Re: New Coal Power Plants


G. R. L. Cowan wrote:
peterwezeman@xxxxxxxxxxx wrote:

I heard on the radio that utilities building new coal fired plants
generally prefer conventional steam plants rather than the newer design
where the coal is gasified and then burned in a gas turbine with steam
bottoming cycle. I find this somewhat surprising as the gasification
plants have a thermal efficiency of 45 percent versus 30-35 percent for
steam plants, and gasification plants have been operating successfully
in this country for over a decade. Are the costs of a gasification
system so great as to offset the increase in efficiency?

I'm not sure there *is* any increase in efficiency. At
http://pepei.pennnet.com/Articles/Article_Display.cfm?Section=Archives&Subsection=Display&ARTICLE_ID=152601
are efficiency data for some power plants
that were going into service in 2000;
percentage efficiencies were over 40.

Thank you for your reply. My information on the efficiency of
coal-steam plants was out of date, as was my information on the size of
modern utility turbines. I had thought that large marine steam turbines
(no longer used in new ships except for nuclear powered ones), at about
100 KW, were similar to generator plant units, but I see from the link
you posted that modern utility turbines are up to 1 GW. Maximum size of
current gas turbines (GE-90 class) is about 100 KW, so it would take 10
gas turbines to replace a big steam turbine, likely at much greater
initial cost, to say nothing of the number of separate generators
needed.

Is it practical to fuel the boilers for a 1 GW turbine with coal gas?
Smaller boilers, down to locomotive size, have used coal gas with
greater efficiency that direct coal burning. There is also a great
reduction in emission of mercury and other trace contaminants.

At some point presumably the new steam technology will filter down to
the size of turbines used in gas turbine bottoming cycle units and
result in higher efficiency for this type of plant as well. Also, large
gas turbines are basically aviation engines, and use air cooling for
the turbine blades. General Electric claims that for utility service
this could be replaced with steam cooling, making the turbine serve as
a superheater. They say this could bring thermal efficiency for a
natural gas fired gas-turbine/steam turbine plant to 60 percent from
the present 55 percent. At a guess, these two developments would boost
the efficiency of a coal gas/ gas turbine/ steam turbine plant to
between 50 and 55 percent.

Thank
you again,

Peter Wezeman

anti-social Darwinist

.

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