Emission Characteristics of Ammonia at Bituminous Coal Power Plant

Abstract

This study developed a NH₃ emission factor for bituminous coal power plants in South Korea in order to investigate the NH₃ emission characteristics. The NH₃ concentration analysis results showed that emissions from the selected bituminous coal power plants were in the range of 0.21–0.99 ppm, and that the difference in NH₃ concentration was affected by NOx concentration. The NH₃ emission factor was found to be 0.0029 kg NH₃/ton, which demonstrated that the difference in the values obtained from the research conducted in South Korea was lower than the difference in the emission factor provided by the U.S. EPA, which is currently applied in the statistics of South Korea. NH₃ emissions were compared by using the NH₃ emission factor developed in this study alongside the EPA’s NH₃ emission factor that is currently applied in South Korea’s statistics; the difference was found to be 206 NH₃ ton/year. This implies that an emission factor that reflects the national characteristics of South Korea needs to be developed. The uncertainty range of the NH₃ emission factor developed in this study was between −6.9% and +10.34% at a 95% confidence level.

1. Introduction

In 2016, Ultrafine (≤2.5) Particulate Matter (PM2.5) concentration in South Korea was 26 µg/m³, which was higher than that of Europe, the United States, and Japan. From 1990 to 2015, the average PM2.5 concentration in South Korea was 29 µg/m³, which was the highest among all the member countries of the Organization for Economic Co-operation and Development (OECD) except Turkey. Moreover, South Korea fared worse than Vietnam, Mongolia, Japan (13 µg/m³), and Singapore [1].

One of major causes of PM2.5 is the increase in secondary sources of particulate matters (PM), such as NOx, SOx, VOCs, and NH₃ [2,3,4,5]. In South Korea, NOx and SOx are controlled by the “Air Pollutant Emission Limit Regulation”, with many studies using it for research [6,7,8]. However, few studies have focused on the emission factor and emission estimation of NH₃ (ammonia).

Among the secondary sources of PM, emission estimation and emission sources of NH₃ are important with respect to air pollution management because emission reduction of NH₃ is closely related to the changes in PM2.5 concentration. A previous study analyzed PM2.5 concentration changes in South Korea based on the reduction in air pollutants (NOx, SOx, NH₃ and PM) using an air quality model (CMAQ) and concluded that a reduction in NH₃ emissions leads to a greater reduction in PM2.5 concentration as compared to any other pollutant [9,10]. Accordingly, there has been an increased focus on research related to NH₃ emission sources and emission estimation [11].

South Korea constructs NH₃ emission inventories using various categories, including energy industry combustion, non-industry combustion, manufacturing industry combustion, production process, off-road mobile sources, waste treatment, agriculture, other area sources and biomass combustion. In the case of energy industry combustion, bituminous coal power plants comprise the majority of power plants [12].

The NH₃ emission factor of bituminous coal power plants is difficult to obtain for South Korea because the U.S. Environmental Protection Agency (EPA) value for the year 1994 is used. Therefore, this study aims to analyze the NH₃ emission of bituminous coal power plants in South Korea and conduct research on the emission characteristics, including the development of an emission factor and an analysis of uncertainty. Furthermore, the differences in NH₃ emissions are examined by using the NH₃ emission factor developed for this study, which reflects the characteristics of South Korea, the EPA’s value currently applied in South Korea, and an emission factor value developed previously in South Korea.

2. Method

2.1. Selection of Objective Facilities

This study collected NH₃ samples from three bituminous coal power plants to investigate their NH₃ emission characteristics.

Table 1. Characteristics of the investigated bituminous coal power plant.

Site	Capacity (MW)	Fuel Type	Sampling
Power Plant A	1020	Bituminous Coal	16
Power Plant B	1050	Bituminous Coal	10
Power Plant C	500	Bituminous Coal	19

shows the power generation capacity, fuel consumption, and frequency of sampling conducted at the power plants. Sampling was performed at least three times at each power plant, with ten or more samples collected.

2.2. Analysis of Ammonia at Bituminous Coal Power Plant

This study employed the indophenol method presented in the “Odor Analysis Method” and “Standard Methods for the Measurements of Air Pollution” of South Korea to measure the NH₃ emission concentration of bituminous coal power plants [13]. The indophenol method adds phenol-sodium nitroprusside solution and sodium hypochlorite solution to the sample solution for analysis and measures the absorbance of indophenols, which reacts with ammonium ions, to quantify NH₃. To collect NH₃ samples, an ammonia absorbing solution (50 mL 0.5% boric acid solution) was put into two 50 mL capacity flasks, and a mini pump was used to pump in 80 L of emission gas for 20 min at 4 L/min. A moisture absorption bottle containing silica gel was installed in front of the NH₃ sampling device to remove the moisture in the gas emitted from the power plants. Figure 1 shows a schematic diagram of NH₃ sample collection. Furthermore, a spectrophotometer (Shimadzu 17A, Japan) was used to measure the absorbance of the ammonia absorbing solution at a wavelength of 640 nm.

2.3. Development of NH₃ Emission Factor

The NH₃ emission factor calculation is shown in Equation (1). CleanSYS data from the three bituminous power plants were used for the flowrate data required in the development of an NH₃ emission factor, and one-day cumulative flowrate data were used for the flowrate. In the case of fuel usage amount, the data were obtained from the power plants.

$$E{F}_{N{H}_3}=\left[C_{N{H}_3}\times \frac{M_w}{V_m}\times Q_{day}\times {10}^{-6}\right]/F{C}_{day}$$

(1)

where $EF$ is emission factor (kg NH₃/ton); $C_{N{H}_3}$ is NH₃ concentration in exhaust gas (ppm); $M_w$ is molecular weight of NH₃ (constant) = 17.031 (g/mol); $V_m$ is one mole ideal gas volume in standardized condition (constant) = 22.4 (10⁻³ m³/mol); $Q_{day}$ is daily accumulated flow rate (Sm³/day) (based on dry combustion gas); and $F{C}_{day}$ is daily fuel consumption (ton/day).

2.4. Uncertainty Analysis by Monte Carlo Simulation

This study used Monte Carlo simulations to estimate the uncertainty of the NH₃ emission factor and performed the analysis in four stages, as shown in Figure 2 [14,15]. First, in the model selection stage, a NH₃ emission factor estimation worksheet was constructed. Second, the probability density functions of input variables needed for the development of the NH₃ emission factor were tested through fitness tests. The level of significance was set to 5% for the hypothesis test, and the probability density functions were calculated through the fitness tests using the data required for NH₃ emission factor development, such as NH₃ emission concentration, emission flowrate, and low calorific value of fuel. Third, when performing the Monte Carlo simulations, random sampling simulations were performed using a “Crystal Ball”. Fourth, the uncertainty range of 95% confidence interval was calculated through the simulation results.

“Crystal Ball” constructs the probability density function of the emission factor as the result of each calculation performed through an iterative process using simulation. It also gives a range of 95% confidence intervals (±$Z_{a/2}$) in the generated emission factors. We can estimate the uncertainty through that range.

3. Result and Discussion

3.1. Characteristics of NH₃ Emission

Table 2. NH₃ concentration of the investigated bituminous coal power plants.

Site	NH3 Concentration (ppm)	SD(Standard Deviation) (ppm)	Sampling	NOx Concentration (ppm)
Power Plant A	0.21	0.14	16	20
Power Plant B	0.99	0.56	10	14
Power Plant C	0.27	0.21	19	23

shows the results of the NH₃ concentration analysis for three bituminous coal power plants. The mean NH₃ concentration of bituminous coal power plant A was 0.21 ppm, with a standard deviation of 0.14 ppm. The NH₃ concentration of bituminous coal power plant C was 0.26 ppm, which showed a similar concentration band as the bituminous coal power plant A. Moreover, it showed a standard deviation of 0.21 ppm, which was higher than that of the bituminous coal power plant A. The NH₃ concentration of bituminous coal power plant B was 0.99 ppm, which was approximately five times higher than that of bituminous coal power plants A and C, and the standard deviation was 0.56 ppm. High NH₃ concentration exhibited by the bituminous coal power plant B was related to NOx concentrations [16]. In the case of coal-fired power plants, NH₃ is injected in the SCR (Selective Catalytic Reduction) in order to reduce NOx concentrations; NH₃ that does not completely react is emitted through the final emission outlet. Therefore, the NH₃ concentration emitted through the final emission outlet will be high in proportion to the amount of NH₃ injected to reduce NOx concentrations. To confirm this, a comparison of the NOx data acquired during the measurement period from the three selected bituminous coal power plants was conducted. The results demonstrated that the NOx concentration of power plant A (20 ppm) and the NOx concentration of Unit No. 6 of power plant C (23 ppm) were higher than that of power plant B (14 ppm). Therefore, it was estimated that the NH₃ concentration of power plant B was high because a large amount of NH₃ was injected to reduce its NOx concentration.

The correlation of NH₃ concentration and NOx concentration was examined in detail, using the daily average NH₃ concentration and the daily average NOx concentration of the selected power plants, as shown in Figure 3. The analysis revealed that as the NH₃ concentrations decreased, the NOx concentrations increased, thus exhibiting an inversely proportional relationship. Therefore, as the amount of NH₃ was increased for NOx reduction, NH₃ slip increased, leading to high NH₃ emission concentration.

3.2. NH₃ Emission Factor and Comparison of NH₃ Emissions

This study developed the NH₃ emission factor by collecting 45 NH₃ samples from three bituminous coal power plants. The results are shown in

Table 3. NH₃ emission factor of the investigated bituminous coal power plant.

This Study (kgNH3/ton)	US EPA(1994) (kgNH3/ton)	NIER (2019) (kgNH3/ton) [18]
0.0029	0.00028	0.0054

.

The NH₃ emission factor development result was found to be 0.0029 kg NH₃/ton, which is approximately ten times larger than the currently-applied EPA NH₃ emission factor of energy industry combustion that is used in South Korea’s national statistics (0.00028 kg NH₃/ton) [17]. This value is about two times lower than the emission factor of 0.0054 kg NH₃/ton for a bituminous coal power plant, which was analyzed in a 2019 South Korean research report [18]. It was also found that this figure is significantly lower than the range of ammonia emission factor (0.07 kg NH₃/ton to 1.17 kg NH₃/ton) for household stoves, which is among the combustion partial ammonia emission factors studied more recently than studies done by the U.S. EPA [19].

Considering these results, the difference in the emission factor obtained from the research conducted in South Korea is lower than the difference in the emission factor of the EPA, which is currently applied in the statistics of South Korea. Therefore, it is important to develop an NH₃ emission factor, which reflects the characteristics of South Korea.

The emission factor that was developed in this study and the EPA’s emission factor, which is currently applied in the statistics of South Korea, were used to compare the difference in NH₃ emissions at the selected bituminous coal power plants. Figure 4 shows the results.

When the emission factor developed in this study was applied, the NH₃ emission was calculated to be 228 NH₃ ton/year. When compared to the NH₃ emission estimate (22 NH₃ ton/year), which is calculated by applying the conventional EPA emission factor, the difference was approximately 206 NH₃ ton/year. Therefore, it is necessary to develop a NH₃ emission factor to improve the confidence level of inventory.

3.3. Uncertainty of NH₃ Emission Factor

Monte Carlo simulation was used to estimate the uncertainty of the NH₃ emission factor of bituminous coal power plants selected for this study. Figure 5 shows the estimation results. The probability density function of the NH₃ emission factor of bituminous coal power plants developed in this study had lognormal distribution. The mean was 0.0029 kg NH₃/ton at a 95% confidence level; the lower 2.5% showed 0.0027 kg NH₃/ton and the upper 97.5% showed 0.0031 kg NH₃/ton. The uncertainty range of the NH₃ emission factor was estimated using these values from −6.9% to +10.34% at 95% confidence level. At present, the values and range are not available for NH₃ uncertainty. Therefore, comparison with relevant cases is difficult. However, in the case of greenhouse gas, uncertainty range and values are available.

When the uncertainty of the NH₃ emission factor of bituminous coal power plants from this study is compared with the greenhouse gas uncertainty range provided by the Intergovernmental Panel on Climate Change (IPCC), the NH₃ emission factor is found to be much lower than the basic uncertainty range, that is 50–150% for CH₄ emission factor. Moreover, the uncertainty is 1000% of the uncertainty of the N₂O in the stationary combustion sector of energy provided in the 2006 IPCC guidelines, but is larger than the uncertainty (−1.0 to +1.04) of the carbon emission factor of bituminous coal [20]. In South Korea, the uncertainties in air pollutants are expressed in ranks and evaluated by experts. If the uncertainty range of air pollutants is provided, as in the case of greenhouse gases, it is possible to evaluate them quantitatively.

4. Conclusions

This study developed a NH₃ emission factor for bituminous coal power plants in South Korea to investigate the NH₃ emission characteristics. Furthermore, three different emission factors were used to compare the NH₃ emissions, namely the EPA value, which is currently applied in South Korea, a previously developed emission factor value in South Korea, and the emission factor value developed in this study, which reflects the characteristics of South Korea. Three bituminous coal power plants were selected to compare the NH₃ emission characteristics, including NH₃ emission factor development.

The NH₃ concentration analysis results showed that emissions from the selected bituminous coal power plants were in the range of 0.21–0.99 ppm, and that the difference in NH₃ concentration was affected by NOx concentration. The NH₃ emission factor was found to be 0.0029 kg NH₃/ton, which demonstrated that the difference in the values obtained from the research conducted in South Korea was lower than the difference in the emission factor from the EPA, which is currently applied in the statistics of South Korea. Furthermore, when NH₃ emissions were compared by using the NH₃ emission factor developed in this study alongside that of the EPA’s NH₃ emission factor that is currently applied in South Korea’s statistics, the difference was found to be 206 NH₃ ton/year. This implies that an emission factor needs to be developed which reflects the national characteristics of South Korea.

The uncertainty range of the NH₃ emission factor developed in this study was between −6.9% and +10.34% at a 95% confidence level. At present, numerical values of uncertainty are not available for air pollutants, thus making their comparison difficult. When compared with the uncertainty of greenhouse gas, the NH₃ emission factor’s uncertainty was higher than that of the carbon emission factor of bituminous coal and lower than that of the emission factors of CH₄ and N₂O. If the uncertainty ranges are provided for air pollutants, like those of greenhouse gas, quantitative evaluation will be feasible.

This study investigated the NH₃ emission factor and emission characteristics for only three bituminous coal power plants. In the future, if a NH₃ emission factor is developed for a larger number of facilities by considering the seasonal effects, the confidence level of NH₃ inventory in South Korea will significantly improve.