Low-excess-air firing

Low-excess-air firing (LEA) is a simple, yet effective technique. Excess air is defined as the amount of air in excess of what is theoretically needed to achieve 100% combustion. Before fuel prices rose, it was not uncommon to see furnaces operating with 50 to 100% excess air. Currently, it is possible to achieve full combustion for coal-fired units with less than 15-30% excess air. Studies have shown that reducing excess air ft-om an average of 20% to an average of 14% can reduce emissions of NO, by an average of 19%. 

Techniques involving low-excess-air firing staged-combustion, and flue gas recirculation are effective in controlling both fuel NO, and thermal NO,. The techniques of reduced air preheat and reduced firing rates (from normal operation) and water or steam injection are effective only in controlling thermal NO,. These will therefore not be as effective for coal-fired units since about 80% of the NO, emitted from these units is fuel NO,. 

One of the most effective methods for controlling NO production in combustion chambers is called low-excess-air firing. As the name suggests, this approach makes use of just enough air to allow combustion of the fuel, reducing to a bare minimum the amount of oxygen available for the conversion of nitrogen to NO,. ... 

Boilers Low-Excess Air Firing (LEA) This marginal technique has been widely used for mote than a decade. 10-30 0-10... 

Low-excess-air firing is effective in reducing NO emissions during the combustion of fossil fuels. As the term implies, low-excess-air firing uses the minimum amount of excess air required for oxidation of the fuel, so that less oxygen is available for the reaction... 

Corrosion of metals by fuel ashes only occurs where the fuel ash contains a liquid phase. Temperatures at which the first liquid will form are inversely proportional to the oxygen partial pressure. Thus, when firing fuels at high excess air ratios, fuel ash corrosion occurs at lower temperatures than when firing fuels with low excess air ratios. 

More modem industrial boiler designs also provide facilities for easy fuel changeover with simultaneous firing, low NOx burners, high turndown ratios, and low excess air capabilities (down to only 10% excess air). 

The feed rate for powders injected into the economizer outlet for boilers firing high-sulfur oil (2% or greater S) and operating with low excess air is 165-621 lb MgO equivalent per 1000 barrels of oil. The feed rate for oil-based magnesia dispersions is 127 lb MgO equivalent per 1000 bbl of oil. Coal-fired boilers, using a high-sulfur bituminous coal, typically require... 

Additives can be helpful in controlling corrosion, particularly in conjunction with firing in low excess air. The effectiveness of the additives varies. The most useful additives are based on organic... 

It is important to remember that although a burner can be fired at low rates it is probable that the efficiency at low fire will be reduced because the excess air is invariably higher at turndown. This will affect the selection of controls (e.g. on/off or modulating). 

Biomass has some advantageous chemical properties for use in current energy conversion systems. Compared to other carbon-based fuels, it has low ash content and high reactivity. Biomass combustion is a series of chemical reactions by which carbon is oxidized to carbon dioxide, and hydrogen is oxidized to water. Oxygen deficiency leads to incomplete combustion and the formation of many products of incomplete combustion. Excess air cools the system. The air requirements depend on the chemical and physical characteristics of the fuel. The combustion of the biomass relates to the fuel bum rate, the combustion products, the required excess air for complete combustion, and the fire temperatures. 

---