DESIGN AND TESTING OF A LOW-COST, PILOT-SCALE BATCH GASIFIER FOR FOOD PROCESSING BYPRODUCTS T. J. Bowser, P. R. Weckler, K. N. Patil, C. DeWitt ABSTRACT. Byproduct disposal from food processing operations is an increasing problem. Gasification is an alternative, value-added process for handling some food processing byproducts. A low-cost, pilot-scale, updraft, batch gasifier was designed, fabricated, and tested at the Oklahoma State University to test feasibility of gasification of food processing byproducts. A complete description of the design and schematics of the gasifier are given. The gasifier system demonstrated simple operational requirements and stable performance when gasifying food processing byproducts. The food processing byproducts tested were: dried sludge obtained from the discharge of a dissolved air flotation unit at a value-added meat processor; wood pellets; and mixtures of wood pellets and dried sludge. Cold gas efficiency of the gasifier ranged from 47% to 60%. Ash production ranged from 6% to 16% of the feedstock input (mass basis). Composition of the producer gas (volume basis) included 2% to 3% H2; 12% to 17% CO; and, 1% to 4% CH4. The study accomplished the design and validated the operation of a low-cost, batch gasifier by successfully demonstrating its capability to gasify food processing byproducts. Keywords. Batch, Gasifier, Food, Waste, Byproducts, Processing, Pilot.
B
yproduct disposal from food processing operations is an increasing global problem (Brandt and Martin, 1996). Food processing byproducts (FPBs)include trimmings from animal carcasses, animal and vegetable oils, wastewater, wood from pallets, corrugated and other paper products, vegetable and fruit waste, nutshells, plastic barrels, bulk food containers and other packaging materials. Animal processors waste an average of 5.2 kg of fats, oils and grease (FOG) to the sewer for every 1000 kg of live weight kill (Ockerman and Hansen, 2000). This equates to about 90 million kg of lost FOG for cattle alone. Land filling, incineration, rendering, and microbial decomposition are examples of traditional means of byproduct disposal; however, these methods are sometimes unavailable, expensive and cumbersome. Gasification is an alternative, value-added process for handling some FPBs that merits consideration. Gasification is the production of combustible gases from solid, organic material by the application of heat, or pyrolysis (Mayer, 1988). It is accomplished by burning the feedstock material with limited air to produce an exhaust gas (producer gas) which contains enough carbon monoxide, hydrogen, acetylene, and other combustible hydrocarbons (Richey, 1984). Gasifiers have been in use for over 200 years. During the industrial revolution, large quantities of coal were coked
Article was submitted for review in September 2003; approved for publication by the Food & Process Engineering Institute Division of ASAE in April 2005. The authors are Timothy J. Bowser, ASABE Member, Associate Professor, Paul R. Weckler, ASABE Member Engineer, Assistant Professor, Oklahoma State University, Stillwater, Oklahoma; Krushna N. Patil, ASABE Member Engineer, Post-Doctoral Researcher, Biosystems and Ag. Engineering, and Christina A. Mireles DeWitt, Assistant Professor, Animal Science Department, Oklahoma State University, Stillwater, Oklahoma. Corresponding author: Timothy J. Bowser, Biosystems and Ag. Engineering, Oklahoma State University, 110 FAPC, Stillwater, OK 74078; phone: 405-744-6688; fax: 405-744-6313; e-mail:
[email protected].
prior to use in smelting operations. The combustible gas driven off during the coking process was used for gas lighting. Subsequently, gasifiers were designed for use with internal combustion engines. During World War I, the German military used bolt-on gasifiers to produce fuel for motor vehicles when oil imports were blockaded (Mayer, 1988). The efficient conversion of FPBs to usable heat energy requires a different gasifier technology than the well-developed methods used for petroleum products and coal. The major differences are (Richey, 1984): S Many FPBs are not free-flowing and require complex handling and feeding equipment. S Some FPBs contain a high proportion of volatile material (up to 80%) which may vaporize rapidly, causing excess smoke and loss of energy if combustion is not controlled and completed. S FPBs may contain silica which produces an ash that can fuse into an adherent slag when heated to temperatures above 800°C. S FPBs often have higher moisture content than other gasifier feedstocks and require natural or artificial drying before use. Direct combustion of biomass materials usually results in smoke and ash pollution unless special filtering equipment is used. For example, smoke from wood-fired household stoves or fireplaces may cause tar (creosote) deposits in the chimney which may eventually ignite, emit dangerous sparks, overheat chimneys, and cause fires. Heat exchangers are often needed in order to avoid contamination of the area or the material being heated. Possible advantages of gasification over direct combustion processes include (Richey, 1984): S Minimum air pollution and direct-drying of the product without the necessity of using a heat exchanger, which reduces efficiency and adds equipment expense. S More efficient conversion of biomass to heat, with 80% to 90% of its heat recovered. S Control of combustion rate by regulating primary air flow.
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E 2005 American Society of Agricultural Engineers ISSN 0883−8542
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DESCRIPTION OVERVIEW The objectives of this study were to: (1) design, (2) construct, and (3) test a low-cost, laboratory-scale gasifier that could be used to test the feasibility of gasification of FPBs. An updraft, batch gasifier configuration was selected for its simplicity, low cost, and versatility. Updraft gasification is perhaps the oldest of the gasification technologies (Li, 2002) and is well-known for its high charcoal burnout and ability to handle a variety of feedstocks at high conversion efficiencies (Quaak et al., 1999). Many modern, commercial biomass gasification systems (e.g. Primenergy, LLC, Tulsa Okla.; VIDIR Machine, Inc., Clermont, Fla.; and Carbona Corporation, Atlanta, Ga.) have successfully utilized updraft technology. Batch processing was deemed suitable for feasibility testing since materials handling properties of FPBs could be obtained by other experimental means and gasification behavior was the primary information needed for feasibility studies. The basic design of the gasifier constructed in this study was inspired by the work of Patil and Rao (1993). Improvements to the Patil and Rao gasifier included a motorized scraper blade, improved sensors, off-the-shelf pipe and pipe fittings for body components, portability and quick disassembly. A photograph of the new gasifier design is given in figure 1 (shown without automated scraper and insulation), and a schematic is shown in figure 2. The gasifier consisted of three basic components: gasifier body, scraper and scraper drive, and support frame. The entire unit was constructed in
Figure 2. Schematic diagram of pilot-scale batch gasifier, showing vertical section of side view (dimensions in m).
the Biosystems and Agricultural Engineering fabrication shop at the Oklahoma State University.
Figure 1. Photograph of the low-cost, pilot-scale batch gasifier for gasification of food processing byproducts (insulation and scraper drive removed).
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GASIFIER BODY All major body components of the gasifier were fabricated of mild steel, 15.2- and 7.6-cm diameter, schedule-40 pipe, and pipe fittings that were welded or bolted together. The gasifier was insulated with a calcium silica insulation blanket (McMaster Carr, Chicago, Ill.) to reduce heat loss and improve efficiency of operation. Heat-resistant gaskets cut from the same insulation were used to seal all non-welded joints of the gasifier. The flanges were forged carbon steel, 1034 kPa, slip-on, and weld-neck type. The flanges were included to allow gasifier disassembly for biomass charging, maintenance, and cleaning. The height of the unit (from ash grate to top opening) was about 1.0 m. The upper section of the gasifier body was designed as a reservoir for FPB feedstock, which was charged into the gasifier through the opening in the top. The top opening was sealed with a 6.3-mm thick, mild-steel disk, matched to a slip-on flange which was in turn welded to the gasifier body. A thrust screw was used to securely hold the steel disk against the mating flange and gasket to seal the top opening of the gasifier. The reduction in body diameter of 16.9 to 8.9 cm (the outside dimensions for a standard 15.2- × 7.6-cm pipe reducer) was included to reduce pressure on the lower column of FPB in the gasifier body and to provide some
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headspace where the flue pipe was attached to the gasifier body. The midsection of the gasifier body included the combustion chamber, thermocouple ports, producer gas exhaust pipe (flue), and access port. The combustion chamber was fabricated from a 7.6-cm Tee with flange ends. The projecting end of the Tee was used as an access port similar to that of the upper section, except that the steel disk was bolted in place. Thermocouple fittings were pass-through, 6.3-mm compression fittings from Omega Engineering (Stamford, Conn.) that were welded to the combustion chamber at the locations shown in figure 2. The producer gas flue pipe was fabricated from 3.8-cm pipe and included a ”dirt leg” to help remove condensed tar and particles from the producer gas, a sampling port, and a diffuser to develop the flare. The diffuser was a 3.8- × 5.0-cm pipe reducer. The lower portion of the gasifier included an ash grate, rotating motorized scraper assembly, ash receptacle, compressed gas inlet, and ash cleanout port. Figure 3 shows a plan view of the ash grate and scraper (The ash grate and scraper assembly are described later.). Ash particles fell through the grate and accumulated in the ash receptacle, which consisted of a 7.6-cm pipe elbow. One end of the elbow was joined by a flange to the combustion chamber and the other end was threaded to accommodate a pipe cap. After a run was completed, ash was cleaned out of the gasifier by removing the cap from the elbow. The compressed gas fitting was a hose barb sized to connect to a 6.3-mm (internal diameter) pneumatic hose. Thermocouples were inserted radially through the compression fittings into the gasifier body, with their tips positioned 1 cm inside the gasifier body. Thermocouples were located vertically at 7 and 18 cm from the surface of the ash grate.
steel, with thicknesses of 19.1 and 6.4 mm, respectively. Blade rotation was powered by a reversible gear motor (unlabeled unit from surplus supply) through a chain drive with 5:1 speed reduction. The scraper rotated at approximately 5 rpm in the clockwise direction and was operated intermittently during an experimental session. Packed ceramic insulation material (calcium silica fiber blanket, McMaster Carr, Chicago, Ill.) was used to provide a seal between the scraper blade drive shaft and the gasifier body.
SCRAPER AND DRIVE The ash scraper and grate are illustrated in figure 3. The scraper blade and grate were custom fabricated from mild
TEST PROCEDURE
Figure 3. Schematic diagram of plan view of ash grate and scraper (inside gasifier), dimensions in cm.
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SUPPORT FRAME The pilot scale gasifier system was supported by a welded framework of painted, 25-mm square tubing. The framework provided a level of safety by guarding operators from the hot body of the gasifier during operation. It also supported the scraper drive system. Swiveling casters were fixed to the base of the framework to facilitate movement and transportation of the system. SENSORS AND DATA COLLECTION System sensors included an air flow meter (model 810M-DR-13, Sierra Instruments, Inc., Monterey, Calif.) and two type-K thermocouples (Omega Engineering, Stamford, Conn.). A laboratory scale was used to weigh FPBs and ash. All temperature and air flow values were automatically recorded during system operation using a data logger (model Hydra 2635A, Fluke Corp., Everett, Wash.) connected to a laptop computer. Producer gas was analyzed with a gas chromatograph (model CP-3800, Varian, Inc., Palo Alto, Calif.). A bomb calorimeter (model 1261 ISOPERIBOL, Parr Instrument Company, Moline, Ill.) was used to measure the heating value of the dried sludge, using the procedure described in the operator’s manual.
FOOD PROCESSING BYPRODUCT (FPB) A major, value-added meat processor donated the FPB material tested in this study. The 13,500-m2 facility produces over 1.8 million kg of sliced bacon and frankfurter products per week. Two FPBs from their plant were considered in this study: sludge from their dissolved air flotation (DAF) units and wood from scrap pallets. Wastewater from the plant was routed through a coarse screen to remove large particulates; then it entered the DAF system. The pH of the wastewater was adjusted and a chemical conditioner was added to assist with the flotation process. Extremely fine air bubbles attached to suspended materials in the wastewater (primarily pork fat) and floated it to the surface where it was skimmed off (Komline Sanderson Engineering Corp., 1996). The sludge had the appearance of a gray-colored mud with a lighter texture as a result of the introduced air. The meat processor considered the sludge as one of the most difficult and expensive waste streams to handle and dispose of. Scrap wooden pallets were plentiful at the plant, and when taken in combination with the sludge, would possibly help to compensate for the excess moisture of the sludge. Table 1 shows results of a proximate analysis of the sludge (before drying) as received on a single day and used in this study. Proximate analysis was performed at the Oklahoma State University Soil, Water, and Forage Analytical Laboratory, using their standard procedures (Zhang et al.,
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Table 1. Typical analysis of sludge from DAF (as received). Component Weight (% ) Moisture Ash Protein Fat Carbohydrate Heating value
72.57 2.89 5.63 13.84 5.06 26.0 MJ/kg
2002). Sludge composition at the processing plant was somewhat variable during the period of this study because of seasonal variations in operations and product mix. In addition, DAF system upgrades that included new holding tanks, pH controls, and a centrifuge for sludge moisture reduction, were being installed throughout the period of this study. During the study, the sludge was air dried on trays to approximately 10% moisture content (dry basis) and classified with a standard 3-Mesh sieve having a 6.7-mm nominal opening. The drying-screening process made the sludge shelf-stable and resulted in improved handling characteristics. Hardwood pellets (Pennington premium quality oak wood pellet fuel, lot 505F2, Greenfield, Mo.) were substituted for the scrap wood from pallets. The wood pellets were 6.3 mm in diameter and varied in length from 0.5 to 4.0 cm. Three experimental sessions were performed to assess the design of the gasifier and the feasibility of gasifying FPBs. The first session used wood pellets alone; the second used sludge alone; and the third used a 50/50 mixture (mass basis) of sludge and wood pellets, which were gasified together to test the feasibility of operating a gasifier with mixtures of FPBs. Figure 4 shows the screened sludge particles and wood pellets. SYSTEM OPERATION AND DATA COLLECTION After preliminary testing, the gasifier was operated for the three experimental sessions described above. Each session was repeated three times. Standard system startup and operation procedures were followed for each gasifier session: S Eight charcoal briquettes (The Kingsford Products Company, Oakland, Calif.) weighing approximately 200 g (total) were soaked with about 50 mL of charcoal lighter fluid (The Kingsford Products Company, Oakland, Calif.), placed onto the grate through the lower access port and ignited with a flame. The biomass charging port remained open.
Figure 4. Representative samples of dried, classified sludge (left) and wood pellets (right), that were gasified in this study.
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S Compressed air (at about 34.5 kPa, gage) was supplied to the gasifier at a rate of about 3 to 5 m3/h. S The charcoal was allowed to burn for about 20 min until the briquettes were completely covered with white ash and glowed cherry-red in the center. S One (1.0) kg of feed material was manually added to the gasifier from the top opening; then the opening was covered and sealed. S The data logger was initialized to continuously record temperatures during gasifier operation. S The scraper blade was operated for a few seconds every 5 to 10 min during the experiment after the feed material was added. S The flare was ignited and the air flow rate was adjusted (within 0.2 m3/h) to optimize the flame for the largest size and most uniform (unwavering) appearance. S Gas samples were taken after the bed temperature stabilized. S The gasifier was allowed to cool after all of the feed material was gasified; then the ashes and tar were removed and weighed. The weight of the charcoal ash was neglected. GAS SAMPLING AND ANALYSIS Capture of producer gas samples was accomplished with a 250-mL gas sampler (Article 653100-022, Kimble Kontes, Vineland, N.J.). After the operating temperature of the gasifier stabilized in the range of 700°C to 750°C, producer gas was allowed to flush through the sampler for at least two minutes before its valves were closed. Three gas samples were obtained for each run at approximately 5, 10, and 15 min after the feed material was added to the gasifier. The samplers were stored in a refrigerator to condense the water and tar vapors from the gas onto the walls of the samplers. A gas-tight syringe (Cole Parmer, Vernon Hills, Ill.) was used to remove a volume of the producer gas from each sampler, through a lid fitted with a septum. The producer gas was then injected into the gas chromatograph for analysis with the instrument setup reported by Cateni et al. (2003).
RESULTS AND DISCUSSION The gasification system demonstrated stable performance for all experimental sessions. Table 2 gives a summary of the average conditions and results of each session. The air input rate (which was set to optimize the flare appearance) varied from 3.1 to 4.7 m3/h. The flare ignited and burned reliably and cleanly for the all of the experimental sessions, without black smoke, particles or offensive odor, indicating that the FPBs tested gasified well. Ash recovery ranged from 60 to 160 g, with the ash recovery from the 100% wood pellets and the mixture of wood pellets and dried sludge at 60 and 70 g, respectively. Ash quantities from the dried sludge were higher because of the chemical conditioner added during the separation process at the wastewater treatment plant. Ash generated from all runs consisted of hard, granular, black or grey particles, indicating a lower likelihood of bed agglomeration in a commercial gasifier. Tar recovery was highest for the 100% wood pellet product and lowest for the mixture of wood pellets and dried sludge. This unexpected result may have been caused by an unknown interaction between the wood pellets and dried sludge during the gasification process. Percent of feed
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Table 2. Summary of results of three experimental gasifier sessions (Rao, 2004). 100% 100% 50% Sludge Sludge Parameter (average value) Wood Pellets Air input rate (m3/hr) Ash recovery (g) Tar recovery (g) Feed material gasified (%) Cold gas efficiency (%)
3.9 60 300 64 58
4.7 160 200 64 47
3.1 70 140 80 60
of packed or loosely compressed, fibrous insulation that would fail under increased pressure. Operation of the gasifier took place outdoors, or in a well-ventilated space, that was equipped with carbon monoxide warning sensors. Operators worked in pairs for safety, and wore protective bump-caps, and eyewear.
CONCLUSIONS AND DISCUSSION
material gasified was highest for the sludge mixture, and virtually the same for the other sessions. We believe that dried sludge and wood pellets gasified more completely because the dried sludge particles filled air gaps between the wood pellets, holding in heat and allowing more contact time with air and gas as it moved through the bed. Cold gas efficiency was calculated as the output energy divided by the input energy. The output energy was calculated as the sum of the chemical energy content of the producer gas. The input energy was determined by multiplying the amount of feed material gasified by its heating value. Heating value for wood pellets, at 5% moisture content, was taken as 19.3 MJ/kg (Schlesinger et al., 1996). Cold gas efficiency for the 100% dried sludge sessions was lower because of the higher level of moisture present in the sludge. Table 3 gives the average analysis of the producer gas samples taken during steady-state operation of the gasifier. The chemical energy content of the producer gas was determined for steady-state operating conditions with the calculated heat of combustion of the producer gas given the composition data reported in table 3. Producer gas composition, when compared to results obtained by other researchers (Craig, 1999; Cateni et al., 2003) who have gasified various biomass materials (switchgrass, cotton gin trash, and wood) in air-blown systems, was similar, except that it was about 2% to 6% lower in carbon monoxide. Lower carbon monoxide content can be attributed to differences in gasifier types (e.g. downdraft as compared to fluidized bed) and feedstocks.
The pilot-scale gasifier was: (1) designed by the authors, based on past experience, (2) constructed at the Biosystems and Agricultural Engineering Machinery Shop, and (3) successfully tested for gasification of FPBs. Experiments with the gasifier demonstrated the feasibility of quickly gasifying FPBs and combinations of FPBs at low cost and minimal effort. Gasification tests for the FPBs and combination of FPBs tested revealed: stable gasification bed temperatures, no tendency for slagging or bed agglomeration, stable flare behavior, gas composition, and cold gas efficiencies. The ash grate and automatic scraping system worked well. No residual biomass remained on the grate after each run was completed. Waste products from a commercial food processor were gasified into a valuable producer gas, which could potentially be used to fire an onsite boiler to make steam for direct use or to generate electricity. Cold gas efficiency of the gasifier ranged from 47% to 60%. Ash recovery ranged from 6% to 16% of the input FPB (mass basis). Combining FPB waste streams for gasification is possible. It is hypothesized that combinations of FPBs may reduce handling requirements (e.g. pre-drying feedstock); particularly when a high-moisture byproduct (sludge) is mixed with low-moisture byproducts, such as wood, hot-dog casings and cardboard. Future experiments are required to test this hypothesis. An improved gasket material is needed for future gasifier designs, since the silica gaskets were time-consuming to cut and handle and could not be reused.
SAFETY Safety considerations were an important factor of the gasifier design and operational procedures. Hot surfaces were insulated, or guarded to prevent burn hazards. The gasifier was not designed for pressurized (above 35 kPa, gage) operation. In the event of an explosion, the headspace above the combustion chamber was directly vented to the atmosphere through the flue pipe. Seals around the scraper driveshaft and on the access and charging ports were formed
ACKNOWLEDGEMENTS This research was supported by a grant from the Oklahoma Food and Agricultural Products Research and Technology Center. Appreciation is extended to the following people at the Oklahoma State University: students Amit Kadam and Neelesh Kale (Industrial Engineering Department), shop manager Wayne Kiner, machinist Robert Harrington, and research engineer Bruno Cateni (Biosystems and Ag. Engineering Department) for their significant contributions in the design, construction and testing of the gasifier.
Table 3. Summary of results of the producer gas composition (% volume) from three gasifier sessions (Rao, 2004). 100% 100% 50% Wood Pellets Sludge Sludge Parameter H2 CO CH4 C2H4 CO2 N O2 [a]
2.1 11.9 0.7 5.3 7.2 50.7 1.9
3.4 13.3 1.0 [a]
11.8 59.3 0.8
2.9 16.8 4.2 0.3 [a]
54.2 [a]
REFERENCES Brandt, R. C., and K. S. Martin. 1996. The Food Processing Residual Management Manual, 2nd ed. Ithaca, N.Y.: Northeast Regional Agricultural Engineering Service. Cateni, B., D. Bellmer, R. Huhnke, and T. Bowser. 2003. Effect of switchgrass moisture content on producer gas composition and quality from a fluidized bed gasifier. ASAE Paper No. 036029. St. Joseph, Mich.: ASAE.
Not detected.
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Craig, J. D. 1999. Completion of final report and gas analysis for a biomass gasifier. Submitted in partial fulfillment of contract 55018a of the Western Regional Biomass Energy Program. Tahoka, Tex. Komline Sanderson Engineering Corp. 1996. Corporate website. Available at: www.komline.com. Accessed 21August 2003. Li, X. 2002. Biomass gasification in a fluidized bed gasifier. Ph.D. dissertation. Vancouver, Canada: University of British Columbia, Department of Chemical and Biological Engineering. Mayer, E. F. 1988. Gasifier apparatus. U.S. Patent No. 4,764,185. Ockerman, F. J., and J. D. Hansen. 2000. Animal byproduct utilization. Boca Raton, Fla.: CRC Press. Patil, K. N., and C. S. Rao. 1993. Updraft gasification of agricultural residues for thermal applications. In Proc. of IV International Technical Meet on Biomass Gasification and Combustion. Bangalore, India: Interline Publishing. Quaak, P., H. Knoef, and H. Stassen. 1999. Energy from biomass. A review of combustion and gasification technologies. World Bank Technical Paper No. 422. World Bank, Washington, D.C.
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Rao, B. R. 2004. Gasification of food processing byproducts – an economic waste handling alternative. M.S. thesis, Stillwater, Okla: Oklahoma State Univ., Biosystems and Agricultural Engineering Dept. Richey, C. B. 1984. Downdraft channel biomass gasifier. U.S. Patent No. 4,452,611. Schlesinger, M. D., K. C. Baczewski, G. W. Baggley, C. O. Velzy, R. S. Hecklinger, and G. J. Roddam. 1996. Fuels and furnaces. In Marks’ Standard Handbook for Mechanical Engineers, 10th ed., eds. E.A. Avallone and T. Baumeister III. New York: McGraw Hill. Zhang, H., M. Kress, and G. Johnson. 2002. Procedures used by OSU Soil, Water, and Forage Analytical Laboratory. Oklahoma State University Cooperative Extension Service Fact Sheet F-2901. Stillwater, Okla.
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