Toxicity of Ashes Produced During the Combustion and Co-combustion of Coal and Meat and Bone Meal in a Fluidized-Bed Reactor

The replacement of fossil fuels by renewable fuels can contribute to improve the environmental performance of the power production and to move forward in the sustainability way. The experience has shown that the availability of alternative fuels can be an obstacle for its extensive use for energy production, since biomass is not always available. The use of non-hazardous wastes may be a good alternative to biomass, mainly if they are economically unattractive for recycling or if they present a high cost for land-filling. The co-firing of non-hazardous wastes with coal is, therefore, a subject of great interest for the sustainability of energy production and the reduction of the emissions of fossil greenhouse gases. The use of these wastes for energy is promising if they combine well with other fuels during the conversion process for energy production and have no negative effect on the combustion system, on the ash quality and on the gaseous emissions. The DEECA/INETI team has performed three combustion assays in a pilot fluidizedbed combustor: a) combustion of coal; b) co-combustion of coal (85%) and meat and bone meal (MBM) (15%); c) combustion of MBM. Each combustion test has produced three types of ashes: 1) bottom ashes; 2) fly ashes collected in the first cyclone; 3) fly ashes collected in the second cyclone. The UBiA/FCT/UNL team has analyzed of the bulk content of the fuels and of the ashes. The quantification of the bulk content was performed according with the USEPA Method 3051A. The ashes were also characterized for chemical and ecotoxicological parameters. The ashes were submitted to the leaching test defined in the European Standard EN 12457-2. The eluates were also characterized for chemical and ecotoxicological characterization. The chemical characterization of the eluates included the following parameters: pH, dissolved organic carbon (DOC), cyanides, chlorides, sulfates, phenolic compounds, total dissolved solids (TDS), fluorides, As, Sb, Se, Hg Cd, Cr, Cr(VI), Cu, Ni, Pb, Mo, Zn, Ba, Ca, Na and K. The ecotoxicological characterization included the following parameters: i) luminescent inhibition of V. fischeri, after 15 and 30 min exposure; ii) growth inhibition of P. subcapitata, after 72h exposure; iii) mobility inhibition of D. magna, after 48 h of exposure. According to Council Decision 2003/33/EC, the fly ashes produced in the three combustion tests need stabilization prior to landfilling, except the first cyclone ash produced in the co-combustion test of coal and MBM. This fly ash could be landfilled in a hazardous waste landfill. Based on this same Council Decision, the bottom ashes produced during the combustion tests were classified as non-hazardous residues. The ecotoxicological characterization of the eluates followed the principles defined in a modified version of the Criterion and Evaluation Methods for Waste Ecotoxicity CEMWE (1998). The chemical and ecotoxicological characterizations have the same significance. Therefore, if at least one chemical or ecotoxicological parameter has a value higher than the limit fixed on the French regulation, the residues shall be classified as ecotoxic. Otherwise, it must be concluded that there is no significant evidence to classify the residue as ecotoxic. The bottom ashes and the first cyclone ashes produced during the combustion of coal and during the co-combustion of coal and MBM have not shown evidences of ecotoxicity. All the ashes produced during the combustion of MBM are ecotoxic, due to the results of the chemical characterization.