Research On The Transformation Of Alkali Species During Biomass Thermal Conversion

Biomass is a promising fuel for its low CO2emission and renewability. However,due to the release of the inherent alkali metals during its thermal conversion, slaggingand fouling problems occur to the thermal treatment systems, which greatly constrainthe application of biomass. This thesis focuses on the transformation of the alkali metalsduring biomass thermal conversion, aiming to provide fundamental supports to solvethe technical problems arised by using biomass.Firstly, the influence of atmosphere on the transformation of alkali and alkalineearth metallic (AAEM) species during rice straw thermal conversion was studied by afixed bed experimental system. The experimental results revealed that, at1073,1173,and1273K, the release of AAEM species increased with the temperature; oxidizingatmosphere facilitated the release of K, but suppressed the release of Na, Ca, and Mg.By analyzing the solid residues of rice straw after pyrolysis and combustion,respectively, it is found that oxidizing atmosphere greatly promoted the formation of thesilicate of the AAEM species.Secondly, a calibration method for the Laser-induced Breakdown Spectroscopy(LIBS) system was developed, which is applicable to high concentration (0~20ppm) Kmeasurement in flame condition. The ICCD intensifier gate delay and width during Kmeasurement were optimized. It is proposed that by keeping an oxidizing flameenvironment and using a proper K injection method, the absorption effect on the KLIBS signal arised by the atomic K around the plasma in the flame can be maintained ata lower level. A correction method was also proposed for the absorption effect during KLIBS calibration. By this calibration method, the calibration fitting curve for K LIBSmeasurement in flame condition achieved a correlation coefficient of0.9996.With the calibration method developed above, an in-situ measurement system forthe combustion of pinewood particle was built, using LIBS for the measurement of theK concentration in the plume above the burning particle, together using two-colorpyrometry for the measurement of the particle temperature. Experimental resultsrevealed that the4particles with different initial mass (20~50mg) possessed similar Krelease behavior. During devolatilization stage, the K release rate increased firstly thendecreased, and the cumulative fractional K release was lower than4%; during char combustion, the K release rate increased monotonically, and the cumulative fractional Krelease was around20%; during ash stage, the K release rate decreased with time, andthe influence of particle initial mass was insignificant. Combining the experimentalresults of the fixed-bed system with the in-situ measurement system, a K transformationmechanism was proposed for the combustion of pinewood particle. During charcombustion, the Char-K releases mainly by converting to the inorganic K when theattached char is burnt and then release to gas phase as the inorganic K. Based on thismechanism analysis, a kinetic model for the release of K during pinewood charcombustion was developed. With the kinetic analysis of the experimental data using thismodel, the correlated initial proportion of inorganic K in pinewood char was0.37, thecorrelated pre-exponential factor and the the activation energy for K release were12.51/s and89.9kJ/mol, respectively.