Fuel Gas Firing – Heating Value
Most fuel gas systems gather gas from a number of sources, perhaps from several of these:
• A natural gas pipeline or utility company
• A butane vaporizer
• A propane vaporizer
• Process-plant excess gas
Heating values for the first three sources are about 1000, 3300, and 2500 BTU/cubic foot, respectively. Process-plant-gas heating values can vary widely, depending on how much hydrogen or inerts are present.
Heating value variations limit the range of heat releases that can be handled by each burner. If the variations are too wide, then the operator will have to light or extinguish additional burners to match the total heat release to the burners’ limits. Where this is needed, the operator should have a control room trend of gas gravity to follow fuel gas heating value swings.
Process plant excess gas is frequently the major source of heating value swings, as its heating value can change rapidly.
If the fuel gas mix drum pressure controls are designed to vent excess gas to a relief header, then they should preferentially vent process gas rather than natural gas or LPG. Thus for this case the vent valve should connect to the process gas line upstream of where the process gas joins other gases. The pressure controls should be designed to block makeup of natural gas to the mix drum when venting of process gas is needed.
Best of all, of course, is a burner that can inherently handle the needed range of heat releases with the available range of fuel gas heating values.