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    光催化氧化废气处理在涂装中处理应用和工程实践

     

    1、废气处理简介

       金属涂装废气中的组分比较复杂,主要成分有:甲苯、二甲苯、醋酸乙脂、环己酮、丁酮、异丙醇、丁醇等,针对铝涂装含VOCs的废气处理,国内不少环保企业研发出多种设备及工艺,主要工艺方式有:
     

    1.1 催化燃烧法

    该法的工作原理是:利用废气中的在催化剂的作用下可以有效降低氧化转化温度的特点,在设备内增加催化剂作为触媒来降低氧化转化温度。

    首先将废气通过外来热源加热到催化起燃最低温度280℃导入到装有催化剂的催化床层,使得含有废气与催化剂充分接触,并保持足够的停留时间。被加热后的依靠催化剂中的活性成分铂Pt、钯Pd的活性作用,使废气中的有机物转化氧化为无害的水和二氧化碳从而达到净化废气的目的。

    该法的缺点是随着设备使用周期的延长,催化剂的性能逐步降低,使得处理效果逐渐下降,同时存在二次维护费用较高,非甲烷总烃残留量超标等缺点。

     
    1.2 热力燃烧法

    该法的工作原理是:利用废气中的在高温下分子链容易被击破产生氧化转化的原理,直接采用外来热源将含有废气加热到最低氧化转化温度750℃。

    首先把含有废气加热到最低氧化转化温度750℃,并且停留足够的分解时间,使废气中的充分转化氧化为水和二氧化碳,从而达到净化废气的目的。

    且在合适的条件下可进行余热利用,不仅净化废气保护环境,同时余热利用产生一定的经济效益。

    该方法适用于任何废气温度的场合,废气浓度在中、高低的场合均能获得较理想的处理效果,由于废气的裂解温度高,燃料消耗较多,对于热量不能充分利用的场合,造成运行成本高。
     

    1.3 活性炭(碳纤维)吸附脱附

    该法的工作原理:利用活性炭(碳纤维)内部微孔的物理吸附作用,把废气中吸附下来,从而达到净化废气的目的。

    由于活性炭吸附只是把吸附下来,并没有把真正转化为无害的物质,并且吸附到一定程度会达到饱和,所以必须进行脱附再生。

    脱附的方法有饱和蒸汽脱附溶剂溶剂回收法、常压过热蒸汽脱附催化净化法和直接催化燃烧热空气连续脱附。

    采用活性炭吸附/脱附的方法适用于温度<50℃且废气浓度<1,000mg/m3的场合。若废气温度较高,有机物分子热运动增强,则活性炭(碳纤维)的吸附效果明显下降;废气浓度较高则活性炭(碳纤维)会迅速饱和,则必须采用连续脱附的方式。该法的缺点是对酮类物质可以吸附但是再生效果很差,活性炭容易饱和,相对净化效率较底。

     
    1.4 蓄热式热力燃烧

    其废气处理的原理和热力燃烧方法相同。不同的是蓄热式方法安装了蓄热材料代替了热力燃烧方法的换热器,由于蓄热材料具有较大的传热面积和热容量,所以依靠对废气进气/排气方向的不断切换,从而使蓄热材料不断的吸收热量和放出热量,达到节能效果。

    该方法的优点在于安装了蓄热材料有效提高了换热效率,通常高达95%以上,远高于普通热交换器的60%以下的换热效率,在较低的废气浓度时可达到最低的运行费用,废气浓度达到2g/m3即可实现无功运行,是目前国际通用且能源消耗最低的新型处理的工艺方法。

    根据以上各种含废气的处理方法、原理并结合铝涂装行业的废气中种类(苯类、酯类为主)、性质(有机物)、浓度(6g/m3),结合国家最新标准GB16297-1996(二类地区)要求,并考虑到未来国家可能进一步提高相关污染物排放要求将重要污染物排放标准提高10倍以上选择工艺设备。
     

    通过对上述处理工艺进行比较,能够处理6g/m3、能够达到表格中的标准且排放浓度低于现行标准十倍以上的工艺仅有热力燃烧及RTO,而RTO对比热力燃烧更有能耗低的特点,非常适合24小时连续生产的铝涂装行业。经过良好的沟通,毫不犹豫拍板敲定采用该工艺来处理铝涂装线的节能减排计划。

     
    2、RTO设备工作原理

    160℃左右待处理有机废气进入蓄热室1的陶瓷介质层(该陶瓷介质“储存”了上一循环的热量),陶瓷释放热量,温度降低,而有机废气吸收热量,温度升高,废气离开蓄热室后以较高的温度进入氧化室。

    在氧化室中,有机废气再由燃烧器加热升温至设定的氧化温度850℃,使其中成分分解成二氧化碳和水。由于废气已在蓄热室内预热,燃料耗量大为减少。氧化室有两个作用:一是保证废气能达到设定的氧化温度,而是保证有足够的停留时间使废气中充分氧化,停留时间为1sec。

    升温后进入氧化室氧化,成为净化的高温气体后离开氧化室,释放热量,降温后排出,而吸收大量热量后升温(用于下一个循环加热废气)。
     

    RTO氧化室抽取部分高温气体经换热器加热涂装室废气自20℃升温至400℃送入烘箱。830℃高温烟气降温后返回氧化炉。

    风机由变频器控制,以适应不同的工况全自动运行。



    3、节能经济

    3.1热平衡计算

    废气量50,000m³/h(26,000Nm³/h),废气温度160℃;溶剂排放150-400kg/h。

    混合溶剂燃烧热值:7,000kcal/kg。

    废气中VOCs氧化产生的最大热值为:Q1=450×7000=315×104kcal/h;考虑废气的去除率是99%,散热损失5%,那么实际最大热值为Q2=296×104kcal/h

    RTO进出口温差最小为50℃左右,则RTO系统正常运行需要的最小热量:Q3=26000×0.31×50=41×104kcal/h

    当Q2≥Q3,正常运行时不需要补充燃料。

    则可理论余热回收热量:

    Q4=Q2-Q3=255×104kcal/h

    按余热回收热量250×104kcal/h考虑,返回热风温度420℃;新风温度20℃。

    则返回热风量:19300Nm³/h。

    总上所述,最高VOCs浓度时,RTO系统不用补充燃料,并回收余热250万kcal/hr。

    3.2RTO工艺简介

    蓄热式热力焚化炉英文名为“RegenerativeThermalOxidizer”,简称“RTO”。是一种高效有机废气治理设备。原理是把有机废气加热到760摄氏度以上,使废气中的VOCs被氧化分解成二氧化碳和少量的水蒸气。氧化产生的高温气体流经特制的陶瓷蓄热体,使陶瓷体升温而“蓄热”,此“蓄热”用于预热后续进入的有机废气。与传统的催化燃烧、直燃式热氧化炉(TO)相比,具有热转换效率高(≥95%)、运行成本低、能处理大风量低浓度废气等特点。

    该法主要特点有:

    (1)适用大风量,低浓度的有机废气处理。(5,000-100,000m³/h风量,有机废气浓度在100ppm-4,000ppm之间。)

    (2)设备采用PLC程序控制,自动化程度高,适合长期运行。

    (3)二室RTO的有机废气去除率在95%以上;三室RTO的有机废气去除率在99%以上。

    (4)符合国家提倡的节能减排产业政策,是国外工业发达国家普遍采用的环保设备,与直燃式或催化焚烧炉相比,不仅节约能源90%以上,还可以大大减少温室气体的排放。

    (5)应用广泛,适用行业有:彩铝、汽车、摩托车、印铁制罐、彩钢板、集成电路、化工、塑料、制药行业的有机废气、恶臭、Nox、等废气的焚烧处理。

    3.3RTO部件简介

    氧化室、蓄热室、废气进出风室、陶瓷蓄热体、燃烧系统、控制系统、风机、气动阀门、新风过滤器箱、混合箱、换热器、烟囱。

    3.4RTO运行能耗测算

    铝涂装行业废气中VOCs浓度一般为6g/m3,下面针对该废气源做热平衡测算:

    有机溶剂经过RTO净化后可利用的热量:

    设计按照零污染为基本原则,将所有烘箱排气均接入RTO进行环保处理,按照设计要求配备50,000m3/h风量处理能力,废气浓度按照6g/m3(C8H10)估算如下:

    RTO的换热效率95%,氧化室温度设定为850℃,废气温度150℃,RTO排放温度设定为t,则:

    η=(850-t)/(850-150)=0.95,则RTO净化后的排放温度t=160℃

    假设最终排放温度为200℃,那么系统外排气体中的热量:Q1=77.5×104kcal/h

    有机溶剂为:50000×6=300kg/h

    平均热值为10,000kcal/kg,那么废气中VOCs氧化后释放热量:

    Q2=300×104kcal/h

    考虑去除效率98%,那么有效氧化热:Q3=300×104×98%=294×104kcal/h

    考虑RTO系统5%散热率,则:实际废气氧化热Q4=0.95xQ3=279×104kcal/h

    Q4>Q1

    即RTO系统可实现无功运行,不消耗天然气。

    可利用能耗

    则可余热回收的热量为Q5=Q4-Q1=201×104kcal/h

    高温850℃,新风20℃,最佳送入烘箱气体温度高于烘箱控制温度150℃,即260+150=410℃,错流热交换最佳温度差为150℃,则换热后高温850℃降低为510℃,则换热器效率为:

    ∆t=(∆t1-∆t2)÷ln(∆t1÷∆t2)=500℃

    (0.31×500×m)÷(0.31×850×m)=58.8%




    4、结论

    金属涂装线外排废气采用蓄热式焚烧工艺设备进行环保处理是非常合适和理想的,不仅处理效果优异,能够完全达标排放,而且运行能耗最省,是金属涂装企业环保处理的最优选择。


     
    1, the exhaust gas treatment Introduction

       Coated metal components in the exhaust gas is more complex, the main ingredients are: toluene, xylene, ethyl acetate resin, cyclohexanone, methyl ethyl ketone, isopropanol, butanol, etc., waste gas treatment for the aluminum coating containing VOCs, the domestic many companies developed a variety of environmental protection equipment and technology, the main process methods are:


    1.1 Catalytic combustion

    The working principle of law: the use of exhaust gas in the role of a catalyst can effectively reduce the oxidation characteristic transition temperature, increase in the equipment as a catalyst for the oxidation catalyst to reduce the transition temperature.

    First, the exhaust gas passes through heat source external to the catalytic ignition temperature of 280 ℃ Minimum introduced into the catalytic bed containing the catalyst, making full contact with the catalyst containing exhaust gas, and to maintain sufficient residence time. Depend on the catalyst was heated in the active component of platinum Pt, palladium Pd active role, so that the exhaust gas in the oxidation of organic matter into harmless water and carbon dioxide in the exhaust gas clean purpose.

    The disadvantage of this method is that with the extension of equipment life cycle, gradually reduce the performance of the catalyst, so that the treatment effect gradually decreased, while there is a higher secondary maintenance costs, NMHC excessive residues of other shortcomings.


    1.2 thermal combustion method

    The working principle of law: the use of exhaust gas at high temperatures easily break the molecular chains to produce oxidative conversion of principle, the use of foreign direct heat source containing the gas is heated to a minimum temperature of oxidative conversion of 750 ℃.

    First, the heated exhaust gases containing oxidative conversion to a minimum temperature of 750 deg.] C, and the residence time is sufficient decomposition, the full conversion of the exhaust gas is oxidized to water and carbon dioxide, exhaust gas clean purpose.

    And in the right conditions can be waste heat utilization, protection of the environment not only for purifying exhaust gas, waste heat utilization at the same time have some economic benefits.

    The method is applicable to any case where the exhaust gas temperature, gas concentration, the level of the occasion can get better treatment effect, due to the high temperature exhaust gas cracking, more fuel consumption for heat can not take full advantage of the occasion, resulting in operating costs high.


    1.3 activated carbon (carbon fiber) adsorption-desorption

    Works of the law: the use (carbon fiber) physical adsorption of activated carbon inside the pores, the gas adsorption down, so as to achieve the purpose of purifying the exhaust gas.

    Since only the activated carbon adsorption adsorption down and did not really into harmless substances, and adsorbed to a certain extent will reach saturation, it must be desorption regeneration.

    Desorption method of saturated steam desorption solvent solvent recovery method, the catalytic desorption pressure superheated steam and direct desorption continuous catalytic combustion hot air.

    Activated carbon adsorption / desorption method adopted for temperature <50 ℃ and the gas concentration <1,000mg / m3 occasions. If the exhaust gas temperature is high, thermal motion of enhanced organic molecules, the activated carbon (carbon fiber) adsorption effect decreased; the higher exhaust gas concentrations of activated carbon (carbon fiber) quickly saturated, must be a continuous desorption method. The disadvantage of this method is to ketones can absorb but poorly regenerated activated carbon easily saturated, the relative cleaning efficiency than the end.


    1.4 Regenerative thermal combustion

    The same principle of its gas treatment and thermal combustion method. The difference is that the regenerative method of installing a heat storage material instead of the heat exchangers in the combustion process, due to the heat storage material having a large heat transfer area and heat capacity, so rely on the gas intake / exhaust constantly switching directions, so that the heat storage materials continue to absorb heat and release the heat to save energy.

    The advantage of this method is that the installation of the heat storage material to effectively improve the heat transfer efficiency, typically up to 95%, much higher than the heat transfer efficiency of 60% or less of the ordinary heat exchanger, at a lower exhaust gas concentration reaches the lowest operating costs, waste gas concentration 2g / m3 can be achieved without power operation, and is the internationally accepted minimum energy consumption of the new treatment process method.

    According to the above method of processing a variety of exhaust gas, and the principle of combining aluminum coating industry exhaust gas species (benzene, ester-based), nature (organic matter), concentration (6g / m3), combined with the latest national standard GB16297-1996 (Region II) requirements, taking into account the future of the country could further improve the relevant pollutant discharge requirements will be important pollutant emission standards more than 10 times the selection of process equipment.


    By comparing the above-described treatment process, capable of handling 6g / m3, to reach the table and standard emission concentrations more than ten times lower than the current standard of combustion and thermal processes only RTO, RTO contrast and low energy consumption of thermal combustion more It makes it ideal for 24-hour continuous production of aluminum coating industry. Through good communication, did not hesitate to use the process to finalize the green light to deal with aluminum coating line of energy saving plan.


    2, RTO equipment works

    About 160 ℃ organic waste into the regenerator ceramic dielectric layer (the ceramic dielectric "Save" on a heat cycle), ceramics release heat, lower the temperature 1 to be treated, while the organic waste gas absorption heat, temperature, exhaust gas leaving after the regenerator at a higher temperature into the oxidation chamber.

    In the oxidation chamber, the organic exhaust gas from the burner heated to oxidation temperature setting 850 ℃, which makes components break down into carbon dioxide and water. Since the exhaust gas has been preheated in the regenerative chamber, fuel consumption is greatly reduced. Oxidation chamber has two effects: first, to ensure that the exhaust gas can reach the set temperature oxidation, but to ensure there is sufficient residence time for the full oxidation of the exhaust gas, the residence time of 1sec.

    After the after-heating chamber into the oxidation oxidizes a high temperature gas leaving the purification chamber oxidation, release heat, is discharged after cooling, heating and characterized by high heat (exhaust heat for the next cycle).


    RTO oxidation chamber portion extracted by the high temperature gas heat exchanger heating the coating chamber exhaust gas from 20 ℃ to 400 ℃ into the oven to heat up. 830 ℃ high temperature flue gas returned to the oxidation furnace after cooling.

    Fan controlled by the inverter, to adapt to different conditions fully automatic operation.



    3, Energy-Efficient Economy

    3.1 Heat balance calculation

    Amount of exhaust gas 50,000m³ / h (26,000Nm³ / h), the exhaust gas temperature of 160 ℃; solvent emissions 150-400kg / h.

    A mixed solvent of heat of combustion: 7,000kcal / kg.

    Maximum exhaust heat generated by oxidation of VOCs is: Q1 = 450 × 7000 = 315 × 104kcal / h; consider gas removal is 99%, 5% heat loss, then the actual value of the maximum heat Q2 = 296 × 104kcal / h

    RTO Importers minimum temperature difference between the minimum heat about 50 ℃, the RTO system uptime required: Q3 = 26000 × 0.31 × 50 = 41 × 104kcal / h

    When Q2≥Q3, do not need to refuel during normal operation.

    Theoretical heat recovery heat can:

    Q4 = Q2-Q3 = 255 × 104kcal / h

    By waste heat recovery heat 250 × 104kcal / h considerations, return air temperature of 420 ℃; fresh air temperature 20 ℃.

    It returns amount of hot air: 19300Nm³ / h.

    On the total, the highest concentrations of VOCs, RTO system without refueling and recovering waste heat 2.5 million kcal / hr.

    3.2RTO Process Overview

    Regenerative thermal incinerator English called "RegenerativeThermalOxidizer", referred to as the "RTO". It is a highly efficient organic waste gas treatment equipment. The principle is the organic waste is heated to 760 degrees Celsius, so that the exhaust gas VOCs are oxidized and decomposed into carbon dioxide and a small amount of water vapor. High-temperature oxidation of the gas flowing through the special ceramic regenerator, heating the ceramic body and the "regenerative", this "regenerative" is used to preheat the follow-up into the organic waste. And conventional catalytic combustion, direct-fired Thermal Oxidizer (TO), compared with a high thermal conversion efficiency (≥95%), low operating cost, capable of handling low wind exhaust gas concentration and other characteristics.

    The method main features are:

    (1) Applicable wind volume, low concentration of organic waste gas treatment. (5,000-100,000m³ / h air flow, the concentration of organic waste 100ppm-4,000ppm in between.)

    (2) equipment uses PLC control, a high degree of automation, suitable for long-term operation.

    (3) two-compartment RTO organic waste removal rate above 95%; three rooms RTO organic waste removal efficiency above 99%.

    (4) in line with national industrial policy to promote energy conservation, environmental protection equipment is widely used foreign industrial countries, compared with direct combustion or catalytic incinerator, not only to save energy more than 90%, but also can significantly reduce greenhouse gas emissions .

    (5) a wide range of applications for industry are: color aluminum, automobiles, motorcycles, Yintiezhiguan, color steel, integrated circuits, chemicals, plastics, pharmaceutical industry, organic waste, stench, Nox, and other waste gas incineration.

    3.3RTO member profile

    Oxidation chamber, regenerator exhaust gases out of the air chamber, ceramic regenerator, combustion systems, control systems, fans, pneumatic valves, new air filter box, the mixing tank, heat exchanger, chimney.

    3.4RTO energy consumption calculation

    Aluminum coating industry exhaust VOCs concentration is generally 6g / m3, do the following for the exhaust gas source heat balance calculation:

    The organic solvent can be used after purification RTO heat:

    Designed in accordance with the basic principles of zero pollution, will all have access RTO exhaust oven for environmental treatment, in accordance with the design requirements with 50,000m3 / h air volume processing capacity, gas concentration according to 6g / m3 (C8H10) is estimated as follows:

    RTO heat transfer efficiency of 95%, the oxidation chamber temperature was set to 850 deg.] C, the exhaust gas temperature of 150 deg.] C, the discharge temperature set RTO is t, then:

    η = (850-t) / (850-150) = 0.95, the discharge temperature RTO purified t = 160 ℃

    Suppose final discharge temperature of 200 ℃, then out of the system in the exhaust gas heat: Q1 = 77.5 × 104kcal / h

    Organic solvent: 50000 × 6 = 300kg / h

    The average calorific value 10,000kcal / kg, then the oxidation of VOCs in the exhaust gas after the release of heat:

    Q2 = 300 × 104kcal / h

    Consider the removal efficiency of 98%, then the effective heat of oxidation: Q3 = 300 × 104 × 98% = 294 × 104kcal / h

    Consider RTO system cooling rate of 5%, then: actual exhaust gas heat of oxidation Q4 = 0.95xQ3 = 279 × 104kcal / h

    Q4> Q1

    That RTO system can run without power, do not consume natural gas.

    Available energy

    Heat may be waste heat recovery for the Q5 = Q4-Q1 = 201 × 104kcal / h

    High temperature 850 ℃, fresh air 20 ℃, the optimum temperature is higher than the gas fed into the oven temperature controlled oven 150 ℃, that is 260 + 150 = 410 ℃, cross-flow heat exchange optimal temperature difference of 150 ℃, the temperature after the heat exchanger 850 ℃ reduced to 510 ℃, the heat exchanger efficiency:

    Δt = (Δt1-Δt2) ÷ ln (Δt1 ÷ Δt2) = 500 ℃

    (0.31 × 500 × m) ÷ (0.31 × 850 × m) = 58.8%
     


    4 Conclusion

    Metal coating line exhaust efflux using regenerative incineration process equipment for environmental treatment is very appropriate and desirable, not only an excellent treatment effect, be able to fully discharge, and energy consumption most provinces, is the most environmentally friendly metal coating business processing excellent choice.
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