As a renewable energy fuel, the molding process of energy-saving biomass burning pellets plays a key role in combustion performance.
First, the raw material pretreatment link in the molding process is crucial. Biomass raw materials are usually diverse and uneven, such as wood chips and straw. Before molding, the raw materials need to be crushed and dried. The degree of crushing affects the density and compactness of the pellets, and the appropriate particle size can make the raw materials better fit each other during the molding process. For example, too coarse raw materials may cause loose pellets, which are easy to fall apart during combustion and affect combustion stability; while too fine raw materials may increase processing difficulty and cost. Drying is to control the moisture content of the raw materials, which should generally be controlled at 8% - 12%. Too high a moisture content will cause the pellets to stick to the mold during the molding process, and a large amount of heat will be consumed for water evaporation during combustion, reducing combustion efficiency, and may also lead to incomplete combustion and produce more harmful gas emissions.
Secondly, molding pressure and temperature are the core parameters of the molding process. Molding pressure directly determines the density and strength of the pellets. Higher pressure can make biomass pellets more compact, improve their resistance to breakage, and facilitate storage and transportation. However, excessive pressure may destroy the fiber structure of biomass, affect its air permeability during combustion, and thus hinder the combustion reaction. For example, in some experiments, it was found that when the pressure exceeds a certain limit, the flame propagation speed during combustion will slow down. The molding temperature should not be ignored. The appropriate temperature can soften the lignin in the biomass, act as a binder, and promote the molding of pellets. If the temperature is too low, the lignin cannot be fully softened, and the pellet molding is difficult and the strength is low; if the temperature is too high, the lignin may be excessively decomposed, which also affects the pellet quality and combustion performance.
Furthermore, the design of the molding mold has an indirect effect on the combustion performance. The size and shape of the pellets are determined by factors such as the aperture, length and shape of the mold. The diameter of the pellets is generally 6-12 mm. Pellets with smaller diameters have a larger specific surface area, are fully in contact with the air during combustion, and burn faster, but may increase dust emissions during combustion; particles with larger diameters burn relatively slowly, which may lead to incomplete combustion. The length of the mold affects the residence time of the pellets during the molding process, which in turn affects their molding quality and density. For example, a longer mold can keep the raw materials under high temperature and pressure for a longer time, which is conducive to pellet molding, but it may also increase the energy consumption of the equipment.
Finally, the comprehensive impact of the molding process on the combustion performance is reflected in the stability, efficiency and environmental protection of the combustion. A good molding process can ensure that the pellets release heat continuously and stably during the combustion process, improve combustion efficiency and reduce energy waste. At the same time, a suitable molding process can reduce the emission of harmful gases such as carbon monoxide and sulfur dioxide during the combustion process, improve the environmental performance of energy-saving biomass burning pellets, and make them more valuable and have more development potential in the energy field.
