Process optimization for densification of water hyacinth pellets fuel

Abstract: Water hyacinth has been identified as one of the top worst water weeds over the world. Due to its characteristics of rapid growth rate and broad environmental tolerance, it has widely spread in most waterways in 17 provinces of south areas of China since 1930's. However, water hyacinth has a strong ability to absorb nitrogen, phosphorus and other harmful heavy metal elements from water, so it has been widely used in the projects of ecological rehabilitation of water bodies in recent years over the world, which has made the problem of resource utilization of water hyacinth more important and urgent than before. Because water hyacinth is high in cellulose and hemicellulose content, it has the potential to be transformed into biomass fuel. Using mechanical force, water hyacinth can be extruded or compressed into biomass pellets, and could be an important way to utilize water hyacinth as an energy source. In the process of biomass densification, different chemical compositions of biomass can result in different compressing process parameters of biomass pellets. As an aquatic plant, the difference in the chemical composition of water hyacinth from other terrestrial plants can result in different compressing process parameters of water hyacinth pellets from other biomass pellets. Among all the compressing process parameters of biomass pellets, compressing force, temperature, moisture content and particle size of material are the 4 important process parameters that greatly influence the quality of biomass pellet fuel. In order to improve the densification quality of pellet fuel made from water hyacinth, the densification process of water hyacinth pellets was experimentally studied by using a compressing apparatus in the laboratory. Firstly, the single-factor tests were carried out, in which the variables were compressing force (1.5, 3.0, 4.5, 6.0 and 7.5 kN), temperature (80, 90, 100, 110 and 120℃), moisture content of material (8%, 10%, 12%, 14% and 16%), and particle size of material (0.43, 0.58, 0.74, 0.89 and 1.07 mm) respectively. After that, the orthogonal test was also carried out, which was four-factor and three-level (compressing force of 5, 6 and 7 kN, temperature of 80, 100 and 120℃, moisture content of 10%, 12% and 14% and particle size of 0.58, 0.74, and 0.89 mm). The regression analyses and modeling equations between the 4 process parameters and the pellet density and diametric compression strength of water hyacinth pellets were performed with SPSS 17.0 statistical analysis software, and the optimal process parameters were obtained by the optimal toolbox of MATLAB. The results showed that the compressing force was the biggest effect factor of pellet density, followed by particle size, temperature and moisture content. The temperature, compressing force and particle size of material had almost the same effect on the diametric compression strength, while the moisture content had the least effect. The optimal process parameters were compressing force of 6 kN, temperature of 100℃, moisture content of 12% and particle size of 0.58 mm. Under those optimum conditions, the pellet density and diametric compression strength of water hyacinth pellets could reach 1362.21 kg/m3 and 1.44 kN respectively. The results will provide a reference for the industrial production of high-quality water hyacinth pellets.