Inorganic matter partitioning in boilers with grate burners and rated output below 25 kW: Ash type and particle forming elements

The inorganic matter of solid biofuels can be categorized into three different ash types: type-S (Si, Al, Fe, Ti), type-K (K, Na, P, Cl, S) and type-C (Ca, Mg, Mn). A total of 9 different biofuels (3 for each ash type) has been analyzed and then combusted in a 25 kW grate fired boiler. Emission factors and partitioning of typical particle forming elements (Ca, K, Na, P and Zn) were determined and show a strong correlation (R 2 = 0.88) with their content in the feedstock. Additionally, gaseous emissions, particle size distribution and emission factors of other major and trace elements were also established. The total ash content of the tested biofuels varied from 0.4 % for spruce up to 32.9 % for paper, however most fuels contained between 5 and 10 %. The emission factors show that the most prevalent element in the flue gas was K (generally contributing over 25 % to total particulate emissions). The release of K into the flue gas varied, with type-K fuels reaching values over 10 %, while type-S fuels only around 6 %, most likely due to the formation of refractory aluminosilicate phases. Moreover, with growing K release, the particle size distribution gradually shifted from 0.14 up to 0.59 μm. • Biofuels prone to slagging show lower KCl and K 2 SO 4 emissions than those without slagging tendencies. • The aerodynamic diameter of particles emitted from small-scale boilers does generally not exceed 1 μm • Zn is among the most volatile contributors to total PM emissions • Ca, Mn and refractory compounds (Fe, Al, Ti, Mg) contribute little to total PM emissions • Total inorganic matter emissions are linearly proportional to the content of K, Cl, S, P, Na and Zn in the feedstock