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v5 Gasifier Hearth Explained

Page history last edited by jim mason 10 years, 2 months ago

 

Background to the Problem

 

If tar is the Achilles heel of gasification, then “hearth packing” is its club foot.  The tar problem gets all the attention, as it is always the first and most dramatic failure (i.e. it’s dirty, smelly and sticky). But “hearth packing” is the next and equally major problem that always appears.  Keeping the charcoal train moving through the hearth, not packing up with fines and choking the reduction zone and grate, is a persistent problem that must be solved before long run times are possible.  This packing problem gets progressively worse as the gasifier is fed with smaller fuels or chunky fuels with lots of fines mixed in—a common situation with real biomass byproduct feedstocks in the world.

 

Tar is primarily a thermo-chemical management issue, while bed packing is primarily a solid material handling issue.  The packing problem follows from gravity and gas going in the same direction in a downdraft gasifier (i.e. down), and packing things into a cake above the grate.  This is accelerated by the physical size reduction of charcoal during chemical reduction, and the resulting mass of char/ash left at the end of the process.  It is common for these fines to get stuck at the grate.  And once the flow stops, fines will quickly back up through the bed, filling the reduction bell, and ultimately the combustion bowl. 

 

The first result of hearth packing is restricted gas flow, but soon the concentrated mineral ash in the combustion bowl will start clinkering, and quickly the entire hearth is plugged with glass rocks.  This usually requires a shutdown and the need to physically dig out the hearth before the gasifier can run again.

 

 

Summary

 

The v5 solves the hearth packing and fines sensitivity problem in a downdraft gasifier by reversing the direction of gas flow during the reduction stage.  Instead of the gas flowing downward against and through the char bed, the flow is reoriented to flow upward against the charcoal and ash, keeping it more "fluffed", not packing it into a cake.  

 

This “Reversed Flow Reduction” is achieved by a compound projected hearth that runs combustion down the center, and reduction rising back upwards around the annular sides of the hearth.  Gas flows downward through the inner bed for combustion, then reverses and flows upwards through an elevated char bed on the outside of the hearth for reduction.

 

The char bed is raised upwards by a novel combination of mechanical agitation and motivated gas flow dynamics.  A high wrap perforated grate basket controls the height of char rise, and creates an open spillway at the top to purge any fines that make it to the end of the elevated bed.  The grate basket is agitated with a washing machine type, sharp “side-to-side shake” motion, which both size separates the char pieces via the “Brazil nut effect” (big pieces up, little pieces down), and purges the fines out the perforated basket base and sides.  A cone activator under the hearth in the center of the grate, helps to keep things moving by eliminating the usual flat grate for solids materials (whether large or small) to stack against, and keeps the char sliding out of the combustion bowl.

 

The above combination of features results in a new hearth architecture that is highly resistant to fines packing, while at the same time creating over 3x increase in reduction time, for improved gas energy density.  The result is a wider range of fine fuel flexibility and generally increased power output from an attached engine.

 

 

 

 

 

Detailed Description of Concepts and Methods in the v5.0 GEK Gasifier Hearth

 

 

1. Compound gas flow pathway. Hybrid down/updraft flow

  • Downward oriented combustion.
  • Horizontal and upward oriented reduction.

 

The v5 solves the common hearth packing problem of downdraft gasifiers by a novel down/updraft gas flow pathway.   Combustion is established with the typical downward flow through the bed.  But for reduction the gas flow is reversed and run both horizontally and upwards against the char bed.  This keeps the fuel more “fluffed” and not packing into a cake, as is typical with downward oriented reduction.

 

Downward oriented combustion is less of a problem than downward oriented reduction, as the char size has yet to decrease during combustion, and fines are still at a minimum.  Reduction is where the char physically shrinks as it is chemically reducing, with the end state of the process being fine char and ash.  It is these finer particles that are most likely to pack together with gas flow, and thus why the reversed flow is primarily needed for the reduction zone.

 

 

 

2.  Projected hearth architecture

  •  Standard downdraft type combustion bowl with restriction down center.
  • Truncated reduction bell at base and expanded reduction area rising up around sides of hearth.
  • Insulated combustion bowl to minimize heat loss from inner combustion area to outer reduction area. 

 

The v5 achieves “Reversed Flow Reduction” via a compound projected hearth architecture that runs combustion down the center, and reduction rising back upwards around the annular sides of the hearth.  Gas flows downward through the inner bed for combustion, then reverses and flows upwards through an elevated char bed on the outside of the hearth for reduction.   See Figures #1, 2, 3 and 4. 

 

    

 

 

 

The inner combustion zone is a typical bowl with perimeter nozzle ring tapering to a center restriction.  The dimensions are the same we've used for years with the v4.x 20kw gasiifers.  However, with the v5 the reduction bell under the restriction is truncated to a very short length, leaving mostly a short restriction section.  This restriction section continues to focus the heat and gas flow for tar cracking, as is typical in a standard downdraft gasifier hearth.  Post restriction the gas first flow outward horizontally, then upward through the rising char bed, for an extended reduction residence time. See Figures #2 and #4.

 

The v5 thermally insulates the inner combustion zone from the surrounding outer reduction zone via an insulation annulus between the inner and outer hearth. See Figure #5.  The “combustion inside reduction” architecture means that heat leakage from combustion is largely captured and helpfully put to work in reduction.  However, while the total thermal budget might be preserved with this waste heat capture, we would rather achieve the highest temps possible in combustion, and the lowest temps possible out the end of reduction.  Maximizing this temp spread requires minimizing the heat leakage between inside and outside hearth zones.  This is achieved by constructing the hearth as a double walled vessel, with high performance insulation filling the annulus between the walls.
 

 

        



 

3.  High wrap perforated basket grate with “high-side” spillway

  • Spillway allows excess fines to dump out top without needing to pass back through bed.
  • Char rise height is controlled by height of perforated grate basket.
  • Perforated grate basket enables "omni-directional" exit gas flow for increased reduction residence time and higher energy density gas.

 

   

 

The rising char bed is formed between the outside of the projected hearth and the inside of a high wrap perforated grate basket, as seen in Figure #6.  Unlike typical flat grates, which spread char spread all the way to the cowling wall, this high wrap grate basket creates an open passageway around the sides of the char bed, where both gas can solids can helpfully pass. 

 

This high wrap grate basket creates an open spillway at the top to dump excess char and/or fines out the end of the elevated reduction bed (Figure #6).  It also actively controls the char rise height, and decouples char bed height from gas flow rate or mechanical agitation particulars.  Grate baskets can also be changed to tune in different rise heights for different fuels.

 

With the spillway configuration, char always rises to the same height and cannot over rise and fill into the airline heat exchange section.  This prevents fines accumulation and packing at the upper end of the bed, as would otherwise be the case with a char bed spreading all the way to the cowling wall.  The top spillway exit eliminates the need for fines produced late in reduction to pass back all the way through the bed to the bottom grate.

 

This fully surrounding char bed produces a char volume and working cross sectional area for gas flow that is significantly increased over typical “downward bell” reduction scenarios.  After the gas exits from the inner hearth restriction, gas can flow near omni-directionally through the char bed, across a wide swath of cross sectional area.  Gas residence time is significantly increased as a result, which drives more complete reduction and a higher energy density gas output.

 

 

4. Cone activator in center of grate prevents static char pile and keeps the char train moving through the hearth
 

 

    

 

The v5 combines the projected hearth and high wrap grate basket with a cone activator in the center of the grate, directly under the hearth. See Figure #7.  The cone eliminates the usual flat grate surface under the hearth on top of which char and fines can stack and back up into the hearth.  The cone creates a slick ramp at approximately the angle of repose of the char, which keeps the char sliding out of the hearth, and not stacking on char pieces otherwise immobilized by the flat grate.  The high velocity gas flow out of the restriction also motivates this flow down over the cone.

 

This grate cone activator is similar to common hopper activators used throughout the material handling industry to prevent bridging or arching at a flat bottom exit of vertical vessels.  What is novel here is combining this activator cone with the other physical features and agitation methods in a gasifier hearth to prevent fines packing, promote char rise, and maximize size separation (see discussion below).

 

 

5.  Grate motion is a washing machine type "side-to-side shaky shaky" movement.  Sharp back and forth agitation on a center rotation point

  • Motivates char rise with minimum of char abrasion.  
  • Sharp agitation to purge fines.
  • Mechanical size separation via the “Brazil nut” effect (chunks move up, char fines move down).
  • Variable shake amount allows active purging of char through bed.
    • Biochar offtake.
    • Partial reduction of high mineral content fuels to control slagging.

 

 

    

 

The grate basket is agitated with a washing machine type, sharp “shaky shaky” motion, which achieves multiple desired outcomes: char rise, char size separation, and fines purge.  In the v5 we are using a side rotary drive with three point star linear reciprocator to create the sharp back and forth motion.  See Figure #8.

 

The back and forth motion is around10 degrees of travel, which is around 3/4"of travel on the grate edge.  The cycle frequency is not terribly important, so long as it does not get high enough to enter the bed into an enlivened state.  If the bed is over vibrated an enlivened bed state will form and the char will circulate around the bed in a toroidal manner, like seen in a vibratory tumbler.  This state destroys the desired size separation as discussed below, and can also result in increased bed packing via particle alignment.  The desired motion is more importantly sharp starts and stops, or high acceleration and deceleration, to impart the high agitating energy to the bed.  This sharp motion is critical to purge the fines out of the holes in the grate.

 

The char bed is raised upwards by a novel combination of the above mechanical agitation and motivated gas flow dynamics.  The mechanical motion alone, in combination with the center cone and weight fuel in the inner reactor column is sufficient for significant char rise on the outside of the hearth.  A good agitation will “loosen” the bed, and weight of the inner fuel column will tend to push fuel out of the hearth and up on the outside.  If the bed was fully fluidized, these inner and outer fuel columns would level off.  With the desired agitation described above there is not a full level equaling, but there is adequate transfer of gravity forces to lower the inside char column and raise the outside char annulus. 

 

We’ve tested many movement types (full rotary, linear side to side, up and down with sharp stop at top or sharp stop at bottom, mechanical ramps, rotary center scroll plate with outer popping grate annulus, and combinations thereof) and the above washing machine type motion has proved superior.  It uniquely results in excellent char rise with a minimum of motion thus char abrasion is kept to a minimum.  The built specifics to realize the grate assembly and desired motion are also relatively simple to accomplish.

 

The described washing machine motion, in combination with the grate cone, is also uniquely effective in size separating the char pieces and fines via the “Brazil nut effect”.  This is the phenomenon whereby agitation of a mixed sized bed of solids will tend to size separate, with big pieces moving upwards, and smaller pieces moving downward.  This results in useful char chunks tending to stay elevated in the char bed, and finer pieces and end of process char/ash tends to work its way to the bottom of the bed.  Many movement types were tested, as listed above, and the washing machine motion uniquely optimized this size separation in the bed.

 

The upward motion of the char is also encouraged by gas flow through the bed.  The gas is flowing at relatively high velocity and is pushing the char forward as it passes through the bed.  The problem is this also preferentially pushes the fines forward and upward in the bed, which is in contradiction to the directional size separation we want via the Brazil nut effect.  This can lead to fines ending up at the end/top of the bed, and thus why the v5 includes the top spillway.  Without this spillway the fines that have been pushed forward by gas flow have nowhere to go and will collect at the top of the bed until they pack.  This failure state was observed repeatedly in testing with a rising char bed that stretches all the way to the vessel walls.  Once we solved the packing in the center under the hearth with the compound down/updraft architecture and grate cone, a new packing problem appeared at the end of the rising char bed.  The addition of the spillway scenario was needed to resolve this issue.

 

The combination of vigorous char rise and open top spillway means we can also purposely over drive char through the hearth and dump it over the spillway.  We do not have to consume all the char in reduction down to the finest possible char/ash.  By increasing the time and vigor of the washing machine motion, we can purposely over purge char from the hearth, without it have to physically pass through the grate as is typical.  This is an exceedingly useful feature as it allows for extra functions like purging clinkers without direct hearth dig out, producing extra char vs gas for charcoal off-take scenarios (i..e. biochar), or simply reducing the fullness of reduction on high mineral content fuel/char so as to lessen the concentration of minerals exposed in the hearth and reduce the clinkering problem.

  

 

Minimize particulate entrainment in output gas

  • Rising bed reduction acts as first "packed bed filter" stage.
  • Gas out annulus is sized for lower gas velocity and reduced particulate entrainment.
  • Use airlines or baffles in gas out rising annulus for turbulence and particulate separation.

 

The above hearth and grate architecture also minimizes the fines entrainment in the exiting gas flow. 

 

The rising char bed acts as a first stage “packed bed filter” to minimize fines exiting the bed.  Fines entrained in the gas flow tend to fall out via impaction and tortuous path direction changes while traveling through the bed.  The gas is also exiting away from where the fines are directed and concentrated (downward), so the flow has less opportunity to pick up fines.  In the downward bell scenario, the gas flow exits at the bottom, exactly where the fines concentration is highest, thus more particulates end up in the gas, with increased filtering problems downstream.

 

Any fines that do leave the bed with the gas flow are further encouraged to settle out by the large buffer volume between the top of the char bed and the beginning of the air/gas heat exchange section.  See figure #2.  Beyond there the spiral air lines of the air/gas heat exchange section create turbulence in the rising gas flow, and work to further separate fines from the flow.  See Figure #1 and #2.  This rising heat exchange annulus is sized to keep gas velocities relatively low to minimize the particulate entrainment forces and maximize the ability of turbulence separated fines to fall back down and out the spillway to the ash catchment under the grate.

 

 

 

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