Wood gas, science fair project, Gasified Questions 1

[a31ford]

Our son is doing research on next years science fair project and has now got me intrested in it, below are some concerns of mine, that I want to address, BEFORE, he attempts to build his prototype.

1) In the art of creating "wood gas" from biomass, is the gas "lighter than air" or heaver ?

2) Can one use the "bottom up" method of combustion (ie: fire is at the bottom of stacked wood, with combustion air below it, and a containment "can" surounding the stack of wood)?

3) If one "opened" the top of the "can" in Q2 while the fire was burning, would it "flare up" opon oxygen entry ? ie: the continment "can" in Q2 has a opening top, as to "stoke" more wood.

We (son & I),have scowered the net, found a lot of info, but no answers to these questions. G.

 

[dangerous]

I'll give it a try. First, since this is for a science fair, I will assume your gasifier will be small - table top size - and that it will not actually be operated during the fair. BTW, I have judged several science fairs and the most successful projects are those that are a solid demonstration of the scientific method. I suggest you begin by concentrating on developing a hypothesis to test empirically, determine what the biases are and what the control(s) will be, what data will be gathered and how the data will be used to test the validity of the hypothesis, etc. For example, a hypothesis might be that the species of wood will affect the ultimate btu value of the off-gas. Students that simply build something, without conducting any science, do not do very well in a science fair. Also, starting with the science end of things will drive the design of your gasifier. Now, on to your questions:

1) As I understand your design, you will be burning wood in a combustion chamber (fire box) that will in turn heat a container of wood. (Here is a simple sketch of an underfire design that is an integral gasifier:

http://www.fao.org/docrep/t0512e/T0512e0a.htm#2.3

types of gasifiers . In this design the combustion and production gases are mixed and separated later.)

If my understanding of your design is correct, I presume you will have two exhaust trains, one for combustion air and one for production gases, and that you will somehow collect/store/use the production gas. The device you describe works by bringing the wood up to a high enough temperature to convert the solids and liquids in the wood to gas, in low-oxygen environment. The important gasses (those with fuel value) that will be generated are CO, hydrogen and methane (depending upon how much air gets into your gasifier, the largest constituents could be nitrogen and CO2). All of these gasses have a specific gravity less than 1 so they are lighter than air (at STP). Since your biomass will be wood and you will be operating at low temperatures, in an oxygen deficient environment, with poor mixing, etc., another component of your off-gasses will be the products of incomplete combustion. Therefore, you will also get varying amounts of 'tar" in your production gas, which consists of a number of uncombusted hydrocarbons. Tar is heavier than air and will settle out once the production gas cools. This sounds good but actually tars are difficult to deal with and, as discussed later, may be a safety problem.

2) Yes, an updraft device will work - plus, it is the simplest design. You also mention a 'stack' of wood. Actually, it is more effective to use small chunks - pellets, chips, or small cubes, max 4 cm or so. (This could be another hypothesis to test!)

3) I am confused about what you will be opening. It would be unsafe to open the top of the combustion chamber during operation - you'd be releasing smoke and heat. You will need to feed your 'combustion' wood supply from the top or side, probably using a hopper and chute, directly onto the grate. The fuel feed rate will be determined by many factors and you will need to do some experimentation to determine this The air supply will come from under the grate so that should not be an issue and your exhaust gases should be going out of a chimney. Still, you will need to control your fire so you will need a damper in your chimney or air supply, or both. On the other hand, why not just use an electric hot plate (or a Bunsen burner) to heat your gasifier box - this would simplify your design/operation, and eliminate the need to worry about exhausting the combustion smoke.

If you are suggesting a batch process, whereby the wood being gasified would be stoked, gasifed, then stoked again, then you will need to wait until the system cools between batches. Otherwise the temperature of the contents will be above combustion temperature and there will indeed be flames as soon as the gasifier box is opened, which introduces outside air. If you want a continuous operation, you must design some sort of automatic feeder to replenish the wood in the gasifier and remove the ashes.

Safety is a big concern. Make sure that your construction is airtight to prevent leakage of combustion gasses (smoke) outside the device. Just in case, I suggest you only operate it outdoors. When you first start up, there will be quite a bit of CO produced, which is odorless and dangerous. Be sure that your chimney exhausts to the outside, above the breathing zone. Another concern is the possibility of tars (commonly called creosote) building up in the chimney. Chimney fires are common in wood stoves where the fire is 'damped down', which is similar to what you will be doing. Once again, using a hot plate would be a much safer way to go.

This is a very ambitious project. I'm curious as to what you intend to do with the gas? How will you collect and store it?

 

[a31ford]

Your answers have started my old head "swiming" with ideas.
Thank You very much !!

Now to answer your questions (I better work on my method of asking questions).

Correct to the fact that is will NOT be operated indoors or at the fair.

Yes, the model is a very small "table top" size.

Yes, the method is the science, NOT the building of the model (that is mainly my department, because of fire hazard, welding etc. we aggreed on that) (Spoke with his teacher, and there is no problem with this).

He has opted for the "Co-current" downdraft style, as his model, and the batch method, of production (that's original Q2 / Q3 questions expanded on since we now know the correct words to use (been really involved with this) and just as I thought, Q3 would produce a "flare-up", this has prompted the batch method.(and a flue fan, rather than forced air). (remember this is a model).

The "stack of wood" is "larger twigs" standing upright, we have 8 acres of "aspen", and I got a hand in this saying "could the model also prove a usefull design of "loose pack" rather than mfg. pellets or such", and what would be the ratios of burn (loose pack vs. pellets)(side note: dad has extra motive in this, will discuss later).

Since then, we have added the above to the "objectives" side of things. (Both of our heads are now swiming)

The basic objectives (at the teachers meeting) was the ability to:
1) produce production gas. (Volume basis only, content & impurities excepted).
2) control and limit the residue content (ash, tars) and account for why they are there. (what didn't burn, and why).
3) can the heat source also be the gas source ? (the "can" I spoke of). (Co-current)
4) based on size, calculate what size would be needed for a needed volume of production gas.
5) can heat transfer be used, to gain a "spin-off" of this unit, and where if it can be done, in the process would be the best extraction point
6) what is the ability of a unit like this and where could it be used ?
7) man made fuel vs. raw biomass, what are the differences

these are the objectives of his project (prodded by the teacher & myself), (side note continued: remember dad's other motive, it appears that the teacher got quite interested also, and we have been talking quite a bit, both of our homes are rural "in the woods" so to speak, and where we live, it gets quite cold in winter -50f Jan. Feb. and the thoughts of outdoor district (local homestead) biomass/hydronic systems are evolving from this (yes I'm a metal worker, as well as computers), what is funny about all this is what started it all was the son saw a site on the web, where a tractor was powered by production gas..... and.... you know how son's are How and why he found it I have no idea...

http://www.gengas.nu/byggbes/index.shtml

is the main page.

G.

 

[boilerdoc]

Hi a31ford(computer),
check

http://www.crest.org/index.html

This is one of the most competent sites about gasification.

 

[a31ford]

Setra, thank you, we have been to that site many times (both before, and after your post). This "project" has spawned some spin-offs, as the sons model works GREAT! and now, his teacher & I are building a larger prototype for OUR own evaluation (see above side note about district heating). I want to thank those , that have responded, (here and in e-mail) as this has turned out to be a fantastic project !!

Greg

 

[a31ford]

Just an update on this project >

We have had great sucess in designing the "full sized" unit (basing it on the son's model), Hope to have more factual info. on the unit as we get it into opperation, and hope it is as good as the sons is, at efficency. will keep all posted as time permits.

Greg

 

I'm researching this topic with an eye towards using the fuel gas to heat a swimming pool and if possible run an electric generator. I am convinced that a downdraft gasifier burning wood pellets or charcoal as it's primary fuel is the only way to fly. The "reducing zone", a region of glowing coals between the burning fuel and the grate, converts CO2 and H2O into CO and H2 and will convert all tars and dioxin precursors into simple fuel gasses. I wouldn't try to run an internal combustion engine on any other kind of gasifier, nor would I be satisfied that I was getting anywhere near optimal amounts or purity of fuel gas. I believe that almost any kind of combustible refuse can be safely converted to fuel gas by this genre of gasifier, even old tires and plastics, which constitute a stupendous potential for nearly free energy. The problem of how to suck the gas out of the bottom of the gasifier is neatly solved by using compressed air to create a venturi effect in the same way that compressed propane pulls air into a torch, or by the suction created by an internal combustion engine.

 

[a31ford]

Update:

Well, well, a small set back, but not overcomeable. It appears that batch process is somewhat of a waste (of heat), as 2x times now, the unit has been left unatended at the time of "final burn" (gone cold so to speak). Trying to get it back up to temp in our weather (Minus 40 c) has been an uphill battle, at these temp's the unit beltches smoke for about 3/4's of an hour until everything is up to temp... Once running, emissions are back down again.

Thoughts are looking at a "Chipped wood version" with a isolated container feed... in the spring..

G

 

Each of those questions can be answered by means of an experiment. From each of them, you can form a hypothesis, design an experiment to challenge that hypothesis, perform the experiment, observe the resulting phenomena, interpret them to the best of your ability and tell us what you have determined to be true. That is the scientific process, which is what your son should be engaging in if he is studying science.

Go get'm, tiger! And keep a fire extinguisher and some Unguentine handy just in case!

 

[a31ford]

LMAO ! how true...

Been doin more thinking (This is now past the son's project, (see above)). The chipped wood attachment is sounding better, and better. I'm thinking about a double door (Gate) system gravity feed, with only one door (gate) open at any one time, therefore there is no chance of flare-up or cross ignition into feedstock, motorized auger feed to the top gate with a load cell to weigh "portions" then open that gate to "dump to 2nd gate" then close 1st, and "TADA" open 2nd gate and Poof, "more fuel on the fire" kinda thing.... whole thing air or hydraulic cylinder driven....hum..... this IS do-able! Now where is that old chunk of grain auger ?

G

 

[ProEDesigner00]

a31ford

You emailed me about a year ago in interest in my sawdust buriner project I am rereading the post here and am wanting you to know I am becoming very successful with what I am doing. Moisture level in the wood is a stickler at the moment but as I experiement I learn. Wish I had a good understanding of how to test what I am doing so its not just blind observations. any ideas?