Carbon negative pyrolysis system for the valorization of agricultural waste into syngas

Abstract

High cost of biofuel production remains a major barrier to replacing fossil fuels, largely because of the low energy density of feedstocks. The utilization of non-edible biomass waste is a viable strategy for cost-effective biofuel production. Here, we explored the pyrolysis of walnut husk (WH) as a model non-edible biomass, employing CO2 as a reactive gas to enhance sustainability and economic feasibility. Under CO2-rich atmospheres, single-phase reactions involving CO2 and volatiles released from WH resulted in distinct syngas (H2 and CO) production trends compared to N2 atmospheres. The introduction of a Ni/Al2O3 catalyst further amplified these effects, increasing CO production by 162.6% relative to the reference case. Notably, the CO2-assisted catalytic pyrolysis of WH has been shown to generate sufficient energy to sustain the walnut industry while sequestering CO2 at levels that could offset its emissions. Given the CO2 consumption during the process, the resulting H2 and CO can be classified as carbon-negative syngas, highlighting the potential of WH as a renewable feedstock in a circular bioeconomy.