Effect of mixed metal salts on pyrolysis characteristics of rice straw

Abstract: To explore the effects of mixed metal salts on rice straw (RS) pyrolysis behavior, pyrolysis process of RS mixed with basal additives, i.e. carbonates Li2CO3-Na2CO3-K2CO3 (LNK), added with metal chlorides or sulfates (MCS) were investigated by Thermogravimetry and Fourier Transform Infrared Spectroscopy (TG-FTIR), respectively. Coats-Redfern method was adopted to calculate the kinetic parameters of pyrolysis process. The results showed that the activation energy of pyrolysis decreased under the action of carbonates, and further decreased as metal chlorides were added, which indicated that the reaction needed less energy to support the pyrolysis process. FTIR analysis indicated that the constituent concentrations (CO, CH4, toluene, phenol, formic acid, H2O) of RS samples decreased after LNK was added, while CO2 concentration increased. Compared to LNK-RS, the concentration increased when MCS were added to LNK-RS, which indicated MCS promoted the conversion of RS into light species. Particularly, LNK promoted Boudouard reaction when temperature was over 550℃, and the LNK added with MCS was capable of facilitating this reaction. Among these MCS, NiCl2 and CoCl2 had better effectiveness on the conversion of RS. The research provided the theoretical reference for better utilization of biomass resources. The rice straws were obtained from the countryside of Hangzhou City, Zhejiang Province. The rice straws were dried under sunlight, with physical impurities separated, and ground in a rotary cutting mill and screened into fractions less than 100 mesh (particle size <0.15 mm). The spectrum scope was in the range of 650-4 000 cm-1 and the resolution factor was 4 cm-1. The sample was dried at 90℃ for 24 h prior to the experiments. Pyrolysis was conducted at the heating rate of 70 K/min from 30 to 650℃. According to the thermogravimetric/derivative thermogravimetric (TG/DTG) curves, RS pyrolysis in the temperature range of 30-650℃ was characterized by three mass loss stage: 30-208, 208-376, 376-650℃. The results showed that the values of maximum mass-loss temperature and maximum mass-loss rate both decreased after adding additives. The kinetic analysis showed that the activation energy was lowered under catalyzed pyrolysis added with metal additives, compared to that without extra cations and anion, and metal chlorides had better catalytic effectiveness than metal sulphates. FTIR analysis showed that peaks of CO2 and H2O corresponding to RS added with CuSO4 and CuCl2 were stronger than that of RS in the first peak. It was known that the carboxylates in RS would undergo decomposition at relatively low temperatures. In this way, CuSO4 and CuCl2 were more likely to associate with the -COO group at the outer particle surface, resulting in the release of CO2 if the bond between the -COO group and the RS matrix was broken first. It should be pointed out here that the NiCl2-form sample behaved quite differently from other samples. The study here did not exclude the possibility that trivalent cations (Fe3+) had more sites to connect with the-COO group. In fact, further studies are needed to make sure of the forms of cations during pyrolysis. With increasing temperature, not only the characteristic absorbance bands of CO2, but also the absorbance bands of H2O and CO emerged. However, the absorbance of C-Hυs, C=O, C=C, and C-O(H) apparently decreased in catalyzed pyrolysis. The reduced CO2 and tars (C-O(H), C=C, C=O) were responsible for the decrease of mass loss.