6. Appendices
6.4 Appendix D – Biomass feed rate
Figure D-6.6 illustrates the technique adopted to determine the biomass feed rate in the reactor. For each biomass type, a prepared sample (milled and completely dried) was introduced in the reactor feeding point. The rotation of the screw (19 rpm) carried the sample towards the other end of the pipe where it fell into a bowl placed upon a digital balance. The weight variation indicated by the balance while biomass sample falls into the bowl was timed. According to the mass balance, the weight variation along time obtained in the bowl quantifies the biomass feed rate along the reactor during its operation.
For each biomass type, a prepared sample of 50 g was used. The screw transported the samples with a constant velocity of 19 rpm while the reactor was kept “cold” (heating resistance turned OFF) and there was no nitrogen flow. The digital balance was a Sartorius balance (model CP6201, ± 0.05 g). The weight variation was recorded with a simple stopwatch: 5 g marked the beginning of the timing and 45 g marked the end (50 g is a rare or even impossible occurrence due to mass losses along the screw). The procedure was repeated three times for each biomass type in order to obtain a more trustworthy result. Figure D-6.7 shows the weight in the bowl (mean value) as a function of time obtained for the various biomasses with a velocity of 19 rpm.
In order to obtain a weight variation value in time, which actually represents the biomass feed rate, a linear regression was estimated for each scatter. Each biomass feed rate is the own slope of each respective linear regression. Table D-6.1 summarizes the linear regressions and the estimated feed rates of the biomasses with a velocity of 19 rpm. The estimated feed rates are consistent with the different density of the four biomasses.
It has been estimated an error of ± 0.3 for all the biomass feed rates based upon the uncertainty of the balance and the timing of reaction time (stopwatch).
Figure D-6.6 – Technique adopted to determine biomass feed rate.
Feedstock Liner regression (t-time) Biomass feed rate (g/s)
Biomass feed rate (g/min) Pinewood 0.127 t + 4.47 ; R2 = 0.998 0.127 7.6 Olive bagasse 0.181 t + 5.57 ; R2 = 0.996 0.181 11 Wheat straw 0.122 t + 6.01 ; R2 = 0.996 0.122 7.3 Rice husk 0.083 t + 5.90 ; R2 = 0.997 0.083 4.9 Table D-6.1 – Feed rates of the biomasses with a screw velocity of 19 rpm.
0
5 10 15 20 25 30 35 40 45 50
0 60 120 180 240 300 360 420 480 540
Weight (g)
Time (s)
Pinewood Olive bagasse Straw
Rice husk
Figure D-6.7 – Weight in the bowl as a function of time for the various biomasses with a screw velocity of 19 rpm.
6.5 Appendix E – Estimation of the hot vapours residence time
By the mass conservation law, the mass flow rate of nitrogen (mNTP) at the flow meter at NTP conditions (ρNTP = 1.25 x 10-3 g cm3) is equal to the mass flow rate of nitrogen in the reaction zone (the reactor is sealed). Estimating an average density (ρR) of the nitrogen flow based on its average temperature (TR) in the reaction zone allow the estimation of a volumetric flow rate on the reaction zone (VR). With that volumetric flow rate one can estimate the residence time of nitrogen in the reaction based on the useful volume of the reaction zone (VU). Assumptions: 1) steady-state condition, 2) constant properties, 3) ideal gas behaviour, 4) negligible pressure variations (atmospheric pressure), 5) nitrogen carries perfectly the hot vapours (same spatial velocity) 6) the reaction occurs totally in the reaction zone and 7) despising the presence of biomass. The T-connection placed at right of the reactor (orientation of Fig. 2.2) was disassembled and the screw was kept on its position inside the reactor. A reactor temperature of 550 ºC was set as calibration temperature since it is the average temperature of the covered spectrum of temperatures (480 - 620 ºC, in the trial tests), and the system heated up. Varying the flow rate (VNTP) with the gas flow meter upstream of the nitrogen circuit, a type- K thermocouple KMQSS-IM025U-300 (Omega) was carefully introduced within the reaction zone (between the wall and the screw) and the temperature of nitrogen was recorded for each flow rate, at steady state. The density of nitrogen in the reaction zone was determined based on that temperature (using a common table of thermophysical properties for nitrogen at atmospheric pressure). The volumetric flow rate of nitrogen in the reaction zone was calculated with the following mass balance equation:
VR= mNTP
ρR (E-6.1) The useful volume is the volume between the inner diameter of the main pipe (2 cm) and the screw volume along the length of the reaction zone (15 cm), which was found to be VU = 25.1 cm3.
The estimated residence time of the hot vapours was then obtained with the equation:
Hot vapours residence time =
VVU
R
(E-6.2) Table E-6.2 summarizes the estimated volumetric flow rates on the reaction zone to the various flow rates of nitrogen controlled upstream. Table E-6.3 summarizes the consequent estimated residence time of the hot vapours in the reaction zone.
In order to impose a residence time on the order of fast pyrolysis (~ 2 s), a minimum nitrogen flow rate of 526 mL/min is needed.
VNTP (mL/min) mNTP ∙!"! (g/s) TR (ºC) ρR ∙!"! (g/cm3) VR (cm3/s)
267 44
5.57 329 1.03 5.40
526 10.9
15.7
337 1.03
10
10.6
755 15.7 339 1.02 15.4
938 19.5 343 1.00 19.5
1200 25 349 0.96 25.8
VNTP (mL/min) VR (cm3/s) H. v. residence time
267 44
5.40 4.65
526 10.6 2.37
755 15.4 1.63
938 19.5 1.29
1200 25.8 0.97
Table E-6.2 – Estimated volumetric flow rates on the reaction zone to the various flow rates of nitrogen.
Table E-6.3 – Estimated residence time of the hot vapours in the reaction zone.
6.6 Appendix F – Trial tests on the reactor
Trial tests were carried out in the screw reactor with the different biomasses in order to attain a reasonable operation condition that yields significant bio-oil conversions and ensures a fast pyrolysis process. The tests were performed with biomass samples of 50 g properly prepared as explained in section 2.1. The experiments were carried out in two series and followed the experimental procedure stated in section 2.3. For both series the condensation temperature was held at - 5 ºC in order to ensure an efficient condensation, and the velocity of the screw was kept at 19 rpm (feed rates refereed in section 2.3.2). The first series was carried out to determine the effect of the reactor temperature on the pyrolysis yields. The nitrogen flow rate was held constant at 526 mL/min (2.4 s), once it is the largest flow rate value that does not carry any ashes and char into the condensers, and by the estimation made (Appendix F) is on the order of fast pyrolysis residence time (~ 2 s). Four reactor temperatures were examined: 480, 530, 580 and 630 ºC. Figure F-6.8 shows the product yields from the pyrolysis of the pinewood, olive bagasse, wheat straw and rice husk in relation to the reactor temperature (N2 flow rate of 526 mL/min, 19 rpm). The second group of experiments was performed in order to establish the effect of carrier gas (nitrogen) flow rate on the pyrolysis yields. The reactor temperature was kept at 580 ºC, based on the results of the first group of experiments. Four nitrogen flow rates were experienced: 1200, 755, 526 and 276 mL/min.
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Figure F-6.8 – Product yields from the pyrolysis of the pinewood, olive bagasse, wheat straw and rice husk in relation to temperature (N2 flow rate of 526 mL/min, 19 rpm).
For both group of experiments the yields of the by-products were obtained as described in section 2.4.
Figure F-6.9 shows the product yields from the pyrolysis of the pinewood, olive bagasse, wheat straw and rice husk in relation to the nitrogen flow rate (reactor temperature of 580 ºC, 19 rpm).
For the wheat straw special case, for a nitrogen flow rate of 1200 mL/min and 756 mL/min the ashes and some char penetrated into the first condenser and its cleaning was difficult. The bio-oil weighting with these flow rates had to account this portion of char/ashes.
Temperature was expected to have the largest effect on the pyrolysis yield and chemical composition, and for this reason a range from 480 ºC to 630 ºC was used to cover the typical temperature range of pyrolysis. As the temperature was increased, the yields on bio-oil and char were reduced as a result of the increased gasification regime. The condensable vapours are further cracked into low molecular weight organic compounds and gaseous products. The increased amount of char at lower temperatures could result from either incompletely or unpyrolysed biomass. The yields obtained were consistent with previous works.
The effect of residence time is clear evident for pinewood and rice husk. The char maintained a constant average yield, while bio-oil reached a maximum at 526 mL/min. The gas yield increased at lower nitrogen flow rates as a result of the higher residence time of vapours on the reaction time
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Figure F-6.9 – Product yields from the pyrolysis of the pinewood, olive bagasse, wheat straw and rice husk in relation to N2 flow rate (reactor temperature of 580 ºC, 19 rpm).