Nowadays, the relationship between energy and quality of life is evident [1], therefore, in view of the high consumption of fossil fuels (non-renewable resource), obtaining fuels from new alternative energy sources is necessary, using residual biomass and economically viable urban waste that contribute to mitigate the adverse environmental implications of fossil fuels [2].

In the scientific literature, there is a large number of previous works in which the energy from waste materials, such as lignocellulosic materials, is investigated. These include biodegradable materials such as palm shell residual biomass that is obtained during palm oil extraction process. Oil palm is the most productive oleaginous plant on the planet; one hectare sown produces between 6 and 10 times more oil than other species. Colombia is the fourth largest producer of palm oil in the world and the first in America. Municipal solid waste and other low-grade fuels, as well as high energy concentration materials such as rubber and plastic [3] are also produced.

The residues that negatively stand out by their characteristics include Waste Tyres which represent a serious environmental risk because rubber is an artificial polymer and is not biodegradable [4]. Additionally, when this material agglomerates, it lends itself as a shelter for rodents, insects and other animals that can transmit diseases. Another negative feature is that they generate visual pollution, for example, in Bogotá approximately 2,050 tyres are discarded every day, which end up invading public space. This problem generates approximately little more than 800,000 tyres annually [5]. It is certainly a problem whose impact increases more and more every day, and that is the main reason why tyre rubber is used in this project, because in addition to complying with the necessary energy characteristics, it also contributes to the environment by recycling and reducing the contamination that tyre rubber can generate [6].

Thermochemical transformations are an attractive method to recycle tyres, which have been the subject of renewed interest in multiple investigations. Pyrolysis of tyres can produce oils, coals and gases, in addition to steel wires, which have the potential to be recycled. Liquids (oils) from this process have been found to be a mixture of paraffins, olefins and aromatic compounds that have a high gross calorific value (GCV) of around 41- 44 MJ/kg, which would encourage its use as substitutes for conventional liquid fuels [1, 7-15].

The advantage of pyrolysis when compared to other thermochemical transformation processes is that it allows to obtain three types of products (solid, liquid and gas phase). The liquid product is important because it is easy to store and transport, and therefore it is necessary to establish the best operating conditions to maximize its production while minimizing the environmental impact [16]. It has been found that liquid products from biomass are unstable due to their tendency to react with oxygen and the amount of water present, while the pyrolysis of tyres produces liquid fractions of greater stability [17]. Different sources and systems of energy production have been used by people through the stages of society development [18]. These systems and sources of energy production include, for example, stubble (lignocellulosic material) from rural areas, as well as firewood. Wood is used to generate electricity, and there is a sustainable forestry industry that allows the forest to be renewed and its wood exploited [19].

At a global level, companies and governments are making intense efforts to present biofuels and transgenics as environmentally friendly alternatives that would help combat climate change by replacing a portion of oil consumption [20], resulting in less environmental pollution. It is necessary to implement and investigate on new characteristics of biomass for the benefit of the environment and to advance in the scientific developments required by this area. This investigation is aligned with the above because it represents an alternative that intends to make significant contributions for both lines of research at Universidad Libre: Mechanical Engineering and Alternative Energy Program.

In this work, the characterization of the raw materials was carried out using physicochemical tests, such as proximate and ultimate analyses. Subsequently, these materials were subjected to TGA tests for the determination of the appropriate working temperature. After completing the pyrolysis process, the samples were characterized again by FTIR analysis, percent determination of solid, liquid and gas fractions obtained by the mixtures and measurement of calorific power of the solid phase.

The following scheme was followed to conduct the project.

Figure 1

Tyre samples were obtained from the Biollantas Company with a particle size of 0.5 mm; samples of palm shell were obtained through the National Federation of Biofuels (Fedebiocombustibles). Samples of raw materials were characterized by proximate analysis, ultimate analysis, Fourier Transform Infrared Spectroscopy (FTIR), Thermogravimetric Analysis (TGA) and calorific value. Once this stage was completed, both raw materials were mixed at different proportions in a mechanical stirring system for 20 minutes according to the amount of raw material used, and then subjected to pyrolysis in a 1-liter capacity fixed bed reactor, coupled to a vapor condensation system. In all pyrolysis reactions a constant mass of 200 g, and a constant flow rate of inert gas (CO₂) of 40 cm³/s, were used. This flow rate is required to keep the system free from oxygen for the duration of the test. Mixtures of 20%-80%, 50%-50% and 80%-20% of palm shell and used tyre respectively were used in each pyrolysis run. The liquid product was cooled and weighed to be characterized by FTIR; the solid product was evaluated for its calorific value and the functional groups were determined by FTIR.

The proximate analyses performed on the raw material were carried out according to the ASTM-D 3173, 3174, 3175 and 3172 standards for the determination of humidity, volatile material, ashes and fixed carbon, respectively. The ultimate analysis (determination of the amounts of carbon, hydrogen, oxygen, nitrogen and sulfur) was determined in an elemental microanalyzer Thermo Finnigan Flash 1112 Series, with carbon being the predominant element in raw materials, and sulfur the lowest in percentage.

The calorific value test was performed on a Parr 1341 Plain Oxygen Bomb Calorimeter pump, according to DIN 51900.

Regarding the thermogravimetric analysis (TGA), the appropriate temperature was determined in order to carry out the pyrolysis process in the reactor under a CO₂ atmosphere, with a heating rate of 10 ºC/min in a STA 7200 Thermal Analysis System- HITACHI, according to ASTM D 7582.

In the pyrolysis tests, an air purge was first performed by supplying an amount of CO₂ equivalent to 10 times the volume of the reactor before starting to heat at a heating ramp of 10 °C/min, until it reached a temperature of 500 °C, where it remained for 2 hours. Then the heating was stopped and the inert flow was kept constant until room temperature was reached, to proceed to carefully collect samples of solid and liquid products, establishing by difference the amount of gaseous product formed.

Table 1 shows the values obtained through the proximate analysis; the left side shows the results for palm shell and the right side shows the results for Waste Tyres.

Table 1

Table 2 shows the calorific value data of raw materials and solid products. It is observed that the mixture of Waste tyres at 80% and palm shell at 20% was the one that generated the largest energy power and the mixture in equal proportions of raw materials showed the least calorific power. This is due to the fact that a large percentage of used tyres are preserved, which alone have the highest calorific value of pyrolysis runs in the solid portion.

Table 2

Figures 2 and 3 present the results from a TGA performed to determine the temperature conditions in the reactor.

Hemicellulose is known to be the least stable of the three constituents of lignocellulosic materials [21, 22], the weight loss recorded between 200 °C and 350 °C, is caused by the degradation of this constituent, generating CO, CO₂ and some volatiles. Cellulose decomposes at a temperature ranging between 350 °C - 400 °C [23]. Lignin is the most stable component of lignocellulosic materials and exhibits a very slow decomposition between 200 °C and 500 °C [24], generating a large amount of charred material, between 25% and 37%.

When performing the TGA analysis, it was determined that the appropriate temperature for the 5 pyrolysis runs should range between 450 °C - 550 °C. For palm shell, as shown in Figure 2, starting at 400 °C the raw material begins to lose a considerable amount of weight and after 500 °C the material shows the greatest decomposition.

Figure 2

Hemicellulose is the least stable of the three constituents of lignocellulosic materials, the weight loss recorded between 200 °C and 350 °C, is caused by the degradation of this constituent, generating CO, CO₂ and some volatiles. Cellulose decomposes at a temperature ranging between 350°C - 400 °C [23]. Lignin is the most stable component of lignocellulosic materials and exhibits a very slow decomposition between 200 °C and 500 ° C [24], generating a large amount of charred material between 25% and 37%. The waste tyres decomposition exhibits three main stages of degradation, which are defined by their maximum peaks. The first stage is between 50 °C and 100 °C, water loss occurs in this area. The second stage occurs between 250 °C and 350 °C, at this point the decomposition of volatile compounds as well as low molecular weight hydrocarbons occurs. The third stage is between 350 °C and 520 °C, finally in this stage the decomposition of butadiene occurs, at which point the decomposition of waste tyres is complete, as shown in Figure 3.

According to the studies, 500 °C was determined as the temperature to carry out the pyrolysis runs because at this point the palm shell loses most of its weight fraction and the decomposition of waste tyres is complete.

Figure 3

The weight fractions obtained in each pyrolysis test performed are shown in Table 3.

Table 3 /

The analysis of the solid, liquid and gas fractions is performed based on 200 g of sample that was the standard in each of the pyrolysis runs. After each thermochemical decomposition, the fractions obtained were weighed (see the table above for the corresponding data). The results shown in Table 3 indicate that the liquid fraction obtained is not significantly affected by the percentage in the initial mixture between the palm shell and the tyres used. The largest amount of liquid fraction was obtained from the mixture consisting of 80% waste tyres and 20% palm shell with a value of 18.6%

For the mixture composed of 50% of each component, the amount of liquid generated was only 2.4% lower.

The analysis of functional groups present in the structure of raw materials and liquid and solid products was carried out in IR Prestige 21 SHIMATZU equipment, according to ASTM WK 24875; the results of this analysis are shown in Figures 4 and 5.

Figure 4

Figure 5 /

Table 4

According to the information shown in Figures 4 and 5, from the analysis carried out in Table 4, it can be concluded that the mixtures obtained from the palm shell and waste tyres, are mostly made up of functional groups associated with alkenes, aromatics and ketones.

Aromatic compounds were evidenced by vibration bands from 1520 cm^-1 to 1500 cm^-1 and 1100 cm^-1 to 900 cm^-1, attributed to this class of organic structures. Stretches presented between 1740 cm^-1 to 1720 cm^-1 together with the vibrations manifested at 1300 cm^-1 and 1350 cm^-1 are associated with oxygen-containing keto structures to carboxylic bonds. These results have been reported in similar works [25]

The analysis and formula determination of the mixed compounds was carried out by ultimate analysis of carbon, hydrogen, oxygen, nitrogen and sulfur.

Table 5 and Table 6 show the procedure to obtain the empirical formula of raw materials. The CHONS elemental analysis was carried out by "Calderon Asistencia Técnica Agrícola Ltda" laboratory, to which the raw material samples were supplied.

Table 5 /

The empirical formula is obtained by dividing the weight present in the sample by the atomic mass of the element, to find the molar fraction, which is then normalized to moles of carbon.

Then, the resulting empirical palm shell formula is:

CH _1.6085 N _0.0205 S _0.00156 O _0.7135

Table 6 _/

To determinate the empirical formula of waste tyres, the above procedure is performed, then the resulting empirical formula of the waste tyres is:

CH _1.0441 N _0.0302 S _0.00751 O _0.0396

From the results shown in tables 5 and 6, it can be established that the empirical formulas for the Palm Shell and the Waste tyres are: CH _1,6085 N _0,0205 S _0,00156 O _0,7135 Palm Shell CH _1,0441 N _0,0302 S _0,00751 O _0,0396 Waste Tyres