Coal

Improve Coal Flow by Using Insert Technology

Coal-fired power stations rely on storing coal in large bunkers or silos. Over time, changes in the type and quality of coal can lead to poor flow during discharge, creating production inefficiencies and significantly reducing storage capacity. Manual intervention is frequently used to promote flow; however, this is a hazardous process for operators and fails to provide a long-term solution to the problem.

One solids-handling equipment company has developed a multiple-stage insert system to overcome coal flow problems. The design, developed by Ajax Equipment, creates a more even discharge, spreading the flow to previously “dead” storage areas and optimizing flow toward the outlet. These insert systems are usually tailor-made to suit the installation. One has been used for a number of years by Tata Steel Europe at its plant in Scunthorpe, U.K., to overcome coal bunker flow problems.

Coal Handling

Coal is crushed and blended on the Scunthorpe site and transported to Appleby Coke Ovens via a series of belt conveyors. It is then stored in a large concrete service bunker.

Built in 1937, the coal bunker (Figure 1) is divided into two rectangular sections, one section holding 3,000 tonnes (t) of coal and the other holding 1,000 t of coal. It is about 17.5 meters (m) tall and the 1,000-t section is 8 m x 13 m, while the 3,000-t section is 20 m x 13 m. Coal is fed into the top of the bunker, where it is distributed to one of the two sections and stored ready for discharge under gravity. Coal is discharged via a number of rows of outlets at the base of the bunkers into the charge cars.



1. Service bunker at Appleby Coke Ovens—Scunthorpe, U.K. Courtesy: Tata Steel

In 1968, half of the outlets were blanked off and lightweight concrete was used to build up a steeper approach to the remaining outlets with smooth glass tiles laid on top to encourage flow. The outlets are arranged in five rows of four outlets on the 3,000-t side and two rows of four outlets on the 1,000-t section. Each row of four outlets operates together to fill a charge car, which feeds the oven. Each outlet has a 640-mm diameter steel throat cast into the concrete. Slide gates are fitted to each outlet, and the charge cars are filled with 17.5 t of coal in, hopefully, 1 minute.

The bunker was originally designed to store local Lincolnshire and Yorkshire coal, but today it holds blended, imported coal from around the world. A typical blend may consist of 60% Australian and 40% North American coal. The particularly cohesive properties of the imported coal have exacerbated the bunker’s flow problems, making the material more difficult to handle.

Poor Bunker Flow

Although the huge bunker has a potential capacity of 4,000 t, the true useable capacity of the 3,000-t section was nearer to 1,500 t. This was due to the phenomenon of “rat holing” (Figure 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 bunker’s contents was retrievable—that which could flow through the 64-cm diameter outlet via a flow channel that flares to approximately 1-m diameter over a depth of up to 10 m. The bunker geometry and construction caused the residue to remain, even when the central flowing channel was emptied (rat holing). Occasionally, the narrow flow channel itself would arch, preventing flow completely.



2. Rat holes formed in the bunker. Courtesy: Tata Steel

Health and Safety Risk

Manual poking to stimulate flow exposed the operators to hazards and unhealthy working conditions associated with the collapse of arches and rat hole walls. It involved manual intervention, using a long scraper at the outlet, to promote coal flow—a task that could be quite physical. Poking for coal accounted for 16% of Appleby Coke Oven injuries in 2004. A number of other issues became apparent, including underfilling 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 as well as 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 the Coal Flow Problem

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 toward the outlet. 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 was to place inserts inside the bunker.

Inserts offer the opportunity to generate slip more easily at the walls, producing a more favorable 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 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 ensuring 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 coal through the final outlet was to have a mass flow section, which needed to have requisite mass flow wall angles but also a shape favorable for flow through the existing limited outlet size.

Bunker Insert System

The results of powder tests and model insert trials identified a critical aspect of the insert system design—the ability of adjacent outlets 2.5 m apart to provide draw over a sufficient area, such that the minimum rat hole size that could form would be so large that any coal remaining would be substantially less than in the original bunker and be more prone to collapse when the central flow channel emptied.

Of course, modifying the flow pattern in a bunker of this size brings with it other concerns associated with the loads acting on the structure. Tata was very keen to avoid the possibility of repeating the failure of the Grange Coke Oven Service Bunker at Port Talbot in 1961. There, a 3,000-t concrete service bunker failed within two years of being built, due to stress cracking of the reinforced concrete walls. While the bunker had been designed for mass flow, it was unable to withstand the pressures generated under the dynamic conditions occurring during mass flow.

Tata calculated the loads on the Scunthorpe bunker walls due to mass flow. The results of this analysis confirmed that extreme care had to be taken not to generate mass flow in the bunker, due to the vulnerability of the hopper walls to the increased loads that total mass flow—if it occurred—would bring. Of course, there was still the requirement to provide the maximum useable storage capacity, but that would have to be satisfied by a combination of mass flow in the vicinity of the outlets and avoidance of mass flow further up the bunker.

At Scunthorpe, a three-stage design with an insert was devised by Ajax. The first mass flow sections produced plane flow converging to the existing outlets. The form of these sections was such that two adjacent outlet sections were joined together to provide one slot.

The second mass flow stage expanded the flow channel to a reliable flow width and connected a full row of four outlets together. To destabilize any large rat hole that might form, a third stage was added, which would not be mass flow, but instead would provide self-clearing of the remaining hopper contents. Due to the offset outlet construction, a further set of inserts was fitted to reduce compacting pressures in the outlet region and encourage flow from the shallow side of the hopper (Figure 3).



3. Drawing of bunker with insert system. Courtesy: Ajax Equipment

Installation and Performance of Inserts

The first row of inserts made such a huge difference to improving the flow of coal from the bunker that a financial case was put together to line three more rows of outlets. There are now two rows of inserts in the 3,000-t section and two rows in the 1,000-t section (Figure 4).

4. Two rows of inserts installed in the 1,000-tonne section. Courtesy: Tata Steel

Prior to fitting the inserts, poking for coal was required at least once per shift from November to March. This is no longer necessary. Moreover, no reportable injuries have resulted from poking activities in the service bunker since the inserts were installed in 2009.

When poking for coal, there was a tendency for the charge cars to be underfilled. This resulted in the ovens not being filled to the required standard, lower coke yield, and subsequent refractory damage to oven chambers. Since the inserts have been installed, better filling of the charge cars has occurred, resulting in improved oven filling and a higher coke yield. The live capacity of the 1,000-t section is now approaching 80%.

In conclusion, powder testing for flow, insert system development using a model, and careful consideration of the bunker’s structural integrity have enabled Tata Steel and Ajax Equipment to deliver a successful solution to overcoming previous flow problems at the Scunthorpe coal bunker.

Dr. Eddie McGee, technical director, Ajax Equipment Ltd., and Ken Picking, materials handling engineer, Tata Steel.

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