METHOD FOR REDUCING NOx DURING COMBUSTION OF COAL IN A BURNER BY OPTIMIZING COMBUSTION AIR FLOW

Abstract

An initial coal feedstock comprised of primary or second coal is cleaned so as to reduce its ash content by at least about 20% in order to yield a refined coal product that produces fewer NOx emissions. The reduced NOx emissions result primarily from the ability to use less primary combustion air in order to maintain the pulverized refined coal in a suspended condition within the feeder pipes of a coal burner compared to the minimum quantity of air required to maintain pulverized coal from the initial coal feedstock in a suspended condition within the feeder pipes. Reducing the primary combustion air reduces the amount of oxygen in the primary combustion zone that would otherwise be available for converting fuel nitrogen into NOx. Instead, more of the fuel nitrogen is converted into N 2 . Reducing the primary combustion air also reduces the temperature of the core flame, reducing thermal NOx formation. Increasing the amount of secondary and/or tertiary combustion air to compensate for the reduced primary combustion air results in an overall decrease in NOx formation, as thermal NOx formation is reduced as combustion is completed in the cooler secondary and/or tertiary combustion zones.

1 . A method for reducing NOx emissions during combustion of coal within a burner, comprising,
 obtaining a refined coal product derived from an initial coal feedstock, in which the refined coal product has an ash content that is less than the ash content of the initial coal feedstock;   pulverizing the refined coal product to yield pulverized refined coal; and   conveying the pulverized refined coal through feeder pipes to the primary combustion zone of a coal burner while maintaining the pulverized refined coal in a suspended condition within the feeder pipes using a reduced quantity of air that is less than a minimum quantity of air required to maintain pulverized coal from the initial coal feedstock in a suspended condition within the feeder pipes, the reduced quantity of air resulting in a lower air/fuel ratio in the primary combustion zone and reduced NOx emissions.   
     
     
         2 . A method as in  claim 1 , the refined coal product having an ash content that is less than about 80% of the ash content of the initial coal feedstock. 
     
     
         3 . A method as in  claim 1 , the refined coal product having an ash content that is less than about 60% of the ash content of the initial coal feedstock. 
     
     
         4 . A method as in  claim 1 , the refined coal product having an ash content that is less than about 40% of the ash content of the initial coal feedstock. 
     
     
         5 . A method as in  claim 1 , the reduced quantity of air resulting in an air/fuel ratio in the primary combustion zone of less than about 2. 
     
     
         6 . A method as in  claim 1 , the reduced quantity of air reducing NOx emissions when burning the refined coal product compared to NOx emissions when burning the initial coal feedstock in the burner using the minimum quantity of air required to maintain pulverized coal from the initial coal feedstock in a suspended condition within the feeder pipes of the burner. 
     
     
         7 . A method as in  claim 6 , the reduced quantity of air reducing NOx emissions by at least about 10% compared to NOx emissions when burning the initial coal feedstock using the minimum quantity of air required to maintain pulverized coal from the initial coal feedstock in a suspended condition within the feeder pipes of the burner. 
     
     
         8 . A method as in  claim 6 , the reduced quantity of air reducing NOx emissions by at least about 20% compared to NOx emissions when burning the initial coal feedstock using the minimum quantity of air required to maintain pulverized coal from the initial coal feedstock in a suspended condition within the feeder pipes of the burner. 
     
     
         9 . A method as in  claim 6 , the reduced quantity of air reducing NOx emissions by at least about 35% compared to NOx emissions when burning the initial coal feedstock using the minimum quantity of air required to maintain pulverized coal from the initial coal feedstock in a suspended condition within the feeder pipes of the burner. 
     
     
         10 . A method as in  claim 6 , the reduced quantity of air reducing NOx emissions by at least about 50% compared to NOx emissions when burning the initial coal feedstock using the minimum quantity of air required to maintain pulverized coal from the initial coal feedstock in a suspended condition within the feeder pipes of the burner. 
     
     
         11 . A method as in  claim 6 , the reduced quantity of air reducing formation of NOx from fuel nitrogen as a result of reducing available oxygen in the primary combustion zone. 
     
     
         12 . A method as in  claim 1 , further comprising increasing at least one of secondary or tertiary air that is introduced into the burner. 
     
     
         13 . A method as in  claim 2 , the secondary and/or tertiary air causing combustion at a lower temperature, thereby reducing thermal NOx emissions. 
     
     
         14 . A method for reducing NOx during combustion of coal within a burner, comprising,
 obtaining an initial coal feedstock;   cleaning the initial coal feedstock to reduce dry basis ash content by at least about 20% to yield a refined coal product;   pulverizing the refined coal product to yield pulverized refined coal; and   conveying the pulverized refined coal through feeder pipes to the primary combustion zone of a coal burner while maintaining the pulverized refined coal in a suspended condition within the feeder pipes using a reduced quantity of air that is less than a minimum quantity of air required to maintain pulverized coal from the initial coal feedstock in a suspended condition within the feeder pipes, the reduced quantity of air resulting in a lower air/fuel ratio in the primary combustion zone and reduced NOx emissions.   
     
     
         15 . A method as in  claim 14 , the initial coal feedstock comprising a primary coal feedstock having a dry basis ash content of at least about 15%, the refined coal product having a dry basis ash content of less than about 10%. 
     
     
         16 . A method as in  claim 14 , the initial coal feedstock comprising a secondary or waste coal feedstock having a dry basis ash content of at least about 40%, the refined coal product having a dry basis ash content of less than about 20%. 
     
     
         17 . A method as in  claim 14 , the initial coal feedstock being cleaned by at least one of dry jigging or wet processing. 
     
     
         18 . A method as in  claim 14 , the reduced quantity of air for maintaining the pulverized refined coal in a suspended condition within the feeder pipes being at least about 5% less than the minimum quantity of air required to maintain pulverized coal from the initial coal feedstock in a suspended condition within the feeder pipes. 
     
     
         19 . A method as in  claim 14 , the reduced quantity of air for maintaining the pulverized refined coal in a suspended condition within the feeder pipes being at least about 10% less than the minimum quantity of air required to maintain pulverized coal from the initial coal feedstock in a suspended condition within the feeder pipes. 
     
     
         20 . A method as in  claim 14 , the refined coal product having a mercury content of less than about 50% of the mercury content of the initial coal feestock. 
     
     
         21 . A method for reducing NOx during combustion of coal within a burner, comprising,
 obtaining an initial coal feedstock having a dry basis ash content of at least about 30%;   cleaning the initial coal feedstock to yield a refined coal product having a dry basis ash content of less than about 20%;   pulverizing the refined coal product to yield pulverized refined coal;   conveying the pulverized refined coal through feeder pipes to the primary combustion zone of a coal burner while maintaining the pulverized refined coal in a suspended condition within the feeder pipes using a reduced quantity of air that is less than a minimum quantity of air required to maintain pulverized coal from the initial coal feedstock in a suspended condition within the feeder pipes, the reduced quantity of air resulting in an air/fuel ratio in the primary combustion zone of less than about 2; and   introducing a quantity of secondary air and/or tertiary air into the burner in order to maintain a combustor exit oxygen level of at least about 2%.