Travelling by car, train or bicycle, playing a game of tennis or golf, or racing down a ski slope on a pair of modern carbon fibre skis – without coal, none of these things would be possible. Few people realise that when coal is coked the process creates a range of by-products that are used in the manufacture of various everyday items. How exactly is coke created from coal? How, where and when did people start converting coal into coke? And would it be possible to substitute coke and its chemical by-products in the steel, foundry or chemical industries with some other raw material?
The first patent for the "coking" of coal was granted in England in 1568, and the first coking licence, issued in 1590, described the purpose of coking as “to purify pit coal and free it from its offensive smell.” That principle remains, and coking can be simply characterised as ridding coal of its volatile compounds. "The resulting coke consists of carbon, a measure of ash material and a maximum one percent of residual volatile compounds,” explained Petr Mokroš, Deputy Director of Production at NWR’s coking subsidiary, OKK Koksovny (OKK).
A brief history of coking in the Czech lands
From the 16th century shortages of firewood for use in iron mills became commonplace across Europe. The desulphurising of coal in kilns originated in the Czech lands under the reign of Rudolph II at the turn of the 16th and 17th centuries. However, the Thirty Years War cut short most of the early experiments in this field of economic activity. By the end of the 18th century, attempts were being made, with various degrees of success, to produce coke in kilns for the production of iron.
The dawn of 19th century was marked by a dynamic expansion of coal mining and iron production. In 1830 in the town of Vitkovice (now a district part of Ostrava) construction began on the first blast furnace to use coke both as a fuel and as a reducing agent. In 1843 the Rothschild family became the owners of the Vítkovice ironworks, with the purchase agreement citing 26 English-type coke ovens (one being comparable to just the coking chamber of a modern-day coke oven).
In the Ostrava part of the coalfield, coke ovens were usually situated next to the mines. The construction of the Emperor Ferdinand Northern Railway, of which the main stretch between Vienna and Ostrava began operating in 1847, was a key influence in the development of the local metallurgy and coking industries. For a time, coke was the fuel of choice for steam locomotives. That is why coke ovens were built next to every important railway station, with a total of 18 in the Ostrava region by 1850. Coke constituted 42 percent of the overall amount of fuel for locomotives. In time the use of coke was gradually replaced by coal.
All operations involving coke ovens during that period were conducted by hand and were very physically demanding. The ovens were built from hard-baked brick, while the parts directly exposed to the heat were made from refractory materials (firebrick).
Historical records show that Vítkovice steelworks had 64 Gobiet-type coke ovens (horizontal flue ovens with conduits in the dividing walls for the exhaust of gases) producing more than 22,600 tonnes of coke annually. There were also coke ovens located within the coalfield near mines such as the Anselm, Karlina, Hlubina and Terezie mines.
“The Ostrava-Karvina coal field was the coke production base for the Austro-Hungarian Empire. We can say that it also ranks as such a base for this country today: the only coke ovens in the Czech Republic are in the Moravian-Silesian Region. Apart from ourselves, the steel giants ArcelorMittal and Třinecké železárny operate coking plants in the region, but they use the coke they produce as a reducing agent for their own blast furnaces,” said Mokroš.
In 1885 Vítkovice steelworks' coking plants were modernised, making it possible to capture the valuable chemical by-products that arise in the coking process. At the time, coke production in the Ostrava-Karvina coalfield supplied 75 percent of demand in the Austro-Hungarian Empire. Moreover, thanks to the Vítkovice upgrade, a coking plant became the first such facility to meet the entire growing consumption of local blast furnaces. Over 430,000 tonnes of coke were produced in the coal district that year.
Coke in the Austro-Hungarian Empire
With the further development of coking facilities, the proportion of coal destined for conversion to coke increased, rising from 13 percent in 1882 to 38 percent in 1917. At the turn of the 19th and 20th centuries the Ostrava-Karvina mine district boasted more than 830 coke ovens that in 1900 produced more than 1.1 million tonnes of coke.
||Read about the new coking
battery of OKK Koksovny
in Open Mine 2/2011
Chemical products released in the coking process, such as coal tar, ammonium sulphate and benzene, were gradually harnessed and put to use. “The volatile compounds released in the course of the coking process are given off in the form of gases. Initially the compounds were simply emitted into the air. Only at the turn of the 19th and 20th centuries did the utilisation of these chemical by-products start. At the time there was no question of the kind of strict production controls to limit the impact on the environment that we are now familiar with,” said Mokroš.
Coke demand grew rapidly: in 1917 the coking plants of the Ostrava-Karvina coal field produced more than 2.5 million tonnes of coal in over two thousand coking chambers, while the coking plants in the region employed more than 4,300 workers.
The predecessor of the present Svoboda coking plant was built in 1908 and was acquired in 1911 by the Emperor Ferdinand Northern Railway. In 1917 it produced 505,500 tonnes of coke. It is this Svoboda coking plant, now owned by OKK, which is currently the only independent coke producer in the Czech Republic, selling its coke to external customers.
Different kinds of coal, better quality coke
It is impossible to produce high-quality coke from one grade of coal. “In the coke industry the rule of indirect proportion applies. For example, coke of a higher grade is produced out of eight different kinds of average-quality coal rather than out of three kinds of premium-quality coal with excellent parameters. Coal has variable properties and is never the same even if it comes from different parts of the same mine. We, on the other hand, require the quality of coke to be stable. We therefore need as many different types of coal with the right parameters as possible. It is thus of great advantage for us that OKD has four separate mines with coking coal. That is why we take most of our coal from here and only need to import a minimum amount from the US or Poland. The critical threshold for the production of high-quality coke often mentioned by experts is five to six different kinds of coal,” explained Mokroš.
Dependent on the properties of the individual types of coal, a mix is composed and is then ground to a point where 90 percent of the grains are less than 3 mm in size. To achieve this, OKK deploys hammer crushers with hammers freely rotating on a shaft. The coal is pulverised and only properly ground grains seep through the mesh and out of the machine. In order to ensure the coke's constant quality the coal mixture is homogenised prior to its transfer to the coal towers, where the mix is prepared for the coking process.
The coal mix is tamped down to a density of 1,100 kg per cubic metre before it is pushed into the coking chamber. Then the chamber is hermetically sealed and connected to the coking plant's chemical section, and the coking process begins.
Each chamber in the coking plant holds 20 tonnes of coal mixture. During the coking process, 20–25 percent of the coal's mass finds its way into the chemical section of the plant – mainly as coke oven gas, which is subsequently reused as the fuel that powers the process. At the end of the coking process, 14-16 tonnes of coke result from the original 20 tonnes of coal.
“The technically clean coke oven gas, also known as coal gas, used to be the first kind of gas people turned to for illumination and heating. In the former Czechoslovakia it was as late as the 1980s before coke oven gas was phased out completely from all households and replaced by natural gas,” recalled Mokroš.
Coking takes place at a temperature of 1,100–1,200°C. The walls in the coking chamber gradually heat up the coal mass, which at 250–300°C starts to break down. At 350–400°C the coal enters a plastic state in which it practically liquefies, losing gases at an intensive rate. At temperatures of 450–500°C the mass re-solidifies, turning into semi-coke. At temperatures above 500°C the degasification enters its final stage, with the coal mass contracting and coke-formation setting in. The entire process is completed the moment that the plastic layers with a thickness of 10-30mm, which have formed adjacent to the oven's walls, move towards the centre of the oven where these layers eventually meet. The temperature at the centre of the oven reaches approximately 950°C, while it can reach up to 1,200°C at the walls. The coking process takes 32–38 hours.
The coke is finally pushed out of the coking battery, quenched by water spray and cooled to below 100°C, after which it is moved to a wharf for 20 minutes of further cooling. Chunks of coke larger than 70mm pass through a screening station and loaded onto railway cars and sent to customers. Smaller pieces are sent to a fine screening station and, after classification, are stored in separate containers before being delivered to customers.
In the rough screening station, coke is separated on screen grates or vibrating screens. Smaller grains are dispatched to the fine screening station for separation into breeze coke (0–10mm), pea coke (10–20mm), nut coke II (20–40mm), nut coke I (40–60mm) and heating coke (60–90mm).
OKK Koksovny produces several types of coke
Foundry coke is used in the production of cast iron and insulating materials. It is typical for its high strength, lumpiness and abrasion resistance.
70 –130 mm
60 – 90 mm
Blast-furnace coke is used as a reducing agent and a heat source in blast furnaces, and as a bearing and filling material ensuring gas circulation in the blast furnace charge.
40 – 90 mm
25 –90 mm
An environmentally friendly fuel due to low contents of noxious combustion products.
< 40 mm
Nut 1: 40 – 60 mm
Nut 2: 20 – 40 mm
Pea: 10 – 20 mm
Dust: 0 – 10 mm
|Chemical products of the coking process
Coke-oven gas, tar, benzole, ammonium sulphate, solid and liquid sulphur.
Foundry coke: added value
More than 500 million tonnes of coke are produced annually in the world, 99 percent of which is blast furnace (metallurgical) coke. The simple principle underlying the blast furnace has in the course of time proven indispensable for the production of crude iron in terms of both cost and environmental impact. That is why the blast furnace method will preserve its dominant position for the foreseeable future.
OKK, however, specialises in the production of foundry coke, of which 1.3–1.5 million tonnes a year is used in Europe. With an output covering one third of European demand for foundry coke, OKK is the continent's biggest producer.
Mmetallurgical coke is distinguished from foundry coke by, among other things, grain-size. Whereas the size of a lump of metallurgical coke stops at 90mm, the size of foundry coke starts with lumps of at least 100mm. Metallurgical coke is, moreover, produced under different conditions (a higher temperature) and from other kinds of coal, usually with a higher content of volatile compounds. That is why lumps of this type of coke fail to make it to sizes as large as those associated with foundry coke.
The function of metallurgical coke as a fuel is diminishing. This coke is primarily used in blast furnaces as a reducing agent and it also secures gas permeability across the entire height of the blast furnace. Foundry coke has a higher real density, a lower porosity and acts as a fuel which, when it touches a piece of cast iron, should not break up.
It is generally the case that foundry coke fetches a higher price on the market than metallurgical coke. The largest buyers of foundry coke use about 50,000 tonnes a year, so it would not be cost-effective for a foundry to build its own coke ovens.
Historically, coke has been transported to the customer by rail. However, in the course of time many smaller companies have abandoned their factory rail sidings so nowadays, in most cases, transport is by road. A vast proportion of the foundry coke produced by OKK is exported, particularly to Germany.
A factory within a factory
About 20–30 percent of the total mass of the coal used as raw material for coking consists of volatile compounds. These are further processed in the chemical part of the coking plant and are subsequently used in a variety of industrial sectors.
The most important by-product of the processing of coal is gas that is purified step by step after being released from the coal and in the process is fractionated to recover ammonia, benzene, hydrosulphate and other fractions. Thus, the technically clean coke oven gas does not burden the environment when burned. OKK's operations have been heavily desulphurised to a value of 50mg of hydrosulphate per cubic metre. “This is a value 10 times lower than that set by law. More than 50 percent of clean coke oven gas consists of hydrogen which turns into water vapour when burned,” said Mokroš. When the gas is burned this predominately results in water vapour and CO2.
Coke oven gas is mainly used to heat the coke oven batteries; any surplus is used to fuel heating plants. Where OKK is concerned this involves the neighbouring heating installation operated by Dalkia Česká republike a.s., which uses coke oven gas in supplying heat to Ostrava households.
Another important by-product is crude coal-tar, of which OKK produces around 25,000 tonnes annually. Through subsequent distillation, a variety of oils are recovered from coal tar, as well as solid coal tar pitch that is used in steel and aluminium production. Soot (carbon black) that is used in tyre manufacturing also has its origins in coal-tar. Crude benzene is likewise an important coking by-product from which, after distillation, there is a recovery of pure benzene, one of the main components used in the chemical industry. When crude coke oven gas is purified, ammonia is recovered which subsequently ends up in ammonium sulphate, used as a fertiliser in agriculture.
Although we may not realise it, every day we come across objects that could only have come about thanks to coal and coke. There is currently no alternative to coal. What is more, coke production is no longer the dirty job it once was. The new coking battery in the Svoboda coking plant, which received its final building approval in spring 2011, fulfils the most stringent environmental requirements. The levels of emitted agents are continuously and directly monitored within the coking battery itself, within the surrounding area and also in the area surrounding the entire OKK complex.