Charcoal from anaerobically digested dairy fiber for removal of hydrogen sulfide within biogas

Anaerobically digested fibrous solid (AD fiber) is an abundant material that offers potential to produce value-added products such as biochar. The objective of this paper is to better understand how thermochemical processing conditions affect the capacity of biochars derived from AD fiber to adsorb H₂S from biogas. AD fiber was pyrolyzed in an electric tube reactor at temperatures up to 600 °C and 60 min. The chars were employed for H₂S scrubbing tests from a synthetic biogas. Results showed that the chars’ capacity for H₂S removal is comparable to that of activated carbon. An additional step consisting of impregnation of the chars with Na₂CO₃ resulted in an improved capacity for H₂S removal. To study the effect of ash, the AD fiber was also subjected to an alternative thermal treatment, hot water extraction (HWE), at 200 °C for 60 min. The resulting HWE material showed no removal of H₂S from biogas, indicating that the ash and the environment employed for the thermal treatment of AD fiber play an important role in the char's performance for H₂S removal. Results also suggest that a portion of the S in the charcoal after the H₂S sorption process exists as free or adsorbed S (i.e., not chemically bonded to the charcoal).