Supercritical Fluids as a Tool for Green Energy and Chemicals

Introduction

Large amounts of waste biomass produced offer a great opportunity to convert this volume of plant material and animal waste to a huge amount of energy. The conversion of biomass to biofuels and biobased chemicals has attracted a lot of attention recently, largely due to the environmental and socio-economic problems associated with the use of fossil fuels. In recent time, many novel technologies have been introduced for conversion of biomass to energy and chemicals. Biomass can be converted to energy by biochemical and thermochemical processes. The two main thermochemical conversion processes to produce biofuel from biomass are the pyrolysis process and the hydrothermal process as promising technologies for the conversion of biomass into biofuels and biobased chemicals. In recent years, hydrothermal energy was accepted as a possible energy resource with a high potential already in the beginning of the previous century. Hydrothermal conversion of biomass has consequently attracted considerable attention. This chapter will therefore focus on the waste biomass conversion for production of biofuels (liquid or gaseous) and biochemicals (furfurals, organic acids, aldehydes, ketones) by applying sub- and supercritical fluids at pressures up to 1000 bar. Operating at high pressures may result in processes that require much lower amounts of energy and maybe also in new processes which are capable of performing new types of reactions that produce completely new bio-based products.

The production of value-added products from waste biomass, such as waste from agriculture, food or forestry industry, will be mostly considered. A topic of the intense research is to reduce the operating temperature and energy requirements by processing biomass with water at much higher pressures or adding supercritical CO₂ to the reaction mixture. Nevertheless, biomass conversion processes at high-temperature and pressures exceeding 300 bar are, for now, a completely new aspect of high-pressure technology for biomass conversion and literature regarding the reactions taking place in such systems practically does not exist. There are also several advantages of supercritical fluid processing under “real” high pressure like new product formulations and economic production of high value products due to the fact that the solubility of several dense gases in low soluble substances increases with increasing pressure and temperature. Most of the commercial supercritical water processes today are limited to pressures, where the complete solubility of water and CO₂ is not achievable. By elevating pressure, the total miscibility of water and CO₂ is already achieved at considerably lower temperatures. Another possibility is lowering the energy barrier by the addition of catalysts. Such treatment of biomass would be a completely new process. However, the data on the mechanisms and kinetics of reactions are scarce. Additionally, the total yields of the final products are practically unknown for these unconventionally high-pressures and temperatures for the multi-component systems.

Considering the above mentioned facts, it is highly important to investigate how of water and CO₂ in such a system would affect the biomass conversion reactions and what products would form, how different ratios and conditions affect the conversion and if it is possible to perform the reactions that normally occur above critical point at milder conditions. This knowledge will open completely new possibilities in biomass conversion, which could also lead to new types of reactions and products. Fundamental data obtained from such research could therefore revolutionize the concept of biomass conversion facilities or could open routes to completely new types of process intensification. Limitations of processes operating at the mentioned high temperatures and unconventionally high pressures are their investment costs that are significantly higher compared to ambient pressure technologies. Nevertheless, many recent studies have suggested that classical large-scale reactors needed for biomass conversion are maybe not necessary for processes involving supercritical water as reaction medium. It has been proven, that reaction kinetics of near-critical water and supercritical water with organic compounds are extremely fast and that long contact times should therefore be avoided.