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Optimisation of thermal treatment of invasive alien plants (IAPs) for char production for use in combustion applications

Mundike, Jhonnah (2018-03)

Thesis (PhD)--Stellenbosch University, 2018.


ENGLISH SUMMARY: Due to the popular worldwide demand for need to use cleaner fuels, lignocellulosic-derived char is gaining importance as a possible component in co-firing with coal. In order to avoid deforestation of indigenous forests in Zambia for char production, possibilities of using alternative feedstocks from invasive alien plants (IAPs) were investigated. In the present study, torrefaction and slow pyrolysis were used for char production from IAPs for energy applications. Both processes were optimised individually at milligram-scale in a thermogravimetric analyser (TGA) for char yield and higher heating value (HHV), through manipulation of the temperature, heating rate and holding time. Two IAPs, namely Lantana camara (LC) and Mimosa pigra (MP), from Zambia were used as feedstock materials. The feedstock particle size distribution (PSD) used was from 425 to 600 μm. The optimisation results for torrefaction and slow pyrolysis showed that temperature majorly influenced char yield and HHV. In case of torrefaction, operating at temperatures ≤ 300 ˚C, heating rate and hold time also influenced char HHV, while neither parameters had a statistically-significant influence on char yield and HHV during slow pyrolysis. During torrefaction at 300 ˚C, LC recorded a higher char yield of 43 wt.%, and a corresponding HHV of 27.0 MJ kg-1, mainly due to increased hemicelluloses content, compared with MP that had a char yield of 52 wt.% with HHV of 24.4 MJ kg-1. In case of slow pyrolysis, MP recorded the highest char HHV of 31.0 MJ kg-1 at 580 ˚C, due to increased lignin, in comparison with LC that had a highest char HHV of 30.0 MJ kg-1 at 525 ˚C. Based on optimised conditions from milligram-scale, LC and MP samples of PSD from 850 to 2800 μm were used for char production at gram-scale in a bench-scale reactor. Scaling-up promoted secondary char formation due to mass and heat transfer limitations in larger particles and increased sample size, thereby increasing char yields for both biomasses. Char yields were increased by 4 and 2 wt.% for MP and LC, respectively, due to scale-up. The highest HHVs at bench-scale were 30.8 MJ kg-1 (614 ˚C) and 31.6 MJ kg-1 (698 ˚C) for LC and MP, respectively. For the purposes of coal substitution and co-firing, a combustion study was conducted in a TGA reactor using LC and MP chars (torrefied and pyrolysed) from gram-scale of PSD from 850 to 2800 μm. LC and MP chars were blended with three South African coals between 5 to 90 wt.% (biomass char). The combustion characteristic results showed that LC chars were more reactive than MP chars, with significantly lower combustibility temperatures than the coals. During co-combustion, the combustion indices for blends < 30% were similar to those of the individual coals, showing that partial coal substitution could be done without significant modifications to existing equipment. There was better combustion performance through increased combustion indices for blends > 60%, though probably with a likelihood of modifications to existing reactors that were initially designed for coal combustion, as the conversion was much faster. In summary, this study has shown that LC and MP IAPs could be valorised through torrefaction and slow pyrolysis to produce char for direct energy applications and co-firing with coal. LC samples torrefied at 300 ˚C were found to be equivalent to high volatile bituminous C coal, while pyrolysed chars for LC and MP were equivalent to high volatile bituminous B coal. To confirm the practicality of co-firing possibilities, it is recommended that scale-up studies to pilot-scale be conducted in order to assess overall energy efficiency, techno-economics, operating conditions of industrial reactors and a life cycle assessment.

AFRIKAANSE OPSOMMING: Weens die populêre, wêreldwye behoefte om skoner brandstof te gebruik, is daar ʼn toename in die belangrikheid van houtskool afkomstig vanaf lignosellulose as ʼn moontlike komponent in die gesamentlike-verbranding met steenkool. Om ontbossing van inheemse bosse in Zambië vir houtskool produksie te voorkom, die moontlikheid om indringer uitheemse plante as grondstof te gebruik, is ondersoek. In die huidige studie is die lae-temperatuur rooster, ook bekend as torrefaksie, en stadige pirolise benut vir die produksie van houtskool wat in energietoepassings gebruik kan word. Die eerste teiken was om houtskool produksie deur torrefaksie en stadige pirolise vir energietoepassings te optimeer, deur die houtskool-opbrengs en hoër-verhittings-waarde (HVW) te maksimeer, deur die optimering van temperatuur, verhittingstempo, en hou-tyd op milligram-skaal in ʼn termo-gravimetriese analiseerder (TGA). Twee indringer uitheemse plante in Zambië, naamlik Lantana camara (LC) en Mimosa pigra (MP), was as grondstof gebruik. Vir die grondstof was ʼn partikel-grootte-verspreiding (PGV) van 425 tot 600 μm gebruik. Die optimiserings-resultate vir torrefaksie en stadige pirolise het getoon dat temperatuur ʼn groot invloed op houtskool-opbrengs en HVW gehad het. In die geval van torrefaksie was dit bevind dat vir temperature ≤ 300 ˚C, die verhittingstempo en hou-tyd ook die houtskool-HVW beïnvloed, terwyl vir stadige pirolise het beide veranderlikes geen statistiese merkbare invloed op die houtskool-opbrengs of HVW gehad nie. Gedurende torrefaksie by 300 ˚C het LC ʼn hoër houtskool-opbrengs van 43 massa% en ʼn ooreenstemmende HVW van 27.0 MJ kg-1 gehad, grootliks as gevolg van die verhoogde hoeveelheid hemisellulose. Dit is in vergelyking met MP wat ʼn houtskool-opbrengs van 52 massa% en ʼn HVW van 24.4 MJ kg-1 gehad het. In die geval van stadige pirolise het MP ʼn hoër maksimum HVW van 31.0 MJ kg-1 by 580 ˚C getoon as gevolg van verhoogde lignien inhoud. Dit is in vergelyking met LC wat ʼn optimale houtskool HVW van 30.0 MJ kg-1 by 525 ˚C gehad het. Gebaseer op die ge-optimiseerde kondisies op milligram-skaal, was LC en MP monsters met PGV van 850 tot 2800 μm gebruik vir houtskool produksie op gram-skaal in ʼn laboratoriumskaal reaktor. Opskalering het sekondêre-houtskool-produksie bevorder as gevolg van massa-en-hitte-oordrag beperkinge in groter partikels en groter monster-groottes en gevolglik het beide monsters verhoogde houtskool-opbrengs getoon. Verhoogde houtskool-opbrengs verskille tot 4 en 2 massa% was verkry vir MP en LC onderskeidelik. Die opgeskaleerde optimale HVW resultate was 30.8 MJ kg-1 (614 ˚C) en 31.6 MJ kg-1 (698 ˚C) vir LC en MP onderskeidelik. ʼn Verbrandingstudie was gedoen op LC en MP houtskool (vanaf torrefaksie en pirolise) met gram-skaal-PGV van 850 tot 2800 μm in ʼn TGA reaktor met die doel van steenkool substitusie. LC en MP houtskool was gemeng met drie Suid-Afrikaanse steenkool in ʼn verhouding van 5 tot 90 massa% (biomassa houtskool). Die verbrandingseienskappe-resultate het getoon dat LC houtskool meer reaktief is as MP houtskool, aangesien dit die verbrandingstemperatuur van al die steenkool aansienlik verlaag het. Gedurende gesamentlike-verbranding was dit bevind dat vir mengsels < 30% is die verbrandingsindekse soortgelyk was die van die individuele steenkool, wat wys dat gedeeltelike substitusie moontlik is sonder om merkbare veranderinge aan die bestaande toerusting hoef te maak. Vir mengsels > 60% was die verbrandings-bedrywe beter met verhoogde verbrandingsindekse, maar aangesien die verbranding baie vinniger was, sal daar waarskynlik veranderinge aan die bestaande toerusting gemaak moet word. In opsomming, die studie het gewys dat indringer uitheemse plante, veral LC en MP, gebruik kan word deur torrefaksie en stadige pirolise om houtskool te produseer vir direkte energietoepassings deur gesamentlike-verbranding met steenkool. Dit was bevind dat LC monsters van torrefaksie by 300 ˚C, gelykwaardig is aan hoogsvlugtige bitumeniese C steenkool, terwyl gepiroliseerde houtskool van LC en MP gelykwaardig was aan hoogsvlugtige bitumeniese B steenkool. Om die praktiese moontlikehede van gesamentlike verbranding te bevestig, word dit voorgestel dat studies rakende die opskalering na loodskaal gedoen word om die algehele energie-doeltreffendheid, tegno-ekonomiese, en bedryftoestande van industriële reaktore te evalueer, sowel as ʼn lewens-siklus-evaluasie.

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