ARTICLE pubs.acs.org/EF
Factors Affecting NO Reduction during O2/CO2 Combustion Hao Liu,*,† Ying Yuan,‡ Hong Yao,§ Siwei Dong,‡ Takashi Ando,|| and Ken Okazaki|| †
)
School of Energy and Power Engineering, University of Shanghai for Science and Technology, Shanghai 200093, People’s Republic of China ‡ College of Energy, Soochow University, Suzhou 215006, People’s Republic of China § State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan 430074, People’s Republic of China Department of Mechanical and Control Engineering, Tokyo Institute of Technology, Tokyo 152-8552, Japan ABSTRACT: O2/CO2 combustion is a promising technology to facilitate carbon capture in pulverized coal-fired power plants. The reduction of recycled NO, influence of CO2 concentration on the conversion ratio of fuel N to NO, interaction between recycled NO and fuel N, and other factors were experimentally investigated, and correlations were quantitatively obtained. The conversion ratio from fuel N to NO increased with an increasing CO2 concentration in the absence of coal but decreased in the presence of coal. The reduction ratio of NO in the recycled gas depended upon the oxygen/fuel stoichiometric ratio, λ, and increased with the NO concentration in the recycled gas when λ = 0.7 and 1.0 but decreased when λ = 1.2. The conversion ratio from fuel N to NO decreased with an increasing NO concentration in the recycled gas, because of the interaction between fuel N and recycled NO. The global conversion ratio from fuel N to exhausted NO in O2/CO2 combustion was derived from the experimental results and analysis of the system. The relative importance of different factors in the low conversion ratio from fuel N to exhausted NO in O2/CO2 combustion has been quantified and found to depend upon λ. Under all conditions investigated, the reduction of recycled NO was the major factor (over 70%) contributing to the low NO emission in O2/CO2 combustion.
1. INTRODUCTION O2/CO2 combustion has been recognized as a promising technology for pulverized coal-fired power plants to control CO2 emissions.1 This process uses pure oxygen instead of air, recycles most (around 80%) of the flue gas, but exhausts a small fraction of the total flue gas. The CO2 concentration in the flue gas may be enriched up to 95%, thus facilitating CO2 recovery. Experiments by Croiset and Thambimuthu2 revealed that combustion with recycled flue gas led to lower NOx emissions than for once-through combustion in O2/CO2 mixtures, because of the reduction of recycled NO. Nozaki et al.3 conducted experiments on O2/CO2 combustion with low- and mediumvolatile bituminous coals and concluded that the recycled NO was rapidly reduced in the combustion zone. Experiments have been carried out by Hannes et al.4 with lignite as well as bituminous coals, showing an overall NOx reduction capability of about 2050% depending upon fuel type and stoichiometry at the burner. The experimental data on recycled NO reduction by Hu et al.57 indicate that the reduction efficiency increased with the fuel equivalence ratio and recycling ratio. In the fuel-rich region, the reduction efficiency reached as high as 80% at a fuel equivalence ratio of 1.4. Normann et al.8 investigated the possibility of high-temperature reduction of nitrogen oxides (NOx) in O2/CO2 combustion. Combustion characteristics for both air and O2/CO2 conditions were numerically investigated by Cao et al.9 and compared to experimental data obtained from a pilot-scale test facility for an Australian sub-bituminous coal. The NO emission under the O2/CO2 condition was predicted to be significantly lower than that in air combustion, even without recycling of NO. Kimura et al.10 r 2011 American Chemical Society
conducted experiments on coal combustion with O2/recycled flue gas on a bench-scale test facility with a coal feed rate of 100 kg/h. The experimental results indicated that the NOx conversion ratio from fuel N in O2/recycled flue gas combustion was less than 10%, much lower than the 30% in air-blown combustion. Results from pilot-scale experiments show a 45% increase in the concentration of NOx in the flue gas compared to the air-firing NOx concentrations, resulting from the recycling of NOx in the flue gas back to the combustion chamber and a reduction in the total gas flow. However, the mass of NOx released per energy generated is significantly less for oxy-fuel combustion, around one-third of the total NOx produced by air combustion.1 Emission characteristics of a 0.03 MW oxy-fuel (O2/CO2) combustor and 0.2 MW liquefied natural gas (LNG) oxy-fuel combustor have been experimentally investigated by Kim et al.11,12 Their experimental results suggest that oxidizer velocity at the oxy-fuel combustor could be one of the crucial design parameters to control the NO emission. Emissions of SO2 and NOx during oxy-fuel circulating fluidized-bed (CFB) combustion were investigated by Jia et al.13 in a mini-CFB combustion reactor, and emissions of CO or NOx were found to be lower or comparable to those from air firing. Chemical thermodynamic analysis based on the Gibbs energy minimization principle was used by Zheng et al.14 for the environmental assessment of coal combustion in the Received: February 24, 2011 Revised: May 11, 2011 Published: May 26, 2011 2487
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Table 1. Experimental Conditions maximum flame temperature, Tmax (K)
Figure 1. Schematic diagram of the experimental facility.
O2 þ CO2 mixture compared to that in air. For the former case, the calculations predict higher emissions of CO and lower emissions of NOx. Experiments carried out in a 20 kW (thermal) down-fired, refractory-lined furnace have shown that oxygen enrichment can achieve benefits of improved carbon burnout with a positive impact on NOx emissions over and above the primary aim of increasing the CO2 concentration in the flue gas for enhanced capture efficiencies.15 In O2/CO2 combustion, the CO2 volume fraction in exhaust gases can be up to 0.95. This CO2-rich atmosphere is likely to alter many features of the process, which remain to be studied in detail. Examples include conversion from fuel N to NO, reduction of recycled NO in the reducing atmosphere in the combustion region of volatile matter from coal, and interaction between recycled NO and fuel N. Although there are some reports about NO formation and reduction in O2/CO2 combustion,1621 the detailed mechanism of NO reduction and the factors that contribute to low NO emission are important issues still to be clarified. The NO emission in an O2/CO2 combustion system and contributions of various factors, including the effect of reduction of recycled NO, influence of the CO2 concentration on the conversion ratio of fuel N to NO, and interaction between recycled NO and fuel N, were investigated experimentally and analytically in this work.
2. EXPERIMENTAL SECTION An electrically heated, one-dimensional premixed reactor (Figure 1) was used to conduct the experiments on NOx reduction. The reaction tube is ceramic (35 mm in inner diameter and 200 mm in height). The concentrations of CH4, O2, CO2, Ar, NO, and NH3 were controlled by flow meters. The gas and particles were fed independently to the reactor to simulate combustion conditions corresponding to actual O2/CO2 coal combustion. A vibrating feeder was used to feed coal particles. The maximum flame temperature, Tmax, was maintained at 1450 K for all experiments by changing the power of the electric heater. To form a flat CH4 flame, a honeycomb structure was placed at the upper edge of the reactor. The honeycomb also facilitates heating of premixed gases. A sampling probe with an axially adjustable position was used to collect gas samples at various residence times. The sampling probe contained a thermocouple to measure the temperature of the gas. A gas chromatograph and a chemiluminescent NOx detector were used to measure the concentrations of O2, CO2, CO, CH4, N2, and NO in flue gas.
1450
initial O2 concentration (vol %)
21
volume ratio of CO2/(CO2 þ Ar) in recycled gas
0.160.8
NO concentration in recycled gas (ppm)
01580
replacement ratio for CH4 by coal, β fuel N concentration (N atom mass, wt %)
0 and 0.2 0 and 1.22
A flat CH4 flame doped with NH3 for fuel N was formed under the honeycomb to simulate coal combustion by separating the gas-phase reactions and carbon combustion. A small amount of anthracite coal was fed to the CH4 flame to simulate carbon combustion. The effects of CO2 and NO concentrations in the recycled gas were examined. An initial O2 concentration of 21 vol % was adopted in the experiments to compare to the case of conventional coal combustion in air. The experimental conditions were described in Table 1, in which β is the fuel replacement ratio for CH4 by coal, defined as β ¼ ðO2 consumption by coalÞ=ðO2 consumption by coal þ O2 consumption by CH4 Þ
ð1Þ
In the absence of coal (β = 0), fuel N was simulated entirely by NH3 fed to the reactor. The experimental facility is a “one-pass” reactor, which does not have flue gas recycle equipment. The effect of gas recycling was considered through an approach of system analysis based on the experimental results obtained in this work; i.e., the effect of flue gas recycling was considered through system analysis.
3. RESULTS The existence of CH4 was the main reason for NO reduction in our study, just as in usual cases. On the other hand, the atmosphere of oxy-fuel combustion is different from conventional combustion in that the CO2 concentration is high and recycled NO also exists. There also exists the interaction between fuel N and recycled NO. All of these factors may affect the effect of NO reduction by CH4. Therefore, all of these factors were studied through experiments. To derive these influences, in our experiments, we only changed the concentration of CO2 or recycled NO, while keeping the concentration of CH4 unchanged. To obtain reliable data, all of the experiments were conducted for several times to ensure the repeatability. 3.1. Influence of the CO2 Concentration on the Conversion Ratio (CR) from Fuel N to NO. To obtain the effect of the
CO2 concentration in the recycled gas on CR, experiments were conducted in atmospheres of various CO2 concentrations, at Tmax = 1450 K, fuel N = 1.22 wt % (as N atom mass), and recycled NO = 0 ppm. For β = 0, no coal was fed and only gasphase reactions occurred. In this case, the CR value increased slightly with an increase of the CO2 concentration in the recycled gas. The experimental data were regressed to be a linear correlation with the CO2 concentration. CR ¼ 0:3633 þ 0:0833CCO2
when λ ¼ 0:7
ð2aÞ
CR ¼ 0:563 þ 0:0833CCO2
when λ ¼ 1:0
ð2bÞ
CR ¼ 0:5667 þ 0:1167CCO2
when λ ¼ 1:2
ð2cÞ
However, in the presence of coal (β = 0.2), the conversion ratio from fuel N to NO decreased with an increasing CO2 2488
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of coal. ΔCR ¼ 0:02 25:0CNO, re
Figure 2. Scheme of fuel N to NO conversion considering the effects of coal and CO. I = intermediate species containing N, and RO = oxygencontaining species.
concentration. CR ¼ 0:3367 0:0833CCO2
when λ ¼ 0:7
ð2dÞ
CR ¼ 0:5167 þ 0:0833CCO2
when λ ¼ 1:0
ð2eÞ
CR ¼ 0:5467 þ 0:0833CCO2
when λ ¼ 1:2
ð2f Þ
The difference above between β = 0 and 0.2 can be explained through a fuel NO formation scheme, as shown in Figure 2, with the effect of coal burning on the reduction of NO in the presence of CO considered. When only gas-phase reactions occurred, the concentration of RO in Figure 2 increased with an increase of the CO2 concentration, and consequently, CR increased slightly. However, in the presence of coal, the lower proportion (ii) in Figure 2 increased under the influence of coal þ CO. Furthermore, because of the increase of the CO concentration with an increasing CO2 concentration, CR tended to decrease. 3.2. Reduction Ratio (RR) of Recycled NO. Experiments were conducted at CO2/(CO2 þ Ar) = 0.48 by volume with no fuel N added to study the reduction of recycled NO in a CH4 flame. We changed the NO (representing the NO in recycled gas) concentration. At λ = 1.2, RR is about 0.3 and decreased a little with the recycled NO concentration, whereas at λ = 1.0 or 0.7, RR increased slightly with an increase of the recycled NO concentration. As much as about 60% of recycled NO was reduced to N2 at λ = 0.7, which corresponds to volatile matter combustion in a pulverized coal combustor. The experimental results demonstrated that the reduction ratio of NO in recycled gas increased with the NO concentration in the recycled gas when λ = 0.7 and 1.0 but decreased when λ = 1.2. RR ¼ 0:53 þ 66:7CNO, re
when λ ¼ 0:7
ð3aÞ
RR ¼ 0:32 þ 58:3CNO, re
when λ ¼ 1:0
ð3bÞ
RR ¼ 0:3 16:67CNO, re
when λ ¼ 1:2
ð3cÞ
3.3. Interaction between Fuel N and Recycled NO. The interaction between fuel N and recycled NO was also investigated through experiments at Tmax = 1450 K, fuel N = 1.22 wt % (as N atom mass), and CO2/(CO2 þ Ar) = 0.48 (vol). The experiments were conducted at various NO concentrations to clarify their effects on the reduction of recycled NO. The conversion ratio from fuel N to NO decreased with an increasing NO concentration in the recycled gas. The amount of decrease was the same in the absence (β = 0) and presence (β = 0.2)
ð4Þ
4. APPROACH OF SYSTEM ANALYSIS If the feed rate of coal is GC (g/s), then the stoichiometric amount of O2 necessary for burnout of the coal is GC WC WH WS WO R O2 ¼ þ þ ðmol=sÞ ð5Þ 100 12 4 32 32 where WC, WH, WS, and WO represent the carbon, hydrogen, sulfur, and oxygen contents of coal (wt %), respectively. Here, a dry recycle is considered; i.e., the moisture is removed and not recycled with the flue gas. The amounts of various species in the flue gas, after moisture removal, are CO2 :
GC WC þ RE x 100 12
O2 : RO2 ðλ 1Þ
ð6Þ ð7Þ
where RE refers to the flux of recycled gas (mol/s), λ is the oxygen/fuel stoichiometric ratio, and x represents the fraction of CO2 in the recycled flue gas. Similarly, the flux of recycled gas must satisfy λRO2 CO2 ¼ λRO2 þ RE x 100
ð8Þ
where CO2 refers to the O2 concentration at the entrance of the furnace before combustion. Combining eqs 7 and 8 yields RE ¼
λRO2 ð100 CO2 Þ CO2 ð1 zÞ
ð9Þ
where z represents the fraction of O2 in the recycled flue gas. Additionally, x and z must satisfy GC WC þ RE x = ½RO2 ðλ 1Þ ¼ x=z ð10Þ 100 12 Combining eqs 9 and 10 gives qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ða þ c þ RE Þ ( ða þ c þ RE Þ2 4RE c z¼ 2RE
ð11Þ
where a¼
GC WC 100 12
c ¼ RO2 ðλ 1Þ The NO originating from fuel N (NOf) is then given by ! WN NOf ¼ CR 3 GC ðmol=sÞ 14 3 100
ð12Þ ð13Þ
ð14Þ
The recycled NO remaining in the flue gas after reduction in the furnace is NOr ¼ ð1 RRÞRE 3 CNO 3 106 ðmol=sÞ
ð15Þ
where CNO is the NO concentration in the flue gas. Because NO in the flue gas is the sum of NO originating from fuel N and 2489
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recycled NO remaining in the flue gas (NOf þ NOr), the NO concentration in the flue gas is CR 3 GC
WN þ ð1 RRÞRE 3 CNO 3 106 1400 106 ¼ CNO a þ c þ RE ð1 zÞ
ð16Þ
5. DISCUSSION From a practical point of view, the amount of fuel N exhausted from the system as NO is important. For the whole system, the global conversion ratio from fuel N to exhausted NO is defined as CR ¼ ðN atom number of NO in exhaust gasÞ= ðN atom number of fuel NÞ
ð17Þ
Using our experimental results (eqs 24) and the approach of system analysis described in the previous section, parameters such as the NO concentration and global conversion ratio of fuel N to NO can now be obtained (on the basis of the coal described in Table 2). The gas recirculation ratio was decided according to coal property, O2 concentration at the entrance of the furnace before combustion, and oxygen/fuel stoichiometric ratio. Table 2. Coal Propertiesa proximate analysis (wt %, as received)
a
ultimate analysis (wt %, dry)
moisture
ash
volatile matter
fixed carbon
C
H
O
N
S
0.94
18.34
10.5
70.22
73.34
3.4
3.23
1.2
0.33
Changzhi, a Chinese coal.
Figure 3. Global conversion ratio from fuel N to exhausted NO at an oxygen/fuel stoichiometric ratio of λ = 0.7.
Different values of the gas recirculation ratio, derived at different levels of the oxygen/fuel stoichiometric ratio, were used to calculate CR*. 5.1. Global Conversion Ratio of Fuel N to NO. Figure 3 shows the global conversion ratio of fuel N to NO (CR*) in various cases at an oxygen/fuel stoichiometric ratio of λ = 0.7. The factors considered in the various cases are listed in Table 3. Case A corresponded to air combustion. In case B, the global conversion ratio of fuel N to NO (CR*) was the same as in case A, because in case B, although the recycling of flue gas was considered, the reduction of NO was not considered. The global conversion ratio of fuel N to NO (CR*) decreased to some degree from case B to case C, when the replacement ratio for CH4 by coal increased from 0 to 0.2. This result was attributed to the enhancement of NO reduction at the solid surface of the coal particle (or “heterogeneous effect” of coal). The global conversion ratio of fuel N to NO drastically decreased from case C to case D when the reduction of recycled NO was included. When the influence of CO2 on CR was included, the global conversion ratio of fuel N to NO decreased from case D to case E. Moreover, when the influence of recycled NO on RR was considered, the global conversion ratio of fuel N to NO decreased somewhat from case E to case F. In comparison to case F, case G considered the interaction between recycled NO and fuel N, in addition to the other factors; i.e., case G included all factors believed to contribute to NO reduction. It was revealed that the CR* value in case G was lower than in case F. This result suggested that CR* decreased because of the interaction between recycled NO and fuel N. Figure 4 shows the global conversion ratio of fuel N to NO in various cases at an oxygen/fuel stoichiometric ratio of λ = 1.0. The trends are similar to those shown in Figure 3 (λ = 0.7).
Figure 4. Global conversion ratio from fuel N to exhausted NO at an oxygen/fuel stoichiometric ratio of λ = 1.0.
Table 3. Factors Considered in Various Cases interaction air combustion case A
recycle of flue gas
heterogeneous
reduction of
influence of
influence of recycled
between recycled
effect
recycled NO
CO2 on CR
NO on RR
NO and fuel N
case B
case C
case D case E
case F
case G
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Figure 5. Global conversion ratio from fuel N to exhausted NO at an oxygen/fuel stoichiometric ratio of λ = 1.2.
Figure 6. Contributions of various factors at an oxygen/fuel stoichiometric ratio of λ = 0.7.
Figure 5 shows the global conversion ratio of fuel N to NO for various cases at an oxygen/fuel stoichiometric ratio of λ = 1.2. As shown in Figure 5, the effects of factors including the heterogeneous effect of coal, reduction of recycled NO, influence of CO2 on CR, and interaction between recycled NO and fuel N were similar to the results shown in Figures 2 and 3 (λ = 0.7 and 1.0). However, the influence of recycled NO on RR at λ = 1.2 enhanced the global conversion ratio of fuel N to NO, in contrast to the effect at λ = 0.7 and 1.0. 5.2. Contributions of Various Factors. To clarify the mechanism of efficient NO reduction in O2/CO2 coal combustion, the contributions of various factors were quantified. Figure 6 shows the results at an oxygen/fuel stoichiometric ratio of λ = 0.7. The contributions were based on the overall decrease of the global conversion ratio of fuel N to NO, from air combustion to O2/CO2 coal combustion. It was revealed that the reduction of recycled NO contributed the most, i.e., 79.6%. The contributions of various factors increased in the following order: influence of recycled NO on RR, interaction between recycled NO and fuel N, influence of CO2 on CR, heterogeneous effect, and reduction of recycled NO. The contributions of various factors at an oxygen/fuel stoichiometric ratio of λ = 1.0 are shown in Figure 7. The reduction of recycled NO is still the major contributor, although not as dominant as at an oxygen/fuel stoichiometric ratio of λ = 0.7. The contributions of various factors increased in the following order: interaction between recycled NO and fuel N, influence of recycled NO on RR, influence of CO2 on CR, heterogeneous effect, and reduction of recycled NO. The contributions of various factors at an oxygen/fuel stoichiometric ratio of λ = 1.2 are shown in Figure 8. Again, the reduction of recycled NO is the major contributor. The contributions of various factors increased in the following order:
ARTICLE
Figure 7. Contributions of various factors at an oxygen/fuel stoichiometric ratio of λ = 1.0.
Figure 8. Contributions of various factors at an oxygen/fuel stoichiometric ratio of λ = 1.2.
influence of recycled NO on RR, heterogeneous effect, influence of CO2 on CR, interaction between recycled NO and fuel N, and reduction of recycled NO. In fact, the contribution of the influence of recycled NO on RR was negative, because it enhanced the global conversion ratio of fuel N to NO at λ = 1.2. The reduction of recycled NO was demonstrated to be the main factor accounting for the low global conversion ratio of fuel N to NO in O2/CO2 combustion, over the range of λ = 0.71.2. However, the relative importance of various factors changed with the oxygen/fuel stoichiometric ratio (λ).
6. CONCLUSION The effects of a range of factors (reduction of recycled NO, influence of the CO2 concentration on the conversion ratio of fuel N to NO, interaction between recycled NO and fuel N, etc.) on the conversion of fuel N to NO in O2/CO2 combustion of coal were experimentally investigated, and the correlations were quantified. The global conversion ratio from fuel N to exhausted NO in O2/CO2 combustion was derived from our experimental results and analysis of the system. The following conclusions were reached: (1) The conversion ratio from fuel N to NO increased with an increasing CO2 concentration in the absence of coal but decreased in the presence of coal. (2) The reduction ratio of NO in recycled gas increased with the NO concentration in the recycled gas when λ = 0.7 and 1.0 but decreased when λ = 1.2. (3) The conversion ratio from fuel N to NO decreased with an increasing NO concentration in the recycled gas, because of the interaction between fuel N and recycled NO. (4) The relative importance of various factors to the low global conversion ratio from fuel N to exhausted NO in O2/CO2 combustion changed with the oxygen/fuel stoichiometric ratio (λ). Nevertheless, under all conditions investigated, the reduction of recycled NO was the major contributor (over 70%). 2491
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Energy & Fuels The results of this work revealed that the reduction of recycled NO was the main mechanism for the low NO emission in O2/ CO2 combustion.
’ AUTHOR INFORMATION Corresponding Author
*Fax: þ86 21 5527 5542. E-mail:
[email protected].
’ ACKNOWLEDGMENT This work is supported by the National Natural Science Foundation of China (50936001) and the Foundation of the State Key Laboratory of Coal Combustion (China).
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’ NOMENCLATURE CCO2 = CO2 concentration CNO = NO concentration in the flue gas CNO,re = NO concentration in the recycled gas CO2 = O2 concentration CR = conversion ratio from fuel N to NO CR* = global conversion ratio from fuel N to exhausted NO GC = feed rate of coal (g/s) NOf = NO originating from fuel N (mol/s) NOr = recycled NO remaining in the flue gas after reduction in the furnace (mol/s) RE = flux of recycled gas (mol/s) RO2 = stoichiometric amount of O2 necessary for burnout of the coal (mol/s) RR = reduction ratio of NO in recycled gas W = mass fraction of some element in coal (%) x and z = fractions of CO2 and O2 in the recycled flue gas β = fuel replacement ratio for CH4 by coal λ = oxygen/fuel stoichiometric ratio ΔCR = amount of CR decrease Subscripts
C, H, O, N, and S = carbon, hydrogen, oxygen, nitrogen, and sulfur
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