Znd. Eng. C h e m . Res. 1988,27, 256-264
256 80
Acknowledgment
~
The authors are grateful to Energy, Mines and Resources, CANMET of Canada, for providing the financial support during the entire study. Registry No. CH4, 74-82-8; CH,OH, 67-56-1.
-
, i
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601
Literature Cited
c
-
;
-
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v a)
I 0
m
5
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c
2
4
6
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8
%I
'0
Figure 3. Effect of oxygen concentration in the feed gas on methan01 selectivity: (symbol) pressure (in atm), temperature (in "C);( 0 ) 35, 411; (0) 36, 449.
concentration of 5% in the feed gas and a residence time the methanol selectivity was found to deof 10 s (NTP), crease from 81% at 50 atm to 25% at 125 atm. However, those studies were conducted in a stainless steel reactor. Hence, it may be worthwhile to vary the pressure in the glass-lined reactor to beyond 65 atm so as to determine the optimum pressure for highest methanol selectivity. Conclusions In this study we showed that methanol selectivities of 75-80% at 8-10% conversion levels per pass could be obtained during the partial oxidation of methane in a glass-lined reactor operated at about 65 atm, 450 O C , and a residence time of about 2 min. The methanol selectivity was observed to depend significantly on the oxygen concentration in the feed gas and reaction pressure. Oxygen concentrations less than 5% and reaction pressures higher than 50 atm were found to be conducive for higher methanol selectivity.
Boomer, E. H.; Broughton, J. W. Can. J . Res. 1937, 15B, 375. Boomer, E. H.; Thomas, V. Can. J . Res. 1937a, 1 5 4 401. Boomer, E. H.; Thomas, V. Can. J. Res. 1937b,15B, 414. Brockhaus, R.; Franke, H. J. German Offen 2 743 113, 1979. Chou, T. C.; Albright, L. F. Ind. Eng. Chem. Process Des. Dev. 1978, 17, 454. Edwards, J. H.; Foster, N. R. Fuel Sci. Technol. Zntl. 1986, 48 365. Foster, N. R. Appl. Catal. 1986, 19, 1. Gesser, H. D.; Hunter, N. R.; Morton, L. A.; Prakash, C. B. "The Production of Methanol by the Controlled Oxidation of Methane at High Pressures". Final Report submitted to Energy, Mines and Resources Canada, 1984. Gesser, H. D*; N*R.;Morton, A. Patent 4618 73% 1986. Gesser, H. D.; Hunter, N. R.; Prakash, C. B. Chem. Rev. 1985, 85, 235. Liu, H. F.; Liu, R. S.;Liew, K. Y.; Johnson, R. E.; Lunsford, J. H. J. Am. Chem. SOC.1984,106, 4117. Lott, J. L. "The Selective Oxidation of Methane at High Pressures". Ph.D. Thesis, University of Oklahoma, 1965. Lott, J. L.; Sliepcevich, C. M. Ind. Eng. Chem. Process Des. Deu. 1967, 6, 67. Mahajan, S.;Mezies, W. R.; Albright, L. F. Ind. Eng. Chem. Process Des. Deu. 1977a, 16, 271. Mahajan, S.; Nickolas, D. M.; Sherwood, F.; Menzies, W. R.; Albright, L. F. Ind. Eng. Chem. Process Des. Dev. 197713, 16, 275. Newitt, D. M.; Haffner, A. E. Proc. R. SOC.London, Ser. A 1932, A134, 591. Newitt, D. M.; Szego, P. Proc. R. SOC.London, Ser. A 1934, A147, 555. Pichler, H.; Reder, R. Angew. Chem. 1933,46, 161. Pitchai, R.; Klier, K. Catal. Rev.-Sci. Eng. 1986, 28, 13. Wiezevich, P. J.; Frolich, P. K. Znd. Eng. Chem. 1934, 26, 267. Yarlagadda, P. S.; Morton, L. A.; Hunter, N. R.; Gesser, H. D. Fuel Sci. Technol. Zntl. 1987, 5 , 169.
Received for review December 9, 1986 Revised manuscript received September 17, 1987 Accepted October 8, 1987
The Hydrogasification of Wood Mahesh Garg, Jan Piskorz, Donald S. Scott,* and Desmond Radlein Department of Chemical Engineering, University of Waterloo, Waterloo, Ontario, Canada N2L 3Gl
I t is demonstrated that wood can be gasified over an alumina-nickel catalyst in 1 atm of hydrogen pressure to give a 75% carbon conversion to methane at optimal conditions. These dptimal conditions correspond to those which are used in the Waterloo Fast Pyrolysis Process to obtain m a x h u m liquid yields. At higher temperatures (above 600 "C), equilibrium considerations greatly reduce the yield of methane. The success of this hydrogasification step suggests the possibility of an integrated gasification process which consists of hydrogasification followed by steam reforming and membrane separation of product gas to yield a CO/H2 mixture and a H2-rich stream which is returned to the hydrogasification step. In effect, the process converts wood to its equivalent yield of CO and H2 with no added reagents except the moisture in the wood. Natural gas and petroleum-derived hydrocarbons are widely used as raw materials in the manufacture of synthesis gas for the production of chemicals such as ammonia, methanol, and other valuable chemicals. However, as the future supplies of natural gas and crude oil diminish, the technology for the conversion of renewable resources will become important. During the last decade, there has been renewed interest in the extraction of fuels and chemicals from wood or other renewable biomass resources. 0888-5885/88/2627-0256$01.50/0
The use of biomass as a possible source of producing synthesis gas (CO + H2) has been particularly attractive. The most important merits of biomass are ita renewability, its low sulfur content, and its reactivity in thermal processes. The possible technologies for the production of synthesis gas from biomass are partial combustion, pyrolysis-gasification with steam or oxygen, or anaerobic fermentation to produce methane for subsequent reforming to synthesis gas. 0 1988 American Chemical Society
Ind. Eng. Chem. Res., Vol. 27, No. 2, 1988 257 Present biomass-gasification processes are often uneconomical or unsuitable for many process applications because of their poor thermal efficiency or because of the nature of the product. Conventional fixed-bed gasifiers usually require high temperatures (more than 800 "C) for good gas yield, and channeling often occurs in the fuel bed. Fluid-bed gasification with or without catalysis may serve to alleviate some of these drawbacks by allowing operation at lower temperatures and by producing higher yields of the desired gaseous products. The application of fluidized-bed technology to gasification also permits direct use of high-moisture biomass and allows good yields at low operating temperatures. The added benefits from catalytic gasification are usually an increase of reaction rates, a lowering of operation temperatures, and cracking of heavier hydrocarbons to form desirable gaseous products. If gasification is carried out in a hydrogen atmosphere, it is generally known as hydrogasification. Gasification of biomass in the presence of catalysts has been studied extensively in recent years (Sealock et al., 1978; Mudge et al., 1980; Tanaka et ai., 1984). Generally alkali catalysts (e.g., Na2C03, K,CO,, and CaC03) and transition metals (Ni, Cu, and Mo) on a mineral-type support (Si02/A1203)have been used to catalyze the gasification process to produce synthesis g?. In most of the studies, gasification is carried out using air or oxygen and/or steam; for example, see Baker and Mudge (1984). Robertus et al. (1982) and others have studied the gasification of biomass in the presence of multiple catalysts. The addition of alkali carbonates dry mixed or impregnated on the biomass feed was reported to increase the total gas production by 100% at 550 "C in comparison to uncatalyzed steam gasification under identical conditions. Rai and Tran (1980) also found that the presence of nickel catalyst in addition to potassium carbonate promoted the gasification and methanation reactions, resulting in a high heating value product gas. Gasification rates in the presence of multiple catalysts were found to be 4-400 times greater than those of the uncatalyzed reaction (Brown et al., 1985). K2C03was found to be far more reactive than Na2C03. Recently, Figueiredo et al. (1984), Terman and Sekher (1985), and Otake et al. (1984) investigated catalytic gasification of biomass chars obtained from various sources under different atmospheres and reaction conditions. They all reported a significant increase in the rate of char gasification in the presence of catalysts. More recently, Suzuki et al. (1984,1985) employed a new technique for catalytic hydrogasification of wood. Carbonized birchwood with added nickel salt impregnation as a catalyst showed a significant increase in total gas production and selective methane production in comparison to uncarbonized (raw) wood. Scott et al. (1985) suggested an alternative method for gasification of wood via methane production. They reported some preliminary experiments that indicated that by catalytic hydrogasification of wood at relatively low temperatures and atmospheric pressure, a high proportion of the carbon in the wood can be converted to methane at a hydrogen partial pressure of about 0.55 atm. The present work was undertaken to investigate more fully this relatively simple process for converting wood to methane and to evaluate its potential usefulness as an alternative gasification method.
Experimental Section Materials. During this study, International Energy Agency (IEA) poplar standard sawdust was chosen as biomass feed for the gasification process. This sawdust
Table I. Analysis of Feed Materials Used Avicel cellulose element % bv wta C 42.96 H 6.3 0 (by diff.) 50.74 N 0.0 ash 0.0 av particle size 100 m higher heating value, map
poplar wood % bv wtb
46.46.37 46.14 0.69 0.4OC +lo0 to -250 m 19.5 MJ/kg
"Including 2.9% moisture. bIncluding 4.32% moisture in the feed. 'Taken from the report by Beckman (1983). Table 11. Active Metal(s) Content of the Catalysts Used manufacactive metal(s) catalvst turer (as w t 70 of catalvst) catalvst sumort G-65RS United Ni, 27% refractory Catalysts G-33RS United Ni, 36% A1203/Si02 Catalysts G-65 United Ni. 25% refractory Catalysts Cll-9-02 United Ni. 12% ceramic Catalysta Ni-3266 Harshaw Ni, 50% proprietary Chemicals silica-alumina Ni-0901 Harshaw NiO, 7.5% alumina-silica Chemicals CoMo-0603 Harshaw non-silicate c o o , 3% Chemicals Moo3, 12% alumina Lab(1) Ni in house Ni, 13% alumina" Ni, 17% alumina" Lab(2) Ni in house Lab(3) Ni in house Ni, 8.8% alumina" Lab(4) Ni in house Ni, 3% alumina" Ni, 30% alumina" Lab(5) Ni in house Ru, 1.5% alumina" Lab(6) Ru in house "Alumina A1-1404 (Harshaw Chemicals).
was prepared from clean, debarked, hybrid poplar wood from the Ontario Ministry of Natural Resources plantation in Brockville, Ontario, Canada. The wood was obtained from trees 7 years old, harvested in 1982,12.34-cm diameter (BH), and designated as Populus Deltoides, Clone D-38. It was chosen as a standard test wood by IEA for use in comparative work in its Biomass Thermal Liquefaction Project. The elemental analysis of this sawdust is given in Table I. The IEA hybrid poplar wood was used in all runs in this work except one. For comparison, in one test (G-17), a highly purified microcrystalline cellulose was used, Avicel PH-102. The elemental analysis of this material is also given in Table I. The procedures for using both materials (poplar wood and cellulose) were identical. Catalysts Used. Both commercial catalysts and catalysts prepared in our own laboratory were used in the present studies. The catalysts (designated "Lab" series) were prepared by an impregnation method. The physical properties and compositions of all the catalysts are given in Table 11. Experiment. The hydrogasification apparatus is shown in Figure 1, and the details of the catalytic reactor and cyclone assembly are shown in Figure 2. The catalyst (approximately 50 g) was reduced in the reactor at 550 "C for a period of 1 h by hydrogen. For most runs, 90% reduction of the catalyst could be achieved during this period. The water produced during the reduction of the catalyst was collected in an adsorption column and weighed. The solid feeder was about half filled with the feed material, weighed, and placed in position. Details of this feeder construction and performance characteristic have been reported by Scott and Piskorz (1982).
258
Ind. Eng. Chem. Res., Vol. 27, No. 2, 1988
Figure 1. Process flow diagram for the catalytic hydrogasification of wood. ~
-32
C
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i
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20.6
/I
,
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m: A L L DIMENSIONS IN M M
Figure 2. Diagram of the catalytic reactor and cyclone assembly.
The reactor was stabilized a t a desired reaction temperature. About 30% of the gas flow entered the reactor down the central tube as entrainment gas carrying feed particles, and the balance entered at the bottom of the reactor after being preheated in the furnace to reactor temperature. The feed inlet was located in the middle of the fluidized bed of catalyst particles. To avoid pyrolysis of wood in the reactor feeding line, this line was air cooled and was also used to control reactor temperature during a run. All gasification products exited from the reactor to a cyclone, also at reactor temperature, which separated all the solid products including catalyst particles, if any, from other products. Condensable gasification products, mainly water, were collected in a series of ice/water condensers and glass wool filters. All uncondensed vapors were trapped by the filter. All noncondensable product gases, together with any excess hydrogen fed into the reactor, were measured and collected in a large polyethylene bag. The product gas mixture was analyzed continuously for CH4 and CO by a dual-channel infrared analyzer. At the end of the experiment, the feeder, reactor, and all vessels
and lines were dismantled and weighed and/or solvent (methanol) washed to allow quantitative recovery of water, tar, and char fractions. The normal time for a run was 30 min, and the normal feed rate of wood, containing 4.3% moisture, was 70 gjh. Analysis of Products. The reactor, charpot, adsorption column, filters, and liquid collecting vials were all weighed before and after a run, as was the feeder. A major fraction of liquid product was collected in the condensers. The methanol solution, recovered after washing of condensers and lines, contained water, tar, and char. After filtration, the char recovered was washed and dried at 105 "C for 30 min and weighed. Water content of the liquid products and filtered methanol solution was determined by Karl Fischer titration. The balance was assumed to be tar. Char was also recovered from the charpot, and the carbon deposited on the catalyst surface during a run was also accounted for as char. The gaseous products were analyzed for CO, COz, CH4, HzO, and other gaseous hydrocarbons and oxygenated compounds by gas chromatography. Light, gaseous hydrocarbons such as ethylene, ethane, propane, and butane were also present in small quantities (less than 0.5% by weight of moisture-free (mf) wood fed). Traces of acetaldehyde, methanol, acetic acid, and furans were also found in the product gas mixture, particularly a t higher feed rates.
Results Hydrogen was consumed with solid feed during the hydrogasification experiments, but the yield of products is reported on the basis of moisture-free wood fed. Therefore, for most experiments, the recovery of products was more than 100%. A number of the terms used in this section are defined as follows for convenience: residence time = The residence time of gases and vapors in the reactor, calculated by dividing the net empty volume in the reactor by the gas volumetric flow rate at the reactor conditions; F/C ratio = the ratio of the weight of moisture-free wood fed per hour (F) and the mass of catalyst (C); reformable carbon in gas = the percentage of carbon present in gas, not associated with CO,; selectivity for CH, = the percentage of gas carbon associated with methane; SCM of CH,/feed = the amount of standard cubic meters of methane produced per kilogram of the moisture-free feed. Initial tests were carried out in a hydrogen atmosphere using six different commercial nickel catalysts. Results from all these runs, carried out at essentially atmospheric pressure, are given in Table 111. Above 500 "C, most of the catalysts containing more than 25% nickel on an alumina support converted more than 70% 'feed carbon to gaseous products. Other catalysts such as (211-9-02 and Ni-0901 did not perform as well, and a major fraction of the pyrolytic tar was not gasified. The yield of 54% uncracked tar with Ni-0901 catalyst was probably due to the very low surface area (1m2/g) of this catalyst and its low (7.5% NiO) nickel content. Comparison of the commercial catalysts from different sources that have a high nickel content on an alumina support indicates that they would give substantial yields of high CHI content gas, with 70%-80% carbon conversion to reformable gases. All the commercial catalysts contained small amounts (up to 4% of catalyst weight) of carbon which was generally associated with the compound used as binder for making catalyst tablets. This carbon was found to cause major difficulty in closing the carbon balance for the tests. A nickel-on-alumina catalyst [named Lab(1) Nil, containing
Ind. Eng. Chem. Res., Vol. 27, No. 2, 1988 259 Table 111. Results of Experiments with G-65 and G-65RS Ni Catalyst (Section 1) and with G-33RS Ni Catalyst (Section 2) G-3 G-4 G-8 G-12 G-5 G-6 G-7 run G-65RS fresh G-65RS fresh G-65RS fresh G-65 fresh G-33RS fresh G-33RS fresh G-33RS fresh catalyst 27.00 27.00 25.00 36.00 36.00 36.00 27.00 catalyst, Ni % 550.00 565.00 500.00 520.00 510.00 500.00 450.00 temp, "C 0.48 0.48 0.72 0.40 0.55 0.59 0.60 residence time, s 29.17 42.47 42.25 28.21 51.74 23.49 28.81 total feed, g 105-250 250-590 105-250 105-250 250-590 105-250 250-590 feed particle, pm 4.32 5.22 4.32 4.32 5.22 4.32 5.22 moisture, % 50.08 50.15 29.79 50.00 50.50 50.20 50.06 catalyst wt, g 250-590 250-590 250-590 105-250 250-590 250-590 250-590 catalyst particle, pm 30.00 30.00 30.00 24.00 26.00 30.00 total run time, min 30.00 1.60 1.60 1.40 1.10 2.30 1.10 1.10 F/C, l / h total yield of compon., % w t feed, mf 6.21 1.44 3.59 4.39 1.16 13.98 0.76 co 1.84 5.05 5.73 8.08 1.22 2.37 22.72 COP 45.41 44.42 40.15 43.39 28.62 34.50 44.56 CHI 0.01 0.02 0.02 0.34 0.01 0.07 0.00 C2H4 0.01 0.01 0.20 0.01 0.01 0.05 0.01 C2H6 0.00 0.14 0.00 0.13 0.03 0.01 0.00 c3 0.00 0.00 0.12 0.03 0.06 0.01 0.07 c,+ 47.71 53.54 48.67 60.44 48.11 65.90 36.58 total gas 10.34 7.23 6.91 10.29 9.26 9.12 10.57 char 2.82 1.93 1.26 6.51 7.16 4.24 0.79 tar 45.89 35.22 36.65 39.64 42.22 44.30 18.28 water 102.44 101.92 111.62 99.25 102.46 100.46 93.61 total yield 106.91 106.71 108.55 99.06 110.29 112.01 104.09 total carbon yield 74.25 70.36 81.52 68.25 70.63 70.34 54.36 total gas carbon 68.05 67.10 70.23 62.09 44.57 68.26 carbon in CH, 52.86 0.66 0.61 0.67 0.69 0.67 0.44 0.52 scm CH,/feed 99 96 94 96 98 62 99 reformable C in gas, '70 97 90 91 86 97 63 97 selectivity for CH,, % 1.34 1.27 1.26 1.16 1.28 0.90 0.99 HHV gas/feed
13% nickel, was prepared in this laboratory. This catalyst gave yields of gasification products which were similar to the nickel-on-aluminacommercial catalysts. The problem of not knowing the exact composition and nature of a particular commercial catalyst, together with the comparable performance of our own Lab(1) Ni catalyst, led to the decision to use catalysts prepared in our own laboratory. The use of these catalysts also allowed investigation of support activity and metal content effect as independent parameters. Experiments were conducted using different particulate materials (sand, alumina, and a nickel catalyst) in the fluidized-bed reactor under a hydrogen atmosphere at 550 "C. The results of these experiments are given in Table IV. The yields of gases, char, and water were significantly higher with alumina than with sand, and the tar yield was much lower, indicating the high tar cracking activity of alumina. Almost complete cracking of tar was observed with 8.8% nickel-on-alumina catalyst. More than 75% of the feed carbon was gasified with the nickel catalyst, whereas sand or alumina alone could gasify less than 30% of the feed carbon. It appears that the cracking activity of alumina is greatly enhanced by the addition of nickel. The nickel catalyst was also very selective for methane production. The char production was slightly higher with nickel catalyst than with sand (9.34%),suggesting that only a small additional fraction of char may be formed during the cracking of tar, and the char formation occurs mainly in the primary thermal pyrolysis step. The effect of nickel content of the catalyst was studied with our catalysts containing different amounts of nickel (3-30%) on alumina support (Al-1404, Harshaw). The reaction conditions and the yield of products obtained during these experiments are presented in Table V. Lab(2) Ni catalyst was used at 560 "C, whereas all other catalysts were used at 550 "C. The char yield decreased with an increase in nickel loading on the catalyst. The tar
Table IV. Hydrogasification with Different Bed Materials at 550 OC run G-42 G-36 G-28 fluid.-bed mater. sand alumina support Lab(3) Ni cat., 8.8% Ni bed part. size, pm 250-300 250-590 250-590 0.37 0.44 residence time, s 0.44 F/C ratio, l / h 1.82 1.17 1.69 total yield of compon., % wt feed, mf co 10.66 12.95 6.21 7.34 8.25 11.21 co2 42.27 CH, 1.63 2.57 c2+ 2.97 4.55 0.18 total gas 23.52 31.28 55.99 char 9.34 21.86 10.70 tar 51.87 8.15 0.98 water 14.91 28.83 39.81 107.48 total yield 99.63 90.12 reform. gases 15.27 20.07 48.65 75.28 total gas C 21.67 29.47 65.37 C in CH, 2.53 3.97 selectivity for CH,, 12 13 87 90
yield was in the range of 1-3% with all these catalysts, indicating that they were active enough to crack most of the tar. The yield of CO and C02 decreased and that of CHI increased with an increase in the nickel content of the catalyst. The change in the yields of total reformable gases (gases other than COz)and the key gaseous components are shown in Figure 3. The conversion of feed carbon to gaseous products was only 58.5% with 3% nickel-on-alumina catalysts, but all other catalysts with higher nickel contents gasified more than 75% feed carbon. The selectivity for methane production also increased from 52% with 3% nickel-on-alumina catalyst to more than 87% with catalysts containing at least 8.8% nickel. The results shown in Table V suggest that more than 8.8% nickel on alumina only marginally improved the
260 Ind. Eng. Chem. Res., Vol. 27, No. 2, 1988 Table V. Effect of Nickel Content of Catalyst (Residence Time, 0.44 s ) run G-44 G-28 G-13 G-45 cat. descript. Lab(4) Ni Lab(3) Ni Lab(2) , . Ni Lab(5) Ni 3.0 8.8 17.0 30.0 cat., Ni % 550 temp, 'C 550 550 560 1.41 1.22 F/C ratio, l / h 1.12 1.69 total yield of compon., % wt feed, mf 4.51 co 20.91 6.21 5.32 2.77 5.00 10.79 7.34 CO, 46.21 44.78 42.27 CH, 19.63 0.03 0.00 C2H4 0.34 0.03 0.03 0.00 C2H6 0.31 0.02 0.08 0.00 c3 0.62 0.06 0.25 0.00 C,+ 0.85 0.07 53.88 55.10 53.46 55.99 total gas 6.64 9.01 17.36 10.70 char 1.75 1.33 3.07 0.98 tar 46.15 41.63 32.39 39.81 water 108.42 107.07 106.28 107.48 total yield 51.11 50.10 48.65 reformable gases 42.67 77.68 76.76 58.54 75.28 total gas C 71.47 69.25 30.36 65.37 C in CHI 98 96 reform. C in gas, 90 95
Table VI. Effect of Residence Time on Gasification Products (Lab(2) Ni Catalyst (17% Ni)) run G-22 G-13 G-23 temp, "C 550 560 550 residence time, s 0.32 0.44 0.55 F/C ratio, l / h 1.87 1.41 1.89 total yield of compon., % wt feed, mf 5.32 6.00 co 4.79 11.74 5.00 5.45 COZ 38.22 44.78 41.62 CH4 0.01 0.00 cz-c4 0.39 55.97 55.10 total gas 52.29 10.47 9.01 char 8.15 0.76 1.33 tar 0.99 33.02 41.63 water 41.46 100.22 107.07 total yield 102.88 44.23 50.10 reform. gases 46.83 71.03 76.76 total gas C 72.39 59.11 69.25 C in CH4 64.37 91 96 96 reform. C in gas, 70 90 83 89 selectivity for CH,, %
7c
selectivity for CH4, 70 HHV gas feed total mol gas produced/kg of wood
52
87
90
92
0.72 22.71
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1.30 31.02
1.35 31.21
5
,
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I LOO
500
450
550
600
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Temperature, OC
Figure 4. Effect of temperature on overall yields: 17% Ni on alumina. 0.44 s.
35-
cc 3 CCI
2
5
iC
15
20
25
30 -
3C
-- in Ca*oys* Figure 3. Effect of nickel content of the catalyst on yields. h'ckel
E
L
25-
3.
cracking and reforming activity of the catalyst, and therefore, a catalyst should contain a minimum of about 9% nickel on an alumina support for the hydrogasification process. The range of residence time which can be obtained in a fluidized-bed reactor is limited by the need for maintaining good bubbling fluidization conditions. Results were obtained over a limited range of residence times (0.32-0.55 s) and are presented in Table VI. More than 0.44-9 residence time appears to adversely affect the methane production and increase the production of CO and C 0 2 presumably via steam reforming of methane and the water gas shift reaction, both of which decrease the reformable carbon in the gas and the selectivity for methane. Despite the relatively narrow range of residence times covered, the results of Table VI show that a residence time of 0.44 s or shorter is probably near the optimum for the wood particle size used. The effects of temperature on gasification products were studied in the range of 400-650 "C at a residence time of
,u
20-
LOO
45C
500
550
6C.C
55C
-empera'ure, 'C
Figure 5. Effect of temperature on CO and CO, yields: 17% Ni on alumina, 0.44 s.
0.44 s in the presence of Lab(2) Ni (17% nickel-on-alumina) catalyst. The yields of total gas, tar, char, and water with temperature are shown in Figure 4. No significant decrease was observed in the yield of tar above 500 "C, but the char yield decreased slowly over the entire temperature
Ind. Eng. Chem. Res., Vol. 27, No. 2, 1988 261
.
tcs c a r m n 100
ac
2
\-
-3 60 r L
K
E ._in
40
I
20 Reformable Cor Corbon
-/
/
0 Selectivity For CH,
0
0 Reformable Gores
C
0 CH,
do0
450
500
550
600
650
400
450
Temperature, OC
Figure 6. Effect of temperature on carbon conversion and CH4 yield: 17% Ni on alumina, 0.44s.
range. The gas yield increased steadily with temperature, but a sharp decrease in water production was observed above 550 OC. The increasing yields of CO and COPwith temperature are shown in Figure 5, and the methane yield as temperature increased is shown in Figure 6 along with the total gas production and percent conversion of feed carbon to gas and carbon conversion to methane. The yield of methane increased with temperature up to 550 "C to a maximum of about 71% C fed and then started decreasing. Since the total gas carbon is almost constant above 600 OC but the water yield decreases sharply, it is probable that the secondary reactions of CHI with HzO, and of CO with HzO catalyzed by the metal, have become important. Equilibrium calculations based on the experimentally measured C, H, and 0 content of the gas showed that the experimental yields of CH4, H20, CO, and COz at 650 "C were nearly in their equilibrium ratio in the gasification product mixture. The increased yield of COS at higher temperatures indicated a more rapid approach to equilibrium by the shift reaction, although the equilibrium yield itself was a little less favorable for COz production. As shown in Figure 6, the production of methane is favored in the range 500-600 "C in the presence of nickel catalyst. Similar behavior was observed by Suzuki et al. (1985) in hydrogasification of birchwood with added nickel as a catalyst. Figure 7 shows that the selectivity for methane production decreased sharply above 550 "C, although the fraction of reformable gas carbon (percent of total gas carbon associated with all gases except C02)was only slightly affected by temperature. The actual production above 400 "C of all the hydrocarbon gases other than CH4 (Cz+)was below 0.2%, on the basis of mf wood fed. Scott et al. (1985) reported that during the flash pyrolysis of wood, above 550 OC the COz yield became relatively constant at about 7.5% on the basis of wood fed. Only slightly higher yields of COzwere obtained during catalytic hydrogasification of wood, indicating that most of the C 0 2 was formed during the thermal pyrolysis step and only an additional small fraction was formed by other reactions during hydrogasification. Interestingly, the optimal conditions for maximum liquid (tar) yield found by Scott et al. are also the same for obtaining maximum reformable gas production with a nickel catalyst. Apparently, then, the wood first undergoes thermal pyrolysis and the tar produced is further cracked and hydrogenated in the
500
550
650
600
Temperature, OC
Figure 7. Effect of temperature on selectivity for CHI: 17% Ni on alumina, 0.44 s.
0
05
1
15
2
25
3
35
4
Feed to Catalyst Ratio, l/hr
Figure 8. Effect of feed/catalyst ratio on product yields, 500 "C.
presence of the nickel catalyst. The distribution of gasification products was found to be very dependent on the ratio of wood feeding rate to mass of the catalyst particles in the bed (F/C ratio). Figure 8 shows that the fractional conversion of wood to char and tar increased, whereas the conversion to gas and water decreased, with the increase in the F/C ratio. A sharp increase in the amount of uncracked tar is evident above an F/C ratio of 2.0. Figure 9 shows the variation in the rate of production per unit mass of catalyst of the key gas components CHI, CO, and COz with the F/C ratio. Above an F/C ratio of 1.5, the production of CO and C 0 2 increased sharply with F/C ratio. However, the production of CHI remained high (more than 0.6 g/g of catalyst/h) in the F / C ratio range of 1.5-3.2 l/h. In Figure 10, the measured rate of production of reformable gas carbon per unit mass of catalyst expressed as a fraction of the feed carbon is shown. A straight line representing the maximum possible production rate is also shown, assuming total conversion of feed carbon to reformable gases at each corresponding F/C ratio. It is clear from the plot that the carbon associated with the reformable gases per unit mass of the catalyst per hour increased only marginally above the F/C ratio of 2.3. The difference between the two plots represents the sum of the carbon associated with COz plus the carbon in char
262 Ind. Eng. Chem. Res., Vol. 27, No. 2, 1988
-/
A i' 15
1
05
Table VII. Effect of F/C Ratio on Lab(3) Ni Catalyst (17% Ni) (500 "C; Residence Time, 0.44 s) run G-51 G-49 G-52 G-50 F/C ratio, l / h 0.84 1.56 2.27 3.68 total yield of compon., % wt feed, mf co 5.89 7.54 11.01 15.84 6.74 11.60 12.95 3.08 COZ 40.82 29.67 11.96 CH, 48.08 t0.97 2.94 0.38 C2-C4 0.41 total gas 57.46 55.46 53.25 43.68 char 6.31 11.31 13.12 13.88 tar 0.71 0.48 2.66 18.40 water 48.91 40.33 31.95 20.26 total yield 113.38 107.58 100.97 96.21 reform. gases 54.38 48.72 41.65 30.73 total gas C 81.99 74.20 63.80 44.77 C in CH, 74.36 63.13 45.89 18.49 reform. C in gas, % 98 95 90 84 selectivity for CHI, % til 85 7: 41 HHV gas/feed 1.41 1.21 0.93 0.50
CH,
3 co 0
25
2
3
cs,
35
4
Feed to Catalyst Ratio, l/hr
Figure 9. Effect of feed/catalyst ratio on yields of individual gases: 550 "C.
3. A large fraction of CHI and, a t higher temperatures, some COz may be produced from CO via secondary reactions
CO CO
+ 3Hz
CH4 + HzO
(1)
+ HzO + COz + H2
(2)
4. Higher gaseous hydrocarbons (Cz+)are probably also primary gasification products which subsequently convert to CH4 and/or CO on the nickel catalyst aC,H, + bHz + xCH, (3) C,H,
+ nH,O + nCO + (2n + m)/2Hz
(4)
5 . Some water formation is likely a primary pyrolysis product, but the majority appears to be a product of both the methanation of CO and of other secondary processes during gasification. Therefore, the water yield appears to depend inversely to some degree upon the extent of conversion of CO. 1
I
0
35
1
' 5
2
25
3
35
4
Feed to Catalyst Ratio, l/hr
Figure 10. Variation of carbon conversion to product gases with feed/catalyst ratio: 550 "C.
and tar, and this quantity increased sharply above F/C ratios of 1.9. An increase in the production rate of C2 + gaseous products with F/C ratio is shown by the data in Table VI1 together with other results. On the basis of the results presented in Figures 8-10 and Table VII, an F/C ratio in the range 1.5-1.9 is likely to be near an optimal value for wood hydrogasification employing the type of nickel on y-alumina catalyst used in this work. Depending upon the nickel content and surface area of the catalyst, the optimum range for F/C ratio may be somewhat different for other nickel catalysts. The hydrogen supply in this work was constant, and therefore the tests of increasing F/C ratios represent decreasing excess amounts of hydrogen. The approximate stoiochiometric equilvalence point for 100% carbon conversion to CH4 would be at an F/C ratio of about 6.8 for the tests described here. The major points arising from these results can be summarized as follows. 1. There is some evidence that the catalytic wood gasification process involves surface reaction or surface adsorption equilibrium, particularly of those steps involving tar decomposition. 2. Most of the COz produced during the gasification below 600 "C is a primary gasification product.
Discussion of Mechanism of Hydrogasification From the preceding results, as well as from our previous results reported for the fast pyrolysis of wood using essentially the same apparatus (Scott et al., 1985), some tentative conclusions can be drawn concerning possible mechanisms of the hydrogasification process. The initial step is assumed to be a rapid thermal pyrolysis of the wood, with the fluidized catalyst particles acting mainly as an agent for rapid heat transfer in the same way that sand particles do in a normal fast thermal pyrolysis. Further, the optimal reaction conditions in hydrogasification for maximum methane yield and selectivity correspond to those required for maximum liquid yield in fast thermal pyrolysis. The yield of char in hydrogasification at 500 OC is very nearly the same as that obtained in fast thermal pyrolysis (13% vs 12%), and the COz yield is also nearly the same as the primary COzproduction in fast thermal pyrolysis. The second stage of reaction would require adsorption of the organic molecules volatilized from the decomposition of the wood onto the catalyst surface. Accelerated cracking of these molecules then occurs, on alumina reaction sites, with a decarbonylation step appearing to play a major role. Hydrogen adsorbed on nickel sites is now able to react with adsorbed CO or with small molecular fragments containing carbonyl moieties to produce the 'CH2 group. This step has been identified as a rate-controlling step in the methanation of CO by Hz by Klose and Baerns (1984). Further hydrogenation yields CHI, with very minor
Ind. Eng. Chem. Res., Vol. 27, No. 2, 1988 263 Table VIII. Hydrogasification of Wood vs Cellulose Lab(%) Ni Catalyst (17% Ni) (500 OC; Resiqlence Time, 0.44 8 ) G-17 run G-14 cellulose feed material wood 100 105-250 feed particle, pm 0.95 F/C ratio, l / h 1.45 total yield of compon., % wt feed, mf 4.33 co 2.10 2.14 COB 1.89 46.21 CH, 43.71 0.66 c,+ 0.18 53.33 total gas 47.87 0.92 char 12.44 5.04 tar 4.78 45.91 water 46.45 105.20 total yield 111.54 51.19 total reform. gases 45.98 77.61 total gas C 70.82 71.46 C in CHI 67.60 reform. C in gas, % 98 98 92 selectivity for CHI, % 95
amounts of Cz+ hydrocarbons. The very low yields of these higher hydrocarbons are consistent with the assumption of the importance of a decarbonylation step followed by methanation. The high yields of CH, up to 550 OC and the subsequent reduction in yield above this temperature is clearly an equilibrium effect. Not only do the gaseous components approach their equilibrium ratios at higher temperatures, for example, at 650 OC, but the incremental increase of yield of CO + COz above 550 "C corresponds very closely to the incremental decrease in yield of H20,indicating two main secondary reactions at higher temperatures (eq 1and 2). Since nickel is known to catalyze both of these reactions, the approach of the gas phase to equilibrium at 650 OC for the two reactions given above is not surprising. The high yield and high selectivity of the nickel catalyst for methane production is possible, apparently, because of the favorable equilibrium at the low operating temperatures (500-550 OC) at which maximum tar yields are obtained in this process, together with the short reaction time required. Secondary carbon can be expected to form also due to the cracking reactions, and a small amount apparently was produced as indicated by the slightly higher char yields when using a catalyst. In additional tests, the nickel catalyst was regenerated by burning off any carbon deposit and, after reduction, was reused as a hydrogasification catalyst. After three such cycles only a slight loss in catalyst activity was found to occur. As an aid in supporting some of these speculations, one hydrogasification test was carried out using a pure microcrystalline cellulose (PH-102,Avicel) as feed in the place of wood. The elemental analysis of the cellulose feed was given in Table I. This cellulose was gasified in the presence of the 17% nickel-on-alumina catalyst at a temperature of 500 "C. The gasification yields from this experiment are given in Table VI11 and are compared to the yields from a similar experiment with the wood at 500 "C. Less than 1%of the cellulose feed converted to char, whereas wood produced 12.44% char at the same reactor conditions. The major sources of the char formed are believed to be the lignin and hemicellulose fractions of the wood. Some unpublished results from our laboratory in which xylan from larch wood was used as a feed showed that high char yields are obtained from this hemicellulose under conditions of fast pyrolysis, with high heating rates. The low char yield from the celllulose during hydrogasification also supports the assumption that very little of the char
formed during the hydrogasification of wood is secondary carbon, inasmuch as the cellulose fraction of the wood is a major source of the volatile compounds formed in the initial thermal decomposition step. This conclusion is also borne out by the similarity of the tar yields from the wood and the cellulose, showing that the volatiles produced in the two cases are probably quite similar in their behavior during the catalytic step.
Conclusions A unique hydrogasification of wood directly to high methane content gases has been shown to be feasible at relatively mild conditions. The success of this hydrogasification step suggests an alternative gasification process for biomass which would lead to either a high heating value gas with a high CH4 content or to a synthesis gas. The basic concept of this approach can be summarized in the three idealized stoichiometric equations below, assuming cellulose as a model compound: CeHloO5 6CH4
+
catalyat
12H2 ~ ~ o - 5 s.Co
+
6H20
750.C- 6 C O 6CO
6CO
+
18H2
6CH4 -t 5H20
catalyst
+
+
(5)
18H2
6H9 (oroduct gas)
(6)
