High conversions of miscanthus using sub- and supercritical water above 400 °C

Miscanthus conversion to bio-fuels was investigated using sub- and supercritical water at 410 and 460 °C with high mass ratios of water to biomass to ascertain the maximum yields that could be achieved. The results indicate that conversions increased with increasing miscanthus to water mass ratio at high water ratio and 410 °C to give the highest conversions of ∼90 wt.%, producing ∼21 wt.% heavy oil, ∼27 wt.% gas, ∼28 wt.% water and ∼12 wt.% light oil. At 460 °C, the maximum conversion was similar but the liquid product was lighter with only 7% heavy oil and more hydrocarbon gas being obtained. The oxygen contents of heavy oil recovered ∼12-16% (for 410 °C and 460 °C), with H/C ratio of 1.09 at 410 °C. Under equivalent conditions with tetralin as a model hydrogen-donor solvent, similar overall conversions were achieved but with much lower solvent to miscanthus mass ratios were needed and the liquid products have lower oxygen contents (∼13%). Reduced CO and increased CO2 yields provide evidence that water hydrogen is being utilised but the estimated amounts of ca. 0.5% at 410 and 460 °C are considerably lower than with hydrogen-donor solvents which account for the lower yields of bio-oils and their higher oxygen contents.