Modernisation project at National Cement leads to a unique Air Pollution Control solution

When National Cement Company Dubai, recently completed an extensive modernisation project, a big challenge was meeting the increased gas flow capacity to be filtered within the limited physical space available. National Cement turned to FLSmidth Airtech, the original kiln filter supplier, to develop a solution.

The National Cement Company plant commands an impressive view of Dubai’s skyscrapers. Close proximity to the city centre, coupled with the United Arab Emirates stringent emission requirement, places high demands on the plant’s air pollution control equipment. National Cement’s extensive modernisation project included replacing the existing five-support kiln and satellite cooler with FLSmidth’s ROTAX-2 (two-support) kiln and a Multi-Movable Cross-bar
cooler. In addition, the plant began introducing alternative fuel burning (tyres) in the precalciner. With these modifications, the plant now required an additional airflow capacity of 183,000 m3/hr while achieving an outlet emission of less than 10 mg/Nm3. FLSmidth Airtech engineered a unique design solution that increased the area of the original FabriClean™ filter supplied in 2005 by 50 percent.

Reliability and performance
Plant and project management were very satisfied with the FabriClean filter’s performance – so contacting FLSmidth Airtech to upgrade the filter to meet the plants changing needs was a natural choice. Project manager Radwan Mouakat says, “We chose FLSmidth Airtech because we wanted the most reliable supplier who could guarantee low emissions and reliable, trouble-free operation of the entire filter system in the future. These requirements are of the utmost importance due to the location of the plant near Dubai city centre. And we had good experience with the performance of the original FabriClean filter.”

The FabriClean filter is an advanced low-pressure jet-pulse filter that uses 6-10 metre long filter bags. Designed with a unique proprietary inlet gas distribution system, the FabriClean optimises gas flow across the entire system, resulting in a lower pressure drop. It is also equipped with the SmartPulse Controller™, which operates the normal pulsing system and has many innovative features for performance monitoring. One of these is an automatic diagnostic feature that analyses pulse duration, interval and pressure with relation to filter differential pressure. It automatically adjusts these parameters to minimise the use of compressed air, ensuring the most efficient operation.

New challenges lead to new solutions
To develop the new solution, FLSmidth Airtech conducted a Computational Fluid Dynamics (CFD) study to determine optimal gas-flow patterns of the new filter.

Focusing on velocity distribution, compartment-to-compartment mass flow balance, temperature distribution and pressure loss, FLSmidth Airtech engineers completed a numerical model of the upgrade. This included:

• Full-scale geometry of the fabric filter inlet ductwork, compartment inlet and outlet manifolds, and outlet ductwork (approximately 2,000,000 computational cells)
• A model for installing guide vanes in ductwork design
• Three fresh-air inlet dampers
• Twelve filter compartments modelled as ‘boxes’, including filter bag resistance based on a fixed-resistance coefficient, but variable mass flow
• Bag pressure loss estimated at 75mmWG.

The study included a worst-case scenario based on a ‘raw mill off’ operating condition, where the fresh air inlet dampers open, cooling the gases below the maximum allowable temperature of 260°C. The resulting velocity distribution of inlet manifolds, as well as the flow balance through the new filter’s twelve compartments, is illustrated to the right.

The CFD study uncovered a technically sound solution to connect the new filter inlet to the original filter inlet via a double wedge-formed ducting piece.

A new ID fan was incorporated in the design to ensure operational flexibility. The fan, which accounts for one third of the total flow through the new FabriClean filter, operates at a fixed speed. The original ID fan accounts for two thirds of the total flow and operates at variable speed, minimising energy consumption and allowing for changes in production.

The FLSmidth Airtech solution included:

• The design and supply of key parts for a new four compartment FabriClean, including fibreglass filter media with PTFE laminate supplied by Advanced Filtration Technology, an FLSmidth company
• Designing a solution to connect the new inlet to the four compartments with the original filter via a double wedge-formed duct
• Relocating three ambient air inlet dampers, and supplying engineering for a new stack
• Extending the existing Cement Kiln Dust material handling system

Smooth implementation
FLSmidth Airtech delivered engineering documentation to a Dubai-based contractor in April 2008. After the filter’s steel parts were completed in a workshop next to the National Cement factory site, the new filter was ready for assembly. To keep plant downtime to a minimum during the assembly and construction phase, a removable blind was welded onto the end of the original filter’s inlet gas ductwork – and once the filter upgrade was in place, the blind was removed.

Exceeding expectations
National Cement’s new air pollution control system is proving a resounding success. The plant now has increased kiln-filter capacity and acquired a neatly integrated add-on to the existing filter construction and ductwork. This unique solution was made easy by the design of the original FabriClean filter: The gas inlet and outlet were located at the same end – opposite the new filter extension.

With plant emission levels even lower than originally forecast, the FabriClean filter upgrade exceeds National Cement’s outlet emissions requirement. And the view of downtown Dubai from the National Cement Company is looking better than ever.

Related links:
Go to FLSmidth Airtech