Microwave drying performance of spent coffee grounds briquette coupled with mineral additives

Abstract

The microwave drying performance of spent coffee grounds (SCG) briquette in a bench-scale setup was evaluated in terms of the influences of power levels (119–700 W) and three mineral additives (10% sodium chloride, 10% sodium sulfate and 10% lignite). During the drying process, a short warm-up period, a first and a second falling rate period were detected. Compared with raw SCG briquette, the required drying times of SCG with Na₂SO₄ and NaCl additives at 385 W decreased by 38% and 18%, respectively, while the drying time increased by 9% with the addition of the fine lignite. The effective moisture diffusivities of raw SCG in the first falling rate period were 7.89 × 10⁻⁹ to 1.75 × 10⁻⁸ m² s⁻¹, while that in the second falling rate period were 1.09 × 10⁻⁹ to 1.81 × 10⁻⁸ m² s⁻¹. The average activation energy of raw SCG in the falling rate period was 24.43 W g⁻¹. The apparent activation energy in the falling rate periods for SCG/sodium carbonate and SCG/sodium chloride was less than that of the raw SCG, which indicated that sodium chloride and sodium sulfate could improve the microwave drying performance of SCG briquette. The lignite had a negative effect on the microwave drying kinetics of SCG briquette.

Keywords:

• Spent coffee grounds
• additives
• briquette
• microwave drying
• kinetics

Additional information

Funding

This work was supported by the National Natural Science Foundation of China under no. 51376017.