Torrefaction procedures

4.3.1 Torrefaction using the bench scale reactor

The bench scale reactor was used for torrefaction experiments in Chapter 5. Four biomass fuels shown in Figure 4.1 were treated in a three zone tube furnace as displayed in Figure 4.2. It has an internal diameter of 75 mm and is 750 mm long and contained a reactor tube, with an internal diameter of 60 mm and 800 mm in length. Approximately 100 g of biomass was packed inside the reactor tube and placed in position between two glass wool plugs as shown in Figure 4.3. Figure 4.4 displays the positions of three thermocouples at about 20 cm intervals inside the reactor tube with a purpose to record the inlet gas temperature, bed temperature and outlet gas temperatures. Nitrogen, with a flow rate of 1.2 mL min-1, was supplied to the reactor to ensure a continuous inert atmosphere throughout the experiment. Samples were initially dried at 150ºC for 60 min, followed by further heating at a rate of 10ºC min-1 to the final temperature. The final temperatures and residence times used are listed in Table 4.1. Here, the residence time is taken as the time at which the treatment dwells at the maximum reaction temperature, after which the samples were rapidly quenched under nitrogen flow to prevent further reaction. However, it was noted that the cooling stage exhibited a sample dependency, and cooling to below 200ºC could be of significant duration. Therefore, strictly speaking the residence times were between 10-20 min longer than intended. The final temperature in the centre of the bed was also noted to be higher than the set point (up to 20°C higher), indicating the torrefaction process can be exothermic. The resulted torrefied product was weighed and the mass yield, ŋm was calculated as percentage of the original mass sample, as follows

ŋm = (mtreated) x 100 (4.1)

mraw

where mtreated is the mass of the torrefied product and mraw is the mass of the untreated biomass (Bridgeman et al., 2010).

Figure 4.2. A longitudinal rig furnace that is equipped with three temperature zones to allow

maximum temperature control used for torrefaction.

Figure 4.3. How biomass fuels are positioned inside the reactor tube. The biomass shown in

the above picture is eucalyptus.

Figure 4.4. How thermocouples are arranged in the tube. The longest (1) thermocouple is

located near to the glass wool on the left. The second (2) goes to the other wool and the last (3) one is 20 cm away from the second as pointed out in the figure.

1

2

Table 4.1. Conditions used in this study.

Condition Temperature/ºC Residence time/min

A 270 30

B 270 60

C 290 30

In this research, the overall mass balances for the torrefaction of the investigated biomass fuels were investigated. Figure 4.5 illustrates the traps used to collect liquid products and how the last trap was connected to a gas chromatography (Perkin Elmer Autosystem XL GC) as shown in Figure 4.6, where methane, carbon dioxide and carbon monoxide were detected (unfortunately, it is important to note that these gaseous products were unsuccessfully quantified due to technical errors). The condenser that was connected to the other end of the reactor tube was also connected to a chiller (that was set to 0°C), where it was then attached to three types of traps. Traps were filled as described in Figure 4.6 to trap tars and volatiles that could cause blockage to the GC. The GC was connected to this last trap and was set to operate 5 min before the drying period ended.

Each of the collecting round bottom flasks for the first, second and third traps were weighed, together with their respective stoppers before and after torrefaction. For health and safety reasons, the next steps were all done in the fume cupboard. After weighing, liquid contents were poured into a 100 mL separating funnel. Any leftover liquids in the flasks were washed with 10 mL of dichloromethane (DCM) and poured into the funnel. Added volume of DCM was used to wash the condenser as some tars were seen stuck on the sides and later poured into the same funnel. These mixtures were shaken for a few seconds and left to stand to allow the separation of two layers. The bottom layer represents the organic phase while the top represents the aqueous fraction. A 100 mL of beaker was weighed and filled with the bottom layer of the mixture. The top layer was transferred into a weighed glass vial. Both the beaker and the vial were left in the fume cupboard for evaporation to ensure that no DCM was contained in these liquids. It took about three to four days for the evaporation to complete. Light-weighted volatiles may also be lost at this stage. Beakers and vials were weighed every day until the weights appeared constant. Analysis of these liquids will be revisited in Section 4.6.

Figure 4.5.Apparatus used for the collection of liquid products.

Figure 4.6. Perkin Elmer Autosystem XL Gas chromatograph that was used to detect the

permanent gases (CH4, CO2 and CO). The other end of the reactor

tube.

Condenser that is connected to the one that is constantly chilled throughout the experiment.

1 shows a condenser that is constantly chilled

throughout the experiment to below 0ºC and also connected to the first trap. 2 and 3 are traps that were filled with dry ice and acetone.

4 represents the last trap that was filled with cotton wool.

Another trap that was filled with granules of calcium chloride and molecular sieves beads.

This tube was connected to

the GC. 1

2 3