**1. Introduction**

The increase in the worldwide population, which is expected to exceed 11 billion by the end of the century, together with economic and social development, has led to an increase in energy demand [1,2]. Despite progressive growth and the falling costs of renewable energy sources, coal, oil, and gas remain the mainstay of the steadily growing energy consumption on a global scale [3]. The energy transition to a low-carbon economy is also threatened by geopolitical and commercial tensions, and by declining investment in clean energy, according to a study published by Capgemini in collaboration with De Pardieu Brocas Maffei and Vaasa ETT. As a result, the concentrations of the main greenhouse gases (GHG) that trap heat in the atmosphere once again reached record levels in 2018: carbon dioxide (CO2) increased by 147%, methane (CH4) by 259%, and nitrous oxide (N2O) by 123%. These increases make climate change more acute as temperatures rise and extreme weather events multiply [4].

Climate change represents by far one of the major threats facing our society [5]. On 12 December 2015, at the Conference of Parties 21 (COP21) in Paris, all signatory countries came to a historic agreemen<sup>t</sup> to promote and accelerate the necessary measures in the fight against climate change, which included accelerating the energy transition to a sustainable low-carbon economy. The purpose of the Paris Agreement was to establish a coordinated global response to the threat of climate change and to conclude a global agreemen<sup>t</sup> on the reduction of greenhouse gases (GHGs), which intends to limit the increase in global temperature this century to below 2 ◦C compared to pre-industrial levels and to maintain ongoing e fforts to further reduce the rise in temperature to 1.5 ◦C. Furthermore, the agreemen<sup>t</sup> seeks to enhance the capacity of countries to address the e ffects of climate change and to ensure that economic streams produce low greenhouse gas (GHG) emissions [6,7].

According to the International Energy Agency, more than a third of the world's energy is consumed in buildings, 70% of which is used to meet heating and cooling requirements [8]. Buildings are one of the main sources of greenhouse gas emissions, and for this reason, more e ffort is needed to understand the energy consumption patterns in buildings and to establish strategies for energy saving at the district level [9,10]. To reduce these emissions (up to 20% since 1990) and promote the use of renewable energies, among other measures, the European Directive 2010/31/EU introduced the definition of a nearly zero energy building (NZEB) and appeals for deployment in public institutions by 31 December 2018, whereas it extends to the identical date in 2020 for privately owned buildings [11–13]. The principal feature of low-energy buildings is that their energy demand must be equal to their energy generated, that is, they must be able to produce the same energy or more than they will consume over a whole year. Furthermore, the energy produced must be in situ or in the closest environment and using renewable energies [14].

Mexico has clear goals regarding the use and exploitation of renewable energies, and Mexican universities, in the context of their social and environmental liability, cannot remain on the sidelines of this commitment. According to the Law for the Use of Renewable Energies and the Financing of Energy Transition, Mexico's goal for 2024 is to generate 35% of its energy from non-fossil sources. To achieve this goal, Mexico would have to increase the share of renewables by 400 percent in less than 10 years. In Mexico, there are a large number of renewable resources with enormous potential for energy generation. We can highlight the high levels of solar radiation, a multitude of dams and reservoirs for the generation of hydroelectric energy, potential for the development of geothermal energy, areas with intense wind activity, and the generation of large quantities of agricultural and industrial waste, as well as solid urban waste [15,16].

Renewable energies are almost unlimited resources that nature provides us with and that we must take advantage of due to their sustainable nature. Another important advantage is that these resources are indigenous and increase a country's energy resilience by decreasing its dependence on energy imports. They also increase the diversification of the energy supply and boost the growth of new technologies and job creation [17]. Under the term of renewable energy, di fferent heterogeneous categories of technologies can be grouped. Some technologies can generate electricity and heat, as well as mechanical energy, while others can produce solid, liquid, or gaseous fuels that meet multiple energy needs. On the other hand, while some technologies can be implemented within rural and urban environments at the same place of consumption (centralized), others are generated at a distance and therefore require large transport networks (decentralized) [18,19].

Among the available renewable energies, the use of biomass for heating has experienced grea<sup>t</sup> growth in the last decade. Biomass is becoming more and more important as an alternative to traditional fossil fuels, such as coal or oil derivatives, since it is a more accessible and a better-distributed geographical resource, its theoretical balance of CO2 emissions is considered to be zero, and it is renewable, among other advantages [20]. Biomass can be defined as the organic substances that have their origin in the carbon compounds formed in photosynthesis and that can be used as an energy source. Under this definition, many materials can be considered biomass. Solid biofuel sources are usually divided into those of primary nature and those of secondary nature. In the former, which includes energy crops and forest biomass extracted for energy purposes, their managemen<sup>t</sup> and use are mainly oriented toward energy production. In the sources of secondary origin, in what is generally called residual biomass, we find agricultural by-products (straw, cane, and fruit prunings), forest by-products (remains from forestry interventions, such as branches and trees with no commercial value), and industrial by-products (sawdust, fruit shells and stones, etc.). Both primary and secondary biomass can be used either via direct combustion or thermochemical processes, such as pyrolysis and gasification [21]. Additionally, both primary and secondary sources can be used for the generation of thermal energy in boilers and district heating networks, as well as for the generation of electrical energy in biomass plants.

Heating and domestic hot water generation are the most widespread thermal applications of biomass, though it is also used for cooling and power generation. Biomass can be used to generate the thermal energy necessary to power an air-conditioning system (heating and cooling), either at a centralized level (single-family house or block of flats) or at a decentralized district level (several buildings) [22,23]. Implementing a distribution network to supply not only housing estates and other residential dwellings but also public buildings, industries, etc., is known as "district heating", and involves transporting the thermal energy over a distance through ducts that are not structurally integrated into a single building [24].

A recent Reuters Business Insight report estimates that the contribution of bioenergy to the world's primary energy supply could reach 50% by 2050. Bioenergy projects are largely conditioned by the location and availability of biomass resources, although in recent years, the market for standardized biofuels has developed considerably. Within the applications of biomass for heating, its use in boilers to replace fossil fuels has experienced a grea<sup>t</sup> boom in the last decade, mainly due to the significant energy and economic savings obtained [25].

Among the most commonly used fuels for biomass boilers are pellets, mostly from the wood and forestry industries that generate waste in the form of sawdust and chips. However, due to the increase in the price of this biofuel in recent years, it is necessary to investigate new forms of bioenergy that allow for reducing costs on the one hand and alleviating the pressure on the wood industry on the other. The European standard EN 14961-1 promotes the use of new forms of biofuels, and studies have already shown the potential of certain fruit shells and stones for generating heat [26–29].

Today, Mexico is a country that depends heavily on fossil fuels for the production of its primary energy. If we pay attention to its energy production between the years 2007 and 2017, this amounted to 8935 PJ, of which 88.2% was referred to fossil fuels, 7.2% was renewable energy (1.31% obtained from biomass), 3.4% was from coal, and 1.2% was nuclear energy [30].

If we focus on primary energy production in Mexico from biomass, the main sources are firewood, occupying 5.2%, and sugarcane bagasse with 0.9%. A large part of the energy production from sugar cane bagasse (50%) is used by the sugar industry for the production of its energy [31].

According to the Mexican Secretary of Energy (SENER), 45% of the waste obtained from crops is used as fertilizer or for cooking and heating houses in rural areas [31].

Honorato-Salazar et al. [32] studied the available agricultural residues in Mexico from twenty crops. They estimated that there is between 17.5 and 58.1 megatons of dry matter produced per year (Mt DM/yr). The corresponding bioenergy potential ranges from 313.4 PJ/yr to 1039.4 PJ/yr.

The zapote mamey (*Pouteria sapota*, also called *Lucuma mammosa*) is a tree species of the Sapotaceae family. The zapote mamey tree is an ornamental, perennial tree that can reach a height of up to 40 m and a diameter at breast height of more than 1 m. The trunk is straight and may have buttresses. The external bark is cracked and falls o ff into rectangular pieces, brownish-gray to brownish-black in color, with a thickness of 10 to 20 mm. The leaves are arranged in a spiral and agglomerate at the tips of the branches. The fruits are berries that are up to 20 cm long, ovoid, reddish-brown in color, and rough in texture. The mesocarp is sweet, fleshy, orange to reddish, with small amounts of latex when unripe. It usually contains a seed that is up to 10 cm long, ellipsoid, and black to dark brown. The percentage in weight of the seed ranges from 8% to 16% of the total weight of the fruit [33].

Its exact origin is di fficult to determine since it was already being cultivated throughout tropical America before the arrival of Europeans. Its natural range probably extends from southern Mexico to Nicaragua, Belize, and northern Honduras. In Mexico, the original areas of distribution are likely to have been in Veracruz, Tabasco, and northern Chiapas; however, it is now found in all southern Mexican states. It is cultivated from Florida (USA) to Brazil and Cuba. It has also been introduced to the Philippines, Indonesia, Malaysia, and Vietnam. It is a plant that is widely distributed in its place of origin, from southern Mexico to Guatemala and Belize, where it is found both wild and cultivated [34].

In Mexico, production has been concentrated since 2006, mainly in the states of Yucatan and Guerrero, but the states of Chiapas, Michoacan, Veracruz, and Campeche are consolidating their participation in production every year. The trend in the production of Mamey in Mexico has had a greater momentum from 2006 to 2008; after that period of growth, the upward trend has been constant but slow [33]. Figure 1 shows the zapote production in the main states of Mexico.

**Figure 1.** Zapote production in the main states of Mexico.

The fruit is eaten raw or made into smoothies, ice cream, and fruit bars. It can be used to produce jam and jelly. Some beauty products use pressed oilseed to produce what is known as sapayul oil. However, the food and cosmetics zapote industries produce large amounts of waste, which is thrown away without making environmentally responsible use of it.

The main purposes of this study were to determine the elemental chemical analysis of the zapote seed and its energy parameters to further evaluate its suitability as a solid biofuel in boilers for the generation of thermal energy in a tropical climate. Additionally, energy, economic, and environmental assessments of the installation were carried out.

#### **2. Case Study**

The heavy chemistry laboratory of the Faculty of Engineering of the University of Veracruz in Mexico was taken as a case study (Figure 2). The Veracruzana University (UV) was founded in 1944 and became autonomous in 1996. It is located in the state of Veracruz on the Gulf of Mexico. It has five regional headquarters: Xalapa, Veracruz, Orizaba-Córdoba, Poza Rica-Tuxpan, and Coatzacoalcos-Minatitlán. In terms of the number of students it serves, it is among the five largest public state universities of higher education in Mexico.

**Figure 2.** Location of the Faculty of Engineering of the University of Veracruz.

The dimensions of the chemistry laboratory (Figure 3) are 35 × 15 × 3 m, and among the equipment, the laboratory has two evaporators and a reactor for the dehydration of food. These equipment are fed by one 200 kW liquefied petroleum gas (LPG) boiler to produce steam. It consumes 200 L of LPG to produce 400 kg/h of saturated steam. Figure 4 shows the evaporator–condenser assembly.

**Figure 3.** University of Veracruz heavy chemistry lab.

**Figure 4.** Evaporator–condenser assembly.

Figure 5 shows the LPG boiler used to produce the steam needed.

**Figure 5.** Liquefied petroleum gas (LPG) boiler used for steam production.

Initially, an energy audit was conducted, which served as the basis for the technical study. Subsequently, all the required data was gathered to assess the feasibility of substituting the current LPG boiler with a biomass boiler that would use zapote seeds as fuel.

#### *2.1. Climatological Data*

The city of Coatzacoalcos in the state of Veracruz (Mexico) has a tropical, warm, and humid climate characterized by a short dry season and heavy rainfall. The average annual temperature is 25 ◦C and the average rainfall is 2471 mm per year. Table 1 shows the most relevant climatic data for the city of Coatzacoalcos in the state of Veracruz.


**Table 1.** Climatic data of Veracruz state.

#### *2.2. Overview of the Existing Heating System*

The 200 kW LPG boiler operates for approximately 4 h daily and an annual average of 300 days. Therefore, the thermal power demanded is 240,000 kWh.

Figure 6 shows the operating diagram of the LPG boiler.

**Figure 6.** Running chart of the LPG boiler.

#### **3. Materials and Methods**

Regarding the use of zapote seeds as biofuel, 2000 g of this waste was collected for the determination of its chemical composition and the evaluation of its energy parameters (Figure 7).

**Figure 7.** Zapote mamey (*Pouteria mammosa L*).

The UNE-EN 14961-1 standard "Solid biofuels–Specifications and fuel classes—Part 1: General requirements," was used to assign the biomass quality parameters and the standard UNE-EN 15148 was used for the determination of the volatile matter. These standards have been developed by the Spanish Association for Standardisation and Certification (AENOR).

Table 2 shows the standards and measuring equipment used.


**Table 2.** Standards and the measuring equipment used in this study.
