Combustion Characteristics of Single Particles from Bituminous Coal and Pine Sawdust in O₂/N₂, O₂/CO₂, and O₂/H₂O Atmospheres

Abstract

Burning fuels in an O₂/H₂O atmosphere is regarded as the next generation of oxy-fuel combustion for CO₂ capture and storage (CCS). By combining oxy-fuel combustion and biomass utilization technology, CO₂ emissions could be further reduced. Therefore, this work focuses on investigating the combustion characteristics of single particles from bituminous coal (BC) and pine sawdust (PS) in O₂/N₂, O₂/CO₂ and O₂/H₂O atmospheres at different O₂ mole fractions (21%, 30%, and 40%). The experiments were carried out in a drop tube furnace (DTF), and a high-speed camera was used to record the combustion processes of fuel particles. The combustion temperatures were measured by a two-color method. The results reveal that the particles from BC and PS all ignite homogeneously. Replacing N₂ by CO₂ results in a longer ignition delay time and lower combustion temperatures. After substituting H₂O for N₂, the ignition delay time is shortened, which is mainly caused by the steam gasification reaction (C + H₂O → CO + H₂) and steam shift reaction (CO + H₂O → CO₂ + H₂). In addition, the combustion temperatures are first decreased at low O₂ mole fractions, and then increased at high O₂ mole fractions because the oxidation effect of H₂O performs a more important role than its volumetric heat capacity and thermal radiation capacity. At the same condition, particles from PS ignite earlier because of their higher reactivity, but the combustion temperatures are lower than those of BC, which is owing to their lower calorific values.

1. Introduction

Coal will continue to be one of the major energy resources over the next few decades due to its low price and rich reserves. It is reported that coal’s share of the global primary energy consumption is 28.1% in 2016 [1]. However, the emission of CO₂ generated by coal-fired power plants is one of the most important factors leading to global warming [2,3,4]. In order to reduce the emission of CO₂, strategies need to be devised.

Oxy-fuel combustion is regarded as a promising technology for facilitating CO₂ capture and storage (CCS) by using the mixture of O₂ and recycled flue gas as an oxidizer, instead of air. Thus, a high concentration of CO₂ can be obtained in the flue gas, and only a small amount of follow-up work is required to achieve the capture and storage of CO₂. Therefore, oxy-fuel combustion has attracted considerable attention in recent years [5,6,7,8,9].

In 2010, Seepana and Jayanti [10] proposed a new variant of oxy-fuel combustion, called steam-moderated oxy-fuel combustion (SMOC). It burns fuel in a mixture of O₂ and steam, the exhausted gas mainly contains steam and CO₂, which can be sent for compression and sequestration after the condensation of steam. Compared with the O₂/CO₂ recycled combustion, the most significant advantage of SMOC is that the flue gas recirculation system is avoided, therefore, the layout of the plant is simplified and the overall system is easy to start and shut down. Furthermore, the pumping costs are lower because the transmission medium is water, instead of hot flue gas [11]. Finally, the formation of NO_X and SO_X is limited because of the presence of steam [10]. Owing to the advantages of SMOC, researchers have started to study the combustion behaviors of pulverized coal in an O₂/H₂O environment [11,12,13,14,15,16,17].

Another method to reduce CO₂ emission is the utilization of biomass [18,19,20]. Firing biomass for electricity generation can not only ease the energy crisis, but also reduce the emission of CO₂ because the carbon contained in biomass comes from CO₂ in the atmosphere. In addition, the emission of NO_X and SO_X can also be limited. Using biomass in existing coal-fired power plants only requires minor modifications, making the co-firing of biomass with coal an easier and less costly method for generating power [21]. However, the power output of a power-plant is reduced because of the lower calorific value of biomass, and the preparation of biomass fuels is influenced by many factors, such as the seasons. Furthermore, biomass contains high levels of K, Na, and Cl, therefore, the use of biomass may result in ash-related operational problems [22]. Although using biomass for electricity generation has some limitations, it is still a competitive technology for reducing the emission of CO₂ [23,24]. By combining oxy-fuel combustion and biomass utilization technology, the CO₂ emission could be minimized. Therefore, oxy-fuel combustion of biomass has attracted much interest [7,21,25,26].

For O₂/CO₂ recycled combustion, the combustion characteristics of coal and biomass are supposed to be different from O₂/N₂ combustion, because the physical properties of CO₂ are different from those of N₂, especially its volumetric heat capacity and lower diffusivity of O₂. Khatami et al. [6] and Molina et al. [27] reported that the ignition of pulverized coal and biomass occurred later in O₂/CO₂ atmospheres at the same O₂ mole fraction, which resulted from a lower gas temperature in the furnace caused by the higher volumetric heat capacity of CO₂. Khatami et al. [7], Zhang et al. [8], and Bejarano et al. [9] claimed that single coal and biomass particles burned at higher mean temperatures and shorter combustion times in O₂/N₂ than in O₂/CO₂ atmospheres. Riaza et al. [21] studied the combustion behaviors of four different pulverized biomasses in air and oxy-fuel conditions and observed that there was an obvious ignition delay of char upon extinction of the volatiles flame, and replacing the background N₂ gas with CO₂ reduced the combustion intensity. Compared with coal, the volatiles flame of biomass was dim due to its low calorific value.

For SMOC, the presence of steam will greatly change the physicochemical properties of the background gas, especially its reactivity [12]. Therefore, the combustion behaviors of coal and biomass are expected to be different from those in conventional or O₂/CO₂ recycled combustion. Zou and Cai et al. [11,12] numerically and experimentally studied the ignition characteristics of BC in O₂/N₂ and O₂/H₂O atmospheres and reported that the larger volumetric heat capacity and higher thermal radiation of H₂O would lead to a longer ignition delay time, whereas the stronger oxidation effect of H₂O would shorten the ignition delay time of BC. Cai et al. [13] then numerically investigated the ignition behaviors of BC in O₂/CO₂/H₂O atmospheres and found that the lower volumetric heat capacity, higher diffusivity and stronger oxidation effect of H₂O, compared to CO₂, would lead to earlier ignition, while the higher thermal radiation of H₂O would lead to ignition delay. Prationo et al. [14] claimed that the promotional effect of H₂O on ignition is more significant for low-rank coal, which is because the volatiles of low-rank coal contain more CO that can readily react with H₂O. Gil et al. [15], Yi et al. [16], and Zou et al. [17] studied the effects of H₂O on the combustion behaviors of coal and biomass chars by thermogravimetric analysis and reported that the ignition of chars was delayed and the comprehensive reactivity of chars was improved with the addition of H₂O.

Previous studies were focused on O₂/CO₂ recycled combustion and the ignition characteristics of coal in an O₂/H₂O environment. In particular, so little has been reported on the entire combustion processes of coal or biomass under an O₂/H₂O environment, especially of biomass. However, this is very important for designing the oxy-steam coal-fired or biomass power plants in the future. In this paper, the combustion characteristics of single particles from BC and pine sawdust (PS) in O₂/N₂, O₂/CO₂ and O₂/H₂O atmospheres at different O₂ mole fractions (21%, 30% and 40%) were studied experimentally. The entire combustion processes of single particles were recorded by a high speed camera and the combustion temperatures were measured by a two-color method.

2. Materials and Methods

2.1. Fuel Samples

Hengshan BC and PS were used in this study. The raw materials were air-dried (AD) before being used. Prior to experiments, the raw materials were ground and sieved to a size of 75–100 μm. The proximate, ultimate, and calorific value analysis of prepared fuel samples are given in

Table 1. Properties of fuel samples (daf, dry-ash-free).

Fuels	Proximate Analysis (wt %)				Ultimate Analysis (wt %)					HHV (MJ/Kg)
	Mad	Vad	FCad	Aad	Cdaf	Hdaf	Odaf	Ndaf	Sdaf
BC	7.99	32.68	50.07	9.26	67.77	4.14	25.35	1.01	1.73	25.74
PS	4.76	80.61	14.13	0.50	48.74	6.88	44.30	0.08	0.00	18.73

. It is clear that the volatile content of PS is higher than that of BC, whereas the higher heating value (HHV) and fixed carbon content of PS are lower. In addition, PS has a higher content of hydrogen and oxygen, indicating that PS has a higher reactivity and is easier to achieve ignition [28,29].

2.2. Experimental Method

The combustion experiments were carried out in an electrically-heated drop tube furnace (DTF) at a constant wall temperature of 1273 K. The DTF was equipped with a transparent quartz tube with an inner diameter of 5 cm and 80 cm in length. A slidable observation window was set to the DTF. The window can be slid to a suitable position where the combustion of fuel particles could be observed. A schematic of the experimental setup is given in Figure 1. The fuel particles were introduced into the furnace from the top entrance of the furnace.

The combustion behaviors of fuel particles were investigated in O₂/N₂, O₂/CO₂ and O₂/H₂O atmospheres under different O₂ mole fractions (21%, 30% and 40%). The total flow rate was set to 1 L/min. The flow rates of N₂, CO₂ and O₂ were controlled by mass flow controllers, while the flow rate of steam was controlled via a syringe pump. The syringe pump can control the flow of distilled water, which determines the flow rate of steam exiting from the steam generator. The steam generator and heating cords were heated up to 473 K to achieve complete vaporization and to avoid the condensation of vapor.

A high speed camera (MIKROTRON Eosens mini1 champion, Unterschleissheim, Germany) equipped with a Nikon 24–85 mm f/2.8-4D AF Zoom-Nikkor long-distance microscope lens (Nikon, Tokyo, Japan) was used to monitor the combustion processes of fuel particles. A frame speed of 1000 fps was chosen and the exposure time was set to 800 μs when recording the flame images.

2.3. Measurement of Temperature

The combustion temperatures of single particles were measured by a two-color method. Detailed theory of this method is given elsewhere [30,31]. The two-color measurement of temperature is based on Planck’s law of radiation. During the combustion process of coal and biomass particles, the flame radiation wavelength ranges from 300 nm to 1000 nm and the temperature ranges from 800–2000 K. Thus, Planck’s law of radiation can be replaced by Wien’s law of radiation:

$$E_{\lambda }(T)=\epsilon (\lambda )\frac{C_1}{{\lambda }^5}\exp (-\frac{C_2}{\lambda T})$$

(1)

where E_λ (W/m²) refers to the radiative energy, ε is the emissivity, λ (m) is the wavelength, C₁ and C₂ are the Planck constants (C₁ = 3.742 × 10⁻¹⁶ W·m² and C₂ = 1.4388 × 10⁻² m·K), and T (K) is the temperature. Therefore, the monochromatic radiative intensity can be given as:

$$I_{\lambda }(T)=\frac{1}{\pi }\epsilon (\lambda ,T)\frac{C_1}{{\lambda }^5}\exp (-\frac{C_2}{\lambda T})$$

(2)

According to chromatic theory, the flame image photographed by the high speed camera is a three-color (RGB) image under their respective wavelengths (λ_r = 7.00 × 10⁻⁷ m, λ_g = 5.46 × 10⁻⁷ m, λ_b = 4.35 × 10⁻⁷ m). The three primary colors red (R), green (G) and blue (B) were calibrated by coefficients K_r, K_g and K_b, and in combination with the Equation (2), we have:

$$I_{{\lambda }_r}(T)=K_{r}R=\frac{1}{\pi }\epsilon ({\lambda }_r,T)\frac{C_1}{{\lambda }_r^5}\exp (-\frac{C_2}{{\lambda }_{r}T})$$

(3)

$$I_{{\lambda }_g}(T)=K_{g}G=\frac{1}{\pi }\epsilon ({\lambda }_g,T)\frac{C_1}{{\lambda }_g^5}\exp (-\frac{C_2}{{\lambda }_{g}T})$$

(4)

$$I_{{\lambda }_b}(T)=K_{b}B=\frac{1}{\pi }\epsilon ({\lambda }_b,T)\frac{C_1}{{\lambda }_b^5}\exp (-\frac{C_2}{{\lambda }_{b}T})$$

(5)

where the R, G, and B are the intensity values of red, green, and blue of each pixel, respectively. Then, the flame temperature can be deduced from the ratio of the monochromatic radiative intensity at two different and nearby wavelengths. Red (R) and green (G) were selected to calculate the flame temperature in this study. In consideration of the narrow variation range of wavelength and temperature, it is reasonable to assume that the emissivity is constant. From Equations (3) and (4), the temperature expression can be derived as:

$$T=-C_2(\frac{1}{{\lambda }_r}-\frac{1}{{\lambda }_g})/\ln (\frac{K_{r}R{\lambda }_r^5}{K_{g}G{\lambda }_g^5})$$

(6)

where K_r/K_g is the only parameter that needs to be determined, and the expression of K_r/K_g is given in Equation (7):

$$\frac{K_r}{K_g}=\frac{{\lambda }_g^5}{{\lambda }_r^5}\frac{G}{R}\exp [-\frac{C_2(1/{\lambda }_r-1/{\lambda }_g)}{T}]$$

(7)

A blackbody furnace was used to calibrate K_r/K_g, a frame speed of 500 fps was selected for temperature measurement, and the exposure time was set to 30 μs to avoid the flame overexposure and the interference of the background color of the furnace. The typical radiation images captured from the blackbody furnace with temperature ranges from 1423–1873 K are shown in Figure 2. The temperature of each radiation image can be read from the blackbody furnace and the RGB values are obtained by MATLAB (MathWorks, Natick, MA, USA), thus the value of K_r/K_g can be calculated. The average K_r/K_g value of the ten images was used in this study. By this method, the temperature of each pixel in the flame image can be gained, and the average temperature of all pixels in a flame image is used to present the instantaneous temperature of this moment. According to the preliminary experiment, the absolute error of this method is less than 30 K, which is accurate enough for this study.

3. Results and Discussion

3.1. Cinematographic Observations Analysis

3.1.1. Ignition Mode and Combustion Process

The typical flame images of BC and PS particles in a simulated air condition (21%O₂/79%N₂) are presented in Figure 3. It is clear that particles from BC and PS ignite homogeneously in gas mode. The devolatilization starts first when fuel particles are introduced into the furnace, then the escaped volatiles are ignited earlier than chars because of their lower ignition temperature. After ignition, a two-stage combustion process is observed. For BC, the particle is surrounded by a bright and very large diffuse flame during the volatiles combustion stage. Upon extinction of the volatiles flame, the char combustion stage starts and the flame becomes a bright spot. Unlike BC, the volatiles flame of PS is low-luminosity and almost transparent, which is due to the lower calorific value of PS. These phenomena are in agreement with the results reported by Khatami et al. [6,7] and Riaza et al. [21].

3.1.2. Effect of Diluent Gas (N₂, CO₂ and H₂O)

The typical flame images of BC and PS in O₂/N₂, O₂/CO₂ and O₂/H₂O atmospheres at a 21% O₂ mole fraction are shown in Figure 4. Upon replacing N₂ with CO₂, the combustion intensity of BC and PS particles is significantly reduced at the same O₂ mole fraction. This is mainly contributed by the lower diffusivity of O₂ in CO₂ than in N₂ [7]. Furthermore, the volumetric heat capacity of CO₂ is 460 J/K·m³ at 1273 K, which is much larger than that of N₂ (325 J/K·m³). Larger volumetric heat capacity of background gas causes a lower gas temperature in the furnace under active flow conditions [6]. This causes the lower devolatilization rate and lower quantity of volatiles of fuel particles, therefore, resulting in a lower combustion intensity in O₂/CO₂ atmospheres. These results are in agreement with previous studies which were carried out for coal and biomass particles [7,8,9,21]. After substituting H₂O for N₂, the similar phenomena are observed. The volumetric heat capacity of H₂O is 427 J/K·m³ at 1273 K and is also larger than that of N₂. Thus, a high concentration of H₂O in the furnace will reduce the background gas temperature. In addition, the emission capacity of H₂O is higher than that of N₂, and this will cause higher radiant heat transfer and decrease the temperature of fuel particles [17]. Therefore, a high concentration of H₂O will decrease the combustion intensity in O₂/H₂O atmospheres.

3.1.3. Effect of O₂ Mole Fraction

As the O₂ mole fraction increases, the combustion intensity of BC and PS is greatly improved, as is seen in Figure 5 (the typical flame images of BC and PS particles in O₂/N₂ and O₂/CO₂ atmospheres at different O₂ mole fractions are given in the Supplementary Materials). For BC, when the O₂ mole fraction increases from 21–40%, the flames of volatiles and chars become brighter and larger, and the fragmentation of coal particles occurs more frequently. Furthermore, the two-stage combustion process becomes less obvious. This is because coal chars are ignited while the volatiles are still burning. For PS, the volatiles flames change from nearly transparent to opaque as the O₂ mole fraction increases. In addition, the brightness and size of flames of volatiles and chars are also improved. Unlike BC, an obvious two-stage combustion process is still observed at high O₂ mole fractions.

3.2. Combustion Temperature

The peak temperatures of volatiles combustion and char combustion of BC are shown in Figure 6. In order to minimize the error, the error bars were added to the final results [32]. Every experiment was repeated 10 times. The two-color method is not suitable for measuring the temperature of the biomass volatiles flame, which is because the gray-body emissivity assumption may not be valid for such low-soot flames [21,33]. Therefore, only average peak combustion temperatures of PS chars are given in Figure 7. It is clear to see that the peak combustion temperatures depend heavily on the O₂ mole fraction, as well as the type of diluent gas. The peak temperatures of BC and PS increase as the O₂ mole fraction increases. The lowest peak temperatures are obtained in O₂/CO₂ atmospheres at the same O₂ mole fraction, which is due to the lower diffusivity of O₂ in CO₂ and the larger volumetric heat capacity of CO₂.

At a low O₂ mole fraction (21% and 30% cases), the peak temperatures of BC and PS in O₂/H₂O atmospheres are lower than those in O₂/N₂ atmospheres. However, the peak temperatures are highest in O₂/H₂O atmospheres when the O₂ mole fraction reaches 40%. There are two kinds of effects of H₂O on the combustion temperatures of BC and PS particles. First is the physical effect, the volumetric heat capacity of H₂O is larger than that of N₂, and the emission capacity of H₂O is higher: these factors will decrease the combustion temperature. Second is the chemical effect: the oxidation effect of H₂O is stronger than that of N₂ or CO₂ [13], thus, the oxidation effect of O₂/H₂O atmospheres is stronger than those of O₂/N₂ or O₂/CO₂ atmospheres, which is beneficial for combustion. Combining with the experimental results, it is clear that the global effect of H₂O is associated with the mole fraction of O₂. At low O₂ mole fractions, the H₂O concentration is high, and the effect of volumetric heat capacity and emission capacity performs a more important role. Thus, the peak temperatures are lower in O₂/H₂O atmospheres. However, when the mole fraction of O₂ reaches 40%, the effect of volumetric heat capacity and emission capacity of H₂O is minor, and the oxidation causes a greater effect; therefore, the combustion temperatures are increased. At the same condition, the average peak temperatures of char combustion of PS are lower than those of BC, which is because the calorific value of PS is lower.

According to the previous studies [7,8,9,21], the combustion temperatures of coal and biomass in an O₂/CO₂ environment are close to those in air (21%O₂) when the O₂ mole fraction increases to 30~35%, which is in agreement with the results obtained in this study. However, when substituting H₂O for N₂, the combustion temperatures are reduced less and are even higher when the O₂ mole fraction reaches 40%.

3.3. Ignition Delay Time

The ignition delay time is defined as the time from the particle enters into the furnace until the first visible flame is detected. The error bars were also added to the results of the ignition delay time, and every experiment was repeated 20 times. The ignition delay time of BC and PS under different atmospheres is shown in Figure 8. At the same condition, the ignition of PS particles occurs earlier than BC, which is owing to the lower ignition temperature of PS volatiles. The ignition delay time of BC and PS decreases with an increasing mole fraction of O₂, because the oxidation effect of background gas is improved. The ignition delay time of O₂/CO₂ atmosphere is longer than that of an O₂/N₂ atmosphere at the same O₂ mole fraction. This is mainly caused by different physical properties between CO₂ and N₂. In an O₂/CO₂ atmosphere, the gas temperature is lower than that in an O₂/N₂ atmosphere because of the larger volumetric heat capacity of CO₂. Therefore, the ignition occurs later in the O₂/CO₂ environment.

On the contrary, replacing N₂ with H₂O leads to the earlier ignition of fuel particles. This is caused by the different physical and chemical properties between H₂O and N₂. In terms of physical properties, the volumetric heat capacity of H₂O is larger than that of N₂ at 1273 K, similar to the O₂/CO₂ environment, the higher volumetric heat capacity of H₂O will result in ignition delay in the O₂/H₂O environment. In terms of chemical properties, there are two aspects. During the heating stage of fuel particles in the O₂/H₂O environment, the steam gasification reaction and steam shift reaction will start [11,12]. The chemical equations of the steam gasification reaction and steam shift reaction are given in Equations (8) and (9), respectively:

$$\mathrm{C}+{\mathrm{H}}_2\mathrm{O}\to \mathrm{CO}+{\mathrm{H}}_2,\Delta \mathrm{H}=130\mathrm{KJ}/\mathrm{mol}$$

(8)

$$\mathrm{CO}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{CO}}_2+{\mathrm{H}}_2,\Delta \mathrm{H}=-40.9\mathrm{KJ}/\mathrm{mol}$$

(9)

Firstly, the steam gasification reaction is an endothermic reaction and will reduce the particle temperature, which is detrimental to ignition. However, the products of this reaction (CO and H₂) are beneficial to shorten the ignition delay time because of their lower ignition temperatures. Secondly, the steam shift reaction is an exothermic reaction and this reaction also produces H₂. Therefore, the steam shift reaction is beneficial to ignition.

From the above analysis, it can be seen that the influence of H₂O on ignition delay is diversified. Combining with the experimental results, it can be inferred that the factors leading to earlier ignition have a greater effect in the O₂/H₂O environment. Previous studies reported that the ignition of coal and biomass chars would be delayed with the addition of H₂O [15,16,17]. However, the ignition of BC and PS particles is promoted in this study. Therefore, it can be inferred that the volatiles of BC and PS perform a crucial role during the ignition process in the O₂/H₂O environment. For BC and PS, their volatiles contain CO that can readily react with H₂O [6,14,21]. Thus, the steam shift reaction has a greater effect on the ignition of BC and PS particles in this study. However, the coal and biomass chars contain no volatiles, therefore, there is no steam shift reaction. The steam gasification reaction has a small effect on ignition because of its low reaction rate. Therefore, the physical properties of H₂O have a greater effect of the ignition of chars and result in a longer ignition delay time.

4. Conclusions

In this study, the combustion characteristics of single particles from BC and PS in O₂/N₂, O₂/CO₂ and O₂/H₂O atmospheres at different O₂ mole fractions (21%, 30% and 40%) were investigated. The results reveal that the particles from BC and PS all ignite homogeneously in O₂/N₂, O₂/CO₂ and O₂/H₂O atmospheres. As the O₂ mole fraction increases, the ignition delay time becomes shorter and the combustion temperatures are increased. By replacing N₂ with CO₂, the ignition delayed time is extended and the combustion temperatures are decreased. By replacing N₂ with H₂O, the ignition occurs earlier, and the combustion temperatures are decreased at a low O₂ mole fraction, while increased at a high O₂ mole fraction. Under the same condition, the ignition of PS particles occurs earlier, while the combustion temperatures are lower than those of BC.

Comparing with O₂/CO₂ recycled combustion, O₂/H₂O combustion can not only achieve the capture and storage of CO₂, but also maintains a higher combustion temperature of BC and PS. In addition, the ignition delay time is shortest in the O₂/H₂O environment. Therefore, burning BC and pine sawdust in an O₂/H₂O environment is a more competitive technology for facilitating CCS.