Proximate Analysis

Proximate analysis was carried out in a laboratory simultaneous thermogravimetric analyser (TGA) and differential scanning calorimeter (DSC), SDT Q600 from TA Instruments (Melbourne, Australia). Air-dried sawdust, 6-12 mg, was heated in an Alumina, Al2O3, crucible with a diameter of 5 mm and height of 4 mm according to the

procedure in Table 3-5. Analysis runs were done with and without a lid as both methods have been used in literature (Hayward, 2011; L. Wang, Skreiberg, Grønli, Specht, & Antal, 2013).

Table 3-5. TGA procedure for proximate analysis.

Step Atmospherea Temperature in °C Heating rate

in °C/min

Hold time at final temperature in min 1 N2 RoomWHPSHUDWXUHĺ 5 30 2 N2 ĺ 5 0 3 Air 900 Isothermal • 4 N2 25

Note. The procedure is the one typically used by the New Zealand Biochar Research Centre (Calvelo Pereira et al., 2011; Hayward, 2011).

a_{applied gas flow rate is 20 ml/min.}

The equipment was calibrated with respect to three variables (a) weight (reference and sample beam have been calibrated from room temperature, RT, to 1250 °C with known weights), (b) temperature (calibration against pure metals (Sn, Pb, Zn, Al) with known melting points), and (c) heat flow (calibrated against sapphire standard) for a heating rate of 5 °C/min (M. Bretherton, personal communication, March 27, 2013). A heating rate of 5 °C/min was applied throughout this research as it represents slow pyrolysis for the manufacture of biochar.

Typically the water contained in the sample is defined as weight-loss till 107 °C (Hayward, 2011) or 110 °C (Calvelo Pereira et al., 2011; Narayan & Antal, 1996). However, looking at the derivative weight-loss curves in Figure 3-1 reveals that this temperature did not coincide with the end of the peak that is associated with water loss (1stpeak in Figure 3-1).

3.2 Materials and Methods 3-9

Figure 3-1. Average derivative weight curve of the proximate analysis experiments with and without a lid. It is important to note that each curve includes an extra pyrolysis run till 700 °C for which reason this graph is only plotted till 700 °C and not 900 °C. The “no lid average-lid average curve” illustrates the difference between the two cases lid and no lid.

The minima between the first two peaks in Figure 3-1 were determined as the temperature where drying is completed. They are 152 and 126 °C for the runs with and without a lid respectively. The moisture peak for the run with a lid is shifted to the right compared to the run without a lid. This indicates that the drying step is prolonged in the case of a lid due to reduced mass transfer and/or increased thermal lag. In contrast, the good overlap of the pyrolysis peaks indicates that the pyrolysis step is not affected by the aforementioned processes. Transfer limitations are discussed in more detail in 3.3.5.

The amount of water (moisture),ܯ, in % (wt/wt) present in the sample was then calculated according to:

ܯ =(݉௜௡െ ݉ௗ)

݉_௜௡ ή100 , (3.4)

where ݉_௜௡ is the initial feedstock weight in kg at RT, and ݉_ௗ the weight of the dried feedstock in kg at 152 and 126 °C in kg for the case lid and no lid respectively. It is important to note that ܯ in equation (3.4) is different to the moisture content in equation (2.6) in section 2.5.2, which is divided by the oven-dry sample weight.

-0.2 0 0.2 0.4 0.6 0.8 1 0 100 200 300 400 500 600 700

Deriv

. W

eigh

t in %/°C

Temperature in °C

The volatile matter, ܸܯ, in % (wt/wt) is determined accordingly as the weight- loss from the dried sample to the introduction of air at 900 °C (Table 3-5) and expressed by equation (3.5):

ܸܯ = (݉ௗ െ ݉௔௜௥)

݉_௜௡ ή100 . (3.5)

In equation (3.5)݉_௔௜௥ is the weight of the sample in kg just before the introduction of air at 900 °C.

The fixed carbon, ܨܥ, in % (wt/wt) is defined as the difference in weight between the introduction of air till no more weight-loss occurs:

ܨܥ =(݉௔௜௥െ ݉௥)

݉_௜௡ ή100 , (3.6)

where ݉_௥is the weight of the residue in kg.

The results of proximate analysis with and without a lid are shown in Table 3-6 and Table 3-7 respectively.

3.2 Materials and Methods 3-11

Table 3-6. Proximate analysis with lid of air-dried radiata pine in % (wt/wt) on an air-dry basis.

Moisture Volatile matter Fixed carbon Ash

ȝin

% (wt/wt)

9.409 72.438 17.879 0.274a

ıin pp 0.384 0.569 0.319 0.019

CV 0.041 0.008 0.018 0.063

Note. The results represent averages of 5 samples except ash which was averaged over 9 samples. CV= coefficient of variation;ʅ= average; ʍ= standard deviation.

a_{determined by Residue on Ignition with and according to Bridges (2013).}

Table 3-7. Proximate analysis without lid of air-dried radiata pine in % (wt/wt) on an air-dry basis.

Moisture Volatile matter Fixed carbon Ash

ȝin

% (wt/wt)

9.307 77.229 13.190 0.274a

ıin pp 0.206 0.355 0.492 0.019

CV 0.022 0.005 0.037 0.063

Note. The results represent averages of 5 samples except ash which was averaged over 9 samples. CV= coefficient of variation;ʅ= average; ʍ= standard deviation.

a_{determined by Residue on Ignition with and according to Bridges (2013).}

It was important to check the validity of the moisture determination method, set as 152 and 126 °C for the TGA analysis for the trials ‘with’ and ‘without’ a lid, respectively. To do this, wood samples (cut wood rods of varying length with a diameter of 20 mm) were placed in a Series 5 Contherm Digital Series Oven (Contherm Scientific, Upper Hutt, New Zealand) at 105 °C till no more weight change occurred, Table 3-8.

Table 3-8. Moisture contained in air-dried radiata pine in % (wt/wt) on an air-dry basis as determined by oven-drying.

Weight-loss when dried at

105 °Ca

Weight-loss when dried at

110 °Cb

ȝin % (wt/wt) 9.434 10.535

ıin pp 0.536 0.386

CV 0.057 0.037

Note. The results represent averages of 6 samples. CV= coefficient of variation; ʅ= average;

ʍ= standard deviation.

a_{determined till no more weight-loss occurred.} b_{weight-loss when dried at 110 °C for 114.6 h}

Table 3-8 shows that the moisture determined at the commonly used drying temperature of 105 °C agrees with the values obtained by proximate analysis in Table 3-6 and Table

3-7, confirming the applied method. The weight-loss for drying wood at 110 °C for a long period of time (114.6 h) has been included to illustrate that this results in an increased weight-loss indicating that some drying still takes place above 105 °C.

The results in Table 3-7 agree with Cetin, Moghtaderi, Gupta, and Wall (2004),

who also conducted proximate analysis on radiate pine wood. Comparing the proximate analysis results done without a lid (Table 3-7) to the results employing a lid in Table 3-6 it becomes evident that this small change has a relatively large impact on the proximate analysis results. That is, in the case of a lid, the VM decreases and the FC increases,

which agrees with the findings of L. Wang et al. (2013). This reveals that the “property”, in this case the VM and FC content, is dependent on the measurement

method, which is a harbinger of the intrinsic difficulty of separating primary and secondary reactions during pyrolysis (Morgan & Kandiyoti, 2013).

It is important to note that the proximate analysis results in Table 3-6 and Table 3-7 have been adjusted for the ash content determined by Residue on Ignition, ROI, according to Bridges (2013), which uses large samples (a1-5 g). This method was necessary as there was a large scatter in the ash content determined by the TGA, where the coefficient of variation (ܥܸ) was 2.720 and 1.974 for the proximate analysis experiments with and without a lid respectively (see Appendix B.1 for the original TGA proximate analysis data). The large TGA ash CVs occur both because the ash content is

very small (a0.3 %) and due to the compounding of errors in ܯ, ܸܯ, and ܨܥ. Dependencies of ash contents on analysis methods have also been reported in literature (L. Wang et al., 2013).