An experimental study on ignition of single coal particles at low oxygen concentrations

Abstract

An experimental study on the ignition of single coal particles at low oxygen concentrations (X_{{\mathrm{O}}_2}<21%) was conducted using a tube furnace. The surface temperature (T_s) and the center temperature (T_c) of the coal particles were obtained from the images taken by an infrared camera and thermocouples respectively. The ignition processes were recorded by a high-speed camera at different X_{{\mathrm{O}}_2} values and furnace temperatures T_w. Compared with literature experimental data obtained at a high X_{{\mathrm{O}}_2} value, the ignition delay time t_i decreases more rapidly as X_{{\mathrm{O}}_2} increases at the low X_{{\mathrm{O}}_2} region. The responses of T_s and T_c to the variation of X_{{\mathrm{O}}_2} are different: T_s decreases while T_c remains nearly constant with increasing X_{{\mathrm{O}}_2} at a low X_{{\mathrm{O}}_2} value. In addition, t_i is less sensitive to T_w while the ignition temperature T_i is more sensitive to T_w at a low X_{{\mathrm{O}}_2} value than in air. Observations of the position of flame front evolution illustrate that the ignition of a coal particle may change from a homogeneous mode to a heterogeneous or combined ignition mode as X_{{\mathrm{O}}_2} decreases. At a low X_{{\mathrm{O}}_2} value, buoyancy plays a more significant role in sweeping away the released volatiles during the ignition process.

Graphical abstract

Keywords

coal particles / low oxygen concentration / ignition / ignition temperature / ignition modes

1 Introduction

Coal ignition at a low oxygen concentration (X_{{\mathrm{O}}_2}<21%) popularly exists in industrial applications. For example, in a coal-fired boiler, when the primary air flow carries coal particles into the furnace, it entrains the surrounding flue gas, making the ignition of most coal particles happen in an environment with X_{{\mathrm{O}}_2}<21% rather than in air (X_{{\mathrm{O}}_2} = 21%) [1]. Similarly, in the moderate or intense low-oxygen dilution (MILD) combustion, which is regarded as a new clean coal technology due to its advantages in NO_x reduction and flame stability [2], coal ignition occurs in low X_{{\mathrm{O}}_2} conditions due to the strongly dilution of the flue gas [3]. Besides, in a circulating fluidized bed (CFB) boiler with a good fuel flexibility, high concentration mm-sized particles mix with secondary air in the dense zone, causing a low X_{{\mathrm{O}}_2} condition for combustion and SO₂ absorption [4]. Therefore, understanding the ignition of coal particles in low X_{{\mathrm{O}}_2} conditions is essential for the coal-fired burner and combustor design, CFB boiler operation, and pollutant removal.

The effect of oxygen concentration on coal ignition has been extensively studied before, but mostly under air or oxyfuel combustion conditions [5,6], and not much researches have been conducted at low X_{{\mathrm{O}}_2} values systematically. Two important parameters, i.e., the ignition delay time (t_i) and ignition temperature (T_i) are often used to describe the ignition characteristics of a fuel [7–9]. It could be straightforward that T_i increases as X_{{\mathrm{O}}_2} decreases because of the decrease in the burning intensity. However, it is found that this hypothesis is not always true, namely, the variation of T_i with X_{{\mathrm{O}}_2} is still controversial. Du and Annamalai. [10] has found that when coal particles experience homogeneous ignition, T_i remains roughly constant and independent of X_{{\mathrm{O}}_2}. However, Liu et al. [11] and Zhou et al. [12] have experimentally found that T_i decreases with increasing X_{{\mathrm{O}}_2}. Moreover, Annamalai and Durbertaki [13] found that T_i also decreases with increasing X_{{\mathrm{O}}_2} in a homogeneous mode but it increases with increasing X_{{\mathrm{O}}_2} in a heterogeneous mode. However, in air or at a high X_{{\mathrm{O}}_2} value, the experiments consistently shows that T_i increases as X_{{\mathrm{O}}_2} decreases for single coal particles [14,15]. At the same time, most researches have discovered that t_i decreases with increasing X_{{\mathrm{O}}_2} [14,16]. Illustrated by Ponzio et al. [17], t_i is more sensitive to X_{{\mathrm{O}}_2} at a low X_{{\mathrm{O}}_2} value than at a high X_{{\mathrm{O}}_2} value.

In addition to the influences on T_i and t_i, the change in X_{{\mathrm{O}}_2} value could also lead to the change in the ignition mode. On the one hand, a lower X_{{\mathrm{O}}_2} value could cause a weaker O₂ diffusion onto the particle surface, making heterogeneous ignition more difficult to occur. On the other hand, a lower X_{{\mathrm{O}}_2} weakens the pyrolysis and volatile burning processes, making homogeneous ignition more difficult to occur as well [7]. Besides T_w and X_{{\mathrm{O}}_2}, the ignition mode can be different for different coal particle sizes (d_p). The ignition modes for mm-sized coal particles at a low X_{{\mathrm{O}}_2} value and different T_w values remain unclear.

t_i is often determined by using a set of continuous images or a temperature variation curve of the coal particle obtained during the ignition period [7,17]. For example, using three-color measurement, Gat et al. [18] have measured the temperature of a single coal particle in the range of 1500–2700 K heated by laser at three bands of 0.92, 1.1, and 1.2 mm. Fletcher [19] has measured the pyrolysis temperature of coal particles in a swirl burner higher than 850 K at 1.36 mm and 2.2 mm by using a two-color method. Zhu et al. [20] have used a monochromatic method to measure the T_i of a single coal particle heated by an electric heating furnace in the visible band. T_i is determined by the surface temperature (T_s) of coal particles at the moment when a visible flame appears. Actually, for single coal particles, the temperature gradient inside it cannot be neglected. Therefore, some researchers [14,15,20] have adopted thermocouples to measure the center temperature (T_c) of coal particles. Based on the two temperature measurements, T_i of single coal particles can be more accurately determined.

In this paper, an experimental system was establsihed to study the ignition of single mm-sized coal particles at low X_{{\mathrm{O}}_2} values by using a tube furnace. Both the surface and center temperatures of the coal particle were measured. Besides, the effect of T_w was assessed. The objectives of this study are to obtain the variation of t_i and T_i with X_{{\mathrm{O}}_2} at low X_{{\mathrm{O}}_2} values, and to assess the possible changes of ignition mode of a single coal particle at low X_{{\mathrm{O}}_2} values.

2 Experimental approach

2.1 Experimental equipment and methods

The experimental system, similar to that used in Ref. [14], is schematically shown in Fig. 1. A high-speed CMOS camera was used to record the ignition and combustion process of the single coal particle. Meanwhile, the temperature variations at the center and the surface of the coal particle were detected by thermocouples and a SWIR camera respectively. A beam splitter divided the radiation from the coal particles into two parts, 90% of the visible light was reflected to the CMOS camera, and 80% of the infrared spectrum was transmitted to the SWIR camera.

A typical Chinese bituminous coal, Datong coal (DTC) was used and its proximate and ultimate analysis is given in Table 1. Hand-made spherical coal particles with a diameter of approximately 2 mm (d_p=2mm) were used. The raw coals were crushed and sieved, and particles of approximately 2.0 mm in size were manually selected. Then the particles were drilled through a 0.25 mm hole and shaped into spherical. A vernier caliper was used to measure the particle size. When the error of two randomly selected particle sizes of the same particle is within 5%, it is considered that the spherical particles are qualified, otherwise, grinding would continue. To measure the temperature at the center of the coal particle, a fine bare-wired K type thermocouple (with a wire diameter of 0.125 mm and a joint diameter of 0.25 mm) was installed in the drilled hole, as illustrated in Fig. 2. The joint of the thermocouple was placed at the center of the coal particle. The hole was then sealed with high-temperature glue. The coal particle with thermocouple installed was supported by a ceramic probe with two tiny holes for the thermocouple wires. The probe was mounted to a linear motor, which could carry the probe into the furnace at a speed of 20 cm/s through a hole opened in the middle of furnace wall. The furnace was electrically heated and preheated to a preset temperature before the coal particle was inserted into. The size of the furnace was f 60 mm × 90 mm (length). More details about the experimental system could be referred to in Ref. [14].

Experiments were conducted at a series of oxygen concentrations (X_{{\mathrm{O}}_2} = 10%, 15%, and 21%) and furnace temperatures (T_w = 923 K, 973 K, 1023 K, and 1073 K). At the beginning of each experiment, the furnace was filled with N₂-O₂ gas at a preset X_{{\mathrm{O}}_2} and T_w value. Though there was an 8 mm hole on the furnace to inject and retract the coal particle probe, there should be no leakage of external air into the furnace since the gas inside expanded only during ignition. When the furnace was filled with the hypoxic air at the lowest testing X_{{\mathrm{O}}_2} value (X_{{\mathrm{O}}_2} = 10%), the excess air coefficient calculated was approximately 2.3. Therefore, oxygen was sufficient for burnout.

The ignition delay time (t_i) was defined as the time interval from the moment when coal particle was sent into the furnace and the moment when ignition occurred, based on the color images recorded at 500 Hz. The ignition moment was referred to the time when a bright image showed up. The t_i equaled to the summation of the residence time of the coal particle at the furnace center before ignition and the translation time in the furnace. In the experiment, the coal particle was inserted into the center of the furnace at 20 cm/s by a programmed linear stepper motor and the translation time in the furnace was about 0.2 s [14]. If a flame with a size larger than that of the coal particle was observed before the particle surface turned luminous red, the ignition was considered as homogeneous. On the opposite, the ignition was regarded as heterogeneous. If one or a few bright spots on the particle surface and at the same time a flame around the particle were observed, the ignition was regarded as combined ignition. T_i was defined as the T_s of coal particle right before the appearance of a visual flame, determined by the infrared images recorded by a SWIR camera at 50 Hz using the monochrome method.

To reduce the error in T_s measurement, relatively long wavelengths were selected to increase the measurable radiation energy. Given that drying and pyrolysis processes could produce a certain amount of vapor and CO₂, which were highly absorptive in the wavelength range of 2.4–2.7 mm, the associated wavelength range was excluded. Based on the previous experiments [21], coal particles do not behave as a gray body in the wavelength range of 2–6 mm, due to the low and fluctuating emissivity. Therefore, the infrared spectrum selected to measure the radiant energy from the coal particle surface was in the range of 0.9–1.7 mm. According to the optical design in the experiment, the relationship between the radiation energy captured by the SWIR camera and the T_s of coal particles is [22]

(1)G(\lambda ,T_{\mathrm{s}})=K(T_{\mathrm{s}})·{\displaystyle {\int }_{\mathrm{900}}^{\mathrm{1700}}\epsilon ·C_{\mathrm{1}}{\lambda }^{-\mathrm{5}}/[\exp ({\displaystyle \frac{C_{\mathrm{2}}}{\lambda T_{\mathrm{s}}}})-\mathrm{1}]}·g(\lambda )b(\lambda )\mathrm{d}\lambda .

where G(l,T_s) represents the grayscale, the radiation intensity; K(T_s) is a conversion coefficient of the optical system at T_s; g(l) and b(l) represent the spectral curves of photo-sensitive elements and beam splitter, respectively; and C₁ is the first constant of Planck radiation while C₂ is the second constant of Planck radiation. During the heating process, the coal particle was assumed as a gray body and its surface emissivity ε remained unchanged. In the calculation, ε was assumed to be 0.85, as used in Refs. [14,23]. T_s calibration was done using a blackbody furnace in the range of 600–950 K, with an interval of 25 K. The typical calibration images are demonstrated in Fig. 3.

Based on Eq. (1), the unqiue relationship between K(T_s) and T_s can be established using a black-body furnace calibration. The relationship was in an exponential form, similar to the one reported in Ref. [14]. Besides, the value of exp(C₂/(lT_s)) is much greater than 1, and thus, G(l,T_s) can be expressed as an exponential function of T_s regardless of l

(2)G(\lambda ,T_{\mathrm{s}})=\mathrm{2.7856}{\mathrm{e}}^{\mathrm{0.0063}{T}_{\mathrm{s}}}.

When the coal particle was just placed at the center of the furnace, the radiation emitted by the coal particle was much weaker than that emitted from the furnace wall and reflected by the surface of the coal particle. The radiation energy received by the SWIR camera was mainly the background, which was subtracted to obtain the net radiation energy emitted by coal particles.

2.2 Error analysis

The total measurement error of the temperature at the coal particle center (T_i,c) by the thermocouples (denoted as s_c) was introduced by three parts. One part of the error was due to the offset (DL) of the joint of thermocouples from the center of the particle, denoted as s₁. The maximum offset was estimated at 10% of the particle diameter, i.e., 0.2 mm. According to the measurement data, the temperature difference (DT) between T_i,s and T_i,c was about 100 K. The radial temperature variation inside the particles could be linear as the intra-particle thermal conduction was considered. Calculated by Eq. (3), s₁ was nearly 20 K, with a relative error of less than 5.0%.

(3){\sigma }_{\mathrm{c}}={\displaystyle \frac{\Delta L}{r_{\mathrm{p}}}}(T_{\mathrm{i,s}}-T_{\mathrm{i,c}})\mathrm{.}

The second part of the error was due to the accuracy of the thermocouple (s₂). The thermocouple provided by the manufacturer was 0.75%, introducing a systematic error was nearly 6 K. The third part of the error could be induced by the high-temperature glue (s₃). Both the thermal conductivity and amount of the high-temperature glue were much less than those for the coal particle. Thus, heat transfer caused by high-temperature glue in the ignition process was little. Namely, s₃ could be be neglected [24]. Besides, some error could be introduced by temperature acquisition. Given the temperature acquisition frequency was 20 Hz and the variation of temperature was 100 K/s, this error was about ±0.1 K. Such a small error could be neglected in the measurement of T_i,c [25].

Therefore, calclauted by Eq. (4), s_c was nearly 20 K.

(4){\sigma }_{\mathrm{c}}=\sqrt{{\sigma }_{\mathrm{1}}^{\mathrm{2}}+{\sigma }_{\mathrm{2}}^{\mathrm{2}}+{\sigma }_{\mathrm{3}}^{\mathrm{2}}}.

The error of surface temperature measurement s_s depended on the accuracy of the black-body calibration process, and the maximum value was estimated at ± 10 K. In our experiments, the error caused by the optical system was neglected [14].

3 Results and discussion

3.1 Ignition delay time at low O₂ concentrations

Figure 4 depicts that the ignition delay time t* (t_i/t_i,air) decreases with X_{{\mathrm{O}}_2} under low X_{{\mathrm{O}}_2} conditions, which decreases obviously faster compared with the results obtained by Liu et al.[14] at a high X_{{\mathrm{O}}_2} value. Similarly, Ponzio et al. found that the decreasing rate at a low X_{{\mathrm{O}}_2} value is faster than the result obtained in this paper because they have adopted larger coal pellets with a diameter of 15 mm and a height of 35 mm in their experiments [17], which may strengthen the effect of O₂ on t_i at a low X_{{\mathrm{O}}_2} value. The reason for this is that X_{{\mathrm{O}}_2} becomes the dominant factor influencing t_i at a low X_{{\mathrm{O}}_2} value for homogeneous ignition of cm-sized coal particles.

Figure 5 exhibits the variation of t_i with T_w in air and low X_{{\mathrm{O}}_2} conditions. It can be seen that t_i is less sensitive to T_w at a low X_{{\mathrm{O}}_2} value than in air, especially at a higher T_w. This can be explained by the ignition theory of the flammability limit criterion proposed by Zhang and his coworkers [26]. Based on the theory, ignition occurs only if the content of volatiles generated by pyrolysis reaches the flammability which is inversely proportional to T_w and X_{{\mathrm{O}}_2}, but the influence becomes weaker as X_{{\mathrm{O}}_2} increases.

3.2 Ignition temperature at low O₂ concentrations

Shown in Fig. 6, in which T* is equal to the ratio of T_i to T_i,air, T_s decreases with increasing X_{{\mathrm{O}}_2} at a low X_{{\mathrm{O}}_2} value, which is consistent with that found by Zhou et al. [12]. As X_{{\mathrm{O}}_2} increases, more O₂ diffuses onto the particle, which accelerates the pyrolysis reaction. Therefore, more volatile matter is released to reduce T_i. These phenomena can also be found in air and high X_{{\mathrm{O}}_2} atmosphere [11,14]. However, similar to the results found by Annamalai and Durbetaki [13], T_s is hardly influenced by X_{{\mathrm{O}}_2} when using center temperature (T_c). Given that the internal temperature gradient cannot be neglected for large coal particles, T_c can better reflect the ignition of coal particle. At a low X_{{\mathrm{O}}_2} value, X_{{\mathrm{O}}_2} barely influences the heating rate of coal particles and thus it plays little role in T_c.

Figure 7 shows that T_i decreases with T_w, but the decrease at a low X_{{\mathrm{O}}_2} value and air is different. T_i is more sensitive to T_w at X_{{\mathrm{O}}_2} = 10%, which is consistent with the result reported by Zhou et al. [27]. At a low X_{{\mathrm{O}}_2} value, the flammability limit for coal ignition is high since the volatile content released is low. Therefore, T_i is more sensitive to T_w under a low X_{{\mathrm{O}}_2} condition than in air. When T_w increases, the devolatilization rate accelerates. Even at a low X_{{\mathrm{O}}_2} value, the surrounding volatile content is high enough to reach the flammability limit. Therefore, the difference in T_i is not significant. In air, the flammability limit can be met at different T_w values. Therefore, the decreasing rate of T_i is nearly constant.

3.3 Ignition modes at low O₂ concentrations

The transient combustion processes of a single coal particle at two low X_{{\mathrm{O}}_2} values and three T_w values are displayed in Fig. 8. It can be seen at the ignition that there is a large amount of soot surrounding the coal particles. It can be also seen that the burning intensity of the flame after ignition varies non-monotonically with T_w at the same X_{{\mathrm{O}}_2} value. For example, when X_{{\mathrm{O}}_2} = 10%, the brightness of the flame at T_w = 1023 K is between that at T_w = 973 K and that at T_w = 1073 K. When T_w is low (e.g., at 973 K), the evaporation of tar, which is regarded as the precursor of the volatile matter, into combustible gases is slow, and O₂ intends to diffuse onto the particle surface. Ignition occurs at the surface with a bright flame and gas phase combustion finishes more quickly as the evoluted volatile matter is depleted. When T_w increases to a certain value (e.g., at 1023 K), more tar is converted into combustible gases. The out-diffusing gases impede O₂ diffusion onto the surface of coal particles. However, the temperature is still not high enough to initiate the ignition while the volatile matter released could be swept by the buoyancy. Thus, a weaker instead of a stronger flame appears even at a high T_w value. As T_w further increases (e.g., to 1073 K), the coal particle changes into homogeneous ignition because more volatile is released. Both the concentration of combustible gases X_{{\mathrm{O}}_2} and temperature meet the flammability limits in the near space surrounding the coal particles.

To better reflect the evolution of flame during ignition, a dimensionless standoff distance r* is introduced, which is defined as the ratio of the distance of flame front standing away from particle center (x) to particle radius (r_p). Shown in Fig. 9, r* decreases with the increasing X_{{\mathrm{O}}_2}, especially for homogeneous ignition at a high T_w value, because a high X_{{\mathrm{O}}_2} value accelerates O₂ diffusion onto the particle surface. This could be attributed to the fact that a higher T_w promotes the pyrolysis of coal particles to produce more volatiles, but the volatiles inhibit the diffusion of oxygen to the surface of coal particles.

According to the flame standoff position, it can be inferred that heterogeneous ignition may occur at 973 K and a X_{{\mathrm{O}}_2} value of 15%. Figure 10 illustrates the ignition modes for single coal particles of 2 mm at different X_{{\mathrm{O}}_2} values. Four regions including unburned, heterogeneous, homogeneous and combined ones are divided. For large sizes, coal particles tend to ignite homogeneously. However, at a low X_{{\mathrm{O}}_2} and T_w value, heterogeneous ignition may occur because the volatile matter released could be swept away by buoyancy. At a higher T_w and a low X_{{\mathrm{O}}_2} value, combined ignition occurs. The reason for this is that more volatiles accumulate around coal particles with insufficient X_{{\mathrm{O}}_2}, leading to a vigorous flame with larger flame standoff but shorter duration, especially obvious at 973 K and a X_{{\mathrm{O}}_2} value of 10%. As T_w further increases, the balance between T_w and X_{{\mathrm{O}}_2} is broken, and ignition intends to be homogeneous again. The critical T_w between homogeneous ignition and heterogeneous ignition decreases with X_{{\mathrm{O}}_2}.

4 Conclusions

Experiments on ignition of single coal particle at low X_{{\mathrm{O}}_2} values were conducted at different T_w values. The combustion process was recorded by a high-speed camera while T_s and T_c were obtained by an infrared camera and thermocouples, respectively. It is found that the ignition delay time (t_i) decreases more rapidly as X_{{\mathrm{O}}_2} increases at a low X_{{\mathrm{O}}_2} value than at a high X_{{\mathrm{O}}_2} value. The surface temperature (T_s) for coal particles decreases with X_{{\mathrm{O}}_2} while the center temperature (T_c) remains constant with X_{{\mathrm{O}}_2} at a low X_{{\mathrm{O}}_2} value. t_i is less sensitive but T_i is more sensitive to T_w at a low X_{{\mathrm{O}}_2} value than in air. The flame front locates further away from coal particles at a high T_w value but approaches to particle surface at a high X_{{\mathrm{O}}_2} value. Observations of the position of flame front evolution illustrate that the ignition of a coal particle may change from a homogeneous mode to a heterogeneous or a combined ignition mode as X_{{\mathrm{O}}_2} decreases. At a low X_{{\mathrm{O}}_2} value, buoyancy plays a more significant role in sweeping away the released volatiles during the ignition process.

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