David Wardale's "Red Devil" book explains the "grate limit" of a coal-fired steam locomotive. Namely, as you fire more coal on a given size grate, more of it gets entrained in the draft and carried out the stack before it finishes burning. At the maximum output of a locomotive limited by this grate limit, fully half of the coal consumed does not contribute to the fire but rather gets carried out the stack and sprinkled about the right-of-way.
Livio Dante Porta's Gas Producer Combustion Systems (GPCS), which Wardale implemented on the Red Devil in South Afric and was less successful implementing on the QJ class in China, is intended to improve efficiency by dramatically reducing carryover. The idea is to limit the air flow through the grate, using that primary air to gasify the coal rather than burn it completely. This reduced primary air flow cuts down on entrainment of the coal particles as they burn down to small size. Secondary air is then introduced above the firebed so as to complete the combustion of the gasified coal.
Wardale completes his book by expressing frustration at getting the gas producer combustion process to work reliably, and strongly hints that the ultimate solution would lie with pulverized coal rather than trying to burn lump coal on a grate. With pulverized coal, the coal is ground so fine that it burns on contact with the hot firebox air much as the atomized oil in an oil burning firebox or furnace.
Pulverized coal on a locomotive has its host of problems, an important one is that it is pulverized before being put into the tender, creating the problem of how to safely store it without and explosion hazard. Pulverized coal has had mixed success in the few instances it has been tried.
But oil burning steam locomotives have been used wherever there was a cheap enough supply of the heavy "bunker" oil traditionally used for this purpose.
An oil burning steamer should work like a hypothetical pulverized coal locomotive? By atomizing the oil with steam, the droplets should burn on contact with the air, and there should be minimal carryover of unburnt fuel, or at least no soot accumulation on the scale of coal flying out the chimney of a coal-fired engine operating at the grate limit?
What is the historical record on the fuel efficiency of oil-fired steam compared to coal? If pulverized coal is so great, assuming that you can do it safely and economically, shouldn't oil-fired steam offer the same combustion efficieny with its different fuel, at least to set a base for comparison?
Maybe oil burners didn't save many btu's per HP-hour over coal? Would this mean that the carryover problem was not the big problem in the total coal bill with steam? Would this mean that coal-fired locomotives were rarely fired at the high power levels where half the coal is going out the stack?
If GM "killed the electric car", what am I doing standing next to an EV-1, a half a block from the WSOR tracks?
Paul Milenkovic David Wardale's "Red Devil" book explains the "grate limit" of a coal-fired steam locomotive. Namely, as you fire more coal on a given size grate, more of it gets entrained in the draft and carried out the stack before it finishes burning. At the maximum output of a locomotive limited by this grate limit, fully half of the coal consumed does not contribute to the fire but rather gets carried out the stack and sprinkled about the right-of-way. Livio Dante Porta's Gas Producer Combustion Systems (GPCS), which Wardale implemented on the Red Devil in South Afric and was less successful implementing on the QJ class in China, is intended to improve efficiency by dramatically reducing carryover. The idea is to limit the air flow through the grate, using that primary air to gasify the coal rather than burn it completely. This reduced primary air flow cuts down on entrainment of the coal particles as they burn down to small size. Secondary air is then introduced above the firebed so as to complete the combustion of the gasified coal. Wardale completes his book by expressing frustration at getting the gas producer combustion process to work reliably, and strongly hints that the ultimate solution would lie with pulverized coal rather than trying to burn lump coal on a grate. With pulverized coal, the coal is ground so fine that it burns on contact with the hot firebox air much as the atomized oil in an oil burning firebox or furnace. Pulverized coal on a locomotive has its host of problems, an important one is that it is pulverized before being put into the tender, creating the problem of how to safely store it without and explosion hazard. Pulverized coal has had mixed success in the few instances it has been tried. But oil burning steam locomotives have been used wherever there was a cheap enough supply of the heavy "bunker" oil traditionally used for this purpose. An oil burning steamer should work like a hypothetical pulverized coal locomotive? By atomizing the oil with steam, the droplets should burn on contact with the air, and there should be minimal carryover of unburnt fuel, or at least no soot accumulation on the scale of coal flying out the chimney of a coal-fired engine operating at the grate limit? I have always understood that it was necessary to throw sand into the firebox from time to time to scour the soot (carbon) that collected in the flues. Perhaps there was much more unburnt pulverized coal than there was soot from the burnt oil? What is the historical record on the fuel efficiency of oil-fired steam compared to coal? If pulverized coal is so great, assuming that you can do it safely and economically, shouldn't oil-fired steam offer the same combustion efficieny with its different fuel, at least to set a base for comparison? Maybe oil burners didn't save many btu's per HP-hour over coal? Would this mean that the carryover problem was not the big problem in the total coal bill with steam? Would this mean that coal-fired locomotives were rarely fired at the high power levels where half the coal is going out the stack?
An oil burning steamer should work like a hypothetical pulverized coal locomotive? By atomizing the oil with steam, the droplets should burn on contact with the air, and there should be minimal carryover of unburnt fuel, or at least no soot accumulation on the scale of coal flying out the chimney of a coal-fired engine operating at the grate limit? I have always understood that it was necessary to throw sand into the firebox from time to time to scour the soot (carbon) that collected in the flues. Perhaps there was much more unburnt pulverized coal than there was soot from the burnt oil?
Johnny
Paul,
A good thought provoking question.
LaMassena's article on the "Big Engines" in the June 1968 issue of Trains hinted that for a given grate area, that oil firing would generate less steam than firing with a high quality bituminous coal.
The coal particles flying around the firebox should have been excellent sources of radiant energy, presumably more so than with the cleaner flame from oil firing. I would not be surprised if radiant heat transfer was the limiting factor in steam production as opposed to how much energy was released by combustion. (It is quite possible that I may be way off base on this supposition.)
- Erik
Oil as a fuel certainly has advantages over coal, such as more BTU's, easier handling, no ashes to dump, no problems with spontanious combustion as coal can have from the dust.
But there's a good story from "Classic Trains" that'll show you everything wasn't always "hunky-dory" on oil-fired steamers.
Go to the "Classic Trains" site, select "The Way It Was" and scroll down to "The Challenges Of Firing An Oil Burner" by Barry Anderson.
A good read.
Coal fire carryover appeared in the form of raining cinders and hot sparks. Oil fire carryover (the result of a too-rich fuel-air mixture) appeared in a towering black cloud that blew away on the wind.
There are plenty of photos of steam locomotives, both coal and oil fired, fouling the air with dirty exhaust. Nor are diesels exempt. A lot depended on the quality of the fuel. Even more depended on the skill of the fireman.
Chuck
DeggestyAn oil burning steamer should work like a hypothetical pulverized coal locomotive? By atomizing the oil with steam, the droplets should burn on contact with the air, and there should be minimal carryover of unburnt fuel, or at least no soot accumulation on the scale of coal flying out the chimney of a coal-fired engine operating at the grate limit? I have always understood that it was necessary to throw sand into the firebox from time to time to scour the soot (carbon) that collected in the flues. Perhaps there was much more unburnt pulverized coal than there was soot from the burnt oil?
When I was a teen, I rode on a CB&Q steam excursion in 1962 or 63 from CUS to Starved Rock State Park (on the line to the Twin Cities). It was pulled by 5632, an oil-burning 4-8-4. Much of the great trip, I rode in the open on a trailing gondola. When I returned home, I was largely covered in oily soot. http://www.rrpicturearchives.net/pictures/16736/CBQ5632-630428%20Hinsdale%20(Highlands),%20IL%201.jpg
C&NW, CA&E, MILW, CGW and IC fan
The smoke from a coal-fired steam engine is not the same thing as the cinders (also called sparks), which I believe is the carryover of carbon from the stack on to the right-of-way. According to Wardale, a locomotive can smoke badly, but this is mainly visual and does not represent much wasted fuel. The carbon carryover is largely invisible, except at night when the bigger "sparks" are visible as glowing embers.
The shower of cinders from sitting in the gondola behind a coal-fired steamer is the carbon carryover. I suppose there could be that much oily soot behind an oil burner, but in excursion service, the try to smoke things up for an impressive photo run by.
No one seems to have any fuel consumption figures of any modern oil burner -- say those huge AT&SF 4-8-4's?
erikemLaMassena's article on the "Big Engines" in the June 1968 issue of Trains hinted that for a given grate area, that oil firing would generate less steam than firing with a high quality bituminous coal. The coal particles flying around the firebox should have been excellent sources of radiant energy, presumably more so than with the cleaner flame from oil firing. I would not be surprised if radiant heat transfer was the limiting factor in steam production as opposed to how much energy was released by combustion. (It is quite possible that I may be way off base on this supposition.)
If I'm not mistaken, there is good data in the Farrington 'Santa Fe Big Three' regarding oil firing rate and conditions. Specifically I remember a table with the pressure in the firebox space and chamber for, I believe, the 3460 class Hudsons. Note that most of the pressures are below atmospheric, and yet characterized by high rates of combustion. The references I have on oil firing indicate that a minimization of excess air (to not more than a few percent) over the range of firing is desirable. All this sums up to unburned carbon.
An advantage of oil firing is that the 'atomization' of the fuel is more easily accomplished, and entrainment and carburetion of the fuel in the primary and secondary air is easier... up to a point. When the hydrogen has flashed off the fuel droplets, you are back to carbon -- hot carbon, yes, but not acting much like a liquid. After that point expect it to act just like similarly-sized levitated coal particles.
There are two things going on at a given point in the combustion gas plume. There is radiation from the heat of each particle, and there is combustion from reaction of oxygen at the surface of each particle. The amount of available oxygen is limited, and the amount of 'scrubbing' that moves carbon dioxide away from the combusting surface and swaps in oxygen may be limited. Meanwhile, there is a temperature below which oxidation of carbon slows down or effectively ( at least during the time of flight of the fuel in the radiant section of the gas) stops -- the inner firebox sheet is well below this temperature (with water near the mid-300s F on the other side of a reasonably conductive plate) and so is the front tubeplate and flue/tube walls. Combustion that radiates away heat too quickly, or particles that come into contact with these walls, are likely to 'quench' -- and not be reignited in a meaningful sense until ejected into the air from the front end.
Meanwhile, the heat transfer from the imperfectly-combusted carbon is also dependent on gas contact (convective and turbulent flow) against surfaces. A USGS (of all agencies!) study about 1910 demonstrated much greater heat transfer in firetubes at very high gas speeds. The problem with high gas speeds is that they allow a shorter time for all the reactions involved in combustion to go to completion. And it is difficult to speed up those reactions in a typical Stephenson firebox. Certainly one of the points of Besler tubes is to speed up gas flow and constrain it closer to the tube wall.
GPCS does the reactions in the gas phase, which gives better mixing and carburetion. But the gases are optically transparent over a fairly wide spectrum of emission (and absorption/reradiation) so the net effect of heat transfer by radiation is lower than that of luminous particles ... I think. (Another point of the Besler tube is that the 'core' absorbs heat at the 'peaky' emission peaks of the gases involved, mainly CO2 and H20, but reradiates it at some reasonable approximation of a blackbody radiator, orthogonal to the wall of the tube itself.
It shouldn't be surprising that decreasing the speed of gas flow through the firebed (which is one big purpose of Lima-style large grate area) and increasing the length of the gas plume in the radiant section (which is what the brick arch and a modern combustion chamber do) will increase the effective amount of fuel that is properly combusted. But it should also not be surprising that sticking a large amount of black-as-night-to-combustion heat-transfer area in the radiant section -- which is what Nicholson syphons do -- can contribute to a certain amount of quench if flow patterns aren't optimal...
I was not clear. Sorry. #5632 class O-5B was a modern (1940) oil-burning 4-8-4. TE = 67541 lbs. Most of the run, it ran quite clean. Only in the obligatory run-bys for the debarked passengers at Starved Rock did it put on a 'show' of heavy black smoke
schlimmI was not clear. Sorry. #5632 class O-5B was a modern (1940) oil-burning 4-8-4. TE = 67541 lbs. Most of the run, it ran quite clean. Only in the obligatory run-bys for the debarked passengers at Starved Rock did it put on a 'show' of heavy black smoke
It doesn't take long for an oil-fired smoke show to coat you in fallout! (I suspect that more than a little sanding of the flues would produce similar effect, perhaps with some grit mixed in...)
As I understand it, typical firing practice involved just a little 'overfiring', to produce a light gray haze at the stack. (This is what the stack light would show at night.) I do not know how this might tend to 'plate out' downwind...
Wizlish schlimm I was not clear. Sorry. #5632 class O-5B was a modern (1940) oil-burning 4-8-4. TE = 67541 lbs. Most of the run, it ran quite clean. Only in the obligatory run-bys for the debarked passengers at Starved Rock did it put on a 'show' of heavy black smoke It doesn't take long for an oil-fired smoke show to coat you in fallout! (I suspect that more than a little sanding of the flues would produce similar effect, perhaps with some grit mixed in...) As I understand it, typical firing practice involved just a little 'overfiring', to produce a light gray haze at the stack. (This is what the stack light would show at night.) I do not know how this might tend to 'plate out' downwind...
schlimm I was not clear. Sorry. #5632 class O-5B was a modern (1940) oil-burning 4-8-4. TE = 67541 lbs. Most of the run, it ran quite clean. Only in the obligatory run-bys for the debarked passengers at Starved Rock did it put on a 'show' of heavy black smoke
Yes, I understand. Sanding the flues was, as I was told, the tactic used to produce a show of black smoke. However, I was standing rather far from the rails during the run-by. The accumulation I had was mostly from the trip out, as I rode inside a coach on the return to Aurora.
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