Ash Fouling Under Co-Firing in a Pulverized Fuel Combustion Rig

Co-firing of coal and biomass in existing coal-fired power stations is a cost-effective method to reduce CO₂ emissions in energy generation. Nevertheless, the introduction of biomass has to be carefully considered since it could significantly modify combustion and heat transfer phenomena and enhance fouling and corrosion inside the boiler. This paper investigates the effect of substituting a fraction of coal by biomass on the heat transfer and ash deposition rates, by performing pilot tests under different operating conditions in a pulverized fuel combustion rig. Fouling rates have been characterized by means of air-cooled deposition probes installed at one tube bank, reproducing the performance of a large-scale superheater. Heat transfer has been simulated coupling thermal radiation models with semi-empirical approaches for the tube bank behaviour. Ash samples compiled from the wind- and the lee-side of the probe has been collected and analysed by SEM (Scanning Electron Microscopy). Low-to-moderate fouling rates have been typically observed for the tested coal and coal + biomass blends, but with somewhat potassium enrichment at the lee-side deposits when biomass is introduced. As a matter of fact, sootblowing manoeuvres in utility boilers should not be affected when co-firing the tested fuels. Furthermore, chlorine-induced corrosion on heat transfer surfaces is not expected to be significant since the concentration of chlorine in the sampled deposits has been always found to be negligible.