Thermal Valorization of an Animal Sludge for Energy Recovery via Co-Combustion with Olive Kernel in a Fluidized Bed Unit: Optimization of Emissions

The thermal valorisation of an animal sludge was investigated through combustion and co-combustion experiments with an agricultural residue, olive kernel, in a fluidized bed unit. The performance of the fuels and their blends in terms of efficiency and emissions was tested under different operating conditions. The thermal behavior and the reactivity of the fuels as well as the distribution of product gases were provided by thermogravimetric mass spectrometric experiments. The effects of excess air ratio, fuel loading, and blending ratio were examined, and a model was applied in order to provide optimal values of process variables for minimizing emission costs. During fluidized bed combustion, both fuels burned mostly within the bed with a high efficiency. CO emissions were low, SO2 emissions were negligible, and NO emissions of animal sludge were similar or lower than those of olive kernel. An increase in excess air or high fuel feeding increased CO and NOx levels. Co-combustion showed an additive behavior. The CO emissions of the mixtures were higher, whereas the NOx emissions were similar or lower than those produced from combustion of olive kernel. The model of emissions cost indicated that the optimum emission performance and cost were accomplished when the combustor was fed with a fuel mix of olive kernel animal sludge 70/30 by weight, at a rate of 0.72 kg/h and excess air 30%.