Following
Copenhagen, many nations will be gearing up to significantly reduce
their greenhouse gas emissions over the coming decades. With
approximately one-quarter of worldwide annual GHG emissions coming from
electricity and heat generation, this will mean that – in order
for coal and gas to stay in a picture where levels of atmospheric CO2
are stabilized at or below 450 ppm –the carbon emissions of such
plants must somehow be slashed.
Typically, “clean” coal or gas refers to a reduction of CO2 emissions
by at least 90-95%. Technically speaking, what options are
available for reaching or approaching this benchmark?
More Efficient Plant
Since combustion of 1 tonne of coal produces (on average) about 2
tonnes of CO2 (for gas, the figure is about 1:1 on a mass basis), the
power producer can save both on fuel costs and on emissions by getting
more Kwh per tonne, i.e., by increasing the thermal efficiency of the
burn.
Coal plants vary widely in efficiency. While the worldwide
average efficiency is about 30%, this figure accounts
for a wide range, from more than half-century old Pulverised Coal
plant running in the low 20s to new super- and ultra-supercritical
plant and IGCC units whose efficiencies can exceed 50%.
Simple open-cycle gas plants are usually more efficient than similarly
configured coal units, with new-built units running at about 40%.
Building more efficient plant usually takes the form of : raising the
boiler temperature to levels above 374 degrees C (‘supercritical’
boilers); or “combining cycles” by using the waste heat
from one turbine assembly to drive another turbine.
Combined cycle technology exists for both gasified coal (Integrated
Gasification Combined Cycle) and standard gas (Gas Turbine Combined
Cycle), and in the future such assemblies could have efficiencies
possibly nearing 70%, although this may be approaching thermodynamic
limitations.
While efficient plant will have lower fuel costs, the capital and
operations and maintenance costs – from special alloys needed to build
supercritical boilers, for instance, or the maintenance needed for
specialized turbines – will tend to be much higher.
Carbon Capture and Storage
Beyond increasing efficiency, it is technically possible to abate stack
CO2 emissions in order to get a power plant below the 5-10%
net-CO2 “clean” limbo bar. The aggregate process,
involving separation of CO2 from the emissions stream, capture,
transport and long-term storage, is called carbon capture and storage
(CCS).
While CCS is at this point nowhere applied at a commercial scale to
power production, it is expected that higher future carbon prices (the
estimates range from $50 to hundreds of dollars per tonne) would make
CCS a commercially viable proposition. In the meantime,
subsidization has been ongoing, if anemic, at the national level and
some governments – including the UK – are mandating that all future
coal build be “CCS ready”.
Current CCS projects are limited almost exclusively to coal, as the
carbon emissions of gas are low enough to make CCS for such plant
unprofitable under likely near-term carbon price regimes.
Essentially, the capturing at the plant is done in one of three ways –
post-combustion, pre-combustion, and oxy-fueled. In
post-combustion CCS, the effluent gas from the coal burn is cleaned of
sulfur and nitrogen oxides then passed through a series of catalytic
and/or non-catalytic scrubbing agents which chemically trap the CO2,
which is then released as the catalytic agent is “regenerated”,
compressed, injected into storage tanks or a pipeline , and transported
to a storage site.
With oxy-fueling and pre-combustion, the issue of nitrogen oxide
contamination is sidestepped by burning the coal in pure oxygen
(oxy-fueling) or by gasifying the coal (pre-combustion), which yields
carbon monoxide and water, which are then treated to produce heat,
hydrogen gas, and carbon dioxide. In either case, a purer stream
of CO2 is produced for separation, compression, injection and eventual
storage.
Because CCS involves extra electricity and heat expenditure - to
separate and pump pure oxygen , regenerate catalysts, run
compressors, and so on – it somewhat counters the effects of other
clean combustion modes by introducing an efficiency penalty of between
5 and 15 percent. Aside from expense, there are other major
pieces of the CCS puzzle that need to be fitted into place – including
the financing and construction of pipeline networks, certification of
the long-term safety and security of underground storage sites,
assigning of long-term risk and monitoring responsibilities, and
obtaining of land and water use permits, where necessary.
Nevertheless, CCS is the only technology out there right now which can
reliably deliver lower than 10% CO2 “clean” fossil fuel
emissions. With countries such as China building a
new coal plant every week, the US sitting on probably at least a
century of reserves, and even the EU starting to look anew at its
still-considerable lignite and brown coal reserves, it seems likely
that fossil fuels will not go so "quietly into that good
night". If so, it hopefully can be
insured that, at the very least, fossil fuel stations will be built
from here on with at least a firm technical and planning inclination
toward CCS build or retrofit.
If you have more questions about carbon limiting technologies, please contact Energy Edge’s Karl Schultz today.
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