Patent Publication Number: US-2023144467-A1

Title: Method for operating a gas burner and gas burner for performing the method

Description:
The present invention relates to a method for operating a gas burner and a gas burner, in particular a gas burner for a heating system. The gas burner includes a lambda probe arranged in the exhaust gas flow. 
     European patent application EP 3 064 937 A1 discloses a generic method directed at detecting errors in the lambda probe and thus preventing a dangerous operating state, for example of a gas burner. 
     Lambda sensors are used to measure the air ratio of an exhaust gas stream from a combustion process. The air ratio quantifies the residual oxygen content in the exhaust gas flow. Depending on the air ratio measured, the combustion process may be controlled such that a desired residual oxygen content in the exhaust gas is achieved. 
     The air ratio is also referred to as the combustion air ratio A and is a dimensionless variable that is calculated as the ratio of the air mass actually available to the minimum air mass required. A=1 is called the stoichiometric combustion air ratio, at which all fuel molecules can react completely with the oxygen in the air without any oxygen required for combustion missing or unburned fuel remaining. At λ&gt;1 there is an “excess of air” which is also referred to as a lean mixture. At λ&lt;1, there is an “shortage of air”, which is also referred to as a rich mixture. 
     The operational principle of a broadband lambda sensor is generally known and is only briefly summarized with reference to  FIG.  1   .  FIG.  1    shows a schematic sectional view of a broadband lambda probe  1  including a pump cell PZ, a measuring chamber MK (or measuring space or measuring gap) and a measuring cell NZ (or reference cell or Nernst cell). The pump cell PZ is positioned adjacent to an exhaust flow. The measuring chamber MK is located adjacent to the pump cell PZ. The measuring cell NZ for measuring a measuring voltage is arranged next to the measuring chamber MK. Accordingly, the lambda probe  1  is formed in a layered structure of pump cell PZ, measuring chamber MK and measuring cell NZ, The pump cell PZ and the measuring cell NZ each have electrodes  3  which are separated by a solid electrolyte, for example zirconium dioxide. In particular, the lambda probe  1  includes a pump cell PZ with a diffusion channel  4  and a controller VSR. The pump cell PZ is supplied with a pump current by a power supply so that the pump cell PZ pumps oxygen from the exhaust gas into the measuring chamber MK or out of the measuring chamber MK. 
     The pump cell and the measuring cell are brought to an operating temperature of, for example, about 800° C. by means of a heater  2 . At this temperature, the solid electrolyte of the pump cell PZ and the measuring cell NZ is permeable to oxygen ions. 
     The oxygen content of the measuring gas in the measuring chamber MK is influenced, on the one hand, by the exhaust gas entering the measuring chamber MK through the diffusion channel  4  and, on the other hand, by the pump current IP. Depending on the polarity, the pump current IP pumps oxygen from the exhaust gas through the solid electrolyte into the measuring chamber MK or from the measuring chamber MK to the exhaust gas. 
     The pump current IP may be closed-loop controlled by an external controller in such a way that the air ratio λ in the measuring gas exactly balances the oxygen flow through the diffusion channel  4 , so that a constant value of λ=1 is present in the measuring chamber MK. A lambda value of 1 is always given when the voltage VS at the measuring cell NZ is 450 mV. The pump current IP resulting from this closed-loop control may be used as a sensor signal that is characteristic and significant for the oxygen content in the exhaust gas flow. 
     Such a broadband lambda probe is used, for example, to monitor the combustion process taking place in a burner of a gas boiler. The burner may then be controlled depending on the measuring values from the lambda probe. A error-free function of the lambda probe is important, If there is a defect in the sensor, the contacts or the driving of the sensor, the lambda probe may provide erroneous signals, which may lead to erroneous burner control. Then, the measured air ratio no longer corresponds to the actual air ratio in the exhaust gas flow and unwanted and potentially dangerous concentrations of carbon monoxide or unburned hydrocarbons may be emitted. If the air ratio in the exhaust gas flow is very high (λ&gt;&gt;1), the combustion efficiency may also be dramatically reduced. 
     Since such errors or defects may also be caused by aging processes in the lambda probe. it is necessary to reliably detect errors in the lambda probe as early as possible. The present invention is based on the object of providing a method with which an error in a lambda probe can be reliably detected in order to prevent the occurrence of a dangerous operating state of a gas burner. 
     According to a first aspect of the invention, the object is achieved by a method according to claim  1 . According to a second aspect of the invention, the object is achieved by a gas burner according to claim  9 . According to a third aspect of the invention, the object is achieved by a heating system according to claim  10 . Further aspects of the invention are the subject matter of the dependent claims, the drawings and the following description of exemplary embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further advantageous developments are described in more detail below with reference to an exemplary embodiment illustrated in the drawings, to which the invention is not restricted, however, 
       In the figures: 
         FIG.  1    shows a sectional view of an exemplary broadband lambda probe. 
         FIG.  2    illustrates an exemplary control scheme of a gas burner with a broadband lambda probe in controlled mode. 
         FIG.  3    illustrates an exemplary control scheme of a gas burner with a broadband lambda probe in the probe test mode. 
         FIG.  4    shows an exemplary flow chart of a method for operating a gas burner. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION BASED ON EXEMPLARY EMBODIMENTS 
     In the following description of a preferred embodiment of the present invention, the same reference symbols designate the same or comparable components. 
       FIG.  4    shows a flow chart of an exemplary method for operating a burner, for example a gas burner of a heating system, which includes a broadband lambda probe  1  for monitoring exhaust gas. The method steps are to denoted by numbers S 1  to S 29 . Transitions between process steps are shown as solid arrows. Dashed arrows illustrate storing or reading of values measured and/or calculated in the method, which are denoted by the letters a to i. 
     The flow of the method can be divided into six function blocks as follows: 
     
       
         
           
               
               
             
               
                   
               
             
            
               
                 F1 
                 Burner on standby (Step S1) 
               
               
                 F2 
                 Burner start with probe calibration in air (steps S2 to S7) 
               
               
                 F3 
                 Closed-loop controlled burner operation (steps S8 to S17) 
               
               
                 F4 
                 Controlled burner operation during active probe test mode 
               
               
                   
                 (steps S18 to S25) 
               
               
                 F5 
                 Burner shutdown with probe calibration in air (steps S26-S28) 
               
               
                 F6 
                 Safety shutdown in the event of an error (step S29) 
               
               
                   
               
            
           
         
       
     
     First, an overview of the method, in particular of the function blocks F 3  and F 4 , is given on the basis of the control schemes shown in  FIGS.  2  and  3   . A detailed description of the individual method steps S 1  to S 29  follows with reference to  FIG.  4   . 
     The function blocks F 1 , F 2 , F 3 , F 5  and F 6  are substantially similarly implemented in methods according to the prior art as well. The function block F 4 , i.e. the probe test mode, is the subject matter of the present invention. The exemplary method according to the invention described below may make it possible to use a commercially available, inexpensive, not intrinsically safe broadband lambda probe, as is also used in the automotive sector, in a safety environment such as a gas burner, without the use of additional sensors. 
     The measuring method of the broadband lambda probe  1  is based on operating two cells (measuring cell VZ and pump cell PZ) and a measuring chamber MK in a separate closed control loop, which is illustrated in  FIG.  2   . 
       FIG.  2    shows a control scheme of a gas burner with a broadband lambda probe in closed-loop controlled operation (first operating state). The block labeled A represents a higher-level control circuit R 1  of a automatic firing unit of the gas burner. It controls the residual oxygen content in the exhaust gas O2 Abgas  and includes an O2 controller O2R, a controlled system RS and a function IP, which provides a target value IP soil , which is calculated by means of a sensor characteristic f(O2) from a target value O2 Soll  for the residual oxygen content in the exhaust gas. The target value of the residual oxygen content in the exhaust gas may be determined, for example, via a desired air ratio λ. A gas burner is usually operated with excess of air, i.e. with an air ratio λ&gt;1. From the desired air ratio λ, the target value O2 Soll  of the residual oxygen content in the exhaust gas may be determined as a volume concentration. 
     The first operating state (closed-loop controlled operation) of the gas burner includes in particular the steps of: measuring (step S 12 ) a residual oxygen content (e) in the exhaust gas; comparing (step S 14 ) the measured residual oxygen content (e) with a predetermined target value (f) and determining a deviation; controlling (step S 15 ) an opening degree of the gas control valve as a function of a deviation; measuring the measuring voltage at the measuring cell NZ; comparing the measured measuring voltage with a predetermined target voltage and determining a deviation; and controlling the residual oxygen content in the exhaust gas via the measured value of the lambda probe  1 . In particular, the pump current may be controlled as a function of the deviation. The individual steps are described in more detail below, 
     A gas burner is usually operated with a substantially constant amount of air. Using a gas control valve, the automatic firing unit may control the volume flow of the supplied gas (fuel) via the O2 controller O2R as a function of the deviation between lP soll  and IP 1st . With the amount of air being constant, the ratio of fuel (gas) to air may be controlled by means of the gas control valve. 
     The controlled system RS in  FIG.  2    summarizes the transfer behavior of the burner from the gas control valve to the measurement of the residual oxygen content in the exhaust gas O2 Abgas . The controlled system RS may be influenced, inter ilia, by flame bodies, combustion and heat exchangers. 
     As illustrated in  FIG.  2    (block B), the broadband lambda probe forms its own closed control loop R 2  for oxygen measurement. Exhaust gas having the residual oxygen content O2 Abgas  enters the measuring chamber MR through the diffusion channel  4  and contributes to the oxygen content O2 MK  in the measuring chamber MK with an oxygen quantity O2 in . The cell voltage VS ist  is measured by means of the measuring cell NZ. As a function of the pump current IP ist , the oxygen quantity O2 pump  is pumped through the pump cell PZ into the measuring chamber MK or out of the measuring chamber MK. A controller VSR controls the measured cell voltage VS ist  to be the target value VS soll =450 mV. When the cell voltage VS ist  is 450 mV, the measuring gas in the measuring chamber MK has the air ratio λ=1. 
     When an error occurs within the control circuit R 2  of the oxygen measurement, which may be caused, for example, by the broadband lambda probe aging, crack formation in one of the cells or blockage of the diffusion channel  4 , the automatic firing unit receives an incorrect measuring value IP ist  and then adjusts to an incorrect residual oxygen content in the exhaust gas O2 Abgas . Within the control circuit R 2  of the oxygen measurement, an equilibrium may be established again and again, so that no error can be detected on the basis of the measured variables typically present (lP ist  and VS ist ). Both the pump current IP and the cell voltage VS are controlled variables that are always controlled towards the target value by the two controllers (VS controller VSR and O2 controller O2R). 
     In order to recognize probe errors and the associated deviation in the residual oxygen content of the exhaust gas O2 Abgas , the operating mode of the gas burner is changed cyclically according to the invention so that additional measuring values can be derived from the control circuit R 2  of the oxygen measurement. The second state of operation is referred to as the probe test mode. The control scheme of the probe test mode is described with reference to  FIG.  3   . 
     In  FIG.  3   , the O2 controller O2R is deactivated. This may be achieved, for example, by virtually equating IP ist  and OP soll . This means that the residual oxygen content in the exhaust gas O2 Abgas  is kept constant. In other words, the degree of opening of the gas control valve remains unchanged and the burner is operated in open-loop controlled operation. 
     Moreover, the VS controller VSR is deactivated in the probe test mode by virtually equating VS ist  and VS soll . In addition, the pump cell PZ is deactivated (see step S 18  in  FIG.  4   ). This is achieved by switching off the voltage supply of the pump cell PZ so that no pump current flows. Consequently, with a pump current IP of 0 mA, the transport of oxygen molecules via the pump cell PZ into the measuring chamber MK (or out of the measuring chamber MK) is stopped (O2 pump =0). The inactive elements are shown hatched in  FIG.  3   . 
     These process steps result in the oxygen partial pressure in the measuring chamber MK matching the oxygen partial pressure in the exhaust gas (O2 MK =O2 Abgas ) and the cell voltage VS ist  drops significantly. Since the cell voltage VS ist  is dependent on the oxygen partial pressure in the measuring chamber O2 MK  via a specific relationship with sufficient resolution, conclusions can be drawn about the residual oxygen content in the exhaust gas O2 Abgas . 
     After a defined dwell time (step S 19 ), a process value VS test_akt (g) is derived from the cell voltage VS ist  by averaging (S 2 ), said value being used for calculations in further process steps (S 24  and S 25 ) and then monitored for limits. A plausibility check is carried out thereby so that errors in the broadband sensor are detected when the process value is not within the specified limit values. 
     In particular, errors in the pump cell PZ and in the diffusion channel  4  can be detected with this method. However, the method requires an error-free measuring cell VZ. This is monitored separately and permanently by measuring the internal resistance Rivs. The method required for this is specified by the manufacturer of the broadband lambda probe. When this value is within a tolerance range (S 5 ), then the measuring cell VZ is error-free. 
     A possible drift in the cell voltage is also adapted to ensure that this method is absolutely error-free. This is done by comparing the pump current IP in air before and after each burner operation (probe calibration S 6  and S 26 ). When the deviation of both values is within a defined tolerance range, then VS test_akt  or VS test  from the last burner operation is included proportionately in the target value VS test_soll  (S 28 ), resulting in a new adapted target value for VS test . 
     Method steps S 1  to S 29  are described in detail below with reference to  FIG.  4   . 
     In step S 1 , the burner is error-free and in standby mode. 
     In step S 2 , it is checked whether there is a burner request from a higher-level temperature controller. When there is a burner request (yes in S 2 ), this is the start signal for burner operation. When there is no burner request (no in S 2 ), the burner remains in standby mode. 
     In step S 3 , the broadband lambda probe  1  is activated. Activating the broadband lambda probe  1  is a standard method that is carried out according to the specifications of the manufacturer of the broadband lambda probe  1 . Here, the broadband lambda probe  1  is first heated by means of the heating electrodes  2  and then the cell voltage is adjusted to 450 mV. 
     When the broadband lambda probe  1  is ready for operation, the cell resistance R IVS  is measured in step S 4 . This measurement is also a standard method that is carried out according to the specifications of the manufacturer of the broadband lambda probe  1 . For this, the measuring cell current I CP  is switched on and off and the cell voltage VS is measured and the internal resistance of the cell (cell resistance) R IVS  is calculated therefrom. The cell resistance R IVS  is output as measuring value d and stored. 
     In step S 5 , the cell resistance Rivs measured in step S 4  is compared with a limit value set at 400Ω, for example. When the cell resistance R IVS  is less than the limit value (yes), then the method continues to step S 6 . A limit value violation (no), i.e. when a cell resistance Rivs is measured to be greater than the limit value, leads to a safety shutdown in step S 29 . 
     In step S 6 , a calibration of the lambda probe  1  is performed in air. 
     When the pump current IP is adjusted and constant, a first calibration factor Kal21 1  (measuring value a) is calculated using a calculation formula. 
     
       
         
           
             
               Kal 
               ⁢ 
               
                 21 
                 1 
               
             
             = 
             
               
                 
                   
                     ( 
                     
                       
                         4 
                         · 
                         A 
                         · 
                         
                           ( 
                           
                             
                               21 
                               ⁢ 
                               % 
                             
                             - 
                             C 
                           
                           ) 
                         
                       
                       + 
                       
                         B 
                         2 
                       
                     
                     ) 
                   
                   - 
                   B 
                 
               
               
                 2 
                 · 
                 A 
                 · 
                 
                   ( 
                   
                     Ip 
                     - 
                     Offset 
                   
                   ) 
                 
               
             
           
         
       
     
     The variables A, B, C and Offset are coefficients of the sensor characteristic which are stored in an automatic firing unit. 
     In step S 7 , it is checked whether the calibration factor Kal21 1  (value a) calculated in step S 6  is within a defined tolerance range, for example by comparing the deviation between the calibration factor Kal21 1  and a target value with a limit value Kal21 obs  (value c). When the calculated calibration factor Kal211 is not within the tolerance band (no in S 7 ), a safety shut down of the burner is performed in step S 29 . When the calculated calibration factor Ka21 1  is within the tolerance band Kal21 obs  (yes in S 7 ), the method continues to step S 8 , 
     Calibration in air is required to compensate for manufacturing tolerances, aging effects of the sensor, and environmental influences such as atmospheric pressure. A deviation of the calibration factor Kal21 1  from the nominal value, defined as the tolerance band Kal21 obs , is interpreted as a sensor error, so that a safety shutdown (step S 29 ) is carried out here. 
     In step S 8  it is checked whether the burner request is still active. If the burner request is active (yes in S 8 ), then the burner is operated in closed-loop controlled operation (continue to step S 9 ). If the burner request is no longer active (no in S 8 ), the method is continued in step S 26  with a renewed probe calibration in air. 
     The closed-loop controlled normal operation (first operating state) of the burner corresponds to steps S 8  to S 17 . First, the burner is ignited. The power modulation then starts in S 9  according to the specification of the higher-level temperature controller. 
     In SW, the probe test mode is activated according to the parameterized cycle duration. The cycle duration results, for example, from the EN 12067 standard and is derived from the risk potential of an erroneous state of the system (gas burner) and its duration. For example, a typical cycle duration for a gas burner is 120 seconds. 
     In method step S 11 , it is queried whether the test mode of the lambda probe  1  is requested according to the cycle duration from the previous step S 10 . If this is not the case (no in step S 11 ), the method continues to step S 12  and the burner is continuedly operated in closed-loop controlled normal operation. If the query shows that the test mode is requested (yes in step S 11 ), the burner changes to the active test mode and the method continues in step S 18 . 
     In step S 12 , the burner is operated in closed-loop controlled normal operation, with the regular probe control circuit R 2  being active and supplying a measured pump current IP ist , which is transmitted to the automatic firing unit. The actual value of the residual oxygen content (e) in the exhaust gas may be calculated from the measured pump current using a characteristic curve. 
     In step S 13 , the automatic firing unit calculates the actual value of the residual oxygen content O2 ist  (measuring value e) in the exhaust gas from the measured pump current IP ist  and the sensor characteristic using the following formula: 
         O 2 ist   =A ·( IP −Offset) 2   ·Kal 21 2   +B ( IP −Offset)· Kal 21 +C  
 
     The variables A, B, C and Offset are coefficients of the sensor characteristic, which are stored in the automatic firing unit. 
     In step S 14 , the O2 controller in the automatic firing unit compares the target value O2 soll  (value f) with the calculated actual value of the residual oxygen content O2 ist  (value e) from step S 13  and determines a deviation. The target value O2 soll  is specified via the parameterization of the automatic firing unit. If the two values match (yes) or if the deviation is less than a predefined limit value, the method continues to step S 16 . If the values are not equal or if the deviation is greater than the specified limit value, the gas quantity V Gas , in particular a degree of opening of a gas control valve as the manipulated variable, is readjusted as a function of the determined deviation in step S 15 . The gas quantity 
     In step S 16 , the internal resistance of the measuring cell R IVS  (value d) is continuously measured during normal operation of the burner. This measurement may be performed using the same method as in the measurement in step S 4 . 
     Similarly to step S 5 , the measured cell resistance R IVS  (d) is compared with the limit value in step  17 . If the limit value is adhered to (yes), the method returns to step  8 . If the measured cell resistance Rvs is greater than the limit value (no), the safety shutdown takes place in step S 29 . 
     In step S 18 , the test mode of the O2 probe is active, with the regular operation (O2 controller) of the burner no longer being active. The burner is therefore no longer in a closed-loop controlled but in an open-loop controlled mode. This state may typically last about five seconds. The voltage supply of the pump cell is switched off in the probe control circuit. 
     In step S 19 , an O2 partial pressure equalization between the combustion chamber and the measuring chamber of the lambda probe  1  is carried out. This process may typically take less than a second. A longer time of two seconds may ensure that partial pressure equalization can also be carried out under disturbed conditions (e.g. blocked diffusion passage). 
     In step S 20 , the cell voltage Vs is measured. This is now typically 60 mV instead of 450 mV (depending on the residual oxygen content of the exhaust gas). 
     In step S 21 , the measured cell voltage VS ist  is averaged over one second in order to compensate for any signal noise. The mean value of the cell voltage in test mode VS test_akt  (g) is stored as an intermediate result. This value is not used directly, but in a weighted manner (see step S 24 ) for monitoring in order to make the process more robust against disturbing variables. If the residual oxygen content O2 ist  in the combustion chamber at the time of the probe test deviates slightly from the target value O2 soll  due to external conditions such as wind or gas pressure fluctuations, this should not affect the result of the probe test too much. 
     In step S 22 , the voltage supplies of the pump cell PZ and of the VS controller are activated again so that the pump current IP returns to the initial level and the cell voltage VS of 450 mV is thus adjusted. 
     It takes about one second to adjust the cell voltage to 450 mV. In step S 23 , there is a pause for an extended time of, for example, two seconds in order to ensure increased robustness since the control behavior may be slower for an aged lambda probe  1 . 
     In step S 24 , the intermediate result from step S 21  VS test_akt  (g) is incorporated into the test result VS test  (h) via a sliding average with a weighting of, for example, 20%. 
     In step S 25 , the result from the probe test VS test  (h) is compared with a dynamic limit value VS test_soll  (i). If the test result VS test  is below the limit value (yes), the closed-loop controlled mode is continued in step S 8 . If the dynamic limit value VS test_soll  (i) is violated, an immediate safety shutdown follows in step S 29 . 
     When the burner request is canceled by the higher-level temperature controller, the method continues to step S 26  after the test in step S 8  (no). In step S 26 , first the flame is switched off and the gas burner or the lambda probe  1  is ventilated subsequently, During the subsequent ventilating, the lambda probe  1  is again calibrated in air. The result of the calibration is a second calibration factor Kal21 2 (b). 
     In step S 27 , it is checked whether the second calibration factor Kal21 2  (b) after the burner run differs from the first calibration factor Ral21 1  (a) from the burner start-up (see step S 6 ). If the deviation (that is, the difference between the values a and b) is greater than a permissible limit value (no in S 27 ), a safety shutdown is carried out in step S 29 . If the deviation of the calibration factors a and b is too large, this indicates an error in the lambda probe  1  or the burner system (e.g. exhaust gas recirculation), so that safe operation can no longer he guaranteed. 
     When the difference between Kal21 1  (a) and Kal21 2  (h) is less than the defined limit value (yes in S 27 ), an error-free lambda probe  1  is assumed, so that a new target value VS test_soll  (i) for the probe test can be determined in step S 28 . This is necessary because the cell voltage VS may drift over the lifetime. In particular, the robustness of the method can be increased in this way. The target value VS test_soll  (i) is corrected or included by 20% with the last result of the probe test VS test  (h). The result is a weighted mean value that allows long-term correction of the target value so that short-term interference does not falsify the target value too much. The robustness of the method can thus be further increased. The new target value VS test_soll  is stored as value i and used in the next probe test (step S 25 ). 
     The features disclosed in the above description, the claims and the drawings may be important both individually and in any combination for the implementation of the invention in its various embodiments. 
     LIST OF REFERENCE SYMBOLS 
     
         
         λ air ratio 
         IP pump current 
         VS cell voltage at the measuring cell 
         VS ist  actual value of the cell voltage 
         VS soll  target value of the cell voltage 
         Kal21 1  first calibration factor (a) 
         Kal21 2  second calibration factor (b) 
         Kal21 obs  tolerance band (c) 
         R IVS  internal resistance of the measuring cell (d) 
         O2 ist  actual value of the residual oxygen content (e) in the exhaust gas 
         O2 soll  target value of the residual oxygen content (f) in the exhaust gas 
         VS test_akt  mean value of VS ist  in test mode (g) 
         VS test  probe test result (h) 
         VS tst_soll  target value for the probe test (i) 
         O2 pump  transport of oxygen molecules through the pump cell 
         O2 MK  oxygen partial pressure in the measuring chamber 
         O2 Abgas  oxygen partial pressure in the exhaust gas 
         O2R O2 controller 
         RS controlled system 
         VSR VS controller 
           1  broadband lambda probe 
           2  heater 
           3  electrode 
           4  diffusion channel 
         PZ pump cell 
         MK measuring chamber 
         NZ measuring cell (Nernst cell)