Synergistic pyrolysis of rice and chili straw under N2/CO2 atmosphere: Nutritional elements (N/P/K) migration and transformation from straw to pyrolysis products

Biochar is rich in nutrients and can be used as a soil amendment. This study analyses the migration path of nutritional elements(N/P/K) from biomass to pyrolysis products based on the synergistic pyrolysis of rice straw and chili straw under N2 and CO2. Biochar's structure and nutrient elements were characterized and analyzed by infrared spectroscopy, elemental analysis, and X-ray photoelectron spectroscopy. The results showed that the N content of biochar decreased significantly in both pyrolysis atmospheres, and a large amount of volatilized pyrolysis gas flowed to a small part of the pyrolysis oil. In contrast, the relative contents of P and K increased. The fixation rate of nutrient elements in biochar under a CO2 atmosphere at 500 °C was significantly higher than that under an N2 atmosphere, and the total N/P/K increased by 50.7 %, 64.4 %, and 85.4 %, respectively. With the increasing of pyrolysis temperature, nitrogen, phosphorus and potassium in biochar gradually changed from organic state to inorganic state. At 700 °C and CO2, biochar has the best effect on inorganic enrichment of nutrient elements. Finally, this study summarized the migration and transition mechanisms of nutrients during straw pyrolysis to provide a theoretical basis for biochar nutrient recovery. [Display omitted] •The pyrolysis oil in CO2 atmosphere has more nitrogen compounds.•Potassium content of biochar under CO2 atmosphere at 700 °C was 23.1 g/kg.•High fixation rate of nutritional elements (N/P/K) in biochar in CO2 environment.•Nutritional element (N/P/K) transport pathways from straw to biochar.•Towards efficient nutritional element recycling in agriculture.