Effects of ambient temperature on the combustion processes

Effects of ambient temperature on the combustion processes of single pulverized coal particle

1. Introduction

Coal is one of the important primary energies that supplies about 30% of primary world energy and about 40% of

power generation. On the other hand, concerns about the usage of coal arising from the view point of the amount of the combustion exhausts including carbon dioxide (CO2), NOx, SOx, and particulate matters. CO2 is one of the greenhouse gases, and its reduction is required. Carbon Capture and Storage (CCS) is studied as a technology that reduce the CO2 emission in coal-fired power generation. There are three methods for capturing CO2: a pre-combustion capture, a pot-combustion capture, and an oxy-fuel combustion (Toftegaard et al., 2010). The oxy-fuel combustion has attracted attention for high efficiency, NOx reduction, and CCS, etc. The oxidant of the oxy-fuel combustion is a mixture of oxygen and CO2 recycled from exhaust gas. Since coal is mostly used as pulverized coal, combustion characteristics of pulverized coal in oxy-fuel conditions have been investigated in practical power plants (Yin et al., 2002; Belosevic et al., 2006; Asotani et al., 2008). Tan et al. (Tan et al., 2006) investigated combustion characteristics of several different types of lignite coals under oxy-fuel conditions in a laboratory-scale combustor. Experiments and computational fluid dynamics (CFD) modelling demonstrated that the oxy-fuel combustion significantly reduces NOx generation while maintaining excellent fuel combustibility. Edge et al. (Edge et al., 2011) conducted large eddy simulation (LES) of the pulverized coal combustion in a 0.5 MW furnace under air and oxy-fuel. In their simulation, the radiative heat transfer from coal

Shinya SAWADA*, Daisuke OKADA*, Noriaki NAKATSUKA*, Kazuki TAINAKA**, Tsukasa HORI*, Jun HAYASHI*,*** and Fumiteru AKAMATSU* *Department of Mechanical Engineering, Osaka University

2-1 Yamadaoka, Suita, Osaka, 565-0871, Japan E-mail: [email protected]

**Energy Engineering Research Laboratory, Central Research Institute of Electric Power Industry (CRIEPI) 2-6-1, Nagasaka, Yokosuka, Kanagawa, 240-0196, Japan

***Department of Energy Conversion Science, Kyoto University Yoshida-Honmachi, Sakyo-ku, Kyoto, 606-8501, Japan

Received: 24 May 2020; Revised: 18 August 2020; Accepted: 23 September 2020

Abstract In the pulverized coal combustion, coal particles cross over a steep temperature gradient formed by a diffusion flame. This temperature gradient affects the particle temperature. This study has experimentally investigated effects of field temperature and residence time in high-temperature regions on the flame structure of single coal particles, since the substances of the devolatilization process varied due to the particle heating rate. The inlet velocity and the oxygen concentration of a laminar couterflow vary to control the residence time and the temperature gradient, respectively. A magnified two-color pyrometry was carried out to understand flame structure and the time series of flame and particle temperature. The results showed that the increase of oxygen concentration raises the volatile matter combustion temperature and flame diameter, and prolongs duration of the volatile matter combustion. The char combustion temperature decreases as the flow velocity increases. The maximum effective flame diameter increases linearly with increasing volatile matter combustion temperature regardless of particle size. This suggested an increase in flame interference distance. The maximum flame diameter increases monotonically with increasing volatile matter combustion temperature.

Keywords : Single coal particle, Volatile matter, Soot, Two-color pyrometry, Char combustion, Counterflow field

Bulletin of the JSME

Journal of Thermal Science and TechnologyVol.16, No.1, 202

Paper No.20-00262© 2021 The Japan Society of Mechanical Engineers[DOI: 10.1299/jtst.2021jtst0011]

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particles was also taken into account. Results implied that LES could be used to investigate instantaneous flow characteristics of turbulent coal flames such as the recirculation zones, and the prediction of radiation from the coal particle surface. Hees et al. (Hees et al., 2019) examined the effect of oxygen concentration on the flame structure using a 40 kW pulverized coal swirling flame, and found that the flame structure remains the same for all oxygen concentrations. Besides, the combustion reactions near the nozzle exit were enhanced with the increase of oxygen concentration.

The combustion process of coal particles mainly consists of volatile matter combustion and char combustion. In volatile matter combustion, the temperature of particles rises, and volatile matter generated by pyrolysis is released and burned. Since volatile matter combustion is the first step in pulverized coal combustion and affects its subsequent phenomena. The investigation of the reaction kinetics of coal and optical measurement was conducted in previous studies (Ahn et al., 2016; Hashimoto et al., 2016; Lee et al., 2015). Ahn et al. (Ahn et al., 2016) investigated reaction kinetics of the volatile matter from coal particle based on analyses with reduced chemical mechanism. They showed that hydrocarbons such as aromatics, methyl and ethyl groups play important role in the ignition and flame propagation processes. Hashimoto et al. (Hashimoto et al., 2016) investigated soot particle size distributions in a coal flame with the time-resolved laser induced incandescence (TiRe-LII) method and the thermophoretic sampling (TS) method. They showed that soot volume fraction and the primary soot particle diameter increases with increasing the height above the burner in any radial distance from the ensemble-averaged TiRe-LII images. Lee et al. (Lee et al., 2015) focused on the initial stages of coal combustion and undertook visual observation of burning coal particles in a hot flowing gas environment. They showed that increasing the oxygen concentration shortens the heat up time and reduces the effective radius of the volatile flame. The effect of oxygen concentration on the flame structure and combustion temperature of pulverized coal has been investigated in previous studies. Shaddix and Molina (Shaddix and Molina, 2009) showed that the soot cloud size and temperature are strongly influenced by the oxygen content of the bulk gas. The results show that the diluent CO2 delays the ignition of single-particle coal and prolongs the duration of volatile matter combustion. Kim et al. (Kim et al., 2014) carried out an experiment with a Hencken burner in an oxy-fuel environment and simulation of burning coal char particles. The simulation showed that it is essential to consider the CO2 gasification reaction when simulating char combustion in an oxy-fuel combustion environment. Köser et al. (Köser et al., 2015) performed highly repeated OH-LIF measurements on single coal particles. They used a laminar flow reactor that provided a hot oxygen-rich exhaust gas environment. Time-resolved imaging of the OH distribution at 10 kHz allowed to identify post-reaction and post-combustion zones and visualize the time evolution of coal particles during combustion. These studies show that the increase in oxygen concentration causes a decrease in flame size and an increase in combustion temperature.

Although these studies showed general combustion characteristics of pulverized coal under oxy-fuel combustion, the detailed combustion processes of pulverized coal particles were still required to verify the numerical simulations in this field. To understand the combustion process in a particle cloud, it is important to investigate single particle combustion. The investigation of a single coal particle flame would complement existing data on coal properties and ultimately leads to the development of the coal combustion model. Investigation of a single coal particle flame would complement existing data on coal properties and ultimately leads to the development of a coal combustion model that is closely related to the formation and stability of large-scale coal flames.

The current study aims to extend previous attempts by observing the combustion of individual coal particles and quantifying underlying properties. Specifically, information on the coal temperature, which is an essential parameter for ignition and radiative heat transfer, is limited. During the oxy-fuel combustion, the temperature of the field changes because the O2 concentration and the specific heat of gas are different from air. Also, the residence time of the coal particle varies because the amount of exhaust gas decreases. The effect of field temperature on the flame temperature of coal particles has been investigated and reported by Jeffrey et al. (Jeffrey et al., 2006). Although oxygen-enriched combustion was found to increase the char combustion temperature and to reduce the char burnout time, the effect of residence time is not considered. The flame structure and temperature interact with each other. Both of them change due to changes in field conditions. Therefore, the aim of this study is to clarify the effects of residence time and field temperature on the flame structure and temperature of the coal particle simultaneously. The temperature of the field and the residence time were changed by the O2 concentration diluted with the inert gas nitrogen and the flow velocity. This study measured the time-series soot temperature, particle temperature, and particle diameter during pulverized coal combustion.

2© 2021 The Japan Society of Mechanical Engineers[DOI: 10.1299/jtst.2021jtst0011]

Sawada, Okada, Nakatsuka, Tainaka, Hori, Hayashi and Akamatsu, Journal of Thermal Science and Technology, Vol.16, No.1 (2021)

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2. Experimental setup and methods 2.1 Experimental setup and measurement apparatus

In an actual pulverized coal burner, unburned pulverized coal particles enter a high-temperature region and experience the steep temperature gradient during combustion process. In this study, therefore, pulverized coal particles issued into a laminar counterflow field with a hydrogen diffusion flame to simulate particle crossing a steep temperature gradient. Figure 1 shows a schematic of the experimental apparatus and image of laminar counterflow flames. The experimental apparatus comprises a counterflow burner, a pulverized coal particle feeding system, a blue backlight system, and a high-speed digital CMOS color camera (Phantom V12.1, Vision Research, Inc.). Figure 2 shows a schematic and an image of a laminar counterflow burner. The top and bottom ports are coaxially mounted opposite to each other. The diameter of the port exit is 25.4 mm, and the separation distance between the top and bottom ports is 30 mm. A pulverized coal particle feeding system is set on the top of the top burner port. Pulverized coal is fed using a screw feeder. The main oxidizer flow of the top port is rectified by two punched metal plates and mixed with the pulverized coal to form a well-mixed flow of oxidizer and the premixed coal particles. The main oxidizer flow carries the pulverized coal particles and supplies them to the combustion region as a two-phase stream. The main fuel flow (XH2: 34.5%, XN2: 65.5%) of the bottom port is rectified using two punched metal plates. For both the top and bottom ports, the co-axial nitrogen stream with the same velocity as that of the mainstream is issued from the outer annular as a curtain flow. The top and bottom ports are maintained at 120ºC using an oil-chiller (EZ-101, TAITEK) to prevent condensation of steam in the burned gas. The volumetric flow rates of gases are maintained constantly using the mass flow controllers. After establishing coal flames in the counterflow field, we measure the average and instantaneous spatial distribution of the soot and coal char temperature with two-color pyrometry and the particle shape with backlit imaging (Sawada et al., 2020). A high-speed camera enables to measure the temperature of soot and solid surface with high temporal resolution (Densmore et al., 2011a, b). A two-color pyrometry requires taking images at two wavelengths. The procedure of the two-color pyrometry using a color camera is selected in this study because it can perform the temperature measurement with only one camera and lens without requiring complicated optical systems. Since heated soot can emit the visible radiation during coal combustion, the simultaneous measurement of particle shape could distinguish the temperature of coal char and soot by carefully selecting two wavelengths in two-color pyrometry. Backlit imaging is suitable for particle visualization (Mock et al., 2017). When the coal particles heated up to the high temperature, the coal particles emit the visible radiation in red during coal char combustion. Therefore, previous research used a blue light (peak wavelength at ~525 nm) and mentioned that it has little effect on two-color pyrometry (Adeosum et al., 2019). In this study, the temperature of a pulverized coal particle measured by the two-color pyrometry was defined as the particle temperature. In this study, continuous light from the blue LED (central wavelength = 440 nm, FWHM = 60 nm) is used as the backlight source. The output of the blue LED is 45 W. The optical diffusion board distributes the LED light homogeneously. A magnified high-speed imaging was performed using a high-speed CMOS camera (10000 fps, exposure time 99 μs) equipped with a long-working-distance lens (UWZ300F, Union Optical). Measurements area is 15~20 mm from the exit plane of the top port (Figure 1). The measurement area is 17.8 mm2 with a spatial resolution of 6.02 μm/pixels. Since the materials in the coal particle can emit ion emissions of 589 nm from sodium (Na) and 760 nm from potassium (K) (Wu et al., 2019), a notch filter (central wavelength = 590 nm, FWHM = 60 nm) and UV-IR cut filter are attached on the long-working-distance lens. Measurements were conducted in a time series of 200,000 images. The temperature of soot and coal particles are analyzed for all images in each condition. In the analysis, the blue channel (B image) was used for the particle position, and green and red channels (G and R images) were used for calculating the temperature distribution. Image analysis was performed for removing the defocused particles in the following steps. First, noise signals on each pixel were subtracted from the B image. Then, the binarized process is proceeded by detecting the outline of the pulverized coal particle.



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Fig. 1 Schematic of experimental apparatus and image of laminar counterflow flames.

Fig. 2 Schematic and image of counterflow burner.

2.2 Experimental conditions

Bituminous coal, is used as fuel. The mass-based median diameter measured by the laser diffraction particle size analyzer is 44 μm, and the mean volume diameter (D30) is 48 μm. The properties and the supply conditions of each component from both the top and bottom burner ports are listed in Table 1 and 2, respectively. The cross-sectional average flow velocities are 41.1 cm/s from the top port and 50.6 cm/s from the bottom port. The strain rate based on the carrier gas under the unburned condition is 30.6 s-1 under Cases 1~3. The momentum of the gas from the top and bottom ports is set to equal. To investigate the effect of the residence time in the high-temperature region on the time series of the temperature of coal particles, the flow velocity in Case 4 is doubled compared to those of Cases 1~3. To highlight the effect of particle size on soot formation of individual pulverized coal flames, the flow rate of the pulverized coal is set at 0.37 g/min (overall equivalence ratio of pulverized coal fcoal = 0.306 (Case1), 0.215 (Case2), 0.128 (Case3), 0.153 (Case4)). Since the flame structure of single coal particle was focused in this study, the effects of the overall equivalence ratio of pulverized coal on a flame structure were assumed to be negligibly small in that range.

FuelFuel

OxidizerOxidizer

N2

N2

N2

N2

Coal

Oil tank

Oil tank

Oil tank

Mesh

Mesh

30 mm

Imaging area

Top port25.4 mm

Bottom port25.4 mm

Oxidizer + Pulverized coal

Fuel

N2

O2

N2

H2

Pulverized coalsupply device

Mass flowcontrollers

Mixing tanks

Counterflowburner

PC

High speed CameraLED(Blue)

Optical diffusion board

Long focus lens

Filter

DC power supply



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Table 1 Coal properties.

Proximate analysis [wt %]

Moisture 0.7 Ash 14.2 Volatile matter 33.5 Fixed carbon 52.3

Elementary analysis [wt%db]

C 70.5 H 4.64 N 1.66 O 8.6 S 0.46

Heating value [MJ/kg] Higher heating value 28.830

Lower heating value 27.780

True specific gravity 1.42

Particle size [µm] Mean volume diameter (D30) 48

Mass-based median diameter 44

Table 2 Experimental conditions: chemical composition and flow velocity.

Case Top port (Oxidizer + Coal)* Bottom port (Fuel)

XO2 [%] XN2 [%] Cross sectional average flow velocity [cm/s] XH2 [%] XN2 [%] Cross sectional average

flow velocity [cm/s] 1 21 79

41.1 34.5 75.5 50.6 2 30 70 3 50 50 4 21 79 82.2 101

* Coal feeding rate: 6.1 mg/s (fcoal = 0.306(Case1), 0.215(Case2), 0.128(Case3), 0.153(Case4))





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3. Results and discussion 3.1 Overall characteristics of flame structure and soot formation

The particle size affects the overall structure of the flame and soot formation in the multi-phase combustion (Hayashi et al, 2011; Hayashi et al. 2013; Li et al., 2017). The time series of soot formation process and the time series of particle temperature was examined with the different sizes of pulverized coal particles in order to obtain a detailed flame structure and temperature profiles of pulverized coal particles each particle size (dp). dp is the effective diameter from the area of individual particles. Typical features of the combustion behavior of pulverized coal ((A) image and (B) spatially temperature distribution) are shown in Fig. 3. The combustion behavior of pulverized coal particles is shown every 1 ms in Fig. 3. The coal particles pass through the diffusion flame. The coal particles receive heat from the diffusion flame and release volatile matters. The time zero (t=0) was set when the soot emitted thermal emission and the luminance exceeded the temperature measurement threshold (T=1300 K). Figure 3 indicates that the transition from the combustion process of volatile matter accompanied soot formation to the char combustion process. This figure also shows that soot is not distributed symmetrically around the coal particle, since the volatile matter is non-uniformly released around the coal particle.

The soot formation area shrinks and disappears after the end of releasing volatile matter. The char combustion starts because the oxidizer can reach the particle surface. The time scale of soot formation observed was in the range from 2 to 4 ms, although this time scale in the combustion of pulverized coal particle was reported as the order of 10 ms (Mock et al., 2017). Besides, the existence time of soot increases with the increase of the particle size as shown in Fig. 3. The combustion of volatile matter requires less time for smaller coal particles because the amount of volatile matter is roughly proportional to the volume of particles. The release rate of volatile matter increases with the heating rate increase (Hashimoto et al. 2012). Thus, an increase in the heating rate increase due to a decrease in heat capacity increase the release rate of volatile matter. The release rate of volatile matter may increase with decreasing particle size because the heat capacity of the small coal particle is lower than that of the large particle.

Fig. 3 Combustion behavior of pulverized coal under various experimental conditions. (A) Image of the particle. (B) Temperature distribution. (Volatile matter combustion starts at 0 ms.)

Figure 4 shows (A) the time series of spatially-averaged temperature, and (B) the effective flame diameter normalized by particle size (Df/dp). Khatami and Levendis reported that there are two peaks of temperature during the combustion of

(A)200 μm

(B)

ⅰⅱⅲⅳⅴ

ⅵⅶⅷ

ⅰⅱⅲⅳⅴ

ⅵⅶⅷ

(A)200 μm

(B)

ⅰⅱⅲ

ⅳⅴ

ⅵⅶ

ⅷ

ⅰⅱⅲ

ⅳⅴ

ⅵⅶ

ⅷ

(A)

200 μm

(B)

ⅰⅱⅲ

ⅳⅴⅵ

ⅶ

ⅷ

ⅰⅱⅲ

ⅳⅴⅵ

ⅶ

ⅷ

dp = 76 μm dp = 96 μm dp = 132 μm

Volatile matter combustionChar combustion

ⅰ: t = 0 msInterval 1 ms

30 mm

Imaging area

Top port25.4 mm

Bottom port25.4 mm



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coal particle; one for volatile matter combustion and the other for char combustion (Khatami and Levendis, 2011). As shown in Fig. 4, the spatially-averaged temperature showed the first peak just before the end of the combustion of the volatile matters. Since soot mainly emits visible radiation during the volatile matter combustion, the temperature of the luminous flame measured by the two-color pyrometry was defined as the volatile matter combustion temperature. The second peak of temperature appeared in the middle of char combustion. Those results support the previously observed tendencies of temperature. Figure 4 also shows that the value of Df/dp reaches from 2 to 5 in the volatile matter combustion. On the other hand, the value of Df/dp does not change in the char combustion. The increase of oxygen concentration raises the volatile matter combustion temperature and flame diameter, and prolongs duration of the volatile matter combustion. The increase in oxygen concentration shortens the period from the end of volatile matter combustion to the start of char combustion. The effective flame diameter and the volatile matter combustion temperature decrease with increasing flow velocity. These results could be due to the difference between the field temperature and residence time in the high temperature region, which affected the temperature history of the particles. Since the coal particle size affects the amount of volatile matter, the flame size of volatile matter would vary with the coal particle size. Also, the temperature around the particles affects pyrolysis and release of volatile matter.

Figure 5 shows the effect of dp and volatile matter combustion temperatuire (Tvolatile) on the maximum diameter of the flame diameter (Dfmax). It is found from Fig.5 (A) that Dfmax increases monotonically with increasing Tsoot. The increase in ambient temperature promotes the pyrolysis of coal. Also, the diffusion coefficient increases with the gas temperature. This indicated that the temperature and volatile flame structure interacted with each other. Interestingly, Dfmax has little effect on particle size as shown in Fig. 5(B). This result indicated that the particle sizes are enough small to heat up even in the steep temperature gradient.

Fig. 4 Time series examples of temperature measurement from volatile matter combustion to coal char combustion (Particle size is 90-100 μm); (A) Time series average temperature, and (B) Time series effective flame diameter normalized by particle size (Volatile matter combustion starts at 0 ms).

1500170019002100230025002700290031003300

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0

T

K

Time ms

Case1 Case2Case3 Case4

0

1

2

3

4

5

6

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0

Df/dp

-

t ms

(A)

(B)



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Fig. 5 Effect of particle size and Tvolatile on Dfmax under conditions of varying oxygen concentration and flow velocity. (A) Relationship between Tvolatile and Dfmax. (B) Relationship between dp and Dfmax. 3.2 Char combustion temperature

Figure 6 shows the relationship between particle size and the char combustion temperature. When the temperature at the particle became constant, the temperature at that area was defined as the char combustion temperature. The char combustion temperature increased with increasing oxygen concentration. This is probably due to the temperature increase in the high-temperature region. These results are in good agreement with the previous study (Shaddix and Molina, 2009). Moreover, the char combustion temperature decreases as the flow velocity increases comparing the results obtained from Case 1 and Case 4. In Cases 1 and 4, the temperature in the high-temperature region does not change significantly. Thus, it is considered that the shortening of the residence time in the high-temperature region is an influence.

Fig. 6 Relationship between particle size and char combustion temperature under conditions of varying oxygen concentration and flow velocity, where Tchar denotes the char combustion temperature.

(A) (B)



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4. Conclusions

This study investigated the effect of the residence time and field temperature on the flame structure of coal particles. The ambient temperature and residence time were changed by the oxidizer diluted with N2 of inert gas and flow velocity. We measured the soot temperature, particle temperature, and particle size during the combustion of volatile matter. The results are shown below.

(1) The increase in the particle size causes the increase in the duration of volatile matter combustion. (2) The increase of oxygen concentration raises the volatile matter combustion temperature and flame diameter,

and prolongs duration of the volatile matter combustion. The effective flame diameter and volatile matter combustion temperature decrease with increasing flow velocity. The char combustion temperature decrease as the flow velocity increases. The maximum effective flame diameter (Dfmax) increases linearly with increasing volatile matter combustion temperature regardless of particle size.

(3) The increase of oxygen concentration raises the char combustion temperature. The char combustion temperature decreases as the flow velocity increases.

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