Physically activated wheat straw-derived biochar for biomass pyrolysis vapors upgrading with high resistance against coke deactivation

Abstract

Wheat straw-derived biochars (produced through slow pyrolysis at 500 °C and 0.1 MPa) were physically (with CO2) and chemically (with K2CO3) activated to assess their performance as renewable and low-cost catalysts for biomass pyrolysis vapors upgrading. Preliminary cracking experiments, which were carried out at 700 °C using a mixture of four representative model compounds, revealed a clear correlation between the volume of micropores of the catalyst and the total gas production, suggesting that physical activation up to a degree of burn-off of 40% was the most interesting activation route. Next, steam reforming experiments were conducted using the most microporous material to analyze the effect of both the bed temperature and gas hourly space velocity (GHSV) on the total gas production. The results showed a strong dependence between the bed temperature and the total gas production, with the best result obtained at the highest temperature (750 °C). On the other hand, the change in GHSV led to minor changes in the total gas yield, with a maximum achieved at 14500 h−1. Under the best operating conditions deduced in the previous stages, the addition of CO2 into the feed gas stream (partial pressure of 20 kPa) resulted in a total gas production of 98% with a H2/CO molar ratio of 2.16. This good result, which was also observed during the upgrading of the aqueous phase of a real biomass pyrolysis oil, was ascribed to the relatively high coke gasification rate, which refresh the active surface area preventing deactivation by coke deposition.