http://en.wikipedia.org/wiki/Combined_cycle ,    
About  Combined Cycle Conversions - Babcock & Wilcox pdf , 
Good repowering text about SOAPP's "Do It Yourself" software:  MSR - Repowering Existing Fossil Steam Plants - SEPRIL .pdf  ( from:  http://soapp.epri.com/ )
Some coal burning power plants have been converted to natural gas simply by replacing their coal burners with gas burners. 

Combined Cycle Natural Gas at Bayside (Tampa FL) has a CO2 Intensity of 0.694.  (The author thinks this is almost too good to be true.  The best coal plants deliver CO2 intensities of 2.1 or so, a direct natural gas, 1.3 or so.)

Another approach is "Carbon Capture and Sequestration."  CCS Retrofitting of Coal-Fired Power Plants for CO2 Emissions Reductions - MIT - meeting-report .pdf 

Part  4:  Costly Carbon Emissions Compliance and Small Town America.
(This story is about a year old now but it still sticks in my craw.)
 

What the EPA's new pollution rules are doing to almost every small town in America.
Here is the EPA's "Fact Sheet" about this issue:  Diesel Engine Emissions - EPA fact sheet - 2006.pdf

(Right) Thousands of small towns around the world have a few diesel generators for emergency stand-by and peak power nearby.
 
These MSCPA semi truck trailer diesel-powered electricity generation modules are near the town of Coldwater, Michigan. 

Combined, they produce about 12 MegaWatts of electricity. 
 

(Left) MSCPA diesel in nearby Hillsdale.

According to an article by Don Reid (dwreid@aol.com) in the Coldwater the "Daily REPORTER" ( http://www.thedailyreporter.com/ ) Michigan South Central Power Agency (MSCPA) has to decide whether sell the above portable diesel generators for about $4.4 million or spend just under $1 million to bring them up to federal pollution standards by 2013.  Likely, they would be shipped overseas and not be subject to the retrofit, Coldwater Board of Public Utilities (CBPU) Director Paul Beckhusen said.  He expects the selling price to drop as the deadline for conversions nears.

Glenn White, MSCPA general manager, said even with the pollution upgrades they can be operated only 15 hours a year unless the entire power grid goes down.  "You are going to have to meet capacity requirements somehow.  The question is how much you are going to want to keep (of local generation) for emergencies, if any at all" White said. 

Not only do the five member communities that own MSCPA have these diesel units, each community has other, older, diesel units they operated before joining the MSCPA power generating co-operative.

Coldwater also has almost 12 MegaWatts (MW) in three units ($250,000 to upgrade), Hillsdale has five older diesels producing nearly 20 MW ($425,000 to upgrade).  Total diesel generation for MSCPA is 74.6 MW plus their Litchfield 55 MW coal-powered steam plant (Top of Page).

Worse, sometime in the near future, the EPA will only allow 25,000 tons of Litchfield's annual 700,000 tons of coal CO2 to be emitted without penalty.  With current estimates of penalties running as high as $25 per ton of CO2 (Currently carbon credits are going for 19 Euros per tonne or about US$25 per ton.), this could add over $15 million per year to MSCPA's customer's power bill.

Hillsdale and Coldwater are not alone.  At least 311 such diesels are under the gun in Michigan alone.

Michigan Generating Unit Types and their Energy Sources
For DOE energy source abbreviations see:  Michigan Electricity Generation Unit Inventory .pdf

    Unofficial Inventory of Michigan's Generating Sources Connected to Utility Grids  (Nuclear not included)

Notice the large number of diesels: (IC) 311 in Michigan alone, perhaps 9,000 in all of the United States, perhaps 30,000 worldwide.

Unit Type Abbreviations:
CA = Combined Cycle Steam Plant;
CC = Combined Cycle Total Unit (use only for plants/generators that are in planning stage, for which specific generator details cannot be provided);
CE = Compressed Air Energy Storage;
CS = Combined Cycle Single Shaft (combustion turbine and steam turbine share a single generator);
CT = Combined Cycle Combustion Turbine Part (type of coal must be reported as energy source for integrated coal);
FC = Fuel Cell;
GT = Combustion (Gas) Turbine (includes jet engine design);
HY = Hydraulic Turbine (includes turbines associated with delivery of water by pipeline);
IC = Internal Combustion (diesel, piston) Engine;
NA = Unknown at this time (use only for plants/generators that are in planning stage, for which specific generator details cannot be provided.);
OT = Other;
PS = Hydraulic Turbine - Reversible (pumped storage);
PV = Photovoltaic;
ST = Steam Turbine, including nuclear, geothermal and solar steam (does not include combined cycle);
WT = Wind Turbine;