By Li Xing and Chen Junqi
Q: Coal-fired power generation plays a dominant role in the electricity mix in China, but it is also considered to be a significant source of air pollution. What is the current overall status of the emissions from coal-fired power generation in China?
A: For a long time, coal-fired power generation has accounted for 75% of total power generation and the coal consumption for power generation accounts for half of the total coal use in China. Emissions of sulfur dioxide, nitrogen oxides, particulate matter, and other atmospheric pollutants from coal-fired power plants have been closely monitored by environmental protection authorities and have resulted in the most important limitations for the continued development of the power industry. Since the 1970s, with the increasingly heightened environmental protection efforts by the Chinese government and the rapid and constant growth of the power industry, coal-fired power plants have continuously improved their efficiencies and emissions control. This became particularly true after nearly a decade of the widespread adoption of emission control facilities; emissions have been under control, with the emissions per kilowatt hour catching up with that of world’s most advanced countries.
In terms of energy efficiency, by continuous construction of high-parameter, large-capacity units as well as elimination of small thermal power units, the nationwide average net coal consumption for coal-fired plants 6 MW and larger has been reduced from 471 gce/kW·h in 1978 to 326 gce/kW·h in 2012. The average energy efficiency, in terms of HHV, has increased from 27.6% to 37.7%.
The total annual discharge of particulate matter has been reduced from 4 million tonnes in the early 1980s to 1.5 million tonnes in 2012, and the emissions per unit power generation were consequently reduced from 16.5 g/kW·h to 0.4 g/kW·h. Particulate matter controls have also been upgraded to include higher efficiency electrostatic precipitators (ESP), fabric filters, as well as ESP and fabric filter hybrid systems compared to the formerly used, less efficient mechanical and water film-based systems. The average dust collection efficiency was increased from 90% in 1985 to 95% in 1995, 98.5% in 2005, and 99.6% today. Currently, the efficiency of new particulate matter collectors is generally better than 99.9%; ESPs make up approximately 90% of installed particulate control systems, while fabric filter and hybrid systems account for the other 10%.
The rapid decrease in sulfur dioxide emissions began in 2005 when desulfurization facilities were widely applied. Sulfur dioxide emissions were reduced from 13 million tonnes in 2005 to 8.83 million tonnes in 2012; sulfur dioxide emissions per unit electricity were reduced from 6.4 g/kW·h to 2.26 g/kW·h, which is better than the 2.8 g/kW·h recorded in the U.S. in 2011. Presently, desulfurization is applied to approximately 90% of all coal-fired power plants in China, which is approximately 30 percentage points higher than that of the U.S. in 2011. If we take into account circulating fluidized bed boilers, which include integrated desulfurization, as well as subtracting units that are slated to be closed, desulfurization will be applied to nearly 100% of coal-fired power plants in China.
Since 1996, China’s coal-fired power plants have operated with low NOX combustion techniques to reduce emissions. Starting in 2010, as required by the pollution control program and the newly revised Emission Standard of Air Pollutants for Thermal Power Plants, power plants increased the installation of flue gas denitration systems, and the discharge of nitrogen oxides has decreased from 10.03 million tonnes in 2011 to 9.48 million tonnes in 2012. NOX emissions per unit electricity have decreased from 2.6 g/kW·h to 2.4 g/kW·h. By the end of 2012, China had put into operation flue gas denitration units on a total power plant capacity of approximately 230 GW, which accounts for approximately 28.1% of all coal-fired power plants in China. The power plant capacity where addition of flue gas denitration units are under planning and construction exceeds 500 GW.
Q: From the second half of last year to the first half of this year, a wide range of hazy weather has prevailed in many parts of China. Reports have stated that this hazy weather was related to PM2.5, and the source of PM2.5 is mainly from coal-fired power plants. Can you frame the relationship between coal- fired power plants and PM in a scientific perspective?
A: The hazy weather is indeed related to PM2.5, but the increase in the number of hazy days in China has shown a growing trend, which is related to coal-fired power plant emissions and also to increased deployment of motor vehicles and the growth of urbanization. In general, the haziness is caused by pollution associated with urbanization and industrial pollution.
Unquestionably, coal-fired power plant emissions will significantly impact the environmental quality in a region; the effects are felt most prominently near the emissions source. However, the impact of coal-fired power plant emissions should be assessed and evaluated scientifically, or it could mislead decision making directed toward controlling the haze pollution. I believe that the atmospheric pollutants produced from coal- fired power plants are not the primary cause of the hazy weather. First, it’s extremely unscientific and irresponsible to calculate coal-fired power plants’ emission ratio based on the coal’s proportion of combustion, because the quantity of emissions mainly depends on the effectiveness of the emissions control. Second, the quantity of emissions discharged will not directly determine the impact to the environment or the impact to creating haze; the matter of where and how the emissions are discharged should be considered as well. For example, emissions to the air and hazards to human health from the pollutants emitted from running vehicles on city roads are most certainly orders of magnitude higher than the equal quantity of pollutants produced from thermal power plants distant from the city. Therefore, the nearby emissions in the city would be more conducive to the forming of PM2.5. It also can be proven that coal-fired power plants are not the main source of PM2.5 according to the following several points:
First, regarding the fine particle matter produced directly from coal-fired power plants, even if everything produced by the plants was PM2.5, the total annual discharge would be about 1.5 million tonnes, which would make little contribution to the forming of haze. For example, the coal use for power plants in is only 4.13% of the total coal use city-wide. The annual PM10 emissions from coal-fired power plants (including PM2.5 emissions) amount to only 0.005% of total PM10 discharged in the area.
Second, regarding sulfur dioxide and nitrogen oxides, which indirectly form PM2.5, in recent years the discharge of sulfur dioxide has been substantially reduced with basically flat levels of nitric oxide, but on the contrary, the hazy weather is increasing.
Third, thermal power plants often have dispersed emissions due to the height of the stack, which is usually 210 m or 240 m in height, with an additional plume rise of 400–500 m. Plants with such tall stacks play a minor role in impacting environmental quality and the forming of hazy weather.
Fourth, over the years, China has banned the construction of new coal-fired industrial boilers except plants that provide heat to urban areas. This is despite the fact that coal-fired power plants built in old towns are used as a substitution for most scattered coal-fired heat sources and create a significant opportunity for improving the city’s environmental quality. Today, there are approximately 600,000 industrial boilers in China that still use coal-fired boilers and direct coal firing for heating; most of these are in residential areas in urban centers in north China. Taking Beijing as an example, there are still 44,000 households with coal stoves in the western district within 2nd Ring Road; the impact of urban environmental pollution caused from these coal stoves is direct and severe.
Q: The Ministry of Environmental Protection of the People’s Republic of China (MEP) issued the revised Emission Standard of Air Pollutants for Thermal Power Plants at the beginning of 2012. Currently, this standard has been formally implemented for power plants. Emission limits for nitrogen oxides, sulfur dioxide, particulate matter, etc., in the new standard are close to or have reached the limits set in most developed countries. Furthermore, the new standard includes an emission limit for mercury. Can you discuss the current status of implementation of the standard for the existing fleet of coal-fired power plants?
A: After the standard was issued, the power industry initiated a new round of large-scale flue gas denitration, desulfurization, and particulate matter control technology development and construction. Taking denitration as an example, denitration control was placed into operation in 2012 on nearly 90 GW worth of power plant capacity. Furthermore, in 2012, most units that will require denitration-related improvements began the modifications and construction. In the next three years, there will be a peak in denitration equipment construction. It is predicted that the power plant capacity with denitration could be up to 500 GW. At the same time, based on the requirements set in the emission standards, many power plants will still require desulfurization and particulate control improvement to meet the standard emission limits, including about 500 GW of plants with particulate control efficiency improvements and about 300 GW of desulfurization efficiency improvements.
Even with this progress, implementation of the new standard still faces several challenges. First, the standard is too strict overall, and some requirements are stricter than those in any other nation. It is difficult to meet the standard with existing technologies while maintaining stable power-plant operation. Second, the limits are unreasonable from an economic perspective. From the viewpoint of reducing society-wide pollution, with most other industries having little to no emissions limits, the standard applied only to power plants may result in excessive costs and result in less environmental protection because the standards are applied only to power plants. Third, the revision cycle for the power plant emission standard is too short. Comprehensive benefits may be lessened if control technologies must be reexamined and upgraded every few years. Furthermore, there is technical difficulty associated with implementation of the control technologies. With regards to what is technologically achievable, the standard is unreasonable and unscientific. Fourth, the implementation time to meet the standard is too short. Consequently, the environmental controls industry will have difficulty providing the necessary support to the power producers, there will not be enough time for technology selection, and the quality of the systems will be difficult to guarantee. From the perspective of the implementation schedule, it will be very difficult to complete the upgrades necessary to completely meet the standard before July 2014.
Considering control of mercury emissions from coal-fired power plants, mercury will mainly be controlled through the co-benefits from existing desulfurization, denitration, and particulate removal. Based on mercury removal demonstrations in China, mercury emissions from the coal-fired power plants that have taken an emission test can generally meet the emission standard. In addition, particulate removal and desulfurization in China are competitive with the world’s best. The average denitration efficiency is expected to increase to 80% or more in 2015. Therefore, co-benefit control of mercury emissions will also increase accordingly.
Q: What’s your opinion on the role of coal-fired power generation in medium- and long-term energy supply mix and state environmental protection work in China?
A: A coal-dominated energy mix in China is inevitable to ensure China’s energy security. Therefore, to reduce environmental pollution to the greatest extent, a similar proportion of improvements to coal-fired power plant emissions and implementation of clean coal power generation are also inevitable. Chinese coal consumption for electricity generation accounts for a little more than 50% of all coal consumption, which is far lower than the percentage in developed countries and even the global average. For example, the percentage of coal use for electricity in the U.S. accounts for about 90%, Canada 85%, Germany 81%, the UK 75%, Russia 64%, and the global average is approximately 78%. China should increase the proportion of coal used for electricity, and also increase the proportion of electricity as the source of end-use energy consumption.
As technology continuously progresses and the economy grows, traditional electricity from coal will gradually move toward cleaner coal conversion. The ideas for the future of coal-based electricity will be based on conventional units, supercritical and ultra-supercritical units, and development of IGCC, CCUS, and 700°C cogeneration and polygeneration units, which all take into account carbon dioxide emission reductions and effective resource utilization through higher efficiencies. New coal-fired power plants with higher efficiencies will address carbon dioxide emissions and effective resource utilization. From the perspective of environmental protection, coal-based electricity will play an important role in national environmental protection and further control haze. Theory and practice have proven that in a coal-dominated energy mix, the higher the ratio of emissions from the electric industry, the better the overall energy mix is and the lower total emissions will be (i.e., because more large coal-fired power plants mean fewer small coal-fired boilers and less direct coal-firing for heating). China should promote replacing small and scattered coal-fired power plants and coal-fired heat providers in cities actively, and effectively promote the development of electricity from coal and electricity instead of oil and urban electric vehicles.
Coal resources are set, so China should encourage power plants using low-quality coal and/or high-sulfur coal to simultaneously strengthen their emissions controls systems, and efficiently control emissions while not mandating power plants to use low- sulfur coal. When plants use high-quality coal, emissions could be reduced, but in reality high-sulfur coal will be used by small companies, which is more likely to result in increasing pollution. When coal is converted to electricity, coal-fired power plants should improve emissions control through the application of technology and enhance the operation management of particulate control, desulfurization, and denitration systems to ensure reliable emission removal efficiency.