Valorisation of paper recycling residue contaminated with plastic into valuable fuel products by pyrolysis

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Stellenbosch : Stellenbosch University
ENGLISH ABSTRACT: The recent drive towards a sustainable green economy, along with changing environmental legislation to combat waste pollution, have promoted the search for alternative disposal techniques. This study focuses on the thermochemical conversion namely pyrolysis of three waste streams from a paper recycling mill contaminated with plastic and a sequential processing route where the three waste streams are first subjected to fermentation to produce bioethanol followed by pyrolysis. The primary objective of this study was to assess different valorisation techniques (pyrolysis and fermentation-pyrolysis) for the disposal of real world industrial waste streams, composed of lignocellulosic fibre and plastic that are currently disposed of at landfill sites, to produce fuel products for energy generation at a paper recycling mill as an alternative to landfilling. The first part of this study considered standalone pyrolysis (fast or slow) for the valorisation of three different paper mill wastes, classified as rejects. Slow pyrolysis of paper mill waste favoured the production of an energy dense char with an HHV and yield of up to 32.9 MJ/kg and 77.3 wt.% respectively at a conversion temperature as low as 300 °C. The fast pyrolysis resulted in significant yields of the energy dense condensable (liquid) phase (HHV of up to 41.7 MJ/kg), with a yield of up to 53.6 wt.% which being obtained at conversion temperatures of 550 °C, which was ~ 21 wt.% higher than the yield obtained for slow pyrolysis. Increasing the conversion temperature to 550 °C for both slow and fast pyrolysis had a detrimental effect on the quality of the char product, with the HHV of char produced decreasing to between 16.9 to 21.7 MJ/kg for both pyrolysis processes. The second part deals with converting the lignocellulosic fibrous component in the waste into bioethanol via fermentation. The fermentation of the three waste streams resulted in only one being chosen for scale up production. As the waste streams (W2 and W3) that contained significant amounts of plastic, resulted in clumping of plastic during fed batch fermentation, which prevented subsequent substrate feeds from reaching the fermentation broth. The waste stream W1 that contained the largest percentage of glucan (56.9 daf wt.%) and the smallest amount of plastic < 5wt.% produced satisfactory results, with ethanol concentration of 39.8 g/l being obtained. Pyrolysis processing of the fermentation residue resulted in the production of an condensable (liquid/wax) energy dense phase with yield of up to 13.2 wt.% for slow and 26.3 wt.% for fast pyrolysis with an associated HHV of 35.1 and 32.1 MJ/kg respectively. This was ~17 MJ/kg higher than that produced from the slow and fast pyrolysis of the untreated waste stream. The last part of this study assessed the economic viability of implementing pyrolysis in a waste to energy perspective at a paper mill as an alternative to waste disposal by landfilling, and resulted in a minimum fuel selling price (MFSP) of between 1.12 to 1.48 $/kg which was 2 to 3 times greater than the target value of 0.65 $/kg at current waste production rates. A study on scale revealed that an increase in size from current waste production rates of between 540 to 1378 kg/hr up to 8700 kg/hr greatly reduced the MFSP to between 0.27 $/kg to 0.73 $/kg, which was comparable to the targeted value of 0.65 $/kg. The additional waste required could be achieved by combining the waste streams produced at a mill and by aquireing the waste produced by similar paper mills in the vicinity. The study has shown that pyrolysis of paper mill waste contaminated with plastic can be a viable alternative to landfilling with fuel products being produced that have a similar calorific value to traditional fuels. It is recommended that the study be scaled up to pilot scale along with testing of the combustion behaviour of the products from pyrolysis with traditional waste fuels. Were possible the waste streams from neighbouring industrial facilities can be combined in order to reach the required scale for economic viability.
AFRIKAANSE OPSOMMING: Die onlangse beweging na ’n volhoubare groen ekonomie, tesame met omgewingswetgewing wat verander om afvalbesoedeling te bestry, het die soeke na alternatiewe wegdoeningstegnieke verskerp. Van belang in hierdie studie, is die termochemiese prosesseringsroete, genaamd pirolise, en ’n sekwensiële prosesseringsroete waar die afvalvoerstof eers blootgestel is aan fermentasie om bioetanol te vervaardig, gevolg deur pirolise. Die primêre doelwit van hierdie studie was die assessering van verskillende valoriseringstegnieke (pirolise en fermentasie-pirolise) vir die wegdoening van industriële afvalstrome, wat bestaan uit lignosellulosiese veselstof en plastiek, om brandstofprodukte te vervaardig as ’n alternatief tot opvullingsterreine. Die eerste deel van die studie het die onafhanklike pirolise (vinnig of stadig) vir die valorisering van drie verskillende papiermeul afval, geklassifiseer as uitskot, in ag geneem. Stadige pirolise van papiermeulafval het ’n energie-digte verkoolsel gelewer met ’n HHV en massa-opbrengs van tot 32.9 MJ/kg en 77.3 wt.%, onderskeidelik, by ’n omsettingstemperatuur van so laag as 300 °C. Die vinnige pirolise het ‘n beduidende opbrengs van energie-digte kondenseerbare (vloeistof) fase (HHV van tot 41.7 MJ/kg) tot gevolg gehad, met ’n massa-opbrengs van tot 53.6 % wat behaal is by ‘n omsettingstemperatuur van 550 °C. Dit was ~ 21 wt.% hoër as die opbrengs verkry met stadige pirolise. Die verhoging in omsettingstemperatuur na 550 °C vir beide stadige en vinnige pirolise, het ’n nadelige effek op die kwaliteit van die verkoolselproduk tot gevolg gehad, met die HHV van verkoolsel geproduseer wat tot tussen 16.9 en 21.7 MJ/kg vir beide pirolise prosesse afgeneem het. Die tweede deel handel oor die omsetting van die lignosellulosiese veselagtige komponent in die afval na bioetanol deur middel van fermentasie. Die fermentasie van die drie afvalstrome het tot gevolg gehad dat slegs een gekies is vir vergrote skaal produksie.
Thesis (PhD)--Stellenbosch University, 2019.
Fibers, UCTD, Energy conversion, Waste paper -- Recycling, Pyrolysis, Fuel trade