Figure 3 Examples of pilots from Zeeco firing high hydrogen fuels
fuels. As the H 2 content increases, the air door must be closed further to maintain the optimal fuel and air mixture for flame speed versus exit velocity (see Figure 3 ). Pilot flame detection Many burner pilots used on fired heaters are equipped with ionisation rods for pilot flame detection. Flame rods work through the ionisation/rectification process to complete an electrical circuit. When the flame rod is energised, the current produces a positive “ Any burner designed to operate with a varying fuel composition ranging from natural gas to high H₂ content should have a fully metered combustion control system, coupled with a Wobbe Index meter ” charge that attracts negative ions in the flame. The positive ions produced in the combustion process are attracted to the grounding area of the pilot tip. By attracting more positive ions to the ground, the flow of electrons is rectified and flows in one direction. This produces a direct current signal that indicates the presence of flame. For the ionisation system to function properly, a significant number of ions must be present in the flame. H2 flames produce few ions compared with organic compounds, so a weak current is generated that may not be detected by the flame ionisation module. Therefore, depending on the H2 content, flame rods may not be the best method for flame detection, and
alternative means of pilot flame detection must be considered. One method of pilot flame detection is the use of a flame scanner that is mounted to the rear end of the pilot. The flame scanner is sighted on the pilot tip for detection of the flame. The pilot tip shield can be specially modified so the flame scanner does not detect a signal from the main burner flame. Instrumentation and controls considerations When utilising H₂ as a fuel source, the final topic to be considered is controls and instrumentation required for safe firing. Any burner designed to operate with a varying fuel composition ranging from natural gas to high H₂ content should have a fully metered combustion control system, coupled with a Wobbe Index meter or, in some cases, a specific gravity meter. The Wobbe Index meter monitors the varying fuel stream composition and provides the necessary input to the control system to properly adjust the fuel/air ratio control in the combustion control system. The inability to monitor the fuel stream composition and adjust the combustion control system to those changes can lead to a potentially unsafe, fuel-rich condition. ZEECO, ZEECO, INC., AND FREE JET are registered trademarks of Zeeco, Inc. in the US and other jurisdictions. Reference 1 Irvin Glassman, Combustion , Second Edition, Academic Press, 1987.
Eric Pratchard Eric_Pratchard@zeeco.com
Refining India
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