Thermochemical and Economic Analysis for Energy Recovery by the Gasiﬁcation of WEEE Plastic Waste from the Disassembly of Large-Scale Outdoor Obsolete Luminaires by LEDs in the Alto Alentejo Region (Portugal)

Featured Application: Energy recovery of agroforestry waste mixed with industrial plastic waste through gasiﬁcation processes. Abstract: The recovery of urban waste is a social demand and a measure of the energy-environmental sustainability of cities and regions. In particular, waste of electrical origin, waste of electrical and electronic materials (WEEE) can be recovered with great success. The plastic fraction of these wastes allows their gasiﬁcation mixed with biomass, and the results allow for producing syngas with a higher energy potential. This work allows for obtaining energy from the recovery of obsolete materials through thermochemical conversion processes of the plastic waste from the disassembly of the luminaires by mixing the said plastic waste in di ﬀ erent proportions with the biomass of crop residues (olive). The gasiﬁcation tests of these mixtures were carried out in a downstream ﬁxed-bed drown daft reactor, at temperatures of approximately 800 ◦ C. The results demonstrate the applied technical and economic feasibility of the technology by thermal gasiﬁcation, for the production of LHV (Low Heating Value) syngas with highest power energy (more than 5 MJ / m 3 ) produced in mixtures of up to 20% of plastic waste. This study was complemented with the economic-ﬁnancial analysis. This research can be used as a case study for the energy recovery through gasiﬁcation processes of plastic waste from luminaires (WEEE), mixed with agricultural biomass that is planned to be carried out on a large scale in the Alentejo (Portugal), as a solution applied in circular economy strategies.


Introduction
Energy sustainability based on the use of clean and decarbonized energy sources has become a priority and strategic objective for countries around the world. Currently, countries with environmental sensitivity are considering satisfying their current energy demand by applying environmental and energy integration strategies that allow the use of new sustainable energy models based on renewable H 2 and hydrocarbons, affecting the type of raw material, while the same trends are observed for the concentrations of CH 4 and tar [21].
The studies on the joint gasification of plastics made it possible to analyze the high energy value of the syngas produced, the production of hydrogen obtained in these processes being especially interesting [22]. It is possible to obtain up to 5 times more hydrogen content in the combined gasification of biomass mixed with plastics wastes than in similar processes using pyrolysis at 800 • C [23,24]. And up to seven times more compared to pyrolysis processes at 900 • C [25,26].
More than 27 million tons of waste plastics are generated in Europe each year, representing a considerable potential resource. Plastic waste is a global environmental problem, which reached 36 Mtons in 2019 and causes a very negative environmental impact on ecosystems.
In the present work, the possibility of recovering energy from waste electrical and electronic material (WEEE) was studied, specifically the plastic recovered from luminaires dismantled by the electricity supply company in the Alto Alentejo, Portugal area [27]. The experiments were carried out with a mixture of plastic waste from the Syntra-type luminaire, luminaires mostly replaced in the processes of replacement by light and energy-efficient (LED)-type luminaires in this region of Portugal. The objective is trying to take advantage of and recover these plastic residues mixed with agro-food industry biomass from the olive seed mixed in different percentages. An economic feasibility study was also carried out for the installation of a gasification unit.

Obsolete Luminaires
The EU (Europe Union) is promoting policies that support technological renewal of luminaires, consisting of the massive replacement of obsolete luminaires that use discharge technologies (Metal Halide (HM) or high-pressure sodium vapor (VSAP)) for LED luminaires [28]. They use new, more efficient light and energy-efficient (LED) technology [29][30][31]. Most are obsolete luminaires that will have to be replaced in the short or medium-term by LED luminaires. This will generate a quantity of waste, to which a recycling solution would have to be found. In Europe (EU28), there are more than 1.6 million km of illuminated streets that consume approximately 35 TWh annually at a cost of EUR 4 billion for public authorities [32].
The luminaires, according to the Portuguese Environment Agency, are classified as waste electrical and electronic equipment WEEE [33,34]. Taking into account this classification and considering that the plastic fraction of these residues can be up to 80% by weight, it is necessary to look for solutions that allow for incorporating these residues in new industrial processes or energetically valorizing them without causing environmental impact [35].
According to data from EUCOLIGHT-The European Association of Collection and Recycling Organizations for Lamps and Lighting and AMBILAMP SA, 8843 tons of WEEE waste for recycling were collected in Spain alone, 72.5% more than in 2018. Collecting 6286 tons of waste lighting and 2557 tons of waste electrical and electronic material (WEEE). In total, almost 8843 tons of waste were collected and treated in the last year compared to 5126 in 2018, representing an increase of 72.51%. This represents a record 29,000 tons of recycled lamp waste (equivalent to approximately 220 million luminaire units). This activity only with recycling reverts to multiple environmental benefits, including avoiding the emission of millions of tons of CO 2 into the atmosphere [36].
According to the Portuguese Waste Management Association, there is an exponential growth in the collection of WEEE waste. Portugal in 2018 had a collection of 119,558 tons, which is equivalent to approximately 26.734 million units of WEEE electrical and electronic equipment [37].
The results of the collection of WEEE have increased in weight and in the number of units collected in the last five years. In 2018, there was an increase of more than 17,886 tons compared to 2017. It is possible to observe that the large amount with a tendency to increase WEEE comes from the collection of luminaires. Luminaires that largely correspond to large-scale replacement processes and changes by new LED technologies. Where the percentage of plastic waste is approximately 30% by weight [37,38].

Materials and Methods
The gasification tests were performed with a mixture of plastics and agro-food industry biomass, mixed in different percentages. Biomass residues originate from the extraction of oil from the olive seed. Both fuels were previously characterized and then gasified, under the following conditions: 100% olive seed; 10% WEEE (plastic) + 90% olive seed and 20% WEEE (plastic) + 80% olive seed [7]. For WEEE gasification tests, manual classification was necessary with the help of magnets to remove any small metals that may be present, as these materials could damage the reactor. In the joint gasification, the mixture between olive biomass and WEEE was tested to investigate the composition of the synthesis gas and its heating value at a temperature between 800 and 960 • C [39].
The plastic waste from the study luminaires was provided by the EDP (Energías do Portugal) (Portugal) [40], the company in charge of promoting the "Projeto Illupub-Projeto para la Melhoria energetic e Eficansa Iluminasao Publica" in charge of the electricity supply and dismantling of obsolete luminaires to replace them with LED luminaires in Alto Alentejo (Portugal) (Figure 1) [41]. The process consists of disassembling the luminaires by separating the plastic parts and removing most of the metal present. The most polymeric part is rectified and passes through magnetic mats for the best use of the metal part. Process waste is sent to landfill [42].

Fuel Analysis
The agro-food industry biomass used in the tests was olive seed. This material was supplied by a company located in the Alentejo, which collects and treats olive pomace, removing most of the oil and moisture and produces pellets.
Polymeric residues from urban luminaires were added to the mixtures. This residue was used to improve the thermal gasification process in terms of improving the calorific value of the gas.
For the use of these residues, it was necessary to dismantle, sort and grind the parts of the luminaires manually. The fractions used for the gasification process were between 1 and 4 cm. The metallic fractions in the waste were removed manually with the aid of a magnet, to prevent damage to the equipment [43].
The residues were analyzed in terms of final analysis, a thermogravimetric analyzer and a higher heat value (HHV) [44,45]. The description of the analyses and equipment used are presented in the following sections.

Final Analysis
The final analysis was carried out to determine the elemental composition of the biomass (C, H, N, S and O content), using a ThermoFisher Scientific Flash 2000 CHNS-O analyzer (See Table 2) [46].  With the information provided by EDP, it has been possible to count the total number of light points of conventional luminaires (sodium, mercury and metal Halogen discharge lamps (HM) in the Alto Alentejo area (Portugal) (45,160). The results are shown in Table 1. Of the total number of luminaires analyzed, 78% corresponded to the luminaire model, mainly installed, model Syntra, approximately (35,593).

Fuel Analysis
The agro-food industry biomass used in the tests was olive seed. This material was supplied by a company located in the Alentejo, which collects and treats olive pomace, removing most of the oil and moisture and produces pellets.
Polymeric residues from urban luminaires were added to the mixtures. This residue was used to improve the thermal gasification process in terms of improving the calorific value of the gas.
For the use of these residues, it was necessary to dismantle, sort and grind the parts of the luminaires manually. The fractions used for the gasification process were between 1 and 4 cm. The metallic fractions in the waste were removed manually with the aid of a magnet, to prevent damage to the equipment [43].
The residues were analyzed in terms of final analysis, a thermogravimetric analyzer and a higher heat value (HHV) [44,45]. The description of the analyses and equipment used are presented in the following sections.

Final Analysis
The final analysis was carried out to determine the elemental composition of the biomass (C, H, N, S and O content), using a ThermoFisher Scientific Flash 2000 CHNS-O analyzer (See Table 2) [46]. Thermogravimetry analyses were performed to obtain the moisture content, volatile matter and the fixed carbon content combined with ash. A PerkinElmer STA 6000 type measuring equipment (see Table 3) was used using a nitrogen flow of 20 mL/ min for the inert atmosphere and a temperature growth rate of 20 • C/min. The thermogravimetric analysis was obtained for each one of the analyzed materials, measuring the variation of the mass with respect to the temperature and observing the inflexion points of the derivative of the mass with respect to the time factor [47,48].

Determination of the Higher Heating Value (HHV) or Gross or High Calorific Value and (LHV) Low Heating Value
A digital pump calorimeter (model IKA C2000) was used to measure the calorific value of the samples (Table 4) [49]. The calorific value of the biomass was calculated according to the methodology used in similar studies [50]. Studies have been carried out to predict the calorific value of biomass from the results of the analysis. The HHV (highest heating value) or LHV (lowest heating value) of the biomass were predicted by immediate analysis [51][52][53][54]. For the LHV calculation, Ozyuguran uses a similar model and the results were compared with those obtained in this article [55].

Gasification Tests
The gasification tests were performed on an AllPowerLabs PP20 Power Pallets-a gasifier with a power of 15 kW (Table 5 and Figure 2), a common downdraft reactor that is combined with an electric power generator and an electronic control unit. [56]. The equipment consists of a storage silo, where the biomass is simultaneously dried by recirculating the hot gases produced in the reactor. Fuel is supplied from the top as the air moves downward, being preheated through contact with the reactor's walls. using pressure probes located at the top at the reactor and on the particulate filter (see Figure 2). The coals were deposited at the bottom of the reactor and the tars are retained in the biomass filter. The power generator (3 kW) was running constantly and without stopping for 2 h. and own elaboration.

Methodological Description of Economic Viability
An economic analysis will now be carried out using the usual financial and economic indicators to justify the feasibility of installing the complete equipment (gasifier-engine-generator, includes installation). We will estimate an investment of € 1,200 / kW for the complete electrical energy production equipment with a direct sale to the Electricity Distribution Company in Portugal (EDP). The gasifier was based on a type of fixed bed, mounted with a gas engine and with the possibility of future installation of a module for the recovery of thermal energy connected to the engine.
It was considered that the electrical energy purchased or consumed through the national energy network, managed by (EDP) by the company that is going to exploit the gasifier generating equipment. It will be complemented with the energy produced or sold as a result of the gasification generator process and will be sold to the same company that manages electricity in Portugal (EDP) at the legally established price. The analysis carried out was based on the results of the previous gasification tests, using the amount of the necessary plastic waste and biomass, the calorific value of the synthesis gas and the income that the company will obtain from the use of the electrical energy produced through gasification. This waste is currently sent to the landfill, where the amount currently paid for the deposit is 9.9 €/t, with an annual adjustment of 1.12% per year. The cost of Ash collection is done in a separate tank in the lower part of the reactor, while the synthesis gas produced passes through a cyclone to remove the fine particles. The produced gas is also cleaned through a filter composed of biomasses of various granulometries and can be subsequently collected for analysis or injected directly into the generator. Condensate matter is collected at the bottom of the biomass filter.
The Otto cycle internal combustion engine burns the gas providing kinetic energy for the generator. In order to start the engine according to the manufacturer's instructions, the temperature in the reactor's lower part (reduction zone) must be at least 400 • C.
The tests were carried out using agro-food industry biomass, olive seed, in co-gasification with the polymeric residues, varying between 10 and 20% of incorporation. At the beginning of the experiment, 100% olive seed was used, which was the standard biomass for the tests. Synthesis gas was analyzed by gas chromatography. In particular, they were analyzed in a Varian 450-GC model kit with a thermal conductivity detector (TCD), the one commonly used to identify and quantify CO, CO 2 , H 2 , CH 4 and light hydrocarbons) [57,58].
During the entire gasification process, the temperature and pressure at the top (oxidation zone) and bottom of the reactor (reduction zone) were constantly controlled and monitored. In addition, the pressure of the biomass particle filter, the air flow at the inlet and the fuel consumption were carefully monitored during the tests. Likewise, the internal pressure in the reactor was measured using pressure probes located at the top at the reactor and on the particulate filter (see Figure 2). The coals were deposited at the bottom of the reactor and the tars are retained in the biomass filter. The power generator (3 kW) was running constantly and without stopping for 2 h.

Methodological Description of Economic Viability
An economic analysis will now be carried out using the usual financial and economic indicators to justify the feasibility of installing the complete equipment (gasifier-engine-generator, includes installation). We will estimate an investment of € 1200/kW for the complete electrical energy production equipment with a direct sale to the Electricity Distribution Company in Portugal (EDP). The gasifier was based on a type of fixed bed, mounted with a gas engine and with the possibility of future installation of a module for the recovery of thermal energy connected to the engine.
It was considered that the electrical energy purchased or consumed through the national energy network, managed by (EDP) by the company that is going to exploit the gasifier generating equipment. It will be complemented with the energy produced or sold as a result of the gasification generator process and will be sold to the same company that manages electricity in Portugal (EDP) at the legally established price. The analysis carried out was based on the results of the previous gasification tests, using the amount of the necessary plastic waste and biomass, the calorific value of the synthesis gas and the income that the company will obtain from the use of the electrical energy produced through gasification. This waste is currently sent to the landfill, where the amount currently paid for the deposit is 9.9 €/t, with an annual adjustment of 1.12% per year. The cost of electricity is approximately 0.16 €/kWh [44].
Taking into account the type of gasification unit that was used during the tests, it is estimated that the cost of installing similar equipment will cost € 1200/kW. For calculations of the use of the gasifier, was consider that the company works from Monday to Friday, for 12 h (Table 6) [59]. Next, the economic-financial analysis for the gasifier installation for the three cases studied, was made. (90% olive + 10% WEEE), (80% olive + 20% WEEE) and (100% olive). The evaluation of the period of return on investment (payback) (PP), net present value (NPV), internal rate of return (IRR) and the average cost of index benefit (IB m C), for each case was also made [60-63].

Net Present Value (NPV)
NPV, also called "capital value", consists of updating the casw-flow "Ct" (the difference between income and expenses for each period analyzed.) of the project for the different periods to an estimated discount interest rate "i" (discount rate or calculating interest). Assuming, as usual, the same interest rate, "i", for all periods, N, generally equal to each other and equal to the calendar year. We considered the first time period, the first year of amortization, as year 1. So "t" takes values between 1 and N. We consider the time period t = 0 is related to the investment during the design and construction phase of the gasifier. The NPV considers this discount rate "i" (calculated as WACC-weighted average cost of capital) during the useful life of the project, giving the annual cash flows at current values. NPV can be equated as [63]: where "i" is the financial discount rate, "Ct" is the annual cash flow (Income minus expenses) each year, and "N" is the total number of years. The time period t = 0 is related to the investment during the design and construction phase of the gasifier. The NPV represents the net profit generated by the project, obtained by financial equivalence at time zero. If the NPV is greater than zero, the project is viable for that interest rate. It is, therefore, a necessary condition although it does not have to be sufficient.
The WACC is calculated based on the cost of capital and the cost of debt (borrowed capital), which allow for determining the discount rate applicable to cash flows to calculate the NPV. The WACC is given by [64]: where "E" is the market value of the equity, "D" is the market value of the debt, "K e " is the cost of equity, "K d " is the cost of debt and "T" is the marginal tax rate.

Internal Rate of Return (IRR)
The internal rate of return (IRR) or rate of return that is the type of update or discount that nullifies the NPV. Hence, it can also be called the marginal efficiency of capital. The rate is internal because it does not depend on factors exogenous to the investment.
In this way, the IRR is calculated by obtaining the rate, r, which meets the following equation [60,65]: Only those projects that meet the "feasibility" condition will be viable: r > i, where i is the interest rate that corresponds to the "cost of capital". The higher r, the better the investment.
The calculation is usually laborious unless some computer program is used. Although it is not difficult to do it with the help of a calculator and following an iterative process of testing until you reach the solution.
There has been a lot of controversy about which method is preferable: the NPV or the IRR. However, more than a preference problem, the interesting thing is to find the reconciliation of both methods, if possible, in the event that they can reach different results when we compare several projects.

Payback Period-PBP
The Recovery Period is the number of time periods it will take for the investment to recover with the cash flows generated by the project. If call K the investment made and C T the cash flows in each period, the recovery period, t, will be the one that enforces the expressions.

Index Benefit Average Cost-IBmC
The Average Benefit Cost Index discriminates the lower risk investments associated with the requested investment [60]. The equation is: where "K" is the investment made. Table 7 shows the steps taken to disassemble the luminaires, as well as the respective weights for each material present in them. In Figure 3, olive seed (a) and luminaire residues (b) after pre-treatment are shown. Table 7. Measurement by the weight of the Syntra luminary decompressed.

Designation Mass Fraction (g) Picture
Closed Public Light 8624.1 difficult to do it with the help of a calculator and following an iterative process of testing until you reach the solution. There has been a lot of controversy about which method is preferable: the NPV or the IRR. However, more than a preference problem, the interesting thing is to find the reconciliation of both methods, if possible, in the event that they can reach different results when we compare several projects.

Payback Period-PBP
The Recovery Period is the number of time periods it will take for the investment to recover with the cash flows generated by the project. If call K the investment made and CT the cash flows in each period, the recovery period, t, will be the one that enforces the expressions.

Index Benefit Average Cost-IBmC
The Average Benefit Cost Index discriminates the lower risk investments associated with the requested investment [60]. The equation is: where "K" is the investment made. Table 7 shows the steps taken to disassemble the luminaires, as well as the respective weights for each material present in them. In Figure 3, olive seed (a) and luminaire residues (b) after pretreatment are shown.  The quality of the producer gas is affected by fuel characteristics (composition, size, and moisture content). Compared with great-sized fuels, reduced-size fuels produce a better-quality producer gas. However, smaller sizes have a tendency to produce a higher pressure loss inside of the reactor, due to a lower bed porosity. Moisture content in the feedstock also affects the quality of the producer gas. Fuels with lower moisture content yield better producer gas quality. Table 8 presents the proximate, ultimate and calorific analysis [66]. The results from LHV of olive seed is similar to another studies [67,68] and also typical of dry pellet wood biomass [69]. The LHV obtained in the case of the polymeric fuel, 41.8 MJ/kg, was double the value of olive seed, having results similar to those reported by other studies [70]; in fact, the amount of carbon present in polymeric fuel is responsible for the high energy value [71]. This observation leads us to think about the interesting use of these materials as fuel with high energy The quality of the producer gas is affected by fuel characteristics (composition, size, and moisture content). Compared with great-sized fuels, reduced-size fuels produce a better-quality producer gas. However, smaller sizes have a tendency to produce a higher pressure loss inside of the reactor, due to a lower bed porosity. Moisture content in the feedstock also affects the quality of the producer gas. Fuels with lower moisture content yield better producer gas quality. Table 8 presents the proximate, ultimate and calorific analysis [66]. The results from LHV of olive seed is similar to another studies [67,68] and also typical of dry pellet wood biomass [69]. The LHV obtained in the case of the polymeric fuel, 41.8 MJ/kg, was double the value of olive seed, having results similar to those reported by other studies [70]; in fact, the amount of carbon present in polymeric fuel is responsible for the high energy value [71]. This observation leads us to think about the interesting use of these materials as fuel with high energy power. On the other hand, it is interesting to note that plastic waste has a high energy, which can affect equipment that is not well designed for it. Due to its high LHV, to avoid damage to the equipment and following the manufacturer's instructions, the co-gasification process is used for waste with a high calorific value.

Analysis of the Syngas Produced
The gasification parameters and results obtained for the tests carried out are presented in Table 9. The results demonstrate the feasibility of this type of reactor to gasify the studied fuels. In fact, hydrogen, carbon monoxide and methane are the main gases responsible for the LHV of synthesis gas. After a first analysis of the results obtained, it is possible to demonstrate that the gas produced contains a greater quantity of carbon monoxide than hydrogen. This situation occurred in all experiments of the mixtures tested. A lesser percentage of methane appears in the syngas. The high percentage of Nitrogen is because co-gasification has been carried out with atmospheric air [72].
For the blank test, accomplished only with olive seeds, the results present an expected rich syngas in the first analysis. In water gas shift reaction, CO and H 2 O are consumed, whereas CO 2 and H 2 are synthesized. The consumption rate is higher than the formation rate for CO, especially at higher temperatures [73], because CO is consumed to form hydrogen and methane in the water-gas shift and methanation reactions: CO + H 2 O <-> CO 2 + H 2 (water-gas shift) 2CO + 2H 2 <-> CH 4 + CO 2 (Methane reforming) The amount of H 2 is higher than the other experiments with polymeric mixtures. This observation is due to the relatively high experimental temperatures during the experiments. Besides, the H 2 concentration increase may also be related to the promotion of tar-reforming reactions [74,75]. During the gas chromatographic analysis, small amounts light hydrocarbons, such as C 2 H 4 and C 2 H 6 , were observed. Note that, under similar experimental conditions, for the non-catalytic gasification of olive seed, higher temperatures increased the formation of hydrogen and light hydrocarbons such as methane [76]. The cracking of tars can contribute to the increase in the CH 4 and to the consumption of CO 2 ; however, both can be consumed through the enhancement of methane reforming, in the gasification process with the rise in temperature [77]. This behavior may be the reason for the decreasing tendency observed in the values of CO 2 concentration, compared with the other experiments. Olive seed gasification provides a gas with a high energy density (5.3 MJ/m 3 ) and similar properties than wood gas. However, our obtained values on LHV are lower than those found by Borello et al. [78], but higher than those reported by Vera et al. [79].
An interesting aspect of the presented results is that the addition of 10% plastic to the olive seeds leads to an increase in the calorific value of the syngas regarding the experiences of 100% olive seed and 20% plastic, because of the capitalization in the production of hydrocarbons and mainly the methane [80]. The increase in the temperature, namely the gasification temperature, is the responsibility of the greater presence of volatiles that the plastic bring fuel [81]. However, the temperature in the 10% experiment (708 • C) is not so high compared with the additional experiments; this aspect is related to the literature and indicates that, as the gasification temperature increases, the concentration of the resulting hydrogen and carbon conversion efficiency increases [25]. Meanwhile, the content of CH 4 increased slightly with the decrease in temperature. The main reason for the decrease in CH 4 content in the product gas is the increasing proportion of gasification gas and the decreasing proportion of pyrolysis gas [82]. Another reason was attributed to the thermal decomposition of methane at a temperature greater than 700 • C [83]. On the one hand, the produced tars were further cracked into gas at the higher temperature. The increase in temperature can rapidly activate the carbon atom and breakdown the carbon chain in the aromatic ring, reacting with the CO 2 to produce gas [84]. This aspect can be observed in the 20% plastic experiment, which has a smaller amount of hydrocarbons in gas produced and also a smaller amount of tars, compared to the 10% plastic experiment.
The H 2 concentration is similar in all the experiments performed, and the above result indicates that the concentration of H 2 is not only dependent on the temperature but also on the type of feedstock and gasifying media. The main reason for the increment of H 2 concentration is supposed to be the result of the chemical breakdown (thermal cracking) of heavy hydrocarbons, which favors molar fraction of the permanent gases, such as H 2 and CO, at an elevated temperature. What happens with the experiments with 20% plastic agrees with the experiments conducted by Xiao et al., Kim et al. and Cho et al. [85][86][87].
The increase in polymeric residue favors an increase in temperature. As the reactor temperature increased, concentrations of H 2 and CO increased. The reason for the increase in the concentrations of H 2 and CO can be mainly explained by Le Chatelier's principle. According to this principle, the increase in temperature favors not only the reactants in exothermic reactions, but also the products in endothermic reactions [88]. Therefore, the endothermic reactions in gasification, such as the water-gas shift reaction and Boudouard reaction, will contribute to increasing concentrations of H 2 and CO at higher temperatures. With increasing temperature, however, the concentration of hydrocarbons and tars content decreased in producer gas, due to active thermal cracking. These aspects lead to a decrease in LHV. The very low tar content and the high LHV (5 MJ/m 3 ) of the producer gas obtained in the experiments would enable it to be used as a fuel for devices that require clean gas, such as internal combustion engines.

Energy Analysis
The Gasifier-Generator equipment was running for 2 h (120 min), even though, for the first twenty minutes, it was running in torch mode without connecting the 3-kW motor, connected to the alternator (see Supplementary Materials). With the values of the electrical energy produced by the alternator, we can determine the kWh/kg, that is, the energy produced per kilogram of agricultural biomass and plastics mixture. We can calculate the efficiency of the gasifier from the average volumetric flow of air, with an anemometer. For the calculation of syngas production during gasification tests, an anemometer is used that is connected to the air inlet. Moreover, with the quality of the syngas produced, it is possible to obtain the amount of N 2 present. Another important factor is related to the amount of N 2 present in the air. The data are applied in Formula (2), and the is volume calculated.
where: V syngas volume of syn-gas produced Nm 3 /h; V air is the volume of air introduced in Nm 3 /h; 0.781 is the percentage of nitrogen in the air; N 2 is the percentage of nitrogen in the syngas. Taking advantage of the fact of calculating the volume of synthesis gas, the equivalence ratio was calculated. The equivalence ratio is commonly used to quantitatively indicate whether a fuel oxidant mixture is rich, lean, or stoichiometric. The equivalence relationship is defined as: where: ER is the equivalence relation; (A/F) stoic is the ratio of air/fuel mass under stoichiometric conditions; (A/F) is the same mass ratio but under the experimental conditions that were adopted. Therefore, for fuel-rich mixtures, ER > 1, and for lean fuel, ER < 1, and for stoichiometric mixture, ER = 1.
Gasifier efficiency, total efficiency and engine efficiency are calculated as follows [89]: η eng = ((η tot × η gen )/η gas ) (10) where: η gas is the efficiency of the reactor-gasifier; LHV syngas is the lowest heating value of the syngas, in MJ/m 3 ; m fuel is the fuel mass used during the test, in kg; η tot is the total efficiency; LHV fuel is the lower heating value of the fuel or inlet mixture, in MJ/kg. E el [kWh] is the electrical energy generated; η eng is the motor efficiency (0.3); η gen is the efficiency of the generator (0.8).

Results of the Technical-Economic and Financial Analysis
Next, the results of the technical-economic-financial viability analysis obtained for the three cases analyzed will be presented [90].

Results of the economic-financial analysis (100% olive-pellet)
The results for a mixture introduced into the gasifier formed by 100% olive biomass and applying Formulas (6), (8)−(10) are shown in Table 10. Therefore, from Formula (9) obtains the electrical energy produced for one hour 8.65 kWh/h, totaling the total annual energy produced at 31,145.40 kWh/year and the income from annual energy sales, according to which the expected annual operating hours it will be 6229.00 €/year.
With this power, one will have to install a 10-KW gasifier, so the necessary investment (K) according to Table 10 will amount to 12,000.00 € (applying a 23% VAT in Portugal); the total investment would be 14,760.00 euros. On the other hand, this type of clean energy production equipment in Portugal is 65% subsidized with POSEUR Aid [91], which means that, for an economic study with 10 years of useful life, a subsidy of € 780.00/year is needed.
On the other hand, it will have operating expenses of around 5% of the investment costs (600 €/year), along with personal costs for the supervision of the Gasifier, which amounts to 3150.00 €/year, in addition to the costs of purchase of olive biomass that rises to 561.60 €/year. The results are shown in Table 11. For a mixture introduced into the gasifier consisting of 90% olive biomass and 10% plastic applying Formulas (6), (8)-(10), the results are presented in Table 12. Therefore, based on Formula (9), it will obtain the electrical energy produced for one hour (10.79 kWh/h), and the total annual energy produced at 38,835.08 kWh/year. We expected annual energy sales, of 7767.02 €/year.
With this power, we will have to install a 15-KW gasifier, so the necessary investment (K) according to Table 10 will amount to EUR 18,000.00 (applying a VAT of 23% in Portugal) the total investment would be EUR 22,140.00. On the other hand, this type of clean energy production equipment in Portugal is subsidized with POSEUR Aid by 65%. Which means, for an economic study with 10 years of useful life, a subsidy of 1170.00 €/year. In addition, it will avoid having to send plastic WEEE waste to the landfill with a saving of 125.19 €/year according to Tables 10 and 13. On the other hand, it will have operating expenses of around 5% of investment costs (900 €/year), along with personal costs, which amounts to 3150 €/year, in addition to the purchase costs of olive biomass that rise to 670.68 €/year. The results are shown in Table 13.

Results Economic-Financial Analysis (80% Olive and 20% Plastic)
Finally, the results for the fuel mixture of 80% olive biomass and 20% plastic applying formulas, the results obtained are presented in Table 14. Therefore, it obtains the electrical energy produced for one hour (7.90 kWh/h), totaling the total annual energy produced at 28.430.53 kWh/year and the income from annual energy sales, according to which, the expected annual operating hours it will be 5686.11 €/year.
With this power, one will have to install a 10-KW gasifier, so the necessary investment (K) according to Table 10 will amount to EUR 12,000.00 (applying a 23% VAT in Portugal); the total investment would be 14,760.00 euros. On the other hand, this type of clean energy production equipment in Portugal is subsidized with POSEUR Aid by 65%, which means, for an economic study with 10 years of useful life, a subsidy of 780.00 €/year. In addition, it will avoid having to send plastic WEEE waste to the landfill with a saving of 181.44 €/year according to Tables 9 and 14. On the other hand, it will have operating expenses of around 5% of investment costs (600 €/year), along with personal costs, which amounts to 3150 €/year, in addition to the purchase costs of olive biomass that rise to 432.00 €/year. The results are shown in Table 15.

Discussion
The combined gasification of agricultural biomass waste from the olive seed and industrial plastic waste (basically plastic from luminaires (WEEE), in a mass portion of between 10 and 20%, has been shown to be energetically valorized in a very efficient way, through the production of sintered gas or syngas. The synchronous gas produced in the reactor has a synthesizing calorific value LHV, which is interesting enough to consider the exploitation of this valuable resource, for its industrial exploitation through generation as an alternative form of energy in appropriate rural settings [92].
The results of the economic analysis indicate that there is viability in the construction of a gasification unit [93]. The HLV of WEEE plastics showed good prospects for converting this fuel into energy. For thermochemical energy conversion processes, due to the specificity of the biomass, it is not possible to introduce it only into the reactor. The highly volatile matter makes the plastics of the luminaires analyzed as WEEE more easily not volatilized during pyrolysis and gasification, and lower volatile matter values reduce LHV. More adjacent energy is required for gasification reactions. The high ash content makes WEEE prone to slag in thermochemical conversion systems. As such, downdraft gasification is recommended as this is reported at lower grate temperatures, reducing ash slag trends. The results of this study are perfectly aligned with the results of similar studies in which the joint gasification of biomass and polyethene (PE) residues was observed, where the results showed a synergistic effect on the gas and tar yields when the PE was especially fed with a maximum PE content of 20%. For higher PE content, total gas and H 2 decreased because more chain hydrocarbons with a relatively large molecular size derived from PE volatiles are more difficult to break than oxygenates from biomass pyrolysis [22,94].
The economic viability of a gasification unit depends on the availability of waste and the size of the unit. In fact, the units with the highest energy generation require a greater quantity of raw material that may not be available in the region, which implies waste transportation costs over considerable distances. This is the reason why gasification systems that use the biphasic combination of agricultural and/or forest resources and industrial wastes from sources close to electricity production centers with gasifiers are especially interesting to favor the circular economy of regions with the ability to supply both resources [66,95]. Table 16 shows the comparative results of the economic analysis carried out for the three cases studied. The results in Table 16 show that the best economic results in all economic indicators are for a gasifier that runs on 10% WEEE fuel with 90% olive biomass. In this more favorable case, it would have to supply the reactor with a total of 2484 kg/year of plastic waste (2.48 t/year). On the other hand, the total amount potentially available of the Syntra model luminaire is 35,593 luminaires in the Alto Alentejo region. (Figure 1 and Table 1). Thus, the total plastic coming from the substitutions of this luminaire in the entire Alto Alentejo area would total 63,921.46 Kg. Therefore, only the plastic recovered from the Syntra model luminaires in the Alto Alentejo Region would allow having guaranteed plastic fuel for 25 years. Taking into account that the biomass of the olive tree is close to the production centers, it would guarantee the availability of fuel in the long term.
Indeed, the power of the production equipment has been adjusted to the total power that we are going to obtain based on the energy characteristics of the synthesis gas produced. This allows the equipment to adjust to the power we expect to produce and therefore reduces the "K" investment costs, making the solution and equipment dimensions more in line with the expected production reality. We have observed that the gas with the highest energy quality, for the reasons argued in that section, results in the proportion of 10% of plastic. The consequences are that the total electrical energy that we hope to produce is also for this higher proportion of approximately 10.79 kWh/h, or approximately 38,835.08 kWh/year, which makes us have an annual income of € 7767.02/year, compared with 8.65 kWh/h, and 31,145.40 kWh/year in the case of 100% olive biomass. Additionally, 7.90 kWh/h, and 28,430.53 kWh/year produce 20% plastic, despite having to make a slightly greater investment in a 15-kW gasifier, 5 kW more in the case of 10% plastic. Then, we observe that adding the plastic fraction in a moderate proportion (10%) produces a more energetic synergistic gas with more hydrogen, capable of producing more electrical energy and producing numbers in the form of better economic indicators. In particular, the VPN. However, when the plastic fraction exceeds 10% and we are at 20%, the energy quality of syngas decreases as we analyze in Section 3.2 (an analysis of the syngas produced). It is for this reason that, in order to achieve a better performance of the equipment as a whole, a gasifier with more power has been chosen in the case of 10% plastic, as we expect a higher annual energy production. However, we have also carried out the simulation for a 10-kW gasifier unit, (K Investment (EUR 12,000)), for 10% plastic, observing that even the economic indicators improve even more obtaining a 4-year amortization with an NPV of EUR 3096 and a 10-year NPV of EUR 20,829, with an IRR 33.45% and an IBmC of 0.31. However, it would be advisable to make a design for 15 kW in anticipation of future expansions of the production plant, although the investment is higher and the indicators are slightly lower On the other hand, from the environmental point of view, the establishment of a gasification unit for WEEE will help to reduce the environmental impact of its deposition causes. Compared to traditional combustion technology [96], gasification contributes to less atmospheric emissions produced per kWh; in addition, as mentioned above, increasing the efficiency of a gasification system will also have an environmental impact [97], reducing the amount of lignocellulosic biomass used [98]. However, the higher efficiencies attributed to gasification systems usually come with higher initial capital requirements compared to combustion technology [99]; but, on the other hand, gasification systems will save on operating costs while reducing greenhouse gas emissions, helping the Portuguese government meet increasingly stringent EU pollution standards [100,101].

Conclusions
This work allows us to confirm that there is great potential for energy recovery through the gasification of waste from plastic products of industrial origin, mixed with agricultural biomass. In particular, of plastics of electrical and electronic equipment (WEEE or WEEE) and in particular of the plastic waste of obsolete luminaires dismantled in vials. By mixing these plastics with agricultural biomass, in a suitable proportion, it is possible to obtain a highly energetic synthesis gas for gasification, and then for its transformation into electrical energy.
Gasification products show that WEEE plastics have great gasification potential along with agricultural biomass and, in particular, pellets from biomass residues from olive milling. The fundamental question is to be able to find the exact proportion that allows for obtaining the best results from the energy, economic and environmental points of view. With the correct combination of 90% of agricultural biomass from the olive seed and 10% of plastic from the dismantling of luminaires on a large scale, in the Alto Alentejo area, it has been able to find a fuel that produced rich synthesis gas, with more than 5 MJ/m 3 and a low amount of coal and tar production. Both coal and tar can be reintroduced into the fuel and gasified again.
This work also demonstrates the technical and economic feasibility of using mixtures of industrial plastic waste and biomass of agricultural origin, as fuel for the production of synthesis gas very rich in hydrogen with a very interesting energy value for use in electricity generation by energy transformation and for industrial use.
This work demonstrates the technical feasibility and economic profitability of transforming this type of industrial plastic waste mixed with agricultural biomass into electrical energy. In particular, for a mixture of 10% plastics with 90% olive biomass, the best results were obtained from the technical, energy and economic point of view, according to the analysis of the analyzed indicators. Likewise, the economic-financial viability for the installation of medium-sized gasification equipment (below 15 kW) with amortizations of less than 5 years with returns of around 20% that would justify the investment made was verified.
This study demonstrates the potential that the gasification of the mixture of biomass resources of agricultural origin and industrial plastic waste could have, as future technology and source of clean and renewable energy production, in strategies aimed at the valorization of agricultural, forestry, industrial and municipal residues in environments close to production centers and within the framework of the strategies promoted by the EU oriented towards the circular economy and the approach of the life cycle of agricultural and industrial products.
Supplementary Materials: The following are available online at http://www.mdpi.com/2076-3417/10/13/4601/s1, Gasification practical process, Video S1: Gasification WEEE Plastic-Biomass Olive. Funding: This research received no external funding; however, in this work, PhD Hermoso-Orzáez Manuel Jesús participated as a researcher in the external research at the Center Instituto Politécnico de Portalegre (Portugal) within the ECOCIR international project -project of cross-border cooperation for the introduction of the ecological and circular economy through the prevention of improvement of recycling, the management and valorization of residues in the regions Extremadura and Alentejo.-Interreg Spain-Portugal supervised by PhD Brito Paulo, with the aim of looking for energy recovery of plastic waste and rubber from luminaire waste that serve as a reference for recuperation waste from the massive LED replacements that are expected to be carried out in the short and medium-term in the Alentejo and Extremadura area.