Effects of hydrochloric acid washing and torrefaction pretreatment on the components of bio-oil from sweet sorghum straw pyrolysis

Biomass has drawn much attention, due to renewable and green energy against environmental degradation and energy shortage in recent decades. Pyrolysis of biomass in anaerobic/anoxic conditions can be an important way to produce high value-added biofuels, such as alkanes, hydrocarbons, aromatics, and phenol derivatives. However, the bio-oil from biomass direct pyrolysis is a type of complex oxygen-containing mixture, composed of organic acids, aldehydes, ketones, alcohols, phenols, and sugars. The bio-oil is difficult to be directly used, because of high acidity, low calorific value, and thermal stability. In this study, a combination of hydrochloric acid washing and torrefaction pretreatment was proposed to improve the quality of bio-oil from biomass pyrolysis, in order to deal with the high oxygen content, strong corrosiveness, and a wide variety of compounds in bio-oil. The physical and chemical properties of sweet sorghum straw were measured before and after pretreatment. The results showed that the simple pickling was beneficial to the generation of volatiles, whereas, the torrefaction pretreatment alone reduced the ratio of H/C and O/C, while increased the energy yield up to 95.13%, particularly on the increased generation of carbon. The combination of pickling and torrefaction pretreatment also significantly increased the biochar and non-condensable gases. A pyrolysis test was carried out in a horizontal tube furnace (OTF-1200X). The high purity N2 was used as the protective gas, and the flow rate was 100 mL/min. The program was set to raise the temperature to 500 ℃, and the quartz boat containing 10 g samples was quickly pushed into the central constant temperature section of the tubular furnace. At this time, the temperature changed within ±2 ℃ and returned to 500 ℃ within 1 min. The heating stopped after 30 min. The test results showed that the acid washing, torrefaction, and combined pretreatment reduced 62, 58, and 68 compounds in the bio-oil from sweet sorghum straw pyrolysis, compared with 111, respectively, while the contents of ketones, acids, and furans were all reduced. There was a weak catalytic effect of most metal ions that were removed by pickling on the acetyl group in the pyrolysis process. A significant decrease was observed in the acetic acid content and the acidic components in the bio-oil for a better corrosion resistance of bio-oil. Furthermore, the pickling and torrefaction pretreatment improved the carbohydrate compounds, and the precious D-Allose, a kind of very rare monosaccharide in nature, particularly with the very limited synthesis amount but a high medicinal value. Allosose was expected to inhibit the proliferation of tumor cells, while protecting islet β cells of diabetic animals. In addition, the combined pickling and pretreatment also promoted the formation of nonanoic acid and valeric acid, with a high yield of furfural, where the yield was higher at about 500 ℃. Furfural was a kind of chemical product with high added value, widelyused in resin, varnish, and organic reagent in the food industry. Simple torrefaction was beneficial to increase the relative content of cellulose in biomass, rich in -OH functional group, thereby for the high content of phenols in bio-oil. Although no new substances were formed, the pH value was significantly improved in the bio-oil.