System and method for stack heat recovery

Abstract

A system and method are presented for recovering heat from flue gas produced by a recovery boiler in a pulp and paper mill, the recovery boiler having a flue stack. Flue gas is drawn from the flue stack and passed through a first and second condensing heat exchangers before exiting through a separate stack. The first heat exchanger is used to heat boiler feedwater whereas the second heat exchanger is used to produce process hot water for use in the pulp and paper mill. Steam previously used to heat the boiler feedwater and produce the process hot water can now be used to generate electricity that can be used for the pulp and paper mill operations or to export to the electrical power grid.

We claim: 
     
         1 . A heat recovery system for use with a recovery boiler system in a pulp and paper mill, the recovery boiler system comprising a flue gas stack operatively coupled thereto, the heat recovery system comprising:
 a) a first fan configured to draw flue gas away from the flue gas stack;   b) at least one first heat exchanger operatively coupled to the first fan and configured to receive the drawn flue gas to pass therethrough;   c) at least one second heat exchanger operatively coupled to the at least one first heat exchanger and configured to receive the drawn flue gas to pass therethrough after passing through the at least one first heat exchanger; and   d) a second flue gas stack operatively coupled to the at least one second heat exchanger and configured to receive the drawn flue gas after passing through the at least one second heat exchanger.   
     
     
         2 . The heat recovery system as set forth in  claim 1 , wherein the at least one first heat exchanger is configured to heat boiler feedwater. 
     
     
         3 . The heat recovery system as set forth in  claim 2 , wherein the at least one first heat exchanger comprises two or more boiler feedwater heat exchangers operatively coupled together sequentially. 
     
     
         4 . The heat recovery system as set forth in  claim 1 , wherein the at least one second heat exchanger is configured to heat process hot water for use in the pulp and paper mill. 
     
     
         5 . The heat recovery system as set forth in  claim 4 , wherein the at least one second heat exchanger comprises two or more process hot water heat exchangers operatively coupled together sequentially. 
     
     
         6 . The heat recovery system as set forth in  claim 1 , wherein the at least one first exchanger is disposed above the at least one second heat exchanger, and wherein the drawn flue gas is directed downward through the at least one first heat exchanger and the at least one second heat exchanger. 
     
     
         7 . The heat recovery system as set forth in  claim 1 , further comprising a wash system configured to wash off one or both of precipitate and the condensate off of one or both of the at least one first heat exchanger and the at least one second heat exchanger, the precipitate and the condensate forming on one or both of the at least one first heat exchanger and the at least one second heat exchanger as the drawn flue gas passes therethrough. 
     
     
         8 . The heat recovery system as set forth in  claim 7 , wherein the wash system comprises at least one spray nozzle disposed on one or both of the at least one first heat exchanger and the at least one second heat exchanger, the at least one spray nozzle configured to wash one or both of the at least one first heat exchanger and the at least one second heat exchanger. 
     
     
         9 . The heat recovery system as set forth in  claim 1 , wherein one or both of the at least one first heat exchanger and the at least one second heat exchanger comprises a condensing heat exchanger. 
     
     
         10 . The heat recovery system as set forth in  claim 9 , further comprising a condensation collector disposed beneath one or both of the at least one first heat exchanger and the at least one second heat exchanger, the condensation produced by water vapour disposed in the drawn flue gas condensing on one or both of the at least one first heat exchanger and the at least one second heat exchanger as the drawn flue gas passes therethrough. 
     
     
         11 . The heat recovery system as set forth in  claim 10 , further comprising a wash system configured to use at least some of the collected condensation to wash off one or both of precipitate and condensate off one or both of the at least one first heat exchanger and the at least one second heat exchanger, the precipitate and the condensate forming on one or both of the at least one first heat exchanger and the at least one second heat exchanger as the drawn flue gas passes therethrough. 
     
     
         12 . The heat recovery system as set forth in  claim 11 , wherein the wash system comprises at least one spray nozzle disposed on one or both of the at least one first heat exchanger and the at least one second heat exchanger, the wash system comprising a first pump configured to pump the collected condensation through the at least one spray nozzle, thereby washing one or both of the at least one first heat exchanger and the at least one second heat exchanger. 
     
     
         13 . The heat recovery system as set forth in  claim 1 , wherein one or both of the at least one first heat exchanger and the at least one second heat exchanger are comprised of one or both of stainless steel and titanium. 
     
     
         14 . The heat recovery system as set forth in  claim 13 , wherein one or both of the at least one first heat exchanger and the at least one second heat exchanger are comprised of SAF 2205™ stainless steel. 
     
     
         15 . A method for recovering heat from a recovery boiler system in a pulp and paper mill, the recovery boiler system comprising a flue gas stack operatively coupled thereto, the method comprising:
 a) drawing flue gas away from the flue gas stack;   b) heating boiler feedwater with the drawn flue gas;   c) then heating process hot water with the drawn flue gas; and   d) then exiting the drawn flue gas to the atmosphere.   
     
     
         16 . The method as set forth in  claim 15 , comprising heating the boiler feedwater with at least one first heat exchanger. 
     
     
         17 . The method as set forth in  claim 16 , wherein the at least one first heat exchanger comprises two or more boiler feedwater heat exchangers operatively coupled together sequentially. 
     
     
         18 . The method as set forth in  claim 14 , further comprising heating the process hot water with at least one second heat exchanger. 
     
     
         19 . The method as set forth in  claim 18 , wherein the at least one second heat exchanger comprises two or more process hot water heat exchangers operatively coupled together sequentially. 
     
     
         20 . The method as set forth in  claim 15 , further comprising washing one or both of precipitate and condensate off of one or both of the at least one first heat exchanger and the at least one second heat exchanger, the precipitate and the condensate having formed on one or both of the at least one first heat exchanger and the at least one second heat exchanger after the drawn flue gas has passed therethrough. 
     
     
         21 . The method as set forth in  claim 20 , further using at least one spray nozzle for washing one or both of the at least one first heat exchanger and the at least one second heat exchanger. 
     
     
         22 . The method as set forth in  claim 20 , further comprising washing the at least one first heat exchanger and the at least one second heat exchanger sequentially. 
     
     
         23 . The method as set forth in  claim 15 , further comprising collecting condensation produced by water vapour disposed in the drawn flue gas condensing on one or both of the at least one first heat exchanger and the at least one second heat exchanger as the drawn flue gas passes therethrough. 
     
     
         24 . The method as set forth in  claim 22 , further comprising washing one or both of precipitate and condensate off of one or both of the at least one first heat exchanger and the at least one second heat exchanger with at least some of the collected condensation, the precipitate and the condensate having formed on one or both of the at least one first heat exchanger and the at least one second heat exchanger after the drawn flue gas has passed therethrough. 
     
     
         25 . The method as set forth in  claim 22 , further comprising pumping the at least some of the collected condensation through at least one spray nozzle to wash one or both of the at least one first heat exchanger and the at least one second heat exchanger. 
     
     
         26 . The method as set forth in  claim 22 , further comprising washing the at least one first heat exchanger and the at least one second heat exchanger sequentially. 
     
     
         27 . A heat recovery system for use with a recovery boiler system in a pulp and paper mill, the recovery boiler comprising a flue gas stack operatively coupled thereto, the heat recovery system comprising:
 a) means for drawing flue gas away from the flue gas stack;   b) means for heating boiler feedwater with the drawn flue gas;   c) means for heating process hot water with the drawn flue gas; and   d) means for exiting the drawn flue gas to the atmosphere.   
     
     
         28 . The heat recovery system as set forth in  claim 27 , further comprising means for washing one or both of precipitate and condensate off of one or both of the means for heating boiler feedwater and the means for heating process hot water, the precipitate and the condensate having formed thereon after the drawn flue gas has passed therethrough. 
     
     
         29 . The heat recovery system as set forth in  claim 27 , further comprising means for collecting condensation produced by water vapour disposed in the drawn flue gas. 
     
     
         30 . The heat recovery system as set forth in  claim 29 , further comprising means for washing one or both of precipitate condensate off of one or both of the means for heating boiler feedwater and the means for heating process hot water with the collected condensation, the precipitate and the condensate having formed thereon after the drawn flue gas has passed therethrough.