United States Environmental Protection Agency Municipal Environmental Research^ Laboratory Cincinnati OH 45268 Research and Development EPA-600/S2-82-047 August 1982 Project Summary Co-Gasification of Densified Sludge and Solid Waste in a Downdraft Gasifier S. A. Vigil and G. Tchobanoglous Thermal gasification is a new pro- cess for the co-disposal of densified sludge and solid waste in a co-current flow, fixed bed reactor (also called a downdraft gasifier). The advantages of this technology include lower costs than other incineration or pyrolysis technologies, simple construction and operation, and the ability to use a var- iety of fuels including agricultural wastes and other biomass materials in addition to densified sludge and solid waste. Essentially, the gasification process involves the partial combustion of a carbonaceous fuel to generate a low energy combustible gas and a char. Operationally, fuel flow is by gravity with air and fuel moving co-currently through the reactor. The low energy gas is composed primarily of carbon monoxide, hydrogen, and nitrogen and of trace amounts of methane and other hydrocarbons. Although fixed bed gasifiers are mechanically simpler than other co- disposal reactors, such as multiple hearth furnaces or mass fired incinera- tors, they have more exacting fuel requirements which include: moisture content, <20 percent; ash content, <6 percent; and relatively uniform grain size. Without front end process- ing, neither municipal solid waste nor dewatered sludge meet these criteria. Demonstrating that a suitable gasifier fuel could be made with a simple front end system consisting of source sepa- ration for solid waste, a sludge de- watering system, and fuel densifica- tion system has been one of the objectives of this project. To demonstrate the gasification process, a pilot scale gasifier was con- structed. A broad range of fuels have been tested with the gasifier including an agricultural residue, densified waste paper, and densified waste paper and sludge mixtures containing up to 25 percent sludge by weight. The sludge fuels were made from mix- tures of lagoon-dried primary and secondary sludge and from recycled newsprint (in full scale systems a mixed paper fraction of solid waste could be used). Mixtures were densi- fied using commercially available agri- cultural cubing equipment. The gasifier was operated with each fuel, and measurements of the varia- bles needed to characterize the pro- cess were made. The results of gas, fuel, and char analyses were used to compute energy balances. These data were also used to calculate efficien- cies for each run. Hot gas efficiency. which include the sensible heat of the gas, ranged from 40.0 to 85.2 per- cent. The cold gas efficiency, which does not include the gas sensible heat, ranged from 37.1 to 80.7 percent. The dry low energy gas produced during the tests ranged in a higher heating value (HHV) from 4.52 to 6.79 MJ/m3. This Project Summary was devel- oped by EPA 's Municipal Environ- mental Research Laboratory, Cincin- ------- nati, OH, to announce key findings of the research project that is fully docu- mented in a separate report of the same title (see Project Report ordering information at back). Introduction The co-disposal of sludge (the solid residues of wastewater treatment) and solid waste in a joint facility is accepta- ble from an environmental, economic, and energy standpoint. However, the trend in development of such projects has been towards very large systems. It has been assumed that the economics of scale precludes the use of such tech- nology by small communities (less than 50,000 population). The ever increasing costs of energy and disposal of sludge and solid waste make small scale co- disposal attractive. This report presents the development of a new process for the co-disposal of sludge and solid waste that, unlike existing co-disposal technology, can be implemented on a small scale. This air- blown gasification process has been widely applied to coal, wood, and agri- cultural wastes but has never before been used for the co-disposal of sludge and solid waste. Co-gasification of den- sified mixtures of sludge and source separated solid waste occurs in a simple fixed bed reactor, also known as a mov- ing packed bed reactor. Energy, in the form of a low energy gas (LEG) produced by the process, can be used to fuel boil- ers, heaters, engines, or turbines. Experimental Gasification System To investigate the co-gasification of densified sludge and solid waste, a pilot scale gasification system was designed and constructed. The complete system consists of three subsystems: batch fed downdraft gasifier, data acquisition, and solid waste shredding and densifi- cation. Batch Fed Downdraft Gasifier A pilot scale batch fed downdraft gasi- fier was designed and constructed for the experiments. The design of the gasi- fier is based on laboratory and pilot scale gasifiers built by the Department of Agricultural Engineering at the Uni- versity of California, Davis. As shown in Figure 1, the gasifier is built in three main assemblies, fuel hopper, firebox, and ashpit. The fuel hopper is a double walled cylinder. The inner wall is in the form of a truncated cone to reduce the tendency for fuel Air inlet pipe Gas dispersion zone Air inlet pipe Gas outlet pipe Figure 1. Schematic of a downdraft gasifier. bridging. The double wall acts as a con- denser to remove water vapor from the fuel before gasification. Condensed vapor is collected in a condensate gutter and drained off after each run. The fuel hopper is mounted on the firebox with quick-release clamps to allow easy inspection after experimental runs. The firebox is also a double walled cylinder. The inner cylinder is the actual firebox. Air is supplied by four air tubes to the annular space between the walls that acts as an air plenum to distribute air evenly to the sixtuyeres(air nozzles), which supply air for partial combustion of the fuel. A choke plate acts as a large orifice, replacing the Venturi section previously used in earlier gasifier designs. The firebox assembly is flange mounted to the ashpit. Char is collected in the ashpit during an experimental run. A rotating eccen- tric grate is located in the ashpit imme- diately below the choke plate. The grate supports the fuel bed, and allows pas- sage of char and gas into the ashpit. Gas isdrawn off continuously through a pif on the side of the ashpit. The choke plates and tuyeres we constructed from Type 304 stainle steel. A temperature resistant allc ASTM Type A515*, was used for tl firebox and rotating grate. The remai der of the gasifier was constructed fro Type 1040 mild steel. The rolled cylindrical sections, inn and outer walls of the firebox, ashp and inner and outer walls of the fi hopper were fabricated by commerc machine shops. All other cutting, £ welding, and assembly were done int College of Engineering shops. Full siz gasifiers could easily be constructed relatively unsophisticated machi shops since exotic materials orcomp machining are not required. Data Acquisition The data acquisition subsystem is automated temperature measuremi system. Temperatures are measui with Type K thermocouples located ------- shown in Figure 2. A Type Tthermocou- ple is used in the air inlet line and a Type K thermocouple is installed in the gas outlet pipe. Provision is made for three magnetically mounted Type K thermo- couples for surface temperature mea- surements. Thermocouple number, temperature, and elapsed time are printed on the paper tape output of a Digitec Model 1000 Datalogger. Solid Waste Shredding and Densificat/on Based on the successful cubing test with the John Deere cubing machine at the University, the Papakube Corpora- tion was contracted to prepare sludge/ solid waste cubes. Key features of the Papakube system include an integral shredder, a metering system that main- tains optimum moisture content of the newspaper, and a modified John Deere Cuber. The extrusion dies of the machine have been modified with a proprietary coating and finishing treat- ment that is said to allow the densifica- tion of many materials without binding agents. Experimental Results In the experimental phase of the proj- ect the gasifier was operated at a con- stant air flow rate but fueled with five different types of fuel: wood chips, almond shells, densified source sepa- rated sol id waste (two types), a nd densi - fied mixtures of sludge and solid waste (10, 15, 20, and 25 percent sludge by weight). The characteristics of the fuels, operational data from the test runs, and energy balances for two typical runs (RUNS 11 and 12) are presented and discussed below. Fuel Characteristics All fuels were tested for proximate analysis, ultimate analysis, and energy content (Table 1). In general, the gasifier fuels tested were all relatively high in volatile combustible matter (VCM), low in carbon content, and low in energy content as compared with coal, but sim- ilar to most woods. Both bulk and indi- vidual particle densities of the fuels were also measured (see Table 1). Bulk density as it relates to storage and transportation is a significant parame- ter, and the bulk density of densified fuels is twice that most natural fuels (e.g., wood chips). Operational Data The results of the gasification test series are given in Table 2. All test runs were conducted at the same air flow rate, 0.41 mVmin (1 atmosphere, 0°C). Thus, the flow rate of fuel through the gasifier, the efficiency, and gas quality are a function of the gasification charac- teristics of the fuel. Fuel hopper Condensate gutter Tuyere Choke plate "Mention of trade names or commercial products does not constitute endorsement or recommenda- tion for use Rotating grate Thermocouple Locations Tuyere Reduction zone T3\ Ashpit uSj Fuel hopper [76] Air plenum Figure 2. Cross section — UCD sludge/solid waste gasifier. 3 Grate drive sprocket ------- Table 1 Summary of Fuel Characteristics Item RUN 1 1, 20% Sludge Cubes RUN 12. 25% Sludge Cubes Proximate analyses Volatile combustible matter, % Fixed carbon, % Ash, % Moisture, % Ultimate analyses (Dry basis) C, % H, % S.% 0. % Residue Energy content, MJ/kg (Dry basis. HHV) Densities Bulk kg/m3 Unit, kg/m3 Table 2. Operational Summary 74.54 13.05 3.07 9.34 45.24 5.81 0.13 0.11 46.81 1.90 18.93 536 486 73.66 13.70 4.08 8.56 45.27 5.77 0.42 0.16 44.18 4.20 18.49 row 1014 Item Fuel consumption rate, kg/hr Char production rate, kg/hr Condensate production rate, kg/hr Net run time, min Gas flare ignition time, min Air input rate, m3/min (0°C, 1 atm) Gas output rate, rrP/min (0°C. 1 atm) Average reduction zone temperature, ° C Average gas outlet temperature, °C Volume reduction, % Weight reduction, % RUN 1 1, 20% Sludge Cubes 17.5 2.47 0.50 265 24 .407 .749 779.8 197.6 64 82 RUN 12, 25% Sludge Cubes 16.3 1.71 0.73 262 44 .415 .735 734.7 180.6 74 83 Gas Analyses Gas samples were collected foranaly- sis in Tedlar gas sampling bags and ana- lyzed off-line with a Leeds and Northrup multicomponent gas analyzer system. Gas moisture content was determined by the condensation method. Dry gas composition, gas moisture content, and gas energy content are summarized in Table 3. The dry gas compositions mea- sured during RUNS 11 and 12 were within the normal range expectedforair blown gasifiers. Energy Balances - RUNS 11 and 12 Energy balances were calculated using computer programs "GASEN," "GASHEAT," and "ENERGY." The out- put from the programs "GASEN" and "GASHEAT," the fuel and char charac- teristics, and the operational data from each run are used as input to the pro- gram "ENERGY," which, in turn, is used to compute energy balances. Listings of the programs and printouts for each run are attached as Appendixes A, B, and C to the report. A summary of the energy balances is shown in Table 4. In Table 4, energy balances for each run are given both in energy units (MJ/hr) and percentages, assuming the fuel net energy as 100 percent. Gas chemical energy is the most significant energy output, ranging from 72 to 81 percent of the input energy. Gas sensi- ble heat is relatively minor, contributing only 5 percent to the energy output. The gas sensible heat could probably be increased in insulating the ashpit and gas piping to the flare. Afar more signif- icant energy output is the char energy, which ranges from 16 to 25 percent of the input net energy. As char generation is sensitive to fuel residence time and air flow rate, char energy could be mini- mized by optimizing operation. Conden- sate energy is very minor varying from 0.9 to 1.4 percent of the input net energy. Energy losses for most runs ranged from 9 to 49 percent, with 20 percent being typical. Hot and cold gas efficien- cies were 40 and 37 percent, respec- tively, for RUN 08, and 85 and 81 percent, respectively, for RUN 12. Hot gas efficiencies in the upper 60 percent range are typical for the runs. The negative energy losses shown in Table 4 in RUNS 11 and 12 are likely the result of errors made in determining the amount of char generated during each run. Because of the relatively large stor- age volume for char in the gasifier above the grate, it was difficult to determine accurately the amount of char gener- ated during a short (2 to 3 hour) run. Limitations to the Co-gasification Process Although gasification itself is an ol< technology, the application of gasifica tion to municipal usesisa relatively nev concept. Hardware needed to imple ment the concept is manufactured b several firms, but the equipment sti must be considered to be in the develop mental stage. Questions on the environ mental effects of gasification still nee to be resolved. Finally, the limitation inherent in the prooduction of LEG mus be recognized. The gas should be use onsite, most efficiently in a boiler, a though it can also be used, with a acceptable loss in efficiency, in a ga turbine or internal combustion engine Conclusions and Recommendations The technical feasibility of operating fixed bed gasifier with densified sludge solid waste mixtures has been demoi strated. Densified sludge/solid was mixtures were successfully prepared a full scale pilot facility, and a pilot sea downdraft gasifier was designed ar constructed. The gasifier was operated with va ious fuels including an agricultur waste (almond shells), wood chips.de sif fed source separated solid waste, ai ------- densified mixtures of sludge and source separated solid waste (10, 15, 20, and 25 percent sludge by wet weight). LEG was produced during the tests with an energy content ranging from 4.19 to 6.26 MJ/m3at hot gas efficiencies from 40 to 85 percent. The co-gasification of densified sludge and source separated solid waste may be a new approach to co-disposal that could be used by smaller communities. Before the co-gasification process can be considered operational, how- ever, several key issues must be addressed in future work: 1. The optimum conditions for gasi- fier operations in terms of fuel consumption, air flow, gas quality, and efficiency need to be defined. These parameters can be used to develop loading factors and speci- fications for the design of full scale systems. 2. Conditions causing slagging should be determined. Slag control mea- sures such as steam or water injection, or continuous grate rotation should be investigated. 3. The fate of heavy metals during the gasification process should be determined. 4. Mass emission rates and particle size distributions for particulates in the LEG should be measured to provide data for the design of gas cleaning equipment. 5. Emission data from engines, burn- ers, and boilers fueled with LEG should be measured. Emissions should also be analyzed for poten- tially toxic compounds. 6. Manufacturers of system compo- nents should be identified. The full report was submitted in fulfill- ment of Grant No. 805-70-3010 by the University of California at Davis, under the sponsorship of the U.S. Environ- mental Protection Agency. Table 3. Composition and Energy Content of Low Energy Gas Item RUN J1. 20% Sludge Cubes RUN 12. 25% Sludge Cubes Dry gas composition fby volume) CO.% Hz.% CHS, % Oz. % /V2b, % Gas moisture content (by volume), % Gas energy content MJ/M3 (dry gas, LHV, 0°C, 762 mm Hg) 20.9 14.5 2.3 0.1 11.9 0.3 50.0 14.15 5.11 21.5 13.7 2.5 0.1 11.0 0.3 50.9 12.31 5.17 a Measured as Total Hydrocarbons, (THC), CH* assumed to be 95% of THC, C2H6 assumed to be 5% of THC. b/V2 includes nitrogen, argon, and trace amounts ofnitrcgen oxides. Nz is determined by difference, N2 = 100% - (CO + H2 + THC + C02 + Oz). Table 4. Energy Balances Item RUN 11, 20% Sludge Cubes MJ/hr % RUN 12, 25% Sludge Cubes MJ/hr % Gross energy, dry fuel Latent heat, combined water Latent heat, fuel moisture Net energy, fuel Gas chemical energy Gas sensible heat Heat loss condenser Char energy Condensate energy Energy losses Hot gas efficiency Cold gas efficiency 269.49 18.48 4.15 273.86 197.15 12.37 21.16 69.00 2.38 -28. 19 100.00 71.99 4.52 7.73 25.20 0.87 -10.30 76.51 71.99 268.08 16.26 4.07 247.75 199.93 11.03 19.27 41.45 3.33 -27.25 100.00 80.70 4.45 7.78 16.73 1.34 -11.00 85.15 80.70 S. A. Vigil and G. Tchobanoglous are with the Department of Civil Engineering, University of California, Davis. CA 95616. Howard Wall is the EPA Project Officer (see below). The complete report, entitled "Co-Gasification of Densified Sludge and Solid Waste in a Downdraft Gasifier," (Order No. PB 82-23O 293; Cost: $13.50. subject to change) will be available only from: National Technical Information Service 5285 Port Royal Road Springfield, VA 22161 Telephone: 703-487-4650 The EPA Project Officer can be contacted at: Municipal Environmental Research Laboratory U.S. Environmental Protection Agency Cincinnati, OH 45268 U. S. GOVERNMENT PRINTING OFFICE: 198^559 -092/0463 ------- United States Center for Environmental Research Fees Paid Environmental Protection Information Environmental Agency Cincinnati OH 45268 Protection Agency EPA 335 Official Business Penalty for Private Use $300 RETURN POSTAGE GUARANTEED ------- |