Studies of peat properties for fluidized-bed gasification: Licentiate thesis

The impetus for this study was given by problems due to the formation of ash deposits in the High Temperature Winkler gasifier (HTW), when peat was used as a raw material. These problems were unforeseen, as the properties of peat determined conventionally did not differ significantly from those of lignite, which was used as a raw material without problems. During peat gasification tests in the HTW gasifier, it was found that the formation of ash deposits hampered the operation of the process by glogging cyclones and reactor outlet. This deposit formation was unexpected, as the process temperatures were controlled to be below the ash fusion temperatures. In this licentiate thesis, properties of peat used in the fluidized-bed gasification process were studied in order to understand the problematic behavior of peat ash in the process, and factors affecting it. The subjects of the study were, how ash is formed and how it behaves in gasification conditions. In microscopic studies it was found that the deposits were formed from spherical ash particles, which were sticked together by a neck forming a porous structure. Therefore, the deposit formati on was considered to be a result of fly ash sintering, which followed the mechanism known as the sintering by viscous flow. Consequently, sintering of different peat ashes, effect of ash chemical composition and ash viscosity were studied experimentally in laboratory. The sintering of ash was studied with a method developed for fly-ash sintering. The method is based on strength measurement of sample pellets, which are heat-treated at temperatures between 700 and 900 oC. This method relates to the strength of a deposit formed in a process. With the method it was tested, how the adding of iron, sodium and phosphorus to a known peat ash affected sintering and how the sinterin g tendency of several peat ashes varied. In addition, viscosities of different peat ash slags as a function of temperature were measured with a needle-penetration viscosimeter. The microscopic examinations revealed that gasification does not take place homogeneously throughout peat char particles. In peat char particles, ash appeared locally when the average ash content of the char sample was approximately 40 %. From the sintering measurements made for various peat ashes, it was concluded that temperatures between 700 and 800 oC are already sufficiently high to sinter, in reducing gas atmosphere, peat ash pellets to hard bodies corresponding to the strength of a real deposit. Adding iron to the known ash resulted in an increase in the sintering strength. However, alkali metal and phosphorus addition did not show any increasing effect on sintering according to this method. The chemical composition of peat ash correlated well with sintering, when the composition was expressed as a so-c alled basic-acid oxide ratio (B/A ratio). The correlation between the B/A ratio and the sintering strength was best for the samples measured after heat-treatment at 800 oC. According to the results of the viscosity tests, the viscosity was greatly dependent on temperature, and it varied greatly from one peat ash to another. The ratio of basic-acidic oxides (B/A ratio) in ash correlated well with ash viscosity measured at 1 000 oC.