The time required to gasify the char residue is essential when designing biomass gasification processes. One way of influencing the char conversion time is to change the process temperature. Changing the process temperature, however, will influence the product gas composition. Increasing the temperature may also lead to ash related problems. In a fluidized bed based process, the bed height will influence the pressure at which the char is converted. Although in this process, a less reactive char primarily will lead to an increased char inventory in the bed, understanding the gasification kinetics is still important.This study compares the char gasification reactivity determined using three different techniques: standard TGA, isothermal TGA and a single particle reactor (SPR) gasifier. TGA is often used to determine char reactivity. This method is appealing since it only requires a very small amount of sample. On the other hand, this technique has several drawbacks. First, the heating rate is typically much lower than in a real process. It is known that the heating rate influences the char yield, and consequently, also most likely the reactivity of the char. In addition, care must be taken that mass transfer limitations do not influence the results. The isothermal TGA utilizes a TG, in which the sample suddenly is exposed to the hot gasification atmosphere. In this way, the heating rate closer resembles that of a real process. In these TGA experiment grinded biomass powder was used as well. In the third experimental setup the powder was pressed to a pellet. Also in the SPR the sample is suddenly exposed to the hot gasification atmosphere.The results show that the char reactivity of the SPR is lower than those measured with the low and the high heating rate TGA devices. This is surprising considering that the heating rate in the single particle reactor is higher than those obtained in the TGA experiments.
|Tila||Julkaistu - 2015|