Efficient process heating requires recovery of the largest possible fraction of a fuel’s combustion heat into the material being processed. There are many avenues of loss in the operation of a heating process. Typically, the dominant loss is energy leaving with the offgas (i.e. the flue gas). The temperature and quantity of offgas indicates its energy content, so keeping its quantity low minimizes energy loss.

In a perfect world, the combustion airflow would be matched to the natural gas flow to give each hydrocarbon molecule the exact amount of oxygen needed to cause complete combustion. In the real world, combustion does not proceed in a perfect manner. Unburned fuel (usually CO and H₂) discharged from the system is a safety hazard and represents a heating-value loss. The first principle of combustion management is based on the fact that combustibles are undesirable in the offgas, while the presence of unreacted oxygen presents minimal safety and environmental concerns.

Combustion management principle number one, stated simply, is: Provide more oxygen than you theoretically need to ensure that all the fuel burns up. For methane (CH₄) combustion, something over two molecules of oxygen per molecule of methane is required. However, the extra oxygen enters at ambient temperature and leaves at the offgas temperature. More important, when air is the oxidant, 3¾ molecules of nitrogen accompany each molecule of oxygen. As a result, more fuel is required to bring the excess air to the offgas temperature, thus increasing the offgas loss.

This brings us to combustion-management principle number two: Do not use too much oxygen. The correct amount of oxygen requires three types of measurements: first, active control of air and natural gas flow; second, offgas oxygen measurement; and third, measurement of offgas combustibles. For each heating process, there exists an optimum condition of minimal offgas heat loss with acceptable levels of combustibles concentration. Keeping the amount of excess oxygen to a minimum pays an additional benefit – for a given offgas temperature, the NOx level is lowest when excess oxygen is a minimum.

Positioning Control

A very common and simple method for controlling the amount of excess oxygen is linking the airflow control device to the gas-flow device. This is called positioning control because the airflow is based solely on the position of the fuel-flow device. Since there are no active oxygen or combustible measurements, operators tend to maintain an airflow above optimum to assure low combustibles during fluctuations in combustion air temperature, natural gas composition and furnace loading.

A positioning control point can be defined in two ways: first, by specifying the % O₂ in the furnace atmosphere; second, by specifying the % excess O₂. These two factors are related by the stoichiometry of fuel combustion. For pure methane, stoichiometric combustion occurs when supplying exactly two molecules of O₂ per molecule of CH₄. When using normal dry air, this is an air/methane volume ratio of 2/0.21, or 9.524, where both gases are at the same pressure and temperature. If they are not, a PVT correction is required.

At perfect air stoichiometry, the combustion of one STP volume of methane produces about 10.52 STP volumes of offgas (one of CO₂, two of H₂O and 7.524 of N₂). Attaining 10% excess air requires an air/methane ratio of 10.476, and the offgas contains 0.952 volumes of air. Equation 1 gives the relationship between the air/natural gas volume ratio and the % excess air, and Equation 2 expresses the %O₂ in the offgas.* 

Air/Natural gas = stoichiometric air (% excess air/100 + 1)   [1]

Vol. % 0₂ at 10% excess air = 21(0.952)/10.52 + 0.952 = 1.74%   [2]

* when dried for analysis, the offgas woudl contain 2.11% 0₂

 

These and other stoichiometry calculations are tedious because natural gas is not pure methane (and its composition can vary), and the air doesn’t contain 21% O₂ (it’s less for high humidity air and higher for oxygen-enriched air). Calculations are best done in Excel with built-in formulae. Workbook StoichCalc.xlsx, which can be downloaded at www.industrialheating.com/stoichcalc, contains positioning templates for any natural gas and air composition, along with references for additional information. The results are displayed as tables, equations and charts that link the % excess air, air/gas ratio and offgas %O₂ (actual and dry). IH