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Process design of RTO treatment of VOCs waste gas

release time:

2022-03-02 17:07

"From VOC emission reduction treatment technology"

 

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Regenerative incinerator (RTO) is a device that burns and treats waste gas polluted by organic matter, and uses heat storage body to exchange heat and use it to heat up the waste gas itself. It is an effective measure to effectively treat VOC waste gas at present.
1. Exhaust gas conditions suitable for RTO device incineration
In general, exhaust gas with volatile organic compound concentration below 25% LEL (lower explosive limit of combustible gas) and combustion adiabatic temperature rise above 40°C is suitable for RTO treatment.
The type of exhaust gas whose VOCs concentration is less than 2000mg/Nm³ and causes the adiabatic combustion temperature rise to be lower than 40°C requires combustion support to increase the adiabatic temperature to above 40°C.
2 Process design of the main component system
The regenerative incinerator is composed of a regenerator, a combustion chamber, a reversing valve and a control system. The process design of its main composition system includes: selection of the number of regenerator beds, selection of regenerator materials and types, calculation of regenerator volume, determination of air inlet flow rate of superficial towers; combustion temperature of combustion chamber, residence time of flue gas. , selection of burners; valve switching time; selection and quantity calculation of heat preservation and refractory materials; matching of pretreatment measures and safety measures.
2.1 Overall requirements of the process system
The system design pressure drop is less than 3000Pa. The regenerative combustion device shall be internally insulated as a whole, and the outer surface temperature shall not be higher than 60°C (except for some hot spots). When the ambient temperature is low and the humidity is high, anti-condensation measures such as heat preservation and heat tracing are taken. With anti-burning and purging functions.
2.2 Regenerator
The regenerator is the space for the heat exchange of the incinerator, and its specific structure and size are calculated and determined according to the requirements of heat recovery efficiency, the structural performance of the regenerator, and the pressure drop of the system.
2.2.1 Selection of combustion process and number of regenerators
The regenerative combustion process can be divided into stationary and rotary regenerative combustion. The fixed regenerative combustion process has two chambers, three chambers, five chambers, etc. In theory, the more the number of regenerative chambers, the higher the purification efficiency, but the equipment investment or land occupation will also increase. Rotary RTO devices include rotary cylinder type, disc type and rotary valve type. The structure of rotary RTO is the same as that of fixed type except the drive area and distribution area. Under normal circumstances, the combustion process considers three-chamber fixed regenerative combustion processes, and when there are limited land occupations, rotary valve type combustion processes can be considered.
2.2.2 Requirements for heat recovery efficiency of regenerator
It is required that the heat recovery efficiency of the regenerator is not less than 95%, mainly to control the temperature of the exhaust gas. A relatively simple calculation method for heat recovery efficiency A balance method for dealing with exhaust heat. For example, when the intake air temperature is 30°C, the exhaust temperature is required to be 60°C, and the combustion chamber temperature is required to be 800°C, the heat recovery efficiency is 96.1%, that is ( 800-60)/(800-30)=96.1%.
2.2.3 Regenerator
2.2.3.1 Basic requirements of heat accumulator
①The specific heat capacity is required to be no less than 750J/(kg·K), and it has a high specific gravity.
② There is enough gas circulation cross-sectional area to make the gas distribution uniform and the resistance low.
③High temperature resistance. The operating temperature of the RTO device is generally 750 to 950 °C. Therefore, materials that can withstand temperatures above 1250 °C are generally used as the regenerator, and regular materials such as honeycomb ceramics and combined ceramics are preferred.
④Good thermal shock resistance. The high temperature shock that it can withstand in a short time is required to be above 1250 ℃, and the service life is not less than 20000h.
⑤ It has sufficient mechanical strength at high temperature and has high temperature oxidation resistance and chemical corrosion resistance.
⑥ The price should be as low as possible.
2.2.3.2 Selection of heat storage body material and type
At present, the ceramic raw materials used in RTO devices mainly include clay, corundum, mullite, zircon, aluminum titanate and cordierite. In the regenerator of the RTO device, the main types of regenerative fillers are structured packing (such as honeycomb packing and corrugated plate packing) and random packing (particle packing, such as saddle ring).
(1) Random packing mainly includes ceramic saddle ring
The bed layer has a large void ratio, and most of the beds are arc-shaped liquid channels, which have high specific gravity and acid resistance and heat resistance. The geometric characteristics and related parameters of ceramic moment saddle products are listed in Table 1.

 

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Compared with structured packing, ceramic saddle ring has the advantage of low price, but the disadvantage is that the resistance is larger than that of structured packing, and the edge of the packing is easy to break, which will cause the bed gap to be blocked and make the bed resistance large. For example, in the RTO device, the ceramic saddle ring lin (25.4mm) is used as the regenerator. In order to achieve 95% heat exchange efficiency, a bed with a height of 2.44-2.74m is required, and the switching time is 2min.
(2) Structured packing
①Ceramic honeycomb filler. The air flow in the ceramic honeycomb packing passes through parallel channels without mixing with each other, and flows into a laminar flow state. In this state, the pressure drop is proportional to the gas velocity. Therefore, the pressure drop of the ceramic honeycomb packing is far greater than that of the ceramic saddle ring. The regular ceramic honeycomb filler is generally made into a columnar regenerator with a size of 150mm×150mm×300mm, and the inner wall thickness of the hole is divided into 0.42mm, 0.6mm, 1.0mm, etc., and is built in the regenerator. Hole densities typically vary from 13×13 (169 holes) to 60×60 (3600 holes). A larger pore density provides a larger heat transfer area, thereby increasing thermal efficiency. The ceramic honeycomb column is built in the regenerator. In order to avoid local blockage, one end face of the ceramic honeycomb column is made into a concave surface with open feet.
②Multilayer plate composite ceramic honeycomb filler. The plate composite ceramic honeycomb filler is composed of multi-layer plates. Instead of direct extrusion molding, it is first made into a single plate, and then the multi-layer plates are bonded together and sintered to form a multi-layer plate combination. Type of ceramic honeycomb filler. Each sheet of the packing is provided with a groove, and the two sheets are combined to form an internal communication channel, so that the air flow can pass through the packing laterally and longitudinally. Usually the thickness of each plate is about 1.5mm, forming a block heat storage body of about 305mm×305mm×102mm. BXM plate type honeycomb ceramic models are BXM-125, BXM-160, BXM-180, BXM-200, BXM-S series.
BXM is also a regular ceramic packing. Compared with the traditional ceramic saddle ring under the same application conditions, the bed pressure drop of BXM is reduced by 50%, because its resistance is lower than that of the ceramic saddle ring, and the energy consumption can be saved by 30%. Some regenerators have a relatively strong heat resistance, and it is easy to reach a changing temperature of 400°C in a short time. The physical properties of ceramic random packing (saddle ring) and structured packing (Monolith and BXM) are compared in Table 2. Under the condition of achieving the same thermal efficiency, the volume required by BXM is more than that of regular honeycomb ceramics; under certain gas velocity conditions, its pressure drop is lower than that of moment saddle ring and higher than that of Monolith. After replacing the moment saddle ring with BXM, the amount of filler is half that of the moment saddle ring, the thermal efficiency and switching cycle time remain unchanged, and the processing capacity can be increased by 30%.
2.3 Inlet flow velocity of superficial tower and cross section of regenerator
For an RTO with a fixed cross-sectional area of ​​the regenerator bed, when the flow rate of the inlet superficial tower increases, the pressure loss of the system increases; and the heat recovery efficiency decreases accordingly. In the design, the general regenerator requires that the cross-section empty tower wind speed should not be greater than 2m/s. When the cross-sectional wind speed and waste gas treatment capacity of the regenerator are determined, the cross-sectional area of ​​the regenerator can be determined. If the air volume of the incinerator is 20000Nm³/h, and the cross-sectional wind speed of the regenerator is set to be 1.5m/s, the cross-sectional area of ​​each regenerator is 3.7㎡, and the cross-section can be arranged in a square or rectangle according to the selected filler.
2.4 Regenerator bed height and total pressure drop
When designing the RTO device, the volume of the equipment is mainly determined by the height of the regenerator bed, and the power of the fan is determined by the total pressure drop of the regenerator bed. Table 3 shows the total pressure drop data of several regenerators. From the data in Table 3, it can be seen that under the same operating conditions for different regenerators, the difference in the total pressure drop of the bed caused by the height of the bed is relatively large. Therefore, the choice of the heat storage body is one of the important factors in the design of the device.

 

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2.5 Gas temperature difference between inlet and outlet of regenerator The temperature difference between inlet and outlet of regenerator should not be greater than 60℃.
2.6 The residence time of the high temperature gas in the combustion chamber should generally not be less than 0.75s, and the volume and size of the combustion chamber should be determined according to the residence time; the combustion temperature of the combustion chamber should be higher than 760℃. The burner is configured according to factors such as auxiliary fuel type, combustion chamber structure, pressure, flow rate of exhaust gas to be treated, device startup time, etc., and has the function of automatic temperature adjustment, in line with the relevant provisions of GB19839. Generally speaking, a burner with a capacity of 250,000 to 600,000 kcal can be selected according to the calorific value of the exhaust gas for a gas of 10,000 m³. For example, the amount of waste gas treated is 10,000 m³, and gas fuel such as biogas or natural gas is used as a combustion accelerant, and North American 4425-5 ( 250,000 kcal/h) gas burner with an adjustable ratio of 1:30. If diesel oil is used as combustion support, the American NA5424-5 (200,000 kcal/h) fuel proportional adjustment type burner is used, and the adjustment ratio is 1:10. The main products of burners used in RTO devices are North American, ESApyronics, ECLIPSE, pyronics, MAXON and other brands. The auxiliary fuel is preferably clean fuel such as natural gas, liquefied gas and light diesel.
2.7 Amount of purge gas
The amount of purging gas is generally calculated according to 10% of the amount of treated waste gas, and a fan can be set separately to supply fresh air.
2.8 Valve and switching cycle
(1) The fixed combustion process adopts the switching valve (including the purging) and selects the pneumatic butterfly valve. The switching valve is the key component of the fixed RTO device for cyclic heat exchange. It must be switched accurately at the specified time, and the leakage should be small ( ≤1%), long life (up to 1 million times), quick opening and closing (≤1s). RTO devices generally require a switching period of 90 to 180S. Rotary distribution area waste
(2) Rotary combustion process The rotary distribution valve is the core component of the rotary combustion process, and a rotary air distribution valve is used. Specifically, the appropriate air distribution valve can be selected according to the design processing air volume, air pressure, number of air nozzles and valve core speed requirements.
3 Insulation and refractory materials
It is required to be light in weight, low in thermal conductivity (the thermal conductivity at room temperature is less than 0.03W/(m·k), good in thermal shock resistance, and strong in chemical stability. In general, the combustion chamber and regenerator of the device can be insulated with refractory Aluminum silicate fiber, its heat resistance can reach 1200℃, and the fleece weight is 220kg/m³. When laying, the thickness of the upper part of the combustion chamber and the regenerator is ≤225mm, and the thickness of the air inlet and outlet of the regenerator is ≤120mm. Generally, there are three layers of thermal insulation layer. , which contains two layers of aluminum silicate fiber felt and one layer of aluminum silicate fiber module. The aluminum silicate fiber module is provided with a heat-resistant steel skeleton, which is fixed on the device shell with anchors.
4 Post-processing requirements
When the nitrogen-containing organic matter is treated and the emission of nitrogen oxides in the flue gas exceeds the standard, the selective denitration process is used for post-treatment. When the sulfur-containing or halogen-containing organic matter is treated to produce sulfur dioxide and hydrogen halide, the post-treatment is carried out by adopting processes such as absorption.
5 Requirements for safety measures
①When the concentration of exhaust gas fluctuates greatly, take measures such as dilution and buffering at the front end.
②A flame arrester or fire damper should be installed in the piping system between the RTO unit and the main production unit. When the air intake and exhaust ducts are made of metal materials, take measures such as flange jumper connection and system grounding to prevent the generation and accumulation of static electricity.
③It should have the function of overheating protection, and the thermal insulation design conforms to the relevant regulations of SGBZ-0805.
④The fuel supply system is equipped with high and low pressure protection and leakage alarm devices; it is equipped with a safe and reliable flame control system, temperature monitoring system, pressure control system, etc. The ignition operation of the burner shall comply with the relevant regulations of GB19839.
⑤ The explosion-proof pressure relief design of the piping system and the regenerative combustion device shall comply with the relevant regulations of GB50160. The compressed air system is equipped with low pressure protection and alarm devices. Equipment such as fans, motors and electrical instruments placed on site shall be explosion-proof, and the level shall not be lower than the site level.
⑥With short circuit protection, the grounding resistance should be less than 4Ω. Install lightning protection devices that comply with GB50057.
6 Selection of honeycomb ceramic regenerator
The process design of the regenerative combustion device for the treatment of volatile organic waste gas should focus on the selection of the material and type of the regenerative body to ensure that the heat recovery efficiency and operation stability can be met; Select the appropriate combustion temperature and supporting facilities for the specific conditions of the exhaust gas.
※The high heat capacity of the material is conducive to heat storage;
※The higher the density or the smaller the water absorption rate, the greater the sensible heat; the higher the thermal conductivity, the more conducive to heat storage and heat exchange, and the higher the density, the smaller the water absorption rate, the stronger the slag resistance and corrosion resistance, and the better the utilization The use of heat storage body, the stronger the ability to adapt to working conditions;
※The thermal shock resistance is good, the expansion coefficient is low, the more difficult the thermal storage body is to crack, the more conducive to the application of the thermal storage body, and it is beneficial to prolong the service life of the thermal storage body;
※The larger the specific surface area, the larger the heat exchange area, the higher the heat exchange efficiency;
※The greater the thermal conductivity, the greater the heat transfer rate;
※The larger the porosity of the section, the smaller the back pressure or wind resistance;
※The smaller the water absorption rate of the regenerator, the denser it is, and the stronger the corrosion resistance and slag resistance; the greater the difference between the chemical composition of the regenerator and the slag powder, the stronger the slag resistance;
※The thinner the hole wall thickness, the higher the hole wall smoothness, the better the product regularity, the smaller the wind resistance of the RTO equipment regenerator bed, and the lower the power consumption;
The stronger the anti-corrosion and anti-slag performance, the larger the area, the higher the heat exchange efficiency, and the lower the ability to resist slag and ash clogging;
※The higher the refractoriness of the product, the stronger the high temperature resistance. Generally speaking, the temperature of the RTO furnace is lower, and the requirements for this are not high, but the requirements for thermal shock resistance are higher.
7. Advantages of Boxin honeycomb ceramic regenerator products
●Standard specifications and dimensions, high regularity, anti-clogging grooves, thick outer wall and thin inner wall, which maximizes operability and reduces back pressure.
The domestic manufacturers that can provide a large number of dense material and large-scale integral honeycomb regenerators have low water absorption and stronger anti-corrosion and anti-slag performance.
Diversified materials, can be customized according to customer requirements
●Excellent product technical service support
●Products are cost-effective and stable in quality

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