Alkali metal release in thermochemical conversion of biomass and coal: Optical measurements and modeling

Alkali metals, mainly K and Na, which are present in solid fuels such as biomass and coal, play an important role during their thermal conversion, e.g., in combustion or gasification. At high temperatures, alkali elements will be released in gas phase as alkali atoms, alkali chlorides, alkali hydroxides and alkali sulphates. In biomass/coalfired boilers, the release of these alkali species can cause problems such as corrosion, slagging and fouling, threatening the safe operation of the facilities. The information on the release dynamic is important for developing proper models for alkali metal transformation in solid fuel combustion and gasification. Therefore, accurate quantitative measurements of the release of different alkali species during thermal-chemical conversion processes of biomass/coal are important. In this paper, we review literatures published over the last few decades in the field of quantitative optical measurements of alkali metals performed in combustion/gasification processes, and the release modeling based on those optical measurements. Firstly, the current situation of biomass and coal utilization is discussed, including the speciation of alkali metals in biomass/coal and their adverse effects on facilities. Secondly, requirements for optical measurements as well as several quantitative optical techniques are introduced including the general principles, typical setups, calibration methods and major advantages and drawbacks. In contrast to off-line techniques, these optical techniques provide nonintrusive measurements with high temporal and spatial resolution, which are indispensable for alkali release modeling. Furthermore, the alkali release behaviors based on optical measurements in thermochemical conversion processes are discussed. Based on the experimental results, the kinetic data for alkali release were summarized. Alkali release modeling was fulfilled relying on the knowledge of alkali release mechanisms and the kinetic data. In addition, simulations of alkali metal release with computational fluid dynamics during the biomass/coal combustion processes are also discussed, providing valuable information for industrial processes. Finally, typical examples of industrial applications of optical measurement methods in solid fuel thermochemical conversion processes as well as waste incineration and other processes are presented.