Abstract: Circulating Fluidized Bed( CFB) boiler,featured by the low quality coal as the fuel,is widely used in China. One of the CFB combustion advantage is its lower NOxemission without cost due to its lower and uniform combustion temperature of the alone circulating loop as well as its unique characteristics that the combustion atmosphere is reduced. With the increasingly strict emission requirements for coal fired power plant in China,the original NOxemission of CFB combustion exceeds the emission limit of the updated National Emission Regulation. Therefore,the CFB combustion processing was theoretically analyzed,according to the NOxgeneration and reduction paths under the condition of CFB combustion. It is believed that it is possible to decrease the NOxoriginal emission in advance by controling NOxgeneration and reduction and using the optimized gas-solid two-phase flow pattern. Furthermore,the engineering method to optimize the gas-solid two-phase flow pattern was suggested,which was to improve the bed quality,decrease the course bed inventory and increase the circulating flow rate. The principle of this technology was discussed,including the improvement of bed quality,the decreasing of the course bed inventory and the increasing of the circulating flow rate. It can significantly enhance the reduced atmosphere to decrease NOxgeneration in both dense zone and dilute phase area and promote the reduction of NOxin freeboard even in cyclone. Besides,the reasonable bed temperature and air supply should be considered. The NOxemission of this technology was expected less than 50 mg/m3 without flue gas denitrification. After the key points were confirmed in laboratory one by one,this technique was proved to be effective in CFB boilers with capacity of 150,260 and 560 t/h. The operation results show that the gas-solid two-phase flow pattern optimization decreases the NOxemission obviously and achieves ultra lower NOxemission. In the same time,no obvious decrease of combustion efficiency is found.These engineering cases of original ultra-low emission include bituminous coal,lean coal and anthracite coal. This technical method of reducing NOxemission concentration through flow design optimization provides a new approach to control the NOxemission in CFB boilers.