Screening of Synergetic Catalytic Effects of Salts Dominant in Sewage Sludge on Corn Stalk Derived Hydro and Biochar

Pyrolysis (Pyr) and hydrothermal carbonization (HTC) of corn stalk (CS) catalyzed with sewage sludge salts were investigated as a comparison of two biomass refining processes to produce char. The influence of the following salt-catalysts (SC)- NH4H2PO4, KH2PO4, Na2HPO4, and CaCl2- on the higher heating value (HHV), char yield (CY), energy yield (EY), and energy densification (ED) as well as an elemental analysis (C, H, N, S, O) of the produced chars were determined. Besides, ATR-FTIR spectra were made from the resulting biochar (BC) and hydrochar (HC). For each processing method, two temperatures were set: HTC 180/210 degrees C and Pyr 400/500 degrees C. CS had an HHV of 19 MJ/kg, and the catalyst-free HC and BC came out with 22 MJ/kg. The NH4H2PO4-catalyst demonstrated the maximal efficiency in advancing the HHV for the HTC with 27 MJ/kg (23 MJ/kg for the Pyr), as the Mixed catalysts for the Pyr with 24 MJ/kg, the Pyr-BC increased 2-5 MJ/kg, and the HTC-HC 5-7 MJ/kg. The HC ended up with higher results compared with the BC: HHV 2-3 MJ/kg, CY and EY 5-10%, and ED 0.1-0.2. On the opposite BC marked a higher level of coalification than the HC, the BC came close to that of coal while the HC ranged in the area of peat. Both processing methods were improved by most SC and the Mixed catalysts. Especially the chars of the Mixed SC reached the same carbonization level at lower processing temperature.