Effect of Pulverized Coal Preheating on NOx Reduction during

Electric Power Research Institute, State Grid Hunan Electric Power Corporation, Changsha, China 410007. Energy Fuels , 2017, 31 (4), pp 4436–4444...
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Effect of Pulverized Coal Preheating on NOx Reduction during Combustion Yanqing Niu,† Tong Shang,†,‡ Jun Zeng,†,§ Shuai Wang,† Yanhao Gong,† and Shi'en Hui*,† †

State Key Laboratory of Multiphase Flow in Power Engineering, School of Energy and Power Engineering, Xi’an Jiao Tong University, Shaanxi, China 710049 ‡ Xi’an Thermal Power Research Institute Company Ltd., Shaanxi, China 710054 § Electric Power Research Institute, State Grid Hunan Electric Power Corporation, Changsha, China 410007 ABSTRACT: Although various NOx removal technologies have been commercialized, NOx emissions are still so high that they cannot meet the newest standard and endanger the eco-environment and human health. Meanwhile, some NOx reduction technologies suffer from high costs. Recently, a cheap and pending technology called gas-fired coal preheating has surfaced again. This method introduces hot flue gas to heat a coal stream to high temperatures before the coal stream is injected into a utility boiler for combustion, thus resulting in a reduction in NOx. Considering the complex mechanisms during preheating, the effect of the preheating parameters, including temperature, residence time, excess air ratio, and coal type, are studied in a laboratory-scale drop-tube furnace in detail. Meanwhile, by means of Chemkin coupled with the mature NOx reduction mechanisms of GRImesh3.0 and Dagaut, as well as a CPD model, a detailed kinetics analysis on NO removal, including the migration and transformation of hydrocarbons and nitrogenous compounds during PC preheating, is performed. The research provides intuitive guidance, in practice, and a new scientific interpretation, in theory, for NOx reduction during PC combustion with preheating.

1. INTRODUCTION Coal is the principal energy source in China and plays a dominant role in the sustainable and rapid economic growth of China.1 However, coal combustion produces a large amount of NOx emissions, which induce the formation of smog and acid rain, as well as ozone depletion, severely endangering the ecoenvironment and human health.2 Although varied de-NOx technologies, such as furnace de-NOx technologies, including air staging, low NOx burning, reburning, cofiring, and exhaust gas recirculation, as well as postcombustion de-NO x technologies, such as selective noncatalytic reduction and selective catalytic reduction, have been employed singly or jointly, NOx emissions still do not meet the newest standard or NOx removal suffering from a high cost benefit ratio. The newly implemented emissions standard for thermal power plants in China (GB 13223-2011) expressly stipulates that the maximum exhaust value of NOx is 100 mg·m−3 for coal-fired boilers with a steam capacity of 65 t·h−1 and above.3 This standard forces power plants to seek more effective NO x reduction technologies, such as oxy-fuel combustion,4 moderate or intense low-oxygen dilution (MILD) combustion,5−7 and low-temperature catalysts.8,9 Recently, a pending technology called gas-fired coal preheating, which was proposed and verified by the All-Russian Thermal Engineering Institute (VTI) and subsequently further improved by the American Gas Technology Institute (GTI),10 has surfaced again.2,11,12 It is similar to MILD combustion, where recirculated exhaust gases containing a low oxygen concentration build a high-temperature and inert gases dilution zone inside the combustion chamber and result in low NOx emissions and improve thermal efficiency.5−7 The gas-fired coal preheating method introduces hot flue gas from gas combustion to heat the coal stream to a high temperature © XXXX American Chemical Society

before the coal stream is injected into a utility boiler for combustion, thus resulting in NOx reductions.10,13 Moreover, NOx reductions can be achieved without a loss of boiler efficiency and at more than a 25% lower levelized cost than state-of-the-art SCR technology.13 Related research on gas-fired coal preheating technology indicates that it can be coupled with other de-NO x technologies,2,11−13 such that the NOx reduction could reach 72% with a combination of the gas-fired coal preheating and air staging technology,11 while the residence time in the preheating zone is a key factor for NOx reduction.2,11 With prolonged residence time from ∼0.3 to ∼0.9 s, NO can be decreased by 21−37%.2 Meanwhile, as the principal NOx formation path below 1473 K, the conversion of fuel-N during coal combustion is essential.14,15 As shown in Figure 1, which shows the overall fuel-N-conversion pathways on the basis of the research of Chen et al.16 and Niksa and Cho,17 during the initial devolatilization process, partial fuel-N is mainly liberated as HCN and aromatic compounds (tar) and some remains in char; subsequently, part of the tar-N further converts into HCN. This process mainly occurs in the preheating stage. Then, the char-N meets oxygen and converts to NO directly or undergoes thermal dissociations.18 Meanwhile, HCN is either oxidized into NO that is subsequently reduced by hydrocarbons, soot, and char or directly reduces NO into N2. The coal type is directly related to N-conversion and subsequent NO removal. By means of a 35 kW drop-tube furnace coupled with the gas-fired coal preheating technology, Liu et al.2,11,12 Received: November 14, 2016 Revised: January 22, 2017 Published: February 22, 2017 A

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Energy & Fuels

Figure 1. Overall fuel-N to NO and N2 reaction pathways. Reprinted with permission from ref 11. Copyright 2014 American Chemical Society.

found that for high-volatility bituminous coals with a volatile content of 34.4% and 38.0% (on a dried ash free basis), the NO emissions could be reduced by 74% and 67%, respectively, while NO only decreased by 48% for low-volatility meagre coal with a volatile content of 16.75%. In addition, during coal preheating, a large number of nitrogenous intermediates such as HCN and NH3 and other species containing element N such as soot and hydrocarbons are yielded, which can effectively improve NOx reduction in subsequent combustion.2 However, in oxygen-rich conditions, the nitrogenous compounds are easily converted to NO, and most soot and hydrocarbons are also oxidized before reducing the NO to N22. Moreover, the release of nitrogenous intermediates and subsequent reactions are affected by temperatures. Thus, the oxygen content or air amount and temperature in the preheating process should also be of concern in practice. Consequently, during NOx reduction through pulverized coal (PC) preheating, the preheating temperature, residence time, oxygen content, and fuel properties have significant effects. However, the comprehensive and in-depth research on NOx reduction during PC preheating considering the above parameters is not well studied. Therefore, to provide guidance for practice, the effect of the preheating parameters, including temperature, residence time, excess air ratio (a ratio of actual air amount and theoretical air requirement for perfect combustion), and coal type, are studied in detail in a laboratory-scale drop-tube furnace. Meanwhile, by means of Chemkin coupled with the mature NOx reduction mechanisms of GRI-mesh 3.0 and Dagaut, as well as a CPD model, the NOx removal efficiency is compared with experiments, and a detailed kinetics analysis on the migration and transformation of nitrogenous compounds and hydrocarbons during PC preheating is also performed.

Figure 2. Schematic of the dual-segmented preheating and burning system. The drop-tube furnace (Luoyang Bolaimante Testing Electricity Furnace Co., Ltd., China) is heated by Si−C element and consists of two segments with independent heating controls (SHIMADEN SRS13A, Tokyo, Japan) and an approximate 300 mm flat-temperature zone, respectively. The furnace center is a 980 mm tall corundum tube with an internal diameter of 51 mm. The maximum temperature is 1473 K. The masterly quartz reactor where both PC preheating and burning are conducted is concentrically installed in the corundum tube. The reactor consists of an outer tube with a fixed inner diameter of 30 mm and a center tube with varied inner diameters of 6, 10, 12, and 15 mm. The pulverized coal is first carried into the center preheating tube and then injected into the outer tube for burning. The variable size center tube is designed according to the specified preheating residence time. When the Stokes number (St) is lower than 1.0, the traction force from the gas flow is predominant for a particle smaller than 100 μm, and the particle running velocity is approximately the same as the gas flow velocity.19 Thus, based on the ideal gas state equation, the residence times of preheating and burning are calculated according to eq 1, where τ represents the residence time, L and A denote the reactor length and cross-sectional area, respectively, Q is the standard gas flow, and T is the preheating or burning temperature.

τ=

273LAi Q (Ti + 273)

(1)

Figure 3 shows the temperature benchmark at different preheating temperatures (673, 773, 873, 973, and 1073 K) and the constant burning temperature (1473 K) in the upside and downside of the furnace. Clearly, two isothermal regions exist in the furnace, and the thermostatic preheating and burning zones are approximately 200 and 250 mm in height, respectively. Then, based on eq 1 and the selected tube size, four residence times in the preheating zone (0.17, 0.48, 0.72 and 1.08 s) and a constant residence time of 2 s for burning at 1473 K are settled. The preheating gas flow is fixed at 500 mL·min−1, and the burning gas flow (including the preheating gas) is constant at ∼800 mL·min−1 to maintain a burning residence time of 2.0 s. 2.2. Fuels. In the experiment, two typical coal types in China, Changzhi meagre coal and Huangling bituminous coal, are selected.

2. EXPERIMENTS 2.1. Experimental System. As shown in Figure 2, the experiments are conducted in a 4 kW two-segment electrical heating drop-tube furnace, which is connected by a homemade micro-PC feeder, a gas distribution system, and a testing system that measures the gas temperature and identifies and tests the flue gas components. During the experiments, the combustion gas is controlled by mass flow meters, and the flue gas is identified and measured by FTIR (Gasmet DX4000, Helsinki, Finland). To maintain a stable and precise PC feeding, an entrained flow microfeeder is employed. The microfeeder can feed PC at a rate of 0.1 g·min−1. B

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Energy & Fuels Table 2. Testing Conditionsa parameters

values

T/K τ/s α coal

773, 873, 973, 1073 0.17, 0.48, 0.72, 1.00 0, 0.2, 0.4, 0.6, 0.8 meagre, bituminous

T, τ, and α represent the preheating temperature, preheating residence time, and excess air ratio in the preheating zone, respectively. a

3. RESULTS AND DISCUSSION To quantify the NOx removal efficiency at different preheating temperatures, residence times, and excess air ratios, eq 2 is defined, where η is the NOx removal efficiency (unit, %); CNOx0 denotes the NOx content in the outlet of the reactor without preheating (unit, mg·m−3); and CNOx denotes the NOx content measured at the outlet of the reactor with preheating (unit, mg· m−3). For meagre coal and bituminous coal, the measured CNOx0 values were 1026.5 and 1379.6 mg·m−3 without preheating, respectively.

Figure 3. Temperature benchmark at specified preheating−burning temperature distributions. For example, 673−1473 K represents the settled temperatures in the furnace upside and downside, which are 673 K for preheating and 1473 K for burning, respectively.

⎛ C NO ⎞ η = ⎜⎜1 − 0 x ⎟⎟ × 100% C NOx ⎠ ⎝

The detailed proximate and ultimate analyses are listed in Table 1. It can be seen that the two coals present distinct components. Compared with the bituminous coal, the meagre coal contains less volatiles and more fixed carbon, ash, nitrogen, and sulfur. In the case of bridging in the storing tube and blockage in the feeding tube during the experiment because of the high moisture content and small size distribution, PC is first dried in a muffle furnace at 65 °C for 24 h and then sized to 61−75 μm for air isolated storage and further experiment. 2.3. Experiment Conditions and Procedure. To elucidate the effect of PC preheating on NOx reduction during combustion, as listed in Table 2, the effects of the preheating temperatures of 773, 873, 973, and 1073 K, preheating residence times of 0.17, 0.48, 0.72, and 1.00 s, and excess air ratios of 0, 0.2, 0.4, 0.6, and 0.8 in the preheating zone are comparably studied. During all experiments, the temperature in the burning zone is stable at 1473 K with a length of approximately 250 mm and the burning residence time is constant at 2 s, with a specified quartz reactor diameter of 30 mm. The fuel feeding is rated at 0.1 g·min−1, and the total air excess ratio is settled as 1.2. Moreover, all testing is strictly conducted according to following procedures. (1) Furnace temperature is adjusted to the desired temperature and kept stable for a half-hour. Then, pure N2 is flowed to clean the reactor twice. (2) PC is added into the storage tube which is fully vibrated to make sure that the PC in the storage tube is compacted; i.e., the PC level surface in the tube is unchanged with continued vibration. This is important for stable and continuous fuel feeding in the subsequent experiment. (3) FTIR is calibrated to eliminate environment error. Meanwhile, to consider the larger swept volume of FTIR, a flow meter is installed after the filter, and FTIR testing and recording begin when the experiment is conducted for approximately 10 min. (4) At the beginning, gas flows before PC injection; at the end, PC injection is stopped before the gas.

(2)

3.1. Effect of Preheating Temperature. Figure 4 shows the effect of the preheating temperature on NOx removal

Figure 4. Effect of preheating temperature on NOx removal efficiency: α = 0.2; τ = 0.17 or 1.00 s.

efficiency. It can be seen that with an increased preheating temperature, the NOx removal efficiency increases; meanwhile, as the preheating residence time increases, the NOx removal efficiency increases. Moreover, when the preheating temperature is raised to 973 K, the NOx removal efficiency increases rapidly. For meagre coal with a preheating residence time of

Table 1. Proximate and Ultimate Analyses of Coalsa ultimate anal/(wt %)

a

proximate anal/(wt %)

coal

Cad

Had

Nad

Oad

Sad

Mad

Aad

Vad

FCad

meagre bituminous

65.95 65.87

3.00 3.96

1.16 0.85

4.18 8.63

4.18 0.54

1.66 6.52

19.87 13.63

12.60 30.34

65.95 49.51

ad denotes air-dried basis. C

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Energy & Fuels 0.17 s, the NOx removal efficiency is 0.64% at 773 K, 3.2% at 873 K, and only increases to 5.68% at 973 K, but it increases by 2.6 times in comparison with that at 973 K and to 14.91% at 1073 K. Similarly, for the bituminous coal with a residence time of 0.17 s, the NOx removal efficiency is 2.57% at 773 K, climbs to 11.28% at 873 K, and rapidly increases from 14.23% at 973 K to 29.1% at 1073 K. Thus, the positive effect of a low preheating temperature such as 773 K on NOx removal is slight, but the NOx removal efficiency increases rapidly when the preheating temperature is above 973 K. Meanwhile, it can be seen from Figure 4 that with an increased preheating residence time, the NOx removal efficiency increases significantly. When the residence time is increased from 0.17 to 1.00 s, the NOx removal efficiency increases 2−4 times for meagre coal and 1.4−1.9 times for bituminous coal. With a preheating residence time of 1.00 s, the NOx removal efficiency of meagre coal can reach 29.1% at 1073 K, and it reaches 51.9% for bituminous coal. Namely, the NOx can be reduced by more than half for bituminous coal through preheating, and the reduction is approximately one-third even for meagre coal. During the preheating process, a larger number of reducing products such as hydrocarbons, hydrogen, and monoxide carbon are generated and accompany the production of NOx precursors (HCN and NH3)2. The NOx precursors not only can be reduced by the reducing products homogeneously but can also be reduced by char heterogeneously.15,20 Thus, the NOx removal efficiency increases with increased preheating temperature because of the increased yields of the reducing products and increased reactivity rate with the increase in temperature. Meanwhile, with low oxygen content in the preheating zone the oxidations of reducing products and NOx precursors are reduced and consequently improve reduction. Along with the increased temperature, the unstable aromatic compounds (tar) and nitrogen-containing functional groups may undergo hydrogenation pyrolysis and generate more nitrogenous volatiles.18 The formation of nitrogenous volatiles can improve NOx reduction. Thus, when the preheating temperature is above 973 K, the NOx removal efficiency increases rapidly. In addition, the increased available surface area attributed to high preheating temperature and subsequent high volatile release rate and yield amount improves the heterogeneous reducing reaction between char and NOx. Additionally, according to the Arrhenius expression, the increased preheating temperature accelerates the reaction rate. All of these result in increased NOx removal efficiency with an increase in the preheating temperature. Noted, as discussed in the following sections 3.2 and 3.3, enough residence time and proper oxygen are necessary for NOx reduction by the formation of nitrogenous volatiles. In comparison with bituminous coal, a lower NOx removal efficiency was obtained from tests carried out with meagre coal under the same testing conditions, mainly because of lower volatile content (as shown in Table 1). However, the meagre coal contains more aromatic compounds, which can be thermally decomposed at high temperature.21 Therefore, in comparison with bituminous coal, the NOx removal efficiency of meagre coal increases rapidly at high preheating temperatures. 3.2. Effect of Preheating Residence Time. Figure 5 shows the effect of the preheating residence time on the NOx removal efficiency with different preheating temperatures. It can be clearly seen that, with an increased residence time, the

Figure 5. Effect of preheating residence time on NOx removal efficiency: α = 0.2:T = 773, 873, 973, or 1073 K.

NOx removal efficiency increases. The extended residence time leads to a greater release of volatiles and more complete reactions. Meanwhile, taking 1.00 s, for example, the growth tendency of the NOx removal efficiency with increased preheating temperature becomes bigger when meagre coal is used as the fuel, while it becomes smaller for bituminous coal. The meagre coal contains more aromatic compounds, which undergo thermal decomposition at high temperatures21 and cause the rapid increase in the NOx removal efficiency with the increased preheating temperature. However, because of the lower volatile content in meagre coal, the NOx removal efficiency is lower than that of bituminous coal. 3.3. Effect of Excess Air Ratio in the Preheating Zone. Figure 6 shows the effect of the excess air ratio in the preheating zone on the NOx removal efficiency with different preheating temperatures. Clearly, the bituminous coal presents a higher NOx removal efficiency because of the high-volatility content. Meanwhile, when the preheating temperature (such as 873 K) is low, the NOx removal efficiency peaks at an excess air ratio of 0.2 for both meagre coal and bituminous coal.

Figure 6. Effect of the excess air ratio in the preheating zone on NOx removal efficiency: τ = 1.00 s; T = 873 or 1073 K. D

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Energy & Fuels However, when the preheating temperature is raised to 1073 K, with a peak point at the excess air ratio of 0.2, the NOx removal efficiency increases again with an increase in the excess air ratio from 0.3 to 0.8. For a low preheating temperature such as 873 K, when the excess air ratio is zero, the meagre coal and bituminous coal show NOx removal efficiencies of 8.17% and 23.11%, respectively, because of a strong reducing atmosphere which improves NOx reduction. With the increase in the excess air ratio to 0.2, the NOx removal efficiency reaches its peak (18.51% for meagre coal and 34.64% for bituminous coal). This result may be attributed to the increased active oxygen species O, which improves the formation of reducing compounds, such as H, CO, CHi, NH, and HCCO, as discussed in section 3.4. These reducing compounds easily promote NOx reduction. However, when the air excess air ratio is further increased, the excess oxygen enhances oxidation and results in low NOx removal efficiency. For a high preheating temperature such as 1073 K, the peak point at the excess air ratio of 0.2 is also attributed to the increased active groups (such as O, H, CO, CHi, NH, and HCCO), which improve the formation of reducing compounds and subsequent NOx reduction. However, the NOx removal efficiency is not maximized with the increased excess air ratio, and it tends to increase with an increase in the excess air ratio from 0.3 to 0.8. This result may be because the coal char begins to burn at 1073 K and excess air ratio of 0.3, which generates more CO because of oxygen depletion and improves NOx reduction. 3.4. Reaction Kinetics Analysis. To elucidate the effect of PC preheating on NOx reduction and the removal paths in theory, the SENKIN programming model and PSR (perfect stirred reactor) model in Chemkin is employed. Two PSRs are used to model the preheating and burning processes. There exist numbers of NOx reduction mechanisms, such as GRImesh3.0, GADM98,22 that proposed by Dagaut and Lecomte,23 and the one by Miller and Bowmn.24 Among them, the GRImesh3.0 mechanism considers the number of reactions of CH4 and NOx, and the Dagaut mechanism contains the detailed NO reducing mechanisms by C1−C4. Thus, comprehensive NO removal mechanisms based on GRI-mesh3.0 and Dagaut are used here. Meanwhile, we take bituminous coal as representative and directly employ the pyrolysis gases calculated on the basis of the CPD model as the initial input data in the preheating process, rather than PC properties. 3.4.1. Benchmark. Figure 7 shows the comparisons of the measured and simulated NO x removal efficiencies of bituminous coal under different preheating parameters. It can be seen that except for a slight overprediction, the tendency of the simulated data is approximate with the measured values. The slight overprediction is caused by the modeling hypothesis that all fuel-N is assumed to be released as NO completely and then input as an initial gas compound in the preheating PSR. Meanwhile, it can be seen that the NOx removal efficiency is positively dependent on the release of volatiles. With the increased temperature (Figure 7a) and residence time (Figure 7b), the ratio of volatile release increases and results in the increased NOx removal efficiency. However, when the preheating temperature is above 1073 K, the increase in the NOx removal efficiency becomes slow; and when the preheating residence time is over 1.0 s with a preheating temperature of 1073 K, the release ratio of the volatiles changes little or the releasing stops, resulting in almost unchanged NOx

Figure 7. Comparisons of the measured value and simulated value of the NOx removal efficiency with different preheating parameters (bituminous coal): (a) preheating temperature, α = 0.2, and τ = 1.00 s; (b) preheating residence time, α = 0.2, and T = 1073 K; (c) excess air ratio in preheating zone, τ = 1.00 s, and T = 873 K.

removal efficiency. Therefore, when the preheating temperature is above 1073 K and the residence time is over 1.0 s, they both are no longer the dominant factor for NOx removal; i.e., in practice, an overextended preheating residence time over 1.0 s and continuously increased preheating temperature above 1073 K are not valuable. Additionally, the modeling presents that E

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Energy & Fuels there exists an optimal excess air ratio of 0.2 in the preheating zone (Figure 7c). During the NOx removal process, the reactions between hydrocarbons and volatile nitrogenous compounds are essential. Thus, a detailed mechanism analysis of the migration and transformation of the hydrocarbons and volatile nitrogenous compounds during preheating is performed on the basis of the rate of production analysis (ROP) in Chemkin. As a typical representative, the bituminous coal with a preheating temperature 1073 K, a preheating residence time of 1.00 s, and an excess air ratio in the preheating zone of 0.2 is used for ROP analysis. 3.4.2. ROP Analysis of the Volatile Nitrogenous Compounds. ROP analyses of the dominant reaction paths of the primary volatile nitrogen-containing compounds during PC preheating are summarized in Table 3. It can be seen that NO is mainly reduced by reactions with NH, H, CH2, CH3, CO, and HCCO (reactions R1−R6). During the reduction of NO, H is the primary reducing compound (reaction R2, ROP(NO) = −0.259), HCCO is the dominant hydrocarbon (reaction R5, ROP(NO) = −0.120), and NH is the dominant nitrogenous compound (reaction R1, ROP(NO) = −0.016). Meanwhile, NO reduction by hydrocarbons (reactions R3−R5, ROP(NO) = −0.164) is more significant than that by nitrogenous compounds (reaction R1, ROP(NO) = −0.016). As a major NOx precursor from PC devolatilization, HCN is mainly originated from the reduction of NO by hydrocarbons (reactions R7 and R10), with CH3 as the dominant active group (reaction R7, ROP(HCN) = 0.629). In addition, the reduction of HCNO (reaction R8) and decomposition of H2CN (reaction R9) is non-negligible for HCN formation. HCN can be reduced to N2 during preheating. However, HCN is not reduced to N2 by hydrocarbons and CO directly; it is first oxidized into NCO, NH, HNCO, and CN by reactions R11−R14, and NCO is the major oxidation product (reaction R11, ROP(HCN) = −0.530). Additionally, during both the formation and oxidation of HCN, O, H, OH, CH2, and CH3 are essential active groups. As the major oxidation product of HCN (reaction R15, ROP(NCO) = 0.905), NCO is mainly reduced to HNCO by reactions R16 and R17. The reduction of NCO by H2 is dominant (reaction R16, ROP(NCO) = −0.883). Once HNCO is formed by the reduction of NCO or the conversion of HCNO (reaction R19, ROP(HCNO) = −0.879), it can be rapidly reduced to NH2. Acting as another principal NOx precursor, NH3 is mainly converted into NH2 by reactions R22 and R23. During the conversion process, the OH active group plays an absolutely dominant role (reaction R22, ROP(NH3) = −0.924). Thus, the presence of OH is the key factor for the conversion of NH3. Then, the active groups of NH2 and NH interconvert. Although most NH is converted back to NH2 (reaction R24, ROP(NH) = −0.892), a considerable amount of NH is converted into N2O and N2 by reaction R25 and R26. Finally, most of the N that originated from the reduction of NO by CO (reaction R27, ROP(N) = 1.000) reacts with the remaining NO and is converted into N2 (reaction R28, ROP(N) = −0.339). Meanwhile, N2O is reduced as the main generation path of N2 (reaction R31, ROP(N2) = 0.417; reaction R32, ROP(N2) = 0.272), and the remaining NO reacts with NH and is also converted to N2 (reaction R33, ROP(N2) = 0.232).

Table 3. ROP Analysis of the Main Volatile Nitrogenous Compounds nitrogenous NO

reaction

ROP (unit, 1)

(R1) H + NO + M ↔ HNO + M (R2)

−0.016

CH 2 + NO ↔ H + HNCO

−0.014

NH + NO ↔ N2O + H

(R3)

(R4) HCCO + NO ↔ HCNO + CO (R5)

−0.030

N + CO2 ↔ NO + CO

−0.003

CH3 + NO ↔ HCN + H 2O

HCN

(R6)

CH3 + NO ↔ H 2O + HCN

(R7) (R8)

HCNO + H ↔ OH + HCN

H + HCN(+ M) ↔ H 2CN(+ M)

(R9)

CH 2 + NO ↔ OH + HCN

(R10) (R11) HCN + O ↔ NCO + H HCN + O ↔ NH + CO (R12) HCN + OH ↔ HNCO + H (R13)

CN + H 2O ↔ HCN + OH NCO

(R14) (R15)

HCN + O ↔ NCO + H HNCO + H ↔ H 2 + NCO

HCNO

(R16)

N2

0.191 0.124 0.046 −0.530 −0.132 −0.091 −0.040 0.905 −0.883 −0.112

HCCO + NO ↔ HCNO + CO

(R18) (R19) (R20)

−0.879

0.981 −0.076

HCNO + H ↔ NH 2 + CO

(R21)

−0.046

NH3 + OH ↔ NH 2 + H 2O

(R22)

−0.924

NH3 + O ↔ NH 2 + OH

N

0.629

(R17)

HCNO + H ↔ OH + HCN

NH

−0.120

HNCO + OH ↔ NCO + H 2O

HCNO + H ↔ H + HNCO

NH3

−0.259

(R23)

−0.076

NH 2 + H ↔ NH + H 2

(R24)

−0.892

NH + NO ↔ N2O + H

(R25)

−0.057

NH + NO ↔ N2 + OH

(R26)

−0.017

N + CO2 ↔ NO + CO

(R27)

1.000

N + NO ↔ N2 + O

(R28)

−0.339

N + O2 ↔ NO + O

(R29)

−0.153

NH + H ↔ N + H 2

(R30)

−0.205

N2O + H ↔ N2 + OH

(R31)

N2O(+ M) ↔ N2 + O(+ M)

NH + NO ↔ N2 + OH

(R32)

(R33)

0.417 0.272 0.232

Based on the above analysis, a flowchart of the primary reaction paths is presented in Figure 8. It can be clearly seen that the reduction of HCN and NH3 during preheating is first converted into intermediates such as HCNO, HNCO, NCO, and NH2 and then they are further converted into NH, which reduces NO into N2. In addition, as the reaction product of hydrocarbons, CO play an important role in NO reduction. Although the reduction of NO by CO is relatively slight (reaction R6, ROP(NO) = −0.003), N as the reaction product F

DOI: 10.1021/acs.energyfuels.6b02984 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

Figure 8. Flowchart of the primary reaction paths of volatile nitrogenous compounds during preheating.

can further reduce NO by reaction R28 (ROP(N) = −0.339), which amplifies the reducing action of CO or hydrocarbons. Moreover, it can be seen that during the conversion of volatile nitrogen-containing compounds, O, OH, and H are essential active groups, and the NO reduction by hydrocarbons is more significant than that by nitrogenous compounds. 3.4.3. ROP Analysis of the Hydrocarbons. During the migration and transformation of NO during the preheating process, the active groups, such as H, CH2, CH3, CO, and HCCO, are important. Thus, ROP analyses of the dominant reaction paths of the hydrocarbons during PC preheating are summarized in Table 4. It can seen that CH4 is mainly from the hydrogenation reaction of CH3 (reaction R34, ROP(CH4) = 0.550, and reaction R35, ROP(CH4) = 0.350). Conversely, CH4 is also mainly converted into CH3 by reaction R37 (ROP(CH4) = −0.565) and reaction R38 (ROP(CH4) = −0.435). The active groups such as H, H2, OH, and O are necessary during the interconversion of CH3 and CH4. Although approximately 100% of CH4 is converted into CH3, the production is slight. Most CH3 is from the decomposition of C3H7 (reaction R39, ROP(CH3) = 0.468) and the hydrogenation reaction of CH2OH (reaction R40, ROP(CH3) = 0.224). The decomposition of C2H5 and C3H8 by reactions R41 and R42 is also non-negligible; i.e., CH3 is mainly from the decomposition of C2 and C3 compounds. Inversely, CH3 also can be converted into C2H6 (reaction R43, ROP(CH3) = −0.580). CH3 is not only the intermediate of the hydrocarbon reaction chain but it is also the bridge of the conversions of C1 and C2 and C3. As seen in Table 4, CH2 is mainly from the oxidation of C2H2 (reaction R45, ROP(CH2) = 0.670) and the decomposition of CH2CO (reaction R46, ROP(CH2) = 0.330). It can react with NO directly (reaction R49, ROP(CH2) = −0.036) or be converted into CO (reaction R48, ROP(CH2) = −0.125) and CH3 (reaction R47, ROP(CH2) = −0.839), which subsequently reduce NO by reactions R4 and R6. Moreover, accompanied by the consumption of CH2, the generated H and OH are important active groups for NO reduction, as described in section 3.4.2. Thus, NH2 is a key intermediate for NO reduction (reaction R3). HCCO is only from the oxidation of C2H2 (reaction R50, ROP(HCCO) = 1.000), and the HCCO is mainly converted into CH2CO (reaction R51, ROP(HCCO) = −0.905) and CO (reaction R52, ROP(CH2) = −0.064). CO and the remaining

Table 4. ROP Analysis of the Main Hydrocarbons hydrocarbons CH4

ROP (unit, 1)

reaction

CH3 + H 2 ↔ H + CH4

(R34)

H + CH3(+ M) ↔ CH4(+ M) CH3 + C2H4 ↔ C2H3 + CH4

OH + CH4 ↔ CH3 + H 2O O + CH4 ↔ OH + CH3 CH3

0.350

(R36)

0.100 −0.435

(R38)

H + CH 2CO ↔ CH3 + CO

(R39)

(R40)

2CH3(+ M) ↔ C2H6(+ M)

0.162

(R42)

HO2 + CH3 ↔ OH + CH3O

(R44) (R45)

CH 2 + O2 ↔ OH + H + CO

HCCO

(R46)

(R48) (R49)

O + C2H 2 ↔ H + HCCO

(R50)

H + CH 2CO ↔ HCCO + H 2

−0.125 −0.036 1.000

(R51)

−0.905

(R52) HCCO + NO ↔ HCNO + CO (R53)

−0.061

OH + CH 2CO ↔ HCCO + H 2O

−0.014

HCCO + O2 ↔ OH + 2CO

C

0.330 −0.839

(R47)

CH 2 + NO ↔ H + HCNO

−0.420 0.670

CH 2 + CO(+ M) ↔ CH 2CO(+ M)

CH 2+ H 2 ↔ H + CH3

0.149 −0.580

(R43)

O + C2H 2 ↔ CO + CH 2

0.468 0.221

(R41)

CH3 + C2H 5(+ M) ↔ C3H8(+ M)

CH2

−0.565

(R37)

CH3 + C2H4(+ M) ↔ C3H 7(+ M) 2CH3 ↔ H + C2H5

0.550

(R35)

(R54)

−0.020

C + O2 ↔ O + CO

(R55)

−0.659

H + CH ↔ C + H 2

(R56) (R57)

−0.268

C + NO ↔ CO + N

C + CH3 ↔ H + C2H 2

(R58)

−0.034 −0.017

HCCO further react with NO. Thus, HCCO is also an important reducing intermediate of NO (reaction R5). For the reduction of char carbon, C is mainly converted into CO in the presence of O2 (reaction R55, ROP(C) = −0.659) G

DOI: 10.1021/acs.energyfuels.6b02984 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels and gaseous hydrocarbons (reaction R56, ROP(C) = −0.268, and reaction R58, ROP(C) = −0.017). Thus, the reducing action of C on NO is mainly performed with the conversion to CO and gaseous hydrocarbons. Meanwhile, NO can be reduced by C directly, but the action is slight (reaction R57, ROP(C) = −0.034). The heterogeneous reduction of NO by C mainly occurs in the burning zone rather than in the preheating zone. Similarly, a flowchart of the primary reaction paths of hydrocarbons during preheating is presented in Figure 9.

sequently resulting in a rapid increase in the NOx removal efficiency at high preheating temperatures. However, in comparison with bituminous coal, a lower NOx removal efficiency occurs mainly because of the lower volatile content. During the experiment, NOx can be reduced by more than half for bituminous coal through preheating, and the reduction is approximately one-third even for meagre coal. (3) The kinetics modeling results agree well with the experimental data. ROP analysis shows that the reduction of HCN and NH3 during preheating is first converted into intermediates, such as HCNO, HNCO, NCO, and NH2; then, they are further converted into NH, which reduces NO into N2. The hydrocarbons are also first transformed as reducing intermediates, such as CH3, CH2, HCCO, and CO, which subsequently react with NO. NO reduction by hydrocarbons is more significant than that by nitrogenous compounds. (4) During PC preheating, the active groups, including O, H, and OH, are essential for the conversion of volatile nitrogenous compounds and hydrocarbons. A certain amount of oxygen in the preheating zone can improve NOx reduction (such as an excess air ratio of 0.2 in the preheating zone).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: 86-13709181734. Fax: 86-029-82668703.

Figure 9. Flowchart of the primary reaction paths of hydrocarbons during preheating.

ORCID

Yanqing Niu: 0000-0001-7267-9305 During preheating, undergoing interconversion, the hydrocarbons such as CH4 and CnHm are transformed as reducing intermediates CH3, CH2, HCCO, and CO, which cause NO reduction. Meanwhile, it can be seen that during the interconversion, O, H, OH, and O2 are essential active groups, especially oxygen, which improves the oxidation of C, CH2, and HCCO to generate the stronger reduction of CO. CO accelerates NO reduction. Thus, the NO removal efficiency is high with a certain amount of oxygen in the preheating zone.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The present work was supported by the National Natural Science Foundation of China (Grant No. 51406149) and the National Key Research and Development Program of China (Grant No. 2016YFC0801904).

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4. CONCLUSIONS Aiming at complex NOx reduction mechanisms during PC combustion with preheating, the effect of the preheating parameters, including temperature, residence time, excess air ratio, and coal type, are studied in a laboratory-scale drop-tube furnace in detail. Meanwhile, by means of the Chemkin coupled with mature NOx reduction mechanisms from GRI-mesh3.0 and Dagaut, as well as a CPD model, a detailed kinetics analysis on NOx reduction and a ROP analysis on the migration and transformation of nitrogenous compounds and hydrocarbons during PC preheating were performed. This research provides scientific guidance for NOx reduction during combustion with PC preheating. (1) The experimental results show that the NOx removal efficiency is positively dependent on the preheating temperature and residence time. With increases in the preheating temperature and residence time, the volatile releasing ratio increases and results in an increased NOx removal efficiency. However, when the preheating temperature is above 1073 K and the residence time is longer than 1.0 s, they are both no longer the dominating factors for NOx reduction. Meanwhile, there is an optimal excess air ratio of 0.2 in the preheating zone. (2) Meagre coal contains more aromatic compounds that can be thermally decomposed at high temperatures, and con-

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DOI: 10.1021/acs.energyfuels.6b02984 Energy Fuels XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.energyfuels.6b02984 Energy Fuels XXXX, XXX, XXX−XXX