Probe Measurements of Ash Deposit Formation Rate and Shedding in a Biomass Suspension-Fired boiler

The aim of this study was to investigate ash deposit formation rate, heat uptake reduction and deposit removal by using advanced online ash deposition and sootblowing probes in a 350 MWth suspension-fired boiler, utilizing wood and straw pellets as fuel. The influence of fuel type (straw share in wood), probe exposure time, probe surface temperature (500, 550 and 600 oC) and flue gas temperature (600 - 1050 oC) on ash deposit formation rate, heat uptake by the probe, the fly ash and deposit characteristics, and deposit removal have been investigated.

The results indicated that increase in flue gas temperature increased the ash deposit formation rate. It was also found that probe heat uptake reduction was not strongly sensitive to the deposit mass load on the probe. This indicated that the heat transfer from the flue gas to the probe was dominated by the rear side, where little material was accumulated on the probe if the deposit mass load was very high on the flue gas facing side of the probe.

Results of deposit removal by using sootblowing probe indicated that probe exposure time and surface temperature significantly influence the Peak Impact Pressure (PIP) needed to remove the deposits. The video recordings of all deposit probe experiments revealed that deposit shedding was primarily through debonding from the surface of the tubes in the superheater region.

Chemical analysis of fly ashes indicated that during suspension-firing of straw and wood, the fly ashes were rich in Si, K, Ca and Cl, but the relative percentage of volatile elements (K, Cl and S) was much less compared to grate-firing conditions. The chemical analysis of the deposits showed that the inner most layer was dominated by contents of K, Cl and S compared to the upper layers rich in K, Si and Ca