How to keep a clean plume?

[Poindexter]

Regulated burner here. In general I am pretty safe from the AQ swat teams, err, my friends in the Air Quality division at the EPA, but I have my system pretty well dialed in.

I know I am not the only regulated burner here, and there are other non-regulated burners here starting to look at their stack plumes. I figure a clearing house kinda thread is worth a try just so we have a thread we can link folks to in the future without having to repeatedly regurgitate.

My intent is for this thread to be about running a clean plume. I do NOT intend debate on the relative merits of our various laws. I like breathing clean air. I would not like it if my neighbor ran a pipe to the property line and started dumping raw untreated sewage into my back yard.

My local AQ (air quality) ordinance requires me to operate my stove, as adjudicated by EPA level 9 VEE certification holder, at under 20% opacity at all times; I am allowed 20 minutes at 50% opacity for start ups and reloads, and I am subject to burn bans that don't allow me to add any more wood to my solid fuel burning appliance until the burn ban is lifted.

Fine for non-compliance is $1000. I have done the free online course work for the EPA VEE level nine, but not paid the $300 and given up a weekend to actually be certified. The foundation cert is only good for one year and in broad daylight, to get level 9 certified to read at night is another class, and another $300, every year.

I am kinda motivated to not get a ticket...

 

[Poindexter]

Introduction aside, here is what I have done and observed from running out in the cold to look at my stack plume dozens and dozens of times.

One thing I did by accident is I run spruce only. All I can get up here in quantity is spruce or birch. The wife and I have varying degrees of birch allergy. We intentionally chose to start burning spruce only to reduce our individual annual allergen loads from birch, an unexpected payoff is since I am only burning spruce, when my stove starts acting "funny" I know it isn't because I was burning oak last week and got into some elm this week.

I don't think wood species is a huge variable compared to moisture content. From reading about various wood species here it is abundantly clear to me that switching species _can_ have enough effect to change ideal operation of the wood stove. I am sure many, even most of you guys can handle it. But if you are really struggling to clean your plume up, you might look at limiting the number of wood species in your stacks.

Moisture content is huge. When I upgraded to a catalytic stove it was clear in the first autumn shoulder season that I needed better control of my seasoning process. I am penurious enough to season my own wood, but "dry" isn't good enough when the option is a $1000 fine. I can run my stove on 20%MC, and I can run it clean, but it is a time consuming daily PITA (Pain in the Ampersand) to do so. I found fairly easily the sweet spot for me (my stove, my install, etc) is to run wood fuel at 12-16% MC. 16-20 is usable, and the mfr of my stove has said more than twice that 20-22% is OK, but it is no where near ideal for me at my house.

So I got my seasoning process under control. Target is 12-16% MC, this year every split I have opened to measure the inside, 13-15%. I see 14% on my meter a LOT and my plume is looking good. I don't know what the magic number is for your stove. It is probably some range of numbers between 12 and 20% MC. 11% and under wood can start acting like napalm and cause angina. 20% and over wood acts like water and can cause angina.

So with fuel and MC under control, the next task for me was to learn my stove and chimney. You could try to operate your stove and chimney system in the full spectrum of heating season weather with 14 species of wood with 6 different average moisture contents, but I don't recommend it.

With consistent fuel, I find my catalytic stove has a clean plume when the combustor outlet probe hits a certain spot on the dial- with the caveat the whole stove has to be hot and my chimney has to warmed up to some unknown temperature. I have been outdoors with inadequate clothing dozens, if not hundreds, of times to look at my chimney to see if my plume is clean. The borough (like a county) the state and the feds can all read my stack plume from the street without a search warrant.

What I ended up having to do to meet the letter of the law was step outside the operating instructions in the manual that came with my stove from the mfr.

The last year I heated this house, with three kids home and no wood stove, I spent $5600 on fuel oil. I did have two daughters home, both with long hair and my oil burning furnace makes DHW in the summer months too, but 5600 is a chunk of change. $466/ month is a decent truck payment. A potential fine of $1000 is a problem. FWIW last year I with no daughters home I spent $2441 on fuel oil. I did spend $1600 on green cord wood, but I have given 5 cords of seasoned wood away to various charities, with 8 cords left over to burn here at home.

 

[Poindexter]

Really should remember to @Rangerbait in thread since I was thinking of him when I sat down to compose this.

How hot a stove? How hot a chimney? Dunno.

If we had any daylight up here in the winter months I could put a mirror out on the deck and monitor my stack plume from inside the house, but it's a non-starter. I have to go out to look, when it is cold enough to run the wood stove, it's dark here. If you can rig up a mirror, do that.

For a start point, with fuel at 14% MC I _always_ (never say never) run my stove at full throttle for 30 minutes before I turn the knob down to cruise setting. Always. This is my third season with the same stove now, I have spent a LOT of time laying on the floor in front of the stove just watching the fire through the window. I think it is mission critical to run (mine) on high for 30 (not 25) minutes before turning it down. Under 12% MC, OK, turn it down after 20 minutes on high. I don't have a hypothesis for that one beyond boiling all the free water out of the splits. Even with a +/- 600dF ignition temperature, it takes a while to heat the free water inside a split to boiling, and keep it hot enough to be steam long enough to get out of the wood and up the stack. IIRC wood logs for log cabins have a rated R value around 1R per inch thickness for white spruce, probably even higher for hardwoods.

For me to find a clean plume when I go outdoors, the cast iron cladding on my stove needs to be too hot to touch _and_ the combustor indicator probe needs to be up to my special non-calibrated spot on the indicator dial. I mean tap my finger tip on the stove top near the combustor and have it come back hot enough that I don't want to touch it again, but not blistered or charred. Not quite hot enough to put a Michelin starred sear on a steak, but hot enough to fry an egg over easy in 3-5 minutes.

 

[Poindexter]

I have two remaining unknowns.

1. I don't know the time lapse from igniting kindling in a cold stove to AQ cops in the street seeing 50% plume opacity and starting the 20 minute timer. Clearly a candle sized flame on dry kindling does not instantly a 50% opaque stack plume make, but how long, how many instants, is unknown to me.

2. Chimney temps. I have, a few times, been able to get my stovetop hot "enough" and the cat probe indicator up to my "special spot" and still found plume opacity > than zero, though probably a lot less than < 20%. These have otherwise been especially fast cold starts, so I am attributing these to hot stove / cold chimney as the remaining variable in the soup.

My target plume opacity is zero. In general I want the AQ police driving by my house to think my stove isn't running at all or maybe see a tiny plume of steam with 0% opacity.

The two most important variables for draft are stack height and average exhaust gas temp in the stack. Logically a cold physical chimney is going to absorb heat from the exhaust gas, lowering the average gas temp in the stack. Averaging exhaust gas temps in a stack from end to end is, mmm, not economical for most home owners.

A lower plume temperature should not change the physical makeup of the particulates in the plume, but I _suspect_ losing a bunch of heat to the chimney walls can sort of compress the particles in the plume by virtue of slowing transit times and thereby allowing particulates in the plume to compress, get jammed closer together; thereby increasing plume opacity without materially changing the output of the stove in grams per hour. Once the chimney heats up transit times will diminish, particulate compression will diminish, and visible plume opacity will diminish. Another reason to run on high for 30 minutes on a fresh load, sort of like blowing your nose in slow motion; you could, in continued slow motion, breathe freely for hours afterwards.

Can you see this as a flow problem? Higher throughput will give you a lower measured opacity for any given volume of smoke production. Thus my endorsement of suggestions from both @Highbeam and @lsucet to rangerbait in his recent thread that will give him higher chimney gas temps, higher throughput and thereby lower plume opacity.

I do think my total emissions from my stove would be lower if I just followed the manual.

My nightmare scenario is my wife not turning the stove down to cruise when she leaves for work. On a typical day I load the stove when I get up, char, engage, and leave the throttle on high. When she gets up the house is warm, the stove is raging, and she is really consistent about turning the stove down to cruise when she leaves for work. On the rare days she doesn't, I have two problems.

When I come home to a cool stove with an inactive cat and just a few coals remaining, I have two problems. One is the AQ police out in the street, the other is getting the house warm before my wife gets home. I do have a loop hole. My local ordinance does not specify how many times a day I am allowed 50% opacity for 20 minutes after reloads.

One option is to load up just a few splits on the minimal coals in a cool stove, high throttle for 30-60 minutes, and then a full load into a hot stove on a fair sized bed of coals. I admit I have done this a few times. It does bother me to do it, but when I do I can get the full load to clean plume pretty quick.

But dang it, I got primo fuel, a primo stove, and the pride that cometh before a fall. My other option is to load kindling as part of the load. I can rake the few coals to the front, load a pretty full load from the back of the stove and then pile kindling on the coals at the front to get the full sized splits lit off quicker. It works good and I don't fully understand my resistance to the method, although my opacity is likely > 50% for part of the 20 minute window.

A third option is to push the coals to one side of the floor, load full size splits from the back, kindling in the front and treat it like a cold start. I will have to fool with this some more. It feels cleaner, if the remaining coals are a small enough volume, to push them out of the way, ignore them and light the whole face of the front as if the stove was cold.

Alright, I will play with this for a few days. It should be more interesting than waiting a month for pictures of metal knife edges that haven't yet appeared.

How do you keep your plume clean?

 

[peakbagger]

Simple with storage, not so simple with a wood stove. One of the long term wood gurus at UMaine who passed away last year advocated owning 2 or 3 wood stoves and swap them in as the heating load changed. Ideally the cleanest stove is one that is being operated at full load with the air dampers open. The concept is swap the stove as the heating demand increases.

 

[webby3650]

Simple with storage, not so simple with a wood stove. One of the long term wood gurus at UMaine who passed away last year advocated owning 2 or 3 wood stoves and swap them in as the heating load changed. Ideally the cleanest stove is one that is being operated at full load with the air dampers open. The concept is swap the stove as the heating demand increases.

Rather than learn how to use the stove you have in different weather conditions?

 

[peakbagger]

There is a point of diminishing returns for any stove, a catalyst extends the range of the stove but physics of it is that there needs to be a optimal range of combustion temps with adequate residence time somewhere in the firebox. There is a range of heat output that corresponds with this temperature and residence time. Get outside that range and the stove owner has a choice, switch over to incomplete combustion, inadequate oxygen (dark plume) or put out more heat than required so the stove runs back up to excess oxygen. The solution on some stoves is add a set of hidden air ports that prevent the stove from being turned down too much. The difference between the maximum clean output versus the minimum clean output is called "turn down", every combustion device has a turn down and optimistically a 2 to 1 turn down is pretty good. The only other way around this is to have enough thermal mass in the system and batch burn which is what storage allows to match the output to.

Unfortunately most folks want more turn down than a typical stove requires to burn clean. The standard testing regimes generally test at rated output and a home stove doesn't have a Continuous Emission Monitor (CEM) so most parties look away when the stove is turned down to reduce output. The Alaska policy appears to be a poor mans CEM, equating visible emissions with inefficient combustion. I have seem comments by Hearth members in the past where they locate and cover the hidden air ports with tape or magnets and claim they have much better turn down extending their burn time. That works but no doubt the stove is running at less than optimal air leading to incomplete combustion.

The limits to turn down is where a shoulder season stove and deep winter stove makes a lot of sense. Run the shoulder season stove within its turndown and when it runs out of capacity switch to the bigger stove within its turndown.

 

[Woody Stover]

Great thread, Poiny. I particularly enjoyed how you combined the words peanus and yoorine into 'penurinous' to get em past the forum's "auto-correct." OK, I had to change the spelling a bit to get it to work, but no one will notice.

Really should remember to @Rangerbait in thread since I was thinking of him when I sat down to compose this.

To minimize the extended creo-bomb plume that @Rangerbait and I occassionally generate, I join you in not following the manual to the letter. Manual sez to run the stove meter above the cat up to 250 before closing the bypass. It takes forever to get that meter up to 250, 1) Because the stone takes longer to heat up and 2) That area of the stovetop is shielded by the combustor scoop. Half the wood in my little 1.5 cu.ft. box will be gone before I see 250 there.
On a standard reload start in my E-W loading Keystone, I today closed the bypass when I had about 120 on the stove top meter, and left a decent amount of flame going. This routes heat to the cat (and warms the stovetop above due to the longer exhaust path.) Within twenty minutes the stovetop above the cat was cresting 200 and the plume was getting fairly clean. It would take longer than that to get the stovetop meter to 250 with the bypass open, and all the wood smoke would be exiting the flue. Not to mention that all the creo would have burned out of the box and ended up outside as well through the open bypass. Getting the bypass closed, the cat burning sooner, and leaving a little flame in the box, I think the creo is burned off later in a more controlled fashion and possibly handled better by the cat. And with some flame still going in the box for a good while into the burn, the creo and moisture burning off the wood is more likely to stay in vapor form and not be deposited on the walls of the stove.
Now you, running your stove balls to the wall and striving for 88 degrees in the stove room and with Aspen in the box, are not going to have much creo/moisture to deal with for most of the winter. It's in the upper 40s here right now so I've got the stove running fairly low.
Another approach, which I haven't delved into yet as much as I probably should have, is the top-down start. It will have a minimal amount of wood gassing/smoking earlier in the burn if I shove the coals to the back instead of pulling them forward, and in my E-W stove, flame burning in the top/front of the box will eat the smoke from the wood that is catching below. It will also concentrate heat in the top of the box to raise cat temp sooner.
Obviously, a lot of this is pure conjecture on my part..I'd need some sophisticated instrumentation to quantify it. One thing I will get is a cat probe that actually works. My stock version comes in from the back of the stove and is just too long at 8" to accurately transmit the temp exiting the cat all the way back to the dial.