Rapid Pyrolysis Of Coal, Biomass, And Coal/Biomass Blends, And Nitrogen Evolution During Rapid Pyrolysis

Pyrolysis is not only one of the thermal conversion method, but also an important process in combustion and gasification. In the entrained-flow gasifier, rapid pyrolysis happens immediately when the fuels are injected into the gasifier. As reaction temperature in entrained-flow gasifier is very high and average residence time of reactants is very short, product distribution of the rapid pyrolysis and gasification characteristics of the pyrolysis char are important factors of the subsequent gasification reactions. Especially for the nitrogen containing compounds whose conversion rates are relatively low, thus nitrogen evolution during rapid pyrolysis greatly affects the yields of the final nitrogen pollutants. Therefore, investigation on product distribution and nitrogen evolution during rapid pyrolysis of coal and biomass and gasification characteristics of the derived char has important significances for clean and efficient utilization of coal and biomass.In this study, high-frequency inductive heating method was employed to the pyrolysis experiments, and a high-frequency furnace which has innovation significance was designed. Yields of gaseous and solid products during rapid pyrolysis (600～1200℃) of coal, biomass, and their blends, structure and gasification characteristics of the residual char were investigated. Nitrogen evolution during individual pyrolysis and co-pyrolysis of biomass and coal was also studied. Nitrogen release mechanisms during rapid pyrolysis were analyzed by integrating the results of quantum chemical calculation and nitrogen containing model compound pyrolysis. Main contents and results are summarized as the follows:(1) Product distributions during rapid pyrolysis of the Inner Mongolia Nanlutian lignite, Shenfu bituminous, and Zunyi anthracite were investigated. Results show that, more volatiles released under rapid pyrolysis than slow pyrolysis. The increasing temperature increased the yields of gas but decreased the char yields. The increasing coal rank decreased the gas yields and increased the char yields. H2 and CO were the main components of the pyrolysis gas, followed by CH4 and CO2, some gaseous hydrocarbons (C2～C3) were also formed. As the temperature increased, yields of H2 and CO increased, yields of CO2 decreased, and yields of CH4 and the other gaseous hydrocarbons mainly increased first and deceased then. Low heating values of all the pyrolysis gas decreased with the increasing temperature.(2) Effects of pyrolysis conditions on morphologic and instinct chemical structures and further on gasification characteristics of the char derived from coal pyrolysis were studied. Lignite char presents a very coarse surface and rich porosity. Serious melting happened to the bituminous char which resulted in smooth surfaces but rich internal porosity. Just some gaps formed on the anthracite char. Thus as the coal rank increased, specific areas of the char decrease, graphitization degrees of the char also increased with the coal rank. Therefore, gasification reactivity decreased with the increasing coal rank. During gasification of lignite char and bituminous char, reaction rates increased first and decreased sharply then, which resulted in unsatisfied fitting results of random pore model (RPM). But the shifted modified random pore model (S-M-RPM) could obtain very good fitting results.(3) Nitrogen evolution during rapid pyrolysis of the three coals was investigated. Most of the fuel-N was retained in char after pyrolysis, N2 was the main gaseous nitrogen product, and NH3 was the main nitrogen pollutant. Both HCN and NH3 could be formed in the primary stage of rapid pyrolysis. With the results of quantum chemical calculations and pyrolysis of nitrogen-containing model compound, it was found that N-6 tended to release as -CN radicals and formed HCN mainly, N-5 tended to release as -NH radicals and formed NH3 mainly, and N-Q tended to release as N radicals under the drastic heat impact and formed NH3 mainly. HCN has the highest sensitivity to the secondary reactions.(4) Product distributions under rapid pyrolysis of rice straw, chinar leaves, and pine sawdust were investigated. During pyrolysis order the organic matter conversions are pine sawdust>rice straw>chinar leaves. Conversion of the organic matters to gas could reach as high as 80%. Char yields were very low after rapid pyrolysis of biomass at high temperature, therefore ash contents had important affected on the char yields. CO and H2 were the main components of the pyrolysis gas, which increased with the increasing temperature. As the temperature increased, yields of CH4 increased first and decreased then, those of CO2 decreased gradually to a very low value in the high temperature. Low heating values of the pyrolysis gas increased first and decreased then.(5) Morphologic and instinct chemical structures of the biomass char and their effect on the gasification characteristics were studied. Graphitization degrees of the char increased with the increasing pyrolysis temperature and led to the decrease of gasification reactivity. Char derived from rapid pyrolysis of rice straw and chinar leaves almost remained the original structures. Serious melting happened to the pine sawdust, and resulted in the destruction of the pore structures and low reactivity. RPM performed well under most of the conditions to describe the gasification rates. But under the condition that high gasification rates were remained or drastic decrease of the gasification rates happened in the high conversion range, the S-M-RPM performed better.(6) Nitrogen evolution during rapid pyrolysis of biomass was investigated. Little nitrogen retained in char after pyrolysis of biomass, and most of the nitrogen converted to N2. The increasing temperature could decrease the yields of nitrogen pollutants and promote nitrogen conversion to N2. NH3 was the main nitrogen pollutant during rapid pyrolysis of protein, amino acids, and nucleobases. While yields of HCN were much higher than those of NH3 during rapid pyrolysis of the 3 agriculture and forestry biomass, and the yields of HCN increased with the lignin contents of biomass. The reason might be that "Microcells" composed by the lignin could result in the polymerization reactions between protein, amino acids, etc. and the organic compositions (cellulose, hemicellulose, and lignin) to form heterocyclic nitrogen and lead to the formation of HCN.(7) The high-frequency furnace not only could make the fuel particles obtain high heating rates, but also could make the biomass and coal particles contact well. Under this condition, obvious synergies were observed during co-pyrolysis of biomass and coal to decrease the yields of char and increase the yields of gas. Packing densities of the biomass and the skeleton collapse characteristics during pyrolysis were important impact factors on the synergies during co-pyrolysis of biomass and coal. Synergies could be observed during co-gasification of biomass char and coal char. But scarcely synergies were found during gasification of the co-pyrolysis char, and inherences between biomass and coal during co-pyrolysis and pore block during char crush might be the reasons. These two aspects also increased the structure homogeneity of the co-pyrolysis char, resulted in a gentle variation of the gasification rates.(8) Nitrogen evolution during rapid pyrolysis of biomass/coal blends was also studied. By affecting the heating rate, nitrogen evolution was also affected significantly by the packing densities and carbon skeleton collapse characteristics of biomass. Co-pyrolysis of biomass and coal could decrease the char-N yields, increase the volatile-N yields, but (NH3+HCN)-N in volatile-N was also decreased. NH3-N was decreased in all the conditions. HCN-N was decreased in the high-temperature range, but under the temperature of 600～700℃, HCN-N yields were increased during co-pyrolysis.