IMPROVEMENT IN HOT RE-HEAT TEMPERATURE
Background
BBGS have 2 X 250 MW Natural circulation Double Downshot boilers with low NOx burners. It is provided with Gas Recirculation system for controlling hot Re-heat temperature. The Gas recirculation (GR) duct is provided from ID Fan outlet to rear side of Boiler at 8.5 meter level near bottom ash hopper arch. As per design, GR flow is required for maintaining designed HRH temperature at part load, but no GR flow should be required to attain desired HRH temperature at Full load.
Previous status
BBGS experienced problem of low Hot Re-Heat (HRH) temperature (design 535 °C) for a long period since commissioning. Desired HRH temperature normally could not be achieved either at part load or at full load even with considerable amount of GR flow.
- Low Condenser vacuum (increased steam flow due to low HRH temperature)
- High probability of erosion at end side LP stage turbine blades
- Increase in BFP loading due to high steam flow
- Application of high gas recycling flow to improve HRH temperature,resulted in :
- High dry flue gas loss of Boiler due to high flue gas exit temperature
- Increased Stack SPM level due to increased gas velocity
- Higher ash erosion at boiler back pass
- Increased ID Fan loading
Update
The above problems were overcome through a number of steps, the major of which are as follows :
- Re-heater Pendant zone Soot Blowing system has been commissioned and regularly operated as per requirement, depending on Boiler operational parameters
- Boiler has been put on Auto combustion mode leading to optimization of air and coal flow
- Re-heater biasing valves has been put on auto control resulting in matching of left and right side Re-Heat temperature
- When coal condition is worse than design, 5 mill operation is used instead of previous practice of using 4 mills resulting in better combustion
- Re-heater attemperation system has been commissioned and kept in ‘Auto’ to take care of any excursion in HRH temperature immediately
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