Quantification of Ash Deposit Build-up and Removal in a Straw and Wood Suspension-Fired Boiler
Bashir, M.S., Jensen, P.A., Frandsen, F. J., Wedel, S., Dam-Johansen, K., Wadenbäck, J. , Pedersen, S.T.
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 suspensionfired 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. A systematic mathematical procedure was used to determine derivative deposit formation (DDF) rate. The overall derivative deposit formation (DDF) rates were measured to be between 0 to 3200 g/m2h. Large differences in the DDF rates are observed without the causes presently known. However, the results indicated that an increase in flue gas temperature increases 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 ash material was accumulated while 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 suspensionfiring 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 gratefiring conditions. The chemical analysis of the deposits showed that the inner most layers was dominated by contents of K, Cl and S compared to the upper layers rich in K and Si.
biomass, fly ashes, fouling, straw, wood, slagging, deposits
Biomass Conversion Technologies for Heating, Electricity and Chemicals
19th European Biomass Conference and Exhibition
1019 - 1032
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