Abstract: Circulating fluidized bed( CFB) power generation technology is one of the clean coal power generation technologies developed in recent decades. Because of its good advantages of desulphurization and nitrogen suppression in furnace,it has been widely popularized.With the increasingly serious environmental situation,the CFB boiler can not meet the NO_xultra-low emission requirement by means of the low-nitrogen combustion in the furnace,so it is necessary to study the low-nitrogen combustion theory of the CFB boiler,and the breakthrough should be realized in 660 MW high efficiency ultra-supercritical CFB boiler. In this paper,based on the design concept of flow pattern reconstruction energy-saving CFB boiler,the generation and reduction mechanism of NO_xin furnace and the technical scheme of NO_xremoval in furnace were studied by experiment and numerical simulation. The results show that the factors affecting NO_xemission of660 MW supercritical CFB boiler include burning coal quality,combustion temperature and uniformity,excess air coefficient( operating oxygen),graded combustion and so on. The 660 MW ultra-supercritical CFB boiler adopts the furnace structure of single furnace,single air distribution plate,M-type arrangement structure,four cyclones and four external heat exchangers. The hot primary air of the boiler is fed from the back side of the water-cooled air chamber at 6 points,which ensures the uniform distribution of the static pressure of the primary air of the boiler,and thus ensures the uniformity of the fluidization of the material. The coal feeding mode of " coal feeding on the front wall and sludge for the back wall" is adopted. The front wall is equipped with 12 coal falling ports,the rear wall is equipped with 8 sludge guns and 8 slag discharge ports to ensure the uniformity of coal feeding. The arrangement structure of the 4-cyclone separator ensures the uniformity of the material,the maximum deviation of flow rate between different cyclones is 7.9%,and the uniform arrangement of the four external heat exchangers ensures the uniformity of the bed temperature. The temperature field and excess air coefficient play a key role in NO_xemission. The boiler design bed temperature is 860 ℃,which not only ensures the boiler efficiency,but also reduces the NO_xemission,at the same time,it ensures the reaction temperature of selective non-catalytic reduction( SNCR) denitrification system under low load conditions. The excess air coefficient of boiler is 1.15,which further enhances the reductive atmosphere. The ratio of primary to secondary air is 4 ∶ 6 in staged combustion,and the position and inclination angle of boiler secondary tuyere are adjusted properly to form a large reductive atmosphere to ensure the furnace combustion atmosphere. The comprehensive effect of the above measures can realize high efficiency nitrogen suppression in the furnace,and finally make the original emission concentration of boiler NO_xless than 50 mg/m3,as The SNCR technology with urea as reductant outside the furnace is selected as the assistant denitrification method realize the final NO_xultra-low emission in the flue gas under the condition of low investment,low cost and full load.