Experimental and modelling studies of particle radiation in flames

Doktorsavhandling, 2015

Combustion of solid fuels is an important part of many industrial and power generation processes. The global use of coal in these processes is vast and thus also the related emissions of CO2 to the atmosphere. It is not possible to continue on the path with continuously increasing emissions of CO2 if the global climate targets should be met. Two strategies to reduce the emissions from large scale coal combustion is to apply oxy-fuel combustion, which is one of the proposed Carbon Capture and Storage (CCS) technologies, or to switch fuel from coal to biomass. Solid fuels are often combusted in pulverized form in flames, where radiation is the most important heat transfer mechanism. When firing solid fuels particle radiation is the dominating contributor to the radiative heat transfer. Application of either oxy-fuel combustion or fuel-switching in combustion processes will change the radiative conditions in the combustion chamber, which implies that knowledge about the main heat transfer mechanism is needed when designing or retrofitting furnaces for the new conditions. The aim with this work is to develop a methodology combining measurements and modelling to quantify the radiative heat transfer in flames, with a special emphasis on the particle radiation features. Parameters, which are of particular importance in flame combustion such as particle temperature, particle type and size distribution have been measured, and the influence on the flame radiation has been analyzed using a detailed radiation model. The experimental work was performed in Chalmers 100 kWfuel oxy-fuel test unit and in a 400 kWfuel scale model of a rotary kiln furnace. In the 400 kW unit the influence on flame radiation of co-firing of coal and biomass was studied. The radiative intensity, measured with a narrow angle radiometer, has been used as reference data in all studies. An optical FTIR based system for measurement of the in-flame spectral radiation was tested and a system for extraction of particles from the flames was developed in this work. The particle size distribution and particle type were investigated using a low-pressure impactor and a scanning mobility particle sizer (SMPS). Detailed models describing the gas and particle properties were applied in the modelling work: a statistical narrow band model for the gas properties and Mie- or Rayleigh theory for the particle properties. In all investigated solid fuel flames, particles were found to dominate the radiation. In the investigated lignite flame in the Chalmers unit, char particles were found to be the main contributor with only a small influence of soot. In an analysis of spectrally resolved particle radiation, the temperature of the char particles was estimated to be approximately 200°C lower than the gas temperature in a position corresponding to peak temperature conditions of the flame. The soot volume fraction in a sooting air fired propane flame was determined to be 6E-8 based on measurements with the SMPS instrument, and this concentration resulted in a good agreement between modelled and measured intensity. The results from the 400 kW study showed that it is possible to obtain similar radiation intensity in the co-firing flames as in the coal flames. But, the length of the radiating part of the flames was shorter for the co-firing flames. Radiation measurements in flames of two almost identical coal types for similar combustion conditions revealed a significant difference in the radiative intensity. This result shows the difficulty of predicting flame radiation without performing measurements, since the radiation depends on factors such as soot formation, which is highly dependent on fuel and combustion conditions.