Stirling, Robert John
(2019)
An investigation into the impact of hydrothermal
carbonisation on the suitability of biomass for fuel
switching.
EngD thesis, University of Nottingham.
Abstract
Biomass has the potential to be a useful low-carbon replacement for coal in a number of applications. Hydrothermal carbonisation (HTC) is a pre-treatment technology which has the potential to alleviate many of the drawbacks of replacing coal with biomass, as it improves many of the fuel properties of biomass. This study was conceived as a means to investigate the impact of HTC on the suitability of biomass for this purpose, and to investigate the chemisty of HTC so that the process could be more deeply understood. HTC experiments were performed using a Parr reactor heated by a sand bath, and numerous analytical techniques were used to investigate the composition and properties of the biocoal produced. Major analytical techniques used include thermogravimetric analysis, elemental analysis, gas chromatography, XRF spectrometry, textural analsyis, and NMR. Devolatilisation of samples in a drop tube furnace was also used so that chars generated under high heating rates equivalent to what is seen in pulverised fuel combustion could be investigated.
The impact of HTC process parameters on the yield and composition of biocoal was investigated, and HTC temperuature was found to have the largest impact. Increasing temperature decreased the yield of HTC, and resulted in a biocoal with a lower moisture and volatile matter content, with a higher fixed carbon content. Temperature also had a significant effect on the energy yield and energy densification of biocoal, with increasing temperature decreasing the yield and increasing the densification. A good compromise was found at a midpoint temperature of 225ᵒC, before the energy yield dropped considerably. Anaerobic digestion was calculated to have significant potenital for increasing the energy yield of HTC, especially when there is sinigicant loss of organic matter to the process liquor. The feedstock was also found to have a significant effect on the outcomes of HTC, primarily linked to the biochemical compostion of the feedstock.
The removal of alkali and alkaline earth metals was found to have a strong impact on the char reactivity of biocoal, with surface area and biocoal composition providing secondary impact. Biocoal produced from soft wood biomass was shown to have a char reactivity similar to that of high-volatile bituminous coal, but HTC of biomass with higher levels of alkali and alkaline earth metals resued in a biocoal with a reactivity lower than that of the feedstock biomass, but higher than that of high-volatile bituminous coal. A coal-equvalent fuel was generated by torrefaction of soft wood biocoal.
The role of water in the aromatisation reactions occuring during HTC in comparison to torrefaction was investigated. Lower levels of aromatisation were seen in HTC than in torrefaction, indicating that the water present in HTC suppresses aromatisation.
HTC was found to have little impact on the yield of activated carbon production, and was found to lower the surface area of activated carbon due to deposition of organic matter in the pores of the feedstock.
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