Method for Eliminating Spikes of Mercury Emissions

Abstract

A method for the reduction of mercury emissions from an industrial plant that utilizes a cement or minerals kiln that has a high level of mercury emissions during specific operating conditions. The invention reduces a large spike in mercury emissions typically seen in cement kiln systems when the in-line raw mill is shut-down and all preheater gases are vented to the stack directly. The invention diverts the preheater gases to a cooling device that functions as an adsorption reactor to provide mercury absorption during periods when the raw mill is not in operation.

1 . A method for the reduction of contaminant emissions and contaminant emission spikes from an industrial plant utilizing a kiln to heat treat a raw material, a preheater to preheat the raw material in the kiln's off gases and a raw mill that is in operation intermittently to grind the raw material, with off gases from the preheater being directed to the raw mill and thereafter to an air pollution control device, said off gases from the preheater containing contaminants in vaporized form;
 said method comprising   (i) diverting at least some of the preheater off gases away from the raw mill into a cooling means wherein the preheater off gases are cooled to near the condensation temperature of the contaminants, and (ii) absorbing such contaminants on a sorbent that is entrained in the preheater off gases.   
     
     
         2 . The method of  claim 1  wherein the cooling means is a gas suspension absorber. 
     
     
         3 . The method of  claim 1  wherein the cooling means is gas cooling tower. 
     
     
         4 . The method of  claim 1  wherein the sorbent is inserted into the cooling means. 
     
     
         5 . The method of  claim 1  wherein the contaminants are comprised of mercury. 
     
     
         6 . The method of  claim 5  wherein the preheater off gases are cooled to from about 40° C. to about 200° C. 
     
     
         7 . The method of  claim 6  wherein step (i) takes place when the mill is not in operation. 
     
     
         8 . The method of  claim 7  wherein all of the preheater off gases are diverted to a cooling means. 
     
     
         9 . The method of  claim 6  further comprising:
 directing the off gases in which the sorbent with absorbed mercury is entrained to a separation device wherein the sorbent is separated from the off gases.   
     
     
         10 . The method of  claim 9  further comprising:
 redirecting the separated sorbent with absorbed mercury from the separation device back to the cooling means.   
     
     
         11 . The method of  claim 10  further comprising:
 directing the off gases from which the sorbent has been separated to the air pollution control device.   
     
     
         12 . The method of  claim 6  further comprising:
 utilizing a flow measuring device located in the proximity of the gas inlet of the cooling to assist in calculating water spray requirements and/or sorbent concentration based on gas volume as a feed-forward control.   
     
     
         13 . A system for reducing contaminants in preheater off gases containing contaminants including mercury that are condensable comprising,
 a kiln for heat treating comminuted raw material;   a preheater located, in the direction of material flow, upstream from the kiln for preheating the comminuted raw material with off gases from the kiln;   a raw mill adaptable for intermittent operation for comminuting the raw material;   means to direct the off gases from the preheater to the raw mill and thereafter to an air pollution control device,   a cooling device for cooling preheater off gases to a temperature near the condensation temperature of mercury;   means to direct the preheater off gases, when the mill is not in operation, from the mill to the cooling device, and   means to direct a sorbent for absorbing the mercury into the cooling device.   
     
     
         14 . The system of  claim 13  wherein the cooling means is a gas suspension absorber. 
     
     
         15 . The system of  claim 13  wherein the cooling means is gas cooling tower. 
     
     
         16 . The system of  claim 13  wherein the sorbent is in the form of a slurry. 
     
     
         17 . The system of  claim 13  wherein the sorbent is in a dry form. 
     
     
         18 . The system of  claim 13  wherein the preheater off gases contain dust that is suitable as a sorbent. 
     
     
         19 . The system of  claim 13  further comprising means to direct at least a portion of the preheater off gases in which there is an entrained sorbent containing absorbed contaminants from the cooling device to the air pollution control device. 
     
     
         20 . The system of  claim 13  further comprising (i) a separation device located intermediate the cooling device and the air pollution control device for separating entrained sorbent containing absorbed contaminants from the cooled preheater off gases and (ii) means to directing at least a portion of the separated sorbent back to the cooling device.