Experimental and modelling studies of radiative heat transfer in flames

Licentiatavhandling, 2013

Combustion of solid fuels is used extensively for electricity generation purposes. However, combustion of fossil fuels leads to CO2 emissions which enhance the greenhouse gas effect. Carbon capture and storage (CCS) is a proposed solution where the CO2 is captured and stored instead of being emitted to the atmosphere. One of the main CCS technologies considered is oxy-fuel combustion. In the present work, the radiative heat transfer occurring in both air-fuel and oxy-fuel atmospheres are investigated. The most common technique for combustion of solid fuels is pulverized coal combustion. Coal is milled and fed to the furnace in a number of burners where the coal is combusted. In the furnace radiative heat transfer is the dominating heat transfer mechanism and both particles and gases contribute to this heat transfer. In oxy-fuel combustion, the air is replaced by oxygen and recirculated flue gases, providing a N2-free flue gas. This changes the radiative properties of the gas and enables flexible operation of the furnaces which requires a fundamental understanding of the heat transfer in the combustion chamber. This work is therefore focused on radiative heat transfer with special emphasis on particle radiation which is an important topic both in air- and oxy-fuel flames. This work combines experimental work and modelling with the aim to discuss and answer important questions related to radiation in coal and gas-fired flames. The two topics in focus are particle radiation and Turbulence-Radiation Interaction. The experimental work has been performed in the Chalmers 100 kW oxy-fuel test unit. A new methodology has been developed to study the particle radiation in coal flames. The method consists of a combination of experimental and modelling work. The experiments include measurements of spectrally resolved radiation, total radiative intensity, gas temperature and gas composition. The spectrally resolved radiation was measured with an FTIR-based probe technique which provides simultaneous estimation of the particle temperature and the amount of particles present in the flame. The radiation modelling is based on particle properties from the Mie-theory and a Statistical Narrow-Band model for the gas properties. The radiative intensity was calculated and finally compared with the measured total radiative intensity with a Narrow Angle Radiometer. The modelled and measured intensity agrees well, which demonstrates the potential of the methodology. The methodology also makes it possible to estimate the soot volume fraction and the contribution from soot to the total particle radiation. The results showed that the soot volume fraction in the centre position of the investigated flame is approximately 1e-7 and that the soot contributes with maximum 40% of the total particle radiation. The Turbulence-Radiation Interactions was evaluated with a simplified model where the temperature fluctuations were measured with the FTIR-based system. The investigation was carried out in two oxy-fuel and one air-fired propane flame. Fluctuations in gas temperature of up to 400 K were seen, but the influence from these fluctuations on the radiative heat transfer was negligible. The temperature fluctuations observed in both the air-fired and the oxy-fuel flames were in the same order of magnitude