3.10.1. Estimation of CrM Waste Generation

To estimate the amount of CrM waste generated, the statistics of the *Survey on Areas and Crop Yields (ESYRCE)* of the Ministry of Agriculture of Spain are thus determined the number of hectares cultivated of barley in each region, and their yield [42]. Taking into account the number of hectares of barley used for malt production, and the performance of this process, as specified in the "Guide to Technical Improvements Available in Spain of the Maltese processing sector" [43] obtains the amount of waste obtained in malting, which is identified with the CrM residue. These results are shown in Table 6, separating them by region, and next to a color scale map representing the distribution of crops in the Spanish geography.

**Table 6.** Estimation of CrM waste generation based on data from cultivated barley hectares from [43].


3.10.2. Estimation of Biogas and Methane Generation in Each Region

Once estimated, CrM production is estimated at the amount of gas and methane that would be generated if this residue is digested in anaerobic digesters. For this purpose, the data obtained in the previous sections, summarized in Table 6, are used. The results of the estimation of biogas and methane for each region of the Spanish geography are shown in Table 7. Logically these results are proportional to those of the amount of CrM waste generated, and in those areas where barley production is much higher, more biogas is produced.


**Table 7.** Estimation of the biogas and methane generated with the anaerobic digestion of CrM.

3.10.3. Estimation of the Energy Balance and Energy Available for External Uses in Each Region

The calculations in this section have been developed as explained in Section 2.6. It has been assumed that biogas is generated in anaerobic digesters of 4500 m<sup>3</sup> capacity, and thus determine the heat and electricity needs of the installation. Once covered, it is calculated whether there is excess energy or net available energy. Figure 7 shows the amount of energy that can be extracted from the biogas generated in each region, the needs required by the installation, and the excess energy that is available for external uses. Table 8 collects all this data, determines the power of a plant equivalent to that generation and to give more applicability to the results is estimated how many homes could be sufficient in one year with the available energy.

**Figure 7.** Energy balance of each region when treating CrM residue by anaerobic digestion.


**Table 8.**Estimation of the energy balance of CrM's anaerobic digestion.

As can be observed, the region that is the most energy capable of obtaining is Castille-La Mancha, which with this solution could become a generating power plant of 115 MW, followed by Aragon and Castile and Leon. The regions that would take the least advantage of this solution are Cantabria, Galicia and Canary Islands, which are precisely the regions that produce the least barley. Precisely for this reason the solution is not energy effective, when generating little amount of barley, the substrate level is low, little biogas is generated and is not enough to meet the needs of the reactors. The energy profitability limit can be set in the case of Murcia, that is, the solution is interesting from an energy point of view in regions that cultivate at least around 6000 hectares of barley and collect at least 1500 tons of CrM waste. As for the amount of homes that could be supplied, this solution is especially attractive for the regions with the highest available energy. For example, Castile-La Mancha could supply more than 10,000 homes in a year

This could be a solution to lack of energy supply or difficult access to energy in certain areas. The Energy Poverty report in Spain 2018 [44] of the Association of Environ-mental Sciences places the regions of Valencian Community, Murcia, Cantabria, Andalusia and Castile-La Mancha, in this order, as the areas where there is the most inequality in terms of household warming, so this solution could be of help in these regions, with the exception of Cantabria, which, as has been seen, is one of the regions in which this solution is not profitable. It is also interesting for isolated areas, such as the Balearic Islands, as it is an autonomous generation system that takes advantage of local resources.

With this proposal, renewable energy is obtained from a waste to be disposed of, thus meeting the Sustainable Development Goals (SDG) #1 (in terms of energy poverty) #7 on affordable and non-polluting energy, #10 reducing inequalities in access to energy, #12 in sustainable waste management, and #13 of climate action by providing a renewable energy source that reduces emissions into the atmosphere, as demonstrated in the following section.

3.10.4. Estimate of the Reduction in Emissions Involved in the Use of Biogas Generated Instead of a Conventional Source of Natural Gas

Precisely, for the above reason of the impact on the SDG #13, and since this solution fits perfectly into these goals, it is decided to study the environmental benefit that comes with, in terms of reducing CO<sup>2</sup> emissions equivalent. The calculations have been developed in accordance with Section 2.7 of the methodology, and the results highlight the importance of the use of biogas rather than conventional sources such as natural gas.

The results are shown in Table 9, and it is confirmed that there is a 55.4% emission reduction if biogas methane is used to generate energy, rather than natural gas. Specifically, it is a reduction of 38,060 tons of CO<sup>2</sup> equivalent at the Spanish level, being more notable in the areas where the most biogas has been generated, with Castile-La Mancha at first position.

It is important, in turn, to take into account that straws are considered biomass according to the Commission Regulation (EU) No 601/2012 of 21 June 2012 on the monitoring and reporting of greenhouse gas emissions [ . . . ] [45], so that if they are issued in the combustion process the emissions can be considered zero, which represents a reduction in the impact of global warming in terms of the use of this fuel of 100%, quantified in approximately 68,750 tons of CO<sup>2</sup> equivalent per year less emitted into the atmosphere.


**Table 9.** Estimation of the biogas and methane generated with the anaerobic digestion of CrM.

### **4. Conclusions**

This study has determined that the composition of the CrM residue presents it as a residue formed, for the most part, by carbohydrates. It has good solubility and is relatively resistant to pH changes. Despite these positive aspects, its high content of hemicellulose and lignin may compromise the proper development of AD.

It has been determined that the process develops correctly, with a slight accumulation of VFA without impact when it is dampened by the buffer effect of the released AN. With the novelty introduced of the analysis of the evolution of hydrogen it has been possible to determine that the process occurs in two stages, digesting in the first stage the soluble COD, and in the second phase that of the COD that is particulate and not directly accessible. Methanization has been correct, transforming almost all the digested COD into methane. However, only 15.282% of all available COD is digested, due to the strong particulate characteristic of the substrate and the high content in lignin and hemicellulose, which makes it difficult for the for hydrolytic enzymes to adhere to particle membranes and solubilize the enclosed organic compounds; and is also due to the granular nature of the UASB inoculum. This explains why the methane content of the generated biogas is not very high.

Numerically, 100 g of CrM residue generates 1604.22 NmL of biogas, with a methane content of 27.485%. The disintegration constant is 0.164 days−<sup>1</sup> and only 15.282% of the substrate is digested. This low level of degradation opens the door to process improvement through techniques such as pre-treatments, which improve accessibility to the substrate, breaking the barriers created by lignocellulose.

As for its applicability at the national scale, it is a particularly interesting solution from the point of view of energy. It has been determined that this solution is beginning to

be energy-effective and therefore to produce enough energy available for external uses, in areas that have at least 6000 hectares of planted barley and collect 1500 tons of CrM waste. At best, it can be considered that this solution provides, in a given region, the equivalent of a 115 MW power plant, and could supply 10,000 households per year in that region. It is also considered an energy-efficient solution that complies with the SDGs #1, #7, #10, #12 and #13, and can guarantee access to energy in isolated areas or with supply problems. Not only is it an energy-efficient solution, but also, in the case of SDG #13, it has been estimated that its implementation would result in a 55.4% reduction in emissions if it were to replace a conventional natural gas energy source, reducing 38,060 tons of equivalent CO<sup>2</sup> released into the atmosphere at Spanish level (increased to 68,750 tons if zero emissions are considered from the burning of biogas, being a form of biomass).

**Author Contributions:** Conceptualization, C.M.-P. and M.d.M.C.-C.; methodology, C.M.-P.; validation, M.d.M.C.-C.; formal analysis, C.M.-P.; investigation, C.M.-P., M.d.M.C.-C., M.R.-A., K.H.-K.; resources, C.M.-P., M.d.M.C.-C.; data curation, C.M.-P.; Writing—Original draft preparation, C.M.-P.; Writing—Review and editing, C.M.-P., M.d.M.C.-C.; visualization, C.M.-P., M.d.M.C.-C.; supervision, C.M.-P., M.d.M.C.-C.; project administration, C.M.-P.; funding acquisition, M.d.M.C.-C. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** Materials and equipment used for experiments were provided by Department of Mechanical Engineering. ICAI School of Engineering. Comillas Pontifical University, Madrid, Spain.

**Conflicts of Interest:** The authors declare no conflict of interest.

### **Abbreviations**

