Supercritical water (SCW) exists at pressures higher than 22.05 MPa and temperatures above 374oC. By treatment of biomass in supercritical water (and in the absence of added oxidants) organics are converted into fuel gases and are easily separated from the water phase by cooling to ambient temperature. The produced high-pressure (HP) gas is rich in hydrogen.
Characteristic of the SCW-organics interactions is a gradually changing involvement of water with the temperature. With temperature increasing to 600°C water becomes a strong oxidant and results in complete disintegration of the substrate structure by transfer of oxygen from water to the carbon atoms of the substrate. As a result of the high density carbon is preferentially oxidized into CO2 but also low concentrations of CO are formed. The hydrogen atoms of water and of the substrate are set free and form H2. A typical overall reaction for glucose can be written as: 2 C6H12O6 + 7 H2O => 9 CO2 + 2 CH4 + CO + 15 H2 .
Short process description
The RSW (Reforming in Supercritical Water) process consists of a number of unit operation as feed pumping, heat exchanging, reactor, gas-liquid separators and if desired product upgrading. The reactor operating temperature is typically between 600 and 650 oC; the operating pressure is around 30 MPa. A residence time of up to 2 minutes is required to achieve complete carbon conversion, depending on the feedstock. Heat exchange between the inlet and outlet streams from the reactor is essential for the process to achieve high thermal efficiency.
The two-phase product stream is separated in a high-pressure gas-liquid separator (T = 25 - 300 oC), in which a significant part of the CO2 remains dissolved in the water phase.
Possible contaminants like H2S, NH3 and HCl are likely captured in the water phase due to their higher solubility, and in-situ gas cleaning is obtained. The gas from the HP separator contains mainly the H2, CO and CH4 and part of the CO2. In a low pressure separator a second gas stream is produced containing relative large amounts of CO2, but also some combustibles. This gas can e.g. be used for process heating purposes.
The SCW process is in particular suitable for the conversion of wet organic materials (moisture content 70-95%), which can be renewable or non-renewable. Potential types of biomass feedstock include bagasse, water hyacinth, algae or waste streams like sewage sludge, VGF (vegetable, garden and fruit) waste, vinasse (rest-product ethanol production), trester (residue of wine production), wastewater etc.
Application of the product gas
The primary gas produced by the RSW process differs significantly from the syngas that is produced in common thermal biomass gasifiers:
Gas is produced at high pressure
Hydrogen content is high
No dilution by nitrogen
High content of CO2
The produced gas is clean (no tar, or other contaminants in high pressure gas even if produced in the process). The gas always contains high amounts of hydrogen; the amounts of CO and CH4 depend on the operating conditions. It is observed that complete carbon conversion is achieved after relative short residence time, and significant amounts of CO are found, whereas the content of methane is still low. For long residence times gas equilibrium has been established, CO is almost completely absent, and the methane content significantly increased. Based on these process characteristics three main applications of the gas are identified:
Hydrogen production (maximise H2)
Synthesis gas production (minimise CH4)
Substitute natural gas (minimise CO)
The synthesis gas (or just “syngas”) can be used for different synthesis processes for the production of renewable transportation fuels like Fischer-Tropsch diesel, Methanol, Dimethyl ether (DME, also know as wood ether) etc.
Status of the technology
The reforming of biomass and biological residues in supercritical water is a rather novel process. Significant R&D work will be required prior to implementation and commercialisation. The pictures show the continuous flow unit (10-30 litre/hour) installed at the BTG laboratory.