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Driving a gasifier is hard.   Proper hearth dimensioning and good fuel is important, but not enough to guarantee success.  Most of us learn an informed operator is also needed.  In fact, an informed operator is often the most important variable on the road to success.  Unfortunately, it is also the variable that varies the most.

Things would be much easier if we had the equivalent of a “speedometer” and “tachometer” to guide the operator.  Something like a car where we have a gauge that tells us how fast we are going, and another gauge that tells us how hard we’re pushing the engine.   Or maybe even idiot lights or automated control systems, further reducing the need for “intuitive expertise” from the operator.  How have gasifiers existed for over a century without these sorts of simple gauges and numbers to tell us what to do?

ALL Power Labs has been working for the last year to formalize downdraft gasifier performance, and through this, to find simple measurement points and number guidelines which tell the operator what is going on in the reactor.   The goal has been to find the easy to measure “indicator values” for the min and max pull rate of the reactor, as well as real time signals which correspond to tar conversion success or failure.  Tar is difficult to measure in real time.  But maybe there are combinations of temp and pressure measurements with high enough correspondence to tar production to give us the same information more easily.

The result of this work is “The Masonic Method”– a formal operation and tuning method for an Imbert type downdraft gasifier.

The Masonic Method uses two temperature and two pressure measurement points to guide all critical operational issues with a  downdraft gasifier.  The same 4 points are also used to diagnosis common problems with a gasifier as well as guide configuration refinements of nozzle sizing, hearth geometry and fuel specific optimizations.  How specifics on using these same measurement points for tuning will come in a separate post.

The two temperature and pressure measurement points needed are as follows.  See also the above graphic.

Temperature 1: Hearth restriction
Temperature 2: Bottom of reduction zone
Pressure 1: Pressure drop across nozzles
Pressure 2: Pressure drop across full reactor (nozzles + bed)

For more info on the tests and analysis that generated the correspondences between temperature, pressure/flow rate and tar conversion, see here:


Your Gasifier Speedometer
(aka: the manometer)

The most helpful gauge for informed gasifier driving is a manometer measuring the pressure drop across the reactor.  The pressure drop across the reactor tells you to an approximation how fast you are pulling the reactor– or in other words, how much gas you are making.

For good results, you need to pull gas between the min and max gas rate acceptable for the gasifier hearth dimensions.   Pulling too fast or too slow will create problems.

-Pull too slow and you’ll make a mess of tar.
-Pull too fast and you’ll make soot, weak gas, and ultimately clinkers.

This min and max gas rate band is fairly narrow, thus the need for an experienced operator to find it.  Fortunately we can quantify this “good band” and have the “newbie” read a gauge to approximate “expertise”.

Our tests have shown the min-max pull rate band as 2″ to 10″ of h2o vac across the reactor (before filtering).  Stay within these limits and you’ll have good results.  For even better results, try to stay in the sweet spot,, which we consistently find to be 4-6″h2o vac range.  You can push things up to about 12″ and idle down to about 1″, but you are tempting trouble at these extremes.

Here’s these “speedometer” manometer numbers as a chart.  You might want to print these out and keep them with your gasifier.

12+: Overpull
10: Maximum

8: OK

6: Good
5: Ideal
4: Good
2: OK
1: Minimum
0-1: Underpull

These values can be read manually via a simple manometer, or we can use the same values to inform idiot lights and electronic automation.  In all these cases, the critical new learning here is that we can formalize the min-max rate, and make informed operating decisions in relation.  We do not need to guess, nor do we need to rely on years spent face down in the black goo.

These numbers assume a well configured gasifier with reasonable hearth geometry and nozzle sizing.  We can get close to reasonable dimensions using the standard Imbert charts.  For further refinement of these dimensions and configurations, see the tuning section below.

Yes, different gasifier types, sizes and fuels will introduce some variations in this scale, but they are less than one would fear.  Also, these variations can be quantified with the same formal system, and thus compared between machines.  Refinements and addenda are expected to the above guidelines as others use it.

Your Gasifier Tachometer
(aka: the thermocouple)

Minimum RPM

Tar cracking in an Imbert type gasifier is achieved through creating a high temp combustion lobe across the full hearth area, and forcing tars to pass through it and crack from the high heat.  The goal is to spread cracking adequate temps across the full hearth volume –all the down to the restriction– and not let any tar pass around the edges, center, or other cool spots.

Full and complete temp PROPAGATION is what is important here, not just high spot temps in front of the nozzles.  Actually, the temp in front of the nozzles is somewhat irrelevant. What matters is how well the high temp has filled the entire hearth and propagated all the way to the restriction.

Our hypothesis is that the best way to measure this “fullness of propagation” is the temp achieved at the restriction, NOT the temp right in front of the nozzle.  If the nozzles are sized reasonably, and we find some X adequate tar cracking temp still present at the restriction, we can reasonably conclude that the hearth is filled with a temp somewhat above this.  Tar most likely will be passing through these higher hearth temps, and at the very least will pass through the directly measured temp at the restriction (given the narrow gas passage).

In our recent tests we’ve been very pleased to find there is, in fact, a very strong correlation with temp at the restriction and tar in the output gas.  There is a very clear and rather linear relationship between temperature at the restriction and tar cracking success.   There is similarly a rather poor relationship between temps right in front of the nozzles and success in tar conversion.

You can see test results on the correspondence between restriction temp and tar conversion here:

The takeaway from the tests is that if we have 900c or above at the restriction, tar will disappear “completely” form the output gas.  Tar destruction continues to be reasonable down to about 800C at the restriction.  Below here tar passing will rise to unacceptable levels.

This gives us a very simple probe point we can monitor in real time to see if we are making tarry gas or not.  It is also a somewhat cooler measurement point than the right at the nozzles, thus our thermocouple is less likely to melt.  We can use this measurement point and numeric value as a new rule for helping newbies drive a gasifier successfully:

“Always keep the restriction temp above 800C!  If you let it go below this, you will have lots of tar in your gas”

Maximum RPM

If a thermocouple at the restriction is your best monitoring point to know when things are too cold, a thermocouple at the end of the reduction zone is your best monitoring point to know when things are too hot.

A thermocouple at the end of your reduction zone tells you how fully your reduction is finishing.  Pull too fast and the reduction reactions will not have adequate time to complete.  Gas exit temp will rise as a result.

Gas energy density increases the further we run reduction to it lowest temp potential.  Also, CO reversion to soot and CO2 increases the higher temp the CO leaves the bed.  Run the temps up even higher and the ash will begin to fuse into clinkers.  This is the ultimate “redline” point of the gasifier—when you start to make clinkers.

This “redline” RPM of the gasifier appears to be about 900-950C out the reduction bell.  Above here I anecdotally notice significantly increased soot production and soot filling the cyclone jar.  Go significantly above it and you’ll fill the hearth with clinkers.

Thus we can give the newbie a simple rule to define the max pull rate and temp before things will start melting down.

“Always keep the restriction temp at or below 900C!  If you let it go above this, you will create excess soot, weak gas, and ultimately clinkers that will plug the hearth.”

When to Shake the Grate

As the reduction bell gets packed with ash, the resistance to gas flow increases.  This will increase the pressure drop across the reactor.  At a constant gas flow rate, we can see this as a rising reading on the manometer.

Unfortunately, gas flow rate will also change this reactor vac reading.  We need to find a signal that is independent of, or co-varies with changes in gas flow rate.  The way we’ve discovered to do this is to monitor the RATIO between the pressure drop across the nozzles and the pressure drop across the bed.

The pressure drop across the nozzles is NOT impacted by variations in bed packing.  The pressure drop across the nozzles follows from the nozzle hole size, not anything happening later in the bed. We can measure this with a pressure port somewhere in the reactor around nozzle level, as shown in the graphic above.  The pressure drop across the bed is the total reactor pressure drop reading minus the nozzle reading.  The total reactor reading is measuring the combination of BOTH the nozzle and bed pressure drops.

Once we know the nozzle and bed pressure drop readings separately, we can find the ratio between them, and this ratio will stay about the same independent of flow rate.  Or rather, both of the pressure readings co-vary near proportionally with flow rate changes, thus the ratio between them stays constant.  This ratio will only change when the bed packs and the resistance to gas flow through the bed rises.

When the bed is properly purged and gas is flowing well, we like to see at 1:1 ratio between the nozzle drop vs bed drop.  Said another way, the nozzle pressure reading should be about 50% of the total reactor pressure reading.  (This of course assumes your nozzles are sized correctly in the first place, which is covered below in the tuning section.)

If this ratio changes, we know the bed is packing.  Proportionally more pressure drop is coming from the bed than the nozzles.  When we see this, we know to shake the grate and restablish the correct ratio.   We can use this ratio threshold for manual grate shaking, or as input to an automated grate shaking system.   We can most simply read this ratio off a manual visual manometer.  Or the computer can read this ratio and respond accordingly via a micro-controller.

Note: This “new” pressure reading at nozzle level is easy to take on the GEK by using the lighting tube as your sample tap. Put a barb on the end of the lighting tube (once done lighting) and attach it to the standard manometer.  Put the other channel of the manometer on the usual end of the reactor tap.  Now you have the total reactor pressure drop and just the nozzle pressure drop.  The difference between the two readings is the bed drop.

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