Energy & Fuels 1998, 12, 457-463
457
Kinetics of NO Reduction by Black Liquor Char Sheng Liang Wu and Kristiina Iisa* Department of Chemical Engineering, Oregon State University, Corvallis, Oregon 97331 Received June 16, 1997
The reduction of NO by black liquor char and activated carbon was studied at atmospheric pressure in a fixed bed reactor at temperatures between 450 and 650 °C. Black liquor char has a high Na to fixed C ratio (2.7), and the Na is well dispersed within the carbon matrix. The reduction of NO by both materials was confirmed to be of first order with respect to NO. The reaction rate constant, based on total surface area, for black liquor char was over 3 orders of magnitude higher than that for activated carbon which is consistent with the role of Na in catalyzing the NO reduction reaction. In the temperature range studied, N2 and CO2 were the major reaction products for the reaction by black liquor char. In the presence of CO, the rate of NO reduction by black liquor char was enhanced by CO over the temperature range studied by a factor of 2.5 with 4% CO. The consumption of carbon was reduced to zero and the black liquor char provided a catalytic surface for NO reduction by CO. The activation energies of NO reduction by black liquor char both in the absence and presence of CO and by activated carbon were all within the range 67-73 kJ/mol.
Introduction Kraft pulping is the most common process for the production of pulp for paper manufacture. In it, wood is digested in a solution of NaOH and Na2S, and the cellulose fibers that form the pulp are separated from the solution. The spent solution is called black liquor and it consists of the wood residue not used for pulp together with the inorganic cooking chemicals. Black liquor typically has a Na content of 18-23% on dry solids basis, and a fixed carbon content of 35-40%. Black liquor is burnt in special boilers, called recovery furnaces, to recover the cooking chemicals and generate energy. Emissions of nitrogen oxide, NO, have become an important environmental issue affecting the operation of recovery boilers. NO emissions vary depending on operating conditions and furnace configuration. In recovery boilers, once NO is formed it may be reduced by several different reaction pathways. Experiments in laboratory reactors have shown that the reduction may be significant: in a reactor simulating a recovery boiler, 40% of the NO formed became reduced,1 and in a laminar entrained-flow reactor, up to 20% of NO was reduced in oxidative conditions and up to 90% in reducing conditions.2,3 Homogeneous gas phase reactions and reduction by char are deemed the most important reduction mechanisms in fossil fuel combustion.4 Another possible NO reducing mechanism in * Author to whom correspondence should be addressed. Current address: Institute of Paper Science and Technology, 500 10th St., N.W., Atlanta, GA 30318. E-mail:
[email protected]. Fax: (404) 8945752. (1) Nichols, K.; Lien, S. J. Tappi J. 1993, 76, 185-191. (2) Iisa, K.; Carangal, A.; Scott, A.; Pianpucktr, R.; Tangpanyapinit, V. Prepr., 1995 Int. Chem. Recovery Conf.; Tappi, CPPA: Toronto, 1995, pp B241-B250. (3) Pianpucktr, R. M.S. Thesis, Oregon State University, Corvallis, OR, 1995.
recovery boilers involves reactions with sodium salts. There is a large amount of sodium aerosols present in recovery boilers and they may provide a NO depletion mechanism. The kinetics of NO reduction by molten sodium compounds have been investigated by Thompson and Empie.5,6 Due to its high Na content, black liquor char may be a very efficient NO reducing agent. The study reported here was undertaken to characterize the reduction of NO by black liquor char. A number of studies of the kinetics of NO reduction by carbon have been made. The carbon sources include graphite, activated carbon, coal char, coke, shale char, and soot from hydrocarbons.7-11 A summary of these results is given by Teng et al.12 The reactivities of different carbon materials vary by several orders of magnitude. In general, the materials with higher ash contents are more efficient in reducing NO. Alkali metals catalyze the NO-carbon reaction, and their activity changes in the order Cs > K > Na.13 Black liquor char has a very high sodium content and, in addition, considerable amounts of K. Therefore, black liquor char is expected to be efficient in reducing NO. (4) Goel, S. K.; Beer, J. M.; Sarofim, A. F. J. Inst. Energy 1996, 69, 201-213. (5) Thompson, L. M.; Empie, H. J. AIChE Symp. Ser. 1994, 90 (302), 33-38. (6) Thompson, L. M. Ph.D. Thesis, Institute of Paper Science and Technology, Atlanta, GA, 1995. (7) Smith, R. N.; Swinehart, J.; Lesnini, D. J. Phys. Chem. 1959, 63, 544-547. (8) Watts, H. Trans. Faraday Soc. 1951, 54, 93. (9) Lai, C. S.; Peters, W. A.; Longwell, J. P. Energy Fuel 1988, 2, 586. (10) Chan, L. K.; Sarofim, A. F.; Beer, J. M. Combust. Flame 1983, 52, 37-45. (11) Furusawa, T.; Kunii, D.; Osuma, A.; Yamada, N. Int. Chem. Eng. 1980, 20, 239-244. (12) Teng, H.; Suuberg, E. M.; Calo, J. M. Energy Fuel 1992, 6, 398406. (13) Kapteijn, F.; Mierop, A. J. C.; Moulijn, J. A. J. Chem. Soc., Chem. Commun. 1984, 1085-1086.
S0887-0624(97)00086-8 CCC: $15.00 © 1998 American Chemical Society Published on Web 03/13/1998
458 Energy & Fuels, Vol. 12, No. 3, 1998
Wu and Iisa Table 1. Physical Properties of the Materials Employed material
dp, µm
black liquor char 125-250 activated carbon 125-250
Figure 1. Schematic diagram of the experimental apparatus: (a) rotameter, (b) electric furnace, (c) reactor tube, (d) temperature controller, (e) cooling tube, (f) heating tape, (g) CO nondispersive infrared analyzer, (h) CO2 nondispersive infrared analyzer, (i) chemiluminescence NO/NOx gas analyzer, (j) data acquisition computer, (k) Fourier transform infrared spectrometer, (l) thermocouple, (m) three-way valve, and (n) sintered glass + char bed.
The reduction of NO by black liquor char can be regarded to be a gasification reaction: carbon is consumed and either carbon monoxide or carbon dioxide is formed.
NO + C(s) f 1/2N2 + CO
(1)
2NO + C(s) f N2 + CO2
(2)
The reactivity of black liquor char in ordinary gasification reactions is very high and the gasification rates are faster than those of other carbon sources with similar alkali metal contents. The CO2 gasification rate of black liquor char is an order of magnitude higher than that of coal char mixed with 10-20% Na2CO314 or that of a high surface area activated carbon impregnated with 12% Na2CO3.15 The higher reactivity of the black liquor char is explained by the very fine dispersion of sodium throughout the carbon matrix. The fine dispersion is a result of formation of the char from a liquor precursor.15 In the present study, the kinetics of reduction of NO by black liquor char were investigated and compared to that of activated carbon. Additionally, the effect of CO on the reduction of NO by black liquor char was examined. Experimental Section The experiments were carried out in a fixed bed reactor. A schematic diagram of the experimental apparatus is shown in Figure 1. The reactor was made of quartz glass and had an inner diameter of 13 mm and a height of 1000 mm. A fixed sintered quartz glass plate was situated at the height of 500 mm in the reactor. A shallow layer (approximately 2 mm) of quartz wool was placed on top of the plate to prevent direct contact of the bed materials with the sintered plate. A sample of the carbon source that was weighed to an accuracy of 1 mg (14) Li, J.; Van Heiningen, A. R. P. Can. J. Chem. Eng. 1989, 67, 693-697. (15) Li, J.; Van Heiningen, A. R. P. Ind. Eng. Chem. Res. 1991, 30, 1594-1601.
Fb, g/cm3 Sa, m2/g re, Å Vp, cm3/g 0.20 0.47
14.5 1150
300
3.255
was added on top of the quartz wool as a fixed bed. The temperature in the reactor was measured by a chromelalumel thermocouple located inside the reactor approximately 2 mm above the bed. Mixtures of NO in helium, CO in helium, and helium were passed into the reactor. The inlet and outlet gases were analyzed for NO by a chemiluminescence analyzer, and for CO and CO2 by nondispersive infrared analyzers. Prior to each experiment the gases bypassed the reactor, and the inlet concentrations were measured during this time. Additional analyses of the product gases were obtained by taking samples for a Fourier transform infrared spectrometer (FTIR) for CO, CO2 and NO analysis, and for a gas chromatograph (GC) for N2 analysis. A grab sample was taken for the gas chromatograph. The gas chromatograph was a Carle series 100 ACG instrument equipped with a 6 ft molecular sieve 5Å column and a thermal conductivity detector. The carrier gas was He and the oven temperature was set at 85 °C. All analyzers were calibrated using several (5-11) known concentrations of the species to be measured. Two different carbonaceous solids were used in the experiments: black liquor char and activated carbon. The activated carbon was purchased from Fisher Scientific and it originally had a particle size of 75-300 µm. The fraction 125-250 µm was used in the study. The activated carbon contained 5% ash and had a surface area of 1150 m2/g. The black liquor char had been produced by partial pyrolysis of a black liquor at a temperature of 950 °C in a black-liquor reactor at the Institute of Paper Science and Technology (IPST).16 The liquor had been obtained from Champion International’s Canton mill in North Carolina. The char from the black-liquor reactor was ground and sieved, and the same size range as for the activated carbon (125-250 µm) was used in the experiments. The internal surface area, pore volume, and pore size distribution of the black liquor char were measured by using N2 physical adsorption (BET). The physical properties of the two carbonaceous materials are given in Table 1. The black liquor char had become oxidized during storage and handling, and most of the sulfur was in the form of Na2SO4. In a kraft recovery furnace, however, the char contains mainly Na2S, not Na2SO4. Therefore, the black liquor char was heat treated in the experimental apparatus to reduce Na2SO4 to Na2S. The reactor was heated to 700 °C at a rate of 20 °C/min in a flow of 5% CO. The temperature was held at 700 °C for 40 min, after which the reactor was cooled to the temperature of the experiment (450-650 °C), still in a flow of CO in He. The sulfate reduction efficiency was 95% by this thermal treatment. The elemental and chemical analysis of the black liquor char are given in Table 2. The reduced char represents the char after the thermal treatment. During the thermal treatment additional pyrolysis of the material occurred as well, and the char mass decreased. The mass loss can be estimated by assuming that all of the sodium remained in the sample. The untreated (raw) char contained 34.6% Na and the heat treated (reduced) char 38.8%. This gives an estimated 11% mass loss during the heat treatment. All the results, however, are based on the mass of the raw char. The composition of the char is representative of char that is produced in a recovery boiler. During the experiments when the appropriate temperature was reached after the heat treatment, the flow of 5% CO was stopped and He was passed into the reactor for 5 min to allow (16) Lee, S. R. Ph.D. Thesis, Institute of Paper Science and Technology, Atlanta, GA, 1994.
Kinetics of NO Reduction by Black Liquor Char
Energy & Fuels, Vol. 12, No. 3, 1998 459
Table 2. Chemical and Elemental Analysis of the Black Liquor Char Fraction 125-250 µm Chemical Analysis, wt % material SO42-
S2-
raw char 14.5
