Analysis of common methods for treatment of steel drum exhaust gas
Hebei Institute of Environmental Engineering / Jia Zhixin
In the production of steel drums, the commonly used paints and coatings contain a large amount of volatile organic solvents. These organic compounds (VOCs) are composed of alkane, olefin, aromatic hydrocarbons and other pollutants. Most of these organic substances are toxic and flammable. Explosion, some are carcinogens; some have a destructive effect on the ozone layer; VOCs will form photochemical smog with nitrogen oxides in the air, causing haze and secondary pollution.
There are two types of organic waste gas treatment methods used in steel drum coating production. One is the destructive method, which is to decompose and recover organic waste gas, such as combustion method, to convert organic waste gas into CO2 and H2O; the other is non-destructive. The method is to purify and recover organic waste gas, such as adsorption method, condensation method, absorption method, biological method and the like.
First, the combustion method
That is, the process of burning and purifying, destroying harmful gases by combustion to make them harmless. The chemical reactions that occur in this process are mainly combustion oxidation and thermal decomposition at high temperatures. Therefore, this method is an organic waste gas generated in the production of steel drum paint coating, which can recover heat.
The combustion purification methods currently used in practice include direct combustion and thermal combustion.
1. Direct combustion method
Direct combustion, also known as direct flame combustion, is the direct combustion of flammable hazardous components in the exhaust gas as fuel. This method is only suitable for purifying exhaust gas with a high concentration of combustible gas, or for purifying exhaust gas with high calorific value when burning harmful gas. If the concentration of flammable gas is higher than the upper limit of combustion, it can be mixed with air and burned; otherwise, a certain amount of auxiliary fuel such as natural gas can be added to maintain combustion.
In the steel barrel painting and drying production process, there are often a large number of organic solvents, such as benzene, toluene, xylene and the like. When these vapor concentrations are high, they can be treated by direct combustion. As shown in Fig. 1, it is a direct combustion method for purifying the exhaust gas of the oven. The combustion furnace is set in a large oven, and the vapor containing the organic solvent is taken out from the top of the oven by the fan and sent to the combustion furnace, and is burned at 800 ° C. The combustion gas and the gas in the oven are exchanged by the heat exchanger and then emptied, and the purification efficiency can be Up to 99%.
The direct combustion method is used for the treatment of organic waste gas, which is the use of hydrocarbons to oxidize and burn at high temperatures, and to produce CO2 and H2O when fully oxidized.
Some domestic large-scale barrel-making enterprises, whose paint drying oven produces a slightly larger concentration of exhaust gas, may be considered to be treated by this method.
1-fan; 2-burner; 3-ceramic ring; 4-heat box wall
Figure 1 Direct combustion method to purify the organic waste gas from the paint oven
2. Regenerative combustion method (RTO)
The regenerative combustion method is a purification treatment for exhaust gas having a low combustible organic content. In the regenerative combustion, the purified exhaust gas is not used as a fuel for combustion, but is used as a combustion-supporting gas when the oxygen content is sufficient, and is used as a combustion target when oxygen is not contained. In the case of regenerative combustion, the method of burning other fuels (such as liquefied gas, natural gas, oil, etc.) is generally used to raise the temperature of the exhaust gas to the temperature required for thermal combustion, so that the gaseous pollutants therein are oxidized and decomposed into CO2. , H2O, N2, etc. The temperature required for thermal combustion is lower than that of direct combustion, which is 540 to 820 °C.
The principle of the regenerative combustion method is to heat the organic exhaust gas to above 760 ° C to oxidize and decompose the VOCs in the exhaust gas into CO 2 and H 2 O. The high-temperature gas generated by the oxidation is heated by the special ceramic hot body to "heat store" the ceramic body, and the "heat storage" is used for preheating the organic waste gas that is subsequently entered, thereby saving the fuel consumption of the waste heat. The ceramic regenerator should be divided into two or more areas or chambers, and each regenerator undergoes a procedure of heat storage-exothermic-cleaning in sequence, and continues to work continuously. After the regenerator is “exothermicâ€, an appropriate amount of clean air should be introduced to clean the regenerator (to ensure that the VOCs removal rate is above 95%), and the “heat storage†procedure can only be entered after the cleaning is completed. Figure 2 is a flow chart of the common regenerative combustion method.
Figure 2 Process of regenerative combustion method
Second, catalytic combustion method
Catalytic combustion is actually a complete catalytic oxidation, that is, the complete oxidation of harmful combustible gases in the exhaust gas to CO2 and H2O under the action of a catalyst. Since most of the organic matter is flammable, catalytic combustion is one of the effective means for purifying organic waste gas.
At present, the catalytic combustion method has been applied to the coating production of the steel drum industry to carry out the purification treatment of the organic waste gas, because the temperature of the drying exhaust gas discharged during the coating production process is high, which is beneficial to the combustion reaction and heat recovery, and is effective and economical. Benefits, so this method is the most widely used in domestic barrel-making enterprises.
There are three types of catalysts for catalytic combustion: noble metal catalysts (palladium, platinum), transition metal oxide catalysts, and rare earth metal oxide catalysts.
The first catalyst used in China for organic waste gas combustion is Pb-Al2O3, a honeycomb ceramic supported catalyst. The catalyst has large free space, low wear rate and small bed resistance, and is more suitable for the treatment of steel drum coating exhaust gas. However, due to its large free space, the contact effect between the reaction gas and the catalyst surface is often lower than that of the particulate catalyst, so its activity is lower than that of the granular catalyst of the same active component. At the same time, its mechanical strength is poor, it is easy to be hot and cold broken, and the heat resistance is not good. In order to overcome the strong contact effect, a microporous flat catalyst has been developed in China, and the application effect is very good.
Different types of catalytic combustion processes can be used for different situations of exhaust gas, but the process composition has the following common features regardless of the process form:
(1) The gas entering the catalytic combustion device must first be pretreated to remove dust, liquid droplets and harmful components to avoid clogging of the catalytic bed and catalyst poisoning;
(2) The temperature of the gas entering the catalytic bed must reach the light-off temperature of the catalyst used;
(3) The catalytic combustion reaction releases a large amount of reaction heat, and this part must be recovered.
The catalytic combustion process has two types: split construction and combined operation:
(1) In the split-type process, the preheater, heat exchanger and reactor are all distributed as independent equipment, and the corresponding pipelines are connected, which is generally applied to the coating production line with large gas volume;
(2) The combined process installs the preheating, heat exchange and reaction parts in the same equipment (ie, catalytic combustion furnace), and the process is compact, and is generally applied to a coating production line with a small gas volume.
Exhaust gas treated by catalytic combustion, the concentration of organic pollutants is usually below 0.2%. The economical and reasonable conversion rate of industrial plants is generally considered to be between 85% and 90%.
Figure 3 is a schematic diagram of a typical device (KIC) for treating exhaust gas from a steel drum paint coating line by catalytic combustion. The device collects the exhaust gas and then draws it into the KIC environmental protection equipment, first indirect preheating→combustion heating (primary purification)→catalytic reaction (secondary purification)→indirect preheating→re-into the waste heat utilization equipment or emptying.
Figure 3 Shanghai Overseas Chinese KIC catalytic environmental protection furnace process flow chart
Third, the adsorption method
In some domestic drum production lines with low steel drum production and low organic waste gas concentration, the adsorption method can be used for waste gas treatment. The adsorption method exhibits the following characteristics in use:
(1) The exhaust gas can be purified relatively thoroughly, and the deep purification can be carried out, especially for the purification of low-concentration exhaust gas, which shows a greater advantage than other methods;
(2) Valuable organic components can be efficiently recovered without using cryogenic and high pressure means.
Due to the limitation of the adsorption capacity of the adsorbent on the adsorbed exhaust gas, the adsorption method is most suitable for treating low-concentration exhaust gas, and the exhaust gas with high concentration of polluted exhaust gas is generally not treated by adsorption.
1, adsorbent
As an adsorbent for purifying organic waste gas, it is active, silica gel, molecular sieve, etc., and the most widely used and most effective adsorbent is activated carbon. Activated carbon can adsorb more organic substances and have larger adsorption capacity, and can selectively adsorb organic gases in the mixed gas in the presence of steam. Generally, the adsorption efficiency of activated carbon to organic matter increases as the molecular weight increases.
2. Activated carbon adsorption and regeneration process
When purifying organic waste gas by activated carbon adsorption, the process should generally include the following parts:
(1) a pretreatment section that removes solid particles and droplets in the intake air in advance and reduces the intake air temperature;
(2) adsorption part, usually using 2 to 3 fixed bed adsorbers in parallel or in series operation;
(3) The regeneration part of the adsorbent, the most common one is the steam desorption method to regenerate the activated carbon;
(4) The solvent recovery portion recovers the water-insoluble solvent.
Figure 4 is a fixed bed activated carbon adsorption-recovery process. After the organic waste gas is cooled and filtered to cool down and remove solid particles, the fan enters the adsorber through the fan, and the gas is evacuated after adsorption. Two adsorbers operating in parallel, when one of the adsorptions is saturated, the exhaust gas is passed to another adsorber for adsorption, and the saturated adsorber is passed with water vapor for regeneration. The desorbed gas enters the condenser to condense, and the condensate flows into the separator, and after a period of time, the solvent and water are separated.
1-filter; 2-fan; 3-adsorber; 4-separator; 5-condenser
Figure 4 Fixed bed activated carbon adsorption-reflow process
Fourth, the absorption method
In the method of organic waste gas treatment, the application of the absorption method is not as extensive as the combustion method, the catalytic combustion method, the adsorption method, etc., especially the exhaust gas emitted from the production process of the steel drum coating, which cannot fully reach the industrial application level and affects the application. The main problem is the choice of a suitable absorbent.
There is a commonly used PVF coating in the paint inside the steel drum. When the paint is sprayed and dried at high temperature, it will release toxic gas. This kind of gas is not an organic waste gas produced by ordinary paint painting, but a kind of fluorine-containing exhaust gas. The components are HF and SiF4, both of which are colorless and toxic gases, but have a characteristic that they are very soluble in water to form fluorosilicic acid. Using this property, the treatment of this type of exhaust gas is most suitable by the absorption method. At present, most of these exhaust gases are treated by water absorption at home and abroad.
The water absorption method uses water, an alkaline solution or some salt solution to absorb the PVF exhaust gas, thereby achieving the purpose of purification and recovery. The advantage of this technology is that the purification equipment is small in size and easy to implement, and the treatment process can continuously operate and recover various fluorides, and the treatment efficiency is high and the efficiency is good.
As shown in Fig. 5, a conventional one-room-one-rotation (absorption chamber + cyclone tower) PVF waste gas treatment process is divided into three absorption sections, each of which circulates, thereby obtaining a higher concentration of fluorosilicic acid by-product.
1-absorption chamber; 2-swirl plate tower; 3, 4-pump; 5-sump
Figure 5 PVF waste gas treatment process
V. Biological law
The biological control of coating organic waste gas pollution is a pollution control technology developed in recent years, mainly for the research and development of industrial waste gas which has no recovery value and seriously pollutes the environment. The technology has been applied in large scale abroad, and the organic matter removal rate is mostly above 90%. Compared with the conventional treatment method, the biological method has the advantages of simple equipment, low operation cost, less secondary pollution formation, and especially economical treatment of low-concentration, biodegradable gaseous pollutants.
1, purification principle
The essence of the VOCs biological purification process is that the microorganisms attached to the filter medium use the organic components in the exhaust gas as a carbon source and energy source under appropriate environmental conditions to maintain their life activities and decompose the organic matter into CO2 and H2O. The VOCs in the gas phase body first undergo mass transfer from the gas phase to the solid/liquid phase, and then are degraded by the microorganisms in the solid/liquid phase.
2, purification process
In the biological treatment of exhaust gas, the biological treatment process can be divided into a suspension growth system and an attachment growth system according to the existence form of microorganisms in the system. The suspension growth system, that is, the microorganism and its nutrients are present in the liquid, and the organic matter in the gas phase is transferred to the liquid phase by contact with the suspended matter, thereby being degraded by the microorganism. In the attached growth system, the microorganisms adhere to the surface of the solid medium, and when the exhaust gas passes through the fixed layer formed by the filter medium, it is adsorbed, absorbed, and finally degraded by the microorganism.
According to the form of the tower used, the biological purification process is divided into a biological washing tower purification process, a biological trickling filter purification process and a biological filtration tower purification process.
(1) Biological washing tower purification process (suspension growth system)
In the biological treatment process of the exhaust gas, FIG. 6 is a biological washing tower purification process flow. The purification system is composed of a washing tower and an activated sludge tank. The function of the scrubbing tower is mainly to provide sufficient contact conditions for the gas-liquid two phases, and a perforated plate tower is widely used at present. The role of activated sludge is to degrade organic matter. The organic liquid-accumulated circulating fluid is sprayed from the top of the washing tower, and is in countercurrent contact with the exhaust gas flowing down the tower, so that the organic matter and oxygen in the gas phase are transferred into the liquid phase, enter the activated sludge tank, and are decomposed by the aerobic microorganisms.
Figure 6 Biological washing tower purification process
(2) Biological trickling filter purification process
The biological trickling column purification process is shown in Figure 7. The organic gas enters from the bottom of the tower, is purified by being connected to the biological filter material that has been contacted with the membrane, and the purified gas is discharged from the top of the tower. A microbial-attached filler is added to the drip filter tower to provide conditions for microbial growth and organic matter degradation.
Figure 7 Biological trickling filter purification process
(3) Biological filtration tower purification process
The biological filtration degradation process is shown in Fig. 8. The organic gas enters the filtration tower from the top of the tower, and is cleaned by contact with the biological filter material which has been inoculated into the membrane during the flow process, and the purified gas is discharged from the bottom of the tower. The nutrient solution is sprayed on the top of the tower regularly to provide nutrients, moisture and pH value for the filter microorganisms. The nutrient solution is in a discontinuous phase, and its flow direction is the same as the gas flow direction.
Figure 8 Biological filtration degradation process
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