Co-production of syngas and potassium-based fertilizer by solar-driven thermochemical conversion of crop residues

We report on the thermochemical conversion of inedible crop residues using concentrated solar energy as the source of high-temperature process heat. Experiments were performed using a 5kWth solar packed-bed reactor exposed to radiative fluxes up to 1788 suns. The waste biomass feedstock consisted of unprocessed batches of cotton boll, soybean husk, and black mustard husk and straw, which were pyrolysed and steam-based gasified at nominal temperatures in the range 879–1266°C, yielding high-quality syngas with molar ratios in the range H2:CO=1.43–3.25, CO2:CO=0.28–1.40, and CH4:CO=0.03–0.28. The solar-to-fuel energy conversion efficiency, defined as the ratio of the heating value of the syngas produced to the solar radiative energy input and the heating value of the feedstock, reached 18%. The heating value of the feedstock was solar-upgraded by 7%, thus outperforming autothermal gasification that typically downgrades by at least 15%. The ash contained 23% potassium. The solar-driven thermochemical process offers a sustainable and efficient path for the conversion of agricultural wastes into valuable fuels and soil fertilizers. [Display omitted] •Agricultural waste feedstocks are pyrolysed and steam-based gasified to syngas.•Robust and scalable packed-bed reactor is driven by concentrated solar energy.•High-quality syngas is produced with a solar-upgraded heating value.•The solar-to-fuel energy conversion efficiency reaches 18%.•Residual ash is suitable for potassium-based fertilizers.