Fired Equipment

Fired Equipment | Combustion Air Requirements

Fig. 8-15a shows the scf of dry air needed per scf of paraffinic hydrocarbons for complete combustion in terms of the specific gravity of the fuel. In using this figure any inert components in the fuel, e.g., nitrogen, carbon dioxide, etc., must be excluded. Fig. 8-15b shows the mass of humid air required per mass of dry air at 760 mm Hg and percent relative humidity. Air is about 20.9% oxygen on a dry basis, hence 4.77 mols (or scf) of air supply 1.0 mol (or scf) of oxygen. Applying this to methane, 9.54 mol (or scf) of air are needed for every mol (or scf) of methane.

pic1 35 Combustion Air Requirements

pic1 36 Combustion Air Requirements

Air is about 20.9% oxygen on a dry basis, hence 4.77 mols (or scf) of air supply 1.0 mol (or scf) of oxygen. Applying this to methane, 9.54 mol (or scf) of air are needed for every mol (or scf) of methane.

The effect of water vapor in the air is relatively small at low and moderate temperatures. Saturated air at 60°F contains 1.75% water. Still this should be considered and 2-3% more air is usually added if exact calculations are not made. The water content in saturated air increases rapidly with temperature; e.g. at 100°F saturated air contains about 6.5% water, and at 115°F it contains about 10%.

Some situations may yield a higher amount of water vapor coming from the combustion air and fuel gas. Consider the complete combustion of 1 mol of water saturated methane at 100°F and 15 psig with 20% excess air with air also water saturated at 100°F. This situation introduces 0.79 mol of water from the air and 0.032 mol of water from the gas. Additionally, 2 mols of water from the methane combustion is added water resulting in approximately 21% water in the flue gas of which 30% is from the air and gas humidity. Also, steam or water addition for NOx control introduces more water vapor to the flue gas. These situations increase the wet bulb temperature of the flue gas. Water condensation should be considered in mass and energy balances and excess air calculations. Errors in consideration of water vapor content and air temperature may cancel a 10% excess air calculation, resulting in incomplete fuel combustion. Designs and operations should consider local weather conditions and seasonal changes.

The theoretical air requirement of an arbitrary carbonbased fuel compound, in mols of dry air per mol of fuel, can be calculated with the following equation.

pic1 38 Combustion Air Requirements

Analysis of the flue gases provides useful information about the actual excess air and the efficiency of fired equipment. The following equations provide the excess air percentage for a sulfur and oxygen free, carbon-based fuel combustion without soot formation. Analysis must be molar and on a dry basis.

pic1 39 Combustion Air Requirements

Many forced-draft burners supply a fixed volume of air. Fig. 8-18 shows the effect of ambient temperature and barometric pressure on the amount of air actually delivered.

The air ideal density and ideal relative density changes with the humidity. For the same relative humidity, the hotter the air, the higher the influence of the humidity in the air densities, because of the higher water vapor fraction, as can be seen in Fig. 8-16 and Fig. 8-17. In these graphs the atmosphere pressure is 760 mm Hg; for other conditions use the equation shown in the graph.

pic1 40 Combustion Air Requirements

pic1 41 Combustion Air Requirements

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