Physicochemical changes of rice straw after lime pretreatment and mesophilic dry digestion

• Rice straw biomass was pretreated, and then produced methane in solid-state condition.
• The methane yield was 177.7 mL g−1 volatile solids.
• Low mass transfer limited the lignin modification and polysaccharide decomposition.
• Pretreatment and digestion changed the thermal properties of rice straw.

A better understanding of the physicochemical properties of the material, its changes and the effects of anaerobic digestion, will help to improve methane production efficiency. The physicochemical changes of rice straw after solid-state lime pretreatment and dry digestion were investigated. Those changes were revealed by the composition of the substrate before any treatment, from samples of the material during the process and by the final product. Such compositions were studied with X-ray, Fourier transform infrared spectroscopy and thermogravimetric analyzes. The obtained results showed that low mass transfer limited the lignin modification and polysaccharide decomposition, resulted in a low biogas yield; and that after pretreatment and 60 days of mesophilic dry digestion, the concentration of cellulose decreased (23.4%) as well as hemicellulose (13.1%). In the opposite case, the concentration of lignin increased (59%) as well as ash (108%). The crystallinity index increased by 5.4% after pretreatment and decreased by 4.8% after digestion. The FTIR spectrogram of the raw sample included a peak reflecting absorption of ester linkages. This peak was absent from FTIR spectra of pretreated and digested samples, indicating that such linkages are broken during pretreatment and then digested by the microorganisms during the anaerobic fermentation. Furthermore, lime pretreatment and dry digestion significantly changed the decomposed profiles and rate, the distribution of pyrolysis product. The obtained data showed that polysaccharides and lignin were decomposed during pretreatment and digestion phases, leading to changes in structural and thermal properties.