Release Behavior Of Inorganic Elements During Agricultural Straw Thermal Conversion

The effective utilization of biomass resources has become an important approach tosolve the energy shortage, which has drawn extensive attraction worldwide. Strawresources are significantly abundant in our country as a kind of agriculture waste, whoseutilization technologies have been developed greatly. However, the high content ofinorganic elements such as potassium and sodium has caused many problems during theprocess of thermochemical conversions which inhibit the wide thermochemical utilizationof straws. With the aim to solve the problems engendered by inorganic elements, this thesishas investigated cotton straw, wheat straw and rice straw in a series of experiments, and hasmade a deep insight to the releasing and conversion characteristics of these inorganicelements in the process of thermochemical conversions, thus provided referred solutions torelative problems theoretically.The experiments of pyrolysis of three kinds of biomass corn straw, wheat straw andrice husk were conducted on a fixed-bed to study the releasing patterns of alkali metal andalkali earth metal （AAEM）, nitrogen, sulfide, fluorine and chlorine. At the end of theexperiments, a study on the existing patterns of alkali metal in residual char and thecompositions of nitrogen, sulfide, fluorine and chloride components were carried out. It hasbeen observed that sodium releases most easily in four kinds of straw, and potassiumreleasing components are mainly composed of inorganic potassium and organic potassiumcompounds. In lower temperature, the potassium components primarily were released asdecomposition of organic potassium compounds. The releasing amount of potassium isrelated to some factors such as the absorption of potassium by char, the size of biomassparticles and the content of chloride and silicon in biomass. Within the whole pyrolysisrange, calcium and magnesium components released slightly. The patterns of AAEMs inchar are as follows: some of AAEMs are dispersed in char as chlorides, silicates andsulfates, and the others are present in carbon matrix. Sulfide releasing process undergoeswith two steps in biomass: organic sulfide and inorganic sulfide. Nitrogen is distributed inchar, tar and gas after pyrolysis. The main components of nitrogen in pyrolysis are ammonia, HCN and NOx, and nitrogen-containing heterocyclic compounds are thedominant nitrogen compounds in tar. Besides, the releasing amount and rate of HFpromotes and the peak of releasing profile tends to appear earlier as the pyrolysistemperature rises up.Then, the releasing and transforming patterns of AAEM in the gasification of biomassand biomass char under carbon dioxide atmosphere were studied respectively. The resultsof biomass raw samples and biomass char gasification indicate that sodium componentsrelease dominantly which are followed by potassium, calcium and magnesium werereleased less significantly in gasification. The calculations of chemical thermodynamicequilibrium indicate that K₂Cl₂（g）, KCl（g）、KOH（g）、KCN（g）、K（g） are the primarycompounds present in potassium releasing process, while for sodium, Na₃Cl₃（g）、Na₂Cl₂（g）、Na（g）、NaCN（g） are the most common compounds. Calcium mainly exists incarbonate calcium, and transforms into aluminosilicates as the temperature increases.Magnesium mainly exists in the form of MgSiO₃、MgCO₃、Mg2SiO₄. The impacts ofbiomass species, carbonation temperature, gasification temperature, silicon contents in thebiomass ash, gasification time and reactor patterns （gasification atmosphere） and so forthwould contribute to the releasing of AAEM.In the end, the ash properties of biomass were studied under different ash temperature.In low temperature, the ash components in straws were dominated by KCl、SiO₂、CaCO₃;Whereas in high temperature, KCl content tended to diminish, while aluminosilicatesaccumulated. The ash components in wood are dominated by calcium crystals. No apparentlinks between the ash components and fluid temperature, hemisphere temperature andsoften temperature of ash were discovered in biomass rather than coal, which indicates thatthe slagging indications for coal are not necessarily suitable for determining the slaggingcharacteristics of biomass ash. Thus, the slagging characteristics of biomass ash should bedetermined on the basis of various impacts of biomass ash properties comprehensively.