Environmental Engineering Reference
In-Depth Information
The big idea to clean up coal is the capture and storage underground
of carbon dioxide, a process known as CCS. This technology is especially
applicable to coal-fired power plants - much more so than gas or oil-fired
plants. The considerable extra expense of CCS technology is easier to jus-
tify in the case of coal-fired plants, because they emit a high proportion of
CO
2
in their flue gases. There could also be commercial spin-offs. Surplus
CO
2
could be used to encourage cultivation in closed greenhouses where
plants re-absorb the CO
2
, to enhance oil recovery - one potential nega-
tive spin-off that environmentalists are rightly concerned about - and,
depending on the CCS process, to produce hydrogen for use in vehicle
fuel cells.
More efficient coal
The extra cost of CCS lies not only in the technology, but also in the
power needed to run the technology, which effectively lowers the output
of the generating plant. In fact, the industry has been working for some
time to improve efficiency in power generation, in particular in the pul-
verization of coal so that it burns more rapidly and efficiently and using
“supercritical” and “ultra-supercritical” coal combustion technology. This
technology heats and pressurizes water above its “critical point” (at which
water is still separable into steam), produces a homogenized mix (not only
very hot, but under such high pressure that separate steam bubbles cannot
form) and passes this expanding mix through to rotate a turbine and so
generate electricity.
The goal is to raise efficiency ratios - the output of electricity (meas-
ured in kilowatt hours) as a proportion of the input of primary energy
(measured in joules). The aim is to lift these ratios into the forty to fifty
percent range (and maybe one day above fifty percent), up from the thirty
percent standard of, for example, the older Chinese generating plants, or
the mid-thirties percentage range typefying much UK and US electric-
ity generation. Every percentage-point improvement in generating effi-
ciency equates to a two to three percentage-point reduction in emissions
(because inefficient generators emit disproportionately more CO
2
per unit
of energy input than efficient ones). But the particular relevance of higher
energy conversion efficiency to CCS is that it helps offset the CCS cost
penalty in lost electricity output.
