I have no idea how coal's bottom ash would act as a "soil betterment". Should that actually work, it would be used for that purpose here in the USA. Bottom ash contains anything that doesn't burn, including heavy metal compounds and other toxins. Growing crops in fly ash, even with unacceptable levels of carbon, would result in VERY firm, cement-like soil.
The boiler operator's intention is to achieve complete combustion of carbon (and other hydrocarbons) in the fuel. Throwing away unburned carbon (fuel) is a detriment to efficient operation.
Bottom ash consists primarily of silica, alumina, and iron oxides, along with smaller amounts of calcium, magnesium, sulfates, and other compounds. These are highly dependent on the coal used, and its treatments. Not much fertilization "stuff" there.