Effect of specimen size on ultimate compressive strength of Bio-coke produced from green tea grounds

Abstract

Various sizes of high-density biomass briquette, named ‘Bio-coke,’ were produced from spent green tea grounds. The mechanical properties at room temperature of the Bio-coke samples were investigated using a compression testing machine. From the results, the relationship between the specimen size of Bio-coke and the ultimate compressive strength at room temperature showed that the ultimate compressive strength depends on the specimen size of Bio-coke. The maximum value of the ultimate compressive strength among the different specimen sizes of Bio-coke was at 67 MPa, obtained from the 12-mm diameter sample. In addition, at 12 mm diameter or smaller, there is hardly any difference in the ultimate compressive strengths measured. Hence, the cold compressive strength properties are divided into two groups based on the uniformity of the structure of the main components along the horizontal cross section of a Bio-coke. Results indicate that the state of the structure, composed of cellulose, hemi-cellulose, and lignin, appears to be consistent resulting from uniform permeation conditions at the 12-mm diameter or smaller samples. Meanwhile, at diameters of 20 mm or larger, the condition of the periphery of the samples were not consistent with that of the middle region because of the temperature, stress gradient and number of void occurring inside the Bio-coke caused by specimen size effect.

Keywords

• bio-coke
• room temperature environment
• ultimate compressive strength
• size effect
• thickness of permeation zone