Ash Behavior in a CFB Boiler during Combustion of Salix

Abstract

A study on the combustion characteristics of Salix viminalis, a fast growing willow, was conducted at a 12 MW circulating fluidized bed boiler. The purpose of the study was to increase the understanding of the mineral matter behavior in the boiler and to foresee possible bed agglomeration or slagging and fouling problems that may occur during the combustion of this type of fuel. Special focus was given to the impact of ash chemistry on the slagging, fouling, and bed agglomeration. Samples 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 samples and ash samples were also analyzed with a scanning electron microscope (SEM/EDAX). Deposit samples were collected at the cyclone inlet and from two different locations in the convective path using specially designed surface temperature-controlled deposit probes. All collected probe deposits were photographed and characterized visually. Selected 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 carefully. This included collection of operational data (fuel feed, air distribution, and total air), collection and monitoring of pressure drops in the furnace, flue gas temperature profiles, and emissions. Multicomponent multiphase thermodynamic equilibrium calculations were then performed for predictions of the fly ash thermal characteristics, using the fly ash chemical composition as input data. The thermal characteristics, 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 the bed agglomeration and fouling tendency found during the combustion tests and draws conclusions about their relevance to the operation of the boiler.

Predicting Bed Agglomeration Tendencies for Biomass Fuels Fired in FBC Boilers: A Comparison of Three Different Prediction Methods

“…The sintering test has been used on several occasions before to both predict bed agglomeration tendencies for FBC coal combustion , as well as for slagging and fouling predictions for PC firing systems − and studying the sintering mechanism. , …”

Ash Behavior in a CFB Boiler during Combustion of Salix

“…The cause for the type 4 deposits found in the sampling location L3 may be explained with a mechanism involving the reaction of free lime in the fly ash particles with CO 2 (g). It has earlier been shown 11 that if CaO is found in a fly ash particle as it hits a surface, it may react at the surface with CO 2 (g) and form a deposit. CaO particles recarbonizing to CaCO 3 have further been shown in lab scale to cause significant neck growth between the particles.…”

The Role of Amorphous Material in Ash on the Agglomeration Problems in FB Boilers. A Powder XRD and SEM-EDS Study

“…The occurrence of agglomeration is expectedly both fuel- and bed-dependent. − If ash has a low melting point, it is sticky by itself under the operating conditions of the boiler and can act as an adhesive between bed particles. These kinds of ashes are typical to fuels with a high iron(II) content, i.e., peat .…”

“…Even if the melting point of ash is sufficiently high and it should therefore be unproblematic by itself, it can interact directly with the bed material and form an adhesive medium between the bed particles. Ashes from some biomass fuels, i.e., bark or forest residues, show this kind of behavior. ,, Adhesive material can also be formed in reactions between the bed material and volatile alkali metal compounds that are released from fuels such as plywood waste or straw. ,− …”