Introduction
Process
Status of the technology
Reference projects
Links
Contact
Introduction
Supercritical water (SCW) is obtained at pressure above 221 bar
and temperatures above 374 oC. By treatment of biomass in
supercritical water-but 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 gas is
very rich in hydrogen.
Process
Chemistry
Characteristic of the SCW-organics interactions is
a gradually changing involvement of water with the temperature. With
temperature increasing to 600 ° water
becomes a strong oxidant and results in complete desintegration 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 fro glucose can be written as: 2 C6H2O6 +
7 H2O => 9 CO2 + 2 CH4 + CO + 15 H2 DH
= 1.32 MJ/kg
Short process description
The SCW 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 300 bar. A residence time of œ 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 efficiencies. 
The two-phase product stream is separated in a high-pressure
gas-liquid separator (T = 25 - 300 oC). Due to these conditions
significant part of the CO2 remains in the water phase.
Possible
contaminants like H2S, NH3 and HCl are even more likely
to be captured in the water phase due to their higher solubility, and in
fact in-situ gas cleaning is obtained. The gas stream 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 internal heating purposes. Based on this reaction
stoichiometry the following dry gas composition is obtained:
| H2 | 56 v% | |
| CO | 4 v% | |
| CO2 | 33 v% | |
| CH4 | 7 v% |
Feedstock
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.
Renewable biomass streams can be a.o. bagasse, waterhyacinth, algues or waste
streams like sewage sludge, garden-fruit waste, vinasse (rest-product ethanol
production), trester (rest product wine production), waste water streams
etc.
Application of the product gas
The primary gas produced by the SCW process differs significantly from most
other biomass gasifiers:
- gas is produced at very high pressure
- hydrogen content is high
- no dilution by nitrogen
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. From the work of Antal it seems that complete carbon conversion is achieved after relative short residence time, and significant amounts of CO are found, whereas methane content is still low. For long residence times gas equilibrium has been established and CO is almost completely absent, but methane content is significantly increased. Based on these process characteristics three main applications of the gas are identified:
- Hydrogen production (maximize H2)
- Syn-gas production (minimize CH4)
- Substitute natural gas (minimize CO)
The syn-gas can be used for different synthesis processes for the production of renewable transportation fuels like Fischer-Tropsch, Methanol, DME etc. Schematically, the applications are depicted in Fig. 2.2.
Status of the
technology
The gasification of biomass and biowaste in
supercritical water is a rather novel process. Significant R&D work
will be required prior to implementation and commercialisation. Currently,
the focus is on experimental research in a continuous flow unit (10
- 30 L/hr), see photos. (nieuwe fotos worden nog gemaakt)
- Technical feasibility EET-Kiem
- Development of a fluidized bed reactor for the conversion of biomass in supercritical water, 2000 2003, financially supported by NEDO Japan
- Biomass and waste conversion in supercritical water for the production of renewable hydrogen, 2001 2005, financially supported by EC
- Conversion of wine residues in supercritical water for the production of a hydrogen rich fuel gas, 2001-2003, financial support EC
Links
Project SuperH2: Biomass and Waste Conversion
in Supercritical Water for the Production of Renewable Hydrogen
Contact
Dr.
Ir. L. van de Beld
Tel +31 53 486 22 88
Up | Technologies