硫酸酸雾的形成机制

硫酸(liú suān)酸雾的形成机制
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一、摘自(POWER ENGINEERING, Acid mist causes problems for FGD systems)
1、硫酸(liúsuān)雾形成有两个主要机制。

第一种机制是形成液滴的H2O蒸气(zhēnɡ qì)和SO3蒸发之间的反应。

第二种机理(jī lǐ)是当气流温度降低到H2SO4露点以下时在气相中硫酸(liú suān)蒸汽冷凝。

（There are two primary mechanisms for sulfuric acid mist formation. The first mechanism is the reaction between H2O vapors and SO3vapors that form liquid droplets. The second mechanism is sulfuric acid vapor condensation in the bulk gas phase when the gas stream temperature is lowered below the H2SO4 dew point.）
2、可以从硫酸分压与水蒸气的分压比来确定硫酸蒸汽(zhēnɡ qì)露点的升高。

尽管典型条件下H2SO4的露点为300-355°F，但由于体相温差，非理想条件和壁效应的不确定性，气体温度高达430°F时会产生雾化。

（The sulfuric acid vapor dew point elevation can be determined from the ratio of the partial pressure of sulfuric acid to that of water vapor. Although the dew point of H2SO4 under typical conditions is 300-355 F, mist formation can occur at gas temperatures as high as 430 F because of uncertainties of bulk phase temperature differences, non-ideal conditions and wall effects.）
3、根据气溶胶科学，气体中的蒸汽的饱和比“S”是气体中的蒸气的分压与液体平面上的蒸气的饱和蒸汽压的比率。

当S> 1时，气体被称为蒸汽过饱和；当S = 1时，气体饱和；当S <1时，气体与蒸汽不饱和。

为了发生冷凝，硫酸蒸气必须过饱和（S> 1）。

然而，硫酸的冷凝可以在热力学上不可能的条件下进行。

例如，当烟道气含有飞灰时，飞灰可以作为蒸汽的冷凝核。

当烟气流过排放控制系统的冷却器阶段时，硫酸蒸气在飞灰上冷凝。

（According to aerosol science, the saturation ratio "S" of a vapor in a gas, is the ratio of the partial pressure of the vapor in the gas to the saturation vapor pressure of the vapor over a plane of the liquid. When S>1, the gas is said to be supersaturated with vapor; when S=1, the gas is saturated; and when S<1, the gas is unsaturated with vapor.
For condensation to take place, sulfuric acid vapor must be supersaturated (S>1). However, the condensation of sulfuric acid can take place under conditions where it is thermodynamically impossible. For example, when the flue gas contains fly ash, the fly ash may act as a nucleus of condensation for the vapor. The sulfuric acid vapors condense on the fly ash as the flue gas flows through the cooler stages of the emissions control system.）
4、硫酸(liú suān)冷凝的另一机制（当S <1）在较冷的表面(biǎomiàn)上冷凝时，如果设备壁的温度明显低于气流的温度，则会发生。

这发生在靠近壁表面的非常薄的层状边界层中。

由于层状层的厚度是总气体体积的一小部分，所以即使对于热交换器表面，这种冷凝机理通常(tōngcháng)也不会产生大量的硫酸雾。

Another mechanism for sulfuric acid condensation (when S<1) is condensation on colder surfaces which takes place if the temperature of the equipment walls is significantly lower than the temperature of the gas stream. This occurs in the very thin laminar boundary layer close to the wall surface. Because the thickness of the laminar layer is a small percentage of the total gas volume, even for heat exchanger surfaces, this condensation mechanism will not generally provide a considerable quantity of sulfuric acid mist.
5、然而(rán ér)，高饱和度的层流(cénɡ liú)层可能是产生最大的部分硫酸液滴
的最好机制。

例如，在SCR系统的下游，其中烟道气体温度为约480°F，硫酸蒸气浓度为50ppm，硫酸雾形成比小于1.0。

在这些条件下，已经在层状层中
开始了硫酸的气相冷凝。

However, the high saturation ratio in this laminar layer may be the mechanism that produces the largest portion of the finest sulfuric acid droplets. For example, downstream of an SCR system, where the flue gas temperature is about 480 F and the concentration of sulfuric acid vapor is 50 ppm, the sulfuric acid mist formation ratio is less than 1.0. At these conditions the gas phase condensation of sulfuric acid has already begun in the laminar layer.
6、当发生慢气体骤冷时，例如通过使用间接接触冷凝器，也可能发生层状层中形成雾的过程。

通过缓慢的冷却过程，飞灰的存在是一个(yī ɡè)重要因素，因为它提供了一个较大的表面积，蒸气(zhēnɡ qì)可以(kěyǐ)冷凝。

这种冷却的优点是，大多数酸会在现有的飞灰颗粒和冷凝器壁。

另一方面，大多数FGD
系统使用快速淬火技术，其中热烟气与水的直接接触冷却(lěngquè)。

该过程通过均匀成核固有(gùyǒu)地产生大量的硫酸液滴。

The process of mist formation in the laminar layer may also occur when slow gas quenching takes place, such as through the use of an indirect contact condenser. With a slow cooling process the presence of fly ash is an important factor because it provides a large surface area on which the vapors can condense.4 The advantage of this type of cooling is that most of the acid will be condensed on the existing fly ash particles and the condenser walls. On the other hand, most FGD systems use rapid quenching technologies with direct contact cooling of the hot flue gas with water. This process inherently generates a large number of sulfuric acid droplets by homogeneous nucleation.
二、摘自(Sulfuric Acid Mist Generation in Utility Boiler Flue Gas)
1、Sulfuric acid formation takes place through the reaction steps of: Oxidation of SO2 to produce SO3 (1) followed by reaction with H2O to form H2SO4 (2): SO2 + ½ O2 → SO3 (1)
SO3 + H2O → H2SO4 (2)
由于(yóuyú)SO2和O2反应(fǎnyìng)形成SO 3（反应(fǎnyìng)1）是放热的，所以(suǒyǐ)在高温（1400℃以上(yǐshàng)）下形成很少的SO3。

在600℃时，约70％的SO2可以转化为SO3，但反应速度慢得多。

Since the reaction of SO2 and O2 to form SO3 (Reaction 1) is exothermic, little SO3 forms at high temperatures (above 1400?C). At 600?C, about 70% of the SO2 can be converted to SO3, but the reaction rate is much slower.）
2、在典型的400℃（750F）条件下，SO2氧化为SO3不超过2％。

当燃烧气体从1600-1700℃冷却至约1000℃时，锅炉烟道气中的大部分SO3可能在数秒钟
内形成。

SO3的产生也随着烟道气中氧浓度的增加而增加（反应1），然而，如图1所示，最小化燃烧气体中的氧会增加颗粒。

Under typical conditions of 400?C (750?F), the oxidation of SO2 to SO3 is no more than 2%[5]. Most of the SO3 in boiler flue gas likely forms during the several seconds when the combustion gas cools from1600-1700?C to about 1000?C. SO3 generation also increases with rising oxygen concentration in the flue gas(Reaction 1), however, minimizing oxygen in the combustion gas would increase particulate as shown in Fig. 3、第一种机理是形成液滴的两种蒸气H2O 和SO3（反应2）之间的反应。

气雾形成的第二种机理是通过降低气流温度超过H2SO4露点而在大气相中硫酸蒸汽冷凝。

The first mechanism is the reaction between two vapors H2Oand SO3 (Reaction 2) forming liquid droplets. The second mechanism of mist formation is sulfuric acid vapor condensation in the bulk gas phase by lowering the gas stream temperature beyond the H2SO4 dew point.
4、雾化的第二种机理是通过本体中的蒸气冷凝通过将气流温度降低到低于硫酸露点的气相。

必须指出的是，尽管典型条件下H2SO4的露点为150-180℃，但由于体相温度差异不确定，非理想条件和壁效应，气体温度可高达220℃。

当包含SO3，H2SO4和H2O蒸汽的气流被冷却时，H2SO4蒸气冷凝，并且SO3蒸气和H2O蒸气反应以形成另外的H2SO4。

当气体被更快地冷却时，形成非常细的亚微米雾颗粒，可冷凝的蒸汽可以通过传质，即“冲击冷却”去除。

当含有SO3的干燥气流与湿气流混合时，可以发生相同的效果。

对于这种情况，两种气流的快速混合显着地使H2SO4液滴尺寸最小化。

The second mechanism of mist formation occurs by vapor condensation in the bulk gas phase by reducing the gas stream emperature below the sulfuric acid dew point. It must be noted that, although the dew point of H2SO4 under typical conditions is 150 - 180?C, because of uncertainties of bulk phase temperature differences, non-ideal conditions and wall effects, mist formation could occur at gas temperatures as high as 220?C [8]. When a gas stream containing SO3,
H2SO4 and H2O vapor is cooled, H2SO4 vapor condenses and SO3 vapor and H2O vaporreact to form additional H2SO4. Very fine submicron mist particles
are formed when the gas is cooled faster thanthe condensable vapor can be removed by mass transfer, i.e. “shock cooling”. The same effect can take place when a dry gas stream containing SO3 is mixed with a wet gas stream. For this case, rapid mixing of the two gas streams dramatically minimizes the H2SO4 droplet size.
5、当混合(hùnhé)后的H2SO4浓度(nóngdù)范围为50-200ppm时，平均
(píngjūn)测量的液滴直径小于0.05mk。

但是，在高浓度硫酸的情况下，由于发生了粒子的规则缩合(suōhé)和生长，所以雾滴的大小实际上更大。

When the H2SO4 concentration after mixing ranges from 50 – 200 ppm, the average measured droplet diameter is less than 0.05 mk. But under high concentration of sulfuric acid, where regular condensation and growth of particles take place, the mist droplet size is actually larger.
6、例如，气相中硫酸(liú suān)浓度为3.6％时，雾的平均(píngjūn)直径为
0.5mk。

在高浓度硫酸(liú suān)的表面上发生类似的雾形成(xíngchéng)过程(guòchéng)。

在酸强度低于98.5％时，酸开始产生可测量的水蒸汽压力，这导致亚微米雾形成。

形成的液滴的直径取决于水的平衡分压。

当用硫酸表面蒸发的水分子足以与所有SO3反应时，液滴的直径是恒定的（水蒸气压0.06mm Hg）。

如果酸雾与水饱和的气流接触，酸滴通过吸收水而生长。

如图所示。

最大的液滴含有最低浓度的硫酸。

然而，初始液滴尺寸取决于气相中水蒸气和三氧化硫的相对浓度。

For example at 3.6% concentration of sulfuric acid in the gas phase, the average diameter of mist is 0.5 mk (Table 1). A similar process of mist formation takes place over a surface of high concentration sulfuric acid. At acid strengths below 98.5%, the acid begins to exert a measurable water vapor pressure, which causes submicron mist formation (Fig 4) [8]. The diameter of the formed droplets depends on the equilibrium partial pressure of water. The diameter of the droplet is constant (water vapor pressure 0.06 mm Hg ) when the water molecules which vaporize with the sulfuric acid surface are sufficient to react with all of the SO3 [9]. If the acid mist contacts the water saturated gas stream, the acid droplets grow by absorption of water. As shown in Fig. 5, the largest
droplets contain the lowest concentration of sulfuric acid. However, the initial droplet size depends on the relative concentration of water vapors and sulfur trioxide in the gas phase.
内容总结
（1）硫酸酸雾的形成机制。