On-line Determination of Ash Particle Size in Circulating Fluidized Bed Boilers

Project Number 214

In order to achieve optimum emission and combustion results, it is important for operators of circulating fluidized bed boilers to be aware of the particle size of bed and circulating ash.

The aim of this research project is to show that there is a clear and reproducible correlation between the chronological pressure profiles in the bed and siphon and the mean particle size of the ash.

To this end Professor C. M. van den Bleek, Professor J. C. Schouten and Dr. J. R. van Ommen of the Technical University of Delft developed an evaluation procedure in which the pressure values were measured using a medium frequency and statistically evaluated by multiple differentiation. Operating parameters such as velocity of the primary air, temperature, loading etc. are filtered out.

This evaluation process has already been successfully used to determine ash agglomerations at an early stage in the straw combustion plant at Grenaa in Denmark, taking the siphon as the measuring point.

The process is being refined in the fluidized bed boiler at Bayer's Uerdingen plant by:

  • selecting the combustion chamber of the fluidized bed boiler as the measuring point (larger volume),
  • using a higher gas velocity,
  • broadening the particle size spectrum under investigation to cover smaller values.

The ash and the pressure profile are additionally measured in the siphon.

The approach adopted involves installing two pressure recorders in the combustion chamber of the fluidized bed boiler and two in the siphons to record the pressure pulsation over a period of several months and store them with a frequency of 100 Hz. At the same time operating parameters relevant to fluidization - such as velocity, temperature, loading etc. - are also stored.

To check the accuracy of the particle size determined by the pressure recorders, the actual particle size distribution in the bed and circulating ash is measured three times a day in the laboratory.

If, as intended, a clear and reproducible correlation can be established between the pressure measurement signals and the mean particle size of the ash, the measuring signals "bed ash particle size" and "circulating ash particle size" can in future be made available to operators of circulating fluidized bed boilers. This would give the operating personnel an immediate insight into the particle size balance of the boiler, and enable the operation of the plant to be optimized.

This would be particularly effective:

  • in the case of load changes,
  • when switching or mixing types of coal,
  • when using supplementary fuel in the form of wood chips or meat and bone meal.

The project is carried out during July 2001 to June 2002 by Bayer AG, Uerdingen Plant, Dr. Rüdiger Krüger, with participation of Delft Technical University, Chemical Reactor Engineering, Dr. Ir. J. R. van Ommen and J. Plackmeyer, Consulting Engineer, Remscheid. From the technical point of view, the project is accompanied by the VGB Working Panel "Fluidised Bed Firing Systems".