Influence of OxidationâReduction Layering on Fuel Nitrogen Oxide

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Influence of Oxidation−Reduction Layering on Fuel Nitrogen Oxide Emissions during a Char Grate-Fired Process Li Xu, Jianmin Gao,* Guangbo Zhao, Laifu Zhao, Wei Zhao, Qian Du, Yu Zhang, Shaohua Wu, and Yukun Qin School of Energy Science and Engineering, Harbin Institute of Technology, 92 West Dazhi Street, Harbin, Heilongjiang 150001, People’s Republic of China ABSTRACT: Previous studies have found that the nitric oxide (NO) trend above the fuel seam of a chain boiler is opposite that of a reciprocating grate boiler. We have researched the factors that influence fuel NO emissions during the char grate-fired process by changing the char/oxygen (O2) ratio in a small-scale one-dimensional fixed-bed system. With an increase in the char/ O2 ratio compared to the mass loss of char, the advent of excess O2 is delayed and carbon monoxide (CO) is generated. As a result, oxidation−reduction layering of char occurs. Char grate firing leads to an increase in char mass of the complete oxidation layer, and a thicker ash layer covers the char surface. The reduction layer and low-O2 region of the oxidation layer favor NO reduction; therefore, NO emission is delayed or shows a double-peak trend. The ash layer covering and a long residence time at a high temperature increase the NO generation rate of the oxidation layer. However, the overall NO generation rate is controlled by the reduction and oxidation layer, and an optimal char/O2 ratio exists, which makes the overall NO generation rate the lowest. A difference in the NO trend between the chain boiler and reciprocating grate boiler in and after the O2-deficient zone is because of the influence of the reciprocating disturbance on the reduction layer, ash layer, and char N release.

1. INTRODUCTION Nitrogen oxide (NOx) is a major atmospheric pollutant. In 2014, 2.5 million tons of NOx were emitted from industrial boilers, which accounted for 12.3% of total Chinese NOx emissions. Industrial boilers are the third largest emission source, after power plants and vehicles.1 The main component of NOx is NO. NO emissions from coal grate-fired processes in industrial boilers have become a key focus for point-source control. The author carried out industrial tests on two typical industrial boilers (a chain boiler and a reciprocating grate boiler), and NO and O2 trends above the fuel seam were obtained.2−4 Figure 1 shows the industrial test results. Measuring points were located on the central line of the grate and ∼50 mm above the coal surface. The converted O2 concentration was 6%. As shown in Figure 1a, the NO trend of the chain boiler is high at the edges and low in the center. Only 49 mg/m3 NO exists in the O2-deficient zone (wind rooms 3 and 4). For the reciprocating boiler, the NO trend is opposite that of the chain boiler. There is 279 mg/m3 NO in the O2-deficient zone, which is higher than at the edges. The difference in the NO trend indicates that, besides the O2 concentration, other influencing factors affect NO emissions during the char grate-fired process. Most research at home and aboard focused on the influence of different factors on NO emission, such as rank,5−7 temperature,8,9 particle size,10−12 pressure,13 O2 concentration,14−16 etc. The background was mostly the pulverized coal boiler or fluidized bed, and the research method was the horizontal tube furnace, drop-tube furnace, or fluidized-bed reactor. For the industrial boiler and grate-fired process, a onedimensional fixed bed is the most common method, and it has been used extensively in research on straw, municipal solid © XXXX American Chemical Society

Figure 1. Industrial test results.

waste, and other materials.17−22 In NO research, Ji et al. have used this method to study the effect of air and fuel classification on NO emissions.23 Rashidian et al. have also used this method to study biomass combustion and have focused on the influence of freeboard deflectors on species concentrations (CO/CO2/ O2/NO).24,25 Xu and Gao have also used this method to research the effect of flue gas recirculation (FGR) on NO Received: May 22, 2017 Revised: July 19, 2017 Published: July 21, 2017 A

DOI: 10.1021/acs.energyfuels.7b01237 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

sample and the gas. The measurement section contains a Mettler MS304S analytical balance (11, Zurich, Switzerland) that is placed on the lift (12), and a Testo 340 gas analyzer (14, Schwarzwald, Germany) with four sensors, namely, O2, NO, NO2, and SO2. During operation, the fuel mass and air flux (namely, fuel/O2 ratio) differ at different areas on the grate and affect the O2 concentration above the fuel seam. Research into NO emissions during the coal grate-fired process must consider the fuel/O2 ratio. In this paper, different char/O2 ratios were achieved by changing the char mass and fixing other parameters. All parameters are shown in Table 1. Because of the absence of N2 in the reaction gas, all NO can be considered to be fuel NO. 2.2. Sample Preparation. Shenhua bituminous coal, which is also used in industrial tests, was prepared into a char by slow pyrolysis. Char was obtained in a muffle furnace under an inert atmosphere (Ar, 1.5 L/min) with a heating rate of 20 °C/min and a final temperature of 700 °C that was maintained for 10 min. When the temperature cooled, a type of char was obtained, which is referred to as 700-1416, depending upon the final temperature and initial particle size (14−16 mesh). Other types of char were prepared and named in a similar way. For example, 800-2024 char meant that the final temperature was 800 °C and the initial particle size was 20−24 mesh. 2.3. Proximate and Ultimate Analyses. All proximate and ultimate analyses are shown in Table 2. With an increasing pyrolysis temperature, N decreases and then increases. Hamalainen and Aho speculated that, at a slightly higher pyrolysis temperature, the gaseous products were mainly CO, H2, and H, which resulted in higher residual N in the char.28 Liu found similar results and suggested that, between 800 and 900 °C, with an increasing pyrolysis temperature, the porosity and release rate of the volatiles increased. This lead to an enhancement of adsorption of volatile N, which yielded a higher residual N in the char.29 The particle size was irregular, which means that the degree of pyrolysis is well and the particle size has little effect on pyrolysis.30 2.4. Analytical Methods. The excess air coefficient (EAC) every 0.5 min is the ratio of the actual to theoretical reaction gas volume that is required for char complete consumption and is calculated from

emission during the coal grate-fired process and provided the foundation for the practical application of this technology.26 To find out the law of NO emission during the grate-fired process is the basis of developing NO control technology for an industrial boiler. This is the foothold and starting point of this paper. Because char combustion occupies a dominant position in wind rooms 3 and 4,27 char is the focus of this paper and volatiles are ignored.

2. EXPERIMENTAL SECTION 2.1. Small-Scale One-Dimensional Fixed-Bed System. A small-scale one-dimensional fixed-bed system with high precision and capability of operating under various reaction conditions was built to research the char grate-fired process. Figure 2 shows the system diagram.

EAC =

V (m0.5n + 0.5 − m0.5n)V0

n = 0, 1, 2, 3, ...

(1)

where V0 is the theoretical reaction gas volume that is required for char complete combustion of each milligram (Nm3/mg), V is the actual reaction gas volume in 0.5 min (Nm3), m0.5n is the mass loss of char at an integer multiple time point of 0.5 min (mg), and m0.5n + 0.5 is the mass loss of char, which occurs 0.5 min later than m0.5n (mg). The NO generation rate is the rate of char N that reacts to form NO. It is determined by from

Figure 2. Diagram of the small-scale one-dimensional fixed-bed system. The system is divided into three sections, namely, the gas mixture (1−7), heating and reacting (8−10), and measurement (11−14) sections. O2 (1) and Ar (2) were used and were controlled by mass flow controllers (MFCs, 3−5). The Ar supply was split: part of Ar was mixed with O2 evenly in the mixing tank (6) to form a reaction gas, and remaining Ar was provided as a protective gas, with its flow equaling that of the reaction gas. The reaction and protective gas cannot be fed to the quartz reactor (10) simultaneously but are switched via a four-way valve (7). The stability of the mixed reaction gas is guaranteed, and water hammer is avoided. The heating body of the heating and reacting section is a vertical tube furnace (8) with an 80 mm diameter central hole. A ceramic fiber board is used to seal the base of the hole. The quartz reactor is composed of a quartz tube and a quartz pad. The pad is a porous SiO2 material. It can hold the sample (9) and allow for gas passage; thus, there is good contact between the

α=

14 m 30 NO

m0Nd

(2)

where mNO is the mass of NO in a period of time (mg), m0 is the mass loss of char in the same period of time (mg), and Nd is the N content in Table 2.

3. RESULTS AND DISCUSSION Figure 3 shows the combustion results of 700-1416 char in different char/O2 ratios. At 0 min, protective gas is switched to reaction gas, and thus, the first 1 min of the curves is invalid

Table 1. Experimental Parameters condition reaction temperature reaction gas composition gas flow coal diameter sample mass

unit °C L/min mm mg

parameter 700, 800, and 900 80% Ar and 20% O2 2 0.5−0.6 (30−36 mesh), 0.85−0.9 (20−24 mesh), and 1.25−1.43 (14−16 mesh) 200, 300, 400, 600, and 800 B


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Energy & Fuels Table 2. Proximate and Ultimate Analyses of the Chars proximate analysis (wt %) 700-1416 700-2024 700-3036 800-1416 800-2024 800-3036 900-1416 900-2024 900-3036

ultimate analysis (wt %)

Vd

FCd

Ad

Cd

Hd

Od

Nd

Sd

14.43 14.28 14.33 11.05 10.90 11.19 9.64 9.29 9.00

82.10 82.84 82.99 85.95 86.25 85.98 88.17 88.07 87.76

3.47 2.88 2.68 3.00 2.85 2.83 2.19 2.64 3.24

90.43 89.96 90.26 91.14 90.77 91.07 92.40 93.33 92.18

1.85 1.89 1.85 1.21 1.24 1.21 0.80 0.74 0.80

3.96 3.95 3.85 3.41 3.85 3.69 3.44 2.09 2.62

0.84 0.84 0.83 0.77 0.80 0.77 0.83 0.84 0.81

0.45 0.48 0.44 0.47 0.49 0.43 0.33 0.31 0.34

Figure 3. Influence of the char/O2 ratio on 700-1416 char combustion.

(from −1 to 0 min). All 700-1416 char combustion conditions at 700 °C are referred to as condition 700-1416. The sample masses were 200, 400, 600, and 800 mg. The curves provide the mass loss of char (mg), the excess O2 concentration (%), the NO concentration (mg/m3), and the EAC (dimensionless). At the end of the experiments, because of the excess reaction gas, the EAC values are often excess. Therefore, the upper limit of the EAC is 3, and all excess EAC values have been adjusted to 3. 3.1. Influence of the Char/O 2 Ratio on Char Oxidation−Reduction Layering. 3.1.1. Appearance of the Oxygen-Absent Part and Oxidation−Reduction Layering of Char. Figure 3a shows the combustion result for 200 mg of

700-1416 char, which is referred to as condition 700-1416-200 and depends upon the char type and initial mass. The other conditions are the same. O2 can be detected initially and increases gradually. This result implies that excess O2 is always present under this condition. Panels b, c, and d of Figure 3 show the results of conditions 700-1416-400, 700-1416-600, and 700-1416-800, respectively. For these three conditions, no excess O2 results within the first 0.8, 1.6, and 2.25 min, which means that O2 is depleted during char combustion. The EAC values are less than 1; therefore, CO is generated. Oxygen absence is an experimental phase when O2 cannot be detected, and oxygen presence means that O2 can be detected. As viewed from the time, the experimental phase can be divided C


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Energy & Fuels into oxygen-absent and oxygen-present parts. With an increase in the char/O2 ratio compared to the mass loss of char, the advent of excess O2 is delayed, the existence of the oxygenabsent part is prolonged, and CO is generated. According to the typical division of the reaction region during the coal grate-fired process, the char layer in the O2deficient zone can be divided into reduction and oxidation layers. This division is in space, and the reduction layer exists above the oxidation layer.31 The reduction layer appears because of a lack of O2 and char gasification.32 Layering may occur between particles or within a single particle.33 Thus, for panels b−d of Figure 3, the presence of the oxygen-absent part indicates that an oxidation−reduction layering of the char occurred. The reduction and oxidation layers exist simultaneously. Insufficient char was formed for condition 700-1416; therefore, layering occurs within a single particle. Figure 4 shows the oxidation−reduction layering of a single char particle that is based on the Thring model.33 The

Figure 5. Mass of the oxygen-present part of condition 700-1416.

The oxidation layer contains ash and char. The ash mass increases from 8 to 34.1 mg because of an increase in the initial char mass (from 200 to 800 mg). The ash rate also increases (from 4 to 13.2%), which means that the char surface is covered with a thicker ash layer. The char mass increases from 192 to 223.5 mg with the same trend as the ash rate. The char mass of the complete oxidation layer depends upon whether it can deplete all O2. It is related to the combustion rate of carbon and oxygen. According to the reaction mechanism of carbon and oxygen and the combustion rate formula of carbon,20 the combustion rate is proportional to the char mass and the O2 diffusion coefficient. The O2 diffusion coefficient is inversely proportional to the ash-layer thickness.34,35 At a fixed O2 flow, a thicker oxidation layer ash layer requires more char mass to deplete all O2. According to the principle of a one-dimensional fixed-bed and grate-fired process,36 if we assume that the reduction layer has no effect on the oxidation layer, the result of condition 7001416-200 is equivalent to the first 200 mg char combustion of condition 700-1416-800. The same holds true for conditions 700-1416-400 and 700-1416-600. Therefore, the complete oxidation layer that occurs at 0.8 min in condition 700-1416400 and 1.6 min in condition 700-1416-600 is equivalent to 0.8 and 1.6 min, respectively, in condition 700-1416-800. Figure 5 provides the trend in the complete oxidation layer during the char grate-fired process: the oxidation layer depletes all O2, the char mass increases, and the char surface is covered by a thicker ash layer. When the time point and char mass of the complete oxidation layer of condition 700-1416 are fit, the char mass of the complete oxidation layer at different times is obtained, as shown in Figure 6. As the char grate-fired process proceeds, the char mass of the complete oxidation layer increases and the speed is increasingly higher. For condition 700-1416-200, because of an absence of the complete oxidation layer, its char mass exists below the curve. 3.1.3. Distribution Relationship between Reduction and Oxidation Layers. Figure 7 shows the mass loss rate of the oxygen-absent and oxygen-present parts of condition 700-1416. With an increase in the char/O2 ratio, the mass loss rate of the oxygen-absent part increases (from 0 to 67.8%), whereas that of the oxygen-present part decreases (from 96 to 27.9%). From the analysis of section 3.1.2, with an increasing char/O2 ratio

Figure 4. Diagram of oxidation−reduction layering of a single char particle.

environmental O2 concentration decreases with the gas flow direction to zero. As a result, the char particle is divided into reduction and oxidation layers. The oxidation layer is limited by O2 diffusion, and eq 3 occurs on the char surface. Generated CO diffuses outward and mixes with O2; therefore, eq 4 results, and a flame front is formed. In the reduction layer, because of a lack of O2, eqs 3 and 4 do not result. However, because of CO2 diffusion to the char surface, only eq 5 occurs. 1 C + O2 → CO (3) 2 CO +

1 O2 → CO2 2

C + CO2 → 2CO

(4) (5)

3.1.2. Char Combustion of the Oxygen-Present Part. When O2 becomes excess, the oxygen-absent part is over and the oxygen-present part begins. In this phase, only the oxidation layer exists. In Figure 3, char combustion in the oxygen-present part is oxidation layer combustion. Figure 5 shows the mass of the oxidation layer for condition 700-1416. Except for condition 700-1416-200, all other oxidation layers that could deplete O2 are complete. With an increase in the char/O2 ratio, the mass increases from 200 to 257.6 mg. D


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Figure 6. Trend of the char mass of the complete oxidation layer at different times. Figure 8. Char mass rate of reduction and oxidation layers of condition 700-1416.

by the initial char mass. If there is only an increase in the initial mass (also an increase in the char/O2 ratio), the char mass of the oxidation layer is almost invariant. Only an increase in the reduction layer results. Therefore, the char mass rate of the reduction layer increases, and the oxidation layer decreases. 3.2. Influence of Oxidation−Reduction Layering on NO Emissions. NO emissions during the char grate-fired process are influenced by two factors: NO generation in the oxidation layer, and the reduction effect of the reduction layer on NO. We analyze these two factors in this section. 3.2.1. Reduction Effect of the Reduction Layer on NO. As shown in panels b and c of Figure 3, with an increase in the char/O2 ratio compared to the mass loss of char, NO emissions are delayed by 1.89 and 2.95 min. This behavior is similar to O2. Many conditions exist in the reduction layer, such as a lack of O2, a high temperature, reducing gases (CO and H2), and blazing char. These conditions benefit NO reduction.37,38 Therefore, the phenomenon of NO emission delay emerges. In industrial tests on the chain boiler, the NO concentration above the reduction layer (also the O2-deficient zone) is lowest (Figure 1a). The delay in NO emissions needs a higher char/O2 ratio than for O2. A comparison of panels a, b, and c of Figure 3 shows that, in the first 0.8 min (also the duration of the oxygenabsent part), the NO generation rate is 14.67, 4.43, and 0%, respectively. Therefore, with an increase in the char/O2 ratio, the effect of NO reduction strengthens. NO emissions are influenced by the distribution relationship between the reduction and oxidation layers. As shown in Figure 8, with an increase in the char/O2 ratio, the char mass rate of the reduction layer increases and the NO reduction ability increases until all NO is reduced to N2. 3.2.2. NO Generation in the Oxidation Layer. 3.2.2.1. The Effect of the Low-O2 Region of the Oxidation Layer on NO. As shown for 1.6−1.85 min in Figure 3c and 2.25−3 min in Figure 3d, with excess O2 (0−2.1 and 0−4.7%), there is no NO emission result. This result indicates that char could reduce NO even for low O2 concentrations. Many scholars have noted a similar phenomenon.39−41 Therefore, besides the reduction layer, the low-O2 region of the oxidation layer (always the top area) is able to reduce NO.

Figure 7. Mass loss rate of oxygen-absent and oxygen-present parts of condition 700-1416.

compared to the increase in the rate of the initial char mass (100, 50, and 33.3%), the increase in the rate of the char mass of the oxygen-present part is small (1.3, 4.8, and 9.7%; Figure 5). Therefore, the mass loss rate of the oxygen-present part decreases, whereas that of the oxygen-absent part increases. This means that more char burning occurs in the presence of the reduction layer. Figure 8 shows the char mass rate of the reduction and oxidation layers of condition 700-1416 and the distribution relationship between the reduction and oxidation layers during the char grate-fired process. No curve exists for condition 7001416-200 because of the absence of oxidation−reduction layering. As char grate-firing proceeds, the char mass rate of the reduction layer decreases, whereas that of the oxidation layer increases. This behavior continues until the reduction layer disappears and only the oxidation layer exists. The disappearance time points of these three conditions are 0.8, 1.6, and 2.25 min, which are also the end time points of the oxygenabsent part. Simultaneously, in the oxygen-absent part, with an increase in the char/O2 ratio, the char mass rate of the oxidation layer decreases, whereas the reduction layer increases. When O2 is deficient, the char mass of the oxidation layer depends upon the combustion rate of carbon and oxygen. It is barely influenced E


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char grate-fired process proceeds, the NO generation rate increases from 20.8 to 79%. From an analysis of section 3.1.2, as the char grate-firing proceeds, the ash layer thickens, which hinders contact between the char and external NO and decreases char reduction on NO. Second, a thicker ash layer hinders O2 diffusion and slows the oxidation layer combustion rate. Figure 11 shows the

Figure 9 shows the result for condition 900-3036-300. The NO curve shows an obvious double-peak trend. Zone I (0−

Figure 9. Result of condition 900-3036-300.

0.47 min) belongs to the oxygen-absent part with NO emissions and is similar to the time period of 0−0.8 min in Figure 3b. Zone II (0.47−0.92 min) belongs to a transition from the oxygen-absent part to the oxygen-present part and is similar to the time periods of 1.6−1.85 min in Figure 3c and 2.25−3 min in Figure 3d without NO emissions. Reaction rates of two zones exist in the figure. Zone II has a 27.9% lower rate than zone I. Zone II belongs mainly to the oxygen-present part. Because of excess O2, the combustion rate of carbon and oxygen and the char N emissions decrease. Meanwhile, because of an absence of the reduction layer, the ability of char-reducing NO also decreases. The low-O2 region of the oxidation layer could still reduce NO, and thus, the decrease in ability of char-reducing NO is less than that of char N emissions. As a result, all NO is reduced and the curve is divided into two peaks. 3.2.2.2. NO Generation Rate of the Oxidation Layer. Figure 10 shows the NO generation rate of the oxygen-present part of condition 700-1416. As shown in Figure 5, Figure 10 provides the NO generation rate of the oxidation layer. As the

Figure 11. Combustion rate of oxygen-absent and oxygen-present parts of condition 700-1416.

combustion rate of the oxygen-absent and oxygen-present parts. Except for condition 700-1416-200, with an increase in the char/O2 ratio, the combustion rate of the oxygen-absent part changes little but the oxygen-present part decreases gradually. Because O2 is stable, a decrease in the combustion rate means an increase in the EAC value of the oxygen-present part. Therefore, the oxidizing atmosphere of char is more intense, which promotes NO generation. The increase in the char/O2 ratio also prolongs the residence time of the oxygen-absent part. Because of a higher combustion rate and the greater heat release, the real temperature of the oxygen-absent part is higher than the oxygen-present part.33 A prolonged residence time at a higher temperature does not benefit NO reduction.42,43 In the oxygen-present part, the greater residence time of the oxygen-absent part yields a weaker char reduction ability on NO. 3.2.3. NO Emission Based on Oxidation−Reduction Layering of a Single Char Particle. Figure 12 shows the NO emissions based on Figure 4. The oxidation layer is divided into a high- and low-O2 regions, and the NO emissions in different regions were analyzed. In the high-O2 region, mainly oxidation and partial reduction of char N occurs. NO, N2, and CO2 are the main gaseous products. The reaction paths are as follows,38,44,45 and eq 8 is the N2 source:

Figure 10. NO generation rate of the oxygen-present part of condition 700-1416.

C(N) + O2 → NO + C(O)

(6)

2Cf + NO → C′(N) + C(O)

(7)

C′(N) + NO → N2 + C(O)

(8)

Cf + C(O) + NO → C(O2 ) + C(N)

(9)

C(O2 ) → CO2 + Cf F

(10) DOI: 10.1021/acs.energyfuels.7b01237 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 12. Diagram of the NO emission based on oxidation−reduction layering of a single char particle.

Figure 13. Overall NO generation rates.

where Cf is an active site on the char surface, C(N) is N at the char surface, C′(N) is the nitrogen complex on the char surface, and C(O) and C(O2) are the oxygen complexes on the char surface. These complexes are important intermediates in the char NO reaction. In the low-O2 region, besides eqs 6−10, a low concentration of O2 could refresh Cf and C(O), which benefits NO reduction. The additional reaction paths are as follows:39,46 Cf + O2 → C(O) + COx

(11)

C(O) → Cf

(12)

catalyst

CO + NO ⎯⎯⎯⎯⎯⎯→ CO2 +

1 N2 2

CO + C(O) → CO2 + Cf

(13) (14)

In addition, Ohtsuka and Wu found that, during gasification between char and CO2, the nitrogen functional groups (Nf) on the char surface react with CO2 and generate NO.48 CO2 is a type of oxidizer in this reaction. The reaction paths are as follows:

In the reduction layer, because of a lack of O2, eq 6 barely occurs. NO originates mostly from the oxidation layer. As a result of eq 5, NO reductions are influenced by CO. Besides eqs 7−10, CO can reduce NO by char catalysis and refresh Cf by removing absorbed oxygen on the char surface. The reaction paths are as follows:47

Nf + CO2 → CO + N(O)

(15)

N(O) → NO

(16)

3.3. Overall NO Generation Rate. Figure 13 shows the overall NO generation rates of all combustion experiments, while the oxygen-absent and oxygen-present parts are merged. With an increase in the char/O2 ratio, the overall NO generation rates decrease first and then increase. An optimal G


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4. CONCLUSION When the char/O2 ratio is changed, oxidation−reduction layering of the char during the grate-fired process is simulated and fuel−NO emissions were researched. The following conclusions can be drawn: (1) With an increase in the char/ O2 ratio, all O2 is depleted in the initial stage of char combustion and CO is generated. According to the advent of excess O2, the char combustion process is divided into oxygenabsent and oxygen-present parts. In the oxygen-absent part, the reduction layer and oxidation layer exist simultaneously, whereas in the oxygen-present part, only the oxidation layer exists. (2) As the char grate-firing process proceeds, the char mass of the complete oxidation layer increases from 194.4 to 223.5 mg and a thicker ash layer covers the char surface. Meanwhile, the char mass rate of the reduction layer decreases, whereas the oxidation layer increases. (3) Because the char mass of the complete oxidation layer is barely influenced by the initial char mass, with an increasing char/O2 ratio, the mass loss rate of the oxygen-absent part increases and the oxygen-present part decreases. Meanwhile, the char mass rate of the reduction layer increases, whereas the oxidation layer decreases. (4) The reduction layer is able to reduce NO, and it is enhanced by increasing the char/O2 ratio. In comparison to the char mass loss, NO emissions are delayed but a higher char/O2 ratio than O2 is required. Besides the reduction layer, the low-O2 region of the oxidation layer could reduce NO well. (5) As the char grate-firing process proceeds, the NO generation rate of the oxidation layer increases from 20.8 to 79% because of the influence of the ash layer covering and a long residence time at a high temperature. (6) With an increase in the char/O2 ratio, the overall NO generation rate decreases initially and then increases. An optimal char/O2 ratio exists, which makes the overall NO generation rate the lowest. Under the experimental conditions, the optimal char mass for the 700 and 800 °C conditions is 400 mg and the optimal char mass for the 900 °C condition is 300 mg. (7) The NO trend between the chain boiler and reciprocating grate boiler in and after the O2deficient zone is different because of the influence of the reciprocating disturbance on the reduction layer, ash layer, and char N release.

char/O2 ratio exists, which makes the overall NO generation rate lowest. Under the experimental conditions, the optimal char mass at 700 and 800 °C is 400 and 300 mg for 900 °C. The analysis in section 3.1.3 shows that, with an increase in the char/O2 ratio, the mass loss rate of the oxygen-absent part increases. Condition 700-1416, for example, shows that 47.2% char burns in the oxygen-absent part for condition 700-1416400, which is much less than those for conditions 700-1416-600 and 700-1416-800 (61.5 and 67.8%, respectively; Figure 7). Because of the reduction layer, the NO generation rate of the oxygen-absent part is very low, which means that more char releases little NO in conditions 700-1416-600 and 700-1416800. The overall NO generation rate is also influenced by the oxygen-present part. In the oxygen-present part, the NO generation rate of condition 700-1416-400 is less than those of conditions 700-1416-600 and 700-1416-800 (Figure 10). The char mass of the oxygen-present part of these three conditions is approximately the same (Figure 5). Thus, as an average result of the oxygen-absent and oxygen-present parts, the NO generation rate of condition 700-1416-400 is lowest. No reduction layer exists for condition 700-1416-200; therefore, the overall NO generation rate is higher. During the actual operation, the boiler load is adjusted by the coal-seam thickness or the grate-advancing speed. Collectively, they determine the supply of coal and air. When the load is stable, an increase in the coal-seam thickness and a decrease in the grate-advancing speed implies an increase in the char/O2 ratio and vice versa. When the coal-seam thickness and the grate-advancing speed are adjusted, an optimal char/O2 ratio can be approached, which benefits NO reduction. 3.4. Difference in the NO Trend between the Chain Boiler and Reciprocating Grate Boiler. In Figure 1a, the NO concentration in the O2-deficient zone is low, because of oxidation−reduction layering. The reduction layer and the lowO2 region of the oxidation layer could reduce NO well.2 The increase in the NO concentration at the back arch is influenced by an ash layer covering and a long residence time at a high temperature. From the analysis of section 3.2.2, both factors promote NO emissions. For the reciprocating grate boiler (Figure 1b), the NO concentration in the O2-deficient zone is high, which is caused by the special reciprocating disturbance of the fuel seam. This disturbance destroys the division of the reaction region during the coal grate-fired process, makes the reduction layer unstable, and reduces the char ability for NO reduction. It also provides the fuel seam with a condition for two sides to burn, loosens the fuel seam, enhances the ventilation effect, and peels the ash layer. These effects promote char combustion and make the char consumption and char N release from the back arch migrate forward. As a result, the NO concentration in the O2deficient zone is higher than that at the edges. At the back arch, because of the peeled ash layer, the reduction in char on NO is enhanced. With the char N release migrating forward, NO concentrations at the back arch are lowered. In summary, differences in the NO trend between the chain boiler and reciprocating grate boiler in and after the O2deficient zone are because of the influence of the reciprocating disturbance on the reduction layer, ash layer, and char N release. The disturbance is characteristic of a reciprocating grate boiler.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-13895753587. E-mail: [email protected]. ORCID

Li Xu: 0000-0002-9347-3549 Jianmin Gao: 0000-0001-6385-583X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science and Technology Support Project (2014BAA07B03) and the National Natural Science Foundation of China (51576056).

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REFERENCES

(1) Ministry of Environmental Protection of the People’s Republic of China (MEP). 2014 Environmental Statistics Annual Report; MEP: Beijing, China, 2016. (2) Zhao, L. F.; Gao, J. M. 2011 Int. Conf. IEEE 2011, 2, 1892−1895. (3) Xu, L.; Gao, J. M. 2011 Int. Conf. IEEE 2011, 2, 901−904.

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