Performance improvement of dual fluidized bed gasifiers by temperature reduction: The behavior of tar species in the product gas

• Published in: Fuel (Guildford) 2013-06, Vol.108, p.534-542
• Main Authors: Kirnbauer, Friedrich, Wilk, Veronika, Hofbauer, Hermann
• Format: Article
• Language: English
• Subjects: Applied sciences; Bed material; Biomass; Biomass gasification; Calcium oxide; Concentration (composition); Energy; Energy. Thermal use of fuels; Exact sciences and technology; Fluidized beds; Fuels; Gasification; Gasification temperature; Natural energy; Olivine; Power plants; Reduction
• ISSN: 0016-2361; 1873-7153
• DOI: 10.1016/j.fuel.2012.11.065

► Olivine is modified by the ash during biomass steam gasification which enhances the gasification properties. ► Reducing the gasification temperature is desired to increase the efficiency of the plant and to reduce fouling issues. ► The reduction of the gasification temperature leads to slightly higher tar concentrations in the product gas. ► The condition of the bed material is more important for tar decomposition than the gasification temperature. To meet the aims of the worldwide effort to reduce greenhouse gas emissions, product gas from biomass steam gasification in DFB (dual fluidized bed) gasification plants can play an important role for the production of electricity, fuel for transportation and chemicals. Using a catalytically active bed material, such as olivine, brings advantages concerning tar reduction in the product gas. Experience from industrial scale gasification plants showed that a modification of the olivine occurs during operation due to the interaction of the bed material with ash components from the biomass and additives. This interaction leads to a calcium-rich layer on the bed material particles which influences the gasification properties and reduces tar concentration in the product gas. In this paper, the influence on the gasification performance, product gas composition and tar formation of a reduction of the gasification temperature are studied. A variation of the gasification temperature from 870°C to 750°C was carried out in a 100kW pilot plant. A reduction of the gasification temperature down to 750°C reduces the concentration of hydrogen and carbon monoxide in the product gas and increases the concentration of carbon dioxide and methane. The product gas volume produced per kg of fuel is reduced at lower gasification temperatures but the calorific value of the product gas increases. The volumetric concentration of tars in the product gas increases slightly until 800°C and nearly doubles when decreasing the gasification temperature to 750°C. The tars detected by gas chromatography–mass spectrometry (GCMS) were classified into substance groups and related to the fuel input to the gasifier and showed a decrease in naphthalenes and polycyclic aromatic hydrocarbons (PAHs) and an increase in phenols, aromatic compounds and furans when reducing the gasification temperature. The comparison with results from an earlier study, where the gasification properties of unused fresh olivine were compared with used olivine, underlines the im