Abstract
Waste generated in the agri-food sector is a potential source of biomass and other products of high added value. In this work, the pyrolysis of orange waste and orange pruning was carried out to produce adsorbent biochars and characterise the bio-oils aiming for high-added-value compounds. Pyrolysis was carried out in a vertical tubular furnace on the laboratory scale modifying the temperature (400–600 °C), the heating ramp (5–20 °C·min−1) to reach the previous temperature and the inert gas flow rate (30–300 mL Ar·min−1) throughout the furnace. The most suitable conditions for obtaining biochar were found to be 400 °C, 5 °C·min−1, and 150 mL Ar·min−1 for orange waste, and 400 °C, 10 °C·min−1, and 150 mL Ar·min−1 for orange pruning. Thermogravimetric analysis showed higher thermal stability for orange pruning due to its higher lignin content (20% vs. 5% wt. on a wet basis). The bio-oil composition was determined by GC-MS. Toluene and 5-hydroxymethylfurfural were the main compounds found in orange waste bio-oils, while orange pruning bio-oils were composed mainly of 4-hydroxy-4-methyl-2-pentanone. Finally, the removal of the sulphur content from waste cooking oil was assayed with the biochars from both orange waste and orange pruning, whose BET surface areas were previously determined. Despite their low specific surface areas (≤1 m2·g−1 for orange waste biochars and up to 24.3 m2·g−1 for orange pruning biochars), these biochars achieved a reduction of the initial sulphur content of the waste cooking oil between 66.4% and 78.8%.
Highlights
Among all raw materials in the food industry, fruits and vegetables produce the most waste
The Thermogravimetric Analysis (TGA) of the raw materials showed that orange tree pruning (OP) presents higher thermal stability than orange waste (OW) due to its higher lignin content (20% and 5%, respectively)
This could be responsible for the obtaining of higher percentages of biochar from OP than from OW when both raw materials were subjected to pyrolysis under the same conditions
Summary
Among all raw materials in the food industry, fruits and vegetables produce the most waste. In Europe, Spain leads the orange and orange juice production ranking, and is sixth in the global ranking, according to the Food and Agriculture Organization (2018) [1]. In Spain, this agricultural activity can be found mainly in Valencia and Andalusia, producing almost 2 × 106 t per year of sweet oranges, the fruit of the sweet orange tree (Citrus sinensis). In Spain, more than 5 × 106 t of orange pruning are produced annually, which are considered as waste and must be disposed of to avoid contamination, pest growth, and delay in agricultural practices [4]. Orange-tree pruning is mainly composed of cellulose, hemicellulose, and lignin
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