| dc.description |
Biomass is a promising renewable energy source widely available worldwide, particularly in
developing countries where clean and affordable energy is a major problem. Biomass gasification
is an attractive technology that can transform biomasses into a more versatile fuel known as
syngas, tar (bio-oil) and biochar. Syngas is a hydrogen-rich gas that could promote a clean and
green energy and promote the agriculture sector. The utilisation of syngas would reduce
dependence on fossil-based fuels and eventually reduce the carbon footprints. The gasification
technology faces operational challenges; one of the problems is tar formation, slow char
gasification reaction, and poor performance of catalysts. These challenges are influenced by
inappropriate operating conditions and the precursors employed in catalyst synthesis. In this
study, the optimisation of noncatalytic and catalytic gasification of rice husk is reported. The rice
husk biomass was gasified under subcritical and supercritical water conditions in a batch
autoclave reactor. The effect of temperature (350-500°C), residence time (30-120 minutes), and
feed concentration (3-10 wt%) is optimised. Moreover, the effect of the addition of natural,
calcined and Fe doped limestone and dolomite catalysts on the gasification yield is studied using
a response surface methodology. The catalyst was prepared by a facile incipient wetness process
using chlorine- and sulphur-free iron (III) ammonium citrate precursor. Optimisation of
operating conditions suggests a quadratic model for gasification efficiency, gas volume, char
yield, and gravimetric tar. The findings revealed that higher temperatures, longer residence times
and lower feed concentrations favoured high gas yields. The lowest tar yield obtained was 2.98
wt%, while the highest gasification efficiency and gas volume attained was 64.27% and 423
mL/g, respectively. The findings of the catalyst characterisation revealed that the predominant
reactive site of Fe/limestone catalyst is iron oxide, calcium ferrite, and calcium oxide. Under all
conditions tested, the manufactured catalyst was highly active in promoting char gasification,
gas volume and gasification efficiency. Tar yield was substantially promoted at low temperatures
and high feed concentrations, but at high temperatures and low feed concentrations (500oC, 3
wt%), tar formation was suppressed by 22%, while char gasification was enormously enhanced
by 65%. The maximum gas yield of 560 mL/g biomass was obtained under the catalytic
conditions of 5%Fe/limestone, 15% catalyst loading, 500oC, 120 minutes, and 3 wt% feed
concentrations. Therefore, these findings revealed that the rice husk's energy content could be
harnessed using supercritical water gasification to obtain substantial fuel products. |
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