How to choose VOCs treatment technologies?

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How to choose VOCs control technologies for coal-to-natural gas? Author/Source: Huahua Network Coal Chemical Industry; Date: 2018-07-12; Clicks: 4. The hazards of volatile organic compounds (VOCs) are well recognized, and the Ministry of Environmental Protection has issued a comprehensive plan for controlling VOCs emissions in the petrochemical industry. The coal chemical industry is a major source of VOC emissions. Taking the coal-to-gas industry as an example, the author introduces the relevant VOCs control measures. Currently, coal-to-natural gas production primarily employs fixed-bed coal pulverization pressurized gasification, low-temperature methanol washing for acidic gas purification, and methane synthesis to produce natural gas. Based on the characteristics of the process flow, there are 4 main sources of VOC emissions: the exhaust gas released after acid gas purification in the low-temperature methanol washing unit, the breathing exhaust gas from the atmospheric-pressure storage tanks for gas and water, the foul-smelling gases emitted by the wastewater treatment system, and the breathing exhaust gas from the storage tanks in the oil tank area. The VOC concentration in the exhaust gas emitted from low-temperature methanol washing is approximately 7,000 milligrams per cubic meter. Since the gasification processes used are mostly fixed-bed Ruhr furnace gasification processes, VOCs are not a single component; rather, they contain substances such as methane, ethylene, ethane, propylene, propane, and methanol, making it difficult to recover them. Reaching a purity level that allows for reuse is almost economically unfeasible. Therefore, destructive methods must be employed, that is, converting VOCs into harmless substances before releasing them into the atmosphere. It should be noted that when using combustion treatment technology, the sulfur monoxide emitted after the combustion reaction may lead to excessive sulfur levels, and the combustion of waste gases may result in the formation of nitrogen oxides, causing secondary pollution. To avoid such a situation, considering the integration of exhaust gas treatment technologies is advisable. Before the gas enters the combustion device, the waste gas is pre-treated using adsorption or absorption methods; the \"adsorption concentration + combustion\" process or the \"alkaline washing absorption + combustion\" process can be employed. The breathing exhaust gas from gas and water storage tanks is mainly unorganized exhaust gas emitted by the tank’s breathing valves; it consists primarily of hydrogen sulfide and ammonia, with VOC concentrations of around 900 milligrams per cubic meter. Due to factors such as ambient temperature, the flow rate and concentration fluctuate significantly. This gas has no value for recovery, and direct combustion treatment using a regenerative thermal oxidizer can be considered. Under the condition of meeting the pressure drop requirement, this stream of gas can be transported via pipelines to a regenerative thermal oxidizer for treatment, thereby saving on equipment investment. The exhaust gases from wastewater treatment plants fall into two categories. One type of waste gas consists of the emissions from devices such as the regulation tanks, homogenization tanks, oil separation tanks, and acidification-hydrolysis tanks in the pretreatment unit; these gases exhibit large fluctuations in volume and concentration, with relatively high VOC content ； Another category consists of the gases from aeration tanks and sludge dewatering rooms, which have a strong foul odor, primarily caused by sulfides and volatile phenols. The components of the two types of gases differ, but both are located in the wastewater biological treatment unit area. Leveraging existing biochemical treatment facilities, the biodegradation method can be adopted; microorganisms are used to break down the organic compounds contained in the waste gas into carbon dioxide and water, while effectively removing inorganic components such as sulfur and nitrogen. The emissions from the storage tanks in the oil tank farm mainly consist of the gas released by the tanks’ breather valves and the organic waste gases generated during loading. The breathing exhaust gases from the tank top pass through the water seal tank and can then be connected via pipes to the exhaust gases from loading, so as to be treated together in a centralized manner. Since the exhaust gases emitted contain organic solvents, they have high value for recycling. Moreover, as the storage tanks hold substances such as naphtha and tar, the exhaust gases are similar to those produced in the petrochemical industry; therefore, mature oil and gas recovery technologies used in this industry can be considered, such as combined processes that involve \"condensation + membrane technology + adsorption\" or \"adsorption + absorption\". On the one hand, it can reduce the emission of organic waste gases and minimize the impact on the environment; on the other hand, it can recover the oil components from these waste gases, thereby improving economic efficiency. In summary, a comprehensive prevention and control approach that combines source control, process control, and end-point treatment should be adopted for VOCs. Gas leakage and detection technologies are employed for qualitative and quantitative monitoring of unorganized emission sources, while production technologies and process equipment are improved to reduce VOCs leakage at the source. Efficient recovery and treatment technologies for organic waste gases are employed for end-of-pipe treatment to meet environmental quality requirements.