Article published HYDROTHERMAL CONVERSION
Supercritical water destroys contaminated liquids
How can waste too wet for effective incineration be processed? A solution now exists: hydrothermal conversion using supercritical water. The CEA's new pilot system for radioactive organic liquids optimises this process and has begun to demonstrate its effectiveness on a vast array of effluents, especially those from the chemical and pharmaceutical industries.
Incineration is still used to
destroy most liquid organic waste
from industry and research. But
putting a liquid in a kiln is extremely
costly in terms of energy as
well as toxic for the environment. To
overcome these important societal
challenges, the CEA has developed a
system for processing toxic organic
solvents. It involves water said to be
«supercritical», involving specific
temperature and pressure conditions
(above 374oC and 221 bar).
«Contaminated liquids are soluble
in this fluid, the water acting as an
oxidising agent and waste acting
as the fuel», explains Bruno Fournel
of the CEA.
Already implemented on an industrial
scale, hydrothermal oxidation
allows for waste combustion at relatively
low temperatures. As a result,
it doesn't produce any nitrogen
or sulphur oxides (NOx/SOx),
as does traditional incineration.
What's more, since the mixture is
very homogenous, the oxidation
reaction is extremely rapid as well
as complete. Finally, this technique
does not give off toxic gases, as all
the oxidised species are kept in the
aqueous effluent.
Despite these advantages, this
procedure has so far been limited to
the treatment of waste containing
hydrogen, carbon, oxygen, and
nitrogen. Under such temperature
and pressure conditions salts tend
to form precipitates, which can clog
and corrode equipment. The CEA's
pilot facility at its Pierrelatte site is
able to work around this thanks to
its specially designed structure: the
external shell is resistant to pressure,
the internal shell to corrosion, and
the mixture is continually agitated
to prevent salt precipitation in the
waste material.
This breakthrough opens up new
avenues for the destruction of
wastes with significant percentages
of chlorine, mineral salts, or fluorine,
including solvents, nitrates, and the
like, as well as chemical or biological
agents, contaminated organic
liquids from pharmaceuticals, etc.
CEA researchers are also working on a
supercritical water reactor to enable
«biomass gasification». «Starting from wet biomass, which includes
effluents from the paper industry,
distilling, and even treatment plant
sludge, the goal is to produce heat
and a synthetic gas rich in hydrogen
that could be used as fuel.»
In 2006, the Chamber of Commerce
and Industry in the Drôme area of
France created an association called
Innovation Fluides Supercritiques
(IFS). It brings together the CEA and
half a dozen industrial and research
partners to encourage the transfer
of this type of technology to businesses.
