Gas quenching probe and field measurements of gaseous cadmium in a 12-MWth circulating fluidised bed combustion boiler

A probe-based technique for measurement of gas phase metal species in flue gas at high temperatures has been developed. This paper deals with work carried out to test and evaluate the technique through field measurements of cadmium species during co-combustion of sewage sludge and other solid fuels in a circulating fluidised bed combustion boiler. During the measurements of vaporous cadmium forms, solid fly ash samples were also collected at the measurement positions. Chemical composition data from these fly ash samples were used to discuss the influence that the fly ash filter cake built up on the probe tip can have on the measurements. Additionally, the solid samples were examined by means of X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques to have a better insight into the problem. Using the data from the field measurements and/or from the integrated measurements and weighing devices of the boiler the mass balance of cadmium from the combustion process was calculated by different methods. The calculation results were fairly consistent. In one method, data from the field measurements was used. This method gave the mass balance for cadmium in the process Cdout/Cdin=113.6%, which was a very good result considering that it involved the analysis of trace amount of metals in full-scale measurements.