Plastics, Rubber, and Paper Recycling - ACS Publications - American

Chapter 16

Spouted Bed Reactor for the Advanced Thermal Recycling of Mixed Plastic Waste Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 23, 2015 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0609.ch016

S. J. Pearson, G. D. Kryder, R. R. Koppang, and W. R. Seeker Energy and Environmental Research Corporation, 18 Mason, Irvine, CA 92718

This paper summarizes the pilot-scale (24 tons/day) development and testing of a novel waste gasification system designed for advanced (thermochemical) recycling of wastes that contain mixed plastic polymers. Energy and Environmental Research Corporation (EER), with coordinated funding from the US-EPA, the Gas Research Institute, and the Southern California Gas Company has developed a pilot scale technology based upon the "spouted bed" fluidization regime. The EER Spouted Bed Reactor is a novel method for ablative gasification, in which heterogeneous solids are gasified and aggressively comminuted intofineparticulate. The objective of the subject technology development is the thermochemical conversion of high heat content wastes into environmentally safe recoverable products. The design of the pilot scale facility will be described and the results of preliminary testing on automobile shredder residue will be discussed in this paper. Annual municipal and industrial waste generation in the U.S. is approximately 237 million tons of which nearly 80% is currentiy disposed in landfills. Plastics account for 4% to 11% by weight of the entire municipal waste stream. (1) Automobile shredder residue (ASR) is the one of the largest waste streams in the US, and it contains « 30% synthetic polymers. A recent market assessment completed by EER suggests that the quantity of ASR available in the US may be adequate to support over 100,100 tons/day spouted bed reactors for the production of syn-gas.(2) Like other mixed wastes, ASR is sometimes contaminated with toxic organics and heavy metals which may put additional restrictions and requirements on landfill disposal of this waste. An alternative to landfill disposal for municipal and industrial waste has been first generation thermal destruction technologies which rely upon incineration (oxidation) to destroy the organic compounds in the waste. Many municipal and industrial wastes are contaminated with halogens, such as chlorinated species, and toxic heavy metals, such as lead and cadmium. Incineration of chlorine containing wastes can result in the formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/PCDF).(3, 4) Thus, environmental pollution and public health are major concerns associated with waste incineration technologies, particularly where high energy wastes are concerned. The relatively large gas volumes and high acid gas emissions (i.e., HC1 & NO ) resulting from combustion with air make incineration costly in terms of back end cleanx

0097-6156/95/0609-0183$12.00/0 © 1995 American Chemical Society In Plastics, Rubber, and Paper Recycling; Rader, Charles P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 23, 2015 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0609.ch016

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up. Using incineration methods allows only the recovery of heat, which is a low value resource. It is much more desirable to reclaim the chemical value in the waste in some high value form other than heat energy. Thus, a pyrolysis or gasification process capable of processing a wide range of wastes and recovering the energy content as high value added non-contaminated products (off- setting costs or generating net revenue) would be of significant national importance. World wide, numerous pyrolysis and gasification strategies have been researched and developed during the pastfivedecades. Commercial success has been limited to gasification of traditional feedstocks, primarily coal, and not applied to wastes because of processing difficulties. However, escalating disposal costs and more stringent air and water regulations require new approaches to the problem. The primary objective of the subject Spouted Bed Reactor (SBR) technology for advanced thermal recycling is substantial recovery of product value at costs significantly below incineration and with regulatory compliance of all ash and effluent streams. "Advanced Recycling" is the term coined by the American Plastics Council to describe thermochemical recovery methods, though specifically excluding incineration processes. Advanced Recycling concepts are favored by the general public and are well received by the regulatory and permitting agencies. Technology Overview Energy and Environmental Research Corporation (EER), with coordinated funding from the US-EPA, the Gas Research Institute, and the Southern California Gas Company has developed a pilot scale Advanced (thermochemical) Recycling technology based upon the "spouted bed" fluidization regime. The EER Spouted Bed Reactor is a novel method for ablative gasification, in which heterogeneous solids are aggressively comminuted intofineparticulate within the bed via high shear velocities in the jet and abrasion. The solids circulation properties of a traditional bubbling fluidized bed, a conventional spouted bed, and a jet-spouted bed are shown in Figure 1. Typical fixed, slugging, and bubbling beds have unidirectional, nearly flat gas velocity profile flows through the bed. Spouted beds have core zones with extremely high shear forces and reverse bed flow on the periphery (i.e., internal bed recirculation). These features facilitate the processing of difficult wastes.^) The unique attributes of the "spouting regimes" provides heat transfer rates comparable to traditional fluid beds, while providing robust circulation of highly heterogenous solids, concurrent with very aggressive comminution (size reduction through abrasion.) Synergistic benefits are obtained by using a multi-stage approach. The SBR primary spouted bed is used as the solids processing zone, resulting in the conversion of heterogenous solids and sludges into a gaseous stream laden with fine particulate. The free board column provides residence time for solid separation and further thermochemical processing. The hot cyclone then uniformly elevates the temperature of the particulate laden gas stream, extracting the mineral solids, further cracking the organic gases at high temperature and, optionally, vitrifying the solids. For some waste feeds the addition of slagging capability in the secondary cyclone may be desirable allowing the waste mineral content to be converted into vitrified aggregate having salable properties. The product gas stream is then purified for subsequent use. Operation under reducing (gasification) conditions with steam and partial oxidation significantly diminishes system capital costs by decreasing equipment volumes to l/4th the size of comparable incineration with combustion air (oxidation) systems. The reducing atmosphere, with excess steam present at high temperature, has been proven to be a good method for converting organic compounds into environmentally benign low molecular weight gases with recoverable value. The reducing atmosphere precludes conditions which result in formation of toxic products of incomplete combustion (PIC), such as dioxins, and avoids the formation of NO . The focus of the development activities are on a novel method for partial oxidation of solid wastes into a syn-gas x

In Plastics, Rubber, and Paper Recycling; Rader, Charles P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

Advanced Thermal Recycling of Mixed Plastic Waste


PEARSON ET AL.

(c) Conventional Spouted Bed

(d) Jet-Spouted Bed

Figure 1. Various fluidization regimes.


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product from which high value products can then be derived, such as clean syn-fuel, hydrogen, methanol. Estimated break even processing costs with no tipping fee are $50/ton to $100/ton for 250 to 24 tons per day SBR facilities, respectively, with capital investments less than 1/2 the cost of incineration systems.ffj Improved cost effectiveness is one technology benefit, though improved environmental performance and enhanced public acceptance because of the recycling ability are both considered highly significant advantages of the technology. Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 23, 2015 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0609.ch016

Waste Applicability The SBR advanced recycling technology is particularly suited to wastes with significant heat content (2,000 to over 14,000 Btu per pound) which are contaminated with toxic organic compounds and heavy metals. Hazardous soils contaminated with coal tar residues, chemical wastes, and petroleum refinery wastes are also appropriate for processing in the SBR advanced recycling system. Municipal Solid Wastes (MSW), mixed (radioactive) waste, munitions and rocket propellants, are also candidate feed materials. Energetic materials can be either recycled or incinerated, depending on the project parameters. In either case, primary gasification in the SBR system is highly advantageous (very high energy explosives are first uniformly diluted with inert materials prior to thermal processing). The PVC bound chlorine content in ASR has been reported to average 3%. Gasification of wastes with high chlorine content, such as ASR, using high temperature steam is a preferred method of thermochemical recycling relative to incineration. The impact of waste chlorine content on PCDD/PCDF emissions during waste incineration is the subject of much debate in the literature with some studies showing a strong correlation (3, 4, 6, 7) and others showing no correlation.^—10) Although results of these studies are mixed, it is clear that a chlorine donor is necessary for the formation of dioxins and furans. The reducing atmosphere in the SBR favors hydrogenation reactions whereby chlorine and sulfur are extracted from macromolecules and converted efficiently to HC1 and H2S. These acids then react rapidly with in-situ sorbents to form salts such as calcium chloride. Nitrogen containing polymers in mixed plastics feedstocks can form toxic intermediate compounds under some operating conditions. ASR contains urethane materials which are composed of isocyanates, molded plastics made from ureaformaldehyde resins, polyamides, such as Nylon carpeting, and polyacrymids. All of these polymers contain nitrogen which could form toxic cyanide compounds such as cyanogen chloride and hydrocyanic acid gases during incineration or pyrolytic thermal destruction. Since ASR contains metals, complex cyanides, such as nickel and iron species, are also possible. Hydrogen is produced under high water vapor, reducing conditions by the water shift reaction. Both hydrolysis and hydrogenation reactions are thus promoted by the presence of superheated steam under reducing conditions. Hydrolysis is applicable to the treatment of polyesters, polycarbonates, and polyamides. Hydrogenation reactions also split the polymer chain at reactive sites (unsaturated carbon-carbon bonds) to reduce molecular weight. Detailed SBR Technology Description The initial development and testing has focused on thermochemical recycling of low-end mixed plastic wastes, exemplified by ASR, which has been used in the initial test runs. EER's Spouted Bed Reactor technology has been constructed at the Process Development Unit (PDU) scale of 24 tons per day. The existing PDU (Figure 2) incorporates a waste feeder, the SBR reactor, a hot cyclone, heat recovery section, and a baghouse.


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Advanced Thermal Recycling of Mixed Plastic Waste 187


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Extrusion Feeding. Lightly prepared waste is fed to a high-thrust, extrusion feeder which forces heterogeneous waste materials into the primary reactor. SBR Reactor. In the spouted bed thermochemical reaction zone solids are comminuted and gasified at temperature (1,000-1,600°F). Large solids remain in the bed until they are reduced in size through attrition, pyrolysis, and gasification reactions. The high velocity inlet spout at the exit of the preheat burner causes aggressive size reduction of heterogeneous feedstocks through ablative gasification reactions. Particle size is rate limiting for heat and mass transfer reactions, including pyrolysis and carbon gasification. Therefore, rapid solid size reduction significantly increases reaction rates. Oxygen and steam are added selectively to control the composition and temperature profile within the reactor. In the presence of excess steam at high temperature, toxic organic compounds are reduced to H2, CO, CO2, HC1, metals and metal oxides, and water. Hot Cyclone. Steam and gaseous products elutriate fine particles out of the reactor and into the hot cyclone where the temperature may be water quenched below the ash softening temperature or increased by the addition of oxygen. The ash may be slagged in the cyclonic slagging system under reducing conditions, and recovered as a medium density granular solid in a water quench tank. Heat Recovery. For full-scale applications, the gas stream would be cooled via a waste heat boiler to raise the process steam. In the pilot plant, a free standing boiler is used to generate process steam for injection into the preheat burner. Waste heat recovered after the hot cyclone provides steam super-heat to 600°F. However, most process heat is rejected to atmosphere via an air cooling heat exchanger prior to the 450°F baghouse. Steam used for the spouting fluid is highly superheated by a small inline oxygen-methane burner. Superheated steam provides some heat for endothermic pyrolysis reactions, along with exothermic heat resulting from partial oxidation of wastes which react with sub-stoichiometric levels of oxygen injected into the spouted bed primary. Optional Fines Recycle. The fine particulate is optionally removed in a traditional or hot ceramic baghouse. The fine particulate recovered in the baghouse may be recycled to the SBR. Thus, all the solids may be recovered as a vitrified product with salable properties, within EPA leachability limits for heavy metals and trace organics. When the hot cyclone is operated at slagging temperature, the baghouse ash may be recycled back to the primary spouted bed along with any trace contaminants for incorporation with the slag. Subsequent purification of the H2/CO/CO2 gas stream can be accomplished using conventional or emerging particulate, acid scrubbing, or gas separation technologies. Test Results Preliminary tests were conducted to characterize the waste feed system required, the spouted bed dynamics, and to characterize the end product syn-gas. To date, the operation of the SBR with ASR has been limited to several short duration trial runs. Relatively trouble-free feeding of "raw" unsegregated ASR has been accomplished at a feed rate of 1,400 lb/hr. Typical ASR feed is characterized in Table I. The ASR feed was previously wet shredded through a hammer mill having 6" x 6" grates using traditional auto shredder industrial methods and allowed to air dry. The bulk of the ferrous metals had been extracted magnetically and nonferrous metal had been hand pickedfroma conveyor belt. However, the feed still contained a high tramp metal and wire content which characteristically makes ASR difficult to handle. The ASR was


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Advanced Thermal Recycling of Mixed Plastic Waste

Table I. Auto Shredder Residue Analysis -12 Shredder Fluff Samples (11) Parameter

Mean

Low

High

% H 0 (moisture) % Ash % Ash (dry basis) % Volatiles (fabric, foam, plastics) % Fixed carbon Btu/lb (higher heating value) Btu/lb (dry basis) % Sulfur % Chlorine % Metals > 12 mesh* % Metals 1,300°F) leaving the hot cyclone. The steam was further superheated by an in-line oxygen/methane burner, and then expanded through a 3" spouting orifice. The ratio of the reactor diameter (24") to spouting orifice (3") diameter was 8. The bed depth was variedfrom1 to 3 ft. The bed materials began to radiate significantly at about 1300°F. The vigorous spouting action could be viewed very clearly looking through a sight port located on the top of the reactor. The appearance of the spout (looking downfromthe top) may be analogous to looking into the mouth of a violently erupting volcano. Some small particles impinge on the sight glass located approximately 20 feet above the


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spouting orifice, though most of the bed materials dropfromabout five feet above the expanded bed. The top eight feet of the reactor is increased to 32" ID as a disentrainment section. Process gases exit the primary reactor through a 12" ID duct located on the side at the top of the reactor. Bed temperatures have typically been near or above ash softening temperature resulting in some bottom ash slagging. Slag formation can interfere with ash removal and inhibit bed fluidization. However, the spouting regime of fluidization was relatively tolerant of heterogeneous materials in the bed. The pilot plant was operated inadvertentiy in this mode with a low melting phase. Unlike the traditional fluid beds, the spouting action continues to turn over the bed materials. However, unless the fused materials are removed periodically, large agglomeration of solidified bed materials result which then tend to obstruct the spout and eventually slump the bed. The pilot plant has been operated in this failure mode without any catastrophic results. The reactor was cooled and the agglomerated clinkers were removed and the bed restarted. Gas samples were obtained at the top of the primary spouted bed prior to addition of tertiary oxygen in the hot cyclone. Therefore, the gas samples are very rich in hydrocarbon and have a high Btu content (Table II). The reactions which assure destruction of toxic organic compounds are accomplished in the hot cyclone where the equilibrium temperature is raised above 1,500°F with the addition of oxygen. The beneficial reducing action of steam, through the water shift reaction with CO is carried out above 1500°F wherefreeradical molar concentrations increase to about 1(H. The destruction of toxic organic compounds using high temperature steam under reducing conditions has been well demonstrated by others. (72,13) The initial pilot plant work has focused upon the mechanical issues relative to the spouted bed reactor. Subsequent work will confirm the ability to obtain high destruction efficiency of principle organic hazardous constituents (POHCs) by passing the process gases and particulate through the high temperature zone for reduction of organics to H2, CO, CO2, and H 0 . 2

Table II. Product Gas Analyses Compound Hydrogen (%) Oxygen (%) Nitrogen (%) Methane (%) Carbon Monoxide (%) Carbon Dioxide (%) Hydrocarbons (ppm) Ci C c c c C Heating Value (Btu(LHV)/scf) HC1 (ppm) H S (ppm) 2

3

4

5

6

Preliminary Results*

Optimal Conditions

12.6 2.4 6.4 10.5 20.9 30.6

30-40

Plastics, Rubber, and Paper Recycling - ACS Publications - American

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