Coking Industry VOCs Comprehensive Treatment Solution
Volatile organic compounds (VOCs) are important precursors of O3 generation, and some of their components have strong carcinogenicity. In recent years, the O3 concentration in the global ambient air has shown a trend of increasing year by year, and the problem of O3 pollution has become increasingly prominent, which is closely related to a large number of VOCs emissions. In many VOCs pollution sources, coking production is one of the important pollution sources. In the process of coking production, the emission problem of VOCs exhaust gas is more prominent, with multiple emission sources, multiple pollutants and high toxicity, unorganized emission and other characteristics, serious pollution to the atmospheric environment. In view of the treatment of coking VOCs waste gas, relevant enterprises have tried and explored, the more common are "three stage washing + absorption (removal) attached" process, three stage washing +RTO process, nitrogen sealing + negative pressure recovery process, waste gas return combustion and other treatment processes. With the release of "Volatile organic compounds Emission Control Standards for Industrial Enterprises", "Coking Chemical Industry Pollutant Emission Standards" and other policies, coking VOCs management is very urgent, according to the characteristics of VOCs exhaust gas in different processes to carry out in-depth treatment, to achieve sustainable and stable standards is an urgent problem for enterprises.
Analysis of the main sources of VOCs in the coking industry
VOCs in coking industry mainly come from chemical production recovery process: the main pollution source of cold drum section is the discharge pipe of tar, ammonia and other storage tanks, mainly producing benzo [a] pyrene, hydrogen cyanide, phenols, naphthol, non-methane total hydrocarbon, ammonia, hydrogen sulfide, etc. The main pollution sources of the desulfurization section are the desulfurization regeneration facilities and the discharge pipes of each storage tank, which are mainly ammonia and hydrogen sulfide. The main pollution sources of ammonium sulfide section are ammonium sulfide drying facilities, ammonium sulfide solutions, ammonia storage tanks, etc., mainly producing particulate matter, ammonia, etc. The main pollution sources of benzene washing section are crude benzene rectification unit, discharge pipe of each oil tank separator, refined benzene processing and tar processing, etc., mainly producing benzene and benzene series, hydrocarbons, etc. These VOCs also have toxic and harmful, flammable and explosive characteristics.
Due to the long and complex process of coking industry, VOCs components are diverse and difficult to collect and manage. Usually, the storage tank with good airtightness and low oxygen content adopts the negative pressure recovery balance process;
Because of its high oxygen content, the dispersive gas cannot enter the gas recovery system, so it is mostly treated by the process of dispersive collection and centralized return to coke oven incineration or other oxidation incineration. Long-distance transmission of VOCs waste gas has problems such as low pressure and naphthol in the waste gas is easy to crystallize and block the pipeline. At this time, it is appropriate to build a separate incinerator.
Xinjieyuan Technology VOCs Treatment Comprehensive SolutionsIn the whole coking process, chemical production recovery is the workshop that produces the most VOCs, especially in the recovery area is more serious, so the VOCs management of coking plants is mainly concentrated in the recovery area. There are many equipment involved in the recovery area, and the exhaust gas of various tanks is directly connected to the atmosphere, resulting in serious odor. Ammonia, tar, naphthalene, phenol, cyanide, methane hydrocarbons and other substances will escape into the atmosphere, especially benzene, hydrogen sulfide and other substances, but also has strong toxicity, pollution of the environment, seriously affect the environment and the health of the surrounding workers.
Xinjieyuan VOCs comprehensive treatment solution, according to the coking enterprise coking, gas purification and chemical production recovery, product storage, phenol cyanide wastewater treatment of different processes of VOCs characteristics and discharge sites are different, The VOCs treatment process discharged by the chemical production system is divided into full negative pressure recovery balance, exhaust gas return combustion, RTO rotary heat storage incineration technology to meet different oxygen content, different concentrations, a variety of application scenarios of VOCs in-depth treatment solutions.
Full negative pressure recovery balance process (low oxygen exhaust gas)The collected gas with good airtighness, low oxygen content and high added value, such as cold drum section, benzene eluting section and oil depot section, is treated by oil washing tower in each area, and then sealed by nitrogen and returned to the gas system under full negative pressure. This process requires a good sealing effect of the tank, so that the air is not easy to enter the negative pressure zone, can better control the oxygen content of the gas.
Working principle: Firstly, the open and closed nitrogen sealing device is installed on the closed tank. When the tank is in and out of the material, the nitrogen sealing valve is used for nitrogen supply and nitrogen discharge, and part of the tail gas is discharged with nitrogen gas. Connect the closed tank release valve to the negative pressure system, install a set of micro negative pressure oxygen control device in front of the fan, set a certain pressure, just suck the exhaust gas discharged by these tanks into the negative pressure system, but not too much oxygen into the negative pressure system when the fan pressure changes, thus causing safety risks.
Nitrogen sealing technology is mainly applied to the sealing of tanks in the reservoir area. Nitrogen is used to supplement the gas space in the tank when the liquid level in the tank drops or the temperature decreases. By filling the tank with nitrogen above the liquid level, the liquid medium is prevented from continuous gasification and the medium gasification escape is inhibited. When the tank feed level rises or the temperature rises, the gas pressure in the tank rises, the nitrogen discharge valve opens and the nitrogen escapes, so as to maintain the pressure balance in the tank. After the storage tank is sealed with nitrogen, it can effectively reduce the exhaust emissions of the tank. The gas space in the tank is mainly a mixture of combustible gas and nitrogen, which will not form an explosive gas mixture. It can improve the production safety of the enterprise, reduce the toxic and harmful media in the operation space, and effectively protect the environment and maintain the physical and mental health of the workers. At the same time, the amount of exhaust gas introduced into the gas system by nitrogen sealing treatment is very small (the maximum amount is less than 1000m3), and it will not adversely affect the gas system pipeline.
Process flow
(1) Set the suction of the pipeline in front of the micro negative pressure oxygen control device as -100-200Pa, and the suction of the tank releasing port as 0~-50Pa. According to the length of the pipeline from the tank to the micro negative pressure oxygen control device in front of the blower, with different pipeline length and different resistance, adjust the suction of the tank releasing port as the set value with the opening degree of the valve on the tank;
(2) The micro negative pressure oxygen control device is connected to the blower reflux pipe or the fan front pipe, and the suction of the micro negative pressure oxygen control device is adjusted at -1000~-5000Pa through the regulating valve;
(3) The special internal structure of the micro negative pressure oxygen control device is composed of suction pipe, pressure limiting water seal chamber, gas water seal, vacuum chamber, baffle, regulating valve, reflux pipe, vacuum gauge, overflow pipe, water supply pipe, etc. Under the action of negative pressure, the gas in the inlet pipe breaks through the water seal and enters the vacuum part, and is sucked into the blower gas pipe, and the water seal height changes with the change of suction. The recovered exhaust gas can just break through the water seal to carry out the vacuum system, and keep the suction of the harmful exhaust gas pipeline constant. The water vapor carried away by the gas is condensed at the baffle to enter the reflux pipe and return to the water seal system. Some water vapor is brought into the gas by the gas, so it is necessary to use the water supplement pipe to ensure the height of the water seal. The whole process has no emission, no leakage, safe and reliable, and truly realizes zero emission treatment of chemical exhaust gas.
The suction of each tank is set to -100~0Pa, and the suction of each tank is set according to the length, caliber and resistance of the pipeline. The tank is installed with open and closed nitrogen supply valve and nitrogen discharge valve, and the speed of the fan is adjusted by frequency conversion controller. The suction is adjusted by the regulating valve at each point to ensure that the gas will not escape into the atmosphere, and at the same time, it can ensure that enough suction is maintained.
The process selects equipment with a high degree of automation, and the signal is connected to the central control system of the central control room of the chemical production, which is operated by the personnel of the central control room without adding additional operators. In order to prevent pipeline blockage, steam cleaning pipeline is installed on the exhaust gas collection pipeline, and steam cleaning is carried out regularly.
Process advantages: The VOCs negative pressure balance recovery process developed by Xinjieyuan Technology has no land area, low investment and operation cost, thorough treatment, zero emissions, and VOC recovered tar, ammonia and benzene can be turned into other products after being recycled through processes such as benzene washing tower, improving the recovery rate and greatly improving the economic benefits of the factory.
Discharge type harmful exhaust gas return combustion process (high oxygen exhaust gas)In this process, the VOCs exhaust gas with higher oxygen content and lower added value in the desulfurization section and ammonium sulfide section is introduced into the negative pressure system of the coke oven as air distribution to participate in the combustion of the coke oven and the VOCs components are completely oxidized and decomposed.
Working principle: The collected gas with higher oxygen content and lower added value in the chemical production recovery section is introduced into coke oven combustion as air distribution after acid washing tower, alkali washing tower and water washing tower, so as to achieve the purpose of thorough oxidation and decomposition of VOCs. At present, most of the coking enterprises have installed coke oven flue gas desulfurization and denitration device, this part of sulfur dioxide, nitrogen oxide will be removed in the desulfurization and denitration device, can basically achieve zero emissions of VOCs treatment. In order to ensure safety, a combustible gas detector is set up before the exhaust gas is fed into the coke oven for combustion, which can monitor the change of flammable and explosive components in the exhaust gas in real time and send the signal to the DCS control system. When the component concentration reaches the set limit, the DCS alarms and automatically opens the air distribution valve; When the component concentration reaches the set upper limit, the automatic valve into the coke oven is cut off to ensure production safety.
Process advantages(1) Reduce the construction investment cost by using the original coke oven as a combustion device;
(2) Low operating cost, VOCs in the exhaust gas after combustion heat energy can be recycled, reduce gas consumption, reduce the denitrification pressure of the back-end SCR;
(3) High security, high degree of automation, can realize unattended;
(4) Nitrogen oxide and sulfur dioxide generated after exhaust gas combustion can be directly removed by coke oven flue gas desulfurization and denitration device, without the disadvantages of traditional combustion method.
Rotary regenerative independent combustion process (RTO)
Combustion method is the current purification of organic waste gas (VOCs) more thorough process, has been fully recognized by various industries. Regenerative Thermal Oxidizer (RTO), also known as regenerative incinerator. The technology belongs to a kind of combustion method, heat storage, thermal oxidation integration into one of VOCs purification technology.
Working principle: Use pipelines and induced draft fans to collect the fugitive exhaust gas nearby, and the exhaust gas of each process is classified for washing and pretreatment. NH3 in the exhaust gas is washed by the absorption liquid in the pickling tower and reacts with the H2SO4 in the absorption liquid, and the absorption liquid in the pickling tower is discharged to the mother liquor tank of the ammonium sulfide section. In the alkali washing tower, NaOH solution is used to absorb H2S, HCN and other acidic gases in the exhaust gas, and the absorption liquid in the alkali washing tower is discharged into the mechanized clarification tank. After the waste gas of salt extraction section is washed, the salt particles in the waste gas are washed; The exhaust gas is collected into the exhaust gas main pipe after washing, and then sent to the gas-liquid separator by the relay fan for gas-liquid separation. After a series of online concentration detection and pressure/flow control in the process, the main induced draft fan is sent to the rotary heat storage independent burner (RTO) for exhaust gas purification treatment, and finally achieve harmless discharge.
Process advantages
(1) Waste gas treatment system and coke oven production system do not affect each other, independent of each other and low operating cost, no need to consume coke oven gas;
(2) VOCs purification rate ≥97%(up to 99.5%), small impact on pipeline wind pressure (±25Pa, stable operation), comprehensive heat recovery rate ≥95%;
(3) Do not have an impact on the coke oven body, to avoid the long-term impact of return exhaust gas on the coke oven;
(4) It has higher exhaust gas purification efficiency than return combustion to meet higher emission requirements in the future;
(5) Rotary valve type RTO system set up 30 safety measures, from the root to eliminate safety hazards;
(6) High degree of automation, the whole system can realize automatic operation, unattended.
VOCs treatment case in coal production workshop waste gas treatment project
The original treatment method of waste gas from desulfurization, ammonium sulfate and salt extraction positions in chemical production workshops is to collect it uniformly and then enter the acid washing and alkali washing tower for washing, and then send it to coke oven for mixing and combustion. In view of the increasingly severe environmental protection situation, the original treatment process has been increasingly unable to meet the needs of VOCs in-depth treatment.
After times demonstration and investigation, our company adopts the combined process route of multi-stage washing + gas-liquid separation + rotary heat storage independent combustion technology (RTO) on the basis of fully profiting the old. The exhaust gas of each process is washed and pretreated by classification. NH3 in the exhaust gas is washed by the absorption liquid in the pickling tower and reacts with H2SO4 in the absorption liquid. The absorption liquid in the pickling tower is discharged to the mother liquor tank of the ammonium sulfide section. In the alkali washing tower, NaOH solution is used to absorb H2S, HCN and other acidic gases in the exhaust gas, and the absorption liquid in the alkali washing tower is discharged into the mechanized clarification tank. After the waste gas of salt extraction section is washed, the salt particles in the waste gas are washed; The exhaust gas is collected into the exhaust gas main pipe after washing, and then sent to the gas-liquid separator by the relay fan for gas-liquid separation. After a series of online concentration detection and pressure/flow control in the process, the main induced draft fan is sent to the rotary heat storage independent burner (RTO) for exhaust gas purification treatment. VOCs≤20mg/Nm3 (non-methane total hydrocarbon) after transformation; NOx≤35mg/Nm3; SO2≤15mg/Nm3; Particulate matter ≤10mg/Nm3; NH3≤5mg/Nm3; H2S≤0.5mg/Nm3; HCN≤0.5mg/Nm3, fully meet the relevant requirements of Volatile Organic Matter Emission Control Standard for Industrial Enterprises, Pollutant Emission Standard for Coking Chemical Industry and ultra-ultra-low emission of pollutants, and has the advantages of high removal efficiency, low construction and operation cost, safety and reliability.
