NOx and N2O Emission in Bubbling Fluidized-Bed Coal Combustion

Hideo Hosoda, and Toshimasa Hirama*. Resources and ... Naohiro Azuma, Koji Kuramoto, Jun-ichiro Hayashi, and Tadatoshi Chiba. Center for Advanced ...
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Energy & Fuels 1998, 12, 102-108

NOx and N2O Emission in Bubbling Fluidized-Bed Coal Combustion with Oxygen and Recycled Flue Gas: Macroscopic Characteristics of Their Formation and Reduction Hideo Hosoda and Toshimasa Hirama* Resources and Energy Division, Hokkaido National Industrial Research Institute (HNIRI), AIST, MITI, Tsukisamu-Higashi 2-17, Toyohira-ku, Sapporo 062, Japan

Naohiro Azuma, Koji Kuramoto, Jun-ichiro Hayashi, and Tadatoshi Chiba Center for Advanced Research of Energy Technology (CARET), Hokkaido University, N13 W8, Kita-ku, Sapporo 060, Japan Received May 28, 1997X

Suppression of nitrogen oxides emission by flue gas recycling was experimentally examined for coal combustion in an atmospheric bubbling fluidized bed. An Australian bituminous coal crushed to sizes smaller than 5 mm was burnt at 1120 K and superficial gas velocity of 1.0 m/s in a 0.158 m i.d. and 3 m high combustor. The combustion was performed in two different modes, namely, an exit gas recycling mode (ERM) where CO2-rich flue gas was recycled and fed to the combustor with pure O2 and a once-through mode (OTM) where gas with various compositions as well as air was fed to the combustor without the recycling. In ERM with the inlet concentration of O2 being kept at 21 vol %, the overall fractional conversions of fuel nitrogen into NOx and N2O were 0.0083 and 0.012, respectively, which were respectively equivalent to about 1/9 and 1/6 of those in OTM with air. In OTM, the effect of the inlet gas composition on NOx and N2O emissions was examined at various inlet N2, CO2, and H2O concentrations (CN2(i), CCO2(i), and CH2O(i), respectively). The results showed that the combustion efficiency and the in-bed char concentration are both independent of the inlet gas composition. The fractional conversion of fuel nitrogen into NOx was 0.077 at CN2(i) ) 79 vol % and decreased linearly with increasing CCO2(i) and CH2O(i) down to 0.044 with the respective values of CCO2(i) and CH2O(i) being 79 vol % on a dry basis and 5 vol % on a wet basis. On the other hand, the fuel nitrogen conversion into N2O was independent of CCO2(i) and slightly increased with CH2O(i). The in-bed reduction of NO or N2O added into the inlet gas was also evaluated to estimate the reduction extent of the nitrogen oxides recycled in ERM. The reduction ratios of NO to N2 and/or N2O, NO to N2O, and N2O to N2, which were determined by assuming no interaction between added NO and N2O, were 0.81, 0.04, and 0.80, respectively, regardless of CCO2(i) and CN2(i) at CH2O(i) ) 0. Both conversions of NO to N2 and N2O to N2 increased with CH2O(i). On the basis of the results for OTM, the overall fuel nitrogen conversions in ERM were estimated as 0.76% for NOx and 1.4% for N2O, again assuming that reduction of the nitrogen oxides occurred independent of their formation from the coal. The considerably lower NOx and N2O emissions in ERM than those in OTM with air was reasonably well explained by much higher CCO2(i) and CH2O(i) and the extensive reduction of NOx and N2O recycled in the former combustion mode.

Introduction CO2/O2 combustion system for coal, in which a large portion of CO2-rich exit gas from the combustor is recycled with oxygen addition (ERM operation), gives rise to possibilities not only to concentrate CO2 in flue gas without separation processes but also to suppress NOx and N2O emissions to a level lower than those in the conventional once-through air combustion (OTM operation) systems. The ERM system was first applied * To whom all correspondence should be addressed. X Abstract published in Advance ACS Abstracts, December 1, 1997.

to pulverized coal combustion and NOx and N2O emissions relatively lower than those in OTM systems were experimentally confirmed by several investigators.1-3 The lower emissions might be attributed to a lowered flame temperature caused by a higher heat capacity of CO2 recycled than that of N2 in air as well as reduction of the nitrogen oxides recycled. The emissions would (1) Wolsky, A. M. Proc. 79th Annu. Meeting Air Pollution Control Assoc., Minneapolis 1986, 22-27. (2) Nakayama, S.; Miyamae, S.; Maeda, U.; Tanaka, T. Energy and Resources (J. Japan. Soc. Energy Resources, in Japanese) 1993, 14, 78-84. (3) Okazaki, K.; Ando, T. Energy 1997, 22, 207-215.

S0887-0624(97)00075-3 CCC: $15.00 © 1998 American Chemical Society Published on Web 01/12/1998

NOx and N2O Emission in CO2/O2 Combustion

Energy & Fuels, Vol. 12, No. 1, 1998 103

Table 1. Results of Proximate and Ultimate Analyses of Blair Athol Coal prox anal (wt %) moisture ash 8.7

7.6

volatile fixed matter carbon 25.1

58.6

ult anal (dry ash free, wt %) C H O N S 80.8

4.9

11.9

2.14

0.25

be further lowered when ERM system is applied to fluidized-bed combustors where coal char particles as a reducing agent for NOx and N2O4-7 reside in the bed at concentrations much higher than those in pulverized coal combustors. In fact, Bonn and Baumann8 recently reported a considerable decrease of the emissions from their pressurized fluidized-bed combustor (PFBC). If the decrease was caused by reduction of recycled NOx and N2O by in-bed char particles, improvement would also be expected for atmospheric bubbling fluidized-bed combustors (AFBC) where the char concentration is as high as that in PFBC. However, details of the reduction mechanism and emission characteristics are still unknown with a lack of systematic experimental data. Hence, in an effort to generalize the characteristics of ERM combustion, a series of experiments were performed in an AFBC which was operated in both ERM and OTM for combustion of a bituminous coal. In OTM, combustion was conducted with variations of the inlet concentration of N2, Ar, CO2, H2O, NO, and N2O to examine their effects on formation and reduction of NOx and N2O. Measurements were also carried out to determine the difference of NOx and N2O concentrations in flue gas in ERM from those in OTM. The difference was quantitatively explained based on the observed results in OTM by assuming simultaneous and independent occurrence of NOx and N2O formation from the coal and in-bed reduction of recycled NOx and N2O in ERM.

Figure 1. Schematic diagram of experimental system of bubbling fluidized-bed combustor (BFBC) with exit gas recycling.

Experimental Section Materials Used. Silica sand with a median particle size of 0.45 mm was used as the refractory bed material. Its minimum fluidization velocity with air was 0.13 m/s at ambient temperature. For each experimental run, fresh bed material was loaded in the combustor. Blair Athol bituminous coal was employed as the coal sample after crushed to a size range smaller than 5 mm. Results of proximate and ultimate analyses are listed in Table 1. Apparatus and Procedure. A schematic diagram of the present experimental system is shown in Figure 1. The system consists of a bubbling fluidized bed combustor (BFBC), a gas cooler, a filter equipped with a plastics fiber sheet for dust removal, a gas blower, gas cylinders, and gas analyzers. Combustion experiments were carried out in two different modes, namely, an exit gas recycling mode (ERM) and a oncethrough mode (OTM). In ERM, exit gas from the combustor was cooled down to about 305 K in the cooler, separated from entrained dust by the filter and recycled into the plenum chamber of the combustor at a certain recycle ratio with (4) Bee´r, J. M.; Sarofim, A. F.; Chan, L.; Sprouce, A. Proc. 5th Int. Conf. Fluidized Bed Combust. (FBC), ASME 1977, 577-592. (5) Furusawa, T.; Kunii, D.; Oguma, A.; Yamada, N. Kagaku-Kogaku Ronbun-shu (in Japanese) 1978, 4, 562-566. (6) de Soete, G. G. Symp. Int. Combust., [Proc.], 23rd 1990, 12571264. (7) Moritomi, H.; Suzuki, Y.; Kido, N.; Ogisu, Y. Proc. 11th Int. Conf. FBC, ASME, Montreal, Canada 1991, 1005-1011. (8) Bonn, B.; Baumann, H. Coal Science (Proc. 8th Int. Conf. Coal Sci.) 1995, 1923-1926.

Figure 2. Details of bubbling fluidized-bed combustor with 0.158 m i.d. and 3 m height. addition of pure oxygen from the cylinder at a prescribed flow rate. In OTM, air or mixture of Ar/N2/O2, CO2/N2/O2, or CO2/H2O/O2 was fed into the combustor and the exit gas from the combustor was exhausted without recycling. Steam was generated in a heater connected with a pump for water supply. In the present paper, concentrations of a species X at the combustor inlet and exit are hereafter denoted as CX(i) and CX(f), respectively, the units of which are expressed in volume percent or ppm on a dry basis for CO2, N2, Ar, O2, NOx (or NO), and N2O while on a wet basis for H2O. CO2(i) is 21 vol % in both combustion modes unless otherwise stated. Figure 2 illustrates details of the fluidized bed combustor, which is made of a stainless steel column with an inner diameter of 0.158 m and a height of 3 m and is equipped with a gas distributor, a screw feeder, a water tube immersed in

104 Energy & Fuels, Vol. 12, No. 1, 1998

Hosoda et al.

Table 2. Lists of Gas Analyzers Useda gas

analyzer

sampled from

sampling mode

O2 O2 NOx N2O CO2 CO2, CO, H2, CH4, O2

MF (Horiba MPS-510s) ZS (Shimadzu NOA-7000) CL (Shimadzu NOA-7000) IR (Horiba VIA-510) IR (Horiba VIA-510s) GC (Shimadzu GC-20B)

inlet and exit exit exit exit inlet and exit in-bet and exit

continuous continuous continuous continuous continuous intermittent

detectable lower limit 0.1% 1% 1 ppm 20 ppm 1% 100 ppm (H2), 500 ppm (others)

a MF, magnetic force sensor analyzer; ZS, zirconia sensor analyzer; CL, chemiluminescence analyzer; IR, infrared gas analyzer; GC, gas chromatograph.

the bed, and two cyclones for collection of fly ash and entrained fine char particles. The distributor had 4 mm i.d. bubble caps on a 26 mm triangular pitch, and four 1.8 mm diameter holes equally spaced in a horizontal plane were bored on each cap. The column was wound by electric heaters for start-up and insulated by ceramic fiber sheets with a thickness of 50 mm. Coal particles were fed from the screw feeder into the bed at 0.1 m above the gas distributor. For all the present experiments the height, temperature of the bed, and the superficial gas velocity at the temperature were respectively kept at 0.3 m, 1120 K, and 1.0 ( 0.1 m/s. During each experimental run, increase of the bed height due to accumulation of coarse ash particles was detected but the rate of increase was within 4% of the initial. The bed temperature was automatically controlled by detecting temperature at 0.15 m above the gas distributor for adjustment of the flow rate of cooling water into the immersed water tube. Also, axial temperature and pressure distributions in the combustor were recorded by measuring at 12 different heights together with those in the plenum chamber, at the plenum chamber upstream, and at the cyclones downstream. The inlet gas composition and the feed rate of coal were controlled so as to keep CO2(f) at 3.5 ( 0.5 vol % in both ERM and OTM. In the experiments where steam was added to the inlet gas, combustion was carried out at slightly higher gas velocity, i.e., 1.1-1.2 m/s, to keep the bed temperature at 1120 K due to the heat capacity of H2O being higher than those of CO2 and N2. To calculate the combustion efficiency, particles collected by the cyclones were sampled for analysis of their carbon contents. In-bed char concentration was analyzed at the end of several experimental runs by discharging the whole bed constituents through a tube fitted to the gas distributor. Gas samples were collected by sucking gas slowly (at about 3 × 10-5 m3/s) from the plenum chamber and the cyclones downstream. Here, the former and the latter sampled gases are referred to as the inlet and exit gases, respectively. Gas sampling was also made intermittently from the bed at a height of 0.15 m above the gas distributor. For intermittent gas chromatographic analyses, gas samples were sucked through a dehumidifier packed with high-purity silica fibers and calcium chloride particles and stored in an impermeable bag. The gases were subjected to analyses described below. Analyses. Concentrations of relevant gaseous species sampled were measured with analyzers listed in Table 2: CO2(f) intermittently with a gas chromatograph and continuously with a zirconia sensor analyzer, CO2(i) with a magnetic sensor analyzer, CNOx(f) with a chemiluminescence gas analyzer, and CN2O(f), CCO2(i), and/or CCO2(f) with infrared gas analyzers for the individual gases. The measurements were conducted by confirming CO2(i) being controlled at 3.5 ( 0.1 vol %. A preliminary experiment was made to evaluate the effect of CO2 concentration on the accuracy of measurement of NOx and N2O concentrations using standard gases. In the present analyses, the lowest concentration reliably detectable was found to be 20 ppm for N2O, and their values for other gases are listed in the table.

Table 3. Combustion Efficiency and Char Concentration in the Bed in OTM with air and ERM with Different Inlet O2 Concentrations ERM

combustion efficiency, % char concn in the bed, wt %

OTM with air

CO2(i) ) 21 %

CO2(i) ) 40%

90.0 1.1

90.8 1.2

91.4 1.0

Figure 3. Temperature profiles in combustor as a function of combustion mode.

Results and Discussion Characteristics of NOx and N2O Emissions. Table 3 summarizes the combustion efficiency and char concentration in the bed for OTM with air and ERM at CO2(i) of 21 and 40 vol %. Here, the efficiency was defined by the fluxes of coal, Fc, flue gas, Ff, and elutriated char particles, Fp, and their high calorific values of ∆Hc, ∆Hf, and ∆Hp as

(

η) 1-

)

Ff∆Hf + Fp∆Hp × 100, % Fc∆Hc

It is seen that η and the char concentration are 9091% and 1.0-1.2%, respectively, and seem to be independent of the combustion mode and the inlet gas composition. Concentrations of CO and H2 in the bed, which were never detected in the exit gas at CO2(f) ) 3.5 vol %, were also not affected by the inlet gas composition (see Figure 7). On the other hand, the inlet gas composition influenced temperature distributions in the freeboard as shown in Figure 3. Higher CO2(i) leads to higher temperatures at any heights in the freeboard. The CO2(i) effect may be due to more significant combustion in the splashing zone just above the bed surface. Figure 4 illustrates time dependent changes of CCO2(f), CNOx(f), and CN2O(f) after shifting the combustion

NOx and N2O Emission in CO2/O2 Combustion

Figure 4. Typical variation of CO2, NOx, and N2O concentrations in exit gas with time after shifting combustion mode from OTM with air to ERM.

Figure 5. Fractional conversions of fuel-N into NOx and N2O in OTM with air and ERM with different inlet O2 concentrations from 21 to 40 vol %.

mode from OTM with air to ERM at CO2(i) and CO2(f) being 21 and 3.5 vol %, respectively. CCO2(f) is shown to increase with time after the mode shift and to reach higher than 90 vol %. Though not shown in the figure, CH2O(i) also increased from 1.5 vol % in OTM to 5.5 vol % in ERM. CNOx(f) decreases with time from about 300 to 190 ppm, while CN2O(f) exhibits only a slight increase from about 120 to 135 ppm. The fractional conversions of nitrogen in coal (hereafter referred to as fuel-N) into NOx, XNOx, and N2O, XN2O, at the steady state, which were calculated from molar flux, FN, of fuel-N, CNOx(f) and CN2O(f), and the flow rate, Ff (m3/h), of flue gas (exhausted) by

XNOx ) {(2/22.4) × 10-3}{FfCNOx(f)/FN} XN2O ) {(1/22.4) × 10-3}{FfCN2O(f)/FN} were 0.0083 and 0.012, respectively. These figures are as low as 1/9 and 1/6 of those in OTM with air, respectively. Such low fuel-N conversions in ERM are maintained even at higher CO2(i), as shown in Figure 5. The slight increase in XNOx and decrease in XN2O with CO2(i) may be attributed to the higher freeboard temperature9,10 mentioned in Figure 4. (9) Amond, L.-E.; Leckner, B. Combust. Flame 1991, 84, 181-196.

Energy & Fuels, Vol. 12, No. 1, 1998 105

Figure 6. Exit concentrations of NOx and N2O as a function of heat capacity of exit gas at different inlet gas compositions in OTM (Ar/N2/O2 and CO2/N2/O2, CO2(i) ) 21 vol %).

The above-described low emissions of NOx and N2O in ERM are possibly caused by physical and chemical changes brought about with the increase of CCO2(i). Increase of CO2 concentration at the inlet and consequently in the bed may induce a decrease in temperature in the vicinity of coal/char particles due to a larger heat capacity of CO2 than N2 and thereby suppress NOx formation.9,10 Also, CO2 suppresses NOx formation or promotes NOx reduction through its conversion to CO by reaction with char. These changes were individually investigated simulating ERM combustion by OTM combustion with different inlet gas compositions. First, the effect of the former physical change on CNOx(f) and CN2O(f) was examined in OTM(Ar/N2/O2) experiments where the heat capacity of gas was varied in a range from 27 to 37 J mol-1 K-1 by changing the inlet Ar concentration from 0 to 79 vol %. The result is shown in Figure 6. Both CNOx(f) and CN2O(f) are seen to be independent of the heat capacity for OTM(Ar/N2/O2), although its range does not fully cover the conditions in ERM. The result further indicates that the nitrogen oxides are hardly produced from N2 in the air under the present experimental conditions. The effects of the latter chemical changes, i.e., the suppressed formation of nitrogen oxides from fuel-N and/or the enhanced in-bed reduction of those recycled to N2, were examined in OTM. Figure 7 shows the effect of CCO2(i) in OTM(CO2/N2/O2) combustion on the exit gas composition and CO and H2 concentrations in the bed. Though the concentrations of N2O, CO, and H2 seem to remain invariable with CCO2(i), CNOx(f) decreases linearly with increasing CCO2(i) from 275 ppm at CCO2(i) ) 0 to 205 ppm at CCO2(i) ) 79 vol %. This implies that CO is formed by reaction of CO2 with char which would in turn result in CO concentration around a char particle higher than those observed as an average in the bed due to a higher temperature at char surface than the averaged bed temperature measured11 and the increase of CO concentration would accelerate NOx reduction on char surface.12 Reduction of NOx can also be promoted by (10) Pels, J. R.; Wojtowitz, M. A.; Kapteijn, F.; Moulijn, J. A. Energy Fuels 1995, 9, 743-752. (11) Hernberg, R.; Stenberg, J.; Zethraeus, B. Combust. Flame 1993, 95, 191-205. (12) Furusawa, T.; Tsunoda, M.; Tsujimura, M.; Kunii, D. Fuel 1985, 64, 1306-1309.

106 Energy & Fuels, Vol. 12, No. 1, 1998

Figure 7. Effects of inlet CO2 concentration on exit NOx and N2O concentrations and in-bed CO and H2 concentrations in OTM (CO2/N2/O2, CO2(i) ) 21 vol %).

Figure 8. Effects of inlet H2O concentration (wet basis) on exit NOx and N2O concentrations at CCO2(i) ) 79 vol % and CO2(i) ) 21 vol %.

H2O which would produce H2 through reaction with char.13 This was simulated in OTM(CO2/H2O/O2) and the result is shown in Figure 8. As expected, CNOx(f) is shown to decrease linearly with CH2O(i) while CN2O(f) slightly increases, leaving the CO and H2 concentrations in the bed unchanged. This result might be explained according to Kasaoka et al.,14 who reported, on the basis of their char combustion experiments in a fixed bed at low O2 concentrations, that H2O promotes HCN formation by its reaction with nitrogen in char (char-N) and at the same time suppress NOx formation by those between char-N and O2. Also, HCN is known to be converted to N2O by both homogeneous15,16 and heterogeneous17 reactions. If recycled NOx and N2O would not be reduced in ERM, their exit concentrations should be approximately 6 times those in OTM with CO2/H2O/O2. However, the observed concentrations in the former mode (see Figure 4) are rather lower or slightly higher than those in the (13) Furusawa, T.; Kunii, D.; Tsujimura, M.; Tsunoda, M. Proc. 7th Int. Conf., FBC, Philadelphia 1981, 525-533. (14) Kasaoka, S.; Sasaoka, E.; Ozaki, A.; Nenryo Kyokai-shi (J. Fuel Soc., Jpn., in Japanese) 1983, 62, 53-62. (15) Kilpinen, P.; Hupa, M. Combust. Flame 1991, 84, 94-104. (16) Kramlich, J. C.; Cole, J. A.; McCarthy, J. M.; Lanier, W. S. Combust. Flame 1989, 77, 375-384. (17) Shimizu, T.; Fujita, D.; Ishizu, K.; Kobayashi, S.; Inagaki, M. Proc. 12th Int. Conf. FBC ASME, San Diego, 1993, 611-617.

Hosoda et al.

Figure 9. Exit NOx and N2O concentrations in OTM with air as a function of their respective inlet concentrations.

latter (see Figure 8, at CH2O(i) ) 5%). Thus, an extensive reduction of recycled NOx and N2O was expected to take place in the bed and, in order to confirm this, experiments were carried out simulating the inbed reduction by adding NO or N2O to the inlet gas in OTM. Figure 9 describes CNOx(f) and CN2O(f) in OTM with air as a function of their respective inlet concentrations. In the figure, the broken lines indicate CNOx(f) and CN2O(f) that are calculated assuming no in-bed reduction of NO and N2O added and no influence of the addition on the net formation of NOx and N2O from fuelN. Therefore, the result reveals that about 80% of NO and N2O added are reduced to N2 and 4-5% of NO to N2O. These rates are independent of CNO(i) and CN2O(i). In addition, when both NO and N2O were added simultaneously the same conversions as above were observed, suggesting no interaction between them. NOx and N2O can be reduced by N2 by char or CO on char surface as mentioned above and possibly by a catalytic function of ash surface18 or in the gas phase containing volatiles as reducing agents. Also, NOx can be reduced to N2O by char-N19,20 or NCO15,16 produced from HCN. Effects of CCO2(i) and CH2O(i) on reduction of NO and N2O added were further examined in OTM(CO2/N2/O2) and OTM(CO2/H2O/O2). The results are depicted in Figures 10 and 11 where the extents of reduction of NO to N2 and/or N2O, YNO, N2O to N2, YN2O, and NO to N2O, YNO-N2O, which are defined with concentrations of CNOx*(f) and CN2O*(f) at CNO(i) ) 0 and CN2O(i) ) 0, respectively, as

YNO ) 1 - {CNOx(f) - CNOx*(f)}/CNO(i) YN2O ) 1 - {CN2O(f) - (CN2O*(f)}/CN2O(i) YNO-N2O ) {CN2O(f) - CN2O*(f)}/CNO(i) are plotted against CCO2(i) and CH2O(i), respectively. As can be seen, CCO2(i) appears to have only a small effect on the extents whereas YNO and YN2O slightly increase (18) Johnsson, J. E.; Åmond, L.-E.; Dam-Johansen, K.; Leckner, B. Energy Fuels 1996, 10, 970-979. (19) Mochizuki, M.; Koike, J.; Horio, M. Proc. 5th Int. Workshop Nitrous Oxide Emissions, Tsukuba, Jpn. 1992, 237-244. (20) Tullin, C. J.; Sarofim, D. F.; Bee´r, J. M. J. Inst. Energy 1993, 16, 207-215.

NOx and N2O Emission in CO2/O2 Combustion

Energy & Fuels, Vol. 12, No. 1, 1998 107 Table 4. Fractional Conversions of Fuel-N to NOx and N2O at CO2(i) ) 20% and CO2(f) ) 3.5% OTM with air observed calculated

ERM

NOx

N2O

NOx

N2O

0.077 ( 0.005

0.072 ( 0.005

0.0083 0.0076

0.012 0.014

Figure 10. Extents of reduction of NO and N2O added as a function of inlet CO2 concentration in OTM (CO2/N2/O2, CO2(i) ) 21 vol %).

Figure 12. Fate of fuel-N in ERM in CO2(i) ) 21 vol % and CO2(f) ) 3.5 vol % (numerals and bracketed numerals respectively represent the fractions of nitrogen flux as NOx and N2O based on that of fuel-N and the ratios of the flux in ERM to that in OTM with air).

N2O, XN2O(ERM), are respectively expressed as

XNOx(ERM) )

Figure 11. Extents of reduction of NO and N2O added as a function of inlet H2O concentration (wet basis) in OTM at CCO2(i) ) 79 vol % and CO2(i) ) 21 vol %).

with CH2O(i). Although formation of reducing agents such as CO and H2 from high concentration of CO2 and H2O through char gasification, which contributes to reduction of NOx, is expected to take place in these experiments, the results do not directly confirm the occurrence of gasification. The detailed mechanisms are still unknown for the moment and further work is needed in the future. Estimation of Fuel Nitrogen Conversion into NOx and N2O in ERM Based on OTM Results. The considerably lower conversions into NOx and N2O in ERM than those in OTM with air were estimated on the basis of the results in OTM with the following assumptions for simplification: 1. Both NOx and N2O are formed only from fuel-N. 2. Reduction of recycled NOx and N2O takes place independently of their formation from fuel-N. 3. In ERM, conversions of fuel-N into NOx and N2O are the same as those in OTM at a given set of the inlet concentrations of CO2, H2O and O2. 4. NOx and N2O recycled are reduced in ERM to the same extents as in OTM at given inlet concentrations as above. When fuel-N conversions into NOx and N2O in OTM are denoted as XNOx, fuel-N into NOx, and XN2O, fuel-N into N2O, respectively, the overall fractional conversion of fuel-N in ERM into NOx, XNOx(ERM), and that into

XN2O(ERM) )

XNOx(1 - R) 1 - R(1 - YNO)

{

RXNOxYNO-N2O 1-R XN2O + 1 - R(1 - YN2O) 1 - R(1 - YNO)

}

Here, R represents the volumetric ratio of recycle to exit gas. From the results for OTM at CH2O(i), CCO2(i), and CO2(i) of 5, 79, and 21 vol %, the fractional conversions and the extents of reduction in OTM are estimated as XNOx ) 0.044, XN2O ) 0.072, and YNO ) 0.94, YN2O ) 0.87, and YNO-N2O ) 0.04, and R in ERM is 0.836 at CO2(f) of 3.5 vol %. From these, XNOx(ERM) and XN2O(ERM) are calculated as 0.0076 and 0.014, respectively, which agree reasonably well with those observed in ERM shown in Table 4. Thus, the lower conversions of fuelN into NOx and N2O in ERM are attributed primarily to their in-bed reduction and in part to suppressed NOx formation from fuel-N due to high CCO2(i) and CH2O(i). The fate of the fuel nitrogen in ERM at CO2(i) ) 21 vol % is schematically summarized in Figure 12 from the above estimation and experimental results. In the figure, the numerals represent the fractions of nitrogen flux as NOx and N2O based on that for fuel-N. Also, the bracketed numerals mean the ratios of the flux in ERM to that in OTM with air. It should be noted that the NOx and N2O fluxes exhausted from the combustor in ERM are approximately 1/9 and 1/6, respectively, of those in OTM with air. These figures could further be lowered by increasing bed height to lengthen their residence time in the bed.

108 Energy & Fuels, Vol. 12, No. 1, 1998

Conclusions Characteristics of coal combustion and emission of NOx and N2O were examined in an experimental system of an atmospheric bubbling fluidized-bed combustor which was operated in CO2-rich exit gas recycle mode (ERM) and in fed gas once-through mode (OTM) with various inlet gas compositions. The following conclusions were drawn within the present experimental conditions with the Australian bituminous coal: 1. The combustion efficiency and in-bed char concentration for ERM are essentially the same as those in conventional air combustion simulated in OTM. 2. In comparison with conventional air combustion, the fractional conversions of fuel-N into NOx and N2O in ERM are as significantly low as 0.0083 and 0.012, respectively, which amount to about 1/9 and 1/6 of those in OTM with air. Even with increasing inlet O2 concentrations up to 40 vol %, the conversions vary to a very small extent without giving a detectable difference in the combustion efficiency.

Hosoda et al.

3. The distinct decrease of the conversions in ERM is attributed to enhanced reduction of both nitrogen oxides by char and reducing agents together with suppressed formation of NOx. The extents of reduction range from 80% to 95%, to which inlet CO2 and H2O have a fractional contribution of a few percent. 4. The enhanced in-bed reduction of NOx and N2O recycled and their suppressed formation in ERM can well be explained on the basis of the experimental results in OTM. Acknowledgment. The authors acknowledge the Environmental Protection Agency, Japan, for their financial support. They also acknowledge Ms. Y. Takahashi, Sanyu Plant Service Co Ltd., Japan, for her assistance in the experimental work and Dr. T. Shimizu, Niigata University, Japan, for his useful discussion on the experimental results. EF970075X