Online-grain Size Determination of Ash in Fluidised Bed Combustion Plants

Project Number 253

Knowledge upon the grain size of the bed material is important to fluidized bed combustion plant operators. The power of the internal ash circulation in the combustion chamber is determined primarily by the grain size. This ash circulation again affects the combustion result, i. e. the amount of the nonburned residues and the amount of emissions. In lignite-fired plants, moreover the additional slagging and in circofluid plants, heating surface erosion in the first draught are influenced, too.

Several operators in their mode of operation already do consider the bed grain size, determining the rates by sampling and laboratory analysis. Thus control room personnel gets them delayed and in the form of tables. In the case of online available grain size rates, the control room personnel could immediately adapt the operation mode to amended operating conditions, particularly in the case of load change, conversion and/or blending of coals, additional firing of residues like wood pellets or animal meal.

At the Delft Technical University, an online process for grain size determination of bed material has been developed which requires the bed pressure development as only measuring rate. The average grain size is calculated from the measured pressure development by means of a complicated mathematical algorithm (analogous to the chaos theory). The process has already been applied in two plants by the Delft Technical University in order to predict beginning grain size agglomerations.

This research project is to determine the average grain size in circulating fluidized bed combustion plants by means of the online measuring process. The operating parameters influencing fluidization like velocity, temperature, charging, pressure level etc. are to be determined by tests. For this purpose, throughout the entire measuring period a parallel laboratory analysis of the bed material is realised.

The aim of the project is to verify and present the online measuring process for determination of the average grain size of the bed material for application in circulating fluidized bed plants.

Based upon practical experience from the precursor project 214 that had been executed in 2001 and 2002, for the actual project extensive twin-track pre-tests are scheduled. This is to enable possible disturbances and/or faults to be detected and removed early.

The project will be executed from April 2004 to March 2005 with Sachtleben Chemie GmbH headed by H.G. Hendriks. The VGB Working Panel “Fluidised Bed Firing Systems” will technically support the investigations.