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Coal and Wood Batch Box Proposal

Posts: 1290
Location: Okanogan Highlands, Washington
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We had a call this week from someone interested in a rocket stove that could burn either coal or wood fuel.
We've done some design speculation on coal before - it seems to need a lot more air, and of course it feeds differently (with a shovel or a hopper, not as long sticks).

Here are a couple of threads:
"Will Coal Work for a Rocket Stove?" https://permies.com/t/20896/rocket-stoves/coal-work-rocket-stove#175369
and "First RMH in Mongolia" https://permies.com/t/13385/wood-burning-stoves/RMH-Mongolia

I would be interested in trying something with Peter's batch-box designs, with a grate and adjustable air settings.

(I am currently imagining a rotating or slot-type air supply door, with settings for "Start" - "Coal" - "Wood" - "Embers" - "Finished". Could be a fun design project.)

I don't know whether the narrow port at the back would need to be tuned somehow to the new fuel, or if you could just size the grate to suggest using a smaller load of coal and stick with the same sizes for everything else. I think the goal of a heater with fewer moving parts might be more useful. If the coal is much richer, we should be able to get a comparable heat output by reducing the amount of fuel per batch, and maintain a more consistent volume of exhaust and air for a reasonably simple heater and chimney design.

Anybody got good numbers on how much air coal needs?
Percentage of water in exhaust?
Anyone with experience running a coal-fired stove where they could recommend a good, clean design for comparison?

My only experiences with coal have been blacksmithing with low-tech pan-forges, simple bellows or fans forcing air in to a central spot in the coal to generate extreme heat. Which is a very different task from producing either steady heat or a complete combustion of one entire load of coal; we'd prefer that the grate didn't melt, for example.

Erica W
Posts: 567
Location: Mid-Michigan
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Fascinating proposal! The blacksmiths group here buys a traincar of coal every couple of years, so I have access to some. The idea of burning a coffeecan of fuel in the morning and a coffeecan of fuel in the evening is pretty exciting.

Here's an interesting writeup at iforgeiron, a blacksmithing group:

Forges most often use coal, charcoal, and coke for fuels. Very few use wood. I wanted to figure out if it could be used and find the numbers needed in order to base a opinion. This may get a little long so stay with me.

Volatile matter is material that is driven off when coal is heated to 950°C (1,742°F) in the absence of air under specified conditions - components of coal, except for moisture, which is liberated usually as a mixture of short & long chain hydrocarbons, aromatic hydrocarbons & some sulphur - measured practically by determining the loss of weight Consists of a mixture of gases, low-boiling-point organic compounds that condense into oils upon cooling, & tars.

Burn the volatiles (gases) from coal and you get coke. This is done industrially or when you add coal to a fire in order for it to *coke up* and be raked into the fire to replace the fuel burned to create the heat for forging. This is most often done with bituminous coal (soft coal) but is also done with anthracite coal (hard coal).

Burn the volatiles (gases) from wood and you get charcoal. This can be done industrially and then is bagged, or bulk, for sale in the stores. The same thing can be done to on a smaller scale at home. Either way the result is wood (charcoal) that is light weight, and mostly carbon.

Reference Indiana Center for Coal Technology Research

Anthracite coal is a dense, hard rock with a jetblack color & metallic luster. It contains between 86% and 98% carbon by weight, & it burns slowly, with a pale blue flame & very little smoke. It has a heat value of nearly 15,000 BTUs-per-pound. Most frequently associated with home heating, anthracite is a very small segment of the U.S. coal market. There are 7.3 billion tons of anthracite reserves in the United States, found mostly in 11 northeastern counties in Pennsylvania.

Heat Content (Btu/lb) 13,000-15,000
Moisture < 15%
Fixed Carbon 85 - 98%
Ash 10 - 20%
Sulfur 0.6 - 0.8%
Chlorine (ppm) 340 ± 40ppm

Bituminous coal (in Indiana), contains between 69% & 86% carbon by weight and a heat value of 10,500 to 15,500 BTUs-per-pound.

Heat Content (Btu/lb) 11,000-15,000
Moisture 2 - 15%
Fixed Carbon 45 - 85%
Ash 3 - 12%
Sulfur 0.7 – 4.0%
Chlorine (ppm) 340 ± 40ppm

I have found these numbers to be general guide lines and may differ depending on the source you use for information, and the type and location of the coal. Many people throw numbers about without knowing what they actually mean in the forge. There is no magic number that makes coal, blacksmithing coal. I have found most blacksmiths use coal from the 13,000 BTUs to mid to high 14,000 BTUs. It is what is available to them at their location and it works well for them in their forge. Yes, higher the BTUs are better as it is a more efficient fuel based on only BTUs. That is until you burn the end of your stock and it falls off into the fire because the fire and fuel is too hot.

United States Department of Agriculture Forest Service publication How To Estimate Recoverable Heat Energy in Wood or Bark Fuels brochure lists the following woods with 8,000 BTUs to 12,000 BTUs.

Oregon white oak, Red alder, Western hemlock, and White fir are on the lower 8,000 BTUs range. Cypress, Yellow pine are at 10,000 BTUs and Pitch pine have 12,230 BTUs. Hickory, Red Oak, White ash, White Burch is 9,360 BTUS, and White Oak is 9,510 BTUs. The bark from trees is from 7,385 BTUs to 10,332 BTUs. Anyone that has used wood for a fire knows that certain woods burn hotter than others.

Density of the fuel
Bituminous mine run coal is 40-50 pounds per cubic foot. Wood is about is about 40 pounds per cubic foot (average) and a useful number for this discussion. Let us say that both are 40 pounds per cubic foot but the BTU value of wood is half that of the same volume of coal. This means that you will use two times as much wood for the same heat.

If you can build a coal fire and use it to burn the volatiles (gases) from coal to get coke while you are forging, then why not burn the volatiles (gases) from wood and get charcoal at the same time you are using the charcoal fire for forging? The fuel BTUs of wood is lower than coal meaning you will have to use more wood to get the same heat BTUs than if you used coal for forging.

The fuel size (wood) I found to work best is about the size of a 2x4 and about 4 inches long or about the same size as charcoal fuel. Build a fire and let it burn down into a good bed of embers (hot wood coals). Add more wood so that as you place the metal into the embers (hot wood coals) it forms charcoal and replaces what embers (hot wood coals) that are consumed. This is no different than burning charcoal as a fuel, except you are making more charcoal from wood as you forge. This is the same as if you were burning coal and making coke as you forge.

Can you burn the end off your stock in a good bed of embers (hot wood coals)? Yes as a matter of fact you can. The metal does not care what fuel you use to get it hot. It only matters that it is hot enough to move under the force of a hammer.
Posts: 521
Location: Salt Lake Valley, Utah, hardiness zone 6b/7a
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Erica, Peter has said that primary air coming up from an ash cleanout through a slit or grate does not work with the batch box, but it may be needed for burning coal. If you review the references I posted in the earlier threads, you can see that the modified traditional stove is not that far off from the batch box. To appease Peter and perhaps make for a multifuel stove, you could put in an ash cleanout and grate or slit with an adjustable vent in the cleanout door that could be shut off for wood and opened for coal (you may not need the additional primary air from underneath, but that ought to be easy to test). You would also want to adjust the airflow through the feed door and through the secondary air channel (p-channel) so you can tune the stove for whatever fuel you are using. Coal, like wood, has wide variations in carbon, water, volatiles and ash content.

The quickest way to know where to start is to throw some coal in a batch-box and start fiddling with what you have. Use the startup techniques described for the mongolian stove. You could weld together a rebar grate to lift the coal an inch or so off the bottom.
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Interesting. I am not convinced it could be done but on the other hand nobody tried this yet. Difficult to say whether this will work or not, I simply don't know.

Just quessing: probably it could be done. With the help of a grate in order to lift the coal from the firebox floor to facilitate under air supply. When tried, I am in favor to do so using a 6" system, that would be a safer size to experiment with. The riser need to be filled with ashes up to the level of the grate in order to avoid a dead end, the under air channel have to be blocked in front of the riser and to avoid rolling coals into the riser there need to be a vertical piece of grate to block part of the port. This is all guesswork, nothing I am able to relate with what I've tried before.

Regretfully, I can't use the three barrel tower for this because we moved house in spring and I've sold the stove to the new owner of the house.
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