The application of GPCS or “Gas Producer Combustion System” to locomotive fireboxes is explained in great detail in pages 78 to 92 of Dave Wardale’s book “The Red Devil and Other Tales from the Age of Steam“. Basically it is a 19th century technology involving the blowing of oxygen over heated coal to make “Producer Gas” or a mixture of carbon monoxide and nitrogen through the exothermic chemical reaction:
2C + O2 + N2 → 2CO+N2.
Producer gas was sometimes used for domestic heating, powering motor vehicles and power generation. It is reportedly still widely used in industry because it can be made with cheap fuel (coal, wood or biomass) – see http://www.answers.com/topic/wood-gas#ixzz1ErGW1xwN.
L.D. Porta was the first engineer to apply the principles of producer gas to a locomotive firebox, first demonstrating its advantages on the locomotives of the Rio Turbio railway in Argentina in the early 1960s.
The advantage that the GPCS offers is that it allows the quantity of “primary” air passing through the grate to be significantly reduced perhaps by as much as 70%, thereby reducing the upward air velocity through the firebed that causes lifting of unburned coal particles off the fire surface and discharged through the chimney. The reduction in unburned fuel loss that can be achieved – and consequent increase in thermal efficiency – is significant especially at high steaming rates that approach the grate limit.
Greater quantities of “secondary” air have to be fed into the firebox above the firebed to burn off the carbon monoxide (CO) that is produced. A significant proportion of this can be fed in through the firedoor (which must be left continuously open), but additional quantities are usually required to be fed in through air inlet tubes penetrating the sides and crown of the firebox. The position of each of these secondary air inlet tubes should be selected to:
- Deliver secondary air as uniformly as possible over the over-fire combustion area – i.e. with some of the air penetrating to the centre of the firebed;
- To create mix turbulently with the producer gas (not with other airstreams entering the firebox);
- As far as possible to separate any rising coal particles from the gas stream and to throw them onto the firebed;
- Direct the airstream high enough to prevent it skimming coal particles off the firebed or to interfere with the firing process.
(See page 90 of Wardale’s book for more specific details).
Another important requirement for effective gas production is that the firebed be deep. Ideally its depth should be at least 15 coal particle diameters in order to ensure that the maximum amount of carbon monoxide is produced from the reaction between the carbon in the coal and the available oxygen in the primary air.
However, an increased firebed depth combined with the exothermic (heat producing) reaction results in higher firebed temperatures and therefore an increase in clinker formation from ash fusion. This can be countered by the introduction of “clinker control steam” into the ashpan which has the effect of reducing the firebed temperature as it passes through the coal particles because it undergoes and endothermic (heat absorbing) reactions in the form of:
C + H2O → H2 + CO
C + 2H2O → 2H2 + CO2
Both reaction result in hydrogen gas which gets burned off with the secondary air, and the first reaction produces additional producer gas (CO) which also gets burned off above the firebed.
Based on the above, the main features of a GPCS firebox are:
- Perforated grate with pin-hole perforations giving a free-air area of between 5% and 10%;
- Lowered grate to maximise the depth of the firebed;
- Secondary air holes through the sides, top, front and/or rear of the firebox to allow “secondary” air to enter above the fire;
- Pipework to deliver small quantities of exhaust steam into the ashpan.
Wardale converted the fireboxes of both his SAR developments (Class 19D No 2644 and Class 26 No 3450) to GPCS operation. He achieved considerable (though variable) success with both conversions, as recounted in detail in his book, noting that the cost of converting a firebox to GPCS may not be warranted on a locomotive that is not going to be steamed anywhere near its grate limit.