ABSTRACT: The global market for fuel pellets (FPs) has been steadily growing because of a shift to coal substitutes. However, sustainability and the availability of biomass are the main issues. Various kinds of bio-wastes can be valorized through cutting-edge technologies. In the coffee industry, a valuable organic waste called spent coffee grounds (SCGs) is generated in bulk. SCG can be divided into two components, namely spent coffee ground oil and defatted spent coffee grounds (DSCG). SCG and DSCG can be used to produce FPs with excellent higher heating values. This review highlights that burning FPs composed of 100% SCG is not feasible due to the high emission of NOx. Moreover, the combustion is accompanied by a rapid temperature drop due to incomplete combustion which leads to lower boiler combustion efficiencies and increased carbon monoxide emissions. This was because of the low pellet strength and bulk density of the FP. Mixing SCG with other biomass offers improved boiler efficiency and emissions. Some of the reported optimized FPs include 75% SCG + 20% coffee silverskin, 30% SCG + 70% pine sawdust, 90% SCG + 10% crude glycerol, 32% SCG + 23% coal fines + 11% sawdust + 18% mielie husks + 10% waste paper + 6% paper pulp, and 50% SCG + 50% pine sawdust. This review noted the absence of combustion and emissions analyses of DSCG and the need for their future assessment. Valorization of DSCG offers a good pathway to improve the economics of an SCG-based biorefinery where the extracted SCGO can be valorized in other applications. The combustion and emissions of DSCG were not previously reported in detail. Therefore, future investigation of DSCG in boilers is essential to assess the potential of this industry and improve its economics. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s10668-022-02361-z.

Alternative Fuels Research Laboratory Energy Division Department of Mechanical Engineering Faculty of Engineering Erciyes University 38039 Kayseri Turkey

Department of Biological Engineering College of Engineering Konkuk University Seoul 05029 Republic of Korea

Department of Chemical and Petroleum Engineering United Arab Emirates University Al Ain United Arab Emirates

Department of Chemical Engineering COMSATS University Islamabad Lahore Campus Defense Road Off Raiwind Road Lahore Pakistan

Department of Electronics Faculty of Electrical Engineering and Computer Science VŠB Technical University of Ostrava Ostrava Czech Republic

Institute of Engineering HUTECH University Ho Chi Minh City Vietnam

Mechanical Engineering Department College of Engineering King Khalid University Abha 61421 Kingdom of Saudi Arabia

School of Chemical and Materials Engineering National University of Sciences and Technology H 12 Islamabad Pakistan

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