steamup wrote:This is a highly techinical question. In pure theory, coal combustion will not condense like gas and oil combustion. Why, because coal is carbon and when combined with oxygen will result in carbon dioxide. (C + O2 => CO2 (12 kg C)+(32 kg O) => (34 kg CO2) ). Gas and oil are hydrocarbons. When hydrogen is combusted, you get water. (2 H2 + O2 => 2 H2O (4 kg H)+(32 kg O) => (36 kg H2O) ) So, in pure theory, coal will not condense to form water.
However, real world issues are that coal is neither 100 percent pure carbon nor perfectly dry. Therefore, any moisture in the coal that is evaporated during combustion, could condense if there was sufficiently cool enough combustion chamber walls.
The sulfer in the coal is combusted to form sulphur dioxide. (S + O2 => SO2 (32 kg S) + (32 kg O) => (64 kg SO2) ) While the sulphur will not condense until really low temperature, it could combine with the water to form sulphuric acid. (H2SO4). This would attack any metal surface over time.
Another problem is that any water temperature below 140 deg. F., is not very useful for heat. Another topic of concern would be on of thermal shock on cast iron equipment. ( not so much of a concern for welded steel boilers).
So, in conclusion, the rule of thumb of 140 deg. f return may not be as hard and fast for coal, but is a good number to stick with.
Note - wood is a different animal, more moisture content and creosote problems.
In the case of a coal boiler, aren't you really balancing coal cost vs. electricity cost? Run your temps too high and delta T between water and flue gas is less, so less heat removed from combustion products and more coal used to reach setpoint, but also less electric used distributing heat from larger delta T between water and living space, but also more residual losses from boiler jacket. Run temps low and use more electric to distribute to the living space, but extract more precious BTU's from the exhaust gas.
If I ever built my own house I would install in floor radiant, conduction heat Xfer is the most efficient way to transfer heat and I could run fairly low temps to get the most heat from the coal flu gas. Right now I run about 150F, but my circ pump and large furnace heat heat exchanger draw out the BTU's in a hurry, to the point where a full fire will keep the water around 111-115 at steady state and I'm wasting electric blowing luke warm air into the living space, especially when its this cold. I think I am going to try installing an interrupt aquastat soon that stops the heat call if the temp drops below 125-130 to give the boiler a chance to recover.