As I see it the basic problem is they all essentially do the same thing. Convert some form of carbon to a fuel that can be burnt in an oxygen environment to produce sufficient heat to boil water (under pressure) to propel an engine. All produce CO2 at a rate dependent to the level of carbon content in the fuel. Wood produces less per unit of volume but requires more fuel to produce sufficient heat to run the engine. The various grades of coal plus charcoal and coke produce more per unit of volume because they have a higher percentage of carbon content which allows them to burn hotter but slower. The bottom line is it all comes out pretty close to even in the end. Produce CO2 now or produce CO2 later. Produce CO2 all at once produce CO2 in stages. Produce CO2 fast or slow. You can not get around that chemical equation C + O2 = CO2.
As I said they do it at different rates. The higher the carbon content per unit of mass the slower but hotter it burns. That's why engines that burnt anthracite coal had fireboxes so wide. They need the increased surface area to produce enough heat to make steam at a rate capable of running the engine. Wood is just the opposite as its carbon content per unit of volume is less then soft coal. You either have to have a bigger fire which was usually not possible due to the design of the narrow firebox between the frames or a smaller engine. (Anyone want to try stoking a 4-8-8-4 with wood, lol).
Yes - an engine can be built to burn almost anything containing sufficient carbon content but where is the advantage to make it worth doing? Coal - environmentally bad as it was (and still is) was the best choice at the time for steam engines.
Ben
, weeds pulled up from out of the garden... I mean the list could go on, and on, and on!