WDF solutions – part 2

Read part 1 here.

‘First-in, first-out’ storage solution

The Saxlund solution, a complete turnkey project from design and manufacture to construction, includes silo technology with a state-of-the-art push-floor. First patented by Saxlund in the 1970s and with hundreds of successful installations (frequently in the most onerous environments), the system incorporates a series of hydraulically operated reciprocating ladders fitted into the silo floor. The motion of the ladders pushes the fuel towards the discharge end of the bunker, achieving a mass flow with a ‘first-in, first-out’ fuel delivery, which is critical when handling these fuels, effectively designing out compaction and self-combustion issues.

‘First-in, first-out’ fuel bunker with Saxlund push floor.

The shearing action of the ladders also helps to agitate and break up the material as it moves downstream and the height is monitored by level sensors above each ladder to ensure an optimal feed rate. If the fuel level is too high at the discharge point the ladder will stop, allowing the others to carry on, which in turn will break the pile down. A reliable process here is crucial, helping both to minimise the risk of fuel compaction against the back wall of the bunker, which could block the whole system, and preventing an over feed of fuel to subsequent equipment.

Once the material is discharged from the push floor it falls into a Saxlund chain conveyor, a slow moving twin-strand chain that conveys the material over an access road and into a process tower. Here the fuel is fed through a drum magnet to remove any ferrous material that could cause damage to downstream equipment. The metal recovered is discharged through a chute and collected for recycling.

The fuel is then fed onto a presentation conveyor with a controlled feed into a policing screen to remove out of spec or oversized material. The star screening system that Saxlund has installed offers an aggressive action, breaking down any matted or oversized material to further minimise the risk of blockages. The system is unique in that the screen size can be changed at the push of a button with the flexibility to change the acceptable particle size of the fuel from 10 mm to 50 mm in size to further refine and optimise the process.

Oversized material is transported over the top of the stars and is diverted into a waste recycling skip, while the in-spec fuel falls through the gaps and is collected by a heavy-duty twin screw feeder with two outlets feeding twin FLSmidth Pfister weighing systems.

In-spec fuel passes to twin screw feeder below star screen and then to FLSmidth Pfister weighing systems.

Accurate control systems

Saxlund specified FLSmidth Pfister weighing systems as they provide highly accurate and reliable gravimetric feeding of alternative fuels with an extensive track record in handling these fuel types over numerous completed projects. Flow rates can be controlled with the greatest degree of accuracy over long periods and the FLSmidth Pfister system satisfies the overall design goals for the project to provide a futureproof solution capable of handling changing fuel specifications. With its multi-fuel concept the rotor weighfeeder is designed to dose extremely light or heavy, fine or coarse fuels, or even potentially explosive bulk materials – only the one system is required. A dust-tight solution with high availability and low maintenance are further benefits to Hope.

Bulk waste derived fuel from each screw feeder passes into a surge pre-hopper on top of each rotor weighfeeder. The filling level in the pre-hopper is measured by load cells to ensure a pre-set or constant filling level, and from here the material cascades into each horizontally arranged rotor weighfeeder, helped on its way by an agitator in the pre-hopper.

Control systems continuously measuring the mass of the bulk material inside the rotor wheel, together with its angular velocity at the discharge point, ensure that the mass flow of the fuel remains constant or follows the feed rate set-point values at all times. This effectively eliminates any disturbances from the process, such as varying fuel density or material flow fluctuations out of the pre-hopper. The bulk material is then fed into the pneumatic feeding pipe via a blow-through rotary valve with feeding shoe and from there into the pneumatic injection systems feeding each kiln.

With pipework for each line extending over 70 m from the fuel process tower to the main burner on each kiln, further design considerations were crucial to minimise bends and facilitate easy maintenance, especially in high wear areas of the system. Pipework is supplied by Saxlund’s system partner Kingfisher International, which has vast experience in supplying pipework for use with these fuel types. Bends, in particular, are designed with a long radius to reduce friction and potential blockages and incorporate Kingfisher’s wear-resistant basalt liners. These increase bend life by six to one over standard mild steel.

Low maintenance, high availability

Reliability is a key factor for Hope and the installation has been designed with low maintenance and high availability in mind, with the emphasis at all times on minimising potential restrictions or blockages. Key maintenance areas, where possible, are also external to the system to minimise disruption. This applies especially to the push floor system – here preventative maintenance or complete overhauls can be carried out even while the fuel store is full.

Conclusion

With construction completed and commissioning underway, the new waste-derived fuel system is due to fire up early in 2015 and will run on a 24/7 basis delivering fuel at a rate of up to 5000 kg/hr to each kiln. This means the Hope plant will soon be operating with a significantly larger proportion of waste-derived fuels. The management team fully expects fossil fuel substitution to progress to 50%, in the process diverting in the region of 80 000 t of bulk solid waste from landfill.

The carbon savings alone will be significant to a company that is committed to manufacturing its products in the most sustainable manner possible. But Britain’s leading independent producer of cement, ready-mix concrete and aggregates certainly is not standing still.

Written by Matt Drew, Saxlund International, UK. This is an abridged article from the January 2015 issue of World Cement. Subscribers can read the full issue by signing in. Non-subscribers can view a preview of the issue here.

Read the article online at: https://www.worldcement.com/europe-cis/12012015/wdf-solutions-part-2-82/