Format And Type Fonts CHEMICAL ENGINEERING TRANSACTIONS VOL. 45, 2015 A publication of The Italian Association of Chemical Engineering www.aidic.it/cet Guest Editors: Petar Sabev Varbanov, Jiří Jaromír Klemeš, Sharifah Rafidah Wan Alwi, Jun Yow Yong, Xia Liu Copyright © 2015, AIDIC Servizi S.r.l., ISBN 978-88-95608-36-5; ISSN 2283-9216 DOI:10.3303/CET1545261 Please cite this article as: Mokhtar M.M., Taib M.R., Hassim M.H., 2015, Comparison of pollutant ambient concentration from two air pollution control (APC) strategies in coal-fired power plant, Chemical Engineering Transactions, 45, 1561-1566 DOI:10.3303/CET1545261 1561 Comparison of Pollutant Ambient Concentration from Two Air Pollution Control (APC) Strategies in Coal-fired Power Plant Mutahharah M. Mokhtar, Mohd R. Taib, Mimi H. Hassim* Department of Chemical Engineering, Faculty of Chemical Engineering, Universiti Teknologi Malaysia, 81310 UTM Johor Bahru, Johor, Malaysia mimi@cheme.utm.my In this paper, ambient concentrations of Hg and dioxins/furans from two air pollution control (APC) strategies are compared. The strategies include the existing APC strategy at the studied coal-fired power plant (CFPP) and the proposed APC strategy for compliance with parameters in the Environmental Quality (Clean Air) Regulations (CAR) 2014. The former system consists of electrostatic precipitator (ESP) and flue gas desulphurisation (FGD) which are commonly employed in CFPP in Malaysia, whereas the latter consists of activated carbon injection (ACI), fabric filter (FF) and FGD. It was found that the emissions under the proposed APC strategy of ACI + FF+ FGD have higher margin of limits compared to the existing APC strategy of ESP + FGD. The emissions values were then used as input in AERMOD to predict the ambient concentrations of pollutants. The findings show that the ambient concentrations of Hg and dioxins/furans from both strategies are well below the ambient guideline values, with those emitted from the proposed APC strategy are so low to the point that they are negligible. 1. Introduction The inorganic and organic content of coal results in emission of various air pollutants during coal combustion. In 1980’s and 1990’s, the main focus of worldwide emission standard for coal-fired power plant (CFPP) was to control particulate matter (PM), nitrogen dioxide (NO2) and sulphur dioxide (SO2). In Malaysia, such emission standard is known as Environmental Quality (Clean Air) Regulations 1978 under Environmental Quality Act, 1974 (EQA, 1974). Early CFPPs in Malaysia are equipped with electrostatic precipitator (ESP) and flue gas desulphurisation (FGD) using seawater which are deemed sufficient to comply with the CAR 1978. Evolving studies on air pollutants from CFPP have shown the need to control other pollutants such as acid gases (hydrogen chloride (HCl) and hydrogen fluoride (HF)), heavy metals and dioxins/furans. Therefore, after 36 y, Malaysia has gazetted a new regulation of the Environmental Quality (Clean Air) Regulations 2014, which specifies additional pollutants (HCl, HF, mercury (Hg), carbon monoxide (CO), dioxins/furans) and establishes more stringent emission limits for CFPP. The new and stringent emission standard has prompted the shift in air pollution control (APC) system for CFPP in Malaysia. 1.1 Air pollution control (APC) system for compliance with the CAR 2014 The CAR 2014 specifies emission limit for PM, NO2, SO2, HCl, HF, Hg, CO and dioxins/furans. These pollutants can be controlled as follows: a) Front-end changes This includes controlling coal quality and increasing the efficiency of coal combustion. The front end changes would mainly control the emission of CO and NO2. b) End-of-pipe control The end-of-pipe controls include the available air pollution control technologies such as selective catalytic reduction (SCR), activated carbon injection (ACI), sodium bicarbonate/lime injection fabric 1562 filter (FF), ESP, wet and dry FGD. The end-of-pipe controls would influence the emission of particulate, acid gases (SO2, HCl, HF) and hazardous air pollutants (HAPs) (Hg and dioxins/furans). Generally, the control of CO and NO2 should not be a major concern because supposedly, power plant is commonly operating at high efficiency. In order to achieve high efficiency, complete combustion of coal must take place in boiler and this would avoid formation of CO and NO2. Most of the CFPPs in Malaysia are equipped with ESP and FGD which can control the emission of particulates and acid gases. Nevertheless, for compliance with the CAR 2014, there is a need to control the emission of Hg and dioxins/furans as well. Various technologies are available to control Hg and dioxins/furans separately. Mercury (Hg) can be controlled through the coal quality itself i.e. coal bleaching to reduce Hg content in coal. Injection of ACI into flue gas is the most reliable technology to remove Hg. Nevertheless, particulate control is necessary to re-capture the carbon that has been used once it adsorbs Hg from the flue gas. Derenne et al. (2009) reported 90 % of Hg removal from a combination of activated carbon and fabric filter. In addition, NOx controls such as SCR have a co-benefit for Hg reduction through oxidation of Hg. As for dioxins/furans, a report by Nescaum (2011) shows that ACI could reduce PCDD/Fs emission in a coal–fired power plant while technologies such as SCR, particulate controls and dry sorbent injection have a co–benefit in reducing dioxins/furans emissions. A study by Chi et al. (2005) demonstrated that ACI and bag filter could effectively remove vapour phase and particle phase dioxins/furans up to 98 %. It is of interest to employ a system that can effectively remove both Hg and dioxins/furans simultaneously. Derenne et al. (2009) suggested that a combination of ACI and FF could achieve up to 90 % removal of Hg while Chi et al. (2005) reported that dioxins/furans could be removed up to 95 %. For overall compliance with the CAR 2014 for pollutants of PM, NO2, SO2, HCl, HF, Hg, CO and dioxins/furans, the APC strategies must be able to treat PM, HAPs and acid gases. As such, ACI is proposed for HAPs, FF for PM removal, and seawater FGD for treatment of acid gases. This paper aims to compare the ambient concentrations of Hg and dioxins/furans from the existing APC strategy of the studied CFPP of ESP and seawater FGD and from the proposed APC strategy of ACI, FF and seawater FGD. The objectives of this paper are 1) To establish the CFPP emission data of Hg and dioxins/furans from the existing and proposed APC strategies; and 2) To obtain ambient concentrations of Hg and dioxins/furans from the two APC strategies. 2. Methodology 2.1 Descriptions of the studied coal-fired power plant (CFPP) The studied CFPP is a 3 x 700 MW power plant that employs pulverised coal technology. The three units burn a total of 20,000 t/d coal. The plant practices coal blending before firing. The studied plant receives three types of coal qualities e.g. poor (0.8 weight % sulphur content), medium and good grade (about 0.1 weight % sulphur). The coals are stockpiled in the coal yard according to the different grades. Prior to feeding into furnace, stacker reclaimer will grab and mix the coals before dumping the mixture into conveyor to the feeder of the furnace. The plant is equipped with air pollution control system of electrostatic precipitator (ESP) and flue gas desulphurisation (FGD) using seawater to treat particulate and acid gases as shown in Figure 1. The treated flue gas is emitted to the atmosphere through three chimneys of 200 m tall. ESP Coal Air Bottom ash Fly ash Furnace boiler Chimney FGD 1 2 4 6 7 3 5 Figure 1: Schematic diagram of one generating unit (1 x 700 MW) of the studied coal-fired power plant 1563 2.2 Establishment of emission data The existing and the proposed APC strategies for CFPP are as follows: a) Existing : ESP + FGD b) Proposed : ACI + FF + FGD Emission data were established for the emission of Hg and dioxins/furans from the existing and the proposed APC strategies. The typical expression to relate the emission level and control systems is represented by the following equation (US EPA, 1997): E = AR • EF • (1-ER) (1) Where; E = emission concentration (mg/Nm 3 ) AR = activity rate [coal feeding rate (kg/h) / volume of flue gas (Nm 3 /h)] EF = uncontrolled emission factor (mass rate) ER = overall emission reduction factor The uncontrolled EF refers to the EF developed from emission data without any control of Hg and dioxins/furans. The uncontrolled EF for dioxins/furans from the studied CFPP had been developed by Mokhtar et al. (2014a) whereas for Hg, the EF was developed based on the data published by Mokhtar et al. (2014b). The emission reduction factor was conservatively estimated at 90 % based on published data for Hg (Derenne et al., 2009) and dioxins/furans (Chi et al., 2005). The values for parameters in Eq(1) to calculate emission data for the existing and the proposed APC strategies are shown in Table 1. Table 1:Values for emission calculation Parameter Pollutant Hg Dioxins/furans Coal feeding rate (kg/h) 2.8 x 10 5 Volume of flue gas (Nm 3 /h) 2.4 x 10 6 EF 0.086 mg/kg 0.1 ng I-TEQ/kg ER (existing) 0 0 ER (proposed) 0.9 0.9 2.3 Air dispersion modelling Air dispersion model was used to predict ambient concentration of pollutants. Models such as Safe-Air II and ADMS 5 were used in previous study by Vairo et al. (2014) to predict ambient concentration of pollutant from power plant. In this study, AERMOD model was used for the purpose. The AERMOD modelling system was run with a commercial interface, AERMOD View (Lakes Environmental Software, 1995). The steps involved in AERMOD modelling are shown in Figure 2. Source data Meteorological data Geographic data AERMAPAERMET AERMOD dispersion model Ambient concentration of pollutant Input data Preprocessing Dispersion modeling Model output Figure 2: Flow in AERMOD modelling system 1564 According to the Guideline on Air Quality Models by the EPA (2005), five years of representative meteorology data should be used when estimating pollutant concentrations using an air quality model. Consecutive years from the most recent, readily available 5-years period are preferred. The 5-years (1st January 2008 to 31st December 2012) meteorological data used in this study were generated by Mesoscale Meteorological Model (MM5) and purchased from Lakes Environmental in Samson and TD- 6201 format files. The data were then pre-processed using AERMET (Lakes Environmental Software, 1995). AERMET organised the meteorological data into a format which is compatible with the AERMOD dispersion model. The topographical effects of the site were addressed by employing the elevated terrain option in the software whereby contours lines with resolution of ∼90 m are obtained from the Shuttle Radar Topography Mission (SRTM3) database maintained by the U.S. National Geospatial-Intelligence Agency (NGA) and the U.S. National Aeronautics and Space Administration (NASA). The terrain data were pre-processed with AERMAP (Lakes Environmental Software, 1995) prior to modelling in AERMOD. 3. Results and Discussion 3.1 Emission data of the existing and the proposed APC strategies The emission concentrations of Hg and dioxins/furans obtained from Eq(1) for the two APC strategies are shown in Table 2. The emissions are well below the limits specified in the CAR 2014. Nevertheless, the emissions under the proposed APC strategy have higher margin of limit compared to the existing APC strategy. Table 2: Emission concentrations of Hg and dioxins/furans from the existing and the proposed air pollution control (APC) strategies Pollutant Existing APC strategy Proposed APC strategy Limits as per CAR 2014 Mercury (Hg) (mg/Nm 3 ) 0.01 0.001 0.03 Dioxins/furans (ng I- TEQ/Nm 3 ) 0.01 0.001 0.1 CAR 2014 – Environmental Quality (Clean Air) Regulations 2014 3.2 Ambient concentrations of pollutants from the existing and the proposed APC strategies The predicted maximum ambient concentrations of Hg and dioxins/furans obtained from AERMOD modelling for the two APC strategies are shown in Table 3 and 4. The ambient concentrations were obtained for 1 h, 24 h and annual average. It was found that the ambient concentrations under the proposed APC strategy are much lower than the existing APC configuration to the extent it could be considered negligible. Since the standards for Hg and dioxins/furans are not specified in Malaysia Ambient Air Quality Guidelines (MAAQG), ambient air guidelines from other countries are used as comparison. Table 3 and 4 show that the predicted ambient concentrations are well below the guidelines values. Table 3: Predicted maximum ambient concentration of Hg and dioxins/furans compared with ambient air quality limit for the existing APC strategy Pollutant One (1) hour average concentration One (1) hour ambient air guideline Twenty-four (24) hour average concentration Twenty- four (24) hour ambient air guideline Annual average concentration Annual ambient air guideline Mercury (Hg) (µg/m 3 ) 0.008 1.5 a 0.001 2 b 0.00032 0.33 c Dioxins/ furans (pg/ TEQ/m 3 ) 0.00915 N.A 0.00134 0.1 b 0.00034 N.A a Arizona Ambient Air Quality Guidelines b Ontario’s Ambient Air Quality Criteria (AAQC) c New Zealand Ambient Air Quality Guidelines (guideline for inorganic mercury) N.A – not available 1565 Table 4: Predicted maximum ambient concentration of Hg and dioxins/furans compared with ambient air quality limit for the proposed APC strategy Pollutants One (1) hour average concentration One (1) hour ambient air guideline Twenty-four (24) hour average concentration Twenty- four (24) hour ambient air guideline Annual average concentration Annual ambient air guideline Mercury (Hg) (µg/m 3 ) 0.0038 x 10 -6 1.5 a 0.00055 x 10 -6 2 b 0.00014 x 10 -6 0.33 c Dioxins/ furans (pg TEQ/m 3 ) 0.00375 x 10 -9 N.A 0.00054 x 10 -9 0.1 b 0.00014 x 10 -9 N.A a Arizona Ambient Air Quality Guidelines b Ontario’s Ambient Air Quality Criteria (AAQC) c New Zealand Ambient Air Quality Guidelines (guideline for inorganic mercury) N.A – not available The results show that pollutants from CFPP must be treated for compliance with emission limits of the CAR 2014. In order to achieve this, CFPP must be incorporated with reliable APC technologies and strategies. Even though the existing APC strategy already results in concentration of pollutants that comply with the stipulated stack emission limits and ambient air guideline values, investment in better APC strategy could compensate in the event of increasing coal consumption and changes in coal quality that will influence pollutant emission level. It should be noted that residual pollutants may exist after treatment with APC. Thus, by ensuring that the pollutants comply with stack emission limits, it could be guaranteed that the ambient concentrations will be at safe level. The residual pollutants could be managed by sufficient stack height to disperse the pollutants into the environment. In addition, conducive meteorological factors could further dilute the concentration of residual pollutants before they reach ground level. 3.3 Economic aspects of APC strategies Qualitative economic evaluation of both APC strategies indicates that the proposed APC strategy costs higher than the existing APC strategy. The major cost is contributed from the installation and operation of FF. Hanseni and Rensburg (2006) reported that the operational cost of FF in coal-fired power plant was higher than ESP. Nevertheless, the net benefits of the proposed APC strategy should be the primary target regardless of the cost. (Zhang et al. (2015)) concluded that even though the average control costs for multi-pollutant control strategy are higher than gradual control strategy, the average health benefits are higher for the former than the latter. 4. Conclusion The Environmental Quality (Clean Air) Regulations 2014 impose limits to additional parameters including hazardous air pollutants (HAPs) of Hg and dioxins/furans for coal-fired power plants. To the author knowledge, there is no coal-fired power plants in Malaysia equipped with air pollution control (APC) for treatment of the HAPs. The comparison of ambient concentrations from the existing and proposed APC strategies shows that the ambient concentrations of Hg and dioxins/furans from the latter are negligible. Therefore, the proposed APC strategy will give better assurance for safe ambient concentrations and protection to human health. References Chi, K.H., Chang, M.B., Chang-Chien, G.P. and Lin, C., 2005, Characteristics of PCDD/F congener distributions in gas/particulate phases and emissions from two municipal solid waste incinerators in Taiwan, Science of The Total Environment, 347, 148-162. Derenne, S., Sartorelli, P., Bustard, J., Stewart, R., Sjostrom, S., Johnson, P., McMillian, M., Sudhoff, F. and Chang, R., 2009, TOXECON clean coal demonstration for mercury and multi-pollutant control at the Presque Isle Power Plant, Fuel Processing Technology, 90, 1400-1405. 1566 EPA, 2005, Revision to the Guideline on Air Quality Models: Adoption of a Preferred General Purpose (Flat and Complex Terrain) Dispersion Model and Other Revisions; Final Rule, 40 Federal Register, Environmental Protection Agency. EQA. 1974. Environmental Quality Act (Act 127), Regulations, Rules & Orders. International Law Book Services, Malaysia. Hanseni, R. and Rensburg, R.v. 2006. Cost comparisons between electrostatic precipitators and pulse jet fabric filters and inherent challenges of both technologies at Eskom’s 6 x 600 MW units at Duvha power station. ICESP X. Australia. p. 1-7. Lakes Environmental Software. 1995. AERMOD View: Gaussian Plume Air Dispersion Model. Version 8.2. Lakes Environmental, Ontario, Waterloo, Canada. Mokhtar, M.M., Taib, R.M. and Hassim, M.H., 2014a, Measurement of PCDD/Fs emissions from a coal- fired power plant in Malaysia and establishment of emission factors, Atmospheric Pollution Research, 5, 388-397. Mokhtar, M.M., Taib, R.M. and Hassim, M.H., 2014b, Understanding selected trace elements behavior in a coal-fired power plant in Malaysia for assessment of abatement technologies, Journal of the Air & Waste Management Association, 64, 867-878. Nescaum, 2011, Control Technologies to Reduce Conventional and Hazardous Air Pollutants from Coal- Fired Power Plants, Boston. US EPA, 1997, Procedures for preparing emission factors documents, US Environmental Protection Agency, Research Triangle Park, NC 27711. Vairo, T., Currò, F., Scarselli, S. and Fabiano, B., 2014, Atmospheric Emissions from a Fossil Fuel Power Station: Dispersion Modelling and Experimental Comparison, Chemical Engineering Transactions, 36, 295 - 300. Zhang, H., Zhang, B. and Bi, J., 2015, More efforts, more benefits: Air pollutant control of coal-fired power plants in China, Energy, 80, 1-9.