GPCS – is it the solution for poor coal
December 17, 2016 at 4:50 pm #1324John HindParticipant
Hi All – to start a technical discussion on the message board I am starting this topic and seeing how it goes – both as trial of the message board and a technical discussion.
James Evans in a post a couple of months ago suggested that GPCS was the solution to poor coal quality and it is certainly the mantra among modern steam entusiasts, but is it?.
Please read pages 502,503 and 504 of Dave Wardales book – the Red Devil.
What I have taken away form the book (I have no practical experience of GPCS – other than seeing fires in deep fireboxes full to just under the door) is that when it works its very good, but when it does not, it is worse than conventional firing.
However to work succesfully, it needs the right type of coal, which is not neccessarily poor quality
JohnDecember 20, 2016 at 12:38 pm #1344Jamie KeyteParticipant
I think pulverised fueling is worth another look as it seems to address the issue of poor fuel quality and also some of the issues associated with loco moaintenance (ash disposal) and fire throwing. Some development required to deal with fuel handling or pulverisation on the loco itself.
JamieDecember 22, 2016 at 12:42 pm #1417
I think that the “heat balance” (for want of a better description) between the various chemical reactions in a GPCS grate needs more consideration. My suspicion is that some GPCS grates are being supplied with too much undergrate steam.December 22, 2016 at 8:08 pm #1419Alex PowellParticipant
John, from what I’ve read in Wardale’s book I think you are right, but the reason that it was used was that it started to solve other issues like spark throwing. It was definitely a hindrance on the QJs in China.
I also agree with Jamie that micronized or pulverised firing is worth another look.December 23, 2016 at 8:04 am #1421
I remember many years ago hearing that pulverised fuel was once trialled on the Great Central, but obviously never became widespread. In power stations the coal is crushed using specialised mills, then entrained in a stream of compressed air before being fed into the boiler using purpose designed burners. It sounds a bit complicated for a locomotive.December 27, 2016 at 5:57 pm #1465David PawsonParticipant
The reaction between steam and coal is endothermic so if you blow too much steam through the grate the fire will go out! In the complete absence of any experience or data my guess is that at best you can get no more than 20% of carbon into the gas phase by this process, which defines the maximum flow of steam possible. If anyone has any data I would love to see it. Even 20% will lead to a significant cooling of the fire. Porta as I understand it developed the technology to deal with very high ash coal. If the process keeps the fire below ash fusion temperature that will be a big boon, so you can see why it worked in this context. So if ‘bad’ means high ash, then yes, GPCS could work. And, as we have discussed, this should prolong the life of the grate. But Wardale’s general answer to the ‘bad coal’ question is ‘no’.
A cooler fire will also help reduce unburned coal loss it would seem. I think (having pored, glassy eyed for many hours at the Rugby boiler data) that the increase in unburned losses at very high specific combustion rates (above rates sufficient to allow very 700+lbs/sqft/hr evaporation), is related to the temperature of the fire surface, no hard data on this, certainly no experimentation- the production of char from lump coal is a black hole for data. This would fit with the idea that GPCS reduces unburned losses at high rates. A simpler solution is to increase the size of the grate!
What I would like to know is how a cooler fire impacts radiative heat transfer in the firebox, and whether this also leads to a reduction in superheat. Again, I don’t think there’s data, but if this did happen it would likely negate the effect of reduced coal loss.
I’m not at all sure about pulverized coal. If you pulverize coal in the screw feed of a mechanical stoker, the fines produced disappear up the chimney and boiler efficiency falls. Swindon showed that adding fines to coal also reduce boiler efficiency, because they too went straight up the chimney. So I think we are talking about a very fundamental redesign of the grate and firebox.December 28, 2016 at 10:42 am #1466
I agree entirely with David’s analysis. I am of the opinion that it is necessary to have some form of control system to regulate the quantity of steam being supplied to the grate.
It’s also interesting to consider this analysis with the discussion of various coal types currently being held on the Nat. Pres. forum.
It would be difficult to engineer a durable probe that could monitor the grate temperature directly, but as a half way house, my suggestion would be to provide a valve which shuts down the undergrate steam if the boiler pressure drops below, say 85%, of its nominal value. I’ve been advised that suitable valves are commercially available to achieve this. Admittedly, there are other reasons why the boiler pressure might fall, but I don’t think that shutting off the undergrate steam would be too much of a disadvantage.
The other point that needs considering is whether there is sufficient secondary air to burn the gases produced by the system (i.e. hydrogen and carbon monoxide). We seem to be assuming that sufficient secondary air passes through the firebox door, but is this actually the case? If I remember correctly, for “Red Devil” additional openings were created in the side of the firebox. Gases passing unburned through the chimney obviously represent a loss of usable energy. Perhaps it may be possible to monitor the carbon monoxide content in the smokebox gas.
Rather than cutting holes in the side of the firebox, and thus penetrating the pressure vessel, my preference would be to have secondary air pipes passing up alongside the grate from below the locomotive. This opens up the possibility of considering pre-heating the secondary air, using exhaust steam. (Preheating primary air could lead to further clinkering problems).December 30, 2016 at 10:02 pm #1487Richard ColebyParticipant
My only close experience of pulverised coal firing was with the forge furnaces at Derby Carriage and Wagon Works in the early sixties. It was a menace! Apart from heavy machinery seemingly liable to frequent breakdowns we had a couple of explosions in the system while I was working in the shop, luckily without injury. Even allowing for the fact that the machinery was pretty old I think that any application to a modern steam locomotive would be extremely difficult to get past the H&S authorities. The precautions necessary would be akin to flour mills for a start. They are also noisy in operation.January 4, 2017 at 9:55 pm #1619John HindParticipant
There were applications of Pulverised Fuel/Micronised Fuel firing in Germany and Australia. There are a couple of ILocE papers from the late 1920’s describing the applications. If you are an I MechE member you can get electronic copies from the archive.
If anyone who is not am IMechE member is interested in them, PM me.January 4, 2017 at 11:06 pm #1620Chris NewmanKeymaster
There’s a copy of H. Herma’s paper “BROWN COAL DUST FIRING FOR LOCOMOTIVES” in Victoria, Australia, on the 5AT website. I transcribed it into digital format a few years ago. It’s linked to a page titled “With the prospect of ever-rising oil prices, should the 5AT’s design be based on the use of coal?”
I also have a digital copy of a paper by John L. Buckland titled “PULVERISED BROWN COAL FUEL FOR STEAM LOCOMOTIVES” published in the Australian Railway Historical Society Bulletin Vol XXIII No 417 July 1972 if anyone wants it. Also Chapter 9 from a book by Robert Carlisle and Bill Abbott titled “Hudson Power” published by the Australian Railway Historical Society Victorian Division in 1985, which describes the conversion of three Victorian Railways locomotives to PBC Dust Firing. Both interesting papers if I remember rightly. I can post them onto AST’s website if anyone wants them.January 10, 2017 at 7:57 am #1680Alex PowellParticipant
If you could do that Chris it would be very helpful as this is an area that I am quite interested in. I also think more knowledge on this subject will really help our projects now and in the future.
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