【作者】 张喜来；

【导师】 黄素逸； 靳世平；

【作者基本信息】 华中科技大学 ， 工程热物理， 2012， 博士

【摘要】 目前我国能源消费以煤为主,效率低且污染严重。提高天然气等清洁燃料的消费比例有利于优化我国的能源结构。与煤炭相比,天然气等价格相对较高,需要进一步提高其利用效率,最有效的方法是回收燃烧后烟气的低温余热。国外已经普遍使用冷凝式锅炉及热水器回收烟气中水蒸汽的冷凝潜热,有效地提高了燃气炉的热效率。但冷凝式锅炉和热水器用低温给水作为介质回收冷凝潜热,难以推广到其他类型加热炉上。本文提出利用助燃空气回收烟气的低温余热,该技术路线有更广的应用范围。通过热平衡计算,发现烟气温度低于一个临界值时,可以用助燃空气回收烟气中水蒸汽冷凝潜热,而高于该值时只能回收烟气的显热。过剩空气系数1.1时,天然气烟气的临界值在270℃左右,焦炉煤气烟气的临界值在260℃左右,过剩空气系数增大临界值升高。烟气低温余热的回收需要通过空气预热器完成,其内部属于气-气换热,传热系数低于气-液换热,且传热温差很小,若采用常规的管式、板式、热管式换热器,金属耗量大,成本高,很可能得不偿失。经过对比分析,本文采用蓄热式换热器作为冷凝式余热回收换热器,该换热器结构紧凑、效率高、耐腐蚀、布置灵活,可以满足回收烟气冷凝余热的要求。建立了一台蓄热式冷凝燃气实验炉,在空气预热器烟气进口温度250℃时可以将烟气出口温度降低到25℃左右的水平,水蒸汽冷凝余热得以回收,整个实验炉热效率按低位发热量计算可达106.7%。换向时间对蓄热式加热炉性能有重要影响,换向周期越短,热效率越高,且炉内、蓄热室内的温度波动减小,有利于加热炉稳定运行。预热空气温度升高后燃烧过程中NOx生成量增多,文中对两种空气分级燃烧器进行了实验研究,实验结果均显示空气分级燃烧可以有效降低燃烧过程中NOx排放。但两种燃烧器的最佳一次空气比例有所区别,说明该最佳值与燃烧器结构直接相关,设计中需要根据燃烧器特定结构分析确定。空气分级低NOx燃烧器不需要空气高速射流,阻力损失小。同时蓄热室内也采用了比较大的流通面积,降低了气体流速,减小了蓄热室阻力。整个蓄热式燃烧器阻力降低有效地解决了蓄热式加热炉炉压偏高问题。针对一台管式加热炉的节能改造,将蓄热式冷凝余热回收与传统烟气余热回收技术进行了经济性对比。结果显示蓄热式冷凝换热器比传统管式换热器成本低13.6%左右,且采用蓄热式冷凝换热器的管式炉比采用管式空气预热器的管式炉热效率提高3%,因此蓄热式冷凝余热回收系统具有明显的经济优势。在一台陶瓷梭式窑上进行了烟气冷凝余热回收工业应用,结果显示蓄热式冷凝余热回收技术可实现26.8%的节能量。并且采用蓄热式燃烧后炉内温度分布更均匀,产品质量得以提升。更多还原

【Abstract】 The energy consumption in China is currently dominated by coal with low efficiency and high pollution. It helps to optimize China’s energy structure by promoting the consumption proportion of the natural gas and other clean fuels. Compared with coal, the price of natural gas is relatively high. The most effective way of further improving the utilization efficiency of the natural gas lies in recovering the low temperature waste heat of flue gas after combustion. Condensing boilers and water heaters have been widely used abroad to recover the condensation latent heat of water vapor within the flue gas. The thermal efficiency of the gas furnaces was improved greatly. As low temperature water supply is adopted as medium to recover the condensation latent heat in condensing boilers and water heaters, it is difficult to extend other types of furnaces.Combustion air was used to recover the low temperature waste heat within flue gas in this paper. It holds a wider range of application. Heat balance calculation proved that the condensation latent heat of water vapor within flue gas could be recovered by combustion air when the flue gas temperature was below a threshold value, while the sensible heat was recovered when it was above that value. When the excess air ratio was 1.1, the threshold of the natural gas flue gas was about 270℃and the threshold of the coke oven gas flue gas was around 260℃. Both the excess air ratio and the threshold value increased.The air preheater is the key equipment in the recovery of low temperature waste heat, in which the heat transferred from gas to gas. The gas to gas heat transfer coefficient is lower than gas to liquid heat transfer, with small temperature difference. It doesn’t pay if the conventional heat exchangers (tubular, plate or heat pipe) are used instead, with great metal consumption and high cost. After comparative analysis, the regenerative heat exchanger was adopted to recover the low temperature waste heat. The heat exchanger was compact, efficient, corrosion-resistant and flexible to arrange, as met the demands of recovering the condensation latent heat of flue gas.A regenerative condensing gas-fired boiler was established as the experimental device. When flue gas inlet temperature of the air preheater was 250℃, the flue gas outlet temperature could be reduced to around 25℃. The condensation latent heat of water vapor was recovered. The thermal efficiency of the entire experimental furnace calculated up to 106.7% by low calorific value. Switch time played an important role in the performance of the regenerative furnace. The thermal efficient increased with the decrease of switch time. Besides, small temperature fluctuations within the furnace and the regenerative chamber favored stable furnace operation.NOx production increased when the preheating air heated and combusted. Experiments were conducted on the two air staged burners. The experimental results showed that air-staged combustion reduced NOx emissions effectively during combustion. Though, the optimum primary air ratio of the two burners differed with each other. It indicated that the optimum was directly related to the burner structure and should be determined in accordance with the specific structure of the burner in design.Air-staged low NOx burners were low in resistance loss without high-speed air jet. Meanwhile, the regenerative chamber adopted a relatively large flow area, reducing the gas flow velocity and the regenerative chamber resistance. The high pressure of the regenerative furnace was settled effectively with the decrease of the resistance of the entire regenerative burner.Aiming at the energy-saving modification of a tubular furnace, the economic comparison between the regenerative condensation waste heat recovery technology and the conventional flue gas heat recovery technology was made. It demonstrated the cost of the regenerative condensation heat exchanger was lower than that of the conventional tubular heat exchanger by about 13.6%. And the thermal efficient of the tubular furnace with regenerative condensation heat exchanger was higher than that of the conventional one by 3%. Therefore, the regenerative condensation waste heat recovery system was in evident economic advantage.Industrial application of flue gas condensation waste heat recovery was carried out on a shuttle kiln. The results showed that the regenerative condensation heat recovery technology could achieve amount of energy savings by 26.8%. Moreover, the temperature distribution within the furnace was more uniform and the product quality was improved applying regenerative combustion.更多还原