Thermal treatment of aqueous streams loaded with organics can be efficiently performed at pressures and temperatures above the critical data for water (Pc = 22.1 MPa, Tc = 374 °C). Two applications are under investigation using supercritical water (SCW) as solvent and reactant: supercritical water oxidation (SCWO) and supercritical water gasification (SCWG).
SCWO is typically operated at 25-35 MPa and 600-900 °C, because water, oxygen (or air), CO2 and most of the organics form a single fluid phase with rapid oxidation kinetics. Thus, SCWO can be processed with high space-time yield and in some cases self-sustaining.
Moreover, expensive off-gas treatment is prevented because NOx formation is suppressed. Other heteroatoms form acids like HCl, H2SO4 and H3PO4 or their corresponding salts. However, acids may lead to corrosion, formation or presence of salts to plugs.
To avoid these problems a transpiring wall reactor (TWR) has been developed and installed. Results of SCWO of different industrial effluents are very promising. Destruction of the organic waste compounds was close to 100 %, even for effluents containing solids and salts up to 5%wt., each.
In accompanied studies material tests have been performed. Long time runs clearly indicate that alloy 625 is most suited to withstand the aggressive environment at temperatures higher than about 500 °C. A corrosion mechanism has been proposed.
The SCWG process of biomass is performed under SCW conditions. The aim of this work is to study the conversion of biomass (e.g.) to fuel gas with high energetic value. R&D is focused on process optimization particularly with respect to energy efficiency as well as applicability to extended feedstocks even with high amounts of solids. At SCW conditions organic matter reacts with water to form a hydrogen containing gas. The feed carbon is converted preferentially to CO2, which can be separated by e.g. stripping, and to methane. While the organic carbon is oxidized to CO2, water is reduced to from hydrogen, e.g. for glucose:
C6H12O6 + 6 H2O → 6 CO2 + 12 H2
In basic studies of the biomass gasification, the main reaction pathways were identified. In addition the influence of ingredients of biomass and additives / catalysts was investigated. Here the influence of salts is of special interest, because biomass with an high water content, which is most suitable for the SCWG process, usually contain high salt contents. The changes in selectivity of different reaction pathways observed, opens the possibility to manipulate the chemistry of biomass gasification in supercritical water. Therefore high gas yields with different feedstock of various compositions can be achieved.
SCWG process is performed using efficient heat exchangers at SCW conditions.
Compression work is low since non compressible water slurry is pressurized. The reaction of the organic substances with water proceeds fast and completely, thus with high space-time yield. At about 600 °C and 25 MPa high gasification yield can be achieved. Advantageously, formation of CO, tar and char is low enhancing the efficiency of the SCWG process.
Consequently, based on lab scale plants, a 100 kg/h SCWG plant called Verena has been installed and is operated since few years. A high thermal efficiency of about 80% for diluted educt streams (only 5 wt% OM) has been measured. Experimental results with the Verena plant confirmed the production of a hydrogen rich gas and the high thermal efficiency of the process.