Use of copper metal balls as internal heat distributor in fixed-bed pyrolyzer of spent coffee grounds for biochar production

Abstract

Importance of the work: Small copper balls were used as internal heat distributor in a cylindrical pyrolysis bed of spent coffee grounds (SCG) for biochar production.
Objectives: To investigate the temperatures of the SCG during pyrolysis, including the biochar properties of the SCG beds with and without copper balls.
Materials & Methods: Samples of 300 g of SCG were pyrolyzed at 500°C under a nitrogen atmosphere for 2 hr. Temperature changes and biochar properties around the axes of the beds with and without metal balls and their last positions of decomposition were investigated. Two placements of metal balls (MBs) were considered: as layers in the SCG bed and mixing of the MBs with the same weight throughout the bed. 
Results: With two layers of 8 mm MBs in the SCG bed, the temperature rate around the bed center during decomposition at 200-450°C was 21.32% greater than without the MBs. This improvement was better than that of 4 mm MBs, which produced a temperature rate increase of 9.64% over the normal bed. Mixing the MBs throughout the bed produced the opposite result, due to the high total heat capacity of the MBs and the SCG bed. The biochar properties from the first case were investigated and compared with those from the normal bed. With the MBs, the product properties were similar to those without MBs, except for the porosity and cation exchange capacity (CEC), which were significantly higher. The biochar was suitable for soil amendment, especially in an acidic soil.
Main finding: Placement of 8 mm copper ball layers could be used as an internal heat distributor in the SCG bed. The temperature around the bed axis was closer to that around the heating surface, resulting in a more rapid decomposition rate which produced higher values for porosity, pH and CEC, which are the main parameters for soil amendment.