Figure 18.4 presents a scheme of a typical HTL process. Prior to feeding into the process, biomass is pretreated to ensure that the feedstock has desired properties: rheological properties, water content, degree of fragmentation of biomass components, etc. Feeding biomass water slurries is a particular challenge due to the problems of biomass settling and filtering and blocking of the process lines, particularly for relatively high biomass/water ratios. Heating to the desired temperature in the range of 250-370°C is performed while water is kept in liquid phase by pressure regulation. HTL can convert very wet streams to a gas without paying a huge energetic penalty, if heat exchanging has taken place efficiently. For this, heat exchange between the reactor effluent and the feed stream is essential that requires operation at high pressures to avoid phase transition (see Chapter 20 of this book). The efficiency of the heat exchanger can be high leading to a feed stream outlet temperature of only 50-100°C below the reactor outlet stream. Make-up heat for the reactor has to be supplied externally. In most cases, tubular reactors have been used in continuous installations. Typically, residence times of 10-90 minutes have been applied. In most reported pilot HTC work, the residence time at an elevated temperature was significantly longer than the ca. five minutes for minimal char formation (see chemistry section), resulting in considerable secondary char formation. In pilot plants, the feed stream was heated externally or

by heat exchange with the reactor effluent. For both cases, it holds that the heating trajectory is already taking several minutes because of heat transfer limitations. The only way to heat biomass in less than five minutes is injecting the feed directly into a hot liquid that may have a negative influence on the energy efficiency in case of a very wet feedstock.

The water phase or oil phase can be recycled for use as a solvent and/or as a dilutant. Keeping the concentration of organics low will decrease the char yield. A low concentration of organics can be achieved by using a (very) diluted feedstock, back mixing or by recycling of water (or oil) over the reactor.

Upon cooling the reactor effluent of HTL, three different products, being also three different phases, are present: a hydrophobic organic phase, an aqueous phase with organic compounds dissolved in it and a gas phase, consisting mainly of CO2. Separating gases and the water phase is straightforward. The product gas is made available at a high pressure (> 200 bar), and thus, for its application, expensive gas compression can be avoided. No reported results were found on the separation of oil and char.