My Home-Made Biomass Gasifier

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  • My Home-Made Biomass GasifierMaking your own gasifier is easy

    I've built a lot of alternative energyprojects over the years. See myhome-built solar panel and windturbine pages. I've always wantedto build a wood or biomass gasifiertoo. Why? Well, the internalcombustion engine is really animportant part of our society andthe basis of a lot of ourtransportation and portable powertechnology. It isn't going to begoing away any time soon. I'vemastered making my ownelectricity from the sun and wind,but that doesn't help my truck godown the road, power the lawnmower, or run my generator oncloudy, windless days. Those allhave internal combustion engines,and they all need fuel to run. Ifinally decided it was time tomaster making my own fuel. Whypay the Arabs for it if I can make a

    working substitute myself?

    So what is A biomass gasifier? Basically is a chemical reactor that converts wood, orother biomass substances, into a combustible gas that can be burned for heating,cooking, or for running an internal combustion engine. This is achieved by partiallycombusting the biomass in the reactor, and using the heat generated to pyrolyse orthermally break down the rest of the material into volatile gasses.

    A well built reactor will also convert combustion byproducts like CO2 and water vapor intoflammable CO and H2 by passing them over a bed of hot charcoal where they will getreduced.

    Thus the gasifier converts most of the mass of the wood (or other biomass feedstock) intoflammable gasses with only some ash and unburned charcoal residue. That is the theoryanyway. This is an extreme over-simplification of how the gasifier really works. Wood andother biomass is made of incredibly complex macro-molecules like Cellulose and Ligninthat break down into hundreds or thousands of different smaller molecules as the reactionproceeds. There are thousands of different complex chemical reactions going on inside thereactor. The overall result though, if the gasifier is working well, is represented in thesimple formulas above.

    Ideally, the gasifier would break down biomass into nothing but Methane (and other simple

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  • gaseous hydrocarbons), Hydrogen and Carbon Monoxide. Here in the real world though,things rarely work ideally. The dirty (literally) little secret about biomass gasification is tarproduction. Above I said that the macro-molecules that make up biomass get broken downinto smaller molecules. Some of those smaller molecules are still pretty big though. If thegasifier is working well, these big breakdown by-products will be further "cracked" intosmaller molecules. If the gasifier isn't working so well, these big molecules will wind up inthe gas being produced. They will condense out of the gas as a thick, sticky, black,semi-liquid that very closely resembles roofing or road tar, but is even stinkier. Even awell-built gasifier produces a small amount of tar. Most real-world applications can't handlemuch, or even any, tar. This story of my struggle to design and build a working biomassgasifier could actually be accurately described as a battle to reduce tar production. Sobelow is the most important of all chemical reactions a novice gasifier builder needs toknow.

    Biomass + Poorly Designed Gasifier = Tar!

    A word of warning here. This project is dangerous. Metal working and welding areinvolved in the construction, so all the usual dangers of laceration, burns andelectrocution that go along with them are present. Use all necessary precautions. Also,the operation of a biomass gasifier produces lots of heat, flammable and poisonousgasses. Never operate the gasifier indoors. The gasses produced are flammable andpotentially explosive if allowed to accumulate in an enclosed space, like a building. Also,the Carbon Monoxide the gasifier produces is lethal! Only operate the gasifier outdoorsand try to stay up wind of the unit when it is running. Treat the gas coming out of thegasifier with the same respect as you would for the natural gas that you may have pipedinto your house. It is just as potentially explosive and deadly.

    My original goals with this gasifier project, were to build a compact and simple gasifier,that used inexpensive feedstock (like wood chips or mulch that is available veryinexpensively, or even free), and produced high-quality gas. Little did I know in thebeginning that these goals appear to be largely incompatible. Simple gasifiers don'tproduce good gas, and inexpensive fuel is the most difficult to work with. Only afterworking away at the project for a while, and going through several major redesigns of thegasifier and changes of fuels, did I achieve a system that works reasonably well. So thisweb site will chronicle the evolution of the gasifier, from early failure, to ultimate success. Iwill point out the mistakes I made and blind alleys I went down, so that you won't have tomake those same mistakes.

    As I said above, my original goalswere to produce high quality gasfrom a compact, simple and easyto fabricate design. My researchshowed that the downdraft gasifierdesign generally produced the bestquality gas. However, there are abewildering number of variations onthe downdraft design. Some quitecomplex and difficult to fabricate,others much simpler. So naturally Igravitated toward the simplerdesigns. I originally aimed for asimple open core design, like theone on the far left of the bottomrow of the diagram.

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  • I found out through experience thatthe simple designs just don't seemto work very well. At least I couldn'tget them to work very well. Thereason there are so manycomplicated designs is that theywork so much better. So I startedout building a simple open core

    design. But by the time I had a reasonably well working gasifier, the design had morphedinto something that looks a lot more like the complex J-Tube design on the far right of themiddle row. Fortunately I was able to incrementally modify the original design to get to thefinal design, and didn't have to completely start over again.

    I chose the open core stratifieddowndraft gasifier design becauseit was by far the simplest of all thedesigns I could find. Everything Iread about it (at the time) said itshould work great. I saw vaguereferences to people in Indiahaving great success with thisdesign. So I thought I couldn't fail.Turns out this design sucks. It isreally good at producing tar, butnot so great at making high qualitygas. Unfortunately I had to build itbefore I figured it out.

    So here is my original design for astratified downdraft gasifier. It didn'twork very well, but it provided agood base to build on. I have lotsof photos from this phase of theconstruction, and most of what isshown below wound up in the final


    I did make a few really good decisions at this point. I decided I wanted to make the flametube easily removable, since I figured some tinkering would be necessary. This made latermodifications much easier. I also decided to make a big access door in the side of thedrum for cleaning out ashes. The door also came in handy to give me access for makingmodifications.

    The one big unknown at this point was what I was going to use for a grate and how I wasgoing to make it shake. So that part of the plan is a little vague. I just forged ahead withthe construction and decided to solve that problem later.

    As I said above, I made a lot ofearly mistakes with this project. Iwas fortunate though in startingwith an good foundation that I wasable to modify and build on toultimately make a working gasifier.In building another unit, evenknowing what I know now, I would

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  • start out exactly the same way.

    The basic structure of the gasifieris built around a 5 gallon steeldrum, and a stainless steel tube 41/4 inches in inside diameter, and14 inches long. These dimensions

    are not really critical. The tube could be a little longer or shorter, and a little wider ornarrower in diameter. I got the drum at work. We use a variety of chemicals that come insmall metal drums like these, and always have a lot of empties around. The stainless steeltube came from a scrap yard. It was a little pricey. I have since discovered that many fireextinguishers have stainless steel bodies that are about the right size for use in a gasifier.Old fire extinguishers are easy to find and cheap.

    The purpose of the drum is to be the main body of the gasifier unit. It contains everythingand collects all the gas, ash and char the unit will produce. The smaller of the two bungson the drum will be the gas outlet. The stainless steel tube serves several purposes. Thebottom of the tube will be the reactor where the gasification takes place. The remainder ofthe tube is a hopper for holding un reacted fuel. The tube will be subjected to very hightemperatures and corrosive gasses. Stainless steel is the obvious choice here.

    I started by cutting a large hole inthe top of the drum so the stainlesssteel flame tube can be inserted.The hole was made very oversize,a fortuitous decision as it turnedout. The hole is offset to the side ofthe drum opposite the small bung.The large bung was sacrificed,since I wasn't planning on using it.

    Next I cut a flange from a piece of1/8 in steel for mounting the flametube into the drum.

    I installed clip nuts on the cornersof the hole in the top of the drum,and drilled mating holes in the

    Click on a ladyto learn howto meet her.

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  • above flange. This would allow meto bolt the flange dow


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