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Filtering Tars and Particulates...  RSS feed

 
William Bronson
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Location: Cincinnati, Ohio,Price Hill 45205
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I have been looking into wood gas as a byproduct of biochar. Aside from using the wood gas to heat the on going pyrolysis, the urge is to use it else where.
Storage is not realistic/safe, and thermal storage may not be needed/ useful much of the year.
Fueling an internal combustion engine that would in turn drive a generator, would allow a variety of useful work to be done.
It would also open the door to storage.
Tar and particulates must be eliminated for the gasses to be used in engines without too many destructive issues.
So far, electrostatic filtering for the particles and a vegetable oil bubbler filter for the tars seem like the best solutions. The cracking of the tars via high heat seems to be a sticking point in most designs, so skipping that step in favor of aggressive filtering might be a good direction.
The resulting fluid could be burned in turn,though the emissions that would result are not yet understood by me.
Can one burn oil in a "rocket "? Could the biochar feedstock be infused with this oil, and pyrolysed? Would the tar simply be caught again, or would any and each successive burn/filter/burn cycle pyrolysis a little more tar each time?
Many questions raised on my part, and I am eager to hear more, as well as solutions.
It just now occurred to me that bubbling the gas through the oil will heat the oil a lot,even as it cools and cleans the gas for use in IC.
Great storage medium,oil is, but again with the waste heat!
If there was an easy way to use heat immediately/store it long term, I would just burn the wood gas in the first place!
Steam is dangerous and/or expensive and Stirlings are expensive and/or hard to DIY...
I am not sure about TEG, and I can't see, to find any neutral parties with first hand knowledge of it...


 
Michael Cox
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Location: Kent, UK - Zone 8
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Interesting questions...

It sounds like your proposed filtration system may be rather complex for a home setup (electrostatic filtering etc...) - when done on industrial setup they go through extensive testing and design to make them reliable. The expense is probably beyond what you have available.

Does your biochar making design use forced air? Forced air gives you a greater pressure drop and opens up various options. I've heard of people using a bed of damp wood chips as a filter, forcing the gases up through from beneath. The various tars and particulates condense into the chips. After a batch you can let it cool, empty it out and compost it - the tars and partially chared chips are supposed to make great fungi/microorganism food.

Simply bubbling the gases through water will remove a substantial amount of the tars and particulates - this is how wood vinegar was traditionally collected during a charcoal burn.

Does your design use a closed retort? Air contains 70% nitrogen gas. If you allow air into your biochar making furnace the exhaust pyrolysis gases will also contain lots of nitrogen. If instead you heat your charge of wood in a sealed retort you will get 100% pyrolysis gases and no nitrogen contamination - this will need less volume to store and will burn far more easily. For storage, I know that people have used tractor tyre inner tubes to store methane at fairly low pressure from biogas digestors. You could probably do the same for pyrolysis gases if they are clean and cool before they enter. Then when you are ready to use the gas (eg kitchen cooking hobs, heating domestic hotwater etc...) you have the gas on demand.

I'd be concerned about putting too many energy conversion steps into the whole process
wood -> biochar+gases -> cool the gases (loses energy) -> run through a motor -> battery storage -> final use. You would be lucky to get 1% overall efficiency through that long series of conversions.

It would be helpful to know what your primary objective is - is the biochar the overwhelming goal, and energy harvesting secondary? What scale are you talking about (small batches regularly, a few tonnes at a time etc...). I've made batches of biochar in the garden before using oil drums and a top lit updraft method (nice clean burn, little smoke, ferocious flames). I've also used cook stoves that burn sticks and make biochar.

If I was going to be investing in a biochar making rig now I would probably go along the custom designed stove route. Make the biochar as a biproduct of cooking, heating etc rather than the other way round. Making a kilo or so every day soon adds up. It is much easier to collect and store biochar than it is to store flamable/explosive gases that need complicated processing and treating.

Mike
 
nathan luedtke
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In the currently ongoing geoff lawton online PDC, in one of the earlier segments he mentioned ancient Chinese cooking stoves where the exhaust was hooked up to distilling bottles that were calibrated very accurately, so you could collect various chemicals off the back end of your fire. He didn't cite any sources and I haven't yet found further info, though I have done cursory Google research. But I thought that idea was pretty sweet! Here's your bottle of terpenes, here's your aldehydes, etc etc.

 
Miles Flansburg
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Nathan, I have operated two different sets of industrial distillation machines. It all has to do with the fact that different liquids and gasses fall out of a mixture at different temperatures and pressures. It is pretty neat and pretty accurate.

William, again on industrial scale plants, I have seen cold water used to "quench" the hot gas stream to knock out all of the solids. But it was a continuos flow and had to be treated for clean up.
Seems like many of the folks I have seen on line use things like filters full of wood chips etc.
 
William Bronson
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Location: Cincinnati, Ohio,Price Hill 45205
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Thank you for your replies!
Yes, I have little time or money for this project. My main goal is to make biochar, but waste is only waste when it is wasted, and my reading indicates that any batch of wood will produce more wood gas than will be needed to pyrolysize itself.

Here is some of what led me to ask these questions...

From http://www.fao.org/docrep/t0512e/t0512e0d.htm
During the Second World War a multitude of dry filters containing wood wool, sisal fibre, glass wool, wood chips soaked in oil, and other types of fibrous or granular material were used for removal of the fine dust (average particle size below 60 micron), but success was very limited.

Wet purifiers such as water and oil scrubbers and bubblers are also effective but only within certain limits.

The best cleaning effect is obtained by employing cloth filters. However, normal cloth filters are very sensitive to the gas temperature. In the case of wood or agricultural waste gasification, the dew-point of the gas will be around 70 C. Below this temperature water will condense in the filters, causing obstruction of the gas flow and an unacceptable pressure drop over the filter section of the gasification system.

At higher temperatures normal cloth filters are likely to char and decompose in the hot gas stream. Another of their disadvantages is that they are subject to a rapid build-up of dust and so need frequent cleaning if not used in conjunction with a pre-filtering step.

The disadvantages of cloth filters can be partly offset by using woven glasswool filter bags as proposed by Nordstrom (33). This material can be used at temperatures up to 300°C. By heating (insulated) filter housing by means of the hot gas stream coming from the gasifier, temperatures above 100°C can be maintained in the filter, thus avoiding condensation and enhanced pressure drop. If a pre-filtering step consisting of a cyclone and/or an impingement filter is employed. It is possible to keep the service and maintenance intervals within reasonable limits, i.e. cleaning each 100-150 h. This combination is probably the most suitable for small and medium-sized systems (up to 150 kW electric power), and experience has shown that engine wear is no greater than with liquid fuels (33).

Electrostatic filters are also known to have very good particle separating properties, and most probably they could also be used to produce a gas of acceptable quality. However, such filters are expensive, and it is for this reason that their use is foreseen only in larger installations, i.e. equipment producing 500 kW electric power and more.






OK, so why not repurpose an electrostatic furnace filter, costs in the 60$ range?
Worth a shot, no?

From http://www.mdpub.com/gasifier/
[quote]There is still some tar in the gas when burning wood pellets. I also see some ash and the occasional spark come out of the burner. A look at the burner after a run shows some (not a lot) soot and coke-like material inside. All this stuff will need to be filtered out before the gas 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 be nice to cool the gas before sending it to the engine. Cool gas is denser, and that means more gas could be pulled into the cylinder on each intake stroke. So I need to build a scrubber and cooler for the gas......


My idea here is to use a water spray to clean up and cool the gas. The gas would move up a column packed with either stones or golf balls, against the flow of water. The purpose of the packing material is to increase the wet surface area the gas is exposed to as it passes up the scrubber column. The more surface area, the easier the gas can give up its heat and suspended particles to the water.

Near the top of the column would be a spray nozzle that would spray out a cone of water, creating a falling water curtain that the gas would have to pass up through. This would be the final cooling and scrubbing step. The water would then flow down the column, through the packing material, and drain into the drum. Clean, cool gas would exit 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 the gas. 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 probably need some tweaking and reworking to get it running right, just like the gasifier did. I have only just started building it, so it will be a while before I know if it works. I will post updates here as the work progresses. .....


The results are in. And the answer is... It didn't work

Well, I should say it didn't work very well. This is a photo of the completed scrubber unit. It removed some tar, as I could tell because the water turned brown over time. However, there was still plenty of tar in the gas that passed through the scrubber. I was very disappointed. One bright spot is the fact that the scrubber did a great job of cooling the gas, as I hoped it would. So I had cool, tarry gas, instead of hot, tarry gas.

So I did some more research. I found some information that I somehow missed earlier. It seems that this method of water spray scrubbing just doesn't work very well when it comes to removing tar from gas. Well, I confirmed that.

Water filtering, not good vs. tars?
Then what is?
From http://www.cset.iastate.edu/research/current-research/steamoxygen-gasification-of-biomass-for-fischer-tropsch-synthetic-fuels/

tar compounds condense or dissolve from the gas stream into the heavy gas oil utilized in the oil scrubber. The oil scrubber also removes residual particulate matter missed by the upstream cyclones. Conventional techniques for removing tar and particulate matter use water as the scrubbing liquid, which requires treatment and disposal. Using oil enables further processing of the waste stream for applications such as heat and power generation, in which the tars and particulate matter serve as additional carbon feedstock rather than contamination.


Oil, good for filtering tar and particles!

From http://www.ncbi.nlm.nih.gov/pubmed/20801021
Abstract
Water is the most common choice of absorption medium selected in many gasification systems. Because of poor solubility of tar in water, hydrophobic absorbents (diesel fuel, biodiesel fuel, vegetable oil, and engine oil) were studied on their absorption efficiency of biomass tar and compared with water. The results showed that only 31.8% of gravimetric tar was removed by the water scrubber, whereas the highest removal of gravimetric tar was obtained by a vegetable oil scrubber with a removal efficiency of 60.4%. When focusing on light PAH tar removal, the absorption efficiency can be ranked in the following order; diesel fuel>vegetable oil>biodiesel fuel>engine oil>water. On the other hand, an increase in gravimetric tar was observed for diesel fuel and biodiesel fuel scrubbers because of their easy evaporation. Therefore, the vegetable oil is recommended as the best absorbent to be used in gasification systems.



Vegetable Oil, good for filtering tar!

This description of oil bubbler filters seem to put them squarely into the DIY realm.



Seems doable.
My design has a closed retort and an insulated firebox and retort.


I have not completed my biochar retort, but other similar examples produce wood gas at fairly strong pressures, so I am not too worried about slowing down the gasses to much...
 
Michael Cox
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William,

Seems daft to contaminate an energy dense fuel oil/biodiesel with tar and particulates to collect a few litres of hard to store, hard to clean, hard to make gas. I would suspect a net energy loss over just burning the oil with a lot more work involved. Have you looked at the woodchip scrubbers? The absorbent surface area is supposed to decrease gas velocities and make for more effective filtration of particulates and tar. They act a bit like a cigarette filter.

Why not just make your biochar when you have demand for the heat? Make it a seasonal winter job and dump the heat in a big insulated tank as a thermal store for short term heating use?
 
William Bronson
Posts: 1412
Location: Cincinnati, Ohio,Price Hill 45205
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I doubt that the filtering would use that much in the way of WVO,but I have no way to to know that yet. I think the wood gas can feed a conventional engine, whereas biodiesel needs a diesel engine.
So far, finding gasoline engines at good prices has been easier for me than finding diesel engines in the same price/power range.
Also, I have plenty of wood, oil, not as much, and said oil can still be burned.Heck, the tars and such can be burned pretty cleanly, but we already have a surfeit of heat being made.

Lets talk about efficiency for a minute. To me, trading 1000 excess doohickies for one hard to come by doodad is a great deal. As near as I can tell, burning stuff to make heat is relatively easy.
And there are plenty of things to burn. Charcoal making will accomplish this and have a lot of potential energy left over.
So an excess of heat. If I could use that excess heat to produce something I need, the exchange rate hardly matters in my mind. What does matter is the investment in infrastructure.
To me,surfeit heat is like wind, or sun, or hydro. Its gonna be there whether one uses it or not.

During the World Wars, some things were made in odd ways , "for reason of production economy". Not only was it important to not waste materials, but it sometimes made sense to use brass instead of steel, or milkweed fluff instead of down. I cut the bottoms out of 5 gallon buckets for Sub-Irrigated Planters, and I use blue tarps to wrap furniture for disposal, because I have more of each than I know what to do with, and I got them for free. Better uses can be made of each of these items, and have been, but using 30 dollars worth of tarps saved me from buying X dollars worth of plastic sheeting.
Could I have gotten 30 dollars for the tarps? unlikely, time consuming, and not leading to directly ridding my yard of said mattress.

I have gas engine, a permanent magnet electrical motor and a bunch of wood. I could get some oil and burn it and still have no electric, I could refine it into biodiesel and buy a diesel engine and then make electric, or I could take the oil, use it as filter, and then burn it , and have electric too.

Or maybe I should just get a TEG, I would love to hear the opinions on that...

All that being said, only burning in the cold months and capturing said heat has its own appeal to me. Getting the heat from outside the house to inside the house and storing it is a challenge I am still working on.
Exhaust into the house is a non-starter for me. Water has potential, but could lead to boomsquish. I am even considering heating up the brick wall itself....

I haven't looked into the chip filter yet, but it sound promising.


I was just looking into how much gas can be had out of a given amount of wood, and found This

Gas produced from 1kg of wood: 2.185 standard cubic metres

Gas produced from 1lb of wood: 35 standard cubic feet



Seems like a lot.
 
William Bronson
Posts: 1412
Location: Cincinnati, Ohio,Price Hill 45205
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Thanks for all the feedback on the filtering.

Reading some of the Charcoal Gasification thread, makes me think charcoal is a better fuel for a "gasoline" engine.
It stores safely and has multiple uses.

I am still in the process of creating my biochar retort, but I have a goo example of what I am trying to do:







All these beautiful steampunkish photos from this site:http://charcoalkiln.com/charcoal-retort-330-gallon-oil-tank/


My design is scaled down, with a 55 gallon drum on the outside and a section of water heater as the inner retort. I will be insulating my entire device, so it should be more efficient.
I will also be adding a rocket style insulated exhaust riser, to encourage a clean burn. I began this project before I knew of rocket stoves, otherwise it would probably look a lot different.



I am still seeking the best way to collect, move, and store the heat from an outdoors biochar retort, and would love any input on that. Al has already put me on to some good video...
 
Michael Cox
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I started a reply but my PC restarted and swallowed it!

Anyway, I love those pictures William - wish I had those skills (+ time, tools and workshop!)

Here is a rough sketch of how I would build this.
Biochar and Hot Water

The idea is to fill a drum with packed dry woody matter - doesn't matter what, but small diameter gives more consistent biochar. By lighting the TOP surface of the fuel load (as opposed to lighting the bottom) you get a nice clean burn with an essentially smokeless flame. Once the flue load has burned down and the flames subside you quench the residue of biochar - I typically dump it into a large tub of water.

You don't need the chimney construction at all, as it draws pretty well just from the height of the drum, however the chimney would allow you to put a heat exchanger in the flames to heat water, as well as reflect some of the heat back in to the secondary burn. The area gets uncomfortably hot while it is going and this is definitely not safe in areas at risk of wildfire!

In this design the drum is essentially free standing so you can pull it forward to load it, then slid it back under the chimney once lit. In the one we use in the garden most of the bottom is cut out from the drum, and the drum is raised on some bricks to allow good primary air inflow beneath the fuel. As the fuel gasifies (at the top, where the flame front is) secondary air is drawn in at the top of the drum and the gases burn off in a spectacular flare.

The typical yield from this, burning twiggy branches/hedge trimmings etc... (nothing greater than thumb diameter) is around 1ft depth of biochar in the bottom of the drum. Not bad considering there is quite a bit of air in there to start with.


Benefits
  • very simple construction
  • Retort systems are fairly complex and, while they are supposed to give marginally increased biochar yield, in my experience of running small batches the additional work involved is not effective. As you are talking about processing massive amounts of wood, I'd consider sacrificing absolute biochar making efficiency for speed of processing.

  • Top Lit Updraft Design
  • TLU gives a clean burn with very little smoke, even with slightly damp fuel, sol long as you have a good layer of dry kindling on top. This is a big benefit compared to traditional charcoal kilns which are notoriously smokey.

  • Easy to load and unload
  • Smaller barrel designs (ie a small barrel inside an oil drum) need more processing of the fuel to stack the barrel tightly. A larger barrel can process a bigger batch with less effort. Easy unloading (by dumping the hot biochar from the bottom and quenching with water) gives a rapid turnaround between batches. With two barrels you could be loading one while the other burns off.

  • Massive batch heating
  • If you can get this rigged up to a big thermal store you could probably heat a home with a burn once or twice a week through the winter, just using woody scraps.

    Disadvantages
  • Did I mention how ferociously hot this thing gets?
  • We have had flames 12ft above the top of the barrel and, in one particular batch burning some some straw like garden waste, we have sent little hot embers flying. Not too much of an issue here when done in winter in the damp old UK but it would be a different issue entirely elsewhere. At times it gets too hot to get close and the surrounding area benefits from a hozing down both before during and after. A brick/concrete block chimney would definitely help contain and direct this heat.

    I'm sure that this could be worked on to make it a lot safer and get better use from the heat. You mentioned encorporating a rocket type design - perhaps by ducting the flue gases up in a riser then back down again through a bench. You could then contain the rather wild flames.

    Mike
     
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