Ignition Characteristics of Pulverized Coal under High Oxygen

At present, there is a tendency to burn coal at high oxygen concentrations more than 21% vol due to its merits in energy saving and flame stability en...
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Energy & Fuels 2008, 22, 892–897

Ignition Characteristics of Pulverized Coal under High Oxygen Concentrations Yue-sheng Fan,*,† Zheng Zou,‡ Zidong Cao,† Yingchao Xu,† and Xiaoke Jiang† School of Energy and Power Engineering, Xi’an Jiaotong UniVersity, Xi’an, 710049, People’s Republic of China, and Jimei UniVersity, Xiamen, 361021, People’s Republic of China ReceiVed May 30, 2007. ReVised Manuscript ReceiVed December 25, 2007

In order to reduce overall fuel consumption, or partially substitute a “valuable” fuel with a poor one, in electric power plant boilers, oxygen enrichment of combustion air can be very effective. Combustion characteristics of three Chinese pulverized coals, Shenmu bituminous, Tianhushan anthracite, and Duolun lignite, and three different particle sizes, under high oxygen concentrations more than 21%, have been investigated using thermogravimetric/differential scanning calorimetry analysis (TG/DSC) and a drop-tube furnace. Results showed that the ignitability, the combustion property, and the burnout were largely improved when added oxygen was used, especially for small particles, the influence of oxygen on the bituminous coal was greater than the lignite and the anthracite, and the suitable O2 concentration for the ignition of pulverized coal flow should be controlled below 40%.

1. Introduction In spite of extensive studies,1–9 the ignition of pulverized coal (PC) is still the subject of much discussion. Many experiments at 1 atm10–21 have involved coal particles being heated in preheated gas or a flame,10–17 by direct attachment to a heating element,18,19 or, most recently, by laser irradiation.20,21 The * Corresponding author. Tel.: +86-29-82202729. Fax: +86-29-82202729. E-mail address: [email protected]. † Xi’an Jiaotong University. ‡ Jimei University. (1) Essenhigh, R. H.; Misra, M. K.; Shaw, D. W. Combust. Flame 1989, 77, 3. (2) Wall, T. F.; Gupta, R. P.; Gururajan, V. S.; Zhang, D. K. Fuel 1991, 70, 1011. (3) Gururajan, V. S.; Wall, T. F.; Gupta, R. P.; Trovlove, J. S. Combust. Flame 1990, 81, 119. (4) Howard, J. B.; Essenhigh, R. H. Ind. Eng. Chem., Process. Des. DeV. 1967, 6, 74. (5) Annamalai, K.; Durbertaki, P. Combust. Flame 1977, 29, 193. (6) Ryan, W.; Annamalai, K. J. Heat Transfer 1991, 113, 677. (7) Zhang, D. K.; Wall, T. F. Combust. Flame 1993, 92, 475. (8) Du, X. Y.; Annamalai, K. Combust. Flame 1994, 97, 339. (9) Krishna, C. R.; Berlad, A. L. Combust. Flame 1980, 37, 207. (10) Chen, M. R.; Fan, L. S.; Essenhigh, R. H. Prediction and Measurement of Ignition Temperature of Coal Particles. In Twentieth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1984; p 1513. (11) Gomez, G. O.; Vastola, F. J. Fuel 1985, 64, 558. (12) Wall, T. F.; Gururajan, V. S. Combust. Flame 1986, 66, 151. (13) Wall, T. F.; Phong-Anant, D.; Gururajan, V. S.; Wibberley, I. J.; Tate, A.; Lucas, J. Combust. Flame 1988, 72, 111. (14) Solomon, P. R.; Chien, P. L.; Carangelo, R. M.; Serio, M. A.; Marham, J. R. Combust. Flame 1990, 79, 214. (15) Du, X. Y.; Gopalakrishnan, C; Annamalai, K. Fuel 1995, 74, 487. (16) Karcz, H.; Kordylewski, W.; Rybak, H. Fuel 1980, 59, 799. (17) Tognotti, L.; Malotti, A; Petarca, L.; Zanelli, S. Combust. Sci. Technol. 1985, 44, 15. (18) Tomezek, J.; Mlonka, J. The Parameters of a Random Pore Network with Spherical Vesicles for Coal Structure Modelling. In Twenty-Third Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1990; p 1163. (19) Fuertes, A. B.; Hampartsonmian, A.; Williams, A. Fuel 1993, 72, 1287. (20) Phouc, T. X.; Mathur, M. P.; Ekmann, I. M. Combust. Flame 1993, 93, 19. (21) Qu, M. C.; Ishigaki, M.; Tokuda, M. Fuel 1996, 75, 1155.

influence of experimental techniques has been analyzed.2,22 At present, there is a tendency to burn coal at high oxygen concentrations more than 21% vol due to its merits in energy saving and flame stability enhancement. However, only a few researchers have studied the effect of oxygen on the ignition of pulverized coal.23–25 The ignition and combustion of pulverized coal is more difficult than that of oil and gas fuel, mainly due to a longer ignition time needed and unstable combustion at lower load.27 Therefore, oil-fired methods are in common use in PC boilers. However, the mixing burning of heavy fuel oil (HFO) and PC makes the technical and economical target worse and brings about severe environmental problems, for instance, the release of SO2 and NOX may increased about 40–50%, whereas the total efficiency of boilers decreased about 4–5%. Furthermore, the use of HFO in boilers caused a loss of boiler availability due to external fouling and corrosion of high temperature.28 Although many new fired techniques, i.e., pulverized coal rich-lean separation technology, high-temperature air combustion technology, and preburner technology, have been applied to enable the pulverized coal furnace to burn steadily, the oil waste is still vast26,30. For a 100 MWe electrical power boiler, the ignition and steady-going combustion process would consume 528.6 t y-1 (tons of oil fuel annually).30 Actually, the ignition and steady-going combustion need only a steady high temperature source. The literature23–25,29 indicated that the fuel had a lower ignition temperature and higher combustion rate under oxygen-enriched atmosphere, hereby (22) Zhang, D. K.; Wall, T. F. Fuel 1994, 73, 1114. (23) Sun, C. L.; Zhang, M. Y. Combust. Flame 1998, 115, 267–274. (24) Chen, J. C. Combust. Flame 1996, 107, 291–298. (25) Zhao, Y.; Kim, H. Y.; Yoon, S. S. Fuel 2007, 86, 1102–1111. (26) Tu, J. H.; Yao, Q.; Chen, K. F. Power Equip. 1997, 7, 8–11. (27) Wu, S. D. Elec. Power Constr. 1998, 11, 1–5. (28) Poullikkas, A. Energy ConVers. Manage. 2004, 45, 1725–1734. (29) Bisio, B.; Bosio, A.; Rubatto, G. Energy ConVers. Manage. 2002, 43, 2589–2600. (30) Shi, W. Numerical simulation and experimental studies on the multistage oil less ignition system using the method of electromagnetic induction. Dissertation, Zhejaing University, Hangzhou, PRC, 2003.

10.1021/ef7006497 CCC: $40.75  2008 American Chemical Society Published on Web 02/14/2008

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Table 1. Ultimate and Proximate Analysis of Coal Samples proximate analysis (wt %, ad)

ultimate analysis (wt %, ad)

coal

M

V

A

FC

C

H

O

N

S

THSH anthr SM bit DL lig

2.93 2.60 20.07

2.54 32.76 37.38

32.49 6.56 11.35

62.04 58.08 31.20

61.87 73.63 52.92

0.62 4.54 2.78

1.47 11.38 11.69

0.32 0.95 0.19

0.3 0.34 1

Table 2. Mean Weight Diameter and Specific Surface Area of Coal Samples coal sample

THSH

SM-1

SM-2

SM-3

DL

mean weight diameter (µm) 96.18 71.30 60.68 53.38 181.14 BET surface area (m2 g-1) 34.077 13.176 14.438 16.156 7.239

producing a higher temperature source in boilers to ignite subsequent pulverized coal flow. A number of laboratory techniques have been used to determine the ignition and flame stability of pulverized coal. Thermogravimetric/differential scanning calorimetry analysis (TGA/DSC) and a drop-tube furnace (DTF) were widely used to investigate the reactivity of pulverized coal. In this paper, the ignition of pulverized coal at high O2 concentrations (21-100% vol) was studied with a thermogravimetric analyzer to continuously measure the temperature of the coal during the ignition and combustion, and a DTF was used to simultaneously measure the temperature and the content of the pulverized coal flow. 2. Experimental Setup 2.1. Samples. Three different rank coal samples were taken from Tianhushan (THSH) anthracite, Shenmu (SM) bituminous, and Duolun (DL) lignite of China. Coals were first pulverized to the needed particle size with a ball mill. The ultimate and proximate analyses of the three coals are given in Table 1. The mean weight diameters of samples were measured by the sieving method and listed in Table 2. Three different size distribution samples were prepared for SM coal. The multimode scanning probe microscope was used here to analyze the surface characteristics of pulverized coal. The amplificatory multiple is 500. Photos of the test samples are shown in Figures 1-3. From photos, we can see that the particle size of the THSH anthracite is the smallest, its configuration polyhedron is similar to a globe body, and, most DL lignite with the largest diameters and smallest specific surface areas (SSA), present a kind of spheroid shape. The configurations of SM bituminous present a kind of atactic shape. The SSA of the test samples are listed in Table 2. The samples were first heated and placed under vacuum to remove the moisture and other contaminants. Then when measured, the BET specific surface area demonstrated the order of PC based on SSA from the minimum to maximum to be DL, SM, and THSH, which was different from the order by mean weight diameter of the test samples. The reason is that distinct agglomerating phenomena occurred between the particles of THSH pulverized coal, which can be observed by microscope. Many small particles tended to swarm and be hard to separate by the sieving method, thus easily building up in the feeding pipe of drop-tube furnace apparatus. Table 2 also revealed that the SSA increased as the mean weight diameter decreased for the same kind of pulverized coal, because more and more small pores inside particles are exposed when the particle diameter is reduced. 2.2. Thermogravimetric (TG) Analysis. Temperature-programmed combustion tests were performed in a thermogravimetric apparatus (Netzsch-STA 409C). In order to compare the samples, factors such as sample mass, heating rate, and gas flow rate were well determined to ensure good repeatability between experimental

runs.31,32 About 10 mg of each sample was heated at 10 °C min-1 from ambient temperature up to 1100 °C, with a flow rate of 100 mL min-1 for mixture of nitrogen and oxygen. Data on time, residual weight, temperature close to the sample, rate of weight loss or differential weight loss, and the nature and extent of endothermic or exothermic reactions during thermochemical conversion were recorded simultaneously to produce combustion profile. From the combustion profile, the following characteristic temperatures (Illustrated in Figure 4) were obtained:33,34 Ti, initial temperature where the rate of weight loss accelerates due to the onset of combustion (almost equivalent to the first DSC peak); Tp, peak temperature at maximum weight loss rate; Tb, burnout temperature where DTG reaches a 1% combustion rate at the tail end of the profile; Tcb, complete burnout temperature, the point where the derivative of the DSC curve inflects to zero gradients. It should be kept in mind that the Ti is not the ignition temperature. Experimental factors such as the sample mass, the particle-heating rate, and the condition of surrounding gases influence the initial temperature. Therefore, the initial temperature is not a physical property of a fuel.35 Our aim was not to propose method for initial temperature evaluation. This study was based on comparison purpose, i.e., we kept the same burnoff procedure to see the impact of high oxygen on the ignition and combustion characteristics of pulverized coal. 2.3. Drop-Tube Furnace Analysis. A DTF can simulate the combustion conditions in industrial pulverized coal combustion more closely than a thermogravimetric analyzer.36 A heating rate of 104-105 °C s-1 and maximum temperature of 1300 °C can be achieved in a DTF. In addition, the particles are in a dynamic, dilute phase. Fewer studies on ignition behavior and flame stability of pulverized coal in DTF have been done than in TGA,34 especially under high oxygen concentration. The measurement techniques available in DTFs for studying particle combustion may be divided into “internal” and “external” classes.14 The former one usually uses probes to take samples for analysis. Of the external methods, the optical technique is the most powerful, which is usually nonperturbing and results can be obtained immediately in situ. The two methods above were accepted here. First, at the sidewall of the DTF, 10 optical windows were designed to reveal the actual combustion flame situation. Second, the optical window sections, fitted with an access port, allowed insertion of a sampling tube that can be easily changed for future experiments. Through the sampling tube, reaction products of different combustion stages can be elicited into the sampling probe. The solids were removed by filtration and the exhaust gases were analyzed for CO, CO2, NOx, etc. with the help of an online analyzer (the GASMET Fourier transform infrared gas analyzer, manufactured by Temet Instruments in Finland). Meanwhile, a pump thermocouple is located in the sampling probe to measure the gas temperature. (31) Morgan, P. A.; Robertson, S. D.; John, F. U. Fuel 1986, 65, 1546– 1551. (32) Chen, Y.; Shigekatsu, M.; Pan, W. P. Thermochim. Acta 1996, 275, 149–158. (33) Pisupati, S. V.; Scaroni, A. W.; Stoessner, R. D. The Influence of Weathering and Low-Temperature Oxidation on the Combustion Behaviour of Bituminous Coals. In 13th Annual Energy-Sources Technology Conference and Exhibition. Petroleum Division of ASME: New Orleans, 1990; p 87. (34) Artos, V; Scaroni, A. W. TGA and drop-tube reactor studies of the combustion of coal blends. Fuel 1993, 72 (7), 927–933. (35) Perry, J. H. Chemical Engineers’ Handbook, 4th ed.; McGrawHill: New York, 1963; Vol. 3–93. (36) Su, J. H.; Pohl, D; Holcombe, J. A. Progr. Energy Combust. Sci. 2001, 27, 75–98.

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Figure 1. Microscope image of THSH anthr pulverized coal. Figure 4. Combustion characteristic temperatures.

Figure 2. Microscope image of SM-1 bit pulverized coal.

Figure 5. Experimental apparatus of the drop-tube furnace.

Figure 3. Microscope image of DL lig pulverized coal.

The DTF experimental apparatus is represented in Figure 5. The designed cylindrical reaction chamber is 40 mm in diameter and 1.20 m in length, on which four K-type thermocouples were equipped for monitoring, controlling, and overtemperature protection. The DTF was heated by silica-carbon rods which are perpendicular to the furnace center and with a space between them of 20 cm in the vertical direction. When pulverized coal is injected into the furnace, the calculated heating rate of coal particles is of the order of 104–105 °C s-1,2 depending on the size of a particle. A 60-µm particle may reach the furnace temperature within 0.05–0.1 s. So, a 10 °C min-1 temperature-programmed process was used in the DTF to achieve the ignition temperature of pulverized coal flow under oxygen concentrations of 21, 30, 40, 60, and 80 vol %, respectively.13,22 The solid flow is entrained into the hot zone of the furnace by a gas flow of 40 L min-1 of a mixture of air and pure oxygen, calculated at standard temperature and pressure. The residence time of flow in the DTF is about 1.2 s at 800 °C, and temperature changes of the furnace wall are only 0.2 °C in that period. Therefore, we can consider that the temperature of the furnace wall remains constant while the gas flow containing pulverized coal was passing

through the furnace length. The temperatures of the furnace wall and gas were measured and recorded at 1-s intervals on a computer by the program. The temperature of the furnace wall can be calculated as the average value of the measured data by four thermocouples located in holes drilled on the furnace wall; at the same time, the flue gas temperature was measured by a pump thermocouple located in the sampling probe. A screw fuel feeder was used in the experiment. By changing the rotation speed of the electromotor, the desired feeding rate can be obtained. However, as the coal particle diameter is decreased and the ash content is increased, the fuel feeding uniformity became poor. In actual operation for high ash coal, subsection feeding in the hopper will be needed to ensure the proper fuel feeding uniformity.

3. Results and Discussion 3.1. Thermogravimetric Analysis Results. The TG/DTG/ DSC profiles of THSH, SM-1, and DL pulverized coal under different oxygen concentrations are shown in Figures 6-8. The figures indicated that higher oxygen concentrations shifted the char combustion peaks to a lower temperature zone, and the profiles revealed that the existence of weight loss related to vapor elapsed in the stage from ambient temperature to about 100 °C. In addition, a net weight gain was observed which is due to oxygen chemisorptions before the onset of combustion for SM coal. The flammability of coal was highly correlated with a quantity of oxygen absorbed at about 100 °C.1 Figure 6 demonstrated that the chemisorptions occurred before 350 °C for all oxygen concentrations, and this became more obvious as the O2 concentrations increased. The quantity of chemisorptions of oxygen could be worked out to be 0.73% for 21%

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Figure 6. TG/DTG/DSC profiles of different oxygen concentrations of THSH.

Figure 7. TG/DTG/DSC profiles of different oxygen concentrations of SM-1.

Figure 8. TG/DTG/DSC profiles of different oxygen concentrations of DL.

oxygen, 1.65% for 30% oxygen, 6.21% for 60% oxygen, and 6.88% for 100% oxygen, respectively. The higher the chemisorbed O2, the greater the chemical reaction rate on the surface of coal. Namely, higher oxygen concentrations can improve the ignition characteristic of pulverized coal. This phenomenon can also be observed in Figure 8 of DL but is not obvious for THSH. From Figures 6-8, another conclusion can be obtained: the hole structure of SM bituminous coal is more helpful for oxygen adsorption than DL lignite and THSH anthracite, since it holds the biggest chemisorptions heat peak. 3.1.1. Ignition Mechanism. Extensive research on the mechanism of ignition of coal particles has classified the ignition into three types:2,3 (1) Homogeneous ignition or the ignition of the volatile matter released from coal (2) Heterogeneous ignition or the ignition of the coal particle surface (3) Heterohomogeneous ignition, which results from simultaneous ignition of the volatile matter and the coal particle surface Distinctly, the three samples gave different combustion characteristics. It may be observed that the DL coal (Figure 8) belonged to homogeneous ignition1,23,32 that involved three stages. The

Figure 9. Variation of characteristic temperature with O2 concentration.

primary step was the initial ignition of volatile matter; following this, the combustion of volatile matter, in a circumambient flame like an oil drop, was presumed to prevent char reaction by screening the solid from access by oxygen; secondary ignition, of char, then occurred as pyrolysis terminated.1 Obviously, one-step ignition, by direct attack of reactant gas on the whole coal, occurs in the THSH coal (Figure 6) for its very low concentration of volatile matter, which is true of heterogeneous ignition.1,23,32

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Figure 11. Illustration of critical ignition temperature in DTF. Figure 10. Plot of Tcb with oxygen concentration for three SM samples.

Figure 7 revealed that SM-1 was heterohomogeneous ignition type.1,35 DSC curves shifted to lower temperature levels with increasing oxygen concentrations, and as the oxygen concentration increased further, two exothermic peaks on the DSC curves and two stages on the TG combustion curves became more apparent. These are the same results as shown in the literature.37 However, all samples provided only one sharp DTG peak, where the gas and solid phase combustions were not separated. From Figures 6-8, another conclusion can be gained that the oxygen concentration did not affect the ignition mechanism for all samples, and it only made the combustion curves shifted to a lower temperature zone and enhanced the intensity of released heat. The configurations of DTG/DSC curves for a certain coal did not change under different O2 concentrations. 3.1.2. Characteristic Temperature. Figure 9 showed the variation of characteristic temperatures vs oxygen concentration for the samples of SM-1, DL, and THSH. It is evident that as the oxygen concentration increases, only the three curves of SM coal gradually moved closer; however, the curves for DL and THSH coal stayed nearly parallel all along. The temperature difference between Ti and Te varied a little as the oxygen concentration increased, namely the combustion time from Ti to Te varied a little as the oxygen concentration increased. However, for SM bituminous coal, the combustion time from Ti to Te varied, for instance 18.0 min for 21%, 17.3 min for 30%, 13.8 min for 60%, and 12.4 min for 100%, respectively. In other words, the influence of oxygen on the bituminous coal was bigger than that on the lignite and the anthracite, and obviously, SM bituminous was the most suitable pulverized coal for the oxygen enrichment ignition system. 3.1.3. Effect of Particle Size on Complete Burnout. The effect of particle size was studied using SM bituminous coal. Jiang37 indicated that as the particle size decreased, both the ignition and burnout temperature decreasd. Because the relative SSA of the coal particle was enlarged, therefore, volatile matter was easily released and char was easily ignited. Figure 10 is the plot of the complete burnout temperature vs oxygen concentrations. It is found that as the particle size decreased, the effect of oxygen concentrations on complete burnout of pulverized coal became more obvious. For SM-3, when the oxygen concentrations was 30%, the Tcb was almost the same as that at 100% oxygen concentrations; for SM-2, the same effect was the 60% oxygen concentrations. On the basis of Figure 10, small particles showed better burnout properties and shorter burning times than large ones, as the former had the larger SSA. This meant that small particles had the higher combustion intensity under high oxygen concentrations, which should be used in the ignition system of oxygen enrichment. (37) Jiang, X. M.; Li, J. B.; Qiu, J. R. Proc. CSEE 2000, 20 (6), 71-74.

Figure 12. Plot of ignition temperature in the DTF with oxygen concentrations.

3.2. DTF Analysis Results. 3.2.1. Ignition Criterion in DTF. The ignition temperature of pulverized coal flow can be indicated by a jump of temperature, a rapid weight loss, and the measuring composition of the exhaust gas. Typical plots of the CO2 concentration and the temperature of the furnace wall (Tb) and flue gas (Ty) are shown in Figure 11 versus measuring time for SM coal, which illustrated that when ignition occurred, Ty and the concentration of CO2 increased simultaneously and sharply. However, Wall13 indicated that when ignition occurred, the temperature of the flue gas may be increased sharply, or not, depending on the coal rank. For lignite, the temperature of gas did not increase sharply when ignition occurred. It is reasonable to assume that before the continuous ignition of pulverized coal flow, CO2 concentrations increased very slowly and would never suddenly increase. Therefore, we can take the temperature of the flue gas at which the CO2 concentration shows a jump as the ignition temperature of pulverized coal flow. The criterion for determining the ignition temperature was based on the derivative curves of gases produced. The ignition temperature was taken as the temperature where the derivative curves, normalized with respect to the maximum derivative value, reached a value of 10%.19 3.2.2. Effect of oxygen Concentrations on the Ignition Temperature of PulVerized Coal Flow. By changing the coalfeeding rate, the average mass concentrations of pulverized coal flow in the reactor obtained, which were 0.4 kg coal kg-1 gas for SM bituminous coal and 0.3 kg coal kg-1 gas for DL lignite coal. Figure 12 is the plots of ignition temperature vs oxygen concentrations, which clearly show that the four tested samples have the similar trend that the ignition temperature decreased as the O2 concentration increased. This is the same as in the literature.1,23,24 Because the combustion of released combustible volatile matter accelerated, char ignition easily occurred as the O2 concentration enlarged. The difference between the DTF and TG experiment is that the ignition temperature of DL lignite was higher than SM bituminous in the DTF experiment, and the opposite situation occurred in the TG experiment. This can

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Table 3. Downtrend of Ignition Temperature with Oxygen Concentrations O2 concentration range (vol %) coal DL lig SM-2 bit

21-30 ∆t (°C) β (°C vol %-1) ∆t (°C) β (°C vol %-1)

31 3.44 131 14.56

30-40 40-60 drop in temperature 18 1.8 102 10.2

34 1.7 51 2.55

60-80 47 2.35 140 7.0

also be explained by the effect of mass concentrations of pulverized coal or too much water content (Mar ) 34%) in DL lignite. The evaporation of water would consume the large initial combustion heat of the pulverized coal flow definitely, thus causing the ignition to delay and the ignition temperature to increase. Another conclusion can be made that the trend shown by the three different size SM coals appears to be the same, and the ignition temperature decreased as the particle size decreased. Furthermore, Figure 12 also revealed that as the volume concentration of O2 exceeded 40%, the ignition temperature of the tested SM pulverized coal flow decreased to 600 °C, equivalent to the ignition temperature of the oil fuel.38 The β is defined as the rate of variation of ignition temperature under every 1% oxygen concentration. Table 3 reveals that the degressive rate of ignition temperature of bituminous coal is larger than that of lignite, that the degressive rate of ignition temperature is the largest in the range of 21–30% vol O2, and it is the smallest in the range of 40–60% vol O2. When the O2 concentration increased from 21 to 30%, the ignition temperature of SM-2 decreased by 131 °C, whereas the DL decreased by 31 °C. Thus, O2 affects much more on bituminous than lignite, and the suitable O2 concentration for the ignition of pulverized coal flow may be controlled below 40% vol O2. 4. Conclusion To sum up the above discussions, we can obtain the following conclusions: (1) The ignitability, combustion property, and burnout have all been greatly improved when enrichment oxygen was added, especially for small particles. The ignition mechanism did not change under the higher O2 concentrations, even under the pure O2 circumstance at the lower heating rate. (38) Cao, Y. Q. Energy Eng. 2000, 5, 60–62.

(2) As the O2 concentration increased, the characteristic temperatures decreased, but only the three curves of SM bituminous gradually moved closer as O2 concentration increased. Namely, the influence of oxygen on the bituminous coal was bigger than that on the lignite and the anthracite; the bituminous pulverized coal is the most suitable for the ignition system of oxygen enrichment. (3) DTF experiments indicated that the degressive rate of ignition temperature was the largest in the range of 21-30% O2 and the smallest in the range of 40-60%. Therefore, the suitable O2 concentration for the ignition of pulverized coal flow may be controlled below 40% O2. Since electrical utilities are highly required to reduce costs, improve combustion behavior, control ash deposition, enhance fuel flexibility, and extend the range of acceptable coals, ignition and combustion technology in oxygen enrichment ambience for pulverized coal would become possible in pulverized coal-fired power stations in the near future. Abbreviations

anthr ) anthracite bit ) bituminous DL ) Duolun DSC ) differential scanning calorimetry DTF ) drop-tube furnace DTG ) derivative thermogravimetry; the rate of weight loss with time, as a function of temperature HFO ) heavy fuel oil HFOF ) heavy fuel oil-fired lig ) lignite PC ) pulverized coal OPCF ) oxy-enriched pulverized coal-fired SM ) Shenmu SSA ) specific surface area TGA ) thermogravimetric analysis THSH ) Tianhushan Ti ) initial temperature Tp ) peak temperature at maximum weight loss rate Tb ) burnout temperature Tcb ) completely burnout temperature Tb ) temperature of the furnace wall Ty ) temperature of the flue gas EF7006497