Changes in pyrolysis characteristics of agricultural residues before and after water washing

Abstract Three agricultural residues (peanut straw, sorghum stalk and reed) were first pretreated by water washing, with which the pyrolysis experiments were carried out in a fixed bed reactor to investigate how the pyrolysis characteristics are changed by removing the alkali earth metals (AAEMs) and a part of fibrous components by water washing. The result shows that the water washing removes 52.7%–92.6% potassium and approximately half of neutral detergent solute (NDS) from three agricultural residues. The removal of AAEMs and NDS has a complex influence on the pyrolysis of agricultural residues, especially for peanut straw due to its higher contents of both AAEMs and NDS. However, the removal of AAEMs has an inhibitory effect on the reactions such as decarboxylation, decarbonylation, dehydrogenation and polycondensation, and the elution of NDS directly affects the yields and composition of gas and liquid products. For all three agricultural residues, overall, the water washing promotes the production of bio-oil and bio-char, but it lowers the gas yield. The bio-oils generated from the water-washed agricultural residues have higher proportions of oxygenates, especially sugars and furans, with lower proportions of hydrocarbons and nitrogenous compounds. Besides, after washing, the yields of CO2, CO and CH4 with peanut straw decrease significantly, while the proportion of long-chain fatty acids in bio oil increases.