# Study of the Briquetting Process of Walnut Shells for Pyrolysis and Combustion
## 1 Introduction
Density is another parameter used for analyzing the quality of the briquettes [15]. High density implies the use of high compression pressure, so the higher the briquettes density, the better the mechanical resistance. Moreover, briquette density influences combustion behavior because denser materials have a longer burnout time [16]. For the briquetting of waste furniture and mattress foams, a correlation between the density and quality of the briquettes was observed in previous works [17; 18]. The densification process and quality of the pellets and/or briquettes obtained depend on many factors: operating pressure, material/materials, particle size distribution, biomass moisture, and operating temperature [15; 17; 18; 19; 20; 21; 22; 23]. It is widely known that increasing the operating pressure increases the durability of densified materials. The deformation of biomass under pressure can be elastic and plastic. Higher pressures are needed to undergo plastic deformation, although the temperature is also a variable that affects this deformation [24]. In the densification processes, new bonds between the individual particles are generated due to the high pressure applied and due to the presence of binders, which can be part of the raw material or can be added in order to help produce the compacted materials. In biomass densification, the presence of natural binders (such as lignin, proteins, starch, fats, and water-soluble carbohydrates) produces solid bridges between the particles. In general, the durability of the densified materials decreases as particle size increases [20], although the increase in the production cost due to the fine grinding must also be considered for the optimization of the operational process. The moisture content of the material is also an important parameter in order to obtain good-quality briquettes or pellets. In densification processes, water is essential since it acts as a binding agent. Water exerts Van der Waals forces between the particles, holding them together. However, it is crucial to find the optimum moisture content to produce good quality briquettes, because if the moisture content is higher than this optimum, their durability will be low. This could be explained by the equilibrium moisture; densified materials with moisture higher than the equilibrium moisture show worse durabilities, because the free moisture prevents the formation of durable bonds [18]. Although most of the briquetting processes occur with the raw material at room temperature, the process temperature can have a considerable influence since it helps the materials lose their elasticity. This fact can be observed in biomass materials, i.e., briquetting almond shells at 170-190 \({}^{\circ}\)C [13] and at 70-80 \({}^{\circ}\)C [12] and polyurethane foam at 160-190 \({}^{\circ}\)C [17]. The aim of this work was to propose a simple and low-cost briquetting process for walnut shells in a hydraulic piston press to obtain good-quality briquettes suitable for direct combustion or for pyrolysis to obtain biochar. The parameters used to measure the briquettes' quality were density and durability. The factors tested to obtain the best quality were operating temperature, moisture content, and the presence of small amounts of walnuts. A compaction pressure of 66 MPa was chosen for these experiments after several attempts at different pressures and verifying that this was the most suitable one.