Comparison of ash from PF and CFB boilers and behaviour of ash in ash fields

Abstract

Over 90% of electricity produced in Estonia is made by power plants firing local oil shale and 25% of the boilers are of the circulating fluidised bed (CFB) variety. In 2007 approximately 6.5 million tons of ash was acquired as a byproduct of using oil shale for energy production. Approximately 1.5 million tons of that was ash from CFB boilers. Such ash is deposited in ash fields by means of hydro ash removal. Ash field material properties have undergone changes as a result of CFB ash deposition - the ash field surface is not stabilising, the ash is not becoming petrified and is unsuitable for building dams needed for the hydro technology application. The analytical research was dedicated to determining the reasons for this different ash field behaviour of ash from CFB and pulverised firing (PF) boilers based on changes of ash properties. Comparative CFB and PF ash studies were conducted: chemical analysis, surveys with X-ray diffractometers (XRD) and scanning electron microscopes (SEM), study of binding properties. Bottom and electrostatic precipitator ash was scrutinised, with its typical rougher and finer particles, accordingly. Such ash types comprise the predominant current ash deposit content in ash fields. The ash samples were taken from boilers of the Balti Power Plant. The Estonian oil shale mineral part consists mainly of the following minerals: calcite 44.0%, dolomite 19.5%, quartz 8.7%, orthoclase 10.5%, hydromuscovite 8.6%. These minerals comprise 91.3% of the mineral part and it should be noted that 63.5% of that are minerals in the carbonaceous part and 27.8% - in the terrigeneous part. When oil shale is fired, thermal decomposition of these minerals occur into simpler compounds, coupled with volatilisation of some compounds and formation of novel minerals, and there are changes in the mineral phase state as well. The ash binding properties and behaviour in ash fields depend on these changes, the extent of which is determined by the firing temperature in the boiler furnace. The comparative studies indicate that the introduction of the CFB method of firing has caused several important changes in the mineralogical composition and properties of oil shale ash. The reason behind this is that the CFB boiler furnace firing temperature is approximately 600°C lower than in the PF counterpart, resulting in weaker fuel mineral decomposition and lower novel mineral formation intensity during combustion. The ash solidification tendency is in this instance substantially less than that observed in PF ash. The compressive strength of the electrostatic precipitator ash test samples from CFB boilers was 4.4 N/mm2; from PF boilers -15.3 N/mm2, the corresponding specific surface area: 4 533-9 806 cm2/g and 707-3 966 cm2/g. The obtained data confirms the need to alter the ash storage technology.