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 Post subject: In regards to GPCS...
PostPosted: Fri Jan 17, 2014 8:15 pm 

Joined: Thu Mar 24, 2011 12:07 pm
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Location: Leicester, MA.
For the followers of moderns steam in the room, there's the knowledge of the GPCS system (AKA a Gas Producing Combustion System). For those who don't know what it is, in the words of modern steam expert Hugh Odom;

Quote:
The coal grates are replaced with grates having smaller air openings, so that only about 30% of the air (primary air) required to completely burn the coal enters through the grates. For proper operation, the grates must fit tightly when closed to prevent uneven air flow up through the firebed. A number of air admission ducts are installed through the walls of the firebox, along the sides, back, top, and/or front. These ducts are sized to admit about 70% of the air (secondary air) required to completely burn the coal. Finally, dispersion tubes are installed below the grates to admit steam to the fire. This steam comes from the exhaust nozzle (3-4% of the exhaust flow from the cylinders) and from various other steam-powered accessories on the locomotive. The steam must be evenly distributed and mixed with the primary air to ensure proper operation. The firebed is maintained much deeper than in a conventional firebox.

An integral component of the GPCS is an improved stack/nozzle arrangement in the smokebox of the locomotive. To ensure complete combustion of the firebox gases, the secondary air is introduced through small openings at high velocity into the firebox. This produces turbulence so that the air thoroughly mixes with the burning gases. Because of the small primary air openings in the grates and the small secondary openings in the firebox walls, more energy is required to "pump" this air through the boiler than with a conventional firebox. If a conventional nozzle and stack arrangement were used (as on most U.S. locomotives), a very restrictive nozzle would be required which would produce excessive back pressure on the pistons. This would negate much of the advantage of the increased steam generating capacity of the GPCS. To overcome this problem, the locomotive is fitted with a high efficiency front end such as a Lempor or Kylpor ejector, both of which were developed by Porta. These systems produce the maximum draft for the minimum back pressure, maximizing the power developed in the locomotive's cylinders, even with the increased pumping that is required with the GPCS.

In the GPCS, the coal burns at a lower temperature than in a normal locomotive. The admission of only 30% of the required air combined with the steam flow causes the solid constituents of the coal to burn, while the remaining components are converted to mostly carbon monoxide gas and water vapor. In the space above the firebed, the secondary air ducts provide the remaining air necessary to completely burn this gas. The low velocity of the air through the firebed combined with the thick fire reduces the carry over of coal particles which greatly reduces the sand-blasting effect and the risk of line-side fires. The firebox is inherently maintained at a more-even temperature which reduces thermal stresses. The thick firebed, cooled by the flow of underfire steam, makes the fireman's job easier as it is much less likely to form clinkers or develop thin spots.


With that in mind, I found this interesting little tidbit of "ancient history", that may be an early version of the system, implemented on the Great North of Scotland at it's construction. Quoting the LNER Encyclopedia;

Quote:
The first twelve locomotives ordered for the new GNSR were designed by Mr. D.K. Clark and built by Messrs. W. Fairbairn of Manchester. Seven were passenger 2-4-0s. Five were goods 2-4-0s, but these were originally designed as 0-6-0s. Deliveries were late, and the first goods engines did not arrive until June 1855. These, and many later GNSR locomotives, were fitted with Mr. Clark's patent smoke preventing system. This had a series of holes in the sides of the firebox above the fuel, that allowed jets of steam to be projected. The steam circulated air in the firebox. The better combustion is also reported to have resulted in improved fuel economy. All twelve locomotives were withdrawn between 1879 and 1898. The original passenger locomotives were heavily worked in their early years and tended to be the first to be withdrawn.


I'm curious, but have the comparisons between the modern GPCS system and the GNSR "smoke preventing system" ever been drawn? Some of the experts might have some insight that they'd be willing to share...

Sources:
http://www.trainweb.org/tusp/firebox.html
http://www.lner.info/co/GNSR/locomotives.shtml

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 Post subject: Re: In regards to GPCS...
PostPosted: Fri Jan 17, 2014 9:17 pm 

Joined: Thu May 24, 2012 1:37 pm
Posts: 895
The Clark device is intended to provide additional overfire air to enhance late particulate conbustion -- it is similar to the overfire air devices or 'guns' used on many late American steam locomotives.

GPCS is fundamentally different in that it generates producer gas by passing steam and restricted air (i.e. a reducing atmosphere) through a thick firebed, then induces complete conbustion of this gas in the firebox and chamber volume, with adequate secondary air.

What is being combusted is highly different between the two systems -- it is not just about "getting complete combustion" or 'stoich" operation.

The radiant-uptake characteristics are different for a combustion-gas plume of gases (optically transparent at visible wavelengths) than a plume containing glowing carbom particles -- 'luminous flame'. Part of the logic behind Besler tubes is that the 'peaky' radiant heat transfer from optically-transparent media is re-radiated from the inserted Besler tubes with a smoother (more 'black-body-like') spectrum, and orthogonal to the absorptive inner wall surface of the firetube involved.

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Last edited by Overmod on Tue Jan 21, 2014 7:27 am, edited 1 time in total.

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 Post subject: Re: In regards to GPCS...
PostPosted: Fri Jan 17, 2014 10:57 pm 

Joined: Sun Aug 22, 2004 7:19 am
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Location: southeastern USA
It's been a few years since in an effort to more completely burn oil in a small firebox I toyed with the idea of overfire air inlets in a common US style system......nobody liked the idea back then, but we've learned a lot since. I think that by preheating the air it might have some merit, relative to the usual big secondary draft through the door vent.

dave

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 Post subject: Re: In regards to GPCS...
PostPosted: Fri Jan 17, 2014 11:16 pm 

Joined: Sun Aug 22, 2004 9:54 am
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Location: NJ
Dave's comment reminded me of a company that I briefly worked for (actually loaned to by the parent company) maybe 35 years ago. They made an interesting oil burner called the Blue Flame. I just took a look on the web, and they are still around. http://www.hed.com/blueflame should get you to their site.

Basically, the burner is an oil burner that has a return circuit that uses hot gas to aid in complete atomization and combustion; at least that is my understanding of it. I did witness some tests which were pretty impressive, and wonder how it would work out on an oil-fired steam locomotive today. I was involved with an oil burning steamer back then, the 148, and could see a real difference in the flames that each system produced.


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 Post subject: Re: In regards to GPCS...
PostPosted: Sun Jan 19, 2014 4:31 am 

Joined: Wed Jan 23, 2008 6:12 am
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Location: North Wales and Australia.
I to have seen articles on such system including an American one. In both cases I thought if you just move the steam under the grate! So close even knowing they where also using coal to make gas is a similar way. The rest of what Hugh says is good.

As to oil firing. The situation is different. You need less moisture and water in the combustion area. 5% of the evaporation rate, more and you lower the combustion temperature. The air needs to be mixed in as close to the burner as possible. Secondary air closer to the tube plate just reduces combustion temperature and increases soot formation. Yes air heating would also act as an air dryer which should help combustion. The practice in IC engines is to like cold air because its more dense and easier to get into the cylinder.

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 Post subject: Re: In regards to GPCS...
PostPosted: Tue Jan 21, 2014 3:16 pm 

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The HED 'Blue Flame' is, I believe, also known as the ISOMAX burner. IIRC this uses regeneration but not FGR. The principle involved here:

http://www.backyardmetalcasting.com/gue ... urner.html

is said to be similar. Neither plasma induction nor electrostatic charging of atomized fuel is involved, according to HED personnel. (Notably, this burner was originally developed in the '70s for HED's major business, as an alternative to gas-firing for kilns and similar equipment!)

Nigel's point about secondary air can be related to a combination of transition temperature and fuel-particle characteristics. The 'catch' is that maintaining flame luminosity in the plume for an extended distance through the radiant zone is important -- and that luminosity involves sufficient mass of very hot and unoxidized carbon. If the secondary air is mixed promptly with the flame, and primary and secondary air are preheated, the luminous 'distance' will be a factor of how quickly the flame plume is moving in the 'effective' zone following ejection from the burner -- BUT all the luminosity-inducing particles will have to have progressed to hot transparent gas (i.e. CO or CO2) before incidence and quench on comparatively cold surfaces -- firebox and chamber waterwalls, as well as rear tubeplate.

Remember that 'combustion' should be considered a reversible reaction with a known transition temperature. Much of the 'sooting' is either premature quench or excessively reducing conditions in part of the plume. Adding secondary air below the transition temperature will not result in meaningful combustion heat release.

Meanwhile, I think it's been recognized for quite a few years (with respect to reflex and condensing-type boilers) that the combustion water represents not a loss but a heat transfer. Note, for example, the effect in a Franco-Crosti style economizer when the gas is allowed to fall below the condensation point of water (or 'dirty water') and the phase change occurs in contact with a heat-transfer surface. This effect was specifically documented for the Donlee (TurboFire XL) boiler during its DOE testing. This may be of renewed interest, even though its gas path is much longer than a typical locomotive boiler. Consider, for example, a reflex boiler designed to fit stack-train clearances, with enough vertical separatioo to the dry pipe to give good default steam separation...

In order to meet Tier V air-quality standards NOx has to be reduced to stringent low levels, which means careful control of plume temperature (below about 1436 degrees F if I remember the numbers correctly). On a Donlee boiler this is modulated, in part, with direct steam injection -- which has the armchair thermodynamic 'engineers' screaming murder. As it turns out, if the convection section is designed correctly, and the Rankine cycle sensibly optimized, most of the effective enthalpy absorbed from the plume into the injected steam can be recovered 'further along' -- quite cost-effectively.

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