RE: Research Topics
Dear
Fred,
At the utility I worked at, we
studied conversion of two of our four coal fired units to gas. We determined
that it was a bad idea.
First, the cost of getting gas to those
units was huge, since there was no infrastructure.
You are absolutely right in that one ought to have a
main gas line reasonably close at hand.
In fact, I read that the hottest sites for new
GT-plants, as currently being built all over the place in the US,
are the intersections of main gas trunk lines in order to have competition
in supply!
Second, we modeled a de-rate in those units. It has been too long for me
to remember exactly what parameters worked against us, but I recall something
about the difference in heat transfer of the flame characteristics. I badly
assumed that this problem that I observed held true for all coal plants -- a
careless act on my part.
Don't worry.
I presume that in a gas retrofit,
you'll notice the same as in replacing large quantities of coal with biomass:
the lack of ash!!!
We find in modeling high cofiring percentages (i.e. much higher
than presently possible to feed to the boilers) that the comparatively low ash
content of biomass (as compared to coal) noticeably shifts the emphasis of the
heat transfer in the boiler.
More biomass means more convective heat transfer down below
(evaporation section) and less superheating and reheating, since less "energy
content" travels to the top of the boiler in terms of (radiating) hot ash.
Ash content, since constituting easily 10-15% of the
mass entering the boiler, has its impact even before it leaves the
boiler!
Natl gas -with no ash at all- I presume will have the same effect: relatively more
heat transfer in the evaporative section below, less heat left for superheating
and reheat.
On top of that there will
be difference in radiative heat transfer at the burner
elevations.
In a Combustion Engineering style boiler originally designed
for full dual fuel firing (either coal or gas), we get approx 96% of full
power on gas versus 100% on coal.
Third, the
efficiency -- compared to combined cycle gas -- was just not there.
Bingo. Can't beat that high CC
efficiency.
The best gas fired utility power plant in the world with
an ultra supercritical, but otherwise classic style, boiler (in
Denmark) reaches 47% efficiency.
The mark for state-of-the art combined cycle GT+steam plants
is around 60%, which is still one third better.
best regards,
Andries
