Reduction of mercury from a coal fired boiler

Abstract

A method and system are disclosed for achieving reduced mercury emissions from a power plant at a minimum cost. The parameters of the combustion chamber and a sorbent injector are manipulated to control the residual carbon in ash and the injected sorbent. These two elements combine to reduce mercury in the exhaust gas to an acceptable level.

1 . A system for controlling a power generating facility including a combustion chamber and a post combustion sorbent adding device, the combustion chamber emitting exhaust gas, the system comprising:
 a processor;   one or more storing units for storing signals; and   software program instructions which are stored in one or more of said   storing units and when executed by the processor cause the system to perform a method comprising:
 receiving a measurement indicative of the amount of residual carbon in ash within the exhaust gas; 
 receiving a measurement indicative of the amount of mercury within the exhaust gas; 
 receiving a measurement indicative of combustion chamber efficiency; 
 assigning an economic value to the boiler efficiency; 
 assigning an economic value to a sorbent; 
 assigning an economic value to the residual carbon in ash; 
 setting a predetermined target mercury level in the exhaust gas; 
 determining a target amount of the sorbent to be added to the exhaust gas and a target amount of the residual carbon in ash to be formed in the combustion chamber based upon a balancing of the economic value assigned to the boiler efficiency, the economic value of the sorbent and the economic value of the residual carbon in ash, wherein the target amounts of the sorbent and the residual carbon in ash correlate to the most cost effective ratio of sorbent to residual carbon in ash; and 
 manipulating the controls of the combustion chamber and the post combustion sorbent adding device in accordance with the target sorbent and residual carbon in ash. 
   
     
     
         2 . The system of  claim 1 , wherein the method further comprises assigning an economic value to the fly ash generated as a by-product in the combustion chamber. 
     
     
         3 . The system of  claim 2 , wherein the method further comprises determining a global optimization point wherein the controls of the combustion chamber and the post combustion sorbent adding device are manipulated to reduce the total cost to generate power and maximize the income from the fly ash. 
     
     
         4 . The system of  claim 1 , wherein the method step of assigning an economic value to the residual carbon in ash, further comprises determining the correlation between reduced boiler efficiency and increased residual carbon in ash. 
     
     
         5 . A method of controlling a power generating facility including a combustion chamber and a post combustion sorbent adding device, the combustion chamber emitting exhaust gas, the method comprising:
 receiving a measurement indicative of the amount of residual carbon in ash within the exhaust gas;   receiving a measurement indicative of the amount of mercury within the exhaust gas;   receiving a measurement indicative of the boiler efficiency;   assigning an economic value to the boiler efficiency;   assigning an economic value to a sorbent;   assigning an economic value to the residual carbon in ash;   setting a predetermined target mercury level in the exhaust gas;   determining a target amount of the sorbent to be added to the exhaust gas and a target amount of the residual carbon in ash to be formed in the combustion chamber based upon a balancing of the economic value assigned to the boiler efficiency, the economic value of the sorbent and the economic value of the residual carbon in ash, wherein the target amounts of the sorbent and the residual carbon in ash correlate to the most cost effective ratio of sorbent to residual carbon in ash; and   manipulating the controls of the combustion chamber and the post combustion sorbent adding device in accordance with the target sorbent and residual carbon in ash.   
     
     
         6 . The method of  claim 5 , further comprising assigning an economic value to the fly ash generated as a by-product in the combustion chamber. 
     
     
         7 . The method of  claim 6 , further comprising determining a global optimization point wherein the controls of the combustion chamber and the post combustion sorbent adding device are manipulated to reduce the total cost to generate power and maximize the income from the fly ash. 
     
     
         8 . The method of  claim 5 , wherein the method step of assigning an economic value to the residual carbon in ash, further comprises determining the correlation between reduced boiler efficiency and increased residual carbon in ash. 
     
     
         9 . A system for controlling a power generating facility including a combustion chamber, the combustion chamber emitting exhaust gas, the system comprising:
 a first reaction zone located in the combustion chamber;   a sorbent injector located downstream of the first reaction zone;   a second reaction zone located downstream of said sorbent injector;   a mercury analyzer located downstream of said second reaction zone;   a processor;   one or more storing units for storing signals; and   software program instructions which are stored in one or more of said storing units and when executed by the processor cause the system to perform a method comprising:
 producing residual carbon in ash in said first reaction zone; 
 injecting a sorbent at the second reaction zone; 
 measuring the amount of mercury in the exhaust gas at the mercury analyzer; 
 determining the cost of the sorbent; 
 determining the cost of the residual carbon in ash; 
 setting a predetermined target mercury level for the exhaust gas at the mercury analyzer; 
 determining a rate to inject the sorbent and a target amount of the residual carbon in ash to be formed in the first reaction zone based upon a balancing of the cost of the residual carbon in ash and the cost of the sorbent; and 
 manipulating the controls of the combustion chamber and the sorbent injector in accordance with the determined sorbent injection rate and the target residual carbon in ash. 
   
     
     
         10 . The system of  claim 9 , wherein the method further comprises assigning an economic value to the fly ash generated as a by-product in the combustion chamber. 
     
     
         11 . The system of  claim 10 , wherein the method further comprises determining a global optimization point wherein the controls of the combustion chamber and the sorbent injector are manipulated to reduce the total cost to generate power and maximize the income from the fly ash. 
     
     
         12 . The system of  claim 9 , wherein the method step of determining a cost of the residual carbon in ash, further comprises determining the correlation between reduced boiler efficiency and increased residual carbon in ash.