Hydrothermal processing of biomass and related model compounds

Future energy supplies, as a result of governmental policy and environmental legislation, will increasingly be borne from renewable sources. Use of biomass to provide fuels and chemicals to replace those derived from coal and oil will be crucial in providing diverse, sustainable and secure supplies for years to come. Production of the full range of solid, liquid and gaseous fuels from biomass to replace those from non-renewable sources is achievable, but not in the quantities currently consumed at present, hence demand will increase with an ever-expanding world population, coupled with competing markets for food production.

Biomass used as fuel has environmental benefits, with CO2 emissions being reduced, however, because biomass energy density is much lower than coal or oil, more prone to microbial degradation and because many biomass sources have high water content, transportation is much more expensive, therefore energy densification techniques are required to overcome these hurdles. Various thermal technologies exist to upgrade biomass to fuels; these include gasification, pyrolysis, anaerobic digestion, hydrothermal processing and torrefaction. Hydrothermal processing (HTP) is an environmentally benign method of energy densification and can be used to produce directly liquid and solid fuels and indirectly gaseous fuels. The reactions are carried out in hot, compressed water in temperatures of 160 – 400°C for reaction times ranging from seconds to days. Benefits of HTP reactions include high energy density liquid and solid fuels, low gaseous emissions and high product yields are also claimed. For oil production, homogeneous alkali catalysts have been used with high returns with equally high temperatures.

Biological components of energy crops biomass comprise largely of cellulose, hemicellulose and lignin, with differing ratios and types between plant species. Taking each individual lignocellulosic component and reacting in isolation presents expected data from each when reacting as a whole, allowing comparisons to be drawn with ‘raw’ biomass, Miscanthus and willow in this study. Oil formation under HTL conditions with alkali catalyst was deemed to be the best fuel product, but yields were lower than expected and the catalyst concentration was brought into question. Another surprising development was the formation of oil from lignin under these conditions, though the process of lignin removal may account for this phenomenon. LC-MS analysis of HTL derived aqueous phase lignin indicated the presence of high molecular weight polymers, heteroatomic and substituted polyaromatic compounds, with λmax outside the rage of 190 – 400nm scanned and notable by their absence.

Results from the factorial study in Chapter 5 showed that of all the reaction conditions tested (temperature, reaction time and catalyst) the greatest effect on all lignocellulosic compounds was temperature. Though it was expected that cellulose and xylan would behave in a similar manner due to the likeness of their polymeric composition, this was not the case for many responses compared. At the conditions in this study, lignin was found to be the least affected by any of the variables. Overall, the use of catalyst, though beneficial for increasing the calorific content and yield of cellulose oil, had a detrimental effect on the other components. KOH catalyst reduced aqueous phase acidity through formation of buffers, but post calculation, xylan was found to have produced more acidic species.

Biomass energy crops Miscanthus and willow were reacted in Chapter 6. Evident from this data are similarities with individual biomass components found earlier. Hemicellulose content of Miscanthus was the reason for greater solubility, though with increasing reaction times, rate limiting condensation polymerisation reaction took place to increase char yields. The most energy efficient hydrothermal conversion method was HTC, with catalyst use only beneficial for oil production. Analysis of chars by AES indicated lower alkali metals concentrations than raw analysis (K, Na, Mg, Ca). Aqueous phases contained high concentrations of these leached metals. Implications are reduction of inorganic matter improves fuel quality and less likelihood of combustion boiler problems.

The final part of the study was comparison of HTC and torrefaction. Information from literature sources and pilot plant produced torrefied materials were compared and contrasted. Drying biomass is the largest processing requirement for this process. Suitability of feedstock is largely dependent therefore on water content. Similarities are evident between products and energy yields, but torrefaction is much closer to commercial realisation due to technological advancements needed for large scale HTP systems.