Silicon nitride hot surface igniter as flame temperature sensor (1)
Do you know that it is possible to use a silicon nitride hot surface igniter as a temperature sensor in certain conditions?
In two very good papers published in 2015 and 2019, 2 scientists of The University of Akron, USA, show that it is possible to accurately use silicon nitride hot surface igniter as a flame temperature sensor in premix gas burner equiped with mass airflow sensor. In this condition the igniter can have a dual use (igniter+flame temperature sensor), so it is a very good idea for burner cost reduction as well as burner efficiency improvement. Indeed, if you know the flame temperature, you can accurately calculate the stoichiometric point (optimium efficiency combustion point) of the burner and hence addapt fuel/air inlet to get to this optimal condition. It would be a small revolution in term of gas burner design.
The idea of using silicon nitride element as temperature sensor (as resistance value TR varying with temperature) is not new and multiple attempts have been made to use this property of silicon nitride hot surface igniters, but the accuracy was not good and phenomenon was not fully understood to take advantage of it. This very good papers clearly highlight the correlation with the gas flow and fuel equivalence ratio. With these parameters it could be easy to develop a smart electronic control board to permanently calibrate the igniter to get a very accurate reading of the flame temperature.
Advantages:
- Dual use of the igniter: as igniter and flame tempeture sensor = cost reduction
- Knowing flame temperature allow to accurately calculte the optimum combustion point = better fuel efficiency, less emmission
- Not need of flame monitoring as the temperature can indicate if there is flame or not
- SN igniter are very robust
- Allow flame monitoring in place where ionization method is not possible (ex: hydrogen)
FKK Corporation will continue R&D especially in the purpose to use these property in Green Hydrogen (H2) combustion burner. Indeed in pure hdryogen flame monitoring is very complicate. This new idea can offer plenty of possibility.
Source:
- Title: "Flame Temperature Sensor Based on a Silicon Nitride Hot Surface Igniter"
- Author: R. Shakya, N. Ida; Department of ECE, The University of Akron, Akron, OH, USA
- Documents: Related documents can be view here and here
Here is some interesting extract of the paper:
The maximum theoretically achievable flame temperature in the combustion process is called adiabatic flame temperature, which is specified in either constant volume or constant pressure with no heat transfer to the surroundings. Adiabatic flame temperature is a function of fuel composition, stoichiometry of fuel and air, and temperature and pressure of the reactants. In a constant pressure and constant reactant temperature combustion process, knowledge of the fuel-air ratio gives the temperature of the flame and vice versa.Flame temperature is one of the parameters, which can be used to determine the efficiency of a combustion.
This paper presents the use of hot surface igniters (HSI) that are made up of silicon nitride (SN), for temperature measurements in a premixed combustion system. The use of SN HSI has the distinct advantage of being a dual-purpose device serving as an igniter and as a temperature sensor, and since many combustion system already employs hot surface igniters, their dual use contributes to lowering costs. The resistance of the SN HSI varies with the change in temperature, and this property is utilized for temperature measurements.
Flame temperature is an important parameter of combustion systems. It provides a way to estimate the fuel-air equivalence ratio (φ) for air-fuel mixtures. Knowledge of equivalence ratio permits adjustment of fuel and airflow rates to maximize the efficiency of premixed combustion systems. Since the equivalence ratio is applicable only for premixed combustion systems, the flame temperature sensor discussed is only useful to control the efficiency of premixed combustion systems.
The fuel-air equivalence ratio varies between 0 and 1 for lean mixtures and from 1 to infinity for rich mixtures. Combustion stoichiometry plays a major role in the combustion process as it is directly related to stack losses, unburnt fuel, auxiliary power consumption, and different environmental pollutant formation.Theoretically, there should be stoichiometric ratio of fuel and air for complete combustion. This raises the flame temperature to adiabatic flame temperature.
A method to estimate the adiabatic flame temperature is to use the average specific heat (Cp) method.
These expressions show that the flame temperature is directly related to the equivalence ratio of fuel and air. Theoretically, the maximum adiabatic flame temperature should occur at the equivalence ratio of 1.
The resistance ratio was correlated to the equivalence ratio of the combustion system as shown in Fig. 6. Figure 6 shows that the flame temperatures varied not only with the variations in equivalence ratio but also with the variations in mass airflow rates and gas flow rates for the same value of equivalence ratio.
For the same value of φ, the flame temperature increased with the increase in fuel and gas flow rates. Higher gas flow rate resulted in an increase in heat generation as the combustion was not adiabatic and additional heat generation resulted in an increase in flame temperature. This was the reason for the increase in flame temperature at a specific value of equivalence ratio with increase in gas flow.The mass airflow rate computed using a commercial MAF sensor was used to compensate the effect of increased fuel flow rate in the flame temperature for different equivalence ratios.