Method for controlling the combustion air supply in a steam generator that is fueled with fossil fuels

Abstract

A method for controlling the combustion air supply in a steam generator that is fueled with fossil fuels, the combustion air being supplied gradually in different combustion zones. The combustion air supply is controlled, depending on the NOx and/or CO content in the flue gas, in such a manner that first the air supply is varied between the different combustion zones with approximately constant air volumes. An external control of the overall air volume overrides this type of control.

1 . A method for controlling a combustion air supply of a steam generator fired with fossil fuels, the steam generator including a firing system having a plurality of combustion zones arranged one after another in a direction of flue gas flow the method comprising:
 controlling the combustion air supply as a function of the amount of fuel consumed in the combustion zones;   directing portions of the combustion air supply to each of the combustion zones; and   varying the combustion air supply portions directed to each of the combustion zones as a function of the NOx or CO concentration in the flue gas whereby the combustion air supply is substantially a constant amount of air.   
   
   
       2 . A method of  claim 1 , wherein said variation of the air supply portions is carried out across at least three different combustion zones. 
   
   
       3 . A method of  claim 1  wherein if a NOx limit value is exceeded, the air supply portion in a first combustion zone seen in the direction of flue gas flow is decreased, and the air supply portion in a last combustion zone seen in the direction of flue gas flow is increased correspondingly. 
   
   
       4 . A method of  claim 1  wherein if a CO limit value is exceeded, the air supply portion in a first combustion zone seen in the direction of flue gas flow is increased and the air supply portion in a last combustion zone seen in the direction of flue gas flow is decreased correspondingly. 
   
   
       5 . A method of  claim 3 , wherein if a specified amount of air in the last combustion zone is exceeded, the air supply portion in a next upstream combustion zone is increased. 
   
   
       6 . A method of  claim 4 , wherein the air supply portion in the next upstream combustion zone is decreased as needed. 
   
   
       7 . The method of  claim 1  wherein the air supply portion supplied to each combustion zone is determined as a function of an air/fuel ratio (A number) to be specified for each combustion zone. 
   
   
       8 . A method of  claim 7 , wherein the air/fuel ratio of each combustion zone is specified as a function of the firing rate. 
   
   
       9 . A method of  claim 7  wherein the air supply portion for a last combustion zone is determined by means of the air/fuel ratio of this combustion zone, by means of a fuel-specific air demand, and by means of the fuel mass flow. 
   
   
       10 . A method of  claim 8 , wherein the oxygen concentration in the flue gas downstream from the firing system is calculated on the basis of the air/fuel ratio specified for the last combustion zone. 
   
   
       11 . A method of  claim 10 , wherein the calculated oxygen concentration of the flue gas downstream from the firing system is used as a setpoint for an external control of the total amount of air. 
   
   
       12 . A method of  claim 1  wherein a measured oxygen concentration of the flue gas is used as an actual value for the control of the total amount of air. 
   
   
       13 . A method  claim 1  wherein the fuel and at least one first partial stream of the combustion air is fed into a burner stage as a first combustion stage of a combustion chamber and at least one additional partial stream of the combustion air is fed in as overfire air downstream in the direction of flue gas flow in at least one downstream overfire stage. 
   
   
       14 . A method of  claim 13  wherein two additional partial streams of the combustion air are fed in as overfire air downstream in the direction of flue gas flow in two downstream overfire stages.