Valorisation of paper waste sludge using pyrolysis processing

Thesis (PhD)--Stellenbosch University, 2016.ENGLISH ABSTRACT: Due to depleting fossil fuel reserves and environmental concerns over global warming, alternate sources such as renewable energy are required. One such renewable energy source is biomass which includes plant matter, agricultural residues and industrial wastes. Of interest in this study is the industrial waste paper waste sludge (PWS) which is generated in large quantities by the pulp and paper industry. PWS is mainly landfilled which is costly and environmentally unfriendly, and thus alternative methods of valorisation such as thermochemical and/or biochemical conversion needs to be considered. The thermochemical process of pyrolysis thermally decomposes biomass, in the absence of oxygen, into products of bio-oil, char and non-condensable gas which have various beneficial applications. Alternatively, biochemical conversion of PWS into bioethanol using fermentation can be used as an initial step, followed by pyrolytic conversion of its fermentation residues (FR). The global objective of this PhD project was to assess the full potential of alternative pyrolysis processes, at varying key operating conditions, as part of a biorefinery to maximise the conversion of PWS and its FR, containing variable amount of organic material, into energy, chemical and biomaterial resources. In addition, statistical analysis of the product yields and quality were performed to reveal new mechanistic insights. The first part of the study considered the maximisation of the bio-oil yield from low and high ash PWS (8.5 and 46.7 wt.%) using fast pyrolysis (FP) processing. To do this, both reactor temperature and pellet size were optimised using a 2-way linear and quadratic model. Maximum bio-oil yields of 44.5 and 50.0 daf, wt.% were obtained at an intermediate pellet size of ~5 mm and optimum reactor temperatures of 400 and 340 oC for the low and high ash PWS, respectively. In addition to the above, a thermogravimetric study was implemented to gain insights in the thermodynamic mechanisms behind the increase in bio-oil yield with larger pellet sizes. Results indicated that fewer secondary tar cracking reactions were prevalent due to lower mass transfer limitations leading to greater yields of bio-oil. Vacuum, slow and fast pyrolysis processes were assessed and compared, at varying reactor temperatures and pellet sizes, for their ability to maximize the gross energy conversion (EC) from the raw PWS to the liquid and solid products. A 2-way linear and quadratic model was used for the statistical approach. Comparison of the overall EC, as a combination of the solid and liquid products, revealed that FP was between 18.5 and 20.1 % higher for low ash PWS (LAPWS), and 18.4 to 36.5 % higher for high ash PWS (HAPWS) when compared to slow and vacuum pyrolysis. This finding was mainly attributed to the higher production of organic condensable compounds during FP for both PWS. The calorific values displayed by the vacuum pyrolysis (VP) tarry phase and FP bio-oil for both PWSs, as well as the LAPWS char, were high (~18 to 23 MJ.kg-1) highlighting their potential for industrial energy applications. The capability of vacuum, slow and fast pyrolysis to selectively drive the conversion of raw PWS into chemicals (primarily glycolaldehyde and levoglucosan) and biomaterials (sorption medium or biochar) was assessed. Product yields were optimised according to reactor temperature and pellet size (2-way linear and quadratic model) and their variability quantified using principal component analysis (PCA). Results indicated that the high heating applied by FP significantly promoted depolymerisation and/or fragmentation reactions leading to higher yields of most organic compounds, particularly levoglucosan for both LAPWS (1.5 daf, wt.%) and HAPWS (3.7 daf, wt.%). The char biomaterial displayed by both PWSs were ultra-microporous, and the application of VP significantly enhanced the sorptive properties of the LAPWS char. Sequential PWS fermentation for bioethanol production (separate study), followed by pyrolytic conversion of the FR using alternative processes at varying reactor temperatures, was performed to maximise the recovery of energy, of which the performance was compared to stand-alone pyrolysis. The recovery of energy was maximised by coupling PWS fermentation and FR fast pyrolysis, resulting in gross ECs of between ~75 and 88% for the LAPWS, and ~41 and 48 % for the HAPWS. These gross ECs were up to ~10 % higher in comparison to those attained for stand-alone pyrolysis of PWS. The greater availability of lignin in FR, after fermentation, led to bio-oil products that were phenols-rich. In summary, the present study pointed out the promising potential of pyrolysis processing of PWS/FR as part of a biorefinery for production of fuels, chemicals and biomaterials resources. FP maximised the organic liquid and levoglucosan yields as well as the gross EC from PWS. Sequential fermentation of PWS coupled with FP of FR maximised the gross ECs, which were higher in comparison to stand-alone PWS pyrolysis ECs. To confirm which process option is best in terms of overall energy efficiency and economics additional modelling and economic feasibility studies are recommended.AFRIKAANSE OPSOMMING: Alternatiewe energie bronne soos hernubare energie is nodig as gevolg van verminderde natuurlike energie bronne en omgewingsbewustheid rakende aardverwarming. ʼn Voorbeeld van ʼn hernubare energiebron is biomassa. Biomassa sluit onder andere plantmateriaal, landboureste en industriële afval in. Die pulp en papier industrie genereer groot hoeveelhede papierafvalslik, wat ryk is aan organiese materiaal en vog, en dus die potensiaal het om as ʼn hernubare energie bron aangewend te word. Hierdie energiebron is die fokus van hierdie studie. Weens die hoë kostes en omgewingsbekommernisse wat gepaard gaan met die storting van papierafvalslik, moet daar gekyk word na ander metodes van waarde-toevoeging, onder andere termochemiese en/of biochemiese omsetting. Die termochemiese prosesse van pirolise ontbind biomassa termies in die afwesigheid van suurstof, wat dan bio-olies, houtskool en nie-kondenseerbare (permanente) gasse produseer, wat almal voordelig aangewend kan word as energie, chemikalieë of bio-materiale. Alternatiewelik kan die biochemiese omskakeling van papierafvalslik na bioëtanol, deur ʼn fermentasie proses, gebruik word as ʼn aanvanklike stap, gevolg deur die pirolitiese omskakeling van die fermentasiereste. Die primêre oogmerk van hierdie PhD studie was assessering van die volle potensiaal van alternatiewe pirolise tegnieke as deel van ʼn bio-raffinadery, teen verskillende toestande, om die omskakeling van papierafvalslik en sy fermentasiereste, in energie, chemikalieë en bio-materiale besit. Statistiese analise van produkopbrengs en kwaliteit was ook gedoen om nuwe meganistiese kennis te ontsluit. Die eerste gedeelte van die studie het gekyk na die optimering van die bio-olie opbrengs van lae-as en hoë-as papierafvalslik (8.5 en 46.7 massa %) deur gebruik te maak van die vinnige pirolise proses. Beide reaktor temperatuur en partikel grootte was geoptimeer deur gebruik te maak van ʼn 2- wyse lineêre en kwadratiese model. Teen ʼn gemiddelde partikel grootte van ~5mm en optimale reaktor temperatuur, is ʼn maksimum bio-olie opbrengs van 45 en 50 massa % (droë as-vrye basis) verkry vir lae-as en hoë-as papierafvalslik. ʼn Termogravimetriese studie is ook gedoen om insig in die termodinamiese meganisme te verkry, nadat ʼn verhoging in bio-olie opbrengs met die groter partikels opgemerk is. Resultate wys ook dat minder sekondêre teer-krakingsreaksies gebeur as gevolg van laer massa oordrag, wat lei tot ʼn groter bio-olie opbrengs. Vakuum, stadige en vinnige pirolise prosesse is geëvalueer en vergelyk teen verskillende reaktor temperature en partikel groottes in terme van hulle die bruto energie opbrengste (EO) van die rou papierafvalslik na die soliede en vloeistof produkte. Weereens is ʼn 2-wyse lineêre en kwadratiese model gebruik vir die statistiese evaluering. ʼn Vergelyking van die totale EO, as ʼn kombinasie van die soliede en vloeistof produkte, wys dat vinnige pirolise tussen 18.5 en 20.1 % hoër opbrengste vir lae-as papierafvalslik, en 18.4 tot 36.5 % hoër vir hoë-as papierafvalslik, in vergelyking met stadige en vakuum pirolise kon lewer. Hierdie bevinding word verklaar deur die hoër produksie van die organiese kondenseerbare komponente gedurende vinnige pirolise vir alle tipes slyke. Die verhittingswaardes van die teer-fase vanaf vakuum pirolise en vinnige pirolise bioolie vir beide papierafvalslik, en ook lae-as papierafvalslik houtskool, was hoog (~18 tot 23 MJ.kg- 1) wat die potensiaal vir industriële energie toepassings, beklemtoon. Die vermoë van vakuum, stadige en vinnige pirolise om die selektiewe omskakeling van rou papierafvalslik tot chemikalieë (veral glikolaldehied en levoglukosaan) en houtskool-biomateriale (absorberende stof of biohoutskool) was geëvalueer. Produkopbrengs was geoptimeer volgens reaktor temperatuur en partikel grootte (2-wyse lineêre en kwadratiese model) en die wisselvalligheid gekwantifiseer deur gebruik te maak van basiese komponent analise. Resultate wys dat die hoë verhittingstempo gebruik gedurende die vinnige pirolise proses die de-polimerisasie en/of fragmentasie reaksies bevoordeel het, wat lei tot hoë opbrengste van meeste organiese komponente, spesifiek levoglukosaan vir lae-as papierafvalslik (1.5 droë asvrye basis, massa %) en hoë-as papierafvalslik (3.7 droë asvrye basis, massa %). Die koolstof bio-materiaal verkry met beide papierafvalslyke was ultra-mikroporieus en die gebruik van vakuum pirolise het die absorberende eienskappe van die lae-as papierafvalslik koolstof betekenisvol verbeter. Om die herwinning van energie te maksimeer is opeenvolgende papierafvalslik fermentasie (bioëtanol produksie – aparte studie), gevolg deur pirolise omskakeling van fermentasiereste teen verskillende reaktor temperature, vergelyking met die uitsette van pirolise alleen. Die herwinning van energie was gemaksimeer deur die kombinasie van papierafvalslik fermentasie en fermentasiereste vinnige-pirolise. Die resultaat was ʼn bruto EO van ~75 tot 88% vir lae-as papierafvalslik, en ~41 tot 48% vir hoë-as papierafvalslik. Hierdie bruto EOs was tot ~10% hoër in vergelyking met pirolise van papierafvalslik alleen. Die groter beskikbaarheid van lignien in fermentasiereste na fermentasie het gelei tot bio-olie produkte wat ryk was in fenole. Ter opsomming, die huidige studie wys na die hoë potensiaal van prosessering van papierafvalslik en fermentasiereste deur pirolise as deel van ʼn bio-raffinadery vir produkte van brandstof, chemikalieë en bio-materiale. Vinnige pirolise maksimeer die organiese vloeistof en levoglukosaan opbrengs asook die bruto EV vanaf papierafvalslik. Opeenvolgende fermentasie van papierafvalslik in kombinasie met vinnige pirolise van fermentasiereste maksimeer die bruto EOs. Dit resultate was hoër in vergelyking met papierafvalslik pirolise alleen. Addisionele modellering en ekonomiese uitvoerbaarheidstudies word aanbeveel om te bevestig watter proses is die beste in terme van energie doeltreffendheid en ekonomiese uitvoerbaarheid