Interactions Between Biomass Volatiles And Coal Char During Coal And Biomass Co-gasification

As a novel energy utilization technology, coal/biomass co-gasification has potential industrial applications in addressing the issues related to the CO2 emissions from coal and compensating the hydrogen difficiency during coal gasification only.what’s more, it has the advantages of low CO2 emmision and low water consuming amount. However, in the process of practical applications, tar problem caused by high volatile in biomass,which restrict the development of biomass gasification technology,is always an obstacle in coal and biomass cogasification. Thus, tar problem has become an important problem in the development of coal and biomass co-gasification technology. Biomass is rich in volatiles while coal is rich in fixed carbon, so biomass volatiles are finally released in the form of tars when they are not completely converted into gaseous products. Tar can cause equipment blocking and corrosion in gasification, Therefore, understanding the formation sources of biomass tar, the evolution of the structures under the co-gasification environment and influential factors on the tar release have important guiding significance. During the co-gasification process, the biomass primary volatiles keep interacting with the coal char and thus the amount and properties of tars are largely dependent on the severity of the secondary reactions on the coal char surface.In this thesis, the main components of biomass, i.e. cellulose and lignin, were used as the raw materials. The experiments were carried out in a two-stage fixed bed reactor to study the interactions of biomass-derived volatiles with Jincheng anthracite chars. The temperature of the top stage was fixed at 500 oC for the pyrolysis of cellulose and lignin. Firstly, the effects of secondary reactions on the destruction of cellulose-derived volatiles during coal/biomass co-gasification were studied, over a bottom stage temperature range from 600 to 900 oC. In this section, three operational modes were carried out in the bottom stage, namely thermal cracking(the second stage was left empty to study the effects of thermal cracking, i.e. only homogeneous gas phase reactions), catalytic cracking(2 g PYchar was preloaded in the bottom stage and was regarded as a catalyst accelerating volatiles decomposition.), and catalytic reforming(besides 2 g preloaded char particles, 150 mL/min of steam was injected into the bottom stage). On the basis of the effects of reaction environment on the decomposition of cellulose volatile, the reforming characteristics of biomass volatiles with different chemical compositions on anthracite chars were investigated by comparing the structural evolution of tars derived from cellulose and lignin. The pyrolysis vapors of cellulose and lignin were subjected to interactions with anthracite chars in the presence of steam at temperatures between 600 and 900 oC. In addition, the surface areas and the gasification reactivity of the fresh and used anthracite chars were analyzed. The following conclusions can be drawn from the present study:1. Cellulose-derived volatiles were of high reactivity, at temperatures below 700 oC, the tar yield under the catalytic cracking and reforming modes were lower than the thermal cracking mode, indicating that catalytic cracking and reforming promoted volatile destruction; above 700 oC the tar yield became almost equivalent under all reaction modes, which indicates that homogeneous reactions dominated at higher temperatures.2. Nascent cellulose volatiles were converted into aromatics by dehydration and aromatization reactions with increasing temperature; on the other hand, aromatics were also consumed by the reforming reactions. Then the formation and consumption of aromatic compounds in volatiles occurred in parallel. At high temperature(900 oC), the overall yield of aromatic compounds in the catalytic cracking and catalytic reforming modes were lower than that in the thermal cracking mode, indicating the positive role of anthracite char in the destruction of aromatic compounds.3. The lignin volatiles were more refractory to the reforming/cracking reactions on the coal char than cellulose volatiles. In addition, the yields of PAHs of lignin were always much higher than those of cellulose, which indicates that the lignin tars were more aromatic.4. At temperatures ≤ 800 o C, after the pyrolysis vapors of cellulose and lignin interacted with anthracite chars, the used anthracite chars showed lowered specific surface areas, which indicates that coke deposition toke palce on the char surface. At 900 oC, the surface areas of used chars became larger due to that the gasification rate of the chars was larger than the coke deposition rate of the tars under the catalytic reforming modes.