Lowering emissions that result from fuel combustion in boilers

Abstract

A method for lowering emissions that result from fuel combustion in a boiler may include obtaining a fuel composition, an air composition, and ambient conditions; determining, based on the fuel composition, the air composition, and the ambient conditions, a temperature margin sufficient for reliable combustion, and a minimum adiabatic flame temperature (AFT_LFL) for sustainable combustion within the boiler; establishing a combustion temperature control envelope bounded by the minimum AFT_LFL plus the temperature margin sufficient for reliable combustion, and by a threshold temperature below which thermal NOx formation is minimized; controlling, via a feedforward cascade control algorithm, an injection rate of the fuel into the boiler via a fuel injection system and an injection rate of air into the boiler via an air injection system; and automatically adapting the feedforward cascade control algorithm and associated control setpoints to remain within the combustion temperature control envelope.

What is claimed is: 
     
         1 . A method for lowering emissions that result from fuel combustion in a boiler, the method comprising:
 obtaining a fuel composition, an air composition, and ambient conditions, wherein the fuel composition is for a fuel used in the boiler, and wherein the boiler comprises a furnace and a burner;   determining, based on the fuel composition, the air composition, and the ambient conditions, a temperature margin sufficient for reliable combustion, a lower flammability limit (LFL), and a minimum adiabatic flame temperature (AFT_LFL) for sustainable combustion within the boiler;   establishing a combustion temperature control envelope bounded at a low end by the minimum AFT_LFL plus the temperature margin sufficient for reliable combustion, and at a high end by a threshold temperature below which thermal NOx formation is minimized;   controlling, via a feedforward cascade control algorithm, an injection rate of the fuel into the boiler via a fuel injection system and an injection rate of air into the boiler via an air injection system such that a calculated combustion temperature within the boiler is maintained within the combustion temperature control envelope, wherein an air-to-fuel ratio is dynamically adjusted in response to a real-time change in at least one of a group consisting of the fuel composition, the air composition, the ambient conditions, and process conditions; and   automatically adapting the feedforward cascade control algorithm and associated control setpoints to remain within the combustion temperature control envelope in response to changes in at least one of a group consisting of the fuel composition, the air composition, the ambient conditions, and a boiler configuration, wherein the fuel control system and the air control system are further controlled based on automatically adapting the control setpoints and the temperature margins.   
     
     
         2 . The method of  claim 1 , wherein NOx emissions in a flue gas that exits the boiler after combustion are no greater than 9 ppmvd at 3% O 2 . 
     
     
         3 . The method of  claim 1 , wherein the temperature margin sufficient for reliable combustion is derived from at least one of a group consisting of real-time measurements of radiant heat loss, extent of reaction, and combustion stability indicators. 
     
     
         4 . The method of  claim 1 , wherein automatically adapting the feedforward cascade control algorithm and associated the control setpoints within the combustion temperature control envelope is in further response to changes in stack gas NOx and CO measurements. 
     
     
         5 . The method of  claim 1 , wherein automatically adapting the feedforward cascade control algorithm and associated the control setpoints within the combustion temperature control envelope is in further response to changes in heat demand and efficiency of the boiler. 
     
     
         6 . The method of  claim 1 , wherein the burner operates over a full turndown range of the burner, including at minimum firing rates, without a loss of combustion stability or an increase in NOx emissions. 
     
     
         7 . The method of  claim 1 , wherein, during a startup operation of the boiler, the fuel comprises at least one of a group consisting of a fuel-rich pilot and a fuel-rich core, and wherein the fuel excludes the fuel-rich pilot and the fuel-rich core following the start-up operation of the boiler. 
     
     
         8 . The method of  claim 1 , wherein the burner comprises a first stage of combustion, wherein the furnace comprises a second stage of combustion, wherein the first stage of combustion operates within a first stage operating temperature range, wherein the second stage of combustion operates within a second stage operating temperature range, and wherein the first stage operating temperature range and the second stage operating temperature range fall within the combustion temperature control envelope. 
     
     
         9 . The method of  claim 8 , wherein the furnace further comprises a third stage of combustion that operates within a third stage operating temperature range, and wherein the third stage operating temperature range falls within the combustion temperature control envelope. 
     
     
         10 . The method of  claim 8 , wherein the first stage operating temperature range of the first stage of combustion and the second stage operating temperature range of the second stage of combustion overlap within the combustion temperature control envelope. 
     
     
         11 . The method of  claim 8 , wherein the first stage operating temperature of the first stage of combustion and the second stage operating temperature range of the second stage of combustion are adjustable within the combustion temperature control envelope by adjusting a setpoint of oxygen. 
     
     
         12 . The method of  claim 1 , wherein combustion in the boiler is operated in a distributed, flameless mode throughout a volume of the burner and the furnace. 
     
     
         13 . A combustion control system for lowering emissions that result from fuel combustion in a boiler, the system comprising a controller that is configured to:
 obtain a fuel composition, an air composition, and ambient conditions, wherein the fuel composition is for a fuel used in the boiler, wherein the boiler comprises a furnace and a burner;   determine, based on the fuel composition, the air composition, and the ambient conditions, a temperature margin sufficient for reliable combustion, a lower flammability limit (LFL), and a minimum adiabatic flame temperature (AFT_LFL) for sustainable combustion within the boiler;   establish a combustion temperature control envelope bounded at a low end by the minimum AFT_LFL plus the temperature margin sufficient for reliable combustion, and at a high end by a threshold temperature below which thermal NOx formation is minimized;   control, via a feedforward cascade control algorithm, an injection rate of the fuel into the boiler via a fuel injection system and an injection rate of air into the boiler via an air injection system such that a calculated combustion temperature within the boiler is maintained within the combustion temperature control envelope, wherein an air-to-fuel ratio is dynamically adjusted in response to a real-time change in at least one of a group consisting the fuel composition, the air composition, the ambient conditions, and process conditions; and   automatically adapt the feedforward cascade control algorithm and associated control setpoints to remain within the combustion temperature control envelope in response to changes in at least one of a group consisting of the fuel composition, the air composition, the ambient conditions, and a boiler configuration, wherein the fuel control system and the air control system are further controlled based on automatically adapting the control setpoints and the temperature margins.   
     
     
         14 . The combustion control system of  claim 13 , wherein the air comprises ambient air. 
     
     
         15 . The combustion control system of  claim 13 , wherein the air comprises vitiated air. 
     
     
         16 . The combustion control system of  claim 13 , wherein the controller is further configured to:
 obtain actual emissions data for flue gas resulting from the continued combustion in the boiler;   identify a difference between the actual emissions data and forecast emissions based on the fuel target flow rate and the air target flow rate; and   adaptively modify the feedforward cascade control algorithm based on the difference.   
     
     
         17 . The combustion control system of  claim 13 , wherein the fuel comprises a primary fuel and a secondary fuel, wherein the primary fuel is injected into the burner of the boiler, and wherein the secondary fuel is injected into the furnace of the boiler. 
     
     
         18 . The combustion control system of  claim 17 , wherein the fuel further comprises a tertiary fuel that is injected into the furnace of the boiler. 
     
     
         19 . The combustion control system of  claim 13 , wherein the controller operates based on a software upgrade to existing burner management and control hardware for the boiler, wherein installing the software upgrade requires no physical modification to piping, instrumentation, or final control elements. 
     
     
         20 . The combustion control system of  claim 13 , wherein all of the fuel is combusted in the boiler.