Bituminous Stoker/Lump Coal Grates

I am trying to build a coal fired boiler. Does anyone know of a stoker burner that accepts Illinois bituminous coal? It's called stoker coal and is sized about 1" to 2" diameter lumps.

Also, if you run a bituminous coal (with sulfur in it) burner at 1/3 or 1/4 of it's maximum heat rate, will the exhaust temp drop low enough to condense elemental sulphur out on the boiler tubes?

Thanks for your help.

Welcome Teddy,

Put "bituminous stoker" in the search box at the top right of the page. That will get you some ideas. A lot of the Bit stokers are out of production, but there is nothing wrong with a used one. The old machines were built like tanks to last many decades of constant service.

I think Wilburt is still in business. Winkler, Iron Fireman and Motor Stoker are some of the oldies but goodies. One of our members uses a modified Gentleman Janitor anthracite stoker to burn bit. Another member down in New Zealand built his own stoker and is currently working on a boiler burning sub-bit.

-Don

Another boiler builder...outstanding

I'm the one in New Zealand that StokerDon referred to.....yay i'm world famous

i have no trouble with sulphur dropping out of suspension no matter how low I run my stoker, on my "idle" setting I can hold my hand on the chimney pipe right where it come out of the stove.....the coal I use is 1.7% sulphur

that size of 'stoker' coal you refer to is a bit larger than what most residential/domestic underfeed Bit stokers seem to be normally designed to run on.....most seem to be intended to use pea coal that is about 3/4" to 1"

Thanks for the info Unhippy. I have attached a picture of the grates I intend to use. The question about the grates is, can I drop the 1"+ stoker coal onto these shaker grates, shake them periodically and will they process the ash and clinkers? These grates were designed for a sawdust burner. I do not intend to use the vertical section, just the two horizontal shaker grates. They should give me about 0.7 ft2 of grate area which should be able to burn 6 or 7#/hr of coal. I want to set it up for two fire rates - max and 1/3 to 1/4 max. I can control the rate that the coal is dropped to meet the thermal demand and maintain a relatively consistent coal bed depth. I will be pre-heating the primary air and secondary air and everything will be enclosed in firebrick with insulation on the outside of them and then enclosed in a steel shell. The secondary air will be introduced just above the bed and the primary air will come through the grates. Do you think there is enough air slot area in the grates to burn 6#coal/hr using natural draft and a 32 foot tall 8" stack running at 400 Deg F?

I had intended to control the exhaust temp after the boiler to approximately 400 Deg F to insure no sulphur condensation and provide good draft. My Illinois coal has around 3% sulphur and 7% ash. I designed and built a manure digester gas exhaust gas heat exchanger for a caterpillar engine a few years ago and plugged the heat exchanger tubes solid with elemental sulphur running the exhaust temp down from 1200 Deg F to 250 Deg F with 220 Deg F water in the shell side of the exchanger. I had to plug some of the tubes to get the exhaust temp up to stop the sulpher from dropping out. The biogas had about 1500 ppm H2S in it. The sulphur should condense out at any exhaust temp less than 350 Deg F. I don't know why you aren't having any problems??? I am trying to figure out the best way to bypass some of the heat exchanger tubes on low fire to keep the exhaust temp in the 400 degree range.

The house is built, the insulated 8" stack is installed, The outdoor hopper and 8" auger to bring the coal into the basement is installed. I am trying to confirm that my burner design will work and get any advice I can before I build the burner and the boiler. Thanks for your help and yes "another boiler builder"!!!!

PS I took a pick up load of my coal to the place where I bought the grates and fed it into their complete sawdust burning furnace/boiler using the same grates and a makeshift 20 foot chimney for draft. There were no forced or induced draft fans. It got so hot that it burned the paint off the furnace and the coal in the feed hopper just above the grates melted into a big clump. That is why I want to drop the coal onto the grates to maintain a consistent bed height.

Even without sulfur issues, a solid fuel boiler is likely to accumulate ash, etc. on heat exchange surfaces. The normal remedy is to clean the boiler periodically, not to assume that accumulation can be prevented through boiler design.

That said, boiler design features can help to minimize the adverse impacts of accumulation and the frequency with which cleaning is needed. From my limited experience operating different boilers, I would say that vertical firetubes or plates require less cleaning than do horizontal firetubes (though some horizontal firetube designs look like they might be more efficient).

If you need to run the unit in the summer, it seems like a true stoker would have advantages over the reduced feed rate approach you have described. A stoker can idle at an extremely low effective feed rate, with a small fire maintained as needed using a repeat cycle timer to periodically add a small amount of fuel and keep the fire from going out. On demand (e.g., to make DHW), it can run hard for whatever duration is needed, then go back to idle. I know many people in the lower 48 who run stoker boilers year-round for DHW; I don't know of any who do it by reducing the feed rate to a fire driven by natural draft. (Maybe there are some I don't know about in the hand-fired section.)

Hope this helps.

Mike

P.S. I probably should caveat all of the above by saying my experience is with anthracite. There are some bit burners who may comment here, but you might get a better response from them in the bit section.

Well Teddy.....i was a little puzzled by your problem with the heat exchanger plugging up with sulphur....then the penny dropped.....you mentioned that you had 1500 ppm H2S hydrogen sulfide......the sulphur in coal is plain old S....when burned it results in SO 2 sulphur dioxide.....which won't condense until well below the freezing point of water.....i think thats why I can run my stove so low without any sulphur problems.....

That 0.7 sq ft grate size is almost the same area as the the grate in my coal range cookstove that also heats my domestic hot water....6 to 7 lb of coal an hr is for me a medium fire rate but doesn't result in clinkers, however the firebox is water jacketed and the coal I use has quite a high ash fusion temp.....i can run 20+ lb an hour but that makes clinkers galore and has the cast iron cook top glowing light red almost orange.
If you insulate the firebox with firebrick/refractory etc I would suspect that you would have clinker problems at a far lower fire rate than I do with my water cooled firebox due to that fact that your firebox will be hotter without the moderating effect of the water jacket.

I don't know how that grate will go dealing with clinkers.....all the "fixed bed" rocking grates I've seen (talking industrial sized boilers) have open ended fingers that intermesh with the grates on either side allowing them to crush any clinkers between the fingers as the grates rock back and forward.....they are normally also in constant motion slowly clearing the ash and clinkers.

The feed type you describe sounds like a soup'd up pellet fire type thing....with bit coal that could be a recipe for making smoke on the low fire setting....i think that high fire wouldn't be too much of a problem as the fire should be active enough once stable that a couple of lumps of the size you describe every few minutes won't produce enough volatiles to over-fuel the flame to any great degree.

Some commercial steam boilers have an over fire feed system with a rotary "slinger" or spreader to distribute the coal (fairly) evenly over the grate

Your coal being a "melting type" bituminous is a bit of a downer as it makes life a bit difficult stoker-wise without going to a 'proper' underfeed type for the size of stoker we are talking about here

I know you already have your design mapped out however maybe building something like this http://www.kotly-witkowski.pl/en/palnik-apr might be an option.....building one based off these is my 'fallback' option if I can't get the burner (this one, http://www.kotly-witkowski.pl/en/palnik-ekoenergia) I want for my boiler....lol its looking more likely that i'm going to have to build one as the manufacturers don't seem that keen on selling only one to me ...but I can get an awesome deal on 50 of them ......shipping and import duty would be a killer tho

do you have a drawing or sketch you can put up that shows how you intend to it to work?

Thank you Unhippy for your words of warning. I am leaving in an hour for a motorcycle race in NY and won't be able to continue this discussion until next week. Nothing is built yet, I just have those grates, so the world is my oyster as far as design is concerned. If I need to find a "proper" underfeed grate for my coal, then so be it. Talk to you next week. Teddy

Just a suggestion take it or leave it.

Look around, there will be a good used stoker/boiler in somebody's basement that they would love to have someone take away for free.

There is no good reason to reinvent the stoker

I agree, I wish I could find one that someone wants to give away!!! Do you know of any I could get? I looked at the Polish burners, the cast iron one does look sweet. I don't see how they deal with ash and klinkers.

After your comments, and if I can't find an old manufactured one, I have been thinking about experimenting with using just the two movable grates and start them positioned in a small vee, like 20 Deg included angle with an air slot between them, maybe 1/4" to 1/2" wide. Then periodically rotate them down or vibrate them down to open up the air slot and drop out the clinkers/ash. Or would the klinkers just bridge and stick to the grates and not drop? I like your idea of the intermeshed fingers to break them up, but I would not be able to modify my grates to do that.

Could the fire rate be changed by changing the depth of the coal - the coal is a bigger size and there should be sufficient air passage at the deeper depths on high fire rate? Then for low fire rates, let it burn down, reducing the depth of the coal and drop less coal. I plan to have 4-20ma actuated air dampers on the separated primary and the secondary air inlet pipes. Haven't figured out yet how to control them - by coal feed rate, by combustion temp, by stack temp, by stack O2, all the above?

I big goal here is to NOT have the burning coal smell in the house. I am hoping that a tight furnace, piping in outside combustion air and the 32 foot tall stack will keep the whole furnace under vacuum and accomplish this. Hopefully when opening up the ash door, room air will go into the furnace and no burning coal smell will come out. Or do I need to try to auger the ashes out of the house in a totally closed system?

I miss-wrote!! I am thinking the grates are 10 deg each side off of horizontal. Or maybe 30 deg, 45 deg?

Where's Berlin.

The Polish underfeed stokers push the ash and clinkers out over the side's the same as the EFM, Iron Fireman and Gentleman Janitor etc.....the one with the vee'd bed grate pushes the ash out over the end.....that is the one I was going to replicate as it is the easiest to build.

I don't know what your coal clinkers like but I would be concerned about the clinkers bridging if you didn't have any way of mechanically breaking them up....
my homebuilt stoker clinkers when fired hard.....bad enough that I have drawn up a clinker breaker that is actuated by the feed block.....but not bad enough that I have got off my chuff and built the breaker.

Controlling the fire output by varying the bed depth would I think be problematic as you would have a very slow ramp up and ramp down time as you first wait for the extra coal being dropped on to catch fire and then at the other end as you wait for the now deeper firebed to burn down.....its this second one that would cause you the most problems as you would have a large water temperature overshoot....if you had your aquastat set to account for this overshoot at full boiler output load and then had a warm day where your only using 25% of the boiler available output to heat the house I would think your heat dump circuit would be getting quite a workout......likewise if you set it up to account for the overshoot in warmer conditions with less load on the boiler in a cold snap you could end up with a chilly house.

As long as the outside combustion air is shut off before you open the ash door I can't see how you could get smoke etc out of the ash door as it is below the grate and the chimney draft would pull air into the open ash door like you want.....if you where really concerned, you could make a venturi stack ejector to fit into the top 2 or 3 feet of your chimney that activates as soon as the ash door is opened....an airbed inflator a few bits of steel pipe and fittings, some rubber hose, a microswitch on the ash door and you'd be sorted .....yes it does work as I used the same setup to stop a woodstove smoking back into the room when the door was opened

1. What is the angle of the Polish vee grates?
2. I will hopefully know what the klinkers are actually like. I am planning on building the furnace outside to experiment with the grates and feeding of the coal. I'll use the brick and insulation and preheat the air to try and duplicate the combustion temp. First I will run a front end loader of my coal through my 8" auger to see if it will break it up a little and use what will actually be dropping onto the bed. If the grates I have can't be used because of klinker control, I could drop the coal into a conventional stoker burner hopper - if I could find one. I looked up the Wilburt site and their smallest stoker is for 30#/hr!!! My burner needs to be max 6#/hr with a typical rate of 1.5 to 2#/hr. Do you know of any commercial burners that are that small?
3. Slow ramp up and down will not cause problems I don't believe. I have a 1000 gallon insulated "milk tank" that the hot water will go to, so I have an enormous amount of thermal buffering, in the 600,000 BTU range. All thermal demands are withdrawn from this tank. In the summer my geothermal system cycles the compressors on at 44 Deg F and off at 40 Deg F tank temperature. The house and shop have been heavenly this summer. The house is 6600 ft2 and the shop in 2400 ft2. In heating mode, I have 3 way TC valves on each pump circuit from the tank to control the delivery temp to the radiant floor piping at around 95 Deg F. I had intended to coal heat the tank water at max fire rate to 195 Deg F and then let the demand draw it down to 100 F over a few hours to a few days. However, that means starting and stopping the burner which I am not sure will work well. Then I thought I should design the burner for 2 fire rates - max and 1/4th max. When the tank gets to100 F, go to max fire rate. When it gets to 140 to 160 F, drop to minimum fire rate. If it gets to 180 Deg F on min fire rate, open the shop doors and heat the universe, but keep the burner going. I have a lot of experience with co-generation engines burning biogas. DON'T turn them off, keep them hot and running all the time. The thermal stress of heating and cooling almost triples the maintenance costs of running the system. The joy of this project is I have never burned coal and have no experience, so I am desperately needing help understanding the pitfalls.
4. What is your proposed klinker breaker made of? Stainless? Inconel? Cast?
5. What might make more sense is to build a very small burner and run the heck out of it. If it can't keep up with demand, the geothermal will be there to keep the water at 95 degrees. Last winter the new house was finished and the geothermal did all the heating. At 8 tons or 96,000 Btu/Hr the geothermal did everything. I kept track of the minutes/hr that the compressors were on each day and the absolute worst case it ran about 40 minutes an hour or about 65,000 Btu/Hr. More typically it ran about 10 to 15 minutes per hour or about 15,000 to 20,000 Btu/Hr. I got started with this project because at the age of 73, I didn't want to split wood anymore and because the stupid "greenies" were closing down power plants due to their improper understanding of the carbon cycle, I better buy some coal before you couldn't get it any more. Also, the cost of the Illinois coal delivered to my home was $105/ton, about 1/3 the cost of firewood delivered to my home on a BTU basis. I thought, why not get about 20 years worth of coal and not have to worry about it anymore? Just load it into my hopper with my front end loader on the tractor every few days, build a large ash box and relax. So now I HAVE to finish the project!!!!
6. I love your stack venturi idea, I was thinking about doing the same thing but for a different reason, to start the fire when cold, and I have an angled tee already in the stack to do it.

I am sorry this is so long, I can't wait to get home and start burning some coal. It will take a couple days to get set up. I am at my son's home waiting for a baby to be born, we might be here for another week and then I have another vintage motorcycle race to compete in before I can get to the job at hand!!!! You have been very helpful, I am looking forward to your response of this post.

I should mention that I put a ratchet wrench on the end of my 8" auger bringing the coal in from the big hopper outside. I then put an air cylinder on the end of the wrench with a length of stroke that will rotate the auger about 30 deg or 1/12 a complete rotation. I will first find out how many pounds of coal a 30 degree rotation will drop. It should be a fraction of a pound per stroke. I will program the PLC to operate the air cylinder and control feed rate.

Sadly I don't know of any stokers in your area, however there is another member Dalmaliousgirl? that started looking around for a stoker and ended up finding more than one pretty quickly. That being said.

I am far from a expert in solid fuel combustion however, I am a machinist and have worked on/with three separate stokers an have built many things with just a concept to go by.

With the above disclaimer you need to find out some things about the coal you plan on burning. Like is it going to clinker or will it burn to ash, and is it going to swell an stick together. Those two factors will massively effect the design of the burn pot. Those polish stoker units are built to burn anthracite, lignite, sub bituminous, and wood pellets. All of these fuels do not clinker, do not swell, and burn without much smoke. Some bituminous coal will clinker and will swell an fuse into a mass when burning however some does not but, all has a lot tars that burn with black smoke.

What you need to find out is the ash fusion temperature (this will tell you if it will clinker) and something called the coke button (this tells you how much it will swell)

If you havent already, you need to look at the many different types of stokers made. That will give you a idea of how they are made.

The polish stokers are most likely not going to burn bit coal well at all, it will smoke like a smudge pot and if the coal fuses the fire will drop off into the ash pit. I suggest searching prill stokers(they burn non swelling, non clinkering bit an sub bit. Combustioneer they burn swelling, clinkering bit.

Proceed only after you have a idea of what your getting into, this will not be a 2 week project. Im not trying to discourage you just arm you with knowledge to help you tackle this idea.