Hydrogen production by supercritical water gasification of biomass. Phase 1 -- Technical and business feasibility study, technical progress report Page: 36 of 124

                                
GENERAL
ATOMICS
3264901 N/C
although the maximum duration of approximately 8 hr during SCWO testing was insufficient to
ascertain long-term viability. It should to be demonstrated that the double pipe configuration is
suitable for very thick slurries, as the required pipe length could conceivably result in
undesirably high pressure drops.
A common concern when carrying out heat exchange with high salts-content feed is
precipitation of inverse solubility salts, i.e., salts whose solubility decreases with increasing
temperature. The chief example is CaSO4, although other alkaline earth salts can behave in a
similar manner. Some wet air oxidation plants must shut down on a weekly or biweekly basis to
flush components with dilute acid to remove scale. Fortunately, the sewage sludge
pretreatment processes described above will likely remove most of the inverse solubility salts
prior to their introduction into the gasifier and heat exchanger, minimizing this concern.
However, this needs to be demonstrated during pilot-scale testing.
A second concern is the possible formation of organic chars. The relatively slow feed
heatup that occurs within the heat exchanger may lead to organic tar formation. Xu, et al.,
(1996) report that swirling feed introduction in the heatup zone of their reactor is useful in
mitigating tar formation and extending catalyst life. Thus rapid heatup of the feedstock appears
to be important in order to minimize condensation reactions. For some feedstocks, char
formation may be mitigated by the addition of a small quantity of oxidant to the stream being
heated.
Optimum materials selection also requires investigation. Titanium has proven to be
highly useful for corrosion resistance in the SCWO process, even at temperatures hundreds of
degrees Celsius above its maximum rating in the American Society of Mechanical Engineers
(ASME) Boiler and Pressure Vessel Code. It has also proven to have exceptional corrosion
resistance in commercial wet air oxidation systems, although the addition of small amounts of
oxygen to the feed stream is sometimes needed to allow maintenance of the passivated oxide
layer on the metal. Furthermore, the stability of titanium decreases as temperatures increase
above about 350 C.
SCWG Gas-fired Trim Heater. Development requirements here are essentially the same as for
the heat recovery heat exchanger. Conditions are more extreme in some respects, however,
with some high pressure piping being exposed to flame or near-flame temperatures. As such,
the trim heater operates with a higher temperature feed stream and presents a further
opportunity for organic or inorganic scaling. At trim heater temperatures, even highly soluble
alkaline salts such as NaCI and Na2SO4 may precipitate, potentially degrading heater
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