Spent Coffee Ground Briquettes: A Critical Review

Introduction

To begin with, Coffea are genus of flowering shrubs from Rubiaceae family, indigenous to tropical African and Asian lands; and are primarily cultivated as commodity crops for their seeds, that hold key significance in beverage industries (Brunerová et al., 2019). Though, these plants occur in 70 different species, only Coffea Arabica and Coffea canephora (or) Coffea robusta are cultivated purposely, as they contribute up to 75% and 25% of global coffee production, respectively (Belitz et al., 2009); and are cultivated in large scale by over 80 countries (Murthy & Naidu, 2012). In fact, these coffee seeds have been processed into wide varieties of beverages, especially coffee for more than 1000 years, and are seen as the second largest traded commodity in the world (Peshev et al., 2018). As a result, these seeds are seen as an essential commodity having greater importance in international trade and relationships, economics, and even politics for many developing countries (Potip & Wongwuttanasatian, 2018). Eventually, the cultivation and trading process offers a wide variety of job opportunities, and benefits millions of people during different stages of its production that includes cultivation of plants, harvesting of cherries, processing of beans, packaging and storage, and shipping and handling (Mussatto et al., 2006).

In general, coffee is brewed from ground roasted coffee beans by adding hot water or steam, which extracts their soluble compounds and aromas; leaving behind a large quantity of residual wastes, commonly termed as spent coffee grounds (SCG) (Fehse et al., 2021). The amount of this waste generated depends on their rate of consumption and source, which vary from domestic households to coffee shops or cafeterias, sometimes even large-scale coffee beverage industries (Blinová & Sirotiak, 2019); and on average, constitutes up to 60% of the processed beans (Acevedo et al., 2013; Fithratullah, 2022). Besides SCG residues, other coffee based wastes include their pulps removed from the beans, their silver skin produced during roasting or hulling, and coffee wastewater obtained during handling of their beans (Seco et al., 2020; Nabais et al., 2008). Discouragingly, these wastes have no market value, or scope as value-added products having any direct application (Kourmentza et al., 2018); and are either disposed in landfills, or discarded in open environment or burnt off in incinerators (Lee et al., 2021; Fermoso & Mašek, 2018). However, this simply results in environmental pollution because of their high pollutant potential, harbourage for insects, and odour (Rivera et al., 2020). Despite this, these SCG wastes have promising calorific content with good thermal efficiency and permissible emission range, which makes them ideal for processing into fuels (Allesina et al., 2017); and in general, are valorised into biofuels by undergoing different chemical and biotechnological processes (Karmee, 2018). In specific, these SCG residues exhibit superior performance in their solid form, especially in form of densified briquettes or pellets, and showcases fuel characteristics similar to existing solid fossil fuels, along with high renewability and low ash content (Fehse et al., 2021) at minimal cost (Rajaseenivasan et al., 2016). Besides, these SCG residues are also used as composts and manures for plants, as animal feed, and as raw material for synthesing numerous organic compounds (Colantoni et al., 2021; Atabani et al., 2019; Kang et al., 2017).