# Study of the Briquetting Process of Walnut Shells for Pyrolysis and Combustion
## 1 Introduction  
Nowadays, the use of waste biomass for energy purposes is essential in order to reduce dependence on fossil fuels, and it represents an increasing business sector. Biomass can be used directly in power plants as the main fuel or in co-firing with coal or other wastes. Alternatively, biomass can be subjected to a previous pyrolysis or torrefraction treatment (a kind of mild pyrolysis) to obtain gas, liquid, and solid products. The liquid product can be used as biofuel, as proposed in the literature from catalytic pyrolysis of several types of waste biomass, such as lignin from the pulp and paper industries [1] or peanut shells [2]. On the other hand, the solid product--called _biochar_--has similar characteristics to coal. Torrefraction increases the efficiency of biomass combustion and gasification processes by increasing the energy density and homogeneity of biomass. During torrefraction, biomass is roasted in an oxygen-depleted environment at temperatures between 240 and 320 \({}^{\circ}\)C, resulting in the removal of moisture and some of the volatiles and a reduction in the atomic ratios O/C and H/C [3, 4]. The studies on factors that improve the pyrolysis process from waste biomass such as the use of sustainable catalysts could be applied to pyrolysis from plastics which are difficult to recycle [5]. Recently, waste biomasses from various agricultural by-products have been used as biochar precursors, such as sawdust, date seeds, olive seeds, corncobs, rice husks, straw, coffee beans waste, hazelnut shells, palm kernel shells, or walnut shells [6]. Among them, walnut shells represent a significant amount, considering that almost one million metric tons of walnuts were produced worldwide in 2021 [7]. Shells from walnuts usually come washed from the removal process, so they produce a cleaner final combustion than other waste biomass such as waste wood, which can be mixed with soil or bugs. Therefore, this product could be used in barbecues and other types of combustion intended for food. Apart from energy purposes, walnut shell biochars have been studied to develop voltammetric sensors in order to detect traces of heavy metal ions in water and solid samples, obtaining promising results [6]. However, potential applications of biomass shells are limited, mainly due to their low bulk density. This problem can be solved by pelletization or briquetting, thus significantly reducing the handling, transportation, and storage costs [8]. Pelletization in combination with pyrolysis or torrefaction to increase energy density has received great attention [3; 9; 10]. The manufacturing of pellets implies a previous grinding step, whereas the briquetting process can be carried out without grinding the material, thus presenting the following advantages: (a) saving on the cost of milling, (b) reducing the presence of powder and small particles, which could lead to safety and environmental problems, (c) widening the range of uses, such as special firing or barbecues where the combustion is maintained during a period of time. Most studies are related to biomass pellets or briquettes with previous grinding, as in the work of [PERSON] et al. [11], although good results were obtained for the briquetting of almond shells without grinding [12; 13]. Nevertheless, the success of the process and the quality of the briquettes obtained depend on the specific biomass and selected conditions used and should be deeply studied for each case. Two parameters can be used to analyze the quality of the briquettes obtained. First, briquettes can undergo abrasion and fragmentation during transportation and storage, so they need minimal mechanical resistance to elude these problems. In addition, dust emissions must be avoided during combustion. In order to analyze the mechanical resistance of the briquettes obtained, durability (DU) can be selected as the parameter to study [14]. The objective is to obtain briquettes of high quality (high values of DU) with minimal energy requirements.