*3.1. Humidity*

The moisture content was determined by drying the fuel at a temperature of 105 ± 2 ◦C in a MEMMERT UFE 700 oven and carrying out three to five complete hourly air swaps. After a period of permanence that should not exceed 24 h to avoid releasing volatiles, the sample was weighed again. The difference in mass corresponded to the sample's moisture content.

The percentage of moisture in biofuel is a major factor when determining its energy efficiency. This is because for the heat to be available, the water must first be evaporated, where the greater the moisture percentage, the lesser the calorific value of the fuel. Furthermore, a high moisture content leads to the appearance of slag that tends to accumulate in the flue pipes, leading to corrosion and clogging problems. Most combustion reactions are complete with biomass moisture contents below 30%, with a maximum heat transfer at 10% moisture [35].

#### *3.2. Elemental Composition*

Except for the fraction corresponding to moisture and ash, the biomass is mainly made up of carbon (C), hydrogen (H), nitrogen (N), oxygen (O), sulfur (S), and chlorine (Cl) compounds. The elemental composition provides the relative amounts of these chemical elements.

For the calculation of the carbon, hydrogen, and nitrogen content, a given mass of the sample is burnt in an oxygen-containing atmosphere, resulting in ash and gaseous products of combustion (carbon dioxide, water vapor, elemental nitrogen, sulfur oxides, and hydrogen halides) [36]. Using instrumental techniques and the LECO Tru Spec CHN 620-100-400 analyzer, the mass fraction of carbon, hydrogen, and nitrogen in the gas stream was quantitatively determined.

The ultimate analysis of the biomass is required to evaluate the performance of biofuel and also allows for the estimation of its calorific value through the use of analytical expressions. It is also indicative of its environment-related burden since nitrogen turns to N2 and NOx, the last of which is one of the main greenhouse gases. The carbon content also helps to evaluate CO2 emissions.

#### *3.3. Ash Content*

Ash is the inorganic and non-combustible fraction of biomass formed due to the minerals it contains, some of which are sodium, potassium, chlorine, and phosphorus. The ash is considered an indicator of the e fficiency of the biofuel because the more residue it leaves when burned, the greater the problems of clogging and corrosion in the installation [37].

For its determination, a mu ffle oven (NABERTHERM LVT 15/11) was used, quantifying the residual mass after the sample was burned under controlled conditions of weight, time, and temperature.

#### *3.4. Chlorine and Sulfur Content*

The chlorine and sulfur contents are other indicators of the quality of a biofuel. After combustion, chlorine is transformed into chlorides, which cause corrosion problems in the installation. On the other hand, sulfur is transformed into sulfur oxide, which is one of the main precursors of SOx emissions into the atmosphere [38].

The standard EN 15289 was followed for its determination. A calorimetric pump (Tirator Mettler Toledo G20) was utilized to determine the chlorine content in the combustion washing water using silver nitrate. For the sulfur, a tubular furnace (LECO TruSpec S 630-100-700 Analyzer) was used for the combustion of the sample at high temperature (1350 ◦C) and further gas quantification.

#### *3.5. Higher Heating Value (HHV) and Lower Heating Value (LHV)*

By breaking the bonds of organic compounds, by direct combustion of biomass, or by combustion of products obtained from it through physical or chemical transformations, carbon dioxide and water, among other substances, are obtained and energy is released in the process. The quality of a biofuel depends on the amount of heat it can release in the energy conversion process; this amount of heat relative to the unit of mass is the power or calorific value.

Depending on how the calorific value of the biomass is measured, the higher heating value (HHV) and lower heating value (LHV) can be defined. The HHV is the total amount of heat released in the combustion of 1 kg of biomass considering the energy needed to evaporate the water since it is assumed that the water vapor will be completely condensed, while the LHV is the heat released during the combustion of 1 kg of biofuel without considering the energy consumed to evaporate the water since it is considered that water vapor does not condense and therefore this energy is not transformed into useful heat power and is lost.

Whereas the HHV is determined by experimentation using a calorimetric pump according to the UNE-EN 14918 standard, the LHV can be calculated from it by using the following expression [39–41]:

$$\text{LHV}\left(\frac{\text{kJ}}{\text{kg}}\right) = \text{HHV}\left(\frac{\text{kJ}}{\text{kg}}\right) - 212.2 \times \text{H}\% - 0.8 \times (\text{O}\% + \text{N}\%). \tag{1}$$

#### *3.6. Volatile Matter (VM)*

The volatile matter content is determined by the loss of mass, less that due to moisture, when the biofuel undergoes a process of heating due to high temperature without being in contact with air. It can come from the organic or inorganic fraction of the biomass. Volatile matter conditions the design of the installation. The higher the volatile content of the biomass, the better the ignition at low temperatures will be since the reactivity is improved and the combustion process is enriched.

#### *3.7. Fixed Carbon (FC)*

Fixed carbon is the non-volatile part found in the crucible when the volatile materials have been determined. The quantity of fixed carbon is not obtained from the weight of the waste but is obtained using the equation:

$$FC = 100 - (\%Ash + \,\%VM).$$

The VM/FC ratio is known as the combustibility index. If the VM/FC ratio increases, the degree of reactivity of the biomass also increases; therefore, the greater this quotient, the easier the ignition and the lower time needed for complete combustion.

#### *3.8. RETScreen Analysis*

The analysis was carried out using the RETScreen software, which has been used to analyze and assess several bioenergy projects. RETScreen is software developed by the Natural Resource of Canada, which helped with the analysis of zapote seed as a source of energy [42]. The biomass heating project analysis that RETScreen uses is presented in Figure 8.

**Figure 8.** Bioenergy process in RETScreen.

This process starts by specifying the biomass selected, which in this case was zapote seed. The model calculates the building's (or buildings') heating requirements. The next phase considers the heating load, where in this stage, all information about zapote seed is given. Finally, the energy delivered and fuel consumption is calculated. This final stage uses the load and demand information of the biomass heating system and/or the peak load heating system.

#### **4. Results and Discussion**

In this section, the results of the zapote seed's physical, chemical, and energy parameter analyses are presented and compared with the quality parameters of other standardized solid biofuels to evaluate the use of this agro-industrial waste as a solid biofuel in thermal energy generation systems. An energy, environmental, and economic balance of the thermal system is also presented.

#### *4.1. Zapote Seed Values*

In this section, we present the quality indicators of the zapote seed, for which 2000 g of a sample of this waste from surrounding industries were collected and analyzed in the laboratory. Table 3 shows the mean value, the standard deviation, the maximum value, and the minimum value, which determined the parametric distribution.


**Table 3.** Chemical and energy indicators derived from zapote seed profiling (parameters calculated on a dry basis, except for moisture).

> HHV: Higher heating value, LHV: Lower heating value.

As shown in Table 3, the main problem with zapote seed for use as a biofuel is its high moisture content of over 50%. Due to this, instability in the thermochemical process is produced that diminishes the e fficiency of the combustion since it is necessary to evaporate the water first such that the heat is available; as a result, the net calorific value of the biomass is reduced and causes problems of jamming and corrosion in the thermal installation due to the emission of tars. Furthermore, environmental pollution increases as a result of ine fficient conversion processes in which carbon monoxide and volatile organic compounds are generated.

The endothermic limit of biomass is 65% humidity; above this value, not enough energy is released to satisfy the evaporation requirement and produce heat. Above a 50% moisture content, some combustion systems will require a support fuel, such as natural gas or LPG. Therefore, in most heat transfer processes, humidities below 30% are required, with an optimum performance occurring at 10% humidity.

In our case study, a drying system needed to be implemented to reduce the humidity of the seed to around 10% before it was used for direct combustion in a boiler. The drying of the biomass can be done naturally in piles or forced with industrial drying systems.

On the other hand, the use of biomass via thermochemical conversion generates a solid residue or ash that comes from the mineral matter of the original plants, as well as from that which may have been added during the collection process. As it is an inert fraction, the energy available in any fuel is reduced in proportion to its ash content.

Therefore, ash is the inorganic fraction of biomass that is mainly composed of metallic oxides, such as calcium oxide (CaO), sodium oxide (Na2O), potassium oxide (K2O), phosphorus oxide (P2O5), aluminum oxide (Al2O3), iron oxide (Fe2O3), silicon oxide (SiO2), and magnesium oxide (MgO), with the oxides being either in the form of salts or organically associated compounds.

Calcium oxide (CaO) and silica (SiO2) a ffect the abrasion of the plant components due to the dragging of the ashes toward the gas outlet. Alkaline metals (K and Na) react with phosphorus and silica, which can cause slag deposits that prevent the correct transfer of heat, even generating problems in the correct operation of the installation. Furthermore, alkaline and alkaline-earth metals influence the melting temperature of the ashes, and consequently, the greater or lesser adhesion of the deposits that are formed. Potassium and sodium also react with chlorine in the gas phase (HCl) to form corresponding chlorides that can be deposited on the pipe surfaces and cause corrosion.

In the case of the zapote seed, its ash content varies between 2.49% and 2.35%, which indicates that despite being a moderate value, it is below other standardized solid fuels, such as almond shell pellets (3.35%) or oak wood pellets (3.32%) [43].

The deviation in the results depends on multiple parameters. In addition to the number of repetitions and samples analyzed, it also depends on the part of the plant analyzed. For example, waste fractions, as in the case study, usually have a higher ash content (due to impurities that may be introduced during collection and transport). On the other hand, the reproducibility of the results depends significantly on the type of plant, the growth conditions, the use of fertilizers, and the type of soil, which is related to the level of soluble inorganic elements (e.g., sandy soils give rise to less ash compared to clay soils). It can be seen that C3 plants, such as zapote, tend to have a higher water intake compared to C4 plants, and therefore have a higher ash content.

Table 4 contrasts the physicochemical indicators of the quality of various commercial biofuels and industrial wastes versus those of zapote seed to assess the applicability of this by-product in boilers.


**Table 4.** Comparison between zapote seed and other biofuels.

FC: Fixed carbon, VM: Volatile matter.

As reflected in the comparison, zapote seed had a higher ash content than other standardized fuels, which would sugges<sup>t</sup> that the problems associated with corrosion and maintenance costs of the installation could be greater.

When analyzing the calorific value of the zapote seed, we found that it was in the range of 18.320–18.364 MJ/kg, which was higher than the energy value of other standardized fuels, such as olive stone (17.884 MJ/kg) or almond shell (18.200 MJ/kg), which shows the energy potential of this residual biomass. The last row of the table shows the variation of HHV as a percentage, varying from −2.5% to +9.2%.

Other parameters that indicate the quality of biofuel are its chlorine and sulfur contents. Sulfur has a corrosive e ffect on the installation and causes the emission of SOx gases that contribute to global warming and cause acid rain. The sulfur content of the zapote seed (0.05%) was very low, similar to the sulfur content of almond shell and lower than that of olive stone (0.110%), which shows that corrosion problems and SOx emissions would be lessened by the use of this biofuel.

The chlorine content has a decisive influence on the corrosion of the plant since the reaction with potassium and sodium produces chlorides that have a dissociative catalytic action in the system. The chlorine content of the zapote seed (0.06%) was very low, similar to that of other agro-industrial wastes, such as olive stone (0.06%) or mango stone (0.07%), but lower than that of almond shells (0.2%), which shows that corrosion problems in thermal installations could be lessened by using this biofuel.

Another important quality parameter of biofuel is its volatile matter content. As can be seen, zapote seed had a higher volatile matter content than other solid biofuels. This high volatility and low fixed carbon content sugges<sup>t</sup> that it produces less corrosion and boiler fouling. On the other hand, the combustibility index or VM/FC ratio was significantly higher than that of other solid fuels. This fact was due to greater reactivity of the fuel and facilitated ignition, even at low temperatures, reducing the residence state until the complete combustion.

It is necessary to take into account that not only is the zapote seed interesting because of its energy potential but also for its carbon neutrality. This is because it is assumed that the plant is capable of retaining more carbon during its life cycle than it releases during combustion, making the carbon cycle neutral.

Zapote waste can be collected from industries processing this fruit in the surrounding area in such a way that it both contributes to the sustainable environmental treatment of this waste and a significant fall in CO2 emissions.

#### *4.2. Energy and Environmental Assessment*

Biomass is considered to be theoretically CO2 neutral because its combustion does not contribute to the increase of the greenhouse effect. This is because the carbon that is released during the combustion of biomass is the same that was continuously absorbed and released by the plants for their growth (Figure 9). The CO2 emissions generated from biomass energy do not alter the balance of atmospheric carbon concentration since it comes from carbon removed from the atmosphere in the same biological cycle, and therefore do not increase the greenhouse effect. Its use contributes to reducing CO2 emissions into the atmosphere whenever it replaces fossil fuel.

**Figure 9.** CO2-saving scheme using zapote seed as biofuel.

Once the energy parameters of the zapote seed had been calculated, the next phase of this study was to calculate the savings in CO2 emissions in the heavy chemistry laboratory of the Faculty of Engineering of the University of Veracruz, derived from the use of this seed as biofuel. Furthermore, the savings in CO2 emissions that would be produced in the states of Mexico that produce zapote were calculated.

The first step was to calculate the energy (MWh) that could be obtained from the zapote seeds, considering the residues that would be obtained from all the zapote production in the main producing states of Mexico. This was done using Equation (2) and the results are given in Figure 10:

$$\rm E\_p = RH \times P\_{\rm zapote} \times HHV \times f\_s \times F\_{c\prime} \tag{2}$$

where:

Ep: energy obtained from the zapote seed (MWh);

RH: considering 10% humidity in zapote seed;

Pzapote: zapote production (kg);

HHV: higher heating value (18.342 MJ/kg);

fs: % of seed in a whole zapote (15%);

Fc: unit conversion factor (0.000277778 Wh/J).

**Figure 10.** Bioenergy potential in the main producer states of Mexico using zapote seed as biofuel (MWh).

Once the energy that would be obtained by using zapote seed as a biofuel was calculated, the savings in CO2 emissions that this would represent compared to the use of LPG were calculated. As discussed in a previous section, biomass is a clean, environmentally friendly source of energy since it produces zero CO2 emissions. The mass (kg/year) of CO2 emitted by the combustion of liquefied petroleum gas (LPG) was calculated using:

$$\text{MCO}\_{2\text{ LPG}} = \text{C}\_{\text{LPG}} \times \text{F}\_{\text{LPG}} \tag{3}$$

where:

MCO2 LPG : kg/year of CO2 emitted by the combustion of liquefied petroleum gas (LPG). CLPG: consumption of liquefied petroleum gas (LPG) per year (kWh/year). FLPG : CO2 emission factor of liquefied petroleum gas (LPG) (kg/kWh).

Table 5 shows the CO2 emission factor for biomass and LPG and the total CO2 emissions reduced annually using zapote seed as biofuel.



The use of a biomass boiler in the heavy chemistry laboratory of the Faculty of Engineering of the University of Veracruz in Mexico would save 60,960.00 kg/year of CO2 emissions.

Figure 11 shows the CO2 savings in the main producer states of Mexico using zapote seed as biofuel (t).

**Figure 11.** CO2 Savings in the main producer states of Mexico using zapote seed as biofuel (t).

#### *4.3. Economical Profit*

The economic viability assessment of the project to replace the former gas boiler with a biomass boiler fed by zapote seed was made by taking into consideration the following data:


The thermal demand of the installation was calculated by taking into account the fact that the 200 kW LPG boiler works four hours a day for an average of 300 days a year. Under this scenario, the annual consumption of the installation is 240,000 kW/h.

The market for biomass boilers for heat generation at both residential and industrial levels is booming due to the rising cost of fossil fuels, which are also severely taxed in most countries. To determine the economic savings that would be achieved with the new biomass heating plant compared to the old LPG boiler, it was first necessary to calculate the amount of fuel required in both cases to meet the annual thermal demand. From the values of the calorific value of LPG and its density, it was calculated that 37,696.73 L of LPG are needed annually to meet the energy demand. Considering the current market price of LPG (\$0.39/L) [48], a total annual cost of \$14,701.72 was calculated for the LPG installation. Table 6 shows the economic analysis of the LPG installation and the zapote seed boiler.


**Table 6.** Economic analysis of the LPG installation and the zapote seed boiler.

Once the calorific value of the biomass (LHV) had been determined and knowing the annual thermal demand of the installation (kWh), the amount of biomass necessary to satisfy it could be obtained from the following expression:

$$\text{Biomass consumption} = \frac{\text{Annual thermal demand}}{\text{LHV} \times \text{Boiler efficiency}}.\tag{4}$$

From this expression, it was calculated that 63,424.95 kg of zapote seed will be consumed, and considering that the price per kilo of this residual biomass, once treated and transported to the point of consumption, is \$0.108/kg, an annual saving of \$7,819.79 will be made, which represents a 53.19% saving compared to the old LPG installation.

Some financial parameters were calculated, such as the internal rate of return (IRR), payback, and benefit/cost ratio (B/C).

The IRR represents the ideal interest yield provided by the project, considering all costs and benefits. The IRR can be calculated when all the cash flow is at time 0 and the costs are equal to the incomes.

Payback is the time that the investment takes to recoup its initial investment; this analysis considers the cash flows from its inception, as well as the leverage of the project.

B/C is the ratio of the net benefits to the costs of the project. Net benefits represent the present value oftheannualrevenueandsavingsminustheannual costs,whilethecostisdefinedastheprojectequity.

 Table 7 presents the financial results and its feasibility.


**Table 7.** Financial analysis.

B/C: Benefit/cost ratio, IRR: Internal rate of return.

As seen in Table 7, the financial analysis describes the viability of the project, where the IRR is predicted to be 7%, which is a competitive market rate; the payback period is predicted to be 9.8 years to recover its investment, and the B/C is predicted to be 1.13, which means that the benefits are 13% higher than the costs.

A sensitivity or what-if analysis was also done and is presented in Table 8.

**Table 8.** Sensitivity analysis of zapote seed. The bold amounts indicate the range of profitability of the project.


In Table 8, it can be seen that \$6,849.89 is the base of the project, that is, the feasibility of the project requires 63,424.95 kg of zapote seed at 0.108 USD per kilogram; if the price and quantity increase in di fferent scenarios, then the project can be rejected. The bold amounts indicate the range of profitability of the project.

#### *4.4. Energy Policy*

The implications of the Mexican Law of Promotion and Development of Bioenergetics (LPDB) that can be taken into account in this paper are the promotion and development of bioenergetics to contribute to the energy diversification and sustainable development as conditions that guarantee support for the Mexican countryside. This law has three main objectives [49]:


According to the Secretary of Agriculture and Rural Development, Ministry of Agriculture and Rural Development (SAGARPA), Mexico is the leader in the production of zapote mamey, with only five states in Mexico producing it [30]. The benefits shown in this study can be proposed to SAGARPA and use the LPDB to increase the market opportunity for small producers and increase the production overall. This necessitates an adequate credit policy for this sector, which includes low interest rates, especially during the first few years, as well as flexibility in the terms and forms of payment.

Considering the results of this study, beginning with a sustainable proposal to universities along with the five producer states of zapote mamey regarding their boiler equipment, this experience can be shared with other institutions with similar conditions.
