By Wang Shumin
Chairman, Shenhua Guohua Power Company
As of the end of 2014, China had an overall power generation capacity of 1360 GW, of which fossil-based power made up 66.7% and non-fossil power contributed the remaining 33.3%. China is rich in coal, with relatively small oil and gas reserves. In fact, coal accounts for about 90% of the country’s total energy resources. As such, coal likely will remain the principal primary energy source for the foreseeable future. Although coal-fired power plants have provided the energy necessary to support the rapid and steady growth of China’s economy, these plants also contribute to emissions affecting air quality, including particulate matter (PM), sulfur dioxide (SO2), and nitrogen oxides (NOx).
Shenhua Group has set a strategic target of building a world-class, coal-based, integrated energy company. As part of this goal, and according to the specific characteristics of China’s coal-fired power fleet, Shenhua Guohua Power Company (Shenhua Guohua) has researched, developed, and applied key environmental technologies. The company has been an early adopter of near-zero emissions technologies, demonstrating that high-efficiency, low-emissions technologies are affordable in China. Currently, the focus of near-zero emissions controls includes control of PM, SO2, and NOx with mercury emissions being maintained quite low due to lower-mercury coal and co-benefits from other emission control systems. With continued research and development, the future application of near-zero emissions technologies could be further extended to include CO2 capture.
At present, coal-fired power generation is the most mature, efficient, and economical large-scale source for electricity in China. Thus, continued application of near-zero emissions technologies at coal-fired power plants is consistent with the ultimate goal of efficient, low-emissions, and low-carbon development of energy in China.
EXPLORING THE CONCEPT OF NEAR-ZERO EMISSIONS
In 2011, an emissions standard for thermal power plants (GB13223-2011) was jointly issued by China’s Ministry of Environmental Protection and the State Administration for Quality Supervision and Inspection and Quarantine.1 The emissions limits for coal-fired power units and gas turbine units in key regions are listed in Table 1.
Shenhua Guohua has met the emission regulation limits stated in the standard as well as the “Air Pollution Prevention and Control Action Plan” published by the State Council. To play its role in reducing China’s haze problem, the company has chosen to implement steps to exceed the regulation requirements and achieve near-zero emissions, effectively operating at or below the regulated limits set for natural gas turbine units (see Table 1).2
DEVELOPING A ROADMAP OF EMISSIONS CONTROL TECHNOLOGIES
Based on its emissions limit targets, Shenhua Guohua has evaluated advanced environmental protection technologies from China and abroad and has increased its investment in such technologies. The company has developed an innovation-driven technology roadmap to achieve near-zero emissions for power units combusting Shenhua coal (see Figure 1), which is characterized by a sulfur content of 0.4–0.8%, ash content of 7–16%, and a lower heating value of 21–24 MJ/kg. In addition, Shenhua’s coal includes relatively low mercury content, averaging 0.08 mg/kg compared to an overall average of 0.188 mg/kg in China.
The first link in the emissions control chain is a low-temperature economizer (LTE), which reduces the flue gas velocity and also reduces the resistivity of PM, increasing PM removal efficiency in the electrostatic precipitator (ESP).3,4 A wet electrostatic precipitator (WESP) is also used, with a PM removal efficiency of 70–90%. SO2 emissions are controlled using high-efficiency wet desulfurization (FGD) equipment with a SO2 capture rate of 98–99%. A combination of low-NOx combustion in the boiler and a full-load denitration system, which captures more than 85% of NOx, is also used. The combination of all these technologies comprehensively minimizes the emissions of PM, SO2, NOx, heavy metals, etc. Through applying these technologies, the coal-fired power plant emissions are not only lower than the regulation limits for coal-fired power units in key areas, but are also lower than the limits for natural gas turbine units.
Key Technologies for PM Removal
Using traditional equipment for PM removal, such as an ESP or fabric filter, PM concentration at the stack can be controlled below the required limit (20 mg/Nm3). However, to achieve the near-zero emission target of less than 5 mg/Nm3, multiple technologies with synergistic PM capture must be applied.
The combination of multiple PM-removal technologies as shown in the technology roadmap consists of three stages: traditional ESP (i.e., dry) or fabric filter, synergistic PM removal during desulfurization, and the WESP. The initial ESP (equipped with LTE and high-efficiency power) or fabric filter has a PM removal efficiency of 99.8–99.9%, and thus PM concentration using these options can be controlled to below 20 mg/Nm3. As a co-benefit, the FGD system will remove about 50% of the PM entering the system, although some gypsum droplets will be entrained by the flue gas. Therefore, at the outlet of FGD, PM can be held to 10–15 mg/Nm3. In the final step for PM control, the WESP downstream of desulfurization has a PM removal efficiency higher than 70%, and thus the PM emissions at the stack can be reduced to less than 5 mg/Nm3. As another option, if the FGD tower is equipped with a high-efficiency demister, its PM-removal efficiency will increase to approximately 80%, which would reduce PM emissions at the stack to less than 5 mg/Nm3, even without the WESP.
Technical Solutions for Desulfurization
In order to achieve the SO2 emission target of less than 35 mg/Nm3, a desulfurization technology with removal efficiency higher than 98% is required. A technology consisting of a conventional spray tower, tray design, dual cycle absorption tower, series absorption tower technology, etc., was the research focus. For a conventional spray desulfurization tower, Shenhua Guohua utilized its own patented technology to prevent the flue gas from sticking to the walls. With this technology and an additional layer of spraying, capture efficiency can be greater than 98%, and the target of limiting SO2 emissions to below 35 mg/Nm3 can be achieved.
To further improve on desulfurization, the use of seawater has also been researched and tested. At the company’s Zhoushan power plant, the desulfurization efficiency is higher than 99% using seawater, resulting in SO2 emissions below 2.76 mg/Nm3. The company plans to increase investment in the research and development of this technology and aims for some of its coastal power plants to be retrofit with a seawater-based desulfurization system.
High Efficiency and Full-Load Denitration
Based on research begun in 2010, Shenhua Guohua has decided to implement a combination of low-NOx burners in the boiler and a full-load denitration system for limiting NOx emissions.5 The NOx concentration at the economizer outlet without any emissions control is about 100–200 mg/Nm3. If low-NOx burners and staged combustion are used, as developed through cooperative R&D with partnering companies, the NOx concentration at the economizer’s outlet can be limited to around 100 mg/Nm3. Then a selective catalytic reduction (SCR) system with a designed denitration efficiency of 80–85% is applied and the stack NOx emissions can be limited to around 20–40 mg/Nm3, notably lower than the limit for natural gas turbine units.6,7
Because the company uses relatively low-mercury coal, and as the existing flue gas purification equipment can remove some mercury, the mercury content of purified flue gas is quite low. The total mercury removal efficiency of the ESP and wet desulfurization system is approximately 25% and 50%, respectively, and thus the total mercury removal efficiency is about 75%. By the company’s calculations, the mercury concentration at the stack should be less than 10 μg/Nm3, based on the application of these technologies. According to measured data, the actual concentration can be even lower. For example, Shenhua Guohua’s Sanhe power plant has measured a stack mercury concentration of only 3–5 μg/Nm3, which is an order of magnitude less than the emissions standard limit of 0.03 mg/Nm3 (GB13223-2011).
DEMONSTRATING NEAR-ZERO EMISSIONS
In order to meet its own environmental requirements, Shenhua Guohua launched a “High-Quality Green Power Generation Plan” for existing coal-fired power units and a “Near-Zero Emission Project” aimed at newly built coal-fired power units. In 2014, Zhoushan No. 4 was commissioned to serve as a leading example for near-zero emissions from new coal-fired power plants. After that, Sanhe No. 1 and Suizhong No. 2 conducted comprehensive retrofits to achieve near-zero emissions. To date nine coal-fired power units in six different power plants have applied the necessary technologies to meet near-zero emissions targets (see Table 2).
New-Plant Example: Zhoushan No. 4
Placed into operation on 25 June 2014, Zhoushan power plant unit No. 4 was the first power plant with near-zero criteria emissions in China. The technologies applied at the plant include low-NOx burners, SCR, ESP (upgraded with high-efficiency power, four conventional electrodes, and one rotation electrode), wet ESP, and a seawater desulfurization system. The emissions cuts of PM, SO2, and NOx are 88%, 94%, and 80%, respectively, while the absolute amounts of emissions cuts are 96, 260, and 440 tonnes per year, respectively.
Retrofitted Example: Sanhe No. 1
Sanhe unit No. 1 was the first existing coal-fired power unit in China to be placed into operation (on 23 July 2014) with near-zero emissions retrofits. The technologies applied at Sanhe No. 1 combine low-NOx burners, SCR, ESP (equipped with LTE and upgraded with high-efficiency power and four conventional electrodes), limestone-gypsum wet flue gas desulfurization, wet ESP, and a natural draft cooling tower (NDCT). The relative amounts of emissions cuts of PM, SO2, and NOx compared with the key area emissions limits for coal-fired power units in the standard (GB13223-2011) are 75%, 82%, and 65%, respectively. The absolute amounts of emissions cuts of PM, SO2, and NOx compared with the key area emissions limits for coal-fired power units in the standard are 94, 255, and 403 tonnes per year, respectively.
ECONOMIC AND ENVIRONMENTAL BENEFITS ANALYSIS
According to China’s recently passed regulations and laws, coal-fired power units must be equipped with environmental protection facilities such as PM removal, desulfurization, and denitration. For those coal-fired power units meeting environmental standards, the grid purchase price is subsidized. For PM removal, desulfurization, and denitration, the feed-in tariffs are RMB 0.2, 1.5, and 1.0 fen/kWh (0.032, 0.24, and 0.16 US₵/kWh), respectively. Therefore, coal-fired power units meeting all the environmental standards may have a total feed-in tariff of RMB 2.7 fen/kWh (0.43 US₵/kWh). Generally, retrofitting coal-fired power plants to achieve near-zero emissions increases electricity generation costs by RMB 0.5–2.0 fen/kWh (0.08–0.32 US₵/kWh). Calculating the investments for retrofitting of Sanhe No. 1 and Dingzhou No. 3, the incremental generating cost is RMB 1.0 and 0.6 fen/kWh (0.16 and 0.097 US₵/kWh), respectively.
From the perspective of net costs, while achieving near-zero emissions increases the generating cost for coal-fired power plants, the total generating cost remains far less than that of natural gas combined-cycle units. For example, in Zhejiang Province where the Zhoushan power plant is located, the total generating cost of a natural gas combined-cycle unit is RMB 57.7 fen/kWh (9.3 US₵/kWh) while those for the near-zero emissions Zhoushan No. 4 are RMB 19.3 fen/kWh (3.1 US₵/kWh)—approximately one third of the total generating cost of a natural gas combined-cycle unit.
At present, more than half of China’s overall coal consumption is used in coal-fired power plants. Thus, reducing emissions from coal-fired power units can significantly reduce criteria emissions from the country’s coal utilization.8 Research and application of near-zero emissions technologies by Shenhua Guohua prove that the roadmap is feasible and the environmental benefits are significant. It is hoped that these practical demonstration projects can serve as an example to pioneer a new route for clean and efficient utilization of coal in China. In 2013, China’s total amount of PM, SO2, and NOx emissions were 1.42 million, 8.2 million, and 8.34 million tonnes, respectively. Although coal-fired power plants are responsible for only a fraction of China’s total emissions, if all of the coal-fired power units in China apply near-zero emissions technologies over the next five years, starting in 2015, the total annual reduction of PM, SO2, and NOx emissions would be 0.27 million, 1.55 million, and 1.54 million tonnes, corresponding to reduction rates of 19%, 18.9%, and 18.5%, respectively.9
Shenhua Guohua Power Company was one of the first movers on research, development, and deployment of near-zero emissions technologies for coal-fired power units in China. At present, nine of the company’s units have achieved internal emissions reduction targets and now exceed national regulations for gas-fired turbine units. All remaining units will be upgraded within the next five years. Shenhua Guohua has demonstrated that application of near-zero emissions technologies is technically and economically feasible and the environmental benefits are substantial in China.
- Chinese Research Academy of Environmental Sciences and Guodian Environmental Protection Research Institute. (2011). GB13223-2011 Emission standard of air pollutants for thermal power plants. China Environmental Science Press.
- Xing, F. (2014, 5 August). Guohua coal-fired power plant takes the first step to air pollutants near-zero emission. China Environmental News.
- Cui, Z., Long, H., Long, Z., et al. (2012). Technical features of lower temperature high efficiency flue gas treatment system and its application prospects in China. Power Engineering, 32(2), 152–158.
- Zhao, H., Li, J., He, Y., et al. (2014). Research and application on low-low temperature electrostatic precipitator technology. Electric Power, 47(10), 17–21.
- Zhou, H. (2013). Study on roadmap and schemes of denitration retrofit technology in thermal power plant. Electric Power Environmental Protection, 29(5), 43–44.
- Yang, Q., & Liao, Y. (2014). The strategy on reduction of SCR minimum operation load. Electric Power, 47(9), 153–155.
- Fang, Z., Jin, L., Song, Y., et al. (2014). Performance optimization and maximum denitration efficiency analysis for SCR-DeNOx power plants. Thermal Power Generation, 43(7), 157–160.
- Wang, Z. (2014, 3 July). Coal-fired power generation is a key to solve the problem of haze in China. China Energy News.
- Wang, S., Song, C., Chen, Y., et al. (2015). The technology research and engineering applications of air pollutants “Near-zero emissions” in coal-fired power plants. Research of Environmental Sciences, (4), 487–494.
The content in Cornerstone does not necessarily reflect the views of the World Coal Association or its members.
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