Grate-fired furnace multipath denitration system and denitration method thereof

The invention discloses a grate-fired furnace multipath denitration system and a denitration method thereof. The system comprises a front arch and a rear arch corresponding to the front arch, a water wall of the front arch protrudes towards the inside of a hearth to enable a cavity to be formed between the water wall and the a front arch furnace wall, a front-arch secondary air device is mounted at the upper end outside the cavity, an ejector is mounted at the upper end inside the cavity, gas ejected by the ejector is natural gas, a nozzle orifice of the ejector is horizontal or downwardly inclined, a natural nozzle is mounted below the ejector, gas ejected by the natural gas nozzle is pressurized natural gas, a nozzle orifice of the natural nozzle is horizontal or upwardly inclined, a water vapor nozzle is mounted below the natural gas nozzle and used for ejecting vapor phase or liquid phase of water, and a nozzle orifice of the water vapor nozzle is inclined upwardly; a light tube water wall is adopted on the lower portion of the protrusion position of the water wall, air gaps are reversed between tube bundles, a gas recovery flared opening is arranged in the lower end of the lower portion, and a membrane water wall is adopted on the upper portion of the protrusion position. By the grate-fired furnace multipath denitration system and the denitration method thereof, NOX generation can be reduced actively, and dual technical objectives of energy conservation and emission reduction are realized.