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Ind. Eng. Chem. Res. 2004, 43, 5820-5827
Pilot-Scale Studies of NOx Reduction by Activated High-Sodium Lignite Chars: A Demonstration of the CARBONOX Process Himanshu Gupta,† Steven A. Benson,‡ Laing-Shih Fan,*,† Jason D. Laumb,‡ Edwin S. Olson,‡ Charlene R. Crocker,‡ Ramesh K. Sharma,‡ Ryan Z. Knutson,‡ A. S. M. Rokanuzzaman,‡ and James E. Tibbets‡ Department of Chemical Engineering, The Ohio State University, 140 West 19th Avenue, Columbus, Ohio 43210 and Energy & Environmental Research Center, University of North Dakota, P.O. Box 9018, Grand Forks, North Dakota 58202-9018
Pilot-scale experiments were carried out to quantify the extent of NOx reduction attained by activated lignite chars on simulated and lignite-coal-combustion derived flue gas. Lignite chars, obtained by devolatilizing high-sodium lignite coal in pure nitrogen, were activated by their reaction with a gas mixture consisting of steam and CO2 in nitrogen at 700-750 °C to yield a nitrogen BET surface area of 200-400 m2/g. The effect of gaseous components such as N2, CO2, SO2, and moisture on the extent of NOx reduction was qualitatively examined. NOx reduction exceeded 99% on simulated flue gas at 525-600 °C. The presence of SO2, even at a concentration of 3600 ppm, did not have any detrimental effect on the extent of NOx reduction. Sodiumenhanced char attained >98% NOx reduction on actual flue gas at a relatively lower temperature of 480-560 °C. Introduction Economical NOx reduction from coal-based combustion sources such as utility power plants continues to be a challenge. Numerous primary and secondary measures have been deployed for the reduction of NOx.1 Boiler modifications, such as low-NOx burners and staged combustion, are effected by fuel and air staging to reduce the temperature of combustion.2-5 NO is reduced to N2 by postcombustion processes such as ammoniabased selective noncatalytic reduction (SNCR)6 and selective catalytic reduction (SCR).7 NO can also be oxidized by ozone,8 and by electrocatalytic and photocatalytic processes to NO2, followed by its subsequent absorption in basic slurries. The post-combustion processes, especially SCR, achieve a higher extent of NOx reduction. However, the implementation of these processes in thermal power plants leads to multiple problems, such as ammonia slippage, formation of sticky ammonium bisulfite, and the oxidation of SO2 to SO3 across the vanadium-based catalyst. Deep reductions in NOx (>98%) across a SCR catalyst bed entail excessive ammonia slip. It is necessary to develop processes that can address current and future NOx regulations adequately. Near-complete reduction of NOx can be achieved by the use of carbonaceous materials, which undergo heterogeneous gas-solid reactions with NO to form N2 and CO/CO2. The reactions of interest can be written as
C + 2NO f N2 + CO2
(1)
2C + 2NO f N2 + 2CO
(2)
These reactions can be carried out under process condi* To whom correspondence should be addressed. Tel.: (614) 688-3262. Fax: (614) 292-3769. E-mail:
[email protected]. † The Ohio State University. ‡ University of North Dakota.
tions in which NO is the limiting reactant and carbon is in excess, thereby ensuring that NO is completely consumed by the carbon. The use of relatively large carbon particles (>1 mm in diameter) ensures that the excess unreacted carbon particles remain in the reactor, unlike in SCR, where unreacted gaseous ammonia escapes with the flue gas. Carbon also reacts with other flue gas components such as oxygen, moisture, and carbon dioxide (besides NO), leading to the formation of gaseous products. These reactions can be written as
C + O2 f CO2
(3)
C + 1/2O2 f CO
(4)
C + H2O f H2 + CO
(5)
C + CO2 f 2CO
(6)
Pure structured carbons, such as graphite require, very high temperatures (>900 °C) for the carbon-NO reaction. Since retrofitting a fixed/fluidized bed reactor that operates at such a high temperature is not easy in a thermal power plant, it would be useful to lower the NOx reduction temperature. Inorganic species present inherently in the char matrix or sorbed on the carbonaceous surface catalyze these carbon-NO reactions.9,10 Coal char, derived from bituminous coal, reacts with NO at a lower temperature (620-820 °C) due to the constituents present in the mineral matter.11 Further, the carbon-NO reaction is catalyzed by coal char that is characterized by a high calcium content in its ash.12 Heterogeneous reactions involved in the gasification of carbonaceous materials proceed at a faster rate in the presence of alkali and alkaline-earth metals13 and calcium.13-17 Oxygen plays an important role in the carbon-NO reaction. The extent of the carbon-O2 reaction is higher than the carbon-NO reaction as oxygen is a stronger
10.1021/ie049692g CCC: $27.50 © 2004 American Chemical Society Published on Web 07/21/2004
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oxidizing agent than NO. This detrimental consumption of carbon by oxygen is further enhanced by the relatively higher concentration of oxygen (1-5%) compared to the NO concentration (250-1500 ppm).12 However, oxygen concentration in the 0.1-2% range plays a beneficial role by reducing the temperature required for the carbon-NO reactions.18 A further drop in the reaction temperature to 300-500 °C in the presence of O2 is obtained by carbon impregnated with calcium, copper, and nickel.19 The identification of Cu2O and CuO on the surface of carbon indicates that the mechanism of the transfer of oxygen atoms from the gaseous compounds to the active sites on carbon has been altered by these metallic impregnants. Carbon can also react with the CO2 and moisture present in the flue gas, besides oxygen, thereby increasing its parasitic consumption. While most carbonaceous materials,10 such as phenol-formaldehyde resin, coal chars, graphite, petroleum coke, activated carbon, graphite, and coconut char, have been used in this process, it is imperative that the raw materials be inexpensive, since the carbon is consumed through its reaction with NO. The performance of bituminous coal char has been detailed in an earlier publication.11 This char required a high temperature (620-820 °C) to attain >90% NOx reduction. An excellent choice of raw material that meets the requirements of lower carbon-NO reaction temperature and low sorbent cost is char derived from lignite coals. Lignite coals mined in the Fort Union coalbearing region of North America are characterized by inherently high sodium content, thus obviating the need to go through multiple unit operations required to impregnate the carbon. These coals are also easier to activate, as they do not go through a plastic state upon devolatilization, thereby yielding high-surface-area chars through their physical activation by moisture and CO2. The CARBONOX (carbon-based NOx reduction) process employs porous carbonaceous materials rich in alkali and alkaline-earth metals that catalyze the carbon-NO and carbon-O2 reactions and also provide better utilization of char due to enhanced selectivity of the carbonNO reaction.20,21 Whereas previous experiments were conducted on NO and O2 mixtures in pure helium on 250-1500-mg char samples, this study aims to understand the CARBONOX process on a larger scale, employing 1001000-g char samples. NOx reduction by lignite chars is quantified on simulated flue gas consisting of N2, CO2, SO2, and moisture, as well as on the flue gas generated by combustion of lignite coal. Another objective of this study is to measure the amount of heat liberated from these exothermic gas-solid reactions, which has the potential for on-site use, and thereby to lower the operating cost associated with the CARBONOX process. Experimental and Analytical Section Char Synthesis and Structural Characterization. Samples of activated lignite chars were synthesized from lignite coals procured from Luscar Ltd. (Beinfait mine Ravenscrag lignite) and Basin Electric Power Cooperative (Freedom mine Beulah-Zap lignite). The bulk ash composition of these two coals on a SO3free equivalent oxide basis is shown in Table 1. It is clear from the analysis that these chars have a high sodium and calcium content. These coals were air-dried and crushed to different particle sizes. The synthesis and activation was carried out in a 3-in.-diameter
Figure 1. Pore size distribution of (a) activated Luscar and (b) activated Freedom char samples. Table 1. Bulk Ash Composition of Beinfait Lignite and Freedom Lignite on a SO3-Free Equivalent Oxide, wt % SiO2 Al2O3 Fe2O3 TiO2 P2O5 CaO MgO Na2O K2O
Beinfait
Freedom lignite
32.8 18.1 7.79 0.62 0.44 21.5 4.54 13.8 0.36
7.79 9.9 16 0.29 0.08 27 8.55 16.5 0.5
stainless steel (SS) tube reactor housed in a Thermolyne electric furnace. Lignite coal in the amount of 2000 g was placed in the central hot zone of the reactor and heated to 400 °C in nitrogen until tar ceased to evolve. The resulting char was cooled to room temperature and stored under nitrogen for further activation procedures. The activation of this char was carried out by heating it at 750 °C in a gas mixture consisting of CO2, steam, and N2. The pore size distribution of the activated Luscar char samples as a function of the activating gas mixture composition is shown in Figure 1. The N2 BET surface area and pore volume of these samples, measured by a Quantachrome NOVA 2200 analyzer, is shown in Table 2. Figure 1 indicates that a majority of the pores in the activated char samples are microporous in nature (pore diameter 1000 m2/g, are not essential for this process. Activation of char by a CO2/steam mixture is a relatively easy operation, one which avoids the use of multiple unit operations required by chemical activation. NOx Reduction Setup. Laboratory Studies. Laboratory studies on NOx reduction by Luscar and Freedom chars were carried out at The Ohio State University facilities. Integral mode experiments were conducted in a 2 in. SS reactor, which was electrically heated by a Thermolyne 21 100-tube furnace. The inlet gases, helium, NO, and oxygen were metered into the reactor by variable area flow meters. The gas mixture was heated in the annular region of the reactor before entering a 3/8 in. SS tube containing a porous support plate that houses a fixed bed of lignite char particles. The exit concentrations of NO, CO, CO2, and O2 were monitored by chemiluminescence, infrared, and microfuel-cell-based analyzers. Details of this reactor setup are described in an earlier publication.11 Pilot-Scale Studies. Pilot-scale studies on NOx reduction by activated lignite chars on simulated and lignite coal combustion-derived flue gas were conducted in a gas-solid contact reactor (NOx reactor) retrofitted into the Conversion and Environmental Process Simulator (CEPS) at EERC. The CEPS unit is designed to produce particulate matter containing flue gas samples to test novel pollution control technologies at the pilot scale. The main components of the CEPS include an electrically heated downdraft entrained-flow combustor, followed by a convective pass section and a downstream baghouse. The CEPS unit is capable of combusting about 0-2 kg/h of pulverized coal, thereby generating about 0-8 scfm of flue gas. The flow through the system is induced by a downstream eductor that maintains the entire system slightly below atmospheric pressure. Details of this pilot scale unit are provided in another publication.22 The material of construction of the NOx reactor and its components was SS 304. The reactor consists of a 6 in. pipe consisting of a SS mesh to support the char sample. The schematic diagram is provided in Figure 2. Weighed quantities of char samples can be fed through the feed port welded to the upper portion of the reactor. Temperatures at various points were monitored using K-type thermocouples. Two key locations for temperature measurement include the inlet reactor temperature (denoted by T1) and the outlet reactor temperature (denoted by T2). The inlet and outlet gas concentrations were monitored by a host of online gas analyzers, connected to a data acquisition system that recorded data at 10-s intervals. Chemiluminescence analyzers were used to monitor NOx, while infrared analyzers tracked the concentrations of SO2, CO, CO2,
Figure 2. Schematic diagram of the NO reduction reactor used in the fixed-bed mode.
and O2 gases. Individual gases were metered using mass flow controllers to maintain a total flow rate of 137 slpm. The flow rates of nitrogen and air were adjusted to obtain the desired inlet oxygen concentration in the flue gas entering the NOx reactor. SO2 and NO gases were delivered using variable-area flowmeters. In all the experiments, the gas mixture was preheated in the entrained-flow section so that the inlet temperature to the NOx reactor could be maintained above 375 °C. Weighed quantities of activated char were then introduced in the hot zone of the reactor at preset time intervals. Changes in the outlet gas concentrations indicated the progress of the reactions. The hot char remaining at the end of each experiment was collected in a SS vessel attached below the NOx reactor. The collection vessel was also purged with nitrogen to prevent any further oxidation of the hot char by ambient air.
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Figure 3. Isothermal reduction of NO by 1000 mg of Luscar char at an inlet reaction temperature of 600 °C.
Results and Discussion Laboratory Studies. Previous studies have demonstrated that coal chars are capable of providing nearly 100% NOx reduction.23,24 The reactivity of activated lignite chars used in the pilot-scale experiments was confirmed in the laboratory scale reactor. Activated Luscar char particles of a 1-2 mm size and weighing 1000 mg were loaded in the reactor at room temperature and brought to 600 °C in pure helium flowing at 1 slpm. After preheating, the flow of gases was switched to a gas mixture consisting of 1000 ppm NO and 2% O2 in helium. The inlet gas temperature to the char bed was maintained at 600 °C throughout the reaction. Figure 3 shows the exit gas concentrations for the reduction of NO by this char. The data in Figure 3 show that under these reaction conditions the outlet NO concentration was 99%) over a substantial time period. The CO concentration, which peaked at 3300 ppm, remained in the 2000-3000 ppm range during the first 60 min. The O2 and CO2 concentrations reach ∼0 and 1.65%, respectively, within the first 2 min and remain at these concentration levels for the next 60 min. This indicates a constant consumption of oxygen during a major portion of the residence time. CO2 formed due to the gas-solid reactions can react with the char downstream of it to form CO. Initially, the relatively high carbon loading leads to the formation of more CO. However, as the reaction proceeds, the carbon mass decreases and the extent of the secondary carbon-CO2 reaction decreases, causing a gradual fall in the outlet CO concentration. However, in reality, CO remains rather steady over a majority of the exposure time of char. Hence, the CO2 concentration does not change appreciably over the major duration of the experiment. The O2 concentration eventually starts rising steadily, attaining 0.05% in the 67th minute and 1.8% by the 74th minute due to the continued loss in char as a result of the various gas-solid reactions that consume char. NO reduction in the 74th minute was 26.2%. In an actual implementation of this CARBONOX process, it will be necessary to maintain a minimum loading of carbon in the reactor to ensure that the desired NOx reduction is maintained indefinitely. The data analysis from this and related experiments also indicated that over the first 60 min of the reaction, 83 mg of NO was reduced while 600 mg of char was consumed, providing an overall consumption of 138 mg of NO reduced/g of char consumed. Under these process
Figure 4. Outlet NO and CO concentration trends for condition 1. Table 3. Summary of the Pilot-Scale Studies condition no. 1 2 3 4
objective of the experiment simulated flue gas, effect of N2 simulated flue gas, effect of 873 ppm SO2 simulated flue gas, effect of 3200 ppm SO2 effect of enhanced sodium loading, actual flue gas
maximum NO reduction, % 47.9 55 >99 90, 98
conditions, it can be concluded that 1000 mg char loading is sufficient to reduce 1.383 mg of NO flowing past it every minute, yielding a rate of 1383 mg of NO/ kg of char/min at the onset of the reaction. At the end of 60 min, 400 mg of char was able to attain the same reduction in NO, yielding a maximum reactivity of 3475 mg of NO/kg of char/min. These experiments also indicate that a high degree of NOx reduction (>99%) can be attained if a sufficient gas-solid contact time is provided. Pilot-Scale Studies. This main objective of pilotscale experiments is to quantify the effect of flue gas components such as N2, CO2, SO2, and H2O on the extent of NOx reduction on a pilot scale. The details of the various experiments carried out in this study are given in Table 3. The objective of each experiment and their corresponding highest extent of NOx reduction attained are also provided in this table. The effect of various flue-gas components was studied sequentially and is detailed in the following experimental runs. The data are plotted as gas concentration against the time of data acquisition. Condition 1: Effect of Nitrogen. Nitrogen is a product of reactions 1 and 2, and there is a possibility that the high concentration of N2 in flue gas might hinder these reactions. The first experiment was conducted to quantify any negative impact of N2 and to identify the temperature range necessary to obtain a high degree of NO reduction on the pilot scale. Data from this experiment are provided in Figures 4 and 5. The gas flows were adjusted at 15:20 to obtain an inlet NO and O2 concentration of 1000 ppm and 0.8%, respectively, to the NOx reduction reactor. This gas mixture, heated in the entrained flow section, was sent through the CEPS reactor system to heat it until the temperatures T1 and T2 reached 441 and 392 °C, respectively. At 15:30, a batch of activated lignite char sample weighing 100 g was added to the preheated
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Figure 6. Outlet NO and CO and inlet and outlet SO2 concentration trends for condition 2. Figure 5. Inlet and outlet O2 and outlet CO2 concentration trends for condition 1.
sorbent bed. The carbon-NO and carbon-O2 reactions started immediately, leading to the evolution of CO and CO2 at the exit of the reactor, as evident from Figures 4 and 5. The reduction in the outlet NO and O2 concentrations also attest to the onset of these reactions. The outlet NO concentration remains at around 900 ppm between 15:31 and 15:41. Oxygen also does not react to a high extent in this time period, probably due to insufficient char loading or a low reaction temperature. The inlet oxygen concentration was slowly increased to 2.56% between 15:41 and 15:52 to enhance the rate of the carbon-O2 reaction, which in turn creates active sites on the carbon surface to react with more NO molecules.11 A second batch of 100 g of char was added at 15:57. Since these 100-g char samples are at room temperature, they momentarily cool the reactor, causing an instantaneous drop in the reaction rates and leading to a downward spike in CO and CO2 concentration. However, the heat released due to the gasification reactions raises the temperature of the second char sample as well. Once the entire batch of char is hot, a higher extent of both reactions takes place, causing a higher reduction in outlet NO and O2 concentrations. Outlet NO and O2 gas concentrations dropped to about 600 ppm and 1.07%, respectively, by 16:20. At this instant, T1 and T2 were 558 and 500 °C, respectively. A further increase in the inlet O2 concentration caused NOx to drop to its minimum outlet concentration of 521 ppm at 16:28, signifying a maximum NOx reduction of 47.9% in this experiment. At the same instant, oxygen dropped from 3.58% to 0.82%, yielding a larger drop of 77.1%. In the last stage of this experiment, the continued gasification and loss of carbon led to the escape of unreacted oxygen and NOx from the reactor. The higher extent of the carbon-oxygen reaction compared to the carbon-NO reaction is in line with the laboratory experimental results described earlier in Figure 3. In both these cases, the continued consumption of char manifested in an earlier escape of NO from the char bed, compared to oxygen. This experiment confirms the scaleup capability of the CARBONOX process and indicates the inert nature of nitrogen toward the carbon-NO reaction. While the temperature and the oxygen concentration used in the pilot scale are slightly different from those of the laboratory experiment, the rate of the carbon-NO reaction was of the same order of magnitude. Even though the exact mass of char loading in the NOx reactor at 16:20 is difficult to measure or estimate,
Figure 7. Outlet CO2 and inlet and outlet O2 concentration trends for condition 2.
the rate of the carbon-NO reaction is estimated to be in the 354-1416 mg of NO/kg of char/min range, assuming that the char loading was about 50-200 g at that instant. Condition 2: Effect of Low Concentration of SO2. The second experiment was aimed at quantifying any adverse effect of low concentration of SO2 on NOx reduction. SO2 may be a barrier to the catalysis by sodium because it can potentially react with sodium to form sodium sulfite/sulfate, thereby rendering it unavailable for the further catalysis of the carbon-NO and carbon-O2 reactions. This experiment involved setting up a baseline NOx reduction and then introducing SO2 to the inlet gas mixture. The data obtained from this experiment are provided in Figures 6 and 7. An inlet NO and O2 concentration of 1000 ppm and 2.2% was maintained throughout the experiment. The addition of 550 g of lignite char at 14:59 initiated the gas-solid reactions, causing the NOx and oxygen concentration to drop to around 650 ppm and 0.2%, respectively, by 15:33. SO2 gas was then injected into the system incrementally until the inlet SO2 concentration reached about 190 ppm by 16:02. It can be observed that the outlet SO2 concentration was significantly below that at the inlet. We can also see from Figures 6 and 7 that the introduction of SO2 in the inlet gas stream caused an increase in CO formation, while both outlet CO2 and O2 concentrations fell. At this SO2 level, there was no perceptible difference in the extent of NOx reduction as the outlet NOx concentration rose from 650 ppm to only 700 ppm upon the introduction of SO2. The SO2 gas flow
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Figure 8. Inlet and outlet NO, outlet CO, and inlet and outlet SO2 concentration trends for condition 3.
Figure 9. Inlet and outlet temperatures for condition 3.
was continuously increased to attain an inlet concentration of 873 ppm, and the trend in NOx reduction was maintained. Under this char loading and the reaction conditions chosen, NOx reduction was maintained at about 40% for the majority of the duration of the experiment, peaking at 55%. This experiment confirms that while SO2 plays an undetermined role in the reactions, it does not lead to any adverse effect on NOx reduction. Condition 3: Effect of Higher Concentration of SO2. The third experiment aimed at quantifying the highest extent of NOx reduction possible and determining the effect of higher concentration of SO2 on NO reduction by lignite char. This experiment was carried out to study the NOx reduction by lignite char under a SO2 concentration range typical of flue gas generated by the combustion of high-sulfur coals. Activated Luscar char sized to -8 + 20 mesh was used in this experiment. The pressure drop across the reactor was between 1.5 and 2.35 in. of water column (385-587.5 Pa) throughout the experiment. The gas concentration data are shown in Figure 8 and the inlet and outlet temperatures (T1 and T2) data are provided in Figure 9. The experiment lasted more than 200 min. The inlet O2 and NO concentration were maintained in the 2.51-2.82% and 970-980 ppm range throughout the experiment. Initially, the inlet reactor temperature, T1, is about 485 °C, and the exit temperature, T2, is slightly lower at 430 °C. Prior to the addition of char, outlet CO and CO2
concentrations remained below the detection level of the analyzers. The first batch of 100 g of char was added at 14:26. Data in Figure 9 show that the outlet temperature, T2, drops to 378 °C at 14:32. This is due to the addition of char that was initially at room temperature. Besides the drop in T2, Figure 8 indicates the formation of CO, signaling the onset of the gas-solid carbon-NO and carbon-O2 reactions. Heat liberated from these exothermic reactions causes the outlet temperature to rise even beyond the inlet temperature. Five subsequent additions of 100-g char samples took place at 14:38, 14:45, 14:58, 15:13, and 15:50. Each addition of a char batch leads to a momentary decrease in the outlet temperature T2, thereby lowering the rate of the gasification reactions, resulting in a fall in the outlet CO concentration and a simultaneous rise in the outlet NO concentration. After the addition of the fifth batch of char, the outlet NO concentration falls below 10 ppm starting at 15:16, signifying >99% NOx reduction, as is evident from Figure 8. In the presence of NO and O2 in nitrogen, the CO concentration peaked at 3300 ppm when the temperatures T1 and T2 were at 560 and 615 °C, respectively. This is similar to the peak CO concentration obtained during laboratory testing on Luscar char at a 600 °C inlet temperature as shown in Figure 3. The similarity in the CO concentration trends could be due to similar operating temperatures in the two cases, indicating that the char-NO reaction occurs at a similar rate at pilot scale as well. Having established >99% NOx reduction, the effect of a higher concentration of SO2 is studied, starting with its gradual addition at the inlet of the NOx reactor beginning at 16:10 and its maintenance at 412 ppm until 16:35. The outlet NOx concentration continued to be 80%) was maintained even when the inlet SO2 concentration reached 3600 ppm in the inlet. These gasification reactions ensured that sufficient heat was liberated in the system to maintain the outlet temperature about 100-120 °C higher than the inlet temperature, as evident from Figure 9. This energy, being relatively high grade (T1 and T2 being 550 and 650 °C, respectively, over most of the reaction duration) has potential to be harnessed and integrated into power plant operations. The viability of the CARBONOX process for its use in high-sulfur-coal-based combustion sources is also enhanced on the basis of this experimental data. Condition 4: Effect of Enhanced Sodium Content and NOx Reduction on Actual Flue Gas. Previous experiments demonstrate that while NOx reduction can exceed 99%, the CO concentration usually remains high, often exceeding 3000 ppm. Numerous studies have correlated a lower CO/CO2 ratio with lower gasification temperature, indicating a need to reduce the reaction temperature. One aim of this experiment was to reduce the operating temperature of the reactions through the use of activated lignite char, whose sodium
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Figure 12. Inlet and outlet temperatures for condition 4. Figure 10. Inlet and outlet NO concentration trends for condition 4.
Figure 11. Outlet CO and inlet and outlet SO2 concentration trends for condition 4.
content was increased by contacting it with a sodium hydroxide solution in a procedure similar to our earlier experiments.25 The sodium content of char increased from 0.0147 to 0.0277 g of Na/g of char by impregnating the char in an aqueous solution of sodium hydroxide, followed by drying the char. Whereas the maximum inlet temperature for the previous experiment was 556 °C, it was only 484 °C in this experiment. The second aim of this experiment was to study the NOx reduction on actual flue gas generated by the combustion of coal, rather than the simulated flue gas used in previous experiments. The use of actual flue gas introduces moisture and high CO2 concentration in the inlet gas mixture. Moisture and CO2 can react with carbon as mentioned in the Introduction section, and this reaction has been exploited to activate these lignite chars, albeit at a higher temperature (750 °C). Experimental data on the gas concentrations and temperatures are provided in Figures 10-12. Under these conditions, Figure 10 indicates that in the first part of the experiment, 700 g of lignite char was able to achieve 90% NOx reduction by lowering NOx from 1200 to 120 ppm from 10:36 to 11:30 and to achieve 75% NOx reduction from 11:30 to 13:00. Figure 11 shows that CO concentration peaked at 2230 ppm and remained below 1400 ppm for 80 min after 12:30. This data set shows that the use of a higher concentration of sodium in char successfully lowers CO emission, while maintaining a high extent of NOx reduction. For the second part of the experiment, the flow of inlet gases was switched to initiate actual coal combustion. Powdered lignite coal was fed to the CEPS unit by a screw feeder. This raised the inlet/outlet CO2 concentra-
tion in the neighborhood of 15%, which was well above the 3-4% observed in previous experiments. The feed rate of coal and the air required for its combustion were slowly increased over 30 min starting at 14:00. The oxygen concentration entering the NOx reactor was maintained in the 3-3.75% range throughout the second phase of the experiment. The inlet NO concentration to the NOx reactor due to coal combustion was in the 740-760 ppm range between 14:25 and 15:26 (Figure 10). The inlet SO2 concentration varied with increasing coal feed rate but reached a steady level of about 700 ppm when the coal feed reached a steady state. This is because this lignite coal is characterized by lower sulfur content than the high-sulfur bituminous coal mined in the eastern part of the United States. Hence, SO2 concentration in the flue gas is not as high as that used in the previous experiment. Under these process conditions, outlet NOx concentration dropped to about 15 ppm, indicating a NOx reduction of 98%. The outlet CO concentration fell from >5000 ppm at about 14:40 to about 1700-2200 ppm toward the latter part of the experiment. However, the inlet and outlet oxygen concentrations remained steady throughout the course of the latter part of this experiment. Hence, the drop in CO is not due to the drastic reduction in char loading as a result of a high extent of char consumption. While the exact reason behind this drop in CO concentration cannot be unequivocally established based on this study, the trend in CO concentration mimics the laboratory data shown in Figure 3. The drop in CO toward the end of the experiment parallels the breakthrough of NO from the reaction bed in both cases. The hypothesis used to explain the falling CO trend in Figure 3 can also explain the falling CO concentration in this pilot experiment. A lower extent of carbon-CO2 reaction occurs as carbon is progressively consumed with increasing exposure time due to its gasification. In both cases, Figure 12 indicates that the inlet temperature remained in the 356-482 °C range, while the outlet temperature continued to be significantly higher. The heat released was significant in this experiment; the maximum increase in temperature being 189 °C during the first phase and about 160 °C for the period when actual coal combustion testing was carried out. The high performance of char toward NO reduction in this experiment indicates that neither CO2 nor moisture are barriers for reduction of NO by lignite char. Conclusions It can be concluded from these experiments that activated lignite chars, characterized by high sodium
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content, are capable of attaining a high degree of NOx reduction. Under suitable process conditions, these chars can reduce NOx to single-digit ppm levels on both simulated and actual flue gas. The outlet temperature of flue gas was higher than that of the inlet by about 50-180 °C. This heat release, being of a reasonably high quality, is amenable for harnessing and on-site use. The large quantity of heat liberated can also potentially lower operating costs associated with the CARBONOX process. Although an interaction between the SO2 and the char constituents or its conversion to another gaseous sulfur species can be suspected, the presence of SO2 did not adversely affect the extent of NOx reduction, even at a concentration as high as 3600 ppm. Enhancing the sodium content of lignite char successfully lowered the NOx reduction temperature by 40-80 °C while maintaining a 90-98% NOx reduction level. The experiments carried out in this study provide us with information that could be used for scaling up this reaction system to a commercial-scale facility. A 500-g char loading is able to reduce NO from 1000 ppm to
