Assessment of the emission of air pollutants and greenhouse gases in the flue gas from coal-fired power plants in Vietnam
Coal-fired power plants (CFPPs) are an important energy source and the major source of air pollutants as well as greenhouse gases (GHGs) in Vietnam.
The emission data of air pollutants and GHGs are an important base for setting up control strategies and management policies. This study assesses the emissions of PM2.5, PM10, SO2, NOx, CO, CH4, N2O, and CO2 from CFPPs in Vietnam in 2019, and 2021 and predicts those in 2030 based on coal consumption and emission factors.
A method to estimate PM2.5 and PM10 emission based on the emission of PM and size distribution was proposed to overcome the difficulty of lacking data and measuring those parameters in the flue gas in Vietnam.
1. Introduction
CFPPs are one of the main sources to air pollutants as well as GHGs emissions in Vietnam. In recent years, air pollution has been a major environmental concern in Vietnam (Huy and Oanh, 2017). For example, the concen-tration of PM2.5 in many provinces and cities in Vietnam has exceeded the allowable limit of the national standards on ambient air quality and is several times higher than the recommended levels of World Health Organization guide-lines (Hien et al., 2019; Thanh et al., 2019; Ly et al., 2018).
In 2019, Vietnam had 28 CFPPs in operation with an installed capacity of 19,744 MW, accounting for 35.83% of the total installed capacity of the power system (EVN, 2022). The total capacity of electricity produced from CFPPs has developed rapidly over the past decade. CFPPs have been projected to be the main power source (Roy et al., 2021, 2022).
However, currently, Vietnam has issued a new National Electricity Development Plan for the peri-od 2021 – 2030, vision to 2050 (Power Development Plan VIII – PDP8), which significantly reduce the percentage of CFPPs to 20% of the power source structure in 2030, cor-responding to a capacity of 30,127 MW. This adjustment is considered as one of Vietnam’s efforts to decrease GHGs emissions by 8% in 2030 as agreement made in COP21.
Assessing the emissions of CFPPs is very important to develop strategies and plans to control air pollution and mini-mize the contribution to global warming. There were three publications on air pol-lution emission of CFPPs in Vietnam, in 2010 (Huy and Oanh, 2017), 2015 (Roy et al., 2021), 2019 (Roy et al., 2022) and emission scenarios according to PDP7 revised (Roy et al., 2022). The emission scenarios according to PDP8, which has a significant reduction of contribution of CFPPs in power source structure, are not yet available.
This study aims to develop an emission inventory in 2019 (based year), in 2021, and forecast emissions according to PDP8 in 2030. Compared with Roy et al., 2022, the overall meth-odology in this study is similar, but as this research is conducted later the data on coal consumption, coal characteris-tics are more updated. Besides, there are some different approaches in the selec-tion of calculation parameters.
2. Methodology
2.1. CFPPs and activity dataa.
a. CFPPs
Twenty-eight CFPPs were operating in Vietnam in 2019, of which 19 plants use pulverized coal (PC) boilers and nine plants use circulating fluidized bed (CFB) boilers. The total installed capacity of CFPPs using CFB and PC technology is 3,420 MW (24%) and 15,638 MW (76%), respectively. Most of the CFPPs are located in the significant coal mines and key economic zones of the Northern, only seven and three plants are in the Central and Southern regions of Vietnam.
b. Activity data
Activity data of CFPPs are critical information to estimate annual emissions from each CFPPs us-ing a bottom-up approach. Besides, emissions from coal combustion depend on coal type and its com-position, the design type and capacity of the boiler, the firing conditions, load, the types of control de-vices, and the level of equipment maintenance (US – EPA, 1996).
Most of the activity data used in this study were collected from local and national sources. Location, installed capacity, boiler technology, and total electricity output (GWh) were obtained from local reports. The coal consumption and coal properties including net calorific value (NCV), ash content, and sulfur content were mainly ex-tracted from CFPPs reports. In case, data on coal consumption is not available (2019), the number was calculated based on electricity capacity and coal consumption rate. The activity data for op-erated CFPPs in 2019 and 2021.
It is important to note that the electricity production in 2020 and 2021 were affected by COVID-19. Therefore, the yearly increase in electricity production is lower than in the previ-ous period. The electricity productions in 2019 and 2021 are almost the same.
For the prediction of 2030 emissions, the to-tal electricity production is chosen following PDP8 (designing value of electric production). The emission rate of all pollutants and GHGs per electricity production of total CFPPs in 2019 was multiplied by the electricity prediction to obtain the emission in 2030.
2.2. Emission calculation
2.2.1. Equation
The amount of polluted gas emitted by CFPPs was calculated according to Equation 1:
E = EF × R × ((100 – EC)/100) [1]
Where: E is the amount of emissions (tons/year); R is activity data (coal consumption (tons/year), for air pollutant estimation; the heat of burned coal (KJ/year) for GHGs estimation); EF is emission factor corresponding to the amount of activity (kg/tons) for pollutants; (kg/TJ) for GHGs; and EC (%) is the efficiency of control systems.
2.2.2. Emission factor of gaseous pollutants
For air pollutants including PM (filterable par-ticulate matter), NOx, SO2, and CO, EFs were ex-tracted from the AP-42 document (EPA, 1998). For GHGs including CO2, CH4, N2O, EFs from the IPCC-2006 document (IPCC, 2006) were applied.
2.3. Pollutant control efficiency
Unlike activity data, data about treatment effi-ciencies for air pollutants are not available in terms of operating values but only designing values. However, not all design treatment efficiencies are available. Therefore, we reviewed the current treat-ment systems and determined the best guess num-bers for operating treatment efficiencies based on the types of installed devices or design efficiencies. This method is subject to large uncertainty but is considered to be the most suitable method for the time being.
2.3.1. Particulate matter treatment and effi-ciency
All CFPPs in 2019 installed electrostatic pre-cipitator (ESP) systems with design efficiency of particulate matter treatment higher than 99.7% (Luong, 2018); 99.6% (EVN, 2019) or 99.19% in some plants (gathered from our communica-tion). The operating efficiencies can be affected by several factors (such as the electric voltage, physical conditions of ESPs, fly ashes charac-teristics, and the shifting of operation tem-perature after a long time operation) and nor-mally lower than designing one depending on the maintenances.
In this study, the treatment efficiency for PM of 99.2%, which is equal to US.EPA-AP.42, was applied for all plants. Unfortunately, there is no local available in-formation regarding the treatment efficiencies of PM2.5 and PM10.
2.3.2. SO2 control and efficiency
SOx emissions in current CFPPs in Vietnam were controlled by two types of technologies: controlling in the boiler (applying for CFB boiler); flue gas desulfurization (FGD) (mainly divided into two solutions: SO2 absorption by slurry limestone and SO2 absorption by sea wa-ter).
All CFB plants remove SO2 in boilers by injecting limestone (CaCO3) into the fire cham-bers, where SO2 reacts with limestone to form CaSO4 which can be discharged as ash and slag. Some plants have additional FGD for further treatment of SO2. Most PC plants apply SO2treatment technology with limestone/Mg(OH)2 or seawater. The method of treating SO2 with limestone is very popular in the world and has been tested through manufacturing and opera-tion. This method has a high efficiency of up to or over 90%.
The seawater absorption method is a modern technology, currently applied to new CFPPs in Vietnam such as Vinh Tan 2, Duyen Hai 1, Vung Ang 1, Vinh Tan 4, Duyen Hai 3, etc. This technology uses cooling seawater from condensers to absorb and neutralize SOx in ex-haust smoke. The SOx removal efficiency of this technology can be up to over 90% (EVN, 2018; Luong, 2018).
The treatment efficiency of 90% for SO2 was applied for all PC CFPPs and 0% for CFB CFPPs.
2.3.3. NOx control and efficiency
There are two main methods for controlling NOx emissions in CFPPs in Vietnam: controlling NOx emissions in boilers and treating NOx in exhaust gas. Currently, NOx treatment meth-ods at PC CFPPs in Vietnam vary from using low NOx emission nozzles/burners to equipping selected catalyst reactor (SCR) equipment. The processing efficiency of the SCR device for NOx treatment can reach about 75 – 80%. For CFB plants, NOx treatment devices are not installed due to low combustion chamber temperatures, but emissions can still comply with the National Technical Regulation on Emission (EVN, 2018; Luong, 2018).
The treatment efficiency of NOx was chosen of 0% for CFB CFPPs and 30% for PC CFPPs, except for seven plants with SCR (Vinh Tan 1, 2, 4 and 4 extension, Mong Duong 2, Duyen Hai 1 and 3 with efficiency of 65 – 88.4% extracted from Roy et al., 2022).
2.3.4. CO, CH4, CO2 control and efficiencies
There is no CO, CH4, CO2 treatment for CFPPs in Vietnam. Therefore, the treatment efficiencies of those compounds are considered as 0%.
3. Results and Discussions
3.1. Determination of emission factors of PM2.5 and PM10
a. Proposal of emission assessment methods for PM2.5 and PM10
In Section 2, it was noted that there is no de-signing treatment efficiency for PM2.5 and PM10 in Vietnam. Obtaining information about the emis-sion of PM2.5 and PM10 in Vietnam is challenging and costly due to the sophisticated monitoring methods. However, research on PM emission in Vietnam has yielded controlled emission factors for new CFPPs and a trouble happened CFFP (Dung et al., 2014).
To address the difficulties in estimating PM2.5 and PM10 in Vietnam, a meth-od utilizing the EF of PM and the PM10/PM and PM2.5/PM ratios was proposed. The uncontrolled EF from the US.EPA – AP42 and unified emission control efficiencies of 99.2% for PM were chosen for all treatment processes, including ESP – FGD, SCR – ESP – FGD, and ESP – FGD. The size dis-tributions of particulate matter were determined based on relevant research comparisons.
Shows a wide variation in the size dis-tribution of PM of different coal types and boiler technology with different control devices. It is noted that the data for certain technologies such as PC burning of anthracite with ESP treatment is not available. The lack of size distributions also means a lack of emission factors and treat-ment efficiency information for PM2.5 and PM10as data of PM are more available and the size distribution can be calculated from the emis-sion of PM2.5 and PM10 (if available).
It can be observed that with additional treat-ment devices (higher efficiency) the percent-age of PM>10 reduces followed by an increase of PM10 and PM2.5. Therefore, for the current study the size distribution for particulate matter after ESP treatment of PC using bituminous coal of US.EPA – AP42 (removal percent-age of 99.2% for PM) were chosen as representatives for both anthracite/bituminous coals and PC/CFB tech-nologies. Size distribution can be upgraded in the future based on the available data.
b. Comparison of emissions based on proposal method using different PM size distributions
The calculated emissions of PM10 and PM2.5 emission of CFPPs in Vietnam based on size distribution reviewed in Section 3.1.a are pre-sented . We chose the size distribution in Korea of PC/Bitumi-nous – Subbituminous with treat-ment system of SCR+APH+ESP and in China of PC/Bituminous with treatment of ESP for comparison. The two options were chosen as they are the most relevant data. The treatment efficiency of the Kore-an system is considered higher and the treatment efficiency in China is closer to the Vietnamese condition.
The calculation results are based on size distri-bution US.EPA-AP42 was in the same range as those after ESP in the research in China of Wu et al. (2018) but 2.9 times lower than when calculated with the size distribution in Korea by Jeong et al. (2021) for PM2.5 and 1.3 times lower than when calculated with the size distribution in Jeong et al. (2021) for PM10.
The high sensitives of PM2.5 emissions with size dis-tribution are comparable with the high sensitivity of emission with treatment efficiency. A small change in efficiency (at a high rate) can also lead to a sig-nificant change in emission. For example, changing from the removal of 99% to 98% can lead to an in-crease of 100% in emissions.
3.2. Emission of air pollutants
3.2.1. Particulate matter
Summarizes the emission of PM10 and PM2.5from previous and current studies for the CFPPs in 2010, 2019 and forecasts for 2030. The number of CFPPs in-creased over time (from 12 CFPPs in 2010 to 28 CFPPs in 2019), leading to a significant increase in the amount of coal burning, increasing emissions of air pollutants (Huy and Oanh, 2017; Roy et al., 2022).
According to the results presented , our study has revealed higher levels of PM10 and PM2.5 emissions in 2019 compared to those reported by Roy et al. (2022). It is important to note that both studies are based on local activities of CFPPs and other selected factors such as EFs, removal rate, or size distribution. Therefore, we cannot claim that our data is better.
However, emissions calculat-ed by our proposed methods can be easier updated in the future. It is also noted that corresponding with the select-ed PM size distribution removal efficiency in this study were 97.65% and 96.00% for PM10 and PM2.5, respectively whereas those in Roy et al. (2022) are 98.4% and 98.5%.
3.2.2. Gaseous pollutants
The results of estimating gaseous pollutants (SO2, NOx, and CO) emissions of CFPPs in Vietnam are shown in Ta-ble 8. The values in 2019 are compared to evaluate our data set. The total coal consumption amount of CFPPs in this study and the study of Roy are only slightly different (this study is 6% lower). The emission of SO2 from Roy et al. 2022 is 35% lower than this study mainly because of the lower treatment efficiencies in this study, especially there are two plants with no treatment system.
Besides, the con-version rate of sulfur in the coal to SO2 in flue gas (which affects emission factors) in this study is higher than that in Roy et al. (2022). The NOx emission in this study is compara-ble with those in Roy et al. (2022). The CO emission in this study is 31.4% lower than in Roy et al. (2022), mainly due to the use of lower emission factors and 6% lower coal consumption in this study.
Despite an increase in coal consump-tion, the emissions of SO2 and NOx were reduced compared to 2019. This is because of the increasing use of CFB boilers, which have lower emission factors for SO2 and NOx than PC boilers.
If the exhaust gas treatment system con-tinues to develop at its current rate and no improvements are made, it is predicted that by 2030, most pollutants will be aproxi-mately 1.5 times as high as those in 2019.
3.3. Emissions of GHGs
Displays the emissions of green-house gases (CH4, N2O, and CO2) from CFPPs in 2019, 2021, and projected for 2023. The calculation of GHG emissions was based on emission factors derived from the heat of burned coal. In comparison to the study con-ducted by Roy et al. (2022), the total heat of burned coal in this study is 19% lower. As a result, the emissions of CH4 and N2O in this study are also 19% lower than those in Roy et al. (2022), as the emission factors for these gases are similar between the two studies.
The CO2 in 2019 of this study and the number reported by the Vietnamese Ministry of Natural Resources and En-vironment (MONRE, 2021) are similar. Both are approx-imately 18% lower than the figure reported by Roy et al. (2021). This difference is primarily due to the disparity in the total heat of burned coal.
If CFPPs continue to operate without GHG emissions controls or technological advancements as outlined in PDP8, GHG emissions are projected to increase by 52.6% by 2030 compared to 2019.
4. Conclusions and Recommendations
This study has calculated the emission of main air pol-lutants (PM2.5, PM10 …) in flue gases of CFPPs in 2019 and 2021. Three GHGs (CH4, N2O, CO2) were also estimated. The projected emission of those air pollutants and GHGs based on PDP8 was done for the first time.
Even though this study was based on available local data about coal consumption and coal characteristics of CFFPs, large uncertainties of the emission existed mainly because of the lack of national emission factors. Besides, the lack of information about treatment systems, design treatment efficiencies, and especially operation treatment efficiencies also increases the uncertainties of the results.
The following additional actions are recommended to reduce the uncertainties of emission inventory data of CFFPs: (1) Collection of information about air pollution treatment systems and designed treatment efficiencies of all CFFPs; (2) Determination of controlled emission fac-tors of all CFFPs; (3) Assessment of operation treatment efficiencies of typical treatment devices in different treat-ment systems; (4) Surveying the technologies of boilers and coal size, consumption rates, and characteristics of ad-ditives to the boilers.
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Author: Trinh Thi Phuong Ly, School of Chemistry and Life Sciences, Hanoi University of Science and Technology
Author: Le Thanh Thuy, Duc Minh Investment and Environment Development Joint Stock Company
Author: Pham Thi Bich Thao, Independent researcher
Author: Nguyen Thi Yen Lien, Faculty of Transport Safety and Environment, University of Transport and Communications
Author: Van Dieu Anh, School of Chemistry and Life Sciences, Hanoi University of Science and Technology
Author: Phung Thi Thu Trang, Vietnam Institute of Meteorology, Hydrology and Climate change
Author: Ly Bich Thuy, School of Chemistry and Life Sciences, Hanoi University of Science and Technology
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