Research On Physical And Chemical Characteristics Of Biomass Pyrolysis Char

Biomass gasification technology has been widely used in recent years.Catalytic cracking is an efficient way to eliminate biomass tar, which is thebottleneck of the high efficient utilization of biomass gasification. Biomass charas biomass tar cracking catalysts is attracted by scholars all over the world,because of its lower price, harder deactivation and higher catalytic activity. Thephysical and chemical structures of pyrolysis char were investigated at differenttemperatures. The micro-physical structures, the active functional groups and thealkali and alkaline earth metallic (AAEM) species in biomass char s wereinvestigated, which have significant impacts on the reactivit ies of chars in thecatalytic process.The experimental studies were carried out on the one-dimensional entrainedflow reactor at temperatures ranging from500℃to900℃in N2atmosphere.With the mercury intrusion method and the scanning electron microscopy andX-ray energy spectrum analysis (SEM-EDX) observation, the pore sizedistribution, specific surface area, porosity, surface structures and distribution ofAAEM species were evaluated. Fourier transform infrared spectroscopy (FTIR),Raman spectroscopic techniques were used to identify the active functionalgroups in the chars. With the microwave digestion and inductively coupledplasma atomic emission spectroscopy (ICP-AES), the contents andchemical speciation of AAEM species by the chemical analysis fractionationprocess were investigated.The developed pore structures in the range of1×102-1×104nm are widelydistributed in the pyrolysis chars. The porosity and specific surface area arealmost constant (or decreased) after the first increasing at temperatures ranging from500℃to900℃. The fibrous, cellular and melting structures appear in thechars and the transformation of the three structures happened in turn with theincreasing of the pyrolysis temperature.Micro-chemical structures of oxygen, sugar and aliphatic functional groupsgradually transform with the pyrolysis temperature increasing, and the aromatichydrocarbon structure containing C=C gradually increased, forming differentkinds of aromatic and the degree of graphitization increases. A transition fromthe relatively small to large aromatic ring systems appears in the chars with theincreasing temperature for all samples was achieved. In the case of the charsprepared at500-700℃, the rapid formation of alkyl–aryl C–C bonds as a result ofcross-linking reactions appears, following the decarboxylation reactions or lossof other O-containing functional groups at the temperature from700to900℃.The concentration of alkali and alkaline earth metallic species in the charincrease firstly and then keep constant or decresae, with a maximum value at800℃. The AAEM species were evenly distributed across all the partitions onthe surface and the enrichment of Ca element appears in the melting positions ofthe porous structures. With the pyrolysis temperature increasing, the total AAEMspecies monotonically decreases.K, Na elements existed in the form of water soluble state in their occurrenceand precipitations, while the migrations of Ca, Mg elements are mainly organicstates in the pyrolysis process.