*3.1. Humidity*

The moisture content of the biomass is the ratio of the mass of water contained per kilogram of dry matter. An excess of humidity in the biomass, leads to [23]:


In order to avoid these problems, a solar greenhouse dryer system for loquat seed improvement as biofuel is proposed.

The solar greenhouse dryer built in this study was the feature on conventional greenhouse type tunnel with north-south facing (Figure 7). Figure 8 shows the dimensions of this greenhouse dryer.

**Figure 7.** Scheme of the greenhouse dryer.

**Figure 8.** Dimensions of the greenhouse dryer proposed.

The drying of the loquat seeds in the solar dryer started on 1 October 2019 and was completed on the 17th of the following month. The moisture content was measured using Extech hygrometer M0210.

Before introducing the loquat seeds into the solar dryer to evaluate its operation, the air temperature and relative humidity levels were recorded during a week at different times of the day with an Extech RHT20 recorder, in order to know precisely their magnitude and the conditions under which the seeds would be exposed.

**Figure 9.** Relationship between monthly solar radiation and air temperature in Almeria.

As seen in Figure 9, in Almería the lowest solar radiation is January and December with 4 kWh/m<sup>2</sup>/d, the months with the highest solar resource are July and August with more than 8 kWh/m<sup>2</sup>/d.

#### *3.2. Elemental Composition*

The elemental analysis technique consists of determining the content of hydrogen, nitrogen, carbon and sulphur present in samples of an organic and inorganic nature, both solid and liquid. The UNE-EN 15104 standard has been consider in this analysis and the analyzer LECO TruSpec CHN 620-100-400 was used.

#### *3.3. Ash Content*

The determination of ashes will be based on UNE EN 14775 standard. Ashes are the residue of air incineration of coal and come from the inorganic compounds initially present in carbonaceous substances and associated mineral materials.

The ash content is expressed as the percentage ratio between the mass of the residue after incineration and the mass of the original sample.

The percentage of ash in biofuels is an important parameter to consider in boilers, since it can produce corrosion and accelerated erosion of the metal that makes up each of the elements of the boiler.

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

Titrator Mettler Toledo G20 was used to obtain the chlorine content and Analyzer LECO TruSpec S 630-100-700 to obtain sulfur content and the standard UNE EN 15289 was followed.

Riedl et al. [24] describe the corrosion mechanism in the presence of chlorine as "active oxidation" and assign to it the accelerated corrosion rate observed in boiler tubes. The authors describe the enrichment of alkaline metal chlorides on the surface of pipes by a condensation process. They also sugges<sup>t</sup> that the chlorides react with SO2 and SO3 in the gases to form sulphates with the subsequent generation of chlorine gas.

An important study on this subject has been carried out in Denmark where accelerated corrosion tests were conducted on boilers that were using fuels such as straw and cereals with ash levels in the range of 5%–7%, chlorine levels between 0.3% and 0.5%, and sulfur content less than 0.2%, on a dry basis [25].

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

The higher heating value (HHV) is defined assuming that all elements of the combustion (fuel and air) are taken to 0 ◦C and the products (combustion gases) are also taken to 0 ◦C after the combustion, so the water vapor will be completely condensed.

In the calculation of the LHV, it is assumed that the water vapor contained in the flue gas does not condense and therefore there is no additional heat input due to the condensation of the water vapor. Only the heat of oxidation of the fuel is available. While determining the HHV experimentally in a calorimeter, the LHV is obtained by a calculation from the HHV [26–28].

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

#### *3.6. Biomass Combustion Technology*

According to Wolf & Dong [29], three types of technologies for biomass combustion can be distinguished: fixed bed, fluidized bed (which can be either circulating or bubbling bed), and pulverized fuel. In our case study, considering the characteristics of the biofuel and the boiler output, fixed bed technology will be chosen for the direct combustion of the biomass once it has been dried.

Fixed-bed combustion includes furnaces with grates and feeders (stokers). The biomass is placed on a grate and moves slowly through the boiler. The required air is supplied through holes arranged along the grate. The combustible gases issued by the biomass are burned after the addition of a secondary air, usually in a combustion zone separated from the fuel bed. This technology is suitable for any type of biomass but limited to installations of up to 100 MW.

An important aspect of the grate furnace is that the combustion stages must be obtained by separating the primary and secondary combustion chambers, in order to avoid the mixing of secondary air currents and to separate the gasification and oxidation zones. The better the quality of the mixture between the combustion gases and the secondary combustion air, the less oxygen is needed to achieve complete combustion, thus achieving greater efficiency.

According to the direction of flow of the fuel and flue gases, there are three operating systems for grate-fired boilers:


In our case study the stream flow system will be chosen. The flow in stream is applied for dry fuels or in systems where primary heated air is used. This system increases the residence time of the gases released by the fuel bed, allowing for a reduction in NOx emissions, due to the improved contact of the combustion gases with the bed of carbonized material at the back of the grills [30].

#### *3.7. Economic Analysis*

The economic analysis has been performed at initial conditions of 267 kW as rated power, loquat properties compared to oil fuel. All these processes have been carried out with RETScreen software, which is a clean energy managemen<sup>t</sup> software system for energy efficiency, renewable energy, and cogeneration project feasibility analysis, as well as ongoing energy performance analysis, developed by the Natural Resource of Canada office [31].

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

The physico-chemical parameters of loquat seed were analysed and compared with those of other sources of residual biomass in order to evaluate its usefulness as a solid biofuel. Afterwards, an energy, economic and environmental analysis of the installation was carried out.

#### *4.1. Loquat Seed Values*

To evaluate the quality parameters of loquat seed, 2000 g of samples from the loquat industry were analysed. Table 4 shows the mean value, the standard deviation, the maximum value, and the minimum value which determine the parametric distribution.



As can be seen from Table 4, one of the main disadvantages of loquat seed is its high moisture content, above 30%. This decreases the efficiency of combustion, since water needs to evaporate before heat is available, resulting in a lower heating value. Furthermore, from a technical point of view, the presence of a high moisture content produces corrosion in the equipment and generates the emission of tars that accumulate in the outlet pipes and can cause them to block. As a result, pre-drying processes must be implemented in order to obtain a moisture content below 10%.

Ash is the inorganic fraction of the biomass that remains once the fuel has been burned, mainly in the form of SiO2 and CaO. The formation of ashes is linked to problems such as the formation of agglomerates on the walls of the grids and the formation of slag deposits which accelerate the corrosion of the installation and increase its maintenance costs. With respect to the ash content of the loquat seed, it is in the range of 2.31%–2.43%. If this value is compared with that of other standardized fuels, such as almond shell pellets (3.35%) or oak pellets (3.32%), it can be seen that in spite of its high value, it is below the ash content of other conventional fuels.

Table 5 compares the physicochemical parameters of several commercial biofuels and industrial wastes with those of loquat seed in order to evaluate the use of this by-product in the generation of thermal energy.



As shown in Table 5, the ash content of loquat seed is higher than that of other industrial waste but lower than the ash content of the mango stone. This suggests that the formation of ash deposits will be greater and therefore will need additional maintenance.

As for the higher calorific value of loquat seed (17.205 MJ/kg), it is observed that it is lower than that of other standardized solid biofuels such as almond shells or olive stones. However, if we compare it with other industrial wastes such as wheat straw (17.344 MJ/kg) or pistachio shell (17.348 MJ/kg) [31], it is shown to have similar values, showing the energy viability of this agro-industrial waste.

Another quality parameter to be considered in a biofuel is its chlorine and sulphur content. Sulphur, in addition to having a corrosive e ffect on the installation, is associated with the emission of greenhouse gases in the form of SOx. Loquat seed has a 0.03% sulphur content that is lower than that of other commercially available biofuels such as almond shells (0.050%) or olive stones (0.110%), which means that SOx emissions would be minimised if this biofuel were used.

Regarding the chlorine content, this has a significant e ffect on the corrosiveness of the plant due to the formation of chlorides, which have a dissociative catalytic e ffect on the steel pipes. Loquat seed has a low chlorine content (0.07%), like that of other standardized biofuels such as olive stone (0.06%) and lower than that of almond shells pellets (0.2%). In view of these results, corrosion problems would be minimized by using this solid biofuel.

However, in addition to its high calorific value, the main advantage of using loquat seed as biofuel is its carbon-neutral character. In fact, when the plant grows it fixes carbon from the atmosphere which is then released when it is burned, making the life cycle carbon neutral.

This residual biomass can be obtained from loquat processing industries in the surroundings, helping on the one hand to a better environmental managemen<sup>t</sup> of these wastes and on the other hand to a reduction of greenhouse gas emissions.

#### *4.2. Environmental Benefits*

Biomass as an energy source has several advantages over other alternatives in the fight against climate change and local pollution. Biomass is a source of non-polluting energy. Plants emit CO2 but also absorb CO2 during their growth, so their total balance is zero (Figure 10).

**Figure 10.** CO2 cycle using loquat seed as biofuel.

Once the energy characteristics of the loquat seed are known, it is possible to calculate the CO2 savings in the installations of the UAL indoor swimming pool, as well as the worldwide CO2 savings in loquat producing countries.

Firstly, the potential energy obtained from the use of loquat seed as a biofuel is calculated using Equation (2). This potential energy is calculated considering the worldwide production of loquat for each country.

$$
\mathcal{U}\_p = RH \times P\_{\text{liquid\\_send}} \times HHV \times f\_s \times F\_c \tag{2}
$$

where:

*Up* denotes energy obtained from the loquat seed as biofuel (MWh);

*RH* is relative humidity (10%); seed

*Ploquat seed*: loquat production (kg); *HHV*:higherheatingvalue(17.205MJ/kg);

 *fs*isthepercentageofseed inawholeloquat(15%);

*Fc*factorconversionfor units(0.000277778Wh/J).

Figure 11 shows worldwide bioenergy potential using loquat seed as biofuel (MWh).

**Figure 11.** Worldwide bioenergy potential using loquat seed as biofuel (MWh).

The next step in the study will be to calculate the CO2 reductions that would occur if loquat seeds were used as a biofuel instead of fuel oil.

As mentioned above, Biomass is a source of non-polluting energy, with zero CO2 emissions. The CO2 emission of fuel oil would be calculated as:

$$\text{MCCO2}\_{\text{fuel oil}} = \text{C}\_{\text{fuel oil}} \times \text{F}\_{\text{fuel oil}} \tag{3}$$

where:

M*CO*2 *f uel oil*: mass of carbon dioxide emitted (kg/year). *Cf uel oil*: consumption of fuel oil per year (kWh/year). *Ff uel oil* : carbon dioxide emission factor of fuel oil (kg/kWh).

Table 6 shows the CO2 emission factor for biomass and fuel and the total CO2 emission reduced annually using loquat seed as biofuel.


**Table 6.** CO2 emission factor of Fuel oil and Loquat seed [39].

The change of the boiler to biomass in UAL indoor swimming pool means a reduction of 147,973.8 kg CO2 in emissions into the atmosphere.

Figure 12 shows the worldwide CO2 saving using loquat seed as biofuel (Tn).

**Figure 12.** Worldwide CO2 saving using loquat seed as biofuel (Tn).

The five main countries that would reduce their annual CO2 emissions by using loquat seeds as biofuel are: China (51,184.92 Tn), Spain (10,617.54 Tn), Turkey (3454.98 Tn), Pakistan (3275.83 Tn), and Japan (2621.95 Tn). Further, the annual worldwide reduction of CO2 emissions if loquat seed is used as biofuel would be 76,363.80 tn.

In a society committed to sustainable development, the use of biomass for heat and electricity production is an important source of renewable energy. Its increasing use as a substitute for fossil fuels can significantly reduce CO2 emissions. However, in recent years discussion has focused on the sustainability of biomass, and the environmental implications of its use must be taken into account.

Particularly important is the particle size of the emissions that are produced, which have been identified as a relevant factor of the deterioration of air quality. Not so long ago, the existing legislation for emission control in combustion plants only took into account total particles with a diameter of less than 10 micrometers (PM10). However, particles with a diameter of less than 5 micrometers (PM5) and especially those with a diameter of less than 2.5 micrometers (PM2.5) have the most harmful effect on health. Fine particles (PM2.5) emitted during biomass combustion can be divided into three groups, based on their chemical composition and morphology: particles spherical organic carbon particles, sooty aggregate particles and inorganic ash particles. Because of their small size, these particles are able to reach the pulmonary alveoli and pass into the bloodstream. This is associated with an increased risk of respiratory and cardiovascular disease, especially when the environmental concentration of these particles exceeds 35.4 μg/m<sup>3</sup> [40–43].

This has led to a boost in research activity, both in the characterisation of emissions and in the development of control equipment, and in enacting legislation at national and European level.

For our case study, the R.D. 1073/2002 transposing the Directive 1999/30/EC on air quality, establishes in relation to PM10, that the limit value of 50 μg/m<sup>3</sup> must not be exceeded in 24 h for more than 35 days, on the date of entry into force of 1 January 2005.

In the year 2015, new European legislation has been published (Directive (EU) 2015/2193 of the European Parliament) that involves air emissions from the combustion of solids, such as biomass, in equipment and installations with a rated thermal input of more than 1 MW and less than 5 MW, and that makes a big impact on particle emissions.

Conversely, the European Directive 2009/125/EC establishes a framework for setting ecodesign requirements for energy-related products. The national transposition of this regulation is the Royal Decree 187/2011 of 18 February.

The result of this directive is Regulation 2015/1189 of 28 April on solid fuel and wood biomass boilers of nominal output not exceeding 500 kW, which is mandatory from 1 January 2020. The environmental aspects considered important in this regulation are energy consumption and emissions generated by particulate matter (PM), organic gaseous compounds (OGC), carbon monoxide (CO), and nitrogen oxides (NOx) in the use phase of this equipment. This regulation stipulates that seasonal particle emissions from heating may not exceed 40 mg/m<sup>3</sup> for automatically fed boilers and 60 mg/m<sup>3</sup> for manually fed ones.

In order to reduce the amount of particles emitted into the atmosphere due to the incomplete combustion of the biomass, modifications have been made to combustion equipment in recent years. In this way, for example, special emphasis has been placed on the modulation of the equipment to adapt to thermal demand, lambda probes have been used to ensure control of the most appropriate fuel-air ratio according to operating conditions and secondary and tertiary air has been introduced in di fferent parts of the equipment.

However, the permitted emission limits are becoming increasingly restrictive, making it necessary to use of equipment that can be coupled to the stoves and boilers of biomass in order to reduce the emission of particles. In this way, research is being carried out into di fferent technologies that can be adapted to the residential sector, such as the introduction of additives with the biomass, the use of catalytic filters or the use of electrostatic precipitators.

In low power installations (up to 1 MW) the most e ffective and profitable solution is the use of an electrostatic filter [44]. Its operation is based on electrically charging the particles in order to direct them out of the gas towards plates with an opposite charge, to which they adhere. Therefore, in our case study an electrostatic filter will be installed consisting of a metal rod that rotates inside the metal chimney tube, carrying a voltage of 24,000 volts, so that it ionizes the solid particles, which are attracted by the walls of the chimney tube, where they accumulate until they fall by gravity into the equipment. This technology has proven to achieve e fficiencies of over 90% in PM2.5 abatement [45].

#### *4.3. Economical Benefits*

The economic feasibility of the study of changing the fuel boiler for loquat seed as a biofuel is based on the following:


Starting with a 267 kW boiler that will work approximately 6 h a day with an average of 297 days, the energy required is 475,800 kWh.

The high price of fossil fuels, which is also heavily taxed in many countries, is boosting the market for biomass boilers for heating generation. In order to calculate the economic benefit to be gained from the new biomass installation compared to the original fuel oil installation, the necessary fuel expenditure in both scenarios has been calculated to cover the annual energy demand. As shown in Table 7, the annual fuel oil consumption of the existing facility during 2018 was 52,239 litres, and considering a fuel price of 0.94 euros/litre, a total annual cost of 49,104.67 euros is obtained.


**Table 7.** Economic analysis of fuel oil installation and the Loquat seed boiler.

The annual thermal demand of the installation (kWh/year) can be calculated as the product of the amount of fuel consumed by the lower heating value (LHV) of the same, but considering the e fficiency of the boiler, this is:

> Annual thermal demand = Fuel quantity × LHV × Boiler e fficiency (4)

Taking into account the necessary thermal demand of 475,800 kwh per year, with the new biomass boiler 133,651.7 kg of loquat seeds will be consumed, and taking as a reference price of this residual biomass a value of 0.1 €/kg already treated and transported, there would be an annual saving of 35,739.5 € which means a saving of 72.78% with respect to the previous fuel oil installation.

A sensitivity analysis has been carried out with a threshold of seven years, a range of 25% and the analysis is performed on equity payback, see Table 8.


**Table 8.** Sensitivity analysis based in equity payback.

Table 8 shows that fuel cost will increases in 0.4 in 7 years if costs do not increase, however, in the worst case the costs will increases in 0.7 times if fuel cost increase in 25%.

A risk analysis is performed on net present value (NPV) and 500 combinations. In Table 9 are presented the parameters and in Figure 13 is presented a tornado chart to identify the impact of NPV on these parameters.


**Table 9.** Parameters.

**Figure 13.** Impact – Net Present Value (NPV).

Figure 13 shows the economic impact of NPV on fuel cost of proposed case and base case, as it can be seen, the highest sensitivity variable is fuel cost-base case (fuel oil); debt interest rate, debt ratio and debt term have very small effects and can be ignored their uncertainty.

Financial viability presents the results provide to the decision-maker with various financial indicators for the proposed case, see Table 10.


**Table 10.** Financial viability.

The internal rate return (IRR) is 20.6%, which is higher than European central bank interest rate (0.25%), payback is 1.3 years, and B-C shows that investment has financial viability.

#### *4.4. Biomass Storage*

The Thermal Installations Regulation (2007) [46] in Spain describes a few essential requirements for solid biofuel storage systems (IT.1.3.4.1.4 Solid biofuel storage). In general, the storage site must be exclusively for this use, and this does not change the physical, chemical and mechanical characteristics of the biomass. It must also comply with a series of requirements to prevent the risk of self-combustion. The types of storage can be divided into prefabricated or built storage, either new construction or existing room, either above or below ground. In new buildings, the minimum storage capacity for biofuel will be sufficient to cover two weeks' consumption. The silo can be filled semi-automatically, with direct loading or with a pneumatic system, depending on the type of solid biofuel and size of the silo.

In the case of this study, the existing deposit room will be adapted as a silo, making some slopes of galvanized sheet and having an extraction system of about 2 m. This silo will have a floor area of 3.4 m<sup>2</sup> and a height of 2 m as defined in Figure 14. Therefore, a volume of 6.8 m<sup>3</sup> is available, which complies with the 15-day minimum fuel supply (6.18 m<sup>3</sup> of Loquat seeds required).

**Figure 14.** Location of storage silo.

#### *4.5. Biomass Drying*

An excess of humidity in the biomass, leads to a lot of problems. It is essential for biomass fuel to remain dry, or it will not burn efficiently.

The drying of the loquat seeds in the solar dryer started on 30 October 2019 and was completed on the 17th of the following month. The drying of loquat seeds with an initial moisture content of 35.20% was started and ended at 10% final moisture content after 12 days (Figure 15).

**Figure 15.** Evolution of the relative humidity in wet basis of the loquat seed in greenhouse dryer.

Figures 16 and 17 show the average temperature and relative humidity levels reached at different times of the day inside the greenhouse dryer.

**Figure 16.** Average temperature variation during the day in the greenhouse dryer.

**Figure 17.** Average variation in relative humidity on a wet basis during the day in the greenhouse dryer.
