Improving Flow in a 4000 t Coal Bunker using Insert Technology

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Glatt Ingenieurtechnik GmbH

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Donadon SDD Srl

Ajax Equipment Limited

Poor Bunker Flow

Although the huge bunker has a potential capacity of 4000 tonnes, the true useable capacity of the 3000-tonne section was nearer to 1500 tonnes. This was due to the phenomenon of 'rat holing' (Fig. 2) where a significant quantity of the contents was effectively trapped around the periphery of the bunker with only the central core of coal directly above the outlets flowing from the bunker into the charge cars.

This meant that only a modest portion of the silo’s contents was retrievable – that which can flow through the 640-millimetre diameter outlet via a flow channel that flares to approximately 1 metre diameter over a depth of up to 10 metres. The bunker geometry and construction caused the residue to remain even when the central flowing channel is emptied (ratholing). Occasionally the narrow flow channel itself would arch, preventing flow completely.

Gallery

Manual poking to stimulate flow exposed the operators to hazards and unhealthy working conditions associated with the collapse of arches and rathole walls. It involved the manual intervention, using a long scraper at the outlet, to promote coal flow – it can be quite a physical task. Poking for coal accounted for 16 percent of Appleby Coke Oven injuries in 2004. A number of other issues became apparent including under filling of ovens and adverse affects on the charging schedule.

In addition, severe constraints were imposed upon the filling procedure and operational flexibility by the limited usable inventory of the hoppers. The delays in the filling of charge cars and erratic calls for operator involvement were not efficient for operating cost or production.

The overall effect was to adversely affect production reliability and planning, and health and safety and operating costs issues. To overcome the rat holing effect, Ajax Equipment carried out a detailed review of the bunker design along with flow property tests and practical trials.

Diagnosing Coal Flow Problem

The flow test results indicated the scale of the challenge to deliver reliable flow through the 640-millimetre outlets of the bunker. In general, flow regimes in hoppers show that mass flow – where all the bulk material moves to the outlet – gives the best flow potential for squeezing through the limited outlet sizes in the bunker.

Moreover, slip at the bunker walls (mass flow) and the ability to flow through smaller outlets can be best achieved with the right geometrical approach towards the outlet – planar rather than over axi-symmetric flow. In addition, strength developed by a bulk solid during storage is dependent on consolidating stresses; if these can be limited then the bulk solid can be made to flow more easily as it is in a weaker condition. The solution devised by Ajax, combining these requirements features, was to place inserts inside the bunker.

Inserts offered the opportunity to generate slip more easily at the walls, converting the axial-symmetric flow to planar flow type – a more favourable, flow form – and shielding the outlet region to reduce consolidating stresses. As a result the material flows more readily and there is a reduction in the tendency to the form a stable arch or rat hole.

Flow modifying inserts take many forms from a simple lining system which offers lower wall friction through to multi-stage systems with varying wall profiles and static inserts. Although these may appear to be an obstacle to flow, they actually work by shielding the outlet region and/or ensure flow comes from the side of the hopper rather than establishing a single central flow channel.

For the Tata coal bunkers, a sophisticated combination approach was needed: the best possible chance of squeezing product through the final outlet was to have a mass flow section: this would need to have requisite mass flow wall angles, but also be of a shape favourable for flow through the existing limited outlet size.

(ID:36570330)