Gasification of N-rich fibreboard in an air-blown fluidized bed reactor: A study on the fate of tars, NH3 and HCN during oxidative mild Hot Gas Filtration

Abstract

Wood wastes, such as Medium Density Fibreboard (MDF), are interesting low-cost fuels for gasifiers. The present study examines the effect of oxidative Hot Gas Filtration (HGF) on gasification indicators and pollutants (tars, NH3 and HCN) of producer gas. Pellets of N-rich MDF were gasified in an air-blow bubbling bed reactor at 800 °C coupled with a downstream HGF unit equipped with multilayer metallic filters and operated at mild temperature (400–600 °C). The fate of pollutants was assessed by complementary analytical techniques (HPLC, UV-fluorescence, GC/MS-FID and wet chemical methods for N-compounds). The deposit of the filter was also sampled after gasification trials and characterised by SEM-EDX.For the reference tests, conducted without the HGF unit, tar concentration was 6.5 g/Nm3 (dry gas, benzene-free) while, 43% of Fuel Bounded Nitrogen (FBN) was converted to NH3 and 4% to HCN. Oxidative HGF tests showed that secondary reactions inside the HGF unit undergo in homogeneous (gas-gas) and heterogeneous (gas-solid) phase. At a relatively high secondary air flow rate brought in the HGF unit, the severe oxidation of the dust cake particles led to irreversible melting, aggregation and sintering phenomena of ash deposits on the surface of the filter, as well as, to a drop of 49% in the tar yield. Overall, this work demonstrates that the addition of secondary air inside the HGF unit should be carefully tailored in order to minimise the exothermic oxidation of dust cake particles and, to promote long-term operation of the filter.