Plasma-vernietiging van rubberskroot : teoretiese en eksperimentele onderbou

Abstract

Afrikaans: Die oorhoofse doelwit van hierdie navorsing was die onwikkeling van ʼn reaktorkonsep en vloeidiagam vir ʼn vervoerbare plasma-gebaseerde eenheid vir die prosesering van skrootrubber. Die projek het voor die internasionale Covid-inperkings afgeskop. In daardie stadium het Necsa reeds oor ʼn ontwerp vir ʼn navorsings-plasma-eenheid beskik. Die konstruksie van die toerusting is eers in 2022 voltooi met `n beperkte begroting. Die inbedryfstelling van die toerusting het deel van hierdie studie gevorm. Die algemene teikenvoermateriaal vir die eenheid is egter munisipale afval. Daar is nie gepoog om enige aspek van die sisteem tydens die studie vir die voer van rubber te optimiseer nie. Die gedrag van die navorsingsisteem ‒ soos hy ontwerp, gebou, en in bedryf gestel is ‒ is dus ge-evalueer vir rubber as voermateriaal. Die uiteindelike uitsette is konsep-voorstelle vir ʼn toekomstige rubbervergassingsisteem ‒ ondersteun deur die nuwe kennis geskep tydens die afhandeling van hierdie projek. Kritieke data vir reaktorontwerp behels die kinetika en die termodinamika van die proses. Die kinetika bepaal die minimum vertoeftyd van die material in die reaktor, en die vertoeftyd pen die grootte van die reaktor vas. Kinetika verwys oor die algemeen na die snelheid van die reaksie, of reaksies, en kan chemiese of fisiese meganismes behels. Die termodinamika van die proses bepaal beide die opbrengs en die energie-vereistes. Die nodige termodinamise data kan grootliks teoreties gegenereer word. Eksperimentele werk is egter nodig om die kinetika te bepaal. Hoewel daar ʼn oorvloed van data oor die termiese gedrag en kinetika van rubber in die oop literatuur beskikbaar is, is dit uiters riskant om nie eksperimentele werk op eie materiaal uit te voer nie; en verder was die formulering van die kinetika nie in die vorm wat vir hierdie projek benodig is nie. Twee noukeurige eksperimentele programme is gevolglik aangepak om die kinetika op `n mikro- en makroskaal vas te pen. Die eerste was ʼn termogravimatriese studie op fyn-gemaalde rubberkrummels, ≤ 100 μm. Die tweede was ʼn videografiese studie van die termiese gedrag van rubberblokkies in ʼn buisoond. Onder inerte toestande piroliseer rubber in die temperatuurgebied 200‒500 °C, om koolstofmonoksied, metaan, en verskeie alkane, alkene, en sikliese organiese verbindings te vorm. Die massaverlies is ordelik 70 %. Indien die residu verder na hoër temperature verhit word, vind addisionele massaverlies plaas ‒ maar slegs van ʼn verdere paar persent. Verskeie opsies vir die benutting van die koolstofhoudende residue is beskikbaar. Die konsep wat in die navorsing getoets is, is die gebruik van die tru-Boudouard-reaksie; dit is die reaksie van koostofdioksied met vaste koolstof, om koolstofmonoksied te vorm. Die termogravimetriese studie het isotermiese sowel as dinamiese lopies oor ʼn wye stel temperatuurprogramme behels. Kinetiese triplette, d.w.s. modelle, aktiveringsenergieë, en pre-eksponesiële faktore, is uit die isotermiese termogramme bepaal. Hierdie data is daarna gebruik as beginwaardes vir direkte passing van die dinamiese krommes. Die hoofbevindinge was dat: die eerste pirolisestap deur ʼn 3D-diffusiemodel beskryf kan word; die hoë-temperatuurpirolise onder inerte toestande deur die Mampel-meganisme; en die tru-Boudouard-reaksie deur ʼn chemiesbeheerde krimpende-partikelmodel. Die buisoondeksperimente het belangrike insig oor die termiese gedrag van makroskopiese rubberblokkies verskaf. Die blokkies verkool rofweg binne die tye wat deur die TGA-kinetika voorspel word, by laer temperature, met ʼn sigbare verkolingsfront wat na binne beweeg. By nagenoeg 800 °C en hoër word die prosestempo egter deur die warmte-oordragtempo bepaal. Die totale pirolisetyd kan bereken word as die som van die inherente pirolisetyd en ʼn warmte-oordragkomponent. Verder word die proses so vinnig dat die druk wat binne die rubber opbou a.g.v. die piroliseprodukgasse, hoog genoeg om mikro-ontploffings en verpoeiering van die blokkies te veroorsaak ‒ in granules van 100 μm en kleiner. Die tru-Boudouard se reaksietempo is direk eweredig aan die koolstofdioksiedkonsentrasie; die diffusiesnelheid van die koolstofdioksied na die reaksieoppervlak bepaal eers die reakietempo bo ~1 200 °C. Die plasmavergassingsreeks het die ondersoek uitgebrei na die kg h-1 skaal. Die werk is uitgevoer m.b.v. ʼn nuut-opgerigte 15 kW(e) plasma-vergassingstelsel, ontwerp om organiese materiaal met lug, suurstof en stoom, of kombinasies daarvan, te vergas waar die reaksiekinetika vinnig is. In die geval van die tru-Boudouardreaksie is die kinetika egter relatief stadig en die vereiste vertoeftyd in die reaktor beduidend. Die uitmekaarspat van die monsters, soos tydens die buis-oondeksperimente waargeneem, is bevestig deur die voorkoms van rubberkooks oral op koue oppervlaktes in die toerusting en die versamelde materiaal in die filter. Die endotermiese reaksie by die aanvanklike blootstelling van monsters in die buisoond is bevestig deur die temperatuurskommelings in die plasmareaktor wat tydens pulserende rubbervoer waargeneem is. Onder eksperimentele kondisies en -gasvloeie was die reaktorvolume van 4.7 L onvoldoende vir die verlangde vertoeftyd en dit het duidelik geword dat reaktorontwerp van kardinale belang sal wees vir ’n praktiese vergassingaanleg.

Ten slotte word ʼn prosesvloeidiagram vir plasmavergassing van skrootrubber deur die tru-Boudouardreaksie voorgestel en bespreek, wat ook bedryfsveiligheid, omgewingsveiligheid, statutêre vereistes, en verwysings na prosesmodellering en ʼn aantal tegno-ekonomiese studies uit die literatuur insluit.

English: The main objective of this research was the development of a reactor concept and flow diagram for a transportable plasma-based unit for the processing of scrap rubber. The project started amid the Covid-19 lockdown. At that time Necsa already had a design for a mobile plasma unit. The construction of the equipment was only completed in 2022. Initial start-up of the equipment was part of this program. The equipment was designed for the processing of municipal solid waste, however, and due to the cost involved no effort was made to adapt the equipment for rubber feed. The eventual output of the study is a number of concept proposals for a future rubber gasification facility – supported by new knowledge created during finalization of this project. Critical data for reactor design encompasses the kinetics and thermodynamics of the process. The kinetics determine the residence time of material in the reactor and therefore the size of the reactor. The kinetics refer in general to the rate of reaction or reactions and may include both chemical and physical mechanisms. The thermodynamics determine the yield and energy requirements. Much of the required thermodynamic data can be generated theoretically. However, kinetic data must be determined experimentally. Although there is an abundance of information available in the open literature regarding the behaviour and kinetics of rubber, it is very risky not to experiment with own material. Furthermore, the formulation of the kinetics was not in the form required for this project. Two experimental programs were consequently embarked upon. The first was a thermogravimetric study on fine rubber crumbs, ≤ 100 μm. The second was a videographic study of the thermal behaviour of rubber cubes in a tube furnace. Under non-oxidizing condtions the rubber pyrolyzes in the temperature range 200–500 ℃ with the formation of carbon monoxide, methane, alkanes and alkenes, and cyclic hydrocarbons. The mass loss is roughly 70 %. Further heating causes a few percent further mass loss. Several options are available for further utilization of the carbonaceous residue. One of these, tested in this research, is the reverse Boudouard reaction in which carbon dioxide reacts with carbon to produce carbon monoxide. The thermogravimetric study included both isothermal and dynamic runs over a wide range of temperature programming. Kinetic triplets, i.e. modells, pre-exponential factors and activation energies were determined from the isothermal data. These data were then used as initial values for direct fitting of the dynamic curves. The main conclusions were that: the first pyrolysis step is described by a 3-D diffusion model; the high temperature pyrolysis under inert conditions by the Mampel mechanism; and the reverse Boudouard reaction by a chemically controlled shrinking particle model. The tube furnace experiments gave important insights regarding the thermal behaviour of macroscopic rubber cubes. The cubes char roughly in the times predicted by the TGA-derived kinetics, at lower temperatures with a visible charring front moving inwards. At temperatures above 800 ℃, however the process rate is determined by the rate of heat transfer. The total pyrolysis time can be calculated as the sum of the inherent pyrolysis rate and a heat transfer component. Furthermore, the process becomes so fast that the build-up of gaseous pyrolysis products causes the cubes to shatter into ~ 100μm and smaller char particles. The rate of the reverse Boudouard reaction is directly proportional to the carbon dioxide concentration, with its diffusion to the reactive surface only becoming rate determining above ~1 200 ℃. The plasma gasification series expanded the investigation to the kg h-1 scale. The work was done with newly constructed 15 kW(e) facility designed for the gasfication of carbon containing material with air, oxygen, steam and combinations thereof, where the reaction kinetics are fast. In the case of the reverse Boudouard reaction, the kinetics are slow and the required residence time in the reactor becomes significant. The shattering of samples observed during the tube furnace experiments was confirmed by the appearance of rubber char on cold surfaces everywhere in the equipment and the material collected in the filter. The endothermic event observed upon introduction of the sample into the hot zone of the tube furnace was confirmed by the temperature variations in the plasma reactor caused by a pulsating rubber feed. Under experimental conditions and gas feed rates the reactor volume of 4.7 L was inadequate for the required residence time and it became clear that reactor design would be of cardinal importance for a practical gasification plant. Finally, a process flow diagram for the plasma gasification of waste rubber by the reverse Boudouard reaction is proposed and discussed which also includes process safety, environmental safety, statutory requirements and references to process modelling and a number of techno economic studies from the literature.