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CO2 combustion environments
Chemical Engineering and Processing 49 (2010) 449–459
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Chemical Engineering and Processing: Process Intensification journal homepage: www.elsevier.com/locate/cep
Properties of char particles obtained under O2 /N2 and O2 /CO2 combustion environments Qingzhao Li a,b , Changsui Zhao a,∗ , Xiaoping Chen a , Weifang Wu a , Baiquan Lin b a b
School of Energy and Environment, Southeast University, Nanjing 210096, Jiangsu Province, PR China State Key Laboratory of Coal Resources and Safe Mining, School of Safety Engineering, China University of Mining and Technology, Xuzhou 221008, Jiangsu Province, PR China
a r t i c l e
i n f o
Article history: Received 18 June 2009 Received in revised form 8 March 2010 Accepted 8 March 2010 Available online 20 March 2010 Keywords: Oxy-fuel Pulverized coal combustion O2 /CO2 environment Microstructure
a b s t r a c t Pulverized coal combustion in O2 /N2 and O2 /CO2 environments was investigated with a drop tube furnace. Results present that the reaction rate and burn-out degree of O2 /CO2 chars (obtained in O2 /CO2 environments) are lower than that of O2 /N2 chars (obtained in O2 /N2 environments) under the same experimental condition. It indicates that a higher O2 concentration in O2 /CO2 environment is needed to achieve the similar combustion characteristic to that in O2 /N2 environment. The main differences between O2 /N2 and O2 /CO2 chars rely on the pore structure determined by N2 adsorption and chemical structure measured by FT-IR. For O2 /CO2 char, the surface is thick and the pores are compact which contribute to the fragmentation reduction of particles burning in O2 /CO2 environment. The organic functional group elimination rate from the surface of O2 /CO2 chars is slower or delayed. The present research results might have important implications for further understanding the intrinsic kinetics of pulverized coal combustion in O2 /CO2 environment. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Environmental issues due to emissions of pollutants from fossil fuel combustion processes have become global problems, including air toxics and greenhouse effects. Among these greenhouse gases (GHG), CO2 emission from human activity was on the order of 7 Gt/a in the late 1990s, which is the largest contributor owing to its amount presence in the atmosphere contributing to 60% of global warming effects [1]. In the world, over 85% of energy demand is supplied by fossil fuels. Fossil-fueled power plants are responsible for roughly 40% of total CO2 emissions, coal-fired plants being the main contributor [2]. Therefore, carbon capture and storage (CCS) technology becomes the obvious priority candidate for long term technology policies. This concept includes three different approaches, namely post-combustion capture, pre-combustion capture, and oxy-fuel techniques. The current work is focused on the oxy-fuel option and especially on pulverized coal combustion in O2 /CO2 atmosphere because of its easy CO2 recovery, low NOX emissions and potential high desulphurization efficiency [3]. Oxy-fuel combustion technology (also named O2 /CO2 combustion technology) of pulverized coal is a process which involves the combustion of pulverized coal in the mixture of oxygen and recirculated flue gas (mainly CO2 ) to reduce the net volume of flue gas and to increase the concentration of carbon dioxide (CO2 ). This technol-
∗ Corresponding author. Tel.: +86 25 83793453; fax: +86 25 83793453. E-mail address: [email protected] (C. Zhao). 0255-2701/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cep.2010.03.007
ogy provides new challenges to combustion specialists. Applying a conventional approach (normal air combustion) for boiler design and operating conditions to oxy-fuel operation will lead to modified distributions of temperature, gaseous species, and radiation fluxes inside the combustion chamber and as a result to flame instability and poor burn-out [4]. Therefore, increasing the oxygen concentration in the O2 /CO2 mixtures were proposed to compensate for the higher molar heat capacity of CO2 and keep the same flame temperature as that in air combustion and providing a stable combustion process [5–13]. During the coal combustion process, coal conversion occurs in two stages: devolatilization and heterogeneous reaction of residual char formed from pyrolysis process. The structure of char is complex [14] and heterogeneous and its oxidation involves many physicochemical processes that occur within the pores [15]. It is quite evident that the pore structure and its evolution have major influence on the mechanism of conversion of porous coal chars. According to porous solid/gas reaction kinetics, char combustion under fast heating condition is usually a regime II reaction (kineticsdiffusion combined control), so the ultimate structure of char plays a significant role in determining its reactivity during combustion [16]. Researches have demonstrated that the distribution of micropores and its available reaction surface area of char particle play an important role in the description of its reactivity and carbon conversion [17,18]. Furthermore, the morphology of char affects its overall combustion efficiency [19,20]. Therefore, it is generally accepted that coal’s intrinsic reactivity should be expressed on an available surface area basis [16,21].
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Table 1 The ultimate, proximate analyses of coal samples (Air Dried Basis). Samples
Ultimate analysis (w/w %)
LY anthracite GZ bituminous coal
Proximate analysis (w/w %)
Calorific value (MJ/kg)
C
H
O
N
S
FC
VM
Ash
M
55.65 61.46
1.31 3.57
0.23 3.04
0.52 0.70
2.74 4.26
54.43 49.68
6.62 23.33
38.23 25.52
1.32 1.45
Furthermore, it has been generally accepted that the changes in coal behaviors as a consequence of oxidation are mainly due to the chemical and structural modifications, which maybe plays an important role in their oxidation ability. Fourier transform infrared spectroscopy (FT-IR) is one of the most versatile analytical techniques for obtaining information about changes in chemical structure such as the functional groups that take place during coal oxidation [22–26]. The aim of the present investigation was to assess the physical and chemical structure differences of coal chars obtained from the two different combustion environments (21%O2 /79%N2 and 21%O2 /79%CO2 ) and their influences on coal combustion characteristic. 2. Experimental
24.64 23.95
measurement. In addition, N2 adsorption analyzer (ASAP2020 M) was used for the pore structure measurement and field-emission scanning electron microscopy (FESEM) was used for morphology analysis of chars. In order to characterize the swelling property of coal combustion in O2 /CO2 environments, LS200 particle size analyzer was employed to measure the particle size distributions of the original coal and chars. In addition, due to the advantages of FT-IR (such as sensitivity, superior signal/noise ratio, high speed of operation, energy throughput, long term precision and the availability of powerful data manipulation facilities), this technique has been widely used for elucidation of the coal structure [27] and for the structural variations brought about by the oxidation of coal [28]. In present research paper, BRUKER VECTOR22 spectrometer was used for the chemical structure analysis of coal and chars.
2.1. Materials 3. Results and discussions Two typical Chinese coals, Longyan (LY) anthracite, Guizhou (GZ) bituminous coal in different ranks and origins were selected for the experiments. The raw coal samples were crushed and pulverized in the laboratory using a bench-scale mill. The ultimate and proximate analyses of the samples are summarized in Table 1. 2.2. Apparatus and methods A drop-tube-furnace (DTF) was used to simulate the temperature and heating rate experienced by coal during a PC boiler operation. Fig. 1 shows the schematic diagram of the DTF used for the coal combustion investigation. This facility mainly includes the gas mixing unit, the heating-furnace body, the water-cooled powder feeding system and the sampling probe. The inner reactor tube in the furnace was an alundum tube of 40 mm I.D. and heating section was about 1000 mm in length. The micro coal feeder (MFEV1VO) was manufactured by Sankyo Piotech Corporation of Japan. Coal particles from the coal feeder were injected into the furnace for combustion tests and chars were collected using water-cooled sampling probe. The original coal and partly burnt chars were characterized according to standard method (GB/T212-2001) for ash content
Fig. 1. Schematic diagram of the drop-tube-furnace system.
3.1. Pulverized coal combustion characteristic in two environments Partly burnt chars were drawn from five sampling points along the centerline of the DTF furnace. The burn-out degree (B) of chars was calculated by the ash tracer method [29], which was based on the ash content analysis (according to the national standard method of GB/T212-2001) of fresh and partly burnt chars. B=
1−
A0 1 − Ai × 1 − A0 Ai
× 100%
(in %)
(1)
where the subscript “0” and “i” represents the fresh coal and partly burnt char collected at the point i, respectively. “A” is the ash content in coal or partly burnt char. The char reactivity (RC ), which is the carbon consumption rate per unit remaining carbon in the char, can be calculated as follows [21]: RC = −
d(1 − B) dm 1 1 × × =− m (1 − B) dt dt
(in g/gs)
(2)
where m is the mass of char at time t, which can be related to the char burn-out degree and initial fed coal mass (m0 ) by m = m0 · (1 − B). The residence time of char particle in the DTF were calculated according to the flow behavior, the gas velocity and the length of reaction tube. The calculated burn-out degrees against residence time are plotted in Fig. 2. Results show that coal burn-out degree increases with the residence time. At a certain residence time, GZ bituminous coal shows higher burn-out degree. In addition, the burn-out degrees of two samples in CO2 environment is always lower than that in N2 environment at a certain residence time, which indicates the poor combustion characteristic of coal in O2 /CO2 mixtures. Fig. 3 shows that there are great reactivity differences between two combustion environments. Under the same experimental condition and residence time, the O2 /CO2 chars show lower consumption rate than O2 /N2 chars with the same O2 concentration. Since the CO2 is a well-known radiative gas having a higher specific heat, higher density and lower thermal and mass diffusivity than N2 (Fig. 4), it could cause the surrounding gas
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Fig. 2. Burning processes of two samples in two environments. (a) LY anthracite chars. (b) GZ bituminous coal chars.
Fig. 3. Burning rates of two samples in different combustion environments. (a) LY anthracite chars. (b) GZ bituminous coal chars.
temperature by several hundred degrees difference for coal combustion in various O2 /N2 and O2 /CO2 environments. It can be concluded that the difference in thermo-physical properties and the lower particle burning temperature may be the major reason for the poor combustibility in O2 /CO2 environment [5,30].
Fig. 4. Comparison of the ratio of different thermo-physical properties and nondimensional groups for CO2 and N2 .
Due to the coal rank difference, the combustion reactivity of LY anthracite chars is lower than that of GZ bituminous coal chars with the same residence time in the DTF. Because LY anthracite contains less volatile matter and has thick solid surface, the ignition temperature is higher and burning process is slower, which cause the lower burn-out rate under the same residence time condition. With the devolatilization and combustion promoting, the formation of micro-pores provide much more active surfaces which enhances the combustion rate of LY anthracite char. This means more surface area is available for the combustion. However, for GZ bituminous coal, the change trend of combustion rate shows another condition. In the higher burn-out region, it is seen that the reaction rates of GZ bituminous coal char decreases as the combustion proceeds. In other words, char becomes less reactive during combustion in the higher burn-out region. Actually, the fact of that the char reactivity gradually decrease with the reaction infers that the chemical structure of char is more predominant. During the combustion, the reactive components like amorphous carbon and aliphatic chains will be perfectly removed and the char becomes intrinsically less reactive. Moreover, in the higher burn-out region, the ash content in the char particle is very higher which inhibits the combustion progress of the residual carbon. It can be seen from Fig. 5 that the burn-out degree of char increases with the furnace temperature in the two combustion environments. The effect of higher temperature and heating rate is enhanced the devolatilization and the char particle temperature. It is a well-known fact and no further discussion is given
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Fig. 5. Burn-out degree of two samples under different temperature conditions. (a) LY anthracite chars. (b) GZ bituminous coal chars.
here. However, some further attention to be deserved is the difference of burn-out degrees between two environments, although the differences at higher furnace temperature get smaller. 3.2. Variation in char particle size distribution The structure of a char from the coal is strongly associated with its swelling history during plastic stage. The extent to which coal swells determines not only the size of the char particle, but also its porosity and wall thickness. In this experiment, the original coal and its partly burnt chars obtained from two combustion environments were measured with LS200 particle size analyzer. Fig. 6 presents the particle volume fraction and its cumulative result. Similar to char structure, the char swelling property is strongly influenced by the heating condition through its effect mainly on the rate of devolatilization. Compared with the original coal, partly burnt chars show different swelling property. It can be seen from the figure (Fig. 6) that O2 /CO2 chars exhibit more swelling character even for the GZ bituminous coal, which cause the distribution curves shifted to the larger particle size zone. Maybe it is contrary to the traditional research findings. The reason for this phenomenon is likely to be explained by the differences in particles’ conversion degree. The lower burning rate, lower porosity and compacted surfaces of O2 /CO2 chars inhibit the volatile release, which may enhance the O2 /CO2 chars swelling properties. In addition, SEM was employed to investigate the surface structure of the char. As shown in Fig. 7 that O2 /N2 char shows more abundant pore structure but the O2 /CO2 char has thick surface and compact pores. The surface of O2 /CO2 chars seems to be covered by some molten matter which makes the pores blinded. The SEM results are in good agreement with the results of N2 adsorption analysis. 3.3. Analysis of gas-phase adsorption isotherms The N2 gas adsorption/desorption isotherms were measured at 77 K with a gas adsorption analyzer (Micrometrics ASAP 2020M) after degassing the char samples for 12 h under vacuum. Data were obtained by admitting or removing a known quantity of adsorbate gas into or out of a sample cell containing the solid adsorbent maintained at a constant temperature (77 K) below the critical temperature of the adsorbate. As adsorption or desorption occurs, the pressure in the sample cell changes until equilibrium is established. The quantity of gas adsorbed or desorbed at the equilibrium pressure is equal to the difference between the amount of gas admitted or removed and the amount required to fill the space around the adsorbent (void space).
Fig. 8 shows the nitrogen adsorption–desorption isotherms of samples obtained under different combustion environments. It can be seen that low temperature nitrogen (77 K) isotherms of samples are all type II according to Brunauer, Deming, Deming, Teller (BDDT) classification, but the adsorption capacity of O2 /CO2 chars are always lower than that of O2 /N2 chars which could be attributed to the poor pore structure for the first ones. Generally, the isotherms at lower relative pressures represent behavior dominated by Van der Waals forces, whereas the isotherms at higher relative pressures characterize actions dominated by capillary condensations. Clearly outlined hysteresis loops are characteristic for the isotherms. According to IUPAC classification, they are H3 or H3 and H4 combined Types. These isotherms indicate the possibility of existing both blind holes and slit-shaped pores in the O2 /CO2 char samples and O2 /N2 char samples. The reported surface areas (from the BET-Brunauer, Emmett, Teller model), pore volumes (from the BJH-Barrett, Joyner, Halenda model) and average pore diameter (dpore ) are plotted in Fig. 9. Compared with O2 /N2 chars, O2 /CO2 chars always present smaller pore parameters, such as BET specific surface area (SBET ), BJH specific pore volume (VBJH ) and average pore diameter (dpore ). From the SEM analysis (Fig. 10), it can be seen that the outer surface of O2 /CO2 char particles seems to be covered by some molten substance which makes the pores blinded and specific surface area smaller. The SEM examination results are in good agreement with the results of N2 adsorption analysis. These might have important implications for understanding the intrinsic kinetics of pulverized coal combustion in O2 /CO2 environment. Fig. 11 shows that the pore surface area (SBET ) decreases with the increment of burn-out degree. It can be seen that at a certain burnout degree, O2 /CO2 chars always present smaller pore surface area than that of O2 /N2 chars. According to previous researches, coal char combustion in the DTF is concluded to be controlled by the combination of pore diffusion and chemical reaction (regime II) on the pore surface. For a reaction occurring in regime II, its reaction rate can be expressed as follows [21]: m = SP Ri Psn
(in g/gs)
(3)
where SP is the specific surface area of the char particles, n is the true reaction order respect to oxygen partial pressure. Ps is the oxygen partial pressure at the outer surface of char particle. The term Ri is the intrinsic reactivity coefficient and will be affected by the chemical structure of char; is the effectiveness factor (the ratio of the actual reaction rate to the rate in the absence of limitation due to pore diffusion) and will be affected by the char structure. Therefore, it can be seen obviously that the
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Fig. 6. Particle size distributions of coal/char obtained under different environments (chars residence time in the DTF: 1.15 s). (a) LY anthracite coal/chars. (b) GZ bituminous coal/chars.
Fig. 7. SEM graphs of LY anthracite chars (chars residence time in the DTF: 1.15 s). (a) Original LY anthracite coal and prepared chars. (b) Original GZ bituminous coal and prepared chars. I: 21%O2 /79%N2 . II: 21%O2 /79%CO2 .
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Fig. 8. Adsorption isotherms of two char samples (chars residence time in the DTF: 1.15 s). (a) LY anthracite chars. (b) GZ bituminous coal chars.
Fig. 9. Pore structure parameters of two char samples (chars residence time in the DTF: 1.15 s). (a) LY anthracite chars. (b) GZ bituminous coal chars.
Fig. 10. SEM graphs of LY anthracite chars (chars residence time in the DTF: 1.15 s). (a) Original LY anthracite coal and prepared chars. (b) Original GZ bituminous coal and prepared chars. I: 21%O2 /79%N2 . II: 21%O2 /79%CO2 .
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Fig. 11. Pore surface area vs. burn-out degree of two samples in different combustion environments. (a) LY anthracite chars. (b) GZ bituminous coal chars.
O2 /CO2 chars with smaller SBET will lead to its lower combustion rate. For the pores less than the free path of oxygen molecules (5 × 10−7 m), the oxygen molecules are expected to diffuse in the form of Knudsen diffusion. The effective coefficient of Knudsen diffusion (De ) can be expressed as follows [31]. De = 9.7 × 103 × dpore ×
T 0.5 P
M
×
(in cm2 /s)
(4)
where and are the porosity and tortuosity of the char, TP and M are the particle temperature and oxygen molecular weight. Due to the larger specific heat of CO2 gas, the particle temperature (TP ) of O2 /CO2 chars may be lower than that of O2 /N2 chars. From the comparison of pore size for two kinds of chars, it can be supposed that the gas diffusion in O2 /CO2 chars is expected to be more difficult than in O2 /N2 chars. 3.4. The pore size distributions calculated from the N2 adsorption isotherms Based on the previous N2 adsorption/desorption analysis, Barrett, Joyner and Halenda (BJH) theory was used to obtain mesopore (2–50 nm) distributions from the desorption branch of the N2 isotherm. Coefficient of variation for mesopore volumes, determined from the repeated runs of the sample, was less than 5%. Fig. 12 shows the pore size distribution of the partly burnt char samples obtained under two combustion environments. Due to burn-out degree difference between the original coal and its char, the pore size distribution cure shows remarkable difference. For LY anthracite chars, it can be seen that the curves exhibit lifted initial parts, which corresponds to the contribution of pores less than 3 nm. The other peak is in the range of 3–5 nm. For GZ bituminous coal chars, it can be seen that the greatest contribution to the pore volume are the pores in the range of 3–5 nm, therefore, the curves exhibit a maximum peak in the range of 3–5 nm. For O2 /N2 and O2 /CO2 chars, although its pore size distribution profiles are quite similar, there is still a clear difference. Those pore size distribution curves show that for O2 /CO2 chars, the mesopore (2–50 nm) contribution to the pore volume is smaller. Maybe, that pore size distribution difference reflects some combustion situation of pulverized coal in O2 /CO2 environment. It can be concluded from Figs. 13 and 14 that the pore structure development represents the similar rule under O2 /N2 and O2 /CO2 environments. With the increase in the residence time in the DTF, the char burn-out rate increases. During the coal conversion process, microstructure evolution mainly occurs in the pore region less than 5 nm. And with the burn-out degree increase and the micro-
crystal carbon’s elimination, micro-pores expand and connect each other gradually, which maybe contribute to decrease in the pores less than 3 nm and reduction in the initial parts of the distribution curves. 3.5. Fractal analysis of the isotherms based on FHH model In the molecular-size range, the surfaces of most materials are fractals: geometrically irregular surfaces that are self-similar upon variations of resolution. For such surfaces, a parameter can give the degree of irregularity: the fractal dimension, 2 ≤ DS ≤ 3, of the surface. The higher the DS value, the more undulant and spacefilling the surface is. Fractal BET (Brunauer–Emmett–Teller), fractal FHH (Frenkel–Halsey–Hill) and thermodynamic methods can be used to measure fractal dimensions on the basis of gas adsorption isotherms [32,33]. Among those, fractal FHH method has been proven to be the most effective method. Using the fractal FHH equation for the analysis of gas adsorption data, the fractal dimension can be determined. The derivation of the fractal dimension using this theory is described elsewhere in detail [34–39]. A brief summary is provided below. The classical FHH theory for multilayer adsorption was extended to the fractal surfaces in the following form: ln
N Nm
P 0
= C + A · ln ln
P
(5)
N/Nm is the number of layers adsorbed on the surface, A is the power-law which is dependent on DS , P0 is saturation pressure and P is the adsorption equilibrium pressure of the gas. At the early stage of adsorption, the van der Waals forces between gas–solid interactions are dominant and the liquid–gas surface tension forces are negligibly small. So, the relationship between A and DS can be derived as follows: A=
DS − 3 3
(6)
For higher coverage, the interface becomes controlled by the liquid–gas surface tension forces and the relationship between A and DS changes to the following expression: A = DS − 3
(7)
According to Ismail and Pfeifer [40], the threshold for the dominant forces between the van der Waals forces and the liquid–gas surface tension forces is given by: ˛ = 3(1 + A) − 2
(8)
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Fig. 12. Pore size distribution of two char samples in different environments (chars residence time in the DTF: 1.15 s). (a) LY anthracite chars. (b) GZ bituminous coal chars.
Fig. 13. Pore sizes distribution development of LY anthracite chars. (a) 21%O2 /79%N2 . (b) 21%O2 /79%CO2 .
Fig. 14. Pore sizes distribution development of GZ bituminous coal chars. (a) 21%O2 /79%N2 . (b) 21%O2 /79%CO2 .
If ˛ < 0 the liquid–gas surface tension forces are dominant, but the van der Waals forces are dominant if ˛ ≥ 0. Fig. 15 shows the plots of ln(N/Nm ) against ln[ln(P0 /P)] from the N2 adsorption isotherms measured on samples. For all chars, one can find clearly linear relationship between ln(N/Nm ) and ln[ln(P0 /P)], indicating scaling properties of all samples surfaces. The surface fractal dimensions calculated with Eq. (5) are presented in Table 2. All calculated fractal dimensions (DS ) are between 2 and 3, and the O2 /N2 chars always show higher DS values which indicate more irregularity and roughness of the char surfaces. Although the geometrical irregularity and roughness of the surfaces are the essential reasons for the obtained DS values, it
Table 2 Fractal dimensions derived from fractal FHH model. Samples
Combustion environment
DS
R
LY anthracite chars
21%O2 /79%N2 21%O2 /79%CO2
2.699 2.687
0.9902 0.9899
GZ bituminous coal chars
21%O2 /79%N2 21%O2 /79%CO2
2.633 2.554
0.9916 0.9887
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Fig. 15. ln(N/Nm ) − ln[ln(P0 /P)] plots chars obtained in two different combustion environments (chars residence time in the DTF: 1.15 s). (a) LY anthracite chars. (b) GZ bituminous coal chars.
is known that the absorbed film volume and pore size distributions are used to define the fractal when fractal FHH equation is used. Therefore, the fractal FHH type equation might be sensitive to the pore size distribution, and pore size distribution could contribute significantly to the surface fractal dimension. The next section gives the analysis and comparison of pore size distributions of samples obtained in two different combustion environments.
3.6. FT-IR spectroscopic analysis of char particles Fourier transform infrared spectroscopies (FT-IR) were obtained with BRUKER VECTOR22 spectrometer. The alkali halide (KBr) pellet technique was used and the ratio of coal (char) to potassium bromide was about 1:120 (w:w). The mixture of coal (char) and KBr was finely milled in a porcelain container and the pellet were dried and kept in vacuum desiccator to avoid any moisture absorp-
Fig. 16. FT-IR spectra of chars obtained in different environments (chars residence time in the DTF: 1.15 s). (a) LY anthracite chars. (b) GZ bituminous coal chars.
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Table 3 Band assignments in FT-IR spectra of coal (char) and inertinite maceral in coal [41,42]. Bands/cm−1
Assignments
3400–3320 3050–3030 3000–2800 1700 1610–1560 1510 1450 1430–1420 1370–1360 1317–1315 1270–1250 1060–1030 870 810 780–770 750
–OH stretching Aromatic CH stretching Aliphatic CH stretching Aromatic carbonyl/carboxyl C O stretching Aromatic C C ring stretching Aromatic C C ring stretching Aliphatic CH deformation Aromatic C C ring stretching Aliphatic CH3 deformation Aliphatic CH2 deformation Aromatic CO– and phenolic –OH stretching Aliphatic ether C–O– and alcohol C–O stretching 1 Adjacent H deformation 2 Adjacent H deformation Aliphatic CH2 deformation 3–4 Adjacent H deformation
tion. The peak assignments in this paper are mainly based on the work of Painter et al. [41] and Marchessault [42] (Table 3). The FT-IR spectra for pyrolysis chars and partly burnt chars prepared from two original coals are shown in Fig. 16. The spectra are characterized by the presence of peaks of 1700 cm−1 , 1615 cm−1 , 1430–1420 cm−1 1060–1030 cm−1 and 780 cm−1 . The peak at 1700 cm−1 , due to acid C O groups and the peak at 1031 cm−1 , due to aliphatic C–O–C and alcohol C–O stretching in the range of 1060–1030 cm−1 , are prominent in all char samples. The aromatic C C ring stretching vibration at 1430 cm−1 are almost eliminated in all samples. The peak at 780 cm−1 can be derived from hydrogen deformation in aromatic ring structure and aliphatic CH2 deformation. This peak is mainly presented in the pyrolysis chars especially for LY anthracite chars. The peak in the band range of 3000–2800 cm−1 , due to aliphatic CHX stretching vibration, is largely eliminated during the pyrolysis or the combustion processes. The broad band at 3400–3320 cm−1 is mainly represented the –OH stretching vibration of water. There are marked differences between N2 and CO2 pyrolysis chars along with O2 /N2 and O2 /CO2 partly burnt chars. It can be seen that the absorbance intensity of the aliphatic and aromatic functional groups in CO2 pyrolysis char (or O2 /CO2 partly burnt char) are much higher than that in N2 pyrolysis char (or O2 /N2 partly burnt char), which indicate that the elimination rate of this organic functional group of chars obtained in CO2 environments is slowly or delayed.
4. Conclusions Two typical Chinese coal and its partly burnt chars obtained under O2 /CO2 combustion condition and conventional air combustion condition with same oxygen concentrations were prepared and analyzed for their physical–chemical structure evaluation. Results present that the reaction rate and burn-out degree of O2 /CO2 chars are lower than those of O2 /N2 chars under the same combustion condition, which indicates that a higher O2 concentration in O2 /CO2 mixture than in O2 /N2 mixture is needed to achieve the similar combustion characteristic. The main differences between O2 /N2 and O2 /CO2 chars rely on the pore structure determined by N2 adsorption and chemical structure measured by FT-IR. The O2 /CO2 char has thick surface and compact pores, which contribute to the reduction fragmentation of particles burning in O2 /CO2 environment. The organic functional group elimination rate of chars obtained in CO2 environments is slower or delayed.
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Chemical Engineering and Processing: Process Intensification journal homepage: www.elsevier.com/locate/cep
Properties of char particles obtained under O2 /N2 and O2 /CO2 combustion environments Qingzhao Li a,b , Changsui Zhao a,∗ , Xiaoping Chen a , Weifang Wu a , Baiquan Lin b a b
School of Energy and Environment, Southeast University, Nanjing 210096, Jiangsu Province, PR China State Key Laboratory of Coal Resources and Safe Mining, School of Safety Engineering, China University of Mining and Technology, Xuzhou 221008, Jiangsu Province, PR China
a r t i c l e
i n f o
Article history: Received 18 June 2009 Received in revised form 8 March 2010 Accepted 8 March 2010 Available online 20 March 2010 Keywords: Oxy-fuel Pulverized coal combustion O2 /CO2 environment Microstructure
a b s t r a c t Pulverized coal combustion in O2 /N2 and O2 /CO2 environments was investigated with a drop tube furnace. Results present that the reaction rate and burn-out degree of O2 /CO2 chars (obtained in O2 /CO2 environments) are lower than that of O2 /N2 chars (obtained in O2 /N2 environments) under the same experimental condition. It indicates that a higher O2 concentration in O2 /CO2 environment is needed to achieve the similar combustion characteristic to that in O2 /N2 environment. The main differences between O2 /N2 and O2 /CO2 chars rely on the pore structure determined by N2 adsorption and chemical structure measured by FT-IR. For O2 /CO2 char, the surface is thick and the pores are compact which contribute to the fragmentation reduction of particles burning in O2 /CO2 environment. The organic functional group elimination rate from the surface of O2 /CO2 chars is slower or delayed. The present research results might have important implications for further understanding the intrinsic kinetics of pulverized coal combustion in O2 /CO2 environment. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Environmental issues due to emissions of pollutants from fossil fuel combustion processes have become global problems, including air toxics and greenhouse effects. Among these greenhouse gases (GHG), CO2 emission from human activity was on the order of 7 Gt/a in the late 1990s, which is the largest contributor owing to its amount presence in the atmosphere contributing to 60% of global warming effects [1]. In the world, over 85% of energy demand is supplied by fossil fuels. Fossil-fueled power plants are responsible for roughly 40% of total CO2 emissions, coal-fired plants being the main contributor [2]. Therefore, carbon capture and storage (CCS) technology becomes the obvious priority candidate for long term technology policies. This concept includes three different approaches, namely post-combustion capture, pre-combustion capture, and oxy-fuel techniques. The current work is focused on the oxy-fuel option and especially on pulverized coal combustion in O2 /CO2 atmosphere because of its easy CO2 recovery, low NOX emissions and potential high desulphurization efficiency [3]. Oxy-fuel combustion technology (also named O2 /CO2 combustion technology) of pulverized coal is a process which involves the combustion of pulverized coal in the mixture of oxygen and recirculated flue gas (mainly CO2 ) to reduce the net volume of flue gas and to increase the concentration of carbon dioxide (CO2 ). This technol-
∗ Corresponding author. Tel.: +86 25 83793453; fax: +86 25 83793453. E-mail address: [email protected] (C. Zhao). 0255-2701/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cep.2010.03.007
ogy provides new challenges to combustion specialists. Applying a conventional approach (normal air combustion) for boiler design and operating conditions to oxy-fuel operation will lead to modified distributions of temperature, gaseous species, and radiation fluxes inside the combustion chamber and as a result to flame instability and poor burn-out [4]. Therefore, increasing the oxygen concentration in the O2 /CO2 mixtures were proposed to compensate for the higher molar heat capacity of CO2 and keep the same flame temperature as that in air combustion and providing a stable combustion process [5–13]. During the coal combustion process, coal conversion occurs in two stages: devolatilization and heterogeneous reaction of residual char formed from pyrolysis process. The structure of char is complex [14] and heterogeneous and its oxidation involves many physicochemical processes that occur within the pores [15]. It is quite evident that the pore structure and its evolution have major influence on the mechanism of conversion of porous coal chars. According to porous solid/gas reaction kinetics, char combustion under fast heating condition is usually a regime II reaction (kineticsdiffusion combined control), so the ultimate structure of char plays a significant role in determining its reactivity during combustion [16]. Researches have demonstrated that the distribution of micropores and its available reaction surface area of char particle play an important role in the description of its reactivity and carbon conversion [17,18]. Furthermore, the morphology of char affects its overall combustion efficiency [19,20]. Therefore, it is generally accepted that coal’s intrinsic reactivity should be expressed on an available surface area basis [16,21].
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Table 1 The ultimate, proximate analyses of coal samples (Air Dried Basis). Samples
Ultimate analysis (w/w %)
LY anthracite GZ bituminous coal
Proximate analysis (w/w %)
Calorific value (MJ/kg)
C
H
O
N
S
FC
VM
Ash
M
55.65 61.46
1.31 3.57
0.23 3.04
0.52 0.70
2.74 4.26
54.43 49.68
6.62 23.33
38.23 25.52
1.32 1.45
Furthermore, it has been generally accepted that the changes in coal behaviors as a consequence of oxidation are mainly due to the chemical and structural modifications, which maybe plays an important role in their oxidation ability. Fourier transform infrared spectroscopy (FT-IR) is one of the most versatile analytical techniques for obtaining information about changes in chemical structure such as the functional groups that take place during coal oxidation [22–26]. The aim of the present investigation was to assess the physical and chemical structure differences of coal chars obtained from the two different combustion environments (21%O2 /79%N2 and 21%O2 /79%CO2 ) and their influences on coal combustion characteristic. 2. Experimental
24.64 23.95
measurement. In addition, N2 adsorption analyzer (ASAP2020 M) was used for the pore structure measurement and field-emission scanning electron microscopy (FESEM) was used for morphology analysis of chars. In order to characterize the swelling property of coal combustion in O2 /CO2 environments, LS200 particle size analyzer was employed to measure the particle size distributions of the original coal and chars. In addition, due to the advantages of FT-IR (such as sensitivity, superior signal/noise ratio, high speed of operation, energy throughput, long term precision and the availability of powerful data manipulation facilities), this technique has been widely used for elucidation of the coal structure [27] and for the structural variations brought about by the oxidation of coal [28]. In present research paper, BRUKER VECTOR22 spectrometer was used for the chemical structure analysis of coal and chars.
2.1. Materials 3. Results and discussions Two typical Chinese coals, Longyan (LY) anthracite, Guizhou (GZ) bituminous coal in different ranks and origins were selected for the experiments. The raw coal samples were crushed and pulverized in the laboratory using a bench-scale mill. The ultimate and proximate analyses of the samples are summarized in Table 1. 2.2. Apparatus and methods A drop-tube-furnace (DTF) was used to simulate the temperature and heating rate experienced by coal during a PC boiler operation. Fig. 1 shows the schematic diagram of the DTF used for the coal combustion investigation. This facility mainly includes the gas mixing unit, the heating-furnace body, the water-cooled powder feeding system and the sampling probe. The inner reactor tube in the furnace was an alundum tube of 40 mm I.D. and heating section was about 1000 mm in length. The micro coal feeder (MFEV1VO) was manufactured by Sankyo Piotech Corporation of Japan. Coal particles from the coal feeder were injected into the furnace for combustion tests and chars were collected using water-cooled sampling probe. The original coal and partly burnt chars were characterized according to standard method (GB/T212-2001) for ash content
Fig. 1. Schematic diagram of the drop-tube-furnace system.
3.1. Pulverized coal combustion characteristic in two environments Partly burnt chars were drawn from five sampling points along the centerline of the DTF furnace. The burn-out degree (B) of chars was calculated by the ash tracer method [29], which was based on the ash content analysis (according to the national standard method of GB/T212-2001) of fresh and partly burnt chars. B=
1−
A0 1 − Ai × 1 − A0 Ai
× 100%
(in %)
(1)
where the subscript “0” and “i” represents the fresh coal and partly burnt char collected at the point i, respectively. “A” is the ash content in coal or partly burnt char. The char reactivity (RC ), which is the carbon consumption rate per unit remaining carbon in the char, can be calculated as follows [21]: RC = −
d(1 − B) dm 1 1 × × =− m (1 − B) dt dt
(in g/gs)
(2)
where m is the mass of char at time t, which can be related to the char burn-out degree and initial fed coal mass (m0 ) by m = m0 · (1 − B). The residence time of char particle in the DTF were calculated according to the flow behavior, the gas velocity and the length of reaction tube. The calculated burn-out degrees against residence time are plotted in Fig. 2. Results show that coal burn-out degree increases with the residence time. At a certain residence time, GZ bituminous coal shows higher burn-out degree. In addition, the burn-out degrees of two samples in CO2 environment is always lower than that in N2 environment at a certain residence time, which indicates the poor combustion characteristic of coal in O2 /CO2 mixtures. Fig. 3 shows that there are great reactivity differences between two combustion environments. Under the same experimental condition and residence time, the O2 /CO2 chars show lower consumption rate than O2 /N2 chars with the same O2 concentration. Since the CO2 is a well-known radiative gas having a higher specific heat, higher density and lower thermal and mass diffusivity than N2 (Fig. 4), it could cause the surrounding gas
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Fig. 2. Burning processes of two samples in two environments. (a) LY anthracite chars. (b) GZ bituminous coal chars.
Fig. 3. Burning rates of two samples in different combustion environments. (a) LY anthracite chars. (b) GZ bituminous coal chars.
temperature by several hundred degrees difference for coal combustion in various O2 /N2 and O2 /CO2 environments. It can be concluded that the difference in thermo-physical properties and the lower particle burning temperature may be the major reason for the poor combustibility in O2 /CO2 environment [5,30].
Fig. 4. Comparison of the ratio of different thermo-physical properties and nondimensional groups for CO2 and N2 .
Due to the coal rank difference, the combustion reactivity of LY anthracite chars is lower than that of GZ bituminous coal chars with the same residence time in the DTF. Because LY anthracite contains less volatile matter and has thick solid surface, the ignition temperature is higher and burning process is slower, which cause the lower burn-out rate under the same residence time condition. With the devolatilization and combustion promoting, the formation of micro-pores provide much more active surfaces which enhances the combustion rate of LY anthracite char. This means more surface area is available for the combustion. However, for GZ bituminous coal, the change trend of combustion rate shows another condition. In the higher burn-out region, it is seen that the reaction rates of GZ bituminous coal char decreases as the combustion proceeds. In other words, char becomes less reactive during combustion in the higher burn-out region. Actually, the fact of that the char reactivity gradually decrease with the reaction infers that the chemical structure of char is more predominant. During the combustion, the reactive components like amorphous carbon and aliphatic chains will be perfectly removed and the char becomes intrinsically less reactive. Moreover, in the higher burn-out region, the ash content in the char particle is very higher which inhibits the combustion progress of the residual carbon. It can be seen from Fig. 5 that the burn-out degree of char increases with the furnace temperature in the two combustion environments. The effect of higher temperature and heating rate is enhanced the devolatilization and the char particle temperature. It is a well-known fact and no further discussion is given
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Fig. 5. Burn-out degree of two samples under different temperature conditions. (a) LY anthracite chars. (b) GZ bituminous coal chars.
here. However, some further attention to be deserved is the difference of burn-out degrees between two environments, although the differences at higher furnace temperature get smaller. 3.2. Variation in char particle size distribution The structure of a char from the coal is strongly associated with its swelling history during plastic stage. The extent to which coal swells determines not only the size of the char particle, but also its porosity and wall thickness. In this experiment, the original coal and its partly burnt chars obtained from two combustion environments were measured with LS200 particle size analyzer. Fig. 6 presents the particle volume fraction and its cumulative result. Similar to char structure, the char swelling property is strongly influenced by the heating condition through its effect mainly on the rate of devolatilization. Compared with the original coal, partly burnt chars show different swelling property. It can be seen from the figure (Fig. 6) that O2 /CO2 chars exhibit more swelling character even for the GZ bituminous coal, which cause the distribution curves shifted to the larger particle size zone. Maybe it is contrary to the traditional research findings. The reason for this phenomenon is likely to be explained by the differences in particles’ conversion degree. The lower burning rate, lower porosity and compacted surfaces of O2 /CO2 chars inhibit the volatile release, which may enhance the O2 /CO2 chars swelling properties. In addition, SEM was employed to investigate the surface structure of the char. As shown in Fig. 7 that O2 /N2 char shows more abundant pore structure but the O2 /CO2 char has thick surface and compact pores. The surface of O2 /CO2 chars seems to be covered by some molten matter which makes the pores blinded. The SEM results are in good agreement with the results of N2 adsorption analysis. 3.3. Analysis of gas-phase adsorption isotherms The N2 gas adsorption/desorption isotherms were measured at 77 K with a gas adsorption analyzer (Micrometrics ASAP 2020M) after degassing the char samples for 12 h under vacuum. Data were obtained by admitting or removing a known quantity of adsorbate gas into or out of a sample cell containing the solid adsorbent maintained at a constant temperature (77 K) below the critical temperature of the adsorbate. As adsorption or desorption occurs, the pressure in the sample cell changes until equilibrium is established. The quantity of gas adsorbed or desorbed at the equilibrium pressure is equal to the difference between the amount of gas admitted or removed and the amount required to fill the space around the adsorbent (void space).
Fig. 8 shows the nitrogen adsorption–desorption isotherms of samples obtained under different combustion environments. It can be seen that low temperature nitrogen (77 K) isotherms of samples are all type II according to Brunauer, Deming, Deming, Teller (BDDT) classification, but the adsorption capacity of O2 /CO2 chars are always lower than that of O2 /N2 chars which could be attributed to the poor pore structure for the first ones. Generally, the isotherms at lower relative pressures represent behavior dominated by Van der Waals forces, whereas the isotherms at higher relative pressures characterize actions dominated by capillary condensations. Clearly outlined hysteresis loops are characteristic for the isotherms. According to IUPAC classification, they are H3 or H3 and H4 combined Types. These isotherms indicate the possibility of existing both blind holes and slit-shaped pores in the O2 /CO2 char samples and O2 /N2 char samples. The reported surface areas (from the BET-Brunauer, Emmett, Teller model), pore volumes (from the BJH-Barrett, Joyner, Halenda model) and average pore diameter (dpore ) are plotted in Fig. 9. Compared with O2 /N2 chars, O2 /CO2 chars always present smaller pore parameters, such as BET specific surface area (SBET ), BJH specific pore volume (VBJH ) and average pore diameter (dpore ). From the SEM analysis (Fig. 10), it can be seen that the outer surface of O2 /CO2 char particles seems to be covered by some molten substance which makes the pores blinded and specific surface area smaller. The SEM examination results are in good agreement with the results of N2 adsorption analysis. These might have important implications for understanding the intrinsic kinetics of pulverized coal combustion in O2 /CO2 environment. Fig. 11 shows that the pore surface area (SBET ) decreases with the increment of burn-out degree. It can be seen that at a certain burnout degree, O2 /CO2 chars always present smaller pore surface area than that of O2 /N2 chars. According to previous researches, coal char combustion in the DTF is concluded to be controlled by the combination of pore diffusion and chemical reaction (regime II) on the pore surface. For a reaction occurring in regime II, its reaction rate can be expressed as follows [21]: m = SP Ri Psn
(in g/gs)
(3)
where SP is the specific surface area of the char particles, n is the true reaction order respect to oxygen partial pressure. Ps is the oxygen partial pressure at the outer surface of char particle. The term Ri is the intrinsic reactivity coefficient and will be affected by the chemical structure of char; is the effectiveness factor (the ratio of the actual reaction rate to the rate in the absence of limitation due to pore diffusion) and will be affected by the char structure. Therefore, it can be seen obviously that the
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453
Fig. 6. Particle size distributions of coal/char obtained under different environments (chars residence time in the DTF: 1.15 s). (a) LY anthracite coal/chars. (b) GZ bituminous coal/chars.
Fig. 7. SEM graphs of LY anthracite chars (chars residence time in the DTF: 1.15 s). (a) Original LY anthracite coal and prepared chars. (b) Original GZ bituminous coal and prepared chars. I: 21%O2 /79%N2 . II: 21%O2 /79%CO2 .
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Fig. 8. Adsorption isotherms of two char samples (chars residence time in the DTF: 1.15 s). (a) LY anthracite chars. (b) GZ bituminous coal chars.
Fig. 9. Pore structure parameters of two char samples (chars residence time in the DTF: 1.15 s). (a) LY anthracite chars. (b) GZ bituminous coal chars.
Fig. 10. SEM graphs of LY anthracite chars (chars residence time in the DTF: 1.15 s). (a) Original LY anthracite coal and prepared chars. (b) Original GZ bituminous coal and prepared chars. I: 21%O2 /79%N2 . II: 21%O2 /79%CO2 .
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Fig. 11. Pore surface area vs. burn-out degree of two samples in different combustion environments. (a) LY anthracite chars. (b) GZ bituminous coal chars.
O2 /CO2 chars with smaller SBET will lead to its lower combustion rate. For the pores less than the free path of oxygen molecules (5 × 10−7 m), the oxygen molecules are expected to diffuse in the form of Knudsen diffusion. The effective coefficient of Knudsen diffusion (De ) can be expressed as follows [31]. De = 9.7 × 103 × dpore ×
T 0.5 P
M
×
(in cm2 /s)
(4)
where and are the porosity and tortuosity of the char, TP and M are the particle temperature and oxygen molecular weight. Due to the larger specific heat of CO2 gas, the particle temperature (TP ) of O2 /CO2 chars may be lower than that of O2 /N2 chars. From the comparison of pore size for two kinds of chars, it can be supposed that the gas diffusion in O2 /CO2 chars is expected to be more difficult than in O2 /N2 chars. 3.4. The pore size distributions calculated from the N2 adsorption isotherms Based on the previous N2 adsorption/desorption analysis, Barrett, Joyner and Halenda (BJH) theory was used to obtain mesopore (2–50 nm) distributions from the desorption branch of the N2 isotherm. Coefficient of variation for mesopore volumes, determined from the repeated runs of the sample, was less than 5%. Fig. 12 shows the pore size distribution of the partly burnt char samples obtained under two combustion environments. Due to burn-out degree difference between the original coal and its char, the pore size distribution cure shows remarkable difference. For LY anthracite chars, it can be seen that the curves exhibit lifted initial parts, which corresponds to the contribution of pores less than 3 nm. The other peak is in the range of 3–5 nm. For GZ bituminous coal chars, it can be seen that the greatest contribution to the pore volume are the pores in the range of 3–5 nm, therefore, the curves exhibit a maximum peak in the range of 3–5 nm. For O2 /N2 and O2 /CO2 chars, although its pore size distribution profiles are quite similar, there is still a clear difference. Those pore size distribution curves show that for O2 /CO2 chars, the mesopore (2–50 nm) contribution to the pore volume is smaller. Maybe, that pore size distribution difference reflects some combustion situation of pulverized coal in O2 /CO2 environment. It can be concluded from Figs. 13 and 14 that the pore structure development represents the similar rule under O2 /N2 and O2 /CO2 environments. With the increase in the residence time in the DTF, the char burn-out rate increases. During the coal conversion process, microstructure evolution mainly occurs in the pore region less than 5 nm. And with the burn-out degree increase and the micro-
crystal carbon’s elimination, micro-pores expand and connect each other gradually, which maybe contribute to decrease in the pores less than 3 nm and reduction in the initial parts of the distribution curves. 3.5. Fractal analysis of the isotherms based on FHH model In the molecular-size range, the surfaces of most materials are fractals: geometrically irregular surfaces that are self-similar upon variations of resolution. For such surfaces, a parameter can give the degree of irregularity: the fractal dimension, 2 ≤ DS ≤ 3, of the surface. The higher the DS value, the more undulant and spacefilling the surface is. Fractal BET (Brunauer–Emmett–Teller), fractal FHH (Frenkel–Halsey–Hill) and thermodynamic methods can be used to measure fractal dimensions on the basis of gas adsorption isotherms [32,33]. Among those, fractal FHH method has been proven to be the most effective method. Using the fractal FHH equation for the analysis of gas adsorption data, the fractal dimension can be determined. The derivation of the fractal dimension using this theory is described elsewhere in detail [34–39]. A brief summary is provided below. The classical FHH theory for multilayer adsorption was extended to the fractal surfaces in the following form: ln
N Nm
P 0
= C + A · ln ln
P
(5)
N/Nm is the number of layers adsorbed on the surface, A is the power-law which is dependent on DS , P0 is saturation pressure and P is the adsorption equilibrium pressure of the gas. At the early stage of adsorption, the van der Waals forces between gas–solid interactions are dominant and the liquid–gas surface tension forces are negligibly small. So, the relationship between A and DS can be derived as follows: A=
DS − 3 3
(6)
For higher coverage, the interface becomes controlled by the liquid–gas surface tension forces and the relationship between A and DS changes to the following expression: A = DS − 3
(7)
According to Ismail and Pfeifer [40], the threshold for the dominant forces between the van der Waals forces and the liquid–gas surface tension forces is given by: ˛ = 3(1 + A) − 2
(8)
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Fig. 12. Pore size distribution of two char samples in different environments (chars residence time in the DTF: 1.15 s). (a) LY anthracite chars. (b) GZ bituminous coal chars.
Fig. 13. Pore sizes distribution development of LY anthracite chars. (a) 21%O2 /79%N2 . (b) 21%O2 /79%CO2 .
Fig. 14. Pore sizes distribution development of GZ bituminous coal chars. (a) 21%O2 /79%N2 . (b) 21%O2 /79%CO2 .
If ˛ < 0 the liquid–gas surface tension forces are dominant, but the van der Waals forces are dominant if ˛ ≥ 0. Fig. 15 shows the plots of ln(N/Nm ) against ln[ln(P0 /P)] from the N2 adsorption isotherms measured on samples. For all chars, one can find clearly linear relationship between ln(N/Nm ) and ln[ln(P0 /P)], indicating scaling properties of all samples surfaces. The surface fractal dimensions calculated with Eq. (5) are presented in Table 2. All calculated fractal dimensions (DS ) are between 2 and 3, and the O2 /N2 chars always show higher DS values which indicate more irregularity and roughness of the char surfaces. Although the geometrical irregularity and roughness of the surfaces are the essential reasons for the obtained DS values, it
Table 2 Fractal dimensions derived from fractal FHH model. Samples
Combustion environment
DS
R
LY anthracite chars
21%O2 /79%N2 21%O2 /79%CO2
2.699 2.687
0.9902 0.9899
GZ bituminous coal chars
21%O2 /79%N2 21%O2 /79%CO2
2.633 2.554
0.9916 0.9887
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Fig. 15. ln(N/Nm ) − ln[ln(P0 /P)] plots chars obtained in two different combustion environments (chars residence time in the DTF: 1.15 s). (a) LY anthracite chars. (b) GZ bituminous coal chars.
is known that the absorbed film volume and pore size distributions are used to define the fractal when fractal FHH equation is used. Therefore, the fractal FHH type equation might be sensitive to the pore size distribution, and pore size distribution could contribute significantly to the surface fractal dimension. The next section gives the analysis and comparison of pore size distributions of samples obtained in two different combustion environments.
3.6. FT-IR spectroscopic analysis of char particles Fourier transform infrared spectroscopies (FT-IR) were obtained with BRUKER VECTOR22 spectrometer. The alkali halide (KBr) pellet technique was used and the ratio of coal (char) to potassium bromide was about 1:120 (w:w). The mixture of coal (char) and KBr was finely milled in a porcelain container and the pellet were dried and kept in vacuum desiccator to avoid any moisture absorp-
Fig. 16. FT-IR spectra of chars obtained in different environments (chars residence time in the DTF: 1.15 s). (a) LY anthracite chars. (b) GZ bituminous coal chars.
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Table 3 Band assignments in FT-IR spectra of coal (char) and inertinite maceral in coal [41,42]. Bands/cm−1
Assignments
3400–3320 3050–3030 3000–2800 1700 1610–1560 1510 1450 1430–1420 1370–1360 1317–1315 1270–1250 1060–1030 870 810 780–770 750
–OH stretching Aromatic CH stretching Aliphatic CH stretching Aromatic carbonyl/carboxyl C O stretching Aromatic C C ring stretching Aromatic C C ring stretching Aliphatic CH deformation Aromatic C C ring stretching Aliphatic CH3 deformation Aliphatic CH2 deformation Aromatic CO– and phenolic –OH stretching Aliphatic ether C–O– and alcohol C–O stretching 1 Adjacent H deformation 2 Adjacent H deformation Aliphatic CH2 deformation 3–4 Adjacent H deformation
tion. The peak assignments in this paper are mainly based on the work of Painter et al. [41] and Marchessault [42] (Table 3). The FT-IR spectra for pyrolysis chars and partly burnt chars prepared from two original coals are shown in Fig. 16. The spectra are characterized by the presence of peaks of 1700 cm−1 , 1615 cm−1 , 1430–1420 cm−1 1060–1030 cm−1 and 780 cm−1 . The peak at 1700 cm−1 , due to acid C O groups and the peak at 1031 cm−1 , due to aliphatic C–O–C and alcohol C–O stretching in the range of 1060–1030 cm−1 , are prominent in all char samples. The aromatic C C ring stretching vibration at 1430 cm−1 are almost eliminated in all samples. The peak at 780 cm−1 can be derived from hydrogen deformation in aromatic ring structure and aliphatic CH2 deformation. This peak is mainly presented in the pyrolysis chars especially for LY anthracite chars. The peak in the band range of 3000–2800 cm−1 , due to aliphatic CHX stretching vibration, is largely eliminated during the pyrolysis or the combustion processes. The broad band at 3400–3320 cm−1 is mainly represented the –OH stretching vibration of water. There are marked differences between N2 and CO2 pyrolysis chars along with O2 /N2 and O2 /CO2 partly burnt chars. It can be seen that the absorbance intensity of the aliphatic and aromatic functional groups in CO2 pyrolysis char (or O2 /CO2 partly burnt char) are much higher than that in N2 pyrolysis char (or O2 /N2 partly burnt char), which indicate that the elimination rate of this organic functional group of chars obtained in CO2 environments is slowly or delayed.
4. Conclusions Two typical Chinese coal and its partly burnt chars obtained under O2 /CO2 combustion condition and conventional air combustion condition with same oxygen concentrations were prepared and analyzed for their physical–chemical structure evaluation. Results present that the reaction rate and burn-out degree of O2 /CO2 chars are lower than those of O2 /N2 chars under the same combustion condition, which indicates that a higher O2 concentration in O2 /CO2 mixture than in O2 /N2 mixture is needed to achieve the similar combustion characteristic. The main differences between O2 /N2 and O2 /CO2 chars rely on the pore structure determined by N2 adsorption and chemical structure measured by FT-IR. The O2 /CO2 char has thick surface and compact pores, which contribute to the reduction fragmentation of particles burning in O2 /CO2 environment. The organic functional group elimination rate of chars obtained in CO2 environments is slower or delayed.
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