In my previous series of articles, I concentrated on feed-water heaters as a way of improving fuel efficiency in steam locomotives. I now intend to discuss another major source of thermal wastage – poor and incomplete fuel combustion.
One of the rules for the Rainhill Trials in 1829 was that the contestants “had to consume their own smoke” and this became the law for nearly all early public railways. This had nothing to do with the efficient use of fuel, but was an early ‘health and safety’ issue to ensure that exhaust smoke was not a nuisance to the public. This requirement initially ruled out the straightforward use of coal in early fireboxes, because of the smoke formed by the combustion of the volatile components inherent in most common (bituminous) coals. The answer was to use coke, the by-product of the process to supply town gas.1 For this, coal was distilled – under pressure and with regulated amounts of water – in retorts to drive off the volatiles as coal tar and end up with a gas that was largely carbon-monoxide and a residue of virtually pure carbon. The use of coke as the prime fuel for steam locomotives solved the smoke emission problem, though sparks from the chimney and the deposit of burning lumps from the grate on to the track would remain as objectionable problems, leading to the need for the invention of spark arresters and self-contained ashpans. Exhaust sparks, black smoke and cinders deposited in the smokebox were clear indications of fuel wastage. While grate ash in itself was not, poor ashpan design certainly was, as we will discuss later.
Largely for reasons of cost, there was a huge incentive to devise new designs of fireboxes that would bum coal in a ‘smokeless’ way and at the same time increase the heat output by burning the combustible volatiles as well. The answer – after much experimentation – came with the invention of the sloping brick arch, which in effect divided the firebox into two combustion zones, one for the coal on the grate and the other to deal with the volatiles. Optimising the relative proportions of each became a matter largely of trial and error, while compromises had to be found to deal with different firing rates and demands
