BTU/Pressure Swing Control

Hey everyone!

I'm looking at a project for which the fuel gas to a heater may vary widely in BTU content, because of the PSA unit swings.  Do you know of control loops for this type of swings? Is it a control loop based on a BTU analyzer? A control valve for the fuel gas?

Thanks in advance,

Furnace controls can be as simple or complex as you may desire.  Schemes exist that evaluate the heat content of the furnace feed, the heat content of the heater outlet, the energy content of the fuel, perhaps even the waste heat issues associated with the stack effluent.  Some of the complex schemes have been around for over 25-years.  Similar simplicity and complexity exists for the furnaces (FD, ID, waste-heat recovery, air-preheat, multiple burner, multiple fuel ...) and for the safety shutdown schemes.  I may look for web links later.

John

One way of inferring the calerific value/BTU of the fuel gas (at least to give your controls a 'heads up' that it is changing) is to measure the gas density, possibly via a coriolis measurement. This can then be used to modify the fuel flow controller.

In addition to BTU there are analyzers that measure the Wobbe Index.  The Wobbe Index as saved from one of the manufacturers is the ratio of the heat of combustion of a gas to its specific gravity. For light hydrocarbon gases the Wobbe index is almost a linear function of the gas' specific gravity.  The Wobbe Index is the number obtained by dividing the saturated heating value of the gas by the square root of its specific gravity.  Wobbe index is a measure of the amount of heat released by a gas burner of constant orifice.  It indicates the quality of the gas and is given by the expression Vp0.5 where: V = the gross calorific value in BTU per cubic foot at STP and p = specific gravity.

You can use Wobbe Index or BTU as a measurement instrument in a combustion control scheme.

John