The small edition of Qingdao Aikang ozone generator discusses the application engineering technology of VOCs ozone control

In addition, VOCs is also an important source of PM2.5 and photochemical fog, which is difficult to control. With the country's increasing attention to the environment, following water environmental governance and soil environmental governance, the control and governance of VOCs will be included in the important agenda, therefore, to carry out VOCs governance technology research, especially the applicability of engineering technology demonstration research, is very urgent and necessary, has important practical significance.

1.Research progress of VOCs governance technology

VOCs treatment technology research is generally carried out from three perspectives: (1) improve the production process of products involving VOCs, or use alternative materials or new production processes to control the generation of VOCs from the source; (2) Separation and reuse of VOCs generated in the production process as far as possible; (3) End treatment, that is, independent treatment of VOCs that must be emitted. In view of the (1) and (2) types involve the upgrading of the production process of enterprises, which is difficult for general researchers to reach, therefore, referring to the management technology research of VOCs, usually refers to the end management.

1.1.1 Direct combustion

In view of the fact that VOCs are rich in C and H elements, they can be oxidized by oxygen in the air to produce CO2 and H2O at appropriate treatment temperatures, and their treatment efficiency can reach 95%~99% under appropriate conditions [1,2]. However, the operation conditions of this method are harsh, and it is only suitable for the purification treatment of VOCs with high fuel concentration. When the combustion calorific value is lower than the heat lost by the system, it is necessary to supplement the fuel for thermal combustion to maintain the system in a continuous working state. Combustion temperature, turbulence degree and residence time will directly affect the treatment effect. In addition, the combustion temperature is usually 700~1200, there will be combustion by-product NOx, the formation of secondary pollutants.

1.1.2 Catalytic combustion

For the treatment of low concentration VOCs, the direct combustion method is not competent due to its obvious shortcomings. Catalytic combustion is the use of catalysts to reduce the reaction energy barrier, achieve VOCs oxidation decomposition and release heat at a lower temperature (200-400 degrees). Catalytic combustion is especially suitable for VOCs with large capacity and low gas concentration, such as alcohols, aldehydes, ketones and benzenes. The reaction temperature of the catalytic combustion process is low, 300-500 degrees lower than that of direct combustion, and the combustion is completed. It is not easy to generate secondary pollutants such as nitrogen dioxide and dioxins at high temperatures. It can also recover heat, save energy and improve pollutant removal efficiency.

1.1.3 Adsorption

Adsorption is a method of adsorbing VOCs by using a porous structure, the large surface area of the adsorbent, and the structurally active sites. It is suitable for the treatment of organic waste gas, with a relatively single composition, a relatively stable airflow and a concentration range of 300 * 10-6 to 5000 * 10-6 r/min. Activated carbon and macroporous resins, as common adsorbents, are widely used to adsorb and recover fats and aromatics, most chlorine-containing solvents, commonly used alcohols, ketones and esters. They are widely used as VOCs control methods.

1.1.4 Absorption

Absorption method is a method of using liquid absorbent to absorb and recover VOCs from the air stream, its essence is VOCs enrichment. The VOCs concentration range of its treatment is 300*10-6~5000*10-6r/min, and the efficiency can reach 95 ~98, and the commonly used process equipment has two kinds of packing tower and spray tower. The absorption process is more effective for the treatment of benzene-containing VOCs at room temperature, low temperature, large air volume and low concentration, and the cost is lower. The mass transfer resistance of the absorption process is mainly in the gas-liquid surface and liquid film, and the purification effect depends on the contact efficiency and area of the gas and liquid phase. Therefore, the optimization of the absorber and the reactor is a main direction of this technology.

1.1.5 Coagulation

Condensation method is a method to achieve purification and recovery of VOCs by supersaturating gaseous pollutants according to their vapor pressure differences at different pressures and temperatures. In practical applications, this method is often combined with adsorption, incineration and solvent absorption to reduce operating costs. Commonly used condensing equipment is condenser, atomizing condensation tower and so on.

Commonly used coolants are water, brine (cooling temperature 4.4~34 degrees) and CFC (cooling temperature -34.4~68 degrees). The compression method can also be used to make gaseous harmful substances become liquid at critical temperature and critical pressure, so as to purify or recover harmful substances, but due to the high cost, it is currently used less.

1.2.1生物法

生物反应过程类似于氧化过程，它利用微生物在有氧条件下将有机物氧化成CO2和H2O，以破坏和消除VOCs。 这是一种廉价有效的处理方法，设备简单，常用于处理低浓度VOCs气体，但对温度，pH值，水分含量，气流速度等操作条件的要求更为严格。 如果氧化不完全，它可能产生比VOC更有毒的副产物。 生物法主要用于处理低浓度的VOCs。 处理效率取决于生物活动和操作条件的控制。 不同有机物质的降解效率为40％~98％。 虽然生物法的操作成本很低，但效率低且不稳定，处理设备和面积大，微生物具有选择性，操作条件苛刻。

1.2.2蓄热式燃烧

鉴于通过直接燃烧排出的烟气带走了大量的热能，已经开发了许多节能技术来利用烟气的余热。 其中，陶瓷球等蓄热材料是吸收和释放热量最有效的材料。 再生燃烧包括陶瓷再生床，自动控制阀，燃烧室和控制系统。 再生床分为两部分，底部的自动控制阀分别与进气口和排气口连接; 换向阀由换向阀代替，以储存来自燃烧室的高温气体的热量，然后将VOCs废气预热到再生床中; 预热到一定温度的VOCs废气与燃烧室中的氧化反应以实现净化。

1.2.3等离子法

等离子体由电子、离子、自由基和中性粒子组成，是导电性流体，总体上保持电中性，被称为物质的第四种形态。低温等离子体主要是由气体放电产生的，属于非平衡等离子体，主要产生形式有辉光放电、电晕放电、介质阻挡放电、频射放电及微波放电。而能在常压下产生低温等离子体的只有电晕放电和介质阻挡放电。低温等离子体中存在的电子、离子、活性基与激发态分子有极高的化学活性，能使很多需要很高活化能的化学反应得以发生，使常规方法难以去除的污染物得以转化或分解。

1.2.4膜分离法

膜分离技术的原理就是利用聚合物复合膜对于有机蒸汽与空气的渗透选择性，使有机物透过膜而从废气中分离出来。在实际应用过程中，通过在膜的进料侧使用压缩机或渗透侧使用真空泵来增强膜的渗透力。含有VOCs的气流在压差作用下，VOCs优先透过膜，在膜的渗透侧形成富VOCs气流，而在膜的截留侧形成主要含有氮气、氧气、甲烷等不易渗透气体的贫VOCs气流。膜法最早源于20世纪60年代的脱盐处理和海水淡化，现已用于石油化工、制药等行业。膜法流程简单、能耗低、无二次污染，是一种非常有前途的分离方法，目前已成功地应用于回收多种VOCs。

VOCs control technology has advantages and disadvantages. Taking into account economic and technical factors, the comprehensive application of a variety of control technologies has obvious advantages. Using a new type of adsorption bed and activated carbon fiber, combined with the functions of a heater and cooler, Li Zeqing identified a low airflow, high concentration VOCs treated carbon adsorption, thermal desorption and condensation recovery process. Lord's original gooeiM, etc. Combined with activated carbon adsorption and low temperature concentration to treat VOCs volatilized from point sources. It can not only bring VOC emissions up to standard, but also return to high-quality liquid VOCs. A new indoor air purifier designed by Gao Lixin et al. The nano-tio2 photocatalyst layer was loaded on the activated carbon powder particles to remove volatile organic compounds in indoor air, and the combination of activated carbon adsorption and photocatalytic oxidation technology was realized. Zhang Zengfeng et al. adopted the integrated technology of low temperature plasma catalysis to remove formaldehyde. It is found that the presence of plasma is very important for the removal of formaldehyde at room temperature and atmospheric pressure. In the plasma atmosphere, the photocatalyst of titanium dioxide can produce activity. In addition, regenerative combustion technologies (RTO) that use ceramics or other high-density inert heat storage materials as filled beds absorb and store heat from the exhaust gases in the combustion zone, and then release the heat into a cold inlet gas (instead of using a shell-and-tube heat exchanger for heat transfer between two fluids). Unlike other thermal oxidation technologies, the heat recovery efficiency can reach 98% if the heat recovery rate is increased. The combination of chemical combustion and regenerative combustion constitutes the regenerative catalytic combustion (RCO) composite technology, which realizes flameless combustion and better operating environment on the basis of heat recovery and utilization.

2.VOCs governance technology selection and process applicability

Compared with the emission and disposal of solid and liquid pollutants, the control and treatment of VOCs is more difficult, for the specific emission substances, emissions, concentrations, humidity and other situations, the selection of efficient, economical and applicable treatment technology and process equipment is a very challenging work. The comparative analysis of the processing characteristics of several VOCs is shown in Table 3.

Any technical process has its limitations. For a specific enterprise, the choice of VOCs treatment technology should first consider the flow rate, concentration (including concentration changes) and composition characteristics of the exhaust gas. Second, factors such as capital investment and operational management requirements should be considered. Some advanced technologies have strict requirements for operational management or management techniques that are difficult to maintain. It's not for small businesses.

The author learned that many enterprises in Tianjin are dealing with volatile organic compounds, and the process recommended by the environmental management department is activated carbon adsorption. However, they are not equipped with temperature changes, pressure changes or combined desorption devices. Enterprises have the form of VOC processing equipment. In fact, they simply don't work reliably. A new batch of activated carbon is replaced during the inspection process (discarded activated carbon is also a potential safety hazard), and other times. And that's totally okay.

In fact, for the VOCs processing of small businesses, the absorption method using mature technology is more appropriate, simple operation, and easy to master technology and process maintenance. It is not only suitable for the treatment of high concentration VOC, but also suitable for the treatment of low concentration VOC. Vocs can also be recovered and reused when the absorbent is properly selected, such as solvents recovered during the spraying process, and can also be returned to paint production. The VOCs treatment system, designed by the Tianjin Institute of Renewable Resources of the China National Supply and Marketing Cooperative, uses novel adsorbents to absorb and purify organic volatile organic compounds such as benzene, alkanes and ethers. VOC solvents can be obtained by desorption - condensation or combustion, and VOC can be desorbed directly at the appropriate time. The automatic desorption control of the system can be realized through the micro-pressure sensing technology. In addition, the combination of infrared, catalytic regeneration and regenerative catalysis technologies can meet the VOCs processing requirements of different enterprises and sizes.

3.Closing remarks

VOCs governance is a difficult task with a long way to go, from the perspective of engineering applications, some technologies are still immature, it is difficult to afford the heavy responsibility of VOCs governance, but also need further efforts of technical personnel. There are also some technologies that, due to the constraints of engineering conditions, can achieve their ideal goals over time. At present, the absorption method and the improved thermal storage catalysis technology have relatively precise economic and technical advantages, and it is recommended that enterprises pay more attention to it.