I can’t find a good explanation for why coke burns so much hotter. Charcoal is 98% carbon and burns at a max of about 2700 F. Coke is also nearly all carbon yet burns far hotter, 4500 F. Coal is in between. Both charcoal and coke are porous and lend themselves to efficient mixing with oxygen. One website states that coke is much stronger and implies that this part of the answer.
Less crap:
The industrial production of coke from coal is called coking. The coal is baked in an airless kiln, a “coke furnace” or “coking oven”, at temperatures as high as 2,000 °C (3,600 °F) but usually around 1,000–1,100 °C (1,800–2,000 °F).[19] This process vaporises or decomposes organic substances in the coal, driving off volatile products, including water, in the form of coal-gas and coal-tar. Coke is the non-volatile residue of the decomposition, the cemented-together carbon and mineral residue of the original coal particles in the form of a hard and somewhat glassy solid.
Coke (fuel) - Wikipedia
Coal contains moisture and other impurities that prevent it from as hot as coke. Anthracite is higher density than common bituminous coal and burns much hotter also. Coke and both forms of coal are much denser than charcoal.
Are any of them good for a barbecue grill?
As a heat source, possibly better. Especially coke, which is almost smokeless and burns several hundred degrees hotter for the same draft.
But really, is smokelessness truly a desired trait for barbecue?
So, a good friend of mine has been a hobbyist blacksmith for decades; back in the 1990s, he ran a blacksmith shop at our local Renaissance Faire. One afternoon, my wife and I were over at his house, and the plan was to have a cookout for dinner. He had to run an errand, and asked his wife to get the grill ready while he was gone – he had a barrel of charcoal in the garage (as well as several barrels of coal and coke).
His wife was pretty sure which barrel had the charcoal; I looked at it, and said, “well, I don’t think that’s coal or coke.” So, we used it, lit the fire, and it took. The husband got home, and started grilling dinner…that’s when he asked, “what did you use?” We pointed to the barrel – it was, indeed, coal. (If it had been coke, we probably wouldn’t have been able to get it to light; we used coke at the Faire, and it was a righteous pain to get lit, requiring a lot of airflow.)
Anyway, once the coal fire had really caught, most of the impurities burned off, and the food tasted just fine.
Smoke is important for barbecue, but not for all types of grilling. I suspect he means a charcoal grill, not a smoker. With typical passive non-underflow ventilation, coke and coal may not get much hotter than charcoal. Raise the coals for air to get in underneath and all the materials will approach maximum temperature. You can grill a steak incredibly fast at those heats.
Coke is made from high quality low impurity coal so is nearly smokeless. Some coals can really stink though. I wouldn’t cook over the direct flames or expose food to the smoke but it might work fine with a flat top grill.
I realize all the water and impurities are burned away when making coke. But they are burned away making charcoal also. You state that coke is denser then charcoal, OK. But what if you have a pound of powdered charcoal and a pound of coke. They apparently do not combust the same.
I did a bit more research and this time I found a range of carbon values for charcoal that run from 50% to 95%, so my original statement of 98% is wrong. Now let’s burn the best charcoal we can find and compare it to coke. Is there still such a huge value in the flame temperature?
I have watched enough videos on home iron smelting to see that they crush the charcoal in marble sized chunks. So if coke is denser I can see how chunks the same size would burn differently. But what if we pulverize it the way they do for coal fired turbines and inject it into the air stream. Do they still burn differently?
For what it is worth - back when I used to shoot black powder guns the guys who made their own black powder always lusted after willow charcoal. Made from the willow branches, not the main trunk.
Willow charcoal is exceptionally porous, which is needed for gunpowder. Effective gunpowder is not just a mixture of nitrate, sulfur, and charcoal powders, it’s more like charcoal saturated with nitrate and sulfur.
The hottest charcoal comes from the densest woods. The smelting methods vary as would the chunk size. Not all smelting processed are based on the highest temperatures.
I don’t really know any other details to explain the differences in temperatures except that there is a lot of variance in the materials in terms of impurities, moisture, porosity, and the granularity.
Certainly I think you’ve answered the majority of the question yourself - the initial assumption about carbon content of charcoal is far too high.
even reasonable quality store bought charcoal would be in the range of 65-75% carbon from my reading.
Somewhat anecdotally - in the grilling world, a very popular (and expensive) charcoal is Japanese binchotan aka white charcoal. This burns very hot and with almost no smoke or flavour. It’s used in certain grilling applications which require this.
Binchotan charcoal is processed differently and from higher quality Woodstock. I’m not sure if it burns as hot as coke, but I think it confirms you can get charcoal to burn hotter with appropriate treatment
Although this question looks simple, it really is not. For example; if per the reasoning above, you used activated carbon powder in gunpowder : you’d think it will be better because activated carbon has great porosity and is almost pure carbon; but you will realize that it does not work !!
So lets look at it a bit closer, there are a few factors at play. The first biggest factor to realize is that burning of coal is an heterogenous reaction i.e. there are 2 phases involved : solid and gaseous. In homogenous reactions, like the burning of acetylene or propane or natural gas, its all one phase and diffusion plays no part.
But in solid-gaseous reactions, diffusion and heat transfer plays a big part. Typical oxygen (air) has to diffuse into the pores of the solid for the reaction, and the resulting CO2 has to diffuse out. The solid particle itself has to heat up for the reactions to start and the heat “diffused” from the outside to inside and introduces delay. If it is coal then, then volatiles are released as heat travels inside he particle - thereby creating a different combustion mechanism.
Lets look at some of these factors :
- Adiabatic flame temperature : Many different coals / cokes have been analyzed over a century, As many have pointed out Coal starts as wood->peat ->lignite->sub-bituminous->bituminous ->anthracite with anthracite having the most carbon percent.
The adiabatic flame temperature of coal (also coke) has been regressed to the following equation :
Tad°F = 52009 - 494(C) - 391(H) - 448(O) - 486(H20) - 438(Ash) - 1087(SR) +
0.51(Tca)
Tad°F = Adiabatic flame temperature, in F
(C),(H),(O),(H2O),(Ash) = Carbon, Hydrogen, Oxygen, Water , Ash Percent → obtained from Ultimate Analysis of Coal.
SR = Stoichiometric Ratio of Air (SR = 1, if no excess air is provided. For example the SR=1 ratio for gasoline cars is 14.7, i.e. 14.7 parts of air per part of gasoline. If you give more air then the engine is running lean and if less air then the engine is running rich!)
As you can see from the equation above, for anthracite and charcoals, the H, O, H2O go almost to zero, thereby the adiabatic flame temp is high.
2.Overall Combustion rates The general mechanism for coals for combustion can be divided in 2 phases :
a. Devolatilization or Pyrolysis : You can understand this very well if you burn green wood. The first step is for the water and volatile matter (oils, alcohols, hydrocarbs, etc) to be released. This step is heat transfer limited.
Special note : Charcoal is where you stop the combustion process at this stage. You heat up the coal to drive out the volatiles but do not let the reaction proceed any further. How quickly or slowly you drive out the volatiles, results in a porous or pocorn like coke.
b. Char Oxidation : This stage is diffusion limited but produces the most heat. You will know this phase from lighting a charcoal grill, when the volatiles are driven off and you get that red hot heat. You can make more reaction go at this stage by putting in more air (driving down diffusion limitations)
Finally lets talk about gunpowder as the OP said :
Here is the combustion rates of different coals
https://ibb.co/zP4FMJj
It is true that Anthracite burns the hottest, but it takes a long time to ignite. The volatiles in coal help to combust easily and thereby initiate the reaction quickly. A little bit of volatiles is left behind in charcoal and therefore it makes a great material for gun powder, however activated carbon powder has no volatiles so it does not ignite in gunpowder.
The requirements for a grilling charcoal is totally different than a gun-powder charcoal. The gun-powder charcoal needs to have a quick ignition and a lot of heat generation in a short time; a grilling charcoal does not care so much about quick ignition but it needs sustained high temperature for a long time.
That sounds like the difference between burning wood and charcoal so I’m assuming the answer to the OP is analogous.