Quantifying the “Water–Carbon–Sulfur” Nexus for Coal Power Plants in China

Abstract

China has implemented strict policies for the installation of desulfurization facilities in coal power plants in order to mitigate their negative environmental and human health impacts. However, it is rarely acknowledged that desulfurization processes lead to increased water consumption and carbon emissions from the coal power sector. By using a bottom-up approach, we quantified that the desulfurization facilities in all of China’s coal power plants together avoided emissions of 29.52 Mt of SO₂ in 2014, with expenses of 550.26 million m³ of increased water consumption, and 53.28 Mt of additional CO₂ emissions. Such conflicts were especially pronounced in the North China Grid, where 9.77 Mt of SO₂ emission reductions were realized at expenses of 132.15 million m³ of water consumption, and 14.25 Mt of CO₂ emissions. The provinces in the North China Grid were already facing extreme water scarcity. Furthermore, while more than 90% of China’s coal power plants have installed desulfurization facilities, the application of full desulfurization would further reduce the greatest amount of SO₂ emissions with the smallest amounts of additional water consumption and carbon emissions in the Northwest Grid. Replacing all wet desulfurization facilities with dry ones saves 498.38 million m³ of water consumption in total, and reduces 26.65 Mt of CO₂ emissions; however, this is at an expense of 14.33 Mt of SO₂ emissions. These conflicts are most pronounced in Shanxi Province in the North Grid, and in Guangdong Province in the South Grid.

1. Introduction

Sulfur dioxide (SO₂) is a short-lived gas that has severe and harmful impacts on human health, ecosystems, and the environment. Direct exposure to SO₂ can cause human respiratory system problems. SO₂ emissions also contribute to particulate matter (PM) air pollution [1], which is a leading cause of death and disability worldwide. Together with NOx, SO₂ is also a major pollutant that causes acid rain, which is harmful to ecosystems both on land and under water, and which can cause the corrosion and weathering of infrastructure systems, as well as affect human health. SO₂ is emitted both naturally (e.g., by volcanoes) and by human activities [2]. Globally, the largest source of anthropogenic SO₂ emissions to the atmosphere is from the fossil fuel combustion of power plants and other industrial facilities.

According to NASA’s data [3], during the last 150 years, anthropogenic SO₂ emissions have increased from 2.06 million tons (Mt) in 1850, to 115.51 Mt in 2005, which is an increase of more than 55 times. Global SO₂ emissions peaked in the 1970s at around 140 Mt annually. Since 1993, China has overtaken the United States and has become the world’s largest SO₂ emitter. In 2005, China alone emitted 32.67 Mt of SO₂, which was more than the United States, India, and Russia combined. According to different sources, coal burning contributed the majority (more than 50%) of the anthropogenic SO₂ emissions worldwide. Specifically, coal burning in China contributed one-quarter of the total global SO₂ emissions. Therefore, SO₂ emission reduction in China can generate significant local, regional, and global benefits.

Starting from the late 1970s, acid rain emerged as a major environmental problem for China [4]. The estimates on the economic costs that have been caused by acid rain in China range from USD 13 billion every year, according to the Chinese State Environmental Protection Administration (SEPA), to USD 11–32 billion, according to the World Bank, depending on the different valuation methods [5]. However, despite the large variations, all of the figures were high, which indicates the severity of the problem. In response, the Chinese government has taken a series of measures, starting from the early 1990s, in order to curb the soaring SO₂ emissions [6]. Coal power production is responsible for the largest share of coal consumption in China, which is followed by manufacturing. In 1996, the State Environmental Protection Administration issued the “Emission Standard of air pollutants for thermal power plants”, which was subsequently amended in 2003 and 2011 [7,8,9]. The current standards (GB13223-2011) set the requirement for the SO₂ emissions from Chinese coal-fired power plants at 100 mg per m³, which is low compared to even global standards [10]. Since the “Technical Specifications for Flue Gas Desulfurization Engineering of Thermal Power Plant” was issued by the SPEA in 2005, desulfurization facilities have been widely applied in China’s coal power plants, and the sulfur dioxide emission intensity has been reduced from 6.4 g/kWh to 2.26 g/kWh, which is even lower than in the United States [11]. In 2014, China further introduced an ultralow emissions (ULE) standards policy for renovating coal-fired power-generating units in order to limit the SO₂ emissions to 35 mg/m³. These policies have been effective. Tang’s research found that the annual emissions of SO₂ from the power plants in China were reduced by 65% from 2014 to 2017, on the basis of the results from a national unit-level emission monitoring system [10].

However, desulfurization processes (flue gas desulfurization (FGD)) often create unintended costs for the environment, which have received much less attention. FGD is a set of technologies that are used to remove the SO₂ from the exhausted flue gases of power plants and other industrial processes. Overall, these technologies can be categorized into dry, semidry, and wet systems, where wet FGD accounts for the predominant share. Wet FGD uses water to create limestone slurry to absorb SO₂, while dry FGD injects dry lime ash for the same purpose. Dry FGD methods normally have lower SO₂ removal efficiencies. However, these FGD processes lead to additional carbon emissions and water consumption. In addition to the additional CO₂ emissions that are caused by the parasitic loads, CO₂ is also emitted as a byproduct of using lime or limestone to neutralize the absorbed SO₂ in the FGD process. Furthermore, wet FGD is the largest water-consuming process at coal power plants with open-loop and air-cooling systems, while the second largest water-consuming process at plants with closed-loop cooling systems, following water consumption for cooling purposes [12].

Because China has made ambitious carbon reduction commitments as the world’s largest CO₂ emitter, and because it also faces a national water crisis, reducing anthropogenic SO₂ emissions may create conflicts with these environmental objectives. To better understand the unintended environmental trade-offs that are caused by desulfurization processes, and to inform the decision-making processes going forward, we make novel contributions in this study by: (i) Quantifying the SO₂ emissions that are avoided by the desulfurization processes that have been adopted by China’s coal power plants, and the resultant additional carbon emissions and water consumption on the basis of a desulfurization technology inventory database that was published by the Ministry of Environmental Protection of China in 2014; (ii) Estimating the further carbon emissions and water consumption that are required to install and operate desulfurization facilities at all of the coal power plants; and (iii) Since dry desulfurization technology offers multiple benefits, including reduced water consumption and carbon emissions at the expense of a lower SO₂ removal efficiency, we also conduct a scenario analysis to quantify the CO₂ emission reduction, the water consumption reduction, and the increased SO₂ emissions if all of the wet desulfurization facilities were retrofitted to dry ones.

2. Method and Data

2.1. CO₂ Emissions and Water Consumption for Desulfurization

The generation of CO₂ for desulfurization is due to two mechanisms: (i) CO₂ is emitted through fossil fuel combustion to generate electricity; and (ii) CO₂ is also emitted in the process of desulfurization through chemical reactions. The CO₂ emissions that are generated by the first mechanism can be calculated by the electricity consumption for the desulfurization process and the carbon emission intensity, as in Equation (1). The CO₂ emissions that are generated from the second mechanism can be calculated according to the chemical reaction equation, as in Equation (2), which is the calculation equation that is mentioned in the “Guidelines for Accounting Methods and Reporting of Greenhouse Gas Emissions by Chinese Power Generation Companies”:

$$E=U\times T\times r\times I$$

(1)

$$E_f={\sum }_{k}CA{L}_k\times E{F}_k$$

(2)

where E refers to the CO₂ emissions from the electricity consumption by the desulfurization process; $U$ denotes the capacity of the electricity generation unit; T refers to the running hours of the unit; and r represents the electricity consumption rate for the desulfurization facilities. Moreover, I represents the carbon emission intensity per unit of electricity generated (ton/kWh); EF refers to the CO₂ emissions from the process of desulfurization through chemical reactions; k refers to the types of desulfurizer; CAL refers to the consumption amount of the carbonate in the desulfurizer; and $E{F}_k$ refers to the emission factor of the carbonate in the desulfurizer of type k.

Similarly, the water consumption for the desulfurization can be calculated according to Equation (3) below:

$$W=U\times T\times F$$

(3)

where W represents the water consumption for the desulfurization; U denotes the capacity of the electricity generation unit; T refers to the running hours of the unit; and F represents the desulfurization-related water consumption factor per unit of electricity generated (m³/kWh).

2.2. SO₂ Emissions

In order to calculate the benefits of reducing SO₂ emissions, as well as the additional SO₂ emissions in a hypothetical scenario, where all of the wet desulfurization facilities are retrofitted to dry desulfurization facilities (similar to Tang et al., 2019), we calculate the SO₂ emissions with a bottom-up approach, according to Equations (4) and (5), as seen below:

$$E{F}_{i,h}=C_{i,h}\times V_i$$

(4)

$$E_{S{O}_2,i,h}=A_{i,h}\times E{F}_{i,h}$$

(5)

where EF is the SO₂ emission factor; C denotes the stack concentration in the flue gas on the basis of a standard oxygen level; V is the theoretical flue gas rate; A represents the activity level (i.e., the electricity generation); i represents the different types of desulfurization technologies, which include wet desulfurization facilities and dry desulfurization facilities; and h is the operation hour of the electricity generating units.

2.3. Data Sources

We obtained the inventory of the desulfurization facilities for China’s coal power plants in 2014 from the Ministry of Environment and Ecology. This inventory list includes the capacity and desulfurization technologies of China’s coal power plants, which was 756.9 GW in total, and which occupied 90.94% of China’s total coal power capacity in 2014. The data on the carbon emission intensity of the electricity production are from the National Center for Climate Change Strategy and International Cooperation [13], and they include six electric power system regions’ carbon emission intensities in China (North Grid: 0.8843 KgCO₂/kWh; East Grid: 0.7769 KgCO₂/kWh; Northeast Grid: 0.7035 KgCO₂/kWh; Central Grid: 0.5257 KgCO₂/kWh; South Grid: 0.5271 KgCO₂/kWh; and Northwest Grid: 0.6671 KgCO₂/kWh). The water consumption intensity for the desulfurization process is taken from Liao’s paper (i.e., 0.175 m³/kWh) [14]; the SO₂ emission factors are from Tang’s research [10]; and the operation hours of the coal power plants in the different provinces in China are from the National Bureau of Statistics of China [15].

3. Results

3.1. Water Consumption and Carbon Emissions for Desulfurization

In total, the desulfurization facilities in all of China’s coal power plants together avoided emissions of 29.52 Mt of SO₂ in 2014; however, this was at expenses of 550.26 million m³ of increased water consumption, and 53.28 Mt of additional CO₂ emissions. The largest SO₂ emission reduction was realized in Shanxi Province (4.2 Mt) in the North China Grid (Figure 1). The North China Grid includes the Jing-Jin-Ji megaregion, where China’s capital city of Beijing is located. It is home to a large population and industrial activities that require large amounts of electricity. Since the North China Grid is coal abundant but water scarce, coal power plants supply more than 95% of the electricity in this grid. The SO₂ emission reduction in the North China Grid amounted to 9.77 Mt, which occupied 33.1% of the national total. Similarly, because of North China Grid has the largest coal power capacity, the water consumption that was caused by the desulfurization was also the highest in the North China Grid, where it amounted to 132.15 million m³. Presumably because of its dry natural conditions, the water consumption from the wet FGD processes occupied the lowest share (96.4%) in the North China Grid, compared to 97.4 to 99.3% in the other regional grids, with the additional water consumption resulting from semidry FGD processes.

In terms of the carbon emissions from the desulfurization processes, the largest amount of CO₂ was emitted in the North China Grid (14.25 Mt), which was followed by the East Grid (11.78 Mt), the Northeast Grid (8.33 Mt), the Central Grid (7.25 Mt), the Northwest Grid (6.26 Mt), and the Southwest Grid (5.41 Mt) (Figure 2). Overall, the Southwest, North, and Northwest Grids had the highest water and carbon efficiencies for the SO₂ reduction (i.e., the lowest amounts of water consumption and CO₂ emissions per ton of SO₂ reduction). In the Southwest Grid, the avoidance of one ton of SO₂ emissions leads to 12.32 m³ of water consumption and 1.04 tons of CO₂ emissions, which is followed by 13.52m³ and 1.46 tons, respectively, in the North China Grid. By comparison, a reduction of one ton of SO₂ requires the largest amounts of water consumption (32.19 m³) and CO₂ emissions (3.01 tons) in the East China Grid.

3.2. Water and Carbon Cost of Applying Full Desulfurization

By comparing the inventory list of the desulfurization facilities and the coal power plant list that was compiled by Tang et al. (2019), it is found that the desulfurization rate of China’s coal power plants has reached more than 90% overall, but with substantial provincial and regional differences, with the lowest rate (82.93%) in the northwest, and the highest rate (98.59%) in the southwest. Nearly 66 GW of the coal-power-generating capacity has yet to install desulfurization facilities, nationwide. The application of the full coverage of desulfurization facilities would further reduce 2.21 Mt of the SO₂ emissions nationally, with the highest reduction coming from the Northwest Grid (0.73 Mt), followed by Northeast (0.52 Mt), North (0.32 Mt), East (0.38 Mt), Central (0.18 Mt), and Southwest (0.07 Mt) Grids.

It can be seen from Figure 3 that, while the reduction potential is much higher in the northwest than in the east, almost the same amounts of water consumption (13.53 million m³) and carbon emissions (1.28 Mt) would be induced in the East Grid as in the Northwest Grid. Therefore, priorities should be given to the Northwest Grid, where the largest SO₂ reduction potential can be realized with the smallest amounts of water consumption and carbon emissions. Although, in general, water consumption for desulfurization does not occupy large shares of industrial water allowances, according to China’s relevant water management policies, it makes up nearly 5% of that allowance in Shanxi Province, and nearly 4% in Ningxia Province. Water is extremely scarce in both provinces.

3.3. Water–Carbon–Sulfur Trade-Offs of Applying Dry Desulfurization Process

In water-scarce regions (WSI > 0.5) (i.e., the north and northwest), switching cooling systems from wet cooling to dry cooling reduces over 1.5 km³ of the water consumption. However, because air-cooling systems are faced with efficiency and energy penalties, such a water consumption reduction would be realized at an expense of 31.08 million tons of CO₂ emissions.

According to our analysis, replacing all of the wet desulfurization facilities with dry ones would save 498.38 million m³ of water consumption in total, and would reduce 26.65 Mt of CO₂ emissions; however, this would be at an expense of 14.33 Mt of SO₂ emissions. It can be seen from Figure 4 that it would be most beneficial to retrofit the desulfurization facilities in Jiangsu, Inner Mongolia, and Henan, where 48.70, 43.98, and 36.01 million m³ of water consumption could be saved, respectively, as well as 1.31, 2.48, and 1.98 Mt of CO₂, respectively, which would lead to only relatively small amounts of additional SO₂ emissions (i.e., 0.52, 0.58, and 0.46 Mt, respectively). Although the water-consumption-savings and carbon-emission-reduction effects are also significant in Shanxi and Guangdong (at 34.34 and 27.70 million m³ of water consumption reductions, respectively, and 2.00 and 1.58 Mt of CO₂ emission reductions, respectively), they cause substantial SO₂ emissions in these two provinces, at 3.17 and 2.23 Mt, respectively.

As is shown in

Table 1. Regional summaries of water–carbon–sulfur trade-offs of changing desulfurization technologies.

Region	SO2 Emissions (Mt)	CO2 Emissions Reduction (Mt)	Water Consumption Savings (Million m3)
Central	1.10	−4.45	−80.41
East	1.26	−4.55	−114.87
North	6.54	−6.79	−111.21
NorthEast	1.15	−4.25	−74.33
Northwest	1.12	−3.41	−59.93
Southwest	3.16	−3.20	−57.64

, on the regional grid level, the highest water consumption savings (114.87 million m³) are realized in the East Grid, together with a 4.55 Mt CO₂ reduction, at an expense of merely 1.26 Mt of SO₂ emissions, whereas, in the North Grid, a similar amount of water consumption savings (111.21 million m³) need to be realized at an expense of 6.54 Mt of SO₂. Further cost–benefit analyses that consider the social, environmental, and economic impacts per unit of water consumption, the carbon emissions, and the SO₂ emissions, need to be conducted in order to facilitate these trade-offs.

4. Conclusions and Discussions

4.1. Conclusions

On the basis of the plant-level data and by using a bottom-up approach, we have quantified that the desulfurization facilities in all of China’s coal power plants together avoided emissions of 29.52 Mt of SO₂ in 2014, at expenses of 550.26 million m³ of increased water consumption, and 53.28 Mt of additional CO₂ emissions. Such conflicts were especially pronounced in the North China Grid, where 9.77 Mt of SO₂ emission reductions were realized at expenses of 132.15 million m³ of water consumption, and 14.25 Mt of CO₂ emissions, while the provinces in the North China Grid were already facing extreme water scarcity. Furthermore, while more than 90% of China’s coal power plants have installed desulfurization facilities, the application of full desulfurization would further reduce the largest SO₂ emissions, with the smallest amounts of additional water consumption and carbon emissions in the Northwest Grid. Replacing all of the wet desulfurization facilities with dry ones would save 498.38 million m³ of water consumption in total, and would reduce 26.65 Mt of CO₂ emissions; however, this would be at an expense of 14.33 Mt of SO₂ emissions, with such conflicts most pronounced in Shanxi Province in the North Grid, and Guangdong Province in the South Grid.

4.2. Water Consumption and Carbon Emissions for Desulfurization

The water-for-energy nexus has gained increasing appreciation in the last two decades [16]. It is acknowledged that significant volumes of water are used at thermoelectric power plants, and primarily for cooling purposes [17]. However, it is rarely acknowledged that the desulfurization process leads to around 30% of the water consumption of coal power plants [12]. Similarly, the desulfurization process also leads to additional carbon emissions. According to our calculation, the desulfurization facilities in all of China’s coal power plants together avoided emissions of 29.52 Mt of SO₂ in 2014, at expenses of 550.26 million m³ of increased water consumption, and 53.28 Mt of additional CO₂ emissions. As the world’s largest CO₂ emitter, China has pledged to peak its carbon emissions by 2030 [18]. The reduction in SO₂ emissions introduces conflicts with China’s climate change mitigation agenda.

Similarly, China is not abundant in water, with an average per capita water resource endowment that only amounts to one-third of the global average, with its northern regions especially facing dire water challenges. Among the thirty-one mainland Chinese provinces, eight of them are facing extreme water scarcity, with less than 500 m³ per person. Seven of them are located in the northern regions, and all five provinces in the North China Grid fall into this category [19]. The North China Grid is identified as facing the highest water risks for power generation in the world [20]. Meanwhile, desulfurization led to the largest amount of water consumption in the North China Grid, where it amounted to 132.15 million m³.

4.3. Water–Carbon Trade-Offs and Synergies at Coal Power Plants

Besides the trade-offs between water and SO₂, and between CO₂ and SO₂, there are also trade-offs between water and carbon. Water-saving technologies at coal power plants are often carbon intensive. For example, compared to wet-cooling technologies, which use water as a cooling medium, air-cooling technologies use air for cooling purposes, thereby significantly reducing the water use of coal power plants. However, air-cooling technologies suffer from 5 to 10% efficiency losses, and they therefore lead to additional carbon emissions [21]. On the other hand, technologies that are used to mitigate carbon emissions can be water intensive. For example, while carbon capture and storage (CCS) offer the potential to cut the carbon emissions from coal power plants, they require additional water use because of the parasitic loads [22].

It is, therefore, important to realize the opportunities to reap the cobenefits of water savings and carbon reductions. Transforming the power sector to low-carbon sources, such as solar PV and wind power, will realize such synergies [23]. Our study shows that the retrofitting of desulfurization facilities is also able to reduce the water consumption and carbon emissions at the same time. Replacing all current wet desulfurization facilities with dry ones would reduce 498.38 million m³ of the water consumption and 26.65 Mt of the carbon emissions, in total. Such benefits are especially significant in the provinces of Jiangsu, Inner Mongolia, and Henan, where large amounts of water consumption and carbon emissions could be reduced at an expense of relatively small amounts of SO₂ emissions.

4.4. Outlook and Future Directions

As a cheap and abundant resource, coal power production has fueled China’s development miracle over the last few decades. However, the negative social environmental consequences that are caused by coal power plants are gaining increasing amounts of attention. Coal power plants are not only major contributors to CO₂ and SO₂, but they also generate other air pollutants, such as PM2.5, which lead to negative human health impacts. According to the estimates from the Global Burden of Disease study [24], air pollution is the fourth leading cause of death and disability in China. In recent years, hazy air, which is caused by the particular matters, PM2.5 and PM10, has given rise to increasing social unrest in China. The research estimates that coal pollution cuts the life expectancy in northern China by 5.5 years [25]. It is therefore useful to incorporate other pollutants from coal power plants into the holistic evaluation and trade-offs. Furthermore, in order to facilitate the decision making on technology choices when considering these various trade-offs, economic analyses could perhaps be adopted in order to monetize the social, environmental, and health impacts of various coal power plants. Although it should be acknowledged that not all externalities can be monetized, monetization enables comparisons between the different dimensions, and it can be used in conjunction with other decision-making methods, such as multicriteria analyses.