Foliage and Grass as Fuel Pellets : Small Scale Combustion of Washed and Mechanically Leached Biomass

The high contents of disadvantageous elements contained in non-woody biomass are known to cause problems during small and large scale combustion, typically resulting in a higher risk of slagging, corrosion, and increased emissions. Mechanically leaching the respective elements from the biomass through a sequence of process steps has proven to be a promising solution. The florafuel process used here is comprised of size reduction followed by washing and subsequent mechanical dewatering of the biomass. Densification of the upgraded biomass into standardized pellets (Ø 6mm) enables an application in existing small-scale boilers. The presented combustion trials investigated the performance of pellets made from leached grass, foliage and a mixture of both in two small-scale boilers (<100 kWth) with slightly different technology (moving grate versus water-cooled burner tube) during a 4-h measurement period. Emissions were in accordance with German emissions standards except for NOx (threshold is 0.50 g/m3) in the case of pure grass pellets (0.51 g/m3) and particulate matter (PM) in all but one case (foliage, 13–16 mg/m3). An electrostatic precipitator (ESP) unit installed with one of the boilers successfully reduced PM emission of both the grass and mixture fuel below the threshold of 20 mg/m3 (all emission values refer to 13 vol.% O2, at standard temperature and pressure (STP)). Bottom ash composition and grate temperature profiles were analyzed and discussed for one of the boilers.