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Contenuto archiviato il 2024-05-07

Pollutants reduction in small biomass combustion systems

Risultati finali

The objective of the project was the reduction NOx emissions of a small biomass combustion system, based an innovative technology concept, which is already being used as a commercial technology for heating applications in the domestic sector. The following main results were achieved by the project: - Monitorization of the existing first generation boiler: Monitorization of a first generation 160kWth boiler in operation for house heating was carried out. Grape pips and almond shells were used as fuels. A total of 19 and 23 monitoring days were employed for this purpose (10-12 operating hours/day) with almond shells and grape pips, respectively. The purpose of this work was to obtain detailed results of the emissions and behaviour of the first design boiler in order to be able to detrmine the emissons depletion and boiler improvements achieved with the defined design modifications. With almond shells, very high levels of CO (well above 6000ppm at 11% O2) were measured in the gas emissions, with NO between 100-350ppm, for boiler temeperatures in the dense bed of 950 degrees Celsius to 1200 degrees Celsius. Utilizing grape pips and boiler temperatures of 850 degrees Celsius -1100 degrees Celsius, lower emissions levels (4000- 500ppm) were obtained though the NO emissions increased (200-350ppm). Unburnt particles were observed, particularly in the case of almond shells, in the ciclone ash, which was related with the high CO levels observed with both fuels. Light ash agglomerations were observed with almond shells in the daily collected bed ash. The higher CO emissions with almond shells in comparison with grape pips were associated to the higher density and size homogenity of the last biomass. In all tested days, the boiler showed o good thermal estability. The smokes temperature was of 150 degrees Celsius - 200 degrees Celsius. - Monitorization of the modified design new boiler and comparison between first and new modified design boiler emissions: The CO emissions of the new design boiler prototype varied a lot from one to another tested biomass (eight biomass materials in total were utilized) and were particularly affected by the density and granulometrical composition of the biomass, and the secondary/primary air ratio. The NO emission was significantly influenced by the temperature achieved in the dense bed of the combustor, which, in turn, depended on the biomass haracteristics, such us the heating value and density. The emission levels of the first generation boiler have been dramatically reduced with the new prototype, which, nevertheless has shown much higher emission levels than those obtained in similar process conditions in the experimental boiler, as well as in the calculations with the reactor network. Best emission results were obtained with the more size homogeneous biomasses tested, such us the grape pips and the pine chips. In the best assayed conditions, the average CO emissions were, respectively, of 324 and 1313mg/Nm3, for grape pips and pine chips. The NO emissions observed a small increase with respect the original boiler with grape pips, up to about 450 mg/Nm3, whilst were lower, of 157mg/Nm3, for pine chips. The results achieved with grape pips were well within the limits of the standard EN 303-5 (1998). The efficiency of the boiler was of about 70-75%, which could be improved by improving the isolation of the equipment, as well as by optimising the heat recovery from the flue gases. - New modified design combustor prototype: The result of the project work is a prototype that consists of a 160kWth boiler, with all the design modifications incorporated, and with a flexibility to incorporate further modifications to improve the combustor behaviour.

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