Effect of Pressure on NOx Emission from Char Particle Combustion

The effect of pressure on NOx emission during char particle combustion was examined in chemical-reaction control and diffusion control regimes. A fixe...
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Energy & Fuels 2002, 16, 634-639

Effect of Pressure on NOx Emission from Char Particle Combustion Shiying Lin,* Yoshizo Suzuki,† and Hiroyuki Hatano† Center for Coal Utilization, Japan, 6-2-31, Roppongi, Minato-Ku, Tokyo 106-0032, Japan Received July 31, 2001. Revised Manuscript Received December 14, 2001

The effect of pressure on NOx emission during char particle combustion was examined in chemical-reaction control and diffusion control regimes. A fixed bed was used for batch testing under 0.1-1.6 MPa. It was found that with increasing pressure, NOx emission decreased extremely, when char combustion rate was controlled by reactant gas diffusion into the char particle at high temperature. The extent to which NOx was reduced in char particles strongly influenced the NOx emission from char pressurized combustion. Pressure increased residence time for diffusion of NOx throughout the char particle and consequently further increased the reduction of NOx in the char particle. Both pressure and temperature strongly influenced the conversion of fuel-N in char to NOx. When pressure was raised from 0.1 to 1.1 MPa, the conversion of fuel-N to NOx fell from 0.18 to 0.06 at 973 K, and from 0.68 to 0.13 at 1173 K. NOx emissions were lower when large char particles were combusted than when small ones were combusted. It was also observed that practically no N2O was formed in to any extent in the char particle.

Introduction NOx is contained in coal combustion gas. It is a pollutant and a cause of acid rain. During the past 2030 years, NOx from coal burning in the atmospheric pressure has been widely investigated in efforts to develop the technology to control these emissions.1 Recently, several coal combustion processes, such as PFBC, have emerged that raise operating pressure in order to improve the efficiency of coal utilization. Coal or char combustion under elevated pressure behaves differently than combustion in atmosphere conditions.2-10 The rate of combustion was found to increase with pressure in the chemical-reaction control regime, but is less affected by pressure in the pore diffusion influence and is invariant with pressure in gas-film diffusion control regimes. The mechanism underlying the * Author to whom correspondence should be addressed at Clean Fuel Research Group, AIST, 16-1 Onogawa, Tsukuba 305-8569, Japan. Fax: +81-298-61-8209. E-mail: [email protected]. † National Institute of Advanced Industrial Science and Technology. (1) Soud, H. N.; Fukasawa, K. IEA Coal Research, London, U.K., 1996. (2) Shiao, S. Y.; Warchol, J. J.; Botros, P. E. Proceedings of the 11th International Conference on Fluidized Bed Combustion, Montreal, Canada, 1989; pp 1183-1190. (3) Sellakumar, K. M.; Isaksson, J.; Tiensuu, J. Proceedings of the 12th International Conference on Fluidized Bed Combustion, ASME, New York, 1993; pp 423-429. (4) Monson, C. R.; Germane, G. J.; Blackham, A. U.; Smoot, L. D. Combust. Flame 1995, 100, 669. (5) Bateman, K. J.; Germane, G. J.; Smoot, L. D.; Blackham, A. U.; Eatough, C. N. Fuel 1995, 74, 1466. (6) Tidona, R. J. Combust. Flame 1980, 38, 335. (7) Wallman, P. H.; Carlsson, R. C. J. Proceedings of the 12th International Conference on Fluidized Bed Combustion, ASME, New York, 1993; pp 1517-1522. (8) Essenhigh, R. H.; Mescher, A. M. Proceedings of the 26th Symposium on Combustion, Napoli, Italy, 1996; pp 3085-3094. (9) Saastamoinen, J. J.; Aho, M. J.; Hamalainen, J. P.; Hernberg, R.; Joutsenoja, T. Energy Fuels 1996, 10, 121. (10) Lin, S. Y.; Suzuki, Y.; Hatano, H.; Tsuchiya, K. Chem. Eng. Sci. 2000, 55, 43.

relationship between pressure and NOx emission during coal or char combustion is not yet understood. Richard et al.11 first studied NxOy emission from pressurized char combustion. They reported that NO, N2O, and the total NxOy decreased with pressure, whereas NO2 increased slightly with pressure. No explanation for this was provided in detail. Croiset12 studied NO and N2O emissions during char pressurized combustion and reported that NO and N2O were reduced with increasing pressure. Jensen et al.13 reported NOx reduction in coal and char pressurized combustion in real boiler. The mechanism of NOx emission in fluidized-bed coal combustion systems is very complex, although the reaction of atmospheric nitrogen with oxygen is not significant. In a reactor, coal first pyrolyzes into volatiles and char. The char contains most of the total fuel-N.13,14 NOx can be formed from fuel-N during the combustion of the volatiles and the char,15,16 and it can be reduced by reaction with the carbon in char, which may act as a surface catalyst.17,18 In a char particle, NOx is generated during gasification, then diffuses throughout the pores. A portion of the NOx is considered to be reduced with carbon during diffusion out. Since the gas density (11) Richard, J. R.; Majthoub, M. A.; Aho, M. J.; Pirkonen, P. M. Fuel 1994, 73, 1034. (12) Croiset, E.; Heurtebise, C.; Rouan, J. P.; Richard, J. R. Combust. Flame 1998, 112, 33. (13) Jensen, A.; Johnsson, J. R.; Andries, J.; Laughlin, K.; Read, G.; Mayer, M.; Baumann, H.; Bonn, B. Fuel 1995, 74, 1555. (14) Wang, W.; Brown, S. D.; Thomas, K. M.; Crelling, J. C. Fuel 1994, 73, 341. (15) Chen, S. L.; Heap, M. P.; Pershing, D. W.; Martin, G. B. Proceedings of the 19th Symposium on Combustion, Pittsburgh, 1982; p 1271. (16) Pels, J. R.; Wojtowicz, M. A.; Moulijn, J. A. Fuel 1993, 72, 373. (17) Teng, H.; Suuberg, E.; Calo, J. M. Energy Fuels 1992, 6, 398. (18) Yamashita, H.; Tomita, A.; Yamada, H.; Kyotani, T.; Radovic, L. R. Energy Fuels 1993, 7, 85.

10.1021/ef010198o CCC: $22.00 © 2002 American Chemical Society Published on Web 03/27/2002

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Figure 2. Effect of char density in bed supply on the gas measurement.

Figure 1. Schematic diagram of the pressurized reactor. Table 1. Analysis of Coals and Chars Used in This Study proximate anal. wt %

ultimate anal. wt %

moi.

C

vol.

FC

coal Blair Athol 4.4 30.4 58.4 Wyoming 15.2 46.6 31.2 char Blair Athol Wyoming

ash

H

N

O

6.8 82.05 4.78 0.93 12.24 7.4 70.32 5.13 0.97 23.57

1.2 86.3 12.5 97.68 0.64 0.73 7.9 81.0 11.1 98.22 0.79 0.66

0.94 0.32

is proportional to pressure,19 increasing pressure increases NOx residence time in coal particle, thereby further reducing NOx. Accordingly, in the present study the problem of the formation of both NOx and N2O during char particle combustion was examined under various pressures with different char particle size, coal rank, and temperature. A small fixed-bed reactor was used for batch combustion tests at pressure ranging from 0.1 to 1.6 MPa. The temperature range was 830-1173 K, which covered the chemical reaction control, the pore diffusion influence, and the gas-film diffusion control regimes. Experimental Section Sample Preparation. The parent coals were a bituminous (Blair Athol coal, Australia) and a lignite (Wyoming coal, United States). Their chars were prepared by heating the coals in a fluidized bed at a rate of 10 K/min up to 1273 K in a nitrogen atmosphere. The char was then crushed and sieved to the desired particle size of 0.25-0.5 mm. The results of proximate and ultimate analyses for these coals and chars are summarized in Table 1. Pressurized Reactor. Figure 1 is a schematic diagram of the experimental apparatus. The reactor consisted of a quartz tube, which housed a fixed bed, and a vessel that could be pressurized up to 2 MPa. The quartz tube was divided into three sections. The upper section, with a length of 150 mm and an inner diameter of 24 mm, served as the reacting (19) Hirchfelder, J. O.; Curtiss, C. F.; Bird, R. B. Molecular Theory of Gases and Liquids; John Wiley & Sons: New York, 1954.

section. The middle section, which was 125 mm long and 4.5 mm in inner diameter, was used to move the reacted gas to the lower section rapidly. The lower section was the watercooling portion, and was 30 mm in length and 24 mm in inner diameter. For thermocouple protection, a fine quartz tube, with an outside diameter of 3 mm, was inserted into the fixed bed from the lower section. Gas Supply. O2, diluted to 5% and 21% by N2, was used as a reactant gas. Thermal NOx production was neglected since thermal NOx is formed under very high temperature (T > 1273 K). The total gas flow rate was 4 L/min (STP), causing the gas to pass through the upper section at a rate of 0.15 m/s(STP), and through the middle section at a rate of 7.6 m/s(STP). Figure 2 shows combustion results when gas was supplied with different quantities of char. For char quantities of 5 to 50 mg, the combustion rate did not change significantly. This result confirmed that, when the quantities of char are 50 mg or less, the O2 supply in the gas flow is enough for a reaction and the effect of bed height can be ignored in measuring the rate of char combustion. Experimental Procedure. About 25 mg of char was uniformly dispersed in a bed of 1.5 g of quartz sand that had the same particle size distribution as that of the char. Quartz sand was also used to stabilize the bed temperature within 20 K of the set temperature during combustion. The bed temperature was measured with a K-type thermocouple. A gas stream was introduced from the top of the upper section, passed through the fixed bed, and flowed out through the middle section to the cooling section. With the mixture of char and quartz sand properly placed in the reactor, pressure and temperature were increased in a nitrogen atmosphere. When the pressure and temperature stabilized at predetermined values, the flow of nitrogen was rapidly switched to that of reactant gas. Gas Analysis. The concentrations of CO, CO2, NOx, and N2O in the exhaust gas were continuously measured by infrared analyzers (Horiba VIA-510 series). The results were stored in a networked computer connected to the analyzer. The response rate of the analyzer as a one-dimensional problem was modified by a simple equation,

Y(t) ) Ty′(t) + y(t)

(1)

where, Y(t), y(t) and y′(t) are the real concentration, the concentration shown on the analyzer, and the differential of y(t), respectively. T is the time constant.20 Combustion Rate. CO and CO2 are found to be the major products of char combustion (see Figures 3 and 4). The rates (20) Shugiyama, S. Tsuuron Kagakukogaku, Kyoritsu shiubanshia: Tokyo, 1977; p 242.

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Figure 6. Rates of char combustion under various pressures. of CO and CO2 emissions correspond to the rate of char combustion. Thus, the rate of char combustion was defined as Figure 3. NOx, CO, and CO2 emission during pressurized char combustion (0.1 MPa).

R)

1 dX 1 - X dt

dX (fCO + fCO2)Fg ) dt VfCWchar

(2)

(3)

where X is carbon conversion of char [-], fCO and fCO2 are fractions of CO and CO2 in the exhaust gas [-], fc is the carbon content of the char [mol/g], Fg is the flow rate of the exhaust gas [103 cm3/s, STP)], Wchar is the char supply weight [g], and V is mole volume () 22.4 × 103 cm3/mol, STP).

Results and Discussion

Figure 4. NOx, CO, and CO2 emission during pressurized char combustion (0.6 MPa).

Figure 5. NOx, CO, and CO2 emission during pressurized char combustion (1.1 MPa).

Effect of Pressure. Figures 3, 4, and 5 show the emission results of each of the gases during char combustion at pressures of 0.1, 0.6, and 1.1 MPa, respectively. O2 in supply gas was 5%, and the O2 partial pressures were 0.05, 0.3, and 0.55 MPa, corresponding to the pressure of 0.1, 0.6, and 1.1 MPa, respectively. During char combustion, CO and CO2 emissions increased with time, peaked at 40, 30, and 28 s for 0.1, 0.6, and 1.1 MPa, and then decreased with time. The peak heights of CO and CO2 emissions increased with pressure between 0.1 and 0.6 MPa (Figures 3 and 4), but did not significantly change with pressure between 0.6 and 1.1 MPa (Figures 4 and 5). The char combustion rates were calculated from CO and CO2 emissions as shown in Figure 6. The rate at 0.1 MPa was a volumetric rate, which was roughly constant during the char combustion, and the rates at 0.6 and 1.1 MPa were surface rates, which increased during char combustion. The results also showed that combustion rate tended to increase with pressure between 0.1 and 0.6 MPa but did not change between 0.6 and 1.1 MPa. Previous studies9,10 investigated the effect of pressure on char combustion rate in different control regimes. Their results indicated that the char combustion rate increases proportionally with pressure in the chemicalreaction control regime, exhibits a nonlinear increase with pressure in the pore diffusion influence regime, and is invariant with pressure in the gas-film diffusion control regime. The explicit pressure effect in the gasfilm diffusion control regime was negated by the combination of increasing diffusion resistance and increasing O2 partial pressure. Accordingly, from Figures 3, 4,

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and 5, it can be concluded that char combustion was under chemical or pore diffusion control in the pressure range of 0.1-0.6 MPa, and was under gas-film diffusion control in the pressure range of 0.6-1.1 MPa. Figures 3, 4, and 5 also show NOx emission during char combustion. This NOx emission increased, peaked, and then decreased with time, showing behavior similar to that of the CO and CO2 emissions. This can be explained by the fact that the conversion of char nitrogen to NOx was approximately proportional to the carbon burnoff under certain pressure and temperature.16 Mechanisms of carbon and fuel-N gasification on carbon surface were suggested by Thomas.21 These mechanisms are as follows: For chemisorption of O2,

-Cfas + 1/2O2 f -C(O)

(4)

for CO and CO2 production,

-C(O) f Cfas + CO

(5)

2 -C(O) f -Cfas + CO2

(6)

and for NO production,

-C(N) + -C(O) f -Cfas + -C(NO)

(7)

-C(NO) f -Cfas + NO

(8)

where -Cfas is a free active site, and -C(NO) and -C(O) are carbon surface complex species. These equations show that the carbon-oxygen surface site, -C(O), is the enhancing factor for both carbon oxide and NO production. These equations agree well with the experimental results. However, the variation of NOx emissions with pressure was much different from the variation of both CO and CO2 emissions with pressure in these figures. When pressure was raised from 0.1 to 0.6 MPa, the NOx emission peak increased with pressure, but the power was weaker than the peak increases of CO and CO2. The peak heights increased by 1.36 times for NOx and 2.7 times for CO and CO2 emissions. When pressure was raised from 0.6 to 1.1 MPa, the peak height of NOx emission decreased; in contrast, the peak heights of CO and CO2 were unchanged. If NOx on the carbon surface occurs in proportion to the production of CO and CO2, as discussed above, then the slow increase of NOx from 0.1 to 0.6 MPa, and the decrease of NOx from 0.6 to 1.1 MPa, should be caused by the reduction of NOx in char particles after the NOx forms. For NO reduction on carbon surface, Thomas21 suggested that

-Cfas + NO f -C(NO)

(9)

2 -C(NO) f 2 -Cfas + N2 + O2

(10)

Since the NO adsorption site, -C(NO), is determined by both the collision frequency and collision time of NO with the free active site -Cfas, it follows that the NO partial pressure and the gas density, both of which (21) Thomas, K. M. Fuel 1997, 6, 457.

increase proportionally with pressure, should be the enhancing factors in NOx reduction. Figure 7a shows the variation of the NOx/(CO + CO2) ratio with pressure during char combustion. The NOx/ (CO + CO2) emission ratio evidently decreased with increasing pressure. By plotting the NOx/(CO + CO2) ratio vs pressure as shown in Figure 8, it can be seen that the NOx/(CO + CO2) ratio was approximately proportional with pressure P-0.61-P-.64. However, with carbon conversion, especially at high carbon conversion, the NOx/(CO + CO2) ratio increased in each pressure condition. The reduced diameter of char particles during the combustion seemed to ease for NOx diffusion out, thereby giving less opportunity for NOx to be reduced. This behavior also be reported by many studies under the atmospheric pressure.22,23 However, under elevated pressure, the increase in the NOx/(CO + CO2) ratio with carbon conversion was weaker than that under atmospheric pressure. Pressure enhanced the residence time for NOx diffusion out. Figures 7b and 7c show NOx/(CO + CO2) ratios under different temperatures (1073 and 913 K, respectively). When comparing these with Figure 7a in which the temperature is 1173 K, it can be seen that, as temperature decreases, NOx/(CO + CO2) decreases in each pressure condition. The oxygen fraction in Figure 7d is 21%. Evidently, the oxygen fraction had no effect on the ratio decreasing with pressure. However, the effect of pressure on residence time for diffusion, and consequently the effect on NOx/(CO + CO2) as discussed in Figure 7a, can be also seen in the different temperature and oxygen supply results. The effect of pressure on NOx emission can be seen directly in results for different particle sizes char combustion. The NOx emission peak when large particles (0.5-1 mm) were used was lower than that with small particles (0.25-0.5 mm); and the power of the decrease of NOx emission with pressure was weaker with large particles than with small particles. For example, when pressure rises from 0.1 to 1.1 MPa, the peak height of NOx emission was reduced to about 1/10 with large particles, but only about 1/4 with small particles. Mass transfer coefficient increases with decreasing particle size lead to the NOx produced will more easily be removed from the particle to the bulk flow when the particles are small. According to the discussion above, the diffusion process in particles is an important factor in analyzing NOx emission in char pressurized combustion. It should be considered that O2 diffuses into char to react with carbon and N, and produces CO, CO2, and NOx; and then the NOx diffuses out and reduces with carbon. A reaction-diffusion differential equation system should be solved.

{

( (

)

∂CO2 1 ∂ ‚ De,O2 ‚ r2 ) Rcomb + Rpro-NOx 2 ∂r ∂r r (11) ∂CNOx 1 ∂ 2 ‚ De,NOx ‚ r ) Rpro-NOx - Rred-NOx ∂r r2 ∂r

)

where r is the radial distance from the center of the (22) Harding, A. W.; Brown, S. D.; Thomas, K. M. Combust. Flame 1996, 107, 336. (23) Tullin, C. J.; Sarofim, A. F.; Beer, J. M. J. Inst. Energy 1993, 66, 207.

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Figure 7. (a) Emission ratio of NOx/(CO + CO2) during char particle pressurized combustion (1173 K). (b) Emission ratio of NOx/(CO + CO2) during char particle pressurized combustion (1073 K). (c) Emission ratio of NOx/(CO + CO2) during char particle pressurized combustion (913 K). (d) Emission ratio of NOx/(CO + CO2) during char particle pressurized combustion (O2 21%, 913 K).

Figure 8. Variation of NOx/(CO + CO2) ratio with pressure (data from Figure 7a).

particle, CO2 and CNOx are concentrations of reactant gases of O2 and NOx, respectively. De,O2 and De,NOx are effective diffusion coefficients of O2 and NOx, respectively. Rcomb., Rpro-NOx, and Rred-NOx are the reaction rates for combustion, NOx production and NOx reduction, respectively. Fuel-N Conversion. Figure 9 shows the tendency of fuel-N in char to convert to NOx under various temperatures and pressures. It can be seen that this tendency is strongly influenced by both temperature and pressure. Under atmospheric conditions, with temperature increasing, conversion of fuel-N to NOx increased.

Figure 9. Effects of temperature and pressure on NOx conversion during char combustion.

Under elevated pressure, conversion to NOx increased with temperature but less than it did under atmospheric conditions. Pressure strongly influenced the conversion of fuel-N to NOx. When pressure increased from 0.1 to 1.1 MPa, the conversion of fuel-N to NOx was reduced from 0.18 to 0.06 for 973 K, and from 0.68 to 0.13 for 1173 K. Effect of Coal Rank. Figure 10 shows NOx emissions from lignite char combustion. The content of N in this char was similar to that in the bituminous char, but the reactivity was higher in the lignite than in the

NOx Emission from Char Particle Combustion

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quickly out from the reaction section to cooling section as described in the Experimental Section. This rapid cooling inhibited the reduction of NOx with CO from forming N2O in the gas-phase reaction. N2O was not formed in char particles. Conclusion

Figure 10. NO emission during Wyoming char combustion.

bituminous char (Table 1). It can be seen that the conversion of char nitrogen to NOx from lignite char was much lower than that from bituminous char. With pressure increasing, NOx reduction was also faster for lignite char than for bituminous char. Highly reactive char provides more free active sites, -Cfas, for NOx reduction. N2O Formation. N2O emission was not detected in any of the experiments, although CO existed in the product gas. In each experiment, product gas flowed

The effect of pressure on NOx emission, as well as on N2O emission, was studied experimentally. It was found that with increasing pressure, NOx emission decreased extremely, when char combustion rate was controlled by reactant gas diffusion into the char particle at high temperatures. NOx reduction with carbon in char particles decreased the emission of NOx from char. Pressure increased the diffusion time of NOx in char, and consequently increased the reduction of NOx. Increasing the temperature increased NOx emission from char combustion, but the effect of increased temperature was weak when pressure was high. Other factors influencing NOx reduction, such as char reactivity, temperature, and particle size, can have co-effects on NOx emissions in char pressurized combustion. It was also observed that no N2O was formed in considerable in the char particle. EF010198O