Reaction mechanism for sulfur species during pulverized coal

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Reaction mechanism for sulfur species during pulverized coal combustion Honghe Ma, Lu Zhou, Suxia Ma, Zhijian Wang, Zhigang Cui, Wei Zhang, and Jun Li Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03868 • Publication Date (Web): 23 Jan 2018 Downloaded from http://pubs.acs.org on February 8, 2018

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Reaction mechanism for sulfur species during pulverized coal combustion Honghe Ma1*, Lu Zhou1, Suxia Ma1, Zhijian Wang2, Zhigang Cui1, Wei Zhang1, Jun Li1 1

Department of Thermal Engineering, Taiyuan University of Technology, Taiyuan, Shanxi 030024, China

2

State Kay Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Science, Taiyuan, Shanxi 030001, China

*CORRESPONDING AUTHOR: E-mail: [email protected], Tel.: +86 0351 6010281. KEYWORDS: Reaction mechanism; Sulfur species; Pulverized coal combustion; Kinetic ABSTRACT: Low-NOx combustion technologies are widely applied in pulverized coal-fired boilers. But it promotes the formation of high concentration of H2S, which is one of the main reasons for high temperature corrosion. In order to limit the H2S formation, it is urgently necessary to reveal the evolution behavior of the sulfur species. In this work, the reaction mechanism for sulfur species was investigated using a tube-heating furnace for low-sulfur bituminous coal combustion. In the primary stage of combustion, the O2 concentration decreased sharply. Meanwhile, the sulfur species of SO2, H2S, COS and CS2, and significant amount of reductive gases CO and H2, were generated. After the sulfur release finished, the distribution of the sulfur species in the downstream region depended on only the gas-phase reactions. With the

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reduction of CO and H2, part of SO2 was converted to H2S and COS. There also exited some shift relationships among SO2, H2S, COS and CS2 in the presence of abundant of CO2 and H2O. Based on the experimental results and the principles of Gibbs free energy and chemical equilibrium constant, a new gas-phase reaction mechanism for sulfur species, consisting of 9 reactions, was established. Furthermore, the kinetic parameters were also determined by a strict mathematical optimization process, and the predication errors for sulfur species were within 20%. The new built mechanism was expected to provide great assistance for the control of H2S formation and the prevention of the high temperature corrosion. 1. INTRODUCTION Currently, low-NOx combustion technologies, such as low-NOx burners, and fuel/air staged combustion, etc., are widely applied in pulverized coal-fired boilers as a primary measure to control NOx emissions, because of the low cost and high efficiency.1,2 The basic design idea of these low-NOx combustion technologies is to establish fuel-rich zones so that a part of formed NOx is reduced because of the oxygen deficient condition.3,4 However, a strong reducing atmosphere is also formed in such fuel-rich zones, and it promotes the formation of H2S.5,6 High concentration of H2S is one of the main reasons for high temperature corrosion,7-10 especially in supercritical and ultra-supercritical boilers.11,12 In order to control the H2S formation and prevent the corrosion, it is urgently necessary to reveal the reaction mechanism of the sulfur species during pulverized coal combustion. Sulfur species evolution during coal combustion mainly includes two primary processes: the sulfur release behavior from coal and the subsequent sulfur species gas-phase reactions.13 During sulfur release process, the major sulfur species are converted into the gas phase as the forms of SO2, H2S, COS, and CS2. Then, the subsequent sulfur species gas-phase reactions strongly affect


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the H2S distribution in the boiler.14 Nowadays, a series of studies have been carried out on the reaction characteristic of sulfur species during coal combustion, and the obtained mechanisms are summarized in Table 1. Zhang et al.15 found that part of SO2 was converted to H2S and COS with the reduction effect of CO and H2, and thus the extremely high concentrations of H2S and COS were formed. Tsuji et al.16 observed the H2S concentration distribution had a strong correlation with H2, and proposed the reaction of R1 to describe the formation of H2S. Zhang et al.17 found that H2S was oxidized to SO2 in the oxidizing atmosphere, while SO2 was shifted into COS and H2S by the reductive gases of CO and H2. These processes were described by the reactions of R2-R4. Frigge et al.18 also suggested R3 as one of the gas-phase reactions of sulfur species. Moreover, Abián et al.19 established R6 as the reaction mechanism between COS and H2O, and also R7 between CS2 and H2O, respectively. These studies provide very important reference value for understanding the reaction behavior of sulfur species, while they mainly focus on the reaction characteristics between a single reductive gas and a single sulfur species. In fact, the real sulfur species gas-phase reactions are very complex with the simultaneous actions of various gases, including O2, CO, H2, H2O, SO2, H2S, COS, and CS2. Zhang et al.20 developed a global sulfur species gas-phase reaction mechanism, considering the possible reactions among O2, CO, H2, H2O, SO2, H2S, and COS, which are also listed in Table 1. However, the Gibbs free energy variations (∆Gr) of R9-R12 all exceed 0 at the reaction temperature of 1000-1400°C, the typical temperature range of the flue gas in the furnace. As well known, in the condition of isothermal and isobaric, if ∆Gr is above 0, the reaction can be conducted spontaneously. Conversely, the reaction cannot occur. Therefore, the reactions of R9-R12 may not occur during pulverized coal combustion. In fact, it is insufficient to verify the feasibility of the reaction mechanism only based on ∆Gr. The chemical equilibrium constant of each reaction should also


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be calculated from the partial pressure of the components in the experimental results, and compared with the theoretical value. If the calculated value is always less than the theoretical value, it can be confirmed that the reaction can be carried out. Table 1. Furthermore, Leeds University,21 Cerru et al.

22

and Zhou et al.23 have built the detailed

reaction mechanisms to describe the gas-phase reactions of sulfur species. In order to make it suitable for the pulverized coal combustion case, Bohnstein et al.24 updated the sulfur chemistry mechanism of Leeds University. Among these mechanisms, hundreds of elementary reactions and more than 10 sulfur species are considered. Thus, if directly coupling these mechanisms into the computational fluid dynamic model (CFD), it is not only greatly wasteful for the computing resources, but also still prohibitive for the full-scale furnaces and burners that are characterized by millions of computational cells.20 Therefore, very simple sulfur species reaction mechanisms are used in many studies. For example, Müller et al.14 adopted one global reaction to simulate the oxidation of H2S by O2 during coal combustion. However, the predication accuracy from the reduced mechanism may be not high especially in the low-NOx combustion.20 Apart from the reaction formulas, the kinetic parameters are also necessary for the application of reaction mechanism in CFD. Although the detailed reaction mechanisms contained the kinetic parameters of each elementary reaction, as mentioned above, they are hardly directly adopted in the numerical simulation of the full scale of furnaces and burners. Thus, the kinetic parameters are still needed to be determined when a novel reaction mechanism is established. Fortunately, a detailed mathematical optimization method is provided to determine the kinetic parameters,20 which has important guiding significance for the subsequent research.


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The main aim of this study is to establish a new global sulfur species gas-phase reaction mechanism based on a large amount of experimental data. Then, the feasibility of the reaction formulas is judged by the principles of Gibbs free energy and chemical equilibrium constant. Finally, the kinetic parameters are determined via a mathematical optimization process. The new mechanism should comprehensively, simply and accurately describe the evolution of sulfur species, and provide the basis data for the further development of the detailed mechanism in pulverized coal combustion. 2. EXPERIMENTS 2.1. Coal samples In China, the bituminous coal is extensively used in utility boilers for power generation. In this work, three bituminous coals, Bailu coal (BL), Daheng coal (DH), and Yunfei coal (YF), were selected due to the different sulfur contents and fuel ratios. The properties of coals are shown in Table 2. The fuel ratio is the ratio of fixed carbon to volatile matter, and widely applied as an indicator of coal rank. The combustibility of coal increases with the fuel ratio. The fineness of all the three coal samples was R90=4%. Here, R90 is the particle-size distribution coefficient, defined as the ratio of the weight of pulverized coal whose diameters greater than 90µm to the total weight of the sample. A uniformity coefficient of n=1.1 was used according to the sieve analysis performances of the bituminous coal provided by the miller for boiler use. Table 2. 2.2. Experimental apparatus and methods The schematic diagram of the pulverized coal combustion furnace with the electrical heating power of 18kW is shown in Figure 1. The furnace was a vertical and cylindrical tube made of alumina. The tube was 2200mm in length and 150mm in inner diameter, and heated to the


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desired temperature by 32 silicon carbide rods. The low-NOx burner had a coaxial dual-piping structure, with a main burner port (inner diameter 8mm) and an annular slit (width 3mm) installed outside the main burner port. The center of the burner exit was regarded as the origin, and Z referred the axial distance from the origin. A movable vertical sampling probe equipped with a water-cooled jacket was used to freeze the combustion at any wanted location.

Figure 1. Schematic of the tube furnace platform. 2.3. Experimental conditions and measurement During the experiments, the total excess air ratio (αt), primary air ratio (αp), and secondary air ratio (αs), were determined by Eq. (1)-(3), respectively.

αt =

αp =

Ft Ft0

Fp Ft0

(1)

(2)


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Fs Ft0

αs =

(3) (4)

α t =α p + α s

Where, Fto is the theoretical air flow rate for pulverized coal combustion, kg·h-1; Ft is the actual total excess air flow rate, kg·h-1; Fp is the primary air flow rate, kg·h-1; Fs refers to the secondary air flow rate, kg·h-1. The experiment conditions are summarized in Table 3. Table 3. Next, Rs,Z is the average ratio of sulfur released from the coal to the gas phase, and defined by Eq. (5).

Rs, Z =

Ws, Z × M S × 0.001kg / g Fcoal × S t × 0.01

(5)

Here, Ms is the molar weight of sulfur, 32g·mol-1; Fcoal is the feed rate of coal, kg·s-1; St is the total sulfur content of the coal, wt%; Ws,Z is the average molar flow of sulfur contained in the flue gas, mol·s-1, and calculated by Eq. (6).

Ws,Z =

QV, Z × p × (CSO2 , Z + CH2S, Z +CCOS, Z ) ×10−6 R × (T + 273)

(6)

Where, QV,Z is the volume flow rate of the flue gas through the cross section of Z, m3·s-1; p is the actual pressure in the reactor tube, 91920Pa; CSO2,Z, CH2S,Z, CCOS,Z, and CCS2,Z are the average concentration of SO2, H2S, COS, and CS2 at the position of Z, respectively, ppm; R is the gas constant, J·K-1·mol-1; T is the reaction temperature, °C. The measured gas components were O2, CO, CO2, H2, SO2, H2S, COS and CS2. The gas sampling probe was inserted into the furnace and connected to analyzers via silica fiber filters, which removed particles from the sampled gas. O2, CO, CO2, and H2 were detected by a gas chromatography system with thermal conductivity detectors (SP 3420A), and SO2, H2S, COS


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and CS2 were measured by another gas chromatography system (Agilent 3000A Micro GC). The gas from the sampling probe was sampled in a Teflon bag, where the temperature was kept above 100°C to prevent the condensation of moisture. The H2O concentration was obtained from the mass of moisture absorbed by CaCl2. For clear comparison, all gas concentrations were converted on a 6% O2 dry basis. Each test was conducted twice. When the operating conditions were stable enough, the sampling was performed and measured twice. The average result was plotted as the final value. In addition, the Gibbs free energy change and chemical reaction equilibrium constant of each reaction were calculated by HSC Chemistry 6.0. 3. RESULTS AND DISCUSSION 3.1. Determination of the initial position for gas-phase reactions As mentioned above, the sulfur release from coal into the gas phase and the subsequent gasphase reactions constitute the sulfur evolution process during coal combustion. Therefore, in order to determine the initial position of the gas-phase reactions, the sulfur release behavior of different coals was investigated. As shown in Figure 2, the sulfur release ratios of the three coals increased with the axis distance from burner exit Z, while the increasing amplitude became lower and lower. The ultimate ratio of sulfur release by DH was approximate 87%, and those by BL and YF were less than 66% and 77%, respectively. This may have a connection with the sulfur forms contained in coal. The main forms of sulfur contained in BL and DH were pyritic sulfur and organic sulfur, respectively. Thus, it indicates that organic sulfur was more readily released than pyritic sulfur.25,26 YF had a comparable content of pyritic sulfur and organic sulfur. That is why the ultimate value of sulfur release ratio by YF was between that of BL and DH. Furthermore, it was found that the sulfur release ratios from the three coals all reached their individually ultimate value after Z = 900mm. This means that the release of sulfur all finished at


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this position. So, this position could be regarded as the starting point of gas-phase reactions,13 and the gas composition of this position was assumed as the initialization of the gas-phase reactions. Then, the sulfur species distribution in the downstream region relied on only the gasphase reactions.

Figure 2. Average sulfur release ratio as the functions of axis distance from the burner (T=1200°C, αb=0.8). 3.2. Distribution characteristics of sulfur species in the downstream region The distribution of main sulfur species and possible reactants along the axis of the furnace for DH is shown in Figure 3. Because the combustion was carried out in an oxygen-deficient atmosphere, that is, αb=0.8, the O2 concentration decreased rapidly and disappeared at the initial stage of pulverized coal combustion. Meanwhile, appreciable amounts of CO and H2 were generated via the gasification reaction, and the H2 trend exhibited some similarity to CO. These reductive gases promoted the formation of H2S and COS converted from SO2.27 This is also can be concluded by the phenomenon that the concentration profiles of H2S and COS followed closely with each other and with that of CO and H2. Where the H2S and COS concentrations were high, the SO2 and O2 concentrations were low. Such trends were consistent with the thermodynamic expectation for sulfur species in the combustion gases.12 Moreover, the SO2


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concentration increased initially and then decreased slightly. The initial increase was the combination effect of sulfur released and conversion from H2S, COS and CS2. The subsequent decrease should be explained by the reduction of SO2 into H2S and COS. In addition, the profiles of CO2 and H2O were always kept at a high level. This is because they were mainly generated from the combustion of pulverized coal, and the gas-phase reactions of sulfur species had little impact on their concentration distributions.

Figure 3. Distribution of gaseous products along the axis of the furnace (DH, T=1200°C, αt=0.8). Figure 4 illustrates the variations of gaseous products at the measuring port of Z=1200mm for various air ratios. In fuel-rich conditions (αt < 1.0), CO and H2 increased and CO2 decreased with decreasing excess air ratio due to the lack of oxygen. Accordingly, SO2 dropped and H2S, COS and CS2 rose. At stoichiometric conditions (αt = 1.0), CO2 formed a maximum, while CO was almost zero. Meanwhile, SO2 also reached the maximum, and H2S, COS and CS2 approached zero. Furthermore, the SO2 concentration increased rapidly, and the H2S, COS and CS2 concentrations decreased quickly. This indicates that H2S, COS and CS2 were converted fast to SO2 under the oxidizing conditions.24 Under fuel lean conditions (αt > 1.0), both CO2 and SO2 dropped because of the dilution effect of excess air, while CO, H2, H2S, COS and CS2 were


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further oxidized to zero. These results again suggest that the reductive atmosphere was essential for the existence of corrosive gas H2S, COS and CS2.

Figure 4. Distribution of gaseous products as functions of total excess air ratio (DH, T=1200°C, Z=1200mm). The gaseous product concentrations at the measuring port of Z=1200mm for various reaction temperature is stated in Figure 5. With the increase of the furnace reaction temperature, the concentrations of all gaseous products were raised to some content. This is because the higher temperature could promote more gaseous products released from pulverized coal.25 According to the thermo-kinetic analysis, the higher temperature could enhance the conversion of H2S and COS to SO2. So, the increase of SO2 was the combination effect of more sulfur release from coal and the shift from H2S and COS. However, the shift effect was weaker than that of the release effect from the pulverized coal, thus the concentrations of H2S and COS also rose.


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Figure 5. Measured gas product concentrations at the measuring port of Z=1200mm for various reaction temperature (DH, αt=0.8). Figure 6 shows the impact of coal properties on the gas products distributions at the measuring point of Z=1200mm. The fuel ratios of DH and BL were similar, which means that the concentrations of the reductive gases formed in these two cases were close to each other. This also can be seen from the distributions of CO and H2. However, the distributions of H2S and SO2 varied greatly. It may have a correlation with the sulfur form contained in the coal samples. The main sulfur forms in BL and DH were pyritic and organic sulfur, respectively. Organic sulfur was in favor of the generation of SO2,28 while pyritic sulfur was favorable of the formation of H2S.29 Thus, in the stage of sulfur release, more SO2 was produced in DH case while more H2S was released in BL case. Afterward, in the gas-phase reaction process, the H2S concentration in DH case rose much quickly, although the ultimate value was still lower. Accordingly, the SO2 concentration in DH case dropped faster. This is because the H2S concentration was far less than the value under the gas-phase reaction reaching equilibrium. Both H2S and SO2 concentrations in YF case were in the middle level, due to the moderate content of pyritic and organic sulfur. Moreover, although the total sulfur content of YF was 1.6 times of that of BL, the ultimate value


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of H2S concentration was not significantly higher than that in BL case. This could be explained by the higher fuel ratio of YF, which led to the weaker combustibility. Therefore, less amount of H2 and CO was generated during combustion, which resulted in less SO2 converted to H2S.

Figure 6. Measured gas product concentrations at the measuring port of Z=1200mm for various coal type (T=1200°C, αt=0.8). 3.3. Gas-phase mechanism for sulfur species Both COS and CS2 were easily converted to H2S with the participation of H2 or H2O, so they were also considered as the corrosive sulfur species as well as H2S. As expected, the sum of the corrosive gases concentrations should be strongly related with the concentrations of the reductive gases of CO and H2.15 In this work, the relationship between the concentrations of reductive gases and the concentrations of the corrosive gases was demonstrated in Figure 7. It clearly


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indicates that the stronger the reducing atmosphere, the higher the concentration of the corrosive sulfur species. Thus, in order to prevent the higher temperature corrosion caused by the corrosive gases, it is extremely necessary to dispel the reductive gases on the surface of the water cooled wall in the coal-fired boiler.

Figure 7. Relationship between the reductive gases (CO+H2) and the corrosive gases (H2S+COS+CS2) According to the above results and the previous research,15-18 15 reactions in total, including 3 irreversible reactions and 6 pairs of reversible reactions, were supposed to constitute the sulfur species gas-phase reaction mechanism, as listed in Table 4. Then, the feasibility of these reactions could be judged by the Gibbs free energy principle. As illustrated in Figure 8, ∆Gr was less than 0 for the reactions of R13-R21, while was above 0 for the reactions of R22-R27. So R22-R27 could not take place during pulverized coal combustion. Please note that the reactions were listed in this sequence just for the convenience of the determination of the kinetic parameters in the next section. Table 4.


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Figure 8. The Gibbs free energy changes (∆Gr) of the supposed reaction mechanism for different temperature. As mentioned above, in order to fully verify their possibility, the chemical equilibrium constants of R13-R21 should be calculated from the partial pressure of the components in the experimental results, and compared with the theoretical values, respectively. However, because the reactions of R13-R15 were the irreversible reactions, it is meaningless to calculate their equilibrium constants. Therefore, only the reactions of R16-R21 were further judged by the principle of chemical equilibrium constant. Here, R16 is used as an example to demonstrate the specific process. The calculated equilibrium constant of R16 is obtained from Eq. (7).

K p16 =

pH2S × pCO pCOS × pH2

(7)

Where, Kp16 is the chemical equilibrium constant of R16; pH2S, pCO, pCOS, and pH2 are the partial pressures of H2S, CO, COS, and H2, respectively. The calculated equilibrium constant of R16 compared with the theoretical value is shown in Figure 9. It clearly indicates that the calculated equilibrium constant was obviously lower than the theoretical value. Similarly, it was confirmed that the theoretical equilibrium constants of R17-R21 were also always larger than their calculated values, respectively. This further indicates that the reactions of R16-R21 were going


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on in the experiments. Therefore, the reactions of R13-R21 could be the gas-phase reaction mechanism of sulfur species.

Figure 9. Relationship between theoretical equilibrium constant and equilibrium constant calculated from measured gas composition for R16.

3.4. Determination for kinetic parameters of reaction mechanism Now, it is to determine the kinetic parameters of the proposed sulfur species gas-phase reaction mechanism. The method was introduced in detailed in the previous work,20 and it was also adopted in this work. The reaction rates of r13-r21 corresponding to R13-R21, respectively, were written as

ri = ki × [Cre,1 ]n1 × [Cre,2 ]n2

(8)

Here Cre,1 and Cre,2 are the concentrations of sulfur species and reducer/oxidant, respectively; n1 and n2 are the reaction orders of the reactant concentrations. Compared with sulfur species, the concentrations of CO, H2, O2, and H2O were all in large excess, so it is reasonable to assume n1=1 and n2=0.31,32 ki is the rate constant and expressed in typical Arrhenius form:


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ki =Ai × exp( −

Ei ) RT

(9)

here Ai is the pre-factor; Ei is the activation energy, kJ·mol-1. The determination of the kinetic parameters was via a mathematical optimization process conducted in the software of Matlab r2014a. The experimental results of the gas products distributions along the axis of the furnace under different conditions were used as the benchmark during optimization process. The residence time t (s) was one of the necessary data to determine the kinetic parameters of gas-phase reactions. It was mainly dependent on the reaction temperature, pressure and fuel gas flux, and calculated according to the standard flue wind resistance calculation. The residence time under each condition is listed in Table S1 of Supporting Information. It can be seen that the residence time varied over a wide range of 1.009.54s. The reaction mechanism considered 9 reactions and 9 gas species: SO2, H2S, COS, CS2, O2, CO2, CO, H2, and H2O. However, the concentrations of O2, CO2, CO, H2, and H2O were mainly dependent on the pulverized coal combustion, rather than the sulfur species gas-phase reactions. Therefore, the experimental data of these reactants concentrations was also adopted as the predication value in the calculated process. Then, the evolution profiles of the sulfur species could be obtained by solving the system consisting of 4 equations. dCSO2

dt d C H 2S

dt

= r13 + r14 + r15 − r17 − r18

(10)

= −r13 + r16 + r17 + r19 + 2r20 + r21

(11)

dCCOS = −r14 − r16 + r18 − r19 + r21 dt

(12)


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dCCS2

dt

= − r15 − r20 − r21

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(13)

The solving of the above differential equations was performed by coding in the Matlab r2014a. The numerical method was the Runge-Kutta method using the ode45 function. In this work, as listed in Table S1 of Supporting Information, there were 60 different reaction conditions, including 5 different total excess air ratios, 4 different reaction temperatures, and 3 different coal types. Meanwhile, 3 different sampling points were adopted. Therefore, each concentration profile of sulfur species was represented by 180 data points. For j from 1 to 180, j represented every data point. For the experimental results, the data points of sulfur species concentrations were CSO2,e,j, CH2S,e,j, CCOS,e,j, and CCS2,e,j; while for the prediction results from the reaction mechanism, the data points were CSO2,p,j, CH2S,p,j, CCOS,p,j, and CCS2,p,j. Therefore, the target function fT was defined as the deviation between the experiment results and the predication results, and written as 180 180 180 180 1 1 1 1 (C − CSO2 ,e,j ) 2 + ∑ j =1 (CH2S,p,j − CH2S,e,j )2 + 180 ∑ j =1 (CCOS,p,j − CCOS,e,j )2 + 180 ∑ j =1 (CCS2 ,p,j − CCS2 ,e,j )2 1 180 ∑ j =1 SO2 ,p,j 180 fT = 4 CSO2 ,0 + CH2S,0 + CCOS,0 + CCS2 ,0

(13)

It is clear that fT strongly depended on the kinetic parameters of reactions R13-R21. It was a function of the vector K under each experimental condition, for the reaction orders fixed.

f T = f T (K )

(14)

K = [ k13 , k14 , ⋅⋅⋅, k 21 ]T

(15)

Here the numerical task was to obtain the optimal vector Ko, which made fT reach the minimum value. The direct search method, the classical Hooke-Jeeves algorithm, was used to solve this numerical question. The specific solution procedure is described in detail in the Supporting Information. Then, the optimal vectors, Ko, at different temperatures, from 1100°C to 1400°C were obtained. For each reaction formula, the classic figure with its optimal reaction rate


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ki at different temperatures could be drawn to calculate the activation energy and the pre-factor. The calculation process of the 9 reactions were displayed in Figure 10. It shows that the linearity of data points is very high. Then, the activation energy and pre-factor can be calculated by multiplying the slope of the fitting line with 8.3145. The optimized kinetic parameters of the 9 reactions in the reaction mechanism are summarized in Table 5.

Figure 10. Deterministic process of activation energy and pre-factor. Table 5. To validate the reaction mechanism of sulfur species, Figure 11 summarizes the experimental data and predication results for SO2, H2S, COS and CS2. It can be seen that the prediction errors for the sulfur species are within 20%. This means that the reaction mechanism of sulfur species is credible in actual engineering application, and could provide necessary theoretical basis to prevent the high temperature corrosion.


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Figure 11. Comparison of experimental and predicated data on sulfur species. 4. CONCLUSIONS The reaction mechanism of sulfur species during low-sulfur bituminous coal combustion was investigated using a pulverized coal combustion furnace. In the primary stage, the O2 concentration decreased sharply, and sulfur released into the sulfur species as the forms of SO2, H2S, COS and CS2. Meanwhile, significant amount of reductive gases, CO and H2, were generated. After the sulfur release finished, the distribution of the sulfur species in the downstream region of the furnace relied upon only the subsequent gas-phase reactions. Part of SO2 was converted to H2S and COS under the effect of the reductive gases, CO and H2. Moreover, there also exited some shift relationships among SO2, H2S, COS and CS2, because of the presence of large amount of CO2 and H2O. A novel gas-phase reaction mechanism for sulfur species, consisting of 9 reactions, was developed based on the experimental results and the principles of Gibbs free energy and chemical equilibrium constant. The kinetic parameters were also determined by a strict mathematical optimization process. The calculated data verified that the reaction mechanism could accurately predict the distribution of the main sulfur species, SO2, H2S, COS and CS2, in the downstream region at different temperatures with different excess air ratio for different kinds


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of bituminous coal. The predication errors for sulfur species were within 20%. The sulfur species reaction mechanism was expected to provide great assistance for controlling the formation of H2S, and thus preventing high temperature corrosion.

ASSOCIATED CONTENT

Supporting Information The summary of the resident time of all conditions; the determination method of the kinetic parameters in Matlab r2014a. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected] (Honghe Ma), Tel.: +86 0351 6010281. ORCID Honghe Ma: 0000-0002-2590-7253 Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT

This work was supported by the National Natural Science Foundation of China (51706151); the Basic Research Projects of Shanxi Province, China (2015021109); and the Coal Base Key Scientific and Technology Program of Shanxi Province, China (MD 2014-07).

NOMENCLATURE

Ai = pre-factor of the reaction Ri; CSO2 = mole concentration of SO2, mol·m-3;


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CH2S = mole concentration of H2S, mol·m-3; CCOS = mole concentration of COS, mol·m-3; CCS2 = mole concentration of CS2, mol·m-3; Ei = activation energy of reaction Ri, kJ·mol-1; Fcoal = feed rate of coal, kg·s-1; Fto = theoretical air flow rate for pulverized coal combustion, kg·h-1; Ft = actual total excess air flow rate, kg·h-1; Fp = the primary air flow rate, kg·h-1; Fs = secondary air flow rate, kg·h-1; Kp16 = the chemical equilibrium constant of R16; Ms = molar weight of sulfur, 32g·mol-1; QV = volume flow rate of the flue gas, m3·s-1; R = gas constant, 8.3145J·mol-1·K-1; R90 = particle-size distribution coefficient; Rs,Z = average ratio of sulfur released from the coal to the gas phase; St = total sulfur content of the coal, wt%; T = reaction temperature, °C; Ws,Z = average molar flow of sulfur contained in the flue gas, mol·s-1; fT = target function; n = uniformity coefficient of pulverized coal; n1 = reaction order of sulfur species; n2 = reaction order of reducer/oxidant; p = actual pressure in the furnace, 91920Pa;


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Energy & Fuels

pCO = the partial pressures of CO; pCOS = the partial pressures of COS; pH2 = the partial pressures of H2; pH2S = the partial pressures of H2S; ri = reaction rate of reaction Ri, mol·m-3·s-1; t = residence time, s. Greek symbols αt = total excess air ratio; αp = primary air ratio; αs = secondary air ratio; ki = rate constant of Ri. Abbreviations BL = Bailu coal; DH = Daheng coal; YF = Yunfei coal.

REFERENCES

(1) Liu H.; Liu Y.; Yi G.; Nie L.; Che D. Energy & Fuels, 2013, 27, 5831-5840. (2) Liu, Y.C.; Fan W.D.; Wu M.Z. Applied Thermal Engineering, 2017, 110, 553-563. (3) Bai W.; Li H.; Deng L.; Liu H.; Che D. Energy & Fuels, 2014, 28, 1820-1828. (4) Wang J.; Fan W.; Li Y.; Xiao M.; Wang K.; Ren P. Energy, 2012, 37, 725-736. (5) Frigge L.; Sröhle J.; Epple B. Fuel, 2017, 201, 105-110. (6) Lin Z.; Fan W.; Li Y.Y.; Li Y.L.; Zhang M. Energy & Fuels, 2009, 23, 744-753. (7) Shim H.S.; Valentine J.R.; Davis K.; Seo S.; Kim T. Fuel, 2008, 87, 3353-3361.


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(8) Bohnstein M.; Langen J.; Frigge L.; Stroh A.; Ströhle J.; Epple B. Energy & Fuels, 2016, 30, 9836-9849. (9) Ma H.; Zhou L.; Ma S.; Du H. Applied Thermal Engineering, 2017, 124, 865-870. (10) Kung S.C. Corrosion, 2015, 71, 483-501. (11) Kung S.C. Corrosion, 2014, 70, 749-763. (12) Kung S.C. Oxi. Met., 2012, 77, 289-304. (13) Shirai H.; Ikeda M.; Aramaki H. Fuel, 2013, 114, 114-119. (14) Müller M.; Schnell U.; Scheffknecht G. Energy Procedia, 2013, 37, 1377-1388. (15) Zhang Z.; Li Z.; Cai N. Energy & Fuels, 2016, 30, 4353-4362. (16) Tsuji H.; Tanno K.; Nakajima A.; Yamamoto A.; Shirai H. Fuel, 2015, 158, 523-529. (17) Zhang B.; Ren Z.; Shi S.; Yan S.; Fang F. Chemical Engineering Science, 2016, 152, 227238. (18) Frigge L.; Elserafi G.; Sröhle J.; Epple B. Energy & Fuels, 2016, 30, 7713-7720. (19) Abián M.; Cebrián M.; Millera Á.; Bilbao R.; Alzueta M.U. Combustion and Flame, 2015, 162, 2119-2127. (20) Zhang Z.; Chen D.; Li Z.; Cai N.; Imada J. Energy & Fuels, 2017, 31, 1383-1398. (21) Leeds University. Sulfur mechanism. 2002. http://www.chem.leeds.ac.uk/combustion/sox. htm. (22) Cerru F.G.; Kronenburg A.; Lindstedt R.P. Combustion and Flame, 2006, 146, 437-455. (23) Zhou C.; Sendt K.; Haynes B.S. Proceedings of the Combustion Institute, 2013, 34, 625-632. (24) Sröhle J.; Chen X.; Zorbach I.; Epple B. Combust. Sci. Technol., 2014, 186, 540-551. (25) Tanner J.; Bläsing M.; Müller M.; Bhattacharya S. Fuel Processing Technology, 2011, 92, 511-516.


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(26) Gu Y.; Yperman J.; Reggers G.; Carleer R.; Vandewijingaarden J. Fuel, 2016, 184, 304-313. (27) Monaghan R.F.D.; Ghoniem A.F. Fuel, 2012, 91, 61-80. (28) Yani S.; Zhang D. Proceeding of the Combustion Institute, 2009, 32, 2083-2089. (29) Gu Y.; Yperman J.; Vandewijingaarden J.; Reggers G.; Carleer R. Fuel, 2017, 202, 494-502. (30) Monaghan R.F.D.; Ghoniem A.F. Fuel, 2012, 94, 280-297. (31) Ma H.; Wang S.; Zhou L.; Gong Y.; Xu D.; Wang Y.; Guo Y. Ind. Eng. Chem. Res., 2012, 51, 9475-9482. (32) Mateos D.; Portela J.R.; Mercadier J.; Marias F.; Marraud C.; Cansell F. The Journal of Supercritical Fluids, 2005, 34, 63-70.


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Table 1. Summary of the sulfur species reaction mechanisms Reactions

No.

Ref.

SO2 +3H 2 → H 2S+2H 2O

R1

13, 15, 16, 20

SO2 +CO+2H 2 → COS+2H 2O

R2

15

H 2S+1.5O2 → SO2 +H 2O

R3

17, 20

SO2 +3CO → COS+2CO2

R4

17, 20

COS+H 2 → H 2S+CO

R5

17, 18, 20

COS+H 2O → H 2S+CO2

R6

19, 20

CS2 +H 2O → H 2S+COS

R7

19

COS+1.5O2 → SO2 +CO2

R8

20

H 2S+CO → COS+H 2

R9

20

H 2S+2H 2O → SO2 +3H2

R10

20

COS+2CO 2 → SO2 +3CO

R11

20

H 2S+CO2 → COS+H 2O

R12

20


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Table 2. Properties of coals Coal

BL

DH

YF

Proximate analysis Ash (dry)

%

31.79

33.99

23.33

Volatile matter (dry)

%

28.08

26.99

23.23

Volatile matter (dry ash free)

%

38.64

39.24

36.53

Fixed carbon (dry)

%

40.13

39.02

53.44

1.43

1.45

2.30

Fuel ratio (fixed carbon/volatile matter)

Ultimate analysis C (dry)

%

50.89

48.97

65.24

H (dry)

%

3.41

3.55

4.15

N (dry)

%

0.84

1.14

1.08

O (dry)

%

10.47

8.70

9.20

Total S (dry)

%

0.54

0.68

0.86

Pyritic S (dry)

%

0.45

0.04

0.32

Organic S (dry)

%

0.07

0.64

0.50

Sulfate S (dry)

%

0.02

0.00

0.04

HHV (dry)

MJ/kg

20.64

20.15

26.18

LHV (dry)

MJ/kg

19.87

19.35

25.27

Analysis of sulfur forms

Heating Value


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Table 3. Combustion conditions. kg·h-1

Coal feed rate Total excess air ratio

0.45 0.7-1.1

Mass ratio of primary air flow rate to coal feed rate

(kg·h-1)/(kg·h-1)

2.2

Primary air flow rate

kg·h-1

0.99

Secondary air ratio

Calculated by Eq. (4)

Temperature of primary air

°C

80

Temperature of secondary air

°C

350

Reaction temperature

°C

1100-1400

Actual pressure of reactor tube

Pa

91920


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Table 4. Gas-phase reaction mechanism for sulfur species Reactions

No.

Feasibility

H 2S(g)+1.5O2 (g) → SO2 (g)+H 2O(g)

R13

Yes

COS(g)+1.5O2 (g) → SO2 (g)+CO2 (g)

R14

Yes

CS2 (g)+3O2 (g) → 2SO2 (g)+CO2 (g)

R15

Yes

COS(g)+H 2 (g) → H 2S(g)+CO(g)

R16

Yes

SO2 (g)+3H 2 (g) → H 2S(g)+2H 2O(g)

R17

Yes

SO2 (g)+3CO(g) → COS(g)+2CO2 (g)

R18

Yes

COS(g)+H 2O(g) → H 2S(g)+CO2 (g)

R19

Yes

CS2 (g) + 2H 2O(g) → 2H 2S(g) + CO2 (g)

R20

Yes

CS2 (g) + H 2 O(g) → H 2S(g) + COS(g)

R21

Yes

H 2S(g)+CO(g) → COS(g)+H 2 (g)

R22

No

H 2S(g)+2H 2O(g) → SO2 (g)+3H 2 (g)

R23

No

COS(g)+2CO2 (g) → SO2 (g)+3CO(g)

R24

No

H 2S(g)+CO2 (g) → COS(g)+H 2O(g)

R25

No

2H 2S(g) + CO2 (g) → CS2 (g) + 2H 2O(g)

R26

No

H 2S(g) + COS(g) → CS2 (g) + H 2O(g)

R27

No


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Table 5. Summary of the kinetic parameters of the reaction mechanism No.

A

Ei, kJ·mol-1

R13

1.3×1013

86.3

R14

5.8×1012

41.6

R15

8.9×1010

63.3

R16

1.4×1012

97.7

R17

8.3×1011

112.4

R18

8.6×1012

87.7

R19

6.7×1012

96.3

R20

2.3×1011

117.6

R21

7.5×1010

126.4


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Figure 11. Comparison of experimental and predicated data on sulfur species 93x65mm (600 x 600 DPI)


Reaction mechanism for sulfur species during pulverized coal

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