Ind. Eng. Chem. Process Des. Dev. 1985, 2 4 , 1281-1287
rc = 7.54 x
1012 exp(
-)cc2H:" -23 500 RT
It is expected that such simplified models might also be used to model the coking rates during the pyrolysis of other hydrocarbons. Nomenclature CA = concentration of n-hexane, mol/L CAo = inlet concentration of n-hexane, mol/L CclH4= concentration of ethylene, mol/L Ei = activation energy of the ith reaction, cal/g-mol or kJ/ g-mol F A o = inlet molar flow rate of hexane, mol/s ki = reaction rate coefficient for ith reaction kio = frequency factor for the ith reaction rate coefficient, s-l or g coke/[(mol i/L)" m2 h] m = reaction order for the coking reaction n = reaction order for hexane pyrolysis PT = total pressure, atm R, R, = gas constant, cal/(g-mol K) or L atm/(mol K) -rA = rate of reaction for hexane pyrol sis, mol/(L s) rc = rate of coke formation, g coke/(m 7h) T = temperature, K T,,, = average temperature for all the runs, K V = reactor volume, L XA = conversion of hexane yCzH4 = ethylene selectivity, mol produced/mol of hexane cracked
7
1281
= space time, s
Registry No. CBH14,110-54-3; C2H4,74-85-1; CHI, 74-82-8; C3H8, 115-07-1; Hz, 1333-74-0; C, 7440-44-0.
Literature Cited Aibright, L. F.; McConneil, C. F. "Abstracts of Papers", 175th National Meeting of the American Chemical Society, Annaheim, CA, March 1978;American Chemical Society: Washington, DC, 1978. Albright, L. F.; Yu, Y. C. "Abstracts of Papers", 175th National Meeting of the American Chemical Society, Annaheim, CA, March 1978; American Chemical Society: Washington, DC, 1978. Biba, V.; Macak, J.; Klose, E.; Maiecha, J. Ind. Eng. Chem. Process Des. Dev. 1078, 17,92. Ebert, K. H.; Ederer, H. J.; Isbarn, G. Int. J . Chem. Kinet. 1083, 15, 475. Fernandez-Baujin, J. M.; Solomon, S.M. Paper presented at the First Chemical Congress of the North American Continent, Mexico City, 1975. Frey, F. E.; Hepp, H. J. Ind. Eng. Chem. 1033, 25, 441. Herriott, G. E., Eckert, K. E.; Aibright, L. F. AIChE J . 1072, 18, 81. Hirt, T. J.; Palmer, H. 8.Carbon 1063, 1 , 65. Iiles, V.; Piaszkats, I.; Szepesy, L. Conf. Chem. Proc, Petro/. Natural Gas 1985, 1.
Johnson, G. L.; Anderson, R. C. Proc. Carbon Conf. 5th 1082, 1 , 395. Kinney, C. R.; Del Bel, E. Ind. Eng. Chem. 1054, 4 6 , 548. Kumar, P.; Kunzru, D., Can. J . Chem. Eng., in press. Murata, M.; Saito, S., Amano, A., Macde, S. J. Chem. Eng. Jpn. 1073, 6 ,
252. Murata, M.; Saito, S. J . Chem. Eng. Jpn. 1075, 8 , 39. Oxley, J. 0.; Secrest, A. C.; Veigel, N. D.; Biocher, J..M. AIChE J . 1061, 7 ,
498. Shah, Y. T.; Stuart, E. 6.; Sheth, K. D. Ind. Eng. Chem. Process Des. Dev. 1076, 15,518.
Snow, R. H.; Schult, H. C. Chem. Eng. Proc. 1057,5 3 , 133. Sundaram, K. M.; Froment, G. F. Chem. Eng. Sci. 1070, 3 4 , 635. Sundaram, K. M.; Van Damme, P. S.; Froment, G. F. AIChE J . 1081, 2 7 ,
946. Zdonik, S. 6.; Green, E. J.; Haiiec, L. P. Oil Gas J . 1067, 6 5 , 98.
Greek Letters a = moles of product formed per mole of hexane decomposed 6 = inlet molar ratio of steam to hexane eA = relative volume change during pyrolysis
Received for review June 26, 1984 Revised manuscript received February 20, 1985 Accepted March 5, 1985
Gasification of Sawdust in an Air-Blown Cyclone Gasifier James W. Couslns' and Wllilam H. Robinson Physics and Engineering Laboratory, Private Bag, Lower Hutt, New Zealand
An air-blown cyclone gasifier was designed to gasify sawdust at rates of 30-180 kg/h, giving cold gas outputs of 0.1-0.6 MW. A number of experiments were conducted with the air/wood ratio being varied. As the ratio was increased from 1.2 to 2.5 scm/kg of ovendry wood, the calorific value of the gas decreased from 5 to 2.5 Ml/scm and the cold gas energy yleld from 60% to 45% of the input wood energy. The gas consisted of N,, CO, H,, CO,, CH, and small amounts of higher hydrocarbons. The lowest operating airlwood ratio of 1.3 scm/kg was set by the accumulation of charcoal in the gasifier. The recommended input of 1.7 scm/kg gave a cold gas output of 0.55 MW for a wood input of 180 kg/h. The cyclone gasifier was simple to operate, reliable, the responded rapidly to changes in demand for gas.
The most common way of converting wood to wood gas is by partial combustion in air. Although it is a century-old technique, it is not a well-developed one. For example, most designs of gasifiers contain a slowly moving bed of charcoal or fuel through which gases and vapors have to flow, and so the performance is highly dependent on the porosity of the bed (Palmer et al., 1982). If a suitable level of porosity and a steady flow of solids through the reactor are to be achieved simultaneously, much of the fuel has to be in the form of small blocks ranging in size from 20 to 200 mm. Not all species of wood are useable, and
neither are sawdust, hogfuel, small wood chips, and many pelleted products. Small particle materials like sawdust make up a large part of the residue from a modern sawmill. Wood-free bark and slabwood can usually be sold, the bark as a horticultural potting mix and the slabwood for pulp chips, leaving the sawdust as the waste most readily available for fuel. Sawdust can be burned for industrial heating, but direct combustion is not always the best approach. In the retrofitting of oil- or gas-fired boilers, for example, it is often
0 196-430518511124- 128 1$0 1.5010 Published 1985 by the American Chemical Society
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Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 4, 1985 GQS + Charcoal 4
n
el
LSawdu +Air
SawdusJE +Air Abrasion -Resistant Refractory -Insulation Preheat Burner Air
SIDE VlEWISect~onl
TOP VIEW ISection A-AI
Figure 1. Sectional views of the air-blown cyclone gasifier for sawdust. The capacity of the gasifier is 200 kg (oven-dry)/h, or 1 MW.
preferable to gasify the solid fuel separately and to burn the wood gas in the boiler. There is, therefore, market potential for an air-blown gasifier for sawdust. Two types of reactors, fluid bed and entrained bed, are able to handle small particle materials. Several fluid-bed gasifiers have recently been developed to laboratory or pilot scales, and for 2 years, a 24 MW unit supplied gas to the boilers of a plywood mill (Reed, 1980;B. H. Levelton and Assoc. Ltd., 1981; Palmer et al., 1982; Adamsak, 1984). Specifications were that the fuel should be less than 50 mm in size and 35% in moisture content, although up to 120% moisture could be tolerated in some cases. There has been less experience with entrained-bed gasifiers. Two of this type, the Pillard and the Morbark, are being offered for commercial use, but there seems to be little information on their capabilities (Reed, 1980). The following work is intended to provide some of the missing information.
Materials and Methods The gasifier was a cyclone, 600 mm in diameter, 1000 mm high, and mounted vertically. It was made from 10mm mild steel and lined with castable insulation and abrasion resistant refractory, both 50 mm thick (Figure 1). Connected to it by steel pipes of 80-mm internal diameter were a fuel hopper and rotary metering valve on the entrance side and on the exit side a cyclone for removing charcoal dust from the product gas and a wood gas burner. The fuel was Pinus radiata sawdust that had been dried to 15 30% moisture and passed through a 10-mm screen. It was 49.9 (*0.3)70 by weight carbon, 6.2 (*0.3)% hydrogen (Campbell, 1978), and 0.4% ash. The fuel was blown into the gasifier at velocities of 8-12 m/s. Most of the air needed for partial combustion entered the gasifier with it, but some was also provided through two 25-mm pipes at the top of the gasifier and one 12-mm pipe at the bottom. One of the 25-mm pipes housed a natural gas burner for igniting the gasifier. The ignition procedure was to preheat the gasifier to 700 “C with the burner and then to turn on the fuel and air and turn off the burner. Gasification would start within a few seconds. The wood gas and byproducts left the gasifier through an axial port at its top. Samples of gas for analysis were taken downstream of the charcoal-removal cyclone and passed through a soot filter and a condensates recovery train that have been described previously (Cousins, 1983a). Analysis was by gas chromatography (Cousins, 1983a). Materials and energy flows from the gasifier were not measured directly but were calculated from the analyses of the feed and products with the assumption that C and H were conserved in the gasification reactions. An alter-
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native method based on nitrogen as a tracer was not used because (a) the elements important for energy were C and H and (b) with nitrogen being the major component of the wood gas, the precision of the results would have been unduly sensitive to the precision of the nitrogen measurements. Temperature profiles inside the gasifier were taken 50, 200, 350, and 500 mm below the inside top surface. The thermometer was an inconel-sheathed Type K thermocouple attached to a digital indicator (SKF Type 729117A). The aims of the work were to determine the permissible operating conditions for the gasifier and the nature of the products. The main operating variables studied were the air/wood ratio, the wood input rate, and the axial air flow. The ranges covered were air/wood ratio 1.2 2.5 scm/kg, wood rate 30 180 kg/h (giving energy throughputs for the cyclone of 2 1 2 MW/m3), axial air flow 0 24 scm/h, and total air flow 160 240 scm/h. Note that the unit of gas volume is the standard cubic meter (scm) and that wood weights and moisture contents are given on an oven-dry basis. The effect of varying moisture content was not studied in detail, although the moisture content varied from 15% to 26%. In the discussion that follows, the term “oxygen input” is used in preference to “air/wood ratio” because the reactive constituent of the air was the oxygen. An oxygen input of 1.00 kg/kg is equivalent to an air/wood ratio of 3.33 scm/kg; hence the range covered by the oxygen input during the gasification trials was 0.35 0.76 kg/kg.
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Results (a) Operation of the Gasifier. Once preheated to >600 “C, the gasifier self-ignited as soon as the fuel and air flows were turned on and ran steadily provided the oxygen input was >0.35 kg/kg. At 0.35 kg/kg, the gasifier ran satifactorily when the fuel moisture content was 15%, but at 26% it gradually filled with charcoal and after about 20 min stopped gasifying. The upper limit to the oxygen input was not found. The highest value tested was 2.9 kg/kg, which corresponds to a combustion with 100% excess air. For the production of a combustible gas, a maximum value of about 0.7 kg/kg is recommended because beyond this point the calorific value of the gas was found to be very low. The limits to the air and wood flows were set by the ancilliary equipment. For example, when the air flow was
