Thermochemical conversion of solid digestates: Effects of temperature and fluidizing gas on products composition

Valorizing digestates from methanisation plants through their thermochemical conversion is an important topic. For this purpose, a micro-fluidized bed reactor was operated by varying the reactor temperatures (from 400 °C to 900 °C) and the fluidizing gas (nitrogen, steam or air). All experiments were conducted by keeping a constant fluidization regime (U/U mF = 2.5) and gas-phase residence time in the freeboard (1.2 s). The mass yields of solids (char and ashes), syngas, and liquids (bio-oils or tar) were quantified. The importance of dehydration and decarboxylation reactions on char composition is highlighted by the Van Krevelen diagram. The effect of steam on N/C composition of char is also discussed. Gas composition was analyzed by chromatography, FTIR (for HCN), and spectrophotometry (for NH 3 ). NH 3 is the major N-gas species for all conditions. Its formation is promoted by steam. It is not significantly converted under our conditions even at 900 °C. HCN is an important product increasing from 700 °C and notably at 900 °C. The liquids were analyzed by GC/MS and high-resolution mass spectrometry (Fourier-transform ion cyclotron resonance, FT-ICR MS). Principal component analysis of FT-ICR MS data unravels the effect of air and steam on the composition of liquid products. Air promotes the formation of CHNO species whereas steam does not significantly impact the composition of the liquids (compared to pyrolysis) on the whole range of temperatures. At 900 °C, all liquids present a similar composition (based on FT-ICRMS analysis) highlighting the predominant effect of temperature over the reactive gas at 900 °C. • Solid digestates pyrolysed in a fluidized bed reactor. • Products analyzed by various analytical methods. • Effects of temperature and fluidizing gas studied. • Mass balances provided. • Mechanisms of pyrolysis and oxidation unraveled.