A study of trace element behaviour in two modern coal-fired power plants: II. Trace element balances in two plants equipped with semi-dry flue gas desulphurisation facilities

Two Finnish coal-fired power plants were experimentally investigated with regard to the distribution of environmentally harmful trace elements in the process. The plants were equipped with low NOx burners, an electrostatic precipitator and a semi-dry flue gas desulphurisation unit consisting of a spray-dryer-type reactor and a fabric filter. All the in-going and out-going mass streams were analysed for As, Be, Cd, Co, Cr, Cu, Hg, Mn, Ni, Pb, Se and Tl in the first plant, and for As, Be, Cd, Cr, Hg, Mn, Ni, Pb, Tl and Zn in the other plant. Total atmospheric emissions of trace elements from the plants studied were very low; most of them even orders of magnitudes smaller than 10 μg/m³ (NTP). The vaporous fraction of the trace elements was found to play a predominant role in the atmospheric emissions. However, the total atmospheric emissions of even the strongly volatile Hg were very low, which can be ascribed to the applied plant technology. Enrichment of the elements into the various ash streams—from the bottom ash up to the particulates downstream of the electrostatic precipitator—was found to be in satisfactory compliance with literature data. Element balances were calculated over the whole process and over the desulphurisation unit for both plants: for the whole process the closure (i.e., ratio output/input) of element balances was within ±30% for all elements studied, except for chromium in plant HB, and for the vast majority of elements it was even within ±20%. The results imply that the sampling techniques and analytical methods developed for this work can be well applied to quantitative mass balance studies in this kind of processes.