The domestic waste gas treatment technologies applied in printing and packaging industry mainly include oxidation decomposition method, plasma method, adsorption method, condensation recovery method, etc., and the application effect of these methods is quite different.

At present, the most effective, thorough and reliable treatment method in the field of packaging and printing is thermal decomposition, that is, VOCs are decomposed by high temperature combustion or catalyst at a certain temperature. The corresponding treatment equipment mainly includes regenerative incinerator (RTO) and regenerator catalytic incineration (RCO). Due to the use of high-efficiency ceramic regenerator in the process, the equipment itself has a high thermal efficiency (up to 95%), so it can be more energy-saving operation. In the United States, at the beginning of VOC treatment 30 years ago, adsorption and plasma treatment were also tried, but they were replaced by thermal decomposition. RTO has high thermal efficiency and purification efficiency. When the concentration of waste gas is suitable, the equipment can efficiently complete the waste gas treatment under the condition of relatively energy saving. At the same time, it has high reliability and relatively long service life (more than 20 years). Therefore, in foreign countries, RTO is the most typical equipment used in packaging and printing industry to treat waste gas. However, in some cases, the concentration runner technology can be applied when the exhaust gas volume is large and the concentration is low.

Organized emissions usually have the following characteristics:
The composition of the waste gas is more complex;
Commonly used solvents are: ethanol, ethyl acetate, n-propyl ester, n-propanol, isopropanol, etc., in addition to ink, varnish, primer contains volatile components.
The concentration of waste gas varies with the product or process;
Due to the change of printing products, the type and amount of ink needed to be used will also change, and the air volume and concentration of exhaust gas will also change.
The typical concentration of waste gas is about 2-3g / m3 (imported);
The concentration of waste gas from domestic equipment is usually lower than 0.5g;
The exhaust gas usually contains only VOCs;
And relatively clean, without dust or a small amount of dust;
The exhaust gas temperature is not high, mostly between 30 ~ 60 ℃;
The concentration of unorganized waste gas is usually relatively low, and the air volume to be treated is also large due to the large distribution space.
Organized emission is the main part of exhaust gas emission. If the organized emission is well collected and treated, its emission will account for 85% of the total VOCs emission (but the official generally thinks that this range is about 60%).
2. Determination of waste gas process parameters
Before choosing the treatment method, three key parameters should be determined first: the air volume, the concentration and the composition of the waste gas.
The exhaust air volume can be obtained according to the original design parameters of the equipment or the parameters of the exhaust fan. The most accurate way is to measure on site (if there is a test instrument) or to find a third party to measure.
Secondly, the concentration of exhaust gas can be detected. We suggest using mobile FID equipment to detect the concentration of exhaust gas. The sampling point sampling method specified in the current national standard is not accurate, and the test results are prone to large deviation, which may be lower. Therefore, the treatment method selected according to this result may have larger problems, such as the problem of substandard after treatment. At the same time, in order to ensure the reliability of the concentration data, we also suggest to estimate the exhaust gas concentration, and there should be no big deviation between the results and the test results. In some cases, the treatment scheme can be selected and designed according to the estimated results. Here is a simple estimation method of the concentration of the organized emission:
The concentration of organized exhaust gas can be simply estimated by dividing the amount of solvent consumed per hour (kg / h) by the volume of exhaust gas a (Nm3 / h). An example is given below
A 10 color printing machine consumes 4.8t of ink within 120 hours, and 7.8T of solvent (ethanol, n-propyl ester, ethyl ester, etc.) is added. A certain amount of ink is also used in the process. Since the ink is basically free of VOCs, it is not included. The solid content of ink is about 45%, so the solvent consumption per unit time is s: 1
S=(4.8t ×(145%)+7.8t)/120h
=0.087t
=87kg/h
The exhaust air volume a of the printing machine is about 30000 Nm3 / h. Considering the evaporation loss of solvent, the solvent is estimated as 80% and discharged after entering the equipment, then the exhaust gas concentration C is:
C=87(kg/h)*80%/30000(Nm3/h)
=2.32g/Nm3
This result is an estimated average value of the concentration, and the test result should not be much different from it. In general, we need to know the average concentration and the maximum possible concentration under typical conditions. The maximum concentration can be estimated on the basis of knowing the average concentration and combining with the specific conditions of the process. If a waste heat recovery system is designed, it is better to measure the average concentration, so as to ensure that the calculated value of waste heat has little deviation from the actual value after the completion of the project, so as to avoid investment mistakes.
In addition, the composition of waste gas also affects the choice of waste gas treatment methods. Considering the characteristics of waste gas composition, different treatment methods have different treatment effects. For example, when the composition of waste gas is relatively simple (1-2 kinds of solvents), condensation recovery, catalytic combustion or regenerative combustion can be selected when the amount is large, which can achieve better results; however, if the composition of waste gas is large, combustion is preferred.
3. Selection of treatment methods
When the air volume, concentration and composition of waste gas are known, the corresponding treatment method can be selected, which can be selected according to the following table (for reference only)
The methods recommended in the table above are based on the treatment effect, which can make the waste gas treatment meet the emission standard, and then consider the equipment operation cost and investment. Of course, due to the uniqueness of the exhaust gas and the special requirements of customers, the actual treatment method may be different from that recommended in the table.
4. Cost control
Cost control is an important factor that every enterprise must consider when choosing waste gas treatment equipment!
RTO energy consumption mainly comes from two aspects: one is the power consumption of the fan, which is mainly the main fan and burner fan. The main fan is used to drive the exhaust gas flow, and the burner fan is used to supply air to the burner; the other is fuel consumption, which provides heat for RTO through fuel combustion to maintain the set burning temperature. The following is illustrated by an example calculation:
Take printing industry waste gas as an example. The exhaust gas volume of a printing machine is about 20000 Nm3 / h, the exhaust gas temperature is about 40 ℃, the concentration is about 1.7 g / Nm3 (34 kg / h), the comprehensive combustion heat value of waste gas is about 6500 kcal / kg, the design air volume of RTO is 22000 Nm3 / h, leaving a small amount of margin, and the design thermal efficiency is 95%.
When RTO is in normal operation, the fan power consumption required to drive 20000 Nm3 / h waste gas is about 40 kW, and 7 m3 / h natural gas is consumed at 1.7 g / Nm3 waste gas concentration to maintain the equipment combustion temperature. If the electricity charge is calculated as 0.9rmb/kwh and natural gas is calculated as 3.5rmb/m3, the operation cost per hour is about 60 yuan, which is not high.
If the concentration of waste gas is more than 2.1g/nm3, natural gas is not needed during normal operation, and the burner will automatically shut down. Therefore, only the energy consumption cost of fan is only 36 yuan / hour.
If the concentration is far more than 2.1g/nm3, the waste heat can be recovered. The waste heat can be used to heat the oven, thus saving energy consumption of production equipment. It can also be used to produce hot water and heat buildings. Therefore, in the case of waste gas with more waste heat, full use of waste heat can create benefits. For example, if the waste gas concentration is 4G / Nm3 (80kg / h), the maximum available waste heat is about 500000 kcal. Assuming that the utilization rate is 60%, 300000 kcal can be recovered. If the power required for printing drying is about 600000 kcal (assuming the drying temperature is 80 ℃), 50% energy consumption can be saved. If the concentration reaches 6G / Nm3, the energy consumption can be saved by 75%. Generally speaking, if the gas-gas heat exchange system is used, the benefit generated by the waste heat recovery can recover the equipment investment in about two years.

5. Energy saving and wind reduction
At present, many domestic printing equipment, especially domestic equipment, have large exhaust air volume and low concentration (usually less than 1G). This brings a lot of trouble to the follow-up governance, no matter what kind of treatment, it will face relatively large investment costs or operating costs. The best way is to optimize the air volume to reduce the exhaust air and increase the concentration of exhaust gas. This will not only reduce the operation cost of production equipment, but also greatly reduce the cost of follow-up waste gas treatment equipment. According to the different structure of the oven, the optimal treatment is also different. But the general idea is hot air recycling. Through hot air recycling to reduce the heating energy consumption, can also reduce the fan operation energy consumption, and increase the concentration of exhaust gas.

According to the process conditions, customers can increase the proportion of recycled hot air to reduce the total exhaust air volume and increase the concentration of waste gas emission under the condition of ensuring safety and not affecting product quality. However, if the concentration of exhaust gas is high, LEL monitor should be installed to avoid the risk of explosion.
In addition, the concentration monitored by LEL can be used as the control signal to control the hot air recycling valve in dynamic real-time. Under the condition that the concentration of waste gas is safe, the return air volume is automatically adjusted, which can automatically adapt to the changes of products and processes. This system is usually expensive. If there is no hot air recycling mechanism in the oven, it can also be recycled at the centralized air inlet and outlet, and the air volume can still be optimized; in addition, the new hot air system of ESO system, which adopts centralized air inlet and exhaust and recycling, can also reduce the air volume and increase the concentration.
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