Effect of Coffee Grounds and Waste Engine Oil as Additives in Food-Processing Biomass Pellet Combustion

Year 2026, Volume: 29 Issue: 1 , 37 - 49 , 08.03.2026

Ana Neacsu , Daniela Gheorghe

https://doi.org/10.5541/ijot.1809933

https://izlik.org/JA47YR49ZD

Abstract

The purpose of the present research is to quantitatively assess the combustion characteristics of three types of food-processing biomass – pumpkin shells (PS), bean pods (BP), and cherry stalks (CS) – analyzed both as individual feedstocks and in composite mixtures containing 10% spent coffee grounds (SCG) namely: M1SCG (PS10%SCG); M2SCG(BP10%SCG); M3SCG(CS10%SCG) and 10% waste engine oil (WEO): M1WEO (PS10%WEO); M2WEO (BP10%WEO); M3WEO (CS10%WEO). Additionally, a goal of this study was to evaluate the suitability of two ternary blends M4SCG (30%PS+30%BP+30%CS+10%SCG) and M4WEO (30%PS+30%BP+30%CS+10%WEO) for pellet production according to European solid biofuel standards. Calorific values were measured with a model 6200 combustion calorimeter in accordance with ASTM D5865. Nitrogen content was evaluated through the formation of nitric acid, while sulphur content was examined by converting it to sulphates and determining BaSO4 gravimetrically. Ash and moisture content, bulk density (CEN/TS 15103), porosity index, volatile matter (EN ISO 18123:2015), and fixed carbon (ASTM D3172–13) were determined using standardized procedures. Energy density, fuel value index, and combustion efficiency were computed following the established methods reported in the literature.
The combustion and physicochemical characteristics of all individual biomasses and biomass-additive mixtures were found to be in accordance with the European standards set. The two composite mixtures M4SCG and M4WEO demonstrated improved calorific performance along with advantageous density-related attributes. Both M4SCG and M4WEO fulfilled essential quality standards, which suggests their technical feasibility as pelletizable biofuels and their potential as sustainable alternatives to fossil fuels. This study provides the first systematic characterization of pumpkin shells, bean pods, and cherry stalks as solid biofuel feedstocks. The first comparative evaluation of SCG and WEO as fixed 10% additives under identical experimental conditions is performed. By integrating underutilized food-processing residues with waste-derived additives, the results extend current knowledge on biomass pelletization, additive-assisted combustion enhancement, and circular waste-to-energy strategies.

Keywords

Coffee ground , waste engine oil , biomass mixing , biomass pellet , calorific power , waste management

References

• G. Moiceanu, M.-N. Dincă, M. Chițoiu, G. Paraschiv, and O.-D. Cristea, “Romanian biomass pellet market techno-economic analysis,” INMATEH, pp. 882–890, Dec. 2023, doi: 10.35633/inmateh-71-77.
• R. Clodnițchi and A. C. Chinie, “Factors of impact on the evolution of electricity markets from renewable energy sources: a comparison between Romania and Germany,” Management & Marketing, vol. 10, no. 1, pp. 34–52, Jun. 2015, doi: 10.1515/mmcks-2015-0003.
• “DanuP-2-Gas - Sonata Project.” Accessed: Feb. 6, 2025. [Online]. Available: https://dtp.interreg-danube.eu/approved-projects/danup-2-gas
• A. Stenmarck, C. Jensen, T. Quested, and G. Moates, Estimates of European food waste levels. Stockholm: IVL Swedish Environmental Research Institute, 2016.
• A. Dyjakon, T. Noszczyk, and M. Smędzik, “The Influence of Torrefaction Temperature on Hydrophobic Properties of Waste Biomass from Food Processing,” Energies, vol. 12, no. 24, p. 4609, Dec. 2019, doi: 10.3390/en12244609.
• A. Minajeva, A. Jasinskas, R. Domeika, E. Vaiciukevičius, E. Lemanas, and S. Bielski, “The Study of the Faba Bean Waste and Potato Peels Recycling for Pellet Production and Usage for Energy Conversion,” Energies, vol. 14, no. 10, p. 2954, May 2021, doi: 10.3390/en14102954.
• K. Ishii and T. Furuichi, “Influence of moisture content, particle size and forming temperature on productivity and quality of rice straw pellets,” Waste Management, vol. 34, no. 12, pp. 2621–2626, Dec. 2014, doi: 10.1016/j.wasman.2014.08.008.
• M. V. Gil, P. Oulego, M. D. Casal, C. Pevida, J. J. Pis, and F. Rubiera, “Mechanical durability and combustion characteristics of pellets from biomass blends,” Bioresource Technology, vol. 101, no. 22, pp. 8859–8867, Nov. 2010, doi: 10.1016/j.biortech.2010.06.062.
• University of Debrecen, Faculty of Economics, Institute of Statistics and Methodology, Debrecen, Hungary and M. Csipkés, “THE IMPORTANCE OF PUMPKINS AND OIL GOURDS IN ROMANIA,” AUOES, vol. 32, no. 2, pp. 13–22, Dec. 2023, doi: 10.47535/1991AUOES32(2)001.
• R. N. Gavril (Rațu) et al., “Pumpkin and Pumpkin By-Products: A Comprehensive Overview of Phytochemicals, Extraction, Health Benefits, and Food Applications,” Foods, vol. 13, no. 17, p. 2694, Aug. 2024, doi: 10.3390/foods13172694.
• G. Marian et al., “Calorific value of pellets produced from raw material collected from both sides of the river Prut,” JES, vol. 29, no. 4, pp. 126–137, Jan. 2023, doi: 10.52326/jes.utm.2022.29(4).10.
• P.-M. Galan et al., “Assessment of the Geographic Origin of Romanian Common Bean (Phaseolus vulgaris L.) Landraces Using Molecular Markers and Morphological Traits,” Agronomy, vol. 13, no. 11, p. 2820, Nov. 2023, doi: 10.3390/agronomy13112820.
• E. Barcanu, O. Agapie, I. Gherase, B. Tănase, and G. Dobre, “Vicia faba - Past, Present and Future Breeding Perspectives in Romania,” BUASVMCN-HORT, vol. 79, no. 2, pp. 117–120, Nov. 2022, doi: 10.15835/buasvmcn-hort:2022.0017.
• A. Jasinskas, A. Minajeva, E. Šarauskis, K. Romaneckas, R. Kimbirauskienė, and N. Pedišius, “Recycling and utilisation of faba bean harvesting and threshing waste for bioenergy,” Renewable Energy, vol. 162, pp. 257–266, Dec. 2020, doi: 10.1016/j.renene.2020.08.070.
• University of Craiova, Craiova, Romania, G. Mirela Floriana, P. Ana-Maria Camelia, and University of Craiova, Craiova, Romania, “Statistical analysis and forecasts of some financial indicators from the agricultural sector in Romania using the Arima model,” MNMK, vol. 22, no. 2, pp. 251–281, Nov. 2024, doi: 10.52846/MNMK.22.2.08.
• J. Quero-García, A. Iezzoni, J. Puławska, and G. A. Lang, Eds., Cherries: botany, production and uses. Wallingford, Oxfordshire, UK: CABI, 2017.
• M. Corneanu, I. E. Golache, E. Iurea, I. Mineață, and S. Sîrbu, “Assessment of sweet cherry (Prunus Avium L.) Genotypes grown in conditions of Romanian northeastern area,” CTNS, vol. 10, no. 19, pp. 476–481, Jul. 2021, doi: 10.47068/ctns.2021.v10i19.063.
• P. Pradhan, S. M. Mahajani, and A. Arora, “Production and utilization of fuel pellets from biomass: A review,” Fuel Processing Technology, vol. 181, pp. 215–232, Dec. 2018, doi: 10.1016/j.fuproc.2018.09.021.
• X. Cui, J. Yang, Z. Wang, and X. Shi, “Better use of bioenergy: A critical review of co-pelletizing for biofuel manufacturing,” Carbon Capture Science & Technology, vol. 1, Dec. 2021, Art. no. 100005, doi: 10.1016/j.ccst.2021.100005.
• N. Mišljenović, J. Mosbye, R. B. Schüller, O.-I. Lekang, and C. Salas-Bringas, “Physical quality and surface hydration properties of wood based pellets blended with waste vegetable oil,” Fuel Processing Technology, vol. 134, pp. 214–222, Jun. 2015, doi: 10.1016/j.fuproc.2015.01.037.
• B. Emadi, K. L. Iroba, and L. G. Tabil, “Effect of polymer plastic binder on mechanical, storage and combustion characteristics of torrefied and pelletized herbaceous biomass,” Applied Energy, vol. 198, pp. 312–319, Jul. 2017, doi: 10.1016/j.apenergy.2016.12.027.
• T. Li, J. Cheng, R. Huang, J. Zhou, and K. Cen, “Conversion of waste cooking oil to jet biofuel with nickel-based mesoporous zeolite Y catalyst,” Bioresource Technology, vol. 197, pp. 289–294, Dec. 2015, doi: 10.1016/j.biortech.2015.08.115.
• D. A. Iryani, H. Halimatuzzahra, T. Taharuddin, A. Haryanto, W. Hidayat, and U. Hasanudin, “Physicochemical Characterization of Wood Mixed with Coffee Waste Pellet,” IOP Conf. Ser.: Earth Environ. Sci., vol. 1187, no. 1, May 2023, Art. no. 012007, doi: 10.1088/1755-1315/1187/1/012007.
• S. I. Mussatto, E. M. S. Machado, S. Martins, and J. A. Teixeira, “Production, Composition, and Application of Coffee and Its Industrial Residues,” Food Bioprocess Technol, vol. 4, no. 5, pp. 661–672, Jul. 2011, doi: 10.1007/s11947-011-0565-z.
• A. Brunerová et al., “Valorization of Bio-Briquette Fuel by Using Spent Coffee Ground as an External Additive,” Energies, vol. 13, no. 1, p. 54, Dec. 2019, doi: 10.3390/en13010054.
• “Romania Imports of Coffee, tea, mate and spices- 2026 Data 2027 Forecast 1989-2024 Historical”. Accessed: Feb. 19, 2025. [Online]. Available: https://tradingeconomics.com/romania/imports/coffee-tea-mate-spices.
• “Romania Coffee Industry Outlook 2024 - 2028”. Accessed: Feb. 20, 2025. [Online]. Available: https://www.reportlinker.com/clp/country/464/726388.
• K. Johnson, Y. Liu, and M. Lu, “A Review of Recent Advances in Spent Coffee Grounds Upcycle Technologies and Practices,” Front. Chem. Eng., vol. 4, p. 838605, Apr. 2022, doi: 10.3389/fceng.2022.838605.
• P. Sołowiej and M. Neugebauer, “Impact of Coffee Grounds Addition on the Calorific Value of the Selected Biological Materials,” Agricultural Engineering, vol. 20, no. 1, pp. 177–183, Apr. 2016, doi: 10.1515/agriceng-2016-0018.
• E. Bottani, L. Tebaldi, and A. Volpi, “The Role of ICT in Supporting Spent Coffee Grounds Collection and Valorization: A Quantitative Assessment,” Sustainability, vol. 11, no. 23, p. 6572, Nov. 2019, doi: 10.3390/su11236572.
• S. Park et al., “Investigation of agro-byproduct pellet properties and improvement in pellet quality through mixing,” Energy, vol. 190, Jan. 2020, Art. no. 116380, doi: 10.1016/j.energy.2019.116380.
• T. Ciesielczuk, U. Karwaczyńska, and M. Sporek, “The possibility of disposing of spent coffee ground with energy recycling,” J. Ecol. Eng., vol. 16, pp. 133–138, 2015, doi: 10.12911/22998993/59361.
• A. Lisowski et al., “Spent coffee grounds compaction process: Its effects on the strength properties of biofuel pellets,” Renewable Energy, vol. 142, pp. 173–183, Nov. 2019, doi: 10.1016/j.renene.2019.04.114.
• M. A. Kougioumtzis, V. Filippou, A. Rontogianni, E. Karampinis, P. Grammelis, and E. Kakaras, “Valorization of spent coffee ground by mixing with various types of residual biomass for pellet production: evaluation of solid biofuel properties at different mixtures,” Biofuels Bioprod. Bioref., vol. 18, no. 4, pp. 968–989, July 2024, doi: 10.1002/bbb.2646.
• W. Intagun, W. Kanoksilapatham, A. Maden, and B. Nobaew, “Effect of natural additive on pellets physical properties and energy cost,” in 2019 IEEE 2nd International Conference on Renewable Energy and Power Engineering (REPE), Toronto, ON, Canada: IEEE, Nov. 2019, pp. 130–134. doi: 10.1109/REPE48501.2019.9025154.
• S. Park et al., “Performance Optimisation of Fuel Pellets Comprising Pepper Stem and Coffee Grounds through Mixing Ratios and Torrefaction,” Energies, vol. 14, no. 15, p. 4667, Aug. 2021, doi: 10.3390/en14154667.
• J. Trnka, N. Č. Kantová, M. Holubčík, A. Čaja, T. Najser, and J. Najser, “Comparison of energy properties of pellets from shells of different nut species,” BioRes, vol. 18, no. 1, pp. 2137–2145, Jan. 2023, doi: 10.15376/biores.18.1.2137-2145.
• O. Arpa, R. Yumrutas, and A. Demirbas, “Production of diesel-like fuel from waste engine oil by pyrolitic distillation,” Applied Energy, vol. 87, no. 1, pp. 122–127, Jan. 2010, doi: 10.1016/j.apenergy.2009.05.042.
• “Waste and recycling - Environment - European Commission”. Accessed: Feb. 8, 2025. [Online]. Available: https://environment.ec.europa.eu/topics/waste-and-recycling_en
• M. Danışmaz and C. Demirtaş, “Investigation of the Use of Waste Biomass Fuels in the Production of Syngas in a Downdraft Reactor: A CFD Analysis,” Arab J Sci Eng, vol. 50, no. 17, pp. 13975–13987, Sep. 2025, doi: 10.1007/s13369-024-09654-7.
• M. Dani̇şmaz and C. Demi̇Rtaş, “An Experimental Investigation of Clean Syngas Production from Waste Biomass by Gasification Method,” IJCESEN, vol. 10, no. 3, Jul. 2024, doi: 10.22399/ijcesen.361.
• A. Tiktas, H. Gunerhan, and A. Hepbasli, “Single and multigeneration Rankine cycles with aspects of thermodynamical modeling, energy and exergy analyses and optimization: A key review along with novel system description figures,” Energy Conversion and Management: X, vol. 14, May 2022, Art. no. 100199, doi: 10.1016/j.ecmx.2022.100199.
• A. Tiktaş, H. Gunerhan, A. Hepbasli, and E. Açıkkalp, “Exergy-based techno-economic and environmental assessments of a proposed integrated solar powered electricity generation system along with novel prioritization method and performance indices,” Process Safety and Environmental Protection, vol. 178, pp. 396–413, Oct. 2023, doi: 10.1016/j.psep.2023.08.048.
• A. Tiktas, “Introducing a novel wind-driven passive cooling strategy for polar shelters: backed by flow dynamics and irreversibility mapping with exergy analysis,” Energy Conversion and Management, vol. 346, Dec. 2025, Art. no. 120481, doi: 10.1016/j.enconman.2025.120481.
• A. Tiktas, “Optimization-Based Exergoeconomic Assessment of an Ammonia–Water Geothermal Power System with an Elevated Heat Source Temperature,” Energies, vol. 18, no. 19, p. 5195, Sep. 2025, doi: 10.3390/en18195195.
• M. M. Uyar, A. Çıtlak, and A. B. Demirpolat, “Investigation of performance and emission values of new type of fuels obtained by adding MgO nanoparticles to biodiesel fuels produced from waste sunflower and cotton oil,” Industrial Crops and Products, vol. 222, Dec. 2024, Art. no. 119712, doi: 10.1016/j.indcrop.2024.119712.
• M. M. Uyar, A. B. Demirpolat, and H. Arslanoğlu, “Investigation of performance and emission values of biodiesel fuels produced by adding ZnO nanoparticles as additives to waste sunflower and Köhnü grape seed oil,” Colloid Polym Sci, vol. 301, no. 6, pp. 557–567, Jun. 2023, doi: 10.1007/s00396-023-05085-2.
• A. B. Demirpolat, M. M. Uyar, and H. Arslanoğlu, “Biodiesel Fuels Produced from Poppy and Canola Oils, Experimental Investigation of the Performance and Emission Values of the Samples Obtained by Adding New Types of Nanoparticles,” Pet. Chem., vol. 62, no. 4, pp. 433–443, Apr. 2022, doi: 10.1134/S0965544122020190.
• S. Głowacki, W. Tulej, M. Sojak, A. Bryś, and K. Pietrzyk, “Analysis of thermal properties of coffee grounds left over from coffee percolation,” E3S Web Conf., vol. 154, p. 03004, 2020, doi: 10.1051/e3sconf/202015403004.
• D. Gheorghe, and A. Neacsu, “Heat of some plant biomass species for biofuels production,” Rev. Roum. Chim., vol. 64, no. 7, pp. 633–639, Jan. 2019, doi: 10.33224/rrch/2019.64.7.10.
• “Applications,” Parr Instrument Company. Accessed: Sep. 15, 2025. [Online]. Available: https://www.parrinst.com/products/sample-preparation/oxygen-combustion-bombs/applications/
• I. Onukak, I. Mohammed-Dabo, A. Ameh, S. Okoduwa, and O. Fasanya, “Production and Characterization of Biomass Briquettes from Tannery Solid Waste,” Recycling, vol. 2, no. 4, p. 17, Oct. 2017, doi: 10.3390/recycling2040017.
• I. M. Ríos-Badrán, I. Luzardo-Ocampo, J. F. García-Trejo, J. Santos-Cruz, and C. Gutiérrez-Antonio, “Production and characterization of fuel pellets from rice husk and wheat straw,” Renewable Energy, vol. 145, pp. 500–507, Jan. 2020, doi: 10.1016/j.renene.2019.06.048.
• A. Paykani, H. Chehrmonavari, A. Tsolakis, T. Alger, W. F. Northrop, and R. D. Reitz, “Synthesis gas as a fuel for internal combustion engines in transportation,” Progress in Energy and Combustion Science, vol. 90, p. 100995, May 2022, doi: 10.1016/j.pecs.2022.100995.
• A. Neacsu and D. Gheorghe, “Characterization of Some Co-Fired Agricultural by-products for Energetic Use,” J. Mex. Chem. Soc., vol. 66, no. 4, Oct. 2022, doi: 10.29356/jmcs.v66i4.1739.
• J. L. Gardner et al., “Calorimetric evaluation indicates that lignin conversion to advanced biofuels is vital to improving energy yields,” RSC Adv., vol. 5, no. 63, pp. 51092–51101, 2015, doi: 10.1039/C5RA01503K.
• P. Sommersacher, T. Brunner, and I. Obernberger, “Fuel Indexes: A Novel Method for the Evaluation of Relevant Combustion Properties of New Biomass Fuels,” Energy Fuels, vol. 26, no. 1, pp. 380–390, Jan. 2012, doi: 10.1021/ef201282y.
• D. Deka, P. Saikia, and D. Konwer, “Ranking of Fuelwood Species by Fuel Value Index,” Energy Sources, Part A: Recovery, Utilization, and Environmental Effects, vol. 29, no. 16, pp. 1499–1506, Oct. 2007, doi: 10.1080/15567030600820476.
• M. Mierzwa-Hersztek, K. Gondek, M. Jewiarz, and K. Dziedzic, “Assessment of energy parameters of biomass and biochars, leachability of heavy metals and phytotoxicity of their ashes,” J Mater Cycles Waste Manag, vol. 21, no. 4, pp. 786–800, Jul. 2019, doi: 10.1007/s10163-019-00832-6.
• T. Nussbaumer, “Combustion and Co-combustion of Biomass: Fundamentals, Technologies, and Primary Measures for Emission Reduction,” Energy Fuels, vol. 17, no. 6, pp. 1510–1521, Nov. 2003, doi: 10.1021/ef030031q.
• M. Dula, A. Kraszkiewicz, and S. Parafiniuk, “Combustion Efficiency of Various Forms of Solid Biofuels in Terms of Changes in the Method of Fuel Feeding into the Combustion Chamber,” Energies, vol. 17, no. 12, p. 2853, Jun. 2024, doi: 10.3390/en17122853.
• J. A. Rivera and C. H. Ortega-Jimenez, “Power Generation with Biomass from Coffee: A Literature Review, Current Trend and Scope for Future Research,” MATEC Web Conf., vol. 293, p. 05002, 2019, doi: 10.1051/matecconf/201929305002.
• R. Abro, X. Chen, K. Harijan, Z. A. Dhakan, and M. Ammar, “A Comparative Study of Recycling of Used Engine Oil Using Extraction by Composite Solvent, Single Solvent, and Acid Treatment Methods,” ISRN Chemical Engineering, vol. 2013, pp. 1–5, Jul. 2013, doi: 10.1155/2013/952589.
• N. Patel and K. P. Shadangi, “Characterization of waste engine oil (WEO) pyrolytic oil and diesel blended oil: Fuel properties and compositional analysis,” Materials Today: Proceedings, vol. 33, pp. 4933–4936, 2020, doi: 10.1016/j.matpr.2020.02.679.
• C. P. Artemio, N. H. Maginot, C.-U. Serafín, F. P. Rahim, R. Q. José Guadalupe, and C.-M. Fermín, “Physical, mechanical and energy characterization of wood pellets obtained from three common tropical species,” PeerJ, vol. 6, p. e5504, Sep. 2018, doi: 10.7717/peerj.5504.
• D. Krause, P. Herdel, J. Ströhle, and B. Epple, “HTWTM-gasification of high volatile bituminous coal in a 500 kWth pilot plant,” Fuel, vol. 250, pp. 306–314, Aug. 2019, doi: 10.1016/j.fuel.2019.04.014.
• S. S. Lam, R. K. Liew, C. K. Cheng, and H. A. Chase, “Catalytic microwave pyrolysis of waste engine oil using metallic pyrolysis char,” Applied Catalysis B: Environmental, vol. 176–177, pp. 601–617, Oct. 2015, doi: 10.1016/j.apcatb.2015.04.014.
• Y. Wang, Y. Sun, and K. Wu, “Effects of waste engine oil additive on the pelletizing and pyrolysis properties of wheat straw,” BioRes, vol. 14, no. 1, pp. 537–553, Nov. 2018, doi: 10.15376/biores.14.1.537-553.
• J. Shen, S. Zhu, X. Liu, H. Zhang, and J. Tan, “The prediction of elemental composition of biomass based on proximate analysis,” Energy Conversion and Management, vol. 51, no. 5, pp. 983–987, May 2010, doi: 10.1016/j.enconman.2009.11.039.
• M. Erol, H. Haykiri-Acma, and S. Küçükbayrak, “Calorific value estimation of biomass from their proximate analyses data,” Renewable Energy, vol. 35, no. 1, pp. 170–173, Jan. 2010, doi: 10.1016/j.renene.2009.05.008.
• A. Bryś, J. Zielińska, S. Głowacki, W. Tulej, and J. Bryś, “Analysis of possibilities of using biomass from cherry and morello cherry stones for energy purposes,” E3S Web Conf., vol. 154, p. 01005, 2020, doi: 10.1051/e3sconf/202015401005.
• D. Trejo‐Zamudio, C. Gutiérrez‐Antonio, J. F. García‐Trejo, A. A. Feregrino‐Pérez, and M. Toledano‐Ayala, “Production of fuel pellets from bean crop residues (Phaseolus vulgaris),” IET Renewable Power Gen, vol. 16, no. 14, pp. 2978–2987, Oct. 2022, doi: 10.1049/rpg2.12365.
• “ISO 17225-6:2021,” ISO. Accessed: Dec. 21, 2025. [Online]. Available: https://www.iso.org/standard/76093.html
• F. M. Kuswa, H. P. Putra, Prabowo, A. Darmawan, M. Aziz, and H. Hariana, “Investigation of the combustion and ash deposition characteristics of oil palm waste biomasses,” Biomass Conv. Bioref., vol. 14, no. 19, pp. 24375–24395, Oct. 2024, doi: 10.1007/s13399-023-04418-z.
• T. Mendel, A. Überreiter, D. Kuptz, and H. Hartmann, “Comparison of Rapid Moisture Content Determination Methods for Wood Chips,” Proceedings of the 24th European Biomass Conference and Exhibition, vol. 6-9 June 2016, doi: 10.5071/24THEUBCE2016-2BV.1.5.
• U. Onochie, A. Obanor, S. Aliu, and O. Igbodaro, “Proximate and ultimate analysis of fuel pellets from oil palm residues,” Nig. J. Tech., vol. 36, no. 3, pp. 987–990, Jun. 2017, doi: 10.4314/njt.v36i3.44.
• B. M. Jenkins, L. L. Baxter, T. R. Miles, and T. R. Miles, “Combustion properties of biomass,” Fuel Processing Technology, vol. 54, no. 1–3, pp. 17–46, Mar. 1998, doi: 10.1016/S0378-3820(97)00059-3.
• K. Theerarattananoon et al., “Physical properties of pellets made from sorghum stalk, corn stover, wheat straw, and big bluestem,” Industrial Crops and Products, vol. 33, no. 2, pp. 325–332, Mar. 2011, doi: 10.1016/j.indcrop.2010.11.014.
• S. Thiruketheeswaranathan, “Comparative Characterization Study of Fuel Pellets from Rice Husk and Wood Chips,” Archives of Science, vol. 6, no. 5, pp. 1–3, Sep. 2022, doi: 10.4172/science.1000132.
• R. Vazquez-Duhalt, “Environmental impact of used motor oil,” Science of The Total Environment, vol. 79, no. 1, pp. 1–23, Feb. 1989, doi: 10.1016/0048-9697(89)90049-1.
• S. Widodo, K. Khoiruddin, D. Ariono, S. Subagjo, and I. G. Wenten, “Re-refining of waste engine oil using ultrafiltration membrane,” Journal of Environmental Chemical Engineering, vol. 8, no. 3, Jun. 2020, Art. no. 103789, doi: 10.1016/j.jece.2020.103789.
• A. Rabajczyk, J. Gniazdowska, Ł. Bąk, J. Roguski, P. Stojek, and D. Bąk, “Emission of Hazardous Substances During Fires in Selected Facilities,” Applied Sciences, vol. 15, no. 24, p. 12989, Dec. 2025, doi: 10.3390/app152412989.