airflow through sawdust to make it burn. 2

[ProEDesigner00]

Greetings

Please have patience in teaching this old dog a new trick!

Problem: making green sawdust burn with out any electricity One source of information of someone allready doing it check out

http://www.hernironworks.com

With out going into alot of boring detail I was asked to copy an appliance to burn green sawdust. I built 2 of these appliances one on a whim of no investigation, the second copying exactly what was being produced. I confirmed by witnessing the use of this fellows appliance, I have met with this fellow 3 times to discuss why his worked and mine didnt. He is Amish and cannot explain what he has found, and cannot explain why mine wont work. He has produced a couple hundred of these units and has since quit due to pour health.

Now I was given a formula that does not make sense as I look at his design. Formula is: Theoretically 564 kg of air is needed to burn 100kg of sawdust. Taking density of air = 0.9kg/m³. So theoretically about 6.27m³ of air is required for combustion of 1kg of sawdust. Taking 100% extra, practically you need 12.5m³ of air/kg of sawdust.

I know I need to apply this to a ratio of cross sectional openings for the intake and exhaust. and the amount of air gatting through the fluid bed (material)or around it to make this work. I am certain some sort of velocity needs to be applied. Can any one in plan english explain this to me so I can make sure this works? I am certain his cross sections are to small unless velocity is put into place.

 

[IRstuff]

Why wouldn't there be velocity?

It's fundamentally no different than a fireplace with a chimney; the combustion produces hot air, which rises and causes an inflow of fresh air from the room.

TTFN

FAQ731-376

 

[ProEDesigner00]

True. but I need to know what to expect

 

[25362]

As explained by IRstuff, the normal method of producing a negative pressure within the ignition chamber is by using a natural-draft stack that employs the buoyancy of the hot flue gases.

This draft should be sufficient to overcome frictional losses and leave a net draft available from 0.05 to 0.10 in. water column negative pressure for the inspiration of combustion air. Formulas for the estimation of the draft abound.

Air-ports area requirements are usually based on the assumption that a 0.1 in water column is equivalent to an air flow of 1,265 ft/min. This should enable your estimation of the area of the various ports.

For estimating frictional losses in stacks, F, in. water column, use F = 0.008 HV[sup]2[/sup][÷](D[×]T) for round stacks.

H= height of stack above breaching, feet
V =velocity, fps
D = stack diameter, ft
T = temperature, [sup]o[/sup]R

For rectangular stacks, use F = 0.002 HV[sup]2[/sup][÷](m[×]T)

where m is the hydraulic radius, ft.

A stack velocity usually recommended is ~ 30 ft/sec.

 

[macmet]

100% excess air might be too high for green sawdust. How wet is your fuel? Is your design exactly the same as the one on the Hern site?

 

[rmw]

Did you copy it in exact detail? I notice that his site has a variety of burner grates for sale. Which if any of these grates does he use in his burner?

What percentage moisture is in the sawdust that you are trying to burn? When you compare yours to his, are you starting at the same starting point and going through the same process with the same fuel? Or is your conclusion based on his design being successful and yours not working?

I have seen plenty of sawdust (and bark) at 50-55% moisture content burned in dutch oven natural draft boilers, but their stacks were very tall. Did you use the same stack diameter/height as his unit had?

Did you get your burner good and hot with dry fuel before starting to feed it with what I assume is wet sawdust? The procedure I used is to store some planer savings or dry sawdust to get the fire started and to get the refractory in the hearth good and hot and then begin to admit the moisture laden fuel. Did you go through a similar process?

The burner that I saw on his site is merely a gravity fed sloped grate burner and has the basic elements of primary air and over fire air, so it should work slick if the fuel doesn't plug the chute. That is the only part I raised my eyebrows over.

Tell us more about your "copy."

rmw

 

[rmw]

OH, yes - and a comment on the excess air issue. You don't want 100% excess air because every bit of air that goes through the furnace that isn't required for complete combustion has to be heated to furnace temperature, meaning that a substantial portion of your fuel is devoted to nothing more than heating air that is passing through the boiler.

You should be able to do this with much less excess air.

You have to remember that to successfully burn wet wood fuel, it first must be dried and then heated to its ignition temperature. Excess air robs heat from this process and carries heat away too fast for the process to work.

rmw

rmw

 

[ProEDesigner00]

greetings everyone.

First thanks for the replies.

First assumption on your part is that the furnace I made is not a copy of the hern appliance. I am copying an Amishmans appliance. I copied his to the exact details and did not get it to react like his does however I have witnessed others that use his device and it works wonderfully.

I am going to reread everyones comments tonight and digest what has been said to this point.

Sawdust moisture is what ever the log is when its cut. these people are taking rain and snow soaked material and burning it on a hot grate situation. I have seen them put lumps of iced sawdust in the hopper and it thaws, dries abit and burns.

Norb

 

[macmet]

Sawdust can definitely burn like that. Try to post a picture of your design so we can make some comments. There is obviously something different between your design and/or fuel and the Amishman's if it burns in one but not the other, sawdust does not just decide when it feels like burning.

 

[rmw]

I assumed with my comments that your sawdust was wet and in an "as sawed" condition. I have grabbed handfuls of sawdust going from the sawmill to the boiler fuel bin that you could wring water out of.

There is something different about what you have done but without further information, we are unable to offer any "tips."

rmw

 

[ProEDesigner00]

rmw and all I will post some pics as I get them............you guys I beieve will be some ov my answer

 

[ProEDesigner00]

greetings

Last night I finally had time to fire my stove and I got a successfull burn. I burned 20 gal of sawdust in about 1.5 hrs. The burn was not as clean as I would like, but it burned complete and that was what I wanted. I did this by leaving my front access door open.

Many of you have asked for more info. So I am going to list a general description of what I have.

The over all stove (burn area and heat exchanger) is 16 wide 36 deep and 44 tall. All measurments are in inches. The hopper stands another 8 inches tall when mounted. my opening for the access door is 9.75 tall by 15.5 wide. My burn grate sets horizontally 5.75 above the floor. I can change pitch of this plat so the grate is setting at a 10° angle for material flow changes.

At the back of this unit is where my intake air comes in (or is suppose to) It has a 2.65 X 5.56 rectangle opening. Controlling the airt flow into this opening is a bi-metal manual thermostate (ie. set the dial, bi metal spring opens and closes door according to the length of the spring) On the inside of the furnace immediatly adjacent to this opening is a collection area for this incoming air that in the oposite wall two round tubes of 2.5dia runs to the forward end of the furnace. (aprox 22inches long). These "dump" or enter the area below the burn grate 15.5x16x5 area for the air to permiate up into the fuel bed and thus allow the burn to happen. My secondary air enters into the sides of this burn area. The guy I copied had two 1" water pipes welded to the floor of his unit and called these things injectors to offer secondary air. My first unit I built this way with no success, but his works. The other thing that his is different is that he has two 2.5 spuare openings running aft forward under the burn gate in the botom outside corners of his unit. As was said earlier friction is not easy in a a spuare opening that is why I went to round tubes. My bimetal spring Thermostat is from the same supplier he got his from.

After getting the fire going last night with very dry baled pine saw shavings (very course, temp between the hopper and the heat exchanger wall right above the fire was at 1200°)I could shut the door and it would work with the front door closed. Once the wet dust hit the fire it begain to go out so I just left the front door open and it burned well for about 1.5 -2 hrs using about 20 gal of material.

My conclusion to what is going on is that what the amish guy thinks is primary air intake is not however if you shut his intake door closed it will put out the fire. So the combination of his primary and secondary air supply is being done correctly. Why my copy of that would not I cannot explain.

Second that my current version of this is not tuned correctly, so if I direct primary and secondary air more direclt and closer to the fire it works. I should redesign and modify this intake area to immitate this door open effect. The Amishman told me originally his opening was in the door. But moved it to the rear so embers would not fall out onto the floor and mark up the wood floors and be a fire hazard. So my redesign is will need to accomadate this desire.

My exhaust flue is seven inch single wall pipe 8' tall. The unit is setting outside with in 15 ft of my pole shed just for convenience. I am tyhinking I am going to get admonished by you guys because my flue is not 3' higher then the peak of this building or that its not hooked up inside the building yet with a three wall insulated flue or that its not in an open area so none of that would be a factor.

Final note. He sold one of his 22 inch units to an englishman(ie one of us that uses electricity) and this guy hooked a small fan to his unit and didnt controll it properly and it melted the stove down. It got the stove to aprox 2400°. I seen this stove about 4 weeks after it was returned to the Amishman to be rebuilt. Another note is that this unit has no fire brick in it. He said he tried but could not get a design with fire brick to work. I have not tried this., figure that a model down the road sometime.

please let me know what you think.
Norb

 

[macmet]

Anychance you could post a picture or drawing of your unit?

What is your goal with these? Do you have a certain heat demand or are you just looking to burn enough wood to keep a room warm?

 

[ProEDesigner00]

How do I get the pics to post to this thread?

 

[macmet]

google: how to post pics in engtips

You'll get this link,

http://www.eng-tips.com/faqs.cfm?fid=1161

And that should be enough information to get you through it.

 

[ProEDesigner00]

ProEDesigner00 (Mechanical) Aug 30, 2007
Here is the original idea. you can see the air enters the back of the stove(right side) Follow the description listed in the post 11.

Here is the second idea. The intake is relocated on the side right behind the burn grate

Let me know what you think.

 

[ProEDesigner00]

I have figured it out! Dont take the air throuigh the sawdust take it over the sawdust. It works very well

Next question! What is a 35:1 fuel to air mixture ratio mean?

Thanks for the replies

 

[kenvlach]

Re 'What is a 35:1 fuel to air mixture ratio mean?'

Are the ratio units weight or volume?
Dry wood is about C[sub]6[/sub]H[sub]10[/sub]O[sub]4[/sub] (atomic). The combustion reaction is

C[sub]6[/sub]H[sub]10[/sub]O[sub]4[/sub] + 6.5 O[sub]2[/sub] = 6 CO[sub]2[/sub] + 5 H[sub]2[/sub]O

The weight of O[sub]2[/sub] is 1.42x the weight of dry wood. If the wood is 25% moisture, can reduce the O[sub]2[/sub] for combustion by this amount, to 1.07x weight of wood (wet). As air is about 23.17% O[sub]2[/sub] by weight, the necessary combustion air is 4.62x wt. of wood (wet).
As wet sawdust has a density of about 200 kg/m[sup]3[/sup] while that of air is 1.2 kg/m[sup]3[/sup], the volume of combustion air is 770x that of the sawdust (wet).

So, air/fuel = 4.62 by weight, 770 by volume.
The '35:1 fuel to air' ratio doesn't seem to make sense.

 

[25362]

As I was given to understand the 35:1 ratio is the lowest limit for exemptions granted by the authorities on emissions. Make a Google search for more information. See for example:

http://www.epa.gov/ttn/chief/ap42/ch01/final/c01s10.pdf

 

[kenvlach]

From link given by 25362, some wood pellet-burning stoves are "exempt from 1988 NSPS due to an air-to-fuel ratio > 35:1."

Makes a lot more sense than '35:1 fuel to air'