Possible applications for vacuum pyrolysis in the processing of waste materials

[摘要] ENGLISH ABSTRACT:Current global trends in government, industry and popular OpInIOn indicate thatrecycling will become essential in the future. Vacuum pyrolysis is a new technologywith many recycling applications that have not yet been investigated. This study is acontribution towards the better understanding of the vacuum pyrolysis process andalso towards finding possible economically favourable recycling applications.A batch operated tube furnace, which allowed the controlled heating of differentmaterials in a vacuum, was designed and built. The gases and vapours passed thougha series of progressively colder vacuum traps, condensing the vapours for furtherstudy. The products from the process are charcoal, oil, an aqueous phase and noncondensablegases. The charcoal and oils' possible economic values (R2500-R5000/ton of charcoal, while the plant product oil can be sold as a low sulphur fuel,with a retail value of approximately R1.42 per litre) were determined along with theoils' chemical composition.Several possible feedstocks were studied, including intruder plant species, leatherwastes, sewage sludge and a simplified representation of municipal solid waste. Thethree intruder plants studied were Kraalbos (Galenia africane), Scholtzbos (pteroniapallens) and Asbos (Psilocaolon absimile). These plants yielded 40%, 42% and 48%(charcoal per kilogram dry feedstock) respectively at their maximum oil yieldtemperatures of 380°C, 480°C and 450°C respectively. The maximum oil yields were36%, 32% and 20% respectively (also on a dry feedstock basis). It was found that theplants with ash contents below 10% yielded commercially competitive charcoal, andthat all of the plants yielded oils with heating values in the range of 24MJ/kg,containing several high value compounds. Asbos was the only plant that did notproduce usable charcoal, as its ash content of 40% was double that of commercialcharcoals.The leather wastes represent a previously unrecognised application of the technologythat could bring huge financial rewards to the tanning industry and could provide amore environmentally friendly alternative to lined landfilling. The cost of landfillingfor a medium sized tannery can be as high as RIOOO 000 a year. Apart from thevolume reductions achieved (up to a factor 8) it was found that landfilling might betotally avoided if the chrome contained in the charcoal product could be extracted andreused.Sewage sludge was studied, as it is a hazardous waste that requires costly disposal in alined landfill. It was revealed that volume reductions of up to a factor 3.5 werepossible with corresponding charcoal and oil yields of 40% and 38% respectively at500°C. It was also found that the charcoal product could be used as compost, whichwould then turn a costly waste into a commodity product. The oil from both theleather and sewage sludge had high energy values (26.7MJ/kg and 30.9MJ/kgrespectively) and might either be sold as a bunker fuel or used in the process as amake-up heat source. The value of the oil depends on the problems posed by the oils'high nitrogen content (±5%-6%).A further study was also made of the co-pyrolysis of PVC and wood to determine theinteraction between the feedstocks and as a simplified representation of municipalsolid wastes. It was found that the HCI released from the PVC caused acid hydrolysisof the wood and led to lower charcoal (reduced from 32.6% to 29.7% on dryfeedstock basis, at the maximum co-pyrolysis oil yield temperature of 460°C) andmuch higher oil yields (42.4% for the co-pyrolysis compared to 23.6% for the plantmaterial at 460°C). An existing computer program (CEA by Gordan and McBride)was also employed in order to find explanations for some of the vacuum pyrolysisresults.Although large specialist vacuum pyrolysis plants have been designed in the past(mostly to dispose of used tyre waste) it will be necessary to determine the processeconomics for small-scale applications if the technology is to be applied at the sourceof the problem. Overall vacuum pyrolysis appears to be a very promising technologythat could solve many waste problems in an environmentally friendly andeconomically beneficial manner.

[发布机构] Stellenbosch University