The proximate analysis and ultimate analysis results of SBC.
As one of the most important primary energy, bituminous coal has been widely applied in many fields. The combustion studies of bituminous coal have attracted a lot of attention due to the releases of hazardous emissions. This work focuses on the investigation of combustion characteristics of Shenmu bituminous pulverized coal as a representative bituminous coal in China with a combined TG-MS-FTIR system by considering the effect of particle size, heating rate, and the total flow rate. The combustion products were accurately quantified by normalization and numerical analysis of MS results. The results indicate that the decrease of the particle size, heating rate, and the total flow rate result in lower ignition and burnout temperatures. The activation energy tends to be lower with smaller particle size, faster heating rate, and lower total flow rate. The MS and FTIR results demonstrate that lower concentrations of different products, such as NO, NO2, HCN, CH4, and SO2, were produced with smaller particle size, slower heating rate, and lower total flow rate. This work will guide to understand the combustion kinetics of pulverized coals and be beneficial to control the formation of pollutants.
- bituminous coal
- combustion characteristics
- TG-MS-FTIR technique
- particle size
- heating rate
- flow rate
Coal is one of the most important primary energy for last whole century and future decades. As the foundation of the application of coal, the research on bituminous coal combustion has never been stopped. Different kinds of bituminous coals with multitudinous research instruments and methods for the investigation of coal combustion characteristics have been carried out for years. Peng and Wu  studied the bromine release from bituminous coal during combustion. They used the sequential chemical extraction method to investigate the modes of occurrence of bromine in bituminous coal by a tube furnace. The results showed that the bromine release rate increased with the increase of temperature, and 500–900°C was the main stage of bromine release. They also concluded that water vapor can promote the Br release and the additive of SiO2 can capture the bromine effectively in the process of the coal combustion. Tsuji et al.  studied the combustion emission of one bituminous coal from Australia and two high-ash coals from South Africa on different blending ratios by an advanced low NO
In recent years, SBC has been studied in terms of its burning and gasification properties. In 2003, Sun et al. investigated the thermogravimetric (TG) characteristics of SBC and reported that vitrinite had higher volatile matter yield, maximum weight loss rate, and lower initial decomposition temperature and peak temperature than that of inertinite. Compared to pressure and heating rate, the temperature has a more important impact on the devolatilization of SBC . With TG analysis, Zhao et al. studied the ignition temperature (
2. Materials and methods
2.1. Coal samples
The SBC samples were bought from Shenmu Energy Developments Ltd Company and grinded into powder with different sizes. Before test, the sample was put into drying oven at a temperature of 105°C for 90 min. Then, the coals were sieved to a particle size of lesser than 40, 90–100, 128–180, 280–355, and 355–500 μm. To identify the phases of SBC, X-ray diffraction (XRD) patterns were recorded using Bruker D8 Focus at 40 kV and 150 mA with Cu Kα (
The elements of SBC samples were identified by the coal proximate analyzer as shown in Table 1. Mad, Aad, Vad, FCad, and
|Proximate analysis||Ultimate analysis|
2.2. Comprehensive test
The tests of SBC combustion characteristics were performed with the TG-MS-FTIR system which comprises a TG analyzer (STA-449F3, Netzsch), a mass spectrometer (QMS403C, Aeolos), and an FTIR spectrometer (Tensor 27, Bruker). The combined system was controlled simultaneously and data were recorded by a computer as shown in Figure 2. Due to the TG’s limit of highest heating temperature and heating rate, the highest heating temperature was set at 1200°C and the heating rate were set at 10, 20, 30, and 40°C/min. Considering the oxygen demand for complete combustion of 10 mg SBC samples within the limitations of TG, the total flow rates were set at 50, 100, and 150 sccm. The tests were carried out in four steps. First, an SBC sample of about 10 mg was dispersed on a circular alumina pan and the air in TG chamber was replaced with a gas mixture which consists of 20% O2 and 80% Ar. The supplied O2 was much more than the theoretically needed for the complete oxidation of the SBC sample but was similar to the air. Second, the samples were heated with a heating rate of 10°C/min from 40 to 110°C and kept for 30 min. Third, the samples were heated to 1200°C by four heating rates, namely, 10, 20, 30 and 40°C/min, respectively. Finally, the whole temperature-rising program was finished after an isothermal process of 10 min. Three total flow rates (50, 100, and 150 sccm) containing 20% O2 and 80% Ar were sent into the TG chamber during the whole heating processes. The detailed experimental conditions are summarized in Table 2.
|Condition||Mass (mg)||Flow rate (sccm)||Heating rate (°C/min)||Particle size (μm)|
2.3. Data treatment
2.3.1. Characteristic temperature
The ignition characteristic of SBC is analyzed based on
2.3.2. Kinetic analysis
The combustion kinetic parameters from TG data are calculated by the Coats-Redfern method :
where is the weight loss ratio;
2.3.3. Products analysis
To obtain the volumetric flow rates of the products during the oxidation of SBC, both MS and FIIR were used. In order to provide accurate quantitative analysis, a novel method of equivalent characteristic spectrum analysis was employed and the details of such method can be found elsewhere . In the MS analysis, argon was used as a reference gas to calibrate the products by considering the electron impact ionization cross sections, ion flow intensities, and the partial pressures of different species. The characteristic spectra and relative sensitivity were deduced. The effect of initial coal weight on the formation of products was excluded by normalization of the coal weight.
Derivative thermogravimetry (DTG) curve is a very important result of SBC because DTG can reflect the change of combustion rate directly and it was related to the acquirement of ignition temperature, burnout temperature, and combustibility index. Since the mass loss of sample mainly comes from the decomposition of volatiles in the coal samples, the DTG calculated from MS results of combustion products should be consistent with the DTG results measured by TG analyzer. As a representative, Figure 3 compares the DTG curve calculated from MS results and measured TG results under condition 4 (see Table 2). The results indicate that the calculated MS results are in good agreement with the measured ones. The relative uncertainty was estimated to be ±2.6% .
3. Results and discussion
3.1. TG/DTG results
Figure 4 presents the TG/DTG curves obtained in the combustion of SBC samples. As temperature increases, the coal sample could proceed with several steps, including devolatilization, coke formation, and coke combustion. The peak of maximum weight loss moves toward high temperatures gradually and the peak value decreases slightly as the particle size increases. The combustion rate depends on the diffusion and reaction ability. During the rapid combustion processes, the major factor affecting the combustion is diffusion ability. As the particle size increases, the contact surface area decreases, which would inhibit the diffusion of oxygen and make the devolatilization and combustion difficult. As a consequence, the devolatilization occurs at higher temperatures and the value of weight loss peak decreases.
Compared to particle size, heating rate has more apparent effect on the characteristics of weight loss. As the heating rate increases, the peak of weight loss moves toward high temperatures. In addition, the peak values and residues decrease as well. Since diffusion ability is the dominant factor in affecting the combustion process, longer contact time of sample surface and oxygen could contribute to SBC conversion. Furthermore, longer time is needed to reach the same temperature at slower heating rate relative to faster one. For instance, the time needed to reach 1200°C at 10°C/min is four times as the time needed to reach 1200°C at 40 °C/min. Thus, the burnout time with low heating rate is longer. Sufficient contact of particles and oxygen is beneficial for the combustion process, and the maximum value of weight loss occurred at lower temperatures and the maximum weight loss shifted to larger values . It is clear that there is a small weight loss peak within 600–700°C. This weak peak could come from the decomposition of carbonate in the coal samples. In addition, the TG/DTG results of different total flow rates are observed to be quite similar as displayed in Figure 4c. As the oxygen supplied is much more than that theoretically needed for the complete combustion, the weight loss characteristics are insensitive to the diverse total flow rate changed in experiments.
3.2. Characteristic temperatures
The characteristic temperatures obtained in the combustion of SBC at different particle sizes, heating rates, and total flow rates are shown in Figure 5. As indicated in Figure 5a, both
Figure 5b displays the characteristic temperature as a function of heating rate.
For the total flow rate, a slight increase of
3.3. Activation energy
Figure 6 summarizes activation energy (
3.4. MS and FTIR results
The volumetric flows of different combustion products at different temperatures can be obtained by normalization and numerical analysis of MS results. The maximum volumetric flows of different combustion products as well as the corresponded temperatures (
Figure 7 shows the volumetric flow rates of different species formed under condition 4 (see Table 2). In the combustion of SBC, both NH3 and HCN exhibited two-peak shapes. The peak before
NO2 exhibited a shoulder peak and an asymmetric single peak. The shoulder peak is due to the weak release of NO2 during devolatilization. The asymmetric single peak could result from the conversion of nitrogen-containing heterocyclic species at the end of devolatilization and the beginning of char combustion. However, the other nitrogenous components enriched in char will release NO2 during the rapid combustion of char. Less NO2 emissions were observed at smaller particle size, slower heating rate, and higher total flow rate. Similarly, the emissions of SO2 are relatively less than those formed with slower heating rate and higher total flow rate. However, the amount of SO2 is quite similar for the investigated particle size range, indicating that it is insensitive to the particle size.
By comparing the formation of NO2 and SO2 with different particle sizes, heating rates, and total flow rates shown in Figure 8, it is obvious that the amount of SO2 production is much lower than those of NO and NO2. To summarize, the SBC combustion with smaller particle size, slower heating rate, and higher total flow rate is beneficial to controlling the formation of pollutants. This finding is consistent with the results reported for Hegang, Tiefa, and Zhungeer coals by Wei et al.  and Heshan sulfur coal by Jiang et al. [23, 26].
To visualize the formation of the major species and avoid the effect of ion fragmentation in MS, FTIR with the spectral range of 500–5000 cm−1 was involved in the current work. Figure 9 presents a representative three-dimensional (3D) spectrum measured with a particle size of 90–100 μm, a total flow rate of 100 sccm, and a heating rate of 20°C/min. The maximum absorbance peak is located at the wave number of 2380 cm−1 corresponding to the anomalistic vibration of CO2. Another obvious peak at 700 cm−1 belongs to the bending vibration of CO2. The peaks at 1375 and 3600 cm−1 correspond to SO2 and H2O, respectively. By comparing the standard spectral peaks of different species, overlapped peaks were observed. For instance, the peak between 1500 and 1800 cm−1 should be the overlap of H2O and NO [27, 28]. As can be seen in Figure 9, CO2 becomes detectable at around 140°C. Up to 200°C, a slow increase was observed. Another turning point was measured at about 375°C, which also agrees well with the wave valley in the profiles of HCN and NH3. Besides CO2, the fast production of the major species was achieved within the temperature range of 400–500°C, which is consistent with the rapid combustion temperature revealed in the MS analysis. In order to show gradual change of specific species at different temperatures in Figure 9, Figure 10 presents two-dimensional (2D) spectra. The peaks located at about 2380 and 3600 cm−1 correspond to the CO2 and H2O, respectively. By comparing different spectra obtained at different temperatures, it is obvious that the maximum peak values of different combustion products are located at the temperature of 450°C, corresponding to the rapid combustion temperature. The spectra obtained at temperatures higher than 450°C exhibit very weak peaks since the SBC samples were nearly burned out.
The combustion of SBC was comprehensively studied with the online TG-MS-FTIR system in terms of characteristic temperatures as well as qualitative and quantitative analyses of products. Five particle sizes ranging from 0 to 500 μm, four heating rates from 10 to 40°C/min, and three total flow rates from 50 to 150 sccm were used. To avoid the influence of overlap peaks, signal drift, and dynamic response delay in ion current spectra during MS analysis, argon was used as a reference gas to calibrate the products by considering the electron impact ionization cross sections, ion flow intensities, and the partial pressures of different species. The results indicate that the decrease in the particle size, heating rate, and flow rate lead to lower ignition and burnout temperatures, while the activation energy tends to be lower with smaller particle size, faster heating rate, and lower flow rate. The decrease in the particle size could lead to more contact area with oxygen and better thermal reactivity. Slower heating rate could provide more sufficient time for the reaction. Moreover, a higher total flow rate would reduce the oxygen adsorbability on the coal particle surface at a higher flow speed. From MS and FTIR analysis, lower concentrations of different products were observed to be formed at smaller particle size, slower heating rate, and higher total flow rate. These findings will guide to understand the combustion kinetics of SBC and be beneficial to control the formation of pollutants.
The authors thank the financial support from the Recruitment Program of Global Youth Experts and Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDA0703100). The authors also thank Mr. Kai Wei, Mr. Zhi-Qiang Gong, and Dr. Zhi-Cheng Liu for their help in the experiments.
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