S. R.A. KERSTEN and D. KNEZEVIC, University of Twente, The Netherlands and R. H. VENDERBOSCH, BTG Biomass Technology Group B. V., The Netherlands and University of Twente, The Netherlands

Abstract: The topic of this chapter is hydrothermal conversion of biomass, a thermo-chemical technique especially suitable for conversion of wet biomass streams. The chapter deals with the process chemistry and the product distribution with special attention to de-oxygenation reactions and char formation. Furthermore, the main characteristics of the reaction products are discussed. The chapter shows typical process layouts and gives a brief historical overview of the research and the status of the most important industrial and laboratory scale activities. Finally, a critical view on the status of the technology is offered.

Key words: hydrothermal conversion, liquefaction of biomass, hot pressurized water, deoxygenation, char formation.

18.1 Introduction

Hydrothermal conversion (short form ‘HTC’) is a thermo-chemical conversion technique in which sub — or super-critical water is used as a reaction medium and/ or as a solvent. Although principally all biomass can be used as feedstock, wet (biomass and waste) streams are the obvious choice of feeds from an energetic point of view. The proposed operating regime is broad and ranges from 250°C to 450°C and from 80 to 300 bar, and depends on the type of products it is aimed at. HTC can be used for gasification and liquefaction (see Fig. 18.1), catalytically and non-catalytically. By using (noble) metal catalysts, complete conversion of biomass to methane-rich gas can be reached under hydrothermal conditions. At more severe temperatures (500-700°C), hydrogen-rich gas can also be obtained. Gasification in hot compressed water is discussed in Chapter 20 of this book.

In this present chapter, we will focus on hydrothermal liquefaction (short form ‘HTL’), the conversion of wet streams into condensed products. Typically, HTL is carried out in sub-critical water (temperature < 374°C). Under these conditions, biomass is converted into various components, which, upon cooling to ambient conditions, constitute three different phases: an aqueous phase (comprising water plus dissolved organics), a hydrophobic phase and a gas phase. Extraction of the hydrophobic reaction product results in a solvent-soluble (oil) and a solvent-insoluble part. Acetone is often used as a solvent. The solvent-insoluble product has a char-like appearance

and is solid at room temperature. It is a direct remainder of the feedstock (char formed in the pyrolysis reactions) and a product of secondary reactions of liquid products.1,2 The hydrophobic product has a considerably lower oxygen content (typically 10-30 wt.%) and, consequently, a higher heating value than the feedstock. A direct application of HTL oil (short form ‘HLO’) and solids is as a fuel. In this process option, HTL yields hydrophobic organic products that are easy to separate from the water phase and can be fed as fuel in boilers, furnaces or gasifiers.3,4 After fractionation by suitable solvents, the solvent-soluble fraction (oil) is considered for upgrading to transportation fuel precursors, for example by catalytic hydro-deoxygenation.5-8

For detailed overviews on HTL, the reader can refer to Moffatt and Overend,8 Bouvier et al.,9 Stevens,10 Solantausta et al.,11 Venderbosch et al.12 and Peterson et al.13

In this chapter, the process layout, chemistry, product characteristics and product distribution are further detailed. Process development and demonstration activities are described as well as the current research focus. The chapter ends with conclusion and future prospects.