Supercritical water gasification of wood-­related products: a thermodynamic and experimental study

Louw, Jeanne (2016-12)

Thesis (PhD)--Stellenbosch University, 2016.

Thesis

ENGLISH ABSTRACT: Supercritical water gasification (SCWG) is a method through which energy can be harvested, allowing high-­‐energy gases such as hydrogen and methane to be generated from wet organic materials without prior energy-­‐intensive drying. This thesis provides new insight into and an improved understanding of both the thermodynamic equilibrium and practical kinetic behaviour during SCWG of three wood-­‐derived products, namely wood chips, primary paper sludge and wood-­‐based pyrolysis char. A method, based on thermodynamic equilibrium calculations and the feedstock composition (molar H/C and O/C ratio) was developed to aid in the selection of suitable feed material and operating conditions for SCWG1. It was shown that, thermodynamically, feed material containing less oxygen, such as pyrolysis char, might be a promising feed material to achieve high H2 and CH4 yields as well as a gas product with a high calorific value. Experimental and theoretical thermodynamic equilibrium results are presented for SCWG of primary paper waste sludge (PWS)2, E.grandis wood chips and various related pyrolysis chars3. The effect of various parameters that may influence the kinetic behaviour were assessed, including catalyst type, catalyst loading, reaction time and feedstock composition (specifically O/C ratio and volatile matter content). A gas product relatively close to the calculated thermodynamic composition was produced during SCWG of PWS and E.grandis using a high loading of heterogeneous catalyst (0.5 – 1 g/gfeed,dry Ni/Al2O3-­‐SiO2) and longer reaction times (60 – 120 min). Furthermore, significantly higher conversion of carbon to the gas phase was achieved with Ni/Al2O3-­‐SiO2 than with a homogeneous catalyst (K2CO3). While feed material with lower O/C ratios typically resulted in higher thermodynamic equilibrium CH4 yields and gasification efficiencies, these feed material resulted in the lowest experimental CH4 yields and gasification efficiencies. Furthermore, a linear relationship between the carbon efficiency (CE) and both the volatile matter content and O/C ratio of the feed material was found to hold true in both catalytic and non-­‐catalytic experiments. The results suggest that, although thermodynamic calculations disregard the volatile matter content of the feed material, very useful predictive information can still be obtained from such calculations. It should therefore not be disregarded when considering a specific material as possible feedstock for SCWG. To summarise, this dissertation provides more insight into both the thermodynamic equilibrium and possible kinetic effects associated with SCWG for various wood-­‐related products. The major contributions are encapsulated in three peer reviewed journal publications.

AFRIKAANSE OPSOMMING: Superkritiese water vergassing (SKWVG) is 'n metode waardeur hoë-­‐energie gasse soos waterstof en metaan geproduseer kan word deur van nat organiese materiaal as voer materiaal gebruik te maak sonder energie-­‐intensiewe drogingsproses. Hierdie tesis bied nuwe insig in en 'n beter begrip van beide die termodinamiese ewewig en praktiese kinetiese gedrag tydens SCWG van drie houtagtige produkte, naamlik houtsaagsels, primêre papierafval slyk en houtskool geproduseer tydens pirolise van hout saagsels. 'n Metode, gebaseer op termodinamiese ewewigsberekeninge en die voermateriaal samestelling (molêre H/C en O/C erhoudings) is ontwikkel om te help met die keuse van geskikte voer materiaal en bedryfstoestande vir SKWVG. Termodinamiese ewewigsberekeninge het getoon dat voer materiaal wat minder suurstof bevat (soos houtskool), 'n belowende voer materiaal vir SKWVG kan wees aangesien dit ʼn gasproduk met hoë H2 en CH4 inhoud produseer. Gevolglik het hierdie gasproduk ʼn hoë kalorie waarde. Beide eksperimentele-­en teoretiese termodinamiese ewewigsresultate wanneer primêre afval papier slyk, E.grandis houtsaagsels en verskeie verwante houtskool voer materiale as voer materiaal gebruik is, word in hierdie proefskrif bespreek. Die bespreking sluit die effek wat verskillende faktore soos die soort katalisator, katalisator lading, reaksie tyd sowel as die samestelling van die roumateriaal samestelling (spesifiek O / C-­‐verhouding en vlugtige materiaal inhoud) in. 'n Gasproduk met samestelling relatief naby aan die erekende termodinamiese samestelling kan geproduseer word tydens SKWVG van papier slyk en E.grandis wanneer 'n hoë heterogene katalisator (0.5 1 g/gvoer,droog Ni/Al2O3-­‐SiO2) en langer reaksie tye (60 – 120 min). Aansienlike hoër omsetting van koolstof na die gasfase is behaal wanneer Ni/Al2O3-­‐SiO2 as katalisator gebruik word, in vergelyking met wanneer 'n homogene katalisator (K2CO3) gebruik word. Voer materiale met laer O/C verhoudings het hoër CH4 termodinamiese ewewigopbrengste en vergassingsdoeltreffendheid tot gevoe gehad. Verder is 'n lineêre verwantskap tussen die koolstofvergassingsdoeltreffendheid en beide die vlugtige materiaal inhoud en die O/C verhouding van die voer materiaal is gevind. Die resultate dui daarop dat, hoewel termodinamiese ewewigsberekeninge nie die vlugtige materiaal inhoud van die voer materiaal in ag neem nie, dit 'n belangrike rol speel in hoe maklik 'n sekere voer materiaal in superkritiese water omgesit word na ʼn gas. Dit moet dus nie buite rekening gelaat word wanneer 'n spesifieke materiaal as moontlike voermateriaal vir SKWVG oorweeg word nie. Ter opsomming, hierdie verhandeling bied meer insig in beide die termodinamiese ewewig en moontlike kinetiese effekte wat verband hou met SKWVG vir verskeie hout-­‐verwante produkte. Die groot bydraes is vervat in drie portuurbeoordeelde joernaalpublikasies.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/100099
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