Energy-saving drying strategy of spent coffee grounds for co-firing fuel by adding biochar for carbon sequestration to approach net zero

Drying is an important but energy-intensive industrial process, while spent coffee grounds can be used as an abundant and potential biomass waste to replace part of the coal consumption for green fuel production and circular economy. In this study, an energy-saving strategy for efficiently drying spent coffee grounds (SCGs) by adding hygroscopic water chestnut shell biochar with 422% water holding capacity is developed. It is found that the contributions of the thermal conductivity and hygroscopicity of the biochar on the moisture removal of the SCG exhibit a competitive relationship. The hygroscopicity is dominant when the drying temperature is below 50 ℃, whereas the thermal conductivity reigns over the drying process once the drying temperature is equal to or above 50 ℃. To prevent mildew growth with lower drying cost at 105 ℃, the optimum trade-off outcomes of CSCG (i.e., the mixture of SCG and biochar) are the moisture content, water activity, and HHV of 21.71%, 0.60 aw, and 18.88 MJ kg⁻¹, respectively. The S/C mixing ratio of 1 at 105 ℃ and around 20% moisture content has the lowest drying cost of 2.3 × 10⁻⁵ USD g⁻¹, which reduces 44% cost compared to SCG with the same conditions. Overall, it was demonstrated that water chestnut shell biochar is a good additive to achieve the energy-saving drying process of SCG, and the dried SCG can be used as a coal co-firing fuel.