The invention relates to supplying heat to an externally fired power system.
In direct fired power plants, fuel, e.g., pulverized coal, is burned in a combustion chamber in which combustion air, typically preheated, is supplied. Tubes surrounding the flame zone contain a working fluid (e.g., water) that is heated to boiling and then delivered to a power system (e.g., including a turbine) for conversion to a useful form of energy, such as electricity. Kalina U.S. Pat. No. 5,450,821 describes a multi-stage combustion system that employs separate combustion chambers and heat exchangers and controls the temperature of heat released at the various stages to match the thermal characteristics of the working fluid and to keep temperatures below temperatures at which NO.sub.x gasses form.