This project focuses on evaluating whether lowered bed temperature in FB-boilers for waste combustion gives operational benefits, such as reduced fouling in the convection pass. If so, this mode of operation could reduce the number of unplanned boiler outages, reduce the need for soot blowing, and extend the lifetime of the superheaters at unaltered steam temperature.
The project is based on full-scale experiments performed at Ryaverket in Borås. The plant has two waste-fired 20 MWt FB-boilers. The study is based on a comparison between operational data and measurement results from two different operating conditions of the boilers. In addition to the data that normally are logged by the control system, samples of fuel, ashes, particles, and deposits were taken and subsequently analysed.
The structure of the bed ash was altered by lowering the bed temperature. Under normal boiler operation, the bed ash contains many small agglomerates that disappeared when the bed temperature was lowered. Due to this, the sand consumption of the plant could be reduced by roughly 25 %.
At lowered bed temperature, the concentration of chlorine increased in the bed ash and in the recycled sand while it decreased in the ashes from the cyclone and fabric filter. The concentration of HCl in the flue gas increased as the bed temperature was lowered. This is considered a consequence of less chlorine forming alkali chlorides. Moreover, the particle measurements showed that the amount of submicron particles decreased during lowered bed temperature, which also is an indication of less alkali chlorides in the flue gas.
The deposit probes showed an approximate 20 % reduction of the fouling rate when the bed temperature was lowered from 876 to 714°C. The chlorine content also decreased in the deposits. For the deposit probes at 500°C, (corresponds to a steam temperature of 465°C) significant amounts of KCl were found in the deposits, even when the bed temperature was lowered.
An economical estimation of the consequences from lowering the bed temperature showed a saving of ca 1000 kSEK per year for the boilers in Borås. Please note that this is an uncertain estimate.
The results are of interest for plant owners that may improve the operation of existing plants by adjusting the operating point. Furthermore, the results can be used by boiler manufacturers to further improve the design of new facilities.