Dependence of briquettes quality on their structure and biomass material composition

Ing. Veronika Chaloupková

Doctoral thesis

Abstract:

Improved knowledge about material properties together with process variables may enhance an efficiency of briquettes production and should bring densified products of higher quality with desired properties. The composition of the biomass and fraction size of a material to be pressed are key parameters in the process of densification that govern the formation of briquette structure, which has influence on the mechanical quality of produced solid biofuels. In this Thesis, the dependence of briquettes quality on their structure and material composition was investigated. The miscanthus, industrial hemp and pine sawdust, the representants of compositionally different biomass materials, were grinded by hammer mill into three differently sized test materials (4, 8 and 12 mm). Important input parameters - moisture content, particle size distribution and materials’ bulk density were determined. Apart from the general particle size characterization done by traditional sieve analysis, the more precise approach – photo-optical analysis, considering the irregular shape of the biomass particles, was applied. Selected materials were pressed into the form of cylindrical briquettes by the hydraulic piston briquetting machine. The structure of briquettes was studied by microscopy and special software for image analysis to identify agglomeration and binding mechanisms and especially the locality concentration of larger particles. Effects of particle size as well as lignocellulosic structure (content of cellulose, hemicellulose and lignin) on the mechanical quality of final briquettes were evaluated. Complementary chemical analyses were included as well to present the main fuel properties of selected materials. Obtained data were summarized by descriptive statistics and the data from particle size measurement of the briquette cross-sections were tested by Kruskal-Wallis test. The results showed that there are statistically significant differences in the particle size on the briquette surface. In case of miscanthus and pine sawdust, larger particles are generally concentrated on the front side of briquettes and vice versa smaller are on the rear side as well as larger particles are centred in the briquette cross-sections and smaller particles are located on the briquette bottom. These two materials obtained the highest mechanical durability for the fraction 8 mm. On the other hand, the hemp briquettes exhibited more even size distribution of particles on the surface and reached greatly superior mechanical qualities for all fraction sizes, with the best results for 4 mm fraction. Mechanical quality was ensured by its fibrous nature and creation of inter-locking bonds, rather than by the content of lignin and its ability to excrete and form solid bridges during the densification, as it occurred in pine sawdust briquettes.

Keywords

solid biofuels, agglomeration, bonding, macrostructure, particle size, image analysis, mechanical properties, durability, strength

Language used: English

Date on which the thesis was submitted / produced: 26. 8. 2019

Thesis defence

• Supervisor: doc. Bc. Ing. Tatiana Ivanova, Ph.D.

Czech University of Life Sciences Prague

Faculty of Tropical AgriSciences

Doctoral programme / field:
Agricultural Specialization / Sustainable Rural Development in the Tropics And Subtropics