**1. Introduction**

Agri-food trade in the European Union (EU) is one of the most important in the world economy [1]. In 2019, the EU positioned itself as the world's largest exporter and the second largest importer of agri-food products. The value of exports increased to 14.7 billion euros in 2019 compared to 2018, while import values increased to 10.7 billion euros [2]. Both imports and exports have been growing since 2002, contributing to a monthly trade surplus in the agri-food sector of 4.0 billion euros.

According to EUROSTAT data, the demographic situation in the EU reflects an upward growth; since 2008, the population has increased by 13 million inhabitants [3]. Alongside this population growth, an increase in needs and consumption is associated, especially in the agri-food sector, given the basic need for population feeding, but also in the energy field, as discussed below.

The agri-food industry comprises activities from all economic sectors [1]. The food supply chain (FSC) begins with stages of the primary sector (agriculture and livestock), which generates by-products (i.e., manure, waffle, cornstalk) and food waste and food loss in the form of low-quality products, damaged production, or products with no commercial

**Citation:** Morales-Polo, C.; Cledera-Castro, M.d.M.; Revuelta-Aramburu, M.; Hueso-Kortekaas, K. Bioconversion Process of Barley Crop Residues into Biogas—Energetic-Environmental Potential in Spain. *Agronomy* **2021**, *11*, 640. https://doi.org/10.3390/ agronomy11040640

Academic Editor: Carlos Martín

Received: 5 February 2021 Accepted: 23 March 2021 Published: 26 March 2021

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value [4,5]. In response to the first part of FSC, and primary sector activities, the main crop in the EU is cereal (including rice), which in 2018 was 295.1 million tons, corresponding to 11.3% of global production [6]. That is why cereal production is particularly influential at European level. In Spain, one of the main producers of the agricultural sector in Europe, an average of 6 million hectares of cereals are grown These are distributed in 38.7% corresponding to barley, 28.9% soft wheat, 15.8% maize, 6.2% oat, 5.7% for durum wheat, 2.9% triticale, 1.7% rye and 0.1% sorghum [7].

Agricultural activities, as well as livestock and forestry activities, generate in their various stages variable quantities of by-products and waste whose storage, disposal or disposal represents an additional task and source of costs for the producer [8], as well as constituting an environmental problem by the increasing generation of waste associated with human consumption and population growth. These wastes are considered an underutilized source of resources as they are produced continuously and renewably [9]. These residues are, in a very small part, partially valued at different levels (production of biofuel, food and animal bed, composting or building materials) [10]; however, a significant volume of them is not reused and constitutes a serious problem that negatively affects the overall sustainability of the agricultural sector [10]. The agricultural waste known as industrial, those that must be eliminated because they are not usable in the area in which they are generated, are in the case of cereal and grain crops, straw [11]. In Spain, 35.7% of all agricultural waste produced consists of residues from cereal crops (mostly barley, followed by wheat and maize), with annual quantities exceeding 9 million tons [12].

Within the production of barley in Spain, an important part is intended for its processing into malt. Malting is a process applied to cereal grains, by which they germinate by submerging them in water, then being quickly dried by injection of hot air. During this process, malted grains develop enzymes that convert grain starch into sugar. Barley is the most commonly used cereal for malting because of this reason [13].

According to the results of the authors, in Spain 320,000 hectares of barley are allocated to the production of malt. This means that 18.850% of barley production (See Section 3.10.1) is destined for this purpose and is therefore a potential source of straw waste generation, which can be harnessed in some way.

Taking up the problem of population growth previously introduced, it is also associated with an increase in energy consumption (Figure 1). This growth rate is expected to continue over time. In fact, the International Energy Outlook [14] predicts a 28% increase in energy consumption for the period 2015–2040. Other agencies such as the International Energy Agency estimate this growth by 35% for the period 2010–2035 [15]. Of all the energy consumed in Europe in 2017, approximately 80% came from fossil sources [16] such as coal, oil, natural gas and derivatives. Finding new forms of energy to reduce this dependence on foreign and fossil fuels is strategic. Moreover, given that fossil fuels are known emitters of greenhouse gases, the reduction in their use is not only strategic, but also necessary. In fact, of the 4.66 gigatons of equivalent CO<sup>2</sup> emitted by the EU, 82.8% of emissions came from the energy sector [16]. These two reasons mark the roadmap for a more renewable energy model [17].

Both growth in cereal consumption and production, as well as growth in energy consumption, lead to several environmental problems such as increased waste generation, and the possible increase in greenhouse emissions when fossil fuels are used for energy production [18]. A joint solution for both problems can be provided by Anaerobic Digestion (AD) [19]. Anaerobic digestion allows the bioconversion of the organic matter present in organic substrates, such as a residue, into biogas (a renewable energy source) via microbiological degradation [20]. AD also generates a digestate that can be used as fertilizer. In this way, an energy source is obtained by reducing the waste generated and it also creates a circular economy in that process, solving both environmental problems [21].

**Figure 1.** Gross biogas production mean curve from the anaerobic digestion of 100 g of residue (Crop Barley Residue) CrM, and box diagram of its variation.

AD is a very complex process in which different stages occur simultaneously with different microorganisms [22], each producing the substrates necessary for the next stage to take place, depending on the substrate composition. It is then important to perfectly know the composition of the substrate to be digested, so that the process development can be determined. In addition, because of the complexity of the AD process, in which different stages occur simultaneously with different microorganisms, a medium in which all influential parameters are controlled is required to ensure a stable environment [23]. One of the novelties of this study is the use as development of the process indicator, typical variables such as pH combined with the study of the evolution of the hydrogen content generated and consumed during the process. H<sup>2</sup> is intermediate gas that marks with its appearance the beginning of the stages of hydrolysis, acidogenesis and acetogenesis, and with its reduction the development of methanogenesis since it is transformed into methane via hydrogenotrophic methanogenesis, so that monitoring its evolution can also monitor the development of the process.

For all of the above, the main objective of this research is to propose and study the feasibility and process development of treatment by anaerobic digestion of crop residues generated during harvest of barley that is intended for malt production (barley straw).

To this end, the substrate is completely characterized, the anaerobic digestion process is analyzed through all phases and processes, focusing on the inhibitions and alterations of the process and the potential for biogas and methane generation is determined. Finally, the amount of energy that can be obtained through this route is estimated, against with the reduction of equivalent CO<sup>2</sup> emissions that comes when using this renewable energy source to replace a conventional energy source.

### **2. Materials and Methods**
