By Chen Yinbiao
Chief Engineer, Guohua Electric Power Corporation of
China Shenhua Energy Company Ltd
Coal is the Foundation of Energy in China
In 2011, energy consumption in China reached 3.48 billion tonnes of coal equivalent, out of which 70% was supplied by coal. A slight majority of coal (53%) used in China in 2011 was used for electricity production. The demand for coal for electricity production is expected to further increase; by 2020 it is predicted that coal for power generation will account for about 63% of total coal consumption.
By the end of 2012, the total installed capacity was 1,145 GW, of which 819 GW (71.6%) were supplied by thermal power. According predictions from the China Electricity Council, by the end of 2015, the total installed capacity will increase to 1,437 GW, and thermal power plants will account for 933 GW.1
While it is crucial to fulfill the energy demand of the growing Chinese economy, the environmental impact of energy production must be minimized. Some of the most important environmental concerns from power generation using coal are related to gaseous emissions of pollutants such as particulate matter, sulfur dioxide (SO2), nitrogen oxides (NOx), mercury (Hg), and carbon dioxide (CO2). In 2011, the total emissions of SO2, NOx, and CO2 from coal usage in China were 22.2 million tonnes, 24.0 million tonnes and 5.6 billion tonnes, respectively. Out of these total emissions, for SO2, NOx, and CO2 the amount produced due to power generation was 9.13 million tonnes, 10.03 million tonnes and 2.74 billion tonnes, respectively. In July 2011, China issued emissions standards for thermal power plants, which are summarized in Table 1.
In addition to the emissions limits shown in Table 1, China has proposed that, based on 2010 levels, energy consumption per unit of GDP and CO2 emissions should be reduced by 16% and 17%, respectively, by 2015. In addition, based on the same commitment, the emissions of SO2 and NOx will be reduced 8% and 10%, respectively, compared with 2010 levels by 2015.
Recently, environmental controls have been applied to some coal-fired power plants in China3, but due to the growth in electricity production, total net emissions have not decreased, which highlights the magnitude of the challenge for decreasing power plant emissions while increasing power production.3
Coal-fired power generation will clearly continue to play a leading role in China’s power production for the foreseeable future, therefore, power generators, such as the Shenhua Group, must strive to increase power generation while decreasing the related environmental impacts. The purpose of this article is to highlight the comprehensive strategy of the largest power producer within the Shenhua Group toward tackling emissions reductions. This approach includes implementation of commercial technologies to existing power plants and research, development, and demonstrations to make progress towards the technologies of the future.
Construction of Advanced Units and Improvement of Existing Units
As the largest coal-based comprehensive energy enterprise in China, the Shenhua Group supplied about 10% of the total energy to China in 2011. The electricity generated by the Shenhua Group accounted for 4.6% of total domestic electricity generation, out of which over 85% of electricity generation was supplied by Shenhua Guohua Power. To reduce the environmental impact of electricity production, Shenhua Guohua Power has implemented measures such as:
- The construction of large, high-efficiency power plants
- Conversion of smaller, less efficient plants to cogeneration facilities
- Implementation of environmental controls
In addition to the near term approaches listed above, research, development, and demonstration must also play a role in developing technologies to improve emissions for power plants of the future.
Construction of High-Efficiency Power Plants
High-efficiency power plants are a strategically advantageous option for producing power from coal because such plants can reduce emissions compared to less efficient plants while meeting an increasing energy demand.
Since its foundation in 1999, Shenhua Guohua Power has focused on optimizing its existing units as well as constructing ultra-supercritical and cogeneration units with large capacity. As a result, the improved environmental performance of the large capacity and high-efficiency units along with the improved existing units can be fully combined to demonstrate an overall reduction in emissions per unit power generated.
By the end of 2011, the total installed capacity of Shenhua Guohua Power reached 35.110 MW, including 24 ultra- supercritical units that accounted for 54% of the power generation. The average unit capacity of these supercritical units is 550 MW, while smaller (i.e. ≤300 MW) are employing cogeneration (discussed in a subsequent section). By increasing the fleet efficiency, the amount of coal consumption and emissions per unit energy produced were consequently reduced. For example, after the retrofit of the LP cylinder, the coal consumption of a 500 MW unit in the Guohua Panshan Power Plant dropped by 9 kg/MWh (i.e., an increase of approximately 3% in thermal efficiency). Similarly, the coal consumption of the 2×100 MW units in the Guohua Shenmu Power Plant was reduced by 18 kg/MWh when the plant was retrofit to improve efficiency. In addition to the improvements in environmental impact, the reliability of the units was also improved; the average frequency of annual unplanned shutdown was decreased from 0.82 per unit in 2010 to 0.62 per unit in 2011.
Shenhua Guohua Power has set goals that will further improve the operation of ultra-supercritical units. The designed coal consumption of the newly constructed 1000 MW ultra-supercritical units will be 270-280 kg/MWh, while emissions of SO2, NOx, Hg, particulate matter, and waste water per unit of power will be substantially reduced.
Conversion of Smaller Power Plants to Cogeneration Facilities
Cogeneration units create not only electricity, but also utilize the steam draining out from the steam turbine to supply users with heat by replacing small, inefficient, decentralized heating boilers. Employing cogeneration increases both the thermal efficiency of the units and the environmental quality of the heating district. The cogeneration retrofit projects involved 14 units in five power plants in Shenhua Guohua Power. After the cogeneration retrofit of the Guohua Sanhe Power Plant, a trans-province heating supply over a distance of 25 kilometers was constructed. The annual heating area covered about 7.6 million m2, which correspondingly replaced 91 small-scale boilers, and saved 0.12 million tonnes of standard coal, and the emissions associated with the combustion of the coal, every year. Similarly, after the cogeneration retrofit of No. 1 and No. 2 units at the Guohua Panshan Power Plant, approximately 130 small-scale boilers were replaced, which saves 0.14 million tonnes of standard coal and the associated environmental impacts every year.
Comparison with Domestic Power
The average annual coal consumption for power supply was 317 kg/MWh in Shenhua Guohua Power in 2011, which was 13 kg/MWh lower than the domestic average, and represented the lowest coal usage for any Chinese power producer. The result of the domestic average coal consumption for power supply vs. Shenhua Guohua Power average coal consumption for power supply is illustrated in Figure 1.
Implementing Environmental Controls
For existing power plants that are not suitable for conversion to high-efficiency or cogeneration, advanced and reliable environmental control technologies can be applied to reduce emissions. One example of using environmental technologies to reduce emissions occurred at the Guohua Beijing Thermal Power Plant, which implemented low NOx combustion, selective non-catalytic reduction (SNCR), and selective catalytic reduction (SCR) denigration technology and subsequently reduced NOx emissions to below 100 mg/Nm3. The low-NOx burner retrofit for the No. 2 boiler at the Guohua Taishan Power reduced NOx emissions to 121 mg/Nm3 from 340 mg/Nm3. Since installation of the SCR denigration unit, the emissions concentration of NOx has dropped to less than 50 mg/Nm3, which is at the most advanced technological levels in the world.
Simultaneously, flue gas emissions were decreased gradually by year as illustrated in Figure 2. In 2011, the emissions for SO2, NOx and PM from Shenhua Guohua Power facilities were 0.2 kg/MWh, 1 kg/MWh, and 0.1 kg/MWh, as shown in Figure 2. All emissions have met the required limits.
Research, Development, and Demonstration to Improve the Power Plants of the Future
Several different approaches to reducing emissions have been demonstrated by Shenhua Guohua Power. Still, further improvement will be necessary to achieve the goal of expanding electricity generation while reducing environmental impact. Research, development, and demonstrations must play a crucial role in the advancement of environmental technologies.
Construction of High Steam Parameter Units
Power plant efficiency can be improved by increasing the steam temperature and pressure and adding reheat stages in the power cycle. Based on existing high temperature metallic materials, plant efficiency can be raised by 1.5-2% by utilizing double reheat, compared with single reheat under the same conditions. By raising the new steam temperature to 650-700°C, the cycle thermal efficiency can be raised to 50-55%, which will reduce CO2 emissions per unit power generation by over 30%. Shenhua Guohua Power is working towards commercial scale deployment of 700°C ultra-supercritical coal-fired units before 2020.
Upgrading and Treatment of Low Rank Coal
The washing and upgrading of low rank coal can result in energy savings and emissions reductions. By 2020, the percentage of raw coal that is washed will reach 70% and the use of low calorific value coal will reach 0.536 billion tonnes, which converts to 0.162 billion tonnes of standard coal. The reasonable disposal and highly effective utilization of coal slime and coal gangue can greatly reduce the pollution from waste.
In addition, alternatives to low rank coal are being investigated. Research on the highly effective use of peat in power generation is being conducted. The demonstration of 2×660 MW ultra-supercritical circulating fluidized bed (CFB) units for peat-fired power generation was executed on plan.
Improved Emissions Controls with Reduced Water Consumption
While it is recognized that emissions controls from coal-fired power plants are necessary, some commercially available options, such as wet flue gas desulfurization scrubbers, create a significant water demand. Based on water usage restrictions in China, it is desirable to develop emissions controls that are not as water dependent. Accordingly, Shenhua Guohua Power has begun research and development of a technology for the simultaneous removal of SO2, NOx, and heavy metals. This technology will be demonstrated at the 600 MW scale.
Compared with WFGD technology, at the same power supply efficiency, this novel technology can reduce water consumption by more than 90%. By realizing the resource utilization of byproducts and greatly reducing water consumption in coal-fired power plants, it will play a critical role, especially in western China where a serious water shortage exists.
Characterization of Emissions Profiles
In order to create a strategy for reducing emissions, it is important to understand where the emissions are being created, and sources should be addressed first to improve the air quality in populated or ecologically sensitive areas. For this reason, the characterization of emissions profiles can play an important role in developing an overall strategy to reduce the environmental impact of power production.
Fine Particulate Matter
The increasingly severe regional haze in China, which involves fine particulate matter PM2.5 as the dominating factor, has become a significant concern to public health. To date, there has been too little research regarding the environmental impact of PM2.5 emissions from point sources. Therefore, it is of great importance to develop monitoring, characterization, and emissions reductions for PM2.5 from coal-fired power plants.
Shenhua Guohua Power has launched a study to monitor and characterize PM2.5, heavy metals, ions, organic carbon, elemental carbon and other major gaseous pollutants from power plants. To find an effective way to mitigate PM2.5 emissions, the amount and fractions of different particulate emissions (TSP, PM10, PM2.5, and PM1), as well as PM2.5 removal efficiency of air pollution control equipment will be obtained by means of field sampling and analytical tests.
During coal combustion, most of the mercury enters the gaseous phase and unless controls are in place it will be emitted. It is well known that mercury is a particularly harmful toxin both to humans and other ecological systems.
Shenhua Guohua Power is collaborating with research institutions to characterize mercury emissions and develop a control strategy. Studies evaluating the effectiveness of different control technologies, such as sorbent injection, WFGD, etc. are being conducted. In addition, studies are being conducted on the native (i.e., co-benefit) removal of already installed emissions equipment. To date, only preliminary results have been obtained. An industrial demonstration of mercury removal technology project is expected to be completed in 2015.
There are three major CO2 capture technologies, pre-combustion (for IGCC plants), oxy-fuel combustion, and post-combustion capture. Currently, IGCC and oxy-fuel combustion are being pursued in the Shenhua Guohua R&D program.
Oxy-fuel combustion of coal is a process of burning coal using O2with a recycle stream consisting mostly of CO2 and O2 instead of air. Because nitrogen from air does not enter the boiler, production of NOx is greatly reduced. Oxy-fuel combustion generates a flue gas consisting primarily of CO2 and H2O. After a purification process, including ash separation, desulphurization, denigration, drying, and possibly further CO2 purification, the CO2 can be pressurized, liquefied and stored. To advance the development stage of oxy-fuel combustion technology, Shenhua Guohua Power is actively supporting a program that will resulting a 200 MWe oxy-fuel demonstration in 2016.
Pre-combustion CO2 Capture
Integrated Gasification Combined Cycle (IGCC) power plants dynamically integrate the benefit of coal gasification and the high-efficiency of combined cycle power generation technology. The thermal efficiency of IGCC power plant has been improved to the point that it is the same as ultra-supercritical power units (~43%). Furthermore, with the development of gas turbine technology, the efficiency of IGCC can be further improved.
China is a world leader in the application of gasification-based power plants. Specifically, the Shenhua Group has mastered large-scale gasification, which is often applied in coal liquefaction and the coal to chemicals industry. Some CO2 from coal liquefaction is currently being stored as part of a CCS test. With a goal of developing advanced power generation with “near zero emissions,” IGCC power plants will continue to be the generation system of choice.
It is the responsibility of power producers to improve their end product. This article has described the comprehensive strategy being employed by Shenhua Guohua Power to improve power generation both now and in the future. However, this is a work in progress; in order to improve the utilization efficiency of energy and reduce emissions, Shenhua Guohua Power will continue to develop coal-fired generating units with large capacity and high-efficiency, as well as transform smaller units with lower efficiency to cogeneration units. In addition, research and development of clean and efficient low carbon technologies will continue. All this must be considered an ongoing effort of the utmost importance if power production to grow while the environmental impact is decreased.
REFERENCES AND NOTES
1. China Coal News, “During the Twelfth Five-Year Guideline Coal-fired Units in China Will Continue to Increase, April, 2011.
2. Emission standard of air pollutants for thermal power plants, GB12233- 2011, July, 2011.
3. Jianyu Zhang, Li Pan, Fan Yang, Jia Liu. Study on Current Status of Air Pollution Control for Coal-fired Power Plants in China. Journal of Environmental Engineering Technology, 2011, 1 (3): 185-196.