Flue gas from
coal-fired electric power plants is the main villain in global warming.Cars are not nearly as bad, so vehicle
emission standards would have only a small effect on global climate
change.The real problem is coal, and
our demand for electricity.
The U.S. emitted
6,049 million metric tons of CO2 in 2004, but by now we probably have
yielded the gold medal to China.See
the worldwide list here.
According to the
IPCC Report on Carbon Capture (September 2005):
... the power and
industry sectors combined dominate current global CO2 emissions, accounting for
about 60% of total CO2 emissions.Future
projections indicate that the share of these sectoral emissions will decline to
around 50% of global CO2 emissions by 2050 (IEA, 2002). The CO2 emissions in
these sectors are generated by boilers and furnaces burning fossil fuels and
are typically emitted from large exhaust stacks. ... The largest amount of CO2
emitted from large stationary sources originates from fossil fuel combustion
for power generation, with an average annual emission of 3.9 MtCO2 per source.Substantial amounts of CO2 arise in the oil
and gas processing industries while cement production is the largest emitter
from the industrial sector. ... The ranges of the technical capture potential
relative to total CO2 emissions are 9–12% (or 2.6–4.9 GtCO2) by 2020 and 21–
45% (or 4.7–37.5 GtCO2) by 2050.
indispensable for our electricity.In
the United States, for example, here are the
figures in megawatt hours for the various forms of utility electricity
production in 2006.
unpopular, and hydro is at its limit, with few damsites still available.Non-hydro renewables (wind, solar, and all others
combined) are too small to matter.That
leaves natural gas and coal, both of which are heavy emitters of carbon
So, given our
dependence on electricity and the huge streams of hot and dirty flue gas
pouring into the atmosphere from coal combustion, how do we treat the flue gas
to extract the carbon dioxide?That is
called “post-combustion carbon capture.”
Methods of Post-Combustion Carbon Capture
capture methods are presently known: membranes, compression, and amine
scrubbing.All are unsatisfactory for
dealing with large, hot, and dirty gaseous emission streams like flue gas from
coal-fired power plants.
The National Energy
Technology Laboratory run by the U.S. Department of Energy summarizes
the problem of carbon capture from flue gas:
“The low pressure and dilute concentration
dictate a high actual volume of gas to be treated.Trace impurities in the flue gas tend to reduce
the effectiveness of the CO2 adsorbing processes.Compressing captured CO2 from atmospheric pressure
to pipeline pressure (1,200 - 2,000 pounds per square inch (psi)) represents a
large parasitic load.”
Translation:The presence of nitrogen ballast (N2)
in the flue gas (about three-quarters of its volume) means that carbon dioxide
is protected from compression or chemical contact by a cushion of nitrogen
molecules.NOx and SOx (nitrogen oxides
and sulfur oxides) become heat-stable salts and corrosive acids during amine
recovery, and along with the fine glassy dust (fly ash) of the flue gas these
clog up and damage the equipment.The
energy required for compression is prohibitively wasteful.
Capture by Membrane Separation.
Membranes may work
in laboratory scale experiments, but could not possibly work on flue gas.The immense volumes to be filtered, and the
pore-clogging fly ash and mercury in the flue gas, weigh heavily against
membrane carbon capture.
Capture by Cryogenic Distillation.
capture by compression.Flue gas is hot
and very dirty, and the carbon dioxide in it is shielded from liquefaction by
the presence of a large nitrogen ballast.Cryogenic distillation captures carbon by liquefaction of flue gas and separates
out NOx and SOx by fractional distillation.Nitrogen, however, is very hard to liquefy and compressing it wastes
energy which should go into compressing NOx, SOx, and CO2.The small partial pressure of NOx and SOx in
flue gas (which are very much less than 1% of the volume) and the small partial
pressure of CO2 (10 -15%) are both due to the high nitrogen ballast
Capture by Amine Scrubbing.
That leaves the
third alternative, amine scrubbing, which is the only method presently being
considered seriously for carbon capture from flue gas. Aqueous amine sorbents
have been successfully used to clean carbon dioxide and hydrogen sulfide from
natural gas and industrial waste streams, but the hope that this proven
technology can be extended to flue gas runs into serious difficulties.
1.The science is not mature, and there is no
time for study.
exact mechanism describing the chemical reaction of CO2with amine regents [sic] under the conditions
typical of a CO2 capture plant is the subject of much debate.”International Test Centre for CO2
Capture (ITC) (Report of
May 2005 §4.1.1).
The science is not mature and there
are major unsolved problems.The
situation is an emergency.Academic
research, at its leisurely pace, and with its usual small and obscure results,
cannot be expected to solve the problem of carbon capture from flue gas.
scrubbing is very expensive.
of amine scrubbing to capture carbon dioxide, then compressing it to
pipeline pressure, is about $2000 per kilowatt, which makes it prohibitively
expensive even if it were feasible for high volumes of flue gas, which it is
It is hoped that the operating cost of
amine scrubbing might come
down to about $30 per ton of CO2 captured and compressed, at
Canadian energy costs. For the average source emitting 3.9 million tons per
year, the best case price tag for amine scrubbing is an additional $120
million.That’s assuming it will work,
which it won’t.
The theoretical maximum loading
capacity of amines is a mole of amine for one mole of carbon dioxide.Actual performance is even lower, so you need
even more amines.In addition, there is
the replacement cost of amines which cannot be regenerated, which is an especially
serious problem for flue gas scrubbing (see below) due to the presence of
strong acids from NOx and SOx.So
capturing the carbon dioxide in a typically enormous stream of flue gas (a
million cubic feet per minute) would require an equally enormous purchase of
Such a prospective bonanza to chemical
companies from the plight of the planet should be taken into account in
evaluating the pressures on researchers and policymakers, and the confident
claims of the amine scrubbing proponents.
a serious problem.
Corrosion of the common materials of
construction, such as carbon steel, in an amine scrubbing facility is a serious
problem known from years of experience in natural gas sweetening.Carbon steel corrodes in the amine solution.
Flue gas presents an even more challenging
corrosion problem because it is hot and contains oxygen, unlike natural
gas.The rate of corrosion of carbon
steel is higher in hot amine solutions.Flue gas also contains a significant fraction (~ 4%) of oxygen.The corrosion rate has been found to increase
by 40% when oxygen content is increased from 0% to 10%.
salts and fly ash sludge block the process.
Natural gas does not contain
combustion products like NOx and SOx and fly ash.NOx and SOx turn into nitric acid and
sulfuric acid when combined with water.Alkanoamines react with strong acids (sulfuric acid and nitric acid are
very strong acids) to form heat stable salts, which effectively take the amines
out of use, so more need to be added.During
the process of regenerating the amine for re-use in the CO2
stripper, a crust of heat-stable salts covers the heat exchange surfaces,
reducing efficiency.A dilute sludge of
fly ash and precipitated salts gums up everything.
If the aqueous amine approach from
natural gas production were to be used for flue gas from coal, the additional
acids, heat-stable salts, and fly ash sludge would be operating difficulties in
addition to the already known corrosion problems and would drive up the
operating costs considerably.
5.You have an
even bigger problem with scrubber wastewater.
Amine scrubbing converts an air
pollution problem into a water pollution problem.Large amounts of solution must be sprayed
into the flue gas in order to get around the nitrogen ballast.Carbon dioxide molecules are in low
concentration, like needles in a haystack, so to contact all of them a lot of
amine solution must be used. What comes out is a voluminous stream of toxic
wastewater.Fine solids suspended in
this wastewater require a long time to settle, and there is also the dilute
sulfuric and nitric acid from the NOx and SOx in the flue gas.Storing the wastewater requires lagoons or
tanks, which are a waste of valuable space.
From the foregoing discussion of amine
scrubbing, membranes, and compression, it should be clear that there is in fact
no present technologically feasible solution to carbon dioxide emissions from
coal-fired power plants.
None of the presently known carbon
dioxide capture methods is feasible unless the flue gas is first scrubbed of
its particulates, NOx and SOx, cooled, and stripped of its nitrogen ballast.
That is what we are working on at
Vorsana.The solution is actually
very simple and inexpensive: highly turbulent scrubbing and radial counterflow
in a rotating device.
An Alternative Method of Carbon Capture: Centrifugal
Stripping the nitrogen ballast amounts
to carbon capture, because what is left once the nitrogen and water vapor comes
out is a concentrated stream of carbon dioxide, which can then be economically
scrubbed of its particulates, NOx and SOx and compressed or disposed of by
other means.Stripping the nitrogen
ballast can be done by centrifugal gas separation, although not by presently
A useful illustration of centrifugal
separation is the cream separator, which spins milk so it separates into cream
and whey (skim milk).The cream, having
a lower density than the whey, concentrates at the axis of rotation.If the raw milk were left in a glass without
doing anything, the cream would rise to the top.The cream separator just exaggerates the
acceleration and makes the separation happen faster.Centripetal acceleration caused by rotation
of the milk in a drum squeezes the low density fractions like cream into the
center, just like the acceleration due to gravity makes cream rise to the
top.Heavy fractions, like water,
displace the light fractions in rotation.
Density differences exist for the
fractions in flue gas as well.It turns
out that the bad stuff – carbon dioxide, NOx and SOx, mercury, and fly ash – is
the heavy fractions.The good stuff –
nitrogen and water vapor – is the light fractions.So, theoretically at least, spinning the flue
gas fast enough should develop sufficient centripetal acceleration to separate
the good stuff from the bad stuff.
What’s in coal-fired power plant flue gas?
Nitrogen (N2) is a harmless
gas which constitutes 75% of the volume of flue gas from coal-fired power
plants.This is referred to as nitrogen
ballast.Nitrogen might be safely
discharged to the Earth’s atmosphere, which is already 78% nitrogen.Water vapor (5%) produced by combustion is
another harmless light fraction.That’s
the white plume you see coming out of the smokestack.It is also why your car tailpipe drips
water.So 80% of the volume of flue gas
is harmless and requires no treatment at all, other than separating it from the
carbon dioxide and other heavy fractions.
Once the nitrogen ballast and water
vapor have been stripped, the remaining one-fifth of flue gas can be scrubbed
to remove NOx, SOx, mercury, and fly ash using known methods or new ones.The toxic lagoon problem of known wet scrubbing
methodswill be minimized because less
scrubbing solution will be necessary to contact the NOx and SOx.And once the nitrogen ballast has been
stripped, there is no need for amine scrubbing.
The molar mass of nitrogen gas (N2)
is only 28 g/mol (grams per mole of gas); carbon dioxide (CO2) is
36% denser at 44 g/mol.Sulfur dioxide
is even denser (64 g/mol).Centrifugal
gas separators which might exploit this 36% density difference are of two
classes: mechanically driven and pressure driven.
Mechanically driven centrifugal gas separators.
Mechanically driven gas separators can
exploit gas density differences as low as 1.5% , far beyond the performance
required for flue gas separation.The
ultracentrifuge is a very delicately balanced cylinder rotating at very high
speed and generating very radial acceleration and high G force which radially
stratifies gases by density within the cylinder.Such rotating cylinder gas centrifuges are
used for separation of uranium isotopes to collect fissionable U-235 for bombs
or peaceful purposes.Because of their
extremely high rpm, gas centrifuge imbalances can cause catastrophic
accidents.The separation effects are
small for each device, so the output of one becomes the input of a second
device, and so on, in what is known as a cascade.
Adapting conventional gas centrifuges
of this type to flue gas would be impracticable.The huge volume of flue gas is an insuperable
obstacle, and the fly ash, water, mercury, etc. which would condense under
pressure might easily cause imbalances which would cause the centrifuge to
destroy the facility.
The Vorsana radial counterflow
device is a mechanically driven gas separator which operates on a different
principle to perform separation at the comfortable 36% density difference
between nitrogen and carbon dioxide, making it suitable for processing large
volumes of hot and dirty flue gas with minimal rotation speed.
Pressure driven centrifugal gas separators.
driven devices include inertial collectors (also known as cyclones), and vortex
tubes.Cyclones and vortex tubes are
axial counterflow devices, wherein flow goes in opposite directions and the
device is static.These have no moving
collectors, commonly known as cyclones, are used extensively to process gaseous
emission streams to remove large particles of dust.Tangential feed swirls downward along a tank
wall, then swirls upward to exhaust, meanwhile expelling most of its dust when
it makes the turn.Pressure drives the
flow, from compressing the feed or from sucking the exhaust, or both.The dust collects in a hopper at the bottom
of the tank.Because both the downward
and upward swirl are on the same axis of rotation, this is axial
counterflow.Cyclones, even cascaded,
are ineffective even for the relatively easy job of separating out >2.5
micron fly ash, which is relatively big dust.Nitrogen / carbon dioxide separation by cyclones has not been reported
and would appear to be impossible.
2. Vortex tubes.
driven centrifugal gas separation device without moving parts is the
Ranque-Hilsch vortex tube.The effect of
a vortex tube is to separate a high pressure mixed gas feed stream into a low
pressure cool light fraction stream, which comes out the feed end, and a low
pressure hot heavy fraction stream which comes out the other end of the
tube.Residence time in the vortex tube
is on the order of milliseconds – very short for doing effective separation by
density.There are a variety of opinions
about why thermal separation happens, but no consensus.
What is especially interesting about
the vortex tube, and what has provoked a spirited discussion, is that a thermal
separation happens without any work being put in, as in the thought experiment
which is known as “Maxwell’s Demon.”In
other words, disorder decreases without effort – an apparent violation of the
Second Law of Thermodynamics.
In operation, pressurized feed gas is
tangentially injected into one end of a static tube having both ends open.The feed gas swirls in a first vortex to the
other end, as in a cyclone.At the other
end is a conical central flow impedance.Hot gas exits the tube around the flow impedance.A recirculation flow rebounds from the other
end in a second vortex inside the first vortex and exits the feed end cooler
than the feed gas.Like the cyclone, the
vortex tube is an axial counterflow device.
Application of the vortex tube has
also been made to the separation of liquefied air into nitrogen and oxygen, to
removing condensible vapors from natural gas, and to improving sorbent mixing
for carbon dioxide scrubbing.The
presence of fly ash in flue gas, the high energy requirement for pressurizing
the feed, and the poor separation efficiency of vortex tubes due to their low
residence time would appear to foreclose application of the Ranque-Hilsch
vortex tube to the job of extracting nitrogen ballast from flue gas.
mechanically driven gas centrifuges and conventional pressure driven cyclones
and vortex tubes are inadequate for separating nitrogen from carbon dioxide. What is needed is a new approach.