328 Ind. Eng. Chem. Process Des. Dev., Vol. 18, No. 2, 1979
0
I0
30
20
( k W h - l l ) l00lg
atom C
Figure 10. Lime conversion to carbide in coal runs under different conditions as correlated with the reduced specific energy input [feed mixture: excess lime (A, A); excess carbon ( 0 , O ) ;anode: graphite (A,0);copper (A,0 ) ;quench distance: 5 in. (points without arms); 15 in. (points with arms); residence time: regular (points without tails; extended (points with tails)].
coal runs are all with an excess of lime (mol of lime/atom C of more than 3); hence comparisons of these runs are difficult. The energy requirements in the best run, equivalent to 36 kWh/lb of acetylene producible from the carbide formed, would presumably be improved in larger scale apparatus. In the present method the theoretical energy requirement would be 4.0 kWhr/lb of CzHzwith coal as a carbon source if the reactants were fed at room temperature and the feed C/H ratio was 1/2. Extremely short reaction time and the possibility of producing CaCz directly from coal or hydrocarbons without having first to produce coke appear to make the present method more advantageous than the conventional method of manufacturing CaCz from lime and coke in large
electrothermal furnaces if energy requirements for both processes are about the same. The latter requires an energy input of about 5.0 kWh/lb of CzHz(Miller, 1965, p 181). Conclusions It has been demonstrated that CaCz can be produced from the reaction of CaO with high volatile coal or hydrocarbons in a one-stage reaction by using a magnetically rotated arc reactor. The results illustrate the intense chemical activity of the nascent hydrogen and carbon species concomitantly produced during the intense heating of hydrocarbons or high volatile coal. Literature Cited Bond, R. L., Lander, W. R., McConnell. G. I. T., Adv. Chem. Ser., NO. 55, 650 (1966). Chen, C.J., Back, M. H., Back, R. A,, Can. J . Chem., 53, 3580 (1975). Gannon. R. E., KnJtonk, V., " A V O ArccOai Process Development, Fiml Report", R&D Report No. 34, OCR Contract No. 14-01-0001-493, AVCO, Lowell, Mass. (1972). Graves, R. D., Kawa, W., Hiteshue, R. W., Ind. Eng. C b m . Process Des. Dev., 5, 59 (1966). Hartig, R., Troe, J., Wagner, H. Gg., "Thermal Decomposition of Methane behind Reflected Shock Waves", 13th Symposium (International) on Combustion, pp 147-152, The Combustion Institute, Pittsburgh, Pa., 1971. Kawana, Y., Makino, M., Kimura, T., Int. Cbem. Eng., 7, 359 (1967). Kim, C. S.,Sc.D. Thesis, Massachusetts Institute of Technology, Cambridge, Mass., 1977. Krukonis. V., "AVCO Arc-Coal Process", R&D Report No. 34, OCR Contract No. 14-01-0001-493, AVCO, Lowell, Mass. (1968). Miller, S.A., "Acetylene; Its Properties, Manufacture and Uses", Voi. 1, Academic Press, New York, N.Y., 1965. Nicholson, R., Littiewood, K., Nature (London). 236, 397 (1972). Snell, F. D., Ettre, L. S.,Ed., "Encyclopedia of Industrial Chemical Analysis", Vol. 8, pp 72-114, New York, N.Y., 1969. Stokes, C. S., "Chemical Reactions in Plasma Jets", in "Reactions under Plasma Conditions", Vol. II., pp 259-298, M. Venugopalan, Ed., Wiley. New York, N.Y., 1971.
Received for review July 10, 1978 Accepted November 17, 1978
Pilot Plant Results for Partial Oxidation of Cattle Feedlot Manure Steven R. Beck," Wllliam J. Huffman, Brian L. Landeene, and James
E. Halligan
Department of Chemical Engineering, Texas Tech University, Lubbock, Texas 79409
A small pilot plant (capacity, 450 kg/day) has been constructed to evaluate gasification of manure by partial oxidation. The reactor operates as a countercurrent, fluidized bed in which an air-steam mixture is the fluidizing medium and manure is the only solid phase present. The product gas from this process contains a significant amount of hydrocarbons (CH4 - l o % , C2H4 - 6 % ) in addition to normal partial oxidation products. The net heating value of the dry gas exceeded 8.6 MJ/m3 (1 atm, 0 'C) in all cases. The net heating value of the C0,-free gas (still containing nitrogen) exceeded 11.2 MJ/m3. The pilot plant design and operating procedure are described as are the results of the heat and material balances. This work confirms the technical feasibility and potential scale-up for producing a medium heating value gas suitable for fuel or for feed to a petrochemical production process at relatively high yields from cattle feedlot manure.
Introduction In view of the approaching energy shortage in the United States, new sources of energy must be developed to maintain the standard of living as current energy resources are depleted. Biomass is one of the many possible alternative energy resources. Biomass can be defined as solar energy stored in the form of plant matter or residues which have some positive value as a chemical resource. This is 0019-7882/79/1118-0328$01.00/0
in contrast to wastes which have a negative value; i.e., one must pay to dispose of wastes. Many obstacles stand in the way of development of biomass as an energy source. One prime obstacle is the fact that the energy density of biomass is very low compared to fossil fuels. This results in high collection costs to deliver the biomass to a conversion facility. It is projected (Inman, 1977) that wood can be produced by intensive silvaculture at the rate of 0 1979 American Chemical Society
Ind. Eng. Chem. Process Des. Dev., Vol. 18, No. 2, 1979
22416 dry kg hectare-year. If wood is assumed to have a heating value of 19.8 MJ/dry kg, the available energy is 4.43 X lo5 MJ/hectare-year. On the other hand, consider a reasonable oil reservoir that is 3.05 m thick with a porosity of 25% and an oil saturation of 65%. If 50% of the oil in this reservoir were produced (an optimistic value), the energy available would be 6.8 X lo7 MJ/hectare. For a coal seam that is 3.05 m thick, with a density of 1.40 g/cm3 and heating value of 28.0 MJ/kg, the available energy is 1.2 X lo9 MJ/hectare. One location in which high concentrations of biomass are available is a cattle feedlot. On the High Plains of the Southwest, feedlots containing 50 000-100 000 head of cattle are relatively common. These large feedlots are located in this region because of low land cost, availability of grain for cattle feed, and low rainfall which minimizes the problems associated with water runoff. The total land required for a feedlot varies with capacity, but a reasonable figure for a 20000 head feedlot is 49 hectares (Texas A&M, 1977). If it is assumed that each cow produces 3.6 kg per day of dry manure (-30% ash) with a heating value of 14.0 MJ/dry kg, the energy available is 7.5 X lo6 MJ/ hectare-year. As a result, a conversion facility using manure would require less land for feedstock supply than a wood-based facility. This manure is currently used for fertilizer, but the cost of transporting and applying the manure restricts it use as fertilizer to areas near the feedlot. In 1971 a research project was initiated at Texas Tech University to develop a method by which manure could be converted to a more useful form of energy. Initial calculations showed that thermochemical conversion of manure would be more efficient from a thermodynamic viewpoint than would biological conversion by anaerobic digestion (Halligan and Sweazy, 1972). Based on these calculations, bench-scale studies were performed to study the production of a synthesis gas from manure and thus the title Synthesis Gas from Manure (SGFM) process. The original objective of the research was to use this synthesis gas for ammonia production. The gas produced from the SGFM process is in fact suitable for ammonia synthesis, but could also be used as a medium-BTU fuel gas or as a synthesis gas for other petrochemicals such as methanol or ethylene. The results of the bench-scale study, which have been reported previously (Halligan et al., 19751, were sufficiently promising that a mini-pilot plant was constructed to evaluate the process. This paper reports the results of the pilot-plant study for gasification of cattle feedlot manure by partial oxidation and pyrolysis. The results are not presented in the form of correlations, but rather as the actual data base developed. It is felt by the authors that the results are significant and should be published at this time. Further work is in progress to develop a mathematical model of the reactor which includes the kinetics of pyrolysis and the fluidization characteristics of the reactor. This model will be suitable for use in scale-up of the reactor.
SGFM Pilot Plant The pilot plant which was constructed a t Texas Tech has a capacity of approximately 450 kg per day of asreceived manure (-10% moisture in this study). A schematic of the pilot plant is shown in Figure 1 and a more detailed drawing of the SGFM reactor is shown in Figure 2. The lower section of the reactor is heated with two 60 cm long electrical heaters to minimize heat losses. The reactor is insulated with 10 cm of Cerafelt. Thermocouple (Type K) or sample ports are located every 15.2 cm in the lower reactor section (total of ten measurements)
329
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Bypass to Stack
II
p Removal
Solid Fines
Reactor Prehealer
Water
-
Condensate Removal
x
Ram
Figure 1. Flow sheet of SGFM experimental unit.
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-
( A L L D I M E N S I O N S IN C E N T I M E T E R S )
t
45 7
31 8 - 90011 RF WN FLN6STAINLESS STEEL 31 8 - SCHEDULE 40 PIPE STAINLESS STEEL 20 3 STD WT WELD CAP STAINLESS STEEL 20 3 - SCHEDULE 40 PIPESTAINLESS STEEL
-
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6
20 x 15 CONCENTRIC REDUCERSTAINLESS STEEL
15 -SCHEDULE 40 PIPE STAINLESS STEEL (316-L) 31.1 -COLLARSTAINLESS STEEL
I
&/
15 2 - 9 w X RAISED FACES WELD NECK FLN6 WIBLIND STAINLESS STEEL
Figure 2. SGFM pilot plant reactor.
with two additional ports in the upper section 30.5 cm apart and one port at the exit. The gas distributor in the bottom of the reactor consists of a 0.94 cm thick plate with 0.1588 cm holes drilled on concentric circles around a 2.54 cm hole in the center. Solid feed enters the top of the reactor and falls countercurrent to a gas mixture of steam, air, and product gases. Product gas from pyrolysis and partial oxidation exit (with input fluids) out the top of the reactor. The
330
Ind. Eng. Chem. Process Des. Dev., Vol. 18, No. 2, 1979
product gas is stripped of entrained solids, tar, and a water/organic mixture using a cyclone, impinger, and heat exchanger, respectively. Char is removed from the bottom of the reactor through a center-port opening in the inlet gas distributor. The cyclone was designed according to a standard procedure (Perry, 1963) and was operated at 250-300 "C to prevent tar, organics, and water from condensing out and agglomerating the solids. Free-flowing solids were discharged from the cyclone without any problems using the heated cyclone. A three-stage tar impinger sequence was used to collect tar (mostly long-chain fatty acids) and minimize steam condensation. The first of the impingers was operated at approximately 200 "C, the second at 160 "C,and the third at approximately 110-120 "C.The major problem with the tar collection was carry-over of extremely fine particles from the cyclone at high rates of gas production (21 L/g) which resulted in an extremely viscous mixture that caused plugging. Room temperature product gas (saturated with water) was measured with a Rockwell Model TP-4 5.08-cm turbometer. This meter required periodic cleaning because of the fine, oily mist that was sometimes entrained with the gas. This meter is the basis for the reported gas flow rates. A star-feeder (Model R2-653, Beaumont Birch Co.) was used to meter solids to the reactor. Variations in the bulk density of the solids were sufficient to cause 5-1570 differences in mass feed rate at a given feeder speed which precluded using the feeder speed to obtain actual feed rates. To determine the feed rate, the mass of the solid in the hopper was weighed before and after each run. The average mass feed rate was determined from these measurements. All other liquid and solid streams were collected and weighed during each steady-state run period. The Varian gas chromatograph (Model 2720) used to determine gas composition contained two columns. One column was filled with 50/30 mesh Poropak R (1.20 m X 0.318 cm) followed by 50/30 mesh Poropak Q (3.65 m X 0.318 cm). This column measured methane, carbon dioxide, ethylene, and ethane. A 3.65 m x 0.318 cm diameter column with 45/60 mesh, 5 A molecular sieve was used to measure hydrogen, oxygen, nitrogen, methane, and carbon monoxide. Results from the two columns were related by the amount of methane. Each of the individual components was calibrated using pure component gases. Operation of the reactor system began with heat-up to the desired operating temperature. The results showed that the best data (heat balance, mass balance, product composition) were obtained when the heat-up continued about 6 h. A t this time, the metal had reached a thermal steady state. After reaching this thermal steady state, feed was begun and another steady state reached after 1-2 h. Only after the second steady state were data recorded. Data were generally obtained over a 1-3 h period. Discussion of Results The manure used in this study was collected from the manure storage pile at a cattle feedlot located in Lubbock, Texas. Because this feedlot is dirt, the manure contained a significant amount of ash (-18%) and, in most cases, the moisture content was approximately 1570. This low moisture is due to drying during storage and could be much higher in a commercial operation. Due to compaction during storage, the manure was run through a hammermill with a 0.64-cm screen to break up the large pieces. This resulted in a very free-flowing, relatively dry manure feedstock of particle size less than 0.64 cm. The moisture after grinding was in the range of 7-1070, which indicated
CUMULATIVE WEIGHT PERCENT
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Ind. Eng. Chem. Process Des. Dev., Vol. 18, No. 2, 1979
331
k'
I 0
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50
'00
150
203
250
3W
HEIGHT ABOVE DISTRIBUTOR cm
Figure 4. Temperature profile in the SGFM reactor.
reactor temperature and air-to-manure ratio. Most of the pilot plant runs were conducted at an air-to-manure ratio of approximately 0.25 kg of air per kg of dry, ash-free manure. The last two pilot plant runs were conducted at an air-to-manure ratio of approximately 0.6 kg per kg dry, ash-free manure and the resulting gas yield was much greater than the earlier runs. Even though the gas yields are related to the operating conditions, the gas composition appears to be relatively constant with the exception of the hydrocarbon content. These hydrocarbons (CH4,C2H4,and C2Hs) are readily oxidized in the runs at a high air-to-manure ratio. The CO and C 0 2 content is not significantly changed because much of these components come from the oxygen in the manure. It can be seen that the yield of ethylene varies from 2% to nearly 7 % of the total gas produced and the methane yield from 6% to 10%. This confirms results seen in the bench-scale studies and is approaching a level where it may be feasible to recover the ethylene as a separate product. If the ethylene is not recovered as a separate product, it will contribute greatly to the higher heating value of the gas. In all cases, the lower heating value of the raw gas produced was greater than 8.8 MJ/m3 (1atm, 0 "C). This relatively high heating value for air gasification is due to the presence of significant amounts of hydrocarbon gases ('2,'s). If one looks at the heating value of the produced gas as a function of manure heating value, as shown in Figure 5 , it can be seen that nearly 60% of the heating value of the dry, ash-free manure feed can be recovered in the raw gas although in most cases it was around 40%. Also shown in Figure 5 is the heating value of the produced gas on a C02-free basis. This gas still contains the nitrogen but, as can be seen, has a heating value consistently greater than 12.0 MJ/m3 which is a very suitable medium BTU gas for feed to a boiler. This gas is also suitable for feed to an ammonia synthesis plant or some other petrochemical production process such as methanol synthesis. Heat balance calculations indicate that the overall partial oxidation-pyrolysis reaction is exothermic as indicated by the net heat of reaction shown in Table I. These heat balance results seem to be relatively consistent although the heat loss from the reactor was extremely high. Calculations were complicated by the extreme temperature profile which existed in a reactor. To calculate the heat losses from the reactor, a mixture of steam and air was fed to the reactor without any solids flow. The surface temperature of the insulation and the
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332 Ind. Eng. Chem. Process Des. Dev., Vol. 18, No. 2, 1979
-" 0
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8
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0 C02- Free Gas
a
a a
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1
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I
a
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40
60
80
100
TOTAL HEATING VALUE OF GAS
sb
TOTAL HEATING VALUE OF DAF FEED
Figure 5. Heating value of product gas as a function of thermal conversion. Table 11. Example Calculations Showing Autothermal Character of SGFM Technology run 25
run 32
input (MJ) manure (wet; 10% H,O) air steam
0 0.0536 17.28
1.13 12.68
o u t p u t (MJ) steam gases char + fines tar + liquids
21.51 4.21 1.54 2.75
18.13 23.24 1.09 4.17
10% heat losses of o u t p u t 80% efficiency of o u t p u t net output net o u t p u t - input (AH) water @ conditions potential added water potential moisture content
(3.01) (4.66) (6.02) (9.32) 21.05 32.63 3.24 18.83 0.593 MJikg 0.593 MJikg 31.75 kg 5.46 kg 23% 57%
0
ambient temperature were measured along with the inlet and outlet enthalpy of the steam-air mixture. This allowed the correlation of the heat loss based on temperature difference across the insulation of the reactor. During the reactor operation, the temperature and pressure of the inlet air-steam mixture, manure, and the outlet product gases were measured. The temperature difference across the insulation on the reactor was also measured to determine the heat losses.
The heat balance data can be used to estimate the heat requirements for a commercial reactor. If one assumes that the permanent heat losses are 10% of the total heat out of the reactor and that 80% of the remaining sensible heat in the product gases can be recovered in a usable form, one can project the energy required to operate a commercial reactor. The results of such a calculation are shown in Table 11. These calculations show that a net increase in enthalpy is achieved during the reaction and thus the commercial reactor could be operated autothermally. This excess heat would be available to vaporize additional moisture in the feed so that a feed containing a much higher moisture content (up to 50%) could be reacted autothermally rather than the 10% moisture content present in the experimental feed. Conclusions The results of the pilot-plant study of pyrolysis and partial oxidation of cattle feedlot manure confirms the results obtained in the earlier bench-scale studies. The work confirms the technical feasibility and potential scale-up for producing a medium BTU gas suitable for fuel or for feed to a petrochemical production process a t relatively high yields from cattle feedlot manure. An important result of the pilot-plant study is that ethylene, which was observed in the product from the bench-scale pyrolysis, is also present in the product from the pilot-plant pyrolysis. By appropriate choice of operating conditions, it may be possible to increase the yield of ethylene and thus develop manure as a possible source of this valuable petrochemical feedstock. Acknowledgment The authors wish to thank the U S . Department of Energy (Contract No. EY-7643-04-3779)for supporting this work. In addition, the Pioneer Corporation and the Texas Cattle Feeders Association deserve special thanks for supporting this project from its inception. The U S . Environmental Protection Agency supported this project during construction of the pilot plant. Literature Cited Halligan, J. E., Herzog, K. L., Parker, H. W., Ind. Eng. Chem. Process Des. Dev., 14, 64 (1975). Halligan, J. E., Sweazy, R. M., "Thermochemical Evaluations of Animal Waste Conversion Processes", presented 72nd AIChE Meeting, 1972. Inman. R. E.. "Silvacultural Biomass Farms", Vol. 1, Mitre Technical ReDOfl Number 7347, 1977. Perry, J H , Ed., "Chemical Engineers Handbook", 4th ed,pp 20-68, McGraw-Hill, New York, N.Y., 1963. "Beef Cattie Feedlot Facildies", Texas A&M University, Bulletin No. MP-680 (1977).
Received for reuieu: April 24, 1978 Accepted December 15, 1978
