Assessment of correlations between tar and product gas composition in dual fluidized bed steam gasification for online tar prediction

Thermochemical conversion of biomass feedstock via dual fluidized bed steam gasification is a well-proven technology used to produce a medium calorific product gas for various applications in the energy or transportation sector or for chemical syntheses. At unfavorable gasification conditions, undesirable high amounts of tar, which are aromatic hydrocarbons, are present in the product gas. High tar contents are a major problem, and they lead to uneconomic operation due to sharply diminished quality of product gas or unexpected plant shut downs due to fouling of the product gas coolers. Currently, tar content is measured with a discontinuous wet chemical analysis method, which needs several hours of sample preparation to receive the final tar content. The aim of this study is to establish valid correlations between online measured permanent gas components in the product gas and its tar content. The results show that hydrogen, methane, and ethene concentrations are strongly related to the tar content in the product gas, while the carbon monoxide and carbon dioxide content did not show a clear correlation. Using these correlations with online measured gas components provides the possibility of a direct and prompt response of a plant operator in case of unfavorable gasification conditions. Additionally, an optimization of the plant operation can be conducted and thereby, the operation hours and, consequently, the economic efficiency are improved.