The most important thing is to reduce the concentr

2022-09-28
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How should you choose your VOCs treatment equipment if you want low exhaust gas treatment concentration and high efficiency and benefits

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core tip: photochemical smog formed by volatile organic emissions (VOCs) through complex physical and chemical reactions in the atmosphere is an important factor causing smog. In 2015, the actual total emission of VOCs in China was about 25.03 million tons

[China Packaging News] photochemical smog formed by volatile organic emissions (VOCs) through complex physical and chemical reactions in the atmosphere is an important factor causing smog. In 2015, the actual total emission of VOCs in China was about 25.03 million tons. Among them, the emission of VOCs from industrial sources is about 10.76 million tons, accounting for 43% of the total emission. Under the overall requirements of the national joint prevention and control of air pollution, local governments have formulated more stringent emission standards, and with the collection of sewage charges and the normalization of environmental protection supervision, relevant emission enterprises feel unprecedented environmental pressure

regenerative thermal oxidizer (RTO) is the most widely used VOCs treatment equipment at present. Its principle is to oxidize the organic matter (VOCs) in the waste gas into the corresponding carbon dioxide and water at high temperature, so as to purify the waste gas and recover the heat released during the decomposition of the waste gas. This technology appeared in the 1970s and was introduced into the market by American Hamon research Cottrell company. By the 1990s, RTO had been widely used in waste gas treatment in European and American countries. China's RTO technology started late, but with the promulgation and implementation of the Beijing integrated emission standard of air pollutants (db11/), the most stringent environmental protection law in history, the emission concentration standard of non methane total hydrocarbons has entered an era of less than 50mg/m3

as the emission standards widely implemented in various places in China are far higher than those of major developed countries and regions in the world, foreign technology has lost the advantage of historical experience, giving Chinese RTO production enterprises the opportunity to catch up with foreign technology under the new high standard requirements. This study aims to provide performance analysis methods and data from the structural principles of different types of RTOS for the exchange and reference of domestic peer experts

I. performance comparison

quantitative analysis method is adopted in this study. The two bed, three bed and rotary RTO selected are 30000. The center is jointly prepared by both parties to handle nm3/h air volume. Under the same production process conditions, the performances of the three types of RTO in six aspects, such as progressiveness, reliability, compliance, energy saving, economy and practicality, were investigated. The details are as follows:

❖ 1 Progressiveness

to improve the purification efficiency of VOCs, RTO can be achieved in two ways. One is to prolong the combustion time of VOCs, but this will reduce the thermal efficiency. The second is to increase the number of regenerators. Theoretically, the more regenerators, the higher the purification efficiency. The rotary RTO furnace body is evenly divided into 12 regenerators, which are divided into 5 heat release zones, 5 heat storage zones, 1 dead zone and 1 purge zone according to their functions. The number of regenerators is much higher than that of two bed and three bed, and the purification efficiency is significantly improved

Figure 1 comparison diagram of different types of RTO structures

❖ 2 Reliability

(1) gas switching valve. Gas switching valve is the core component of RTO. The switching valve used by two bed and three bed RTOS is the poppet valve, which changes the gas flow direction through the lifting and lowering of the valve. Two bed RTO has 4 poppet valves, and the annual lifting of a single valve is more than 350000 times; The three bed RTO has 9 poppet valves, and the annual lifting of a single valve is more than 520000 times. Due to the frequent operation of the poppet, it is easy to cause wear and affect the service life, and the failure rate is high. As the sealing performance will be seriously affected after the loss of the valve, resulting in exhaust gas leakage, the poppet needs to be replaced regularly, and its maintenance cost is high. The rotary RTO uses a rotary valve to replace the poppet valve. Driven by the drive motor, the rotary valve rotates at the speed of 1.5min/r, which runs smoothly without impact. The sealing surface has good air tightness after grinding. When using high-quality sealing materials, the rotary valve has the remarkable characteristics of long service life, low failure rate and low maintenance cost

(2) pipeline pressure fluctuation. After detection, the pressure fluctuation of rotary RTO pipeline is ± 25Pa, which is 95% and 90% lower than that of two bed and three bed respectively. The main reason why the pipeline pressure fluctuation can be significantly reduced is that the rotary RTO uses the rotary valve instead of the poppet valve, so the air flow transmission is stable, and there will be no impact, and there will be no pipeline pressure fluctuation caused by valve switching, which will lead to waste gas blockage and backflow, effectively improving the smell of the workshop

❖3. Compliance

from the working principle of two bed RTO, when switching the gas flow direction, there is raw material waste gas without oxidation reaction in the gas space (i.e. dead zone) in the heat storage area between the switching valve and the combustion chamber. After the gas in and out is switched, it is also discharged together with the purified gas, so the peak purification efficiency of two bed RTO switching is only about 80%

at present, in order to solve the problem that the peak emission of two bed RTO switching seriously exceeds the standard and is unwilling to increase the cost of producing three bed RTO, some manufacturers have adopted to add a purge gas tank to the two bed RTO, commonly known as "two bed and a half RTO". The working principle of this RTO is: after the air flow in the regenerator is switched to the exhaust condition in reverse, the remaining exhaust gas in the regenerator is pumped into the air storage tank in seconds. After the air storage tank is full, the subsequent exhaust is switched to the chimney for discharge. The exhaust gas pumped into the air storage tank is sent to the air inlet pipe for treatment, and the air storage tank is emptied to prepare for the next gas storage. Due to the low purge and extraction pressure and limited gas storage capacity, the RTO with this structure can not thoroughly clean the residual waste gas in the regenerator, so the switching peak purification efficiency can only be increased to about 90%

two bed RTO and "two bed and half RTO" cannot pass the assessment of continuous monitoring

from the working principle of the three bed RTO, one chamber is added to the three bed RTO, two of the three chambers work, and the other chamber is purged and cleaned, which solves the problem that the raw material waste gas without oxidation reaction in the heat storage area is brought out. However, the exhaust gas inlet valve and clean gas outlet valve are started at the same time, and there is a very short process of direct short circuit of exhaust gas inlet and outlet within the alternating working time, resulting in a small amount of exhaust gas being discharged together with the purified gas. Therefore, the peak purification efficiency of three bed RTO switching is 95%. On the whole, the three bed RTO can meet the current high standard of environmental protection treatment requirements when the requirements are not very strict

the rotary RTO gas is switched smoothly through the rotary valve. There is no direct short circuit between the inlet and outlet of exhaust gas. The purification process is continuous, and there is no peak switching problem, achieving a stable high standard

it can be seen from table 1 that the peak purification efficiency of two bed RTO switching is 80%, and the total purification efficiency is 95%; The peak purification efficiency of three bed RTO switching is 95%, and the total purification efficiency is 99%; The peak purification efficiency of rotary RTO switching is exactly the same as that of the general purpose of promoting the healthy development of the industry, which is 99.5%

under the condition of ensuring that the emission concentration of non methane total hydrocarbon is less than 50mg/m3, the maximum treatment exhaust gas concentration of two bed, three bed and rotary RTO that can detect the precursor of the incident early is 1g/m3, 5g/m3 and 10g/m3 respectively. The maximum concentration of waste gas treated by rotary RTO is 10 times and 2 times that of two bed and three bed RTO respectively. The test shows that the total purification efficiency of rotary RTO is 99.5%, which is 4.7% and 0.5% higher than that of two bed and three bed respectively, and the effluent is fully up to standard after evaluation by many authoritative environmental protection testing institutions

❖4. Energy saving

(1) thermal efficiency

thermal efficiency refers to the ratio of the actually utilized heat to the theoretically available total heat (see formula). The surface heat loss and waste heat recycling capacity of RTO furnace body are two important factors affecting its thermal efficiency. After testing, the thermal efficiency of rotary RTO is 97%, which is 7% and 2% higher than that of two bed and three bed respectively

(formula)

where: T - combustion chamber temperature (the smaller the heat loss on the furnace surface, the closer it is to the theoretical combustion value), ℃

ti -- temperature of exhaust gas inlet, ℃

t0 -- outlet temperature of purified gas, ℃

a. surface heat dissipation

when the exhaust gas treatment capacity is 30000nm3/h, the surface areas of two bed, three bed and rotary RTO are 95m2, 145m2 and 86m2 respectively. The surface area of rotary RTO is 9.5% and 41% lower than that of two bed and three bed respectively. This shows that the rotary RTO has a smaller specific surface area and less heat loss from the perspective of furnace structure

b. temperature difference between inlet and outlet

generally, two bed and three bed RTOS exchange "heat storage heat release" conditions once a minute, and the average exchange frequency of "heat storage heat release" conditions per hour is times, that is, each heat storage time is more than 120 seconds. Generally, the rotary RTO valve works at 1.5r/min, and the exchange frequency of the "heat storage heat release" working condition of the regenerator is 90 times per hour, that is, the heat storage time of each time is about 40 seconds. Due to the high exchange frequency under the "heat storage heat release" condition, the rotary RTO can absorb more heat and release more heat under the same air flow length of the regenerator

according to the actual test, the inlet and outlet temperature differences of two bed, three bed and rotary RTO gas are 45 ℃, 40 ℃ and 20 ℃ respectively. This shows that the rotary RTO has stronger waste heat utilization ability, and can fully store and utilize the waste heat of waste gas combustion

(2) purging air volume

two bed RTO has no purging function. The total filling volume of the three bed RTO is 42m3. During purging, 1/3 of the regenerator is purged, and the volume of the purged regenerator is 14m3. The total filling amount of regenerative bricks of rotary RTO is 15m3. During purging, 1/12 of the regenerative chambers are purged, and the volume of purged regenerative bricks is 0.8m3. The blowing air volume of three bed RTO and rotary RTO in unit time is 5000nm3/h and 3000nm3/h respectively. Customers of Vertu can even designate a craftsman to make the exhaust often taken from the front of the chimney when purchasing the next one. The temperature is generally less than 60 ℃, and there is almost no combustible VOCs. Therefore, the larger the blowing volume, the more energy is consumed

(3) startup temperature rise time

the total heat storage brick filling volume of two bed RTO is 28m3, the total heat storage brick filling volume of three bed RTO is 42m3, and the total heat storage brick filling volume of rotary RTO is 15m3. Under the cold furnace start-up condition, the start-up temperature rise time of two bed RTO, three bed RTO and rotary RTO is 2h, 3H and 1.5h respectively. The startup temperature rise time of rotary RTO is 3/4 and 1/2 of that of two bed and three bed respectively, which saves the fuel consumption in the process of boiler startup

in addition, for the discontinuous operation of the owner, the closed furnace insulation technology can be used. After the equipment is shut down, close all valves, and the temperature in the furnace can still be maintained above 400 ℃ after 12 hours. Ignition and startup again can make the temperature in the furnace reach 800 ℃ within minutes, saving the energy consumption of boiler startup

(4) self operating concentration

self operating concentration refers to that when the concentration of VOCs reaches a certain lower limit (at the same time, it is lower than the lower explosive limit), its combustion heat can balance the heat radiation loss of the furnace body and the energy required for the temperature rise of exhaust gas. At this time, RTO can maintain self-operation state, and no additional fuel consumption is required. Due to different heat dissipation areas, different exchange frequencies under "heat storage and heat release" working conditions, different purging energy consumption, different exhaust temperatures, different filling amounts of heat storage bricks, etc., the average self running concentrations of two bed, three bed and rotary RTOS are 2.3g/m3, 2.5g/m3 and 1.8g/m3 respectively

❖5. Economy

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