Coal Blending Paper

THE COKE OVEN MANAGERS’ASSOCIATION

______________________________________________________________ Louise Hutson and Chris Wilson are Process Engineers at Corus, Scunthorpe Works

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COST EFFECTIVE COAL BLENDING AT SCUNTHORPE

By

Louise Hutson & Chris Wilson

Presented on 20 May 2004 at a meeting held at Monckton Coke & Chemicals Works, Barnsley

The proposed coal buy for Corus for 2004/2005 is approximately 10 million tonnes. (Fig 1) This is for both carbonising and coal injection. Ijmuiden receives the largest proportion of coal as their plant output is greatest, (27% of the coking coal and 12% of the injection coal). Redcar only receives coking coal (24% of the total) as the blast furnace operates an oil injection system. Scunthorpe and Port Talbot use are 19% and 8% of the coking coal respectively. they each use 5% of the total injection coal.

The Corus coal requirement by source (Fig 2) is only determined

one year ahead (currently for 2004/2005) as there may be unforeseen supply problems. There have been several problems in the recent past with coal supply including China becoming an importer of coal (and not an exporter) due to increased steel production, various problems with Australian mines and Canada’s cold winter causing transport difficulties. The highest proportion of Corus coal is from Australia (49%). Of this, 40% is for carbonising, the remainder being used in South Wales for injection. Australian coals are preferred because the Australian coking coalmines are some of the largest in the world with very large seams of 5-6m depth, making it cheaper to mine. The disadvantage is that the shipments take 7 weeks to reach the U.K. The next largest coal supplier is the U.S. (22% of total Corus coal buy, mainly high volatile coal). The U.S. has smaller seams of


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approximately 2m (max). U.S. mines have the advantage that the U.S. is closer to the U.K. than Australia, with shipments taking 2-3 weeks. This means less oxidation of the coal during transportation. 17% of Corus coal comes from Canada. This is used for both coking and injection. South American mines supply 11%, which is all injection coal and is not suitable for coking. Only 1% comes from Europe. This is in part because of long-term customer supplier relationships - for example, German mines tend to supply German plants. Poland also seems to prefer to supply locally and problems have been encountered with reliability of the supply. There are no really viable coking coalmines in the U.K. The geology dictates that the coal would be expensive to mine and it tends to be particularly high in sulphur so would exceed environmental limits and would affect steel properties.

For Corus as a whole, the blend proportions of the various coal

types are : 51% medium volatile, 29% low volatile, 20% high volatile (Fig 3). The blend proportions at Ijmuiden are slightly different, in that they use more high volatile coal in their blend with less medium and low. This is because they have a market for the gas produced. As the blend volatile increases, the amount of byproducts produced also increases. Teesside operate with a lower blend volatile content, using more low volatile coals and less medium volatile coals to produce a greater coke yield.

Ijmuiden prefer high volatile, semi-soft coals for blast furnace

injection while Scunthorpe favour medium volatile coals. Medium volatile coals are easier to crush (crushing is one factor that limits injection rates at Scunthorpe), the blast furnaces operate well on them, they have a high value in use and because they are relatively soft there is less wear and tear on plant when compared to high volatile coals. South Wales tend to favour low volatile, semi-soft coals because of cost.

The aim of blending coals is to produce coke with the correct

chemical and physical properties for optimum blast furnace operation, while maintaining battery fabric, all at a competitive price.


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Chemical properties – Ash - More ash in coke means less carbon for fuel. Some ash

however is needed for blast furnace operation. Sulphur - It is very important to control the amount of sulphur

as it affects steel properties and has an environmental impact. There may be consent limits for H2S in the end gas, for example.

Phosphorus - This affects iron and steel quality. High

phosphorus content tends to make metal brittle. Alumina - There is a limit on this for slag quality. Slag is used

in road making and alumina content is important to our customers. Alkalis - Can attack refractory brickwork in blast furnaces and

coke ovens. It also attacks coke during the process, significantly affecting coke strength and causing breakdown of the coke in the blast furnace.

Physical properties are critical to blast furnace operation,

particularly at high levels of coal injection. Mean size - The arithmetic mean size is typically around 48-

49mm for Dawes Lane Coke Ovens. It is calculated from the size fractions obtained from screening a representative sample of coke through a series of sieves.

Micum 40 (M40) - This is a measure of the physical strength of

coke which is essential to blast furnace operation. It is the percentage of coke from a dried representative sample that is above 40mm after being rotated 100 times in a standard micum drum.

Coal selection has a significant impact on the coke oven battery

fabric. The two key concerns are centre charge pressures and pushing forces. Centre charge pressures (CCP) are a function of the coal type and blend. The CCP is the amount of force exerted on the Battery walls


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as the coal is being heated and passing through a plastic state as it is changing into coke. The pressure is measured using probes inserted into the oven chamber after charging and blends are generally selected to maintain this level below 1 psi. There are a number of instances over the years of severe damage being sustained by oven refractory as a result of excessive CCP’s. One American plant had to rebuild ovens after only 4 years’ operation as a result of using a particular high-pressure coal.

Pushing force is the resistance of the coke to be pushed out of the

oven. It is measured by an instrument mounted on the pusher which measures the torque on the pusher beam drive shaft during the push. Pushing forces are affected by coal type and blend, oven wall deformation, carbon build up on oven walls and battery heating and schedule control. At Dawes Lane, pushing forces above 20 tonnes are logged as significant on the shift report. A pushing force of 50-60 tonnes would result in a “sticker”. Carbon growth can be removed by draughting the oven for a cycle and is controlled by correct filling of the oven chamber, good control of the heating system and of the bulk density of the charge, avoiding excessively low coal moistures and schedule control.

Different types of coal give different CCP’s and pushing forces, so

both are monitored as the blend used can adversely affect the ovens over a period of time. These are of overriding importance, but blend cost and coke quality must also be considered. Currently coal prices are at a very high level and blend costs demand attention. However, coke quality, battery integrity and effective operation must all be maintained and cost cannot be the only consideration..

There are two methods for blending coals, bed blending and silo

blending, both of which are practiced by Corus. Redcar (Fig. 4) and Ijmuiden use bed blending. The advantages are relatively low capital costs and the fact that an unlimited number of components can be used if required. The disadvantage is that the blend is unable to be changed at short notice as 2-3 weeks supply will already be blended and will need to be used.


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Scunthorpe (Fig 5) and Port Talbot use silo blending. The disadvantages are a relatively high capital cost in comparison to bed blending and the number of components are limited to the number of silos available. The advantage however, is that the blend can be changed immediately if required and is more flexible.

Silo blending at Scunthorpe Coal is delivered by train from Immingham Bulk Terminal and is

off- loaded at the “Merry go Round“ (MGR). The coal is taken via the reception route and is stacked out using one of two stacker/reclaimers(Fig 6) . Wherever possible the same coal type is stocked within reach of both reclaimers. The coals are reclaimed and transferred via conveyors to one of the silos. There are 2 banks of 9 silos at Scunthorpe, A and B side. A side is used for carbonising and B is used for Injection blending (also used as a standby for carbonising if needed). The coal is taken by constant rate feeders (CRF) from each silo in blend proportion. CRF operation is controlled by a SCADA system. The blended coal passes to a crusher where the coal is blended/mixed and crushed to 83% -3.35mm at a rate of 700 tonnes/hour. The coal is then distributed via conveyors to both coke oven plants. hour and is then transported by lorry to the Coal Preparation Plant where it is dried and crushed further. Pre-crushing the

Bed blending at Teesside Coal is unloaded from the ship and is stacked out onto individual

stockpiles using a stacker/ reclaimer. One coal at a time is reclaimed and is stacked out via the surge bunker onto the blended coal beds to a specific depth/tonnage. This process is then repeated with layers of different coals until the composition of the bed is correct. When the coal is required, a barrel reclaimer moves along the length of the bed mixing the coal as it reclaims. The coal is then crushed and transported by conveyor to Redcar Coke Ovens or to South Bank Coke Ovens by lorry.

At Scunthorpe, a model is used to predict the coke properties and

yields from an inputted coal blend. (Table 1). This model is run when developing the standard blend for the year or used for major blend


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changes e.g. for trialling new coals. It is not normally used for routine blend changes. The properties of the coals, operating details and byproducts yields are entered and the model gives a prediction of coke quality and properties, blend properties and yields. The M40 prediction is based on test oven work carried out on Carboniferous and Permian coals and would be inaccurate if blends containing Cretaceous coals were being modelled.

Two important coke quality parameters which are monitored daily (2 results per day, 6x2 & 2x10) are M40 and mean size. If coke quality results deviate too widely from the mean figures (Table 2), then corrective action will be taken at the blast furnaces. This entails reducing coal injection rates which is very expensive, resulting in a reduction in iron output and an increase in coke rate.

Table 2 Limits on M40 for DLCO Coke Mean Caution Outer Micum 40

82.5 81 80

Mean size

47.5 44.3 43

The “normal operating” coal injection rate in Scunthorpe is

approximately 160 Kg/thm. If 3 consecutive plots lie beyond the caution limits stated above, then the coal injection is reduced to 145 Kg/thm. If one plot lies beyond the outer limit, then again, the coal injection is reduced to 145 Kg/thm and extra samples are requested. If the next plot lies beyond the outer limit, the coal injection is reduced to 100 Kg/thm, if it is between the caution and outer limit, the coal injection is increased to 145 Kg/thm. Reducing injection rates entails increased cost as the coal must be replaced by coke which costs more and furnace productivity is affected. It is therefore critical that coke quality is maintained within the given specification wherever possible.


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The coke quality is monitored by plotting the results on a chart (Fig 7). There are a number of factors which may reduce the level of M40. High volatile coals in particular may oxidise in storage. Blending errors may result from incorrect identification of coals at the stocking and blending stages, or from problems with CRFs. Specific coals may fail to perform as predicted or there may be significant variation in quality between different cargoes of the same coal type. There is also the possibility of reclaiming old coal or bed base material when clearing a stockpile.

The M40. test comprises taking a 25 Kg representative sample of

dried, +50mm coke. It is rotated in a standard drum for 100 rotations (25rev/min) and the sample is then resized. The M40. figure is the percentage of coke > 40mm. This test is carried out in duplicate and the figures averaged. If the individual results are significantly different, then a third test is carried out. Significant deviations in mean size for a given blend are more likely to be an effect of battery operation rather than being blend related.

It is of critical importance that in addition to giving acceptable coke

quality, the coke produced from the selected coal blend must be able to be pushed from the oven with an acceptable pushing force. The CCP produced by the blend must be kept below 1psi to ensure that excessive damage to the battery fabric does not occur and so prolonged battery life. CCP’s ( Fig. 8) increase with rank and reflectance, with high volatile (low rank) coals (0.9-1.1% reflectance) producing the lowest CCP and low volatile (high rank) coals (1.4-1.8%) the highest. Australian coals on the whole give lower CCP’s than similar rank U.S. coals. However, U.S. coals usually produce coke with superior physical quality. Selection of a blend will therefore involve some measure of compromise between the imperatives to ensure good coke quality and to minimise pushing forces and CCP’s. The long term future of the battery must, however, never be compromised

In 1998, against a back drop of increasing U.S. coal prices and

decreasing Australian coal prices, a project team was set up with following remit: “To ensure that the coal blending cost at Scunthorpe is at least equal to the best in Corus. Coke quality and oven security


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must be maintained.” At the time U.S. coal prices were increasing partly as a result of environmental legislation, this made it more attractive for the Utility Companies to use the coking coals (low suphur) for steam raising, this in turn meant that whereas the markets for American coking and steam coals had previously been relatively separate they were now in competition. However, Australian coking coal prices were decreasing partly because new mines were opening and partly because prices on the world coking coal market were in decline and unlike the USA there is no significant home market (either coking or steam) to sell into.

In 1998, the Scunthorpe blend was predominately USA coal and

with additions of super crushed breeze (<0.5mm) which was used as a CCP moderator and an aid to coke quality. However, super crushed breeze is very expensive, in the region of twice the price of coal. The Scunthorpe blend was therefore relatively expensive compared to other Corus sites. To get from the 1998 blend to the recommended blend, took 9 months of intensive trials. The recommended blend (Table 3) consisted of entirely Australian coals except for the high volatile proportion which was a USA coal. The USA high volatile coal was preferred as past experience at Scunthorpe with Australian high volatile coals had shown coke quality suffered as the Australian high volatile coal oxidised over time.

. The effect on coke properties (Table 4) resulting from the change to

the ‘recommended’ blend were as follows:- Ash - Still well within specification, although increased slightly.

This was not so significant as to adversely effect blast furnace operation.

Sulphur - Decreased, giving improved steel quality and reduced

environmental impacts. Alumina and phosphorus both increased slightly but were both still

within specification.


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Alkalies - Decreased, giving reduced attack on refractory brickwork and coke within the blast furnace.

The physical properties of the coke critical to blast furnace operation

were not affected by the change to the recommended blend. Centre Charge Pressures were relatively unaffected, they may have

even dropped slightly, even with the removal of the supercrushed breeze.

Experience over the last five years has confirmed the benefits of this

work. The blend in use currently at Scunthorpe is similar to the recommended blend established in the project work in 1999. The one exception is that one of the Australian medium volatile coals stopped being used because of a deterioration in quality combined with a price increase when the mine was taken over.

With the current high demand for coking coals on the world markets

and production problems at a number of mines, there have recently been significant supply problems. This has meant that on occasion the optimum blend has not been available, but it has been possible to maintain an acceptable level of coke quality using available coals.

Future work on the blend may involve reducing ash content to get

more carbon and reducing the phosphorus slightly to benefit iron and steel properties. However with the current availability problems the main challenge is in maintaining an available blend which meets all of our requirements.


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Coal Blending Paper

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