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Effect of Char Particle Size on NO Release during Coal Char Combustion Jie Xu, Rui Sun, Tamer Ismail, Shaozeng Sun, and Zhuozhi Wang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02580 • Publication Date (Web): 09 Nov 2017 Downloaded from http://pubs.acs.org on November 10, 2017
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Energy & Fuels
Effect of Char Particle Size on NO Release during Coal Char Combustion
Jie Xua, Rui Suna*, Tamer M. Ismailb*, Shaozeng Suna, Zhuozhi Wanga a
School of Energy Science and Engineering, Harbin Institute of Technology, 92 West
Dazhi Street, Harbin 150001, P. R. China b
Department of Mechanical Engineering, Suez Canal University, Ismailia, Egypt
*Corresponding author Rui SUN School of Energy Science and Engineering, Harbin Institute of Technology, 92 West Dazhi Street, Harbin 150001, P. R. China Tel.: +86 451 8641 3231 802 Fax: +86 451 8641 2528 E-mail address:
[email protected] (R. Sun)
Tamer M. Ismail Department of Mechanical Engineering, Suez Canal University, Ismailia, Egypt Tel.: +20 01224745463; fax: +20 0226829366 E-mail address:
[email protected],
[email protected] (T. M. Ismail).
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Effect of Char Particle Size on NO Release during Coal Char Combustion Jie Xua, Rui Suna*, Tamer M. Ismailb*, Shaozeng Suna, Zhuozhi Wanga a
School of Energy Science and Engineering, Harbin Institute of Technology, Harbin 150001, China;
b
Department of Mechanical Engineering, Suez Canal University, Ismailia, Egypt
Abstract: In this study, the effect of particle size on the release of NO during the char combustion under particle packed-layer conditions was investigated at combustion temperatures of 900 °C~1400 °C. The results show that after pyrolysis, the small size char favored more pores and specific surface area. No significant variations of the intrinsic reactivity of O2 and NO with char particle size were measured. By increasing char particle size, more NO conversion from nitrogen in char (char-N) was found regardless of the combustion temperature and bulk oxygen concentration. This can be explained as follows. By increasing char particle size, less O2 penetrated into the pores and then more NO formed which was attributed to a less accessible pore surface area and a decreased NO reduction time. The effects of reaction temperature and bulk oxygen concentration were also discussed. A quantified description using a first-order reaction model shows that the char particle size will finally influence the accessible pore surface area and depth of O2 diffusion, which will directly determine the char-N/NO conversion. The evolution of nitrogen functionality with particle size and burn-off were also presented to demonstrate migration tendency of the char-N. The larger ratio of O/C in smaller size particle as determined using XPS (X-ray Photoelectron Spectroscopy) further confirmed higher pore diffusion. In addition, nitrogen was found to be preferentially retained in the char when carbon is oxidized. * Corresponding authors: E-mail addresses:
[email protected] [email protected],
[email protected] (T.M. Ismail).
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(R.
Sun),
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Keywords: Particle size; Char-N to NO conversion; XPS; Accessible pore surface area; Depth of O2 diffusion; Char combustion
1. Introduction Emission of nitrogen oxides (NOx) has drawn the attention of many researchers and the public because of its serious environmental impacts. In practical coal combustion systems, nitrogen content in coal (fuel-N) is generally the major contributor to total NOx emissions. Previous studies1-3 have provided a general understanding of the conversion of fuel-N in the volatile matter (volatile-N) or char (char-N) to NOx. Nitrogen in volatiles has been shown to form HCN, NH3, and soot-bound nitrogen as intermediate species during the pyrolysis process, which is then oxidized to NOx, or NOx can be reduced to produce N2 under a low-oxygen atmosphere. Meanwhile, a large fraction of char-N may be oxidized to form NOx. In certain low-NOx pulverized-coal (PC) burners and in fluidized-bed (FB) coal combustion, a large fraction of char-N may be oxidized to form NOx directly. Especially in modern low-NOx PC or FB coal combustion systems, volatile-N will react to produce N2 by precise control of the reacting atmosphere, while char-N will play the role of the main contributor to NOx emissions at the furnace exit4.
Some research groups, in their studies of char-N conversion, have reviewed the understanding of char-nitrogen conversion. Thomas3 reviewed the influence of coal and char structural characteristics on the release of NOx during coal combustion and noted that more developed pore structures produced less NOx. Aarna and Suuberg5 summarized the kinetics of the char-NOx reaction and noted that this may involve the possible initial chemisorption of NOx and reaction of surface complexes. The
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reduction of pure NO is generally found to be of the first-order with respect to NO concentration. Molina et al.6 focused their research on developing a mechanism and model of nitrogen release from char to the homogeneous phase, its further oxidation to NO, and the reduction of NO on the surface of char. Based on their analysis of experimental results, the mechanism of NO production from coal, which has been proposed by De Soete7, is considered to be the most comprehensive and useful, and they emphasized the importance of the reaction between NO and char surface.
Although numerous studies on the production of NO during coal char combustion have allowed for the preliminary characterization of this process, there is still uncertainty and controversy concerning the particle size. Most studies have focused on the larger particle size of char in the FB combustion system due to the diffusion effect. At single particle conditions, Jensen et al8 and Park et al9 found that the fraction of fuel-N to NO is independent of O2 concentration, and the smaller the particle size is used, the more char-N is converted to NO. This may be contributed to less NO reduction time when NO is released from the pore to the particle surface. Ninomiya et al10 showed that the large particle size resulted in more NO formation under single particle condition at two different reacting temperatures. In Ninomiya’s experiments, the applied particle size was at a relatively large range of 65-1125 µm. Experiments on the fluidized bed system by Tullin et al11 indicated that the fuel-N conversion to NO decreases monotonically with the increase of the particle size. He explained that the larger the particle size is, the more of formed NO during C(N)-O2 reaction inside the particle may be reduced by the C-NO reaction when NO diffuses from the inside of the particle to the surrounding gas.
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Other experiments were carried out at batch particle condition. Contradictory trends were reported by de Soete7 on the effect of particle size on the fuel-N conversion to NO. He found that the fuel-N conversion may either increase or decrease with the increase of particle size depending on different char. However, similar results by Hayhurst and Lawrence12 and Yue et al13 have been obtained under the fluidized bed combustion conditions. The fraction of char-N released as NO decreased when the particle size increased.
Nevertheless, there are different conditions in the practical pulverized combustion system where pulverized coal (PC) particles are injected into a furnace with a relatively fine size produced by the grinding process. The specific size fraction significantly influences the combustion and emission characteristics14. For the NOx emission characteristics with coal particle sizes, many studies2, 15-20 have been carried out to find the relationship between particle size and NO produced. However, very little experimental data has been accumulated regarding the effect of particle size of char on NO conversion after the volatile combustion, which will influence final NOx emission. Thomas3 reported that for the small particles or at the low temperature, the carbon-oxygen reaction is several orders of magnitude faster than the carbon-NO reaction, and diffusion processes must be taken into account. Therefore, most of the NO reduction on carbon will occur in the pores of char. Molina et al21 found that there is not a clear specific trend in the variation of char-N conversion with the particle size (88–108 and < 37 µm). Thus, they conclude that this lack of an observed trend suggests that the setup used in these experiments was not sensitive enough to determine any trend in the conversion of fuel nitrogen to NO with varying particle size. However, in their experimental system, it should be noted that NO background
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was also added.
In general, from the above research, we can find that most studies focus on large particles due to the diffusion at millimeter level at low temperature in the FB combustion system. While in the PC combustion system, a particle size of
