740
Energy & Fuels 1989,3,740-743
amount of lignin in the wood is sufficient to account for all of the up to 23 wt % of the wood tar estimated to be prone to char-induced conversion to char and for about four-fifths of the up to 35 wt % of the wood tar estimated to be susceptible to rapid, char-induced conversion to char plus gases. Further analysis of data of Nunn et al?J3 led to the estimate that pyrolysis of milled wood lignin, a type of lignin less disturbed during its separation from wood than are most lignins, can evolve tar in yields up to 71 wt % of the lignin. Analysis of the present wood type reveals a lignin content of 27.3 wt % (Table I). Thus, assuming no interactions with pyrolysis products from the other wood components, the maximum amount of lignin-derived tar released by pyrolysis of the present wood is estimated to be 19.4 wt % of wood (0.71 X 27.3) or 28 wt % (19.4/ 68.6) of the wood tar. Pyrolysis of cellulose and hemicellulose could also yield aromatics, but aromatization of the substrate or, more probably, of species released by initial thermolysis of the substrate would first be required. Assuming the above estimates of the magnitude of the char-reactive tar fraction are correct, on the order of 7 wt % of the wood tar (35% - 28% estimated maximum tar from lignin), or about one-fifth of the char-reactive fraction, would need to come from non-lignin sources. In summary, consideration and further analysis2 of results from Nunn et al.9J0J3in light of the present mode 2 and mode 4 observations suggest that a fraction amounting to as much as 35 wt % of the tar released from the surface of pyrolyzing wood undergoes rapid (space time as low as 52.5 ms) conversion in the presence of fresh wood pyrolysis char. Carbon dioxide and CO are definite products of this conversion, and additional char (coke) is (13) Nunn, T. R.; Howard, J. B.; Longwell, J. P.; Peters, W. A. Ind. Eng. Chem. Process Des. Deu. 1985,24,844-852. (14)Chang, H.-M. Department of Wood and Paper Science, North Carolina State University, Raleigh, NC, private communication, 1981. (15) Andrews, E. K. M.S. Thesis, Department of Wood and Paper Science, North Carolina State University, Raleigh, NC, 1980.
a highly probable product. The conversion reactions are too fast to determine their kinetics from the present data. The underlying mechanisms of the char-induced tar conversion, including the roles, if any, of active carbonaceous sites or native mineral matter, were not established.
Conclusions A fraction of newly formed wood pyrolysis tars is very reactive (space times of 12.5 ms) in the presence of fresh wood pyrolysis char at temperatures as low as 400 "C. This fraction is highly resistant to vapor-phase thermal cracking up to 600 OC, contains oxygen, is probably more aromatic than the whole tar,and is estimated to amount to as much as 35 wt % of the total amount of tar released at the surface of pyrolyzing wood. Lignin is indicated as a major but not the only source of this tar fraction. Carbon dioxide and CO are definite products of this char-induced tar conversion, and additional char (coke) is a highly probable product. Acknowledgment. Most of the above work was supported by the National Science Foundation, Division of Chemical and Process Engineering, Renewable Materials Engineering Program, under Grant No. CPE-8212308 and CBT-8503664. We also gratefully acknowledge an NSF Fellowship for M.L.B. and financial support by the Edith C. Blum Foundation of New York and the Robert C. Wheeler Foundation of Palo Alto, CA. Drs. D. Bruley, M. K. Burka, J. Hsu, L. Mayfield, and 0. R. Zaborsky have served as NSF technical project officers. Christine Clement and Anne Reeves have made valuable contributions to this work as part of MIT undergraduate research programs. We also thank Professor H.-M. Chang and his colleagues, Department of Wood and Paper Science, North Carolina State University, who specially prepared samples of sweet gum hardwood, milled wood lignin, and other wood constituents for our biomass research program. Registry No. COz, 124-38-9;CO,630-08-0.
Control of NO, Emissions by Selective Catalytic Reduction with Hydrogen over Hydrophobic Catalysts L. Fu and K. T.Chuang" Department of Chemical Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 2G6 Received May 24, 1989. Revised Manuscript Received July 28, 1989 The selective catalytic reduction of NO, with H2 over noble-metal catalysts supported on hydrophobic material has been studied for the control of NO, emission from simulated stationary sources. At a reaction temperature below 140 "C, the hydrophobic catalysts show excellent selectivity, i.e., NO, rather than 02,is preferentially reduced. For a feed containing 1000 ppm NO,, 1%H2, and 3.2% O2 in Nz, the catalysts showed a stable NO, conversion of 60-80% at a temperature above 55 OC and a space velocity of 3000 to 10000 h-l. The introduction of liquid water into the reactor made little difference in catalyst activity. The activity of the noble metal catalysts was found to be in the order Pt > Pd-Ru > Pd > Ru >> Au. The presence of O2 in the feed tends to decrease the NO, conversion. The control of reaction temperature is crucial to the catalyst selectivity due to the competitive reaction between H2 and Oz. Optimum reaction temperature and space velocity lead to the minimum hydrogen requirement. Introduction The control of NO, emission from stationary sources is required to protect the environment. Selective catalytic 0887-0624/89/2503-0740$01.50/0
reduction is one approach that has been studied extensively. A number of gases, H2, CO, CHI, and NH,, can reduce the NO, to elemental N2.'-' However, only NH3 0 1989 American Chemical Society
Control of NO, Emission
has been shown to selectively react with NO, while the other gases also readily react with oxygen and thus are considered to be nonselective reducing agents. The flue gas normally contains 2-5% oxygen and 0.02-0.05% NO,. Obviously, nonselective reaction is not economical because a large amount of reducing agent is consumed. Since the flue gas is also saturated with water vapor, the reaction can only proceed at an elevated temperature where pore condensation does not occur. Typically, reduction is carried out at about 400 "C with NH3 and at >200 "C with Hp2 Although NH3 shows good selectivity, the use of NH3 can cause operating problems, such as the formation of solids in the process equipment. Also, the discharge of unreacted NH3 causes other environmental damage. In contrast, hydrogen is an environmentally benign agent. If a catalyst with good selectivity could be found, reduction of NO, with hydrogen could be a preferred process. Anderson et al.' were the earliest scientists to systematically study the reduction of NO, with hydrogen in the presence of oxygen. The experiments were performed in a small pilot scale plant at 120 "C and 100 psi. The feed gas had a composition similar to that of the tail gas from a nitric acid plant, i.e.: NO
+ NOp
0 2
0.2-0.5% 2.5-5%
H20
saturated under absorber conditions
It was found that NO2was reduced to NO in the presence of excess 02, but further reduction of NO was not reported. To achieve high NO, conversion, high H2 content (>5%) and high reaction temperature (>12OoC)were required. Also, test data indicated the high temperature resulted in a short catalyst lifetime. Shelef and Gandhi2reported that noble metals, Pt, Pd, Ru, and Os, showed good activity at temperatures above 200 "C. Among these, Pd had the best selectivity. Shelef and Gandhi also studied the effect of oxygen on the catalyst activity and selectivity when hydrogen is in excess to oxygen. It was found that higher temperatures were required to maintain the same activity at a higher concentration of O2in the feed. Base-metal oxide catalysts were reported to be active only at temperatures above 200 oC.e Because they are deactivated by the water vapor present in the feed gas, none of the catalysts that have been tested can be used at temperatures below 100 "C. To avoid water vapor poisoning, the reactor must be maintained at high temperatures, requiring extra equipment and energy. It has been reported that the activity of hydrophobic catalysts was insensitive to high humidity and liquid water for CO oxidation and the hydrogen-oxygen reaction under ambient conditions?~~In this study, tests were conducted to assess the activity and selectivity of noble-metal catalysts supported on hydrophobic materials for the reaction (1)Anderson, H. C.; Green, W. J.; Steele, D. R. Ind. Eng. Chem. 1961, 53, 199-204. (2)Shelef, M.;Ghandi, H. S. Ind. Eng. Chem. Prod. Res. Deu. 1972, 11, 393-396. (3)Kobylinski, T. P.; Taylor, B. W. J. Catal. 1974,33,376-384. (4)Stenger, H. G., Jr.; Hepburn, J. S. Energy Fuels 1987,1,412-416. (5)Hepburn,J. S.; Stenger, H. G., Jr. Energy Fuels 1988,2,289-292. (6)Shelef, M.;Gandhi, H. S. Ind. Eng. Chem. Prod. Res. Deu. 1972, 11, 2-11. (7)Bauerle, G. L.;Nobe, K. Ind. Eng. Chem. Prod. Res. Deu. 1974,13, 185-193. (8)Seddon, W.A,; Chuang, K. T.; Holtslander, W. J.; Butler, J. P. Proceedings of the American Society of Mechanical Engineers-EnergySources; Technology Conferenceand Exhibition; New Orleans, LA, 1984; 84-PET-11. (9)Chuang, K.T.; Quaiattini, R. J.; Thatcher, D. R. P.; Puissant, L. J. Proceedings of the 18th DOE Nuclear Airborne Waste Management and Air Cleaning Conference; NTIS: Springfield, VA, 1984.
Energy & Fuels, Vol. 3, No. 6,1989 741
1. Mass flow c o n t r o l e r 2. Two-way valve 3. Three-way valve
4 . Four-way valve 5 . Dry r e a c t o r 6 . Furnace
7. Wet reactor 8. NO-NOx analyzer 9. Recycle pump
F i g u r e 1. Flow diagram of test facility.
of NO, and HP. Because most industrial waste streams are at low temperatures and high humidities, our measurements were carried out at 20-150 "C,thus avoiding the need to preheat the feed gas to high temperatures.
Experimental Section A number of noble metal catalysts supported on styrene-divinylbenzene copolymer (SDB) were tested. They were Pt/SDB (2 wt %), Pd-Ru/SDB (2 wt %-2 wt %), Ru/SDB (2 wt %), Au/SDB (2 wt %), Pd-Ru/SDB (5 wt %-0.5 w t %), and PdRu/SDB (0.5 w t %-5 wt %). These catalysts were prepared at Chalk River Nuclear Laboratories by impregnation of H2PtC&, RuC13, HAuCl,, and PdC13 dissolved in ethanol. The material was rotary evaporated at 95 "C and, under a slight vacuum to remove the ethanol, then heated in air at 105 "C for 1 h. Subsequently, it was reduced in hydrogen a t 200 "C until the p H of the furnace outlet became neutral. The percent loading of the catalyst was based strictly on the initial weight of the chemical complexes. The SDB catalyst support, in the form of 10-20 mesh granules, was prepared by polymerizing divinylbenzene in ethylvinylbenzene (DVB). 2Methyl-1-pentanol and 2,2-azobis(2-methylpropionitrile)(AIBN) were used as solvent and initiator, respectively. The SDB had a BET area of 465 m2/g and an average pore diameter of 1.8 nm. These values were obtained with a Micromeritics Accusorb 2100E surface area analyzer. The support was stable a t temperatures up to 240 "C, as indicated by the thermogravimetric analysis. The apparatus used to test the catalysts is shown in Figure 1. Reactor 5 was a dry 1.5-cm-diameter integral reactor containing 10 mL (2.3 g) of catalyst. It was enclosed in a furnace, and the temperature of the furnace could be varied from 25 to 400 "C. A thermocouple was installed at both the inlet and the outlet of the catalyst bed to monitor the temperature rises due to the heats of reaction. Reactor 7 was a wet reactor. Water could be added to the reactor from either the top or the bottom, and its temperature was controlled with a constant-temperature water bath. The system was designed so that the reaction gas could flow through reactor 5 or through reactor 7 or through both. Compressed air was purified with activated carbon. Matheson mass flow controllers were used to control the feed flow rate and composition. The feed gas a t a rate of 12-100 L / h was prepared by mixing 5 vol % NO in nitrogen with various amounts of diluent gases. The content of NO in the inlet and outlet gas streams was measured with an NO-NO, analyzer (Model 10, Thermo Electron Instruments). The conversion of Hzand O2across the reactor was determined by analyses of the gas streams with a gas chromatograph (HP 5710A). Since the reaction could produce N2, N20, and NH3, N 2 0 and NH3 were analyzed by gas chromatography, and the concentration of N2 was determined by nitrogen balance. N2 and N20 were found to be the major products whereas NH3 was not found, presumably because the formation of NH3 was suppressed by the presence of O2in the reaction mixture. Test data indicated that the product composition is a complex function of experimental conditions and catalyst design. A comprehensive
Fu and Chuang
742 Energy & Fuels, Vol. 3, No. 6, 1989 100
,
$
I
60
-
i
~
70
/
/
10000h' [Hz]= 1 % [Oz]= 3 2 % [NOx]= 0.1 % A:5%Pd-0 5% Ru/SDB
B: 2% Pd-2%
z
8
'
I
3000 h'
Ru/SDB
40
i 2% Au/SDR
0
20
40
60
e0
100
120
I40
160
180
FEED TEMPERATURE ( % )
Figure 2. Effect of temperature on NO, conversion over no-
ble-metal catalysts.
Table I. Effect of Temperature on the Activity and Selectivity of Pd-Ru (2 wt %-2 wt % )/SDB Catalysta Ti&, "C Tout,OC conversion, % selectivity, % 42 48 9 9.6 52 64 18 14 31 15 60 87 74 12 68 130
a[H2J= I % , [02] = 3.2%;SV = 3000 h-l (SV = space velocity).
test program is under way to study the product selectivity. Results and Discussion Activity and Selectivity of Noble-Metal Catalysts. The activity of the catalysts was measured in terms of NO, conversion versus feed temperatures at a constant space velocity of 3000 h-l. The feed composition was 1000 ppm NO,, 1% H2,and 3.2% 02,with the balance being NP The results are summarized in Figure 2. The Pt/SDB catalyst showed the highest activity while the Au/SDB catalyst showed no activity. The catalyst activity was in the order Pt > Pd > Ru >> Au. The activity of the Pd catalyst was greatly improved by the addition of Ru to the catalyst. The Pd-Ru/SDB bimetallic catalyst showed higher activity than Pd/SDB at a temperature above 40 "C. The Pt/SDB catalyst reached the maximum NO, conversion at 45 "C while the Pd/SDB catalyst reached its peak at about 68 "C. From Figure 2, it can be seen that the activities of Pt/SDB and Pd/SDB catalysts increase rapidly in the temperature range 30-60 "C. Part of the increase in activity is due to higher reactor temperature from the heat of the H2-02 reaction. With 1% H2 in the feed, there is a potential for the gas temperature to increase by 80 "C if the H2 is completely consumed by oxygen. Anderson et al.l reported that hydrogen was an effective reducing agent for the treatment of tail gas from a nitric acid plant. At temperatures above 140 "C, Pt, Pd, and Ru could nonselectively reduce NO2to NO, thus avoiding the visibility of NO2 at the stack. Under our experimental conditions, a temperature increase as high as 42 "C was observed. The effect of temperature on the activity and selectivity is given in Table I, where the selectivity is defined as the ratio of the amount of H2 consumed by the reaction with NO, to the amount of H2 consumed by the reaction with O2 Nevertheless it can be seen from Figure 2 that there is a wide range of temperatures, 50-140 "C, at which the catalyst shows high activity and selectivity toward NO, reduction. This temperature "window" allows the process to accept various industrial feeds without further heating or cooling. If the feed is available only at room temperatures, the reaction rate can be increased by
FEED TEMPERATURE ('C)
Figure 3. Effect of temperature and space velocity on the activity of Pd-Ru/SDB catalysts.
spiking the feed with a small amount of hydrogen. The heat of reaction between H2 and O2 can easily raise the reactor temperature to a desirable level. For Pt, Pd, and Pd-Ru catalysts, there is an optimum temperature at which the highest NO, conversion is obtained. Catalyst activity increases when the reaction temperature reaches a peak and then falls off. Higher initial feed temperatures resulted in high rates for both the H2-02 and H2-NO, reactions. Heat from the Hz-O2 reaction raises the temperature in the reactor and leads to higher NO, conversion. However, if too much hydrogen is consumed by oxygen, the NO, conversion will decrease. Therefore, the NO, conversion is affected by feed composition, temperature, and flow rate and the selection of catalyst. For a given feed composition and temperature, there is an optimum selectivity that can be achieved for a given catalyst. Three different Pd-Ru/SDB catalysts, 5% Pd0.5% Ru/SDB, 2% Pd-2% Ru/SDB, and 0.5% Pd-5% Ru/SDB, were tested with two different space velocities, 3000 and 10000 h-l. Figure 3 shows the NO, conversion versus the feed temperature. It can be seen that the NO, conversion over all catalysts has a similar pattern regardless of the differences in metal loading and space velocity. The NO, conversion increases with feed temperature, reaches a peak optimum conversion, and then decreases. This confirms the observations by other researchers that H2is a nonselective reducing agent. Clearly hydrogen loses its selectivity if the reaction temperature is above 200 "C. The increase in space velocity simply shifts the optimum temperature to a higher value. Figure 3 indicates that Pd is active at a temperature below 80 "C whereas Ru is active only at temperatures above 120 "C. Unfortunately, Ru can be oxidized to Ru02in the presence of oxygen at >200 "C, and the catalyst is unstable at these temperatures. Effect of O2 and H2 Concentrations in the Feed. Infrared studylo shows that most of the noble-metal catalyst surface is covered with NO molecules during the reduction of NO with H2. However, the situation is different with the presence of O2 in the gas stream. First of all, part of the catalyst surface will be covered with oxygen: 02 + 2s 20-s Furthermore, oxygen atoms adsorbed on the surface will react with either available NO or hydrogen on the surface, i.e. NO-s + 0-s NOz-s 2H-s 0-s H2O-s
-
+
+
+
(10)Hecker, W. C.; Bell, A. T. J. Catal. 1985,92, 247-259.
Energy & Fuels, Vol. 3, No. 6,1989 743
Control of NO, Emission loo
,
Table 111. Stability of Pd-Ru/SDB (2 wt 70-2 wt W ) Catalyst' time, h 0 22 25 33 55 74 78 conversion, % 76 78 78 76 77 79 79
I
A
Space Velocity = 4200 / h 80
I
/
i[NG] ,= 1500 pppm, 1 Reaction Tem e r a t u r e
65'C ~
3011 20
0 ; 0
1
Pd-Ru/SDB(2.0-2.0 w t % ) I Space Velocity = 1200/h [ @ ] = 3.4 vol% [ N G ] = 1000 p p m Reaction Temperature 7OoC
I 2
6
4
I n l e t [O,] a n d
[I&]
8
10
(volX)
Figure 4. Effect of hydrogen and oxygen concentration on NO, conversion. Table 11. Effect of reactor T, conditions O C 60 dry wet 60
Liquid Water on NO, Conversion SV, % % conversion, h-' H2 O2 % 5000 1 3.0 88 5000 1 3.0 90
These two surface reactions compete with the reduction of NO. The reaction between H2 and O2 over the Pt catalyst can even be carried out at room temperature^.^ Therefore, one should expect that the presence of O2 in the feed stream will result in lower conversion of NO,. NO, conversions were measured over the Pd-Ru/SDB (2 wt %-2 wt %) catalyst at 65 "C with different O2 concentrations in the feed. The NO, concentration in the feed was 1500 ppm. Figure 4 shows the experimental results. The NO, conversion decreased from 70% to 52% when the O2concentration in the feed increased from 2.8% to 9.8%. This clearly shows that the reaction between H2 and O2is a competing reaction with H2 and NO,. When H2 concentration is in excess of 02,a complete NO, conversion can be achieved at a temperature as low as 70 "C. When O2 concentration in the feed is high, e.g. 9.8%, more H2 is consumed and the reactor temperature increases due to the heat of reaction between H2 and 02.Thus, process variables like feed temperature and space velocity must
aTemperature = 70 "C; [H,] = 1%; [NO,]= 0.1%; [02]= 3.2%; = 3000 h-*.
SV
be balanced to achieve optimum NO, conversion. Effect of H20. To study the effect of water on the NO, conversion, liquid water was added from the top of the reactor to the catalyst bed with a recycle pump. Table I1 shows the results for the Pt/SDB catalyst. The NO, concentration in the feed was lo00 ppm. The temperature of the reactor was controlled by circulating hot water through a jacket. It can be seen that the addition of liquid water into the reactor has no effect on NO, conversion. This is the major advantage of SDB-supported catalysts. Being hydrophobic, the catalyst can survive in high humidity-even in liquid water. Thus, it may be possible for the catalyst to be incorporated into an SO2wet scrubber for simultaneous removal of SO2 and NO, from the flue gases. Catalyst Lifetime. A 78-h test showed no deactivation for Pd-Ru/SDB catalyst during the test. This is shown in Table 111. Because the reaction can be carried out with hydrophobic catalyst at relatively low temperatures, the deactivation of the catalyst due to the high reaction temperature is avoided. Conclusion Catalytic reduction of NO, with hydrogen is a nonselective reaction at temperatures above 200 "C. When the reaction temperature is lowered the catalyst becomes selective. For a group of noble-metal catalysts, there is a wide temperature range 50-145 "C, where the selectivity is excellent. To avoid deactivation of catalyst by high humidity, it is imperative that hydrophobic support be used to ensure long catalyst life. Acknowledgment. This study was funded by Atomic Energy of Canada Ltd. Registry No. Pt, 7440-06-4; Pd, 7440-05-3; Ru,7440-18-8;Au, 7440-57-5; Hz,1333-74-0; NO,, 11104-93-1.
