Flue gas recirculation method and system for combustion systems

Abstract

A method and system for improving high excess air combustion system efficiency, including induration furnaces, using a re-routing of flue gas within the system by gas recirculation. Flue gas is drawn from hot system zones including zones near the stack, for re-introduction into the process whereby the heat recovery partially replaces fuel input. At least one pre-combustion drying zone, at least one combustion zone, and at least a first cooling zone exist in these furnaces. At least one exhaust gas outlet is provided to each pre-combustion drying and combustion zone. At least part of the gaseous flow from each system zone exhaust outlet is selectively delivered to an overall system exhaust, the remaining flow being selectively delivered via recirculation to cooling zones. Recirculation flow is adjusted to meet required system temperatures and pressures. The method and system provide efficiency improvements, reducing fuel requirements and greenhouse gas emissions.

1 . A flue gas recirculation system for a combustion system, the combustion system comprising in sequence at least one pre-combustion drying zone, at least one combustion zone, and at least a first cooling zone, the recirculation system comprising
 (i) at least one exhaust gas outlet provided respectively to each pre-combustion drying zone and each combustion zone, and constructed and arranged to remove a gaseous flow from each said zone;   (ii) at least one cooling zone intake means provided to each cooling zone;   (iii) at least one flue gas delivery means each having at least one recirculation intake means and at least one delivery outlet, each exhaust gas outlet being operatively connectable to one of the recirculation intake means to selectively deliver at least part of each gaseous flow as a recirculation flow to the flue gas delivery means, and each delivery outlet being selectively operatively connectable to a selected one of the cooling zone intake means; and   (iv) control means operatively connected to the flue gas delivery means to selectively control and direct the recirculation flow.   
     
     
         2 . A recirculation system according to  claim 1 , wherein the at least one pre-combustion drying zone comprises an updraft drying zone and a downdraft drying zone. 
     
     
         3 . A recirculation system according to  claim 1 , wherein the combustion system further comprises at least one pre-heating zone. 
     
     
         4 . A recirculation system according to  claim 1 , wherein the at least one cooling zone comprises a first cooling zone and a second cooling zone, and the flue gas delivery means is constructed and arranged to deliver the recirculation flow to the cooling zone intake means of the first cooling zone. 
     
     
         5 . A recirculation system according to  claim 1 , wherein the combustion system is for an operational use selected from at least one of curing, drying, induration, heating, annealing and forging. 
     
     
         6 . A recirculation system according to  claim 2 , wherein the combustion system further comprises at least one pre-heating zone. 
     
     
         7 . A recirculation system according to  claim 2 , wherein the at least one cooling zone comprises a first cooling zone and a second cooling zone, and the flue gas delivery means is constructed and arranged to deliver the recirculation flow to the cooling zone intake means of the first cooling zone. 
     
     
         8 . A recirculation system according to  claim 2 , wherein the combustion system is for an operational use selected from at least one of curing, drying, induration, heating, annealing and forging. 
     
     
         9 . A recirculation system according to  claim 3 , wherein the at least one cooling zone comprises a first cooling zone and a second cooling zone, and the flue gas delivery means is constructed and arranged to deliver the recirculation flow to the cooling zone intake means of the first cooling zone. 
     
     
         10 . A recirculation system according to  claim 6 , wherein the at least one cooling zone comprises a first cooling zone and a second cooling zone, and the flue gas delivery means is constructed and arranged to deliver the recirculation flow to the cooling zone intake means of the first cooling zone. 
     
     
         11 . A method of recirculation of flue gas for a combustion system, the combustion system comprising in sequence at least one pre-combustion drying zone, at least one combustion zone, and at least a first cooling zone, the method comprising the steps of
 (i) providing at least one exhaust gas outlet respectively to each pre-combustion drying zone and each combustion zone to allow a gaseous flow selectively to and through each exhaust gas outlet;   (ii) providing at least one cooling zone intake means to each cooling zone;   (iii) providing at least one flue gas delivery means each having at least one recirculation intake means and at least one delivery outlet;   (iv) connecting each exhaust gas outlet to one of the recirculation intake means;   (v) selectively delivering at least part of the gaseous flow from each exhaust gas outlet as a recirculation flow to the flue gas delivery means;   (vi) selectively delivering at least part of the recirculation flow through one of the delivery outlets to the selected cooling zone intake means;   (vii) monitoring at least periodically the temperatures and pressures in each cooling zone receiving the recirculation flow to determine temperature and pressure values; and   (iv) adjusting the recirculation flow in response to the determined temperature and pressure values.