Stack test results: correcting to reference O2 and converting units

Start from the basis of the limit

A limit can be written as a concentration at a reference oxygen level, a mass rate, a heat input rate, or an output-based rate such as g/hp-hr. Before comparing a result, write the limit's basis down exactly, including dry or wet basis, reference O2, averaging time, and the standard conditions the rule uses.

Dry and wet basis

Most limits are on a dry basis. If an analyzer measured on a wet basis, divide by one minus the stack gas moisture fraction (Bws), using moisture measured during the same run. Keep concentration and flow on the same basis whenever you multiply them.

Oxygen correction

Correcting to a reference O2 removes the effect of dilution air. The usual form multiplies the measured concentration by (20.9 minus reference O2) divided by (20.9 minus measured O2), with both O2 values on a dry basis. Use the reference O2 written in your limit, and check whether the rule specifies a different correction.

Concentration to mass rate

Mass rate is concentration times molecular weight times flow, divided by the molar volume at standard conditions. The molar volume depends on the standard temperature and pressure, so use the conditions the rule or test method defines. Many rules express NOx as NO2 for this step.

lb/MMBtu with F-factors

Heat input based limits are compared with stack concentrations through a fuel-specific F-factor, the volume of combustion gas per unit of heat input. EPA Method 19 (40 CFR Part 60, Appendix A-7) tabulates F-factors and explains how to calculate them from fuel analysis. Use the O2 level the concentration is expressed at in the oxygen term.

Common mistakes

The errors that come up most in test report reviews:

• Correcting a wet concentration with dry O2, or the reverse
• Using actual flow (acfm) in a standard-condition formula
• Mixing standard temperatures between the flow and the molar volume
• Reporting NOx mass as NO when the rule expresses it as NO2
• Averaging runs differently from what the subpart specifies