Experimental methods

4.2.1 Raw materials

The biomasses used in this thesis work was Eucalyptus clone E. urophylla (Timor) x E. camaldulensis (VM1) and Finnish pinewood. Their details were presented in Table 4.1.

Table 4.1 Details of the biomass used in this study.

Name Supplied by Description Used

in Eucalyptus clone of E. urophylla

(Timor) x E. camaldulensis (VM1). The trade name of this clone was ‘‘urocam’’

Department of Forest Engineering, Federal University of Vicosa, Minas Gerais, Brazil Debarked stem wood. Paper I

Finnish Pinewood Kuljetusliike

Viikari Oy, Narva, Finland Debarked Stem wood chips Paper II and Paper III

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A proximate analysis of the biomass selected for this thesis is presented in Table 4.2. A detailed compositional analysis of the eucalyptus clone can be found in [88,89]

Table 4.2 Proximate analysis (wt.%) of Eucalyptus clone (‘urocam’) and Finnish pine wood.

Biomass sample Fixed

Carbon

Volatiles Ash Reference

Eucalyptus clone 12.1 87.6 0.27 Adopted from [88]

Finnish Pinewood

13.4 82.5 4 Evaluated using TGA

(Mettler Toledo TGA850 and following the

methodology presented in [90].

4.2.2 Sample preparation

Initially, the biomass samples were dried in an air driven furnace at 105 ⁰C for 24 h. The dried biomass samples were ground using a Retsch ZM 200 centrifugal mill. The grounded biomass was sieved to a particle size of 100 to 125 µm for the TGA and DSC experiments (paper I) in order to avoid the effects of internal heat transfer. A particle size of less than 100 µm was used in the adsorption experiments (paper III).

4.2.3 Torrefaction treatment

The torrefaction process was carried out with three different approaches. For the TGA studies, the torrefied biomass was produced in a Mettler Toledo TGA850. For each run, the furnace temperature was raised from 50 ⁰C to 105 ⁰C at 20 ⁰C/min and kept at that temperature for 30 min to ensure complete drying. Then the reactor temperature was raised at 50 ⁰C/min from 105 ⁰C to one of the three selected torrefaction temperatures, i.e. 250 ⁰C, 275 ⁰C and 300 ⁰C and kept at that temperature for 1 h. There was a nitrogen flow of 80 mL/min which created an inert environment and removed the released volatile gases. A sample size of about 7.5 mg in a 70 μL alumina oxide crucible was used.

For the DSC experiments, the torrefied biomass was produced in a tube furnace located in the university’s (TUT) materials science department. The drying procedure was the same as with the TGA furnace. The torrefaction procedure would have been the same, but owing to the limitations of the equipment, the temperature of the biomass was raised at 20 ⁰C/min instead of 50 ⁰C/min. In each run, a sample size of 10 mg in a ceramic crucible was used for each run.

The torrefaction reactor system presented in Fig. 4.4 was used to produce torrefied biomass and torrefaction condensate for the experiments on

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integrating torrefaction with AD (paper II and paper III). The main reactor and other components such as the volatiles, flow pipes and air-circulating coil were made of stainless steel. A detailed description of the reactor system was presented in paper II and in [91] . Briefly, 1 kg of previously oven-dried biomass was loaded into the reactor for each run. The temperature of the reactor was raised from room temperature to the torrefaction temperature at around 5 ⁰C/min and kept at that temperature for 1 h. Initially, a nitrogen flow of 20 L/min was used to maintain the inert environment. Later, during the isothermal period N2 flow was reduced to 5 L/min.

Fig. 4.4 The torrefaction reactor system used to produce torrefied biomass and torrefaction condensate (paper II)

The volatile gases were condensed using a water-circulated condenser that is connected with a glass bottle submerged in an ice-water bath. It should be noted that, because of the large sample volume and thickness of the wood chips, the sample temperature at some places in the reactor might deviate from the thermocouple reading. The torrefied biomass is stored in a closed container while the torrefaction condensate was stored at 4 ⁰C to avoid further aging reactions.

4.2.4 TGA experiments

The pyrolysis of both the original and torrefied biomass was carried out in a Mettler Toledo TGA850. Initially, the samples were dried according to the procedure explained in section 4.2.3. Later, the TGA furnace temperature was raised from 105 ⁰C to 700 ⁰C at one of the selected heating rates, i.e. 5, 8, 12, 20 ⁰C/min. Finally, the samples were kept at 700 ⁰C for 40 min to ensure that pyrolysis was complete. The nitrogen flow was 80 mL/min. A sample of about 7.5 mg in a 70 μL alumina oxide crucible was used.

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4.2.5 DSC experiments

The Mettler Toledo DSC821e was employed to carry out the DSC experiments. Samples (both dried and torrefied biomass) of 3.5 to 4 mg were loaded into 40 μl aluminum crucibles, and later sealed with a pierced lid. An empty crucible of the same size, i.e. 40 μl, was used as a reference material. A nitrogen flow of 50 cm3_{/min was used to create the inert environment. The drying zone was same as for} the TGA experiments (section 4.2.3). Because of the limitations of the equipment, the pyrolysis temperature was set at 500 ⁰C. The temperature was raised from 105 ⁰C to 500 ⁰C at a heating rate of 20 ⁰C/min.

4.2.6 AD batch assays of torrefaction condensate

Two different types of inoculums were used in this thesis work. These are pre-collected inoculums that were stored at 4 ⁰C. The inoculum used in paper II was sludge from mesophilic (35 ⁰C) and thermophilic (55 ⁰C) anaerobic digesters from our laboratory. These digesters were operated at the respective temperature with pulp-paper industry sludge as a substrate. Before using the inoculum in the AD batch assays, they were incubated at the appropriate temperature for 48 h in order to allow them to adapt them to the mesophilic and thermophilic conditions.

In paper III, the inoculum used was granular sludge collected from the mesophilic up flow anaerobic sludge blanket (USAB) reactor that treats wastewater from an integrated plant, which produces Beta-amylase and ethanol (Jokioinen, Finland).

The bio-methane potential of torrefaction condensate was studied through batch experiments using 120 mL serum bottles at mesophilic and thermophilic conditions. The operating volume was 60 mL. The torrefaction condensate produced at 225 ⁰C, 275 ⁰C and 300 ⁰C was used as a substrate (paper II). The substrate to inoculum ratios, i.e. VSsubstrate:VSinoculum of 0.1, 0.2 and 0.5 were tested. A bottle with water and inoculum only was used as a blank to find out the methane yield from the inoculum. All the experiments were carried out in duplicate and the methane yield presented as mean. In case of paper III, the AD batch assays were carried out with original and detoxified torrefaction condensate produced at 300 ⁰C.

4.2.7 Cyclic batch AD experiments

Cyclic batch experiment were carried out to better understand the type of inhibition that torrefaction condensate exerts on the inoculum. For the cyclic batch experiments, the initial setup was same as for the batch assays as explained in section 4.2.6. However, at selected intervals, the bottles with no gas production were diluted to approximately match the COD of 0.1 VSsubstrate:VSinoculum loading. For dilution, the liquid phase from the batch assays was removed and then the same volume of water and the required amount of substrate were added. The cyclic batch experiments were carried out at 0.1, 0.2 and 0.5 VSsubstrate:VSinoculum using torrefaction

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condensate produced at 300 ⁰C. Two intermittent points, i.e. after 30 days and 50 days were selected and the total operating time was 72 days.

4.2.8 Adsorption

The adsorption of furfural using torrefied biomass was studied at varied torrefied biomass dosages, i.e. 25, 50, 100 and 150 g/L. The batch adsorption experiments were carried out in a volume of 20 mL at room temperature (≈20 ⁰C) and with continuous mixing at 150 rpm. An initial furfural concentration of 6000 mg/L at its initial pH 3.6 was selected. The isotherm studies were carried out by varying the initial concentration from 300 to 6000 mg/L at 50 g/L dosage. The selected residence time was 1, 2, 4, 6, 8, and 12 h. The effect of the pH was tested by varying it between 2 to 9 with an initial concentration of 6000 mg/L and torrefied biomass dosage of 100 g/L.

Batch adsorption studies of torrefaction condensate were carried out at torrefied biomass dosages of 25, 50, 100, 200 and 250 g/L of torrefied biomass. A sample volume of 10 mL was selected. The torrefaction condensate was used at its original pH. After adsorption, the solid liquid separation was achieved by centrifugation at 5018 Xg for 5 min. Supernatants were filtered using 0.45 µm (Chromafill® - PET 45/25) prior to gas chromatography mass spectrometer (GC-MS) analysis. All the experiments were carried out in duplicate.