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A review of the treatment techniques of VOC

Publié en ligne: 15 Apr 2022
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Reçu: 20 Jul 2021
Accepté: 06 Aug 2021
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Abstract

Volatile organic compounds (VOC) are important sources of environmental pollution. Several commonly used techniques for the treatment of VOC are reviewed. Recovery techniques can be classified into absorption, adsorption, condensation and membrane separation. Destruction techniques include biodegradation, photocatalysis, low-temperature plasma and combustion. The basic principle, applicable conditions, advantages and disadvantages of the above technologies are discussed. By contrast, the rotational regenerative combustion method has a broad prospect. Suggestions on the improvement of rotary regenerative combustion technology are presented. The treatment of VOC requires comprehensive consideration of the advantages and disadvantages of various technologies.

Keywords

Introduction

In general, volatile organic compounds (VOC) refer to a class of organic compounds with boiling points in the range of 50–260°C [1]. They mainly include benzene, alkanes, hydrocarbons, esters etc. VOC are widely distributed and can be found ubiquitously across several industries. The main sources of VOC emissions are industrial sources, such as petroleum refining, chemical production, medical and pharmaceutical, food processing, paint production, paper and ink printing industries. Among these, the petrochemical industry accounts for the largest proportion of organic emissions.

VOC are complex and often have a foul odour. Most VOC are flammable and explosive. High volatility is their remarkable characteristic. These compounds are major precursors for the production of PM2.5. Owing to the action of VOC, there is an increase in the rate of destruction of the ozone layer. The ecological environment is affected in some way. The emission of VOC can even lead to climate change and acid rain [2]. The presence of VOC also poses a threat to humans, animals and plants. They can not only come into direct contact with the skin or mucous membranes but also adhere to the surface of the body in many ways. Long-term exposure to them can result in chronic diseases, cancer and other life-threatening conditions. The existence of all these effects makes them an important target for environmental pollution control.

The serious impact of air pollution on human health is a long-term concern [3]. The management of VOC has become a focus of global attention. Countries, organisations, industries and relevant researchers are taking action. China has promulgated a series of laws and regulations regarding the emission of VOC. In 2017, the ‘Thirteenth Five-Year’ VOC Pollution Prevention and Control Work Plan was released. ‘Determination of VOC Release in Coatings’ was released in August 2019 and is scheduled to be implemented on 1 July 2020. In December 2019, the China Environmental Testing Centre formulated the ‘Technical Regulations for Continuous and Automatic Monitoring Quality Control of Volatile Organic Compounds in Ambient Air of the National Ambient Air Testing Network (Trial)’. From the above actions, it can be observed that the current situation concerning the deleterious health consequences and environmental degradation caused by VOC is serious, thus prompting regulatory bodies to impose increasingly stringent emission standards.

With the advancement of technology and the improvement of research, the knowledge of volatile organic exhaust gases has been greatly broadened and the level of understanding of them has been further deepened. Management of VOC is a hotly studied topic due to the widespread cognisation. People have developed a variety of treatment technologies. They can be separated into two categories: recovery technology and destruction technology. Recovery technologies can be classified into absorption, adsorption, condensation, membrane separation etc. Destruction technologies mainly embrace combustion, photocatalysis, biodegradation, low-temperature plasma etc. The combustion method can be subdivided into direct combustion, catalytic combustion, porous media combustion and regenerative combustion. Each method has its advantages and disadvantages. The application depends on temperature, composition and concentration of VOC, emission volume flow rate, space limitations, economic value and operating costs [4].

In this paper, the research progress of the several technologies mentioned above is reviewed. The basic principles, advantages and disadvantages of various processes are briefly introduced. In addition, the rotary regenerative oxidation technology is highlighted. Finally, the future direction of development is put forward.

Recovery technology
Absorption

Absorption methods can usually be classified into physical absorption and chemical absorption methods. It is well known that different types of gases show distinct solubility in the same absorbent. The physical absorption method takes advantage of this fact to selectively absorb harmful gases, thereby purifying the exhaust gas. The chemical method uses the chemical reaction between the waste gas and the absorbent to absorb the waste gas. Comparatively, the physical absorption method is more extensive.

The physical absorption method requires a number of factors to be taken into consideration, such as the type of absorbent and the choice of absorption device. There are three main types of general absorbents: oil-based absorbents (e.g. non-polar mineral oils such as diesel, oil wash etc.), water complex absorbents (e.g. water-oil wash, water-alkali etc.) and high boiling point organic solvents (e.g. phthalates, adipates etc.) [5, 6]. Some special adsorbents are also being investigated. Briefly, the choice of absorbent should be considered in terms of safety, effectiveness, cost and environmentally friendly performance. Common absorption devices include plate columns, packed columns and spray towers.

Absorption is a more mature technology, which has the advantages of simple operation, low investment and low maintenance costs. It is suitable for the treatment of high concentration of organic exhaust gas at room temperature. The method still faces many problems in the selection and development of absorbent and the optimisation of absorption equipment, which needs to be further explored.

Adsorption

Adsorption can adsorb VOC gas molecules selectively, using the inherent characteristics of the adsorbents to purify and treat the waste gas containing VOC. Depending on the characteristics of adsorbent regeneration, it can be classified into two types: regenerated adsorption and non-regenerated adsorption; in terms of adsorption principles, these two types cover physical adsorption and chemical adsorption, respectively. The former is mainly deployed in treatment with organic waste gas, while the latter is mostly used for wastewater treatment. Based on the process conditions, adsorption methods can be categorised into temperature swing adsorption (TSA); pressure swing adsorption (PSA); temperature and PSA; and electric swing adsorption [7].

The choice of the adsorbent is crucial to the adsorption effect. The adsorbent usually selects materials with large specific surface areas, high selectivity, loose structure and long service life, which are beneficial in terms of enhancing the adsorption effect. Most of the commonly used adsorbents are porous materials, such as activated carbon, zeolite molecular sieves, metal–organic framework materials, alumina and resins [8, 9]. Currently, activated carbon is the most effective adsorbent in organic waste gas treatment [1], which is attributed to its advantages such as strong adsorption capacity, large surface area and easy access. However, it has little effect in environments with high humidity or high temperature, and even some safety hazards. Molecular sieves also have certain advantages in exhaust gas treatment, such as good stability, high adsorption activity and hydrophobicity. It is a porous crystalline aluminosilicate material with pore structures [10]. Its adsorption process is mostly physical.

The adsorption method is simple and has a good purification effect. However, there are shortcomings such as adsorbent poisoning and secondary pollution. This method is effective for low concentration and high flow rate organic exhaust gas but is not suitable for high temperature, high humidity and high concentration exhaust gas cleaning.

Condensation

Condensation is one of the simplest methods of exhaust gas treatment. Condensation converts VOC to liquid at lower temperatures or higher pressures, thereby recovering large amounts of VOC [11]. The saturated vapour pressure is related to the type of gas and the temperature at which it is applied. Industries usually use lower system temperature or higher system pressure to separate and purify organic exhaust gas.

Cooling units and refrigerants are critical to the treatment of exhaust gases by condensation. Currently, condensation is mainly divided into surface condensation and contact condensation – and the corresponding condensing units are surface condensers and contact condensers. Refrigerant is an industrial substance used for heat exchange. Refrigerants are usually selected from working fluids having high refrigeration efficiency, high thermal conductivity and high exothermic coefficient.

Condensation is simple when treating high concentrations and high boiling point organic exhaust gas. Theoretically, it is highly effective. If the method is used alone to treat components with low freezing points and low concentration, the system pressure needs to be significantly increased or the system temperature needs to be drastically reduced. The great change in the operating environment is a challenge to the equipment. The method has a high cost and consumes a lot of energy. Therefore, the condensation method is commonly used to treat high concentrations of valuable VOC [11].

Membrane separation

Research on membranes has been performed since the 18th century and has been gradually applied in the industrial field since the 1960s of the last century. With the development of science and technology as well as the continuous improvement of the research level, membrane separation technology has been applied more and more widely. It has made significant contributions to wastewater treatment, food processing, pharmaceutical production and petrochemical refining.

The membrane has selective permeability. Under the push of external force, membrane separation technology can separate and purify organic waste gas. It is well known that the diffusion rate in the same solvent is related to the type of gas; membrane separation technology mainly makes use of this feature and the inherent characteristics of membranes to realise the needed purification. Exhaust gas emission standards are met and then discharged. There are various types of membranes. Generally, based on the separation mechanism and application scope, they can be grouped into microfiltration membranes, ultrafiltration membranes, nanofiltration membranes, reverse osmosis membranes, pervaporation membranes and ion exchange membranes [12].

Corresponding to the classification of membranes, relatively mature membrane separation technologies include microfiltration, ultrafiltration, nanofiltration, reverse osmosis and electrodialysis. The driving force of microfiltration, ultrafiltration, nanofiltration and reverse osmosis is the pressure difference. Driven by pressure, the solvent is facilitated to flow smoothly. Electrodialysis is driven by a voltage differential and allows only ions to pass through.

Membrane separation technology is a relatively advanced treatment technology. It possesses the significant advantages of simple operation, high efficiency and wide application range. It is energy efficient because no thermal driving force is required to separate the mixture [13, 14]. However, the cost of the membrane is high. The membrane separation method has poor stability and can cause membrane contamination problems. The technology is appropriate for treating VOC with low flow rates and high concentrations.

Destruction technology
Biodegradation

Biodegradation is a new technology. Research on the use of this technology to treat VOC began as early as the mid-to-late 20th century. By taking advantage of the unique physiological functions and characteristics of living organisms, harmful substances in VOC are degraded into inorganic substances such as water and carbon dioxide, thereby purifying exhaust gases.

Biological methods can be divided into four types: biofiltration, bio-trickling filtration, bio-scrubber and membrane bioreactors [15].

The main equipment of the biological filtration method is the biological filter tower. It is based on the principle that pre-treated exhaust gases can be transported to a biological bed containing packing materials, where the organic exhaust gases would be broken down into harmless substances.

The biological drip filter method is equipped with a biological drip filter tower, where organic waste gases are purified after contact with a moist biofilm. The organic waste gases enter the tower from the bottom for purification and eventually release from the top. Metabolic wastes are eliminated with the waste liquid [16].

The important equipment of the biological scrubbing method is a biological scrubbing tower, which is sprayed with a circulating scrubbing solution to absorb the organic waste gases. After absorption, organic waste gas is introduced into the regeneration tank. Then the activated sludge degrades it [17], resulting in the purification of organic waste gas.

Membrane bioreactor is a new process that combines membrane separation technology with biological methods. It is composed of three parts mainly: a bioreactor, a membrane module and a control system [18].

The biodegradation has simple equipment. Its operation as well as the management cost is low. There will be no secondary pollution during the treatment. However, this technology is greatly influenced by temperature and humidity. The biodegradation method has a high removal efficiency when treating low concentrations of VOC.

Photocatalysis

Photochemical water-cracking reactions were discovered in the early 1970s, after which photocatalysis was widely used in water treatment and degradation of VOC [19]. The basic principle is that the photocatalyst produces electron-hole pairs under the action of light. By using the strong oxidation of electron holes, VOC will be oxidised into carbon dioxide and water, and other substances. Thus, the purpose of eliminating pollutants can be achieved. The method also includes processes such as adsorption, chemical degradation and desorption of products or intermediates of VOC [20]. Catalytic efficiency is related to the type of catalyst. Commonly used photocatalysts include TiO2, ZnO, WO3, ZnS and CdS [20]. Among these, the most extensively studied catalytic material is TiO2, which is non-toxic and has the advantages of simple preparation, stability, low cost and strong ability to degrade VOC pollutants [21]. TiO2 is capable of being not only used alone but also making the modified TiO2 photocatalysts. Both of these can be utilised for the purification of VOC.

Photocatalysis is a new treatment method with high efficiency in the treatment of low concentration organic waste gases. The reaction conditions are mild. It has good green environmental performance and low energy consumption. The operation and management costs are also low. However, there are problems such as catalyst deactivation. Therefore, in order to enhance the photocatalytic efficiency or activate the photocatalyst, it is usually necessary to develop new catalytic materials [22].

Low-temperature plasma

In 1928, the concept of plasma was first proposed to describe a collection of charged particles [23, 24]. It is also the fourth state of matter. Depending upon thermodynamic equilibrium, plasma can be divided into thermodynamic equilibrium and non-thermodynamic equilibrium [25]. Non-thermodynamic equilibrium plasma is also called low-temperature plasma. Low-temperature plasma has been increasingly studied and applied in the treatment of organic waste gas. Low-temperature plasma technology is a method that uses high-energy electrons to cause inelastic collisions with gas molecules or atoms to cause free radicals, and the free radicals react with organic gas molecules to degrade VOC [26]. The main generation mechanism of low-temperature plasma is discharge, including corona discharge [27], dielectric barrier discharge [28, 29] and glow discharge [30].

The low-temperature plasma method applies to processing large flows of low concentrations of organic waste gas. It is a simple energy-efficient treatment method. In addition, low operating and maintenance costs are another advantage. However, the generation of by-products, safety and severe condition controls are the issues to be considered when applying this method.

Combustion

The combustion method is also known as the thermal oxidation method. It means that under certain conditions, the combustible organic exhaust gases are burned (or oxidised) into CO2, H2O and other harmless substances, achieving the purpose of purifying exhaust gases. The combustion method can be further sorted into direct combustion, catalytic combustion, porous media combustion, regenerative combustion and so on.

Direct combustion

Direct combustion is the direct combustion of combustible organic waste gases as chemical fuels. This method is suitable for the treatment of organic exhaust gases with a high concentration of combustible gas components or high calorific value. The combustion temperature is usually set at around 1100°C. It has demanding combustion conditions. For the exhaust gases to be fully combusted, the highly concentrated exhaust gases must be completely mixed with air in advance; otherwise, harmful substances such as dioxins will be generated. This method is relatively easy and does not need much investment in the initial stage. It has strict equipment requirements and operating conditions. Besides, there are safety hazards and waste of heat energy.

Catalytic combustion

Catalytic combustion is a typical type of gas–solid catalytic reaction. The role of the catalyst is to reduce the activation energy and increase the reaction rate. Catalytic combustion is a process in which the combustible components of the exhaust gas are oxidised at low temperatures to substances such as CO2 and H2O, accompanied by the release of large amounts of heat, under the action of a catalyst.

The performance of the catalyst affects the catalytic effect. Catalysts can be classified into three major groups: noble metal catalysts, non-noble metal catalysts and complex oxide catalysts. The common noble metal catalysts include Au, Pb, Pt etc. Noble metal catalysts have the advantages of high activity, high deactivation resistance and high regeneration capacity [31]. However, the application of noble metal catalysts is restricted by their high cost, low oxidation stability and poor resistance to sulphur and chlorine [11]. Non-noble metal catalysts can be loaded or non-loaded metal oxides [32, 33]. Common catalysts are Co, Ni, Cu, Mn etc. They have the advantages of better activity, recyclability, low price and easy availability, which makes them an ideal alternative to precious metal catalysts. Composite metal catalysts are usually made of a combination of two or more oxides, which can improve the performance of the catalyst and further improve the removal efficiency of VOC.

The catalytic combustion method has superior purification efficiency and good safety performance. However, the catalyst is prone to poisoning and surface coking. The catalytic combustion method applies to a wide range of concentrations and achieves low or zero emission of CO and NOx [34, 35]. This method still needs further research in the development of new novel combustion catalysts.

Porous media combustion

Porous Medium Combustion (PMC) technology is a new type of gas combustion technology. The porous medium has the characteristics of large heat capacity and strong thermal conductivity etc. The VOC enter the porous medium and the heat generated is transferred in the form of heat conduction, convection and radiation, which preheats the upstream gas and recovers the waste heat of the high-temperature flue gas [36]. This combustion technique achieves heat reflux, resulting in a more uniform temperature inside the porous medium.

Porous media material plays a remarkable role in the purification of exhaust gas combustion. It is not only an important component for carrying VOC exhaust combustion but also an indispensable medium for the heat reflow process. Therefore, it is necessary to comprehensively consider the thermodynamic properties, flow properties and heat transfer properties of porous media materials when they are selected [6]. The structure of porous media generally consists of stacked particles, straight-hole grid, foam, and fiber grid type. And cordierite, mullite and SiC are the commonly used porous media materials.

The technology is effective when treating organic exhaust gas with low calorific value and high flow rate. It has the advantages of compact structure, low investment cost, stable combustion and high removal efficiency. However, due to the heat storage properties of the porous material, it may cause heat accumulation and even damage the equipment.

Regenerative combustion

The regenerative combustion method is used to store the heat generated by combustion in a regenerative body after the combustible components of the organic exhaust gases are burned. The stored heat is used to preheat the exhaust gases entering the next cycle, which results in the recycling of heat.

The core device of the regenerative combustion method is the regenerative oxidiser. Regenerative Environmental Equipment Company, Inc. (REECO) (Research-Cottrell) first introduced Regenerative Thermal Oxidiser (RTO) systems to the market in the early 1870s. The early units achieved a heat recovery efficiency of 80–85% [37]. Later, the RTO system was introduced into the country. As the research progressed, its structure was continuously improved and optimised, and the heat recovery could reach >95%. Nowadays, RTO systems are increasingly being widely used in industrial organic waste gas treatment.

The regenerative thermal and the regenerative catalytic combustion methods are the two main technologies available in the regenerative method. The basic principle of the regenerative thermal combustion method is that the exhaust gases flow through the regenerative body and burn in the combustion chamber and then flow again through the regenerative body, transferring most of the heat to the regenerative body. This energy is used to preheat the exhaust gases to be treated. The regenerative catalytic combustion method is an improvement over the original regenerative thermal combustion method. A catalyst is added to the regenerative bed layer. The gas flowing through is catalysed by the catalyst, which reduces the activation energy and allows combustion at a lower temperature. The systems for both treatments are similar and consist primarily of a regenerative chamber, combustion chamber and transfer valve. The heat storage system can be categorised into a single-chamber system, double-chamber system and multi-chamber system depending upon the number of different regenerative chambers. Among these, the dual-chamber and multi-chamber systems are more widely studied and applied.

The regenerative thermal combustion method is suitable for the treatment of organic exhaust gases with low air concentrations. It saves fuel and also achieves heat recovery, while greatly reducing operating costs. It has a high combustion temperature, which easily generates secondary pollutants such as nitrogen oxides at high temperatures. The regenerative catalytic combustion rule concentrates on the advantages of both regenerative and catalytic combustion technologies. Its combustion temperature is lower and the heat loss is greatly reduced. At the same time, the purification efficiency is still very high. The ability to deal with a wide range of exhaust gas concentrations is a significant advantage. However, some factors need to be taken into consideration, such as the type, cost and life of the catalyst.

The traditional regenerative burner has been improved through continuous research and is widely used in exhaust gas treatment. However, there are still many shortcomings, such as large volume, unstable temperature field and instability of continuous operation processes. In order to make the regenerative combustion method more stable and effective in the treatment of exhaust gases, a new type of regenerative combustion device has been developed based on the original equipment. The new apparatus is known as the rotary RTO. This method has advantages such as high exhaust gas treatment efficiency and a large treatment flow rate. However, we have less production equipment, which attributes to the complex equipment manufacturing process and other factors. At present, the main focus is on the technical analysis and research of this equipment.

Rotational regenerative combustion
Structure

The components of the RTO burner include a regenerative body, combustion chamber, rotary valve, fan, control system etc. The regenerative body is the core of the RTO system and plays a key role in storing and releasing heat. Spherical and honeycomb are the main varieties widely used in the domestic and international industries [38]. Ceramic honeycomb regenerative bodies are widely used in RTO systems due to their advantages such as high thermal conductivity, high-temperature resistance and corrosion resistance. The combustion chamber is the main place where the combustible components of the organic exhaust gases are burned and purified. The exhaust gases preheated by the regenerative body are combusted and decomposed after entering the combustion chamber to produce carbon dioxide and water. The rotary valve is set up to help direct the gas distribution. It is also gas-tight, which greatly reduces the possibility of gas leakage.

According to the different airflow switching methods, rotary RTOs can be broadly classified into rotary valve type and multi-valve switching type [39]. Based on the sealing method of the rotary valve, there are three types: gas seal, mechanical flexible seal and the combination of the two. In terms of different purging media, it can be divided into two types: air purge and purge gas purge.

Principle

The regenerative body is preheated to 700–1000°C. The rotary valve is driven by the fan. Under the guidance of the fan and the rotary valve, the VOC flow through the regenerative body and absorb the stored heat. When the gas reaches a certain temperature, it is sent to the combustion chamber for combustion, generating carbon dioxide and water. Meanwhile, it releases a large amount of reaction heat. The purified gas carries a large amount of heat. Heat transfer occurs when it flows through the regenerative body. Most of the heat is transferred to the regenerative body by heat conduction and radiation. The temperature of the purified exhaust gas drops significantly. Finally, it is discharged from the bottom outlet. At the same time, the temperature of the regenerative body increases, storing a large amount of heat. The task of the stored heat is to preheat the organic waste gas that has just entered. In this way, the continuous and efficient treatment of the organic waste gas is realised.

Fig. 1

Schematic diagram of the rotary RTO process [39]. RTO, regenerative thermal oxidiser

The rotating thermal storage oxidation technology has three advantages: First, multiple valves are simplified into one valve, which greatly simplifies the complexity of the device and reduces the flow resistance of gas. Therefore, the intake, exhaust and scavenging process is smoother. Second, the oxidation of the intake air and the exothermic heat of the exhaust occur simultaneously in the oxidation bed. The heat transfer intensity and efficiency are higher. The heat exchange area is saved and the structure is more compact. Third, the device can rely on the oxidative heat release of the fuel to realise self heating to maintain operation, which greatly saves energy.

Related research

As a new technology for the treatment of VOC, many scholars and research departments have seen the huge potential for development, and have initiated a series of research and development.

Chen and Wei [40] introduced the governance technology of VOC in flexible packaging printing enterprises in recent years. The novel rotary combustion method was introduced in the novel VOC end treatment method, showing the structural principle of the method and the process route. A technical comparison between the traditional and new VOC terminal management technologies was also carried out. It was found that the new governance technology has high heat efficiency, low energy consumption and much lower operating costs. A detailed introduction of rotating RTO was carried out by Li et al. [39]. The significant advantages of rotary RTO were emphasised based on the comparison of RTO types and performance. And the rotary RTO were classified and introduced based on five aspects. Guo et al. [41] introduced the working principle and the main performance index of the rotary RTO. It showed that the thermal efficiency of the rotary RTO reaches 97%. It achieved the secondary recycling of heat energy while reducing heat loss in operation. Iloeje et al. [42] proposed an accurate simplified model of the rotating reactor, which greatly reduced the computational cost. And the optimal combination between the variables was determined to meet the needs of reactor design. Cheng et al. [43] proposed a rotating reactor for NOx reduction, using a fixed bed reactor to simulate the dynamic adsorption-reduction process of the reactor. Compared with the conventional fixed bed reactor, it had a high removal efficiency of NOx in an oxygen-rich environment. And it was adapted to a wide temperature range. A rotary regenerative catalytic combustion reactor (RRCCR) was proposed by Sang et al. [35]. The results of their study showed that both the inlet velocity and concentration had a large effect on the performance of this reactor. The reactor performance was found to be more sensitive to the increase in velocity and a decrease in methane concentration. Sun et al. [44] simulated the effect of reaction conditions such as oxygen concentration in the adsorption zone and adsorption to the reduction time ratio on the performance of the rotary reactor during CO reduction of NOx. SO2 and H2O in the flue gas were found to have a severe influence on the NO adsorption-reduction dynamic process.

Improvement

In the current rotating RTO system, the temperature field in the regenerative body is stable, and continuous gas input is achieved. However, most rotary reversing valves have a complex structure, and large flow losses may occur during gas transport. The friction of the plane seal is high, and the high energy consumption of the drive motor is a problem [45]. These problems can shorten the service life of rotary reversing valves and lead to unstable system operation. Therefore, Mao et al. [46] invented a new type of rotary directional valve for a rotary heat storage combustion device. An inlet air distribution plate, an exhausting return air distribution plate and a sweep air distribution plate were correspondingly installed at the top, middle and bottom of the rotary shaft in the valve. Each air distribution plate and the cylinder body were surrounded to form the intake chamber, exhaust chamber and air scavenging chamber. The simple structure of the reversing valve and the reasonable distribution of the three air distribution plates made the process of the air intake, exhaust and sweeping smoother; thus, the flow loss was minimised and the jamming situation could be avoided. The operation was stable and the energy consumption was low.

Current rotating regenerative combustion units are mostly constructed with a separate regenerative chamber and combustion chamber. The combustion chamber needs to rely on an igniter to ignite the inlet air that has been preheated to a high temperature (about 800°C), which makes it difficult for the combustion process to maintain self heating and requires the utilisation of an external heat source. Also, the regenerative body is bulky for a simple heat transfer process in the heat storage body. To remedy the above shortcomings, Mao et al. [45] developed a coal mine spent air rotary heat storage catalytic oxidation device with a combined oxidation bed. The oxidation bed consisted of a catalytic bed body and a regenerative body. There was a heater in the top air collection chamber. After the spent air entered the system, the inlet preheating combustion and exhaust heat storage occurred simultaneously in the combined oxidation bed. This design greatly improves heat transfer efficiency and makes the structure more compact. Under the combined effect of the low-temperature catalytic oxidation of the catalytic oxidation bed and the high heat accumulation and strong thermal feedback of the honeycomb ceramic oxidation bed, the device can achieve self heating to maintain operation and save energy.

Summary

Among the recovery methods, the absorption method has mature technology and low investment, and can deal with high flow rate and medium and high concentration of room temperature waste gas. The adsorption method has a good purification effect, but secondary pollution, adsorbent poisoning and other problems may occur. It can deal with low concentration and a large flow of waste gas. The condensation method is simple to operate, but consumes large amounts of energy and has high operating costs. This method is often used for a high concentration of organic waste gases with high utilisation value. The membrane separation method is highly efficient, widely applicable and can be recycled, but it will produce membrane pollution.

In non-recovery technologies, the biodegradation method has low operation and management costs. The main drawback is that the purification effect is highly influenced by temperature and humidity. The reaction conditions of the photocatalytic method are mild. It consumes less energy. The challenges faced are catalyst deactivation and surface coking. The low-temperature plasma method is energy efficient. It needs to take by-product generation and security issues into account. The combustion method is a very efficient method for waste gas treatment. Among these, direct combustion is more expensive. It has safety concerns and wastes heat energy. In practice, direct combustion of organic exhaust gases is rarely used. Catalytic combustion and regenerative catalytic combustion both have lower combustion temperatures. In contrast, regenerative catalytic combustion is more energy saving. Their common limitation is catalyst poisoning. The porous media combustion method is compact and stable but may suffer from heat build up.

The most promising among the thermal storage combustion methods is the rotary thermal storage combustion method. Due to the compactness of its equipment, it has a small footprint. The equipment has good airtightness and allows for a continuous input of exhaust gases. The existence of the external insulation layer of the device greatly reduces heat loss. The high heat recovery rate allows for essential self sufficiency in heat, saving fuel and improving the high energy utilisation rate. The purification effect of this method, which is up to >99%, is also not inferior to other methods.

Improvement ideas are proposed for the performance of the rotary regenerative combustion device. Most of the rotary regenerative combustion devices have complex rotary reversing valve structures, and the friction of flat seals are large; so, the performance of the device can be improved by simplifying the rotary reversing valve structure and reducing the sealing friction. There were two main problems with the previous device. One of the problems was the bulky volume caused by the separation of the regenerative chamber and the combustion chamber. The second problem was that the combustion process is difficult to maintain by self heating. In order to solve these issues, the heat accumulator combustion design can be considered for incorporation in the same heat accumulator. This results in a more compact structure, a smaller device, self-heating maintenance and improved combustion stability.

In summary, due to the wide variety of sources and chemical properties of VOC, the choice of VOC treatment technology should comprehensively consider many factors, such as the nature of the exhaust gases, the emission concentration, flow rate and the applicable conditions of the technology. The right choice contributes to the dual benefits of efficient treatment of exhaust gases and the maximisation of social benefits.

Fig. 1

Schematic diagram of the rotary RTO process [39]. RTO, regenerative thermal oxidiser
Schematic diagram of the rotary RTO process [39]. RTO, regenerative thermal oxidiser

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