Ind. Eng. Chem. ProcessDes. Dev. 1985, 2 4 , 581-588
581
Reed, T.; Gubblns. K. “Applied Statlstlcal Mechanics”; McGraw-Hill: New Yo&, 1973. Rushbrook, 0 . “Introductlon to Statlstlcal Mechanics”; Clarendon Press: Oxford, 1957. Slttlg, M. “Polyolefln Prcductlon Processes, Latest Developments”; Noyes Data Corp.: Park Ridge, NJ, 1976. Spahl, R.; Luft, 0. Ber. Bunsenges. phvs. Chem. 1981, 8 5 , 379. Spahl, R.; LufI, 0. Angew. Makromol. Chem. 1983, 775, 87. Vera, J.; Prausnltz, J. Chem. Eng. J . 1972, 3. Wohlfarth, C.; Ratsch, M. Acta fowm. 1981, 32(72),733.
Flory, P. Discuss. Faraday SOC. 1970, 49, 7. Golosov, A,; Dlnces, A. “Techndogy of Polyethylene and Polypropylene Productlon”; Hlmia, Moscow (In Russian), 1978. Goebel, S. “Kunststoff-Handbuch, Band I V , Polyoleflne”; Vlewig. R.; Schley, A.: Schwarz. A. Ed., Carl Hauser Veriag: Munchen, 1969. Harmony, C.; Bonner, D.; Helchelhdm. AIChEJ. 1977, 23(7), 758. Kobyakov, V.; Kogan, V.; Retsch, M.; Zernov. V. Mast Massy 1979, 8 , 24. Klelntjens. L.; Konlngsveld, R. Cdlold fo/ym. Scl. 1980, 258. 71 1. Konlngsveld, R.; Staverman, A. J . Pdym. Sci. 1988, fartA-2(6). 305. Konlngsveld. R.; KlelntJens, L. Macromdecules 1971, 4 , 637. Llchtenhaler, R.; Liu, D.; Prausnltz, J. Macromolecules 1978, 7 7 , 192. Uu, D.; Prausnk. J. Ind. Eng. Chem. frmess Des. Dev. 1980, 79, 205. Loos, T.; Poot, W.; Dlepen. G. Macromolecules 1983, 76, 111. Luft, 0.; Llndner, A. Angew. Makromol. Chem. 1978, 5 6 , 99. Luft, G. Chem. Ing. Tech. 1879, 57(10). 960. Maloney, D.; Prausnltz, J. J . Appl. fowm. Sci. 1974. 78, 2703. Maloney, D. Ph.D. Thesis, Unlversh of CaHfornla, Berkeley, 1975. Maloney, D.; Prausnk, J. A I C M J . 1978a, 22, 174. Maloney, D.; Prausnltz, J. Ind. Eng. chem. Rocess Des. Dev. 1978b, 75(1), 216.
Received for review July 18, 1983 Revised manuscript received July 20, 1984 Accepted August 8, 1984
Part of this paper was presented at the International Congress ‘Berliner Polymeren Tage”,Oct 7-9,1982, West Berlin, Federal Republic of Germany.
Liquid Products from the Continuous Flash Pyrolysis of Biomass Donald S. Scott,’ Jan Plskorz, and Demond Radlelnt Depettment of Chemical Engineering, University of Waterloo, Waterloo, Ontario, Canada
A bench-scale continuous flash pyrolysis unit using a fluidized bed at atmospheric pressure has been employed to investigate conditions for maximum organic liquid yields from various biomass materials. Liquid yields for poplar-aspen were reported previously, and this work describes resuits for the flash pyrolysis of maple, poplar bark, bagasse, peat, wheat straw, corn stover, and a crude co”ercla1 cellulose. Organlc liquid yields of 60-70% mf can be obtained from hardwoods and bagasse, and 4 0 4 0 % from agricultural residues. Peat and bark with lower cellulose content give lower yields. The effects of the addition of lime and of a nickel catalyst to the fluid bed are reported also. A rough correlation exists between ash content and maximum organlc liquid yield, but the liquid yield correlates better with the cy-ce#ubsecontent of the biomass. General relationships valid over all reaction conditions appear to exist among the ratlos of final decomposition products also, and this correlation is demonstrated for the yields of methane and carbon monoxide.
In an earlier paper (Scott and Piskorz, 1982), the authors described a bench-scale continuous fluidized bed apparatus for the atmospheric pressure flash pyrolysis of biomass, peat, or coal. In that report, results were given for the pyrolysis of hybrid poplar-aspen wood. A number of other biomass feeds have also been studied in this apparatus, and results for these materials are given here. Some tests are also described using additivites or catalysts in the fluid bed. From theae results some general empirical conclusions may be drawn. The atmospheric flash pyrolysis process to produce organic liquids from biomass appears to have wide applicability. It is a simple process, requiring low capital investment, and may be economic on a relatively small scale. This latter factor is of critical importance when the cost of harvesting biomass waste is high, e.g., as for logging residues, and a processing unit should be sited as closely as possible to the source of the biomass. The process may be of particular interest in third world countries that have no indigenous fossil fuels but do have extensive biomass resources, for example, the Caribbean countries. The liquid product may be used as a source of low grade fuel directly or may be upgraded to higher quality liquid fuels. In addition, the possibility exists that chemicals may be directly recoverable from the liquid product. Also, a medium heating value gas is generated and minor amounts +Departmentof Chemistry, University of the West Indies, Mona, Kingston, Jamaica. 0196-4305/85/1124-0581$01.50/0
of a reactive char, both of which are available as good quality fuels. For these reasons, a number of biomass waste materials were investigated for their behavior in atmospheric flash pyrolysis. Experimental Section Materials. In addition to the hybrid poplar-aspen wood reported on previously, raw materials used in the present work were hybrid poplar-aspen bark, eastern red maple wood, wheat straw, corn stover, bagasse, commercial cellulose, and peat moss. The poplar-aspen wood came from a plantation of the Ontario Ministry of Natural Resources and was cut from a 7-year-old log about 0.19 m in diameter. The poplar bark was taken from the same section of log. The wheat straw and corn stover were obtained as harvesting wastes, that is, after ripening, from Ontario farms. The bagasse was the air-dried trash from normal sugar cane grinding in which the whole cane, both pith and rind, is ground. It was obtained from a sugar mill in St. Catherine, Jamaica. The peat was also Jamaican in origin, from the Negri1 region. It was a heterogeneous, earthy gray-brown material still containing visible plant remains. The commercial cellulose was obtained from Iotech Co. and is prepared by a stream process with alkaline extraction. Samples were air-dried, ground in high-speed rotary cutting mills. and then screened to eive a fraction 105-250 pm iniize. r;lsome cases, fine or &mer material was also tested to see if any significant difference in behavior resulted. However, as there appeared to be little difference 0
1985 Amerlcan Chemlcal Society
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Table I. Materials Used: Typical Values (maf) crude cellulose poplar-aspen lignin 8 16.2 hemice11u1ose 0 31.0 42.3 92 cellulose ash 70 0.39 0.75 C 49.6 41.90 6.11 H 5.48 N 0.46 0.56 0 43.7 52.16 S moisture 70 as fed 4.02 4.93 H/C mole ratio 1.48 1.57
maple 24.0 23.7 44.7 0.59 48.5 6.1 0.50 44.9 nil 5.33 1.51
either in pyrolysis behavior or in elemental analysis for moderate changes in particle size, the 105-250 pm size was used as the standard. Properties of the materials used are given in Table I. Values for lignin, hemicellulose, and cellulose for wheat straw, corn stover, and bagasse are typical literature values. All other data given are from analyses of the actual material used. Because of the high chrbohydrate content, all samples, except possibly the peat, had similar ranges of carbon-hydrogen-oxygen content. However, wide variations in ash content are evident. Again, with the exception of the peat, all are quite highly cellulosic materials, although the hemicellulose to cellulose ratio shows considerable variation. Experimental Procedures. All tests were done with the apparatus described in the previous report by Scott and Piskorz (1982). The low-rate feeder used previously was found to be satisfactory for the various additional materials pyrolyzed, although the entrainment orifice size had to be adjusted as did the ratio of the entrainment gas through the entrainment line and through the bed. The most difficult material to feed was corn stover because of the highly two-dimensional particle shape. This material also pyrolyzed very readily and occasionally caused feed inlet blockages. An attempt was made to feed Iotech lignin also, but the low melting point of this material caused it to decompose in the inlet feed line, and operation of the fluid bed could not be maintained. In all tests reported here, a gas residence time of 0.445 was used with nitrogen as the fluidizing bed gas, and -250, +lo5 pm feed particles, unless otherwise specified. All analyses of products were conducted as previously described, using acetone as the solvent to remove residual reaction products from the reactor and connecting lines. Material balances were generally good, that is, 95% recovery or better. However, in cases where large anibunts of volatile organic compounds were produced, material balances were not as satisfactory because the solvent washmg and evaporation technique used to determine tars also caused nearly total loss of any light materials collected in the condensers. The principal light organics were included in the material balances as determined from the chromatographic analysis of gas samples withdrawn from the reactor free board space. It may be of some interest to observe here that the micro samples required for modem analytical chemistry, e.g., chromatography, elemental analysis, are not ideal for process work, or work with heterogeneous mixtures. The bench scale unit used experienced occasional brief fluctuations in feed rate, or system pressure, and because of the small reaction volume and very short processing time, these oscillations are not damped out. Hence, a 2-mL on-line gas sample sent to the chromatograph could show a considerable scatter. Similarly, sampling a heterogeneous feed material for elemental analysis when a quantity of only a few milligrams is used can lead to substantial variations in results. Some of the
aspen bark 13.7 18.0 34.3 4.38 51.0 6.40 0.53 42.1 nil 7.55 1.51
bagasse 20.2 28.5 37.0 3.92 46.4 5.2
peat 56.5 16.6 12.9 9.2 62.0 4.4 3.8 28.0 1.8 13.0 0.85
0.0
48.2 0.2 5.5 1.34
----
75
corn stover 13.0 43.0 31.0 11.0 50.1 5.01 0.93 33.0 nil 9.0 1.20
-;---. '.
L
406
wheat straw 16.7 41.8 32.4 4.6 48.5 5.13 0.50 41.3 nil 6.5 1.27
LIME ADDED - _ _ NO LIME 800
500 TEMPERANRE 'C
Figure 1. Liquid yields with CaO added; -250, +105-pm aspenpoplar; 5% moisture; N,; 0.44 s.
- LIME ADDED --- NO LIME
-..x
* \
/
/
/'.
,'
'\
\\ CHAR
I
400
I
500
I
600
TEMPEPATURE ' C
Figure 2. Char and gas yields with CaO added: -250, +105-pm aspen-poplar; 5% moisture; N,; 0.44 s.
scatter in the resylta reported here is felt to be due to these analytical difficulties, although results reported are, in most cases, averages of a number of samples which were taken in as representative a fashion as possible.
Results Poplar-Aspen Wood. (a) Effect of Lime Addition. It is well-known that alkali cations can catalyze the thermal decomposition of wood. In order to test this effect in the simplest possible way, CaO was added to the sand in the fluid bed (0.5 g of CaO for 15 g of sand). While this may not be as effective as impregnation from solution, the simplicity of adding dry lime is economically attractive. Aspen-poplar wood was fed continuously to the bed containing the lime. In these tests, the sawdust/lime ratio over the time of the run was about 15:l on a weight basis. Results for the liquid yield as a function of reaction temperature are given in Figure 1compared to results in
Ind. Eng. Chem. Process Des. Dev., Voi. 24, No. 3, 1985 583
+
Table 11. Yields of Light Organics from Aspen Poplar with Lime Added (-250, 105 pm Sawdust, N2 Atomsphere, Moisture 4.93%, Residence Time 0.44 8 ) run no. 80 78 76 77 81 83 82 temperature, OC 400 450 500 550 550 550 600 yield, % of wood weight fed acetaldehyde 0.32 0.49 0.99 1.68 0.75 0.82 1.84 ethanol 0.11 0.13 0.11 0.25 acetic acid 1.65 0.61 2.63 2.21 3.11 1.79 1.05 acetone, acrolein, furan 0.35 0.30 0.68 1.28 1.72 0.93 0.58 methanol + formaldehyde 1.15 0.94 1.10 1.25 1.18 1.35 0.85 water 13.41 10.70 14.06 14.93 15.61 14.14 12.62 tar analysis %
C H
0 char analysis % C
H
54.48 6.66 38.7
53.78 6.70 39.34
54.73 6.74 38.33
53.03 6.63 40.15
56.87
64.70 63.16 4.31 4.39
63.10
71.56
3.62
3.28
4.94
+
Table 111. Poplar-Aspen Sawdust (-280, 105 pm; 0.44 Moisture 4.93% with Girdler Ni Catalyst) run no. 99 86 temperature, OC 350-380 500 product yields as % of wood fed (5% moisture) 56.48 54.89 CHI
co
co2 H20 char tar light org. H2 reacted, % of wood fed % O2 in gas/O2 Fed % C in gas/C Fed
s; 87 600
2.82 4.64 18.36 21.77
4.82 13.16 25.27 2.5
40.7 23.33 19.00 15.89 1.82
nil
nil nil
nil nil
0.47 (C2&) 11.36 50.8 93.8
9.74 5.74 83.0 98.5 102.0 97.4
which no lime was added. Figure 2 shows the corresponding yields of char and gas. Liquid yields are not affected by the addition of lime, but the maximum yield is obtained at about 50" lower temperature, that is, 470 "C rather than 525 "C. Somewhat more gas is produced when lime is added, as expected. The yield of light organics is not seriously affected as shown in Table 11, and also Figure 1,but appears to be somewhat lower. The only other significant effect of lime is to double the yield of ethylene and to increase methane yield by 50%. Comparison of Table I1 with previously published results for aspen-poplar derived tars (Scott and Piskorz, 1982) indicates that the tar produced when lime is added is higher in carbon and lower in oxygen, presumably because of the greater CO + C02production. As was found for ordinary pyrolysis, the best quality of oil appears to be produced a t the point of maximum yield. Depending on the products desired, lime addition to the fluid bed might prove to be beneficial. (b) Effect of Metallic Catalyst. A Girdler nickel catalyst on a refractory support (G-33RS, 35.6% nickel) was crushed and screened to give 15 g of -205 +105-pm material which was substituted for the sand in the fluidized bed. It was activated for 24 h before use by passing a slow stream of hydrogen through the reactor at 500 "C. Test were carried out in an atmosphere of approximately 55% hydrogen and 45% nitrogen by volume. Results of the three runs carried out, all a t essentially atmospheric preasure, are given in Table 3. The only products obtained in measurable amounts were CI&, CO, C02,H20,and char. At low temperatures (350-500 "C), 8595% of the carbon fed forms methane. At 600 "C, only 65% of the carbon goes to methane with the balance appearing as CO and COP. There is also an increase in the amount of pyrolytic water formed over that obtained without catalyst. A t 500
'4 01 400
I
500
I
600
I
650
TEMPERATURE ,T
Figure 3. Tar, gas, and char yields: -250, +105-pm aspen-poplar bark; 7.55% moisture; N2;0.44 8.
and 600 "C,the composition of H20, C02, H2, and CO is close to that required for the water gas shift equilibrium, so that the catalyst appears to also promote this reaction, as well as acting in the expected way as a methanation catalyst. The ready hydrogenation of pyrolysis fragments all the way to methane under these relatively mild conditions suggests that catalytic hydrogenation of the oil product should not be difficult. Pyrolysis of Poplar-Aspen Bark. Results obtained for the pyrolysis of poplar bark are shown in Figures 3 and 4. Product yields are quite different from those obtained with poplar-aspen wood. The maximum tar yield of 43% (mf) occurred at the same temperature (500"C) but was much lower than that from wood. Char yields were much higher (26% to 19%, mf), due in part to the high ash content, and gas yields were also much higher (13% to 43%). Figure 4 gives the yields of C02, CO, and CHI obtained and clearly shows the much larger quantities of C02generated from bark pyrolysis than from wood (compare to Figure 8 in the paper by Scott and Piskorz, 1982). The poplar bark contains a high level of water and alkali extractives, suggesting a high content of tannin-like materials, which are often found in significant amounts in barks. Pyrolysis of tanninsis known to yield polyhydroxy phenols and C02. These very reactive phenols would probably appear as polymeric materials in the tar or char. The fact that the yields of hydrocarbon gases are relatively low also reflects the low lignin content of the bark as shown in Table I. The high COzproduction removes a larger than
564
Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 3, 1985
Ir
4t 500
400
600
TEMPERATURE 'C CH.
TEMPERATURE ,%
Figure 4. CO, COz, and CHI yields: -250, +105-p aspen-poplar bark; 7.55% moisture; N,;0.44 s.
Figure 6. Char and gas yields: -250, +105-pm sugar maple; 5.33% moisture; Nz0.44 s. Dashed lines are for aspen-poplar.
Table IV. Pyrolysis of Iotech Cellulose (N2, 0.445; -250, 105 pm: 4.02; Moisture; 0.75% Ash) temperature, "C 426 532 625 741 yields, % as fed gases 12.1 18.3 21.1 44.1 46.9 50.4 41.4 29.0 tar light organics 0.4 1.8 2.9 4.1 18.9 9.7 char 10.9 8.0 water 12.6 18.0 20.9 13.0
+
total
90.9
98.2
97.2
98.2
gas yields, % of feed H 2
45{
4 TEMPERATURE ' C
Figure 5. Liquid yielda: -250, +105-pm sugar maple; 5.44% moisture; N,;0.44 s. Dashed lines are for aspen-poplar.
usual amount of oxygen from the liquid and char, and although COz, CO, and HzO are only about 22% of the yield at 500 "C, they contain about 50% of the oxygen in the original bark. Pyrolysis of Eastern Maple Wood. Results for maple wood are shown in Figures 5 and 6 with the results for poplar-aspen at comparable conditions shown as dashed lines. The moisture content of the air-dried maple sawdust (5.33%) was very similar to that of the poplar-aspen. Hence, the yields of tar shown in Figure 5 can be directly compared. They are very similar except that the maximum tar yield from maple occurs at 450 "C rather than 500 "C. Gas and char yields (Figure 6) are also very similar, although poplar-aspen tends to produce more char than maple. A somewhat higher yield of CO is obtained from maple, and a somewhat different pattern of COz production. The most strinking difference in product distribution is in the yield of hydrocarbon gases, with yields of CHI and CzH4up to five times greater from maple wood than from aspen-poplar. In addition, considerably larger amounts of ethane and propylene were obtained from maple. The lignin content of maple is much higher than that of as-
0.011 5.19 6.79 0.051 0.016 0.017 0.16
0.054 7.69 9.76 0.34 0.12 0.094 0.40
0.19 9.53 9.88 0.69 0.22 0.17 0.81
0.39 26.73 12.85 1.30 1.30 0.34 1.33
pen-poplar. Since the yield of hydrocarbon gases obtained from all woods at higher temperatures is considered to be derived partially from lignin decomposition, it is therefore not surprising that CHI and CzH4yields are increased at the high temperature. The amounts of light organics formed from maple are very similar to, although somewhat less than, those obtained from aspen-poplar. It can probably be concluded that most hardwood species in flash pyrolysis would give results for product yield and distribution comparable to those obtained with the two hardwoods used here. Pyrolysis of Poplar-Aspen Cellulose. The material used was a residual cellulose remaining after steam treatment and an alkaline extraction, that is, the cellulosics left after removal of the hemicelluloses and partial delignification by the Iotech process. The resulting product from poplar-aspen is about 92% cellulose and 8% lignin. Pyrolysis tests were carried out on this partially purified cellulose in order to obtain some data on the behavior of one component of the wood. Results are shown in Table IV for temperatures from 426 to 741 "C. The maximum yield of organic liquids occurs at 525-535 "C, as it does for hardwoods. Surprisingly, char is somewhat higher and organic liquid yields somewhat lower for this cellulose. Gas yields are also significantly higher with CO and C 0 2 yields up to 50% greater. These results may reflect a greater reactivity of the delignified cellulose or a catalytic action due to residual sodium present from the extraction step which would tend to give higher gas and char yields and less tar at a given temperature. The trends in CO, COz,
Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 3, 1985 585 0
A
TAR TOTAL ORGANICS TOTAL LIQUIDS
Table V. Pyrolysis of Corn Stover-Experimental Yields (Nz, 0.445; -250, 105 pm; 9.0% Moisture; 11.0% Ash) temperature, "C 450 500 550 600 650 yields, % as fed gases 11.0 13.7 13.8 13.4 12.5 tar 19.9 25.5 29.7 29.4 22.0 light organics 1.0 1.8 2.1 2.7 3.3 char 45.7 42.2 34.5 27.7 19.0 water 14.1 18.7 18.4 gas yields, % of feed
+
0 2.45 8.25 0.08 0.02 0.02 0.23 0.43 0.19 0.18 0.41
HZ
co
q
l
0
-1
25 450
500
550
co2
b
, 600
6%
, 700
TEWEMTURE P C )
Figure 7. Liquid yields: -250, +105-pm wheat straw; 6.5% moisture; Nz; 0.44 a. A
CHI
CZH, CZH6
c3+
acetaldehyde furan acrolein acetone
0 3.43 9.75 0.18 0.05 0.04 0.24 0.98 0.43 0.20 0.56
0 3.50 9.72 0.21 0.04 0.04 0.28 1.36 0.29 0.24 0.56
0.07 3.82 8.64 0.42 0.15
0.27 3.87 7.56 0.34 0.18
0.07
0.05 0.26 1.24 0.82 0.93 0.94
0.23 1.12 0.75 0.90 0.50
0 GAS
A CHAR
\
I
I
500
600
TEMPERATURE ,"C
Figure 9. Tar, gas, and char yields, bagasse: -250, +105-pm; 5.5% moisture; N,;0.44 s. I
450
500
I
I
I
1
550
600
650
700
I
TEMPERATURE, 'C
I
I
I
Figure 8. Char and gas yields: -250, +105-pm wheat straw; 6.5% moisture; N,;0.44 a.
CHI, hydrocarbon gases and light organics yields compared to those from wood all confirm that the majority of these materials a t lower temperatures are generated primarily as end products of cellulose and hemicellulose decomposition. Pyrolysis of Wheat Straw. The yields of liquids are shown in Figure 7, and the gas and char yields in Figure 8. Char and gas yields are both much higher for this material than for hardwoods, with a corresponding decrease in organic liquid yield. The temperature for maximum liquid yield is much higher also at 550-600 "C, but the narrow temperature range over which maximum liquid yields are obtained is similar to that found with other biomass feeds. The production of volatile organic liquids is much lower than from wood. Much less CO is formed and proportionally much more COz than from wood, with the result that the liquid a t 600 "C contains only about 36% oxygen as compared to 42% in the feed. Because of the high ash content of wheat straw, which is a common characteristic of the grasses, the char has a lower heating value than that from wood. Char at 600 "C contains only 39.5% C and 2.5% H with an ash content of about 15%. Pyrolysis of Corn Stover. Results from the pyrolysis of corn stover at normal conditions are shown in Table V. Pyrolysis occurred readily with this material and pyrolysis began at very low temperatures, so feed line blockage occasionally happened. Yields of organic liquids are considerably less for corn stover than for wheat straw, al-
500
600
TEMPERATURE, 'C
Figure 10. Gas yields, bagasse: -250, +105-pm; 5.5% moisture; N,; 0.44 a.
though both have similar total carbohydrate contents. However, ash is much higher for the corn stover. Inasmuch as gas yields are of the same order but char yields are much higher for the corn stover than for wheat straw, it is possible that the high ash content catalyzes the polymerization of some of the organic liquid initially formed. The maximum yield of organic liquids (about 36% moisture free basis) occulg at 550-600 "C, in the same temperature range as that of wheat straw. Pyrolysis of Bagasse. The yields of tar,gas, and char from bagasse are shown in Figure 9. Yields of tar are nearly as high as those obtained from hardwoods, and the temperature for optimum liquid yield is the same, 500 "C. Char production is higher, and gas yield lower, than from wood. Yields of various gases are shown in Figure 10. The amounts of the various gases produced and the trends with temperature are much more similar to those from hard-
586
Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 3, 1985
50
fi
-
L*
4 70
s9 w>
40-
lA W
L
0 30-
8
2
20-
B
10-
c W
I
I
I S3OO
I
2
3
4
5 6 7 8 9 IO II ASH CONTENT, % mf.
12 13
14
Figure 13. Maximum yield of organic liquid VB. ash content. 1
I
I
160
- ---
I20
0 so
0 40
0
400
500
600
700
TEMPERATURE, ' C
Figure 12. Gas yields, peat: -250, +105-pm; 13.0% moisture; N2; 0.44 8.
woods than to those from the straws, although it might be expected that bagasse would show thermal decomposition behavior more like the latter. It can be concluded that the ash in bagasse shows little catalytic activity. Pyrolysis of Peat. The yields of tar,gas, and char from Jamaican peat are given in Figure 11. The yields of various gaseous products are shown in Figure 12. The maximum tar yields of about 20% (24.8% maf basis) was obtained at 500 OC. About 2% light organic liquids were produced in addition to this tar. The low liquid yield is typical of earthy peata and brown coals (Tyler, 1979)pyrolyzed in fluid beds. Gas yields increase at higher temperatures at the expense of both tar and char production. The much larger amount of COz produced and the large COz/CO ratio reflect the humification of the vegetable matter in peat and the high content of carboxylic acids. Hydrocarbon gas production is very similar to that from other biomass feeds. Discussion The major objective of the pyrolysis tests done was to determine the optimum conditions for maximum liquid yields from various biomass materials. Although not reported here, as discussed in a previous report (Scott and Piskorz, 19821,the apparent gas residence time of 0.5 s appears to be near the optimum value for high liquid yields for all the biomass materials used. This was checked for peat for residence times from 0.25 to 1.0 s and again the optimum tar yield was obtained at 0.5 s. The maximum organic liquid yields show some interesting trends among the various materials used. Figure 13 shows a plot of maximum organic liquid yield against ash content. A regular trend exists for all except the peat moss (not shown) which has been chemically altered from ita natural state. The result for bagasse and for Iotech cellulose show some deviation, the latter possibly due to the fact that the ash content, although low, was about 13%
CELLULOSE, %
mf
Figure 14. Maximum yield of organic liquid vs. cellulose content.
sodium, which even in small quantities (Shafiiadeh and Stevenson, 1982)can be a significant gasification catalyst. On the other hand, the high ash content of bagasse appears to have very little effect on organic liquid yields. An alternative correlation of maximum organic liquid yield by flash pyrolysis can be obtained by comparison with the a-cellulose content as shown in Figure 14. Although the liquid yield considerably exceeds the a-cellulose content, because liquid products are also derived from the hemicellulose and lignin fractions, the maximum amount of organic liquid obtainable is directly related to the cellulose content for all materials except the alkali extracted cellulose. In some cases,for example, for the hardwoods and wheat straw, total carbohydrate contents have very similar values, but maximum organic liquid yield is nearly 50% larger for the hardwoods, which have a significantly higher cellulose content. How general this empirical relationship is for biomass materials is uncertain; however, it is likely to be generally useful for materials which do not contain excessive amounts of materials other than lignocellulosics, e.g., g u m s or resins. Recently, Funazukuri (1983)has shown that in the rapid pyrolysis of lignocellulosic materials very general approximate relationships exist among the ratios of various final decomposition products such as CO, COz, CHI, and C,H,/C&, and that these ratios apply over wide degrees of decomposition, wide rangea of temperature and heating rate, and for various kinds of reactors, as long as a relatively short residence time pyrolysis is being carried out. Figure 15 shows a plot of data for CO and CH4yields from the pyrolysis of cellulose from Funazukuri's work, as well as from results of other workers (Martin, 1965;Hajaligol
Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 3, 1985 587
where Vi is the yield of product i at time t, and Vi* is the ultimate yield of i at high temperature and long times. For two products 1and 2, one can write the integrated forms of eq 1
t
-
4
I
01
-
10 I O_I
10
I O2
CO YIELD, w t %
Figure 15. Correlations between CH, and CO yields for pyrolysis of cellulose (from Funazukuri). 4.0,
ol\
I
;6 '4 CO YIELD,
kElb 'le
2b
d
BY WEIGHT
Figure 16. Correlations between CHI and CO yields for various biomass materials.
et al., 1981; Bergougnou et al., 1984). This particular correlation is surprisingly good over very yide rahges of reaction conditions. Figure 16 shows the data for CO and CHI yields from this work, as well as those from 06previous paper (Scott and Piskorz 1982), and from unpublished pilot plant results, compared to data given by Funazukuri. Separate correlations are obtained for each material. These data represent all temperatures, various degrees of decomposition, and different particle sizes. All data were obtained in fluidized beds with apparent residence times of 0.25-1.0 s except for the data of Fun-, which are based on results not only from fluidized beds but ale0 from a pyroprobe and a cyclonic reactor. A variety of pyrolysis atmospheres were also employed. While the correlations in Figure 16 are approximate, they appear to apply under essentially all flash pyrolysis conditions and so may have a good deal of usefulneas for predicting results. While these results may also have mechanistic implications, for example, concerning the relative rates of secondary decomposition reactions, this subject is beyond the scope of the present work. Some explanation can be offered for the results shown in Figure 15 or 16, based on the work of Hajaligol et al. (1982), for the pyrolysis of cellulose. In their work, the rate of production of each gaseous pyrolysis product, i, was described by the rate equation
Since a data point in Figure 15 consists of corresponding yields of CO and CHI taken at the same temperature and reaction time, eq 2 becomes In V,*/(V,* - V l ) =
If the activation energies for the formation of a particular pair of gaseous products are similar, then eq 3 represents only a weak function of temperature, and a logarithmic plot of the yield of one product vs. the yield of the other product will give an approximately linear correlation for all times and temperatures. In the case of CO and CHI, the values of E reported are similar at 52 740 and 60 040 cal/mol, respectively (Hajaligol et al., 1982). Kumar and Mann (1982) used the same type of firstorder rate law as that given by eq 1to successfullymodel the yield of gaseous products from the pyrolysis of poplar bark. Although the rate law as given by eq 1was intended by Hajaligol et al. (1982) to apply only to the primary pyrolysis decomposition step, similar rate laws have been used successfully by others, e.g., Kumar and Mann, to apply to the global conversion to a product or to weight loss on pyrolysis. If it is assumed that the form of eq 1 can be applied to the overall rate of formation of a final decomposition product, then the conclusions given by eq 3 are valid and a correlation of yields will be obtained for processes with approximately equal activation energies. Indeed, one of the most interesting aspects of the correlation shown in Figure 15 is its apparent generality, since it covers a range of pyrolysis conditions and conversions which include not only primary but secondary pyrolysis reactions as well for some of the data shown. The temperature history and heating rate in the short residence time fluidized bed unit must have a bearing on the results obtained in this work, especially in view of the very high liquid yields. Heating of biomass particles begins in the feed injection tube, which is normally not cooled. Inasmuch as essentially all the biomass char is blown from the bed,the char residence time is short, although probably somewhat longer than that of the gas and vapor products. An approximate analysis has been made of the temperature history of the wood particles as a function of particle size and Nusselt number, and the results have been reported by Scott and Piskorz (1984). For particles of the size used in the bench scale work reported here (250 pm or less), the outside temperature of the particle was calculated to reach reaction temperature in about 0.2 s. Internal temperature gradients were estimated to be small and therefore the particle can be assumed to have been held at the stated reaction temperature for the majority of the residence time. It is probable, therefore, that heat transfer eonsiderations were not rate controlling in the results given here. The development of processes based on the thermal pyrolysis of biomass have been directed largely to gasification, either for fuel gas or synthesis gas production, and
588
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many such processes have been described. The pyrolysis of biomass to give a maximum direct yield of liquid products had received much less attention, and very few studies have been reported, for example, Duncan et al. (1981), Kosstrin (1981), Roy et al. (1982), Scott and Piskorz (1984). Of these, only the work of Kosstrjn and Scott and Piskorz appears to have both used a fluid bed reactor and been directed to maximizing liquid yields. Roy et al. pyrolyzed batch-wise under vacuum, and Duncan et al. (hyflex process) used a transport reactor under pressure. Comparison of the results reported here with those of others, whether using fluid bed reactors or other types, shows that as high or higher organic liquid yields have been achieved in this work. Although there is an extensive literature on the mechanisms of biomass pyrolysis reactions, a fairly detailed knowledge of primary reaction mechanisms together with a knowledge of the heat and mass transfer behavior in a particular reactor is required to determine why liquid yields may be better in one process than another. Registry No. Ni, 7440-02-0; C02, 124-38-9;CO, 630-08-0; CHSCHO, 75-07-0; CH&H20H, 64-17-5; CH&02H, 64-19-7; CH,COCH,, 67-64-1; CH,OH, 67-56-1; HCHO, 50-00-0; CH4,
1985,24. 588-592
74-82-8; acrolein, 107-02-8; furan, 110-00-9; cellulose, 9004-34-6.
Literature Cited Be[gougnou, M. A.; Graham, R. G.; Mok, L. K.; Freei, 8. A.; Overend, R . P. Ultrapyrolysis: The Continuous Fast Pyrolysis of Biomass”, Bio Energy 84,GGteborg, Sweden, June 1984. Duncan, D. A.; Bodle, W. W.; BanerJee, D. P. ”Production of LiquM Fuels from Biomass by the yrflex Process. Energy from Biomass and Wastes, V”; Inst. Gas Tech., 1981;pp 917-938. Funazukuri, T. Ph.D. Thesis, Department of Chemical Engineering, University of Waterloo, Waterloo, Ont., 1983. HabUgOl, M. R.; Howard, J. B.; Longweii, J. P.; Peters, W. A. Ind. Eng. Chem. ProcessDes. D e v . 1982, 21, 457-465. Kosstrin, H. M. “Direct Formation of Pyrolysis Oil from Biomass”; Proc. Spa cialists Workshop on Fast Pyrolysis of Biomass, Copper Mountain, C SERIlCP 622-1096,Solar Energy Research Institute, US. Dept. of Energy, Oct 1981;pp 105-121. Kumar, A.; Mann, R. S. J. Anal. Appl. pvrolysis 1982, 4 , 219-226. Martin, S.B. “Proceedings, Tenth International Symposium on Combustion”; 1985;p 877. Roy, C.; de Caumia, B.; Chwnet. E. “Liquids from Biomass by Vacuum Pyrolysis-Production Aspects”, Proc. Specialists Meeting on Biomass Liquefaction, Saskatoon, Sask. ENFOR Program, Canadian Forestry Service, Envkonment Canada, Feb 1982;pp 57-74. Scott, D. S.; Piskorz, J. Can. J. Chem. Eng. 1982, 60, 866-674. Scott, D. S.;Piskwz, J. Can. J. Chem. Eng. 1984, 62, 404-412. Shafizadeh, F.; Stevenson, T. T. J. App. Polym. Sci. 1982, 27, 4577-4585. Tyler, R. J. Fuel 1970, 58, 880-686.
Receiued for review August 19, 1983 Accepted August 20, 1984
Effectiveness of Magnetic Water Treatment in Suppressing CaCO, Scale Deposition Davld Haseon” and Dan Bremson Department of Chemical Engineering, Technlon-Israel Institute of Technology, Halfa 32000, Israel
The effecttveness of a commercial magnetic device in suppressing CaC03 scale deposition was investigated in a system consisting of a cast iron pipe through which hard water flowed at ambient temperature. The main variable studied was the supersaturation level of the CaC03-formingions over a range represented by (Ca*+XCO~-)= 20 X lo3 to 65 X lo3 (ppm as CaC03)*. The effect of magnetic exposure on scale suppression was evaluated from measurements of the rate of deposit growth, the extent of the induction period, and the adhesive nature of the incrustation. The accurate rate data showed that magnetic exposure had no effect on deposit growth. Similarly, magnetic exposure exerted no effect on the adhesive nature of the deposits. The less accurate induction period data did not reveal a statistically significant difference either.
Introduction Scale suppression by magnetic treatment consists of passing a potentially scaling water stream through a magnetic field provided by an adequately sized device. Sizing is based primarily on the water flow rate. The intensity of magnetic fields of research and industrial devices that have been used for inducing a scale suppression tendency ranges from 100 to 10000 G and exposure times are of the order of a few seconds, at most. Promoters of magnetic devices claim that this simple operation provides a viable scale control method, even for waters having a marked scaling tendency. It is asserted that magnetic treatment can prevent or markedly reduce the amount of scale precipitated. Moreover, it is said that precipitate morphology is altered. Any deposit accumulating on a flow surface is said to precipitate in the form of an easily washable sludge rather in the form of a trou-
blesome tenacious incrustation. It is also often claimed that magnetic exposure can inhibit corrosion. The intense controversy regarding the effectiveness of magnetic water treatment devices has a long history (Cowan and Weintritt, 1976). Currently, there is a revival of the controversy and renewed interest, stemming apparently from favorable reports published in the Russian literature (Hibben, 1973; Troup and Richardson, 1978; O’Brien, 1979). The present work was initiated as a result of an aggressive promotion drive of magnetic treatment in Israel, by a representative of a US. company. This led Mekorot, the national water supply authority, to sponsor tests of a magnetic device under well-controlled laboratory conditions. The results described in this paper provide unambiguous quantitative data on the effect of magnetic exposure on the rate of CaC03 scale deposition and on the
Ol96-43O5/85/1 124-O588$Ol.50/0 0 1985 American Chemical Society
