Biomass reforming in hot compressed water (T = 230-700°C, pressure high enough to keep water in the liquid/supercritical phase) can convert very wet streams to a gas5,36 without paying a huge energetic penalty for water evaporation. To achieve this, heat exchange between the reactor effluent and the feed stream is essential which requires operation at high pressures.54 Figure 20.7 shows a

conceptual flow sheet of such a process. The efficiency of the heat exchanger is high leading to a feed stream outlet temperature of only 100-150°C below the reactor outlet stream.55 Make-up heat for the reactor can be delivered by e. g. burning of a part of the product gas or exothermic reaction heat in case of methanation. Further promises of the technology are: (i) the product gas is available at high pressure (>200 bar) and thus, for its application, expensive gas compression can be avoided, (ii) the product gas is clean; minerals, metals, and the undesired gases like CO2, H2S, and NH3 (which have a high solubility in compressed water) remain in the water phase and can thus be separated and recovered, (iii) the product gas is not diluted with inert gas, (iv) sequestration of (pure) CO2 seems readily possible.

These promises however go together with a series of problems that need to be solved in the process development. Pumping of biomass slurries to pressures of up to 300 bar is a challenge. The high temperatures and pressures involved put serious demands on the construction materials to be used, especially because corrosion problems are expected. Here separation of functionalities might be a solution: one material to withstand the pressure and another one for the temperature. Heat exchange between the reactor feed and effluent is required to make the process efficient, but heating of biomass slurry is likely to cause fouling and plugging as the biomass starts to decompose already around 200°C. Catalysts, if employed, need to operate under severe and fouling conditions. However, hot compressed water is a good solvent for most organic chemicals and thus especially useful to keep coke precursors dissolved. Ash deposits will cause problems, and an effective ash removal system must therefore be part of the process. At the time of writing several pilot plants are in operation to facilitate the process development. These pilot plants are still moderate in size: maximally 100 kg/hr wet feedstock.

Without catalysis the process suffers from incomplete conversion and an uncontrollable gas distribution.5 Catalysis research for reforming in hot compressed water is discussed below for low (230-400°C) and high (400-700°C) temperature separately. Reviews on reforming of biomass in hot compressed water are those by Matsumura.5 Van Rossum,36 Elliott,55 Peterson,56 Kruze and co-workers.57