Application of the NO<em>_x</em> Reaction Model for Development of Low-NO<em>_x</em> Combustion Technology for Pulverized Coals by Using the Gas Phase Stoichiometric Ratio Index

We previously proposed the gas phase stoichiometric ratio (<em>SRgas</em>) as an index to evaluate NO<em>_x</em> concentration in fuel-rich flames. The<em> SRgas</em> index was defined as the amount of fuel required for stoichiometric combustion/amount of gasified fuel, where the amount of gasified fuel was the amount of fuel which had been released to the gas phase by pyrolysis, oxidation and gasification reactions. In the present study we found that <em>SRgas</em> was a good index to consider the gas phase reaction mechanism in fuel-rich pulverized coal flames. When <em>SRgas</em> < 1.0, NO<em>_x</em> concentration was strongly influenced by the <em>SRgas</em> value. NO<em>_x</em>concentration was also calculated by using a reaction model. The model was verified for various coals, particle diameters, reaction times, and initial oxygen concentrations. The most important reactions were gas phase NO<em>_x</em> reduction reactions by hydrocarbons. The hydrocarbon concentration was estimated based on <em>SRgas</em>. We also investigated the ratio as an index to develop a new low-NO<em>_x</em> combustion technology for pulverized coals. We examined the relation between local <em>SRgas</em> distribution in the fuel-rich region in the low-NO<em>_x</em>flame and NO<em>_x</em> emissions at the furnace exit, by varying burner structures. The relationship between local <em>SRgas</em> value and local NO<em>_x</em> concentration was also examined. When a low-NO<em>_x</em> type burner was used, the value of <em>SRgas</em> in the flame was readily decreased. When the local <em>SRgas</em> value was the same, it was difficult to influence the local NO<em>_x</em> concentration by changing the burner structure. For staged combustion, the most important item was to design the burner structure and arrangement so that <em>SRgas</em> could be lowered as much as possible just before mixing with staged air.