Fate and influence of inorganics and heteroatoms during the hydrothermal carbonisation of biomass

Hydrothermal carbonisation (HTC) is an emerging biomass pre-treatment that works by converting biomass into a coal like material, in the process overcoming some of the inherent limitations of biomass. To date, there have been limited publications looking into the fate and influence inorganics and heteroatoms have on the HTC chemistry. This is surprising given these elements are of critical importance when it comes to the fuel’s utilisation. This work sets out to understand the role and fate of key inorganics and heteroatoms during HTC, but goes on to develop a mechanistic understanding of the HTC process chemistry. This work primarily focuses on the feedstocks Miscanthus, willow, brown kelp (macroalgae) and swine manure. Additionally, this work has also looked into the processing of food waste, secondary sewage sludge, AD press cake, microalgae, municipal solid wastes and oak wood, providing a large database of samples. Reaction parameters investigated as part of this work include how temperature, retention time, particle size, pH and recycling of process waters influence product yields, energy density, combustion properties, the bio-chemical composition of the bio-coal, the process water chemistry and the retention and removal of inorganics and heteroatoms. The results show that under the correct conditions HTC can produce a fuel with a HHV ranging from 25 to 30 MJ/kg (db) and the resulting bio-coal can burn like a coal, grind like coal and can overcome many of the limitations of burning biomass in pulverised coal plant. By recycling the process waters Miscanthus can be made into a fuel with an energy density of 29 MJ/kg (db), with an energy yield of 91 % and fuel properties comparable to a high volatile sub-bituminous coal. The behaviour of the inorganics and heteroatoms during HTC appear dependent on feedstock, the feedstocks inorganic chemistry and the HTC processing conditions. Generally speaking, alkali metals, which are primarily responsible for the slagging and fouling behaviour of solid fuels, are largely removed (>80%) during HTC. Moreover, when processing at 250 °C retention of calcium and reincorporation of phosphorus occurs. The combination of reduced alkali metals and relative increase in calcium and phosphorous in the ash brings about significant improvement in the fuels slagging and fouling propensity as demonstrated by the ash fusion test. It hypothesised that any residual potassium within the fuel should form calcium potassium phosphate complexes in the ash. These complexes are thermally stable and prevents the formation of low melting temperature potassium silicates or the volatilisation of potassium chloride, further reducing slagging, fouling and corrosion beyond that expected for alkali metal leaching. This can be applied to a range of low value fuels such as green harvested Miscanthus and seaweeds. This demonstrates the technologies potential to valorise low quality feedstock and produce a direct substitute bio-coal from an expanded range of terrestrial and aquatic biomass. Recycling process water brings about an increase in bio-coal energy density, energy yield and produces a fuel with more coal-like properties. It is hypothesised that the recycled process waters contain organic acids that hydrolyse the hemicellulose and cellulose to furfural like compounds at a lower temperature and increase saccharide concentrations within the process water at lower temperatures. The increased saccharide concentrations favour aromatization and repolymerisation, which better enables the decomposition products to undergo polymerisation and form the bio-coal before the increasing process temperature brings about their further degradation to organic acids. Once degraded to organic acids these acids appear to only undergo limited reincorporation into the bio-coal, but do appear to play a role in the demineralisation of the fuel. Based on this it is proposed that the slow heating rates followed by an hour retention time is favourable to overcome kinetic limitations otherwise imposed by faster heating rates and shorter retention times. The heteroatom oxygen plays a critical role in the reactions involved in HTC. The energy densification of the bio-coal is largely due to the deoxygenation of the fuel. Removal of this oxygen forms unsaturated compounds that polymerise quickly, and intermolecular dehydration results in polymerisation, condensation and aromatisation of these fragments. Oxygen is also critical in repolymerisation, with aromatic structures initially chemisorbed though reactive oxygen functionalities that dehydrate to form stable oxygen bonds linking a polymeric matrix of cyclic aromatic carbon rings. The retention of calcium also suggests it may play catalytic role in the repolymerisation process with the literature supporting this. There is however evidence that at high calcium concentrations, calcium in the process water can have an adverse effect on carbonisation, binding to surface oxygen functional groups on the biomass feedstock and preventing hydrolysis and decomposition of the feedstock.