Table 2.

Key challenges of rice straw as feedstock for biogas production and alternative solutions

Issue	Description	Suggested solutions	References
Low energy content	In comparison with other feedstocks (e.g., energy crops), rice straw has a low energy content, which might be less effective as a biogas feedstock when used alone	Some pretreatment methods can be involved to enhance the calorific value, such as torrefaction and pelletization	Menardo et al. 2015 Bhattacharyya et al. 2021
Low nitrogen content	The low nitrogen content might inhibit the development of microorganisms during the digestion process and reduce biogas production	Mixing rice straw with another substrate that has a low carbon content is a common method to adjust the carbon/nitrogen ratio of rice straw, enhancing the biogas yield while maintaining the cost low	Atelge et al. 2020 Mothe and Polisetty 2021
High ash content	Rice straw contains a significant amount of ash, which can contribute to equipment blockage and corrosion and reduce the effectiveness of the anaerobic digestion process	Pretreatment of rice straw by washing is a promising method for removing ash; acid solution and hot water are the most commonly used methods	Chen et al. 2019
Seasonal availability	Rice straw is often only accessible during the harvesting period, which is challenging to provide a regular supply of feedstock to the biogas plant	Formation of briquettes and pellets with a suitable bulk density may be an alternative method to get rice straw in non-season times	Bhattacharyya et al. 2021 Hassan et al. 2021b
High variability in quality	The quality varies based on the type, growth environments, and storage practices of rice crops, which might impact the output energy efficacy	Recently, for example, some types of genetically modified rice have more good characteristics, which can be reflected in the properties of straw. Also, some logistical solutions for using the rice straw of each region may be a solution to reduce the conflict in the characteristics	Singh and Patel 2022
High moisture content	The high moisture content can raise drying costs and make the subsequent processes, i.e., storage, more challenging	The most common way to reduce the water content in raw rice straw is drying by sun drying or machine drying	Logeswaran et al. 2020 Shang et al. 2020

Rice straw is an abundant feedstock for biofuel production worldwide, not only in China. However, some characteristics need to be enhanced to increase the efficiency of output products while maintaining economic viability. Low energy and nitrogen contents, seasonal availability, and others obstruct valorizing rice straw efficiently. But some problems have been well addressed, and still, several developments are ongoing