Author: Sonal Gilani
Thermal oxidizers are used to destroy the waste gases containing volatile organic compounds (VOC) and volatile hazardous air pollutants (HAP) by raising the temperature of the material above its auto ignition point in the presence of oxygen. The high temperature is maintained for sufficient time to complete combustion to carbon dioxide and water.
The destruction efficiency depends on design criteria including temperature, residence time, inlet VOC concentration, compound type, and degree of mixing (EPA, 1992 & ICAC, 1999). The energy intensive processes amounts to fuel costs running in hundreds of thousands of dollar.
Emerson’s James Beall shared a story with me where for over a year a chemical production facility had been operating one of their thermal oxidizer furnaces the hard way—in manual control mode. Running the process in manual led to excessive consumption of natural gas along with additional time and attention that the process demanded from the plant personnel. In other words, running in manual was eating into the plant’s operating margins and compromising the performance of process.
James worked with the chemical manufacturer’s staff to address the situation. He started by analyzing the process system and control scheme and found the problem to be with the signal characterizers in combustion control scheme. The issue was the relationship between the air control valve position and the air flow which is used in combustion cross limit control.
The composition of the feed to the thermal oxidizer had changed. This meant the operating conditions had changed such that the signal characterizer, tuned for different set of operating conditions, was no longer accurate. James recalculated the correct characterization points and retuned the key loops using the Entech toolkit, which helped determine the dynamics of the process. With the new PID tuning parameters applied, the operations staff was able to switch the loops over from manual to automatic.
Additionally, to make the operator’s job easier and based on their inputs, the team made a few key changes to the DeltaV system‘s graphical user interface. These improvements helped the operators to better understand the process control scheme and run the furnace in automatic mode with confidence.
These changes delivered a reduction of natural gas usage by 25% or approximately $70,000 in fuel cost savings for the first year. The operations staff predicts these changes will deliver around $100,000 in subsequent years after ramp up. Beyond energy savings, the risks and penalties associated with regulatory noncompliance were significantly reduced.