Grate Area Vs. Soot Buildup

I'm burning Wyoming bituminous coal in my homemade hand-fed. I have three grates made of rebar, patterned after the Harman Mark III. When I first burnt the stove this fall, I started with all 3 grates. I noticed two things. First, the house got too warm with that much grate area. Second, the walls of the stove and glass quickly were covered with a tarry, oily, black, sticky substance. Also, after a few weeks, the walls of the stove and stovepipe were quickly getting a "fuzzy" buildup that reduced my draft, heat transfer, etc.

I decided after a few weeks that I should check my stovepipe for buildup, so I shut the stove down. I cleaned the chimney and stovepipe, which were both getting pretty heavy buildup. While I was at it, I took one of the grates out of service by stacking firebrick on the back grate and removing the rear connector pin from the shaking mechanism (see photo of grate connector). I was hoping that would reduce the BTU output of the stove. For the past 2 months I have been burning this way with 2 grates in service. The result was that my chimney, stovepipe, and firebox have been staying much cleaner. I inspected the stovepipe after 1 month, and it was very clean. I have not cleaned the chimney, and it looks clean as well. The fuzz has burnt off the firebox and I don't have the oily glass and walls.

So now the weather has turned colder. I had a few changes I wanted to make to the stove, like adding some sheet metal under the grates to better funnel the ash to the pan, preheated secondary air, etc. I also thought I should "unreduce" my firebox to get more BTUs back. I did that last night. Today, I am back to burning 3 grates, but the oily tar is back and the firebox is growing "fuzz" again. It also seems like even 6-8 hours after I load coal, not all of the coal has ignited completely, some still as black as when I added it.

Here's my question: I'm wondering if this has something to do with my draft. My 6" SS chimney is marginal, but sufficient to keep the stove going and keep the house at 80*+ if I want to. I have read some threads that claim that regardless of the burn rate, the soot shouldn't change (unlike wood fires where smoldering causes creosote). However, I'm thinking that my setup is causing the soot to form worse if I burn a big firebox at a low rate. I'm thinking of going to a single grate and see what happens. Any thoughts?

One more question: If I do things just right, have a nice deep tight bed, and burn the volatiles off hard and fast, say within an hour or two, I can get many hours of dancing blue flames from the bit. coal. It's a sweet sight. I just about can't quit looking at the dancing blue ladies. They elude my camera pretty well even without a flash though.

Of course anthracite can and should give blue flames. But are others burning bit. coal getting blue flames? Or only yellow flames and then just glowing coal? I can't get much blue flame when I run all 3 grates, maybe because there's not enough airflow through the bed.

Thanks for your thoughts. I've posted several times on different issues. Everyone is helpful. Just hoping to get some answers and leave a trail for others to solve future problems too.

Steinke

BTW - another interesting item about 2 vs. 3 grates - The ash with 3 grates seems to be fluffy/fine and falls throught the grates well. With only 2 grates, I get harder ash clumps and even a few clinkers. With 2 grates running, the ash seems to clog the grates worse and I have to use a poker to clean them some. I'm wondering if I'm reaching a higher bed temperature with only 2 grates, where ash is fusing, volatiles are combusting differently, blue flames are dancing, etc. I'm not sure what to expect with 1 grate only, might be scary.

Steinke

I've seen plenty of blue flames off of Bitum coal, but like you said, only after the volitiles have burn off. As for the two vs three grates, I think you may be on the right track with the limited draft idea. You should have a good hot fire all the way across the coal bed if you have enough draft. What you are seeing is the same as haveing the air inlet control turned too far down, not enough air getting to the fire to get to burn all the coal. The only time I've seen black areas in my hand fed firebox was when clinkers were blocking the airflow under the coal. So your lack of draft would create the same thing: not enough air to the fire.

Greg L

Thanks. I have plenty of (OK, too much) draft if I leave the ash door open for awhile. I was able to be home on the long weekend, and learned alot about the stove. I really think my draft is OK, I may have just not drilled enough openings in the draft plate (see pic below). There are sixteen 1/2" dia. holes. I may build a larger plate with a few more holes. I'll wait awhile and see. One inherent problem with forums (or me) is that my impatience/jumping to conclusions is exposed publicly. I had made the comment above that with a reduced firebox, I get more hard, chunky ash that clogs the grates. From the past 5 days, I think I'm wrong on that one. My grates are doing "great" and I must have just gotten into some rocks/shale earlier that caused some clogging, etc.

Steinke

I went back to just buning 2 grates. It looks to me like I just don't have enough draft to burn 3 grates very well. I'm sure I'll try again sometime.

On a related note, I have been burning my bit. coal 2 different ways. One way is with big chunks (6" size). That large size burns more like wood. I shake the bed down and add the big chunks. The bed is not tight, and the btus are slowly burnt off. But the fire burns alot differently. Longer yellow flames, very few blue flames during the burn cycle, except toward the end when the chunks have begun to disintegrate and become smaller. Seems to me like the volatiles are "metered" off slower, because they aren't exposed since they're are still trapped inside the big chunks of coal. Few clinkers with this burn. I get more even heat, but don't enjoy the show as much. I'm thinking anthracite may not provide the opportunity for this type of burn, because it may die. I have enough volatiles, and I can be pretty sloppy with this method and still get heat. I get 12 hour burns no problem. But I make a mess loading with the big chunks (seems like I shouldn't but I do). I don't really need to open secondary air with the big chunks. Plenty of room to get some top air from underneath.

The other way I've been burning is more classic. Nut coal and smaller. I shake the fire down and add as much coal as I can, leaving a spot in the firebox with old coal exposed to light the volatiles. I open secondary air and the ash door for 5-10 minutes to get things going good. Then, I have shut the ash door and close the primary air down to 1/4 open or the stove gets to 650* or more. 1-2 hours later, after the volatiles are gone, I have a nice bed of glowing coal and blue flames. I fill the hole with new coal, and open the primary air up full open, and shut secondary air off. The stove then stays at 350*-400*. 6 hours later or so, temps slowly fall to 200*-250*, and I know it's time to reload. I need to get my firebox deeper to get more burn time. This morining I tried to go 12 hours between loads, and I just barely had enough coals left.

I notice that with the latter classic method, I get hard clinkers, at least in the very center of the firebox. Some softer clinkers around the firebox edges. So I'm probably getting hotter temps with the classic method.

Question I have is this - is there an advantage to the classic method vs. burning the bigger chunks? Clinkers are interesting, but get old in a hurry. I enjoy the nut/pea coal more, easier to load, nicer flame, funner to burn. But is it worth it? Picking clinkers is not fun after while either. Any opinions on burn efficiency comparisons, soot generation, etc. comparing the two methods? Maybe most coal can't be burnt except the classic way? Maybe I should try avoiding the pea, just use nut. But the "run of the mine" coal has so much size variety, I hate to throw away any more than I already am (fines). Maybe I should buy a stoker too, so I can sort the coal and use the appropriate in each stove.

Steinke

I am a fan of 8" flue pipes I change your 6" to at least a 7" pipe. I burn soft coal in a 7" flue and have no problems. I also have a very good chimney draft so It is easy for me. Look into this as well I bet your coal will burn better!

with smaller coal sizes I do get a very clear volitle flairing stage, carbon burning stage (blue flames of CO) and then it dies to a glowing coalbed. when I burn lump, I get much more even heat, slightly more soot, but the volitile stage lasts pretty much throughout the burn cycle, I much prefer the larger coal sizes simply for ease of maintaining a constant consistant burn time and temperature. with an 8" pipe, your pipe sooting problems will go away.

I never read this thread by Steinke until now. I don't throw anything away...If I'm going to be home for the day and have the time I bring in a bucket(s) of fines/small stuff and feed the stove small amounts throughout the day with them. It's kind of a pain but I don't want any coal to go to waste. I prefer the lump/larger pieces as apposed to using small nut/pea sizes because it's easier to control in my old hand fed stoves. Don't have much soot problems either way and seldom get clinkers.

steinkebunch wrote:Of course anthracite can and should give blue flames. But are others burning bit. coal getting blue flames? Or only yellow flames and then just glowing coal? I can't get much blue flame when I run all 3 grates, maybe because there's not enough airflow through the bed.

The color of the flame depends on when air is mixed with the fuel. If air is mixed when the fuel is in a gaseous state the flame will be blue. Yellow flame is from bits of particulate carbon burning that have received adequate air and heat later in the burn cycle. When you light your acetylene torch you get a long yellow flame which goes blue when you add the right amount of oxygen.

Anything you can do to make the combustion chamber hotter, like shape and firebrick lining, plus secondary air will increase blue flame and less smoke.