Modeling optimization for a typical VOCs thermal conversion process

: Aiming at the current environmental problems, the thermal oxidation treatment for industrial VOCs emission is a common and effective measure. This paper studies on the optimization effect of one optimization method for direct VOCs thermal oxidation of a color aluminum spraying production line based on Aspen-Plus. According to the direct VOCs thermal oxidation process with a 30000 m ³ /h circulating air volume, propose the flue gas reflux and coating room drainage technology. Use the second law of thermodynamics, and the exergy flow analysis shows the methane consumption could be reduced 12%. (cid:28595) Carbon emissions also decreased significantly, with 3.42% reduction. These findings are practical for industrial production cost saving and environmental protection problems solving.


Introduction
Volatile organic compounds (VOCs) generally refer to aromatic hydrocarbons, higher alkanes, olefins and nitrogen-containing sulfur compounds [1]. Most of these substances pollute the air environment and even cause serious harm to human health [2,3]. In recent years, carbon emissions are also receiving increasing attention [4]. Policies on industrial VOCs emissions are also becoming more and more stringent [5]. Therefore, VOCs treatment is of great significance to industrial development and environmental protection.
VOCs of industrial waste gas is usually characterized by large amounts and low concentration. Thermal oxidation technology has become the first choice due to its low cost and high processing efficiency [6]. The combustion treatment rate of toluene, chloromethane, hydrocarbons, and other VOCs can be maintained at more than 95% [7]. Thermal oxidation technology can be divided into direct thermal oxidation, regenerative thermal oxidation, and catalytic thermal oxidation. Thermal oxidation technology needs adding accelerants to meet the ignition and combustion maintenance, because of low concentration of combustible components, and usually using methane [8,9]. regenerative thermal oxidation technology such as two-chamber and three-chamber direction improves the energy utilization efficiency of the incinerator chamber [10,11]. The use of precious metal or Cu, Mn, and other metal oxides as catalyst, can reduce the reaction temperature, become flameless combustion [12,13]. Most factories choose the direct thermal oxidation and build an incinerator to directly incinerate the exhaust gas containing VOCs. Although the structure is simple, the energy utilization efficiency is low. An aluminum spraying factory chooses the direct combustion method and uses the three-stage heat exchanger to utilize the preheating in China [14]. It has the problems of high energy consumption and large carbon emission.
The technology of flue gas recovery and utilization is widely used in the research field of boiler [15]. Flue gas recirculation technology can reduce the energy consumption of coal-fired boilers [16]. The adsorption of VOCs by the zeolite runner can also change the concentration of industrial source exhaust gas [17]. Researchers have studied the adsorption VOCs in high concentration through experimental verification and Aspen-Plus software simulation [18,19]. For direct thermal oxidation technology, the system structure can be changed by flue gas drainage and pre-treatment before waster gas input to be optimized.
In this paper, one method is proposed for the direct thermal oxidation for VOCs in the color aluminum spraying plant to achieve the purpose of improving energy consumption and reducing carbon emission.

Direct thermal oxidation process modeling
The modeling data comes from the operation parameters of the production line of a color aluminum spraying plant in Anhui, China. The production and operation air volume of the system is 22,000m³/h, and the circulating air volume is 30,000m³/h. Circulating air refers to the supply air volume required for internal circulation of the drying oven in the spraying plant. The operation of the system can be understood as the VOCs waste gas provided by the coating room and the drying oven, which is heated by the high, neutral, and low temperature three-stage preheater and burned together with the auxiliary gas methane in the furnace. The drying oven is a unique system step in the color aluminum spraying factory. It is mainly used to dry the aluminum sheet sprayed with paint. It requires a temperature environment of about 450 °C, which will lead to the volatilization of massive VOCs. Paint raw materials are stored in the coating room, and little VOCs are volatilized in a low temperature environment. Figure 1 shows the flowsheet of direct thermal oxidation process. Table 1 shows the parameters of simulation in direct thermal oxidation. There are two sources of VOCs, one is from the drying oven which has high temperature and concentration, the other is from the coating room which has low temperature and concentration. The methane consumption for stable operation of the system is about 180m³/h.  Table 2 shows the temperature of furnace and other heat exchangers. The combustion temperature of the furnace is 750℃. After circulating in the drying oven, the hot air mixed with two sources of VOCs is heated from 250 ℃ to 544 ℃ by the preheater 1 and 2, and sent to the furnace. The exhaust gas temperature is stable at around 140 ℃.

Flue gas reflux and coating room drainage technology modeling
The flue gas reflux technology means part of the flue gas before the water heater is controlled by the valve to enter the furnace. Since the heat of this part of the flue gas can supply the demand of the furnace and balance the energy of the inlet and outlet of the furnace, the consumption of methane to support combustion can be appropriately reduced, to achieve higher economy. The coating room drainage technology means the air carrying little VOCs entering the low temperature preheater is controlled by a valve to replace the supporting air for methane combustion, to achieve the purpose of energy saving and emission reduction from the perspective of reducing furnace load. The modeling of the two methods in Aspen-Plus is shown in Figure 2. Figure 2 Model diagram of flue gas reflux and coating room drainage technology The parameters of feed and heat exchangers are the same as those of direct thermal oxidation type.

Exergy analysis method
The exergy analysis is a numerical calculation based on the thermodynamic parameters of the inlet and outlet of different components for this system, including the physical exergy and chemical exergy of Formula (1) [20]. ( ) Where m is molar flow, i x is molar fraction of each component, and i e is standard chemical exergy of each component.

Sensitivity analysis of flue gas and coating room drainage technology
The reflux rates of Case 2 were changed as independent variables to test whether the model dependent variables were reasonable. Methane consumption, combustionsupporting air amount and CO2 emissions were set as dependent variables. Figure 3 shows the results of that. Figure 3 Sensitivity of reflux and drainage rates: (a) Flue gas reflux; (b) Coating room drainage With the reflux and drainage rate increasing, the methane consumption decreases, the CO2 emissions decrease. The heat of flue gas can supply the demand of the furnace, balance the energy at the furnace inlet and outlet. Air carrying a small number of VOCs replaces the air supporting combustion gas to reduce furnace load. To meet the requirements of hot air system heating aluminum sheet, the combustion air quantity must be around2500 m ³/h. Flue gas reflux rate is set to 0.1, and coating room drainage rate is set to 0.3.

Exergy analysis
After calculation, the exergy flow results of direct VOCs thermal oxidation and flue gas reflux and coating room drainage technology systems are shown in Figure 4 and  The optimization method is to set the flue gas reflux rate of 0.1 and the coating room drainage rate of 0.3, to provide heat recovery for furnace combustion and reduce the supporting air volume for methane, reducing the consumption of methane to 88m³/ h. To meet the heat supply required by the oven, its proportion increases from 71.6% to 76.6% as the input energy of the system decreases, while the methane saving rate is 12%. The exergy loss also decreased from 25.2% to 20.9%, of which the percentage of furnace exergy loss decreased most, from 13% to 9.9%.

Carbon emission analysis
For the color aluminum spraying production line, due to the hot air demand of the oven, the amount of combustion air is guaranteed to be excessive, and the carbon in VOCs and methane is completely burned. Therefore, the carbon emission analysis mainly focuses on CO2. 4 E3S Web of Conferences 385, 03012 (2023) https://doi.org/10.1051/e3sconf/202338503012 ISESCE 2023 Figure 6 Carbon emission of two cases As shown in Figure 6, the optimization methods have significantly CO2 emissions reduction effect, and reduce CO2 emissions by 85.09 tons/year, accounting for 3.42%.

Conclusions
VOCs thermal conversion is a common industrial VOCs processing technology. One optimization method is provided for the direct combustion method in this article. Use flue gas reflux and coating room drainage. The energy demand of hot air circulation is met, the proportion of energy utilization is increased from 71.6% to 76.6%, and the methane consumption is reduced to 88 m³/ h with 12% reduction. Have significantly CO2 emissions reduction effect, and reduce CO2 emissions by 85.09 tons/year.