Effects of waste liquid on the formation and properties of wheat slag pellet fuel

Clean and sustainable energy has been drawn much attention at present, due to the shortage of fossil energy and the prominent environmental pressure in the world. Biomass can serve as one of the greenest alternatives so far, due to its renewable, environmentally friendly, and highly valued energy. Herein, most raw materials (such as wheat straw) can be normally used in the pulping process, where some residues are generated in the section of material preparation, while some raw materials are directly decomposed into the bio-mechanical pulping liquid. Therefore, the wastes of wheat straw and pulping liquid can be collected to prepare the cylindrical pellet fuel using hydraulic compression molding. Moreover, the pulping liquid can greatly contribute to the physical strength of pellet fuel, due mainly to the adhesive properties in the decomposed cellulose, hemicellulose, lignin, and extractives. In this study, a systematic investigation was made to clarify the effects of waste liquid on the formation and properties of wheat slag pellet fuel. The wheat straw wastes were first collected to screen and then blend with the various contents of waste liquid for the uniform mixture. A hydraulic compression molding was selected to prepare the samples. The wastes of wheat straw and bio-mechanical pulping liquid were first mixed and treated in the section of material preparation, and then put into the mold at room temperature, forming pressure of 10 MPa, and holding time of 3 min, further to prepare the rod-shaped granular fuel with the diameter of 13 mm, and finally to balance in a sealed bag. The waste liquid was added at the mixture ratio of wheat residue and waste liquid of 1:1, 1:5, 1:10, 1:15, and 1:20. Some parameters were also measured under various waste liquid amounts, including the density, crushing resistance, transverse compressive strength, radial compressive strength, and water resistance of the granular fuel. At the same time, the combustion performance was evaluated on the ash content, the actual combustion time, and combustion calorific value. The molding effect was considered to optimize the mixing parameters in the preparation of wheat slag pellet fuel. The waste liquid made a great contribution to the density and adhesivity of the pellet fuel, thereby fully meeting the requirements of storage and transportation of the products. The additive amount of waste liquid was positively associated with the shatter resistance and density. Moreover, the transverse and radial compressive strengths of pellet fuels were improved with the increasing addition of waste liquid, but decreased at the ratio of wheat straw waste/waste liquid lower than 1:15. Correspondingly, the water permeability resistances were outstandingly improved with the increasing addition of waste liquid, indicating the better storage performance of pellet fuels. The combustion of pellet fuels performed better, indicating a better ignition property. Specifically, the ash content increased, whereas the combustion heat decreased, with the increasing addition of waste liquid. Anyway, the pellet fuels presented the optimal physical and chemical properties at the mass ratio of wheat residue and waste liquid of 1:10. Meanwhile, the relax density was up to 1.24 g/cm3, the crushing resistance was 98.88%, the transverse compressive strength was 69.71 MPa, the radial compressive strength was 1.23 MPa, and the water seepage resistance was 10 min. The ash content was 22.91%, the combustion flame was extinguished 110 s after ignition, and the calorific value was 14 689 J/g. The findings can provide theoretical guidance and technical support to the reuse of solid waste in the pulp and study-making process, particularly for the reduction of pollution and the industrial preparation of biomass briquette. The biomass solid and organic liquid wastes can be prepared into the briquette in waste utilization and industrialization.