Real-time release of Na, K and Ca during thermal conversion of biomass using quantitative microwave-assisted laser-induced breakdown spectroscopy

Abstract

Power production with thermal conversion has met new challenges due the global pressure to use CO2 neutral and renewable fuels, e.g. recycled fuel and biomass. Many of these fuels contain high concentrations of elements, such as alkali metals and chlorine, that together are harmful for boiler structures and may cause operational problems. Therefore, detailed quantitative information on release behaviour of the problematic elements, potassium and sodium, is required. For this, a new burner, which allows linear calibration of laser induced breakdown spectroscopy (LIBS) measurement towards higher concentrations relevant for the release studies during thermal conversion of biomass, was designed. The analytical performance of conventional LIBS measurement is significantly improved by introducing microwave radiation to the laser-induced plasma. An enhancement of linearity and up to 60-fold improvement of limit of detection (LOD) was observed with microwave-assisted LIBS (MW-LIBS) in comparison to conventional LIBS. The LOD of Na, K and Ca were 10 ppb, 19 ppb and 16 ppb, respectively. In-flame MW-LIBS measurement was applied to record time-traces of K, Na and Ca during thermal conversion of a poplar pellet. This is the first demonstration of Microwave near-field applicator injected MW-LIBS for gas phase measurement. With broad dynamic measurement range, the proposed method can be applied to extensive research of elemental release behaviour of different fuels. In addition to combustion studies, the MW-LIBS method can be extended to study trace-elements in gas phase in different fields of industry and science.