Integration of spent coffee grounds valorization for co-production of biodiesel and activated carbon:An energy and techno-economic case assessment in China

Abstract

Over 10 million tons of coffee were consumed globally every year, resulting in an enormous amount of spent coffee grounds (SCG) waste to be processed. However, in many parts of the world, the SCG is treated as general waste and usually ends up in landfill or incineration. This practice is a waste of resource and also can cause environmental pollution. SCG has a fine cellulosic fiber structure and contains a considerable amount of lipid. It can be considered as a promising feedstock for bioenergy and biomaterials production. Before building a processing plant, it is essential to evaluate the feedstock characteristics and thermal conversion routes to fully understand the process's technical feasibility and economic viability. This work developed a comprehensive process model for the integrated SCG valorization process to evaluate the energy flow, process efficiencies, and costs for co-production of biodiesel (BD) and activated carbon (AC) in Changsha, China. The results showed that the SCG valorization system can co-produce BD, AC and glycerol with product yields of 13.41%, 14.06% and 2.24% (wet feed basis), respectively. The overall process efficiency was calculated to be 30.5%. The fuel gas produced from the pyrolysis process can meet part of the process heat demand, but additional natural gas is required to provide sufficient heat for the conversion process. Significant energy consumption occurs in hexane recovery, char activation, and SCG carbonization subsystems, accounting for 39.4%, 21.0%, and 18.4% of the total energy consumption, respectively. The minimum selling prices of the BD and AC are calculated to be CNY 1.83/kg and CNY 4.42/kg, respectively, which are well below their current market prices. The base case scenario can make the plant profitable, but the investment return and payback time may not be attractive enough to the investors. Process developers should identify optimum plant locations and endeavor to improve the market values of the products in order to enhance the economic viability.