Coal boiler vs wood boiler efficiency

 
Mr.Ark
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Post by Mr.Ark » Sun. Nov. 11, 2018 8:24 pm

Hello folk,
I’m new here but have been reading for awhile as my interest has been sparked in coal heat after I cut my first years worth of wood for my new building. I will need about 25 full cords of wood for a year. It was then that I realized that I might want some alternative heat also. The building is a rather large shop/house combo and I am going to heat it with a wood/coal boiler and propane backup. The boiler is a slightly used Royall 6300. Does anybody have any experience burning coal in a Royall indoor boiler?

My question is if boilers that burn coal can achieve high efficiency like that of a wood gasifier? The wood guys at the hearth.com forum laughed at me and my old school boiler that I bought cheap because it fit my budget. They said it will never be a good unit for me because it is old technology and inefficient. HOWEVER I’m hoping to get the last laugh on them and burn coal. This unit has a deep V shaped base with air coming up through the shaker grates and a little blowing across the top too. It’s a dual purpose wood/coal boiler that can burn both types of coal.

So the question is do units like this need secondary combustion like a wood unit does to be efficient? I don’t see very many coal boilers with secondary combustion.....not even the high end ones. I was going to use this late-model, like new boiler for the season until next year when I could afford a modern indoor gasifier for wood. Until I discovered COAL as an option and now I think I might just get the last laugh on those wood snobs at hearth.com LOL!!
Sorry for the long post and thanks in advance. I’m a little “stoked” right now because the unit will be online after tomorrow and my new slab will have some heat for the first time!!

Thanks
Drew


 
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coaledsweat
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Post by coaledsweat » Sun. Nov. 11, 2018 8:46 pm

A coal boiler runs near the sweet spot most of the time, you can't do that with wood.

 
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Post by Rob R. » Mon. Nov. 12, 2018 5:40 am

Generally speaking, units that are made to burn wood and coal do not do a great job at burning either one. That V-shaped firebox has a tenancy to create clinkers when burning coal.

25 cords is a lot of wood - what kind of building are you heating with this monster? A gassification boiler would burn a lot less wood, provided the wood is seasoned properly.

 
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Post by franco b » Mon. Nov. 12, 2018 11:57 am

Wood gasification sounds fine, but the fact remains that when you heat wood it very quickly gives up half its heat potential in the form of gas. If you expect that load of wood to heat for 12 hours then you will need massive heat exchange surface or mass to take advantage of all that potential heat released so quickly. Absorb the heat produced so quickly, or waste it. In contrast hard coal only has about 5 percent or so of gas to contend with. Some secondary air at this time can help burn it, but a clear grate at tending time can often be all that is needed. There are good solid reasons why coal took over from wood so quickly when it became available. For many centuries it has been well understood that wood needs huge mass in the stove to burn clean and efficiently.

Coal takes up a fraction of the space and has higher BTU per pound , so much less weight and bulk. to carry and handle and store.

Your V shaped fire pot is trouble to do a good job clearing ash. Read posts by Lightning of how he solved the problem with the use of a flat poker just above the grate.

25 cords of wood is a huge pile and to get it well seasoned and keep it dry is a bigger problem that coal just does not have.

 
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CoalisCoolxWarm
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Post by CoalisCoolxWarm » Mon. Nov. 12, 2018 12:22 pm

25 full cords???

Wow. Even if heating 6 months a year, that is still ONE CORD PER WEEK! Even a low 280 pieces of wood per cord, that is FORTY pieces of wood per day.

What do you do about all the ashes and cleanout?

An auger fed stoker or other solution could save you TONS of time ;)

 
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Post by lsayre » Mon. Nov. 12, 2018 1:25 pm

If your wood is guaranteed perfectly seasoned and is a mix of good quality hardwood its efficiency in burning will be about the same as for anthracite coal. But wood is almost never perfectly seasoned (and sometimes it isn't even close to this goal), and is also almost never a perfect mixture of average mixed quality hardwoods either. They always seem to slip in something on the much softer side like poplar. Coal is very consistent in BTU's per pound vs. wood being all over the place. So the odds are greatly in your favor that coal will in the long run be more efficient than wood. Plus coal can go typically 2 to perhaps 3 times longer between stove tendings. vs. wood. And whereas wood gives off a bunch of its store of heat right away, coal spreads the heat out far more evenly over time.

That said, coal generates what seems to me to be roughly ballpark close to 30 to perhaps 50 fold more ash by volume than does wood. If you are used to ash removal roughly every week to every other week with wood, get used to ash removal twice a day with coal.

 
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Post by Mr.Ark » Mon. Nov. 12, 2018 5:31 pm

I have not used this unit yet and have never burned coal. It’s in a new shop/house that is roughly 6000 square feet with 18 feet sidewalks. I might be a little strong on the wood at 25 cords. That was just a quick calculation on a napkin. Here is some pics of the unit and the firebox.
Does it seem like it would burn coal well given the looks of it?

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lsayre
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Post by lsayre » Mon. Nov. 12, 2018 6:40 pm

I could be wrong, but those don't look like shaker grates to me. You need shaker grates in order to burn coal.

 
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Post by Lightning » Mon. Nov. 12, 2018 6:51 pm

lsayre wrote:
Mon. Nov. 12, 2018 6:40 pm
I could be wrong, but those don't look like shaker grates to me. You need shaker grates in order to burn coal.
It's just too hard to tell if those are parallel style shaker grates or just flat stationary grates.

Edit, by the looks of the top picture there is a lever for shaking the parallel grates.

Yeah, if that's the case it will burn coal no problem. Those levers do wear out at the pivot points though.

 
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Post by Mr.Ark » Mon. Nov. 12, 2018 7:08 pm

They are shaker grates however they might be a little worn at the bolts because they don’t move as much as they would seem. Probably an inch tops of total movement.
Does this firebox look like it would be better with hard coal or soft coal?
It’s got most of the air coming from below but a small hole blowing it on top also.

 
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Post by Lightning » Mon. Nov. 12, 2018 7:15 pm

I see the combustion blower, where exactly does it blow into the fire box? Above, or below the fuel bed?

 
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Post by Mr.Ark » Mon. Nov. 12, 2018 7:18 pm

Lightning wrote:
Mon. Nov. 12, 2018 7:15 pm
I see the combustion blower, where exactly does it blow into the fire box? Above, or below the fuel bed?
It blows into the whole entire ash pan and up from there.
Just a little hole that blows over the top.

 
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Post by Lightning » Mon. Nov. 12, 2018 7:36 pm

You may want to squelch the top hole a little bit somehow, but other than that it shouldn't have any problem burning anthracite. I bet it will take a lot to fill it, probably 130+ pounds. I'd use a nut/stove size mix in it.

 
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Post by Mr.Ark » Mon. Nov. 12, 2018 10:02 pm

Lightning wrote:
Mon. Nov. 12, 2018 7:36 pm
You may want to squelch the top hole a little bit somehow, but other than that it shouldn't have any problem burning anthracite. I bet it will take a lot to fill it, probably 130+ pounds. I'd use a nut/stove size mix in it.
Do I have to operate with it full? Would it work with say 40lbs at a time?

 
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Post by franco b » Mon. Nov. 12, 2018 10:45 pm

Mr.Ark wrote:
Mon. Nov. 12, 2018 10:02 pm
Do I have to operate with it full? Would it work with say 40lbs at a time?
You need coal depth to maintain the fire. With nut coal, minimum 8 inches


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