GENERAL CONCLUSIONS

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Biomass is the only renewable source of energy and organic carbon that has the potential to reduce our overreliance on fossil fuels and mitigate environmental issues. Effective utilization of biomass in the current energy system, which is essential to the establishment of a circular, bio-based economy, necessitates the development of new technologies to overcome the limitations of the structural recalcitrance, low bulk density and high moisture content of the biomass. With this perspective, this thesis has been devoted to the combined hydrothermal and pyrolytic conversion of biomass, the implementation of which is expected to achieve a simple and selective production of light oil, clean biochar or/and fuel gas, with sufficiently high recoveries and relatively low temperatures.

In Chapters 2 and 3 described the novel processes for the conversion of typical biowastes of modern biorefineries, that is, solid lignin and aqueous streams, respectively. A Japanese cedar lignin prepared by a Klason method, i.e., hydrothermal treatment with concentrated sulfuric acid, was selectively converted to phenolic monomers and biochar by a particular type of pyrolysis free from external chemicals/catalysts. The pyrolysis enabled heavier portions of the bio-oil, that is, HO, to be sorbed by the parent lignin and then be repyrolyzed with the lignin. The feasibility of the process was successfully examined in a fix-bed reactor by repeating pyrolysis of HO-loaded lignin up to nine times at a peak temperature of 600°C.

As a result, the biochar gained an increase in the production to a more or less extent with near-unchanged elemental composition, volatile matter content, and calorific value. Most importantly, the resultant liquid bio-oil is abundant in phenolic monomers with a molecular mass below 200, in particular, catechol, guaiacol and their derivates. The HO recycling pyrolysis can be technically feasible in industrial practice, such as, a countercurrent moving bed reactor, because the lignin has capabilities of HO capturing in terms of condensation and holding capacity and allows for neither discharge of HO from the system nor accumulation therein. Biomass-derived wastewaters, in general, possess hazardous properties such as corrosivity and high TOC. The conversion strategy that we proposed in Chapter 3 applied the wastewater as not a by-product but instead a leaching agent to remove AAEMs from biomass and biochar. The leachate was then converted under subcritical conditions (350°C, 20 MPa) to a fuel gas rich in CH4. An aqueous phase of bio-oil derived from the pyrolysis of rice straw was used as starting feedstock. The pH of the feedstock as low as 2.8 enabled a removal rate of K up to 95% from the biomass/char. The organic compounds were near completely gasified to mainly H2, CH4, and CO2 and particularly, the

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AAEMs solubilized in the solution suppressed the growth of Ru particles and thus the deactivation of Ru/C catalyst during CHTG. This proposed process is supposed to be applicable in many biorefineries that generate exhaust liquids. Another feature of the process is the recovery of inorganic mattes such as K and Na in their ionic forms, for recycling into cultivation. Chapter 4 in-detail investigated once-through leaching and repeated leaching of char with pyrolytic aqueous phase. The leaching of AAEMs roughly follows pseudo-second order reaction kinetics. Repeated leaching of char with AP up to 18 times enables the internal recycling of the pyrolytic products.

In Chapter 5, application of CHTG to lignite-to-syngas conversion was investigated and discussed. Additional effort was devoted to achieving a high degree of dissolution of the lignite in an alkaline aqueous medium. A Victorian lignite was subjected to hydrothermal treatment in an aqueous solution of NaOH at 250°C and then oxidation with pressurized O2

at 100°C. The sequential treatments solubilized 95% of the lignite on mass/carbon bases.

The resulting solution was further converted by CHTG in a flow reactor at 350°C for 10 h, employing a Ru/C catalyst (16 wt% Ru). The initial carbon conversion to gas was as high as 98% while CH4, CO2 and H2 were produced. The conversion gradually decreased due to coke deposition over the catalyst but was near steady around 83% at 8–10 h. The solubilized lignite consisted of compounds with molecular mass up to 5,000. The heavier portion (molecular mass > 1,000) was responsible for the coke formation and accumulation that caused the catalyst deactivation. Taking into account the dissolution and CHTG of the lignite, total higher heating value recovery of CH4/H2 and insoluble matter was 80%, much higher than those by gas-solid gasification operated at reaction temperatures well above 1000°C.

In conclusion, this work has presented several aspects of combined pyrolytic and hydrothermal conversion of biomass and lignite and it has shown that such technologies are potentially very effective. Nevertheless, it is also quite complex, since there are many aspects to be considered at the same time. Hydrothermal gasification actually needs much more scientific and technical work in order to be fully developed and applied to real scale. First, heterogeneous catalysts are necessary at low temperatures to allow effective gasification.

However, continuous or semi-continuous studies with long periods online often show a significant decline in catalyst activity that is, in general, related to changes in the support and active metal (e.g., leaching, loss of surface are by crystallite growth or sintering). Coking is a serious issue with biomass streams as is fixing of sulfate onto catalysts. The degradation

or oxidation of catalyst support is another key issue under hydrothermal conditions. More research works are needed to design and develop superior catalysts and supports to ensure long-term operations of a full-scale plant. Second, the detailed reactions pathways, mechanisms and kinetics are still unknown or not clear for hydrothermal processing owing to the complex of biomass and lignite.