Sequestering Carbon Using Mass Quantities Of Small Scale Supertorrefaction
SEP 16, 2016 @ 10:11 AM
Many schemes proposed for carbon capture and sequestration (CCS) operate on
a massive scale. The visions are so big that testing them involves
multi-billion dollar investments and project timelines that run into
decades. As a result, there are only a tiny number of demonstrations and
none of them provide a full system proof that CO2 can ever be economically
separated, captured and permanently stored underground.
Frank Shu is a world renowned astrophysicistwho recently turned his
attention to the tightly linked challenge of producing plenty of energy for
human society while also actively reducing — and eventually controlling —
atmospheric CO2 concentrations. Dr. Shu is convinced that if humans can
destabilize the climate by dumping CO2 into the atmosphere, we can
also find and use ways to reverse the instability we’ve added by actively
removing CO2 from the atmosphere.
Shu knows that a large portion of human-produced CO2 comes out of hundreds
of millions, if not billions, of small combustion units. The overall rate
of production is massive. He has determined that removal might be most
effective if the necessary systems can be almost equally well-distributed,
with massive numbers of small-scale operations.
Shu’s invention is a patented process that he calls “supertorrefaction.” It
is a substantial improvement on a long established process known as
torrefaction, which is a method of creating a carbon-rich material — often
called char — by heating biomass to a temperature in the range of 200 ℃ to
280 ℃ in the absence of oxygen. The traditional torrefaction processes burn
some of the biomass to create the heat. They use the resulting combustion
gasses as the heat transfer medium. It can take several days to complete
the conversion of biomass into char.
Note: You probably recognize that torrefaction is a technical-sounding name
for the ancient process of producing charcoal.
Shu’s improvements to torrefaction involve using higher temperatures and
molten salts as the heat transfer fluid. Supertorrefaction (STR) submerges
wood chips or other forms of shredded biomass into a bath of molten salts —
NaOAc/KOAc for example — that is held at an average temperature of 450 ℃.
Direct contact in a fluid that has a volumetric heat transfer capacity that
is about 2000 times that of combustion gasses at a substantially higher
temperature turns the several-day-process of torrefaction into a 10 minute
per batch process. Using multiple chemical reactors each with porous
baskets to hold the biomass and a conveyor system, the STR process can be
turned into a continuous process with a steady throughput.
Shu established a company called Astron Solutions Corporation (ASC) and has
worked with a team of researchers from the University of California and
Academia Sinica (Taiwan) to test and create pilot-scale equipment
demonstrating the process. The machinery is designed to be transportable by
truck to places where there are abundant resources of biomass that is not
useful for other purposes.
One of the biggest disadvantages of biomass as a fuel source is that it is
not very energy dense because it contains water and a number of
non-combustible components. That makes it bulky and difficult to transport;
it is cheaper to move the STR machinery to the source of material than to
gather the material and move it to a much larger, centralized processing
In his ultimate clean energy vision (pg 19-23) the heat required to melt
the salt and maintain it at operating temperature will be provided by
molten salt reactors, either an advanced, two fluid breeder or a simplified
burner like those being developed by Terrestrial Energy and ThorCon.
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