Co-gasification of digestate and lignite in a downdraft fixed bed gasifier: Effect of temperature

Abstract

To improve energy efficiency and biomass utilization in the process of anaerobic digestion, co-gasification is considered as an effective method to post-treat anaerobic digestion residues. In this work, the effect of temperature (650 °C, 750 °C, 850 °C and 950 °C) on the co-gasification of digestate and lignite was thoroughly investigated in a downdraft fixed bed gasifier. The results showed that the increase of gasification temperature increased the gas yield and the lower heating value (LHV) of product gas. Physicochemical properties of biochar were characterized by physical adsorption analyzer, Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy and thermogravimetric analyzer (TG). It was shown that the average pore diameter increased in the range of 650 °C to 950 °C, while specific surface area and pore volume first increased from 650 °C to 850 °C and then decreased at 950 °C. The Raman analysis of biochar indicated that small aromatic rings condensed to large aromatic ring and increased the content of Caromatic-Calkyl and the crosslinking density at higher temperature. The variation of biochar properties at the higher temperature caused a decrease in the gasification reactivity. With the increase of temperature, the content of carbolic oil in the tar increased, but the contents of light oil, naphthalene oil and washing oil decreased. This study comprehensively analyzed the products properties and demonstrated the feasibility of co-gasification of digestate and lignite.