First, as i said above, anthricite is good for most people and it suits thier needs well. Coalman I'm sure you'll agree with me on this.
Their are two types of efficiencies that have to be taken into account;
2. efficiency of the actual fuel burning
the reason that many seem to think that their stove/furnace/etc. are more efficient when burning slow is two fold, one- there is the fact that not as much coal IS being used and two, the STOVE may very well be running more efficiently. this is because there is less excess air entering the burning coal that has to be heated, and the hot gasses are moving more slowly through the combustion chamber thus losing more of their heat to the heat exchanger before being exhausted out the stack.
The efficiency of the actual fire, the actual process of combustion, will never be more efficient when it is proceeding slowly from restriction of the air.
high quality anthricite coal empirical formula: C240H90O4NS
When hard coal burns in enough oxygen; being mostly carbon co2 is formed, however, there is still a sizable amount of CO this is because outside of perfect conditions there will always be CO when burning hydrocarbon fuels. CO is gasous fuel, it can further combust to produce CO2. When burning anthricite rapidly, with ample oxygen, CO is kept to a minimum, however, when the air is restricted to slow the fire the percentage of CO being produced and remaining unburned can reach as high as 90% of flue gasses. So, as you can see even burning anthricite slowly will result in large wastes of fuel.
However, because of the nature and design of some stoves/furnaces the OVERALL efficiency of the appliance may actually increase when coal is burned slowly, anthricite or bituminous; even though the combustion process becomes less efficient.
Bituminous coal empirical formula: C137H97O9NS
Bituminous can contain anywhere from 15% to 45% volitiles. Bituminous coal combustion is extreemly complex depending on whether stoker fed or bulk fed, and how much air is present dramatically changes the ways in which bituminous burns, much more so than anthricite which behaves in a simpler fashon. But basically its like this; The hydrogen fractions of most hydrocarbons will allways attempt to combust first, in soft coal, this means that under incomplete O2 conditions the intial combustion that takes place in a fresh load of coal happens right next to the piece of coal burning in the appliance- which is actually the complex hydrocarbons breaking apart and some hydrogen combining with O2 to form hundreds of different gases including h2o as steam. So simply i will say the the some of the volitile fractions are combusting even in a very oxygen poor environment; but even as the volitiles produce energy by breaking down and partially combusting, a large portion ends up as visable smoke.
based on heating load calculations for my home, outside temps being mild and the btu content per lb of my soft coal, i can maintain 60-70% total OVERALL efficiencies even while burning slowly in my bulk loading stove.
Conclusion is that while soft coal may waste slightly more energy while burning slowly, the differences are not dramatic. The main difference between bituminous and soft coal is that the inefficiency of burning soft coal slowly shows up as partially burned hydrocarbons in the form of dense yellow/grey smoke while hard coal shows little to no visable signs that it's burning inefficiently.
btw, i don't want people to get the wrong idea when i mentioned "smoke and smell" in my previous post; i can very rarely smell the coal burning outside my home, and the smoke is not excessive even though i burn some of the highest volitile coal that can be bought (for those of you that had the picture of a steam locomotive pouring out black smoke in your minds)