凝变除湿推动燃煤烟气水分回收和多污染物协同脱除

并且,从图6可以看出,出口烟气中H2O在0.5μm以上颗粒/液滴中占有较大的体积分数,充分表明水蒸汽主要在该粒径范围内的颗粒物表面凝结。综合对比相变凝聚器进出口的颗粒/液滴尺寸分布(图5)及组分变化(图6)发现,硫酸细液滴由于团聚作用,与其它粒径颗粒/液滴结合,有效改善颗粒/液滴特性,将有利于后续电除尘中的荷电、迁移和脱除。

设计相变凝聚器时,通常在满足压降的前提下,改变换热管束疏密度以使烟气降低不同温度。同时,相变凝聚器内的流动特性也随之发生变化,主要体现在物理流速、停留时间、平均湍动能耗散率等,进而影响到相变凝聚器内的凝结聚并过程。表1列出了烟气下降不同温度时的流动特性参数,分别作为模拟的输入项,以进一步研究不同烟气温降下的凝结聚并规律。

相变凝聚器出口烟气中颗粒、液滴粒径分布如图7所示,结果表明,经过相变凝聚器烟气降低不同温度时,出口烟气中颗粒、液滴粒径分布轮廓结构基本一致,当温度降低3.5和4K,换热面积增加,换热壁面捕集作用增强,并且湍流团聚作用机制增强,同时,水蒸气冷凝产生的“降雨”效应增强,多方面因素导致全粒径范围内的细颗粒/液滴数浓度比降温3K时明显减少;另一方面,水蒸气在换热壁面的凝结量增大,体系内的过饱和度降低,颗粒表面凝水相对减少,并且由于开尔文定律作用,水蒸气在较大尺寸颗粒表面凝结,因此,降低不同温度时,出口烟气中颗粒/液滴粒径分布曲线出现“凹陷”的位置和范围略有差异。表2对比了不同的烟气温降时,相变凝聚器进出口烟气内污染物的

表1 烟气降低不同温度时相变凝聚器内的流动特性参数

Tab. 1 Flowing acteristic parameters within the heat exchanger for different temperature

图7 烟气降低不同温度时颗粒/液滴粒径分布

Fig. 7 Size distribution of particles/lets in the outlet gas with different temperature

表2 烟气降低不同温度时,相变凝聚器进出口烟气内 污染物的分布情况

Tab. 2 The typical pollution in the inlet and outlet flue gas with different temperature

分布情况。结果表明,当相变凝聚器烟气温度分别降低3~4K时,相变凝聚器出口总烟尘脱除率约15%~50%,PM1中烟尘脱除率约67%左右,硫酸脱除率约30%~70%。

进一步地,针对上海某1000MW机组,开展了现场测试,并基于现场测试的温降等数据,进行模拟计算,表3数据表明,模拟输出的硫酸脱除率、烟尘脱除率以及凝水量等结果与现场测试结果基本相符合,证实了相变凝聚器的蒸汽凝结除湿和细颗粒团聚辅助脱除的效果。

表3 某1000MW机组模拟计算结果和现场测试结果对比

Tab. 3 The comparison of the site measurement data and the simulation results on a 1000MW power plant

4 结论

通过分析过饱和度对成核方式的影响及对不同尺度颗粒碰撞频率函数的研究,确定了烟气湿法脱硫后的烟气状态,通过建立颗粒群平衡方程,研究了水蒸气凝结过程中细颗粒物聚并规律及协同脱除效果。主要结论如下：

1）湿法脱硫后烟气呈现高湿状态,通过相变凝聚器换热降温可实现水蒸气的凝结,同时相变凝聚器内发生细颗粒物团聚过程。

2）相变凝聚器内烟气温度降低,水蒸气在烟尘颗粒物表面快速凝结、伴随团聚长大,直径1μm以上的颗粒/液滴数浓度增加,穿透窗口区0.1~1μm颗粒/液滴数浓度减少。

3）现场测试和模拟证实相变凝聚器表现出良好除湿和细颗粒团聚辅助脱除效果,烟气温度降低较大时更加明显。此外,由于硫酸细液滴团聚作用,有效改善颗粒/液滴特性。

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