The Clean Power Plan (CPP) establishes customized targets for states to reduce the carbon pollution produced by power plants that reflect each state’s energy mix
The main objective of the gathering system is to utilize a series of fans along the tunnel to collect CO2 particles from the elevated concentration in the tunnel air and flow these particles into a DAC unit. For the scope of the project, this system has been sized to accommodate 600 to 760 lbs./hr. Assuming the current CO2 levels in the tunnel, this correlates to an air rate range of 60,500 to 62,300 cubic feet per minute. The Wallace tunnel is approximately 3,000 ft long, and three jet fans are to be strategically located to maximize the potential harvesting capability of the CO2. Each designed jet fan is sized for a minimum of 25,000 cubic feet per minute, and they have been individually ducted to a common header for discharge into the DAC system.
Ventilation systems are critical for maintaining air quality and safety in tunnels, particularly in road and rail tunnels where vehicle emissions can accumulate. The primary objectives of tunnel ventilation systems are to remove pollutants such as carbon monoxide (CO), nitrogen dioxide (NO2), and particulate matter, to control smoke in the event of a fire, and to provide fresh air for passengers travelling through the tunnel. Generally speaking, there are three major types of ventilation systems employed within tunnels: (1) Longitudinal Ventilation which uses jet fans to create airflow along the tunnel length, pushing or pulling air through the tunnel, (2) Transverse Ventilation which supplies and exhausts air through ducts running parallel to the tunnel, ensuring even distribution of fresh air, (3) Semi-transverse Ventilation, which is a combination of longitudinal and transverse systems, supplying fresh air at intervals along the tunnel, and (3) Natural Ventilation, which utilizes the tunnel's geometry and natural airflow, typically used in shorter or less trafficked tunnels. Extensive studies have been conducted on optimizing fan placement and operation within tunnels, as well as on how to improve current tunnel ventilation systems to better control car emissions, contaminants, and excessive temperatures
As carbon capture technologies are coupled with ventilations systems within metropolitan tunnels, these technologies can efficiently manage and reduce the environmental impact of vehicular traffic. Integrating ventilation and carbon capture systems in tunnels can enhance air quality by removing both traditional pollutants and CO2 emissions. Research has been focused on improving the efficiency of CO2 capture and reducing the cost of these technologies to make them more viable for widespread use. Studies have also been conducted to explore the potential of integrating carbon capture with renewable energy sources to offset the energy requirements of the capture process
Based on a reported pollutant concentration measurement study from 2020, max concentrations of pollutants have been found to occur in the latter half of a tunnel structure
Defined functional requirements for the study are noted in
The authors also identified and abided to the applicable federal, state, and local codes that had to be met for the project. These are shown in
| Requirement #1 | Ability to move air from inside the tunnel to a DAC system outside of the tunnel | Tunnel jets have been strategically located to maximize gathering capacity |
| Requirement #2 | Collect at least a 25% total current CO2 mass rate | This defines the minimum amount of air required for equipment sizing |
| Requirement #3 | System must be secured to tunnel ceiling to prevent road obstructions | Clearance and safety requirements as per the current highway safety guidelines |
| Requirement #4 | System also gathers entrained smoke and VOC particles | Capable of withstanding concentrations of smoke and VOCs |
| National Cooperative Highway Research Program (NCHRP) Guidelines for Emergency Ventilation Smoke Control in Roadway Tunnels |
| National Cooperative Highway Research Program (NCHRP) Synthesis 415 Design Fires in Road Tunnels |
| National Fire Protection Association (NFPA) 502 |
| US Department of Transportation Federal Highway Administration Publication FHWA-NHI-10-034 Dec 2009 |
| American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Heating Ventilation and Air Conditioning (HVAC) Applications |
| National Highway Institute (NHI) Smoke Control in Roadway Tunnels |
| National Cooperative Highway Research Program (NCHRP) Report 525 |
| Alabama Building Code 2021—Chapter 16 (Structural Design) and Chapter 22 (Steel) |
| Mobile, Al Building Code 2023—Chapter 11 (Industrial Structures within City Limits) |
As previously stated, CO2 levels in the tunnel were estimated to be between 2,300 and 3,000 lbs./hr. on July 1, 2021 (see
Based on the calculated total required air flow rate of 60,300 CFM in the design basis, the authors assumed that tunnel air could be collected and transmitted to the DAC system effectively using three independent zones each at 20,767 CFM along the identified peak potential area in the ladder half of the tunnel. The jet fan sizing was then chosen based on the next available size that was larger than the calculated zone rate. For this purpose, the authors opted to utilize three Novenco AZN 900/350-6 A53 units with an available discharge rate of 25,000 CFM each at 2.83 inWC for a combined total rate of 75,000 CFM. Duct sizing was evaluated for each separate run to allow a minimum of 1 inWC at the main header located at the end of the tunnel up to the suction of the booster fans. Each jet fan unit utilizes approximately 13 hp for a combined total of 39 hp for the entire gathering system.
Since the tunnel air contains a greater concentration of VOC as compared to the open-air source that the DAC was originally designed, there could be additional buildup of soot and hydrocarbons that could develop and render the DAC ineffective. For that purpose, the authors of the study considered a contingency option where an additional set of fans is used downstream of the gathering system as a separate stage to boost the airflow pressure and allow for additional losses for utilization with a pretreatment system. This pretreatment would consist of a wide spot in the feed to the DAC contactor where the flow velocity is reduced to allow for heavier entrained particulate to fall out of the stream. For this purpose, two ZerAx AZN 1000/350-6 A55 units were chosen and placed in a parallel configuration. This modification increases the overall discharge pressure of the system by an additional 3.4 inWC before entering a pretreatment system. Since each booster fan is approximately 25 hp, the overall power requirements increased by an additional 50 hp. Although this modification more than doubles the power requirement from the original design, it does however limit the issue of increasing original duct sizing any further. The overall process has been reflected in
The duct sizing requirements for each run were evaluated for use with the selected fan at the respective lengths of 100 ft, 750 ft, and 1,500 ft based on the three preestablished zones. The required duct diameters were based on a minimum supply pressure of 1 inWC at the contactor assuming the 25,000 CFM rate. A thirty-inch, forty-inch, and forty six-inch ducts were selected, respectively.
For the material selection for the support system, the decision was made to go with a simple but durable approach. An ASTM A193 Grade B7 material was utilized for the bolting and anchor system. This material has a high yield strength and ultimate tensile stress, making it a reliable option for demanding structural
applications. Its durability and simplicity align well with the project requirements. A 5/8” ASTM A193 Grade B7 material was used for the anchor bolt assembly. The embedment depth was based on the manufacturer’s minimum embedment depth chart, which is 2-1/2” for a 5/8” undercut anchor. Each anchor would require 3,323 lbs. for pull out. It was be noted that deeper embedment will yield higher tension and shear capacity. Adhering to the manufacturer's guidelines ensures the anchor’s reliability and performance.
These values were based on 2,500 psi concrete. Normal substantial pressure ranges from 3,500 to 5,000 psi. The final duct hanger design is shown in
The conducted Finite Element Analysis (FEA) in this study employed a 735 lbf external force for the All-thread, calculated from the fan’s total weight divided by 4. In
For the potential forces that could be acting on the tunnel due to the thrust developed by the fans, the thrust forces were evaluated. Through careful consultation with the Tunnel Jet Fan manufacturer, Novenco, it was decided that the AZN 900/350-6 model aligns impeccably with the specific demands of this project. The decision to horizontally mount the fans onto the tunnel’s ceiling was strategic, ensuring optimal performance. Operating at a substantial volume flow rate of 25,000 CFM, the fan’s design incorporates a flexible duct connection on the outlet alongside duct spigots and anti-vibration dampers; thus, reinforcing its adaptability and functionality. Moreover, the fan’s capability to effectively manage fire smoke, CO2, and nitrogen dioxide ventilation underscores its suitability and robustness for project objectives. Furthermore, the fans’ sound emission, as per specifications, is less than the recorded 100.5 decibels inside the George Wallace tunnel. This underscores the project’s commitment to not creating additional hazards in the surrounding area.
For potential flow, the authors utilized PyroSim to conduct a qualitative analysis. PyroSim is typically used to simulate smoke patterns to assess ventilation designs for fires in buildings.
Since the smoke particulate and CO2 are somewhat similar, it was assumed that the model would provide an accurate representation of the intended design. A rudimentary model was generated to assess the quality of the flow to the inlet of one fan. The model cross-section is approximately 23 feet tall to mimic the available overall height of the distance from the roadbed to the ceiling. The model width was set at 26 feet wide to mirror the lanes of traffic. The length was configured to assess the flow of the particulate in a 300 ft section immediately upstream of fan #3 beginning approximately 100 ft from the tunnel exit where the concentration would be the highest.
A traffic speed of 45 miles per hour or 66 feet per second from right to left was assumed and evaluated. To create a scale rate for the CO2 flow, the overall required tunnel air rate was assessed by using the original design basis of 250,023 ft3/min divided by the area of the roadbed of 78,000 ft2 to determine an average constant rate of 3.2 ft3/min*ft2. In the model, an area of approximately 12,300 ft2 was specified. The model area was configured in front of the fan to flow at a rate of 39,360 ft3/min. A fan rate of 25,000 ft3/min was chosen per the fan specifications as previously stated. The resulting velocity vector indicates a clockwise eddy current developing at the roadbed and continuing upwards toward the tunnel ceiling. The model does not account for the semispherical characteristics of the tunnel ceiling. The authors believe this will help the flow converge towards the fan inlet. The vector field analysis results in
The fan shows a significant increase in local velocity, but it does not seem to reflect a large outreaching pattern from the roadbed to the suction, as depicted in
One of the many variables in this project is that the CO2 concentration of the tunnel is constantly changing throughout the day. The models generated by PyroSim require more than 3 hours of computational time for every 60 seconds of
model run time. The available density evaluation tool was utilized to evaluate density based on 1,200 ppm concentration to generate a preliminary rendering based on 60 seconds of run time, as shown in
Tunnel ventilation and direct air capture (DAC) are two separate technologies but can be integrated to address environmental challenges, particularly in the context of air quality and carbon capture. In the case of the Wallace Tunnel located in the City of Mobile, Alabama, the tunnel has enough space to install a system above the roadbed to support an adequate ventilation and air capture combo. In this project, it was determined that the overhead space and structure worked well to allow for the suspension of the ducting system. If the space above the roadbed had been any more constricted, then smaller ducting would have been required, and as a result, it would have taken more power due to the increased losses.
DAC is an evolving technology and will continue to be limited by the amount of available capital to fund it. To help maximize capital impact, the CO2 and VOC concentrations must be understood for the application area in question. Carbon capture projects that utilize tunnel ventilation should be focused on capturing CO2 during the parts of the day when CO2 concentration is at the highest. Additionally, when concentrations are highest the displacement energy of the passing traffic is also high, thus allowing for greater displacement of CO2 particles from their source.