Characterization of hydrochar and process water formed by hydrothermal carbonization of waste wood containing urea–formaldehyde resin

Abstract

Conversion of waste wood containing urea-formaldehyde resin (WW-UF) into solid fuel with lower nitrogen content and better fuel property through the hydrothermal carbonization (HTC) process was studied. With the increase of HTC temperature from 180 to 260 °C, the mass yield and energy yield of the hydrochar decreased from 81.2 to 38.7% and from 81.6 to 62.3%, respectively, while the higher heating values (HHV) of the hydrochar increased from 19.2 to 27.1 MJ/kg. Due to the hydrolysis of urea-formaldehyde resin into urea, the denitrogenation efficiency could reach over 80% at 180 °C. However, the denitrogenation efficiency decreased at elevated HTC temperature because of the condensation/polymerization of urea with carbohydrate derivatives. The combustion and pelletization behaviors of the hydrochar were all improved compared to that of the raw WW-UF. FT-IR spectra confirmed that the UF resin and hemicellulose in the WW-UF could be hydrolyzed at 180 °C, but carbonization of cellulose occurred with HTC temperature over 240 °C. The process water was analyzed by LC-MS/MS to confirm that the mono-saccharides, carboxylic acids, carbocyclic compounds, O-heterocyclic compounds, and N-heterocyclic compounds were presented in process water. The study confirmed that hydrothermal carbonization could be one promising method to covert the WW-UF into clean and efficient solid fuel.