I asked this question before.
David Wardale in "The Red Devil" expressed his frustration with the Gas Producer Combustion System as the answer to the coal-carryover problem that greatly reduces boiler efficiency as you approach the "grate limit" at high firing rates. He thought the answer to be some form of pulverized coal combustion, which had mixed success, at best, in prior experiments, but a combustion-system vendor in South Africa appeared to be interested in having another go at it.
Pulverized coal combustion would then have the advantage of oil firing in that you wouldn't be blasting half of your fired coal up the stack at high firing rates. Or turn this around, oil firing should have the advantages claimed for pulverized coal, were pulverized coal even work under locomotive conditions.
I looked at some data on BTUs per ton mile for (mainly Santa Fe) oil burners against such data for coal-fired steam. On the basis of BTUs fired, oil burners didn't appear to have an efficiency advantage.
Could this be that properly firing an oil burner requires a lot of skill that isn't often applied? Or could it be that coal-burning locomotives aren't as wasteful as you think because they are not operated at their highest firing rate that often?
If GM "killed the electric car", what am I doing standing next to an EV-1, a half a block from the WSOR tracks?
Keep in mind that the GPCS is in no wise a 'pulverized coal' firing system. It uses larger sizes of coal, that will not levitate in primary air or be disturbed by secondary air, and a combination of air and steam to provide a reducing atmosphere to the hot coal bed (thereby inducing 'producer gas' formation rather than radiant combustion). The producer gas is then combusted/oxidized further out in the gas plume, spreading out the heat release and at least theoretically giving better overall heat transfer.
Pulverized coal is usually fired in a method highly like oil firing using a mechanical pressure burner: the primary and secondary air are tightly associated with the burner, and form a carbureted plume at high and turbulent gas velocity (so the CO2 from combustion is scrubbed off the particles continuously and the effective heat release is higher). Note that there is the same hard limit, imposed by the volumetrics of the combustion chamber and achievable induced draft, that there is with an oil burner: any attempt to 'overfuel' (as is done by forcing a conventional solid-fuel boiler) will result mainly in smoke and soot as combustion limits in the plume are reached. It should also be remembered that as this condition is approached, there is no guarantee that there won't be hot spots in the plume or at some points in the radiant section, which can cause problems like DNB and consequent plate overheating, staybolt failure, etc.
Similar to GPCS, except that it uses prompt combustion to a greater degree, is fluidized-bed combustion, as described by Combustion Engineering and Foster-Wheeler in the 1980s. Here, a strong and distributed flow of primary air levitates a mix of relatively small solid-fuel particles in a matrix of other particles that can include clean-coal treatment elements like dolomite; the airflow performs the high-speed scrubbing described above but the particles stay in the bed relatively long, with combustion and heat release as luminous flame (which is the 'best' for Stefan-Boltzman radiant-heat transfer to black inner wrapper surfaces) above it. Problem here is that the effective 'grate limit' is related to the mass in the actual levitated bed, and there is no good way to run a higher mass of fuel at any point in the bed (for example, as a 'heel' is used when building a good coal fire).
Remember that there are restrictions on how hard you can economically fire a Stephenson firetube boiler, and some of them involve the highest effective volume of luminous flame that just fills, but does not quench on, the radiant section of firebox and chamber. You should not be surprised to find that there is not that much difference between good-grade coal firing and oil firing with a somewhat higher nominal heat content relative to mass; in both cases the limit is what the structure will take, not how much fuel you can burn.
You will note that I carefully dance around locomotives like UP 4014 as built, where the fuel is truly awful in heat content but much of it is burned much like pulverized coal, well above and forward of the grates, giving an extended heat release more typical of a higher rank. "Fuel economy" measured as mass flow may be equally awful, but in terms of fuel cost -- and we can include in this the relative absence of ash that needs to be handled at terminals (because it blew, much of it partially burned or unburned, out of the front end).
Overmod Keep in mind that the GPCS is in no wise a 'pulverized coal' firing system.
Keep in mind that the GPCS is in no wise a 'pulverized coal' firing system.
I get that much, that GPCS is a grate-firing of lump coal. The separation of primary and secondary air is meant to have less air coming up from the grate through the coal bed so the coal doesn't levitate and then be carried up and out the stack, or at least until the lump has burnt down to a small particle. The deep coal bed is to get meaningful gasification with the restricted primary air so the secondary air has something to burn. The steam injection into this deep coal bed is less part of the gasification process and more to reduce its temperature so the whole thing doesn't turn into a choked mass of clinker.
The theme of "Red Devil" is Wardale's steadily building frustration of getting GPCS to work effectively. He as much as concludes the GPCS is too tempermental in relation to the conditions and operating practices he was facing. Wardale saw pulverized coal as abandoning lump-coal on grate and trying something entirely different. Wardale claims that pulverized coal would eliminate the grate limit and operate at near 100% combustion efficiency (boiler efficiency remains below this owing to the heat retained in the combustion gases past the flues). I believe he likened it to oil firing.
So if pulverized coal could be made workable on a locomotive, having the effect of oil firing, oil firing should function much like pulverized goal. The whole ACE 3000 oil-crisis inspired idea was to burn coal because it is much cheaper than oil, even the cheap, heavy oil fired in locomotive fireboxes. But the relative efficiency of oil and coal firing, taking into account the higher BTU content of oil, should indicate whether pulverized coal firing offers the improvement over GPCS or any other grate firing Wardale was hoping for.
I understand why pulverized coal never got very far with locomotive -- do you load the tender with fine coal power that is an explosion hazard? Or do you pulverize the coal in the stoker feed system, which involves complicated machinery in a bouncing, shaking locomotive?
I once wondered why fluidized bed combustion was not considered, but then I read that fluidized bed boilers take some time to achieve stable operation and may not tolerate the high "turn-down" of variable power locomotive operations.
Instead of GPCS or pulverized coal firing, two very different approaches with the same objective of reducing or eliminating carbon carryover, what can be done?
One can make the steam circuit use less steam and one can supply this more economical steam demand with a bigger grate. It appears that Woodard's Superpower designs at Lima were working on both concerns, up to the never-build 4-8-6 "Ohio type" locomotive with poppet valves and a very big grate.
Can a locomotive grate get too big that it loses efficiency during drifting or other periods of low steam demand? "Apex of the Atlantics" suggests that too big a grate can have that problem when they tried a much larger grate before settling on the 50 sq ft for the E6 Atlantic?
Let me take some of these points up in order, and comment a bit more inline.
quote user="Paul Milenkovic"]I get that much, that GPCS is a grate-firing of lump coal. The separation of primary and secondary air is meant to have less air coming up from the grate through the coal bed so the coal doesn't levitate and then be carried up and out the stack, or at least until the lump has burnt down to a small particle.[/quote]
The predominant point of the primary air is to maintain the proper 'retort' conditions to give the right combination of combustion heat release and reducing-atmosphere gas generation. Keep in mind that this is still reasonably induced-draft; experiments like the Fuller-Lehigh 'snuff dipper' with forced draft or pressurized combustion, while technically fascinating don't last long in 'commodity' railroad power where bottom-line cost including capital and maintenance has to combine with high mechanical reliability. You can easily understand why high primary-air preheat is valuable here (it can be produced for example as 'bottoming' in a Franco-Crosti style arrangement) and also why 'windboxing' cellular arrangements for the primary air --with more or less dynamic control over flow in those individual cells -- is valuable in implementing this on a locomotive of any particular size. Remember the reducing atmosphere -- it is important later, in both GPCS and a discussion of what conventional fireboxes do.
Let me interject here that the fundamentals of effective boiler design, particularly the advantages of long vertical plume 'rise' (this is found in some discussions of why some narrow fireboxes, like those on 4-4-0s or Chapelon's 4-8-0 conversions, are effective beyond what their GA would indicate). A good GPCS design does not 'replace' combustion-plume generation with gas generation like one of those charcoal car systems; it produces a conversion of some of the carbon to a form that is no longer 'grate-limited' in producing a combustion plume with best heat transfer to the radiant surfaces, in a reducing atmosphere.
Those of you who understand why the radiant section is important have already spotted one of the jokers in this deck: a good part of the Stefan-Boltzmann uptake there involves effective radiation, and the generated gases are transparent across a good range of wavelengths and so require something else to "heat" to optimize the radiant uptake (we will get to convective and conductive in time if you can stand the diet of salad) and for this what you want is enough black reduced carbon in the plume to absorb the peaky gas radiation and re-emit it across a broader emission spectrum. Besler tubes do the same thing in the convection section, which is why they're passive re-radiators and not clever places to put reheat or superheat tubes to kill two birds with one stone. So there is still plenty of conventional combustion in GPCS ... the flame will still be visibly white as far as possible... and the importance of avoiding quench on cold surfaces remains very high, although there is somewhat better chance of quenched particles regaining good emission when there is active combustion continuing around them and not just transferred in the partial vacuum conditions that prevail.
The deep coal bed is to get meaningful gasification with the restricted primary air so the secondary air has something to burn. The steam injection into this deep coal bed is less part of the gasification process and more to reduce its temperature so the whole thing doesn't turn into a choked mass of clinker.
Actually, the steam contributes chemically to the reaction and brings its own heat to add to what the primary air preheater ... which, we may note, is already likely countercurrent heating using steam ... has contributed -- these are nontrivial.
The elimination of clinker is an important point in an undisturbed bed, especially when it can't be addressed with a better rank of coal, or clean-coal-style additives, or better ash composition. My own opinion is that it's better to address this with fuel pretreatment or preconditioning than reducing radiant energy from the plume, but that's only an opinion.
The theme of "Red Devil" is Wardale's steadily building frustration of getting GPCS to work effectively. He as much as concludes the GPCS is too temperamental in relation to the conditions and operating practices he was facing.
Which of course it is when you realize all the different things that happen at different rates when the gas-flow conditions change. Where there has been success in GPCS, it is found in locations like Donna Teresa Christina where there are comparatively long runs of steady-state output near the peak mass-flow requirement of steam generatiion -- conditions that almost never apply in most practical railroading. And when they don't apply, the fireman has a number of prompt-reactor control problems to optimize, in invisible gas formation and combustion, in addition to his existing duties.
The "practical" thing that can be done is to provide assisted draft, which in practice often means going to pure assisted draft with some sort of fan replacing the ejector nozzles in the front end, that can be controlled to slow the bed changes that occur when the throttle is abruptly closed. Problem is, of course, that if you were operating to maintain high boiler pressure for the usual 'thermodynamic' and performance reasons, this is going to result in dramatic popping-off much of the time, precisely when steam generation is least wanted. Remember all the reasons keeping the water rate minimized is important!
Meanwhile, another "practical" thing we can try is coordinated combustion controls, something the Cold War and its inherent forcing of advancement in cybernetic tech has greatly facilitated since the end of the Golden Age of Steam. The great disadvantage here is our old friend 'differential thermal stress' at high pressure, particularly in the non-flexible staybolted regions of the firebox and chamber (and syphons where you have them). To manage this requires much better and probably GPS-or tercom-aware management of steam generation predictive of impending load conditions; we could always throw this on "The Knowledge" of the fireman, but he already had far more on his plate than the job either pays or contributes to job advancement or security.
Wardale saw pulverized coal as abandoning lump-coal on grate and trying something entirely different. Wardale claims that pulverized coal would eliminate the grate limit and operate at near 100% combustion efficiency (boiler efficiency remains below this owing to the heat retained in the combustion gases past the flues). I believe he likened it to oil firing.
Of course it's like oil firing; in fact, most of the considerations in combustion-plume generation are similar. Of course this presumes the oil firing itself is correctly understood, something that generations of designers have gotten confused with right up to the age of Reading 2100 in the Pacific Northwest.
You can also -- in theory -- extend the benefits of dual-fuel overfiring of oil onto a grate-based coal 'baseline' into using transient PC firing over some amount of stoker-fired grate-based combustion (probably a traveling-grate setup). Don't try this trick with a typical fireman, though, or on the other hand try to explain to railroads why they now need explosion control and finicky equipment in addition to the full stoker monty.
But meanwhile, with the sweet comes the bitter, including the effects of prompt quench where the gas contacts surfaces transferring heat to water and all the differential structural boiler changes when heat release changes quickly. We may add to this the substantially greater fun with flameholding, explosions on demand, etc. that go with variable PC firing to a greater extent (with many coals) than with oil. And it's bad enough with oil, particularly with mechanical high-pressure burners in large combustion/heat uptake spaces ... but I'll take that up again in a bit.
So if pulverized coal could be made workable on a locomotive, having the effect of oil firing, oil firing should function much like pulverized coal...
Let's throw in universal world peace and an end to hunger -- those having easier technical solutions.
The issue is not nearly so much whether PC can be made 'workable' as whether it continues to be workable in a commodity situation where 'a million moving parts supplied by the lowest bidder' maintained by wage slaves overseen by bean counters are working with inherently explosive coal dust. There is a long-recognized 'correct answer' to this, and it happens to be just the same as the one to using nuclear fuel for locomotives: burn the PC safely in a baseline thermal plant, and use the power and heat generated either for electric/hybrid propulsion (with any overage or trips in propulsion demand being accommodated via 'the grid') or to facilitate making of better fuels or fuel systems.
The whole ACE 3000 oil-crisis inspired idea was to burn coal because it is much cheaper than oil, even the cheap, heavy oil fired in locomotive fireboxes.
And you see PRECISELY how far that got when Iran and Iraq went to war and the latter needed cash quick. And that's before anyone addressed how you handle the whole fly-ash/"self-cleaning" smokebox thing in a post-EPA world, a fun issue for you to discuss here. Note that we haven't gotten even to the edge of the practical turndown discussion for PC systems yet, but don't forget it.
The ACE 3000 (and the later stillborn 2-10-2 version) were notable for rejecting any sort of PC system in favor of 'mine run' (!) coal in easy-to-handle swap-body coaltainers. Theoretically this could have been used with a GPCS system; in practice... it is almost pathetic to imagine what would have been involved.
In practice the alternative-fueling systems almost always benefit from some fuel pre-processing, even as 'simple' as dousing clean, washed 2" coal with a slurry of clean-coal additives, or using SRC to generate ashless particulates for "PC" handling and combustion. This gets radically clear once you look at the full costs of the power system and not misconcentrate just on cheaper and cheaper fuel cost.
But the relative efficiency of oil and coal firing, taking into account the higher BTU content of oil, should indicate whether pulverized coal firing offers the improvement over GPCS or any other grate firing Wardale was hoping for.
You're looking at the wrong things, and ignoring far more essential things, in making that particular comparison. GPCS has a set of advantages and disadvantages very different from those in PC firing, and you should consider the locomotive as a system, and then as part of a system providing transportation for known commodities at known margins under varied conditions. Once you do that, many of the reasons why both PC and GPCS have 'failed to thrive' will become more apparent to you.
Fortunately some of the experience with this was captured in the early-80s work on fluidized-bed locomotive boilers. The chief issues were that shock and motion threw off the fluidization kinetics -- give the bed a good bang, say from a low joint, and part of the fluidization collapses causing the effect of holes in the fire, and not incidentally seeing some coalescence of the ash-coated non-combustible particles circulating in the bed. It was clear even in the '70s that you'd need the equivalent of a boiler on gimbals controlled for 'negative cant deficiency' to get the bed to wprk correctly on curves. But the killer was heat release: you just can't get the energy density of a good Stephenson boiler out of a fluidized bed because so little of the combustion heat release happens above the bed as luminous flame. You need longer gas passes, with better uptake from combustion gas, to work with this, and testing of this stuff (while done, cf. the testing of the Donlee "TurboFire XL" that you may have to use the Wayback Machine to find now) was decidedly underfunded and underexplored. In practice it involved waaaaay too much critical complexity to be a sensible low-cost and robust solution in competition with the usual suspects.
Continuing with some of the points in #2
Paul MilenkovicInstead of GPCS or pulverized coal firing, two very different approaches with the same objective of reducing or eliminating carbon carryover, what can be done?
The issue is likely better considered in terms of why there is carbon carryover, and what its advantages up to a point (that 'point' being where the gas goes into the firetubes and, within a couple of inches, cools below any transition energy possibility to get 'black carbon' to go to CO/CO2) might be.
The point of the combustion system on a modern large road locomotive is to optimize both radiant heat emission in the plume and radiant-heat uptake to generate steam. Hence you have a relatively large heating surface, with relatively long rise (and good 'folded' gas path via the arch and its contained circulators) and as long a TOF as you can pay for, fed by gas that is as 'hot' (in terms of energy coupled to the water to make and superheat steam) as possible.
Now shoot that in the head by imposing NOx restrictions on combustion. This immediately implies that you shift the heat uptake further 'down' the combustion plume, both by moderating the heat release at any particular point by delaying radiant combustion or by using enhanced 'bottoming' past the convection section (which is not part of this discussion) but it also implies a much more extreme limit on overall combustion (peak flame temperature anywhere pegged to somewhere in the low 1400s F, as the NO generation runs away above about 1431F if I remember correctly) To get adequate steam mass flow then requires much greater uptake area, which in practice means a wider or longer grate to produce the needed amount of gas without forcing combustion or causing problems at any point of that grate.
It was possible to fire 'less' of a large grate to get the effect of better turndown. The two problems here are that the 'rest' of the grate needs to be blanked, or 'windboxed off' with any remaining fire perhaps doing weird things, and that the shape and characteristics of the resulting combustion plume may not give the same heat transfer to the parts of the firebox and chamber as before -- paradoxically perhaps resulting in excessive heat at some places and far too little in others. Problems are likely less than those involved in once-through systems with natural circulation (e.g. the typical once-through Benson boiler fired on PC) but they were already getting severe with the later Lima engines; in addition, most of the weight of the enhanced grate, plates, radiant water mass, etc. do not aid in weight on drivers even if the equalization is good.
Here again we can look at Tuplin's report of a sliding-pressure-fired Niagara doing the work of a 2-8-0 on a 2-8-0's budget of fuel and water. This is nifty, but we then need to ask whether there was additional maintenance or other cost from the firing or pressure excursions, etc. -- the sort of thing Porta tended to forget when he got excited over thermodynamic possibilities.
One can make the steam circuit use less steam and one can supply this more economical steam demand with a bigger grate. It appears that Woodard's Superpower designs at Lima were working on both concerns, up to the never-built 4-8-6 "Ohio type" locomotive with poppet valves and a very big grate.
Of course this equally applies to the school of thought that points out a more efficient combustion system (with Snyder preheaters and Cunningham circulation) at the rear, and a good Berkshire-quality guiding arrangement at the front, will let you do most of the practical work of that 4-8-6 with a 2-8-2. Once WWII was over, many of the reasons for sustained high horsepower at high road speed went away, and this was precisely the sort of market that Col. Townsend's engines were pointed toward.
Can the grate get too big that it loses efficiency during drifting or other periods of low steam demand? "Apex of the Atlantics" suggests that too big a grate can have that problem when they tried a much larger grate before settling on the 50 sq ft for the E6 Atlantic?
There is an obvious answer, partly discussed above, in that there is a minimum thickness that the fuel on the grate can tolerate without burnthrough or holes, and maintaining that via the stoker arrangement produces a minimum fuel delivery rate. Most of the methods to mitigate this involve some approach to reducing the fired grate area, transiently or permanently, and there are issues associated with this. What you want is to increase both the efficiency of combustion and of the Rankine cycle, without spending a fornicaton of money or working your fireman to death in one of the myriad ways, in an engine that is smaller, shorter, and lighter than some overweight technical masterpiece optimized for a world nobody really built by choice.
One interesting peripheral consideration, since you mention PRR Atlantics, is the anecdotal mention of very long firing intervals on some of these engines in service. I have read reports of up to 47 miles in service without the fireman's having to shovel in more coal, this of course not involving any fancy thermodynamic enhancement at all. To me this is evidence of 'rightsizing' the engine and the loads allocated to it in service, but that may be a luxury in a world that wants to extract the last drops of performance out of 'capital investment' by loading it down to make fullest use of the nominal TE at speed or horsepower (take your pick of measurement index).
Proportion is the key here: larger area with good distribution means less tendency to spot-overheat and more gas filling of the large inside space for radiant uptake. If you're proportioning this to maximize high-speed horsepower, the 'worse' consequences for drifting, standing, etc. are going to be, and perhaps more importantly, the worse and less predictable the changes in boiler and firebox structure will be as you transition between power and drifting.
Overmod Now shoot that in the head by imposing NOx restrictions on combustion. This immediately implies that you shift the heat uptake further 'down' the combustion plume, both by moderating the heat release at any particular point by delaying radiant combustion or by using enhanced 'bottoming' past the convection section (which is not part of this discussion) but it also implies a much more extreme limit on overall combustion (peak flame temperature anywhere pegged to somewhere in the low 1400s F, as the NO generation runs away above about 1431F if I remember correctly) To get adequate steam mass flow then requires much greater uptake area, which in practice means a wider or longer grate to produce the needed amount of gas without forcing combustion or causing problems at any point of that grate.
My recollection from my upper division thermo class was the inflection point was 2700F - the professor did a couple of stints in the Cal Air Resources Board (CARB), so would assume he knew what he was talking about. With radiant flux being proportional to the 4th power of absolute temperature, it doesn't take much of a temp reduction to dramatically lower radiant heat transfer, so your basic point still stands.
Erik_MagMy recollection from my upper division thermo class was the inflection point was 2700F
Be just like me to invert F and C, but even so that doesn't explain the disparity. Perhaps the inflection at peak cylinder pressure, which probably corresponds very nearly to peak combustion temperature, comes at a lower temperature. I will look back and see if I can find references to NOx that have better numbers.
Let me remind the class that MUCH higher firing temperature, and peak pressure, and more effective combustion of a boosted charge, become possible when the engine is run so its entire nitrogen-oxide production is knocked down in post-treatment using SCR with DEF. No more EGR and its sapping effects and hideous maintenance problems and costs, either. This carries over into better use of modulated pilot injection, use of higher nominal boost, and better cam timing for power production.
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