GEK Wiki / Pyrolysis Gas Recirculation Ejector Design
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Pyrolysis Gas Recirculation Ejector Design

Page history last edited by bk 11 years, 4 months ago

Pyrolysis Gas Recirculation Ejector Design



Recirculation of pyrolysis gases up through the pyrolysis zone and back into combustion can greatly decrease tar content gasifier gas (see Susanto and Beenackers, 1996). Recirculation of pyrolysis gases leads to an actively formed convection cell which increases the length of the pyrolysis zone. This should reduce the temperatures that pyrolysis occurs at, yielding tars that are less recalcitrant to thermal cracking [citation needed]. Premixing pyrolysis gases with incoming air should yield a premixed flame and not a diffusion flame in the heterogenous conditions of a packed bed. This should lead to more complete combustion.


Given these benefits, we have sought to develop an internally driven design that can recirculate pyrolysis gases within the GEK. This is the motivation of the testing of various designs below.


Discussion of design, testing, and ejector principles is welcome on the forum here.


Idealized System





Nozzle - Modification of original GEK nozzle, wrapping an entrainment tube and and tar inlet tube around it. This architecture is depicted here, and shown below:

T - A design using a 3/8" T and 3/8" cap with jet entering parallel to entrainment tube. Not implementable inside GEK, but allows easy testing/modification of air jet diameter (D_air), position (x_jet), and different entrainment tubes (L_mix, D_mix).

Jet in Riser - After seeing the limiting constraints on entrainment length in the nozzle design, the jet is moved into the vertical riser, with incoming gases entering through a second tube. This yields much larger entrainment areas to work with, but is constrained by the nozzle ejecting the mixed stream. Results forthcoming.



Q_air - Volumetric flow rate of air into the ejector

Q_in - Volumetric flow rate (of tars) into the ejector prior to mixing

Q_mix - Total volumetric flow rate inside and out of the entrainment tube

x_jet - Position of (straight cut tube) jet from center of inlet tube (positive towards entrainment tube)

Q_in/Q_air (Entrainment Ratio) - The desired goal is to achieve greater than 0.5 entrainment, above this, Susanto et al. saw no improvement in tar reduction.

P_air - The static pressure before the nozzle (measured using a manometer and tap off a large tube that should make the kinetic pressure term ~0).


Cold Testing Results

Nozzle Design

Entrainment across different flow rates. Note Q_in/Q_air is linear across different flows (Q_in/Q_air is given by the slope of the lines, m in y=mx+0). L is length of 1" entrainment tube from riser center (subtract 0.75" for length of entrainment from jet to exit).



Here we can see the (approx?) static pressure of the incoming air in relation to the flow. We can see that for all intents and purposes, the pressure is independent of modifications to entrainment length. Pressure developed will be primarily dependent on D_jet, frictional, and exit losses. Note that with the GEK nozzle we stay below 1" of static pressure at up to 5 m3/hr per nozzle.


Q_in/Q_air values for a small selection of varied entrainment tubes, holding P_air at 1"

Q_in/Q_air: 0.14, L: 1.25, D_mix: 0.885 (no extention)

Q_in/Q_air: 0.52, L: 25.25, D_mix: 0.880 (2' plastic extention)

Q_in/Q_air: 0.41, L: 2.0, D_mix: 0.885 (0.75" extention)

Q_in/Q_air: 0.47, L: 4.25, D_mix: 0.625 (1/2" NPT nipple extention)

Note: Q_air was 9.6 m3/hr to achieve 1" P_air as measured. The measurement technique for P_air was flawed. Will be improved for next round, which will lead to lower flows at 1" pressure.


T Design

Effect of jet position and entrainment length on entrainment in the (not implementable) T design.

L is additional entrainment length added to the T (?). Jet is 1/8" ID.


  • Entrainment ratios are dependent on the repeatablity of the orifice flow meter measurements. Calibration curve was done a few days before these measurements. Take ratio with a grain of salt, but ratio w/ entrainment tube was above 0.9. Update: After testing, calibration has not really shifted (given noise in measurement).
  • For D_jet=0.15", P_air (~static) at 3 m3/hr is ~5.5 inH2O (measured, with same jet). This could be reduced with changes to the nozzle design (minimize frictional losses by using a minimum of small diameter tube/hole).
  • First rise at -1.30" is a suspected outlier. 
  • x_jet zero is on center with downcomer, negative away from entrainment.


Literature on Converging/Diverging Nozzles

Most ejectors use a converging/diverging nozzle design. An open question remains as to what increase in performance can be expected by implementing this design over a straight tube/jet design, especially when operating at sub-sonic velocities.

The De Laval nozzle design produces a choked flow caused by the supersonic velocity of the restriction (wikipedia). Quoting "Bob B." on Choked flow occurs when the flow through a restricted area, such as an orifice or nozzle, can no longer be increased by a reduction in the downstream pressure.  This occurs when the sonic velocity is reached at some point along the flow path.



Literature review of optimization results by Kroll, 1947 in Watanawanavet, 2005.


Watanawanavet states "the optimal geometry of a high-efficiency jet ejector was discovered. The research results indicated that optimum length of the throat is 2 to 3 times the inlet diameter. This is approximately 5 to 7 times the throat diameter, which is consistent with the literature. The optimum throat diameter is about 0.44 times the inlet diameter, which allows for complete mixing of the propelled and motive streams before flowing to the divergence section. The flow visualized diagram of the optimized model confirms the complete mixing of both streams. The optimum nozzle position is – 0.05 to 0.05 times the inlet diameter in most cases, which is compatible with the ESDU (1986) recommendation."


Entrainment Ratio at Different Temperatures

A graph in Watanawanavet from Holton and Schultz, 1951 shows a performance drop of 0.77 in entrainment ratio from 90 to 1000°F air (32 to 537°C).


Modelling a jet-pump

MIT Opencourseware provides a good idealized case of a jet pump system, here.

This will be expanded upon.



Susanto, H, and A. A. C. M. Beenackers, 1996. A moving-bed gasifier with internal recycle

of pyrolysis gas. Fuel Vol. 75. No. 11. pp. 1339-1347.


Watanawanavet, Somsak. 2005. Thesis: Optimization of a High-Efficiency Jet Ejector by Computational Fluid Dynamics Software


MIT Opencourseware 

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