Finally got around to have the time to write this (its probably gonna be long) As a generall background i could mention that i work as a PhD student in Energy technology specializing towards combustion and ash chemistry (hence the geekiness)
Before i get to the pyrolizer and gasifier i would like to go over the fuels first to give a suggestion on how we would make the energy content of the different fuels more realistic. The unit i will use is Mj/kg for solids, Mj/dm3 (liter) for liquids and Mj/m3 (cubic meter) for gases. its presented as : Suggested value (typical value). I will go over the fuels quickly with regards to energy density and bulk density
Solids
Coal - 30 Mj/kg (24-30) 600-950 kg/m3,
Lignite - 10 Mj/kg (5-10) 650-850 kg/m3, Moisture content 40% (30-60%)
Dry lignite - 20 Mj/kg (10-20) special, read below,
Coke - 30 Mj/kg (28-30) 500 kg/m3,
Fresh wood - 10 Mj/kg (6-10) 600-1300 kg/m3 moisture content 40% (40-60%)
Dry wood - 20 Mj/kg (15-21) special, read below,
Charcoal - 30 Mj/kg (24-30) 150-200 kg/m3,
Liquids
Pyrolysis oil/creosote - 20 Mj/dm3 (18-22) 1200 kg/m3,
Gas
Low energy syngas - 5 Mj/m3 (3-7) 1 kg/m3,
Medium energy syngas - 10 Mj/m3 (7-14) 1 kg/m3,
High energy syngas - 20 Mj/m3 (15 plus) 1 kg/m3
On moisture content: Both lignite and fresh wood contain a substantial amount of water which reduces the energy content a lot. You can burn them if you already have a hot furnace as you need a large buffer of heat to dry off the water before the wood actually starts to burn. This also makes it almost impossible to use it in a pyrolizer/gasifier as it simply wont produce enough heat. Preferably we should need some kind of industrial dryer to dry the fuel or alternatively leave the lignite/wood out to dry in the sun before it can be used. Since we wont be mining dry fuels the bulk density is of less importance, just remove the weight of the water from the bulk density of the wet wood/lignite and you should have the weight of the dry fuel left. Since ive assumed about 40% moisture content on both lignite and wood just remove 400 kg from its starting bulk density and you should have close to its new bulk density.
You might also notice that we have 3 different qualities of syngas. This is because depending on the fuel and how we gasify we will get substantially different energy content of the gas. Air gasification of biomass for instance produces a very low energy content syngas due to a substantial amount of N2 (from the air) and the high ratio of hydrogen to carbon monoxide so using oxygen will increase the energy content of the resulting gas quite alot (althought the total energy content is the same, it just takes less space). In generall the energy content of syngas from coal will be higher than the syngas from wood and lignite due to higher amount of hydrogen. Also due to the low energy content of the low energy syngas it cannot be used in a turbine but can only be used in a gas burner. Medium and high can be used in a turbine and high can be used in performance turbine.
Before we go on to the gasifier/pyrolyzer i would also like to suggest som vanilla nerfs. Charcoal should still be possible to make in a regular furnace, we could think of this process as a low tech charcoal pit which usually have a horribly bad efficiency. I would suggest that 8 chunks of wood becomes 1 chunk of charcoal or something similar. The thermal efficiency of the process should be about 25% and the mass l should decrease by 85-90% or so (you put in 100 kg of dry wood, you get 10-15 kg of charcoal out with an energy content of 25% of the original fuel)
Now on to the pyrolyzer. This process typically operates between 500- 1100 degrees C depending on what is produced. at 500 degrees it produces syngas, creosote and charcoal/coal. and at 1100 degres we produce coke and syngas (all the fluids evaporate). Its usually a batch process where the syngas produced from the process is burned in a seperate burner to heat the process. for 500 degrees the energy balance is usually 50% solid, 25 % gas and 25% liquid. for 1100 deggrees (coke production using coal or charcoal) it is about 75% solid and 25% gas The efficiency is about 80-90% so if we input 100 Mj of fuel we would get 50 Mj of solid fuel, 25 Mj of creosote and 25 Mj of syngas of which 15 Mj would have to be burned to heat the process (with 87% thermal efficiency 1/0,87= 1.15). or att 1100 it would be 75 Mj of solid coke and 25 Mj of high energy syngas. I think a good implimentation of this machine would be a multiblock structure with an external heater (liquid or gas burner). Higher tiers would be faster with the first tiers being very slow. Time should be a large restriction in this process. The machine uses no air or oxygen for the process but uses the oxygen in the fuel itself (wood is about 40-50% oxygen on a elemental basis) Here is a suggestion of possible recepies
500 degress
dry wood/dry lignite into medium energy syngas, creosote and coal/charcoal something like 3 chunks of dry wood becomes 1 chunk of charcoal and x mb of creosote and y buckets of syngas.
1100 degrees
Charcoal/coal into high energy syngas and coke.
Now for the gasifier. The gasifier partially combusts the fuel into gaseous form using either air or oxygen. It should have a thermal efficiency of about 75% so if you input 100 Mj of fuel you would get 75 Mj of energy out as a gas. Depending on if you use air or oxygen and fuel you get either low, medium and high energy syngas. air gasification of wood/lignite would result in a low energy syngas and charcoal/coal and creosote would yield medium energy syngas. Notice that the total energy content of the syngas would be the same, however the volume would be half of the medium energy syngas. Now if we do the same thing with oxygen we dont dilute the syngas with nitrogen so the energy content is bumped one step. So oxygen gasification of lignite/wood gives medium energy syngas and high energy syngas. The amount of oxygen needed to gasify 1 kg of fuel would be about 0.2-0.4 cubic meters. you usually need about 1 cubic meter of oxygen to completly combust 1 kg of wood for instance and with gasification you would give the process between 20% and 40% of this to gasify it.
The gasifier would preferably be a large tall multiblock structure with higher tiers requiring less oxygen to gasify the fuel( tier 1: 0,4 cubic meter/kg, tier 2: 0,35 cubic meter/kg, tier 3: 0,3 etcetc) with each tier also being a bit faster.
I would also like to point something out about ashes. Ashes should not contain carbon unless you have a really crappy combustion process where you fail to fully combust the fuel. In a decent combustion application the remaining ashes would be the inorganic elements in the fuel that wont burn or evaporate at the combustion temperature. The major ash forming elements during combustion is usually K, Na, Ca, Mg, P, S, Cl, Si Fe, Al and trace amounts of various metals in various concentration depending on fuel type. Also the amount of ashes vary greatly between fuels. In heart wood the ash content would be less than 1% by weight while coal can have substantial amounts of ashes in the order of 3-20% with the major part usually being Si. So ashes should contain a variation of the major ash forming elements rather than carbon.
Greg: If you consider implementing all or parts of this suggestion i wouldn't mind giving you a hand in energy/mass balance calculations and making flowcharts for the process
Sorry for the overly long post
TLDR:
Suggestions for how to have a more realistic energy content of solid fuels in minecraft based on actuall bulk densities and energy density
a few new fuels including 3 types of syngas, creosote and drying of wood and lignite
Nerfs to vanilla so that 8 wood gives 1 charcoal (or something similar)
A indirectly heated multiblock pyrolyzer that can either make biomass and lignite into charcoal/coal, creosote and syngas and charcoal and coal into coke and syngas with realistic efficiency and energy balance
A multiblock gasifier that turns solid or liquid fuels into syngas using either air or oxygen to partially combust the fuel with realistic efficiency
Rant about how ashes should contain trace elements rather than carbon