Environmental Technology ISSN: 0959-3330 (Print) 1479-487X (Online) Journal homepage: http://www.tandfonline.com/loi/tent20 Fine tuning of process parameters for improving briquette production from palm kernel shell gasification waste Alireza Bazargan, Sarah L. Rough & Gordon McKay To cite this article: Alireza Bazargan, Sarah L. Rough & Gordon McKay (2017): Fine tuning of process parameters for improving briquette production from palm kernel shell gasification waste, Environmental Technology, DOI: 10.1080/09593330.2017.1317835 To link to this article: http://dx.doi.org/10.1080/09593330.2017.1317835 Accepted author version posted online: 12 Apr 2017. Published online: 05 May 2017. Submit your article to this journal Article views: 5 View related articles View Crossmark data Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=tent20 Download by: [188.245.130.91] Date: 07 May 2017, At: 14:03 ENVIRONMENTAL TECHNOLOGY, 2017 https://doi.org/10.1080/09593330.2017.1317835 Fine tuning of process parameters for improving briquette production from palm kernel shell gasification waste Alireza Bazargana, Sarah L. Roughb and Gordon McKayc a Department of Civil Engineering, K. N. Toosi University of Technology, Tehran, Iran; bDepartment of Chemical Engineering and Biotechnology, University of Cambridge, Cambridge, UK; cDivision of Sustainable Development, College of Science, Engineering and Technology, Hamad Bin Khalifa University, Qatar Foundation, Doha, Qatar ABSTRACT ARTICLE HISTORY Palm kernel shell biochars (PKSB) ejected as residues from a gasifier have been used for solid fuel briquette production. With this approach, palm kernel shells can be used for energy production twice: first, by producing rich syngas during gasification; second, by compacting the leftover residues from gasification into high calorific value briquettes. Herein, the process parameters for the manufacture of PKSB biomass briquettes via compaction are optimized. Two possible optimum process scenarios are considered. In the first, the compaction speed is increased from 0.5 to 10 mm/s, the compaction pressure is decreased from 80 Pa to 40 MPa, the retention time is reduced from 10 s to zero, and the starch binder content of the briquette is halved from 0.1 to 0.05 kg/kg. With these adjustments, the briquette production rate increases by more than 20fold; hence capital and operational costs can be reduced and the service life of compaction equipment can be increased. The resulting product satisfactorily passes tensile (compressive) crushing strength and impact resistance tests. The second scenario involves reducing the starch weight content to 0.03 kg/kg, while reducing the compaction pressure to a value no lower than 60 MPa. Overall, in both cases, the PKSB biomass briquettes show excellent potential as a solid fuel with calorific values on par with good-quality coal. Received 13 March 2016 Accepted 4 April 2017 KEYWORDS Biocoal; pelletization optimization; ram press; refuse-derived fuel (RDF); gasification residue Abbreviations: CHNS: carbon, hydrogen, nitrogen, sulfur; FFB: fresh fruit bunch(es); HHV: higher heating value [J/kg]; LHV: lower heating value [J/kg]; PKS: palm kernel shell(s); PKSB: palm kernel shell biochar(s); POME: palm oil mill effluent; RDF: refuse-derived fuel; TGA: thermogravimetric analysis Nomenclature Roman D F H diameter of sample [m] maximum force applied to break sample in tensile crushing strength test [N] height of sample [m] Greek s radial tensile crushing strength [N/m2] 1. Introduction The palm fruit contains two different types of oil: palm oil, which comes from the flesh of the fruit (mesocarp); and palm kernel oil, which comes from the seed within the fruit known as the kernel or endocarp. In general, after the fresh fruit bunches are harvested, they must be milled for oil extraction [1,2]. Both liquid and solid wastes are generated in large quantities in palm oil mills. The liquid wastewater CONTACT Alireza Bazargan alirezabazargan@kntu.ac.ir © 2017 Informa UK Limited, trading as Taylor & Francis Group generated from the mills is referred to as palm oil mill effluent (POME). Each ton of palm oil produced requires 5–7.5 tons of water, half of which ends up as the hot acidic brownish colloidal suspension known as POME [1]. The various constituents of POME include cell walls; organelles; short fibers; carbohydrates such as hemicellulose; simple sugars; nitrogenous compounds such as proteins and amino acids; free organic acids; and other organic and mineral components. About 6 wt% of the fresh fruit bunch input ends up as palm kernel shell (PKS) waste [3]. Indonesia is the world’s largest producer of palm oil with production exceeding 20 million tons per year, followed closely by Malaysia. Although it is the 4th most populous country in the world (after China, India, and the US), the land area of Indonesia is slightly less than 2 million square kilometers, making it the 15th largest country by land mass [4]. The island of Sumatra is the largest producer of palm oil in the country (70–80%) followed by recent expansions in Department of Civil Engineering, K. N. Toosi University of Technology, Tehran, Iran 2 A. BAZARGAN ET AL. Borneo; the oldest large-scale plantations were established in the early 1910s. The production of palm oil in the country has increased steadily since the 1990s; there are several years of lag time before planted palms can be harvested for oil and the increase in production seen today is the result of plantation activity in the previous decade. Indonesia currently has approximately 8 million hectares of cultivated plantation area. Production is growing at a rapid pace, and by 2020, Indonesia plans to increase crude palm oil production to 40 million tons per year and add 4 million hectares of plantations to its portfolio [4–8]. Some of the well-known palm oil companies in the country include London Sumatra, Socfindo, and Marihat. Although the government of Indonesia has been successful at generating foreign exchange and promoting the palm oil industry through land concessions, government plantations, and small holder programs, problems such as the loss of tropical rain forests have resulted in serious international concerns. At least half of the current plantations are built on lands which were previously forests. Not only does deforestation lead to the loss of habitat of endangered species, it also leads to air pollution problems such as smog and haze [9]. Eco-sensitive markets such as the EU respond negatively to environmental problems and burdens caused by the palm oil industry. In order to overcome such problems, the Indonesian government has responded by implementing eco-friendly and sustainable practice schemes. The export tax of crude palm oil is an important source of revenue for Indonesia. In 2008, the industry generated more than $12 billion in foreign exchange for the country. More than 3 million people are estimated to be working in the palm oil sector of Indonesia with estimations of 0.4 persons per hectare. The expansion of plantations also facilitates infrastructure developments, such as roads, which the government would otherwise be burdened to provide [9,10]. One process to make the palm industry more environmentally attractive is the use of solid wastes from the process (such as the PKS) for the production of renewable/green energy. For example, prior to this study, the PKS from a palm oil mill in Indonesia was used as feed in a gasifier. High calorific value syngas was produced from the PKS and used to generate electricity using a turbine. Sustainable electricity generation from palm oil biomass wastes has been reviewed elsewhere [11]. After the gasification process, solid PKS residues remain in the form of palm kernel shell biochar (PKSB). This research is concerned with making use of the PKSB and builds on our previous work [12]. These biochars are compacted to form solid fuel pellets/briquettes. In the present study, process conditions are optimized in order to improve the refuse-derived fuel (RDF) production. Soluble starch and water are used as binders to improve briquette strength and quality. Alternatives to using the PKS as fuel are using them as light-weight aggregates in concrete [13], making composite iron-ore pellets [14], producing catalysts for biodiesel production [15], producing aromatic hydrocarbons [16], synthesizing silicon-carbide nanowhiskers [17], and producing activated carbons [18]. 2. Materials and methods 2.1. Feedstock and binder The PKSB, resembling charcoal in texture and color, is in the form of small solid particles in the range of several micrometers to several millimeters. The soluble starch (BDH Laboratory Supplies, UK) is first dissolved and gelatinized in water at 90°C before being mixed with the PKSB. The mixing was done either by hand or by using a planetary mixer (at various speed settings). The method of mixing did not appear to affect the consistency of the paste. 2.2. Characterization Elemental CHNS analysis was performed on the PKSB in order to determine the absolute values of its constituent elements. The CHNS analysis determines the carbon, hydrogen, nitrogen, and sulfur content of a sample simultaneously. For this, the sample is completely combusted, forming CO2, H2O, N2, and SO2, which are subsequently captured, separated by frontal chromatography, and measured by a thermal conductivity probe. The oxygen content cannot be directly measured and is approximated from the difference: O = 100 − (C + H + N + S). (1) Note that Equation (1) does not account for the other elements (such as Ca, Si, and K) within the sample. Nonetheless, although this method for the calculation of the oxygen content is not exact, it is commonplace [19–21]. Thermogravimetric analysis (Perkin Elmer) was employed to observe the PKSB decomposition behavior. In each run, approximately 15 mg of the sample was first heated to 100°C and held for 15 min, followed by heating the sample up to 900°C at a rate of 10°C/min. The weight loss of the sample was recorded in real time via a computer. ENVIRONMENTAL TECHNOLOGY 2.3. Compaction A fully instrumented strain frame (Zwick/Roell, Germany) modified with a load cell (±0.1 N) and displacement transducer (±1 μm) was used for compaction experiments [22]. An estimated 6 g of sample (comprising PKSB, water and starch) was compacted in each experimental run. A stainless steel (316 SS) cylindrical ram fitted with a high-density polyethylene tip (24.9 mm diameter) was used to compact the samples in a stainless steel compaction cell (25.0 mm diameter) to a given applied pressure. The applied pressure was held for a specified duration known as the retention time. The samples were then ejected from the cell by the ram (at a rate of 0.5 mm/s) after removing the base platen. The typical height of each sample was 10–15 mm. 2.4. Quality testing The quality of a briquette/pellet can be measured by various tests, including (but not limited to) tensile crushing strength, impact resistance, and water resistance testing. All the quality tests in this study are performed according to the guidelines and benchmark targets proposed by Richards [23]. An extensive comparison of various briquette qualities produced from biomass is available in the literature [24]. Alternatively, instead of the standards used herein, the more-or-less relevant ASAE/ASABE S269.5 and ASTM D440 – 07 could have been employed. The guideline values proposed by Richards [23] for testing briquette quality are summarized in Table 1. For tensile crushing strength testing, the cylindrical briquette is placed on its side on a stationary stainless steel platen. Using the strain frame setup, another stainless steel platen (40 mm diameter) is screwed onto the ram tip and lowered at a speed of 0.5 mm/s onto the sample. The amount of force applied by the platen on the briquette is recorded via a coupled computer. The radial tensile crushing strength, s, can be determined as follows: s= 2F , pDh 3 from triplicate tests, with range bars indicating the maximum and minimum values. The impact resistance, also known as the shattering resistance or drop resistance, is used to replicate the type of forces the briquette would be subjected to in falls, for example, while being emptied from a truck. In this study, the briquette was dropped from a height of 2 m onto a concrete surface. The drops were repeated until the sample broke into at least two pieces, which were then counted. The water resistance of the samples was measured in order to determine the influence of water (rain and high humidity). Immersion tests were performed, in which the sample was submerged in water at room temperature. After 30 min, the sample was removed and wiped clean of surface water. The sample was weighed and the increase in sample weight was recorded. The water resistance is defined as 100 minus the percentage of water absorbed by the sample [23]. 3. Results and discussion 3.1. Thermogravimetric analysis The decomposition behavior of the PKSB was examined gravimetrically with TGA both under air and inert N2 atmosphere. The TGA profiles are shown in Figure 1, and the data indicate that the remaining ash after the combustion of PKSB is relatively small. Under air, the majority of the mass is lost in the range of 400–520°C. Decomposition under N2 indicates a high amount of fixed carbon, as deduced from the sudden loss of mass at the end of the profile when the inlet gas is switched from N2 to air. Considering the nature of the PKSB, this high proportion of fixed carbon was expected. Since the PKSB has previously undergone gasification, most of the volatile components (2) where F is the force needed to break the sample, D is the sample diameter, and h is the sample height. All plotted data within the manuscript show average values taken Table 1. Guideline values for assessing briquette quality [23]. Parameter Compressive strength Impact resistance Abrasion resistance Water resistance Guideline At least 350 kPa but preferred target value above 375 kPa Impact resistance index of at least 50 More than 95% mass retained on 1/8′′ BS mesh after tumbling test Absorb 5% water (or less) when immersed for 30 min Figure 1. TGA profile of the as-received PKSB under air and inert (N2) atmospheres. Note that in the inert experimental run, the N2 atmosphere is switched to air at 900°C. 4 A. BAZARGAN ET AL. have already been removed and extracted as high calorific value syngas. Hence the as-received material is predominantly composed of fixed carbon. 3.2. Calorific value The calorific value of fuels is an important parameter describing the amount of energy that can be produced from their unit mass. The higher heating value (HHV, also known as the gross calorific value or gross heating value) is indicative of a sample’s calorific value, including the latent heat of its water content. The lower heating value (LHV, also known as the net calorific value) does not include the latent heat of water, and hence is lower than the HHV. Various equations have been developed for estimating the calorific value of samples from their proximate and/or ultimate analysis. In this study, the 11 models displayed in Table 2 were used for calorific value estimation. The ultimate analysis of the PKSB samples showed 81.40 wt% carbon, 1.60 wt% hydrogen, 1.80 wt % nitrogen, and 0.16 wt% sulfur. The oxygen content is calculated at 15.04 wt% from the difference (see Equation (1)). The ash content was 3.0 wt%. The corresponding calorific values as calculated from each model are shown in Table 2. The mean calorific value is 31.29 ± 1.44 MJ/kg. The calorific value of the PKSB is noticeably higher than other biomass samples such as willow tree wood at 20.0 MJ/kg [25], and is on par with the best West Virginia coal samples at 35.66 MJ/kg [26]. The high calorific value and the small ash content hence make PKSB an excellent candidate to be used as solid fuel. which makes combustion in boilers more efficient, reduced handling and storage costs, reduced transportation costs, and improved stability and durability [34– 36]. A laboratory process was initially established whereby briquettes could be formed from the PKSB. The process conditions were: addition of 10 wt% starch binder, ram compaction speed of 0.5 mm/s, final applied compaction pressure of 80 MPa, and retention time of 10 s. Even though the resulting briquettes have excellent strength (tensile crushing strength exceeding 800 kN/m2) and favorable impact resistance, the process parameters do not allow for practical production of PKSB briquettes due to excessive costs. Most noticeably, the ram speed is too low and the retention time is too long for the process to be economical. Assuming 30 mm of ram displacement is needed for the PKSB to reach its target compaction pressure, the current laboratory process will take at least 70 s per briquette (including the retention time, but not accounting for the time required for ram retreat and briquette ejection after compaction). This means the production of approximately 50 briquettes per hour. In order to improve production efficiency, the process parameters need to be modified. Meanwhile, the quality of the product should not fall below the target levels proposed by Richards [23]. Since the current strength of the laboratory PKSB briquettes exceeds the benchmark target by more than 100%, there is scope to manipulate the process parameters. Hence, the aim of the current research is to discern to what extent the process parameters can be modified while keeping the briquette quality above benchmark target levels. 3.3. Initial briquetting process conditions Successful briquetting of the PKSB affords the added advantages of increased energy density (J/m3 fuel), Table 2. Calorific value estimation of PKSB. Calculated calorific value (MJ/kg) Model for HHV estimation Reference HHV = 0.328 C + 1.419 H + 0.0928 S HHV = −3.147 + 0.468 C HHV = −2.907 + 0.491 C − 0.261 H HHV = −3.393 + 0.507 C − 0.341 H + 0.067 N HHV = −5.29 + 0.493 C + 5.052/H HHV = 0.336 C + 1.44 H + 0.105 S − 0.139 O HHV = 0.461 C + 1.443 H + 0.188 S + 0.105 Ash − 11.986 HHV = 0.605 C + 1.352 H + 0.84 N + 0.321 S + 0.275 Ash − 26.29 HHV = 0.2949 C + 0.825 H HHV = 0.00522 C2 − 0.319 C − 1.647 H + 0.0386 CH + 0.133 N + 21.028 HHV = 0.00187 C2 − 0.144 C − 2.820 H + 0.0683 CH + 0.129 N + 20.147 [27] [28] [28] [28] 28.98 34.95 36.64 37.45 [28] [29] 38.00 29.42 [30] 28.18 [31] 27.51 [32] [33] 25.32 32.28 [33] 25.43 Figure 2. The influence of retention time on PKSB briquette tensile crushing strength. PKSB:water:starch weight ratio, compaction speed, and final applied compaction pressure fixed at 70:20:10, 0.5 mm/s and 80 MPa, respectively. ENVIRONMENTAL TECHNOLOGY 3.4. Improvement I: decreasing retention time The retention time, also known as the holding time, is the duration at which the final compaction pressure is held before the ram is retracted from the sample. In general, longer retention times allow for better compaction and agglomeration of particles leading to higher strength [37]. On the other hand, by reducing the retention time, the production rate of the briquettes can be increased. The influence of retention time on the tensile crushing strength of the PKSB briquettes is shown in Figure 2. The PKSB:water:starch weight ratio, compaction speed and final applied compaction pressure are kept at 70:20:10, 0.5 mm/s and 80 MPa, respectively. The data show that reducing the retention time has no significant influence on the PKSB briquette tensile crushing strength. These results are in agreement with the work of Li and Liu on wood residues, who concluded that at higher pressures the effect of retention time is negligible, while the retention time can have a considerable effect at lower pressures [38]. Here, a minimum retention time of 0 s would result in an approximate 14% increase in production rate compared to the initial laboratory process, without adversely affecting the tensile crushing strength. 3.5. Improvement II: increasing compaction speed Another process improvement for increasing the briquette production rate is to increase the compaction speed. The compressive strength of PKS briquettes has been shown to have an inverse relationship with compaction speed, meaning that slower compaction leads to stronger briquettes [37]. Nonetheless, the compaction speed must be increased in order to increase the overall production rate and make the process more Figure 3. The influence of compaction speed on PKSB briquette tensile crushing strength. PKSB:water:starch wt ratio, final applied compaction pressure and retention time are fixed at 70:20:10, 80 MPa and 0 s, respectively. 5 economically viable. For the initial laboratory process, increasing the compaction speed from the original 0.5 mm/s to 1, 5, or 10 mm/s, would reduce the production time for one briquette from 70 to 40, 16, or 13 s, respectively. Coupled with the previous improvement of reducing the retention time to 0 s, a 10 mm/s compaction speed would result in the production of a briquette every 3 s. At this rate, approximately 1200 briquettes could be produced per hour. The influence of the compaction speed on the tensile crushing strength of the PKSB briquettes is shown in Figure 3. The PKSB: water:starch weight ratio, final applied compaction pressure, and retention time are 70:20:10, 80 MPa, and 0 s, respectively. Although increasing the compaction speed to 10 mm/s does lead to a 4% decrease in tensile crushing strength, the strength is still above the minimum benchmark target (namely 375 kN/m2) [23]. 3.6. Improvement III: reducing starch content The next improvement for the process would be to reduce the amount of starch used as binder. Although starch is cheap and environmentally friendly, using less binder is still economically beneficial in terms of the overall expense of the process. The starch binder content was gradually decreased from 10 to 1 wt% to ascertain the effect on briquette tensile crushing strength. The PKSB wt% was kept constant at 70%. The PKSB:water:starch weight ratio of the different tested samples were 70:20:10, 70:25:5, 70:28:2, and 70:29:1. The data in Figure 4 demonstrate that the starch content can be lowered to 3 wt% without the tensile crushing strength falling beneath the target benchmark. Figure 4. The influence of starch content on PKSB briquette tensile crushing strength. Compaction speed, final applied compaction pressure, and retention time fixed at 10 mm/s, 80 MPa and 0 s, respectively. The dashed line shows the acceptable benchmark tensile crushing strength of 375 kN/m2 [23]. 6 A. BAZARGAN ET AL. The compaction speed, final applied compaction pressure, and retention time are 10 mm/s, 80 MPa, and 0 s, respectively. In general, the starch content exhibits more of an effect on tensile crushing strength than the previously modified process parameters of retention time and compaction speed. 3.7. Improvement IV: decreasing compaction pressure Higher compaction pressures are known to lead to the development of solid bridges within the feed material and to facilitate the diffusion of molecules in between particles leading to higher briquette densities [24]. The increase in density usually follows a linear trend with the logarithm of applied compaction pressure [39]. At relatively high pressures, natural binding components in various materials have been reported to be squeezed out from the particles and form bonds. Pressures of 150 MPa and higher are commonly used in industrial compaction processes [24]. On the other hand, applying higher pressures would require more heavy-duty compaction equipment. In addition to higher capital investment, higher pressures would also result in increased operational costs, due to the energy required to attain a higher pressure, as well as maintenance fees, due to the equipment becoming more prone to wear and breakage at higher pressures. The aim is hence to reduce the compaction pressure as much as possible while retaining the corresponding tensile crushing strength of the formed briquettes above the acceptable benchmark. Figure 5 shows the effect of the final applied compaction pressure on the tensile crushing strength of the Figure 5. The influence of compaction pressure on PKSB briquette tensile crushing strength. Compaction speed and retention time are fixed at 10 mm/s and 0 s, respectively. The PKSB:water: starch weight ratio is 70:25:5 (solid fill) or 70:27:3 (hatched). The dashed line shows the acceptable benchmark tensile crushing strength of 375 kN/m2 [23]. The bordered columns are chosen as optimum operating conditions for further quality testing. samples. Compaction speed and retention time are fixed at 10 mm/s and 0 s, respectively. Since the starch binder was found to have an appreciable effect on the final tensile crushing strength (Section 3.6), two different PKSB:water:starch weight ratio formulations, namely 70:25:5 and 70:27:3, are tested. It is expected that the samples with the higher starch content will require lower compaction pressures in order to achieve a given strength. Figure 5 confirms that an increase in compaction pressure leads to an increase in the tensile crushing strength of the briquettes. The results indicate that compaction pressures as low as 40 and 60 MPa (depending on the starch content) are sufficient to provide compacts with strengths above the benchmark value of 375 kN/m2. In Figure 5, the columns with a bold border indicate two possible optimal scenarios. With 5 wt% starch content, the applied compaction pressure can be reduced to 40 MPa (OPT-1), whereas if only 3 wt% starch is employed, the compaction pressure should be at least 60 MPa (OPT-2). The reasons for choosing either the OPT-1 or OPT-2 conditions will depend on the preference of the manufacturer. If starch is readily accessible, then OPT-1 could be chosen to reduce operational costs of the briquetting plant. However, in remote areas where access to soluble starch is challenging, OPT2 may be a better choice. 3.8. Quality of the final product By applying the previously discussed adjustments, it is possible to reduce the cost of PKSB briquette production. However, it is important to ensure that the resulting products meet the required quality benchmarks. So far, it has been confirmed that the OPT-1 and OPT-2 samples meet the tensile crushing strength requirement. Impact resistance and water resistance tests need to be conducted in order to further assess the quality of the briquettes. The impact resistance of the OTP-1 and OPT-2 briquettes was relatively high. During the impact resistance tests, repeated drops of the briquettes did not lead to any notable fragmentation. Even after 10 drops onto concrete, the briquettes retained more than 95% of their initial weight. An interesting observation was that the impact resistance of the OPT-1 and OPT-2 briquettes was higher than that of the original laboratory process, which employed a higher compaction pressure (80 MPa) and higher starch content (10 wt%). The reason behind this observation is stipulated as follows: as the final compaction pressure is decreased, the apparent density of the briquette also decreases. This means that the adjusted process results in PKSB briquettes that have more void spaces between the constituent particles. In the impact resistance tests, when the samples ENVIRONMENTAL TECHNOLOGY are dropped onto a concrete surface, the samples with more voidage are less brittle since part of the impact energy can be dissipated via the air-filled voids. OPT-1 and OPT-2 samples performed poorly in the water resistance tests and began to disintegrate within seconds of being placed in the water. Complete disintegration of the briquettes had occurred after the full 30 min of the test. Although the briquettes produced under the original laboratory process exhibited poor values of water resistance (lower than 50%), they did not completely disintegrate. Thus, for practical purposes, waterproof packaging of the briquettes is advised. 4. Conclusions Solid fuel briquettes produced from PKSB taken from a gasifier have a markedly high calorific value and low ash content, making them strong candidates to be used as solid fuel. The process parameters for PKSB briquette production have been adjusted in order to improve the manufacturing of PKSB briquettes. Variables such as starch binder content, compaction speed, final applied compaction pressure, and retention time have been fine tuned to find the best combination of process parameters. Ultimately, optimizing the process conditions will afford a decrease in capital, operational, and maintenance costs. In one of the fine tuning scenarios, the briquette production rate could be increased by more than 20 times; hence capital and operational costs could be reduced and the service life of the compaction equipment could be increased. Overall, the PKSB biomass briquettes show excellent potential as a solid fuel with calorific values on par with good-quality coal. The quality of the products was compared to the guidelines proposed by Richards [23]. The final tensile crushing strengths exceed benchmark values, and the impact resistance tests yielded excellent results. However, water resistance test failed due to briquette disintegration. Future studies could be focused on the comparative assessment of the operating conditions in a qualitative manner, either in monetary values or with the use of environmental indicators such as Life Cycle Assessment. Disclosure statement No potential conflict of interest was reported by the authors. References [1] Singh RP, Ibrahim MH, Esa N, et al. 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