Download - My Home-Made Biomass Gasifier

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

My Home-Made Biomass Gasifier http://www.mdpub.com/gasifier/

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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

design.

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 down to the top ofthe drum. My idea here was tomake the core of the gasifier easilyremovable for service andmodification.

Next, I made some angle bracketsout of aluminum and bolted theflame tube to the flange. I left 6 1/2inches of the flame tube sticking upabove the flange. The restprotrudes down into the drum. Atthis point in the project I did not yethave access to a welder. Even if Ihad one, I'm not sure I could havewelded the mild steel flange to thestainless steel flame tube anyway.Here the unit is being test fit on topof the drum. The holes in the endsof the angle brackets are over theclip nuts in the top of the drum.

This is my new best friend. I wentthrough several tubes of this hightemperature silicone gasketmaterial. I used it to seal everycrack, crevasse, joint, seam andbolt hole in the gasifier. It worksgreat.


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Here I have used the gasketmaterial to seal the gap betweenthe flange and the flame tube.

Another test fit to make sure all thebolt holes line up with the clip nutsin the top of the drum. I have alsoinstalled a ball valve on the smallbung.

Here is the access door in the sideof the drum. I cut a rectangularhole in the side of the drum just bigenough for me to get my handsinside to clean out the ash andchar. then I cut a larger rectangularpiece out of another drum to serveas the door. The door is held inplace with six more clip nuts andbolts and sealed with lots ofsilicone gasket material.

It was time to tackle the problem ofwhat to do for a grate at thebottom of the flame tube. At a totalloss for ideas, I spotted a stainlesssteel vegetable steamer at a yardsale. Inspiration struck. I decidedto try using the steamer as a


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grate. I wired the petals togetherwith stainless steel wire to keepthe steamer in this bowl shape.

Here the steamer is suspended onchains a little bit below the bottomof the flame tube. The steamer hasbeen formed into a bowl shape alittle larger than the diameter of theflame tube. I suspended thesteamer from the bottom of theflange with chains so that it couldmove back and forth a little. I nexttied a length of stainless steel wire(not shown) to the steamer andran the wire outside the drumthrough a tiny hole drilled in thedrum. Tugging on the wire madethe steamer shake and twist. I

wasn't thrilled with the result, but I figured it would work well enough for a test run or two ofthe gasifier.

Eventually I came up with a much better grate design. It appears further down the page.

This photo shows how the otherends of the chains are attached tothe bolts on the bottom of theflange. I used ring terminalscrimped onto the ends of thechains.


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With the internal parts all sortedout, it was time to button up thegasifier. Here I have mounted tocentral core assembly (flame tube,flange and grate) and used liberalamounts of the high-temp siliconegasket material to seal up the unit.I could hardly wait for the siliconeto cure so I could start the testruns.

Here is the air blower I used to pullair through the gasifier. Thisblower last saw use in myhome-built jet engine project. Mostgasifier projects I have seen use ablower to pull air through the unit.It is usually used to start thegasifier, then once running, thevacuum from the intake of theengine the gasifier is meant topower keeps the gas flowing. Thisblower is a little under powered.However, it is the only all metalblower I could find. Most blowersthese days are full of plastic parts.

The plastic would melt at the temperatures the gasifier operates at. So I made do with myundersized blower. I actually had some success with it, after I modified the design of thegasifier.

I used a flexible metal natural gas line like the ones used on gas ranges and dryers toconnect the gasifier to the blower. The gas line is a little undersized, and probablycontributed to the poor performance of the pump.

My dream fuel for the gasifier isfree wood chips and mulchavailable from lots of placesnearby. I know of at least threeplaces I pass on a regular basisthat have signs offering free mulchto anyone who would come andhaul it away. There are probablydozens of other sources I couldfind with a little research. So I gotmyself a bag of wood chips. Thechips were very wet. So here I amdrying them with a fan. After 2weeks under the fan, they werebone dry and ready to burn in the


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gasifier. I realized that if this worked, I'd have to find a less energy intensive way of dryingthe wood chips in the future.

Now it was time to give somethought to how to start up thegasifier. A lot of people build in aport in the side of their gasifier sothey can stick a blowtorch insideand start the fire. I decided to takea different approach. I decided topre-load the reduction section ofthe gasifier with hot charcoal, andthen I'd fill the hopper up the rest ofthe way with whatever fuel I wasgoing to burn. This startingprocedure actually works quitewell, and I have continued to use it,even through all the redesigns and

fuel changes the gasifier has gone through. With the reduction section full of hot charcoal,the reaction starts almost immediately, and the gasifier (in its final design below) isproducing good gas in only a couple of minutes.

I use hard wood lump charcoal, not briquettes, though they might work too. I pulverize thecharcoal down to pieces no larger than about 1/4 - 1/3 of an inch across.

Here I am lighting the charcoal withmy propane torch. The pulverizedcharcoal is in yet another stainlesssteel vegetable steamer. Thesethings are just so amazingly usefulfor so many non-vegetablesteaming tasks. The small piecesof hardwood charcoal light easilyand burn very hot. Only a few quickpasses from the torch gets themgoing. Once the charcoal is goodand hot, I dump it into the flametube of the gasifier, and fill it up tothe top of the reduction section.Then I fill the flame tube up the rest

of the way with fuel.

Here is a photo of the completedMark I gasifier in operation. Ignorethe hopper extension at the top ofthe flame tube. It was anexperiment I tried to increase thefuel capacity of the gasifier. I nolonger use it. The aluminum foil ison top of it because it was raininga little bit, and I was trying to keepthe fuel dry.

I could never get the gasifier towork well. It would produce some


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flammable gas sometimes, butmainly it just produced a lot of

stinky tar and smoke. I was having a lot of trouble with zone migration. That is where theflame zone (and hence all the other zones too) moves up the fuel column, instead of thefuel moving down. The result was that in short order, the flame would reach the top of thefuel hopper section and there would be no more fuel for the gasifier to gasify. The fuel gotconverted mostly to charcoal, with lots of tar and a little gas as byproducts. If I wanted amachine to make charcoal and tar, this would be a great design. However, as a machineto make gas, it was a flop.

After doing some more research,(research I probably should havedone up front), I came to realizethat wood chips are a difficult fuel.The non uniform size and shape ofthe chips makes them difficult toburn in a simple gasifier. So Iswitched fuels. I wanted to usewood pellets, but they are hard tofind here in Florida. So I settled onhay pellets. I could get them fromfeed stores, and they werereasonably priced. They seemedlike a reasonable substitute forwood pellets.

The result with the hay pellets was only marginally better than with the wood chips. Moregas was produced, but the zone migration problem didn't go away, and the tar problemwas just as bad, and if anything, even stinkier. The gasifier didn't seem to be gettinganywhere near as hot as it ought to be. I knew that low temperature operation would leadto less gas and increased tar. I suspected that poor air flow through the fuel was part ofthe problem. The uniform size, shape and composition of the pellets made them an idealfuel, as far as I could tell. So I had to face the fact that the problem was the design of thegasifier, not the fuel. The final nail in the coffin of this design was when I found the blog ofanother gasifier tinkerer who was complaining about the open core design. He waslamenting that he had built a tar producing machine instead of a gasifier. It was time for amajor re-think.

So I sat down and made a list of the problems I was having with the gasifier, in order oftheir severity. My day job involves use of a lot of statistical analysis. One tool we use a lotat work is Pareto Charting. With a Pareto Chart, you list the problems or defects in yourprocess in order from highest to lowest percentage. Then you start trying to fix theproblems at the top of the list because fixing them first will produce the biggestimprovement in your product. I had a list of defects with the gasifier, and so I listed them inwhat seemed like the most logical order.

Zone MigrationPoor Air Flow


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Low Temperature OperationExcessive Tar ProductionShaker Grate Not Working WellWeak Air Pump

Looking at the list, it was obvious to me that the zone migration problem was the biggestproblem I was facing. The fuel was being mostly converted into charcoal, rather than beingproperly gasified. Solving that problem would make a big improvement in the operation ofthe gasifier. I also realized looking at the list that solving the poor air flow problem wouldalso probably improve or even eliminate other problems on the list like low temperatureand tar production. So I decided to tackle the top two problems first, and tackle the othersas opportunities presented themselves.

So it was back to the drawingboard. I knew that the easiest wayto solve the zone migrationproblem was to cap the top of theflame tube. The flame was simplymoving up-wind toward the sourceof oxygen. Since the original designrequired air to move through thewhole fuel column, the flame simplywent up the column toward thesource of air and created the zonemigration problem. Capping thetube meant I had to find a new wayto get the air into the gasifier.

I studied the various gasifierdesigns. It was clear to me Ineeded to install air inlet tubesnear the bottom of the flame tube.It looked like the easiest way to do

it, and still allow the core section to be removable was to install J-Tubes. The J-Tubeswould also have the added benefit of pre-heating the air, since they would pass throughthe hot gas in the drum before entering the flame tube. I also noticed (maybe a little late)that almost all gasifier designs have a constriction or throat where the unit narrows belowthe flame zone. Further research explained that the constriction helped reduce tarproduction by forcing the volatiles produced in the pyrolysis zone to pass near or throughthe hot flame zone where the tars get cracked into gas. I decided that while I had thegasifier apart I'd add a constrictor plate. Also, even though it wasn't a super high priorityon my Pareto Chart, I'd replace the shaker grate, since I saw a way to make a new oneeasily.

So goodbye simple open core design, and hello complex J-Tube design. Even though thiswas a major re-design, I could build on the already existing hardware. I wouldn't have tostart all over again from ground zero. This allowed the changes to happen fairly easily andquickly.

Here I have installed the sixj-tubes. They are made of 3/8 inchcopper tubing. They are calledj-tubes because they are shapedlike the letter J. I used a large hose


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clamp cinched down tight to holdthe tubes in place. The opening inthe top of the drum needed to havea few notches cut in it toaccommodate a couple of thej-tubes that stuck out too far.

This photo also shows the chainsthat suspended the originalvegetable steamer shaker grate.

They will be reused with a new and improved grate below.

Here is a look up the bottom of theflame tube at the business ends ofthe j-tubes. Copper is probably notthe ideal material to use here,since at least in theory, thetemperature at the point the air isinjected could be high enough tomelt them. So far my gasifierdoesn't seem to get anywhere nearthat hot, and the copper is holdingup well. However, in my nextgasifier, I will probably make atleast the tips of the air inlets out ofsteel. Copper is just so darn easyto bend and work with compared to

steel tubing.

Here is a view of the constrictorplate I made. By this point in theproject I had my own welder(Yahoo!) and was getting proficientat using it. To make the plate I cuta circle out of 1/8 inch sheet steelthat would fit in the bottom of theflame tube. Then I cut a 2 1/2 inchdiameter hole in the center of thecircle. To mount the constrictor inthe flame tube, I welded three1/4-20 nuts to the plate, and drilledpassage holes in the flame tube forthree 1/4-20 bolts.


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Here is a view of the constrictorplate installed in the bottom of theflame tube.

Here I have installed the newshaker grate. I made it by cuttingthe bottom out of a stainless steelcolander I bought cheap at a yardsale. The colander already had alot of holes in it, but I drilled quite afew more in it as well. It issuspended under the flame tube bythe same four chains that held theoriginal steamer grate. I used thesame system of attaching a lengthof stainless steel wire to one cornerof the grate and running it outsidethe drum. Tugging on the wiremakes the grate shake and twist. It

is not an ideal system, but it seems to work. I put a ring on the outside end of the wire tomake it easier to grip.

Here is a photo of the top of there-assembled gasifier showing thetops of the j-tubes poking out of asea of red silicone gasket material.It's a little messy, but to me it wasa thing of beauty.


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Here is a look down the flame tubeof the new and improved design.

Here is a video I shot downthe flame tube of there-assembled gasifiershowing all the new partsand the shaker grate inoperation.

Here is a video I shot of thenew and improved gasifier inoperation. At last, glimmersof hope. The unit is actuallyproducing decent amounts offlammable gas, andsomewhat less tar. Thegasifier is running a lot hotterthan it had been too, but stillnot as hot as I expected.After the run, looking underthe cap showed that the fuelwas moving down the tubeinto the flame zone, as it wassupposed to, and gettingconsumed. No more zonemigration! I was very happyabout that. Still, theperformance left a lot to be


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desired. There was still a lotof tar. Also, I was convincedthe wimpy blower and lossesin my skinny outlet hosewere really preventing thegasifier from reaching highertemperatures and impairingits performance.

After a few runs in this configuration, and burning through a lot of hay pellets, I becameconvinced that the next big problem that needed solving was further improving the air flowthrough the unit. I had tried a few tests where I used a vacuum cleaner I had boughtcheap at a yard sale in series with the blower to increase the draw through the gasifier. Ifound that the temperature of the gasifier shot up with the increased air flow, and thequality of the gas being produced seemed to increase. I quickly tarred up the vacuumcleaner, but I didn't care. I only paid a few bucks for it, and it served its purpose inconfirming my suspicion that I needed better air flow through the gasifier.

What to do? A more powerful blower was the obvious answer. Unfortunately, powerful, allmetal blowers are very expensive. A new one was way out of my price range for thisproject. I finally found a used one at a scrap yard and got it for almost nothing. But when Itried it out, I discovered the motor was burned out. Not such a bargain after all. A newmotor for it would cost over $100! Ouch. So scratch that idea.

Then it hit me. Why not blow compressed air through the gasifier, rather than use a blowerto pull air through. My workshop has a huge air compressor. I have essentially unlimitedcompressed air. I could adjust the pressure and flow rate through the gassifier easily withjust a regulator and a valve. Brilliant, if I do say so myself. Just one little problem. Thecurrent design had six air inlets. How would I join them together to connect them to thecompressed air supply?

Back to the drawing board again.While brainstorming on how to getcompressed air into the gasifier, Inoticed a steel air duct reductionfitting I had previously used as anaborted attempt at a fuel hopperextension earlier in the project. Irealized I could cut it down andmake it a manifold that would coverthe tops of all six j-tubes. Thatwould allow me to inject air into thesystem at only one point on themanifold and feed all six j-tubes.Brilliant, again, and this time itlooked foolproof. The manifoldcame together quickly.

I was sure that improving the airflow through the gasifier wouldmake a big improvement in

performance. However, comparing my design to other gasifiers had me fretting about thesmall size of my reduction zone. It was pretty short compared to all other designs I hadseen. I was worried that the quality of the gas may be suffering due to an inadequatereduction zone. I decided that since I was once again reworking the gasifier, I would


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lengthen the reduction zone. I figured it would be easy to do by simply welding a shortlength of steel tubing to the bottom of the constrictor plate and lengthening the chainssupporting the shaker grate. Unfortunately, I don't seem to have any photos of thatparticular modification. I must have forgotten to take my camera to the workshop that day.I'll get a shot of the reduction zone extension the next time I have the gasifier opened up. Ibelieve I welded a 2 1/2 inch length of 3 1/2 ID inch steel tubing to the bottom of theconstrictor plate, then just lengthened the chains suspending the shaker grate.

My early decision to make the core of the gasifier removable once again allowed for thesemodifications to be made quickly and easily.

Here is the manifold I made tocover the inlets of all six j-tubes. Itwas cut from a 6 in to 4 in steel ACduct reduction fitting. It slips downover the flame tube and getssiliconed to the top of the flange. Asingle air inlet fitting will beinstalled on the side of themanifold.

Here is the new single air inlet onthe side of the manifold. I used aTee fitting. One leg of the Tee goesinto the manifold. One leg has ahose fitting installed that I can useto inject compressed air. The thirdleg of the tee is plugged for now.My idea here was that I could startthe gasifier on compressed air,then once it was running, I couldunscrew the plug, and let enginevacuum pull air through the gasifier(from whatever engine the gasifiereventually gets connected to).


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Here is the new and improved(again) gasifier, all buttoned up andsealed with yet more great gobs ofred silicone gasket material, readyfor a test run. I couldn't wait to tryit. So even though it wasthreatening rain, I set up thegasifier outside and fired it up.

I finally found some wood pellets.On one of my trips to Arizona, Ibought two 40 pound bags of woodpellets. They were dirt cheap too.Less than $6 per bag. I couldn'tfind them to save my life in Florida.Every hardware and homecenterstore in Arizona seems to carrythem though. Now I have plenty ofhigh quality fuel for testing out thenew and improved (again) gasifier.Fortunately I drive out to Arizonatwice a year. So getting 80 poundsof wood pellets back home in mybig truck was not a problem.

At last! The gasifier was workingwell. It is making lots of gas andhardly any tar. Everything wasworking great. The gasifier wasproducing such a huge volume ofgas, I decided I needed a betterway of flaring it off. So I bodgedtogether a quick and dirty gasburner. I just drilled a bunch ofholes in the bottom of an 18 ouncesteel can, and bolted it on top ofthe gas outlet pipe. I then but theold stainless steel vegetablesteamer I originally used as ashaker grate over the open top of

the can. It works great as a burner. The flame doesn't blow out even in very strong windgusts. I'll need to increase the stack height since the heat from the burner is starting tocook the rubber and silicone parts a little.


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Here is a video I shot duringthe first test run of the newand improved gasifier. Ashort while into the run, Ibodged together animprovised burner assemblyto flare off the gas producedby the unit. It was windy andwhen I would light the gascoming out of the outlet tube,the flame would get blownout every few seconds by awind gust. The burnerworked so well, that I plan onmaking a lot less improvisedversion of it, and mountinghigher up so the heat itproduces is away from therubber parts on top of thegasifier.

You can see the dark blackclouds reflected in thewindow behind the gasifier. Abad storm was rolling in. Ihad planned on running thegasifier at a high flow rate, tosee how long it took to burnthrough a full load of woodpellets. Somewhere less thanan hour into the run though,the skies opened up with areal gully washer of a rainstorm that brought everythingto a halt. I was drenched,then trapped inside for overan hour waiting for thetorrential rain to stop. Atleast the gasifier was niceand cool after that drenchingwhen I cleaned it out and putit away. I'll have to run thatparticular test another time.


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Here is a video of anothertest run a week later. Thistime the weather was great,though very hot. I increasedthe height of the stack underthe burner. This time Imanaged to get in the timedrun I wanted to do. It tookabout 50 minutes to use up afull load of pellets in thegasifier.

I was just as pleased as could be with the performance of the gasifier. It was workinggreat. The volume of gas it could produce was amazing. As I cranked up the flow ofcompressed air to the unit, the flow of flammable gas to the burner increased until I had areal flame thrower on my hands. The heat from the burner was so bad that it becamedifficult to approach the unit to make adjustments or shake the grate. I was franklyamazed and thrilled at the volume of gas my little gasifier could produce. This small unitshould be able to power even a huge engine. I had succeeded far beyond my wildestdreams.

So now I had a working gasifier. Time to hook it up to an engine and start producingpower, right? Well, not yet. The bright yellow color of the burning gas tells me that there isstill a lot of tar in the gas. There is not nearly as much as in the beginning, with the olderdesign, when tar would just run out of the pump and puddle around it, but there is stillsome tar in the gas, and tar is bad for engines. So I was determined to reduce the amountof tar being produced. Further research led me to believe that reducing the size of theconstriction in the hearth might reduce the tar production. Most highly efficient gasifiersseem to have hearth constrictions that are on the order of about 1/3 the diameter of thereactor. Mine was closer to 1/2 the diameter.

So I tore down the gasifier andwelded in a new constrictor plate.Now the opening is only 1 1/2 inchin diameter. The theory here is thatby making the restriction smaller,the tar has to pass through thehottest part of the reaction zoneand gets cracked. My originallarger opening was allowing tar tosneak out without passing throughthe hottest zone.


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Here is a video of a night testof the modified gasifier withthe smaller constriction. I amvery happy with this run.There is much less tar. Themodification is working great.Running at night allowed meto see the true color of theflame, and see that there ismuch less tar now.

While I had the gasifier setup and hot, I decided to do atest I had been wanting to dofor a long time. I ran thegasifier until the wood pelletswere used up, then I loadedthe hopper with charcoal. Iran the gasifier for a littlewhile on nothing butcharcoal. The flame was veryclean and almost pure blue(see the video). The gasifierproduces very clean gas oncharcoal. There are twoproblems with running oncharcoal though. First,charcoal burns a lot hotterthan the wood pellets. Thegasifier wasn't designed tohandle such hightemperatures. Somethingwould melt or break prettyquickly if I made a habit ofrunning on charcoal. I'd haveto redesign and rebuild thegasifier to run on charcoalsafely. Second, makingcharcoal throws away a lot ofthe energy in the wood. Iwant to use that energy inmy gasifier. So for now I willcontinue experimenting withburning wood pellets andother biomass. It sure is niceto see what a clean, tar-free


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flame looks like though.

There is still some tar in the gas when burning wood pellets. I also see some ash and theoccasional spark come out of the burner. A look at the burner after a run shows some (nota lot) soot and coke-like material inside. All this stuff will need to be filtered out before thegas gets piped into the intake of an engine, or the engine probably won't survive for long.The valves would get gummed up and the cylinder walls would be scored. It would also benice to cool the gas before sending it to the engine. Cool gas is denser, and that meansmore gas could be pulled into the cylinder on each intake stroke. So I need to build ascrubber and cooler for the gas.

A lot of other people use cyclone separators and radiators to clean and cool the gas fromtheir gasifiers. I considered this option. However, My metal working skills are somewhatlimited. I also wanted to keep the unit as compact as possible. A cyclone and big radiatorswould make the unit huge. I am hoping to make a unit that would eventually power avehicle. Until I get all the bugs worked out, it will only power stationary engines, but I don'twant it to grow too big. I've seen trucks running on wood gasifiers. They are giganticthings sticking high up out of the bed of the truck, or following behind on trailers. I'mhoping to make something more compact than that. Maybe something small enough to fitin the trunk of a car with the trunk lid closed. That's my dream anyway. So I wanted to tryto make a compact scrubber cooler system to mate with my compact gasifier.

So here is my initial rough sketchof of a scrubber cooler system. Myidea here is to use a water spray toclean up and cool the gas. The gaswould move up a column packedwith either stones or golf balls,against the flow of water. Thepurpose of the packing material isto increase the wet surface areathe gas is exposed to as it passesup the scrubber column. The moresurface area, the easier the gascan give up its heat andsuspended particles to the water.

Near the top of the column wouldbe a spray nozzle that would sprayout a cone of water, creating afalling water curtain that the gaswould have to pass up through.This would be the final cooling andscrubbing step. The water wouldthen flow down the column, throughthe packing material, and drain intothe drum. Clean, cool gas wouldexit at the top of the column.

The whole thing would be built around another 5 gallon steel drum, just like the gasifier.The drum would hold a few gallons of water, and collect all the residue scrubbed out of thegas. A fountain pump would be used to pump water up to the spray nozzle.

This is my initial idea anyway. Will it work? I don't know. At the very least, it will probablyneed some tweaking and reworking to get it running right, just like the gasifier did. I have


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only just started building it, so it will be a while before I know if it works. I will post updateshere as the work progresses.

The results are in. And the answeris... It didn't work :-(

Well, I should say it didn't workvery well. This is a photo of thecompleted scrubber unit. Itremoved some tar, as I could tellbecause the water turned brownover time. However, there was stillplenty of tar in the gas that passedthrough the scrubber. I was verydisappointed. One bright spot isthe fact that the scrubber did agreat job of cooling the gas, as Ihoped it would. So I had cool, tarrygas, instead of hot, tarry gas.

So I did some more research. Ifound some information that Isomehow missed earlier. It seemsthat this method of water sprayscrubbing just doesn't work very

well when it comes to removing tar from gas. Well, I confirmed that.

I'm not going to spend a lot more time describing the water spray scrubber since it seemsto be a dead end. Maybe it will come in handy for cooling the gas, but I am going to haveto come up with something else to take the tar out of the gas. Next I am going to look intoways of further reducing the amount of tar produced by the gasifier, and more conventionalmethods of removing what remains from the gas.

In order to try to get someidea why my gasifier is stillproducing so much tar, Iinstalled a thermocouple in itto measure the temperatureat the throat (constrictorplate). The temperature washigher than I expected. Sothe gasifier is getting hotenough to crack the tars.Something else is wrong. Tarmust be somehow be leakingout of the reaction tubewithout passing through thethroat. I have some ideaswhere it could be happening.I will be making some moremodifications and tryinganother run when timepermits.


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Another test run of thegasifier. This time I had takenthe gasifier completely apartand plugged every possiblegap and joint where tar couldleak out of the reaction tubewithout having to passthrough the hot zone and getcracked. The previous testrun showed that the gasifieris getting plenty hot enoughto crack the tar. The onlyexplanation I could see forthe excess tar was leaks.Sure enough, I saw where tarcould sneak out aroundwhere the J-tubes enteredthe reaction tube, and aroundthe outside of the constrictorplate. I slathered on copiousamounts of the red high-tempRTV silicone in all thoseplaces. This is not apermanent fix, since the heatwill quickly breakdown thesilicone, but I figured I shouldbe able tell if the early part ofthe run looked less tarry thenusual. It does seem lesstarry. I need to find a way topermanently plug all thosegaps. If not in this gasifier,then in my next one.

My future goals for the gasifier include of course getting it attached to and running anengine. Preferably an engine that would do something useful like run a generator.

Other goals include automating the grate shaking so I don't have to do it myself every fewminutes. I have some ideas on how to do that. At some point I will probably redesign thewhole shaker system to make it more robust and more amenable to automation. I am alsotoying with the idea of an automatic pellet feeder, to keep the pellet hopper topped off. Anautomatic feeder would allow the unit to run for hours at a stretch, rather than the 45minutes or so I can get out of it on a single load of pellets. The limiting factor would thenbe the build-up of ash and char in the bottom of the gasifier. That of course would lead toa new goal of an automated char removal system. Then there would be no real limit onhow long the unit could run.

This gasifier is a prototype unit. It is rather bodged together, and not terribly robust.Another goal I have is to eventually redesign the gasifier to be more robust and rugged sothat it could survive the rigors of years of hard use, and the banging around it would getbeing used to run a vehicle. So I will probably be working on gasifier related projects foryears to come.

I'll post updates here as the project progresses.


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Download - My Home-Made Biomass Gasifier

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