The Research Of Combustion And NO_x Emission For A 330MW Down-fired Boiler Burning Anthracite Coal With Extra Low Volatile Content

For a kind of anthracite coal from the coal mine region in northern Vietnam,its volatile content(dry ash free basis)usually lower than 5.0%,the combustion characteristics such as ignition,stable combustion and burnout are extremely difficult.It is one of the coal which is the most difficult to burn around the world.It is urgent to develop an effective technology which can achieve good combustion efficiency and lower NO_x emission for this kind of anthracite coal.The research of this thesis is based on a 330 MW down-fired boiler from Uong Bi thermal power plant in Vietnam.Firstly,this thesis does research and analysis on the problems such as unstable combustion,low boiler efficiency and the boiler cannot be operated without oil support during the trial operation period.Hence,a preliminary retrofit of air distribution scheme is proposed.Then,a 3-D numerical simulation model is created to verify the furnace combustion status for the original design scheme and the retrofit scheme.The furnace temperature distribution,furnace gas velocity distribution and oxygen distribution have been calculated and analyzed according to the numerical simulation calculation.Finally,the retrofit scheme has been determined and executed.After the execution of retrofit,the actual boiler operation is excellent and be coincided with the retrofit target.Meanwhile,the research on the combustion and NO_x emission based on the different ratio of up/down tertiary air and the different ratio of OFA air flow were also executed and analyzed in detail.The result of this analysis shows that the boiler design and air distribution design shall keep the center temperature of lower furnace higher than 1500 deg C for this kind of anthracite coal with extremely low volatile content(Vdaf<5%).For the indirect pulverized system with ball mills,the tertiary air flow ratio is normally very high and the temperature of tertiary air flow is normally very low.Therefore,the location of the tertiary air nozzles shall far from the center of combustion zone to avoid the interference to the main flame.The final recommended operation condition is defined as follows: the ratio of separated up/down tertiary air flow is 1:1;the ratio of total up/down tertiary air flow is 1:4;the OFA damper shall be full opened and the ratio of OFA air flow shall be kept around 10%.Under this recommended operation condition,boiler can achieve the best combustion efficiency and NO_x emission.