Coal ash contains more energy than was gained from burning the coal.

coal_vs_coal_ash_energy_content.md

Coal ash contains more energy than was gained from burning the coal.

https://www.world-nuclear.org/information-library/facts-and-figures/heat-values-of-various-fuels.aspx

Natural uranium, in FNR                28,000,000 MJ/kg [so 28e9 J/g]
Hard black coal (IEA definition)            >23.9 MJ/kg
Hard black coal (Australia & Canada)        c. 25 MJ/kg [so 25e3 J/g and 207.25 g/mol according to https://pubchem.ncbi.nlm.nih.gov/compound/1-Anthrylmethanolate]
Sub-bituminous coal (IEA definition)    17.4-23.9 MJ/kg
Sub-bituminous coal (Australia & Canada)    c. 18 MJ/kg
Lignite/brown coal (IEA definition)         <17.4 MJ/kg
Lignite/brown coal (Australia, electricity) c. 10 MJ/kg

mass / molar mass = moles
moles * avogadro = number of molecules

number / avogadro = moles
moles * molar mass = mass

1e6 / 6.022140857e23 * 207.25 = 3.4414672e-16 g of coal = 8.603668e-12 J (25e3 J/g * 3.4414672e-16 g)
1 / 6.022140857e23 * 238.02891 = 3.952563e-22 g of uranium = 1.1067176e-11 J (28e9 J/g * 3.952563e-22 g)
1 / 6.022140857e23 * 232.03806 = 3.8530826e-22 g of thorium = 1.0788631e-11 J (28e9 J/g * 3.8530826e-22 g)

https://pubs.usgs.gov/fs/1997/fs163-97/FS-163-97.html

"2,000 coal samples from the Western United States and approximately 300 coals from the Illinois Basin. In the majority of samples, concentrations of uranium fall in the range from slightly below 1 to 4 parts per million (ppm). Similar uranium concentrations are found in a variety of common rocks and soils, as indicated in figure 2. Coals with more than 20 ppm uranium are rare in the United States. Thorium concentrations in coal fall within a similar 1–4 ppm range"

So at 1 ppm uranium and 1 ppm thorium, a coal power plant releases only (8.603668e-12 / (1.1067176e-11 + 1.0788631e-11)) = 0.3936559286 = 39% of the available energy in coal.

Even so, fission is slow (or not at 5%/m or 200 MW/m down vs 2 MW/m up) and expensive compared to grid-battery-buffered solar. PV is cheaper than algae oil as well:

Photovoltaic panels are 25% efficient at converting sunlight; times 90% for lithium energy storage is 23%. Algae are 98% * 36% generator = 35% efficient, but farming in water takes more expensive manpower, leaving PV the cheapest source of energy. See also Water & Power, Tech Prefs, and Threads.