ARTICLE pubs.acs.org/EF
Gas-Phase Reaction of NOX Formation in Oxyfuel Coal Combustion at Low Temperature Ryo Yoshiie,*,† Takuya Kawamoto,† Daisuke Hasegawa,† Yasuaki Ueki,‡ and Ichiro Naruse† †
Department of Mechanical Science and Engineering, and ‡Ecotopia Science Institute, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan ABSTRACT: Oxyfuel coal combustion with flue-gas recirculation is known to be one of the most promising methods for reducing CO2 emissions from pulverized coal combustion power plants. In the oxyfuel system, the combustion atmosphere consists of only O2 separated from air and flue gas recirculated from a stack. Therefore, the flue gas is dominated by CO2 without any nitrogen. The most attractive potential aspect of oxyfuel coal combustion would be the ability to achieve a carbon capture and storage (CCS) system, which would be a “quick-impact” approach to reducing global CO2 emissions. In the oxyfuel system, several minor components in the flue gases are also recirculated to the combustion zone, together with the CO2. The effects of these additional impurities on the flue gas composition are one of our concerns. In this study, NOX emissions were experimentally monitored in an oxyfuel coal combustion atmosphere. The effect of NOX recirculation on NOX formation in the combustion zone was investigated in particular. In addition, elementary reaction kinetics for NOX formation was numerically analyzed under oxyfuel coal combustion conditions identical to the experiments. All experiments and numerical analyses were conducted at temperatures between 1073 K and 1223 K; these temperatures are relatively low, compared with various coal combustion conditions. In particular, this temperature range corresponds to fluidized-bed combustions. As a result, NO emissions under CO2O2 conditions were confirmed to be lower than those under “air” conditions. In addition, the concentration of NO under oxyfuel conditions was unchanged by fluegas recirculation. In contrast, the concentration of N2O under CO2O2 conditions was higher than that under air conditions, and, furthermore, it was increased by flue-gas recirculation.
’ INTRODUCTION CO2 emissions per unit calorific value during coal combustion are greater than those observed for any other fossil fuels. The oxyfuel coal combustion system with flue-gas recirculation is known to be one of the most promising methods of reducing CO2 emissions from coal combustion power plants. This system has two major advantages. First, in the oxyfuel system, the combustion atmosphere consists of only O2 separated from air and flue gas recirculated from the stack, resulting in flue gases dominated by CO2 without any nitrogen content. Therefore, there is no need to separate CO2 from the flue gas for carbon capture and storage (CCS), which is a “quick-impact” approach to the reduction of CO2 emissions to avoid global warming. Second, it is easy to retrofit such a system to a conventional coal power plant, compared to other advanced coal utilization power plant systems, such as IGCC. Moreover, it has been reported that NOX emissions from oxyfuel coal combustion are less than those from air coal combustion, even if the O2 concentration at the burner inlet is the same in both cases.1 flue-gas recirculation results in a reduction of the available oxygen in the combustion zone and leads to a decrease of the flame temperature, thus reducing both fuel-bound nitrogen conversion and thermal NOX formation.2 The absence of nitrogen in the recirculation gas has a significant influence on NOX emissions. The oxy-combustion emission levels are dependent on the firing conditions, such as the types of coal that are used. The reductions in NOX have been confirmed to be the highest for bituminous coal and less for lignite.3 The effects of CO2 concentration, the reduction of recycled NOX, and the interaction between fuel-N and recycled r 2011 American Chemical Society
NOX on the decrease in the final quantity of NOX exhausted from the coal combustion system with recycled CO2 have been separated for appropriate NOX-reduction mechanisms.4 Less volume of exhausted flue gases, a low oxygenfuel stoichiometric ratio, and an increase in temperature have been highlighted as factors for the reduction of NO emissions in the exhaust gases under O2CO2 coal combustion with heat recirculation.5 In the fluidized-bed combustion of semidried municipal sewage sludge, a reduction in NOX emissions when using flue-gas recycling was observed as a result of a decrease in the partial pressure of O2 and of the combustion temperature in the combustion zone.6 The effects of the flue-gas recycling ratio and the properties of the coal on the reduction of recycled-NOX in coal combustion with O2/recycled flue gas was investigated.7,8 In order to reach the maximum NOX reduction in oxyfuel combustion, a burner that was specifically designed for oxygen/ recycled flue gas (O2/RFG) combustion was determined to be necessary.9 The predictions for NOX in air and oxyfuel combustion that were derived from computational fluid dynamics were compared to the experimental data. The levels of NO emissions under oxyfuel conditions are predicted to be significantly lower than those in air combustion, even without recycled NO.10 However, there have been few reports describing NOX formation under oxyfuel combustion conditions, including the quantitative effect of adding
Received: February 22, 2011 Revised: April 8, 2011 Published: April 15, 2011 2481
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Table 1. Proximate and Ultimate Analytical Results of Tested Coal sample coal Proximate analysis [wt %, dry]
Ultimate analysis [wt %, d.a.f.]
Table 2. Experimental Conditions for CO2O2 and Air Combustion
bituminous coal moisture
atmosphere
2.0
CO2 þ O2
air
CO2:O2
79:21
ash volatile matter
13.7 30.3
primary gas [L/min]
0.22 (O2) 0.83 (CO2)
1.05 (air)
fixed carbon
56.0
secondary gas [L/min]
0.81 (O2)
3.85 (air)
C
80.97
H
9.32
stoichiometric air ratio
N
1.47
coal feed rate [g/min]
O
7.77
sampling distance from injector [mm]
S
0.47
reaction temperature [K]
3.04 (CO2)
residence time [s]
1.2 0.5 1300 1073, 1123, 1173, 1223 ∼2
Table 3. Concentrations of Gas Species in Atmosphere Simulating Oxyfuel Coala gas
concentration (vol %)
O2
21
NO
Rxi1
N2O CO2
Ryi1 balance
Legend: i, recycle time (i = 05); xi, NO concentration in flue gas in the ith recycle experiment (x0 = 0); yi, N2O concentration in flue gas in the ith recycle experiment (y0 = 0); and R, dilution ratio defined by additional O2 to the flue gas. a
Figure 1. Schematic of drop-tube furnace for coal combustion.
additional NOX in the recycled flue gas. In particular, N2O has not been discussed in the previous studies described above. Therefore, our objectives are to estimate the combustion behavior and NOX formation reactions under oxyfuel combustion conditions at low temperature, and to compare the results with those for air combustion. In this study, both NO and N2O are regarded as NOX compounds. The coal combustion behavior and NOX emissions were observed experimentally in a CO2O2 atmosphere. The NOX emissions were also estimated under fluegas recirculation conditions, in which a small amount of NOX was repeatedly added to the inlet gas to simulate the oxyfuel condition. The elementary reaction kinetics for NOX formation was numerically analyzed under conditions identical to those used in the experiments.
’ MATERIALS AND METHODS Coal Sample and Experimental Apparatus. Table 1 shows the results of proximate and ultimate analyses of the coal that was tested in this study. This coal sample is a bituminous coal from Australia, containing nitrogen at 1.47 wt % on a dry ash-free (daf) base. Coal combustion experiments were conducted using a drop-tube furnace equipped with an external electric heater, as shown in Figure 1. This can be divided into three parts for fuel injection, reaction, and sampling. Pulverized coal can be continuously fed into the reactor through the injector with an entraining gas. The reactor is made from a quartz tube,
with a length of 2 m and an inner diameter of 40 mm. The temperature in a reaction zone is controlled to be constant by an external electric heater. A water-cooled sampling probe is inserted from the bottom of the reactor. The combustion gas in the reactor was sampled using this probe, and was analyzed for gas composition and unburned carbon in the particulates. The distance between the injector and the top end of the sampling probe was fixed at 1300 mm in this study, which determined the residence time for coal combustion to be ∼2 s. Table 2 shows the experimental conditions for air and CO2O2 coal combustions. The concentration of O2 was set to be 21 vol % under both air and CO2O2 combustion conditions. In general, the O2 concentration for oxyfuel combustion should be increased to ∼30%, so that the same temperature fields could be produced between oxyfuel and air combustions. In this study, however, the reaction mechanism was of the most interest, while the furnace temperature can be kept constant by an external electric heater. Therefore, we kept the O2 concentration constant at the reactor inlet under both air and CO2O2 combustion conditions. The oxygen fuel ratios were equal at 1.2 under all of the experimental conditions in the present study. The temperature range was 10731223 K, which is relatively low and is close to the fluidized-bed combustion condition. Oxyfuel Coal Combustion Experiment. In the oxyfuel system, several minor components in the flue gas were recirculated to the combustion zone, together with CO2. The effects of these additional impurities on the composition of the flue gas are our main concern. In this study, the combustion atmosphere under oxyfuel coal combustion conditions was simulated, focusing on only NO and N2O as recirculated impurities. Table 3 shows the procedures that were used to comprise the recirculated gas with additional NO and N2O. These were prepared sequentially with gas recycle times i, using the results of the NO and N2O concentrations measured in the previous experiment (i 1). Steam recirculation was not considered in this study. The other experimental conditions were the same as those for CO2O2 combustion shown in Table 2. Figure 2 shows a schematic of the first step to simulate flue-gas recirculation conditions. First, a CO2O2 combustion experiment was 2482
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Figure 2. Schematic of the first step to simulate flue-gas recirculation in the experiment for oxyfuel coal combustion.
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Figure 3. Carbon conversions under air and CO2O2 combustion conditions.
conducted and the resulting flue gas was analyzed for its composition. This composition was then copied as the inlet gas condition with additional O2 in the first flue- gas recirculation experiment. By making these repetitions, the inlet gas conditions approached the steady-state oxyfuel condition with increased gas recycle times. Elemental Reaction Analysis. Elemental reaction analysis was conducted on the basis of GRI-Mech ver.3.0 plus several surface reactions.11,12 It includes 4 solidgas reactions, 52 chemical species, and 323 elemental reaction formulas. Pressures and temperatures were assumed to be constant through the reaction time. The initial O2 concentrations in the present analysis were fixed at 21 vol % to make the ratio of excess oxygen to fuel constant at 1.2 under air and CO2 combustion conditions. Fixed carbon and the volatile components in coal have been assumed to have the same elemental compositions, based on C, H, N, and O. The chemical species in the volatiles from coal were assumed to be C2H4, CO, H2, HCN, and NH3, and their compositions were determined as follows. Nitrogen in coal was distributed between HCN and NH3 at a ratio of 1:1, and the remaining H in coal was distributed between C2H4 and H2 at a ratio of 1:1. The balance of the carbon content was regarded as CO. The elemental compositions of fuel coal and the other combustion conditions are identical to those in the experiments described above. In addition, the procedure that was used to make up the recirculated gas in this simulation was in common with that used in the experiment shown in Figure 2. The flue-gas compositions under oxyfuel combustion conditions can be derived as a convergence of the calculations after several iterations of recycling.
’ RESULTS AND DISCUSSION Carbon Conversion. Figure 3 shows carbon conversion under air and CO2O2 combustion conditions at different furnace temperatures. Carbon conversion was evaluated by investigating the ash tracer analysis of particulates sampled by an isokinetic probe. Carbon conversion under CO2O2 conditions was lower than that under air conditions at all of the furnace temperatures that were tested. This moderate level of combustion under CO2O2 conditions was due to the high partial pressure and high heat capacity of CO2. The former restrains the forward reaction during combustion, while the latter restrains the increase in temperature of the gas around the coal particles.13,14 As a matter of fact, light emission from burning particles under air conditions was confirmed to be brighter than that under oxyfuel conditions. This phenomenon agrees with the previous study, in which the combustion of coal particles was observed in a laboratory-scale drop-tube furnace by a high-speed camera.15
Figure 4. Conversions of input nitrogen to NO and its concentrations with gas recycle times at (a) 1073 K and (b) 1223 K.
In the present study, however, the furnace temperatures in the reactor were controlled to be constant by an external electric heater. The gas temperature, which was measured using a thermocouple with a ceramic sheath, was verified to be constant along the flow direction in the reactor. This means that the momentary increases in the temperature of the coal particles, caused by their combustions under CO2O2 conditions, are less than those observed under air conditions, and it has the effect of decreasing the level of carbon conversion. Conversions of Input Nitrogen to NO and N2O. The concentrations of NO and N2O in the gases sampled by the isokinetic probe were monitored by continuous gas analyzers (NO: Horiba, Model PG-250; N2O, Thermo Environmental 2483
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Figure 6. Variations of CO, CO2, and O2 concentrations with reaction time under air and CO2O2 conditions.
Figure 5. Conversions of input nitrogen to N2O and its concentrations with gas recycle time at (a) 1073 K and (b) 1223 K.
Instruments, Model 46C) and were then utilized to estimate the conversion of the input N into NO and N2O. These were defined by the following formula, where the nitrogen input to the combustion gas from the fuel was assumed to be proportional to the carbon conversion ratio: NO or N2 O emission ½mol fuel N carbon conversion þ additional NO þ 2 additional N2 O ½mol
To be precise, the Horiba Model PG-250 equipment used in this study is an analysis device for measuring a concentration of NOX as the sum of NO and NO2. However, it is known that NO2 has shown little contribution to NOX, compared with N2O and NO under most coal combustion conditions.16 Therefore, the NO concentration is assumed to be equal to the NOX concentration measured by the Horiba Model PG-250 device in this study. Figure 4 shows the conversions of input nitrogen to NO and its concentrations with gas recycle time at 1073 K (Figure 4a) and 1223 K (Figure 4b). It also shows the conversion and concentration of NO under air combustion conditions. The conversion of input nitrogen to NO under flue-gas recirculation conditions was determined to be almost constant with increasing gas recycle times, and was lower than that under air conditions. As shown in Figure 4a, NO concentrations at 1073 K are almost constant with gas recycle times, too. This means that NO emissions at 1073 K hardly accumulated in the flue gas, because the addition of NO to the inlet gas that results from flue-gas recirculation is relatively small. Note that the level of NO emissions under oxyfuel combustion conditions was lower than that under air combustion conditions. As shown in Figure 4b, the NO emissions became greater at higher furnace temperature
Figure 7. Variations of (a) NO and (b) N2O concentrations with reaction time under air, CO2O2, and oxyfuel combustion conditions.
under both oxyfuel and air combustions, in common with many of the previous reports into NO emissions. In addition, NO concentration at 1223 K increased in early gas recycle times until reaching a steady state. This means that NO emissions at 1223 K accumulated in the flue gas, because of large additions of NO to the inlet gas resulting from flue-gas recirculation, despite constant conversions of input nitrogen to NO. The differences between the outcomes of oxyfuel and air combustion became less at higher furnace temperature. 2484
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Figure 8. Reaction paths for NOX formation under air and oxyfuel combustion conditions.
Figure 5 shows the conversion of input nitrogen to N2O and its concentrations, with respect to gas recycle time, at 1073 K (Figure 5a) and 1223 K (Figure 5b). N2O concentration under flue-gas recirculation conditions was observed to reach a steady state after increasing slightly with gas recycle time at 1073 K, despite constant conversions of input nitrogen to N2O. This means that N2O accumulated in the flue gas because of additional N2O to the inlet gas resulting from flue-gas recirculation at 1073 K. The conversion of input nitrogen to N2O under flue-gas recirculation conditions was higher than that under air conditions, and then N2O emission under oxyfuel combustion conditions was greater than that under air combustion at 1073 K. The N2O emissions became much lower at higher furnace temperature under both oxyfuel and air combustions, which is consistent with the observations in many previous reports on N2O emissions. The accumulation of recycled N2O and the differences between the outcomes of oxyfuel and air combustion had practically disappeared at 1223 K. Elemental Reaction Analysis. Figure 6 shows variations of CO, CO2, and O2 concentrations with reaction time under air and CO2O2 conditions, as obtained from elemental reaction analyses. There are two types of reaction periods in each variation. The former is regarded as a hydrocarbon decomposition process that generates CO before the ignition, and the latter is regarded as a combustion process that consumes CO after the ignition. The time to ignition under CO2O2 conditions is slightly longer than that under air conditions. These reaction analyses are in good agreement with the combustion characteristics observed in the experiments shown in Figure 3. Figure 7a shows the variation of NO concentration with reaction time, as a result of elemental reaction analysis. The initial concentrations of NO under air and CO2O2 conditions are naturally zero, while the initial concentration of NO under oxyfuel conditions is equal to the value in the flue gas diluted by
the additional oxygen. The line that represents NO concentration under oxyfuel combustion conditions is regarded as a “convergence line” after several gas recycles under flue-gas recirculation conditions. In all three cases, the combustion reaction progress rapidly over ∼500 ms and terminates in a short time. Simultaneously, the NO concentrations increase to stable values after the combustion reaction. The NO concentration after the combustion reaction under CO2O2 conditions is lower than that under air conditions. In addition, the NO concentration after combustion under oxyfuel conditions is the same as the value under CO2O2 conditions. In other words, flue-gas recirculation process does not result in any accumulation of NO in the combustion zone at 1073 K. These facts are almost consistent with the experimental results described in the previous section. The concentration of NO in the gas at the reactor exit can be determined as a result of the thermal equilibrium of nitrogen compounds in a gas. In Figure 7a, we also plot the experimental results for the NO concentrations in the reactor (1300 mm distant from the injector). These are very close to the analytical results, showing the accuracy of the elemental reaction analyses when estimating NO emissions. Figure 7b shows variations in N2O concentration with reaction time. The N2O concentrations in all three cases increase drastically to peak values after the ignition, and then gradually decompose through the reactor. The N2O concentration at the reactor end under CO2O2 conditions is higher than that under air conditions. Furthermore, N2O accumulation rises due to the additional N2O. Nonetheless, this accumulation behavior converged within several recirculation cycles in this analytical simulation. The convergent concentration can be regarded as the N2O concentration in the steady state under oxyfuel conditions. These facts are also consistent with the experimental results. This means that the N2O concentration in the gas at the reactor exit can be determined by balancing the reaction 2485
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Energy & Fuels kinetics between the nitrogen compounds in the gas. In Figure 7b, the experimental results for the concentrations of N2O in the reactor (1300 mm distant from the injector) are also plotted. These are about half as high as the analytical results. The kinetic parameters for N2O generation need to be readjusted, although the relative magnitudes of the relationships for N2O concentration under air and oxyfuel conditions are similar between the elemental reaction analyses and the experiments. Figure 8 shows the reaction paths for NOX formation under air combustion conditions (Figure 8a) and oxyfuel combustion conditions (Figure 8b), focusing on the balance between the nitrogen-containing compounds. As described in the previous section, the atomic nitrogen in coal is initially allocated to two chemical species, such as NH3 and HCN, in the present elemental reaction analysis. The arrowed lines connecting the chemical species in Figure 8 indicate the directions of nitrogen transfer from one chemical species to another, and the thickness of the line indicates the size of its contribution. Although N2 is only one compound as a terminal species under both air and oxyfuel conditions, there are many intermediate products and reaction paths between the initial and the terminal species. Note that NO is directly reduced to N2 under oxyfuel conditions, while NCO is directly oxidized to N2O. These facts can be considered as reasons for the lower NO and higher N2O concentrations that were observed under oxyfuel conditions in the experiments.
’ CONCLUSIONS The formation of NOX from the combustion of coal in a CO2O2 atmosphere with flue-gas recirculation at temperature between 1073 K and 1223 K was estimated experimentally and numerically. Carbon conversion under CO2O2 combustion was lower than that under air combustion. The concentration of NO from coal combustion in a CO2O2 atmosphere was lower than that in air, and it did not increase, despite additional NO input under oxyfuel conditions at 1073 K. However, NO emissions accumulated under oxyfuel conditions at 1223 K. The concentration of N2O from coal combustion in a CO2O2 atmosphere was higher than that in air, and it accumulated under oxyfuel conditions at 1073 K. The accumulation of recycled N2O and the differences between the outcomes of oxyfuel and air combustion had practically disappeared at 1223 K.
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’ AUTHOR INFORMATION Corresponding Author
*Phone: þ81-52-789-2712. Fax: þ81-52-789-5123.
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