One
of the main barriers for the successful utilization of new
biomass feedstocks in commercial pellet boilers is the lack of knowledge
about the combustion behavior of such fuels, particularly in terms
of ash slagging, which can potentially damage the boiler and limit
combustion efficiency. The VI Framework European project Domoheat studied the combustion of 15 Mediterranean woody and herbaceous
biomass fuels in commercial 60 kW domestic pellet boilers. The performance
of several methods for the prediction of measured boiler slagging
was tested, including ash composition-based slagging indices, standard
ash initial deformation temperature (IDT), and a new slagging laboratory
method, the BioSlag test, based on the sieving of
the ash and slag obtained in the combustion of 250 g of fuel in a
furnace at controlled temperature conditions. IDT values allowed us
to identify slagging in fuels with high silica (e.g., rye straw pellet)
or alkali contents (e.g., almond shell) but failed to discriminate
medium slagging of fuels such as contaminated poplar chip. Slagging
indices based on silica and alkali content of the ashes showed success
for predicting boiler-observed slagging. The BioSlag test predicted observed slagging percentage in the boiler with R
2 = 0.87. This test could discriminate both
high slagging of fuels with high Si content (e.g., rye straw pellet,
pinecone seed shell) and the medium slagging tendency caused by alkali
elements (e.g., almond shell, olive stone) and by moderate Si contents
(e.g., contaminated poplar chip).
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