Wherever coal is burned it is necessary to have an efficient ash handling system, especially in a coal-fired power station environment where large quantities of pulverized fuel ash (PFA) are created. Such ash can be a considerable environmental nuisance as well as being awkward to handle due to its abrasiveness and fine particle size. With the growing environmental awareness that hydraulic ash removal systems are costly in the use of water and land, emphasis has been placed on finding a better system.
The criteria set down for such a system was to be robust, simple, and not requiring a high level of operator or maintenance skill. The Fairfield ‘En Masse’ chain conveying system has proved that it fulfils these requirements. The Fairfield® ‘En Masse’ principle of conveying is now firmly established worldwide in most industries as well as at many power stations for the handling of pulverized fuel ash (PFA).
Movement of the chain when buried in the material will induce the whole mass to move forward GENTLY in a SOLID, PLACID COLUMN ‘EN MASSE’. Material dragging, particle tumbling or rolling DOES NOT occur. Fairfield® conveyors are manufactured in a standard range of sizes from 200mm wide upwards, enabling them to meet every requirement.
A range of haulage members is available with breaking loads of up to 600kN. To optimise the working life of the haulage member, the working load is usually limited to 1/6th of the breaking load.
The haulage members are drop-forged from suitable alloy steels and case-hardened.
The flights are generally of mild steel and to promote efficient conveying are made the same depth as the haulage member. Attachment to the haulage member is by fusion welding process, which gives an extremely strong connection and greatly reduces the risk of flights breaking off due to the entry of tramp material. Connection of the haulage members is made with a case-hardened pin retained in position with a circlip.
Stock Fairfield Chain T-Plate Type
A number of different casing configurations are used to suit different requirements. Normal single run casing is used for general conveying with the return chain being supported by a runner.
Underneath the precipitators and on some other applications the ash has to be initially conveyed in the top run of the machine and the two-way configuration is used. In both single and two-way applications, split leg casing is sometimes used, as this allows better access into the bottom conveying compartment. Casings are constructed to withstand small positive and negative pressures.
Sides and bottom plates are usually manufactured from 6mm thick mild steel plate and Manganese steel wear strips used for the chain runners. All cross section flanges are ground after assembly to ensure perfect alignment and provide air-tightness.
Pulverised Fuel Ash (PFA) is abrasive, but excessive measures do not have to be taken in casing construction on “En –masse” machines to obviate this.
|Referring to Figure, the use of manganese steel strips on the bottom plate up to 10mm thick for the chain to run on, provides a protective layer of ash on the bottom plate itself. Whilst on the casing slides, replaceable wear plates can be fitted. Alternatively, the ash can is used to provide protection if the conveying flights are reduced in width to create a much slower moving area of ash at the sides of the machine.||
Protection against abrasion
One of the major considerations in the design of the driving and tension ends is the prevention of air ingress. Shaft entries and tensioning systems have to be carefully designed with this in mind. Due to the temperature of the ash and the long length of some machines, provision has to be made to accommodate the expansion of the chain. Manual tensioning arrangements are not sufficient and automatic sustained systems such as hydraulic spring or droop tensioning have to be applied. Sprockets are usually provided with replaceable teeth to facilitate easier maintenance.
To prevent the ingress of air into a negative pressure system or the escape of gas form a positive pressure system it is advisable to incorporate sealing valves within the system, such as at positions directly below the hopper or between conveyor tran sfer points. This can be achieved by the inclusion of single or double flap valves which act as a seal but permit the fly ash to pass freely through the valve. Rotary seal valves can also be used and as with the flap valves their motorised operation can be fully integrated into the electrical control system. Additionally, knife valves can be used to provide a positive ‘shut off’ access or sampling points.
The nature of the system requires only a basic and uncomplicated form of control system. Motor starters, isolators, interlocks, fuses etc all mounted in a control cubicle is the minimal need. The addition of a control panel with a mimic diagram of the equipment layout is traditionally supplied to facilitate effective operator control. Should more sophisticated controls be required, then this can be readily applied.
When considering a STOCK Fairfield® En-Masse System to handle Fly Ash in power station applications, the main considerations that have to be made are:
All these items are inter-related and each require careful investigation and consideration.
The preferred chain speed would be between 0.1 to 0.2 meters per second while the capacity and the size of machine is determined by operational requirements.
Consideration must be given to the operational changes that can occur within the collecting hopper, such as (i) a row within the filter bank going ‘offline’ (ii) outputs of two boilers being put through one collecting system, (iii) discharging of an accumulation of material already in the hopper as well as accommodating the normal dust drop-out. For complete flexibility the conveying system is often designed to accommodate all these operational requirements at the same time. Consequently this has a major effect on the installed size and capacity requirements of individual conveyors.
The STOCK Fairfield® En-Masse System is ideally suited to collect ash from virtually all types and forms of dry dust separators and precipitators which have various forms of hoppered bottoms.
Generally these can be divided into two main configurations (1) PYRAMID TYPE (2) TROUGH TYPE. The features of these two types are broadly illustrated below:
Fig. 1 - Pyramid hopper bottom arrangement
Fig. 2 - Stock Fairfield under precipitator conveyors
Trough hopper arrangements
Fig. 3 - Stock Fairfield under precipitator conveyors
Separation of dusts from various parts of hopper
The pyramid type precipitator hopper bottom is the most common arrangement used in Power Stations with each hopper or row of hopper bottoms being located underneath a separate field.
The number of hopper bottoms depends upon the size and duty of the precipitator. A small unit would comprise a single row of four hopper bottoms and a large unit may well comprise six rows each of six hopper bottoms.
The ash drop-out varies along the length of the precipitator with probably 75 % dropping out at the first field and the remainder progressively over the rest of the fields.
Especially on the large precipitators consideration must be made of the drop-out pattern of the dust. Correct arrangement of the conveyors underneath will ensure optimum size selection of the machines and the ability of the conveying system to accommodate any malfunction of a precipitator, such as a field going down or even the malfunction of one of the conveyors.
These are not used as frequently as the pyramid type in Power Stations but are used extensively in other industries such as steel and cement.
The “En-Masse” conveyor, however, has the ability to discharge these long slot type arrangements, without problem, and in this respect is much more versatile than a pneumatic system.
In the situation where material has built-up in the hopper, the “En-Masse” machine has the ability to discharge under a head of material and at a controlled rate.
There are occasions when some form of separation of the dust collected from various parts of the precipitator hopper are required, this can easily be arranged with “En-Masse” conveyor – fig 3.
STOCK Fairfield® En-Masse handling systems have been installed in many power stations in the United Kingdom and overseas. Such systems are being fitted to new power stations as well as being retro-fitted to stations previously employing the hydraulic system. The two illustrations show the STOCK Fairfield® En-Masse System being employed for different applications.
This diagram shows the system designed for one unit only of a 2 x 500MW coal fired power station in the Middle East. This shows the conveyor layout collecting ash from a 6 row precipitator with additional collection from air heater hoppers. Ash is conveyed and ultimately elevated via chain and bucket elevators into storage silo prior to forward disposal.
This illustration shows the configuration of machines situated at C.E.G.B. Drax Power Station in the U.K. Fly Ash is collected from the precipitator via a pneumatic system into a large storage silo. Ash from the silo is then fed into STOCK Fairfield® En-Masse Conveyors for transportation to the ash conditioners.
© 2007 Stock - Schenck Process Group