ASH BEHAVIOR IN A CFB BOILER DURING COMBUSTION OF SALIX

A study on the combustion characteristics of Salix viminalis, a fast g rowing willow, was conducted at a 12 MW circulating fluidized bed boil er. The purpose of the study was to increase the understanding of the mineral matter behavior in the boiler and to foresee possible bed aggl omeration or slagging and fouling problems that may occur during the c ombustion of this type of fuel. Special focus was given to the impact of ash chemistry on the slagging, fouling, and bed agglomeration. Samp les from all ingoing (bed material, fuel) and outgoing solid material streams (secondary cyclone and bag filter) as well as from the bed and the return leg were collected and analyzed chemically. Selected bed s amples and ash samples were also analyzed with a scanning electron mic roscope (SEM/EDAX). Deposit samples were collected at the cyclone inle t and from two different locations in the convective path using specia lly designed surface temperature-controlled deposit probes. All collec ted probe deposits were photographed and characterized visually. Selec ted samples from both windward (front) side and leeward (back) side of the sampling probes were analyzed chemically as well as with SEM/EDAX . In addition to these samples, the boiler operation was monitored car efully. This included collection of operational data (fuel feed, air d istribution, and total air), collection and monitoring of pressure dro ps in the furnace, flue gas temperature profiles, and emissions. Multi component multiphase thermodynamic equilibrium calculations were then performed for predictions of the fly ash thermal characteristics, usin g the fly ash chemical composition as input data. The thermal characte ristics, i.e., the melting behavior, were predicted for the different ash samples and compared with the results from the full scale fouling measurements. The paper discusses the impact of the ash chemistry on t he bed agglomeration and fouling tendency found during the combustion tests and draws conclusions about their relevance to the operation of the boiler.