Law for migration and transformation of carbon and trace elements during the pyrolysis of wheat straw

Abstract: As a kind of clean renewable energy source that can replace coal, such as crop straw and other biomass, have a wide range of applications and great potential. However, the content of alkali metal and other inorganic elements in the crop are high and melting temperatures of which are low. In the process of straw utilization for energy, the precipitation of these trace elements will cause various problems. To study the migration and transformation of carbon and trace elements during the pyrolysis of wheat straw, the contents and existence forms of carbon were calculated and discussed in process of pyrolysis carbonization. HSC Chemistry software was used to simulate the component changes of trace elements in the pyrolysis and carbonization of straw. The migration and transformation of 8 trace elements K, Na, Ca, Mg, Al, Fe, P and S were analyzed. Results showed that the calculation of carbon footprint accords with conservation law. The content of carbon element from high to low during wheat straw to pyrolysis product were pyrolysis carbon, pyrolysis gas, tar, pyroligneous and overflow flue gas. Especially, the content of carbon element in pyrolytic carbon was the most, which was 41.12%. The carbon element in pyrolysis carbon was fixed carbon. 22.83% of the carbon elements migrated to the tar and existed as macromolecules of long-chain hydrocarbon and aromatic compounds. 26.62% of the carbon elements exist in the pyrolysis gas as short-chain hydrocarbon in the six-carbon. 4.71% of the carbon elements transferred to wood vinegar solution for aldehydes, ketones, acids, etc. In the whole carbon balance, the content of CO2 in the overflow fume was low, which was 4.72%. The total amount of CO2 emitted by the system was low, which reduced the greenhouse gas emissions. C, H, O, N and S were the main organic elements in wheat straw. The content of trace elements in wheat straw was analyzed by inductively coupled plasma emission spectrometer. The trace elements could be divided into two classes. The first class was high concentration trace elements with a content of more than 100 μg/L, including Ca, K, Mg, Na, Al, Fe, P and S, the second category was 22 kinds of low-enriched trace elements with contents between 0.1 and 100 μg/L. In all wheat straws with high enrichment of trace elements above 10 μg/L, the migration rules of K and Na, Ca and Mg, Al and Fe, P and S were similar in pairs. The form of migration and transformation of metallic elements were mostly inorganic salts. However, the precipitation of P and S elements in the pyrolytic carbonization process were mainly the decomposition of their organic compounds. The retention rates of K and Na in pyrolytic carbon were 37.41% and 40.17%, respectively. More than half of Ca and Mg were retained in pyrolytic carbon, approximately. These four metal elements mainly exist in pyrolytic carbon in the form of sulfate, phosphate and chloride. However, Al and Fe were mostly retained in pyrolytic carbon in the form of oxides, sulfides and silicon-oxygen eutectics. On the whole, all conclusions provide data support for further research on straw pyrolysis mechanism and research ideas for effectively improving the quality of straw pyrolysis char. Temperature has a significant influence on the migration of trace elements. High temperature (600-1 000 ℃) greatly reduced the retention rate of all trace elements. In a suitable temperature range, reducing the carbonization temperature was conducive to improving the retention rate of trace elements and the porosity of pyrolysis carbon, which effectively improves the quality of pyrolysis carbon of straw.