Reductions in coal-fired power plant volatile organic compound

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Reductions in coal-fired power plant volatile organic compound emissions by combining APCDs and modified fly ash Jie Cheng, Jun Liu, Tao Wang, Zhifeng Sui, Yongsheng Zhang, and Wei-Ping Pan Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b04277 • Publication Date (Web): 21 Feb 2019 Downloaded from http://pubs.acs.org on February 26, 2019

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Reductions in coal-fired power plant volatile organic compound emissions by

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combining APCDs and modified fly ash

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Jie Cheng1, Jun Liu1, Tao Wang1, Zifeng Sui1, Yongsheng Zhang1*, Wei-Ping Pan1, 2

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North China Electric Power University, Beijing 102206, China

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Abstract: The distributions and reductions of volatile organic compounds (VOCs;

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alkanes, aromatics and halogenated hydrocarbons) emitted from two coal-fired power

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plants were compared. The partial removal of VOCs by air pollution control devices

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meant to reduce NOx, particulate matter and SOx was assessed. The data show that VOC

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levels in flue gas were reduced by 5% to 35% after selective catalytic reduction, 0% to

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40% after air preheating and electrostatic precipitation, and 10% to 20% after flue gas

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desulfurization and wet electrostatic precipitation. Adding modified fly ash was found

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to further reduce the effluent concentration of VOCs by 10% to 20%, resulting in an

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overall VOC reduction of 40% to 80%. This decrease in emissions is attributed to

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oxidation, deposition, condensation and water absorption mechanisms.

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Keywords: VOC emission, APCD, modified fly ash, synergistic effect

1

2

*

Key Laboratory of Condition Monitoring and Control for Power Plant Equipment, Ministry of Education, ICSET Solutions, Bowling Green, KY 42104, USA

E-mail: [email protected]

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1. Introduction

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The total amount of volatile organic compounds (VOCs) emitted annually from coal-

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fired power plants in China has been estimated at approximately 500 tons.1, 2 However,

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the concentrations at which these VOCs appear in power plant emissions are lower than

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those in emissions generated by other sources, including vehicle exhausts, the

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evaporation of fossil fuels and petrochemical production.1 For this reason, the release

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of VOCs from coal-fired power plants has received relatively little attention3, 4. This is

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unfortunate, because power plant flue gas emissions are toxic and are released in

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significant quantities. In addition, the Chinese government is reported to be considering

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new regulatory standards to further reduce VOC emissions.5 There is around 10-40%

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of volatile matter in coal. Thus, the combustion of coal has the potential to release

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VOCs6-8. The reported average VOCs concentration in power plant emissions ranges

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from 0 to 5 mg·m-3

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conditions, including boiler temperature, oxygen concentration and the presence of air

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pollution control devices (APCDs).

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Because of the use of APCDs, less than 1% of the VOCs produced during the

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combustion process are actually emitted into the environment. These include alkanes,

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aromatics and halogenated hydrocarbons. Some VOCs are also transformed into

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polycyclic aromatic hydrocarbons (PAHs) by condensation and cyclization reactions,

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as well as decomposition processes during coal combustion.12 These PAHs, as well as

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some portion of the VOCs, tend to condense on the surfaces of fly ash generated during

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coal combustion, while the remainder of the VOCs is released into the air.

9-11.

The range variation is related to differences in combustion


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There have been several studies focused on developing techniques for the adsorption

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for VOCs emitted by chemical industries.13, 14 Only a few have addressed the VOCs

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produced by power plants.9-11 Garcia reported that the most common compounds

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generated by power plants are aldehydes, aliphatic and aromatic hydrocarbons and

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chlorinated hydrocarbons.9 Fernández-Martı́nez discussed the range of VOCs emitted

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during combustion processes in coal-fired power stations10 and Shi studied the VOC

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emission profiles from a power plant in the Liaoning Province of China.11 The majority

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of research studying power plant VOC emissions has used thermal desorption (TD) in

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conjunction with gas chromatography/mass spectrometry (GC/MS) to determine the

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total concentration of various VOCs.15 The most challenging aspects of the sampling

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and analysis of VOCs in power plant flue gases include the low concentrations of such

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compounds and the high moisture and dust levels in these gases. In the work reported

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herein, both sampling and analysis were performed using modified versions of US EPA

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methods 0030 (Volatile Organic Sampling Train) and 0031(Sampling Method for

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Volatile Organic Compounds). In these procedures, sorbent tubes were employed to

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trap VOCs present at low concentrations, in conjunction with the use of an inertial probe

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to avoid interference by dust and a Nafion tube to remove moisture.

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In most power plants, several APCDs, including selective catalytic reduction (SCR),

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air preheating (APH), electrostatic precipitation (ESP), flue gas desulfurization (FGD)

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and wet electrostatic precipitation (WESP), are used to clean the flue gases. Several of

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these methods are typically employed simultaneously to remove pollutants such as NOx,

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particle matter (PM) and SOx.16-19 Sui researched the capture of PM by APCDs in coal3 ACS Paragon Plus Environment

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fired power plants20 and found that agglomeration of PM allowed this material to be

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readily captured via ESP. Water absorption in association with FDG can also reduce

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the PM levels in flue gases, indicating that ultra-low emissions can be achieved by

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taking advantage of the synergistic effects obtained from combinations of different

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methods. Mercury capture from flue gases was once a significant challenge, and studies

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have shown that activated carbon (AC) allows efficient removal of this pollutant.21

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Modified fly ash has also been found to remove mercury in conjunction with a flue gas

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cleaning unit.22 Research at the North China Electric Power University has developed

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a modified fly ash injection system. The comparison of different flue gas treatment

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technologies were summarized in Table 1. In this process, a small portion of fly ash

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(less than 1% of the total fly ash in the flue gas duct) is withdrawn from the ESP ash

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bunker, modified through both mechanical and chemical treatments, and injected back

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into the SCR outlet. This modified fly ash subsequently passes through the APH and

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ESP units, where it absorbs mercury.

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As noted, increasingly stringent environment regulations are driving the removal of

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VOCs from power plant emissions. For this reason, over the last 20 years, power plant

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boilers have been equipped with three to five different flue gas cleaning units so as to

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remove various pollutants. Thus, the implementation of the modified fly ash injection

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technique requires few changes in operating conditions and provides improved mercury

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capture. However, if this technique can also reduce VOCs in the flue gas, this would

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represent a cost effective process. The present work therefore examined the removal of

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VOCs using APCDs in conjunction with modified fly ash. 4 ACS Paragon Plus Environment

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This project is based on previous work on the laboratory scale using a fixed bed reactor

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that investigated the emission of VOCs from coal pyrolysis23 and the effect of heating

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rate on VOC production.5 The work reported herein assessed the extent to which an

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APCD system incorporating the modified fly ash technique to remove mercury also

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reduces VOCs.

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2. Experimental

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2.1 Sample characteristics

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Two raw bituminous coal samples (obtained from operational conveyor belts in power

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plant according to the method provided in ASTM D-2234) were used in this study,

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designated A and B. The major oxides and minor elements in the fly ash (sampled from

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the ESP ash bunker according to ASTM D-2234) employed in this work were

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determined by inductively coupled plasma atomic emission spectroscopy (ICP−AES,

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Prodigy, Leeman Laboratories, USA) and X-ray ﬂuorescence (XRF, RIX 3001, Rigaku,

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USA).

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The modified fly ash was prepared by drying the original material at 100 °C for 1 h,

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followed by ball milling for 1 h. Particle charge analyses at the power plants were

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performed using an electronic low pressure impactor (ELPI+, Dekati, Finland). The

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data obtained from ultimate and proximate analyses of these coals and the fly ash

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compositional analysis data are provided in Table 2. These data represent the averages

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of three replicate analyses.

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2.2 VOC analysis

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In Figure 1, a fixed bed was used to study the VOC adsorption properties of the original 5 ACS Paragon Plus Environment

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and modified fly ash as well as those of molecular sieves (MS, Hisiv 1000, UOP) and

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activated carbon (AC, Chemical reagent, China). This was accomplished by monitoring

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the effluent gases by in-line Fourier transform infrared (FTIR, Frontier, PerkinElmer,

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USA) spectroscopy in conjunction with the TimeBase software package, using toluene

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as a model VOC. In these trials, the concentration of toluene was determined by

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tracking the intensity of the peak at 728 cm-1 relative to the intensity at 762 cm-1 as a

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baseline. A gas flow containing 1000 ppm toluene was diluted and then sent into the

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test bed apparatus at a rate of 200 ml·min-1. This apparatus contained a tube serving as

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the fixed bed that held a 1 g fly ash sample, situated inside a heating jacket. In some

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trials, the tube was heated to 100 °C to simulate the conditions in an exhaust stack,

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although the effect of temperature was also examined in this study. FTIR spectroscopy

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was used to quantify the concentration of toluene in the effluent by diverting a 70

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ml·min-1 flow from the tube exit to the spectrometer. The remaining gas flow was

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released through an activated carbon trap.

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During this work, the VOC emissions from two power plants incorporating ULE units

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were also examined during coal combustion, to determine the effect of the modified fly

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ash. VOC emissions were sampled at the SCR inlet and outlet, ESP inlet and outlet,

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FGD outlet, and WESP outlet for plant A. For Plant B, Samples were collected at the

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SCR inlet and outlet, ESP inlet and outlet, and FGD outlet. VOCs were collected from

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the flue gases via sorbent tubes (filled with Tenax) for 30 min, using a modified version

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of the sampling train in US EPA methods 0030 and 0031. A device to remove fly ash

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was installed in front of the sorbent tube, together with a Nafion tube (Perma Pure LLC, 6 ACS Paragon Plus Environment

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USA) to remove moisture. The contents of each tube were then analyzed using TD-

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GC/MS (Clarus SQ 8T, PerkinElmer, USA), applying a desorption temperature of

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300 °C. The GC incorporated a fused silica dimethyl polysiloxane capillary column

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(Extile-1, 60 m × 0.25 mm × 0.25 µm film thickness) and the column temperature was

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held at 50 °C for 5 min, increased at 10 °C·min−1 to 280 °C, then held at this temperature

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for 5 min. The carrier gas was helium at a flow rate of 1.0 mL·min−1. Mass spectra were

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recorded over the m·z−1 range of 35 to 500 and analytes were identified by both

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retention time and spectral interpretation. The instrument was calibrated with standard

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compounds (Sigma-Aldrich) using total ion current values and each analyte was

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quantified on this basis. VOCs solutions (Sigma-Aldrich EPA VOC Mix, analytical

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standard) were injected to the adsorbent tube with helium carrier gas (99.999%) and

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analyzed by TD-GC/MS to establish the calibration curves for different VOCs. Each

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analysis was repeated at least three times and the average values are reported. The

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standard deviation of the retention times from repeated trials were within ±0.1 min. The

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peak area under GC spectrum was calculated to determine the concentration of VOCs

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using calibration curve which established by standard VOCs solutions. The relative

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standard deviations of the concentration of VOCs were less than 5% in this study.

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2.3 Power plants with APCD units

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The full-scale experiments were conducted in a 300 MW unit at power plant A and a

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1000 MW unit at power plant B. The test platform in plant A employed a slipstream

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pilot to divert flue gases at a rate of 50,000 m3·h−1 (approximately 2% of the total flue

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gas in the duct) from the boiler outlet. Both plants were equipped with SCR (for NOx 7 ACS Paragon Plus Environment

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control), APH, ESP and FGD units (for SO2 control). Plant A also incorporated a WESP

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device to reduce PM emissions.

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The sampling and analysis techniques were identical to those described in Section 2.2.

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The modified fly ash was injected between the APH and ESP units in plant A and

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between the SCR and APH units in plant B. In the modified fly ash injection study, the

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VOC samples were only collected after the injection points such as outlet of ESP, FGD

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and WESP. The temperature profiles determined for each location are shown in Figure

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2. The major differences between these two plants were the temperatures at the SCR

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and ESP inlets; plant A had a higher temperature at the SCR inlet and a lower

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temperature at the ESP inlet.

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3. Results and discussion

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3.1 Adsorption trials with a lab-scale fixed bed

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Figure 3A is shown the capability of different adsorbents as a function of time. Table 3

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is the comparison of adsorption capability for different adsorbents at the 10th second

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during experiments. In the case of the modified fly ash sample 1 (F1-M), approximately

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35% of the original toluene was captured and the 65% was breakthrough. This value

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was lower than the value of 85% obtained from the original fly ash sample 1 (F1). The

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relative concentration of toluene (RCT) of breakthrough for modified fly ash sample 2

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(F2-M) was approximately 70%, and this value again was lower than the 80%

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remaining in the fly ash 2 trial (F2). Thus, both modified fly ash specimens exhibited

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better adsorption of toluene than the unmodified material. The MS showed the best 8 ACS Paragon Plus Environment

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absorption capacity, removing 95% of the toluene, while the AC, which is widely used

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as an absorbent, captured 70%. Thus, both these materials outperformed the modified

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fly ash, while the modified fly ash showed 10% to 20% improvements in performance

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relative to the unmodified fly ash. The mechanisms of the modified and raw fly ash

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samples adsorbing VOCs should be different. The usage of the raw fly ash as a sorbent

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to remove VOCs has been proved to be challenged

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chemical and mechanical treatment which it is on-line to make fly ash through the ball

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mill to produce higher surface energy 24, then injected the modified fly ash into the flue

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duck less than 10 minutes after it made. The modified fly ash with high surface energy

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and more active sites to capture more mercury and VOCs. Although the adsorption

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capacity of the fly ash was not as high as those of the MS and AC, even after

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modification, the cost of fly ash is only one tenth those of the MS and AC. The modified

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fly ash was able to maintain adsorption of the toluene for more than 20 s. In power plant

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applications, the adsorbent is only retained in the flue duct for several seconds less than

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10 seconds. In prior work, modified fly ash acting as an adsorbent has been shown to

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remove 90% of the mercury in flue gases.

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The effect of temperature on the performance of the modified fly ash is shown in Figure

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3B, which demonstrates that increasing the temperature decreased the adsorption values

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(that is, lowered toluene removal). At a test temperature of 300 °C, the fly ash exhibited

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no adsorption after 5 s. The EERC reported the mercury adsorption of fly ash was

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mainly by the unburned carbon in fly ash25. The modification would also increase the

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surface energy and active sites24, 26. In this study, the concentration of the VOCs (like

24.


The modified fly ash with

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toluene) in real flue gas emission was very low compared to the levels at which other

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pollutants are typically found. The new development of adsorbents to reduce the VOCs

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emission will increase the operating cost in power plant. Therefore, these data from this

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study suggested that an APCD system in conjunction with modified fly ash injection to

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capture mercury would be co-benefit to reduce some VOC emissions through the

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majority of physical capture and some chemical effect at no additional cost.

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3.2 Identification of VOCs from coal combustion

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The VOCs identified by GC/MS included alkanes, aromatic hydrocarbons and

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halogenated hydrocarbons, as summarized in Table 4. In the case of plant A, eleven

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different alkanes, five aromatics, one halogenated hydrocarbon and three other types of

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compounds were identified. The plant B compounds included seven alkanes, thirteen

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aromatics and four other substances. Table 4 provides semi-quantitative data regarding

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the VOCs emitted from both power plants, and shows that the compounds produced by

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these facilities were largely the same. The differences between the two plants can be

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attributed to variations in the coal combustion conditions and the process temperatures.

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The lack of benzene derivatives (such as 1,2,4,5-tetramethyl benzene) and the lower

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amount of naphthalene in the effluent from plant A is ascribed to the higher combustion

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temperature and superior combustion conditions in the boiler in this unit. Haiyan Wang

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also reported that the release of PAHs (polycyclic aromatic hydrocarbons) was reduced

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when the temperature increased above the 300oC27. These conditions (higher

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combustion temperature) tend to produce fewer aromatics and polycyclic aromatics in

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the inlet to the SCR system. 10 ACS Paragon Plus Environment

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3.3 VOC emissions from an ultra-low emissions power plant

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As a means of comparing the VOC emissions from different power plants, the total

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concentrations reported for a number of plants (as methane) are plotted in Figure 4.

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These data demonstrate that the average VOC concentrations in power plant emissions

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determined by other researchers are in the range of 0 to 5 mg·m-3.9-11 The boilers in the

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power plants that were assessed in the present study were equipped the ultra-low

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emission (ULE) APCDs. The flue gas sampled between the SCR and the pulverized

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coal furnace is considered to represent the initial flue gas emitted from the furnace, and

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this gas contained from 0.3 to 0.4 mg·m-3 VOCs as methane, which is on the lower end

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of the emissions values. The final VOC concentration after the ULE unit was in the

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range of 0.1 to 0.3 mg·m-3 (indicated by red dots in Figure 4). Thus, based on the

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average 1.8 billion tons per year of coal burned in coal-fired power plants in China, the

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VOC emissions in the associated flue gases are estimated to be on the order of hundreds

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of tons per year.

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The VOC concentration in the flue gas is also affected by the APCDs that are present.

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In Figure 5, the VOC concentration in the initial flue gas was set to 100%. In the case

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of power plant A, this value was decreased by 5% after the SCR, while plant B exhibited

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a much larger drop of 35%. This difference may have been due to the three-layer SCR

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catalyst bed in plant B, as opposed to the two-layer unit in plant A. In a typical SCR

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system, VOCs are oxidized by catalysts such as TiO2 and V2O5. When flue gas reached

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the sampling point prior to the ESP unit, the VOC concentration was only 30% in plant

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A, compared to 55% in plant B. In addition, the flue gas temperature at the ESP inlet 11 ACS Paragon Plus Environment

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was only 105 °C (representing a significant decrease from 250 °C at the APH inlet),

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which was much lower than the value of 130 °C in plant B (from 220 °C at the APH

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inlet). This cooling effect plays an important role in reducing the VOC concentration

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because it promotes the deposition on the baskets and the condensation on the fly ash

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surfaces. Thus, variations in the temperatures between the two plants resulted in

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differences in the extent to which VOCs condensed on the fly ash.

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After the ESP unit, the VOC concentration increased by approximately 25% ~ 30%

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because of the electrostatic effect of the ESP unit. Sui has previously studied the

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electrostatic effect on fine PM (less than 0.03m) and reported that this effect separates

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finer particles from the surfaces of larger ones20. The concentration of fine PM would

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be increased after the flue gas passes through the ESP unit. Normally, the charged

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particle matter were neutralized when flue gas flow through ESP. However, the

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desorption of VOCs and increasing the number of fine PM indicated there were a large

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number of charges on the surface of particulate matters which have an electrostatic

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repulsion effect on VOCs adsorption and fine PM agglomeration. To examine this

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phenomenon, the charge on the surfaces of differently sized particles at the ESP outlet

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were evaluated by ELPI+. Larger particles were found to be more highly charged and

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therefore to provide greater electrostatic repulsion shown in Figure 6. This electrostatic

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repulsion is a negative effect on the adsorption of VOCs.

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Some VOCs adsorbed by the fly ash would be expected to be released as the finer

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particles depart from the surfaces of larger particles, and would then desorb VOCs. This

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explains the increase in VOCs at the ESP outlet. Some VOCs may also dissolve in the 12 ACS Paragon Plus Environment

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limestone slurry employed in the FGD system, while the water mist generated in the

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WESP unit would capture smaller PM to further reduce the VOC levels. Overall, the

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VOC concentration was decreased by 60% in plant A compared to 40% in plant B. This

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difference is attributed to the improved performance of the APCD system in plant A,

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along with the lower flue gas temperature downstream of the ESP inlet and the

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additional WESP unit.

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This study suggests that there are three different mechanisms by which VOCs are

8

removed in a ULE power plant: oxidation, deposition, condensation and water

9

absorption. During SCR, the temperature of the flue gas remains above 300 °C. The

10

SCR contains a bed filled with a catalytic metal and the oxygen content in the flue gas

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in this zone is approximately 3%. Fly ash in flue gas also contains a lot of oxides shown

12

in Table 2. Thus, the oxidation effect is the main factor by which VOCs are oxidized

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during SCR3, 28. Boycheva studied the application of coal fly ash as a heterogeneous

14

catalysts for VOCs oxidation28, while Schwämmle reported that SCR units remove

15

mercury via oxidation in association with a transition between the oxides SO2 and SO329.

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Applying this oxidation effect to VOCs would involve the chemical reaction

17

CmHn + O2

18

The boiling points of most VOCs are higher than that of toluene (110 °C, see Table 4)

19

and, at the APH outlet, the flue gas temperature would be below 130 °C. Belaissaoui

20

studied the condensation of VOCs and found that fly ash in the flue gas primarily

21

removes VOCs after the APH unit by condensation30. The condensed VOCs adsorbed

22

on fly ash are subsequently removed by ESP. During the FGD and ESP processes, the

SCR Catalyst or fly ash, ∆

CO2 + H2O (1)


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flue gas passes through various solutions. Considering that the VOCs partly dissolve in

2

water, the water wash would be the main factor in these two units. Thus, the oxidation

3

is the main function of SCR for VOCs reduction. The condensation due to the

4

temperature drop and the deposition on the baskets and scrubbing by the hot air both

5

are the possible mechanisms to reduce the VOCs when the flue gas through APH. The

6

condensation is the main function of ESP for VOCs reduction. The ESP removes the

7

VOCs adsorbed on the surface of the particle matters. The FGD and WESP contribution

8

for VOCs reduction is mainly through the water wash. Setting the VOC concentration

9

of the initial flue gas to 100% and making a summary for the reduction results from this

10

study, oxidation appears to oxidize approximately 5-35%, deposition and condensation

11

is around 0%-40%, water absorption is around 10-20%. The remaining 30-60% is

12

emitted from the stack, as shown in Figure 7. Therefore, oxidation, deposition and

13

condensation play more important roles in the reduction of VOCs than that of

14

absorption. However, the operating temperature also controls the level of reduction in

15

each APCD unit. The best synergistic effect is obtained from an APCD when applying

16

a temperature of 350 °C in the SCR system to ensure that the catalytic effect occurs. At

17

the same time, the temperature at the ESP inlet needs to be as low as possible to achieve

18

the maximum extent of deposition and condensation. However, this temperature must

19

also allow efficient capture of the fly ash.

20

3.4 Power plant adsorption trials

21

The points in red in Figure 5 indicate the VOC emissions obtained using modified fly

22

ash. In the case of plant A, the total VOCS reduction was approximately 20%. The 14 ACS Paragon Plus Environment

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Energy & Fuels

1

reduction after the ESP unit was 12.7% while the reductions after the FGD and WESP

2

processes were 14.6% and 20.7%, respectively. In plant B, the total reduction was

3

approximately 10%, with a 4.4% reduction before ESP and 17.4% reduction after.

4

Approximately 10.7% of the VOCs was removed by the FGD process.

5

The point in plant B at which the modified fly ash was injected was between the SCR

6

and APH units, which was expected to allow more time for reaction and/or adsorption,

7

although the resulting VOCs removal was lower than that of plant A. The modified fly

8

ash may allow for both physical and chemical adsorption at different locations as it

9

passes through the flue duct, and these processes are also affected by temperature. The

10

temperature at the injection point in plant B was approximately 300 °C and thus much

11

higher than that in plant A (105 °C). The ESP temperature in plant B of 130 °C was

12

also higher than the value of 105 °C in plant A. The modification of the fly ash in this

13

work was primarily a physical modification that served to increase the surface area and

14

concentration of active sites. At higher temperature zones in the flue gas, high

15

concentrations of salts and other compounds would be expected to compete with VOCs

16

for condensation on the fly ash.

17

The concentrations of the most common aromatic compounds were compared before

18

and after the modified fly ash injection, with the results shown in Table 5. These

19

adsorption percentages were calculated using the change in the VOC concentrations in

20

the flue gas before and after injection of the modified fly ash, divided by the

21

concentration before injection. The adsorption values for most of these compounds

22

were in the range of 10 to 30%, although more than 50% of chlorobenzene and 15 ACS Paragon Plus Environment

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

chlorotoluene were removed. It is probable that the more polar molecular structures of

2

these compounds allowed easier adsorption on the active sites of the modified fly ash.

3

Zhang Xueyang summarized that the VOCs with heteroatom had higher opportunities

4

to be adsorbed than that without heteroatom compounds31. The oxides in the present fly

5

ash samples are provided in Table 2. Aguayo-Villarreal reported that iron oxides on

6

carbon surfaces play the most important role in the adsorption of VOCs32. The oxides

7

in fly ash would provide high surface energy and active sites. As noted, the modification

8

process exposed the unburned carbon and increased the number of active sites, both of

9

which would be expected to promote the adsorption of VOCs.

10 11

4. Conclusions

12

The emissions of VOCs determined during coal combustion in this work were in the

13

range of 0.3 to 0.4 mg·m-3, which is in the lower end of the range of previously reported

14

data. Oxidation during the SCR process, deposition and condensation during the APH

15

and ESP steps, and water absorption in the FGD and WESP units were found to be the

16

primary causes of reduced VOC emissions from power plants. The optimum synergistic

17

effect can be obtained from an APCD system by maintaining a temperature of 350 °C

18

in the SCR unit so as to ensure a strong catalytic effect. In addition, the operating

19

temperature at the ESP inlet should be as low as possible to achieve the maximum

20

extent of deposition and condensation, although still sufficient to allow fly ash removal.

21

Overall, the SCR, APH, ESP, FGD and WESP units in a power plant can remove 40 to

22

70% of the VOCs from the flue gas. The concentration of VOCs can be reduced by a 16 ACS Paragon Plus Environment

Page 16 of 33

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Energy & Fuels

1

further 10% to 20% via the addition of modified fly ash.

2 3

AUTHOR INFORMATION

4

Corresponding Author

5

*E-mail: [email protected].

6

ORCID

7

Yongsheng Zhang: 0000-0002-1104-5605

8

Notes

9

The authors declare no competing financial interest.

10

Acknowledgments

11

This work was supported by the National Key Research and Development Program of

12

China (No. 2018YFB0605200) and National Natural Science Foundation of China (No.

13

51706069).

14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

References 1.

Liu, Y.; Shao, M.; Fu, L.; Lu, S.; Zeng, L.; Tang, D., Source profiles of volatile organic compounds

(VOCs) measured in China: Part I. Atmospheric Environment 2008, 42, (25), 6247-6260. 2.

Chagger, H. K.; Jones, J. M.; Pourkashanian, M.; Williams, A.; Owen, A.; Fynes, G., Emission of

volatile organic compounds from coal combustion. Fuel 1999, 78, (13), 1527-1538. 3.

Kamal, M. S.; Razzak, S. A.; Hossain, M. M., Catalytic oxidation of volatile organic compounds (VOCs)

– A review. Atmospheric Environment 2016, 140, 117-134. 4.

Wang, T.; Sun, B.; Liu, H.; Zhang, X.; Wang, Y.; Guo, Y.; Zhang, Y., plasma-assisted catalytic system

for NO removal over CuCe/ZSM-5 catalysts at ambient temperature. Fuel Processing Technology 2017, 158, 199-205. 5.

Cheng, J.; Zhang, Y.; Wang, T.; Xu, H.; Norris, P.; Pan, W.-P., Emission of volatile organic compounds

(VOCs) during coal combustion at different heating rates. Fuel 2018, 225, 554-562. 6.

Fernández-Martínez, G.; López-Mahía, P.; Muniategui-Lorenzo, S.; Prada-Rodríguez, D.,

Determination of volatile organic compounds in coal, fly ash and slag samples by direct thermal desorption/GC/ms. Analusis 2000, 28, (10), 953-959. 7.

Solomon, P. R.; Serio, M. A.; Suuberg, E. M., Coal pyrolysis: Experiments, kinetic rates and

mechanisms. Progress in Energy and Combustion Science 1992, 18, (2), 133-220. 8.

Anthony, D. B.; Howard, J. B., Coal devolatilization and hydrogastification. AIChE Journal 1976, 22,


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(4), 625-656. 9.

Garcia, J. P.; Beyne-Masclet, S.; Mouvier, G.; Masclet, P., Emissions of volatile organic compounds

by coal-fired power stations. Atmospheric Environment. Part A. General Topics 1992, 26, (9), 1589-1597. 10. Fernández-Martı́nez, G.; López-Mahı́a, P.; Muniategui-Lorenzo, S.; Prada-Rodrı́guez, D.; Fernández-Fernández, E., Distribution of volatile organic compounds during the combustion process in coal-fired power stations. Atmospheric Environment 2001, 35, (33), 5823-5831. 11. Shi, J.; Deng, H.; Bai, Z.; Kong, S.; Wang, X.; Hao, J.; Han, X.; Ning, P., Emission and profile characteristic of volatile organic compounds emitted from coke production, iron smelt, heating station and power plant in Liaoning Province, China. Science of the Total Environment 2015, 515–516, 101-108. 12. Liu, K.; Han, W.; Pan, W.-P.; Riley, J. T., Polycyclic aromatic hydrocarbon (PAH) emissions from a coal-fired pilot FBC system. Journal of Hazardous Materials 2001, 84, (2), 175-188. 13. Zhong, Z.; Sha, Q. e.; Zheng, J.; Yuan, Z.; Gao, Z.; Ou, J.; Zheng, Z.; Li, C.; Huang, Z., Sector-based VOCs emission factors and source profiles for the surface coating industry in the Pearl River Delta region of China. Science of The Total Environment 2017, 583, 19-28. 14. Stojić, A.; Maletić, D.; Stanišić Stojić, S.; Mijić, Z.; Šoštarić, A., Forecasting of VOC emissions from traffic and industry using classification and regression multivariate methods. Science of The Total Environment 2015, 521-522, 19-26. 15. Fernández-Villarrenaga Martín, V.; López Mahía, P.; Muniategui Lorenzo, S.; Prada Rodríguez, D., Development of a thermal desorption-gas chromatography-mass spectrometry method for determination of styrene in air. Application to workplace air. Analusis 2000, 28, (8), 737-742. 16. Ma, Z.; Deng, J.; Li, Z.; Li, Q.; Zhao, P.; Wang, L.; Sun, Y.; Zheng, H.; Pan, L.; Zhao, S.; Jiang, J.; Wang, S.; Duan, L., Characteristics of NOx emission from Chinese coal-fired power plants equipped with new technologies. Atmospheric Environment 2016, 131, 164-170. 17. Li, Z.; Jiang, J.; Ma, Z.; Wang, S.; Duan, L., Effect of selective catalytic reduction (SCR) on fine particle emission from two coal-fired power plants in China. Atmospheric Environment 2015, 120, 227-233. 18. Li, Z.; Jiang, J.; Ma, Z.; Fajardo, O. A.; Deng, J.; Duan, L., Influence of flue gas desulfurization (FGD) installations on emission characteristics of PM2.5 from coal-fired power plants equipped with selective catalytic reduction (SCR). Environmental Pollution 2017, 230, 655-662. 19. Bin, H.; Lin, Z.; Yang, Y.; Fei, L.; Cai, L.; Linjun, Y., PM2.5 and SO3 collaborative removal in electrostatic precipitator. Powder Technology 2017, 318, 484-490. 20. Sui, Z.; Zhang, Y.; Peng, Y.; Norris, P.; Cao, Y.; Pan, W.-P., Fine particulate matter emission and size distribution characteristics in an ultra-low emission power plant. Fuel 2016, 185, 863-871. 21. Pudasainee, D.; Kim, J.-H.; Yoon, Y.-S.; Seo, Y.-C., Oxidation, reemission and mass distribution of mercury in bituminous coal-fired power plants with SCR, CS-ESP and wet FGD. Fuel 2012, 93, 312-318. 22. Zhou, Q.; Duan, Y.; Chen, M.; Liu, M.; Lu, P.; Zhao, S., Effect of flue gas component and ash composition on elemental mercury oxidation/adsorption by NH4Br modified fly ash. Chemical Engineering Journal 2018, 345, 578-585. 23. Cheng, J.; Zhang, Y.; Wang, T.; Norris, P.; Chen, W.-Y.; Pan, W.-P., Thermogravimetric–Fourier Transform Infrared Spectroscopy–Gas Chromatography/Mass Spectrometry Study of Volatile Organic Compounds from Coal Pyrolysis. Energy & Fuels 2017, 31, (7), 7042-7051. 24. Zhang, Y.; Zhang, Z.; Liu, Z.; Norris, P.; Pan, W.-p., Study on the mercury captured by mechanochemical and bromide surface modification of coal fly ash. Fuel 2017, 200, 427-434. 25. Hassett, D. J.; Eylands, K. E., Mercury capture on coal combustion fly ash. Fuel 1999, 78, (2), 243248.


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1 2 3 4 5 6

26. Zhang, Y.; Zhao, L.; Guo, R.; Song, N.; Wang, J.; Cao, Y.; Orndorff, W.; Pan, W.-p., Mercury

7 8 9 10 11 12 13 14 15 16 17 18 19 20

28. Boycheva, S.; Zgureva, D.; Václavíková, M.; Kalvachev, Y.; Lazarova, H.; Popova, M., Studies on non-

adsorption characteristics of HBr-modified fly ash in an entrained-flow reactor. Journal of Environmental Sciences 2015, 33, 156-162. 27. Wang, H.; Cheng, C.; Chen, C., Characteristics of polycyclic aromatic hydrocarbon release during spontaneous combustion of coal and gangue in the same coal seam. Journal of Loss Prevention in the Process Industries 2018, 55, 392-399. modified and copper-modified coal ash zeolites as heterogeneous catalysts for VOCs oxidation. Journal of Hazardous Materials 2018. 29. Schwämmle, T.; Bertsche, F.; Hartung, A.; Brandenstein, J.; Heidel, B.; Scheffknecht, G., Influence of geometrical parameters of honeycomb commercial SCR-DeNOx-catalysts on DeNOx-activity, mercury oxidation and SO2/SO3-conversion. Chemical Engineering Journal 2013, 222, 274-281. 30. Belaissaoui, B.; Le Moullec, Y.; Favre, E., Energy efficiency of a hybrid membrane/condensation process for VOC (Volatile Organic Compounds) recovery from air: A generic approach. Energy 2016, 95, 291-302. 31. Zhang, X.; Gao, B.; Creamer, A. E.; Cao, C.; Li, Y., Adsorption of VOCs onto engineered carbon materials: A review. Journal of Hazardous Materials 2017, 338, 102-123. 32. Aguayo-Villarreal, I. A.; Montes-Morán, M. A.; Hernández-Montoya, V.; Bonilla-Petriciolet, A.; Concheso, A.; Rojas-Mayorga, C. K.; González, J., Importance of iron oxides on the carbons surface vs the specific surface for VOC’s adsorption. Ecological Engineering 2017, 106, 400-408.

21


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

Figure captions

2

Figure 1. A diagram of the fixed bed adsorption test platform

3

Figure 2. The locations of VOCs sampling and modified fly ash injection points in the power plants

4

Figure 3. Adsorption of toluene using fly ash, molecular sieves and activated carbon in a fixed bed

5

adsorption apparatus

6

Figure 4. The total concentrations of VOCs in emissions from coal-fired power plants

7

Figure 5. The distributions and reductions of VOCs in ultra-low-emissions power plants

8

Figure 6. The surface charges of differently-sized particle at the ESP outlet

9

Figure 7. Diagram summarizing the process used to reduce VOC emissions from an ultra-low-emissions

10

power plant


Page 20 of 33

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1 2

Figure 1. A diagram of the fixed bed adsorption test platform.

3


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Page 22 of 33

1 2

Figure 2. The locations of VOCs sampling and modified fly ash injection points in the power plants.

3

Legend:

4

and flue gas temperatures in plant B,

5

fly ash injection points in plant B.

VOCs sampling points and flue gas temperatures in plant A,

VOCs sampling points

modified fly ash injection points in plant A,

6


modified

Page 23 of 33

100%

F1 F1-M F2 F2-M MS AC

Adsorption, %

80% 60% 40% 20% 0%

0

10

20

30

40

50

60

Time, s 1 2

A. Data obtained using various adsorption materials.

100%

50oC 100oC 200oC 300oC

80%

Adsorption, %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

60% 40% 20% 0%

0

10

20

30

40

50

60

Time, s 3 4

B. Data obtained using various temperatures with fly ash.

5

Figure 3. Adsorption of toluene using fly ash, molecular sieves and activated carbon in a fixed

6

bed adsorption apparatus.

7

Legend: F1 = fly ash sample 1, F2 = fly ash sample 2, MS = molecular sieves, AC = activated carbon,

8

F1-M = modified fly ash sample 1, F2-M = modified fly ash sample 2.


Energy & Fuels

17

Total concentration as methane, mg·m-3

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 33

16 15 5 4 3 2 1 0

F1

F2

F3

F4

F5

G1

G2

G3

G4

S1

PA

PB

Power Station 1 2

Figure 4. The total concentrations of VOCs in emissions from coal-fired power plants.

3

Legend: F1 = As Pontes reported by Fernandez, F2 = Compostilla reported by Fernandez, F3 =

4

Escatron reported by Fernandez, F4 = Teruel reported by Fernandez, F5 = Litoral reported by

5

Fernandez, G1 = B reported by Garcia, G2 = C reported by Garcia, G3 = D2 reported by Garcia, G4

6

= D4 reported by Garcia, S1 = reported by Jianwu, PA = power plant A in this study, PB = power

7

plant B in this study.

8


700

TVOC TVOC after modified fly ash injection

600 500 400 300 200 100 ISCR

OSCR

IESP

OESP

OFGD

OWESP

Sampling Point 1 2

A. Power plant A

500

Total Mass Concentration, g·m-3

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

The Total Mass Concentration, g·m-3

Page 25 of 33

TVOC TVOC after modified fly ash injection

400

300

200

100

0

ISCR

OSCR

IESP

OESP

OFGD

Sampling point 3 4

B. Power plant B

5

Figure 5. The distributions and reductions of VOCs in ultra-low-emissions power plants.

6

Sampling position: ISCR = SCR inlet, OSCR = SCR outlet, IESP = ESP inlet, OESP = ESP outlet,

7

OFGD = FGD outlet, OWESP = WESP outlet. 25 ACS Paragon Plus Environment

Energy & Fuels

2.0x105

1.5x105

1.0x105

5.0x10

4

0.0 0.01

1 2

Charge (1·particle-1)

4.0E-16

Charge (1· particle-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 33

3.0E-16

2.0E-16

1.0E-16

0.0E+00 0.01

0.1

Dp (m)

0.1

1

Dp (m) Figure 6. The surface charges of differently-sized particles at the ESP outlet.


10

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Energy & Fuels

1 2

Figure 7. Diagram summarizing the process used to reduce VOC emissions from an ultra-low-

3

emissions power plant.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

Table captions

2

Table 1. The compare of different flue gas treatment technology

3

Table 2. Data obtained from analyses of coal and fly ash

4

Table 3. The comparison of adsorption capability for different adsorbents

5

Table 4. Compounds identified by TD-GC/MS at the SCR inlets of two power plants

6

Table 5. The percentage adsorptions of various VOCs by modified fly ash

7


Page 28 of 33

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Energy & Fuels

1

Table 1. The comparison of different flue gas treatment technologies

Flue gas treatment

2 3

Abbreviation

Major

reduction

pollutant

Synergistic

reduction

(speciation

change)

pollutants

Selective Catalytic Reduction

SCR

NOx

Hg

Air Preheating

APH

--*

Hg

Electrostatic Precipitation

ESP

PM

Hg

Flue Gas Desulfurization

FGD

SO2

PM, Hg

Wet Electrostatic Precipitation

WESP

Fine PM

Hg

* -- = not applicable


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 33

1

Table 2. Data obtained from analyses of coal and fly ash.

2

A. Proximate and ultimate analysis data for coal samples (dry basis). Proximate analysis (%)

Ultimate analysis (%)

V

A

FC

C

H

O

N

S

Coal A

29.31

18.48

52.21

63.43

4.21

12.24

0.89

0.75

Coal B

28.80

22.69

48.51

62.36

3.60

9.70

1.02

0.63

Sample

3 4

Legend: V = volatile matter, A = ash, FC = fixed carbon, C = carbon, H = hydrogen, O = oxygen, N = nitrogen, S = sulfur.

5

B. Major and minor components of fly ash (% basis). Al2O3

CaO

Fe2O3

K2O

Na2O

MgO

P2O5

SiO2

TiO2

BaO

MnO2

SrO

F1

18.87

12.52

7.10

2.34

2.18

0.76

0.62

48.53

0.81

0.07

0.13

0.65

F2

13.07

1.84

4.31

0.72

0.55

0.48

0.10

83.72

1.38

0.11

0.08

0.07

6 7


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1

Table 3. The comparison of adsorption capability for different adsorbents Adsorbent

2 3

The adsorption capability in 10th second Adsorption (%)

The RCT* of Breakthrough (%)

Molecular sieves

95

5

Activated carbon

70

30

Fly ash sample 1

15

85

Modified fly ash sample 1

35

65

Fly ash sample 2

20

80

Modified fly ash sample 2

30

70

*RCT = the relative concentration of toluene


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

Page 32 of 33

Table 4. Compounds identified by TD-GC/MS at the SCR inlets of two power plants. Boiling

No

Time, min

Compounds

1

8.03

Benzene

80

1.9%

2.0%

2

8.16

1-butanol

118

4.2%

ND**

3

9.07

Heptane

98

1.8%

ND

4

9.50

1-pentene, 2,4,4-trimethyl-

105

4.3%

1.9%

5

11.35

Toluene

111

8.0%

2.6%

6

11.50

undecane, 5-methyl-

206

1.1%

ND

7

12.08

3-heptene, 5-methyl-

120

4.2%

2.8%

8

12.33

3-ethyl-3-hexene

116

8.9%

7.9%

9

12.73

2-hexene, 3,5-dimethyl-

110

1.7%

2.6%

10

14.61

Ethylbenzene

136

7.4%

1.4%

11

14.91

p/m-xylene

138

3.2%

2.7%

12

15.68

o-xylene

144

ND

1.6%

13

19.52

benzyl chloride

179

9.9%

ND

14

21.04

Acetophenone

202

13.1%

3.2%

15

21.95

Nonanal

191

6.2%

1.9%

16

22.40

benzene, 1,2,4,5-tetramethyl-

197

ND

3.0%

17

22.49


198

ND

6.4%

18

23.27


205

ND

10.5%

19

23.39

benzene, 2,4-diethyl-1-methyl-

214

ND

1.0%

20

23.96

1-undecene

211

1.2%

ND

21

23.96

benzene, pentamethyl-

237

ND

1.0%

22

24.17

naphthalene

218

6.3%

28.9%

23

24.28

decanal

207

2.6%

ND

24

24.67

benzene, 1,4-dipropyl-

225

ND

0.5%

25

24.92

benzene, 1-ethyl-2,4,5-trimethyl-

213

ND

2.7%

26

25.89

Dodecane

234

2.3%

0.8%

27

26.15

naphthalene, 2-methyl-

241

4.5%

10.6%

28

26.40

naphthalene, 1-methyl-

244

1.9%

4.0%

29

27.29

Tridecane

254

4.1%

ND

30

27.96

1-Tetradecene

268

1.2%

ND

point* (oC)

2

* Boiling points were determined from NIST library.

3

**ND = be not detected or below the limit of detection.

Plant A (Percent)


Plant B (Percent)

Page 33 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1

Table 5. The percentage adsorptions of various VOCs by modified fly ash.

Benzene Toluene Ethylbenzene m/p-Xylene o-Xylene Chlorobenzene Methyl isopropyl benzene styrene Chlorotoluene Naphthalene 2

Plant A

Plant B

15.7%

25.6%

23.2%

28.1%

34.6%

ND*

13.7%

19.5%

10.6%

30.6%

ND

53.3%

17.9%

ND

ND

22.7%

ND

65.6%

20.7%

10.8%

*ND = below the limit of detection.

3 4


Reductions in coal-fired power plant volatile organic compound

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