A review of progress on torrefaction, pyrolysis and briquetting of banana plant wastes for biofuels

Abstract

The banana value chain produces over 4 tonnes of waste biomass for every tonne of bananas harvested, including leaves, pseudostems, peels, rejected fruits, rhizomes, and empty fruit bunches. With rising fossil fuel costs and environmental concerns, these wastes present opportunities for alternative biofuel production through thermochemical processing and densification. This review examines the properties of various banana plant wastes, their pretreatments, and suitability for processes like pyrolysis, torrefaction, and hydrothermal carbonization, as well as briquetting. Banana plant wastes vary in physico-chemical properties depending on the biomass type. Their high volatile matter content (70.5–89.1%db) makes them better suited for bio-oil and gas production rather than biochar. Pretreatment methods such as water-washing, alkaline treatment, drying, pressing, chopping, grinding, and milling may be needed before thermochemical conversion of the wastes. Among conversion routes, pyrolysis is the most studied, followed by hydrothermal carbonization and dry torrefaction. The hydraulic press is the most commonly used technology for briquetting banana plant wastes. Depending on factors such as binder-to-biomass ratio, dwell time, and compaction pressure, this method can produce briquettes with compressive strength ranging from 1.33 to 38.39 MPa, which exceeds the minimum acceptable level of 0.38 MPa. However, these briquettes can have ash content as high as > 20%db, which can reduce their calorific value, increase the risk of ash slagging and fouling in combustion systems, as well as lead to increased emission of particulate matter during combustion. While thermochemical conversion and briquetting of banana plant wastes may incur significant costs, these could be offset by the low cost of the raw materials, improved fuel properties, and better handling, transportation, and storage. Research efforts should focus on ascertaining the emission potential of thermochemical conversion and briquetting of banana plant wastes, which could encourage wider acceptance of these technologies, especially considering growing awareness about the need for environmental protection.

Graphical abstract