Thermochemical pretreatments to improve the fuel properties of rice husk: A review

The alarming rate at which the deposit of non-renewable or fossil energy resources are depleting around the world has been nudging us in the direction of green energy for quite a long time. Considering the current and upcoming global energy crisis, we have no choice but to pay our serious attention to renewable resources to meet the energy demand of an ever-growing global population and industry. Rice husk has about 20% mass percentage of rice, with low energy and bulk density. Due to these properties, it is not viable to directly combust rice husk as a fuel. Different thermochemical pretreatment technologies can be effectively implemented to improve the fuel characteristics. However, there have been no comprehensive review article on different pretreatments of rice husk. This paper reviews most common rice husk pretreatments technologies namely, gasification, torrefaction, pyrolysis, and hydrothermal carbonization including multiple sub-categories of each technology. All of these processes are successful in improving the fuel characteristics of rice husk such as higher heating value or calorific value, moisture content, fixed carbon, etc. Each of them serves the purpose of converting a low energy-density biomass into an energy-rich material like coal differently. Location, resources, available technology, etc. should also be considered while deciding which pretreatment would be optimum for a given particular case. A comparison has been presented on how much energy can be extracted from rice husk by implementing different technologies given they can be upscaled to an industrial scale. For 527,534 kg of rice husk produced from a sample location in Bangladesh per annum, wet torrefaction before pyrolysis of rice husk yields 85 MW of power which is the highest among the 4 pretreatments discussed in this paper.