Parametric study of reburning of nitrogen oxide for superfine pulverized coal

Energy Conversion and Management 89 (2015) 825–832

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Parametric study of reburning of nitrogen oxide for superfine pulverized coal Jun Shen, Jiaxun Liu ⇑, Junfang Ma, Hai Zhang, Xiumin Jiang Institute of Thermal Energy Engineering, School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai 200240, PR China

a r t i c l e

i n f o

Article history: Received 6 July 2014 Accepted 25 October 2014 Available online xxxx Keywords: Superfine pulverized coal Reburning NOx abatement Coal combustion

a b s t r a c t An experimental investigation of process parameters design by superfine pulverized coal reburning on NOx reduction is carried out in a one-dimensional bench-scale combustion system. Reburning effectiveness for initial levels ranging from 600 to 650 ppm is evaluated. It is found that for Shenhua bituminous coal, NOx reduction performance of superfine pulverized coal owns its unique advantage, 50% higher than conventional particle size. Medium reburning fuel fraction (RF20RF25) and low oxygen concentration are recommended to be ensured in this experiment. The yield of HCN and CH4 along the furnace axis does not seem to be decisive on the effect of particle size, but is closely related to the effect of reburning fuel fraction. Both exhaust and in-flame measurements are reported in this paper to provide valuable experimental data for practical engineering applications. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction Reburning is a combustion technology which is first originated from the research of Myerson et al. [1], and then this name is coined by Wendt et al. [2]. This combustion modification removes NOx by injecting various kinds of fuel as reducing agent. Similar kinetic mechanism has been found on this technology compared to air-staged combustion. Moreover, reburning has the potential for significant NOx reduction as long as the process parameters (such as reburning zone stoichiometry, temperature, or residence time) are properly under control. The technology of reburning is very promising if it is combined with oxy-fuel combustion to remove NOx and CO2 emissions simultaneously [3–5]. The principle of the reburning process is shown in Fig. 1 and can be summarized into three zones [6]. First, in primary combustion zone, where approximately 80% of the thermal input is added, fuel-lean environment should be guaranteed to make sure of complete combustion. Secondly, in reburning zone, where the gas products of primary zone enter, about 20% of the total fuel is transported downstream to ensure fuel-rich environment. In this zone nitrogen species react with hydrocarbon radicals to form the generation of intermediate gas species such as NH3 and HCN, or to react with char provided by reburning fuel. Thirdly, in burnout zone, where burnout air is added to produce overall fuel-lean combustion, char nitrogen in the fuel will either be oxidized into NOx or ⇑ Corresponding author. Tel.: +86 21 34205681. E-mail address: [email protected] (J. Liu). http://dx.doi.org/10.1016/j.enconman.2014.10.059 0196-8904/Ó 2014 Elsevier Ltd. All rights reserved.

act as an agent to reduce NOx depending on different working conditions. As for the reburning fuels, ideally it is recommended that natural gas is firstly proposed because of minor burnout problem and free bearing of nitrogen and sulfur content when applied to existing coal-fired plants. However, it is reported that coal reburning is also preferred compared to other technologies for the reason that good reserves and low cost of coal is very promising [7]. The technical problem concerned with coal reburning is the transformation of fuel-N in the reburning fuels. Since the oxidation of nitrogen in the coal can produce NOx emissions, there is a trade-off between the choice of reburning coal and the optimization of reburning process to maximize NOx reduction efficiency. Meanwhile, it still remains a question whether the physical and chemical properties of coal/char play an important role in reducing NOx emissions. Moreover, for coal reburning the issue of stable combustion and burnout problem should also be taken into consideration. The usage of superfine pulverized coal as reburning fuel for the control of nitrogen oxide emissions has received great attention in USA since the year of 1999 [8]. The combustion advantage of superfine pulverized coal includes higher combustion efficiency, lower ignition temperature, and better burnout performance than conventional coal. Recently, basic research has been carried out to understand physical structure parameters [9–11] and chemical features such as surface functional groups [12]. In the last two years, the study of quantum chemistry has been introduced to give deeper insights into the migration and depletion mechanisms between NOx and char [13–15].

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Nomenclature ad DTF OC ppm

air dried basis drop tube furnace oxygen concentration part per million

PSD RF SH

Based on technologies mentioned above, the novelty of this present study is then proposed, aiming to combine the usage of superfine pulverized coal with reburning combustion to give new findings about nitrogen oxide abatement. Moreover, regarding to the unique characteristics of superfine pulverized coal, heterogeneous NO-reduction pathway has also been proposed to interpret the phenomenon that occurs in this study. Finally, the data of process parameters provided by this laboratory-scale experiment will be useful for the design of working conditions in our next 1 MW pilot-scale U-shape flame boiler.

particle size distribution reburning fraction Shenhua bituminous coal

Table 1 Ultimate and proximate analysis of the SH bituminous coal. Proximate analysis(wt%)(ad) SH

Moisture Volatile Ash Fixed carbon

Ultimate analysis (wt%)(ad) 11.50 24.22 10.7 53.58

C H O N S

63.13 3.62 9.94 0.70 0.41

ad-on an air dried basis. O content is calculated by difference.

2. Experimental section 2.1. Coal Shenhua (SH) bituminous coal, a typical kind of coal widely applied in the industrial power generation area in China, is used in this study. The ultimate and proximate analysis of SH coal are listed in Table 1. Coal particles are classified into four size levels. The coal particles are produced using an air jet mill and acquired by adjusting the frequency of the classifier in this air jet mill. In this study, fineness of coal sample is determined by average particle size (by volume):

P 4 cd Dav ;43 ¼ P i i3 ci di

ð1Þ

where Dav,43 means average particle size (by volume), di refers to mean diameter of a specific narrow range of particle size, and ci means corresponding mass fraction of each di. The average particle size can be calculated from particle size distribution (PSD) in Fig. 2 using Malvern MAM5004 Laser Mastersizer (Malvern, U.K.). In addition, the definition of coal particle size can be divided into many kinds [16,17], mainly average value (D43, D32), interme-

Burnout Air

Reburning Fuel

Primary Air Primary Fuel

Fig. 1. Schematic of the reburning process principle.

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Fig. 4. Global NOx formation-depletion mechanism. [20].

Fig. 2. Particle size distribution of SH bituminous coal specimens.

diate value D(v,50) and maximum-frequency value. The reason why we choose D43 is that D(v,50) is generally applied in industrial area and D32 is more suitable for the description of sub-micron or nano-scale area. Hence, D43 is used in this study which is automatically acquired by this laser particle size analyzer. 2.2. System The schematic diagram of the experimental 60 mm-i.d. 2.5 mlong isothermal quartz reactor system is shown in Fig. 3. The pre-combustion system/primary combustion zone is simulated in this experiment where flue gas composition from pre-combustion

Primary air Coal feeder Gas preheater

P1 On-way sample gas (NOx, O2, CH4, HCN)

P2

P3

P4

Thermocouple

Drop tube furnace Quartz tube

zone consists of doped gas 15–20% CO2 vol., 2–6% O2 vol. and 600– 650 ppmv NO, which is also called as inlet NO concentration. The 1 m-long reburning zone is injected downstream of the simulated primary combustion zone. A micro-screw coal feeder, controlled by a micro-reducer with the adjustment of power frequency, is precalibrated before each working condition. The ambient gas temperature inside the furnace is set at 1000 °C. 2.3. Measurement The mole fraction of NO, NH3, HCN, CO, CO2, CH4 is quantitatively measured by an on-line Fourier Transform Infrared (FTIR) analyzer which resolves the gas IR absorption spectrum. The mole fraction of O2 is quantified by a paramagnetic-based detector. The apparatus is calibrated every day before the experiment runs. The zero point is acquired with the flush of high-purity N2 calibration gas. Temperature measurement is acquired by a coated Type S Pt–Rh thermocouple which allows the maximum measurement range to 1650 °C. The residence time of coal particle from the top of the furnace to the exit is about 0.3–0.4 s. The concentration of gas species is divided into two categories, that is, the measurement from the exit of the furnace and from different on-way positions (in-flame measurements). It can be seen from Fig. 3 that when the experiment is carried out, the concentration of gas species from the exit is first measured for analysis, and the results are discussed in Figs. 5–7. Secondly, the concentration of on-way gas species (mainly CH4, HCN) is also acquired in this experiment. There are four measuring points (Position 1, 2, 3, 4 (P1, P2, P3, P4)) along the axis of the furnace. The measurement data of different positions is presented in detail in Figs. 8 and 9. 2.4. Method Stoichiometric ratio (SR) in this paper is an important design parameter to control the environment where combustion process takes place. It is defined as [18]:

SR ¼ ððactual atomic oxygenÞ=ðactual fuel usedÞÞ =ððtheoretical atomic oxygen usedÞ=ðtheoretical fuel usedÞÞ Exit gas analysis (NOx, O2, CH4, HCN)

Exhaust Filter Fig. 3. Schematic diagram of experimental apparatus.

It is necessary to give the definition that SR1 stands for the stoichiometric ratio in primary combustion zone. In this experiment, SR1 is kept fixed at 1.05 to make sure of slight fuel-lean combustion. SR2 is the overall stoichiometric ratio ranging from 0.735 to 0.945 in reburning zone, with both primary fuel and reburning fuels taken accounted. SR3 is the overall stoichiometric ratio which is set at 1.1 in burnout zone (that it, for the whole combustion process). The relation between stoichiometric ratio and its

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100

RF_10% RF_15% RF_20% RF_25% RF_30%

40

30

Reduction efficiency (%)

Reduction efficiency(%)

50

20

10

SH_14.71 SH_17.44 SH_21.30 SH_44.26

80

60

40

10% 20

grow th

0~5% growth

0 10

15

20

25

30

35

40

0

45

0

Particle size (µm)

1

2

3

4

OC_2% OC_4% OC_6%

30

SH_14.71 SH_17.44 SH_44.26

30

10

15

20

25

30

6

Fig. 7. Effect of oxygen concentration on global NO reduction; Initial NO concentration = 600–650 ppmv at 20% reburning fuel fraction.

20

10

5

Oxygen concentration (% vol.)

HCN (ppm)

Reduction efficiency (%)

~15 %

35

40

20

45

Particle size (µm)

10

Fig. 5. Effect of particle size on global NO reduction; Initial NO concentration = 600–650 ppmv at 2% vol. oxygen.

1

2

3

4

Postion (-)

40

SH_14.71 SH_17.44 SH_44.26

400

CH4 (ppm)

Reduction efficiency (%)

500

SH_14.71 SH_17.44 SH_21.30 SH_44.26

50

30

50% 20

300

200 Low RF Medium RF High RF

10

100 0 10

15

20

25

30

1

Reburning fraction (%) Fig. 6. Effect of reburning fuel fraction on global NO reduction; Initial NO concentration = 600–650 ppmv at 2% vol. oxygen.

corresponding zone is shown in Fig. 1. In this paper, the focus is put on the reaction in reburning zone, which is crucial to the total NO reduction performance. NO reduction efficiency is defined as below:

2

3

4

Position (-) Fig. 8. Effect of particle size on profiles of HCN and CH4 along the furnace axis.

uNO ¼

exit Cinlet NO  CNO

Cinlet NO

 100%

ð2Þ

where uNO indicates the total NO reduction efficiency, and C inlet NO indicates the NO concentration at the inlet of the furnace. As is

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The nitrogen content in the fuel (Fuel-N) is generally composed of volatile-nitrogen (HCN, NH3) in the process of pyrolysis and char-nitrogen obtained after pyrolysis. Fig. 4 illustrates the global formation of NOx formation-depletion mechanism [20]. It is found that HCN is the center of the whole reaction loop. The result of Solomon [21] reveals that NH3 and HCN originate from the Fuel-N, and that NH3 can be converted into HCN provided that large quantity of NO exists in the fuel-rich environment, while HCN cannot directly be turned into NH3. In addition, HCN also comes from the hydrocarbon species:

40

RF_15% RF_20% RF_30%

HCN (ppm)

30

20

CHi þ NO ! HCN þ . . .

2

3

4

Position (-) 500

RF_15% RF_20% RF_30%

CH4 (ppm)

400

ðR1Þ

Moreover, according to different concentration of free radicals such as O, H, the reaction pathways also vary. The NO formation pathway is summarized as the following steps [22]:

10

1

829

HCN þ  O !  NCO þ  H

ðR2Þ



ðR3Þ

NCO þ  O ! NO þ CO

Or

HCN þ  H !  NH þ CO

ðR4Þ

And then 300



NH þ  O ! NO þ  H

ðR5Þ

This reaction pathway is dependent on the concentration of free radicals such as O, H, and the depletion pathway also relies on the participation of O, H free radicals. That is to say, R(2) and R(3) are fundamental, and NH from R(3) plays an important role in NO reduction if there are abundant H radicals in the environment [23]:

200

100

1

2

3

4

Position (-) Fig. 9. Effect of reburning fuel fraction on profiles of HCN and CH4 along the furnace axis.

mentioned, the inlet NO concentration is properly controlled at certain concentrations (about 600–650 ppmv) before each working condition is carried out. Cexit NO represents the measured concentration at the exit of the furnace. For the readers’ reference, the definition of reburning fuel fraction is also given here:

Rmeasure RF ¼  100% Rdesign

ð3Þ

where RF stands for reburning fuel fraction and Rdesign is the design value of coal-feeding rate. In this paper, the design value of coalfeeding rate is fixed at 1 g/min because the calculation of air flow rate is acquired needing this parameter. Rmeasure is measurement value of each working condition from the coal feeder. For example, RF20 shows that the coal feeding rate is adjusted to 0.2 g/min to meet the experimental request. 3. Mechanisms of nitrogen chemistry NO is produced mainly from thermal and prompt formation with the reactive participation of N2 in the air, and from fuel nitrogen formation. Among these mechanisms fuel-NO contributes most to the formation of NO about more than 80% [19]. Therefore in this paper fuel-NO is stressed to be discussed of the experimental data.



NH þ  H !  N þ H2

ðR6Þ



N þ NO ! N2 þ  O

ðR7Þ

These above two pathways compete with each other, and the reaction rate is decided by the oxygen and NO concentration in the flame. The situation of NH3 is in a similar way, and the relevant reaction pathway is listed as below:

NH3 þ O2 ! NO þ . . .

ðR8Þ

NH3 þ NO ! N2 þ . . .

ðR9Þ

4. Results and discussion 4.1. NO reduction efficiency behaviors 4.1.1. Effect of particle size Fig. 5 shows the effect of particle size on NO reduction efficiency. From the viewpoint of reburning fuel fraction, when the reburning fuel fraction ranges from RF10 to RF15, the influence of particle size on the reduction efficiency is not obvious. While as the reburning fuel fraction continues to rise from RF20 to RF25, the advantage of superfine particle size of 14.71 lm, 17.44 lm begins to appear. That is to say, the reduction efficiency floor of 21.30 lm, 44.26 lm reaches about 20%, while that of 14.71 lm, 17.44 lm ranges from 30% to 40%. When the reburning fuel fraction comes to RF30, the trend of all particle size is similar. As for the change of oxygen concentration, when oxygen concentration decreases from 6% to 4% vol., the effect of particle size hardly changes the reduction efficiency. When the oxygen concentration reaches 2% vol., the reduction efficiency of 14.71 lm, 17.44 lm is clearly higher than that of 21.30 lm, 44.26 lm.

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From the above result, it is found that either the increase of the reburning fuel fraction or the decrease of oxygen concentration is favorable to the promotion of reduction efficiency. These two working mechanisms are similar in that the reduction environment should be ensured in the reburning zone where the intermediates such as NH3, HCN, and NCO help to reduce NO. Detailed explanation is given including homogeneous and heterogeneous mechanisms. From the viewpoint of homogeneous mechanism, with the decrease of coal particle size, the heating rate of coal particle increases, which favors the thermal decomposition of coal. Within the same residence time, the yield of volatile content of superfine particle is higher than conventional size [24]. Therefore, the finer particles tend to yield more volatiles content such as CH4, HCN and NH3 which favor NO reduction performance in the reducing environment. From the viewpoint of heterogeneous mechanism, as the particle size decreases, the high yield of volatiles leaves the surface of char more irregular, and the porosity and specific surface area both increase at the same time, which enhances NO reduction efficiency. The heterogeneous reaction rate between char and NO obeys the following formula [25–27]:

V ¼ k  S  PNO

ð4Þ

where V denotes the reaction rate, k is the reaction rate factor; S stands for the specific surface area and PNO for NO partial pressure. The reaction rate factor k in the above formula is closely related to temperature which also plays an important role in the heterogeneous reaction; however in this paper the temperature of each working condition is constant. We are more interested in particle size effect during the combustion process; therefore the effect of temperature will not be discussed in this paper. It can be seen that the increase of S promotes the whole NO-char reaction rate. Moreover, due to the rapidness of heating rate and short diffusive path in the pore, the finer particle size is easier to establish NO-char heterogeneous reaction process. There are other possible reasons for the performance advantage of superfine particle size. For instance, the rapid consumption of oxygen for the fine particle in the combustion process, or the relatively higher content of metal oxide favors NO reduction. 4.1.2. Effect of reburning fuel fractions Fig. 6 indicates that as the reburning fuel fraction increases, the NO reduction efficiency increases. However, the whole trend of reburning fuel fraction should be analyzed stage by stage. From RF10 to RF15, all the particles exhibit a gradual increase in the reduction performance. From RF15 to RF20, the reduction performance of 44.26 lm stops to increase. From RF20 to RF25, reduction performance of both 21.30 lm and 44.26 lm stops to increase. That is to say, there occurs a platform of about 20% NO reduction efficiency for 21.30 lm and 44.26 lm. Moreover, the efficiency of fine particle (14.71 lm and 17.44 lm) becomes prominent in that NO reduction performance is promoted by about 50% (from 20% level to 30% level) when fine particle is applied. Finally from RF25 to RF30, all the particles reach satisfying efficiency at about 50%. However, high reburning fuel fraction may result in high unburned carbon ratio, corrosion at inner furnace wall and low ash melting point. This finding is new and different from that of Mereb and Wendt [28] and Overmoe [29]. The former team reveals in their report that the optimum NO reduction efficiency is achieved at RF22 under SR1 = 1.0. The latter finds that the optimum reburning fuel input should be controlled between RF10 and RF20. It is also found that high RF25 is not necessarily favorable to NOx abatement. The existence of oxygen can explain the reason why reduction performance of high reburning fuel fraction in the reburning zone. When the reburning fuel fraction is high, the yield of volatiles (CH4, HCN, and NH3) also expands following large of oxygen, at this

moment the oxygen concentration in the gas environment gets lower, which favors the volatiles to participate in the NO reduction reaction. When the reburning fuel fraction decreases, the amount of oxygen begins to be abundant. In this scenario, the volatiles that used to reduce NO will be consumed by oxygen, which weakens the whole NO reduction process. Meanwhile, the existence of oxygen weakens the ability of char which catalyzes NO-char reduction by oxidizing CO into CO2. 4.1.3. Effect of reburning zone oxygen concentration The effect of oxygen concentration in reburning zone on NO reduction is shown in Fig. 7. As the oxygen concentration increases, the NO reduction also increases. As the oxygen concentration decreases from 6% to 4% vol., NO reduction efficiency changes a little at to reach about 20%. As the oxygen continues to drop to 2% vol., the reduction performance sensitivity of 21.30 lm and 44.26um is not obvious, while that of 14.71 lm and 17.44 lm is improved to below 40%. As the oxygen concentration finally decreases, reduction efficiency of all particles reaches nearly 100%. The working condition of zero-oxygen gas environment is ideal and can only be taken as reference. This working condition is not advised to be used in the area of practical engineering application. The change of oxygen concentration in reburning zone can affect the role played by volatiles (CH4, HCN and NH3) in the process of pyrolysis. These volatiles will participate in the homogeneous reduction with NO, as mentioned in Section 4.1.2. Meanwhile, small portion of oxygen is helpful for the heterogeneous reduction between NO and char. Since de-volatilization of coal particle into char only takes milliseconds in the early stage of combustion [30], there is abundant time for heterogeneous reduction among NO, O2 and char. Yamashita et al. [31,32] has put forward the pathway mechanism that adsorption of NO and O2 takes place on the surface of char active sites (C*), resulting in the formation of surface carbon–oxygen complexes (C(O), C–O), which is also conducive to NO abatement. 4.2. On-way gaseous species profiles 4.2.1. Effect of particle size Fig. 8 reflects the data of volatiles (CH4, NH3) in reburning zone along the axis of the furnace. It can be observed that the change of HCN is not very obvious. In the upstream of the furnace, the yield of HCN presents in this order: 21.30 lm > 44.26 lm > 14.71 lm. As the combustion process goes on, HCN concentration goes up first for 14.71 lm, and then decreases. The trend for 21.30 lm is different, the evolution of HCN is very fast in the early stage of combustion and so does its depletion. For 44.26 lm, the data of HCN is very steady, which indicates that the formation and depletion is in balance. The change of CH4 is similar to HCN. In the early stage of combustion, the yield of CH4 complies with such order: 21.30 lm > 44.26 lm > 14.71 lm. It is known that the volume concentration of CH4 for 14.71 lm and 44.26 lm rises in reburning zone, which indicates that the speed of the formation of CH4 is faster than that of the consumption. The case is different for 21.30 lm where high concentration of CH4 appears first, followed by a sharp decrease along the furnace. The pyrolysis process of coal macromolecule results in fracture of some weak bonds in certain temperature, accompanied by the evolution of lightweight substance such as tar, then the residual molecular bonds recombine to form new carbonic macromolecule, namely char. In the early stage of pyrolysis, the weakest molecular bond is prone to be broken into smaller molecular bonds. That is to say, the element H is peeled off from hydro aromatic compounds or aliphatic chain, mainly CH4.

J. Shen et al. / Energy Conversion and Management 89 (2015) 825–832

4.2.2. Effect of reburning fuel fraction Fig. 9 shows the profiles of HCN and NH3 along the furnace axis for different reburning fuel fractions. As reburning fuel fraction increases, the overall HCN content also increases by a different degree. As for RF15, the change of HCN concentration is not obvious. The evolution profile of HCN is smooth before P3, followed by the decline downstream the furnace. When reburning fuel fraction increases to RF20, rapid evolution of HCN takes place at first before P2, then HCN slowly declines until the exit of the furnace. When reburning fuel fraction continues to increase to RF30, the quantity of HCN becomes larger in the initial stage, then maintains at a high level and depletes quickly at last. In regard to CH4, the trend is slightly different from that of HCN. The global trend of CH4 concentration is upward from P1 to P3. For RF15 and RF30, the concentration of CH4 continues to rise, but for RF20, the trend turns downward at P3. It can be found that as the reburning fuel fraction increases, the concentration level of HCN and CH4 both increases, this is due to the increase of total nitrogen and carbon input in the process of pyrolysis. In the initial stage of reaction where oxygen concentration is relatively higher, HCN/CH4 is easily oxidized into NO/CO2. When the oxygen continues to be consumed, then HCN/ CH4 begins to participate in the NO reduction reaction. It is observed that HCN gradually decreases along the axis of the furnace, while CH4 gradually increases in despite of the consumption of NO reduction. 5. Conclusions Based on data and analysis acquired through this study, conclusions can be drawn as follows: (1) For SH bituminous coal, superfine pulverized coal owns its unique advantage, such as 14.71 lm and 17.44 lm. To achieve this goal, parameters of reburning fuel fraction and oxygen concentration should be controlled at certain values. (2) The increase of reburning fraction is favorable to NO reduction performance for superfine pulverized coal. Under low reburning fuel fraction working conditions (RF10RF15), the superiority of fine coal is not obvious. Under high fraction (RF30), NO reduction performance of four particles is similar, yet in this condition high unburned carbon should be taken into consideration. Under medium fraction (RF20RF25), reduction efficiency of fine particle (14.71 lm and 17.44 lm) become prominent in that NO reduction performance is promoted by about 50% (from 20% level to 30% level) when fine particle is applied. Hence, the data of reburning fuel fraction acquired in this paper can be used for reference in the design of practical engineering applications. (3) The reduction of oxygen concentration in reburning zone is helpful for NO reduction. In this paper it is found that at high oxygen concentration environment (4–6% vol.), NO reduction performance of all particles is not very good. When oxygen concentration drops to 2% vol., the performance of regular particle is still poor. However, the performance of fine particle (14.71 lm and 17.44 lm) is rather satisfactory. (4) For different particle sizes, the yield peak of HCN for fine particle is relatively later, and the effect of HCN concentration on NO reduction performance does not seem to be decisive. The influence of CH4 is similar to HCN. This may be due to the impact of physical structure or surface chemistry on the penetration of HCN and CH4. For different reburning fuel fraction, the yield of HCN and CH4 is related to NO reduction.

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With the increase of reburning fuel fraction, HCN and CH4 concentration in the flue gas increases, resulting in the improvement of NO reduction efficiency.

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