Production of an upgraded bio-oil by catalytic pyrolysis of forest residues

Thesis (MEng)--Stellenbosch University, 2019.ENGLISH ABSTRACT: Forest plantations generate solid residues which are usually disposed of by combustion. Sometimes these residues are simply left in the forest, where they contribute little to soil fertility, and yet pose a significant fire hazard. Unlike combustion, which produces heat and is limited to onsite use only, pyrolysis can be used to convert the forestry residues to produce a bio-oil that is easier to transport and use offsite. However, bio-oil has limitations in its use as a liquid fuel because its high oxygen content (about 40 wt. %), gives it undesirable qualities such as high acidity, oxidative instability and low energy content. The aim of the project was to catalytically upgrade the bio-oil by lowering its oxygen content so that it can be co-processed in a conventional crude oil refinery to produce transportation fuels. Eucalyptus grandis was chosen as the feedstock based on it being the most abundant species used in the regions with the most forestry residue in South Africa. From literature, 3 catalysts: CaO, MgO and Al2O3 were selected based on their ability to promote deoxygenation. Screening tests were carried out in a batch reactor, under intermediate pyrolysis conditions at 550 °C and 30 wt.% catalyst concentration. The improvement in the bio-oil HHV in the catalyst screening was similar for all the catalysts; it increased from 21.8 MJ/kg without catalyst to between 26.3 – 26.8 MJ/kg for all 3 catalysts. Optimisation experimental designs (CCD) were then carried out for each of CaO and MgO to maximise bio-oil quality in terms of HHV at an acceptable yield. Temperature was varied from 444 – 656 °C and the catalyst concentration from 1.7 – 58.3 wt.%. For MgO deoxygenation occurred mainly via decarboxylation reactions and the maximum HHV was at 26.9 MJ/kg at 560.0 °C and 33.8 wt.% catalyst concentration, at a yield of 19.4 wt.%. For CaO, dehydration reactions were dominant and the maximum HHV was 27.5 MJ/kg at 490.0 °C and 59.0 wt.% CaO concentration. The bio-oil yield was 13.4 wt.% which was low to achieve the target bio-oil blending ratio. A statistical desirability function was then used, and the desirable optimum conditions were found to be those of the catalyst screening. The best performing catalyst was found to be CaO based on energy conversion assessment and the better applicability of char derived from CaO catalytic pyrolysis in soil amendment. A pilot reactor with a 1 kg/hr capacity was then used to scale up the process from bench scale. Better contact between the solid catalyst and organic volatiles in the pilot reactor meant that the optimum reaction at bench (550 °C) had to be reduced to 500 °C to limit the effect of severe catalytic cracking. The resultant bio-oil had an oxygen content of 12.6 wt.%, a water content of 19.7 wt.% at a yield of 15.6 wt.%, which meet the specifications required for successful co-processing in a crude oil refinery at a 10 wt.% blending ratio. However, it is recommended that the bio-oil be tested for co-processing in a Fluid Catalytic Cracking unit (FCC).AFRIKAANSE OPSOMMING: Bosplantasies genereer soliede residu’s wat gewoonlik deur verbranding verwyder word. Soms word hierdie residu’s gewoon in die woud gelos, waar dit min bydrae tot grond vrugbaarheid, maar ’n beduidende brandgevaar inhou. Anders as verbranding wat hitte produseer en beperk is tot gebruik slegs op die perseel, kan pirolise gebruik word om die bosbou residu om te skakel na bio-olie wat makliker is om te vervoer en te gebruik weg van die terrein. Bio-olie het wel beperkinge in sy gebruik as ’n vloeistof-brandstof as gevolg van sy hoë suurstofinhoud (omtrent 40 wt.%) wat dit onwenslike kwaliteite soos hoë suurheid, oksidatiewe onstabiliteit en lae energie-inhoud gee. Die doel van hierdie projek was die katalitiese opgradering van die bio-olie deur sy suurstofinhoud te verlaag sodat dit in ’n konvensionele ru-olie raffinadery geko-prosesseer kan word om vervoer brandstof te produseer. Eucalyptis grandis is gekies as die voermateriaal omdat dit die oorvloedigste spesie is wat gebruik word in die omgewings met die meeste bosbou residu in Suid-Afrika. Uit literatuur is drie kataliste — CaO, MgO en Al3O2 — gekies gebaseer op hul vermoë om deoksigenering te bevorder. Siftingstoetse is uitgevoer in ’n lotreaktor, onder intermediêre pirolise toestande by 550 °C en 30 wt.% kataliskonsentrasie. Die verbetering in die bio-olie hoër warmte waarde (HHV) in die katalissifting was soortgelyk vir al die kataliste; dit het verhoog van 21.8 MJ/kg tot tussen 26.3 en 26.8 MJ/kg vir al drie kataliste. Optimering eksperimentele ontwerpe (CCD) is toe uitgevoer vir elk van CaO en MgO om bio-olie kwaliteit in terme van HHV by ’n aanvaarbare opbrengs, te maksimeer. Temperatuur is gevarieer van 444 °C tot 656 °C en die kataliskonsentrasie van 1.7 wt.% tot 58.3 wt%. Vir MgO het deoksigenering hoofsaaklik voorgekom via dekarboksilasiereaksies en die maksimum HHV was 26.9 MJ/kg by 560.0 °C en 33.8 wt.% kataliskonsentrasie, by ’n opbrengs van 19.4 wt.%. Vir CaO was dehidrasiereaksies dominant en die maksimum HHV was 27.5 MJ/kg by 490.0 °C en 59.0 wt.% CaO-konsentrasie. Die bio-olie opbrengs was 13.4 wt.% wat te laag was om die doelwit bio-olievermengingverhouding te bereik. ’n Statistiese wenslikheidsfunksie is toe gebruik, en die gewenste optimale toestande is bevind om dié van die katalissifting te wees. Die beste presterende katalis is gevind om CaO te wees gebaseer op energie-omsetting-assessering en beter toepassing van verkoolsel wat van CaO katalitiese pirolise af kom in grondwysinging. ’n Loodsreaktor met ’n 1 kg/hr-kapasiteit is toe gebruik om die proses op te skaal vanaf banktoetsskaal. Beter kontak tussen die soliede katalis en organiese vlugtige stowwe in die loodsreaktor het beteken dat die optimale temperatuur by banktoetsskaal (550 °C) verlaag moes word na 500 °C om die effek van ernstige katalitiese kraking te beperk. Die resulterende bio-olie het ’n suurstof inhoud van 12.6 wt.%, ’n waterinhoud van 19.7 wt.% by ’n opbrengs van 15.6 wt.% gehad, wat die spesifikasies vereis vir suksesvolle ko-prosessering in ’n ru-olie raffinadery by ’n 10 wt.%, haal. Dit word egter aanbeveel dat die bio-olie getoets word vir ko-prosessering in ’n vloeistof katalise breking (FCC)-eenheid