*4.5. Physical Properties Analysis of the Biofuel Briquettes from Different Particle Sizes of Pecan Pericarp Residues*

## 4.5.1. Density

The density was determined according to the UNE-EN-16127 standard [58]. Equation (1) was used: *m*

$$
\boldsymbol{\upvarphi} = \frac{\boldsymbol{m}}{\mathbf{V}} \tag{1}
$$

where ϕ is the density of the briquettes, m the mass of the sample, and v the volume of the sample.

The volume of the samples was determined by Equation (2):

$$\mathbf{V} = \pi r^2 \mathbf{h} \tag{2}$$

Measurements were made after seven days of conditioning at 20 ◦C and 65% relative humidity, so that the briquette stabilized and that the dilation effect did not affect the results [59].

#### 4.5.2. Hardness

The hardness consisted of estimating and analyzing the briquette ability to resist during the storage, transport and/or compression processes. It is based on weighing each sample, dropping it three times from a height of 1.85 m, and weighing again the piece that remained larger, the calculation of this property was made as described by Kaliyan and Morey [42] with Equation (3):

$$\text{Hardness} = 100 - \frac{\mathcal{W}\_{\text{s}} - \mathcal{W}\_{p}}{\mathcal{W}\_{\text{s}}} \times 100\tag{3}$$

where *Ws* is the weight of the briquette sample, *Wp* is the weight of the piece that remained larger.

#### 4.5.3. Swelling

The swelling index was used to see the ability of agglomeration between the particles during the briquetting process. It was determined by Equation (4):

$$\text{Swelling} = \frac{W\_{\text{s}} - W\_{p}}{W\_{\text{s}}} \times 100\tag{4}$$

where *Ws* is the weight of the briquette sample, *Wp* is the weight of the piece that remained larger.

#### 4.5.4. Impact Resistance Index (IRI)

The impact resistance test simulates the forces encountered during emptying of densified products from trucks onto ground, or from chutes into bins. The IRI was calculated as described by Richards [60] with Equation (5):

$$\text{IRI} = (100 \ast \text{N}) / n \tag{5}$$

where *N* is the number of drops, and n is the total number of pieces after *N* drops. Small pieces weighing less than 5% of the original weight of the logs were not included in the IRI calculation.

#### *4.6. Statistical Analysis*

Since the data resulting from the proximate analysis are percentage values, they were transformed with the square root function of the *p* arcsine, where *p* = the proportion of the dependent variable [61]. Subsequently, data normality tests were performed for each variable, using the Kolmogorov–Smirnov test. For all the results, a completely randomized experimental design was applied, evaluating 4 treatments with 7 repetitions: T1. N◦ 12 (1.60 mm), T2. N◦ 20 (0.841 mm), T3. N◦ 40 (0.420 mm), T4. N◦ 60 (0.250 mm), and the statistical package used for the analysis of the data obtained was Origin 2021b. An analysis of variance was performed to verify the significant differences between the variables evaluated, with a 95% confidence interval.

#### **5. Conclusions**

The proximate analyses carried out on the materials of different particle sizes obtained from pecan (*Carya illinoinensis*) pericarp allowed us to determine the moisture content, volatile matter, ash content, and fixed carbon, which turned out to be adequate to define these materials as good quality biofuels. Its physical transformation through briquetting significantly increased its bioenergetic potential, with calorific values going from 17.00 MJ/kg for the base material to 18.00 MJ/kg for briquettes, due to the high density reached, its hardness and strong impact resistance, giving a solid biofuel that is easy to transport, store, and handle. In this way, the particle size was illustrated as a determining factor in the quality of the briquettes, since the finer materials presented higher density (1.25 g/cm<sup>3</sup> for granulometry 0.42 mm from a number 40 sieve), and higher hardness (99.85 for biofuel briquettes made from number 60 sieve particle size materials) than larger materials (obtained using number 12 and 20 sieves). Likewise, a greater bioavailability of the main functional groups was registered in the finest materials, which increases the briquettes' energy characteristics. Based on the results obtained, the studied biomass can produce suitable densified biofuels.

**Author Contributions:** Conceptualization, L.R.S.C., L.D.-J., M.N.H. and A.L.R.H.; methodology, M.N.H., A.L.R.H., L.R.S.C., A.C.P., M.S.H. and L.F.I.P.; software, M.N.H., M.S.H., A.C.P. and L.F.I.P.; validation, L.R.S.C., L.D.-J., M.N.H., A.C.P. and R.F.P.; formal analysis, M.N.H., A.C.P., R.F.P., M.S.H. and L.F.I.P.; investigation, M.N.H., A.C.P. and M.S.H.; resources, A.L.R.H., L.R.S.C., A.C.P., M.S.H. and L.F.I.P.; data curation, M.N.H., A.L.R.H., A.C.P., L.F.I.P. and M.S.H.; writing—original draft preparation, M.N.H., A.C.P., M.S.H. and L.F.I.P.; writing—review and editing, all the authors; visualization, all the authors; supervision, L.R.S.C., L.D.-J. and M.N.H.; project administration, L.R.S.C. and M.N.H.; funding acquisition, L.R.S.C. and A.L.R.H. All authors have read and agreed to the published version of the manuscript.

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

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** We thank Sara Catalina Garza Rodriguez, a student in the Agricultural Engineering program at the Faculty of Agronomy UANL, for her valuable contribution in the preparation of this work, within the framework of the Scientific Summer 2021 (PROVERICYT) program. We also thank Guillermo Cristian G. Martínez Ávila for his observations for the improvement of the manuscript.

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

## **References**

