Calculation of combustion gas flow rate and residence time based on stack gas data

In many situations, it is desired to estimate the combustion chamber gas residence time of operating combustion systems. This is typically accomplished by performing a mass and energy balance around the combustion chamber. Unfortunately, the detailed physical, chemical, and thermodynamic data needed for each of the feed streams, effluents, and combustion gases are often not readily available. Further, a rigorous mass and energy balance calculation can be time-consuming unless a computerized routine is available. It is possible, however, to calculate the combustion gas flow rate and the gas phase residence time of a combustion chamber when only stack gas data and the combustion chamber temperature are available. The technique presented is applicable to systems that incorporate adiabatic saturation cooling of the flue gas using direct water evaporation in a quench chamber or similar device. The technique can be extended to systems in which adiabatic saturation cooling is not achieved (i.e. partial quenching) or those systems incorporating external heat removal (i.e. boilers, indirect scrubber water cooling, etc.). The procedure for determining the combustion chamber flow rate utilizes the concept of a mass and energy balance (in a simplified form) to relate stack gas data to combustion chamber conditions. In the case of a system using adiabatic saturation cooling, the energy in the hot combustion gas is used to directly evaporate water sprayed into the combustion gas stream. The temperature of the combined combustion gas and water vapor stream decreases as energy (expressed as sensible heat and heat of vaporization) is transferred from the combustion gas to the water. This temperature decrease reaches a practical limit when the combustion gas stream becomes saturated with water (the adiabatic saturation temperature). Therefore, assuming that there is negligible leakage of air into the system, the mass of stack gas is equal to the mass of combustion gas plus the amount of water added for cooling. Further, since the water vapor and combustion gas are combined, no energy has left the system, thus the total enthalpy of the cooled stack gas stream is equal to the total enthalpy of the hot combustion gas stream. A simplified mass and energy balance is used to determine the moisture added to the combustion gas stream for cooling, and then the mass flow rate of combustion gas is determined by subtracting this amount of moisture from the measured stack gas mass flow rate. The paper describes the theory behind this calculation technique, presents the formulas needed to perform the calculations, discusses the sensitivity of the calculations to errors in the assumptions used and the data measurements, and describes how the technique can be extended to systems which achieve cooling of the combustion gases through means other than adiabatic saturation.