In
this work, slow pyrolysis of sawdust of Eucalyptus
pilularis biomass and ternary molten carbonate eutectic
[Li2CO3, 43.5%; Na2CO3, 31.5%; and K2CO3, 25% (mole percentage)]
in thermogravimetric analysis at three different temperatures, 600,
750, and 900 °C, was studied. These salts affect the slow pyrolysis
process, including changes in the volatile release mechanism and the
morphology of remnant char material. The initial results show that,
in the presence of molten carbonate, biomass particles make bubble-shaped
larger particles, which result in less volatile emissions and more
char residue. It is suggested that the ternary eutectic has a chemical
diluent and catalytic role, particularly in the case of higher salt
doping. Results from scanning electron microscopy images give strong
evidence that molten carbonates capture volatiles inside swelling
carbon particles, which causes the generation of various sizes of
pores as well as char-making reactions, and at a higher temperature,
the bubble-shaped particles will rupture. Swelling of this nature
has previously only been observed clearly in coal precursors; however,
this is the first observation in a biomass-based system. Also, at
a temperature above 750 °C, decomposition of molten carbonate
generates CO2 and carbon/carbonate gasification produces
CO as well as a more “activated” biochar.
This
study forms the fundamental foundation for the development of a novel
carbon arrestor process to produce a functionalized biochar. In this
study, an experimental investigation was carried out on the production
and characterization of biochar produced using a novel carbon arrestor
process, which operates under the principle of in situ pyrolysis of biomass with lime (CaO) to produce a functionalized
biochar product for use as an agronomic soil amendment. Two biomass
sources were used, a woody biomass, Eucalyptus pilularis (or blackbutt) sawdust, and a herbaceous biomass, wheat stem. Characterization
of the biochars produced as well as the gaseous products was completed
via thermogravimetric analysis, micro gas chromatography, solid-state
Fourier transform infrared (FTIR) spectroscopy, nitrogen adsorption,
and scanning electron microscopy. The addition of CaO to the pyrolysis
process resulted in a significant reduction in CO2 evolution
via the carbonation reaction of CaO and the formation of H2 and CH4 at lower temperatures in significantly higher
quantities in comparison to biomass pyrolysis alone. This was achieved
via carbonation and hydration reactions of CaO with pyrolysis gases
and catalytic effects of CaO. Biochar produced with no CaO from blackbutt
had the highest surface area (201 m2/g), while the wheat
stem was considerably lower (6.7 m2/g), and both had morphology
resembling the parent biomass. The addition of CaO resulted in a drop
in the surface area (37 m2/g) for blackbutt biochars, with
wheat stem biochar presenting similar surface areas to the respective
blackbutt biochars. Greater swelling of char particles, a significant
reduction in the particle size, and considerable fracturing of the
CaO particles were evident. Biochars produced with the addition of
CaO resulted in a reduction in oxygenated functional groups on the
surface, determined via FTIR and, to a lesser extent, elemental analysis,
which may be beneficial for char stability and longevity in soil.
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