Gasification of plastic wastes

M. Gebauer and D. Stannard, Gasification of Plastics Wastes in J. Brandrup, M. Bittner, W. Michaeli, and G. Menges, eds.. Recycling and Recovery of Plastics, Hanser/Gardner, Cincinnati, OH, 455-480 (1996). 

Mastellone ML, Arena U. Olivine as a tar removal catalyst during fluidized bed gasification of plastic waste. AIChE J 2008 54 1656-1667. 

Using clean polyolefine fractions in pyrolysis, the toxic substances and the solid residues are essentially lower. Also the fluid bed sand is possible to recycle partly. At gasification of plastics waste solid residues are accumulated as slag. This integrates the advantage that a displacement of toxic substances into other mediums does not occur. 

This reports on the developments by a German company in the use of gasification in the chemical recycling of plastics waste. Brief details are given. 

High-temperature pyrolysis (650-800°C) of plastic waste, fed into the rotary kiln via a screw feeder. Solid cokes and pyrolytic vapours are sent to further treatments in gasification or hydrogenation plant... 

In 1996, the so-called economic circulation and waste law was introduced in Germany. It was orientated towards different possibilities of plastic waste exploitation recycling by thermoplastic processing, conversion into low molecular weight substances, or alternatively gasification in order to use the synthetic gases or to use... 

Most of the processes so far proposed for the gasification of polymeric wastes have been directly derived from earlier processes developed for the conversion of coal, natural gas and heavy petroleum fractions. However, certain details must be taken into account when processing plastic and rubber wastes in the gasification units, for instance the heterogeneity of the starting material, the problem of feeding the highly viscous melted plastics, and the possible formation of corrosive compounds, mainly HC1 from PVC. 

Closely related to the previous process is the plastic waste gasification facility being set up at Rotterdam with a capacity of 150 tonnes day-1 of plastic wastes.13 The feed of this plant will be a mixture of polyethylene, polypropylene and polystyrene with minor amounts of other polymers (2.4 wt% of PVC) and a significant proportion of cellulose. The plan is that an injection of ammonia into the gasifier will neutralize the chlorides entering with the plastic wastes, which will lead to the formation of ammonium chloride salt as a by-product. A production of 350 000 m3 day -1 of synthesis gas is estimated, which will be used in chemical synthesis. 

Among the large number of gasification procedures only a few have been found to be suitable for the conversion of plastic waste and only one is being considered for use in the recovery of chlorine from PVC waste. 

Details on the principles and chemistry of gasification as well as recent industrial applications for treatment of plastics wastes in Europe can be found in [16]. The following section gives details on some of the earlier work in this area. 

Fractions of plastics waste which are not possible or not meaningful to recycle in material (mechanical) recycling processes are used in blast furnace or recycled in feedstock (chemical) recycling. Here hydrogenation has the priority followed by gasification and pyrolysis. Pyrolysis of type clean plastics waste will be suited only in exceptional cases. 

The thermal degradation of mixtures of the common automotive plastics polypropylene, ABS, PVC, and polyurethane can produce low molecular weight chemicals (57). Composition of the blend affected reaction rates. Sequential thermolysis and gasification of commingled plastics found in other waste streams to produce a syngas containing primarily carbon monoxide and hydrogen has been reported (58). 

Texaco gasification is based on a combination of two process steps, a liquefaction step and an entrained bed gasifier. In the liquefaction step the plastic waste is cracked under relatively mild thermal conditions. This depolymerisation results in a synthetic heavy oil and a gas fraction, which in part is condensable. The noncondensable fraction is used as a fuel in the process. The process is very comparable to the cracking of vacuum residues that originate from oil recycling processes. 

Coal used in power stations has the potential to be partly replaced by fuels derived from pre-treated plastics and paper waste, reducing both dependency on fossil fuels and reliance on landfill. APME reports on a project in the Netherlands which it co-sponsored to develop a substitute fuel from plastics. The environmental assessment of the project compared the environmental impacts of coal substitution with other plastics recovery methods, including gasification in feedstock recycling and energy recovery from plastics waste in cement kilns. The study also compared coal substitution with the generation of power from burning biomass. 

The present state of technology is reviewed (mainly from German literature of 1993 -4) in the Add of three principal thermal methods used for plastics wastes, namely pyrolysis (high-temperature carbonisation, coking), hydrocracking and gasification. 36 refs. Articles from this journal can be requested for translation by subscribers to the Rapra produced International Polymer Science and Technology. 

Feedstock recycling is examined as a method of plastics recovery. The range of techno logics currently employed are described, and include pyrolysis, hydrogenation, gasification, and chemolysis. Methods for the recycling of mixed plastics wastes are discussed, which include work by BP Chemicals, VEBA Oil, Shell Chemicals and Leunawerke. 

Current methods of feedstock recovery are reviewed. Brief details are given of pyrolysis, hydrogenation, gasification, and chemolysis. Activities of some European companies are briefly discussed in the areas of recycling mixed plastics waste and closed-loop recycling. 

Pyrolysis or gasification, as processes, are both much easier to control than direct firing of plastics. The latter is impossible on mechanical grates, eqnipping conventional incinerators for mnnicipal solid waste (MSW). On the other hand, thermal conversion is feasible by means of flnidized bed technology. The few percent of plastics, as in traditional MSW is nnproblematic and the calorific content is converted into heat and often into power, albeit at a disappointing level of conversion efficiency, of the order of 15-25%. 

Sorting of plastics is often manual and can cause allergic and health problems. Remarkably, plastic waste is not without a smell, and air extracted from storage and handling is thermally deodorized, e.g. at the Ube Industries gasification plant. Part of the pyrolysis products can be regarded as toxic. 

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