Introduction
Haze pollution is one of the major environmental issues in China [
1,
2]. Rapid economic development and urbanization have caused China’s energy consumption to increase by 120% from 2000 to 2010. Coal accounts for over 60% of China’s primary energy consumption and its use results in large quantities of pollutants being emitted into the atmosphere. In 2013, the Chinese government announced its “Air Pollution Prevention and Control Action Plan” [
3], the first plan with air quality in China. These new policies have already had an impact. The Beijing-Tianjin-Hebei (Jing-Jin-Ji) Region, with an annually average particulate matter (PM
2.5) concentration of 106
mg/m
3 in 2012, was one of the most polluted regions in China. The average PM
2.5 concentration in 74 cities was 55
mg/m
3 in 2015 under similar meteorological conditions. This is a 24% reduction compared to the 2013 levels, indicating that these control policies have achieved initial success.
China is the largest coal-consuming country in the world. Emissions from coal-fired power contribute more than 30% of the total NOx and SO2 emissions in the Jing-Jin-Ji Region. The Chinese government has implemented ultra-low emissions (ULE) standards for PM, NOx, and SO2 through the “Reformation and Upgrading Action Plan for Coal Energy Conservation and Emission Reduction.” The targets for PM, NOx, and SO2 in the Jing-Jin-Ji Region are now 5 mg/m3, 35 mg/m3, and 50 mg/m3, respectively.
To meet the ULE standard, the pollution controls systems have been transformed in several power plants. Nowadays, conventional ULE technologies like selective catalytic reduction (SCR) for NOx, electrostatic precipitators (ESP) for PM, and flue gas desulfurization (FGD) for SO2 have been installed widely in power plants with an intention to reduce the emissions. ULE technologies also include low temperature economizers (LTE) installed at the inlet of the ESP to collect the heat generated from flue gas, and wet electrostatic precipitators (WESP) to capture PM.
This paper compares the haze reduction potential of coal-fired power fitted with ULE to that of natural gas-fired power. Natural gas-fired power is an alternative to coal-fired power equipped with ULE technologies. However, the cost of gas-fired power is much higher than that of coal-fired power in China. This paper focuses on the Jing-Jin-Ji Region, and uses the GEOS-Chem model system to model PM air pollution. Three cases are compared: a “standard” scenario using emissions data for coal-fired power plants from 2012 which pre-dates ULE adoption [
4,
5], a “best case” scenario which assumes emissions consistent with full adoption of ULE at all coal-fired power plants across the Jing-Jin-Ji Region, and a “natural gas” scenario assuming emissions factors consistent with natural-gas-fired power.
Atmospheric model
The current global-multi-regional nested grid GEOS-Chem model is built on version 11 of GEOS-Chem developed by Wang et al. [
6] and Chen et al. [
7]. The simulation was driven by meteorological data assimilated by the Goddard Earth Observing System (GEOS) at the NASA Global Modeling and Assimilation Office (GMAO).
The structure of the nested-grid GEOS-Chem model involves a window with a uniform horizontal resolution of 0.5° × 0.667° embedded in a low-resolution (4° × 5°) global background. The nested-grid GEOS-Chem retains the generic high horizontal resolution of the GEOS-5 data over the nested regional domain. For the present paper, the nested domain is set at 70° E–150° E and 11° S–55° N and includes all of China, its neighboring countries, and a significant portion of the north-western Pacific. The high resolution regional simulation is coupled dynamically to the low-resolution global model through lateral boundary conditions that are updated every three hours.
Compared with WRF-Chem, GEOS-Chem has a high computational efficiency. The model is driven by off-line weather field. Wang et al. [
8] presented a detailed description and evaluation of the model mechanism. However, based on Lin et al. [
9] and Yan et al. [
10], the chemical mechanism has been changed as follows:
1) Anthropogenic emissions of NO
x, CO, SO
2, and VOCs over East Asia were taken from the bottom-up inventory of Streets et al. [
11,
12] for 2000 and Zhang et al. [
5] for 2012.
2) The concentration of propane emission was improved by 3 times than that of before.
3) Sulfate is a major component of fine haze particles. Chen et al. [
13] found that NO
x served as an oxidant to form sulfate. As a result, the system also introduced coupling with this reaction.
This paper focuses on winter transport of pollutants over a particulate region of China, the Jing-Jin-Ji Region. Taking advantage of the increased horizontal resolution, it is possible for the nested-grid GEOS-Chem model to resolve the heterogeneous transport and emission patterns over this area [
14]. The adjoint GEOS-Chem model was developed specifically for inverse modeling of PM
2.5 observations with explicit inclusion of gas-phase chemistry, heterogeneous chemistry, and treatment of the thermodynamic couplings of the sulfate-ammonium-nitrate-water aerosol system; it is thus uniquely capable of exploiting aerosol-phase measurements in novel ways. The adjoint model is used to calculate gradients of the error weighted squared difference between model predictions and observations with respect to emissions [
15].
The pollutant emissions characteristics of coal-fired power plants, ULE power plants, and gas-fired plants were collected and input into the model to determine the maximum environmental benefits for each scenario. The ULE system developed by Shenhua Group can reduce the average emissions of NO
x, SO
2 and PM by 81%, 71% and 84%, respectively. Emission factors for NO
x, SO
2, and PM from gas-fired power plants were taken to be 1.6%, 69.0% 1.0% of the respective emission level of coal-fired plants, following published statistical data by Spath and Mann [
16]. Figure 1 shows the emission factors for NO
x, SO
2, and PM under the three scenarios of power generation. The relative reductions of the ULE vs NG cases differ, but they are both significantly reduced relative to the standard. The average power generation costs are also shown in the Fig. 1.
In the winter of 2012–2013, a period of persistent, severe haze occurred in the Jing-Jin-Ji Region. This episode attracted the attention from all sectors of society, and represents an important case for analysis. In the 2012 Jing-Jin-Ji regional emission inventory, emissions from power source contributed to 36.4%, 35.2%, and 0.1% of the total NOx, SO2, and black carbon (BC) emissions, respectively. The meteorological data of Nov. 2012 to Feb. 2013 was obtained from NASA.
Results and discussion
Environmental benefits analysis
During winter 2012, five haze pollution episodes were identified in the Jing-Jin-Ji Region. During these episodes, the maximum hourly PM2.5 mass concentrations in Beijing ranged from 530 to 680 mg/m3. The simulations in this paper only took into account the effects of pollution controls of power plants located in the Jing-Jin-Ji Region. The effects of contributions from power plants outside this region were not considered. PM only contributes to a small part of the total PM emissions, the influence is marinally.
Figure 2 demonstrates the regional NOx and SO2 emissions distribution for the three scenarios. The heat maps show the simulated spatial distribution of the NOx and SO2 pollutants during a severe haze episode.
For standard scenario, the regional averaged emission concentrations of NOx and SO2 are 27.6 and 34.5 ppbv, while those for ULE scenario are 17.0 and 24.1 ppbv, respectively. If all power plants in the Jing-Jin-Ji Region use natural gas instead of coal, the regional averaged emission concentrations of NOx and SO2 drop to 18.9 and 25.7 ppbv. It also can be deduced from Fig. 2 that the region-averaged reduction percentage of NOx and SO2 for ULE scenario is 38.4% and 30.2%, respectively, but for natural gas scenario, NOx and SO2 emissions are reduced by 31.5% and 25.5% relative to the baseline.
To verify the reliability of the simulation results, the PM2.5 concentration observed by the Environment Monitoring Center (EMC) of Beijing was compared with the simulated “standard” results. In the winter of 2012–2013, the seasonal averaged PM2.5 in Beijing is 107.3 µg/m3 while the simulated PM2.5 concentration is 100.9 µg/m3 as listed in Table 1. The relative error is about 6.0%, it, therefore, can be deduced that the results in this paper are reasonable.
Figure 3 displays the PM2.5 concentration and PM2.5 pollution reduction distribution in the Jing-Jin-Ji Region by the simulations. Table 1 further illustrates the quantitative environmental benefits of ULE and natural gas repowering. The seasonal averaged PM2.5 concentration and regional averaged reduction ratio under the three scenarios are listed. The regional averaged PM2.5 concentrations of the 3 cases in the Jing-Jin-Ji Region are 94.5, 86.8, and 85.6 mg/m3 for standard, all ULE, and all gas-repowered scenarios, respectively. It can be shown by simulations that if all power plants had installed ULE, the PM2.5 pollution during the winter of 2012 could have been mitigated by 8.1%. In comparison, complete repowering by natural gas could have reduced the PM2.5 concentration by 9.4%. In the regions of highest population density in Beijing, these improvements in seasonal-averaged haze concentration could reduce haze impacts by 7.8% or 9.3%, respectively.
Cost analysis
Both the ULE technology and gas repowering have the potential to help improve the haze situation. However, the cost also matters. In China, natural gas resources are scarce. Since the cost of gas-fired power is almost twice that of coal-fired power, widespread repowering by natural gas is still impractical.
Figure 1 shows the average cost of electricity for three scenarios. According to “The 2015 Annual Report on the Regulation of Electricity Prices” released by the National Energy Board,” the average network electricity price for coal-fired power was 38.8 CNY cents/kWh in 2015. According to the research conducted by the Coal Economy Group in North China Electric Power University, the average generation cost is about 30 CNY cents/kWh in China. Retrofitting a power plant with ULE equipment will add extra cost. This extra cost can be partially offset by sewage charges and environmental subsidies by the government. In the Jing-Jin-Ji Region, the government credits for desulfurization, denitrification, and dust are 1.5 1, and 0.2 CNY cents/kWh, respectively. Taking these into account, ULE deployment would add about 1.0–2.0 CNY cents/kWh to the net cost.
In comparison, the average network electricity price of natural gas-fired power generation is 79 CNY cents/kWh. Because of the high price of natural gas, gas-fired power plants are only economical in the eastern area of China where significant regional government subsidies exist. In the Jing-Jin-Ji Region, the net cost of gas-fired power would be about 55 CNY cents/kWh, much higher than that of coal-fired power, which is only 38.8 CNY cents/kWh. Given this economic situation and the comparable environmental benefits, ULE is competitive with natural gas repowering as an approach for controlling haze pollution in the Jing-Jin-Ji Region.
Conclusions
Atmospheric modeling scenarios were analyzed for three cases to provide a better understanding of the environmental benefits of the ULE technology compared to natural gas repowering. The results offer a reference for both policymakers and industry to make relevant and effective decisions regarding the economics of different emissions reduction options.
The results simulated indicate that the widespread adoption of both ULE technology and natural gas power plants can improve the air quality. The seasonal averaged concentration of PM2.5 in Jing-Jin-Ji Region can be reduced by 8.1% and 9.4%, respectively. At current fuel pricing, the generation cost of ULE-equipped coal-fired power plants is lower than that of gas-fired power plants. The cost of ULE installation should add an extra 1–2 CNY cents/kWh to the cost of electricity, which would still be significantly lower than the cost of natural gas power plants. The results suggest that ULE technologies can play a role in improving the air quality over the Jing-Jin-Ji Region. In the long run, ULE also could be a practical way for pollutions reduction in future coal consumption in China.
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