| For boilers burning biomass,natural gas and other high-moisture fuels,the flue gas they emit contains a large amount of water vapor,and the latent heat contained in this water vapor is also wasted along with the exhaust flue gas.Although this part of heat is low in temperature and grade,but the total amount is very large.If it can be used,it can significantly improve the thermal efficiency and reduce fuel consumption of boiler,which has high significance in energy saving and emission reduction and economic.Based on the common direct mixing and indirect contact total heat recovery devices,this thesis proposes a new total heat recovery technology that combines direct heat exchange with the upper circulating water spray,and indirect heat exchange through the liquid film on the surface of the tube bundle.This kind of heat exchanger uses the circulating water to spray to form a liquid film outside the tube bundle,which can greatly increase the heat transfer coefficient outside the tube,and at the same time avoid the problem of fouling of the heat exchange tube bundle.Since the working fluid is heated in the tube bundle through the partition wall,clean hot water can also be obtained,which solves the defect that the direct mixing heat exchanger pollutes the working fluid.Through thermodynamic analysis,the mathematical model between the flue gas temperature,relative humidity,pressure,absolute humidity,and specific enthalpy was obtained,and the flue gas humidity-temperature diagram and enthalpy-temperature diagram were drawn.With these two figures,the thermal process of the flue gas in the deep waste heat utilization process,and the calculation method of the recovered heat,are explained.Process parameters such as spray density,spray height,flue gas flow rate in empty tower,filler and atomization pressure were analyzed trough the pilot test bench of a biomass power plant in Jiangsu.It can be concluded that the heat transfer coefficient outside the tube increases as the spray density increases.The spray height mainly affects the gas-liquid contact in the spray area.If the spray height is too low,the temperature of the liquid film covering the surface of the tube bundle will also decrease,which reduces the heat transfer temperature difference,resulting in a decrease in the heat transfer coefficient.The flue gas flow rate in empty tower is related to the spray density.Under the same spray density,an excessively high empty tower flow rate will cause the tube bundle surface to fail to form a stable liquid film.The filler and atomization pressure mainly affect the gas-liquid contact in the spray area,the use of filler can greatly reduce the longitudinal space of the device.According to the experimental data,the fitting empirical formula for this type of heat exchanger is=0.01417-0.012284.78925-1.38058,which is convenient for the subsequent design before large-scale use and manufacturing work.Taking a biomass power plant in Jiangsu as an example,the annual heat obtained through deep waste heat recovery is about 2.41×1011 k J,which can be equivalent to 8,212 tons of standard coal.In this part of the heat,the latent heat of water vapor accounts for 70.30%.For the corrosion problems found during the experiment,based on the ion concentration of the circulating water,the morphological characteristics of corrosion,and the composition of the rust,it is judged that the corrosion is caused by chloride ions.In the same chloride ion concentration solution,the corrosion rate of Q235 is one order of magnitude higher than that of304 and 2205.Among 304 and 2205,2205 has relatively better resistance to high-concentration chloride ion corrosion.When the chloride ion concentration is high,due to the decrease of the dissolved oxygen concentration in the water,starting from the chloride ion concentration of50,000 ppm,the corrosion rate decreases with the increase of the chloride ion concentration.At the same time,the ZS-711 inorganic anticorrosive coating was tested.The coating showed good resistance to chloride ion corrosion and good thermal conductivity.The double-layer coating has little effect on the overall thermal conductivity,which is ideal anti-corrosion coating.In the calculation of the inverse balance thermal efficiency of the boiler,the test method of the carbon content in the biomass fly ash was studied.The study revealed the main processes that affect weight loss during the heating and burning of biomass fly ash:burnout of combustibles,decomposition of magnesium carbonate,decomposition of calcium carbonate,evaporation of alkali metal chloride,vitrification,and evaporation of sulfate.In order to avoid the test error of carbon content,it is proposed to use the difference in ignition loss at 600°C in air atmosphere and nitrogen atmosphere as the combustible content in biomass fly ash.After obtaining the accurate combustible content,it is calculated that the thermal efficiency of a biomass power plant in Jiangsu is 80.84%,and the recoverable heat from deep waste heat utilization is about 7.89%of the heat incoming from fuel.At the same time,according to the results of technical and economic analysis,if Q235 and ZS-711 are used,the initial investment is only 210,000 yuan;and the initial investment of 304 and 2205 are both about 900,000 yuan,which is very low compared to fluoroplastics heat exchangers.When the investment is recovered,the annual net income will be more than 3 million yuan,which has good economic benefits. |
