NO_x and H₂S formation in the reductive zone of air-staged combustion of pulverized blended coals

Abstract

Low NO_x combustion of blended coals is widely used in coal-fired boilers in China to control NO_x emission; thus, it is necessary to understand the formation mechanism of NO_x and H₂S during the combustion of blended coals. This paper focused on the investigation of reductive gases in the formation of NO_x and H₂S in the reductive zone of blended coals during combustion. Experiments with Zhundong (ZD) and Commercial (GE) coal and their blends with different mixing ratios were conducted in a drop tube furnace at 1200°C–1400°C with an excessive air ratio of 0.6–1.2. The coal conversion and formation characteristics of CO, H₂S, and NO_x in the fuel-rich zone were carefully studied under different experimental conditions for different blend ratios. Blending ZD into GE was found to increase not only the coal conversion but also the concentrations of CO and H₂S as NO reduction accelerated. Both the CO and H₂S concentrations inblended coal combustion increase with an increase in the combustion temperature and a decrease in the excessive air ratio. Based on accumulated experimental data, one interesting finding was that NO and H₂S from blended coal combustion were almost directly dependent on the CO concentration, and the CO concentration of the blended coal combustion depended on the single char gasification conversion.Thus, CO, NO_x, and H₂S formation characteristics from blended coal combustion can be well predicted by single char gasification kinetics.

Keywords

blended coal combustion / NO_x formation / H₂S formation / air staged combustion

Introduction

China is a rapidly developing country,whose demand for electricity is continually increasing. In China, more than 60% of electricity generation relies on coal combustion in power station boilers [¹]. Currently, blended coal is widely used in power station boilers because the supply of the designed coal for boilers is not sufficient, and the price of the designed coal is often too high [²–⁴]. Generally, there are two ways to blend coal: out-furnace blending and in-furnace blending [⁵,⁶]. Out furnace blending involves blending coal before pulverizing and feeding the pulverized coal into the boiler. In-furnace blending involves pulverizing coal individually and feeding it into the boiler separately. To make it compatible with the existing design, most boilers are using out-furnace blending. Many experimental studies have been reported on blended coal combustion characteristics. Experiment studies have not only been conducted on the laboratory scale including thermogravitic analysis (TGA) [⁷–⁹], entrained flow reactors (EFRs) [¹⁰,¹¹], and drop tube furnaces (DTFs) [⁸,¹²,¹³] but also on a pilot scale involving actual boilers [²,¹⁴,¹⁵]. There is a wide range of research targets forblended coal combustion. Researchers are most interested in the blending effect on ignition [²,⁷,⁸,¹⁰,¹³], flame stability [²,¹³], char burnout and slagging [²,⁷,⁸,¹¹,¹⁵], and pollutant emissions [⁸,⁹,¹²,¹⁴]. Based on experimental data, computation fluid simulation (CFD) models can be established to predict the blended coal combustion in boilers [¹⁶–²⁰]. From previous studies, it has been recognized that the properties related to fuel composition, such as proximate and ultimate analysis data and the heating value, remain additive after blending [⁷], while many characteristics related to combustion are non-additive, e.g., ignition, the combustion and flame stability, slagging, and fouling, which can not be predicted by additivity [²].

Coal-fired boilers often currently use the air-staged combustion technology [²¹–²⁴]. Air-staged combustion involves a decreased amount of air in the primary zone and the formation of a reductive zone in which O₂ is depleted and NO_x is reduced by a reductive gas [²⁵]. However, corrosive gases, such as H₂S, are formed simultaneously with CO and H₂ and produce corrosion problems from water on the wall of the boiler [²⁶,²⁷]. Most blended coal combustion research has been conducted using normal combustion; thus, the reactions in the reductive zone have been neglected. The general characteristics of gas-phase species upon changing the blending ratio, temperature, or excessive air ratio in the reductive zone are still unknown. To evaluate NO_x reduction and H₂S formation, it is of vital importance to study their behavior in the reductive zone of blended coal combustion, and to investigate the underlying mechanisms.

In this paper, coal conversion and NO_x and H₂S formation in the reductive zone of air-staged combustion of pulverized blended coals were focused on. Using abituminous coal, i.e., Zhangdong(ZD), and a lean coal, i.e., Commercial(GE), and their blends in different blending ratios, experiments were conducted on an electrically heated drop tube furnace, which could achieve the air-staged combustion of blended coal. Combustible conversion, CO and H₂S formation, NO reduction, and the reaction distance were studied in detail. Based on the experimental results, the relationships among char gasification conversion, CO concentration, NO concentration, and H₂S concentration were established. Based on accurate gasification kinetics of single coal, blended coal gasification could be predicted; thus, CO, H₂S, and NO could be well predicted.

Experiments

Coal properties

The proximate and ultimate analysis of the abituminous coal, i.e., ZD, and a lean coal, i.e., GE, is presented in Table 1. Before being used in the drop tube furnace (DTF), each coal was first grounded and then sieved using anultrasonic vibrating screen classifier. The fine pulverized coal was removed to achieve smooth coal feeding without aggregation [²⁸–³⁰]. Besides the pure ZD and GE coals, blended ZD and GE coals of three different mass blending ratios of 1:3, 2:2, and 1:3 were tested. The particle size distributions of the five coal samples were measured using a laser diffraction particle size analyzer (Malvern 2000), as shown in Fig. 1. Most of the pulverized coal fell in the well-defined range of 20–110 mm, and the volume percentage of the particles whose diameter was below 2 mm was less than 3%. There were no obvious differences in particle size distributions between pure and blended coals; thus, possible influences that would occur due to differences in the particle size distribution could be neglected.

DTF

An electrically heated DTF was used as illustrated in Fig. 2. Primary air and over fire air (OFA) were supplied by high pressure cylinders and controlled by mass flow meters. The feeding rate of the pulverized coal was controlled by a scraper coal feeder. Primary air carried the pulverized coal into the reaction tube by a water-cooled injection gun. The reaction tube was a cylindrical aluminatube with an inner diameter of 60 mm. A small alumina tube with an inner diameter of 6 mm was inserted in the reaction tube and used as the OFA tube. The OFA tube injected burnout air directly into the burnout zone. The heat required for maintaining a high temperature was supplied by 24 electrically heated silicon carbide rods which were distributed uniformly around the reaction tube and were controlled by 10 thermocouples. A vertically movable probe with a water-cooled jacket could freeze combustion at any location of interest. Once removed from the probe, the samples were collected in a cyclone and filtered while the flue gas was pumped into the gas analyzers to measure the gas components. The details of the DTF are demonstrated in Fig. 2. By introducing the OFA tube, a mean combustion zone, reduction zone, and burnout zone in the air-staged combustion of the pulverized coal could be achieved inside the DTF, as observed in Fig. 2.

The total excessive air ratio (abbreviated as SR_t hereafter) was maintained at 1.2 in all cases. The feeding rate of each coal sample was determined based on the ultimate analysis data to ensure an air flow rate of 20 L/min corresponding to a SR_t of 1.2. The flow rate of the primary air was varied from 10 to 20 L/min, corresponding to a primary excessive air ratio (abbreviated as SR₁ hereafter) of 0.6-1.2. The flow rate of the OFA varied from 10 to 0 L/min, corresponding to a burnout excessive air ratio introduced by the OFA (abbreviated as SR₂ hereafter) of 0.6–0. The detailed test conditions are provided in Table 2. Stable operation of the DTF was required for accurate data, which relied on stable coal feeding. Typical variations in CO₂ and CO and the end of the reductive zone under different conditions were reported in Refs. [¹²,³¹,³²]. The maximum relative variations of CO₂ and CO are no more than 3% in reductive zone, including the error of the analyzer, which shows that the combustion is rather steady.

Sample analysis

Solid samples were collected while the gas component was analyzed at four sampling locations inside the DTF (0.35 m, 0.6 m, 0.85 m, and 1.1 m), which were all located in the reductive zone. Based on the samples collected from these sampling locations, combustion characteristics along the reaction distance were investigated. The collected unburned coal samples were analyzed by TGA (TA, Q500) to obtain combustible conversion. The concentrations of gas components, including CO, CO₂, SO₂, NO, and NO₂, were measured using a Fourier transform infrared spectrometer (FTIR, Nicolet 670) while the concentrations of H₂S, COS, and H₂ were measured using a gas chromatography (GC, Shimadzu, GC-2015), and the O₂ concentration was measured using a paramagnetic oxygen analyzer.

Experiment data analysis

The combustible conversion, X_coal, was calculated from the TGA data by using the ash-trace method,

(1)X_{\mathrm{coal}}=\mathrm{1}-{\displaystyle \frac{\xi A_{\mathrm{0}}}{({\mathrm{FC}}_{\mathrm{0}}+V_{\mathrm{0}})A}},

where A₀ and A represent the initial and remaining ash contents in the samples, FC₀ represents the initial fixed carbon in the samples, V₀ represents the initial volatiles in the samples, and \xi represents the remaining combustibles in the samples.

In the reductive zone, char gasification can be described as a global reaction mechanism [³³]

(R.1){\mathrm{C\; +\; CO}}_2\to \mathrm{2CO}

(R.2){\mathrm{C\; +\; H}}_2\mathrm{O}\to {\mathrm{CO\; +\; H}}_2

Thus, the char gasification conversion, X_g, can be expressed as

(2)X_{\mathrm{g}}={\displaystyle \frac{\mathrm{0.5}([\mathrm{CO}]+[{\mathrm{H}}_2])\stackrel{·}{V_{\mathrm{flue}}}{M}_{\mathrm{C}}}{\mathrm{22.4}\cdot \mathrm{FC}\cdot {\stackrel{·}{m}}_{\mathrm{coal}}}},

where \stackrel{·}{V_{\mathrm{flue}}} represents the volume flow rate of the flue gas, M_C represents the molar mass of carbon, FC represents the mass ratio of fixed carbon in the used coal, and {\stackrel{·}{m}}_{\mathrm{coal}} represents the mass flow rate of the coal.

Results

Blending effect on coal combustible

Establishment of a reductive zone in air-staged combustion can promote NO_x reduction as O₂ is depleted in this area and coal conversion is delayed; therefore, it is necessary to investigate the combustible conversion characteristics in the reductive zone. Figure 3 depicts the combustible conversion as a function of the reaction distance for SR₁ = 0.6 and T = 1400°C. It can be observed that the combustible conversion increased along the reaction distance, and the conversion was higher when the ZD blending ratio was higher. In the reductive zone, char was continuously gasified; thus, the coal combustible conversion increased. When the reaction distance increased, the residence time of char particles also increased; thus, more char was gasified. At the end of the reductive zone, X_coal was 90.9% and 59.5% for ZD and GE coal, respectively. Obviously, ZD had a higher char gasification reactivity than GE. Blending ZD into GE accelerated char gasification; therefore, more combustible was consumed when the ZD blending ratio was higher. Figure 4 exhibits the effect of the ZD blending ratio and temperature on the combustible conversion at the end of the reductive zone (L = 1.1 m). With a higher temperature, more combustibles were converted. When SR₁ = 0.6, at the end of the reductive zone, the combustible conversions for ZD at 1200°C, 1300°C, and 1400°C were 75.7%, 84.7%, 90.9%, respectively, and those for GE were 59.5%, 63.7%, and 65.6%, respectively. At higher temperatures, the char gasification kinetics were enhanced based on the Arrhenius law; therefore, more char was consumed, and combustible conversion increased. From 1200°C to 1400°C, the combustible conversion increased by 15.2% for ZD coal and increased only by 6.1% for GE coal. Thus, a higher temperature contributed more to the combustible conversion of ZD, which had a higher char gasification reactivity.

Figure 5 illustrates the combustible conversion at the end of the reductive zone for different SR₁ when T = 1400°C. When SR₁ = 1.2, O₂ was excessive, and a reductive atmosphere did not exist; the combustibles were almost fully converted for all ZD blending ratios. A blending effect was not able at SR₁ = 0.6 and SR₁ = 0.8. In the reductive atmosphere, ZD had a higher gasification reactivity; thus, blending ZD into GE increased the combustible conversion. When SR₁ = 0.6, the difference between the combustible conversion of ZD and GE pure coal was 25.3%, and that at SR₁ = 0.8 was 21%. At a lower SR₁, combustible conversion relied more on gasification than on oxidization; thus, the difference between ZD and GE was higher, and blending ZD into GE was more effective in increasing the combustible conversion.

Blending effect on CO and H₂

In a reductive zone of air-staged combustion of pulverized blended coal, CO and H₂ are produced by the gasification of char with CO₂ and H₂O. These reductive gases not only accelerate corrosive gas formation but also have an important effect on local ash slagging and fouling [¹²]. It is important to investigate the reductive gases in the reductive zone. Figure 6 displays the CO and H₂ concentrations as a function of reaction distance at SR₁ = 0.6 and T = 1400°C. Both CO and H₂ increased with increasing reaction distance because more char gasified with increasing residence time. CO and H₂ also increased with an increasing ZD coal blended ratio because the reactivity of ZD char was higher than that of GE char. For all blending ratios, the CO concentration was almost five times the H₂ concentration, which showed that CO was the main reductive gas, representing a reductive atmosphere.

Figure 7 presents the CO and H₂ concentrations at the end of the reductive zone at different temperatures and ZD blending ratios for SR₁ = 0.6. With a higher temperature or ZD blending ratio, both CO and H₂ concentrations increased. Increasing the ZD blending ratio can promote char gasification, and gasification reaction kinetics increased with an increase in temperature. When the temperature was increased from 1200°C to 1400°C, the CO concentration increased 3.2 vol% for GE and 7.4 vol% for ZD; while the H₂ concentration kept increasing to approximately 0.65 vol% for all blending ratios. Thus, CO was more sensitive to temperature at different blending ratios.

Figure 8 shows the CO and H₂ concentrations at the end of the reductive zone for different excessive air ratios and ZD blending ratios at 1400°C. When SR₁ = 1.2, O₂ was excessive, and the formation of the reductive zone was inhibited; thus, almost no CO and H₂ were produced. When SR₁ = 0.6 or 0.8, O₂ was depleted and considerable amounts of CO and H₂ were produced in the reductive zone. At a lower SR₁, the atmosphere shifted earlier from oxidization to reduction, and some char particles were gasified by CO₂ and H₂O; therefore, a large amount of CO and H₂ were produced. When the excessive air ratio decreased from 0.8 to 0.6, the CO concentration increased 11.3 vol% for ZD and 5.7 vol% for GE; as the H₂ concentration increased 1.6 vol% for ZD and 1.3 vol% for GE, increasing ranges of CO and H₂ concentrations for blended coals were found to be between those of GE and ZD. Blending ZD into GE promoted gasification; thus, more CO and H₂ were produced. Of note, combustible conversion, CO concentration, and H₂ concentration did not maintain linear relations with the ZD blending ratio. A study has confirmed that there is inhibition of char gasification via the production of CO and H₂ [³⁴]. In char gasification, ZD and GE compete for CO₂ and H₂O. Besides, ZD has a higher reactivity and reacts faster, and its reactions inhibit GE char from gasification. Both competition and inhibition effects lead to the nonlinear behavior during blended coal combustion.

Blending effect on NO

NO is reduced in the reductive zone via both homogenous and heterogeneous reduction [³⁵]. The NO concentration as a function of the reaction distance at SR₁ = 0.6 and T = 1400°C is shown in Fig. 9(a). The NO concentration remained below 50 ppm as a function of the reaction distance for ZD coal combustion, whereas the NO concentration decreased from 377 ppm to 68 ppm as a function of the reaction distance for GE combustion. The NO concentration in blended coal combustion was between the NO ranges of the two single coals. With the higher blending ratio of ZD coal, the NO concentration was lower because ZD char had a high gasification reactivity and a large amount of reductive gas could be produced, which accelerated NO reduction.When the SR₁ increased to 0.8, there was an obvious increase in the NO concentration as a function of the reaction distance, as shown in Fig. 9(b). At the end of the reductive zone, NO for GE increased from 67 ppm to 463 ppm, and that for ZD increased from 13 ppm to 120 ppm. At a higher excessive air ratio, the formation of the reductive zone was inhibited with more oxygen, and less NO was reduced for pure coal and blended coal in a weaker reductive atmosphere. The NO concentration also decreased with the increasing ZD blending ratio.

The NO concentration as a function of the reaction distance for SR₁ = 0.6 and T = 1200°C is shown in Fig. 10. Upon comparing Fig. 9, when the temperature changed from 1200°C to 1400°C, NO moderately decreased. At the end of the reductive zone, NO produced from ZD was approximately 10 ppm, and NO produced from GE decreased from 122 ppm to 67 ppm.The NO concentration for blended coals was between the NO ranges for ZD and GE. For GE, at a higher temperature, the gasification reaction rate was enhanced, and more reductive gases were produced; thus, the reducing atmosphere was stronger, and more NO was reduced. Because the reduction ability of ZD was higher, it could fully reduce NO even at lower temperatures. NO reduction in the reductive zone was affected by the blending ratio, excessive air ratio, and temperature.

Blending effect on H₂S

In the reductive zone, O₂ was depleted as H₂S formed simultaneously, which might have resulted in the corrosion of the water wall [³⁶]. Figure 11(a) shows the H₂S concentration as a function of the reaction distance at 1400°C and SR₁ = 0.6. ZD produced most H₂S, and GE produced little H₂S; the H₂S concentration increased with increasing ZD blending ratio. For higher ZD blending ratios, the char gasification reactivity was higher, and the reductive atmosphere was stronger. With a stronger reductive atmosphere, more SO₂ was reduced to H₂S. Of note, GE produced little H₂S due to its weak reducing atmosphere. Figure 11(b) shows the H₂S concentration as a function of the reaction distance at 1200°C and SR₁ = 0.6. Upon comparing the experimental results at 1400°C and 1200°C, there was a notable H₂S concentration decrease, especially for ZD. At the end of the reductive zone, the H₂S concentration for ZD decreased from 544 ppm to 350 ppm. The H₂S concentration for blended coals also decreased at lower temperatures. A higher temperature could accelerate the gasification rate, the reducing atmosphere was stronger, and more H₂S was produced.

Discussion

Based on the experimental data, some characteristics of the reductive zone for air-staged combustion of pulverized coals can be summarized. First, as far as the relationship between char gasification conversion and CO concentration is concerned, as shown in Fig. 12, the CO concentration remains directly proportional to char gasification, which does not change with blending ratio, excessive air ratio, or temperature. CO was produced by char gasification in the reductive zone. It can be concluded that char gasification controls CO production. This is an important result for CFD modeling of air-staged combustion of pulverized blended coal combustion. By using accurate gasification kinetics of single coal, blended coal gasification conversion can be well predicted; and, in turn, the CO concentration can also be well predicted.

Second, as far as the relationship between CO concentration and NO concentration is concerned, as shown in Fig. 13, at different temperatures, there was a logarithmic relationship between NO and CO in the reductive zone. With an increase in the CO concentration, more NO was reduced; thus, the NO concentration decreased. This relationship is also important for CFD simulation. In the detailed reducing mechanism of NO, hydrocarbon radicals (C_iH_j) [³⁵,³⁷,³⁸] may effectively reduce NO; however, hydrocarbon radicals are difficult to measure, and the calculation of hydrocarbon radicals by using adetailed mechanism require a long CFD modeling time. The CO concentration can be measured well in experimental studies or predicted well in CFD simulations. This paper confirmed that a direct relationship can be established between CO and NO, which prevents complex measurements or calculations of hydrocarbon radicals; thus, once the CO concentration is known, the NO concentration can also be obtained using the reduction mechanism.

Finally, as far as the relationship between CO and H₂S concentration is concerned, as reported in the literature, there is astrong relationship between reductive atmosphere and H₂S concentration [³⁶]. The global conversion mechanism from SO₂ to H₂S and COS can be illustrated as [³⁹]

(R.3){\mathrm{SO}}_2\mathrm{+CO}\to {\mathrm{COS+CO}}_2

(R.4){\mathrm{SO}}_2{\mathrm{+3H}}_2\to {\mathrm{H}}_2{\mathrm{S+2H}}_2\mathrm{O}

(R.5){\mathrm{COS+H}}_2\mathrm{O}\leftrightarrow {\mathrm{H}}_2{\mathrm{S+CO}}_2

For a higher concentration of CO, the reduction of SO₂ is mainly due to CO, and sulfur-containing gas mainly exists as H₂S. Figure 14 shows the local CO and H₂S concentrations at 1200°C, 1300°C, and 1400°C. At different temperatures, the H₂S concentration has a good linearity with the CO concentration. In the reductive zone, SO₂ and other oxidative sulfur species were reduced to H₂S. With a stronger reductive atmosphere, the CO concentration was higher; thus, more H₂S was produced. In the CFD simulation, there exists a dilemma regarding the accuracy of the H₂S concentration and computational cost because detailed reaction mechanisms for sulfur species usually include hundreds of reactions [⁴⁰,⁴¹]. Based on the direct relationship between CO and H₂S and avoiding calculation of the complex sulfur species transformation mechanism, H₂S can be rapidly and accurately calculated at a low computational cost.

Conclusions

Blended coal combustion (ZD and GE) in a DTF was studied, and combustible conversion, reductive gas species (CO, H₂), and pollutant gases (NO, H₂S) were measured to investigate the effect of blending ratio, temperature, excessive air ratio, and residence time on the combustion characteristics in the reductive zone. Combustible conversion of both ZD and GE and their blends increased with an increase in temperature and excessive air ratio. It is found that char gasification involves important reactions in the reductive zone, and both gas-phase species and solid-phase conversion are directly dependent on the char gasification reactivity. For reactive ZD char, the concentrations of CO, H₂, and H₂S are higher, whereas the concentration of NO is lower than that for low-reactivity GE. When the blending ratio of ZD is increased, the concentrations of CO, H₂, and H₂S decreases, whereas the concentration of NO increases. When the temperature is higher, the char gasification rate is enhanced; thus, a reductive atmosphere is stronger, and more H₂S is produced when NO is reduced. At a lower excessive air ratio, oxidation is inhibited and gasification is promoted; the reductive atmosphere is stronger, more char is gasified, and more CO and H₂ are produced. More NO is reduced in the stronger reductive atmosphere. Interestingly, there is a direct relationship between NO, H₂S, and CO, and the concentration or reaction rates of NO and H₂S can be predicted based on the CO concentration, which is based on an accurate char gasification model. This provides a strategy for CFD model development of pulverized blended coal combustion.

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