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The Ultimate Steam Page

The Gas Producer Combustion System

This page provides information on the construction and operation of the Gas Producer Combustion System (GPCS).


Normal coal-fired steam locomotives burn coal on a (relatively) flat grate, usually consisting of cast iron segments which can be rocked back-and-forth with a mechanism to shake the ash down into the ash pan. Air holes are provided in the grates which allow most of the air required to burn the coal to come up through the coal bed. Locomotives typically generated much more steam for a given physical size boiler than a stationary boiler would (for example, a power plant boiler). They accomplish this by using a power draft-inducing mechanism (the smokestack and nozzle) which draws huge amounts of air through the grate to allow coal to be burned very rapidly.

While this allowed locomotives to produce ample power, it was not very efficient. The main reason for this is the velocity of the air which comes up through the fire. As the lumps of coal on the fire burn, they shrink in size. Eventually, they become sufficiently small that the force of the airstream lifts them off of the firebed, carrying them through the firebox, into the flues, and eventually out of the smoke stack where they are discharged as cinders. In a locomotive operating at high power output, more than 50% of the coal that was fed into the firebox could be ejected from the stack in this way before it could be completely burned. This effect was somewhat worse on stoker-fired engines as the coal was ground up in the stoker and blown into the firebox with steam jets. Many particles would be caught in the airstream without ever landing on the firebed. This represented a tremendous loss of efficiency in the boiler.

Besides the efficiency loss, this carryover of unburned coal causes several problems. The coal particles act much like sand-blast grit as they fly through the boiler at high velocity. This causes wear on the surfaces in the boiler, including the rear tube sheet, rear tube ends, superheater ends, and internal parts of the smokebox. The cinders, if of sufficent size, can ignite line-side fires along the railroad tracks. A conventional steam locomotive firebox is illustrated below.

firebox

This drawing shows a simplified cross-sectional view of a typical steam locomotive firebox. Most of the air required to burn the coal (about 90%) enters through the ashpan and comes up through the grate. A much smaller amount of air (about 10%) enters the firebox through holes in the firedoor, and sometimes through openings installed in the sides of the firebox (such as over-fire jets).

Another problem with conventional coal combustion was clinker formation. All coal contains non-combustible components. Some of these components can melt at the temperatures attained in the coal bed. When this happens, the molten substance flows together to form a clinker. Since the clinker can't burn, it blocks off a portion of the firebed, reducing the engine's output (sometimes by extreme amounts). The fireman has to attempt to break it up manually using a steel rod and then shake the engine's grates to get the broken pieces to drop into the ash pan. This was a laborious task, especially on a moving train.

A final problem with conventional coal combustion is uneven heating. A good fireman tried to maintain his fire "light, level, and bright". To do this, he had to keep the firebed relatively thin (a few inches thick). Problems with the stoker or with his shoveling technique could cause the fire to be thicker in some areas and too thin in others. This produced uneven heating in the firebox, causing stress on the hot firebox surfaces. Occasionally, the fire could actually burn out in a thin location. This would immediately allow a stream of cold air to come up through the grate causing further firebox stresses. Burning coal (not fresh coal) had to be spread over this spot and then fresh coal added.

All of these problems with conventional coal combustion made the fireman's job difficult, increased maintenance on steam locomotives, and severely limited the efficiency which could be attained.


GP firebox

The illustration above illustrates the same firebox after conversion to a GPCS configuration. 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. On the Rio Turbio engines, the stoker steam jets are not normally used; the coal just spills out of the stoker and is allowed to spread across the firebed.

As a comparison, the efficiency of a typical modern locomotive boiler with a huge combustion chamber was less than 50% at maximum output. Porta's 2-10-2's, built in the late 1950's and early 1960's, attained 78 to 80% efficiency at high output under documented tests.

The GPCS can be adapted to virtually any solid fuel, and has been successfully tested with wood, charcoal fines mixed with oil, and sawmill waste. Porta has recently been working in Cuba to adapt the GPCS to burn baggasse, the discarded waste left when sugar cane is crushed to produce sugar.

Porta has equipped one engine with a refinement of this design, known as the cyclonic gas producer firebox. This locomotive has the air ducts arranged to produce a swirling effect in the firebox gases, augmented through the use of steam jets. This causes the air to more completely mix with the firebox gases for even more complete combustion, and centrifugally separates the few airborn coal particles to allow them to completely burn before exiting the firebox. New firebox designs shown in Porta's technical papers would have a different shape to maximize this cyclonic affect.

The GPCS has been applied to locomotives in Argentina, Paraguay, Brazil, and Cuba by Mr. Porta, to locomotives in South Africa and China by David Wardale, and to locomotives in England and South Africa by Phil Girdlestone. The GPCS has even been applied to miniature steam locomotives (7-1/2 inch and 15 inch gauge) in England, South Africa, and the USA.

While the GPCS is a simple concept, it requires careful attention to its design and tuning to ensure its proper operation. The GPCS is another example of advanced steam locomotive engineering which requires Porta's philosphy of detailed engineering analysis and calculation- rather than the good old "trial-and-error" method- for optimum operation.




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