It's 4 below here tonight and supposed to be 15 below in the morning. When it gets below zero we switch from wood to anthracite stove coal in our furnace. We have a central forced-air Daka high efficiency combination wood/coal furnace, built in Pine City, Minnesota.
If you've ever tried to burn anthracite, you know it's almost impossible to get the stuff to light and burn. It's like 86% pure carbon - a few more million years and it would diamonds. And when it burns it has no flame (except for some blue flame right at first when the coal gas gets driven off it). It just glows.
Anthracite is the cleanest burning fossil fuel known to man. It emits no smoke, no particulates, nor any odor when it burns. There's only two problems with it - getting it to light, and extreme BTU output once it is burning.
I had a nice hot bed of hardwood coals on the grates in the furnace tonight so I threw in three shovel fulls of anthracite, one at a time in layers. You have to "light" it in layers using the bed of hardwood coals with full draft under the grates. It takes about 30 minutes to get each layer of coal lit, then you add another layer and spend another 30 minutes getting that hot enough to start glowing. I usually put in three layers for a new coal bed. It's important for burning coal to have draft air both above and below the coal bed. The draft air from underneath provides combustion air, the top draft provides combustion air for the coal gas that gets driven off while the coal is heating up. Once the bed is glowing good, the top air is shut off and only bottom draft is used.
The furnace has to be on manual control for draft to get it going. The stuff burns the chimney slick and clean as a whistle. If the chimney is coked up from burning wood in the furnace, it usually gets a chimney fire going first. I let the chimney burn until it gets to critical temperature, then close the air off and kill the fire in the chimney, let it cool down, then turn the air back on to get my coal going again.
The blower on the furnace has to be running all this time to keep the firebox temperature at a safe level. By the time I spend roughly two hours getting the chimney burned out , and the new coal bed burning and stable, it's so hot in the house my wife has the windows open. This is why I don't even mess around with anthracite in warmer weather.
Once it's going, and stable, life is good. Only have to fire the furnace once a day in 20 below weather. And firing it is a 30 minute job every day. Shake the coal bed down, put the furnace on manual and add a new layer, clean the ash pan out, and leave it on full draft until the new coal is going and glowing red all the way thru. Then close the draft down, put the furnace back on automatic, walk away and she's good for another day.
Last year, with two solid months of sub-zero weather I started a coal fire in January and it burned until the February when we finally switched back to wood. This year, with Global Warming kicking in, this is the first day we've burned coal this winter.
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Chris