Luffa cylindrica Slow Pyrolysis and Solar Pyrolysis: Impact of Temperature and Heating Rate on Biochar Properties and Iodine Adsorption Performance

Recently, Luffa cylindrica has been drawing lots of attention in adsorption applications. However, the contaminated biomass needs to be properly disposed. Pyrolysis is a process capable of turning this type of residue into valuable product. Luffa cylindrica pyrolysis produces biochar which has been used as adsorbent for various cationic and organic species. Additionally, the use of solar power to heat the reactor reduces the environmental impact of pyrolysis. In this work, a lab-scale solar pyrolizer was built in a 40-dollar budget. This biomass was previously subjected to slow pyrolysis in an electrical reactor at various temperatures (300, 400, and 500 °C) and heating rates (2, 10, and 20 °C min−1) to assess the influence of these parameters on biochar properties. Further, the Luffa sponge sample was subjected to solar pyrolysis. The characterization methods of TG/DTG, FTIR, SEM, and HHV analysis were employed to assess biochar properties. Biochar adsorption performance was assessed by iodine adsorption experiments. Highest HHV (29.3 MJ kg−1) was obtained for the biochar from the 500 °C, 2 °C min−1 pyrolysis. Maximum iodine adsorption (162.9 mg g−1) was observed on the biochar produced at 400 °C, 2 °C min−1. Solar biochar had a 24.3 MJ kg−1 HHV and a Iodine adsorption of 115.2 mg g−1.


Introduction
Due to the ever-increasing global environmental concern, biomass pyrolysis studies have been broadly developed in recent years. Biomass global production is estimated to be around 100 billion tons per year, which is enough to supply 10 times the world's current energy demand [1,2]. Biomass is capable of providing not only clean renewable energy in the form of heat and fuels but also chemicals and other products [3]. One of the most abundant types of biomass is lignocellulosic biomass, which comes from plant matter and represents a natural renewable chemical feedstock that can be used to produce high value-added products [4]. Lignocellulosic biomass is mainly composed of three major components: cellulose, hemicellulose, and lignin [5]. Other than these three components, biomass also comprises some extractives and inorganic components [6]. In plant walls, cellulose microfibrils are hydrogen-bonded to high molecular weight hemicellulose molecules, while the inner space is packed with lignin linking material [7]. Properties of lignocellulosic biomass are highly dependent on its specific composition and feedstock type [8].
Luffa cylindrica is a herbaceous creeper, whose length may reach 10 m or more, usually cultivated for its 15-150 cm long, oblong cylindrical fruit [9,10]. As a lignocellulosic biomass source, it is mainly composed of cellulose, hemicellulose, and lignin. The chemical composition of Luffa fibers (Table 1) depends on several factors, such as plant origin, weather conditions, soil nature, and others [11]. Sponge gourd fibers have a higher cellulose content than most lignocellulosic biomasses [12].
The Luffa fruit, when harvested early is an edible vegetable, but as it ripens, it becomes fibrous and suitable for use as a cleaning sponge. The pharmacological properties of substances found in Luffa cylindrica have been discussed in the literature [14,15]. Luffa fibers have been studied as a reinforcement material for polymeric matrix composites [11,16]. Luffa sponges have been used as stable support in a catalyst system [17]. Due to its stable porous structure and large surface area, adsorption systems based on the Luffa fibers have been proposed for the removal of dyes, phenols, cationic surfactants, and heavy metal ions [18][19][20][21][22][23]. Many studies have proven the efficiency of Luffa cylindrica biochar on various wastewater adsorption applications. Great results have been obtained on the adsorption of harmful heavy metal ions, such as uranium, copper, thorium, and samarium [24][25][26][27]. Adsorption of organic compounds, like Norfloxacin and Caffeine, has also been studied with encouraging results [28,29].
Although very abundant and promising, lignocellulosic biomass quite often requires some pretreatment to be used in its diverse final applications. Lignocellulosic biomass is processed into solid, liquid, and gaseous products through either biochemical, physicochemical, or thermochemical routes [30,31]. Biochemical conversion allows for the decomposition of biopolymers into sugars using biological agents such as bacteria and enzymes [32]. Physicochemical processing of biomass is typically linked with the esterification and transesterification of bio-oils into high-density bio-fuels, like biodiesel [30]. Thermochemical biomass conversion comprises the processes of liquefaction, gasification, and pyrolysis. Biomass liquefaction utilizes a high-temperature and high-pressure solvent to decompose solid biomass into liquid bio-oil [33]. Gasification is a process in which biomass is converted to produce syngas, a gaseous mixture, mainly composed of methane, hydrogen, carbon monoxide, carbon dioxide, and water vapor [34]. Lastly, there is pyrolysis, the thermochemical process studied in this work.
Pyrolysis is defined as the thermal decomposition that occurs in the absence of oxygen [35]. It has been vastly developed as a promising platform to produce fuels, chemicals, and other products from various types of carbonaceous matter, including lignocellulosic biomass. The heating of plant biomass in an inert atmosphere result in the degradation of biopolymers, generating an organic vapor, which can be condensed to a liquid known as bio-oil [5]. The noncondensable portion of the gaseous products is called pyrolytic gas, which contains, among other products, methane, hydrogen, and carbon monoxide, compounds with high calorific power [36]. The remaining solid carbon-rich material is called biochar. Biochar has been drawing increasing attraction from academia due to its fine properties, diverse applications, and ecological appeal. Besides been used as a solid fuel due to its high carbon content and calorific power, biochar has been widely used in environmental applications such as soil remediation, carbon sequestration, water treatment, and wastewater treatment, given its high surface area, structural stability, and good adsorption [37].
Depending on the heating rate and solid residence time, biomass pyrolysis can be divided into two main types 1 3 including slow (conventional) pyrolysis and fast pyrolysis [38]. Slow pyrolysis, also known as carbonization, is the method traditionally applied to obtain high proportions of charcoal. The thermal decomposition of biomass under a very low heating rate allows sufficient time for repolymerization reaction maximizing solid yields [2]. Therefore, it presents a long residence time, ranging from hours to days, and it does not require fine biomass feedstock particle sizes. Pyrolysis temperature significantly influences the distribution and properties of products. Raising the temperature in pyrolysis affects the biochar yield negatively, as it allows the thermal cracking of heavy hydrocarbon materials, leading to the increase of liquid and gaseous products and a decrease in the biochar yield. Generally, the bio-oil yields reach their peak concentrations at temperatures between 400 and 550 °C and then decline after proceeding with heating due to the dominant secondary cracking reactions that produce noncondensable gases [2]. The growing demand for alternative energy sources is driving not only the investigation of new and renewable alternative feedstocks but also clean production mechanisms. The application of solar energy for biomass pyrolysis is a promising technology for converting biomass to energy, fuels, and other chemical substances with neutral CO 2 emissions. The main challenge for the usage of solar energy as a heat source for chemical processes is the need to increase the solar radiation flux density, given the dilution of terrestrial solar radiation [39]. Optical reflective concentration devices, such as parabolic troughs, parabolic dishes, heliostat fields, and linear Fresnel reflectors have been successfully used to focus incident solar radiation [40].
In this work, a solar dish pyrolizer was built and used to perform pyrolysis on a Luffa cylindrica sample. A parabolic dish is a surface generated by a parabola revolving around its axis. Parabolic dish solar concentrators are systems that make use of this geometry to reflect solar radiation toward the thermal receiver located on the focal point of the dish collector [41]. Therefore, it is necessary to ensure a good radiation reflectivity on the solar collector surface, which is usually achieved by covering the supporting parabolic device with a highly reflective material. BoPET (biaxially-oriented polyethylene terephthalate) is a polymeric film that, when coated with aluminum by vacuum metallization, becomes highly reflective, with a reflectance index of 89.5% [42].
Luffa Cylindrica fibers were also subjected to pyrolysis at various temperatures and heating rates in an electric pyrolizer to investigate their effects on biochar properties. To the best of the author's knowledge, this is the first time Luffa Cylindrica pyrolysis is studied to this extent with emphasis on its derived biochar. Therefore, this work is tailored to be a useful reference for future Luffa Cylindrica pyrolysis and solar pyrolysis researches. Thermogravimetric Analysis, Infrared Spectroscopy, Scanning Electron Microscopy, and High Heating Value determination were applied to the Luffa fibers and all biochar samples. Iodine adsorption experiments adapted from ASTM D4607-14 (Standard Test Method for Determination of Iodine Number of Activated Carbon) were performed to evaluate the impact of pyrolysis parameters on biochar adsorption performance.

Materials
Samples of dried Luffa cylindrica fibers were purchased at a popular market (Mercado Central) in Belo Horizonte, Brazil. Chemicals used in this work (iodine, potassium iodide, hydrochloric acid, sodium thiosulfate, potassium dichromate, sodium carbonate, and starch) were of analytical grade, except for starch, which was food grade.
The materials used in the construction of the solar dish solar pyrolizer were: a 400 mL aluminum can, a standard 60 cm parabolic antenna, a commercial BoPET film, a high temperature resistant black spray paint (Rust-Oleum Black Automotive High Heat 2000 °F), and a commercial 128 kg m −3 ceramic fiber insulator.

Luffa cylindrica Slow Pyrolysis
In each pyrolysis experiment, approximately 5 g of clean and dried Luffa fibers were added to a reactor within a temperature-controlled electrical furnace. For greater temperature precision, the thermocouple was placed entwined in the loofah fibers inside the reactor. The biomass was not milled, since in its gourd form it already presents a very high surface/volume ratio. The reactor, initially at room temperature (25 °C), was heated under a 1 L/min nitrogen gas flow. Once the final pyrolysis temperature was reached, the furnace was promptly turned off and the reactor was removed from it, so it could naturally be cooled down to ambient temperature. Figure 1 exhibits a graphic representation of the used electrical furnace pyrolyzer loaded with a loofah gourd. The obtained biochar masses were weighed and reserved for future analysis. Volatile products were not collected, given that this work focuses solely on solid products.
Three final temperatures (300, 400, and 500 °C) were investigated at three heating rates (2, 10, and 20 °C min −1 ), totaling 9 pyrolysis experiments. To make this work more comprehensive, biochar samples will be called as shown in Table 2 according to their pyrolysis temperature and heating rate.

Luffa cylindrica Solar Pyrolysis
The built parabolic dish solar collector consists of a reused satellite antenna covered with a commercial Aluminized BoPET reflective film with a cylindrical reactor placed at one of the two parabolic focal points. The built parabolic dish solar pyrolizer can be seen in Fig. 2.
The reactor, which was made of a reused 400 ml aluminum can, had one of its plane surfaces facing the parabolic dish. This surface was painted with a commercial high temperature resistant black paint to increase irradiation adsorption. All reactor surfaces but the one facing the reflective parabola were insulated with 25 mm, 128 kg m −3 ceramic fiber insulator. A gas inlet and a gas outlet were drilled onto the reactor. Figure 3 shows a graphic representation of the produced solar reactor.
A solar pyrolysis run was performed on 26/07/2021 in Belo Horizonte (Latitude: −19.5°). The solar elevation angle varied between 45 and 50 degrees during the experiment. The solar collector was aligned to the Sun ray's incidence to maximize energy input. A clean sponge gourd (approximately 5 g) was placed inside the reactor. A 1 L/min nitrogen flux was used to guarantee a non-oxidative atmosphere. A thermocouple was placed entwined in the loofah fibers inside the reactor via the gas outlet to evaluate the inner temperature. The experiment was conducted for one hour of solar irradiance (12:30-13:30 h). The resulting biochar was collected, weighed, and reserved for posterior analysis. Volatile   products were not collected, given that this work focuses solely on solid products.

Characterization Experiments
All Biochar samples and a Luffa gourd sample were subjected to Thermogravimetric analysis (TG/DTG), Fourier Transform Infrared spectroscopy (FTIR), and High Heating Value (HHV) determination. For the Scanning Electron Microscopy (SEM), were chosen for analysis the Luffa cylindrica raw biomass, the biochar with the highest iodine adsorption (B400-2), the biochar with the highest HHV (B500-2), and the solar biochar. Ultimate analysis was also performed on Luffa cylindrica on duplicates in a Eurovector 3100 instrument. Thermogravimetric and Derivative Thermogravimetric analyses were performed in a TA Instruments Q50 equipment. Curves were obtained in the temperature range from 35 to 900 °C with a 10 °C min −1 heating rate under 60 mL min −1 N 2 flux atmosphere.
FTIR absorption spectrums were obtained in the range of 550-4000 cm −1 by ATR (Attenuated Total Reflectance). An ARIS-ZONE ABB Bomem-MB Series equipment with a diamond cell was utilized to acquire 32 reading, 4 cm −1 resolution scans.
HVV determination was performed in accordance with ASTM D5865-19 (Standard Test Method for Gross Calorific Value of Coal and Coke) on a Shimadzu EDX 800 equipment.
SEM Images were obtained by secondary electrons detection in a 40 FEI Quanta FEG 3D equipment with a 15 keV electron beam. The samples were prepared on conductive carbon adhesive tape. Due to the non-conductive character of the biomass and its biochars, it was necessary to apply a carbon sputter coating onto the samples to avoid the surface charging effect caused by the electron beam.

Iodine Adsorption Experiments
Iodine adsorption experiments adapted from the experimental procedure of ASTM D4607-14 (Standard Test Method for Determination of Iodine Number of Activated Carbon) were performed on all samples. A CABOT Norit GAC 1240 activated carbon was tested as well to establish a standard for this methodology. Biochars ground samples (< 149 μm) were dried on a stove for 3 h at 120 °C and cooled down to ambient temperature on a desiccator. Approximately 0.5 g of each biochar was weighed and transferred to an Erlenmeyer, where they were acidified with 10 ml hydrochloric acid 5 wt%. The mixtures were allowed to boil for 30 s on a hot plate to remove any sulfur that could interfere with test results. 100 ml of a standardized 0.05 N iodine solution were added to each flask with subsequent agitation for 30 s. The mixtures were filtered by gravity, while the filtrates were collected into a flask. The first 20 ml of each filtrate were discarded. 20 ml of each filtrate were then titrated in triplicates with a 0.05 N sodium thiosulfate standardized solution until a pale-yellow color was obtained. 2 ml of a 1wt% starch solution were added to the solutions as titration indicator. The titration proceeded until a colorless solution was obtained. The average iodine adsorption in mg/g of each biochar sample was calculated.

Luffa cylindrica Slow Pyrolysis
Thermal degradation had a visible impact on the solid phase as the final pyrolysis temperature increased. Luffa cylindrica's color progressed from a pale yellow to a dark brown at 300 °C, to a deep matte black at 400 °C, and, finally, to a greyish black at 500 °C. Progressive volume shrinkage was observed as well as final temperature increased. Biochar visual aspect was not altered due to variations in the heating rate. Figure 4 compiles images of all biochars obtained at 2 °C min −1 and a raw Luffa cylindrica gourd sample.
After pyrolysis, biochars were weighed and their percentage yield (Fig. 5) was calculated. For every investigated heating rate at 300 °C, around 75% of the initial mass was present in the solid product. Below this temperature, hemicelluloses are the only major component of lignocellulosic biomass to thermally degrade in large proportions [9,12,38]. This result indicates that the pyrolysis of hemicelluloses is a fast kinetics event, which is in agreement with observations reported in previous studies, which describes it as a low activation energy process, due to the weak heteropolymeric glycosidic bonds and the amorphous branched structure of hemicelluloses [43,44].
At the 400 °C final pyrolysis, the greater the heating rate is, the fewer volatile products are produced, resulting in larger biochar yields. This effect is especially observed in B400-20, whose yield was significantly greater than B400-2 and B400-10. In the temperature range of 300-400 °C, two major biomass pyrolysis events occur, degradation of cellulose, the major component of Luffa cylindrica, and the primary reactions of depolymerization of lignin [43,45,46]. Considering that the cellulose/ lignin ratio in Luffa cylindrica is usually around 6:1 or lower [9,11,13] it is safe to say that most of the mass loss in the 300-400 °C range is due to the pyrolysis of cellulose. Furthermore, primary pyrolysis reactions of lignin below 400 °C involve partial polymer depolymerization by the cleavage of ether and ester polymeric bonds, while saturated C-C polymeric bonds are still preserved, resulting in a conservation of up to 60% of lignin initial mass 1 3 [47,48]. Cellulose degradation, therefore, is a kinetically slower event when compared to hemicellulose thermal degradation. Although cellulose monomers are linked with glycosidic bonds similar to those found in hemicelluloses, the organized homopolymeric crystalline structure of cellulose grants it higher stability, increasing pyrolysis activation energy, which influences the reaction kinetics [12,49].
Finally, in 500 °C final pyrolysis temperature experiments, further mass loss due to the formation of volatiles was observed. An especially greater mass loss was observed in B500-2, which indicates that not only hemicellulose and cellulose may have been degraded, but also lignin. At this temperature, some saturated C-C bonds of lignin side-chains crack, resulting in a large production of guaiacols/syringols monomers, that are further degraded due to secondary reactions [50,51]. Biochar B500-10 presented a small mass loss when compared to B400-10. This result suggests that the shorter residence time was not enough for the complete thermal degradation of all three major biopolymers. Biochar B500-20 had a similar mass loss to B500-10, indicating that the pyrolysis of cellulose, which was incomplete on B400-20, happened to a greater extent at 500 °C.

Luffa cylindrica Solar Pyrolysis
The pyrolysis experiment had a maximum temperature of 417 °C, and a final temperature of 402 °C resulting in average heating of 5.5 °C min −1 . The experiment was restricted to 1 h to keep it consistent with the pyrolyzes conducted in the electric furnace reactor. In the furnace, the energy input was cut off after the maximum temperature was reached. In the solar reactor, on the day the pyrolysis run was conducted, the average max temperature was just above 400 °C. Once this temperature was reached and stabilized, the experiment was terminated. Figure 6 shows the temperature evolution over the pyrolysis experiment.
The biochar yield of the solar pyrolysis experiment was of 50.8%. This result indicates that initial hemicellulose content is fully degraded, while cellulose content is partially preserved. Similar to what was observed in the B400-10 and B400-20 biochars, there was not enough time for cellulose to fully degrade. In fact, given the obtained solar biochar yield, it is concluded that the residual cellulose content of the solar biochar is higher than B400-10 and lower than B400-20.

Ultimate Analysis
Ultimate analysis of Luffa cylindrica fibers is available on Table 3.
The high oxygen content indicates that Luffa cylindrica is comprised of highly polar structures, which is beneficial to the ion-exchange adsorption mechanism. However, the adsorption performance of raw biomass is usually poor when compared to activated biochars due to the lower surface area.
A high oxygen content is also detrimental to biomass HHV. Pyrolysis processes lower the ratio of this element, increasing the heat of combustion on biochars. The low nitrogen and sulfur content on the sample is a very important result since the combustion of these elements generates pollutant oxides (NOx and SOx), which have adverse effects on the ozone layer in the troposphere.
A recent study has proposed an ultimate analysis correlation to the higher heating value of lignocellulosic biomass with absolute percentage errors lower than 3% [53]. According to this correlation, which is available in Eq. 1, the HHV of Luffa cylindrica fibers is predicted to be 17.0 MJ kg −1 .

Thermogravimetric Analysis (TG/DTG)
The typical thermogravimetric behavior of Luffa cylindrica fibers is well documented in the literature [9,13,16,[54][55][56]. There are four major observable events on the TG/DTG Luffa curves. First, at low temperatures (ambient-120 °C), the free water content is evaporated. Hemicellulose degradation usually happens between 250 and 320 °C, peaking at around 280 °C. The mass loss related to cellulose pyrolysis is commonly observed between 320 and 360 °C, peaking at around 340 °C. Lastly, mass loss due to lignin pyrolysis is perceived on TG curves as a slow process, which stands up to very high temperatures (900 °C or more). Therefore, TG/ DTG analysis is a great methodology to study the thermal degradation of Luffa cylindrica, since the pyrolysis of each major biopolymer on different biochars can be easily identified. As mass loss events are far more visible on differential analysis, Fig. 7 portrays DTG curves for all produced biochars, as well as for a Luffa sample.
B300 biochars had similar DTG curves. No hemicellulose peak is observable, while the prominent cellulose peak is still well preserved after 300 °C pyrolysis. Signals related to lignin primary and secondary pyrolysis at around 400 and 550 °C are visible on B300 curves [48]. These peaks are not as clear on the Luffa DTG curve, due to the low lignin (1)  1 3 content of this biomass. As other polymers pyrolyze, lignin proportion on the solid phase becomes more significant. The heating rate had a great impact on B400 biochars DTG curves. No cellulose peak is found on the B400-2 curve, which indicates that 2 °C min −1 is a slow enough heating rate to complete cellulose pyrolysis at 400 °C. Lignin content is well preserved at 400 °C since its peaks are prominent on the B400-2 curve. Pyrolysis experiments up to 400 °C with 10 and 20 °C min −1 heating rates could not completely degrade cellulose content. Cellulose on B400-20 is still particularly well preserved.
At the final temperature, 500 °C, the only observable, although small, event on the B500-2 curve is related to lignin secondary pyrolysis. Therefore, pyrolysis to 500 °C at 2 °C min −1 is capable of degrading all Luffa cylindrica major biopolymers to great extent. No cellulose or lignin primary pyrolysis peaks were found on the B500-10 curve. On the other hand, there is still cellulose content on B500-20. Lignin is also preserved on this biochar.
No hemicellulose peak can be found in the solar biochar DTG curve since the pyrolysis experiment was conducted at temperatures up to 417 °C. On the other hand, the characteristic cellulose peak is still present, however quite diminished when compared to the Luffa cylindrica DTG curve. The cellulose content in the solar biochar is between that of the B400-10 and B400-20 biochars, as the cellulose peak in the solar DTG curve is also of intermediate proportions. Lignin content is still well-preserved, as both peaks associated with lignin pyrolysis are present.

Fourier Transform Infrared Spectroscopy
Infrared spectra of Luffa cylindrica fibers are well documented in the literature [9,11,13,54]. As most lignocellulosic biomasses, the main absorbance signals are related to functional groups present on cellulose, hemicellulose, and lignin. FTIR spectra of all produced biochars and a Luffa cylindrica sample are shown in Fig. 8. As was expected, infrared absorbance signals of biopolymers functional groups fade out as pyrolysis temperatures rise. As the heating rate increases, infrared absorption peaks require higher temperatures to vanish or to be softened. Some new peaks Interpretation of FTIR peaks present on the studied spectra is given in Table 4. The wide hydroxyl peak at 3335 cm −1 is softened even after 300 °C pyrolysis, mainly by the decomposition of hemicelluloses. At 400 °C, a high enough temperature to degrade cellulose molecules, the peak is quite soft for all heating rates and for the solar biochar. At 500 °C lignin is partially degraded and no hydroxyl peak can be discerned in any B500 spectra. A very similar trend can be observed on the 2905 cm −1 C-H stretching peak to the carbonization of the solid phase.
At the 1733 cm −1 C = O stretching peak, a different phenomenon is observed: the initial C = O bond content initially decreases on B300 biochars, due to the degradation of hemicelluloses, biopolymers with some carbonyl groupcontaining monomers [12]. However, as the final pyrolysis temperature rises to 400 °C, this peak is rather enhanced due to the formation of C = O bonds. The thermal degradation mechanisms of all three major components of lignocellulosic biomass have carbonyl-containing intermediate products  Asymmetric bridge C-OR-C stretching (cellulose) 1098 Anhydroglucose ring 1027 C-OR stretching (cellulose and hemicellulose) 1 3 [44,45,57]. Once pyrolysis reactions mechanisms come to an end, these intermediate products are further degraded resulting in a decrease in the 1733 cm −1 C = O stretching peak on the B500-2 spectrum. B500-10 and B500-20 spectra still have pronounced 1733 cm −1 C = O stretching peaks. The 1642 cm −1 absorbed water peak is present only on the Luffa spectrum due to the high pyrolysis temperatures biochars were subjected to.
The C = C stretching of aromatic compounds peak at 1597 cm −1 in the Luffa spectrum is mainly due to lignin, the only aromatic major component of lignocellulosic biomass. For all B400 and solar biochar spectra, however, this peak is enhanced indicating the formation of aromatic species. Thermal decomposition of cellulose produces great quantities of furanoses (furfural, hydroxymethylfurfural, etc.), which are aromatic heterocyclic compounds [45]. On B500-2 spectra, this peak is softened once again by the volatilization of furanoses and decomposition of lignin. The C = C stretching of aromatic compounds peak is preserved in B500-10 and B500-20 spectra, evidencing the lower extent of secondary pyrolysis and the preservation of lignin.
Peaks from 1506 to 1325 cm −1 fade as temperature increases due to the degradation of Luffa fibers and the volatilization of products. The 1239 cm −1 C-O-C stretching peak, however, which is correlated to the glycosidic bond on hemicelluloses, is only found at the Luffa spectrum, once hemicelluloses have already been degraded on all biochars.
At 1205 cm −1 , on B400 and solar biochar spectra, a new peak is observed. This peak is assigned in literature to the C-O bond in phenolic structures [58]. This result can be correlated to the primary pyrolysis reactions of lignin, which produce phenolic compounds, like coniferyl alcohol, coniferyl aldehyde, isoeugenol, etc. [48]. This peak is still present on B500-10 and B500-20 spectra, while absent on B500-2. The slower heating rate was enough so lignin primary pyrolysis products could be further degraded or volatilized.
1156, 1098, 1027, and 898 cm −1 peaks are all correlated to hemicellulose and cellulose. Therefore, they are softened on all B300 spectra, due to hemicellulose pyrolysis. They cannot be observed on the B400-2 spectrum, while still being present on the B400-10, B400-20 and solar biochar spectra. In this sense, special attention is given to the strong 1027 cm −1 peak, relative to the C-OR stretching of cellulose molecules.

High Heating Value
High Heating Value is a critical property for chars and other fuels. Once this work focuses on Luffa cylindrica pyrolysis with emphasis on its biochars, it was of utter importance to investigate the energetic potential of these products. Figure 9 presents HVVs for the Luffa cylindrica sample and all produced biochars. HHV of produced biochars is considered relatively high. For comparison, eucalyptus charcoal, one of the most widespread solid fuels in Brazil had an average 31.3 MJ kg −1 high heating value in a recent study [59].
Luffa cylindrica's experimental HHV (17.5 MJ kg −1 ) is very close to the prediction made via the ultimate analysis correlation (17.0 MJ kg −1 ), with an absolute error of only 2.8%. This low HHV is mainly due to its high cellulosehemicellulose/lignin ratio. Calorific values of cellulose and hemicellulose are reported to be 18.60 MJ.kg −1 , while the lignin is 23.26 to 25.58 MJ/kg [60]. It can be observed as a general trend that the greater the pyrolysis temperature is, the greater is also the HHV, which is in agreement with observations reported in previous studies [61,62]. HHV results had also a great correlation with pyrolysis heating rate. Slower heating rates resulted in greater thermal degradation of lignocellulosic biopolymers, producing biochars with higher calorific power.
High heating values of studied biochars greatly agreed with TG and FTIR results. As it was seen on the other characterization methods, B300 biochars were very similar, despite the heating rate difference. B400-2 was the only biochar subjected to 400 °C pyrolysis with no TG and FTIR peaks related to cellulose. Similarly, it had the highest HHV among B400 biochars. B500-2, the biochar with the highest HHV (29.3 MJ kg −1 ), was also the one with the lower pyrolysis solid yield and overall highest thermal degradation. Solar biochar had an intermediate higher heating value (24.3 MJ kg −1 ), which was expected due to the incomplete cellulose and lignin pyrolysis at the final temperature for this pyrolysis run, 402 °C. Scanning Electron Microscopy Figure 10 shows scanning electron micrographs of the surfaces of Luffa cylindrica, B400-2, B500-2, and the solar biochar. B400-2 was chosen for analysis since it demonstrated higher iodine adsorption, while B500-2 was chosen for it is the biochar with the HHV and the lower pyrolysis solid yield.
Luffa cylindrica has a rough solid surface with no visible pores in the micrograph. This surface morphology is not optimal for adsorption systems, since it results in a low surface/volume ratio. The solar biochar, as well as B400-2, presented some cracks on the surface, unveiling their inner porosity, what was crucial for the increase in adsorption performance. B500-2 presented a highly cracked surface due to its pyrolysis temperature. Figure 11 are micrographs of cross-sections of the same samples.
Luffa cylindrica has a compact inner structure with sparce visible porosity. The solar biochar presents tubular inner pores with thick walls. B400-2, which had a higher thermal degradation than the solar biochar, presented the same tubular pores, but with thinner walls. This effect is enhanced in B500-2 due to its higher pyrolysis temperature.

Iodine Adsorption Experiments
ASTM D4607-14 covers the determination of the relative activation level of unused or reactivated carbons by adsorption of iodine from an aqueous solution [63]. It also is a relative indicator of porosity in activated carbon, being used even as an approximation of surface area for some types of activated carbons. As activation processes were not the focus of this study, a simplified version of the ASTM D4607-14 methodology was employed to assess the potential use of

3
Luffa cylindrica biochars in adsorption systems with or without later activation. Figure 12 presents the iodine adsorption performance of all produced biochars. The greatest iodine adsorption, 162.9 mg g −1 , was observed on the B400-2 biochar. For comparison, CABOT Norit GAC 1240 commercial activated carbon had an iodine adsorption result of 878 mg g −1 . These much higher numbers can be explained by the highly optimized activation process which was employed on the commercial char, while Luffa cylindrica biochar was tested untreated.
A similar trend is observed on the Iodine adsorption of biochars produced in 2 and 10 °C min −1 pyrolysis. There is an improvement in adsorption performance when the final pyrolysis temperature is increased from 300 to 400 °C, followed by a subsequent decrease when the temperature is raised to 500 °C. Literature supports that surface area in biochars has a positive relationship with pyrolysis temperature in the range of 300 to 500 °C [64][65][66], which explains the initial adsorption increase. However, as pyrolysis progresses to higher temperatures, surface functional groups are removed by charring reactions, resulting in a negative impact on adsorption performance. The solar biochar had the third best iodine adsorption, 115.2 mg g −1 , which shows that it has a good balance between porosity and surface functional groups.
Another trend observed in iodine adsorption results is the decrease in performance as the pyrolysis heating rate increases. It has been reported in the literature that higher heating rates produce biochars with lower surface areas [67,68]. While lower heating rates allow for sufficient time for diffusion of volatile pyrolysis products, at higher heating rates, these products accumulate on empty spaces, decreasing porosity. Therefore, biochars produced at 20 °C min −1 did not develop good porosity, resulting in poor iodine Fig. 11 Luffa cylindrica and its biochars cross-section micrographs: a Luffa cylindrica (× 10,000); b Solar biochar (× 5000); c B400-2 (× 5000); d b500-2 (× 5000) adsorption performance at lower temperatures. B500-20, however, had iodine adsorption equivalent to other B500 biochars. This result indicates that, at this higher temperature, volatiles accumulated on porous medium have been evaporated, increasing biochar porosity and hence iodine adsorption.

Conclusion
Concentrated solar power is a promsing energy source for biomass pyrolysis, as it lowers the environmental and economic impact of the process, while still being capable of sustaining temperatues high enough to produce biochars with competitive calorific (24.3 MJ kg −1 ) and adsorptive (115.2 mg g −1 -iodine) properties. Studied slow pyrolysis parameters: final temperature, and heating rate showed a significant influence on the properties of Luffa cylindrica biochars, as well as their combustible and adsorptive performances. Greater final temperatures and lower heating rates resulted in higher thermal degradation of lignocellulosic biopolymers, producing biochars with less functional groups. This progressive thermal degradation was observable on TG/DTG results, as well as on FTIR scans, through which even some pyrolysis products. Higher final temperatures and lower heating rates also improved the HHV of studied biochars. The highest HHV (29.32 MJ kg −1 ) was observed for the biochar produced by the 500 °C, 2 °C min −1 pyrolysis experiment. Biochar produced by the 400 °C, 2 °C min −1 pyrolysis experiment had the highest iodine adsorption (162.9 mg g −1 ). Adsorption performance peaked at 400 °C due to the balance of porosity and surface functional groups for biochars produced at 2 and 10 °C min −1 heating rates. Pyrolysis final temperature influenced positively the iodine adsorption of biochars produced at 20 °C min −1 due to the progressive clearance of pores obstructed by volatile products.