硫酸酸霧的形成機(jī)制

1、一、摘自(POWER ENGINEERING, Acid mist causes problems for FGD systems)1、硫酸霧形成有兩個(gè)主要機(jī)制。第一種機(jī)制是形成液滴的h2o蒸氣和SO3蒸發(fā) 之間的反應(yīng)。第二種機(jī)理是當(dāng)氣流溫度降低到h2so4露點(diǎn)以下時(shí)在氣相中硫酸 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

2、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、可以從硫酸分壓與水蒸氣的分壓比來確定硫酸蒸汽露點(diǎn)的升高。盡管典型 條件下H2SO4的露點(diǎn)為300-355F，但由于體相溫差，非理想條件和壁效應(yīng)的 不確定性，氣體溫度高達(dá)430F時(shí)會(huì)產(chǎn)生霧化。(The sulfuric acid vapor dew point elevation

3、 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-

4、ideal conditions and wall effects.)3、根據(jù)氣溶膠科學(xué)，氣體中的蒸汽的飽和比“S”是氣體中的蒸氣的分壓與液體平 面上的蒸氣的飽和蒸汽壓的比率。當(dāng)S 1時(shí)，氣體被稱為蒸汽過飽和；當(dāng)S = 1時(shí)，氣體飽和；當(dāng)S 1)。然而，硫酸的冷凝可以在熱力學(xué)上不可能的條件下進(jìn)行。例如， 當(dāng)煙道氣含有飛灰時(shí)，飛灰可以作為蒸汽的冷凝核。當(dāng)煙氣流過排放控制系統(tǒng)的 冷卻器階段時(shí)，硫酸蒸氣在飛灰上冷凝。(According to aerosol science, the saturation ratio S of a vapor in a gas, is the ratio of th

5、e partial pressure of the vapor in the gas to the saturation vapor pressure of the vapor over a plane of the liquid. When S1, the gas is said to be supersaturated with vapor; when S=1, the gas is saturated; and when S1). However, the condensation of sulfuric acid can take place under conditions wher

6、e 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、硫酸冷凝的另一機(jī)制(當(dāng)S 1)在較

7、冷的表面上冷凝時(shí)，如果設(shè)備壁的溫度 明顯低于氣流的溫度，則會(huì)發(fā)生。這發(fā)生在靠近壁表面的非常薄的層狀邊界層中。 由于層狀層的厚度是總氣體體積的一小部分，所以即使對于熱交換器表面，這種 冷凝機(jī)理通常也不會(huì)產(chǎn)生大量的硫酸霧。Another mechanism for sulfuric acid condensation (when S1) is condensation on colder surfaces which takes place if the temperature of the equipment walls is significantly lower than the tempe

8、rature 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 q

9、uantity of sulfuric acid mist.5、然而，高飽和度的層流層可能是產(chǎn)生最大的部分硫酸液滴的最好機(jī)制。例如， 在SCR系統(tǒng)的下游，其中煙道氣體溫度為約480F，硫酸蒸氣濃度為50ppm，硫 酸霧形成比小于1.0。在這些條件下，已經(jīng)在層狀層中開始了硫酸的氣相冷凝。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 exa

10、mple, 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、當(dāng)

11、發(fā)生慢氣體驟冷時(shí)，例如通過使用間接接觸冷凝器，也可能發(fā)生層狀層中 形成霧的過程。通過緩慢的冷卻過程，飛灰的存在是一個(gè)重要因素，因?yàn)樗峁?了一個(gè)較大的表面積，蒸氣可以冷凝。這種冷卻的優(yōu)點(diǎn)是，大多數(shù)酸會(huì)在現(xiàn)有的 飛灰顆粒和冷凝器壁。另一方面，大多數(shù)FGD系統(tǒng)使用快速淬火技術(shù)，其中熱 煙氣與水的直接接觸冷卻。該過程通過均勻成核固有地產(chǎn)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 us

12、e 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 particle

13、s 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 Utili

14、ty 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):+ 切 O2 - SO3 (1)+ H2O - H2SO4 (2)由于SO2和O2反應(yīng)形成SO3 (反應(yīng)1)是放熱的，所以在高溫(1400C以上) 下形成很少的SO3。在600C時(shí)，約70%的SO2可以轉(zhuǎn)化為SO3，但反應(yīng)速度 慢得多。Since the

15、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400C(750F)條件下，SO2氧化為SO3不超過2%。當(dāng)燃燒氣體 從1600-1700C冷卻至約1000C時(shí)，鍋爐煙道氣中的大部分SO3可能在數(shù)秒鐘內(nèi) 形成。S

16、O3的產(chǎn)生也隨著煙道氣中氧濃度的增加而增加(反應(yīng)1)，然而，如圖 1所示，最小化燃燒氣體中的氧會(huì)增加顆粒。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 al

17、so 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、第一種機(jī)理是形成液滴的兩種蒸氣H2O和SO3 (反應(yīng)2)之間的反應(yīng)。氣霧 形成的第二種機(jī)理是通過降低氣流溫度超過H2SO4露點(diǎn)而在大氣相中硫酸蒸汽 冷凝。The first mechanism is the reaction between two vapors

18、 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、霧化的第二種機(jī)理是通過本體中的蒸氣冷凝通過將氣流溫度降低到低于硫酸 露點(diǎn)的氣相。必須指出的是，盡管典型條件下H2SO4的露點(diǎn)為150-180C，但由 于體相溫度差異不確定

19、，非理想條件和壁效應(yīng)，氣體溫度可高達(dá)220 C。當(dāng)包含 SO3, h2so4和H2O蒸汽的氣流被冷卻時(shí)，h2so4蒸氣冷凝，并且SO3蒸氣和H2O 蒸氣反應(yīng)以形成另外的h2so4。當(dāng)氣體被更快地冷卻時(shí)，形成非常細(xì)的亞微米霧 顆粒，可冷凝的蒸汽可以通過傳質(zhì)，即沖擊冷卻”去除。當(dāng)含有SO3的干燥氣流 與濕氣流混合時(shí)，可以發(fā)生相同的效果。對于這種情況，兩種氣流的快速混合顯 著地使H2SO4液滴尺寸最小化。The second mechanism of mist formation occurs by vapor condensation in the bulk gas phase by reduci

20、ng 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 tem

21、peratures 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

22、 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、當(dāng)混合后的H2SO4濃度范圍為50-200ppm時(shí)，平均測量的液滴直徑小于0.05mk。 但是，在高濃度硫酸的情況下，由于發(fā)生了粒子的規(guī)

23、則縮合和生長，所以霧滴的 大小實(shí)際上更大。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、

24、例如，氣相中硫酸濃度為3.6%時(shí)，霧的平均直徑為0.5mk。在高濃度硫酸的 表面上發(fā)生類似的霧形成過程。在酸強(qiáng)度低于98.5%時(shí)，酸開始產(chǎn)生可測量的水 蒸汽壓力，這導(dǎo)致亞微米霧形成。形成的液滴的直徑取決于水的平衡分壓。當(dāng)用 硫酸表面蒸發(fā)的水分子足以與所有SO3反應(yīng)時(shí)，液滴的直徑是恒定的(水蒸氣壓 0.06mm Hg)。如果酸霧與水飽和的氣流接觸，酸滴通過吸收水而生長。如圖所 示。最大的液滴含有最低濃度的硫酸。然而，初始液滴尺寸取決于氣相中水蒸氣 和三氧化硫的相對濃度。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