electricity industry were approximately 9.48 million tonnes, accounting for 40.6% of NOx emissions nation-wide. The total emissions of particulate matter from the electricity industry were approximately 1.51 million tonnes. According to estimates, if ultra-low emission technologies are applied to all of China’s coal-fired units, the emissions from the coal-based fleet will be effectively reduced to 530, 760, and 80 thousand tonnes for SOx, NOx, and particulate matter, respectively. Such reductions in criteria emissions from the coal-fired power industry would result in a 90% decrease in total emissions from the electricity sector (compared with 2012 levels). Undoubtedly this will have a significant impact on improving air quality in China.
For newly constructed and retrofitted coal-fired units with different levels of various emissions as well as different plant sizes, the required investment costs and operating costs differ for new construction versus retrofitting. The increase in the cost for power generation with ultra-low emission retrofitting is 0.01–0.02 yuan/kWh, while the increased power generation cost of new construction of ultra-low emission units is even lower, at 0.005–0.01 yuan/kWh.
For some time, there have been active calls for power generation in China to switch from relying primarily on coal to gas. However, if current coal-fired power plants are converted to gas-fired power plants, power generation costs will increase significantly. Take natural gas combined-cycle power generation as an example. If the natural gas price increases from 2.0 yuan/Nm3 to 5.0 yuan/Nm3, the power generation cost would increase from 0.59 yuan/kWh to 1.23 yuan/kWh, while the power generation cost of coal-fired power plants is approximately 0.4 yuan/kWh. Therefore, although there has yet to be comprehensive investment in achieving ultra-low emissions coal-fired power plants, the environmental and economic benefits are worthwhile.
Q: The use of coal for power generation only accounts for a little over 50% of the coal consumption in China. A significant amount of coal is still used in industries such as the production of cement, steel and plate glass, and in numerous, dispersed, and small coal-fired boilers. How can the issue of environmental impact be resolved for the smaller, dispersed, coal consumers?
A: Presently, due to the large amounts of coal consumed, the percentage of nation-wide emissions from coal-fired power remains the highest among key coal-consuming industries. However, the regulation and subsequent emissions reductions at large coal-fired power plants are relatively easy. Industrial boilers are smaller, more numerous, and widely dispersed, making regulating, supervising, and the overall control of emissions more difficult. The emissions from burning one tonne of coal in an industrial boiler could result in emissions that are dozens of times larger than the amount released from burning the same amount of coal at a large coal-fired power plant.
In 2013, the electricity industry accounted for approximately 58% of coal consumption in China, which is far lower than the 92% in the U.S. or 80% in Germany in 2010. Compared with these other major coal-consuming countries, the coal-consuming industries in China are too widespread, making it challenging to manage emissions.
Instead of directly controlling emissions, small boilers, especially coal-fired industrial kilns that use 10 tonnes or less per hour of steam, can substitute fuels such as coal-derived natural gas from staged coal conversion to limit emissions. Thus, the staged coal conversion approach can transform power plants and ensure low-emissions power generation while simultaneously generating syngas or coal-derived natural gas for these smaller, dispersed industrial kilns. With regard to industrial parks and other areas with numerous medium and small boilers, centralized heat supply could also be adopted. In doing so, ultra-low emission technologies would be in place at the boilers responsible for centralized heat supply so that the net emissions could meet emission requirements for natural gas units. For example, flue gas emission controls are being implemented at five 220 tonnes/hr combined heat and power boilers at Jiaxing Xinjia Aisi Thermoelectricity Co., Ltd. The efficiency of existing particulate removal, desulfurization, and denitration systems is being improved; high-efficiency simultaneous emission removal technologies are being adopted so the major criteria emissions (i.e., particulate matter, SO2, and NOx) in the flue gas of coal-fired units can meet the national standards for natural gas-fired power plant emissions.
Q: Zhejiang University began research relatively early in the field of staged utilization of coal in China. What progress has been made to date? What are the prospects for commercialization?
A: Zhejiang University has been researching staged coal utilization for over 20 years. Our researchers proposed the concept of polygeneration processes for heat, electricity, and fuel gas as early as 1987. Supported by the original National Board of Education Doctoral Fund, National 8th Five-Year Science and Technology Research Plan, National High-tech R&D Program of China (863 Program), and National Program on Key Basic Research Project of China (973 Program), experimental 1-MW syngas and steam cogeneration apparatuses were built—much experimentation and theoretical research have been since conducted on the key technologies of the proposed approach. The research indicates that staged coal utilization offers extensive fuel applicability, high rates of fuel utilization, and low emissions. A series of national invention patent applications have been submitted. In June 2007, Zhejiang University and Huainan Mining Industry (Group) Co., Ltd. collaborated to transform a 75-tonne/hr coal-fired power unit into a 12-MW staged conversion power generation unit. Thermal commissioning and operation showed that without using high-pressure or purified oxygen, the calorific value of the gas mixture from coal pyrolysis was over 20 MJ/Nm3. The major components of the fuel gas were CH4 and H2, with a tar yield over 10%. The system operation was stable and could be modified relatively easily. Generally, operation was safe and reliable. The production of tar and fuel gas was stable. The production of multiple high-value products in an organically integrated system with coal as the raw material was accomplished.
Based on this, Zhejiang University and China Guodian Corporation’s Xiaolongtan Power Plant collaborated to convert a 300-MWe lignite-burning circulating fluidized bed boiler into a 300-MWe circulating fluidized bed heat, electricity, and fuel gas polygeneration facility (still using lignite). The project was constructed in two phases. To date, the conversion project’s first stage of construction has been completed, with the experimental apparatus designed to use 40 tonnes/hr of lignite. The 72-hr evaluation operation and performance parameter testing were completed in June 2011.
Building on existing research and development, Zhejiang University and Dongfang Boiler Co., Ltd. are collaborating to carry out the design of 350-MWe and 600-MWe supercritical circulating fluidized bed pyrolysis and combustion staged conversion facilities, which are intended to lay a foundation for future large-scale industrial application. Such technology could be used in the new construction of power plants as well as retrofitting older power plants. The technology’s economic and environmental benefits are considerable and the application prospects are broad.
Q: In November 2014, China and the U.S. issued the U.S.–China Joint Statement on Climate Change, announcing the actions of each country for dealing with climate change after 2020. China plans to peak CO2 emissions around 2030 and will attempt to reach this peak early. China also plans to increase non-fossil energy’s share of primary energy consumption to 20% by 2030. In light of these goals, how should low-carbon coal utilization be realized in China?
A: The main paths to low-carbon utilization of coal include high-efficiency, low-emissions utilization and the development of CO2 capture, utilization, and storage (CCUS) technologies. Achieving high-efficiency clean utilization of coal mainly relies on the integrated innovation of technologies. Ultra-supercritical power generation units, IGCC technology, and staged coal conversion are the principal approaches for high-efficiency, low-emissions coal utilization.
Another path to low-carbon coal utilization is to develop CCUS. CCUS technologies are necessary to realize the large-scale control of coal-derived CO2 emissions. In 2007, China surpassed the U.S. and became the world’s largest carbon emitter. For a very long time, fossil fuels, especially coal, will continue to dominate the energy mix in China. Since 2006, China has funded a series of research and development projects to support the development of CCUS technologies, including the 863 Program, 973 Program, and National Key Technologies R&D Program. These projects include key technologies in areas such as CO2 capture, bioconversion, geological storage, and enhanced oil recovery. With the support of the government and corporations, many demonstration projects have been completed in recent years.
Although significant progress has been made in the research and development of CCUS technologies in China in recent years, there are challenges that have yet to be fully overcome. Such issues include high cost and energy consumption as well as proving the technology is safe and reliable over the long term. Therefore, the large-scale implementation of CCUS will continue to require the combined efforts of the government, research institutions, and corporations.
The content in Cornerstone does not necessarily reflect the views of the World Coal Association or its members.
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