Firstly, LWA is half the weight of stone or gravel allowing considerable savings in delivery to site and in the laying costs. Concrete produced using LWA has a density of about 1,800 kg/m³ compared to 2,400 kg/m³ for normal weight concrete. As a result, the labour required to place a cubic metre is substantially reduced.
Secondly, the coefficient of thermal expansion for LWA concrete is lower than for normal weight concrete. For practical purposes it can be taken as 7 x 10-6 /°C. Thermal conductivity is 50 per cent that of normal weight concrete and is typically 0.35 W/mK at 500 °C, increasing to 0.58 at 1,000 °C. As LWA is produced by heating to over 1,000 °C, it has excellent resistance to high temperatures and is ideal for use in refractory concrete. It has no tendency to spall and also exhibits better crack resistance from shrinkage, creep and thermal expansion movements.
Since LWA is produced as a spherical pellet, it flows easily and quickly into position, reducing the need for specialist placing techniques. It will not breakdown, clog or impede drainage. It is non abrasive and, because of its rounded shape, does not cause damage to plastic pipes. LWA concrete can be easily drilled yet has high energy absorption properties.
It can also be used as a filter medium, in vehicle arrestor beds and in other applications where its light weight, spherical shape and resistance to degradation and fire damage are useful.
The down draft sinter process
PFA is usually pneumatically conveyed from the power station to the sinter plant, which is normally located nearby. PFA is then mixed with coal and pelletised with water in inclined disc pelletisers. The pelletising operation is closely monitored to ensure the production of a consistent feed to the sinter machine. The design of the plant closely integrates the pelletising and sinter machine sections to maximise control of the process and to minimise degradation of the pellets.
Handling and storage of the pellets is kept to a minimum. The production of a strong pellet, which will burn out effectively in the sintering process, is a prerequisite for satisfactory operation. If the carbon levels in PFA are below about 2.5 per cent, additional pulverised coal is added to the pelletiser feed. In the case of very high levels of carbon in PFA (above 10 per cent), the PFA may have to be blended with low carbon fines.
Suitability of feed
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Pelletising and sinter pot tests are carried out to ensure the suitability of the design for the available PFA. However, in practice the operation has been found to be extremely resilient to changes in the quality of the feed.
The pellets are fed onto the sinter strand via a hopper. The strand is a continuous band of pallets that carry the pellets through the ignition hood and over the windboxes where the carbon is burnt out and the bed is sintered. The bed of pellets is supported on gratebars, which are carried in a pallet frame. The discharge of pellets on to the sinter strand is carefully controlled to minimise impact and to ensure that a permeable bed of pellets is produced. This allows the free flow of air through the bed, ensuring the even combustion of the fly ash in the pellets and the production of a sinter cake with the required characteristics. |
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Sinter Machine supplied to
South African project
- shows sinter strand of
pallets | |
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| Bed of Pellets passing under levelling plate and entering the ignition hood | |
The pellets pass under a levelling plate to give a constant profile to the bed before it enters the ignition hood. Oil- or gas-fired burners in the ignition hood heat the top surface of the bed as it passes under the hood. The top surface is heated to approximately 1,300 °C which starts the flame front in the bed. The combustion gases from the burners are drawn through the bed by suction from the windboxes under the pallets.
These gases dry out the remainder of the bed and start to heat the pellets immediately under the flame front. This is slowly drawn through the bed as the pallets move down the sinter machine towards the discharge end. The speed of the flame front through the bed is affected by the permeability of the bed. This in turn is a reflection of the quality of the pellets fed onto the sinter strand. |
Variable machine speed
The machine speed is adjusted by the operator to keep the breakthrough (maximum waste gas temperature) to about the seventh windbox. The machine speed is variable between 0.25 and 1.8 metres per minute, a speed of 1.2 metres per minute would be typical for a throughput of 40 tonnes per hour. Air is drawn through the sintered bed for the remaining three windboxes cooling down the bed. Waste gases, drawn through the windboxes by a waste gas fan, are dedusted before being discharged to atmosphere.
Rubber seals are provided between the pallets and the windboxes to give a more consistent load on the drive motor and, unlike grease seals, are not prone to abrasion damage from sinter particles. Sinter is then discharged from the sinter machine and crushed to 14mm with the fines, and 4mm screened out for separate sale. The products are then discharged by conveyor to storage bins for direct loading to road or rail transport.
This down draft sinter process is very similar to that used for the production of blast furnace feed from iron ore fines. The PFA sinter machine has a bed area of typically 33m². This is sufficient to produce over 250,000m³ per year of LWA. The sinter product from the PFA machine is much easier to crush and is usually produced at 12mm top size. And the total production from the sinter machine is saleable.
Daqing Petroleum project in China
| The Daqing Petroleum project in China, awarded in January 2001, involved equipment being supplied from Europe with fabricated sections then being built in China. The installation was supervised by a specialist engineer from AK Engineering Services, based in Stockton-on-Tees in the UK and part of the Aker Kvaerner group. Installation was completed in December 2001 and the sinter machine was commissioned by AK Engineering Services in February 2002, with a 24 hour test run completed without any faults. The burner operating system was commissioned by the suppliers, Bloom Engineering. |
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| Feed End of Daqing Sinter Machine | |
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