Users of a gaseous fuel are naturally interested in its heating value. The heating value is usually expressed as available energy per unit volume, such as Btu/ft3or (in the SI system) J/m3. In the worst case, no mention is made of temperature and pressure. It’s likely that the volume referred to is at a standard temperature and pressure (STP), which may be stated in the fuel specification. Using STP allows comparisons to be made between fuel properties across a range of actual temperature and pressure. But what exactly is the STP for a gas? Is there a clear and universally accepted definition?
The answer is no. There is a variety of alternative definitions for the standard conditions of temperature and pressure, so users need to know exactly what STP means for accurate combustion calculations. The difference between using one set of STP versus another isn’t important if accurate heating-value calculations aren’t necessary. But if they are, especially if you’re trying to resolve differences between cited data, you’ll want to know what is meant by the STP of a fuel or the combustion air. We’ll look at the two most commonly accepted definitions of STP, and we’ll show how to convert actual volumes from one STP to another.
- The IUPAC (International Union of Pure and Applied Chemistry) STP definition is 0°C (273.15K, 32°C, 491.67°R) and 105 Pa (1 bar, 0.98692 atm, 14.504 psia). At STP, one kmole of an ideal gas occupies 22.711 m3, and the gas constant R = 8.3145 m3·Pa/(mole·K).
- The AES (American Engineering System) STP definition is 60°F (519.67°R, 15.56°C, 288.71K) and 14.696 psia (1 atm, 101325 Pa, 1.01325 bar). At STP, one lb-mole of an ideal gas occupies 379.48 ft3, and the gas constant R = 10.7316 ft3·psi/(lb-mole·°R).
Conversion of Actual to STP Gas VolumeThe volume of a gas at any temperature and pressure can be converted to STP by assuming ideal gas behavior. Then, where T and P are in absolute units (Fig. 2).
For 1,000 ft3of air initially at 15.5 psia and 75°F (534.67°R), the STP volume is 1,025 ft3(at 14.696 psia and 60°F) and consists of 2.701 lb-mole. The conversion between actual and STP volumes in IUPAC versus AES systems is a bit more complex and requires a conversion formula.
If you need to do these calculations often, an Excel template is useful. Figure 1 shows a portion of such a template for initial conditions noted in the blue-shaded cells. You can download the workbook at www.industrialheating.com/volcalc for your own use. The template also includes informational references for STP.
One way to avoid the complexity and confusion of varying gas temperature and pressure on gas amount is to use mass, or even moles, to specify the quantity of fuel and air. Mass and molar amounts are unaffected by temperature and pressure, so there is no confusion about standard states. It’s a little more difficult to convert mass to STP volumes, but an Excel template can be developed to do it easily.
The Assumption of Ideal Gas BehaviorEquation 1 is based on the assumption that gases behave ideally. Is this valid? Deviation from ideality is indicated by comparing the compressibility Z of an ideal versus an actual gas. Equation 2 (Fig. 2) gives the compressibility equation, and for a perfectly ideal gas, Z = 1.
From 0-300°C (30-600°F) and 1-10 atm (15-150 psia), Z for air varies between 0.994 and 1.004. From 0-40°C (30-100°F) and 1-5 atm (15-80 psia), Z for CH4 varies between 0.988 and 0.999. Assuming ideal behavior for air is reasonable, but accurate calculations at higher pressures may require correction for non-ideality for natural gas. Workbook VolCalc.xlsx contains charts of Z for air and CH4as a function of temperature and pressure. You can see the workbook by clickinghere.IH