Application of the NO_x Reaction Model for Development of Low-NO_x Combustion Technology for Pulverized Coals by Using the Gas Phase Stoichiometric Ratio Index

摘要

We previously proposed the gas phase stoichiometric ratio (SRgas) as an index to evaluate NO_x concentration in fuel-rich flames. The SRgas 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 SRgas was a good index to consider the gas phase reaction mechanism in fuel-rich pulverized coal flames. When SRgas 1.0, NO_x concentration was strongly influenced by the SRgas value. NO_x 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_x reduction reactions by hydrocarbons. The hydrocarbon concentration was estimated based on SRgas. We also investigated the ratio as an index to develop a new low-NO_x combustion technology for pulverized coals. We examined the relation between local SRgas distribution in the fuel-rich region in the low-NO_x flame and NO_x emissions at the furnace exit, by varying burner structures. The relationship between local SRgas value and local NO_x concentration was also examined. When a low-NO_x type burner was used, the value of SRgas in the flame was readily decreased. When the local SRgas value was the same, it was difficult to influence the local NO_x concentration by changing the burner structure. For staged combustion, the most important item was to design the burner structure and arrangement so that SRgas could be lowered as much as possible just before mixing with staged air.