外文翻译--动力学研究与二氧化硫反应在低温和氢氧化钙在一固定床

翻译部分

英文原文

Kinetic study of the reaction between sulfur dioxide and calcium hydroxide at low temperature in a

fixed-bed reactor

Abstract

A quantitative study of the influence of inlet sulfurdioxide concentration (600–3000 ppm),relative humidity (20–60%), reactortemperature(56–86℃)and different amounts (0–30 wt.%) ofinorganic additives(NaCl, CaCl2 andNaOH) on gas desulfurization has been carried out in acontinuous downflow fixed-bed reactor containing calcium hydroxide diluted with silica sand.Results show that the reaction rate does not depend on sulfur dioxide partial pressure (zero-order Kinetics) and that the temperature and the relative humidity have a positive influence on reactionrate. An apparent activation energy of 32 kJ/mol Ca(OH)2 has been estimated for the reaction.

An empirical reaction rate equation at 71.5℃ and 36.7% relative humidity that includes thetype and amount of additive is proposed. It has been found that

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calcium chloride is the bestadditive studied because it allows for a higher degree of sulfur dioxide removal. 2000 ElsevierScience B.V. All rights reserved.

Keywords: Desulfurization; Sulfur dioxide; Calcium hydroxide; Kinetics; Inorganic additives

1. Introduction

The increasing concern during the last few years on the protection of the environmenthas had its influence on the design and operation of power plants, especially on thereduction of sulfur dioxide and nitrogen oxide emissions from them. They are the mainpollutants from coal and fuel-oil combustion in power plants. Both gases are responsiblefor acid rain.

In USA and Europe, new power plants that use fuels with significant quantities ofsulfur have to meet severe standards to reduce these air pollutants. One of the majorproblems facing older power plants is that they were designed prior to the presentstandards for pollution control and therefore have no facilities on space to incorporatesuch controls.

The technologies to control sulfur dioxide emissions can be distributed into threegroups by considering if the treatment is done before, during or after the

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combustion. Itseems clear that the last group of technologies cited is the most advantageous, fromvarious points of view, for power stations which have been in operation for many years.These are called FGD technologies (Flue Gas Desulfurization),and among them, themost usedare: IDS (In-Duct Scrubbing, developed by General Electric); E-Sox(developed by US EPA, Babcok and Wilcox, Ohio Coal Development Office and OhioEdison), EPRIHYPAS (Hybrid Pollution Abatement System, developed by ElectricPower Research Institute), DRAVO HALT (Hydrate Addition at Low Temperature,developed by Dravo), CONSOL COOLSIDE (developed by Consolidated CoalCom-pany)and ADVACATE(developed by Acurex and US EPA). These processes are basedon the injection of a solid sorbent plus water by spraying or injecting a slurry into theduct situated between the air preheater and the particulate collection system. Calciumhydroxide or limestone are usually used as sorbents to capture sulfur dioxide and acalcium sulfiter/sulfate mixture is obtained as the reaction product.

Klingspor and Stromberg proposed a mechanism to explain the reactionbetween sulfur dioxide and calcium hydroxide or calcium carbonate in the presence ofwater vapor. According to them, when the relative humidity is low (below 20%), sulfurdioxide and water can be adsorbed on the solid surface, however, no reaction occursuntil there is at least a monolayer of water molecules adsorbed on the surface. As therelative humidity increases, less sulfur dioxide can be adsorbed on the surface becausewater adsorption on the solid occurs preferentially due to intermolecular forces. Thus,sulfur dioxide has to be absorbed on the adsorbed water, forming complexes where thesulfur atom is

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bound to the oxygen atom of water. This fact leads to the formation of apositive charged hydrogen atom that can combine with hydroxide or carbonate ions fromthe sorbent to form reaction intermediates and products. Experimental findings show thatthe reaction rates for lime and limestone are similar. Consequently, the complexformation SO2 nH O is considered to be the rate-determining step, since all further reactions are different for the two types of sorbents. The initial rate of the process isindependent of sulfur dioxide concentration when the relative humidity is below 70%.Above this value, the reaction rate becomes gradually more and more dependent on thesulfur dioxide partial pressure. This fact can be attributed to the formation of stableconfigurations of water ligands around the sulfur dioxide molecules. Also, it has beenfound that the initial reaction rate is a very weak function of temperature but increasesexponentially with relative humidity, for both hydrated lime and limestone.

Jorgensen also studied this reaction in a bench-scale sand bed reactor. Someof their conclusions point out that the calcium hydroxide conversion has a very strongdependence on relative humidity. The conversion rate is increased moderately withtemperature in agreement with activation energy of 25 kJ/mol. However, there is noclear indication of increasing conversion with increasing sulfur dioxide concentration.

Ruiz-Alsop and Rochelle found that the relative humidity is the most importantvariable affecting the reaction of sulfur dioxide and calcium hydroxide. The chemicalreaction taking place at the surface of the unreacted calcium

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hydroxide presentszero-order kinetics in sulfur dioxide. At high relative humidity and/or high SO2concentration, the chemical reaction at the surface of the unreacted calcium hydroxidesolid controls the overall reaction rate. At low relative humidity and/or low sulfurdioxide levels, diffusion of sulfur dioxide through the solid product layer becomes therate-controlling step. The reaction rate has a weak temperature dependence. Theactivation energy of the reaction was estimated to be 12 kJ/mol.

Experimental data by Krammer showed that the reaction ratedepends onthe sulfur dioxide concentration but only at low concentrations and not so obvious athigher concentrations. In contrast to other publications, they found that the influence ofSO2 concentration on the reaction rate is rather linked to the conversion than to the 2relative humidity, which has a major impact on the conversion throughout the entirereaction as usually reported in literature. But they found out that the initial reaction rateseems to be independent of relative humidity and sulfur dioxide concentration, whichhad not been reported yet. They postulated that the reaction can be divided into thefollowing foursteps. During the initial stage, a chemisorption process of the sulfurdioxide on the particle surface seems to be important and the reaction rate decreasesexponentially with increasing conversion. Simultaneously, a nucleation processdominates the formation of the consecutive product layers where the reaction rateincreases with increasing relative humidity. The rate of reaction increases untilproduct layer diffusion takes over and reaction rate decreases again with conversion. Itshould be noted that only relative humidity

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has an impact on product layer diffusion. Beyond a conversion of around 9%, reaction rate drops significantly, which can be dueto pore closure.

Irabien consider the adsorption of sulfur dioxide on calcium hydroxideacting as a nonideal solid sorbent is the rate-limiting step. They use a parameterreferring to this nonideal behavior of the solid surface as independent of temperature butexponentially dependent on relative humidity. The authors obtained activationenergy of75 kJ/mol for the reaction.

All published work thus far indicates that relative humidity has the greatest impact onthe reaction rate between sulfur dioxide and calcium hydroxide. The relative humidity isin turn correlated with the moisture content of the solids. Additives that will modify themoisture content of the calcium hydroxide solids in equilibrium with a gas phase of agiven relative humidity would then be expected to enhance the reactivity of calciumhydroxide towards sulfur dioxide in FGD processes.

Organic and inorganic additives have been tested in spray dryer systems to improvethe

desulfurization

power

of

calcium

hydroxide

and

calciumcarbonatew.It seems that inorganic hygroscopic salts such as barium, potassium, sodium and calciumchlorides and also cobalt, sodium and calcium nitrates would be the most effective ones.Some researchers also consider sodiumhydroxide as an effective additive due to itsalkaline and hygroscopic properties.

Ruiz-Alsop and Rochelleindicated that deliquescence alone does notexplain

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thepositive effect of some salts. They contend that for an additive to be effective, it is alsonecessary that the hydroxide of the cation be very soluble, otherwise, the cation willprecipitate out as the hydroxide and the anion will form the calcium salt which could notbe hygroscopic. The effectiveness of a certain salt also depends on the relative humidity.This could be expected because when therelative humidity of the gaseous phase islower than the water activity in a saturated solution of the salt, it would not absorb waterand so, it would not enhance the calcium hydroxide reactivity. These researcherscontend that chlorides and sodium nitrate modify the properties of the product (half-hy-drated calcium sulfite) layer that is formed as the reaction takes place, therebyfacilitating the access of sulfur dioxide to unreacted calcium hydroxide, which remainsin the interior of the particle.

The scope of the present work is to quantify the influence on the reaction rate ofsulfur dioxide concentration, relative humidity, temperature and type and amount ofadditive. An empirical equation, which relates the reaction rate with these variables, hasbeen obtained and an apparent activation energy value for the reaction has also beendetermined from kinetic constants at different temperatures by using the Arrhenius plot.

2. Experimental section

This equipmentconsists of a continuous feeding and humidification system of a gaseous stream, afixed-bed reactor and an analytical system. The apparatus is operated with a personalcomputer using LabView software (National

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Instruments), which allows programmingand control of the experimental conditions, namely, nitrogen and sulfur dioxide flowrates, humidification temperature and electric resistance heating of the pipes to avoidcondensations and also provides the experimental data acquisition, in particular nitrogenand sulfur dioxide flow rates, reaction temperature, pressure, relative humidity andsulfur dioxide concentration, vs. reaction time.

Simulated flue gas was obtained by mixing sulfur dioxide and nitrogen from separatecylinders in appropriate amounts using mass flow controllers Before mixing, pure nitrogenwas passed (by switching on valve 1 from thecomputer)through the humidificationsystemthat consisted of three cylindrical flasks with 200 ml of watereach submerged into a thermostatic bath. Each flask contains small glass spheres toimprove the contact between gas and water. After the humidification system, thetemperature and the relative humidity of the wet nitrogen were measured by using aVaisala HMP 235 transmitter .At the same location, the pressure was alsoMeasuredto calculate the flow rate of water vapour generated. Thewet nitrogen by-passed the reactor until the desired experimental conditionswerereached and then valve 2 was opened from the computer to allow thegaseous streamflow through the reactor. The bed was always humidified for 15 min while the sulfurdioxide analyser was set to zero. At this time, the desired flow of sulfur dioxide wasintroduced by a mass flow controller and the experiment began. Data generated duringthe experiment were stored in an EXCEL format computer file.

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The glass reactor, a jacketed Pyrex tube (450 mm height, 12 mm i.d.)with a porousplate to hold 1 g of dry calcium hydroxide (Probus, 99% purity and particle size smallerthan 0.05 mm in diameter)or calcium hydroxide–additive mixtures(all additives weresupplied by Fluka, 99% purity and particle size smaller than 0.05 mm in diameter) diluted with 8 g of silica sand (Merck; 0.1–0.3 mm in diameter)to assure isothermaloperation and to prevent channelling due to excessive pressure drop, was thermostatedby pumping a thermal

fluid

(water–ethyleneglycol

mixture)

from

an

external

thermostaticbath.The reacted flue gas is passed through a refrigeration systemin orderto remove water because it interferes with the SO2 analyser measurement.The output from the analyser was continu-ously collected by the computer for 1h (experiment time)and the concentration (ppm) of sulfur dioxide stored as a function of time (experimental curve). Each experiment wasconducted in the same manner except a reactive solid was substituted for the

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10 g ofinert silica(‘‘blank’’ experiment) to obtain a reference flow curve. The reaction rate wascalculated as SO2 mol removed/h mol OH- from the area enclosed by the two curves (experimental and ‘‘blank’’). Some experiments were replicated to estimate the experimentalerror in reaction rate.

3. Conclusions

In this research, the quantitative influence of sulfur dioxide concentration, temperature,relative humidity and the type and amount of the three inorganic additives on thereaction rate between calcium hydroxide and sulfur dioxide have been determined.

The SO2 concentration (0–3000 ppm)was shown to have no significant

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influence on the reaction rate at a relative humidity of 38% and at 71.5℃. These results agree withthose of Ruiz-Alsop and Rochelle who indicated that sulfur dioxide concentrationdoes not influence the reaction rate at temperatures ranging from 30℃ to 90℃; 17–90%relative humidity and sulfur dioxide concentration varying from 0 to 4000 ppm. Sinceour experiments are within the range of these experimental conditions, we assume thatsulfur dioxide concentration will not influence the reaction rate at our other experimentalconditions also.

An empirical rate equation, which allows us to quantify the influence of temperatureand relative humidity on reaction rate has been developed and an apparent activationenergy of 32 kJ/mol Ca(OH)2 has been calculated. This value, relatively high, demonstrates the weak influence of temperature, but the reaction order of 1.2 withrespect to the relative humidity shows its strong influence on reaction rate.

Three inorganic additives were tested to evaluate their quantitative influence onreaction rate. An empirical equation for each additive at 71.5℃and a relative humidityof 36.7% was developed.

The kinetic rate constants for calcium chloride, sodium hydroxide and sodiumchloride were found to be respectively, 9, 5 and 0.81 times the rate constant for calciumhydroxide without any additive. The reaction orders for the weight ratio of the sameadditives were 0.6, 0.52 and y0.12, respectively. Calcium chloride is the best additivewhereas sodium chloride is an inhibitor.

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中文译文

动力学研究与二氧化硫反应在低温和氢氧化钙在一

固定床反应器

摘要

一个入口二氧化硫浓度（600-3000百万分之一），相对湿度（20-60％），反应器温度（影响的定量研究56-86℃）和不同的金额（0-30%重量）ofinorganic添加剂（氯化钠，氯化钙和氢氧化钠）对气体脱硫已进行acontinuous下行流了固定床反应器含有氢氧化钙与氧化硅sand.Results稀释表明，反应速度不依赖于二氧化硫分压（零阶动力学），而温度和相对湿度对reactionrate积极的影响。一个32千焦耳/摩尔中Ca（OH）2已反应表观活化能的估计。

实证反应在71.5℃，相对湿度36.7％，其中包括thetype和添加剂量，提出了速率方程。它已经发现，氯化钙是bestadditive研究，因为它是二氧化硫去除程度较高允许。 2000 ElsevierScience B.诉保留所有权利。

关键词：脱硫，二氧化硫，氢氧化钙;动力学;无机添加剂

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1、介绍

期间对环境的保护近几年越来越多的关注，特别是已对二氧化硫和氮氧化物排放量减少了他们的设计和电厂运行的影响。他们来自煤炭和燃油燃烧电厂的主要污染物。这两种气体对酸雨负责。

在美国和欧洲，新电厂的使用必须符合硫严重标准，以减少这些空气污染物的大量燃料。老电厂面临的主要问题之一是，它们被设计为污染控制之前，目前的标准，因此对空间没有任何设施，以纳入此种管制。

该技术来控制二氧化硫的排放量可分为三个考虑如果治疗完成之前，期间或之后燃烧，群体分布。它似乎很清楚的是，引技术，最后一组是最有利的从不同的观点，电力已经营了许多years.These站被称为烟气脱硫技术（烟气脱硫），其中，最使用的：入侵检测系统（内部管洗涤，由通用电气公司开发的）;电子红袜队（美国环保局，Babcok和威尔科克斯发展，俄亥俄州煤炭开发办公室和OhioEdison），电力科学研究院HYPAS（混合污染治理系统，由电力科学发展研究所），德雷沃省电（水合物加低温，由德雷沃开发），康寿COOLSIDE（由综合CoalCom -帕尼发达国家）和ADVACATE（由Acurex和美国环保局制定）。这些进程是3208一个稳固的吸附剂加水注射液喷洒或注入theduct之间的空气预热器及微粒收集系统位于泥浆。Calciumhydroxide或石灰石通常用作吸附剂捕捉二氧化硫和acalcium sulfiter /硫酸混合物作为反应产物获得。

克林斯波尔和斯特罗贝格提出了一个机制来解释李丰水蒸气的存在之间的二氧化硫和氢氧化钙或钙碳酸盐反应。据他们说，当相对湿度较低（低于20％），二氧化硫和水可以在固体表面吸附，但是，没有发生反应，直到

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至少有一个水分子的单分子膜的表面吸附。随着相对湿度的增加，减少二氧化硫，可吸附在表面，因为水对固体吸附时优先由于分子间力。因此，二氧化硫已被吸收水的吸附，其中硫原子形成一定的氧原子复合体的水。这一事实导致了一个带正电的氢原子可以结合从吸附剂与氢氧化物或碳酸盐离子，形成反应中间体和产物的形成。实验结果表明，石灰和石灰石反应率是相似的。因此，该复合物的形成二氧化硫被认为是速率决定步骤，因为所有的进一步反应的两种不同的吸附剂。该过程的初速度是二氧化硫浓度无关，当相对湿度低于70％，超过这个值。时，反应速率逐渐变得越来越依赖于二氧化硫的部分压力。这一事实可以归因于对水分子稳定构型分子周围形成二氧化硫。此外，人们发现，最初的反应速率是温度的功能非常弱，但指数相对湿度增加，为熟石灰和石灰石。

约根森也研究这一条长凳规模砂床反应器的反应。其结论部分指出，氢氧化钙的转换有一个相对湿度非常强烈的依赖。转换率温和上升，在与活化25千焦耳/摩尔能源协议的温度。但是，没有增加转化率随二氧化硫浓度明显迹象。

鲁伊斯艾尔索普和罗谢尔发现，相对湿度是最重要的变量影响二氧化硫和氢氧化钙反应。化学反应以在未反应的氢氧化钙表面发生介绍二氧化硫的零级动力学。在高相对湿度和/或高浓度的二氧化硫，在未反应的氢氧化钙固体表面化学反应控制的总体反应率。在低相对湿度和/或低二氧化硫水平，二氧化硫的扩散层，通过了坚实的产品成为速率控制步骤。反应速度有微弱的温度依赖性。该反应的活化能估计为12千焦/摩尔。

由克拉默实验数据表明，反应速率取决于二氧化硫浓度，但只能在低

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浓度的，不是那么明显的高浓度。相较于其他刊物，他们发现，对反应速率的影响，而二氧化硫浓度与比的2相对湿度，这对整个转换为通常在文献报道的整个反应产生重大影响的转换。但他们发现，在最初的反应速度，似乎是相对湿度和二氧化硫的浓度，该尚未见报道的独立。他们推测，该反应可分为以下四个步骤分。在初始阶段，对粒子的表面化学吸附二氧化硫的进程似乎是重要的，反应速率下降指数随转换。同时，一核进程的主导产品连续层形成时，随着相对湿度的反应速率增加。增加的反应产物层的扩散速度，直到接管和反应速度下降再次转换。应当指出，只有相对湿度对产物层扩散的影响。除了约9％的转化率，反应率显着下降，这可能是由于气孔关闭。

Irabien考虑钙作为一个非理想的固体吸附剂acting氢氧二氧化硫的吸附是速率控制步骤。他们使用一个参数是指这个作为独立的温度相对湿度butexponentially依赖固体表面的非理想行为。作者们获得激活75千焦/摩尔的反应能量。

所有发表的作品迄今表明，相对 湿度对反应速率影响最大 二氧化硫与氢氧化钙。该 相对湿度又与相关 水分含量的固体。添加剂会 修改氢氧化钙含水量 在平衡固体与气体的某一阶段 相对湿度然后将可望提高 氢氧化钙反应对二氧化硫 在烟气脱硫过程。

有机和无机添加剂已经在喷雾干燥系统测试，以提高氢氧化钙和钙carbonatew.It脱硫权力似乎如钡，钾，钠，钙氯化物以及钴，钠和钙无机硝酸盐吸湿性盐将是最有效的ones.Some研究人员还考虑氢氧化钠作为一种有效的添加剂，由于其性能的碱性和吸湿性。

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鲁伊斯艾尔索普和罗谢尔表示，潮解不足以解释某些盐类产生积极影响。他们争辩说，为有效的添加剂，也是必要的，该阳离子氢氧很溶于水，否则，将沉淀的阳离子和阴离子进行氢氧化将形成钙盐而无法被吸湿性。某盐的成效也取决于可以预料的，因为当气相相对湿度比1的盐饱和溶液水分活性较低，不会吸收水务等相关humidity.This，也不会加强氢氧化钙反应。这些研究人员认为，氯和钠硝酸盐修改的产品（半海鹰drated亚硫酸钙）层，是发生反应，从而促进二氧化硫获得未反应的氢氧化钙，它仍然在内部形成了物业粒子。

本工作范围是量化对二氧化硫的浓度，相对湿度，温度和添加剂的类型和数量反应速率的影响。一个经验公式，其中涉及与这些变量的反应速度，并已取得明显活化反应能源的价值也已确定从动力学常数在使用阿列纽斯情节不同的温度。 2、实验部分

该设备包括一个连续喂养和加湿的气体流，固定床反应器和分析制度。该仪器操作与个人使用LabView软件（美国国家仪器公司），它允许编程和实验条件，即二氧化硫和氮流量，加湿温度和管电阻加热控制，以避免浓缩，并提供电脑实验数据采集，特别是二氧化硫和氮流量，反应温度，压力，相对湿度和二氧化硫浓度，低于反应时间。

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模拟烟气混合，得到了在适当的数额由分缸二氧化硫和氮的使用质量流量控制器。混合前，纯氮通过（通过从计算机一阀门开关）通过加湿系统表示，包括200毫升的水每3瓶为圆柱形恒温浴淹没。每一瓶含有小玻璃球，以提高天然气和水之间的接触。后加湿系统，温度和相对湿度的湿氮测定使用维萨拉等候英女皇发落235发射机在相同的位置。，压力也测计算水的流量产生的蒸汽。湿氮，通过实验反应堆，直到所需的条件达成了2阀，然后从电脑上开启允许通过反应器气体流的流动。床总是加湿15分钟，而二氧化硫分析仪设置为零。在这个时候，二氧化硫的理想流介绍了一个质量流量控制器和实验开始。在实验过程中产生的数据在一个Excel格式的计算机文件中。

玻璃反应器，一护套耐热管（450毫米高，12毫米的ID与多孔板）持有1克干氢氧化钙（普罗比斯，99％纯度，粒度直径小于0.05毫米）或氢氧化钙，添加剂混合物（所有添加剂是由铵，99％的纯度，粒度直径小于0.05毫米附带），8硅砂（默克克稀释，直径0.1-0.3毫米），以保证等温

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操作，防止由于过度输送压力下降，由泵从外部恒温bath.The热流体（水乙二醇的混合物）恒温反应烟气是通过一个制冷系统为了消除水，因为它与SO2的干扰分析仪测量。从分析仪的输出是连续统ously由1小时（实验时间）和浓度（ppm的二氧化硫）作为时间（实验曲线）功能存储计算机收集。每个试验相同的方式进行，除了被动的固体是为惰性二氧化硅（'' ''空白实验10克代替）来获取一个引用流量曲线。反应率计算去除二氧化硫摩尔/小时摩尔羟基由两条曲线（实验和''空''）围成的区域。一些实验复制到反应速率估计在实验误差。

1、结论

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在本研究中，二氧化硫的浓度，温度，相对湿度和类型以及之间的三个氢氧化钙与二氧化硫反应速率的定量影响无机添加剂量已确定。

二氧化硫浓度（0-3000 ppm）的结果显示在一个有38％的相对湿度对反应速率没有显着影响，在71.5℃。这些结果与鲁伊斯艾尔索普和罗谢尔那些谁表示，二氧化硫浓度在不影响反应速率的温度范围内从30℃至90℃，相对湿度17?90％，二氧化硫浓度从0变化到4000 ppm的。由于我们的实验范围内，这些实验条件范围内，我们假设二氧化硫浓度不会影响我们的实验条件外，还有其他的反应速率。

一个经验速率方程，这使得我们可以量化的温度和相对湿度对反应速率的影响，并已制定了32千焦耳/摩尔中Ca（OH）2，计算表观活化能。此值较高，显示了温度影响较弱，但对于相对湿度反应级数为1.2显示其对反应速率的强烈影响。

三无机添加剂进行了测试，以评估其对反应速率的定量影响。每一种添加剂的经验公式为71.5℃，36.7％的相对湿度为发展。

为氯化钙，氢氧化钠和氯化钠动力学速率常数分别被发现，第9，第5和0.81倍的速率常数为氢氧化钙无任何添加剂。对于同一添加剂的重量比例的反应级数为0.6，0.52和y0.12分别。氯化钙是最好的，而氯化钠添加剂是一种抑制剂。

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