Integration Optimization And Performance Analysis Of The Lignite Pre-drying System In Coal-fired Power Plants

In recent years,the high-quality bituminous and anturacite in central and eastern China decreased sharply and coal mining became more and more difficult.In this case,more and more coal-fired power plant began to burn or mixed burn lignite.Lignite began to play an increasingly important role in the power industry.For a lignite-fired power plant,there is normally additional drying equipment for lignite pre-drying,due to that lignite is low in calorific value,with high water content and easy to weathering.In this paper,we focused on that how to efficiently integrate the drying unit and the entire thermodynamic system.The effect of lignite pre-drying process on the thermal performance of unit heat,boiler efficiency and power consumption rate was theoretically analyzed based on thermodynamics,heat transfer,technical economics and other related theories.Then a calculation model of lignite pre-drying power plant was established,and based on the model a detailed calculation and analysis of traditional flue gas drying and steam drying system were carried out from the view of thermodynamics and economics.Calculation results show that: the traditional lignite pre-drying system has a significant energy saving effect.For the case unit,when adopting the flue gas pre-drying system and the steam pre-drying system,the unit net efficiency could increase by 0.8 percentage points and 1.0 percentage points respectively,and the net income could increase by about 20 million yuan and 29 million yuan.However,there are also a lot of limitations in the traditional lignite pre-drying system.For example,the exhaust flue gas heat is limited and the drying degree of the lignite in the flue gas pre-drying system is always quite low.The amount of steam required for drying in steam pre-drying system is large and the high-grade energy is always degraded for drying.All these shortcomings would limit the further improvement of the efficiency of the lignite-fired power plant.In view of the shortcomings and limitations of the traditional pre-drying system,an optimized two-stage lignite pre-drying power plant with integrated flue gas drying and steam drying system is proposed in this paper,which is based on the principle of energy level matching.The optimized system arranged two stage drying units and applied a steam ejector,which could fully use the boiler exhaust heat and the low-grade extraction steam,thereby reducing the flow and grade of steam required for drying.Besides,it can greatly reduce the work loss of steam turbine caused by the drying steam in the case lignite is fully dried.Related calculation results show that: for the case unit,when applying the two-stage pre-drying system,the unit net efficiency can increase by 1.5 percentage points and the net income can increase by about 37 million yuan.Compared with the traditional flue gas and steam pre-drying system,the two-stage lignite pre-drying system has better thermal performance and economic performance.The research result of this paper can lay a firm foundation for the optimi zation design and efficient operation of the lignite-fired power plant in China,and it indicates the direction for efficient integration of lignite pre-drying unit and the thermal system of power plant.