Visiting a coking plant

This blog post accompanies the one I wrote about visiting Scunthorpe Steelworks.  Many steelworks have their own coking plants, but this post is an account of a visit to a self-contained coking plant, which I undertook with a group of sixth form chemistry students when I was their teacher.

Alan Murray-Rust, Monckton Coking Plant - geograph.org.uk - 442347, CC BY-SA 2.0

Location of the plant

The plant in question was the Monckton coking plant at Royston on the outskirts of Barnsley.  It is no longer there, having closed in 2014.  A description I came across when I was checking the closure date states, "...one of the last remnants of the coal industry in Yorkshire."  I can't overstate the historical influence of the coal industry in Yorkshire, and South Yorkshire in particular.  Anyone born after 1990, say, will have few memories of how the industry dominated the landscape and shaped the working class culture of this part of the world.  I was born a while before this, and it was something I simply took for granted.  As a child growing up in Barnsley in the 1970s and 80s it was simply part of the scenery, and hard to imagine it might not always be there.  I could see the pit head gear and slag heap of North Gawber pit from my junior school.  My secondary school stood alongside the railway line to Dodworth pit, and coal trains passed several times a day.  There are some places where the scene in 1980 would literally be unrecognisable now.  One such place is the current Old Moor RSPB reserve, which is now a haven for wildlife but up until the mid-1980s was the site of the huge Wath Marshalling Yard for rail freight, mainly coal across the Pennines.  Nearby the Manvers Business Park occupies the site of what was once Europe's largest coking plant, Manvers, with Manvers Colliery next to it and a far more complex network of railways than is currently in evidence.  Only a few miles away the view from the front of Ecclesfield School was dominated until the late 1980s by the huge Smithywood coking plant across the valley.  Without rambling on too much about this, coal really was king round here for a very long time.  

Monckton was a good place to visit because, unlike Scunthorpe Steelworks, the site could be walked around on foot as it wasn't too big.  It was also quite close to the school where I was teaching at the time, Minsthorpe Community College in South Elmsall.  Hence I could borrow the college mini-bus and a visit to Monckton could be undertaken in an afternoon.  I also had a friend, John, who worked there in a supervisory role which also involved some scientific analysis, so this meant he would be a suitable guide and arrangements for the visit were relatively straightforward.  

The process of coke making

Coke making, as I have written in my Scunthorpe blog post, is heavy industry at its dirtiest.  The dirt can produce the illusion that the process is somehow simple or unsophisticated.  At their heart, most industrial processes, even nuclear power generation, can be made to appear simple on paper, but the devil is in the detail, and though coke making is in theory not a difficult process, controlling it on a large scale, protecting the environment as much as possible and maintaining economic production require a lot of careful planning and plant construction.  

Coal is mainly carbon, but has a lot of other substances in it due to the fact that its origin is prehistoric plant matter.  It is, therefore, a complex mixture of things, and for a long time in the UK it was a valuable source of organic chemicals, before the UK discovered its own crude oil under the North Sea which could be used instead.  This image taken from the Ladybird book "The Miner" gives some sense of the wide range of products which at one time owed their existence to coal.  

Burning coal in air produces oxidation products of the carbon and the other elements in the coal.  These oxidation products mainly include oxides of carbon, sulfur and nitrogen, the last two of which are highly polluting as they are acidic gases and will readily dissolve in atmospheric water, turning it into acid rain.  If the coal is heated in the absence of air, a lot of the 'impurities' can be distilled off, collected and put to use or disposed of in a more acceptable way.  What is left behind is virtually pure carbon, a porous low-density solid that we call coke.  It burns at a hotter temperature than coal, and what's more it does so without any smoke.  Although no longer a fuel for domestic use, it does have specific industrial uses such as in steel-making in blast furnaces.  The coke made at Monkton was, by the time of my visit, used virtually entirely by the chemical company Brunner-Mond (now Tata) to make soda ash, which is the common industrial name for sodium carbonate.  Sodium carbonate is a valuable feedstock for various things including glass making.

To make coke requires coal to be heated to a temperature of over 1,000 degrees Celsius in the absence of air for several hours - I think 18 hours was the duration at Monckton.  It is a batch process, and in order to maintain a flow of production a battery of furnaces, or coke ovens as they are called, are always built together with their operating times on a staggered cycle.      

During the heating in the coke oven the coal actually becomes plastic and swells, and this is potentially problematic.  If it swells too much, for instance, the coke will simply get stuck fast in the oven and ultimately it has to be dismantled which is a very expensive job - the oven is destroyed, effectively.  There are, therefore, only certain types of coal that are suitable for coke-making.  The swelling behaviour, which is related to the moisture content of the coal and other factors, can be quantified by the Gray-King scale, and it's important to know the Gray-King index of a coal to judge its suitability for coke making (or coalite making - see my blog post on my industrial placement).  

Coke ovens

For coke-making to be economical, not only is it desirable to be able to get the coke out of the oven when it's finished 'cooking', but the oven has to be maintained at a high temperature for its entire lifespan - effectively, it has to be always 'switched on'.   The interior of a coke oven has to withstand extremely high temperatures, so it is important that it does not destroy itself through heating, and it's also essential to ensure as much energy goes to the coal as possible, rather than the surroundings.  To ensure these outcomes prevail the inside of the oven is lined with a refractory material which can withstand high temperatures and provide insulation.  What this material cannot tolerate however is repeated heating and cooling; it would simply become cracked and unusable, and the furnace would require rebuilding.  This is why they are always switched on.  

Coke ovens are fuelled by natural gas and gas which is produced in the coke making process itself.  Originally, before North Sea gas was discovered, each large town had its own gasworks (essentially just a coking plant but with the emphasis on gas production) and the gas produced was commonly called 'town gas'.  It is a mixture, but the main flammable components are hydrogen, carbon monoxide and hydrocarbons such as methane and ethene.  Town gas was therefore toxic due to the carbon monoxide content, and it was not unknown for people to commit suicide by putting their head in the oven and inhaling the gas mixture.  Nowadays this would not work as methane is not toxic.  The ethene content caused town gas to have a different side-effect.  It was noticed that trees near gas leaks tended to drop their leaves prematurely.  Ethene has since been recognised as the cause of this, and it has in fact been exploited for a related purpose - to artificially ripen fruit so that its ripeness is timed to coincide with its period of sale in shops.   

Coke ovens are tall and narrow, and built in rows side by side.  One self-contained set of ovens is called a battery, and as already mentioned, these operate on a staggered cycle to maintain a flow of production.  The picture below shows a battery of coke ovens.  Each vertical strip, of which there are many, is an individual oven.

 

It is possible to see some flames at the top of some of the oven doors, where the seal is clearly not so good, and some of the coal is actually burning.   

Each coke oven is filled with coal from above, through a set of manhole-size openings on the top of the battery.  After filling, these openings are sealed shut with a mortar mix.  

A hairy moment on top of the battery

One of the most memorable aspects of the visit to Monckton was when John, our guide, took us on to the top of one of the batteries.  He told me afterwards that he shouldn't really have done this, and he got a bit of a telling off from management afterwards, though not while we were there.  The top of a battery of coke ovens is a pretty hazardous place for anyone to be, and especially a group of sixth form students, albeit pretty well-behaved ones.  I'm not sure whether the school's insurance policy would have covered this, because it is somewhat beyond the nature of ordinary hazards encountered on school visits.  The danger became all too apparent as we walked across the battery top, specifically because one of the coal filling holes was open.  The battery top was quite a large flat area, and this opening was not immediately apparent, just as you would not notice a manhole cover missing if you were walking along a street, until you were quite close to it.  However it became visible as we filed across, and we could see the bright orange glow from inside, and feel the heat from the oven.  There was no barrier, and I still shudder at the thought of what would have happened if we'd not been paying attention, or if somebody had started chasing around, being silly.  It would have been so easy to just drop into the coke oven.  There would have been one fewer person on the minibus back, basically.  Except of course we'd have not been using the minibus if a student had dropped in the coke oven, because I'd have definitely been too traumatised to drive back to school.  Kids do die on school trips, fortunately very rarely, but this would have unquestionably made headline news and have been a most gruesome way to die, albeit quick.  I think it would have been the end of my teaching career.  

Production and environmental challenges

There is another illusory aspect to the dirt of heavy industry, and that is that the process must be highly polluting and environmentally very damaging.  Well, I'm not going to pretend that coke making is good for the environment, but I was always very impressed just how far these industries actually go (or went) in controlling things like harmful emissions and in reducing unnecessary waste.  (For more examples of this, see my blog post on my Teacher Industrial Placement).   One of the more challenging aspects of coke-making is dealing with the quantity of gas (the 'coke oven gas') which is produced.  This is hard to predict for any given sample of coal.  When the quantity of gas is modest it can be fed straight back to the coke ovens as fuel, or stored in large 'gasometers'.  When the quantity is large, it is produced at a rate which exceeds the ability to do either of these things, and as it would be unsafe and environmentally unacceptable to simply vent it to the atmosphere, there is no option but to 'flare' it.  This is the process often observed at oil refineries or platforms, in which there is a thin chimney from which emanates a large orange flame.  This is simply excess flammable gas from the industrial process, which cannot be economically used or stored.  The flare is lit by firing a flare gun, similar to the type used by ships in distress, into the gas stream from a safe distance on the ground.   According to the Wikipedia page, the coke ovens at Monckton produced up to 276,000 cubic metres of gas each day.  Monckton actually went one step further in using this, and had its own small power station in which it was burnt, and feeding electricity into the national grid.

When the coke is pushed out of the oven it emanates as a spectacular flaming cascade of red hot solid material.  It lands in a special heat-resistant wagon, called a coke car, that runs on a short railway track at the side of the battery.  The wagon is slowly pushed by a locomotive so that the coke spreads out along the length of the wagon.  Once the push has finished, the burning coke is then pushed further to the nearby quenching tower, where a large quantity of water is unceremoniously dumped on it to put out the fire and cool it.  This sends up a huge white cloud of condensing water vapour, visible from miles away, and a handy signal, from a distance, as to what has just happened. 

When we do things at home we tend to take it for granted that we can just put things in the dustbin, flush the toilet, or rinse stuff away down the sink.  This is an easily overlooked facet of an industrially advanced society.  Do we ever think about where this stuff ends up?  Industrial plants such as Monckton, producing waste of a quite different nature to domestic households, cannot do what we do at home.  They have to be very careful to monitor their waste, because just rinsing it away into a drain could cause a lot of environmental damage.  Hence the water used to quench the coke, as well as that used to 'scrub' the coke oven gas before it was used or flared, had to be monitored so that an appropriate disposal method could be chosen.  Common substances tested for were ammonia and sulfur compounds including thiocyanates.  Too high a concentration of these would render the water unsafe to discharge into a culvert for instance, as it would find its way into local rivers and threaten ecosystems.  The testing to determine the concentrations of these things was done using specific titration techniques, in principle no different to the ones carried out by school students, and using mostly the same equipment.  Should the concentrations be too high then often the only satisfactory solution was to send the effluent to storage ponds.  This is still how a lot of industrial effluent is dealt with.  The water containing the undesirable materials sits in a pond which is situated on top of ground which ensures it will not soak too quickly into natural water courses.  This might be an old mine slag heap, for instance.  The water will eventually percolate through the ground, but the act of filtering through the very thick layer of other material will result in the chemicals either becoming less harmful as they react with other naturally occurring chemicals, or being effectively locked into the ground itself.  This can, however, result in land such as this being considered 'contaminated', and it is one of the reasons why 'brownfield' sites need careful consideration before having houses built on them for example.   

Concluding thoughts

I have conflicting views on the coal industry.  Having grown up surrounded by it there is an inevitable nostalgia for sights, sounds and smells which have now disappeared.  More objectively, we're better off without them, and green spaces are much better than slag heaps.  There is little to be said for burning coal from an environmental perspective.  Hence, I think Britain is better off without coal in the long run.  What I think we shouldn't forget, however, is how much we owe to coal - economically and culturally.  Coal fuelled Britain through the Industrial Revolution, and helped make it the country it was and is.  Many towns and villages across the land are only there because of coal, and many of their traditions reflect a common origin in coal.  What this sheds light on is the human dimension of the coal industry, and it shouldn't be overlooked what a gruelling and dangerous job being a coal miner was.  I don't think we should ever forget the lives lost to coal, such as those in the Oaks Disaster of 1866.  I wonder how many people, even if they notice the memorial at the side of the A635 on the way out of Barnsley, know what it commemorates  - the lives of 361 miners who died in one of Britain's worst industrial disasters.     

I still teach about coal and coke when I'm teaching about fossil fuels, even though crude oil is the big focus on the formal curriculum.  Coal doesn't get a mention, perhaps understandably.   Quite recently a mature student from London told me he had never even seen a piece of coal.  This would have been unthinkable a couple of generations ago.  I brought a piece for him to see.  It's not surprising either that most students today have no idea what coke is - they think it's either something you snort or drink from a can.   I think it's important, and interesting for them, to know about the other sort.