| Summary: | Thesis (PhD)--Stellenbosch University, 2012. === ENGLISH ABSTRACT: Gasification provides a proven alternative to the dependence on petroleum for the
production of high value products such as liquid fuels and chemicals. Syngas, the
main product from gasification can be converted to fuels and chemicals via a number
of possible synthesis processes. Coal and natural gas are currently the main
feedstock used for syngas production. In South Africa (SA), Sasol operates the largest
commercial coal-to-liquids conversion process in the world, based on updraft fixed
bed gasification of low grade coal to syngas. Co-utilizing alternative and more
sustainable feedstock (such as biomass and wastes) with coal in existing coal-based
plants offers a realistic approach to reducing the costs and risks associated with
setting up dedicated biomass conversion plants.
An experimental and modelling investigation was performed to assess the impacts of
co-gasifying two of the most commonly available agricultural wastes in SA
(sugarcane bagasse and corn residue) with typical low grade SA coals, on the main
products of updraft fixed bed gasification, i.e. liquid condensates and syngas.
Condensates are produced in the pyrolysis section of the updraft gasifier, whereas
syngas is a result of residual char conversion. An experimental set-up that simulates
the pyrolysis section of the gasifier was employed to investigate the yield and
composition of devolatilized products at industrially relevant conditions of 26 bars
and 400-600°C. The results show that about 15 wt% of coal and 70 wt% of biomass
are devolatilized during the pyrolysis process. The biomass derived condensates
were determined to comprise of significantly higher quantities of oxygenates such as
organic acids, phenols, ketones, and alcohols, whereas coal derived hydrocarbon
condensates were dominated by polycyclic aromatic hydrocarbons, creosotes and
phenols. Results of investigation into the influence of coal-biomass feedstock mix
ratio on yields of products from pyrolysis show limited evidence of non-additive or
synergistic behaviour on the overall distribution of solid, liquid and gas yields. On the
other hand, in terms of the distribution of specific liquid phase hydrocarbons, there
was significant evidence in favour of non-additive pyrolysis behaviour, as indicated by the non-additive yield distribution of specific chemicals. Synergistic trends could
also be observed in the thermogravimetric (TGA) study of pyrolysis under kinetically
controlled non-isothermal conditions. Model free and model fitting kinetic analysis
of the TGA data revealed activation energies ranging between 94-212 kJ mol-1 for the
biomass fuels and 147-377 kJ mol-1 for coal. Synergistic interactions may be linked to
the increased presence of hydrogen in biomass fuels which partially saturates free
radicals formed during earlier stages of devolatilization, thereby preventing
secondary recombination reactions that would have produced chars, allowing for the
increased formation of volatile species instead.
Analysis of char obtained from the co-pyrolysis experiments revealed that the fixed
carbon and volatile content of the blended chars is is proportional to the percentage
of biomass and coal in the mixture. CO2 reactivity experiments on the chars showed
that the addition of biomass to coal did not impose any kinetic limitation on the
gasification of blended chars. The blended chars decomposed at approximately the
same rate as when coal was gasified alone, even at higher biomass concentrations in
the original feedstock blend. Based on these observations, a semi-empirical
equilibrium based simulation of syngas production for co-gasification of coalbiomass
blends at various mix ratios was developed using ASPEN Plus. The model
showed that H2/CO ratio was relatively unaffected by biomass addition to the coal
fuel mix, whereas syngas heating value and thermal efficiency were negatively
affected. Subsequent evaluation of the production cost of syngas at biomass inputs
ranging between 0-20 wt% of coal reflected the significant additional cost of pretreating
biomass (3.3% of total capital investment). This resulted in co-gasification
derived syngas production costs of ZAR146/tonne (ZAR12.6/GJ) at 80:20 coalbiomass
feedstock ratio, compared to a baseline (coal only) cost of ZAR130/tonne
(ZAR10.7/GJ). Sensitivity analysis that varied biomass costs from ZAR0 ZAR470
revealed that syngas production costs from co-gasification remained significantly
higher than baseline costs, even at low to zero prices of the biomass feedstock. This
remained the case even after taking account of a carbon tax of up to ZAR117/tCO2.
However, for range of carbon tax values suggested by the SA treasury (ZAR70 tCO2 to ZAR200 tCO2), the avoided carbon tax due to co-feeding biomass can offset between
40-96% of the specific retrofitting cost at 80:20 coal-biomass feedstock mass ratio.
In summary, this dissertation has showed that in addition to the widely recognized
problems of ash fouling and sintering, co-feeding of biomass in existing coal based
updraft gasification plants poses some challenges in terms of impacts on
condensates and syngas quality, and production costs. Further research is required
to investigate the potential in ameliorating some of these impacts by developing
new high value product streams (such as acetic acid) from the significant fraction of
condensates derived from biomass. === AFRIKAANSE OPSOMMING: Vergassing bied 'n beproefde alternatief vir die afhanklikheid van petroleum vir die
produksie van hoë waarde produkte soos vloeibare brandstof en chemikalieë.
Sintese gas, die belangrikste produk van vergassing, kan omgeskakel word na
brandstof en chemikalieë deur 'n aantal moontlike sintese prosesse. Steenkool en
aardgas is tans die belangrikste grondstowwe wat gebruik word vir sintese gas
produksie. In Suid-Afrika (SA) bedryf Sasol die grootste kommersiële steenkool-totvloeistof
omskakelingsproses in die wêreld, gebaseer op stygstroom vastebed
vergassing van laegraadse steenkool na sintese gas. Die gebruik van alternatiewe en
meer volhoubare grondstowwe (soos biomassa en afval) saam met steenkool in die
bestaande steenkool-gebaseerde aanlegte bied 'n realistiese benadering tot die
vermindering van die koste en risiko's wat verband hou met die oprigting van
toegewyde biomassa omskakelingsaanlegte.
'n Eksperimentele en modelleringsondersoek is uitgevoer om die impak van
gesamentlike vergassing van twee van die mees algemeen beskikbare landbouafvalprodukte
in Suid-Afrika (suikerriet bagasse en mieliereste) met tipiese
laegraadse SA steenkool op die vernaamste produkte van stygstroom vastebed
vergassing, dws vloeistof kondensate en sintese gas, te evalueer. Kondensate word
geproduseer in die piroliese gedeelte van die stygstroomvergasser, terwyl sintese
gas 'n resultaat is van die omskakeling van oorblywende houtskool. 'n
Eksperimentele opstelling wat die piroliese gedeelte van die vergasser simuleer is
gebruik om die opbrengs en die samestelling van produkte waarvan die vlugtige
komponente verwyder is by industrie relevante toestande van 26 bar en 400-600°C
te ondersoek. Die resultate toon dat ongeveer 15% (massabasis) van die steenkool
en 70% (massabasis) van die biomassa verlore gaan aan vlugtige komponente tydens
die piroliese proses. Daar is vasgestel dat die kondensate afkomstig van biomassa uit
aansienlik hoër hoeveelhede suurstofryke verbindings soos organiese sure, fenole,
ketone, en alkohole bestaan, terwyl koolwaterstofkondensate afkomstig uit
steenkool oorwegend bectaan uit polisikliese aromatise verbindings, kreosote en fenole. Die resultate van die ondersoek na die invloed van die verhouding van
steenkool tot biomassa grondstof op piroliese opbrengste toon beperkte bewyse van
nie-toevoegende of sinergistiese gedrag op die algehele verspreiding van soliede,
vloeistof en gas opbrengste. Aan die ander kant, in terme van die verspreiding van
spesifieke vloeibare fase koolwaterstowwe, was daar beduidende bewyse ten gunste
van 'n sinergistiese piroliese gedrag. Sinergistiese tendense is ook waargeneem in die
termogravimetriese (TGA) studie van piroliese onder kineties beheerde nieisotermiese
toestande. Modelvrye en modelpassende kinetiese analise van die TGA
data het aan die lig gebring dat aktiveringsenergieë wissel tussen 94-212 kJ mol-1 vir
biomassa brandstof en 147-377 kJ mol-1 vir steenkool.
Ontleding van die houtskool verkry uit die gesamentlike piroliese eksperimente het
aan die lig gebring dat die onmiddellike kenmerke van die gemengde houtskool die
geweegde gemiddelde van die individuele waardes vir steenkool en biomassa
benader. CO2 reaktiwiteitseksperimente op die houtskool het getoon dat die
byvoeging van biomassa by steenkool nie enige kinetiese beperking op die
vergassing van gemengde houtskool plaas nie. Die gemengde houtskool ontbind
teen ongeveer dieselfde tempo as wanneer steenkool alleen vergas is, selfs teen
hoër biomassa konsentrasies in die oorspronklike grondstofmengsel. Op grond van
hierdie waarnemings is 'n semi-empiriese ewewig-gebaseerde simulasie van sintese
gas produksie vir gesamentlike vergassing van steenkool-biomassa-mengsels vir
verskeie mengverhoudings ontwikkel met behulp van Aspen Plus. Die model het
getoon dat die H2/CO verhouding relatief min geraak is deur biomassa by die
steenkool brandstofmengsel te voeg, terwyl sintese gas se verhittingswaarde en
termiese doeltreffendheid negatief geraak is. Daaropvolgende evaluering van die
produksiekoste van sintese gas vir biomassa insette wat wissel tussen 0-20%
(massabasis) van die hoeveelheid steenkool het die aansienlike addisionele koste van
die vooraf behandeling van biomassa (3.3% van die totale kapitale belegging) gereflekteer. Dit het gelei tot 'n produksiekoste van ZAR146/ton (ZAR12.6/GJ) vir sintese gas afkomstig uit gesamentlike-vergassing van 'n 80:20 steebkool-biomassa grondstof mengesl, in vergelyking met 'n basislyn (steenkool) koste van ZAR130/ton (ZAR10.7/GJ). Sensitiwiteitsanalise wat biomassa koste van ZAR0 - ZAR470 gevarieër
het, het aan die lig gebring dat sintese gas produksiekoste van gesamentlike
vergassing aansienlik hoër bly as die basislyn koste, selfs teen 'n lae of nul prys van
biomassa grondstof. Dit bly die geval selfs nadat koolstof belasting van tot
ZAR117/tCO2 in ag geneem is.
In opsomming het hierdie verhandeling getoon dat, bykomend tot die wyd-erkende
probleme van as besoedeling en sintering, die gesamentlike gebruik van biomassa in
bestaande steenkool stygstroom vergassingsaanlegte groot uitdagings inhou in
terme van die impak op die kwaliteit van kondensate en sintese gas, asook
produksiekoste. Verdere navorsing is nodig om die potensiaal te ondersoek vir die
verbetering van sommige van hierdie impakte deur die ontwikkeling van nuwe hoë
waarde produkstrome (soos asynsuur) uit die beduidende breukdeel van kondensate
wat verkry word uit biomassa.
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