Characterization and Quantification of Deposits Buildup and Removal in Biomass Suspension-Fired Boilers

Utilization of biomass as wood or straw in large suspension­fired boilers is an efficient method to reduce the use of fossil fuels consumption and to reduce the net CO2 formation. However, the presence of chlorine and alkali metals in biomass (straw) generate ash with a low melting point and induce large problems of ash deposit formation on the superheater tubes.

Full scale studies on biomass ash deposition and removal had been done on biomass grate boilers, while only limited data is available from biomass suspension­firing. The aim of this study was to investigate deposit mass uptake, heat uptake reduction, fly ash and deposit characteristics, and deposit removal by using an advanced online deposit probe in a suspension­fired boiler using wood and straw pellets as fuel.

The influence of fuel type and probe exposure time on the ash deposition rate, the heat uptake, the fly ash and deposit characteristics, and deposit removal have been investigated. The final deposit mass signal after a residence time of 3 to 5 days region was 1041, 1475, 1520 and 1670 g/m2 for 35, 65, 80 and 100% straw share respectively in the superheater region (flue gas temperature, 800­900 oC), while the mass uptake was very small in the tube bank region (flue gas temperature, 550­605 oC) during pure wood­firing.

It was found that during suspension­firing of pure straw at low boiler load, the overall weight uptake is comparable with grate­firing, even though the amount of fly ash generated was significantly higher during suspension­firing. Deposit removal through debonding was the main mechanism of deposit shedding when no plant sootblower was in operation.

Elemental analysis of fly ashes and deposit samples was made in order to determine concentrations of the major elements Al, Ca, Fe, K, Mg, Na, P, Si, S and Cl. It was identified that the straw suspension­firing fly ashes contain high contents of Si and Ca, while grate­firing fly ashes contain higher contents of volatile elements K, Cl and S.