Energy & Fuels 1996,9, 680-684
680
Reduction of Nitric Oxide over Magnesium Oxide and Dolomite at Fluidized Bed Conditions Birgitta Olanders” and Dan Stromberg Department of Inorganic Chemistry, Chalmers University of Technology and University of Giiteborg, S-412 96 Giiteborg, Sweden Received November 21, 1994@
The reduction of nitric oxide over lime and char at fluidized bed conditions has been studied by many authors, but less attention has been paid to other surfaces existing in the boiler. This investigation concerns the reduction of nitric oxide by carbon monoxide, with or without oxygen present, and also the effect of small amounts of water, The reactions were studied in a fixed-bed quartz reactor with quartz, magnesium oxide, magnesium hydroxide, and dolomite as bed materials in the temperature interval 600-950 “C. The residence time and the concentrations of nitric oxide, carbon monoxide, and oxygen were varied. Magnesium oxide formed from heated magnesium hydroxide and calcined dolomite were more active than magnesium oxide of analytical grade, which indicates that the origin of the surface is important for the reaction. The reaction between nitric oxide and carbon monoxide is very sensitive t o the presence of oxygen. When oxygen was present the reduction of NO almost ceased. However, if the gases were thoroughly dried, the effect of oxygen addition was not that severe.
ash.2s3*7When dolomite is present as bed material MgO also may act as a catalyst. The emissions of NO, from fluidized bed combustion, Addition of lime or dolomite to the bed of sand FBC, are low compared to other combustion techniques. increases the fuel-N conversion to NO, while N2O is This is partly due to the low combustion temperatures decreased.1° Both lime and dolomite are catalysts for of 800-900 “C and partly to the staged combustion in the heterogeneous oxidation of HCN, which has a high FBC. However, the relatively low combustion temperselectivity for the formation of N0,11-14and catalyze the ature increases the emission of N2O. The emissions of decomposition of N20.14 Lime, on the other hand, is a NO and NzO have proved t o be very much influenced good catalyst for the decomposition of NO with reducby operating conditions,la2and many heterogeneous and tants as C0,2p3H2,15 and NH3.2J6 Sulfated limestone, homogeneous reactions are important for the formation though, is a poor catalyst for the reduction of N0,416 and destruction of NO and N20. while Cas is a good catalyst.12 Cas may be formed in The bed material in FBC consists of quartz sand, fuel, reducing areas in the fluidized bed c o m b ~ s t o r . ~ , ~ J ~ char, ash, and an adsorbent for S02. The most used Dolomite has shown the same influence on the fuel-N adsorbent is limestone (CaCOd, but also dolomite conversion to NO as lime has.1° When dolomite is used (CaMg(CO3)a) may be used. Lime and dolomite are as a sulfur capture material, Cas04 is formed, while calcined to the corresponding oxides in the temperatures almost all magnesium is expected to be in the form of valid for FBC conditions. MgO. Therefore, it would be possible that the MgO part Among the surfaces mentioned above the following of the dolomite could catalyze the reduction of NO. ones have been shown to catalyze the reduction of NO Typical for the circulating FBC is the occurrence of with CO: calcined l i m e ~ t o n e , ~ sulfated -~ l i m e ~ t o n e , ~ ~ oxygen-rich ~ (oxidizing)as well as oxygen poor (reducing) carbon: char2 Ca0,3,4,7 CaS04,427,8quartz and zones. It is therefore of interest t o investigate the
Introduction
@Abstractpublished in Advance ACS Abstracts, June 1, 1995. (1)h a n d , L. E.; Leckner, B. Proceedings ofthe 24th Symposium (International) on Combustion; The Combustion Institute; Pittsburgh, 1992; p 1407. (2) Johnsson, J. E.; Dam-Johansen, K. Proceedings of the 11th International Conference on Fluidized Bed Combustion (Montreal, Canadaj; ASME: New York, 1991; p 1389. (3) Tsujimura, M.; Furusawa, T.; Kunii, D. J . Chem. Eng. Jpn. 1983, 16, 132. (4) Furusawa, T.; Koyama, M.; Tsujimura, M. Fuel 1985, 64, 413. (5) Dam-Johansen, K.; Amand, L. E.; Leckner, B. Fuel 1993,72(4), 565. (6) Hansen, P. F. B.; Dam-Johansen, K.; Johnsson, J. E.; Hulgaard, T. Chem. Eng. Sci. 1992, 47(9-11), 2419. (7) Abul-Milh, M.; Nilsson, 0. 1991 International Conference on Coal Science Proceedings (Newcastle-upon-Tyne,England); IEA Coal Research Ltd.; Butterworth-Heinemann: Oxford, England, 1991; p 440. (8)Allen, D.; Hayhurst, A. N. Conf. Fluidized Bed Combust. (London, England); 1991, December, 221.
(9) Wittler, W.; Schiitte, K.; Rotzoll, G.; Schiigerl, K. Fuel 1988,67, 438. (10)Gavin, D. G. NO,: basic mechanisms of formation and destruction and their application to emission control technologies, Proceedings; Imperial College of Science: London, UK; 1993; CRF. (11)Jensen, A.; Johnsson, J . E.; Dam-Johansen, K. 12th Int. Conf. Fluid. Bed Combust. (San Diego, USA) 1993, 447. (12) de Soete, G. G.; Natsoll, W. “Catalytic nitrogen chemistry on CaO and CaC03 a t fluidized bed conditions”. Report IFP, No 39362, Institut Francais du Petrole, 1991. (13)Gavin, D. G.; Dorrington, M. A. Fuel 1993, 72, 381. (14) Johnsson, J . E. Presented at the 23rd IAE-Meeting (Firenze) November 1991. (15)Tsuiimura, M.; Furusawa, T.; Kunii, D. J. Chem. Eng. - Jpn. . 1983, 16, 524. (16)h a , K.; Salokoski, P.; Hupa, M. Proceedings of the 11th International Conference on Fluidized Bed Combustion (Montreal, Canada); ASME: New York, 1991; p 1027. (17)Mattisson, T.; Lyngfelt, A. To be presented at The 13th International Conference on Fluidized Bed Combustion (Florida, USA); May 7-10, 1995.
0887-0624/95/2509-0680$09.00/00 1995 American Chemical Society
Reduction of NO over MgO and Dolomite
Energy & Fuels, Vol. 9, No. 4, 1995 681 Table 1. Bed Materials
Gas tubes
wt= of wt loss by the active calcination BET (m21g) compd (g) (%) 112.3 Jr 0.2 3 m2/30 m2
bed material magnesium oxide
magnesium hydroxide 27.0 f 0.1 before calcination after calcinationb 204.7 f 2.4 calcium oxide 3.6 f 0.0 dolomite, Myanit D before calcination 0.4 f 0.0 5.8 f 0.1 after calcination quartz 0.02 f 0.00
Mass flow controllers
Thermocouple,
r
0.0310.2 7 5.5 m2 0.04 0.03 0.84 0.4 m2/3 m2 0.13l0.94 0.07010.52 8.00
30.6
44.8
The magnesium compounds and calcium oxide were diluted in Si02 to a total bed weight of 8.000 g. * Calcination to 950 "C. Table 2. Chemical Analysis of the Dolomite (wt
L I1 spectrometer
Quartz tube Gasout:
/"v
I
1
Figure 1. Experimental setup.
heterogeneous reactions, e.g., NO with CO, in both oxidizing and reducing atmospheres although the average oxygen concentration in the flue gas is 4-6%. In this investigation the reduction of NO with CO over dolomite has been studied and the results are compared with those of CaO, MgO, and MgO from the heating of Mg(0H)z. The effects of oxygen and small amounts of water on the reduction reaction was checked. The literature reports both increasing reaction with addition of small amounts water and decreasing ratesz1 because of the poisoning of the surface due to addition of water. The crystalline structure of the different bed materials has been investigated too.
Experimental Section A diagram of the fixed bed reactor is presented in Figure 1. The reactor consists of a quartz tube (28 mm i.d. x 1000 mm height) fitted with a sintered quartz filter used to support the solid particles of the fixed bed. The gas supply, set at a desired flow rate by using mass flow controllers, could pass through (plug flow) or bypass the tube reactor as directed by three-way taps. The temperature over the surface of the fixed bed was measured and kept constant by using a Pt-Rh thermocouple connected to a temperature control unit, which delivered the appropriate voltage output to the furnace housing the tube reactor. The operating temperature in the bed, over the range 600-950 "C, was maintained with reasonable stability ( 4 1 "C). The reaction product concentrations were measured by using a quadrupole mass spectrometer (Baltzer QMS 311). After the mass filter the ion current is measured by an (18)Hadman, G.; Thompson, H. W.; Hinshelwood, C. N. Proc. R. SOC.(London) 1932,A137, 87. (19)Verdurmen,E. A. Th. J.Phys. Chem. 1967,71, 678. (2O)Yetter,R. A,; Dryer, F. L.; Rabitz, H. Combust. Sci. Technol. 1991,79, 97. (21) Bank, C. A,; Verdurmen, E. A. Th. J . Phys. Chem. 1963,67, 2869.
Ca as theelement as theoxide
21.2 29.7
Mg
Si
10.6 2.55 17.6 5.5
%)a
Fe
AI
K
Na
0.485 1.39
0.452 1.8
0.073 0.13
0.00 0.00
Loss on ignition 44.8%. electrometer from which the concentration levels can be followed visually using a chart recorder. Background signals possibly arising from adsorbed species on the walls of the ionization chamber were checked by periodically flowing pure argon through the apparatus. The mle 3 0 signal was monitored for the NO decomposition by CO and the mle 28 signal for the oxidation of CO with 0 2 at the different experimental conditions. The concentrations of the gases were 380-1000 ppm for NO, 400-7600 ppm for CO, and 0-1% for 0 2 . The effect of small amounts of water was investigated by drying the gases with phosphorus pentoxide (Sicapent). The reaction time was between 50 and 100 ms. The residence time is defined as the time it takes for the gases t o pass through the bed of catalyst. The weight of the bed was 8 g for quartz sand, while the beds of magnesium oxide, magnesium hydroxide, and calcium oxide were diluted with quartz sand t o avoid a pressure drop in the reactor tube, and the bed of dolomite was diluted with quartz because of the high reactivity. The total weight of the bed was always 8 g. Magnesium oxide, magnesium hydroxide, and quartz sand were of analytical grade and used as supplied. The dolomite comes from Glanshammar (Myanit D). MgO with the active surface areas of 3 and 30 m2 was chosen. The active surface area chosen was 5.5 m2 for Mg(OH)2, 3 m2 for CaO, and 3 and 0.4 m2 for dolomite. The surfaces of Mg(OH)2 and dolomite were for the oxides formed after heating. The materials are presented in Table 1,where the specific surface areas are presented, and the composition of dolomite is given in Table 2: The specific surface area of the different bed materials was determined by using the BET method with nitrogen as the adsorbate, except for Si02 where krypton was used. The calcination of magnesium hydroxide and dolomite was done by heating the material together with the quartz sand t o 950 "C and monitoring the mle 1 8 and the mle 44 signal, respectively. When the signals had reached the background levels the calcination was regarded as complete.
Results and Discussion The reduction of nitric oxide by carbon monoxide was studied over dolomite and the results were compared to the reduction over calcium oxide, magnesium oxide, and magnesium oxide originating from the heating of magnesium hydroxide. The results are presented in Figures 2-5. Also the effect of oxygen (Figure 6 ) and small amounts of water was investigated.
Olanders and Stromberg
682 Energy & Fuels, Vol. 9, No. 4, 1995
Table 3. Reaction Parameters for the Estimation of Reaction Rate for the Reduction of NO with CO over Dolomite 3 surface area of dolomite (m2) 8 total weight of bed (the dolomite diluted in Si021 (g) 400-1000 concentration of NO (ppm) 400-1000 concentration of CO (ppm) 50-100 residence time within the bed of catalyst (ms) 5.8 bed volume (cm3) 0.45 bed voidage fraction temperature interval ( " C ) 600-950
80
E 0
2
60
.-c?
P
40
Empty reactor. SOz
20
\
0 600
700
800 900 Temperature ("C)
1000
Figure 2. Reduction of NO with CO. The dolomite was calcined by heating to 950 "C before the experiment. Total weight of bed: 8 g. Concentration: 400 ppm NO, 400 ppm CO. Residence time within the bed of catalyst: 50 ms (Si02, dolomite), 75 ms (dolomite), and 100 ms (CaO, MgO). (U) Empty reactor; ( 0 )8 g of Si02; (A)3 m2 dolomite, 50 ms; (0) 3 m2 dolomite, 75 ms; (+) 3 m2 CaO; ( 0 )3 m2 MgO.
loo 80
3
regularities lead to a higher activity, since almost 6 times smaller surface area is needed for the same reduction as compared to MgO (analytical grade) (Figure 3). Dolomite consists of the double salt MgC03CaC03, which after heating shows the two oxides CaO and MgO in the X-ray analyses. When dolomite is used as the adsorbent for SO2 in fluidized bed combustion, only the calcium oxide part is sulfated while MgO is almost not affected and could act,as a catalyst. However, it can be seen from these experiments (Figure 2) that CaO has a much higher catalytic effect on the NO reduction than MgO. The catalytic effect of dolomite is rather close to that of CaO. This is somewhat surprising since only half of the 3 m2 surface ought t o consist of CaO and the rest is MgO. It seems as if the activity of the MgO part of the dolomite is more active than the pure MgO. This supports the idea of the importance of the origin of the catalytic material. Another possible reason is that perhaps CaO and MgO are not evenly distributed on the surface. CaO could be enriched on the surface. In a real boiler, sulfurization of the oxides to sulfates will occur. Sulfated limestone is a poor catalyst for the reduction of NO3 compared t o calcined limestone. The same is not expected for dolomite, since the magnesium sulfate is not stable at this temperature and thus MgO should be present. One thing that could reduce the activity of magnesium oxide in the dolomite is the filling of the pores with sulfate and thereby lowering the surface area. The reaction rate was estimated for the reduction of NO with CO over dolomite. The reaction parameters used are presented in Table 3. The rate was analyzed by assuming the surface catalyst reaction to be first order for both nitric oxide and carbon monoxide:
1
2o0600
700
800
900
1000
Temperature ("C)
Figure 3. Reduction of NO with CO. The dolomite and Mg(OH)2 were calcined by heating to 950 "C before the experiment. Total weight of bed: 8 g. Concentration: 380 ppm NO, 7600 ppm CO. Residence time within the bed of catalyst: 100 ms. (W) Empty reactor; (0)8 g of SiOz; (A)30 m2 MgO; (+) 5.5 m2 Mg(OH)2; ( 0 )0.4 m2 dolomite.
The products from the reduction of NO with CO are N2 and CO2, which was seen by comparing the mass scans obtained for the initial and product gas mixture. N2O was not measured, but Hansen et a1.6 found that less than 2-3% of the NO was converted to N2O over CaO and Cas. In Figures 2 and 3 the reduction of 400 ppm NO with 400 and 7600 ppm CO, respectively, over the bed materials is presented. The origin of the oxide plays an important role as can be seen for the reduction of NO with CO over the magnesium oxides. MgO (analytical grade) is less active than MgO from the heating of Mg(OH)2 (analytical grade). X-ray analyses of MgO (analytical grade) showed broader peaks before compared to after heating, from which it might be concluded that a more ordered crystalline structure is obtained. The spectra of heated MgO and MgO from heated Mg(OH12 were similar, but the results from the NO reduction indicate a difference maybe due to oxygen vacancies or some other irregularities of the surfaces formed during the heating of Mg(OH)2. These ir-
-d[NOl/dt = k[NOI[CO]
(1)
Introducing a as the ratio of the inlet concentration of carbon monoxide to that of nitric oxide, and XNO,the fractional conversion of NO, calculated directly from the inlet and outlet concentrations, and integrating, eq 1 results in In
a - xNO = k(a - l)[NOI,t a(l - XNo)
(2)
The residence time, t , is defined as t = Vlv, where V is the void volume of the bed and v is the volume flow rate. The expression t o the left was plotted versus (a - l)[NO]otas shown in Figure 4. The rate constants at a given temperature were evaluated from the slopes in Figure 4 by using the least-squares method (average correlation coefficient = 0.96). The Arrhenius equation
Energy & Fuels, Vol. 9, No. 4, 1995 683
Reduction of NO over MgO and Dolomite 4001
I
3800ppm O2
3 2
2
-
x 21
I\ /-
8
950 ppm O2
X
ir
-
T O
I
-1
-2 -1
-0.8 -0.6 -0.4 -0.2 0 0.2 0.4 (a-l)[NOlot {mole.s/m3)
0.6
0.8
0
Figure 4. Testing of the dolomite data for the second-order rate equation (eq 1): (m) 600 "C; ( 0 )700 "C; (A)800 "C; (e) 900 "C;(O) 950 "C. 1.5
1
0.5
rc h
-
0
-0.5
-1
8
8.5
9
9.5
10
10.5
11
11.5
12
io4m ( K 1 l
Figure 5. Arrhenius diagram using the second-order rate constant for dolomite.
(eq 3) could then be applied (Figure 5), giving the activation energy E , and the preexponential factor A.
k = Ae-En'RT(m3/(mol*s))
20
30 40 Time (min)
50
60
70
Figure 6. Decrease in the reduction of NO with CO as the concentration of 0 2 is increased. Bed: 30 m2 MgO diluted in Si02. Total weight of bed: 8 g. Concentration: 380 ppm NO, 7600 ppm CO, 0-3800 ppm 0 2 . Residence time: 100 ms. Temperature: 850 "C.
The influence of oxygen and small amounts of water on the NO reduction reaction was investigated over MgO. In Figure 6 the qualitative decrease in the reduction of NO with CO is shown as the oxygen concentration increases. When equivalent amounts of oxygen and carbon monoxide were added, the reduction of NO decreased to nearly zero. At this temperature the oxidation of CO was complete (7600 ppm CO, 3800 ppm 0 2 , 100 ms, and 850 "0. When the gas mixture with 380 ppm NO, 7600 ppm CO, and 3800 ppm 0 2 was dried with phosphorus pentoxide, the oxidation of CO decreased slightly, giving some CO available for the NO reduction reaction. The reduction was increased from approximately 4% without t o 10-15% with phosphorus pentoxide at 700 "C. At 850 "C no difference could be seen with or without phosphorus pentoxide. The influence of water on the reaction between nitric oxide and carbon monoxide, without oxygen present (380 ppm NO, 7600 ppm CO, 100 ms), wasnot significant at 850 "C. At 700 "C the reduction of NO increased 5% without compared to with phosphorus pentoxide.
(3)
The activation energies and the preexponential factors obtained were 600-814 "C Ea = 85.8 kJ/mol,
A = 7.14 x l o 7 m3/(mol*s) 814-950 "C E , = 1.3 kJ/mol, A = 6.29 x
IO
lo3 m3/(mol*s)
The break in the Arrhenius plot at 814 "C suggests that the reaction mechanism was changed. The Ea at temperatures above 814 "C is too low, though, which makes it unrealistic. Effects of a nonuniform temperature distribution in the bed from the heat of reaction were neglected due to the low concentrations of NO used. The kinetic results were not compensated for the empty reactor and the quartz sand used to dilute the dolomite. The catalystic effect of the quartz at the concentrations studied, however, is rather low (
