Re: Coal cooking summary

[Carefreeland@aol.com]

 Tami, Stovers, 
       We are all more familiar with wood and charcoal.  We now want to apply 
our skills to coal to improve the combustion.  Let's look at the primary 
differences between charcoal and rock coal.  
      1. Rock coal has more ash, similar to a briquette with clay binder. 
      2. Rock coal has higher density, more like the ceramic stove than the 
ticanium stove. 
      3. Rock coal has almost zero porosity unless chipped or powdered. 
      4. Rock coal can be high or low volatility, but generally is lower in 
volatile than wood or char.
       The first feature gives more heat retention as the resulting ash 
insulates the burning embers much more quickly. This also enables reflection 
back into the burning coal. 
       The higher density holds more heat once properly warmed. Higher 
density acts like a heat sink while trying to ignite, requiring more initial 
heat.  
       Lower porosity allows less surface area which slows the burning rate 
without sufficient movement of oxygen. The higher heat retention 
characteristics somewhat offset this in open burning. This is because of the 
higher resulting internal temperatures producing CO instead of CO2. The CO is 
then emitted and burned as more oxygen becomes available away from the ember. 

       The more volatile rock coal has, the more hydrocarbon gas emitted at a 
lower temperature.  This gas can be useful or not just like the gas from 
wood.  The exception is that rock coal produces sulfur compounds, nitrogen 
compounds and heavy metal oxides.  For this reason I also recommend outside 
ventilation of flue gasses. 
       The Chinese undoubtedly have used some sort of bellows or even 
blowpipes just to establish their pit fires.  The lack of air circulation 
would probably kill the flames from CO2 suffocation without extra oxygen.  A 
sizable fire built from wood would also give sufficient air circulation. 
       Since early man, bellows of all sorts have been used.  Some made of 
skins, some just blowpipes to enhance the lungs. By directing the air into 
the coals, less starting fuel can be used. 
       There are two kinds of reflectivity spoken of here. 
       1. Direct reflectivity, where the infrared and visible light are 
reflected off of a shiny surface. 
       2. Secondary reflectivity. This is where the radiation from combustion 
of fuel heats a surface such as the inside of a stove. This surface then 
reradiates the infrared light waves back to the fuel. The heat retention is 
required to steady the intermittent pulses of combustion. 
       With the rock coal, the combustion tends to be self contained once 
started.  The problem is the lack of initial combustion heat to sufficiently 
raise the temperature of the entire grain of coal.  If powdered coal is used 
it helps, but still does not match the porosity of wood char or the volatile 
content of wood. 
       Anyway you look at it you need more air.  Direct reflectivity would 
help, but how do you keep a white or polished surface?  High temp firebrick 
has a high alumna content keeping it's walls white during combustion. 
       Thin cast iron seems to be a compromise. 
      1. It heats quickly enough not to conduct too much heat away from the 
starting fire.
       2. It retains enough heat to be secondary reflective. 
       3. It insulates enough not to draw heat away from the beginning fire.  

       4. It can be thin and still not burn through unlike any other material 
known to man. 
       I return to the thought that we threw away the perfect material for 
combustion stoves when we forgot how to make certain types of iron materials. 
 Every time I start my potbellied stove I am reminded of this. My stove has 
both of the iron and brick materials and was made very long ago.  In our 
stoves today, the secret is all in the grate. It is the only high carbon 
silicon iron (wrought iron) used in combustion. 
       Rock coal was the common term until 1900.  The "rock" name was dropped 
after charcoal was almost made obsolete. That was back when the "Collier" was 
the most important person around.  He was the skilled tradesman who made your 
coal for you. 
       The best stoves I have known for coal burning, had the these same 
features.  Secondary air is not as important for coal once up to temperature 
because the burning temp is hot enough to completely combust everything.  
This is provided  enough primary air is present.  Draft is far more 
important. 
           Dan Dimiduk 

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