Combustion of Refined Renewable Biomass Fuel (RRBF) in a bubbling fluidized bed
The outlet of a mechanical biological treatment plant for mixed municipal solid waste is further processed to produce RRBF (Refined Renewable Biomass Fuel) within the frame of the EU Life+ project MARSS (Material Advanced Recovery Sustainable Systems). The input for the MARSS processing is dry and has small particle sizes below 40 mm, which makes it impossible to be burnt in standard grate firing systems. The main purpose of this study was to examine whether RRBF is a suitable fuel for bubbling fluidized bed combustion for the decentralised production of combined heat and power. RRBF was fed into a bubbling fluidized bed combustion plant with a nominal fuel input of 100 kW in 3 combustion tests. Proximate and ultimate analyses were performed for original RRBF and fly ash. The fuel analyses showed high ash contents between 25 and 37 weight-%, while the lower heating value lay in the range of 10.5 - 12.9 MJ kg-1. The ash softening temperature was above 1,150 °C and therefore no bed agglomeration was observed. Combustion at around 900 °C could be maintained without preheating of the combustion air. The carbon content of the fly ash was about 1 %, which indicates complete combustion in the bubbling fluidized bed with sufficient residence time. The content of phosphorous in the fly ash was above 1 % and therefore this is an interesting material for prospective phosphorous recovery, potentially together with sewage sludge or ash from sewage sludge combustion. RRBF is a suitable solid fuel for fluidized bed combustion.