IMPACT OF FUEL QUALITY AND OPERATING PHASE ON EMISSIONS OF A LOG WOOD BOILER

Recent statistical surveys (2012) indicate that firewood corresponded to 23.8% of the total thermal energy consumption of households in Greece. This high percentage is a fairly recent development due to the increased taxation of heating oil and the lifting of a long-standing ban on biomass combustion in the urban centers. Good environmental performance of biomass heating systems require the use of a suitable fuel supply, however lack of experience means that customers are often unaware of the impact of fuel quality on the performance of their heating system. The purpose of this paper is to present efficiency and emission results of a 17 kW log wood boiler operated with three different varieties of wood coming from beech, olive and oak trees. The three fuel batches differ significantly in terms of main characteristics: beech: 9.5% / 0.7 %, olive tree: 13.9% / 0.6%, oak: 18.0% / 3.6% weight percentage for the as received moisture and ash on a dry basis respectively. The boiler is installed in a laboratory test stand and subjected to a common test procedure including the initial heating up phase, constant load operation and cool down phase. Emissions are continuously monitored and results indicate that during constant load operation CO, OGC, NOx and dust are generally at levels that respect the emission limits of EN 303-5 / Class 3 for all tested fuels, while NOx emissions are closely related to the fuel-N content. High CO emissions are observed for all tested fuels during the start-up phase and cool down phase, while high dust emissions during the heat-up phase are common. Overall results indicate that beech and olive tree yield quite similar environmental performance results, while oak has the higher unburnt emissions taking into account both the constant load operation and the calculated annual emissions.