Purpose

 

We intend to develop a temperature and pressure based diagnostic and configuration system for a downdraft gasifier.  Assuming reasonable geometry in the hearth, we conjecture there is a minimum temperature we need to keep at the top of the reduction bell that indicates sufficiently higher temps have been maintained in the combustion lobe above, and and all tar has been forced to pass through sufficiently high temps, at sufficient residence time, to crack pyrolysis products and yield a clean gas.  The temperature maintained at the constriction (T_tred) might be a simplification of this larger complex of variables, and one we can directly measure and control the reactor in relation to.  Pressure drop across the reactor is closely associated with blast rate out the nozzles, which approximates a measure of penetration of the bed.  We want to explore whether we can use simple temperature at top of reduction and pressure drop across the whole bed to define the min flow rate for clean gas, and the max flow rate before we overpull the reactor. 

 

Other tests in this series are linked to from the GEK User Pages and Run Reports page.

 

Methods

 

Pictures, left to right:

• Highly organized wiring and tubing
• Ignition through the V3 ignition port (seal improved by using aluminum tape between pipes)
• Terminal screen with real-time CSV (Comma Separated Values) datalogging (if anyone wants to develop a Processing app for better real-time visualization, it would be highly appreciated)
• Stirring assembly

 

Data

Temperature

(note: temp scale tops out at 1250C.  The flat spots on the top of the graph for combustion temp are where the temp went above 1250C).

 

Pressure

Flow

Pyrolysis/Combustion/Reduction Contour Plot

version c - x-axis labelling/grid corrected

 

Conditions

The conditions plot is intended to represent other conditions that are not shown in other plots, but will have an impact on the performance of the reactor, not all metrics are direct measurements and are still being developed.

Auger On is a direct measurement of the switch state of the mechanical fuel sensing device (black = on).

Fuel Low is based on pyrolysis temperature measurements. As fuel level drops, sensed temperature increases (see 47 min, 82 min, and 110 min on contour plot) (black = low).

     - The equation used needs confirmed, since correspondance between contour and fuel low strip is low.

          - Confirmed, issue was with contour plot x-axis which is now fixed.

Hopper Open is based on a ratio of air to gas flow, since a significant amount of air enters through the hopper when open, this ratio decreases when open (black = open).

 

Tar Sampling

 Methodology

"Tar" samples were collected after the cylone but BEFORE the packed bed filter.  We were trying to remove as much of the large particulate as possible, but not muddy the sample with fine filtration, which will also remove tars.  We are trying to gauge reactor performance in converting tar-- not the effectiveness of the filter train in removing tar.

 

The gas for the tar sample was taken from the bottom bung on the filter vessel (before filter), through the 1/2" NPT port provided for testing. A short length (<4") of thin neoprene tube was used to connect to a spot sampling apparatus consisting of 2 plastic plates aligned and clamped over a piece of 1" wide ceramic insulation strip as a filter medium. A 60 mL syringe was used to pull in and push back (purge) the sample gas through the spot. Pullside refers to the darker side, first to come into contact with the gas, pushside is the opposite side and was also recorded.

 

The Tar Value Index is the value of the printed greyscale reference (0 -> 100%, white->black, in 5% increments) that best matches the darkness of the tar sample spot, on visual inspection.

The Tar Value Index, with methodology currently used, is also a measure of the soot content in the gas.

 

A modified version of this apparatus was used for the test

 

Tar strips and greyscale 

 

Suggestions for future improvements to the methodology:

Use standard greyscale (for reproducability)

Use hole through greyscale chart for comparison (value was gauged by covering half the spot with shade next to it which may introduce visual noise)

Provide standard apparatus for sampling

Develop method to differentiate between soot/tar

Align method to those used in the literature

 

Correlating Gas Tar/Soot Content to Reactor Conditions

Time series plot of spot samples taken at 100 second intervals throughout the test. Black line is temperature at top of reduction (scale not shown, but index 0 = 600°C, index 100 = 900°C).

 

 Gas sample darkness is seen to decrease with increasing top of reduction temperatures.

 

Scatterplot showing relationship of top of reduction temperatures to gas tar/soot content. Temperatures taken from single data point at sample time.

 

On this and the two following charts: Lines are linear regression lines, red includes points when fuel level was "low", black only includes points when fuel level was "normal". Low fuel levels would decrease quantity of pyrolysis products, leading to cleaner gas independent of the tar cracking capability of the reactor.

Scatterplot showing relationship of combustion temperatures to gas tar/soot content. Temperatures taken from single data point at sample time.

The direct effect appears to be insignificant.

 

Scatterplot showing relationship of air blast velocity to gas tar/soot content. Velocities taken from single data point at sample time. Air blast velocity is air velocity through air nozzles at standard conditions (room temp. air density, not the faster velocity that would be seen with lower density pre-heated air). Nozzle hole diameter was 5.5 mm with 5 air nozzles. Calculated as:  Air Blast Velocity [m/s] = Q_air_in [m3/hr] /(NozzleCount*pi*((NozzleDiameter [mm] *0.001)/2)^2))/3600 [s/hr].

 

These correlations are suggestive and merit further exploration in future tests, but should not be taken as significant until repeated.

 

Air and Gas Flow vs. Reactor Vacuum

We can start to determine flow rates that can be expected at different reactor pull rates. One of the largest dependencies on flow rate will be packing of the reduction cone which will reduce flow through it for any given reactor pressure. While we should not extrapolate this information to other fuels, these plots start to provide a baseline set of flow data.

Measured airflow into the reactor through air cowling. Red cluster when reactor is hot (after 30 min), black when reactor is cool (before 30 min).

Measured gas flow out of the reactor through air cowling. Accuracy of gas flow measurements may be low (repeatability has been low). Gas flow measurement is compensated for temperature and density, with an assumed gas density of 0.95 kg/m3 and temperature measured just before the flow meter. Red cluster when reactor is hot (after 30 min), black when reactor is cool (before 30 min).

 

Thermal Characteristics

Close-up of top of reduction temperature as reactor pressure is varied during the test. The five minute test length is too short to achieve stable temperatures. Top of reduction cools off at rates around 20°C/min.

 

Appendix

 

Run Description
Run Name:	Instrumented Walnut Shell Run
Run Location:	Shipyard, Berkeley, CA
Operators:	Bear, Charlie
Date:	08/03/09
Fuel
Type:	Walnut Shells
Moisture Content:	ND
Angle of Repose:	ND
Ash Content:	ND
GEK
Version:	v3
Reactor Type:	Imbert
Air Nozzle Size:	3/8" street 90°C with 3/8" cap drilled on center with X dia. hole, 2.75" above reduction top
Reduction Cone Height:	6
Top Reduction Diameter:	3
Bottom Reduction Diameter:	6
Tar Fence:	yes
Tar Fence Height:
Details:
Filter:	v3
Filter Media:	Char, run before, remixed
Fill Height:
Details:
Gas Motive Force
Ejector	yes
Jet Nozzle Type (eg barb, plug):	drilled plug
Jet Exit Position:
Fan	no
Power Source/Voltage:
Engine	no
Make:
Model:

 

GCU Setup Form
Run Name:	Instrumented Walnut Shell Run
Date:	08/03/09
GCU Version:	1.0
Firmware:	in development (PID control of reactor with servo controlled ejector air, timed grate shaking (1 min intervals, ~40° fwd/back rotation)
	Use
Thermocouples:
TC0	T_bred - bottom of reduction - 1" in from cone bottom, through manometer port
TC1	NA
TC2	T_tred - top of reduction - inside 1/4" mild steel pipe quarter round, welded closed and welded to the top edge of the reduction cone
TC3	T_air_in - air in - inside air riser, through hole drilled co-axially with the riser tubethrough street 90°. 1 1/2" below top of riser.
TC4	T_comb - combustion - first TC of profile assembly. centered by ring rod assembly, 1" in front of air nozzle hole, at same elevation.
TC5	T_1in - 1" above combustion. 2nd TC of assembly...
TC6	T_2in - 2" above combustion.
TC7	T_3in - 3" above combustion.
TC8	T_4in - 4" above combustion.
TC9	T_6in - 6" above combustion.
TC10	T_8in - 8" above combustion.
TC11	T_gas_out - installed in cowling gas exit port
TC12	T_flare - installed in 1 1/2" tangential entrance tube
TC13	T_gas_flowmeter - installed in 1 1/2" to 1/2" reducing T just after filter, before union based gas flowmeter
TC14	NA
TC15	NA
Drivers:
FET BANK 1:
VOLTAGE	12 V
FET0	Grate Fwd Relay (30A automotive) (FET # not confirmed)
FET1	Grate Rev Relay (30A automotive) (FET # not confirmed)
FET2
FET3
FET BANK 2:
VOLTAGE
FET4
FET5
FET6
FET7
Servos:
SERVO1	Ejector Air Control
SERVO2
SERVO3
Pressure:	Part (7002,5004,4006,7007,5010,7025,5050)
P0	7007 - P_comb - pressure at combustion, via profile assembly
P1	7007 - P_reactor - taken from reactor manometer port
P2	7002 - P_gas_out - gas out flowmeter differential pressure
P3	7002 - P_air_in - air in flowmeter differential pressure (flowmeter installed on V3 air cowling inlet)
I/O:
RS-232
CAN-BUS
ANALOG0	Additional 7007 pressure sensor input
ANALOG1	Experimental CO2 sensor input -  not sampling this run
ANALOG2	Auger plunger on sense (12V motor line, through voltage divider)
ANALOG3
Frequency Counter: