This project adopts indirect thermal desorption soil remediation technology. "Indirect thermal desorption" refers to the process of heating organic pollutants in the soil to the boiling point temperature of pollutants through indirect heat exchange under vacuum conditions or when a carrier gas is introduced, causing organic pollutants to evaporate and separate from the soil, and effectively collecting and treating the emitted pollutants.

Specifically for this project, the technical essentials are:

a. This set of indirect thermal desorption equipment uses high-temperature flue gas produced by Italian imported combustion engines burning clean fuel natural gas as a heat source to heat polluted soil to boiling point temperature, causing pollutants to evaporate and achieve remediation effects.

b. The thermal desorption reaction is carried out in specialized thermal desorption equipment, and the high-temperature flue gas does not come into contact with the soil during the heating process.

c. The equipment comes with an air lock device, which basically isolates the soil from the atmosphere after entering the equipment, and forms a slight negative pressure state through the induced draft fan.

d. The smoke generated by combustion does not contain pollutants and can be directly discharged.

e. The thermal desorption gas produced by soil volatilization is collected and processed by self-contained equipment.

f. During the process of thermal desorption gas treatment, secondary wastewater will be generated and needs to be treated before discharge.

Process flow

The indirect thermal desorption soil remediation process includes three parts: pretreatment, heating, and exhaust gas purification.

(1) Preprocessing

As the first step in the indirect thermal desorption soil remediation process, pretreatment is crucial but often overlooked. The purpose of preprocessing is to regularize and homogenize the soil to meet the feeding conditions of the thermal desorption equipment. It is a prerequisite for ensuring the stable operation of the thermal desorption system and achieving good repair results.

Pre treatment usually needs to be carried out in a relatively enclosed space. Generally, pre-treatment greenhouses need to be set up on construction sites, and the greenhouses need to be equipped with corresponding exhaust gas circulation and purification facilities.

Pre processing includes three steps: desiccant mixing - self-made inclined sieve coarse sieve - AULL fine sieve crushing

a. Desiccant mixing

The soil excavated from contaminated sites generally has a moisture content of around 30%. The project is located in Pujiang County, Chengdu City, Sichuan Province, adjacent to a river, with underground water tributaries responsible, and a rainy climate. There is water seepage during excavation of the foundation pit, resulting in a much higher moisture content than usual. Initially, it appears as a block with high viscosity. Directly entering the thermal desorption equipment can cause uneven heating, high energy consumption, and easy bridging and blockage. Therefore, pre-treatment is needed to reduce the moisture content.

The usual practice is to mix desiccants such as hydrated lime. The mixing ratio depends on the condition of the original soil. After mixing, the soil moisture content should be ≤ 20%, and the soil should be in a loose state.

Due to the soil moisture content being much higher than the conventional standard, during the excavation of the foundation pit, the surface layer was sandy soil, which was loose and had good water absorption; The middle and lower layers of soil are clay with severe compaction and high moisture content. The conventional mixing method is unable to reduce the moisture content and particle diameter, which ultimately leads to a decrease in the production capacity of subsequent equipment, an increase in failure rate, and an increase in energy consumption.

Based on the experience of repairing other polluted sites and combined with the local site conditions, a pre treatment plan for mixing has been developed after multiple attempts: a. After excavation, the soil is centrally piled up, and block lime with particle size between 5mm ≤ block lime particle size ≤ 8mm is used for one-time mixing, with a reaction time of not less than 12 hours. b. After the block lime fully reacts with the soil, it is stirred once and the second reaction time is not less than 8 hours. c. Before screening, mix a small amount of powdered quicklime powder twice, and after 2 hours of reaction, the soil can be screened. At this point, the soil has reached a loose state. For clay compaction with high moisture content, it can be effectively treated.

b. Self made diagonal sieve coarse sieve

The purpose of coarse screening is to remove large stones and debris from the soil, typically including concrete, stones, and household waste on the surface of retired sites. Coarse screening can effectively remove large stones, concrete, and household waste from the soil, and also provide necessary conditions for subsequent treatment.

c. AULL screening bucket fine screen crushing

AULL screening bucket fine screening crushing is a further screening and crushing of materials based on coarse screening. After fine screening, the soil particles should be ≤ 50mm, and it also plays a role in secondary mixing, making the soil more loose. This can ensure that the soil is heated evenly, less prone to material accumulation, and has a good treatment effect.

(2) Heating up

The basic principle of thermal desorption is to heat the soil to achieve gas phase and solid phase separation, therefore heating is the core process of indirect thermal desorption soil remediation.

The heating source usually uses high-temperature flue gas generated by the combustion of natural gas.

Soil, as a solid substance with extremely poor fluidity, usually has much lower heat transfer efficiency than liquid and gaseous substances, and requires specialized equipment for related operations.

Indirect thermal desorption requires that the soil and heat source do not come into contact, heat transfer occurs between the walls, and the soil needs to be in a relatively enclosed space for the collection of volatile gases.

(3) Exhaust gas purification

After being heated, the moisture and pollutants in the soil evaporate into the gas phase, which separates from the soil. The main components of the separated gas, in addition to moisture and pollutants, also carry dust and require purification treatment.

The indirect thermal desorption exhaust gas purification process method is: water spray cycle washing+activated carbon adsorption.

a. Exhaust gas purification process

Due to the indirect thermal desorption process, high-temperature flue gas does not come into contact with soil, which is relatively isolated from the atmosphere. Therefore, the main components of the thermal desorption gas are high-temperature steam containing water and pollutants, and cooling is an effective purification measure.

This project adopts the method of water spray circulation washing, which rapidly cools high-temperature steam into liquid state and dissolves dust into water. Circulating water absorbs moisture and organic matter from the thermal desorption gas, achieving the effect of treating exhaust gas through water circulation.

b. Activated carbon adsorption

After the water spray cycle washing, this device is equipped with activated carbon adsorption and finally discharged to the high-temperature combustion chamber for high-temperature incineration, ensuring thorough purification of the exhaust gas.

The water and organic matter in the hot desorption gas continuously accumulate in the circulating water, and the total amount of circulating water gradually increases. The excess circulating water becomes secondary wastewater, which needs further purification treatment.

Due to differences in the composition of pollutants and sewage discharge indicators among different polluted sites, further purification treatment of secondary wastewater is usually not within the scope of this project construction.

Specialized equipment solution

The 3t/h indirect thermal desorption soil remediation equipment is used as a specialized equipment for the construction of this project.

Indirect thermal desorption technology is designed for small and medium-sized organic pollution sites and composite organic pollution sites in China, with complex pollution situations. It has high efficiency, low energy consumption, strong maneuverability, and can ensure a pollutant removal rate of over 90%. It comes with exhaust gas purification equipment and low secondary pollution risk, which can meet the needs of the chemical, pharmaceutical and other organic pollution site remediation industries. The core technology of indirect thermal desorption provides important technical support for this project.

Composition of specialized equipment

The 3t/h thermal desorption equipment mainly includes: feed/discharge equipment, hot air generation device, thermal desorption body, exhaust gas purification unit, automatic control system (including container control room), and auxiliary equipment such as pre-treatment screening equipment.

The main equipment adopts a modular integrated design, with all equipment integrated into seven main modules: combustion chamber, thermal desorption body, exhaust gas purification, circulating buffer sedimentation water tank, circulating pump unit, air cooler, and central control room.

The size of a single pry block should not exceed 12196mm × 2438mm × 2896mm, making it convenient for transportation and assembly.

Multiple layers of stacked installation are used between the pry blocks, which is convenient to install and saves space. The equipment body occupies approximately

a. Incoming/outgoing equipment

The feeding equipment mainly completes the buffering, metering, and transportation of soil, including silos, screening and filtering screens, quantitative feeders, belt feeders, etc.

The discharge equipment mainly completes the transportation and cooling of high-temperature soil after thermal desorption treatment, including the discharge screw, humidifier and its supporting pipelines.

b. Hot air generating device

The function of the hot air generation device is to generate high-temperature hot flue gas through the combustion of natural gas. The device includes a combustion chamber, a burner, a combustion supporting fan, etc. The high-temperature flue gas generated by the hot air generation device is connected to the thermal desorption body through the outlet, providing a heat source for the thermal desorption body.

c. Thermal desorption body

The thermal desorption body is the core equipment for the desorption reaction of soil when heated. It adopts a partition spiral structure as the basic reactor model, consisting of a driving device, a screw auger, a gas lock device, a feeding conveyor, etc.

d. Exhaust gas purification unit

The soil is heated in the thermal desorption reactor, producing a large amount of thermal desorption gas, which is collected and processed by the exhaust gas purification unit. The exhaust gas purification process method is: spray washing+activated carbon adsorption. The equipment includes: spray washing room, defogger, activated carbon adsorption device, induced draft fan, circulating buffer sedimentation water tank, circulating pump unit, air cooler, etc.

e. Automatic control system

This equipment is equipped with an automatic control system independently designed and developed by Zhongchuan Oasis. The system includes operation control system, alarm system, and auxiliary engineering control system. It can automatically complete various functions such as data collection, parameter display, over limit alarm, and report printing for the entire treatment process, including the feeding amount and equipment operation status of polluted soil, achieving intelligent human-machine interface.

The entire automatic control system consists of on-site instruments and meters, on-site control boxes, transfer boxes, main distribution boxes, central control boxes, upper computer, etc.

Operators can view the real-time operating status of the system, query historical data, and control the start and stop of on-site equipment, as well as set main control parameters such as heating temperature on the upper computer. The system automatically adjusts the operating status of the equipment based on the set parameters.

f. Preprocessing equipment

This equipment is equipped with one self-made inclined screen and one AULL screening bucket for soil screening pretreatment.

After coarse screening of soil with a particle size of less than 200mm and AULL crushing treatment, the soil particle size reaches below 50mm, which can meet the requirements of subsequent thermal desorption construction.

g. Accessory equipment

The external heat rotary desorption soil remediation equipment includes auxiliary equipment such as chimneys, platforms, inclined ladders, and non integrated pipelines. The height of the chimney shall comply with the requirements of GB16297 "Comprehensive Emission Standards for Air Pollutants", and the exhaust outlet shall not be less than 15m. There is a smoke sampling port on the chimney, and a sampling platform shall be set up around it for the convenience of smoke sampling and monitoring operations.

Repair construction method

The processing work site is set up in a non polluted area within the bidding scope, and the work site is a closed rigid structure greenhouse. Covering an area of approximately 300 square meters, it includes a contaminated soil pretreatment area and a mixing area.

Soil circulation in the field: After excavating contaminated soil, it is transported to the pre-treatment area for lime mixing and coarse screening. Then, one ALLU screening bucket is equipped in the pre-treatment area to excavate the contaminated soil for desiccant mixing (block quicklime mixing), coarse screening, and AULL screening bucket crushing and screening (particle size less than 50mm). The soil is then transported by a loader to the maintenance and inspection area for maintenance, waiting for repair.

1) Site anti-seepage design

All work sites are treated with anti-seepage hardening, and the anti-seepage structure is to clean the original ground and compact it with plain soil, with a compaction coefficient of not less than 0.93; Lay two fabrics and one film anti-seepage layer (600g/㎡ non-woven geotextile+1.5mm thick double rough HDPE anti-seepage film+600g/㎡ non-woven geotextile) on it, and then pour 300mm thick C30 plain concrete for hardening treatment on top.

2) Site structure design

The work site adopts a steel structure enclosed greenhouse, and the enclosed area includes soil pretreatment and treatment zones.

3) Drainage system design

The site is sloped from the center to both sides with a slope coefficient of 0.5%. Brick drainage ditches with a specification of 300 × 300mm are set up around the site to mainly intercept external rainwater and prevent it from entering the work site and eroding the soil. A rainwater collection tank is set up along the sloping side of the site and connected to the drainage ditch to collect rainwater. The collected rainwater is regularly sampled, tested and analyzed. If it exceeds the standard, integrated treatment equipment is used to treat it and reuse it or discharge it through pipes.

Soil excavation

After excavating contaminated soil, transport it to a pre-treatment greenhouse.

Preprocessing

Pre treatment plan: a. After excavation, the soil should be piled up in a centralized manner, using block lime with particle size between 5mm and 8mm for one-time mixing, and the reaction time should not be less than 12 hours. b. After the block shaped lime fully reacts with the soil, it is stirred once and the second reaction time is not less than 8 hours, followed by coarse sieving. c. Before conducting fine screening, crushing and sieving, mix a small amount of powdered quicklime powder again for 2 times. After 2 hours of reaction, crushing and sieving can be carried out. At this time, the soil has reached a loose state. The particle diameter is less than 50mm, which meets the production requirements.

Heat treatment

Preheat the thermal desorption equipment in advance.

Use a loader to feed fine soil into the dedicated thermal desorption equipment bin.

Soil undergoes thermal desorption reaction when heated in specialized equipment for thermal desorption.

After about 40 minutes, the organic pollutants in the soil reach the boiling point temperature, and the concentration of pollutants decreases below the remediation target value. They are discharged from the specialized equipment for thermal desorption, and then sprayed with water to cool and humidify, becoming the treated soil.

Transfer the processed soil to the inspection area using a loader.

Self inspection and external inspection

After high-temperature remediation, the soil must undergo self inspection and third-party quality inspection monitoring and acceptance processes before further disposal can be carried out.

Divide the inspection area into zones every 500 tons, collect one representative sample from each zone, and first select 30% of the representative samples for self inspection. After passing the self inspection, apply for the owner and authorized department (institution) to organize acceptance.

Self inspection: For the treated polluted soil, the construction unit must conduct self inspection analysis by sampling and sending it to the laboratory for testing and analysis. After reaching the corresponding remediation target value, backfilling treatment can be carried out.

External inspection: Before backfilling the treated soil, sampling must be conducted under the supervision of the supervising unit and sent to a third-party testing unit for analysis. The sampling frequency is one mixed sample every 500 tons.

Security landfill

After passing the self inspection, a third-party acceptance unit shall conduct acceptance testing on the excavation pit bottom, side walls, and repaired soil, issue an acceptance report, and backfill the repaired soil after confirming that the repair is qualified.

Service Hotline
Email: luzhou@nj-lzep.com
Phone: +86-13951765609

Customer Service
Email: services@nj-lzep.com
Phone: +86-4000169188