In a low NOx boiler of the present Invention, a high-temperature reductive combustion zone is provided to an upper portion of a rectangular combustion chamber, and a second-step combustion zone is provided to a middle stage of the combustion chamber. A wall provided below the second-step combustion zone is tapered to narrow the combustion chamber, with a taper angle of approximately 35° or so, relative to a vertical line. An ash discharge port is provided to a furnace bottom portion, and a gas outlet port communicated with a rear pass water wall is provided to a lower side face of the second-step combustion zone. This rear pass water wall is connected with a further post-treatment step, via a super-heater tube and an economizer.
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