Preliminary experiments to develop pretreatments

Preliminary experiments were conducted by pretreating biomass to remove/reduced the three catalysts naturally present in biomass (inorganics, water, and organic acids). Table 4-1 indicates the target for each experiment and the change in the biomass composition following the pretreatment. In terms of the organic acid reduction, uronic acid groups breakdown to form formic acid, acetone, and methanol during pyrolysis and hydrolysis [24, 30, 31], these were not detected in the biomass carbohydrate analysis and only minimal amounts of formic acid relative to acetic acid were detected in the bio-oil; therefore pretreatments targeted primarily acetyl compounds in biomass. Reduction/removal of biomass catalysts was accomplished following procedures given in Appendix 4.1, but in brief:

 Moisture was removed through complete drying at 105 °C overnight

 Inorganics were reduced by leaching with 1% nitric acid (HNO3) for 4 h at 30 °C. The mild leaching

conditions were not strong enough to alter the biomass’s structure or to significantly reduce the acetyl content

 Acetyl groups were removed by leaching with 1% sodium hydroxide (NaOH) for 4 h at 30 °C. Acetyl and formyl side-branches are much more easily cleaved by alkali than by acid [32]. This also targets uronic acid removal [33].

Table 4-1: Combinations of biomass catalysts removed/reduced

Exp. no.

Catalyst/s targeted Sample pretreatment Inorganics (wt%) Moisture (wt%) Acetyl content1 (wt%) Biomass analysis Lignin (wt%) Cellulose (wt%) Hemi. (wt%)

1 Raw biomass Raw biomass 0.41±0.04 10.2±1.0 1.51±0.03 28.2±0.8 43.0±2.3 26.1±0.6

2 Inorganics 1% HNO3 leached 0.12±0.01 9.8±1.8 1.48±0.05 28.1±1.7 43.6±1.7 26.8±3.6

3 Water Dried 0.41±0.04 0 1.51±0.03 28.2±0.8 43.0±2.3 26.1±0.6

4 Acids 1% NaOH leached 1.35±0.10 9.9±0.3 0 27.5±0.7 45.7±2.3 26.9±2.3

5 Inorganics and water 1% HNO3 leached and dried 0.12±0.01 0 1.48±0.05 28.1±1.7 43.6±1.7 26.8±3.6

6 Inorganics and acids 1% NaOH and 1% HNO3 leached 0.08±0.02 9.2±0.9 0 27.5±2.0 44.2±2.8 26.1±3.2

7 Water and acids 1% NaOH washed and dried 1.35±0.10 0 0 27.5±0.7 45.7±2.3 26.9±2.3

8 Inorganics, water, and acids

1% NaOH leached, 1% HNO3 leached, and dried

0.08±0.02 0 0 27.5±2.0 44.2±2.8 26.1±3.2

1

99

4.4.1 Removal of one catalyst by leaching or drying

The experimental runs 2-4 in Table 4-1 reduced/removed only 1 biomass catalyst. This was to determine the influence of the individual biomass catalysts. Leaching with HNO3 reduced inorganic content from

0.41±0.04 to 0.12±0.01 wt% and reduced the acetyl content of the biomass slightly; but did not alter the biomass’s lignin or sugar composition. Leaching with NaOH removal all acetyl branches and slightly reduced the lignin content, which is characteristic for alkali pretreatments [34, 35]. The biomass structure may also have been altered by glucoside bond breakage and bond breakage between polymers. The inorganic content increased significantly to 1.35±0.10 wt% as Na was incorporated into the biomass after leaching, which could not be removed through rinsing.

Biomass samples with one catalyst reduced/removed were pyrolysed at the standard pyrolysis conditions given in Section 3.3. Yields from pyrolysis and key bio-oil properties are displayed in Table 4-2. Reducing the inorganic content in biomass gave a higher bio-oil and lower char yield. The acetic acid and water content in the bio-oil decreased. These results indicate secondary reactions were reduced. Pyrolysis of dry biomass produced a similar trend but to a lesser extent, indicating moisture in biomass enhances secondary reactions slightly. P. radiata has a naturally low inorganic content compared to other biomasses; therefore the moisture in biomass had a more significant catalytic effect than reducing the biomass’s thermal conductivity when dried (causing an increase in the particle heating time). Finally, when acetyl braches were removed through NaOH leaching, the bio-oil yield decreased while the water content increased from 24.0 to 42.6 wt%. Since Na ions became incorporated into the biomass structure following leaching, secondary reactions increased significantly and the effect of reduced organic acids could not be determined. Nevertheless, the acetic acid content in the bio-oil decreased compared to pyrolysis of raw bio-oil, confirming that acetic acid is a combination of acetyl branches and products of secondary reactions. Results were comparable to Wang et al [35] who pretreated loblolly pine with NaOH and noticed the inorganic content increased from 0.39 wt% for raw biomass to 2.49 wt% after leaching. This decreased the bio-oil yield from 54 to 49 wt% and increased that char from 19 to 24 wt%.

Table 4-2: Summary of yields and bio-oil properties from pyrolysis of pretreated biomass

Exp. no.

Catalyst/s targeted Bio-oil (wt% dry) Char (wt% dry) NCG by diff (wt% dry) pH Acetic acid in bio-oil (wt%) Water in bio-oil (wt%) Organic yield (wt%) 1 Raw biomass 46.9±0.5 15.4±0.7 37.6±1.2 2.5±0.1 3.5±0.4 24.0±1.2 35.5±0.3 2 1% HNO3 leached 56.8 10.1 33.1 3.0 1.0 14.3 48.7 3 Dried 49.6 14.9 35.5 2.3 3.3 12.0 43.7 4 1% NaOH leached 44.8 19.3 35.9 2.7 2.7 42.6 25.7

5 1% HNO3 leached and dried 54.7 14.7 30.6 2.9 2.0 12.9 47.6

6 1% NaOH and 1% HNO3 leached 50.2 7.5 42.3 2.9 0.6 16.4 42.0

7 1% NaOH washed and dried 43.5 22.6 33.9 3.0 3.1 27.4 31.6

8 1% NaOH leached, 1% HNO3

leached, and dried

100

4.4.2 Removal of two or more catalysts by leaching and drying

The experimental runs 5-7 in Table 4-1 aimed to reduced/remove two or more catalysts from the biomass. When biomass with reduced inorganics was subsequently dried, the bio-oil yield decreased and the acetic acid and pyrolytic water production increased compared to biomass that was solely leached. This indicated that either water reduces secondary reactions for low inorganic biomasses or that the catalytic effect of organic acids was more pronounced for dry biomass. The same relationship was observed for NaOH leached and dried biomass: the yields and bio-oil quality were lower when biomass was leached and subsequently dried. This was likely due to the enhanced effect of the Na ions incorporated into the biomass after NaOH leaching for dry biomass.

When NaOH leaching was followed by HNO3 leaching, the bio-oil yield was lower than for solely HNO3

leached biomass, while the water content was higher, signifying increased secondary reactions. This indicates that NaOH may negatively alter the biomass structure. It is likely that the slightly disturbed biomass structure following NaOH leaching reacted to a further extent compared to solely HNO3 leached

biomass, thermally enhancing cracking of primary vapours. The acetic acid content in the bio-oil was lower as acetyl branches were removed during the pretreatment.

When all three catalysts were reduced/removed in the experimental run 8, the organic yield was the highest measured while the char, acetic acid, and water content were significantly lower than for all other experiments. These experiments demonstrate the complexity of pyrolysis reactions and the ease at which the reaction mechanism can be altered. Therefore, even though the effect of individual catalysts could not be established without divulging into detailed pyrolytic mechanisms, it is clear all three catalysts enhance secondary reactions and reduction/removal of these catalyst from biomass would improve the bio-oil quality and quantity from pyrolysis.