Combustion Process and Nitrogen Oxides Emission of Shenmu Coal

2548

Energy & Fuels 2007, 21, 2548-2554

Combustion Process and Nitrogen Oxides Emission of Shenmu Coal Added with Sodium Acetate Yang Weijuan,* Zhou Junhu, Liu Maosheng, Zhou Zhijun, Liu Jianzhong, and Cen Kefa State Key Laboratory of Clean Energy Utilization, Institute for Thermal Power Engineering, Zhejiang UniVersity, Hangzhou 310027, Zhejiang, P.R. China ReceiVed NoVember 9, 2006. ReVised Manuscript ReceiVed May 23, 2007

Shenmu bituminous coal with 4% sodium acetate added was used to investigate the characteristics of combustion and nitrogen oxide (NOx) release in a fixed bed reactor heated by a tube furnace. The composition of the flue gas was analyzed to investigate the effects of sodium acetate on the combustion process and NOx emission. The experiments were carried out in a partial reductive atmosphere and a strong oxidative atmosphere. The O2 valley value in the partial reductive atmosphere was reduced by the added sodium acetate. Sodium acetate accelerated the combustion and shortened the combustion process. The experimental results showed that the emissions of NO, NO2, and N2O were affected by the reacting atmosphere and the combustion temperature. In the strong oxidative atmosphere, sodium acetate resulted in a slight NOx reduction. In the partial reductive atmosphere, sodium acetate reduced both the peak value of NO concentration and the total NO emission significantly. An over 30% NOx reduction efficiency was achieved at 900 °C in the partial reductive atmosphere, which decreased with the increase in temperature. Sodium acetate was decomposed into hydrocarbon radicals and sodium hydroxide, which can both reduce NOx emissions due to their special reactions with the nitrogen component.

1. Introduction Coal is still a major energy resource in our society today. But its combustion releases large amounts of nitrogen oxides (NOx), which are harmful to humans and the biosphere. It is important to study the production regularity of NOx during coal combustion and to develop good methods to reduce NOx emissions.1-3 Many kinds of catalysts can improve the coal combustion characteristics, and some of them can also reduce the emissions of NOx and SO2 simultaneously. Many researchers have devoted much attention to the study of catalysts for coal combustion.4-8 Lee et al.9 studied the effect of sodium on NOx reduction in the * Corresponding author. Tel.: +86-571-87952885. Fax: +86-57187951616. E-mail: [email protected]. (1) Tomita, T. Suppression of nitrogen oxides emission by carbonaceous reductants. Fuel Process. Technol. 2001, 71, 53-70. (2) Glarborg, P.; Jensen, A. D.; Johnsson, J. E. Fuel nitrogen conversion in solid fuel fired systems. Prog. Energy Combust. Sci. 2003, 29, 89-113. (3) Beer, J. M. Combustion technology developments in power generation in response to environmental challenges. Prog. Energy Combust. Sci. 2000, 26, 301-327. (4) Zhiheng, W.; Sugimoto, Y.; Kawashima, H. Catalytic nitrogen release during a fixed-bed pyrolysis of model coals containing pyrrolic or pyridinic nitrogen. Fuel 2001, 80, 251-254. (5) Tsubouchi, N.; Ohtsuka, Y. Nitrogen release during high temperature pyrolysis of coals and catalytic role of calcium in N2 formation. Fuel 2002, 81, 2335-2342. (6) Zongbin, Z.; Wen, L.; Jieshan, Q.; Baoqing, L. Effect of Na, Ca and Fe on the evolution of nitrogen species during pyrolysis and combustion of model chars. Fuel 2003, 82, 1839-1844. (7) Xiao, Y.; Defu, C.; Tongmo, X. Effect of rank, temperatures and inherent minerals on nitrogen emissions during coal pyrolysis in a fixed bed reactor. Fuel Process. Technol. 2005, 86, 739-756. (8) Maly, P. M.; Zamansky, V. M.; Ho, L.; Payne, R. Alternative fuel reburning. Fuel 1999, 78, 327-334. (9) Lee, S.; Park, K.; Park, J.-w.; Kim, B.-H. Characteristics of reducing NO using urea and alkaline additives. Combust. Flame 2005, 141, 200203.

process of selected noncatalytic reduction. They found that the reduction efficiency could be improved 30% by adding NaOH into a urea solution. In their experiments, NaOH had the best effects on NOx reduction compared with other sodium-containing compounds. Liu et al.10 investigated the effects of indigenous minerals and sodium additives on the release of coal nitrogen during combustion on a thermogravimetric analyzer using Yibin coal from China. They obtained the following results: Sodium compounds reduced NO and HCN releases remarkably in the combustion process. Sodium chloride had a better effect on NO reduction than sodium carbonate. The catalysis of sodiumcontaining compounds in the reactions between HCN precursors and NO might lead to the drastic reduction of both species. Nimmo et al.11,12 investigated the potential of calcium magnesium acetate (CMA) as a substance for the simulataneous control of NOx and SOx. NO reduction by CMA reburning was tested in an 80 kw pulverized coal-fired furnace. A CMA solution was sprayed into the furnace at primary zone stoichiometries (λ1) of 1.05∼1.4. The feed rate of CMA (Rff) was expressed as a percent of the total combustibles fed to the furnace. NO reduction increased with the increase of Rff, and the best NO reduction efficiency was 75% under the conditions of Rff ) 17.3% and λ1 ) 1.05. Patsias et al.13 studied the performance (10) Liu, Y.-h.; Liu, Y.-h.; Che, D.-f.; Xu, T.-m. Effects of minerals and sodium addition on nitrogen release during coal combustion. Proc. CSEE 2005, 25 (4), 136-141. (11) Nimmo, W.; Patsias, A. A.; Hampartsoumian, E.; Gibbs, B. M.; Fairweather, M.; Williams, P. T. Calcium magnesium acetate and urea advanced reburning for NO control with simultaneous SO2 reduction. Fuel 2004, 83, 1143-1150. (12) Nimmo, W.; Patsias, A. A.; Hampartsoumian, E.; Gibbs, B. M.; Williams, P. T. Simultaneous reduction of NOx and SO2 emissions from coal combustion by calcium magnesium acetate. Fuel 2004, 83, 149-155. (13) Patsias, A. A.; Nimmo, W.; Gibbs, B. M.; Williams, P. T. Calciumbased sorbents for simultaneous NOx/SOx reduction in a down-fired furnace. Fuel 2005, 84, 1864-1873.

10.1021/ef060558d CCC: $37.00 © 2007 American Chemical Society Published on Web 07/25/2007

Combustion Process and Nitrogen Oxides Emission

Energy & Fuels, Vol. 21, No. 5, 2007 2549

Figure 1. A schematic diagram of the experimental system.

with the increase of the initial oxygen concentration in the reacting gas. In this work, the combustion process and nitrogen oxide emissions of Shenhua coal with sodium acetate (NaAC) additives were studied in a tube furnace whose temperature could be controlled. In the process of coal combustion, the major source of NOx production is fuel-bound nitrogen. Fuel NOx transformed from fuel-bound nitrogen is generally 80%∼90% of the total NOx. Only fuel NOx was targeted in our experiments, while prompt NOx and thermal NOx were excluded due to the absence of nitrogen gas in the furnace. 2. Experimental Methods Figure 2. O2, CO2, and CO concentrations in PRA without NaAC at 900 °C.

Figure 3. O2, CO2 ,and CO concentrations in SOA without NaAC at 900 °C.

of carboxylic salts of calcium as dual NOx/SOx reducing agents. The salts used as reburning fuel were sprayed and combusted to generate a fuel-rich reburning zone for NOx reduction. The results showed that CMA and calcium propionate were the best dual NOx/SOx reducing agents followed by calcium benzoate and calcium acetate magnesium acetate and calcium formate. Hall and Williams14 investigated the reduction of SO2, HCl, and NOx using CMA as a sorbent. CMA could capture 60% SO2, 61% HCl, and 26% NOx simultaneously. The reduction of individual NOx could be up to 94% at 850 °C with dry CMA particle injection. The efficiency of NOx reduction decreased

The experimental system is shown in Figure 1. An electric-heated tube furnace was used to create certain high-temperature conditions for coal combustion and was controlled by an automatic temperature controller. A 77-mm-long porcelain vessel was put into the center of a quartz tube whose diameter was 18 mm. About 0.2 g of pulverized coal sample was used in every case. Argon was chosen as a carrier gas and was mixed in a mixer with oxygen. The gases flowed into the quartz tube and reacted with the coal sample, which had been put in the vessel beforehand. After filtration, the product gases were analyzed by a gas analyzer (Rosemount Analytica NGA 2000). The species analyzed were O2, CO2, CO, NO, N2O, and NO2 with a precision of 1%. The gas concentrations were collected and displayed by the data collector. Shenmu bituminous coal with particle sizes ranging from 150250 µm was used in the experiments. The ultimate and proximate analyses of the coal are shown in Table 1. Some quantity of sodium acetate dissolved in distilled water and the pulverized coal were put into the sodium acetate solution for 2 h. The volume of sodium acetate solution was controlled to add 4% sodium acetate into the coal. After sufficient mixing, the mixture was kept at 110 °C for 24 h to desiccate it. Hence, the pulverized coal could be used as a coal sample to carry out tests. The furnace was heated up to the required temperature first. The carrier gas and oxygen whose fluxes were adjusted to the required value flowed into the quartz tube. Then, the vessel with a 0.2 g coal sample was put into the reactor quickly. The test began, and the gas analyzer and data collector started to operate at the same time. Testing continued until the gas concentration remained constant. Only oxygen and argon were entrained into the experimental system, and the reacting atmosphere originally excluded nitrogen gas. Therefore, there was no thermo NOx or prompt NOx in all of the tests. In the experiments, two kinds of reacting atmospheres were adopted. One was the partial reductive atmosphere (PRA), which was 100 mL/min of oxygen and 400 mL/min of argon, and the other was the strong oxidative

Table 1. Ultimate and Proximate Analyses of Shenmu Coal ultimate analysis weight % (as dried and ash-free basis)

proximate analysis weight % (as air-dried basis)

coal type

C

H

O

N

S

moisture

ash

volatile

fixed carbon

Shenmu

77.45

4.92

16.31

1.05

0.27

3.12

4.67

32.33

59.88

2550 Energy & Fuels, Vol. 21, No. 5, 2007

Weijuan et al.

Figure 4. O2 and CO2 concentrations in PRA with and without NaAC. The temperature was (a) 900 °C, (b) 1000 °C, and (c) 1100 °C.

atmosphere (SOA), which was 400 mL/min of oxygen and 100 mL/min of argon.

3. Results and Discussion 3.1. Discussion on the Reacting Atmosphere. The coal combustion was affected greatly by the reacting atmosphere.

Figure 5. CO2 concentrations in SOA with and without NaAC. The temperature was (a) 900 °C, (b) 1000 °C, and (c) 1100 °C.

Not only the combustion process but also the combustion products were different in varying combustion atmospheres. In this work, two kinds of atmospheres, PRA and SOA, were considered, as described in detail in the above Experimental



Table 2. Maximum CO2 Concentration during the Entire Combustion Process conditions A ) max CO2 (without NaAc) % B ) Max CO2 (without NaAc) % B/A - 1 %

900 °C in PRA

1000 °C in PRA

1100 °C in PRA

900 °C in SOA

1000 °C in SOA

1100 °C in SOA

14.87

9.19

11.03

22.63

25.80

27.64

17.45

12.52

12.30

24.16

26.99

30.81

17.35

36.24

11.51

6.76

4.61

11.47

Methods section. The O2, CO2, and CO concentrations during the combustion of Shenmu coal without NaAC in PRA at 900 °C are shown in Figure 2. There was a CO concentration peak at 1.2% and a minimum O2 concentration valley at 8% in the earlier stage of the combustion process. The CO peak and the O2 valley lasted a short time and made coal burn in the reductive atmosphere. During the combustion anaphase, CO vanishes and the concentration of oxygen becomes higher, resulting in an oxidative atmosphere. Hence, it created a partial reductive atmosphere. The O2, CO2, and CO concentrations during the combustion of Shenmu coal without NaAC in SOA at 900 °C are shown in Figure 3. The maximum CO concentration was only 233 ppm. The oxygen concentration was high and over 30% during the entire combustion process. Hence, the entire combustion was in a strong oxidative atmosphere. The maximum limit of O2 concentration for the gas analyzer was 30%, so the displayed value over 30% in Figures 2 and 3 only indicated that the actual O2 concentration was over the maximum value. 3.2. Experimental Results for the Combustion Process. In PRA and SOA, the combustion experiments were both carried out at 900, 1000, and 1100 °C separately using the original coal and coal enriched with sodium acetate. Figures 4 and 5 show the O2 and CO2 concentration curves during the coal combustion process in PRA and SOA. With and without sodium acetate, the burnout time of the pulverized coal was much shorter in SOA than in PRA. The O2 valleys became lower in PRA and the CO2 peaks appeared higher in PRA and SOA when sodium acetate was added. By adding sodium acetate, the O2 valley decreased from 7.9% to 4.6% in PRA at 900 °C, and from 6.1% to 3.4% at 1000 °C, and from 3.3% to 1.4% at 1100 °C. The concentration of O2 was over 30% throughout the combustion process in SOA, and accurate values were not obtained due to the gas analyzer’s limit. Because carbon was the major combustible matter in coal, and CO2 was the main combustion product, the instantaneous CO2 concentration in product gases could express the combustion velocity of coal in this work. The higher- and shorter-duration CO2 peak indicated that sodium acetate accelerated combustion and shortened the combustion process. Sodium salts are the active catalysts for the CO2 reaction, and mechanisms classified as electron-transfer theories or oxygen-transfer theories are proposed to explain the action of sodium salts.15 Table 2 gives the maximum CO2 concentration during the combustion process, which can present the maximum combustion velocity for the entire combustion process. The data indicated that the combustion velocity was much higher in SOA than in PRA while the velocity improvement was lower in SOA than in PRA by adding NaAC into coal. The maximum combustion velocities could be enhanced by about 5∼35% under different conditions. 3.3. Experimental Results on Nitrogen Oxide Emissions. The N2O concentration curves in PRA at 900, 1000, and (14) Hall, W. J.; Willaims, P. T. A novel additive for the reduction of acid gases and NOx in municipal waste incinerator flue gas. Waste Manage. Res. 2006, 24, 388-396. (15) Mckee, D. W. Mechanisms of the alkali metal catalysed gasification of carbon. Fuel 1983, 62, 170-175.

1100 °C are given in Figure 6. N2O production decreased as the reacting temperature rose. It is indicated in Figure 6 that N2O production was reduced by adding sodium acetate into the original coal. The N2O production peak was reduced by about 30%, and the value changed from 23 to 16 ppm at 900 °C after adding NaAC. The effects of sodium acetate on N2O reduction became weaker as the temperature rose. The N2O concentration reduction was only 1 ppm at 1100 °C. In the partial reductive atmosphere, the concentration of NO2 was only several parts per million in each case, so it could be ignored. Figure 7 shows the NO concentration curves during the coal combustion process in PRA at 900, 1000, and 1100 °C. It is known that the coal combustion process can be divided into two stages: volatile combustion and char combustion. Correspondingly, the NO concentration curve should have two peaks: one from the volatile-N and the other from the char-N. There are obviously two NO peaks by volatile-N and char-N in Figure 7a. As the temperature rose, the NO peak from volatile-N was more distinct, while the NO peak from char-N was weaker and could not even be identified, as Figure 7c shows. When sodium acetate was added into coal, the NO emission could be reduced during the coal combustion process. The reductive effectiveness became weaker at higher temperatures. The NO reductive effect of sodium acetate worked during the whole combustion process at 900 °C, and the reductive effect of the NO peak value from volatile-N was 31 ppm and that of the NO peak from char-N was 52 ppm, as shown in Figure 7a. The total NO emission can be obtained by integrating the NO concentration with the combustion time. The NO reduction efficiency was 22% at 1000 °C and 24% at 1100 °C. At 900 °C, a 32% reduction of the total NO emission was obtained, which was the best result in the experimental temperature range. The NO reductive effect of adding sodium acetate into coal was obviously lower than that with CMA as the reburning fuel11-12 or a sorbent14 and was equivalent to that with the carboxylic salt of calcium13 as the reburning fuel, where the reburning fuel fraction was lower than 4%. Figure 8 shows the NO and NO2 concentration curves in the strong oxidative atmosphere. The NO concentration curves had

Figure 6. N2O concentration in PRA with and without NaAC.


Weijuan et al.

Figure 8. NO and NO2 concentrations in SOA with and without NaAC. The temperature was (a) 900 °C, (b) 1000 °C, and (c) 1100 °C. Figure 7. NO concentration in PRA with and without NaAC. The temperature was (a) 900 °C, (b) 1000 °C, and (c) 1100 °C.

two peaks formed from volatile-N and char-N in coal; the same peaks were in PRA. The NO peaks from char-N in SOA were much higher than those in PRA. Compared with the NO peaks

of the original coal, the NO peaks from char-N were reduced slightly after adding sodium acetate, and the NO peak values from volatile stage combustion with sodium acetate were nearly equivalent to those without sodium acetate. Adding sodium acetate can prevent the conversion of coal’s nitrogen to NO



Figure 9. N2O concentration in SOA with and without NaAC.

after the NO peak from volatile-N. NO2 showed the same phenomena as NO. Adding sodium acetate can also prevent NO2 production after the NO pinnacle from volatile-N. The N2O concentration curves in SOA at 900, 1000, and 1100 °C are given in Figure 9. N2O production decreased as the temperature increased. N2O production was reduced by adding sodium acetate into the original coal. The N2O production peak was reduced and the value changed from 65 to 57 ppm at 900 °C by adding NaAC into the coal. The reduction effect of sodium acetate on N2O emission became weaker with increasing temperature, and the concentration reduction was less than 2 ppm at 1100 °C. 3.4. NOx Reduction Reactions Analysis. Figure 10 shows the N2O, CO2, CO, O2, and NO concentrations during the entire combustion process at 900 °C with NaAC in PRA and SOA. The combustion process in SOA lasted only about 250 s, while that in PRA was 600 s. In both PRA and SOA, the N2O peak appeared and ended earlier than the NO peak. The CO2 peak appeared between the NO peak from volatile-N and that from char-N. The CO peak appeared earlier than the O2 valley, and the O2 valley appeared earlier than the CO2 peak in PRA, which might suggest that CO could work on NO release from volatile-N. Sodium acetate decomposes at high temperatures, and one of the main products is alkyl (CHi). CHi can react with NOx and create HCN, which has an effect on NOx reduction in the coal combustion process.16 HCN can react with O2 and create NOx, but it can also react with NOx and create N2. In a reductive atmosphere, the oxygen concentration was low, and the reductive reaction of HCN and NOx was enhanced; therefore, sodium acetate appeared to have a strong reductive effect on NOx. In an oxidative atmosphere, the reaction of HCN and O2 was enhanced by higher oxygen concentrations, which made sodium acetate appear to have a weak reductive effect on NOx. This mechanism is shown in Figure 11. That is the reason why the NOx peak from volatile-N was not reduced significantly by NaAC in SOA. Alkyl from NaAC decomposition can be consumed quickly by O2 and NOx at high temperatures, and the reacting atmosphere becomes more oxidative, so its effect on NOx reduction is important in volatile burning but negligible (16) Liang, X.-m.; Gao, Z.-y.; Yan, W.-p. Analysis on reaction mechanism of HCN and NH3 during coal reburning. North China Electric Power 2004, 4, 19-21.

Figure 10. Gas components in the combustion process with NaAC at 900 °C. (a) in PRA and (b) in SOA.

Figure 11. NaAC reaction mechanism in the coal combustion process.

in char burning. Vitali et al.17 researched sodium salt’s effect on NOx release in combustion, and they considered that sodium salt produced NaOH at high temperatures and NaOH consumed O and OH radicals by the chain reactions (eq 1). NOx formation strongly depends on the local combustion environment, and a reduction in the concentration of O and OH radicals results in an increase of reductive radicals and the NOx reaction, which results in a decrease in the NOx concentration. In conclusion, sodium acetate decomposed to CHi and Na in combustion, and both of them had a NOx reduction effect through special mechanisms. CHi reduction played an important role in PRA, (17) Lissianski, V. V.; Zamansky, V. M.; Maly, P. M. Effect of metalcontaining additives on NOx reduction in combustion and reburning. Combust. Flame 2001, 125, 1118-1127.


Weijuan et al.

and its effect was weak in SOA during the volatile combustion process, and Na reduction worked throughout the whole combustion process.

NaOH + H f Na + H2O Na + O2 + M f NaO2 + M NaO2 + OH f NaOH + O2

(1)

4. Conclusions Shenmu bituminous coal with an addition of 4% sodium acetate was employed to combust in a tube furnace, and the components of the flue gas were analyzed to investigate the effects of sodium acetate on the combustion process and NOx reduction. The following conclusions were drawn: (1) Sodium acetate accelerated the combustion. The CO peak and O2 valley in the partial reductive atmosphere were lowered by the added sodium acetate. The CO2 peak was heightened in PRA and SOA by the added sodium acetate. The maximum combustion velocity can be enhanced by about 5∼35% for different conditions.

(2) Adding sodium acetate into the original coal can reduce NO, N2O, and NO2 production in combustion. NOx reduction was affected by the combustion atmosphere and temperature. Greater than 30% of the NOx reduction efficiency can be obtained at 900 °C in a partial reductive atmosphere by adding sodium acetate into the coal, which was the best NO reduction efficiency in all cases. (3) In a partial reductive atmosphere, the NOx peak from volatile-N decreased obviously, while the reduction effect was weakened greatly in the strong oxidative atmosphere. Sodium acetate can also prevent converting char-N into NO after the NO peak from volatile-N in both atmospheres. (4) Sodium acetate decomposed to CHi and Na in coal combustion, and both of them had a NO reduction effects through special mechanisms. Acknowledgment. The authors express their acknowledgment of the financial support from the National Natural Science Foundation of China (50606030), Program for New Century Excellent Talents in University (NCET -04-0533), and National Key Technology R&D Program (2006BAA01B06). EF060558D

Combustion Process and Nitrogen Oxides Emission of Shenmu Coal

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