Determination of active sites during gasification of biomass char with CO2 using temperature-programmed desorption. Part 1: Methodology & desorption spectra

Based on a carbon conversion mechanism for the gasification of carbon with CO2, a method for the determination of active sites during gasification of biomass char is presented. Beech wood char was partially gasified in CO2 followed by a temperature-programmed desorption (TPD) in order to determine total and stable surface complexes as a function of carbon conversion degree. The experiments were conducted in a temperature controlled quartz glass reactor coupled with a mass spectrometer for the detection of desorbed gas species (CO and CO2). Similar CO2 signals for total and stable surface complexes are observed for all carbon conversion degrees. Increased release of CO during the determination of total and stable surface complexes is detected for XC = 0.9 carbon conversion degree. The desorption spectra of CO and CO2 during TPD cannot completely be explained by the underlying mechanistic model. The measured concentration profiles indicate that the gas species released during TPD may originate from decomposition of surface complexes but also from decomposition of ash components. CO and CO2 arising from of ash components or surface complexes must be differentiated in order to determine reactive surface area (RSA) as a function of carbon conversion degree which is then transferred into a kinetic rate expression for the specific conversion rate Rm. This paper describes the methodology applied for the determination of RSA and discusses the raw data obtained during TPD. In part 2, a detailed analysis concerning the origin of the released gases during TPD is conducted.