Cooking and carbonisation plant
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Cooking and Carbonisation Plants as a class are subject to co-ordination arrangements as outlined in the relevant Practice Note to this Section.
2.1 History of the industry
The year 1735 saw the successful use of coke in a blast furnace and since then the coke industry has developed in conjunction with the iron industry.
Early attempts were made to heat coal in open mounds which allowed the volatile matter with its valuable by-products to be lost.
Enclosed ovens of a beehive shape were designed circa 1860 but it was not until 10 years later that serious attempts were made to recover by-products. The basic design of the battery oven had evolved by the early part of this century and with improvements and modifications has continued to be built in similar form up to the present day. From 1952 most major steelworks expanded their own coke oven plants so that by the early 1960s the capacity of the cooking industry as a whole far exceeded market demands. This coupled with many industrial users converting to oil firing of boilers further depressed the market and as a result the NCB embarked on a planned demolition programme starting with the small uneconomic plant.
The discovery of natural gas and its subsequent availability to the domestic user has led to a steady decline in the demand for domestic coke. The more recent introduction of other new types of smokeless fuels such as Phurnacite and Ancit has further increased this decline.
The demand for blast furnace coke is also being reduced through the recent introduction of the technique of direct coal/oil injection into blast furnaces.
This has resulted in a gradual reduction in the coke production capacity through plant closures throughout the 1980s. By April 1988 considerable excess capacity existed in the industry which has resulted in an acceleration of plant closures. See the relevant Practice Note for details of the state of the industry as at the time of Revaluations.
2.2 General
The object of coke manufacture is to convert coal, which has caking properties unsuitable for use in blast furnaces or foundry cupolas, into a fuel which is non caking, rich in fixed carbon and of a suitable size and hardness.
Coal is crushed and graded into small pieces and “cooked” at high temperatures for a fixed time. The product is rapidly cooled thus forming coke. The end use of the coke determines the method of production. This can be divided into 3 main groups:
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Blast Furnace Coke.
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Industrial/Domestic type coke (Boilers).
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Domestic type coke (fires).
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Blast Furnace Coke
Requires to be made from hard coal and none is available in the UK. This coal is mainly imported from Australia, USA and Poland. Coal is cooked and screened to the size of “pea grit” and fed into a blending bunker. The charge car hopper running on top of the furnace takes a fixed charge from the bunker and positions itself over the oven. Usually 4 filling holes over a distance of 16m (approx). Note the deposited charge is not level in the oven. The ram car top pusher levels the oven charge. Gas is mixed with air and burnt in the adjacent flues. The hot gas rises up and across into the next flue which at the bottom exhausts to a waste heat chimney. After a set period of time the gas flow is reversed and the burner flue becomes the exhaust flue and vice versa.
Gas is extracted from the coke and this is taken away for treatment and process. The clean gas is returned to the oven and fired in the heating flues. The remainder goes to the by product plant. The coal “cooks” at 1300°C for 16 hours (varies on the product required) and then the coke side door is removed and the coke car is placed in position. The ram side door is removed and the bottom ram pushes the coke out of the chamber. The oven is some 400mm opening up to 450mm wide at the coke side. 1. Industrial/Domestic Type Coke
This process is similar to the one described above but the ovens are usually smaller (ie. batteries are not as large) but the oven sizes are similar. 450mm wide x 5.3m high x 14m long. The coal used is mainly UK coal for the end product requires some tars left and the softer UK coal is ideal for this. The temperatures reach only 800°C with a cooking time of about 16 hours. 1. Domestic Type Coke (Coalite)
This process takes place in a retort which consists of approximately 22cm diameter tubes fixed together in a cast iron mould. The retort is some 8.3m high and set in a fire brick surround in groups of 40. The product requires less working time as many of the tars are retained. Temperatures may be as low as 450°C and working time is some 4 hours.
2.3 Cooking Plant Process
General
The Guide to Referencing and the Table and Diagrams (Appendices 1, 2 & 3) at the end of the section should be studied in conjunction with this section.
Section A - coal preparation
Coal is normally received having already been washed and crushed to under 6mm mesh at coal washery plants and is brought to the site by rail, road or direct conveyor systems. The coal intake point will usually consist of a tippler with a hopper feed to belt conveyers which distribute the coal to rod mills for further crushing before passing to the blending bunkers or to the service bunker. Where coal is still taken directly from a mine (“run of mine coal”) both primary and secondary crushers are used together with screen houses, and possibly a coal washery plant, but this is rapidly becoming an obsolete practice making much of this plant redundant.
Section B - blending
Coal received at the blending bunkers via conveyor systems from the stock pile or intakes will have been crushed to 3mm mesh or under. Blending bunkers consist of a series of reinforced concrete silos each of about 200 tons capacity served by overhead shuttle conveyor in a reinforced concrete housing. Each silo has a hopper bottom fitted with a scraper discharge on to a conveyor system feeding the service bunker. As the name implies the blending bunker enables various qualities of coal to be mixed and also acts as back up storage for the service bunker.
Section C - coke oven block
Prepared or blended coal is moved by conveyors to the service bunker which consists of a reinforced concrete structure usually mounted centrally above the coke oven batteries. A bunker may be divided into two compartments each of which has outlets arranged to feed directly into the coal charging car (“Larry Car”). Within the main structure of the bunker below oven roof level will normally be ablutions, mess rooms, switch rooms and control machinery for reversing the gas flow to the ovens.
A coke oven battery consists of a series of ovens normally 46cm mean width built as a unit on reinforced concrete piers and slab. Coal is charged into the ovens through gas tight inlets in the roof of each oven. The coal within an oven is carbonised at approximately 1350°C by transfer of heat through the oven walls from gas burned in the flues which is controlled by a reversible gas flow system. The coal within the oven is baked not burned, so that the volatiles are driven off leaving a red hot mass of coke. The volatiles in the form of raw gas are taken off at roof level by ascension pipes into a collecting main for processing in the primary by-products plant. Waste heat after passing through the regenerators is discharged to atmosphere via flues and chimney. Discharge is by ram which rushes the coke through the oven into a coke car which is then moved to a quenching tower for cooling by water sprays.
Ovens are fired by cleaned coke oven gas (rich gas) or by producer gas or, at iron and steel works, blast furnace gas (these being lean gases). The calorific value of producer and blast furnace gas is about one-fifth that of coke oven gas so that considerably more lean gas must be burnt to produce the same heat and it follows that ovens fired by lean gas will have flue and regenerator arrangements constructed differently from those of rich gas ovens. Hence, single fired ovens (rich gas) or dual fired ovens (rich gas or lean gas or in combination) will be found. There is no substantial visible difference in the outward appearance to distinguish oven construction although some indication can be gained from the damper arrangement in the gas galleries and gas mains, but the method of firing should be ascertained by on site enquiry.
The life of conventional coke ovens was originally about 20 years and the NCB in 1971 started a rebuilding scheme as most modern type ovens were built from 1951 onwards. At that time the gas supply in the UK was based on gas produced by coke ovens at gas works or at separate cooking plants, and to conserve and sell rich oven gas, an alternative method of under-firing the ovens by producer gas (lean gas) was used and the regenerators and under-flues were constructed for this purpose. Following the introduction of natural gas the demand for coke oven gas for domestic use has ceased and all coke ovens (other than at steelworks) are now fired only on rich oven gas produced by the plant; surplus gas is either burnt to atmosphere or where possible supplied to industrial users by pipe-line.
Another modern practice is to build a small service bunker with large blending bunkerage compared with older plants where very substantial service bunkers exist, and comparatively small blending bunkers. Future practice is directed towards feeding pre-heated coal directly into ovens and so dispensing with convential bunkerage.
Section D - coke screening
After initial cooling in the quencher tower the chilled coke is spread on a sloping reinforced concrete structure with fire brick surface known as a coke wharf. When sufficiently cooled the coke is moved by conveyer to a screening plant for sizing and separation of breeze (small coke usually 16mm to 9mm and below). From the screening plant the sized coke is passed by conveyor to storage hoppers for loading road or rail vehicles or transfer to stockyard. Breeze is normally transferred to the power plant for firing boilers.
Section E - primary by-products
Raw gas must be cleaned before it can be used and the following processes are essential and will be found at all coke oven plants.
The volatile products of carbonisation pass from the ovens via the ascension pipes where a spray of weak ammonia liquor is introduced to reduce temperature and assist in the condensation of tar. The coal tar and ammonia liquor contained in what has now become saturated gas settles out in catch tanks for separation and recovery later.
The gas is drawn by exhausters through a primary system of water tube coolers which further reduces temperatures from about 85°C to about 25°C and then to ammonia saturators or scrubbers and napthalene washers each of which extract feed stocks for further processing. Modern practice tends to use electrostatic detarrers in the gas stream before passing to the exhausters. Up to this point the exhausters have drawn raw gas from the coke ovens and they now pass it under pressure through the remainder of the by-product plant.
On leaving the exhausters the gas stream passes through final coolers and benzole scrubbers. Both coolers and scrubbers are large cylindrical columns packed with wooden hurdles in which the gas is forced upwards against a descending spray. The water spray in the coolers also washes out the remaining tar and naphtha. In the benzole scrubbers the descending spray is wash oil which absorbs benzole from the gas and this “benzolised oil” then passes to the crude benzole plant which extracts benzole in a live steam still and recycles the de-benzolised oil (wash oil) through rack coolers to the benzole scrubbers.
The gas stream on leaving the last benzole scrubber passes to the gas holder, to be used for under-firing the coke ovens, or to further treatment plant in the secondary by-products (Section G).
Section F - power services
Power services at coke oven plants vary from simple Lancashire boilers primarily for process steam to sophisticated multi-purpose water tube boilers providing power for turbo alternators which produce a substantial part of the electricity required in the plant; the pass out steam being used for process purposes (see RM 4:3).
Section G - secondary by-products
The most common use of secondary by-products plant is second stage cleaning of rich gas by purifiers so that it can be sold on the commercial market. At some plants the secondary by-products plant is extensive and is akin to a chemical works for further processing the feed stocks.
Section H - gas producers
Gas producers consist of a form of retort in which coke or coal is heated to a high temperature and then sprayed with water producing a geaseous vapour. The moisture in the vapour is condensed out leaving a lean gas which can then be used for underfiring the coke ovens. Gas producers may still be found at some plants but are now mostly disused as the majority are underfired by rich gas.
2.4 Domestic carbonisation plants
General
Carbonisation plants for the production of domestic smokeless fuel are in essence similar to coke oven plants except that the actual process of carbonisation is as described below. Coal preparation, blending, screening and primary by-products follow the general pattern on coke oven plants.
Coalite plants
The oven blocks, constructed of refractory brickwork, are divided into a number of batteries usually from 6 to 12 in number. Each battery contains 40 retort sections arranged in two rows of 20 across the width of the oven block. Each retort section is of solid cast iron construction and measures approximately 1.15m x 0.41m x 0.23m in length with 12 N° 13mm diameter holes through the length. The ‘tubes’ so formed have a coal capacity of about 330kg. Charging of the retorts is from the top by mechanical charger running across the width of the battery and serving each group of retorts.
Discharging is from the bottom of the tubes on to a chain conveyor leading to a quenching point.
Heat from gasses circulates through multiple flues in each battery and is thus transferred to the retort walls. Firing is by gas produced by the processes. Carbonisation takes place at 650°C and the cycle is 5-6 hours in duration.
As with conventional cooking plants, effluents are drawn off in the process and dealt with as normal by-products, principally crude coal oil and tar distillants.
After quenching, conventional screening and primary by-products processes are used similar to those in coke oven practice.
Phurnacite plant
Phurnacite is a favourite fuel for boilers and room heaters and consists of an Anthracite based brickette produced at Immingham on Humberside by the mild heat treatment process, a completely new method which uses less energy and is environmentally clean. After binding and pressing Phurnacite brickettes are cured in a travelling bed oven. Should information be required on this type of plant enquiries should be made to the SVU.
Home fire plant
Home Fire is a distinctive hexagonal brickette, and is made at Keresley near Coventry from a blend of bituminous coals which produce a smokeless fuel without additives or binders. After crushing and drying the blend is carbonised in a fluidised bed and the resulting “char” is extruded into brickettes, cooled and quenched.
Ancit plant
Ancit is a newer, high heat output fuel made from Anthracite and cooking coals at Aberanan, Mid Glamorgan. No binders are used. The Ancit process involves heating the raw materials in a reactor and at the critical moment mixing and bricketting them with the heated cooking components in a roll press.
A guide to the referencing of cooking and carbonisation plants has been distributed to Technical Advisor’s and to those local offices having hereditaments for which the SVU is responsible. Additional copies of this guide are available upon request to CEO (Rating).
Cooking and carbonisation plants are highly specialised hereditaments rarely, if ever let, and the appropriate method of valuation is the contractors basis (see RM 4:7).
5.1 Plant and machinery
Cooking and carbonisation plants contain a considerable number of items of plant and machinery named in the relevant Plant and Machinery Regulations. See the relevant Practice Notes for details. Rateability should be tested in accordance with the principles set out in RM 4:3.
5.2 Site
Historically coke ovens were sited at colliery pit heads with direct delivery of “run of mine” coal. Quality control requirements have become very much more stringent and few suitable “run of mine” supplies are available. Plant operators are now forced to blend coals from a number of pits so that siting of plants at pit heads is no longer crucial. However the site should be within reasonable distance of a source of raw material and adequately served by road and rail.
5.3 Methods of comparison
Methods of comparison on a broad basis include:-
a. Rated capacity (when figures available)
b. Notional standard capacity
c. Coal charge capacity
Comparison of one plant with another may be made by all or any of these methods and the reason for any variations should be sought. This may well stem from differerences in types of oven, but all salient features of the respective plants should be examined by a more detailed analysis which follows the functional sections referred to in section 2.3.
Rated capacity denotes the optimum output of coke production of the plant, conditioned by the actual carbonisation cycle.
Notional standard capacity is based upon an average cooking time of eighteen hours with a coke product equal to 70 per cent of the wet coal charge (normally 70% of wet coal charge is converted into saleable coke).
The formula for Notional Standard Capacity is therefore:
(70 x Wet Coal Charge ) x (365 x 24) x number of ovens
(100 ) ( 18 )
Coal charge capacity is the tonnage of wet coal charge per oven.
5.4 Case law
Birchenwood Gas and Coke Co Ltd V Hampshire 1959 LT 52 R & IT 226 is the only reported case relating to cooking plants and three points emerged:-
a. The NAV of industrial premises is not affected by the imminence of the undertaking of heavy periodical repairs for which the hypothetical tenant will be liable.
b. When applying the contractors basis any inherent disadvantages should be reflected by a final adjustment of the NAV.
c. The prosperity and prospects of the industry and cost of raw materials are factors which would affect the mind of the hypothetical tenant.
Major works within this class often represent significant rate income to be collected by the Billing Authority who should therefore be informed, in advance, of likely significant changes of value.
Referencing Prefix Letter |
Production Section |
Included Buildings and Plant |
A |
Coal preparation |
(Coal intake and stocking (Washery (Primary and secondary (crushing |
B |
Blending |
(Tertiary crushing (Blending bunkers |
C |
Oven block |
(Ram track (Coal bunker (Oven block (Quenching (Coke wharf |
D |
Coke screeing |
(Coke screening (Storage and despatch |
E |
Primary by-products |
(Colling (Scrubbing (Stripping (tar (ammonia (naptha (crude benzole (Exhauster house (Storage and despatch (Gas holder |
F |
Power services |
(Boiler-house (Power-house (Water treatment and cooling |
G |
Secondary by-products |
(Gas purification (Gas holder (Other fractionating plant (eg. tar plant - batch (distillation |
H |
Producer gas plant |
(Producer plant (Gas holder |
J to N |
Unclassified other activities |
|
P |
Central administration |
|
Q |
Central maintenance |
##Appendix 2 : Cooking and Carbonisation Plant