An advanced-type compact water-tube boiler has been designed on the basis of the new concept of cooling flame by water-tube bank in the furnace, and is referred to as "tube-nested combustor". It realized drastic reduction in boiler size as well as in the NO x emission. In this present study, aiming at further improvement of boiler efficiency and reduction of NO x emission, the combustion characteristics in the furnace were investigated by using the test boiler of 0.5 t/h steam output. Experimental results indicated that the NO x formation was restricted in the narrow area close to the burner exit, and that the CO-concentration decreased drastically with the combustion gas stream through the tube-bank. These facts were closely related to the experimental evidence of the cross-sectional distribution of gas temperature being rather uniform owing to the agitation by the tube-bank. These experimental results verified the soundness of the boiler design and, in addition, gave relevant information on local combustion characteristics available for the R&D of the next generation tube-nested combustor.
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