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Oxygen Analysers

                OXYGEN MONITORING FOR  COMBUSTION CONTROL.

COMBUSTION

Combustion is an exothermic reaction of fuels with oxygen
when oxygen from air is used, for every cubic meter of oxygen, four cubic meter of nitrogen is present.

COMMON COMBUSTION REACTIONS

    2C     + O2     —> 2CO
    2CO     + O2     —> 2CO2
    S     + O2     —> SO2
    2H2     + O2     —> 2H20
    2S     + 3O2     —> 2SO3   

COMBUSTION KINETICS

    Combsution Reaction is faster, when

    - Temperature is higher.
    - Proximity of fuel molecules with oxygen.
    - Availability of oxygen near the burner Heat Release is faster.
    - Away from the burner, availability of oxygen and the temperature  reduces and hence reaction slows  down.


NEED FOR AIR FUEL RATIO CONTROL

    The burner design / furnace configuration determines mixing of fuel & air and also the temperature of the zone
    The only controllable  factor is oxygen availability
    Lower  the  oxygen combustion is incomplete.
    Higher the oxygen, excess oxygen with  nitrogen  in air takes away heat
    Both cases causes flame temperature reduction , resulting in reduced heat transfer
    Poor burner condition increases the optimum  air required for complete combustion
   
COMBUSTION PRODUCTS ANALYSIS

    Fuel can be measured
    Air infiltration cannot be measured
    Hence combustion product analysis is used for  air fuel ratio control
    Carbon-di-oxide
        No reliable continuous monitoring
        Optimum varies with  fuel type
    Carbon monoxide
        Excess air is not indicated
        Only "indirect"  methods of measurement available
    Nitrogen
        Does not indicate combustion levels
    Oxygen
         Direct physical principle  based measurement
        Other indirect methods reliable to large  extent
    Oxygen measurement
        chemical / orsat apparatus
        electro chemical sensors based on oxygen availability
        para megnetic and infra red variation of other gases can alter              oxygen reading
    Zirconia based sensor
        Based on first principle
        Not affected by other gases
        Very high repeatability and reliability

CONVENTIONAL ZIRCONIA SENSORS
   
    Located at low temperature as materials of construction cannot
          withstand high temperature
    Requires external heater to enable  Ionic conductivity
    Presumtion of "Constant Temperature" discounts thermal stresses.

LOCATION OF ZIRCONIA SENSORS

    Conventional location is well alter combustion gases are cooled to less
     than 500°c.
    Combustion reactions slowdown drastically below   600°c.
    Any infiltration of air would be reflected in sensor reading but would
     not be part of combustion.
    Locating sensors at temperature above 700°c over comes this handicap    
     and provide  reliable reading for oxygen trimming.
    When combustion is in complete due to deficient air,  co monitoring is    
       required with conventional sensor.


NET AND GROSS OXYGEN

    If the high temperature oxygen sensor shows oxygen values as if
     combustion reaction's are completed co sensors would not be required
    Let us call the un reacted total oxygen as "Gross" and the oxygen which
     would be present after completion as net.
    Assume gross oxygen is                      5%
    Consumption  by  un burnt fuel is         3%
                                    ----
    Net oxygen                                       2%
                                             ----


IDEAL  OXYGEN SENSOR

    Zirconia sensors work from first principles of physics based on nernst
    equation

        mv = kT ln(pa/pc)
 
    k is cell constant of zirconia which does not vary with composition/
     ageing
    T is the absolute temperature of the cell - which is measured, and can be independantly verified
    mv is the Cell millivotls, which is measured
    Pa is the oxygen content in the reference air, which does not vary unless measured in high altitudes

    Pc is the actaul volumentric oxygen percentage( as partial pressure) , calculated from the above variable.

    There is no value, which might change with time, and thus, any calibration need  would be only for compensating for air
     infiltration acreoss the sensor - or mixing of reference air and the gas being analysed for oxygen.
    If this infiltration is eliminated "calibration" is a myth.

SECO'S CONSTRUCTION


picture


ADVANTAGES OF SECO'S CONSTRUCTION

    No Joints - No leak - No calibration
    Net oxygen instead of gross oxygen
    Simple verification of sensor functioning by increasing reference air pressure
    Scientifically designed problem reporting format identifies, combustion problems also, along with diagnosis of sensor.