> Biomass gasification in supercritical water
> 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 ˝ 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)
> Reference projects
- 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
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