# Feasibility Analysis of Brewers' Spent Grain for Energy Use: Waste and Experimental Pellets
## 1 Introduction  
In 2014, the European Commission formulated an ambitious program that was named the Circular Economy Package, in order to establish productive systems that adapt to the environment. That is, these systems should be self-sustaining through the continuous reuse of different raw materials. Currently, the circular economy is a key factor for the modernization of the European economy, which opts for carbon emission reductions and the effective use of available resources. For this reason, the recovery of waste is of key importance. The implementation of steps and procedures that assure the correct recycling of wastes and their conversion to byproducts is necessary. The brewing industry is no stranger to this philosophy [1]. The brewing industry in Spain occupies a prominent place in its state economy. At international level, Spain is ranked fourth in Europe and 11 th in the world, with a production exceeding 38 million hectoliters in 2018 (9.36% of the total in Europe) [2]. During beer brewing, the main waste generated is brewers' spent grain (BSG), which represents 85% of the total byproducts for this industry [3]. Some authors established that 20 kg of BSG is obtained per 100 L of beer produced [4]. However, other authors stated that it is possible to generate up to 45 kg per 100 L [5]. It is an interesting waste because it can be used in different areas, whereas it is cheap and available year-round, with a valuable chemical composition from a nutritional point of view [6, 7]. Among its disadvantages, it shows high moisture values (around 70% wb); therefore, its removal or management should be prompt in order to prevent fermentation problems or spoilage of the product [8]. Most research works have been devoted to the nutritional value of BGS, especially concerning its effect on the diet of animals [9, 10, 11, 12]. In other cases, it has been studied in an effort to improve organic matter content and, thus, increase crop productivity, whileit has also been explored as a graphene alternative for the production of screen-printed electrodes [13; 14]. Nevertheless, this waste has also been assessed for energy production [15]. In fact, there are research works about biodiesel [16], biogas [17; 18; 19], ethanol [20], biohydrogen [21], or biobutanol [22] production from this waste. Although scarce, there are some research works dealing with the possibility of the use of this waste as a solid biofuel. Thus, [PERSON] et al. [23] analyzed the electricity production of a turbine which was fed by a boiler with BSG. [PERSON] et al. [24] carried out a mechanical pretreatment with a screw press. Afterward, they mixed the solid phase with wood chips and carried out different combustion tests, obtaining low \(CO\) emissions, although high \(NO_{x}\) and dust particle emissions were found. Within the brewing industry, craft beer production is still being developed, providing variety to the sector due to the increase in demand by consumers. In this sector, small brewers have increased by 31% in 2019 compared to 2018, producing around 400,000 hectoliters [25]. Craft beer production is characterized by its limited production in microbreweries. The management of the wastes generated during production implies a considerable limitation for small brewers, who cannot manage them properly as they do not have the room or conditions for this purpose. In addition, the productive process of microbreweries is characterized by considerable thermal consumptions, such as those generated during maceration and the subsequent filtration (heating water and steam generation). Thus, one of the most important options for BSG valorization consists of its energy yield in microbreweries, through a pre-conversion of this waste into a high-quality biofuel. For this purpose, it is necessary to resort to a densification process in order to obtain dense and homogeneous products, overcoming the drawbacks related to the direct use of this waste [26; 27]. In addition, the use of densified products reduces hygienic risks (lower dust generation) [28; 29].