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Sunday, March 17, 2013

Explaining How Wood Gasification Works Part4

Now if you read my article on how wood gas works then you should have a good idea on how it works. In this I will describe in better detail more on how to build a unit and the many different kinds of units and ways to build one.
Keep in mind this is just what I know about wood gas units and taken from my own testing and testing some of the different options. When I say testing I mean I built and tried and run and made changes to see if by combining options make any difference. Let me tell you some of my good and bad test so you can get an idea of how different ones work and the downfall of some of them and some of the benefits of them too.

Let’s start with the updraft design it’s a very old design most people these days use a down draft. How the updraft is made the fire is started at the bottom like how the downdraft is but the big difference here is the gas flows up threw the material that is in waiting to be gasified. I have seen one big difference with this kind of unit that has one benefit that is if you have real heavy tar wood that you are using in it. This works as a filter with one nice added thing the collected tar then on the next wood to be burned so the tar gets burned up. The one big drawback is pulling the gas threw the wood as it get filled with more tar it gets harder to pull the gas threw. Now on most updraft designs they never did build a good air inlet control. I made one where I had full control of the inbound air and found another big issue that is reloading the unit due to the gas going out the top when you open it gas gets mixed and you have a flammable gas. It also stops the gas flow I did find a way around this with using a wood pellet stove auger setup. This allowed me to keep feeding the unit from a side mounted hopper and not inter fear with gas flow now to keep fresh air from mixing I made a sliding door for the feed auger pipe on the side of the top of the burner tube. Now I know most knock the updraft setup without ever having tried one but I was impressed with just some modification to the updraft unit on how well it really worked.

There is also a top burn unit used in a lot of third worlds for cooking on, but there are a few big issues with these as the unit burns down the tube the ash builds up. This has a small smothering effect on the burning area another big thing is if you try to reload on of these then you get another smothering effect and I saw a dual burn meaning the burn kept moving down but also moved up. This was very odd as the ash pile built up the flame kept getting smaller and smaller. On most these units they have a small fan at the bottom to push in air to fan the fire. This also made it work more like a blast furnace and not like a gasification unit.

There is one kind of unit I want to talk about that is the units that use a hearth ring they look nice and look like they should work nice. Most setups with a hearth ring also or most have a fresh air in preheat this is due to how the air inlets run on the outside of the main burner chamber. The only thing I like about some of this is that the inbound air is more controlled so it may give a nice burn without over oxygenating the fire keeping it more in control. The big thing I don’t like about how most these are setup is they have so much reduction in the hearth ring that right above it the unit has an outward lip some 3” some up to 6” to me this just looks like it would stop or hinder the dropping wood and cause a void spot. The only thing I can see to help this is to manually break up the wood or have to keep poking at it. Some I have seen electric vibrating setups on them I think this is to shack the crap out of the unit to keep the wood moving down. Now this is just my own opinion of these hand crafted units. Now there have been a few that I have seen made on YouTube that back yard builder have tried to add in a style of hearth ring I like how they did it. One I saw that was slick is he used this old mobile home tire rim and he drilled holes in it on the lower side so the holes face down to let the fresh air in towards the fire and keep wood chips out. Then he wrapped a flat plate of metal around the rim and welded it in and this was so he could control the air going in. Then he cut some holes in the flat plate and added on some pipe and put on them ball valves. This kind of hearth ring doesn’t have that big step out and is a small reduction in size but also gives the user a good way to control air in to the unit. He also just built the rest of the unit from that rim making the outside of the burner chamber even with the outside of the rim.

Now with this way I can see how more people could build this way or use a similar way of making this. If one would take a piece of pipe, flat stock and add some kind of indent or lip on the lower end by the burn area. The idea here is one to control the air inbound and make it so the air inbound is closer to the burning area. Once that’s in place I could see a few ways to enclose the drilled holes in on the outside the burner tube one could use a sheet of medium metal to make a bubble and wrap over the holes leaving a half pipe like shape and then put one or 2 pipes welded to it to let the fresh air in and have a ball valve or a butterfly on or in the pipe. One thing that a lot of people don’t make a cover for the top of the burn tube and I see that as a very big mistake due to once your unit is fired up. The wood chips or chunks let a lot of air into the unit making it an uncontrolled I don’t know if they think it’s still controlled due to they have a smaller pipe outlet or what. I do know it has no air control this way I know this from I have seen a few people that posted videos on YouTube and I can see when they have it running smoke out the top on some not all but some.
One big thing if you build a gasification unit never ever push air into the burner unit always pull it threw with a fan. The reason is if you push more air in than can be pushed through your setup. What you really are doing is mixing wood gas with air and when you light the wood gas on the end of your burner tube it will back flash threw your whole setup and if your gas is at the right air to fuel mix I have seen covers and stuff blow off. So never ever push air in to your burner unit it’s just way to dangerous.

Let’s get into the down draft units this is the most common way to make a unit these days don’t ask why I have do real good answer to give it just is. The nice thing I can say is if you make a down draft right you can load material in it as its running and they work nice. How they work is just like most you set up a burner tube or area with a stainless steel bowl for a shaker grate having holes drilled in it for the size of material you plan to use. What I mean by the hole size is if you are going to use real small stuff then make smaller holes if you plan on using 2” diameter chunks than you need a lot bigger holes. Why the hole size is so important is if you are using 2” diameter chunks but only have a lot of ¼” holes drilled you will get a backup of bio char and it won’t madder how much you shake the grate. You will get a backup that the burning area will than work its way up and at some point you won’t be able to add more material to keep it working. This will make it so you will have to shut it down or let it run out of burnable material and clean it out. The next part i will talk more on how the basic burner is setup.

Time to talk more on the burner this is the one spot most people try to over complicate it and it doesn’t have to be. When I say that I mean it you can make the burner tube just by taking a tube and putting a stainless steel bowl at the bottom with holes drilled in it. Now if you make it this way you also need to use some good wire or chain to hold the bowl just under your tube. Now put this into a barrel or make a container to hold the burner tube now once that’s made you need a way to control the air input so you add a tube welded to the side of your burner tube and have a hole into the burner tube. It’s very important that you keep this simple for a few reasons to keep it air tight so you only let in the amount of air you need to. Now you need to make some kind of cover or lid for on the top of the burner tube it needs to be air tight. I have used lids for frying pans and other stuff a bowl turned over with some high temp sealant. When you make this I like to weld a door hinge on one side and make a lock on the other I have used stuff like a bolt welded to the burner and a wing nut for easy and fast take off and open up to add more fuel. It’s also good to add a way to shake the bowl grate. That’s all it really takes to make a burner for wood gasification those people that add or do all the extra welding and fabrication work it may make it look better.

Really all they are doing is if they ever have a problem they have to cut or grind a lot to get it apart to fix what’s wrong. Some things I have had over long use is metal burn threw and tar build up now with all that extra crap it’s a lot to get that fixed. Now it’s not hard to fix a simple made one takes only a little bit to change out a burner tube or bowl grate but with a complex one you have to cut out the bad and may even take out more than you really need to just to get to it and then fabricate the part and weld it in. For anyone thinking about buying a premade unit keep this in mind ask them for a contact on someone that uses there unit every day running a generator and ask them if they have any videos of their unit running a generator on normal wood and showing it running for more than a few minutes. Also ask them do they have a test unit they can let you use to try it out. Most of the answers they will give is NO. I love to ask what are you trying to hide? Most will try to make it sound like they are not hiding anything but the fact is they have never run their own units for more than just short test run they never have done any real world test like running a generator for a few month every day. And do you want to know why that is in their short test they have gummed up motors and so they have never done any long test. Another reason is a real test would cost them way too much money but why you can get free scrap wood all over. Well they don’t use just any wood they only use store bought hard wood pellets. That’s the reasons why they won’t show you any real runs using their units or have any real data to show you any long term use age of their units. So don’t buy any premade units and waste $3,000 to $10,000 when you can build a nice for around $2,000 or less on your own now if you design one and have someone build it like a metal shop then yes your cost can go up by a lot. I have made units including welding cost and gas to get parts for as little as $500 keep in mind they may not have looked all fancy but they got the job done. The cheapest one I have ever help build was a friend had everything to build one and the only cost was the welding wire to weld it up and it was about $100 but that’s not normal unless you have a nice collection of goodies.

Keep in mind too if you plan on building one look around for what’s in your area like if you have a saw mill where they just give away sawdust and scrap wood. Go look at what kinds of wood they cut up most will cut up everything and if they do than you need a unit with really good filtering. You need that due to they will have pine and other woods in the mix that make more tar. The good thing is if you can get a lot of sawdust you have a good filter material and just need to use it and have a few sawdust filters to catch all the tar and water. Let’s say the only thing by you is a real metal shop see if you can buy or get some of their metal turning and use filters filled with them so as you can see there are many different ways to make one and there is just no one way to do it. But as long as you get to the same good end result that is taking biomass and making it into clean gas that’s all that madders.