9 replies to this topic

[jimmason]

many of us are interested in methods for continuous feed into a closed top gasifier.  as this type of gasifier operates under sealed conditions, some type of air lock feeding method is needed.  problems ensue. . .

feed air locks are long known a sensitive and expensive piece of machinery.  even when they do work, maintenance is usually high, and jamming not infrequent.  here's a bit fo summary and some new thoughts on tolerable solutions.

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the usual go to solution for a feed airlock is either a rotary star valve or a double dump flap valve.

double dump valves are typically considered more forgiving and less fuel fussy, but they are very large, usually in the upwards direction, and the actuating mechanism can get complicated.  you also need to control the fill into them or the top flap will get stuck, and control the stack up below, or the bottom flap won't open.

rotary valves promise more compact forms and drive, without the need to control fuel in feed, but always have the problem of fuel pinching at the entry between rotor and housing.  the second main problem seems the high maintenance needed to keep the tolerances adequate for seal.  some of the more interesting ones use a tapered rotor in housing so gap tolerance is an "easy" linear movement of the shaft inwards or out.

the pinching problem is most "easily" dealt with by only passing small size solids through it, and or raw power to cut the fuel.  both of these routes are not attractive for feeding wood chips in small scale gasification apps.

two lesser used routes i've recently found that seem interesting-

1. controlling the feed rate to the rotary valve so the the Vs only partially fill.
usually this is done with an auger feeding the rotary valve, but other other methods are possible.  either way it usually gets rid of the passive feed from the overhead hopper into the rotary valve.  

here's one nice example of an auger fed rotary valve:
http://www.heizomat....llenradschleuse anyone know of more of these?  and yes, know you are back to having to control the feed as in a double dump valve.

2. using an side feed rotary valve with a controllable cut to similarly prevent the full filling of the rotor V.

i only recently learned of this category of side feed star valves that try to fix the pinch through changing the fill level of the V by fill path geometry change.  you can see some of these units here: http://www.carolinac...ry_airlock.html

the idea is to get the fill to the side of the rotor on the upswing. once the rotor vanes get angled up enough the material falls into the V, which is set up before the V gets to the formal housing, and thus the pinch issue at the edge is moot.  no metering solution is needed at the inlet.  the outlet can still have a metering/overfill problem.

either the side fill rotary or top fill rotary can have an added cut ledge that helps to control the amount of fill into the Vs of the rotor.   this cut plate is set quite far above the rotor circumference so it itself does not become a binding point.  angle of repose is assumed between the cut plate and rotor.

you can see an example of a side entry rotary valve with a cut plate on page 7 in the drawings in the pdf attached or at this link:


https://files.pbwork...RotaryValve.pdf



looking at these geometries, i don't see any reason we can't use a regular top to bottom rotary valve, and mount it on it sideways to the desired angle.  this will achieve the same cut off and gravity fill of the V cups.  to this we can then add a cut plate to further increase fuel flow happiness.

mounting a standard rotary valve on its side will complicate the hopper and reactor flanges, but potentially less so than the major cost of the more exotic side entry rotary valves.   if we can use a regular star valve, there are lots of cheap sources for them, as they are commodity material handling tech around the world.  whether their maintenance is tolerable is another issue.

neither will likely make a rotary valve suddenly wonderful, though it might move things towards tolerable.

anyone else have any other ideas and/or examples of interesting air tight feeding systems?


j

Attached Files

•  SideEntryRotaryValve.pdf   3.23MB   9 downloads

[fuel spiller]

Hi Jim

   A few thoughts from WAY outside the box. The same way a smart pump motivates fluid through a tube could stop chip flow and pinch seal below in a straight dump style application. (I'm thinking of those 'dry bags' used in rafting and how well they seal in air with a few folds like the end of a toothpaste tube) Rather than folding to seal, a few of those pinch off flow controllers in line on a vert section of dry bag like material would make for a cheap grav-feed air lock. Magnets could help clamp things air tight thru the fabric.

Neal

[jimmason]

neal,

that's not too way out of the box.  there are some valves made in a manner similar to this.  they are called trickle or vac valves.  here's an example of one i saw recently.

http://www.dustcolle.../vacuvalve.html

i still don't see how these can get the material out of an off the sealing surface to reestablish the seal.  it seems that material would easily get stuck between the flaps as they start to close.  maybe if the duckbill is long enough the top starts to shut and the bottom is able to empty out.  if material would stop at the top but without vac established the bottom might be able to empty, then the duck bill would start sealing up from the end.  ok, maybe i see how it works.

j

[HarryN]

Most valves have a lifetime, and that lifetime is based on "cycles".  More than one well intentioned design failed due to misunderstanding this basic principle, and gasifiers are not exception.

My suggestion is to use something where the valved off volume is relatively large compared to the feed.  For example - rather than trying to valve off a 1 qt volume per cycle, try to valve off 50 gallons per  valve cycle.

Another approach commonly used is a ram rather than a valve.

[Sparker]

What about something like a large ball valve with one side of the ball closed off to form a bucket on one side.  One rotation empties one batch.  It could be filled by an auger on a timer so that the auger won't completely fill the bucket.

[fuel spiller]

Hey Sparker,
I was thinking of that the last time I saw a baby diaper can I think was called 'Diaper Genie'.
A 'bucket ball valve' sits atop an airtight trash can, one half turn dumps the load in with no air out.

[fuel spiller]

From Jim
Quote   " i still don't see how these can get the material out of an off the sealing surface to reestablish the seal. it seems that material would easily get stuck between the flaps as they start to close."

Jim,  How about a seal area far bigger than chip size, say 6" top to bottom. Make them out of a memory type foam that could seal around any odd chips to create air lock (like food in a seal a meal bag)

Or maybe two brushes could 'rub' the surface (like rubbing palms / hands together before the seal began to close

[Biobale]

The rotary airlocks look like the simplest method, as long as you only part fill the V as already outlined. This should be manageable by using a screw auger feeding the airlock at a speed set to deliver the correct rate of wood chips to only partly fill the V. Whether it's a side entry to vertical rotary airlock shouldn't matter in this case. Also, I would suggest that the auger and rotary airlock should be on a timer control system that would operate for say 5 - 10 minutes each hour to refill the GEK hopper to a suitable level. This would significantly reduce the amount of air entering the system through the airlock compared to running continuously. This means the feed system would need to be sized to deliver the required fuel in only 10-15% running time.

The next challenge would be to work out the filling rate depending on the fuel usage in the hopper. Some type of high and low level sensors in the GEK hopper would be useful here to turn the auger-air lock system on, but this is a bit out of my area of expertise. Maybe someone else has an idea for this component?

These comments apply to wood chip or pellet fuels or reasonable consistant size. Variable sized material should be screened to get an even particle size to suit the size of the airlock V sections.

[fuel spiller]

Perhaps a pair of sight glass fixtures mounted high and low on the hopper, allowing a  laser or light beam and optic sensor to "see" the level?

[HarryN]

One sort of non standard method is to use a water seal.  I know this sounds strange to consider putting your fuel into water, but in some strange way it can make sense.

- Consider a hopper filled with very hot water - perhaps engine radiator type temperatures
- As you add chips / fuel, it will eventually be forced to sink to the bottom
- For very dry chips, they would become slightly wetter, but for green wood or similar, the moisture level might actually drop
- Use an auger to pull the fuel from the bottom of this water filled hopper.

The auger would form a natural "wet seal" with the hopper and tend to keep air from getting into the gasifier.

If you want to further reduce the moisture level, then you could make a second "wet seal" hopper setup, but use a vegetable oil, such as safflower oil to reach even higher temperatures.  At this stage, you could reach temperatures approaching torrified wood type conditions, but certainly the fuel would come out of this stage very dry.

Some of the water will flash off, and some will sink to the bottom, but either way, air and moisture levels will be much reduced.

I have been playing with this a little at home, and it seems to work.  Each fuel does have its own drying time though, so this needs to  be considered.  The other area that needs consideration is that fully torrified wood burns very rapidly and can self - ignite under certain conditions.