Migration And Catalytic Characteristic Of Intrinsic AAEMs During Pyrolysis And Gasification Process Of Biomass

As a low-sulfur, low-nitrogen and carbon-neutral renewable clean energy, biomass has attrated worldwide interests, becoming research focus in the renewable energy field. In view of the notable feature of lignocellulosic biomass which has higher alkali and alkaline earth metallic species (AAEMs) contents, the typical Chinese agricultural and forestry residues are selected, and systematic study on intrinsic AAEMs removal by leaching process, their migration and catalytic characteristics during biomass pyrolysis and gasification process have been conducted in this thesis.Firstly, the release characteristic of AAEMs was studied, and the release mechanism of them was determined during pyrolysis and char gasification process. The results indicated53～76%of alkali metals and27～40%of alkaline earth metals released during pyrolysis process, comparing to12～34%of alkali metals and12～16%of alkaline earth metals evaporated during char gasification process. The release of AAEMs during pyrolysis process was due to substitution reaction between char matrix and free radicals which were generated from initial pyrolysis. However, the release of AAEMs during gasification process was created by the evapotation of AAEMs presented as inorganic salts, which release mechanism is different from that of pyrolysis process.The biomass samples were leached by six solutions, then the removal efficiency of AAEMs and the influence on physicochemical structure of samples were assessed by a series of detection methods, finally the catalytic role of intrinsic AAEMs during biomass pyrolysis was investigated. The results showed that most of alkali metal (K and Na) were water-soluble comparing to most of acid-soluble alkaline earth metal (Ca and Mg), thus the intrinsic alkali metals and alkaline earth metals of samples could be respectively removed by water and hydrochloric acid leaching, which provided a necessary tool in research on their catalysis and reaction mechanism during pyrolysis and gasification process. The thermal decomposition of hemicellulose component was promoted by alkali metal; however thermal decomposition of cellulose component was inhibited by Ca metal. In addition, comparing to physicochemical structure changes created by leaching process, the influence of removal of minerals played the dominant role in biomass thermal behavior. The biomass samples leached by deionized water and hydrochloric acid solutions were selected to investigate the catalytic role in pyrolysis characteristics of biomass samples. The results showed the removal of AAEMs led to reduction of char yield and increasing of tar yield, and decreased the yield of H2and CO2and enhanced the yield of CO. Alkali metals played a leading catalytic role in biomass pyrolysis comparing to alkaline earth metals. UV fluorescence spectroscopy (UV-F) measurement also showed that AAEMs had a catalytic role in thermal cracking of large PAHs of which promoting effect was more-significant than that of temperature elevating.Steam gasification experiment of above samples was conducted, and the catalytic role of the intrinsic AAEMs in the characteristics of steam gasification was studied. The results indicated external steam had negligible effects on promoting further thermal cracking or reforming of tar under900℃. The char-steam heterogeneous reaction, hydrocarbons reforming reaction, water-gas shift reaction were promoted by the presence of AAEMs, which indicated the AAEMs released in the form of gaseous participated in the steam reforming reactions and enhancing them. Alkaline earth metals had more significant catalytic effects on H2producing comparing to alkali metals. The UV-F measurement also proved that the addition of steam had no obvious promotion to secondary reforming of tar, and AAEMs had a significant catalytic role in thermal cracking and reforming of tar.Finally, combining with the distribution of pyrolysis and gasification products, the promoting of AAEMs in reaction pathway of pyrolysis and gasification was determined. The results showed the products of furans, saccharide, etc. were yielded from thermal decomposition of cellulose and hemicellulose. The products of hydroxyl-, methoxy-, alkyl-containing phenols and ketones, alcohols, etc. were yielded from thermal decomposition of lignin. The formation of the important carbohydrate products of biomass pyrolysis, such as1,6-anhydro-beta-D-glucopyranose (levoglucosan, LG) and1,4:3,6-dianhydro-α-D-glucopyranose (DLG), were suppressed by the presence of AAEMs. In addition, the formation of phenol and its derivatives were promoted by the presence of AAEMs.