Hydrothermal Gasification of Biomass - Biomass Gasification, Pyrolysis and Torrefaction - page 350

9.8.2.1 Contact Avoidance
The following are some innovative options that may be used to reduce con-
tact between corroding species and the reactor wall:
A transpiring wall on which water constantly washes down, preventing
any corroding material's contact with the wall surface.
A centrifugal motion created in the reactor to keep lighter reacting fluids
away from the wall.
In a fluidized bed, neutralizing or retaining of the corrosive species by
the fluidized particles.
9.8.2.2 Corrosion-Resistant Barriers
Corrosion-resistant liners are used inside the reactor to protect the vessel
wall. These are required to withstand the reactor's high temperature but not
its high pressure. Titanium is corrosion resistant, but in large quantities, such
as required for the reactor shell, it is not recommended because of the risk of
fire if it comes in contact with high concentrations of oxidant, particularly
when pure oxygen is used in an SCWO. In much smaller quantities, titanium
can be as a liner; alternatively, some type of sacrificial liner can be used.
9.8.2.3 Process Adjustments
Changes in process conditions may reduce or even avoid corrosion in some
cases, but they may not be practical in many situations. For example, if the cor-
rosion is as a result of acidic reaction, the addition of a base to the feed may
preneutralize the reactant. Since most of the corrosion occurs just below critical
temperature, the water without the feed may be preheated to a sufficiently high
temperature such that on mixing with the cold feed the reaction zone quickly
reaches the design reactor's temperature; then the biomass may be fed directly
into the reactor to reduce the corrosion in the feed preheat section.
9.8.2.4 Corrosion-Resistant Materials
If corrosion cannot be avoided altogether, it can be reduced by the use of
highly corrosion-resistant materials. Choosing one of these as the primary
construction material in an SCWO system is the simplest and most basic
means of corrosion control. The following materials have been tried in super-
critical environments. Of course, no single material can meet all design
requirements, so some optimization is required. The materials listed are
arranged in the order of least-to-most corrosion resistant.
Stainless steel
Nickel-based alloys
Titanium
Tantalum
Niobium
Ceramics