The waterproof membrane industry, as an important sector for asphalt-based products, has received increasing attention regarding the treatment of asphalt fumes and organic waste gas generated during production. Due to the complex composition of the exhaust gas, which contains not only volatile organic compounds (VOCs) but also large amounts of asphalt fumes, particulate matter, and odorous substances, higher requirements are placed on the stability and adaptability of Waste Gas Treatment Equipment.

RTO (Regenerative Thermal Oxidizer), as one of the most widely used waste gas treatment technologies in the field of industrial VOCs control, offers advantages such as high destruction efficiency, stable operation, high heat recovery efficiency, and strong adaptability to complex operating conditions. Through high-temperature thermal oxidation, RTO completely decomposes organic pollutants into CO₂ and H₂O, while ceramic heat storage media recover thermal energy, achieving a heat recovery efficiency of over 95%. For the medium- and low-concentration, high-air-volume organic waste gas generated in the waterproof membrane industry, the RTO waste gas treatment system can achieve both efficient purification and energy-saving operation.

I. Overall Technical Solution for VOCs Treatment in the Waterproof Membrane Industry

1. Overview

The main air pollutant generated by waterproof membrane manufacturers is asphalt fume. Asphalt fume is a heterocyclic mixture containing a large amount of polycyclic aromatic hydrocarbons (PAHs) and a small amount of oxygen-, nitrogen-, and sulfur-containing compounds, usually existing in the air in the form of aerosols. Asphalt fumes contain various pollutants harmful to humans, animals, and plants. Benzo[a]pyrene in asphalt fumes is a highly carcinogenic substance.

Asphalt fumes mainly consist of gas and liquid phases. The liquid phase consists of extremely fine volatile condensates, with particle sizes mostly ranging from 0.1 to 1 μm, the smallest being only 0.01 μm and the largest approximately 10.0 μm. The gas phase is a mixture of different gases. For such low-concentration and highly dispersed asphalt fumes, conventional dust collection and purification methods cannot completely purify and treat them.

The production of waterproof membranes mainly includes two processes: asphalt batching and asphalt coating. After entering the plant, asphalt is transferred to asphalt storage tanks and heated by a thermal oil system to maintain a fluid state. The heated asphalt is then transported through pipelines to mixing tanks in the asphalt batching workshop, where the temperature is maintained at approximately 180°C. Auxiliary materials such as waste oil, rubber powder, and stone powder are added and mixed to form a blended material.

The blended material is subsequently transported to the dipping tank in the coating workshop. During membrane production, the reinforcement fabric enters the dipping tank for impregnation, followed by coating, cooling, film lamination, and other processes before the finished waterproof membrane products are stored in the warehouse.

During the above production processes, VOCs waste gas and asphalt fumes are generated to varying degrees from asphalt storage, heating, mixing, conveying, coating, and cooling processes. Due to the multiple emission points and elevated exhaust gas temperatures, a closed collection system is generally adopted for centralized collection, followed by pretreatment and treatment through an RTO system. Through the proper design of the exhaust gas collection system, pretreatment equipment, and regenerative thermal oxidizer (RTO), asphalt fumes and VOC pollutants can be effectively removed while reducing system energy consumption and improving the environmental performance of the enterprise.

2. Selection of Treatment Technology Route

Considering both environmental protection requirements and operating cost savings, the technical route of "Cyclone Oil Removal + Tower Filtration + RTO" is proposed for waste gas treatment. The process flow is shown in the figure below.

Process Description:

a. As shown in the simplified process flow diagram, the waterproof membrane workshop consists of two sections: asphalt batching and asphalt coating. The waste gas from the asphalt batching section contains high concentrations of carbon black particles and oily components. It first passes through a cyclone separator for dust and oil removal, separating large particulate dust and oil droplets.

b. After passing through the cyclone separator, the waste gas from the asphalt batching section is mixed with the waste gas from the asphalt coating section in a tower filter and undergoes deep dust and oil removal through wire mesh filtration.

c. Manual valves are installed at the bottom of the cyclone separator and tower filter for the periodic removal of waste oil and dust.

d. A differential pressure gauge is installed on the wire mesh filter to remind operators to clean or replace the filter when the pressure difference reaches the preset limit.

e. Periodic reverse blowing is carried out to remove oil contaminants adhering to the bottom ceramic beds by using high-temperature flue gas. The specific implementation method is to reduce the rotational speed of the rotary valve significantly, causing the temperature of the bottom heat storage bricks to gradually rise to approximately 180°C. The oil contaminants adhering to the bottom heat storage bricks are then blown off and discharged through the oil outlet at the bottom of the rotary valve. This method can effectively solve the problem of oily substances clogging the bottom heat storage bricks.

f. A high- and low-temperature mixing chamber is installed to utilize high-temperature gas from the furnace to preheat the waste gas. The high-temperature gas continuously enters the 12 chambers of the RTO, carrying the oily substances from the bottom heat storage bricks into the RTO combustion chamber, where the oil contaminants are fully oxidized and decomposed before compliant discharge. This method can effectively solve the blockage problem caused by oily substances in the bottom heat storage bricks while ensuring compliant emissions.

g. An oil discharge outlet is installed at the bottom of the RTO rotary valve to allow periodic removal of accumulated oil contaminants.

h. An oil discharge outlet is also installed at the bottom of the RTO main fan for periodic oil removal. The impeller of the RTO main fan is designed to be manually detachable, facilitating regular cleaning of oil deposits on the impeller.

i. After preheating, the waste gas enters the rotary RTO. It is first preheated to above 760°C through five chambers of the rotary RTO and then fully mixed turbulently in the oxidation chamber, where the temperature reaches 850°C with a residence time of no less than 1.0 second. After the VOCs are oxidized and treated to meet emission standards, the gas passes through another five chambers, where heat is stored in the ceramic heat storage media before being discharged from the lower part of the RTO.

j. A bypass pipeline and bypass valve are installed on the top of the RTO. When the furnace temperature exceeds the set limit, the bypass valve automatically opens to quickly discharge high-temperature flue gas and rapidly cool down the furnace.

k. Considering the fire risk caused by long-term accumulation of oily substances in pipelines, fire sprinkler systems, rupture discs, and cleaning/inspection openings are installed at multiple locations in the ductwork. A fire damper is installed at the RTO inlet.

l. Inspection ports are provided between the bottom layer and the second layer of heat storage bricks in each zone of the rotary RTO (12 inspection ports for 12 zones). Since asphalt fumes are most likely to condense on the bottom layer and cause blockage, the condition of the heat storage bricks can be monitored through these inspection ports, and the bottom layer of heat storage bricks can be conveniently replaced when necessary.

2.2 Air Volume Planning

Reference air volume for batching tanks (reactors): 550 m³/h (each)

Reference air volume for 2-meter asphalt coating production line: 22,000 m³/h (each line)

Reference air volume for 1-meter asphalt coating production line: 11,000 m³/h (each line)

Reference air volume for 1-meter asphalt coating production line without reinforcement substrate: 7,000 m³/h (each line)

Reference air volume for asphalt storage tanks: 1,000 m³/h (each tank)

II. Factors for Equipment Selection

1. Selection of Pretreatment Equipment

Most waterproof membrane manufacturers have already installed flue gas purification systems. The commonly used technology in the industry is a "three-stage spray cooling and dust removal + electrostatic precipitation" system. However, due to low purification efficiency, unstable system operation, and unsatisfactory treatment performance, as well as the generation of large amounts of wastewater, this solution is not ideal.

Based on the principles of economy, environmental protection, and investment cost reduction, a cyclone separator can be selected to remove dust and oil from the waste gas, followed by a tower filter for deep oil removal, and finally the gas is sent to the RTO system for complete purification.

2. Selection of Backend Equipment

Common purification methods used in the waterproof membrane industry include electrostatic precipitation, adsorption, and absorption. However, due to low purification efficiency, incomplete odor removal, and the tendency to generate secondary pollution, it is difficult for these technologies to meet emission standards.

Although the composition of asphalt fume exhaust gas is extremely complex, its primary components are combustible hydrocarbons. Under certain temperature conditions and in contact with air, they can be completely combusted. When the temperature reaches above 790°C and the residence time exceeds 0.5 seconds, hydrocarbons can be almost completely burned.

The mainstream thermal oxidation technologies currently available are RCO and RTO. Since asphalt fume exhaust gas generally has a relatively low concentration and extremely complex composition, catalyst poisoning and deactivation can easily occur in RCO systems. In addition, the investment cost of large-air-volume RCO systems is high, and the catalyst service life is generally only 8,000–100,000 hours, resulting in excessive replacement costs. Therefore, a rotary RTO with lower energy consumption is selected as the backend treatment method.

III. Case Study

Taking the organized waste gas emissions of a certain enterprise as an example, the air volume of the asphalt batching section is 6,000 m³/h, and the air volume of the asphalt coating section is 12,000 m³/h. One 20,000 m³/h rotary RTO unit is selected, and the process flow is shown in the above diagram.

According to this technical solution, a 20,000 m³/h rotary RTO should be selected.

According to the relevant data, based on a concentration of 350 mg/m³, an air volume of 18,000 m³/h, and an asphalt calorific value of 37.7 kJ/g:

Waste Gas Concentration (mg/m³)	Exhaust Air Volume (m³/h)	Oxidation Heat Release (kcal/h)	Heat Required for Self-Sustained Operation (kcal/h)	Supplemental Heat (kcal/h)	Natural Gas Consumption (m³/h)
350	18,000	56,820	200,235	143,415	14.5

The calculation shows that the natural gas consumption is 14.5 m³/h. Assuming a natural gas price of RMB 3/m³, the operating cost is RMB 43.5 per hour, achieving complete combustion.

Cold Start (2 h)

Energy	Consumption	Unit	Unit Price (RMB)	Time (h)	Cost (RMB)
Electricity	35	kWh	0.8	2	56
Natural Gas	34	m³/h	3	2	204

Total: RMB 260 per start

Hot Start (0.8 h)

Energy	Consumption	Unit	Unit Price (RMB)	Cost (RMB)
Electricity	35	kWh	0.8	22.4
Natural Gas	40	m³/h	3	120

Total: RMB 142.4 per start

Normal Operation (Full Load)

Energy	Consumption	Unit	Unit Price (RMB)	Time (h)	Cost (RMB)
Electricity (70%)	49	kWh	0.8	1	39.2
Natural Gas	14.5	m³/h	3	1	43.5

Total: RMB 82.7 per hour

IV. Social Benefits

The application of rotary RTO equipment for the treatment of waste gas generated in waterproof membrane production can achieve the following results:

1. Asphalt fume, non-methane hydrocarbons, and other emission indicators can meet national emission standards;
2. Odorous substances in the exhaust gas can be completely removed.