4.4 Discussion
5.1.3 Initial methanol heat exchanger reformer performance test
5.1.3 Initial methanol heat exchanger reformer performance test
5.1 Initial methanol reformer system design
are started. The fuel ow introduced by these two pumps is shown in gure 5.7. The initial high water ow is needed to empty air out of the water fuel pipe in the system.
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0 0.5 1 1.5 2
Methanol and Water Flows [L/hr]
Time [s]
Methanol Water
Figure 5.7: Methanol and water ows during system experiment.
At around 800s a constant water ow of 0.8 L/hr and a methanol ow of 0.4 L/hr is started. This corresponds to a steam-to-carbon ratio of 4.5, and introduces a hot steam gas into the reformer by heating in the evaporator. After this constant fuel ow, a step change in the water ow is made, reducing it to 0.6 L/hr (S/C 3.4), and afterwards the sizes of the water and methanol ows are switched for a short while, i.e. the methanol ow is 1 L/hr and the water ow is 0.5 L/hr. This change is made in order to see a short dramatic change in the gas composition response, which is much dependent of steam-to-carbon ratio. A large increase in methanol ow and simultaneous decrease in water ow is expected to decrease the water-gas-shift activity and hereby increase the amount of CO in the reformate gas. The gas composition is measured using a mass spectrometer, and clearly states this change in fuel and water ow which can be seen in gure 5.8, where the CO shortly increases and the CO2 decreases.
The time axis in the gas composition measurement shown in gure 5.8 is a bit dierent due to the dierence in start time of the datalogging started on the mass spectrometer, and the other reformer states. While using the mass spectrometer, dier-ent calibration procedures were conducted in order to measure the contdier-ent of dierdier-ent
10000 1500 2000 2500 3000 3500 4000 4500 5000 5500 10
20 30 40 50 60 70 80 90 100
Time [s]
Molar fraction [-]
CO2 CO H2
Figure 5.8: Dry volumetric gas composition during reformer operation.
species in the reformate gas. Particularly with measurement focusing on CO in hy-drogen, it was found that the ion current calibrations were somewhat nonlinear, i.e.
calibrations using many dierent CO concentration gasses were needed in order to yield the correct gas concentrations. Due to this, an uncertainty in the CO measurement shown in gure 5.8 is expected in the concentration more than a few percent away from the 5% calibration gas used in the calculation of the volumetric fractions. After the rapid switch in ows, a steady steam-to-carbon ratio of ≈4 is kept for 1300s, also yielding a steady reformer output concentration of 65% H2 30% CO2 and 5% CO.
The evaporator itself initially heats the liquid fuel mixture to the boiling point, and afterwards superheats it before it enters the reformer. The temperatures in and out of the reformer during the test is presented in gure 5.9.
The burner input temperatureTBurner,in, is constantly at ambient temperature, and the output burner temperatureTBurner,out follows the behaviour of the surface mounted temperature measurements, initially rising during heating, to around 300 oC during start-up and afterwards slowly rising throughout the experiment. The reformer input temperature is controlled in the evaporator, which is the reason for the step changes seen in the measurements. This is due to changes in the evaporator temperature set point, to examine the response of the implemented controller. Oscillations are encountered in the
5.1 Initial methanol reformer system design
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0 100 200 300 400
Gas Temperatures [ C]
Time [s]
TReformer,in TReformer,out TBurner,in TBurner,out
Figure 5.9: Temperatures if the gasses entering and exiting the heat exchanger reformer during operation.
evaporator exit temperature, and they seem to be decreasing in amplitude at increasing set point values. This behaviour could indicate stability problems with the implemented controller, but in the experiments conducted the PI controller parameters could not be found to stabilize this behaviour. During close examination of the step response of the evaporator temperature a few seconds of dead time were found, i.e. a time delay exists from when the evaporator electrical power is turned on until the evaporator temperature start rising. In order to compensate for this dead time a Smith-predictor is implemented, using a derived model of the system and the time delay. The principle of this predictor is shown in gure 5.10.
The principle of the Smith-predictor shown in gure 5.10 is to use a linear model of the evaporator, in this case a rst order system as shown in the gure. The used PI controller output is fed into the evaporator model and the model output is used as a negative feedback which is subtracted from the dierence between the reference tem-perature and an outer feedback signal. This outer feedback signal is the model response of the PI controller output including the estimated system dead time subtracted from a measurement of the controlled state. The dierence is used as a negative feedback together with the temperature setpoint, and creates part of the error signal for the PI
5. METHANOL REFORMER BASED HIGH TEMPERATURE PEM FUEL CELL SYSTEM
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©SJA 2007
Figure 5.10: Smith-predictor used as dead time compensator.
controller. In this way, if the system model is correct, a dynamic compensation for the dead time will be made. This strategy works, and avoids the oscillations shown in gure 5.9, but a precise linear system model, and dead time estimation is needed. The means, that some areas of operation could exist, where the system shows more nonlinear behaviour, than the linear model predicts. Figure 5.11 shows the top gas temperature and bottom solid temperature of the evaporator using the Smith predictor, during a constant feed ow and using and dierent temperature set points.
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50 100 150 200 250 300 350 400 450 500
Temperatures [ C]
Time [s]
0 25 50 75 100
Duty Cycle [%]
PWM Duty Cycle TEvaporator,top TEvaporator,bottom
Figure 5.11: Evaporator temperature as a function of time using a PI controlled Smith-predictor to compensate for system dead time.
5.2 Integration of heat exchanger methanol reformer system with fuel cell