Energy & Fuels 1995,9, 962-965
962
Decomposition of N H 3 over Calcined and Uncalcined Limestone under Fluidized Bed Combustion Conditions Tadaaki Shimizu," Eisuke Karahashi, Takuya Yamaguchi, and Makoto Inagaki Department of Chemistry and Chemical Engineering, Faculty of Engineering, Niigata University, 2 Ikarashi, Niigata, 950-21, Japan Received March 13, 1995@
In this work, decomposition of NH3 to N2 over limestone was investigated as a mean to reduce NO, emission from fluidized bed combustors. A fxed bed study was conducted and the effect of C02 and H2O on the products of NH3 decomposition over limestone was studied. In the absence of C 0 2 and HzO, NH3 was decomposed to N2. From a NH3-CO2 mixture, (NH2)zCOwas formed through NH3 decomposition over both calcined limestone at a COz concentration of 15% and uncalcined limestone at a C02 concentration of 75%. However, from the NH3-CO2-HzO mixture, (NH&CO was not formed. The presence of H2O is necessary to evaluate the rate and the products of NH3 decomposition over limestone under commercial fluidized bed combustion conditions.
Introduction Oxidation and decomposition of NH3 over limestone have been extensively investigated to clarify the formation pathways of NO, within fluidized bed combustors (FBCS).'-~ NH3 is formed through pyrolysis of fuels. Limestone (CaC03) is fed into FBCs to capture SO2 as CaS04. In atmospheric FBCs, CaC03 is calcined to CaO whereas in pressurized FBCs it remains as CaC03 due to high C02 partial pressure.6 Both CaO and CaC03 are known to catalyze NH3 oxidation to NO, in the presence of 02.1-5 On the other hand, both CaO and CaC03 are also known to catalyze decomposition of NH3 in the absence of 02.1-4 The rate of NH3 decomposition over CaC03 was reported to be higher than the rate of NH3 ~ x i d a t i o n . Thus, ~ it is possible to reduce NO, emission by decomposing NH3 to N2 over limestone if an oxygen-deficient zone is formed in FBC reactors. To design such a combustion system, the rate of NH3 decomposition must be evaluated. However, the effect of coexisting gases such as C 0 2 and HzO is not yet fully known. In this work, the effect of coexisting gases, COz and HzO, on catalytic decomposition of NH3 over limestone was investigated. Urea was found to be formed from @Abstractpublished in Advance ACS Abstracts, October 1, 1995. (1)Hirama, T.; Kochiyama, Y.; Chiba, T.; Kobayashi, H. NenryoKyokai-Shi ( J . Fuel SOC.Jpn.) 1982,61,268-275. ( 2 ) Furusawa, T.; Tsujimura, M.; Yasunaga, K.; Kojima, T. Pmceedings of the 8th International Conference on Fluidized Bed Combustion (Houston, TX);DOE/METC-85/6021;US.Department of Energy, Office of Fossil Energy: Morgantown, WV, 1985; pp 1095-1104. (3) Lee, Y. Y.; Sekthira, A.; Wong, C. M. Proceedings of the 8th International Conference on Fluidized Bed Combustion (Houston, TX); DOE/METC-85/6021; US. Department of Energy, Office of Fossil Energy: Morgantown, WV, 1985; pp 1208-1218. (4) Lin, W.; Johnsson, J. E.; Dam-Johansen, K.; van den Bleek, C. M. Proceedings of the 7th International Conference on Coal Science (Banff, Canada); Canadian National Organizing Committee: Devon, Canada, 1993; pp 554-557. (5) Shimizu, T.; Tachiyama, Y.; Fujita, D.; Kumazawa, K; Wakayama, 0.;Ishizu, K.; Kobayashi, S.; Shikada, S.; Inagaki, M. Energy Fuels 1992,6,753-757. ( 6 )Ljungstrom, E.; Lindqvist, 0.Proceedings of the 7th International Conference on Fluidized Bed Combustion (Philadelphia); DOE/METC/ 83-48; U.S. Department of Energy: Morgantown, WV, 1982; pp 465472.
the NH3-CO2 mixture but it was not formed from the NH3-C02-H20 mixture. NH3 decomposition rate is overestimated if the rate is evaluated using dry NH3C02 mixture and only gaseous NH3 concentration is measured. A conclusion is that H2O is necessary to evaluate the rate and the product of NH3 decomposition over limestone in the presence of C02.
Experimental Section A fixed bed study was conducted using a quartz reactor of 2 cm inner diameter. The details of the experimental apparatus are described e1sewhe1-e.~The experiments were conducted under atmospheric pressure conditions. The temperature in the fixed bed was measured by a chromel-alumel thermocouple covered with high-purity alumina and it was controlled by heating the reactor with an electric heater. The temperature was fixed a t 1123 K, which is the typical operating temperature of FBCs. Chichibu limestone from Japan was employed as a sample. Its composition (wt %) was CaC03 96.9, M&03 1.4, Si02 0.6, A1203 0.8, Fez03 0.3. The particle size was 0.42-0.59 mm. In the present study, the catalytic activity was evaluated for both calcined limestone (CaO) at COZconcentrations of 0 and 15% and uncalcined limestone (CaC03) at a COz concentration of 75%. At a COz concentration of 15%,which is as high as the COz concentration in the flue gas of FBCs, limestone is known to be calcined to CaO. A COZ concentration of 75% was sufficiently high t o suppress the calcination of CaC03, as previously observed by use of a thermogravimetric a n a l y ~ e r . ~ For the reactions over calcined limestone, 5 g of quartz sand was used to dilute the limestone. The particle size of the sand was 0.3-0.5 mm. Since the heat transfer affects the calcination rate and thus the pore evolution of calcined limestone, the limestone was calcined in a similar way as FBCs. The limestone sample was injected into the quartz sand bed fluidized by 0 2 stream at 1123 K. For uncalcined limestone, for which pore evolution does not occur, the sample was packed in the fixed bed reactor and pure COZwas fed into the reactor during heat-up. After the reaction temperature was attained, the reactant gas was introduced from the top of the reactor downward through the fixed bed. Blank tests were conducted using quartz sand packed in the quartz fixed bed reactor. ~
~~
(7) Tonsho, M.; Shimizu, T.; Inagaki, M. Prepr. 58th Annu. Meet. SOC.Chem. Eng., Jpn. 1993,$203
0887-0624/95/2509-0962$09.00/00 1995 American Chemical Society
Decomposition of NH3 over Limestone
Energy & Fuels, Vol. 9, No. 6,1995 963
NH3,1~=900ppm,Temperature-llZBK, Oz=O%
12,
1
I
COZ 0% HzO 0% Limestone 1000mg inventory
0% 0% 250mg
15% 0% 250mg
15%
Mo/. 250mg
Figure 1. Material balance of NH3 decomposition over calcined limestone. NH3(g):NH3 in dry produced gas NH4+(1): NH4+ in solution. (NH2)zCO: NH4+ produced by urease addition to solution. Nz: Nz in produced gas. Deposit: NZ+ NO N2O detected during 02-He mixture feed to spent catalyst bed.
+
Negligible change in NH3 concentration was observed between the inlet and the outlet; thus the following results are caused by the catalytic activity of limestone. In the present study, the effect of COz and HzO on catalytic decomposition of NH3 was investigated. To establish the material balance of nitrogen, helium was used as a diluent. The reactant gases, except for HzO, were fed from cylinders. H20 vapor was produced by feeding water using a pump into a heated vaporizer into which He or COz was also fed, and then the Hz0-He mixture or HzO-COZ mixture was fed into the reactor. The gas feed tube was heated by flexible heaters to avoid condensation of water in it. Total feed rate of reactant moVs; i.e., superficial velocity was 60 gases was 2.05 x c d s at 1123 K. NH3 concentration was fixed at 900 ppm. Concentration of NH3 in the dry gas was measured by Kitagawa detector tubes8 For the experiments using an COz-HzO mixture, for which the detector tube method was not applicable due to condensation of water, the produced gas was washed using 0.8 mol/dm3 HzS04 solution in a gas washing bottle, and then the concentration of NH4+ ion in the solution was analyzed using an ion electrode. Also, gas chromatography with thermal conductivity detector (TCD-GC) was used for the NH4+ analysis for some samples. Before sample solution was injected into TCD-GC, NaOH was added t o the sample solution to convert NH4+ to NH3. The results obtained by use of the ion electrode agreed well with the results by use of TCD-GC. The concentration of NO was measured by an on-line analyzer using chemical luminescence. Concentrations of NZ and NzO in the gas were measured by TCD-GC. The concentration of NO2 was measured by Kitagawa detector tubes.8 The accuracy of the NH3 analytical methods, the detector tube, the ion electrode, and the TCDGC, was evaluated by using standard NH3 gas mixture or standard NH&l solution. The errors of these methods were less than 10% of the measured value. The NO analyzer was calibrated using standard NO gas mixture and the error was only few percent. Although the accuracy of Nz analysis was not so good due to the leakage of the air into the system, the error of Nz analysis was less than 50 ppm (100 ppm as N) for the worst case and it was less than 11%of the inlet NH3 concentration. Therefore, the error of the overall nitrogen balance due to the analytical methods is considered to be less than 10%.
m-
Results and Discussion Figure 1shows the material balance and the products of NH3 decomposition over calcined limestone. The concentrations of NO and N2O at the outlet of the reactor were within the range of noise level of the ' 1 0 ppm) under all conditions without analyzers ( oxygen. In the absence of coexisting gases, the amount (8) Japanese Industrial Standard K 0804-1985 (DetectorTube Type Gas Measuring Instruments),Japanese Standards Association,Tokyo, Japan, 1985.
of produced N2 agreed fairly well with the difference in NH3 concentration between a t the inlet and the outlet. By increasing the limestone inventory from 250 to 1000 mg, more NH3 consumption was observed but the nitrogen balance was established by NH3 and N2. Therefore, NH3 was decomposed to N2 when neither C02 nor H20 was present. For the NH3-C02(15%) system, the outlet NH3 concentration in the gas was far lower than that for NH3 decomposition without C02, referring to the results at a limestone inventory of 250 mg in Figure 1. However, the material balance of nitrogen was not established by the total amount of NH3 and N2 in the produced gas. For the NH3-CO2 system, solid deposition was found at the exit of the reactor. It was possible that (NH2)2CO, (NH4)&03*H20, and NH4HC03 were formed from NH3-CO2 mixture as follows:
2NH3 + CO,
-
2NH3
(NH,),CO
- N, + 3H,
+ H,O
+ CO, - H,O + CO 2NH3 + 2H,O + CO, - (NH4),C03*H,0 NH3 + H,O + CO, - NH4HC03 H,
(1) (2) (3) (4) (5)
To determine the amount of the products for the NH3C02 system, the produced gas was washed by HzSO4 solution. First, the concentration of NH4+ ion in the solution was measured using an ion electrode. the amount of NH4+ ion in the solution is the total amount of nitrogen in gaseous NH3, solid (NH4)2C03*H20,and solid NH4HC03. The amount of urea was determined by decomposing urea using urease as follows:
(NH,),CO
+ 2H,O - 2NH4++ C0:-
(6)
Before urea was decomposed, NH4+ ion was removed from the solution since NH4+ inhibited the activity of urease. NaOH was added to the sample solution t o convert NH4+ ion to NH3, and NH3 was swept out from the solution by feeding Nz. The NH3 concentration in N2 at the outlet was intermittently measured using detector tubes until the NH3 concentration became sufficiently low. After the solution was neutralized by adding HzS04, urease was added to decompose urea; then the concentration of the produced NH4+ in the solution was measured. As shown in Figure 1,urea was the major product of NH3 decomposition for the NH3-CO2 system; the selectivity of consumed NH3 to urea was 43%. The difference between the amount of NH4+ in the liquid and that of NH3 in the gas, NH4+(1) - NH3(g),was 23% of the consumed NH3. This is explained by the formation of (NH4)&03*H20(eq 4) or NH4HC03 (eq 51, both of which are solid a t a room temperature. In this work, identification of these products was not conducted. By taking account of urea and total NH4+ in the solution, total nitrogen at the exit of the reactor was 87% of the nitrogen in the feed gas. However, the error of the nitrogen balance of 13% is more than the error of the analytical methods. There are two possible explanations of the error in the nitrogen balance: (1) coking of organic material
(urea) occurs over limestone or sand in the reactor and (2) biuret, (NH~COIZNH, was formed through condensation of urea a t ca. 430 K as9 2(NH2),C0
- (NH,CO),NH + NH3
;:im-t Shimizu et al.
984 Energy & Fuels, Vol. 9,No. 6, 1995
(7)
NH3 IN=900ppm. COz=75%, Temperature=llPBK
(NHZ)zCO
06
>8
"1
04
W9)I
2 02
To evaluate the nitrogen loss due to coking, the used catalyst was burnt using O2-He mixture. N2, NO, and a small amount of NzO were detected in the produced gas. Thus a part of consumed NH3 for the NH3-CO2 system was found to form solid deposit over the calcined limestone or quartz sand. The amount of nitrogen in the solid deposit was calculated from the concentrations of NO, N2, and N2O in the produced gas. By taking account of the nitrogen in the deposit, the nitrogen balance was improved as shown in Figure 1. The error of N balance was 8%and this is within the range of error of analytical methods. Biuret formation, another possibility of nitrogen loss, could not be proven in this work. CuSO4 and NaOH solutions were added t o the standard and sample solutions, but the sensitivity of this method (biuret reaction) was too low t o detect biuret under the present experimental conditions. Second, urease addition to standard solution of biuret was conducted and the conversion of nitrogen in biuret to NH4+ was found to be less than 10%. Thus the formation of biuret increases the nitrogen which can not be measured by the present analytical methods. According to the present results, a conclusion is deduced that the rate of NH3 decomposition to N2 is overestimated if NH3-CO2 mixture is employed for the experiment and only outlet NH3 concentration in the gas is measured. The products of NH3-CO2 reaction over limestone are not only N2 but also urea, (N&)zCOyHzO or NH4HC03, and solid deposit (coke). Though the present analytical method could not detect the biuret, it is still possible that biuret is formed through condensation of urea. In the presence of H20, however, only a negligible amount of (NH2)zCOwas produced as shown in Figure 1. The mechanism of the inhibition of urea formation shall be discussed later. In FBCs, there exists a large amount of H20 vapor from vaporization of moisture and combustion of hydrogen in the fuel. Therefore, urea formation is considered t o be inhibited in FBCs. It is concluded that the presence of H2O is necessary to evaluate the catalytic activity of calcined limestone for NH3 decomposition in the presence of C02, since the products of the NH3-C02-H20 system is different from those of the NH3-CO2 system. Uncalcined limestone was found t o behave in a similar way as calcined limestone for NH3 decomposition as shown in Figure 2. For the NH3-CO2 system without H20, (NH2)zCO was produced. The amount of NH4+ ion in the solution was more than the amount of NH3 in the produced gas, and thus (NH4)2C03-H20or NH4HC03 was considered to be formed. The error of nitrogen material balance for the NH3COz system was 18%. To evaluate the nitrogen loss due to coking, 02-COz mixture was fed immediately after the NH3-CO2 mixture feed was stopped. The total ~
~~
(9) Encycl. Chim. (KyoritsuShuppan (in Japanese)) 1964, 7, 287288. (10)Shimizu, T.; Inagaki, M. Energy FueEs 1993, 7, 648-654.
WdB)
00 02
2 7%
HzO 0% Limestone inventory 5g
NH3(Q) 0% 0%
0%
59
log
10%
Figure 2. Material balance of NH3 decomposition and NH3 oxidation over uncalcined limestone at a COz concentration of 75%. NO, NzO, Nz: in produced gas. NHdg): NH3 in dry produced gas. NH4+(1): NH4+ in solution. (NHhCO: NH4+ produced by urease addition to solution.
amount of the produced NO, N2, and N20 was less than 1%of the total nitrogen fed as NH3. Thus the coke deposition was found to be negligible over the uncalcined limestone for the NH3-CO2 system. Another possibility of nitrogen loss is biuret formation but the present experimental procedure could not detect biuret in the sample solution. Thus the mechanism of the nitrogen loss is a problem to be solved in the future experiments. In the presence of HzO, the formation of (NHhCO was negligible. In this work, the consumption of NH3 for the NH3-CO2-H-20 system was so little in comparison with that for the NH3-COz system; the result at a limestone inventory of 10 g is shown in Figure 2. For the NH3-CO2, system the result a t a limestone inventory of 5 g is shown since sufficient difference in NH3 concentration was observed between the inlet and the outlet. A tentative mechanism of the formation of (NHd2CO from a NH3-COz mixture and the inhibition of urea formation by HzO is discussed as follows: for uncalcined limestone (CaCOd, urea is formed as follows: CaCO,
+ 2NH3 CaO
-
CaO
+ (NH2),C0 + H 2 0
+ CO, - CaCO,
(8)
(9)
In a similar way, urea is considered to be formed over calcined limestone (CaO) as follows:
+ CO, - CaO-CO, + 2NH3 - CaO + (NH,),CO + H20 CaO
CaO-C02
(10)
(11)
where CaO-C02 denotes a C02 molecule adsorbed on an active site of limestone. C02 is known to adsorb on CaO surface a t 1123 K.l0 The inhibition of urea formation by H2O is considered to enhance the reverse reaction of eqs 8 and 11. Another explanation of the inhibition is that HzO adsorbs on active sites, and thus the number of active sites available for the NH3-CO2 reactions decreases. Indeed, H2O is known to inhibit decomposition of N2O over calcined limestone due t o adsorption.1° However, these explanations are only tentative and detailed mechanisms should be elucidated in future works. In Figure 2, the results of NH3 oxidation over uncalcined limestone without H2O are also shown. In the presence of 0 2 , the products were NO, NzO, and N2, total amount of which balanced with the amount of consumed NH3. The concentration of NO2 was also measured but it was negligible. By comparingthe outlet concentration
Decomposition of NH3 over Limestone of NH3 between oxidation (NH3-0z-CO2 system) and decomposition (NHB-CO~system), consumption of NH3 for the decomposition was more than that for the oxidation. These results agree with the report by Lin et a1.,4 in which only the conversions of NH3, NO, and Nz0 were reported using dry feed gas. According to the present results, it is possible that (NH2)zCO and (N&)2C03°Hz0 or NH4HC03 were formed for the experiments by Lin et al.4 and the decomposition rate was overestimated.
Conclusion The effect of coexisting gases, C02 and Hz0, on the products of NH3 decomposition over limestone under fluidized bed combustion was investigated. A conclusion is deduced as follows: H2O is necessary to evaluate the
Energy & Fuels, Vol. 9, No. 6,1995 965 rate and the products of NH3 decomposition over both calcined limestone (CaO) and uncalcined limestone (CaC03) in the presence of COz. If the rate is evaluated using a NH3-COz mixture and measuring the NH3 concentration at the reactor outlet, the reaction rate will be overestimated. Also, urea is formed for the NH3COZsystem whereas it is not produced in the presence of Hz0.
Acknowledgment. The authors express their thanks to Hatakeyama Foundation for financial support. The authors thank Mr. Yoshinao Niitsu, Mr. Masao Saito, and Mr. Hirokazu Yamamoto, undergraduate students of Niigata University, for their assistance during the experiments. EF9500503
