Study On The Transformation Mechanism Of Nutrient Element During The Preparation Of Biochar

Biochar refers to the solid product with high carbon content and stability after pyrolysis of biomass under the condition of completely or partially exclusion of oxygen. Rice straw, which is renewable and available in large quantities all over the world, has been considered an attractive raw material for the production of biochar. As a potential soil amendment, the effect of biochar returning on the soil and crop growth has been widely studied. However, the effect and mechanism of preparation condition affect ing biochar nutrient characteristics is unknown.In this experiment, rice straw was employed as raw biomasses. Biochars were preprared under various pyrolytic conditions and their nutrient properities were characterized by yield calculation, elemental analysis, XPS, SEM-EDS, BET, GC-MS. In this experiment,which revealed the transportation and transformation mechanism of nutrient element during the preparation of biochar.Therefore, the main conclusions are drawn as follows:（1） The yield of biochar, pH and nutrient elements in CO₂ atmosphere were higher than that of N₂ atmosphere; The yield of biochar and nutrient elements decreased with the increase of temperature, while the pH decreased; The yield and nutrient elements were not affected by the air flow rate, but the pH decreased with increasing of the air flow rate; Heating rate increases, the yield of biochar, pH and the total amount of nutrient elements showed a decreasing trend; APH was used to analyze the yield, pH and nutrient elements under different conditions, obtained that the preparation of biochar is better under CO₂ atmosphere-400 oC-20 oC/min- 0 L/h.（2） With the increase of temperature, the content of C and N in bio-oil first increased and then decreased, in which phenolic compounds were the main components, N in bio-oil is converted from heterocyclic nitrogen to other substances; the C and N in the gas products showed an increasing trend; the relative content of Phenols and amide in bio-oil at CO₂ atmosphere were higher than that of Ar atmosphere; The total amount of C and N in the gas products under CO₂ atmosphere was higher than that of N₂.（3） Under CO₂ atmosphere, the total amount of C and N in the biochar was higher than that in Ar atmosphere and the carbon frame structure of biochar is sparse, the combination of C and O is obviously increased; the proteins, free amino acids, amines, pyrrole, alkaloids and inorganic N in the rice straw were converted into amino compounds, amide, cyanide, amine, inorganic N and other substances in the biochar during pyrolysis.（4） The P and K in rice straw remained in biochar, The organic P in rice straw was converted to K2HPO3,（NaPO3）3, P4O10 and other substances which were retained in the biochar, the part of halides-K, KNO3, KNO₂ in rice straw remained in the biochar, part of that converted to complex-K, K-HOPG intercalation compounds in the biochar.（5） With the increase of temperature, the specific superficial area and pore volume of the biochar had been enlarged, indicating that biochar was consisted of micropore and mesoporous; the relative total amount of C, N, P and S decreased, by contrast, the relative total amount of K increased, the relative amount of inorganic N, P and S increased, but the relative amount of available K decreased; however, the relative total amount of inorganic N and P as well as available K element showed a trend of reduction. Therefore, it is showed that the quality of 400 oC is better.（6） The specific superficial area and pore volume of biochar at CO₂ atmosphere were higer than that of the biochar under N₂ atmosphere, meanwhile, the relative amount of inorganic N, P, S and available K of the biochar were also higher than that of biochar in N₂ atmosphere; in summary, it could been seen that the quality of biochar under CO₂ atmosphere was better than that of biochar in N₂ atmosphere.