Research On Waste Heat Recovery And Water Recycling In Lignite Drying Process And Performance Of Super-hydrophobic Surface Condensation

Lignite has high moisture content, causing low direct combustion efficiency of boilers and combustion instability and other issues. The storage of lignite in the world is huge, and therefore it is very important to use the lignite economically. Current usage of lignite is mainly through pre-drying lignite to reduce the moisture content to achieve the purpose of economic utilization. There is a lot of water in the pre-drying process of lignite. In China, most of lignite is distributed in arid regions, Collection and utilization of this kind of water will greatly alleviate using water problems of local power plants. And lignite drying process will generate a lot of heat, recovery of part of this waste heat while collecting the water will help power plants reducing energy consumption.Taking a 600 MW lignite fired units with a pipe steam lignite drying equipment as an example, the effect of water recovery and heat recovery is analyzed under different recovery plan and different amount of recovery energy and water using equivalent enthalpy drop method and heat balance of steam-water method. Heat transfer area of different amounts of waste heat recovery under different recycling plan is calculated using heat transfer equation and designing an iterative algorithm. The increasing of flow resistance and the increasing of the electrical consumption of the fan is calculated using hydrodynamic equations. At last comprehensive waste heat recovery proceeds is calculated considering the initial equipment investment, operation and maintenance costs, equipment depreciation and bank interest rates and other factors to find the optimal recovery plan and the amount of waste heat recovery and water recovery.Condensing heat exchanger is used in recovery of exhaust heat and water from exhaust gas of lignite drying process. The efficient of heat transfer of drop-wise condensation is several times larger than the efficient of film condensation under the same conditions. The heat transfer area of the condensing heat exchanger will reduce greatly if drop-wise condensation can be achieved. The super-hydrophobic surface due to its super-hydrophobic properties is expected to achieve drop-wise condensation. Two kinds of super-hydrophobic surface are created using chemical etching and carbon depositing method, and their contact angle and roll angle are measured. Measurement shows that both of these two surfaces have good super-hydrophobic properties. Experimental device are designed and installed to get the experiments of pure steam condensation and 60 ? saturated wet air condensation done. Experimental results show that under the conditions of pure steam both two surfaces are film condensation, but at 60 ? saturated wet air condition, both two surfaces can form a stable drop-wise condensation. Experimental results show that the exhaust gas of the lignite drying process which is constituted by vapor and air can form a stable drop-wise condensation on the surface of a condensing heat exchanger using super-hydrophobic surface, therefore the amount of heat transfer area of the condensing heat exchanger will reduce greatly.