In practice, combustion conditions never are ideal, and additional or excess air must be supplied to completely burn the fuel. The correct amount of excess air is determined from analyzing flue gas oxygen or carbon dioxide concentrations. Inadequate excess air results in unburned combustibles; too much excess air results in heat lost due to the increased flue gas flow, thus lowering the overall boiler fuel-to-steam efficiency. Table 1 relates stack readings to boiler performance.
On well-designed natural gas-fired systems, an excess air level of 10% is attainable. An often stated rule of thumb is that boiler efficiency can be increased by 1% for each 15% reduction in excess air or 40°F reduction in stack temperature.
An example demonstrates these principles. Suppose a boiler operates for 8,000 hr/yr and consumes 500,000 MBTU of natural gas while producing 45,000 lb/hr of 150 psig steam. Stack gas measurements indicate an excess air level of 44.9% with a flue gas less combustion air temperature of 400°F (204°C). From table 1, the boiler combustion efficiency is 78.2%. Tuning the boiler reduces excess air to 9.5% with a flue gas less combustion air temperature of 300°F (149°C). The boiler combustion efficiency increases to 83.1%. Assum-ing a steam value of $4.50 per MBTU, annual cost savings are:
= Fuel Compensation times
(1 - E1/E2) times Steam Cost
= 29, 482 x $4.50
= $132,671 annually
Because boilers often operate at excess air levels higher than optimum, OIT suggests periodically monitoring gas flue compensation and tuning boilers to maintain excess air at optimum levels.