Page 322 - Wastewater Solids Incineration Systems

Appendix A Combustion Fundamentals 283

2.2.5 Composition and Properties of Air
The air in the atmosphere that surrounds us is composed of several gases, including
some not previously discussed. Table A.5 gives the normal composition of bone dry
air (water vapor not included), both in volume fraction and weight fraction.
For combustion problems, it is generally acceptable to consider the small frac-
tions of argon and carbon dioxide as nitrogen, thus giving a bone dry air composi-
tion of 79.05% nitrogen and 20.95% oxygen by volume (76.86% nitrogen and
23.15% oxygen by weight). The molecular weight of bone dry air is 28.996. From
this, the density of air at any pressure and temperature can be calculated using the
ideal gas law.
The term relative humidity often is used to describe atmospheric moisture condi-
tions. Relative humidity is defined as percentage of moisture in the air compared
with the maximum amount the air could hold at saturation at the same pressure and
temperature. The term psychrometrics is used to describe the science of air-water
vapor mixtures. In combustion problems, absolute humidity is typically used.
Absolute humidity has the units of lb H O/lb dry air. Table A.6 gives an abbreviated
2
range of typical values of absolute humidity found in combustion problems, tabu-
lated in terms of relative humidity and temperature. In Table A.6, all tabulated values
are expressed as lb H O/lb dry air (note that lb/lb 1000 g/kg).
2
As an illustrative example based on Table A.6, the absolute humidity of air at
atmospheric pressure, 90°F and 75% relative humidity is calculated as follows:

Absolute humidity 0.75 0.031 18 (lb H O/lb dry air)
2
0.023 39 lb H O/lb dry air
2

TABLE A.5 Normal composition of dry outdoor air*.

Component Fraction by volume Fraction by weight
Nitrogen 0.7809 0.7553
Oxygen 0.2095 0.2315
Argon 0.0093 0.0128
Carbon dioxide 0.0003 0.0004
Totals 1.0000 1.0000
*Air also contains slight traces of neon, helium, krypton, ozone, and other components.