Experimental Study On The Bed Agglomeration Characteristics Of Biomass Combustion In Fluidized Bed

In order to satisfy the biomass energy needs in China, direct combustion of biomass forpower generation is one of the easiest and feasible ways of biomass utilization. Fluidized bedcombustion becomes the most potential technologies among many combustion technologiesof biomass due to its flexible fuel adaptability, high combustion efficiency, and low emissions,etc. However, the fluidization of biomass is significantly different from that of coal, thus hasimpacts on the temperature distribution and combustion in the furnace. In addition, the highcontent of alkali compounds in biomass can cause the bonding and agglomeration of bedmaterials, which leads to the defluidization. Therefore, Study on the fluidization of biomassand characteristics of agglomeration of bed material during biomass combustion providesguidance to solve problems in practice. The fluidization of biomass and agglomerationproblems during combustion was mainly researched in this thesis. The characteristics ofagglomeration of bed materials was studied intensively through combination of theoreticalanalysis and experimental study with the help of analysis approaches such as SEM/EDX,XRD, XRF etc., and chemical equilibrium software HSC Chemistry6.0.At first, the fluidization of four kinds of biomass fuels was studied on the cold fluidizedbed test facility. It was observed that biomass performed a poor fluidization, because therange of fluidization for biomass was very narrow. The bed material was mixed with tobaccostem to investigate the impacts of mixing ratio and particle size of biomass to the fluidizationof the mixtures. The results indicated that the range of fluidization became wider when theproportion of bed material increased. When the particle sizes of bed materials and biomassare similar, the addition of bed materials had little improvement on the fluidization; when thedifference of the two particles was large, the addition of bed materials improved thefluidization significantly.The agglomeration characteristics of biomass fuels with different bed materials duringcombustion was investigated in a bubbling fluidized bed reactor. It is observed that, thepossibility of agglomeration increased as the temperature increased. The results showed thatthe difficulty of bed agglomeration for the three kinds of bed materials was as follows:alumina aggregate>lime stone>cinder. The distribution of elements in biomass ash was studied after combustion. It is observed that the potassium and calcium compounds weremainly found in fly-ash and semi-coke, but little amount of these compounds were found inbed materials.Then, the combustion of tobacco stem with quartz sand and cinder as bed materials wasconducted in a circulating fluidized bed facility. The results indicated that the high contents ofalumina and iron could combine the alkali compounds to form compounds with high meltingpoint, coating on the surface of the bed materials, thus reduced the possibility of conversionof alkali elements to silicates with low melting point, and the occurrence of defluidization asa result. Meanwhile, the calculation of chemical equilibrium indicated that iron compoundshad significant effects on the mitigation of bed agglomeration.