Mass balance determination for trace elements at coal-, peat- and bark-fired power plants: Dissertation

Determining the distribution of trace elements in a large-scale power plant is a challenge for both sampling and analysis. Besides the accuracy determination of the element content, the quantity flow rate of each stream of the plant is also a challenge because there are many streams without any flow recorders. This thesis describes a method to determine the steady-state mass balance of trace elements with highly satisfactory closures. When the method first developed for coal-fired power plants is applied to bark- and peat-fired power plants equipped with a fluidised bed, in order to attain a high closure of balance, some critical factors must be considered (running parallel tests, analysing parallel samples, sampling the main fuel and added fuel separately, including at least one element that is determined well above the detection limit in all the streams, waiting a long time to start sampling after the fuel supplying change etc.). This thesis presents the results and evaluation of the mass balances for 14 trace elements (i.e. As, Be, Cd, Co, Cr, Cu, Hg, Mn, Mo, Ni, Pb, Tl, Se and Zn) determined in three coal-fired, three peat-fired power plants, and one bark-fired power plant in Finland. Each plant is equipped with specific low-NOx burning technology (i.e. low-NOx burners, staged over-fire air suppliers, selective catalytic NOx reduction or fluidised bed combustor) and an electrostatic precipitator. In addition, the coal-fired power plants were equipped with SO2 control technology including a fabric filter or gypsum production unit. Some of the results pertain to the co-combustion of peat and bark with wooden waste material or sludge from the wastewater treatment plant. The outcoming trace element flows of the power plants were low. Most of the trace elements exit the plant via the pulverised fly ash, removed by electrostatic precipitator, except for mercury. The collection efficiency of the electrostatic precipitators (ESP) was from 92 % to over 99 % for As, Be, Cd, Co, Cr, Cu, Mn, Ni, Pb and Zn at the coal-fired power plants. In respect of the atmosphere though, the ESP is followed by a flue gas desulphurisation plant (FGD), which removes the majority of the penetrated trace elements. The collection efficiency of the ESPs were from 94 % to over 99 % for As, Cd, Cr, Mn, Mo, Ni and Pb at the fluidised bed plants. The mercury in the flue gases was removed effectively by a semi-dry flue gas desulphurisation plant in the coal-fired power plants, but it was released mainly in the vapour phase to the atmosphere in the other plants. The results of multivariate data analysis showed that the plants operating at maximum load was able to run well repeatable. The low-NOx burning technique had no significant effects on the enrichments of the trace elements in the outcoming streams compared to those of the conventional burning technique reported in the literature. No significant differences were found in the relative enrichment of trace elements in the outcoming streams of the plants between the coal-fired power plants equipped with semi-dry and wet flue gas desulphurisation. The case fluidised bed plants demonstrate appropriate techniques for co-combustion. The co-combustion tests revealed differences between pure bark and peat combustion according to the differences in the contents of Cd, Cr, Ni and Pb in the outcoming streams of the plant and in the relative enrichment factors of these trace elements. In addition to the relative enrichment factor of trace elements, the ratio of trace element content in the ash samples to the content in the fuel was found to be useful for element characterisation. The peat-fired power plants equipped with bubbling and circulating fluidised beds showed differences in the ratio of Cd, Hg and Pb in the outcoming streams compared to the contents in the fuel.