"Sludge for heat" – European project presentation / S. Kandefer, E.M. Bulewicz, M. Pilawska, J. Baron, W. Żukowski, J. Wrona, J. Porzuczek, M. Olek, S. Chrupek, J. Dzikowska // W: Integration and Optimization ... więcej

"Sludge for heat" – European project presentation / S. Kandefer, E.M. Bulewicz, M. Pilawska, J. Baron, W. Żukowski, J. Wrona, J. Porzuczek, M. Olek, S. Chrupek, J. Dzikowska // W: Integration and Optimization of Sanitation Systems in Urban Areas : proceedings of Polish-Swedish seminars : Cracow, March 17-18, 2005 [Dokument elektroniczny] / eds. E. Plaza, E. Levlin. – Stockholm : Land and Water Resources Engineering, KTH, 2007. – (Trita-LWR Report, ISSN 1650-8610 ; 3018). – S. 61-70 : fot., rys., schem., tab., wykr. – Tryb dostępu: http://energiomiljo.org/kth/Polishproject/rep13/SFH_CUT61.pdf. – Abstr. – Joint Polish-Swedish reports No. 13. – ISBN 978-91-7178-826-9

For small/medium towns there is no standard method for the utilisation of sewage sludge. The problem could be solved by burning the sludge (only mechanically dewatered) together with other locally available ... więcej

For small/medium towns there is no standard method for the utilisation of sewage sludge. The problem could be solved by burning the sludge (only mechanically dewatered) together with other locally available combustible wastes, of higher calorific value. Some of the very damp sludge can be partly replaced by the second waste fuel, irrespective of its physical state (solid, liquid or gaseous). Candidate supporting fuels are e.g. the biomass by-products of the maintenance of vegetation within the city (street trees, parks, etc), partly segregated municipal solid waste (MSW), biogas or even animal waste. The technology selected is to burning sewage sludge in a bubbling fluidised bed (very tolerant of low grade or ”difficult” fuels). It has the advantage that the process can be carried out at the site where the sludge originates, at a relatively low temperature but with good efficiently, avoiding the energy consuming drying stage. Designing a system for the thermal utilisation of sewage sludge was preceded by extensive laboratory tests and was backed by many years of research and industrial experience of the members of the group. A pilot installation, 1 MWt has been designed, with some of the heat produced employed in the gas cleaning system and for heating the premises used by the technical staff and for providing hot water. The combustor is equipped with feed systems for the sewage sludge and for the supporting fuels. On the basis of the experimental results, an automatic process control system has been developed. The effect of bed temperature and air excess on the flue gas composition and the degree of mineralisation of the sewage sludge has been assessed. Burning sewage sludge and supporting fuels on laboratory (5 kW) and pilot (100 kW FBC boiler, type KFD-s14u, own design) scales has shown that, on account of the flue gas concentrations of CO, VOC and NOx, the optimum operational temperature range should be 880-950 o C. In the bed the air excess coefficient should be below 1.6, but it can be increased in the freeboard (the secondary combustion zone). The flue gas concentrations monitored were: O2, CO, NO, NO2 and SO2 (using ECOM®-SG Plus instrument, with electrochemical sensors), VOCs (JUM®, working on the FID principle). The air excess in the freeboard was also continuously recorded. The presence of SO2 and NOx was due to the presence of combined sulphur and nitrogen in the fuel, most likely in organic form. It can be assumed that all organic S is converted to SO2, so that the SO2 concentration is a measure of the S content in the sewage sludge. If necessary, emissions of SO2 could easily be reduced by adding crushed limestone to the fluidized bed, even with the S content of the fuel very variable. Fuel nitrogen may be gasified to either HCN or NH3 or remain in the char. Its subsequent oxidation can lead to the formation of NO, N2O or N2, depending on the combustion conditions. The NOx levels observed with sewage sludge are comparable to those with coal and to reduce them would require a system capable of coping with rapidly changing concentration levels. If the co-combustion fuels are very low in S and N, the flue gas concentrations of SO2 and NOx will fall. Wood chips are nearly free of S and their N content is a factor of 2-3 lower than in the sludge. Flue gases from burning segregated MSW (paper, cardboard, polymers, e.g. PE, PP, PET) are similar to those from wood waste and burning biogas is intrinsically clean. Thus the supporting fuels cannot have an adverse effect on the flue gas emissions, which are determined mainly by the elemental composition of the wastes. The use of supporting waste fuels will improve the degree of oxidation of CO and VOCs. There is practically no unburned material in the ash, which is also fully mineralised. The results obtained for selected metallic elements are relevant to the problem of the fate of these elements during the combustion process and particularly their distribution between the flue gases and the ashes – most metals are retained in the ash. Thus the results obtained show that in a bubbling fluidized bed the combustion can be effective and potential pollutant emissions can be kept down.