Combustion characteristics of cotton stalk in FBC

The present work reports studies on the mixing and combustion characteristics of cotton stalk with 10-100 mm in length in FBC. Experiments on a cold model show that cotton stalk cannot fluidize, and adding bed material can improve the fluidization condition. Cotton stalk can mix well with 0.6-1 mm alumina at fluidization number N = 3-7. However, when the fluidization number is higher more than 7, the mixing bed will exist a little segregation comparing with N = 3-7. Thermogravimetric experiments show that cotton stalk can be ignited easily at a lower temperature, and its devolatilization and combustion are quick. Fluidized-bed combustion of cotton stalk was tested in a 0.2 MWth test facility. According to the temperature distribution along the bed height, when the primary and secondary air is adapted cotton stalk can be burned stably in the fluidized bed. During pure cotton stalk combustion tests, silica sand and alumina are used as bed material to compare their agglomeration characteristics. SEM/EDX analysis on agglomerate samples after combustion about 38 h suggests that the high alkali metals content causes the formation of the coating around silica sand particles. The coating consists of compounds with low-melting temperature results in agglomeration of silica sand particles. By contrast, alumina is difficult to react with alkali metals from biomass ash, and the agglomeration of alumina was not found at 910 °C. It is found that alumina is more favorable than silica sand particle for use in a fluidized bed in cotton stalk combustion.