Treatment of organic waste gas

Catalysts used for catalytic combustion are mostly precious metals such as Pt and Pd; these catalysts exhibit good activity, a long service life, and stable performance. Research on rare earth catalysts has achieved certain results to date. The gas entering the catalytic combustion device must first undergo pretreatment to remove dust, droplets, and harmful components, thereby preventing clogging of the catalytic layer and catalyst poisoning. Compared to other types of combustion, the advantage of this method is that catalytic combustion is a flameless process, making it safe; it requires a low combustion temperature, results in low costs for auxiliary dyes, imposes few restrictions on the concentration and calorific value of the combustible components, generates little secondary pollutant NOX, features smaller combustion equipment, and achieves a high rate of VOC removal ； The disadvantage is that the catalyst is expensive, and the exhaust gas must not contain components that could deactivate the catalyst.

I don’t know what this post means! Are you interested in catalytic technology? Or something else! I don’t know what the intention is behind the post made by the original poster!

Advances in Catalytic Combustion Technology for Organic Waste Gas (I) Source: Internet Author: Li Guangming Abstract: This paper provides a review of the basic principles, characteristics, and economic efficiency of catalytic combustion in the treatment of organic waste gas, as well as the research progress regarding catalysts, kinetics, and process flows. Keywords: catalytic combustion; organic waste gas; catalyst. Organic waste gas is a common pollutant emitted by industries such as petrochemicals, light industry, plastics, printing, and coatings. It often contains hydrocarbon compounds (aromatics, alkanes, alkenes), oxygen-containing organic compounds (alcohols, ketones, organic acids, etc.), as well as nitrogen-, sulfur-, halogen-, and phosphorus-containing organic compounds. If these exhaust gases are discharged directly into the atmosphere without treatment, it will cause severe environmental pollution and harm human health. Traditional methods for purifying organic waste gases include adsorption, condensation, and direct combustion. These methods often have drawbacks such as the tendency to cause secondary pollution, high energy consumption, and sensitivity to the concentration and temperature of the organic waste gases. The emerging catalytic combustion technology has moved from the experimental stage to practical application, and is gradually being used in industries such as petrochemicals, pesticides, printing, coatings, and wire processing [4]. 1 Basic principles of catalytic combustion Catalytic combustion is a typical gas-solid phase catalytic reaction, whose essence lies in deep oxidation involving active oxygen species. In the catalytic combustion process, the role of the catalyst is to reduce the activation energy. Meanwhile, the catalyst surface possesses adsorption properties, which cause reactant molecules to accumulate on the surface and thereby increase the reaction rate, accelerating the progress of the reaction. With the help of a catalyst, organic waste gases can undergo flameless combustion at lower ignition temperatures, being oxidized and decomposed into CO2 and H2O, while releasing a large amount of heat energy. The reaction process is as follows: 2 Characteristics and economic benefits of catalytic combustion 2.1 Characteristics of catalytic combustion 2.1.1 Low ignition temperature, energy savings: Compared to direct combustion, catalytic combustion of organic waste gases features a low ignition temperature as well as reduced energy consumption. In some cases, no external heating is required once the ignition temperature is reached. 2.1.2 Wide range of applications: Catalytic combustion can handle almost all types of hydrocarbon-based organic waste gases and malodorous gases; in other words, it is suitable for treating various organic waste gases with wide concentration ranges and complex compositions. For waste gases with low concentrations, multiple components, and no recycling value, emitted by industries such as organic chemicals, coatings, and insulating materials, the adsorption-catalytic combustion method yields better treatment results. 2.1.3 High treatment efficiency with no secondary pollution: The purification rate of organic waste gases using catalytic combustion is generally over 95%, and the final products are harmless CO2 and H2O (organic compounds containing heteroatoms produce other combustion products as well); thus, there is no issue of secondary pollution. Furthermore, due to the low temperature, the formation of NOX can be significantly reduced. 2.2 Economic efficiency of catalytic combustion The main factors affecting the economic efficiency of the catalytic combustion method are: catalyst performance and cost ； Concentration of organic compounds in exhaust gas ； Heat recovery efficiency ； Level of management and operational skills. Although catalytic combustion cannot recover useful products, it can reuse the heat generated by the reaction, thereby saving energy and reducing treatment costs, making it economically viable. 3 Catalysts and Combustion Dynamics 3.1 Key Performance Indicators of Catalysts Under conditions of high space velocity and low temperature, the conversion rate of organic waste gas in the combustion reaction approaches 100%, indicating that the catalyst has high activity. The activity of a catalyst goes through three stages: induced activation, stabilization, and aging-induced deactivation; it has a certain limited lifespan, with industrially useful catalysts generally lasting more than 2 years. The length of the usage period is related to the stability of the optimal active structure, which in turn depends on heat resistance and toxicity resistance. Catalysts used for catalytic combustion are required to have high heat resistance and resistance to poisons. The catalytic combustion of organic waste gases is generally not carried out under very strict operating conditions, as the concentration, flow rate, and composition of the waste gases are often unstable; therefore, the catalyst is required to have a wide range of tolerance for such operating conditions. The catalytic combustion process operates at a high space velocity, resulting in strong impacts of the gas flow on the catalyst. Additionally, fluctuations in bed temperature cause thermal expansion and contraction, which can lead to the breakdown of the catalyst carrier; therefore, the catalyst must possess high mechanical strength as well as good resistance to thermal expansion and contraction. 3.2 Types of Catalysts There are currently a considerable number of catalyst types, which can be broadly divided into 3 categories based on their active components. 3.2.1 Precious metal catalysts: Precious metals such as platinum, palladium, and ruthenium exhibit high catalytic activity for the oxidation of hydrocarbons and their derivatives. They have a long service life, a wide range of applications, and are easy to recover; therefore, they are the most commonly used catalysts for exhaust gas combustion. The Pt-Al2O3 catalyst, which was first adopted in our country, belongs to this type of catalyst. However, due to its scarce resources, high cost, and poor resistance to poisoning, efforts have been made to find alternatives or to minimize its use. 3.2.2 Transition metal hydride catalysts: As alternatives to precious metal catalysts, transition metal oxides with strong oxidizing properties are used; they exhibit high activity toward hydrocarbons such as methane as well as carbon monoxide, while also reducing the cost of the catalysts. Common examples include MnOx, CoOx, and CuOx catalysts. The MnO2-containing catalyst developed by Dalian University of Technology is capable of removing methanol vapor at 130°C and an space velocity of 13,000 h-1; it is also effective in removing acetaldehyde, propanol, and benzene vapor. 3.2.3 Mixed oxide catalysts It is generally believed that, due to interactions such as structural or electronic modifications among mixed oxides, their activity is higher than that of the corresponding single oxides. There are mainly two categories: (1) Perovskite-type composite oxides. Under certain conditions, rare earth and transition metal oxides can form composite oxides with a natural perovskite structure, with the general formula ABO3; these compounds exhibit significantly higher activity compared to their corresponding single oxides. In the structure, A generally adopts a tetrahedral geometry while B takes on an octahedral geometry; thus, A and B form an alternating three-dimensional structure that is prone to substitution, resulting in lattice defects. These defects serve as catalytic active sites, and the surface lattice oxygen provides highly active oxidation centers, thereby enabling deep oxidation reactions. There are several common types, such as BaCuO2, LaMnO3, etc. (2) Spinel-type complex oxides, as an important structural type of complex oxides, are represented by the formula AB2X4. Spinels also exhibit excellent catalytic activity for deep oxidation; for example, their catalytic combustion of CO occurs at low temperatures (around 80°C), and hydrocarbons can be completely oxidized at low temperatures as well. Among these, CuMn2O4 spinel, which has received the most research attention, shows particularly outstanding activity toward aromatic compounds – complete combustion of toluene can be achieved at only 260°C, enabling low-temperature catalytic combustion, which holds great practical significance. 3.3 Catalyst Loading Method The active components of the catalyst can be deposited on the carrier in the following ways: (1) Electrodeposition onto wound or compressed metal carriers ； (2) Deposited on granular ceramic material ； (3) Deposited on ceramic materials with a honeycomb structure. Metal support catalysts generally involve forming the metal into a mesh or strip shape, and then depositing the active component on it. The advantages of metal-supported catalysts are good thermal conductivity and high mechanical strength, while the disadvantage is a relatively small specific surface area. Ceramic carrier structures fall into two main categories: granular and honeycomb, with ceramic materials typically being silicon-aluminum oxides. The advantage of granular carriers is their large specific surface area, while the disadvantages are high pressure drop and wear loss of the active components due to friction between the carriers. Honeycomb carriers are a relatively ideal type of carrier, featuring a high specific surface area, lower pressure drop compared to granular or columnar types, as well as high mechanical strength, wear resistance, and heat shock resistance. 3.4 Catalyst Deactivation and Prevention 3.4.1 Catalyst Deactivation As catalysts are used over time, their activity gradually decreases until they become deactivated. There are mainly three types of catalyst deactivation: (1) complete catalyst deactivation. Substances that deactivate catalysts fall into two categories: fast-acting and slow-acting poisons. Fast-acting poisons mainly include phosphorus, arsenic, etc., while slow-acting poisons include lead, zinc, etc. Normally, catalyst deactivation is caused by poisons combining with the active components or forming an alloy. For fast-acting poisons, even trace amounts can rapidly deactivate the catalyst. Below 500°C, chronic toxicants slow down the alloying of the active substances considerably. (2) Inhibit catalytic reactions. Compounds of halogens and sulfur readily bind to active centers, but this binding is relatively loose, reversible, and temporary. When such substances in the exhaust gas are removed, the catalyst’s activity can be restored. (3) Deposition covers the active centers. The presence of unsaturated compounds leads to carbon deposition; in addition, ceramic dust, iron oxides, and other particulate substances clog the active sites, thereby affecting the catalyst’s adsorption and desorption capabilities and resulting in a decrease in its activity. 3.4.2 Prevention and control of catalyst deactivation To address the decline in catalyst activity, the following measures can be taken: properly control the reaction conditions in accordance with the operating procedures ； When carbon deposits form on the catalyst surface, fresh air is blown in to raise the combustion temperature, thereby burning off the carbon deposits on the surface ； Pre-treat the exhaust gas to remove toxins and prevent catalyst poisoning ； Improve the preparation process of the catalyst to enhance its heat resistance and resistance to poisons. 3.5 Combustion kinetics When organic waste gases burn on metal oxide catalysts, the oxidation of hydrocarbons occurs through a cycle of surface redox reactions. This mechanism was proposed by Mars-Van Krevelen, and the reaction mechanism is as follows: where Ri represents hydrocarbon species i. The corresponding reaction kinetic model equation can be expressed as: where ki and koi are the reaction rate constants for hydrocarbon species i and oxygen, respectively; Ci and Coi are the concentrations of hydrocarbon species i and oxygen, respectively; and Vi is the number of moles of oxygen required per mole of hydrocarbon species i to be completely oxidized. Experiments show that the reaction order of the hydrocarbon oxidation reaction with respect to hydrocarbons lies between 0 and 1. 4 Catalytic combustion process flow: Based on the methods of waste gas preheating and enrichment, the catalytic combustion process flow can be divided into 3 types. 4.1 Preheating type The preheating type is the most basic form of catalytic combustion process. The temperature of organic waste gas is below 100°C and its concentration is also low; it cannot generate heat on its own. Therefore, it needs to be heated in a preheating chamber before entering the reactor. After combustion and purification, the gas exchanges heat with untreated waste gas in a heat exchanger to recover some of the heat. This process typically uses gas or electric heating to raise the temperature to the ignition temperature required for the catalytic reaction. 4.2 Self-heat balance equation: When organic waste gas is emitted, its temperature is relatively high (around 300°C), above the ignition temperature, and it contains a high amount of organic substances. The heat generator in the heat exchanger recovers some of the heat produced by the purified gas, allowing heat balance to be maintained under normal operating conditions without the need for additional heat supply. Typically, only an electric heater is required in the catalytic combustion reactor for use during ignition. 4.3 Adsorption-Catalytic Combustion When the flow rate of organic waste gas is high, its concentration is low and the temperature is low, and catalytic combustion would require a large amount of fuel, adsorption can be used first to capture the organic waste gas on an adsorbent thereby concentrating it. Subsequently, hot air is used to desorb the organic waste gas, resulting in concentrated organic waste gas with a concentration 10 times higher or more, which can then be subjected to catalytic combustion. At this point, normal operation can be maintained without the need for an additional heat source. The choice of catalytic combustion process for organic waste gases mainly depends on: the heat release during the combustion process, that is, the type and concentration of combustible substances in the waste gases ； Ignition temperature, namely the properties of the organic components and catalyst activity ； Heat recovery rate, etc. When the recovered heat exceeds the heat required for preheating, self-thermal balance operation can be achieved without the need for an external heat source, which is the most economical approach.