2038
Energy & Fuels 2007, 21, 2038-2043
Effect of Mineral Matter on NO Reduction in Coal Reburning Process Zhihua Wang, Junhu Zhou,* Zhengcheng Wen, Jianzhong Liu, and Kefa Cen State Key Laboratory of Clean Energy Utilization, Institute for Thermal Power Engineering, Zhejiang UniVersity, Hangzhou 310027, Zhejiang, China ReceiVed October 2, 2006. ReVised Manuscript ReceiVed March 24, 2007
The effect of mineral matter on the NO reduction efficiency during the coal reburning process was studied in an entrained flow reactor. Apart from the homogeneous reactions between hydrocarbon radicals and NO, the heterogeneous reaction between NO and char is also important in the coal reburning process. The discrepancy in NO reduction between two kinds of bituminous coals with similar volatile matter and inherent nitrogen contents was investigated. Shenhua coal enriched with Na, K, Ca, and Fe in ash has more than a 15% predominance over the Huainan coal. The mineral matters in the coal ash were studied by the demineralization and reimpregnation process. The original coal, demineralized coal, and impregnated coal were evaluated as reburning fuels. Results show that the mineral matter in the coal has great influence on the NO reduction efficiency during the coal reburning process. The additional mineral matter can greatly improve the NO reduction efficiency of the coal reburning process. The effective sequence of the mineral matter is Na > K > Fe > Ca. The corresponding CO concentration in flue gas validated the catalyst mechanism of metal additives on NOchar reactions.
1. Introduction The emissions of nitrogen oxide during coal combustion pose serious problems for coal-fired power plants which need to comply with increasingly stringent NOx emission legislation. This has been the driving force behind the development and commercialization of NOx control methodologies involving modifications to the combustion process itself such as low NOx burners, over fire air, flue gas recirculation, and reburning technology and the application of post-combustion flue gas cleanup technology such as SNCR/SCR.1-4 Reburning is a promising NOx control technology for high efficiency and low cost especially for retrofit applications, which has been demonstrated in laboratory reactors and in full-scale boilers.5,6 In reburning, NO reduction occurs within the furnace by injection of a second fuel downstream of the primary combustion zone. The reburning fuel creates a local fuel-rich zone where NO can be reduced by hydrocarbon radicals and chars. Air called over fire air (OFA), which is injected downstream of the primary combustion zone, burns out the * Corresponding author. Tel.: +86-571-87951668. Fax: +86-57187951616. E-mail:
[email protected]. (1) Nimmo, W.; Patsias, A. A.; Williams, P. T. Enhanced NOx reduction with SO2 Capture under Air-Staged Conditions by Calcium Magnesium Acetate in an Oil-Fired Tunnel Furnace. Energy Fuels 2006, 20, 18791885. (2) Radojevic, M. Reduction of Nitrogen Oxides in Flue Gases. EnViron. Pollut. 1998, 102 (S1), 685-689. (3) Wendt, J. O. L.; Linak, W. P.; Groff, P. W.; Srivastava, R. K. Hybrid SNCR-SCR Technologies for NOx Control: Modeling and Experiment. AIChE J. 2000, 47 (11), 2603-2617. (4) Zabetta, E. C.; Hupa, M.; Saviharju, K. Reducing NOx Emissions Using Fuel Staging, Air Staging, and Selective Noncatalytic Reduction in Synergy. Ind. Eng. Chem. Res. 2005, 44, 4552-4561. (5) Smoot, L. D.; Hill, S. C.; Xu, H. NOx Control through Reburning. Prog. Energy Combust. Sci. 1998, 24, 385-408. (6) Hampartsoumian, E.; Folayan, O. O.; Nimmo, W.; Gibbs, B. M. Optimisation of NOx Reduction in Advanced Coal Reburning Systems and the Effect of Coal Type. Fuel 2003, 82, 373-384.
reburning fuel. Gaseous hydrocarbon fuels are usually more attractive than pulverized coal as reburning fuels for higher NOx reduction efficiency and lower retrofit cost. Additionally, there is no serious burnout problem. However, for most coal-fired utility boilers, pulverized coal is always the preferred choice for its being on-site and having a low operating cost. But it is usually difficult to achieve the current Chinese NOx emission standards of 450 mg/Nm3 (at 6% O2 conditions) not to say 250 mg/Nm3 in some strict areas such as Beijing and so forth. It is difficult to achieve 50% NOx reduction efficiency by the original coal reburning technology. Therefore, many studies have been conducted to improve the NOx reduction efficiency by coal reburning technology. The mechanism of coal reburning is more complicated than that of reburning with gaseous hydrocarbon fuels, due to the heterogeneous character of coal combustion and effects of coal rank, ash composition, and fuel-bound nitrogen content. Highly volatile coals with a low nitrogen content are usually considered effective reburning fuels.7 But Liu et al.8 found that, under fuelrich conditions, coals with rich nitrogen content could increase the NO reduction efficiency, whereas at fuel-lean reburning zone conditions, the increasing fuel N might be marginal or adverse. With three kinds of U.K. bituminous coals as reburning fuel, the volatile played a predominant role in the NO reduction compared with char. But Chen and Ma9 found that the heterogeneous mechanism between NO and char was more important than the homogeneous mechanisms when lignite was used. Bituminous coal char impregnated with CaO could achieve higher NO reduction efficiency than the original. The fresh(7) Maly, P. M.; Zamansky, V. M.; Loc Ho, R. P. Alternative Fuel Reburning. Fuel 1999, 78, 327-334. (8) Liu, H.; Hampartsoumian, E.; Gibbs, B. M. Evaluation of the Optimal Fuel Characteristics for Efficient NO Reduction by Coal Reburning. Fuel 1997, 76 (11), 985-993. (9) Chen, W. Y.; Ma, L. Effect of Heterogeneous Mechanisms during Reburning of Nitrogen Oxide. AIChE J. 1996, 42 (7), 1968-1976.
10.1021/ef0604902 CCC: $37.00 © 2007 American Chemical Society Published on Web 05/08/2007
NO Reduction in Coal Reburning
made “young” char had more activity than the premade “old” char.10 Garcia et al.11,12 found that potassium-containing briquette coal could reduce NO efficiently by catalyzing the carbon-NO reaction. Zhong et al.13 investigated the kinetic parameters between NO-char reactions by adding KOH as a catalyst. Zhao et al.14,15 found that adding some Ca-Fecontaining catalysts to the char particles can greatly improve the NO reduction efficiency over coal chars. Gomez et al.16-20 fond that the NO reduction process over coal char could be greatly improved by using K, Ca, Fe, and various transition metals as catalysts. However, most of the catalytic studies for NO reduction are performed in fixed-bed reactors with prepared devolatilized coal chars. The overall NOx reduction efficiency by coal reburning with mineral matter or catalyst added should be studied in more practical flow reactors. This paper investigates the influence of inherent and extra added mineral matter on NO reduction in coal reburning in an entrained-flow reactor. First, three kinds of coals were evaluated as the reburning fuel for NO reduction. The discrepancy between the two kinds of bituminous coal with similar volatile content was investigated in detail by ash analysis. After that, the effect of several kinds of mineral matter such as Na, K, Ca, and Fe in the coal ash was studied by the demineralization and reimpregnation process. Finally, the effectiveness of the impregnated mineral matter on NO reduction efficiency was studied and compared in detail.
Energy & Fuels, Vol. 21, No. 4, 2007 2039
Figure 1. A schematic diagram of the entrained flow reactor for coal reburning.
2. Experimental Section 2.1. Experimental Setup. Reburning experiments were carried out in an electric heated entrained-flow reactor as shown in Figure 1. The main feature of the reactor is a corundum tube with an inner diameter of 50 mm and an overall length of 1.0 m. The center flow tube was heated by an outside electric heating element which is a 1.2-m-long spiral silicon carbide tube (Shandong Bajian Co.). The inner temperature of the reactor can achieve up to 1400 °C. A retrofitted gas burner was used to generate the primary combustion zone products by burning a petrogas/NH3 gas mixture. Part of the combustion products with appropriate oxygen and NO levels was (10) Chen, W. Y.; Lin, T. Variables Kinetics and Mechanisms of Heterogeneous Reburning. AIChE J. 2001, 47 (12), 2781-2797. (11) Lopez, A. B.; Garcia, A. G. Combined SO2 and NOx Removal at Moderate Temperature by a Dual Bed of Potassium-Containing Coal-Pellets and Calcium-Containing Pellets. Fuel Process. Technol. 2005, 86 (16), 1745-1759. (12) Lopez, A. B.; Garcia, A. G.; Lecea, C. S. M. d.; McRae, C.; Snape, C. E. Low-Cost Potassium-Cotaining Char Briquettes for NOx Reduction. Energy Fuels 2002, 16, 997-1003. (13) Zhong, B. J.; Zhang, H. S.; Fu, W. B. Catalytic Effect of KOH on the Reaction of NO with Char. Combust. Flame 2003, 132, 364-373. (14) Zhao, Z.; Qiu, J.; Li, W.; Chen, H.; Li, B. Influence of Mineral Matter in Coal on Decomposition of NO over Chars and Emissions of NO during Char Combustion. Fuel 2003, 82, 949-957. (15) Zhao, Z.; Li, W.; Li, B. Catalytic Reduction of NO by Coal Chars Loaded with Ca and Fe in Various Atmospheres. Fuel 2002, 81, 15591564. (16) Garcia, A. G.; Gomez, M. J. I.; Solanom, A. L.; Lecea, C. S. M. d. NOx Reduction by Potassium-Containing Coal Briquettes. Effect of Preparation Procedure and Potassium Content. Energy Fuels 2002, 16, 569574. (17) Garcia, A. G.; Gomez, M. J. I.; Solanom, A. L.; Lecea, C. S.-M. d. Potassium-Containing Briquetted Coal for the Reduction of NO. Fuel 1997, 76 (6), 499-505. (18) Gomez, M. J. I.; Brandan, S.; Lecea, C. S. M. d.; Solano, A. L. Improvements in NOx Reduction by Carbon Using Bimetallic Catalysts. Fuel 2001, 80, 2001-2005. (19) Gomez, M. J. I.; Brandan, S.; Solano, A. L.; Lecea, C. S. M. d. NOx Reduction by Carbon Supporting Potassium-Bimetallic Catalysts. Appl. Catal., B 2000, 25, 11-18. (20) Gomez, M. J. I.; Pinero, E. R.; Garci, A. G.; Solano, A. L.; Lecea, C. S. M. d. Catalytic NOx Reduction by Carbon Supporting Metals. Appl. Catal., B 1999, 20, 267-275.
Figure 2. Temperature distribution in the reactor at 830 °C.
pumped through a water-cooled probe into the upper zone of the reactor. A screwed coal feeder with the assistance of air can deliver coal particles into the reactor steadily. The coal particles just act as the reburning fuel. Along with the feeded reburning coal particles, the flue gas goes down through the reactor. The whole reactor just simulates the reburning zone without additional OFA added. The reburning heat input was determined by the ratio of flue gas volume between ideal reburning coal combustion products and the overall flue gas in the reactor. The gas compositions of O2, NO, N2O, NO2, CO2, CO, and SO2 were monitored by continuous emissions monitoring systems (Rosemount Analytical NGA2000, Emerson Process Management Co., Ltd.). Figure 2 is the temperature distribution profile in the reactor along the center line. The temperature profile is a traditional trapezoidal distribution with a center uniform temperature range from 40 to 70 cm. The 830 °C in the figure caption is the display value in the temperature-controlling device when the temperature is calibrated. Most of the experiments were conducted at a temperature of 1200 °C. Coals were fed at 1 g/min with same weight of sands as feeding assistant materials. The purpose of adding sand is to improve the stability of the coal feeding rate through the screwed feeder. The resident time is about 0.75 s in most of the testing cases in this paper. Figure 3 shows the gas composition of the simulated primary combustion zone products generated in the gas burner. In the continuous 4.5 h, the contents of oxygen and NO are relatively stable generally speaking, which can match the experimental values needed. The time required for one case is only 5-10 min. The initial NO concentration was set to around 500 ppm (converted to 6% oxygen conditions) in every case. At the beginning and ending of each testing case, the gas compositions were recorded and averaged. The overall NO reduction efficiencies were determined by the average initial NO concentration and
2040 Energy & Fuels, Vol. 21, No. 4, 2007
Wang et al.
Table 1. Proximate and Ultimate Analysis of the Coals proximate analysis ad (wt, %)
ultimate analysis ad (wt, %)
coal type
moist.
ash
volatile matter
fixed carbon
C
H
N
S
O
SH coal HN coal JC coal
6.39 0.69 0.52
14.55 29.65 22.16
26.93 25.45 7.32
52.13 44.21 70.00
62.8 60.74 68.98
3.93 3.82 3.02
0.91 1.13 1.09
0.41 0.35 0.43
11.01 3.62 3.80
Table 2. Results of the Demineralization Process proximate analysis (wt, %) coal type
moist.
ash
volatile matter
fixed carbon
SH SH-dem
6.39 2.89
14.55 0.96
26.93 21.05
52.13 75.10
Table 3. Ash Content of the SH and HN Coals
SH coal HN coal
Fe2O3 %
MgO %
CaO %
TiO2 %
Al2O3 %
SiO2 %
K2O %
Na2O %
3.08 0.88
0.64 0.48
2.81 1.09
0.46 0.26
29.28 32.23
37.68 49.07
6.17 1.87
2.08 1.08
average NO concentration in the exit of the reactor. The NO reduction efficiency is defined as ηNO )
[NO]0 - [NO] 100 [NO]
(1)
where ηNO is the NO reduction efficiency, [NO]0 is the average initial NO concentration (including the effect of dilution by adding coal carrier gas), and [NO] is the NO concentration at the reactor exit during reburning. The stoichiometric ratio λ is defined as the ratio between the oxygen content in flue gas (including the coal carrier gas, O2, and CO in the simulation flue gas) and the oxygen needed for complete combustion of the reburning coal: λ)
0.21V1 + ([O2] - 0.5[CO])V2 0.21V0
(2)
where V0 is volume of air needed for the complete combusiton of reburning coal, V1 is the amount of coal carrier air, V2 is the amount of simulation flue gas, and [O2] and [CO] are the oxygen and carbon monoxide contents in the simulation flue gas. 2.2. Demineralization and Reimpregnate Procedure. Three coals were selected as the samples for this study with different coal ranks, volatile matter, ash contents, and so forth. All of the coals were air-dried, ground, and sieved to a size of 54∼77 µm. The proximate and ultimate analyses of the coal samples are listed in Table 1, where all of the coals are represented as the corresponding code names throughout the paper. The selected coals are typical Chinese coals widely used in the power plants: SH means the Shenhua coal, HN means the Huainan coal, and JC means the Jincheng coal. The SH coal was selected for the demineralization and was reimpregnated with extra mineral matter. In every case, about 30 g of the sample was first drenched with 100 mL of deionized water in a plastic bottle, and then 250 mL of 36∼38% HCl (Hangzhou Chemistry Reagent Co., Ltd.) was added to the sample. The mixture was stirred with a magnetic stirrer for 3 h, and the mixture was kept at a temperature of 80∼90 °C. After natural cooling, the sample was separated from the solution by filtration and washed with deionized water. After that, the sample was drenched in 100 mL of deionized water and 250 mL of 40% HF (Hangzhou Yingguang Chemical Engineering Co., Ltd.) solutions. The procedure was the same as the HCl treatment. Finally, the demineralized sample was washed with deionized water repeatedly to remove the remaining Cl- and F- ions with 0.1 M of a HgNO3 solution as an indicator. After that, the samples were dried in a vacuum oven at 60 °C for 3 h. The suffix “dem” is the added to the code name to denote the demineralized coal. This demineralization process can remove more
than 90% of the inherent coal ash, which can be found in Table 2 with the proximate analysis. The ash content in the final SH-dem coal can be controlled below 1%. Extra mineral matter was added to SH-dem coals with different concentrations of KOH, NaOH, Fe2(SO4)3, and Ca(OH)2 solutions. The actual values of Fe, Ca, K, and Na in the original coal were Fe2O3, 0.45%; CaO, 0.4%; K2O, 0.9%; and Na2O, 0.3%. Therefore, the content of extra mineral matter added to the SH-dem coal was designed as 1%, 2%, and 3% in this work. Certain concentrations of mineral matter solutions were first made with deionized water. After that, the coal samples were added and stirred with the solutions. Finally, the samples were dried in a vacuum oven at 60 °C for more than 12 h.
3. Results and Discussion 3.1. Reburning Process by Different Coals. Three kinds of typical coals including two kinds of bituminous and one kind of anthracite were first selected for evaluation of NO reduction efficiency by a traditional coal reburning process. JC coal is a kind of anthracite with a volatile matter content smaller than 10%. SH coal and HN coal are two kinds of bituminous with similar volatile contents of 26.93% and 25.45%, respectively. The inherent nitrogen contents are similar for the three coals, which are all around 1%. Generally, coals with high volatile matter and low nitrogen contents are believed to be good reburning fuels. Figure 4 shows the NO reduction efficiency of these coals at different reburn zone stoichiometric ratios λ. It is obvious that the NO reduction efficiencies are all improved with the decreasing of the reburn zone stoichiometric ratio λ. The anthracite JC coal can also achieve a 25.8∼28.5% NO reduction efficiency, which is not sensitive to a change of λ. Compared with JC coal, bituminous SH and HN coals are relatively good reburning fuels which can achieve 30.6∼50% NO reduction efficiency but are sensitive to a change of λ. The homogeneous reactions between NO and hydrocarbon fragments are strongly correlated with the reburn zone atmosphere. Under fuel-rich conditions, the hydrocarbons will still first react with oxygen rather than reducing NO into N2 in the local area of coal particles. Whereas, the heterogeneous reactions between NO and char are little influenced by the reburn zone atmosphere. From this point of view, the NO reduction by JC coal is mainly controlled by heterogeneous reactions between NO and char, which means the heterogeneous mechanism is important under some conditions. The SH and HN coals with similar volatile and inherent fuel-N contents should have similar performances under the same reburning conditions. But from Figure 4, we found that the SH coal is preponderant versus HN coal by about a 5% NO reduction efficiency under all the reburn zone stoichiometric ratio conditions. The SH coal can have more than a 15% improvement in NO reduction efficiency over the HN coal. It is interesting and meaningful to find out the fundamentally different reasons. The main reason may be the difference of heterogeneous reactions between NO and char, which can be catalyzed by the mineral matter in the coal ash. Therefore, the ash compositions of the two coals were analyzed and listed in Table 3. From Table 3, we found that the Fe, Ca, K, and Na
NO Reduction in Coal Reburning
Figure 3. Gas composition of the simulated primary combustion zone products.
Figure 4. NO reduction efficiency by different coal types in coal reburning.
Figure 5. NO reduction efficiency influenced by K additions in coal reburning.
mineral matter are more enriched in SH coal than HN coal, which may be the reason for the different performance. So, in the follow studies of this work, the SH coal was selected for demineralization and additional K, Na, Ca, and Fe were reimpregnated into the SH-dem coal samples. 3.2. Effects of Reimpregnate K, Na, Ca, and Fe. The original SH coal, SH-dem coal, and 1∼3% (w/w) K, Na, Ca, and Fe impregnated SH-dem coals were selected and compared as reburning fuels for NO reduction. Figure 5 shows the influence of K additive on the NO reduction efficiency. The performance of SH-dem coal is the most ineffective one compared with others, even the original SH coal, when the mineral matter in coal ash is removed. It is obvious that the NO reduction efficiency is greatly improved when amounts of K are reimpregnated into the SH-dem coal. With an increasing amount of K added, the NO reduction efficiency is also
Energy & Fuels, Vol. 21, No. 4, 2007 2041
Figure 6. NO reduction efficiency influenced by Na additions in coal reburning.
Figure 7. NO reduction efficiency influenced by Ca additions in coal reburning.
Figure 8. NO reduction efficiency influenced by Fe additions in coal reburning.
improved, especially from 2% to 3% where about a 30% increase is observed. Compared with the original SH coal’s 36.2%, about 51.9% of a NO reduction efficiency can be achieved by SH-dem-K-3% coal at a reburn zone stoichiometric ratio λ ) 1.2. The heterogeneous reactions between NO and char are greatly enhanced by the added K mineral matter. Figure 6 shows the influence of a Na additive on the NO reduction efficiency and as compared with that for SH and SHdem coals. It is a little different with K; only a 1% Na additive can greatly improve the performance of coal reburning. The difference of NO reduction efficiency between 1%, 2%, and 3% is moderate and gentle. About 73.32% NO reduction efficiency can be achieved at λ ) 1.0 by SH-dem-Na-3% coal compared with 44.12% by original SH coal. Figure 7 shows the effect that a Ca additive has on NO reduction in the coal reburning process. Ca is a kind of metal ion popular for its ability to remove SO2 in the furnace. Although in this paper, this is
2042 Energy & Fuels, Vol. 21, No. 4, 2007
Wang et al.
Figure 9. NO reduction efficiency by different mineral matter additives in coal reburning. The amounts of additives are (a) 1% w/w, (b) 2% w/w, and (c) 3% w/w.
not a matter of concern, but it is well-known for SO2 absorption by CaCO3 or CaO sorbent in furnace applications.21 The performance of Ca is obviously different from that of others. The demineralized SH-dem coal is less effective than the original SH coal. But the 1% Ca additive condition is not as good as those of K and Na, even less effective than the original SH coal. With 2% Ca added to the SH-dem coal, the NO reduction efficiency can be greatly improved from 47.54 by SH coal to 73.49% by SH-dem-Ca-2% at λ ) 0.8. At 3% Ca added conditions, the performance of reburning has a great turn, sharply decreasing from the top performance at 2% to the worst conditions, even less effective than the demineralized SH-dem coal. The fundamental reason should be investigated in the future. Figure 8 shows the influence of Fe additive on the NO reduction efficiency in the coal reburning process. Like the performance of K, the NO reduction efficiency keeps improving with increasing amounts of Fe added. But when Fe e 1%, the performance is still less effective than the original SH coal. At 3% Fe conditions, the NO reduction efficiency can also achieve 86.15% at λ ) 0.6, as well as those of K and Na. 3.3. Comparison of the Added Mineral Matter. Figure 9 compares the performance of NO reduction by different mineral matter additives at the same dosage. Figure 9a shows results under conditions of 1% w/w additives at different reburn zone stoichiometric ratios λ. As observed, 1% Na and K impregnated coals can greatly improve the NO reduction efficiency over the original SH coal. Na is the most active mineral matter additive. Only small amounts of additives can effectively improve the
NO reduction efficiency by catalyzing the NO-char heterogeneous reactions. Fe and Ca are always less effective than the original SH coal at small amounts of additives (1%). Figure 9b shows results under 2% additive conditions. All the impregnated coals have better performance than the original SH coal. Na and Ca are the most effective ones at all of the λ conditions. Fe and K are sensitive to changes in the reburn zone stoichiometric ratio. Figure 9c shows results under 3% additive conditions. Na, K, and Fe all have good performance, which means that the NO reduction efficiency by the impregnated coal reburning process can be greatly improved. But, Ca at 3% is special, as mentioned above, which is less effective than the original coal. The overall effective sequence of the mineral matter can be generally listed as Na > K > Fe > Ca. The performances of Na and Ca are relatively stable with the change of λ. Whereas, K and Fe are sensitive to the reburn zone atmosphere. Ca has an optimized addition dosage. But additives of K and Na may increase the possibility of sintering and fouling in the boiler, which still need to be evaluated for the used dosage. 3.4. Mechanism of the Catalysis Reburning Process. The above results show that the mineral matter in the coal ash and additional mineral matter additives can greatly improve the NO reduction efficiency of the coal reburning process by catalyzing the NO-char heterogeneous reactions. The catalysis mechanism has been widely studied. Lee and Park22 believed that there must be a chemical reaction cycle. First, NO is absorbed at the active site of the char surface; the metal ions can rob the O atom from the NO molecule, resulting in the formation of metal oxide and
(21) Cheng, J.; Zhou, J.; Liu, J.; Zhou, Z.; Huang, Z.; Cao, X.; Zhao, X.; Cen, K. Sulfur Removal at High Temperature during Coal Combustion in Furnaces: A Review. Prog. Energy Combust. Sci. 2003, 29, 381-405.
(22) Lee, Y. W.; Park, J. w. Studies on the Surface Chemistry Based on Competitive Adsorption of NOx-SO2 onto a KOH Impregnated Activated Carbon in Excess O2. EnViron. Sci. Technol. 2002, 36, 4928-4935.
NO Reduction in Coal Reburning
Energy & Fuels, Vol. 21, No. 4, 2007 2043
concentrations are increased corresponding with the improvement of NO reduction efficiency at different levels of K additives, which can prove the catalysis mechanism discussed above. 4. Conclusion
Figure 10. Change of CO concentration in the coal reburning process.
an N radical. The metal oxide can be subsequently reduced into metal ions by the char. Therefore, the metal ions are recovered and keep on taking part in the chemical reaction cycles. The N radicals in the char surface will be combined together to form N2 molecules, so the NO can be reduced continuously with the catalysis effect of metal ions. But the mechanism of Ca additives is still unknown. The chemical reaction cycles such as those for K can be listed as
K+ + NO ) KO- + N
(3)
The overall reaction is
KO- + C ) K+ + CO
(4)
N + N ) N2
(5)
2NO + 2C ) 2CO + N2
(6)
From the above reactions, we found that, if the heterogeneous reactions are catalyzed by the metal ions, the concentration of CO in the flue gas should be increased. Figure 10 shows the NO reduction efficiency and CO concentrations in the flue gas at three levels of K additives from 1-3%. Obviously, the CO
The effect of coal-ash-inherent mineral matter and extra additives on the NO reduction efficiency by the coal reburning process has been evaluated by the demineralization and reimpregnation process in an entrained-flow reactor. The anthracite JC coal can also have 25.8-28.5% NO reduction efficiency, which mainly contributes to the heterogeneous reactions. The difference of SH and HN bituminous coals with similar volatile and inherent fuel-N contents on the NO reduction efficiency by coal reburning was investigated in detail. The coal ash content was compared between SH and HN coals by ash analysis. The enrichment of Na, K, Ca, and Fe in SH coal ash may be the reason forcatalysis of the NO-char heterogeneous reactions. The SH coal was demineralized and reimpregnated with about 1∼3% (w/w) Na, K, Ca, and Fe additives. The original SH coal, SH-dem coal, and impregnated coal were used as reburning fuel for the NO reduction under the same conditions. Compared with the original SH coal, the demineralizaed SH-dem coal is always less effective in all of the tests. The addition of mineral matter can greatly improve the performance of coal reburning. The effective sequence of the mineral matter is Na > K > Fe > Ca. The performances of Na and Ca are relatively stable with the change of λ. Whereas, K and Fe are sensitive to the reburn zone atmosphere. Ca has an optimized addition dosage. The corresponding CO concentration in the test confirms the theory of a metal catalyst reaction mechanism. Acknowledgment. The authors express their acknowledgment of the financial support of the National Natural Science Foundation of China (50476059), Key Project of Chinese National Programs for Fundamental Research and Development (2006CB200303), and National Science Foundation for Distinguished Young Scholars (50525620). EF0604902
