FEED FORWARD FIRED HEATER CONTROL
Tighter emission regulations and high-energy costs pose new challenges to control systems for fired heaters. Quick changes in fuel gas heating value, air demand and composition are typical for applications in oil refineries,chemical plants and many other sites. Traditional feed-back control based on temperature and stack oxygen and combustibles measurement are not quick enough to deal with fast changes effectively. This shortcoming is typically addressed by controlling the excess air set point with a certain safety margin. Unfortunately, whereas this approach prevents the emission of unburned components, it increases the CO2 emission due to poorer fuel economy: air is heated unnecessarily and heat transfer efficiency is reduced. Moreover, NOx formation is promoted as a result of the higher oxygen level in the combustion process. For these reasons feed forward control of the air/fuel ratio is gaining more attention. Proper selection and installation of the fuel gas property analyzer and using the right control parameters is essential to get the best results.