Multi-stage combustion system for externally fired power plants

A method for supplying heat to an externally fired power system comprising the steps of:

• supplying a stream of pre-heated air and a first amount of a total amount of combustion fuel combusted in said method to a first combustion chamber, resulting in a mixture of said pre-heated air and said combustion fuel;
• combusting the mixture of said pre-heated air and said combustion fuel in said first combustion chamber to form a first flue gas stream having a first flue gas temperature,
• said first amount of said combustion fuel supplied to said first combustion chamber being chosen such that said first flue gas temperature is below a temperature at which NO.sub.x gases form;
• transferring heat from said first flue gas stream to a working fluid stream from an externally fired power system;
• combusting a mixture of said first flue gas stream and a second amount of said total amount of combustion fuel in a second combustion chamber to form a second flue gas stream having a second flue gas temperature,
• said second amount of said total amount of combustion fuel supplied to said second combustion chamber being chosen such that said second flue gas temperature is below the temperature at which NO.sub.x gases form;
• transferring heat from said second flue gas stream to a working fluid stream from an externally fired power system;
• combusting a mixture of said second flue gas stream and a third amount of said total amount of combustion fuel in a third combustion chamber to form a third flue gas stream having a third flue gas temperature,
• said third amount of said total amount of combustion fuel supplied to said third combustion chamber being chosen such that said third flue gas temperature is below the temperature at which NO.sub.x gases form;
• transferring heat from said third flue gas stream to a working fluid stream from an externally fired power system,
• wherein a sum of the temperature rises associated with each combustion step being equal to the temperature rise associated with combusting all of said total amount of combustion fuel in a single combustion step using a minimum amount of air necessary for complete combustion, and
• using heat from said second or third flue gas stream to provide heat to an incoming air stream to generate said stream of pre-heated air.

Apparatus for supplying heat to an externally fired power system comprising:

• a pre-heater for pre-heating an incoming air stream and generating a pre-heated air stream;
• a first combustion chamber for combusting a mixture of said pre-heated air stream and a first amount of a total amount of combustion fuel combusted in said apparatus to form a first flue gas stream having a first flue gas temperature;
• a meter for selecting said first amount of said combustion fuel supplied to said first combustion chamber such that said first flue gas temperature is below a temperature at which NO.sub.x gases form;
• a first heat exchanger for transferring heat from said first flue gas stream to a working fluid stream from an externally fired power system;
• one or more additional combustion chambers arranged in series for combusting one or more further amounts of said total amount of combustion fuel in a series of one or more successive combustion steps to form in each step a flue gas stream having a successive flue gas temperature using the flue gas stream created in the immediately preceding combustion step;
• one or more meters for selecting each said further amount of combustion fuel in each said successive combustion step such that each said successive flue gas temperature is below the temperature at which NO.sub.x gases form,
• wherein a sum of the temperature rises associated with each combustion step being equal to the temperature rise associated with combusting all of said total amount of combustion fuel in a single combustion step using the minimum amount of air necessary for complete combustion;
• and
• one or more additional heat exchangers for transferring heat from each successive flue gas stream to a working fluid stream from said externally fired power system,
• said pre-heater using heat transferred from one of said flue gas streams.

Apparatus for supplying heat to an externally fired power system comprising:

• a pre-heater for pre-heating an incoming air stream and generating a pre-heated air stream;
• a first combustion chamber for combusting a mixture of said pre-heated air stream and a first amount of a total amount of combustion fuel combusted in said apparatus to form a first flue gas stream having a first flue gas stream temperature;
• a first meter for selecting said first amount of combustion fuel supplied to said first combustion chamber such that said first flue gas temperature of said first flue gas stream is below the temperature at which NO.sub.x gases form;
• a first heat exchanger for transferring heat from said first flue gas stream to a working fluid stream from an externally fired power system;
• a second combustion chamber for combusting a mixture of said first flue gas stream and a second amount of said total amount of combustion fuel to form a second flue gas stream having a second flue gas temperature;
• a second meter for selecting said second amount of combustion fuel supplied to said second combustion chamber such that said second flue gas temperature is below the temperature at which NO.sub.x gases form;
• a second heat exchanger for transferring heat from said second flue gas stream to a working fluid stream from an externally fired power system;
• a third combustion chamber for combusting a mixture of said second flue gas stream and a third amount of said total amount of combustion fuel to form a third flue gas stream having a third flue gas temperature;
• a third meter for selecting said third amount of combustion fuel supplied to said third combustion chamber such that said third flue gas temperature is below the temperature at which NO.sub.x gases form;
• and
• a third heat exchanger for transferring heat from said third flue gas stream to a working fluid stream from an externally fired power stream,
• wherein a sum of the temperature rises associated with each combustion step being equal to the temperature rise associated with combusting all of said total amount of combustion fuel in a single combustion step using the minimum amount of air necessary for complete combustion,
• said pre-heater using heat transferred from one of said flue gas stream.