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Characterization of biomass and pyrolysis products

We have tested an analytical method of characterization needed to produce and commercialise pyrolysis oil from biomass. These are new products potentially harmful and with a variable energetic content that must be carefully assessed before large-scale use.

Three biomasses have been chosen as representative of different kind of feedstock for the fast pyrolysis that has been carried out by a rotating cone reactor: herbaceous energy crops (Switchgrass), woods (Beech) and residues (Pine sawdust). As expected, the feedstock resulted quite differently in morphology and chemical composition. From the chemical point of view, Switchgrass differs from the other two in the higher content of organic extractives (8.2% vs. about 2%) and ash (7.7% vs 2.2 of Beech and 0.27 of Pine sawdust). The high content of silica suggested that the high ash content in switchgrass came from soil contamination during harvesting. The content of Ca in Beech was relatively high (6600 ppm vs. 525 of Switchgrass and 768 of Pine sawdust). Pine sawdust had the highest content of cellulose (44.9% vs. 30.9 of Switchgrass and 34.1 of Beech) to which corresponded a relatively low content of xylose (5.2 vs. 25.2 and 18.7 respectively).

Beech wood had the highest calorific value (21.1 Mj/kg, vs 19.1 of Switchgrass and 19.9 of Pine sawdust). This parameter seemed to be correlated to the lignin content (31.1% vs. 24.8 and 27.9 respectively). Although the data too little for a meaningful statistical analysis, it is worthwhile to notice that the correlation becomes even better by using the percent of lignin on the organics, to separate the null influence of ash on the heating value. Three oils obtained by fast pyrolysis have been investigated; they were respectively produced from chips of beech wood, pine sawdust and switchgrass.

The thermal behaviour of such pyrolysis oils has been investigated by TGA in air. The high content of volatile molecules has been pointed out by the weight loss at room temperature that reached 40% in the case of switchgrass oil. After an optimisation study, the heating rate of 10 degrees Celsius/minute has been selected as reference. The weight loss under heating reached 50% below 150 degrees Celsius for the three oils, as a general trend, the weight loss rate has showed to maximise around 50 and 500 degrees Celsius.

Compared to the TGA of the starting biomasses, the oils loose weight more regularly and quickly below 400 degrees Celsius but at higher temperature, the residue appears more resistant to the oxidation than the starting biomass, probably because of char formation. The heating values of the three oils are comparable if evaluated on the wet basis (HHV of 17.5, 17.3 and 16.9MJ/kg respectively for beech wood, pine sawdust, switchgrass) while higher differences are pointed out if calculated on the dry basis (24.6, 23.2, 26.7 MJ/kg respectively).

The elemental composition of the pyrolysis oils has been assessed (C, H, N, S, Cl and by difference O). The content of Mg, Ca, and K has been determined by oil combustion and AAS and resulted considerably lower than in the starting biomasses. The water solubility of the three oils has been investigated in a wide range of oil to water ratio. The water-soluble fraction increases in the order: beech wood (66%), pine sawdust (76%), switchgrass (80%). The whole oils and the water insoluble fractions have been analysed by FTIR, it has been pointed out that the water insoluble has a spectrum very similar to that of lignin.

In the water-soluble fractions it has been determined Formic acid, Acetic acid, Hydroquinone, Furfuril alcol, 5HMF, Catecol, 2Furfural, 4benzaldehyde, and 3,5DM-4Hba. The water content in the oils has been determined by Karl Fisher method, switchgrass oil contained 36.8%, while beech and pine oils respectively 28.8 and 25.5%. The stability of the oils has been investigated at different temperature (room temperature, 60 degrees Celsius and 90 degrees Celsius) by measuring the viscosity, density, water content and water solubility.

It has been concluded that viscosity is the most sensitive parameters affecting aging. The three oils showed different behaviours on aging. Pine sawdust oil seemed the most stable one as its viscosimetric aging rate is 0.45 cst/day at room temperature and not phase separation would occur after 2 years, as predicted by accelerating tests. Although Beech wood oil showed a lower viscosimetric aging rate (0.18 cst/day) after seven months undergoes a phase separation.

The worst behaviour has been pointed out in the case of the Switchgrass oil that after a few days at room temperature undergoes an evident phase separation, probably because of the high water content. To slow down the oil aging, different solvents were added to them. By considering the viscosimetric index, methanol resulted the best stabilizing solvent for pine sawdust and for beech wood oil. None of the examined solvent was able to stabilize the switchgrass oil; on the contrary, the addition of solvent accelerated the pitch formation.

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75026 Rotondella
Italy
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