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利用热泵技术回收天然气烟气低温余热的研究

作者:优质期刊论文发表网  来源:www.yzqkw.com  发布时间:2019/9/4 8:23:02  

摘要:天然气烟气中含有水蒸汽,其容积比高约20%,含有大量汽化潜热,如果排烟温度下降到低于水蒸气露点温度,则可充分回收显热和潜热,经济效益显著。目前,国内的燃气锅炉排烟温度一般能达到200℃左右,热损失过大,而且还对局部环境造成了严重污染。针对该问题,本文提出一种天然气锅炉低温烟气二级余热回收系统,利用热管式换热器和烟气源热泵技术相结合,将排烟温度降至常温20℃,起到回收烟气全热和“烟气脱白”作用,锅炉效率可达到109.2%。

首先,对天然气进行燃烧分析计算,分析天然气烟气的特性,进行烟气焓的计算,分析了烟气潜热和显热回收潜力,并计算燃气锅炉的效率,发现排烟温度降到露点以下时,同时回收显热与潜热能使锅炉效率发生飞跃。

其次,提出了低温烟气二级余热回收系统,描述了该系统的工作流程,第一级为热管换热器,第二级是热泵,热管对烟气显热进行回收利用,热泵对烟气潜热进行回收利用。以10t/h的燃气锅炉为例,对第一级换热器进行设计计算,确定热管换热器的工作液和管壳材料,热管中传热量以及热管具体的结构。接着进行第二级烟气源热泵的设计,选择了制冷剂,蒸发器采用了波纹板冷凝式换热器发明专利技术,对波纹板蒸发器进行设计计算,冷凝器采用壳管式结构,并进行设计计算,再进行压缩机,节流阀,气液分离器等部件的选型。

再次,利用Ansys Fluent软件进行波纹板蒸发器中烟气侧冷凝换热的数值模拟分析,运用C语言编写了烟气冷凝的UDF程序,得出烟气温度场,速度场,冷凝水浓度场,分析烟气特性以及波纹板的结构参数对湿烟气冷凝换热的影响,结果表明当空气过剩系数较小,烟气质量流速为2.3-3.45kg/m2·s,烟道宽度为10-12mm,波纹高度为18-20mm,波纹节距为36-40mm时,能带来较好的换热效果。还发现了烟气的冷凝规律:水蒸气含量不一样的烟气充分冷凝后的终状态是一样的。并将波纹板换热器和平板换热器进行对比,得出波纹板的冷凝换热效果优于平板,为工程设计提供理论基础和参考。

最后,对二级余热回收系统进行经济性分析,和常规的供热方案对比,二级余热回收系统具有优越的经济性,经计算得出其静态投资回收期为一年左右。与此同时,系统回收绝大多数的冷凝水,溶解部分NOx氮氧化物,减少污染气体的排放,因此具有良好的环保效益。

There is water vapor in the flue gas ofnatural gas. The volume ratio of water vapor is up to about 20%. Flue gascontains a large amount of latent heat of vaporization. If the exhaust glue gastemperature falls below the dew point of water vapor, the sensible heat andlatent heat in the flue gas can be fully recovered, bringing significanteconomic benefits. At present, the exhaust gas temperature of domestic gassteam boilers can generally reach about 200℃, the heat loss of exhaust gas istoo large, and it also causes serious pollution to the local atmosphericenvironment. Aiming at this problem, this paper presents a two-stage waste heatrecovery system for low-temperature flue gas of natural gas boiler, whichcombines the heat pipe heat exchanger and the flue gas source heat pumptechnology to reduce the exhaust gas temperature to the environment temperatureof 20℃,recovers the flue gas full heat and plays the role of “flue gasde-whitening”.The boil efficiency can be up to 109.2%.

Firstly, the paper conducts the combustionanalysis and calculation of natural gas, analyzes the characteristics ofnatural gas flue gas, calculates flue gas enthalpy, analyzes the latent heatand sensible heat recovery potential of the flue gas and calculates theefficiency of the gas boiler. We find that when the exhaust gas temperaturedropped to the dew point, sensible heat and latent heat can be simultaneouslyrecovered, making a significant leap in boiler efficiency.

Secondly, a two-stage waste heat recoverysystem for low-temperature flue gas of natural gas boiler is presented. Theworkflow of the system is described. The first stage is a heat pipe heatexchanger and the second stage is a heat pump. The heat pipe heat exchangerrecovers the sensible heat of the flue gas and the heat pump recovers thelatent heat of the flue gas. Take 10t/h gas boil for example. The design andcalculation of the heat exchanger are conducted, determining the working fluid,shell material of the heat pipe heat exchanger, the heat transfer amount in theheat pipe and the specific structure of the heat pipe. Then the paper conductsthe design calculation of the second-stage flue gas source heat pump, choosethe refrigerant. The patent for inventions—corrugated plate condensing heatexchanger is used as the evaporator. The paper designs the corrugated plateevaporator. The paper chooses the shell and tube heat exchanger as thecondenser and designs the condenser. Finally, we do the model selection ofcompressor, throttle, gas-liquid separator, etc.

Thirdly, the numerical simulation of fluegas condensation heat transfer in corrugated plate evaporator is carried out. CLanguage is used to write UDF of flue gas condensation. The temperature field,velocity field and condensate water concentration field of flue gas areobtained by using Fluent software. The paper analyzes the influence of flue gascharacteristics and structural parameters of corrugated plate evaporator oncondensation heat transfer. It is found that when the equivalent excess aircoefficient is small, the flue gas mass velocity is 2.3-3.45m/s, the flue gaspassage width is 10-12mm.the height of corrugation is 18-20mm and the pitch ofcorrugation is 36-40mm,the heat transfer effect is better. Through the analysisof flue gas condensation, it is also found that if flue gas containingdifferent ratio of water vapor is condensed fully, the final state is same.Compared with flat plate heat exchanger, the condensation heat transfer effectof corrugated plate is better, which provides theoretical basis and referencefor engineering design.

At last, economic analysis is conducted forthe two-stage waste heat recovery system. Compared with conventional heatingschemes, the two-stage waste heat recovery system has supreme economicadvantage. By calculation, its static investment return period is about oneyear.

关键词:烟气余热;热管换热器;波纹板蒸发器;烟气源热泵;数值模拟

Flue gas waste heat; Heat pipe exchanger;Corrugated plate evaporator; Flue gas source heat pump; Numerical simulation

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