Analyzing Coal to Gas Conversions

 

Many utilities are considering burning natural gas in a boiler constructed for coal-firing. TRAX has assisted customers by performing studies on full and partial gas-firing conversions to predict the effects of natural gas combustion on draft system pressures.

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WIth natural gas, units will see more severe furnace pressure excursions during an MFT. The figure at right compares a unit MFT from full load while burning 100% coal vs. 100% gas. The more severe pressure excursion is due to the kinetics of gas combustion, and to a control system that was designed for coal only. TRAX looks at gas shut-off timing as well as control system response in order to design the most favorable set-up for gas burning. Along with control changes, we examine stroke times for gas shut-off valves and ID fan IGVs. The ID fan IGV stroke time is critical in making sure the ID fan can react quickly for the faster fireball collapse seen with gas burning.

Testing various fuel trip scenarios enabled TRAX to make recommendations for gas valve stroke times and ID fan IGV stroke times to minimize negative pressure excursions that will be seen in the furnace on a loss of fuel.

TRAX also made controls recommendations that will both assist with the minimum negative pressure in the furnace and also prevent any positive furnace pressure during recovery from the fuel trip.

TRAX can conduct similar  studies based  on other plant changes such as fan upgrades, FGD modifications, or other process changes.  These studies can also be combined with TRAX CFD analysis in areas of special interest.