Laboratory investigations on chemical hot gas cleaning of inorganic trace elements for the “UNIQUE” process
Highlights
► In-bed removal of trace elements in fluidized bed steam gasification of biomass. ► Alkali concentration below 100 ppbv using aluminosilicates. ► H2S and HCl concentration below 1 ppmv using a new Ba containing sorbent.
Introduction
In recent years, the importance of alternative energy sources using renewable raw materials has increased. Biomass gasification is one of the most efficient technologies for biomass energy conversion. It offers the advantage of product flexibility, e.g. heat, power or synthesis gas for production of synthetic fuels. Syngas derived from biomass, treated in a fluidized-bed gasifier, suffers from contaminants released during thermal conversion. These contaminants can harm downstream equipment, e.g. fouling, filter plugging and poisoning of catalysts. The most important detrimental inorganic contaminants are alkalis and sulphur compounds with other contaminants including HCl, ammonia, particles, etc. [1]. In order to avoid the mentioned problems, nowadays, the product gas is quenched as soon as it leaves the gasifier. Depending on the follow up process, the product gas is heated up again for subsequent upgrading. Therefore, one of the major points for higher efficiency is an effective hot gas cleanup technology [2].
The “UNIQUE” project [3] aims at a compact version of a gasifier by integrating the fluidized bed steam gasification of biomass and hot gas cleaning and conditioning into one reactor vessel (Fig. 1). The concept is based on the Güssing gasifier in Austria [4]. It consists of two connected fluidized bed systems. Biomass is gasified at about 850 °C with steam. A part of the remaining coke is transported to the combustion chamber together with the bed material (olivine), which is also used as heat carrier. Thus, the heat of combustion is utilized to run the endothermic steam gasification. The integration of gasification and hot gas cleaning will be obtained by placing a bundle of catalytic ceramic candle filters in the gasifier freeboard and furthermore, by using a catalytically active mineral substance for primary tar reforming and by addition of sorbents into the bed for removal of detrimental trace elements. The main objective is to develop an innovative technology for the production of syngas with the specifications required for use in fuel cells (H2S, HCl, KCl < 1 ppmv) in a cost-effective way, while keeping high the thermal efficiency of the whole conversion process as no cooling step is included.
Several investigations on chemical hot gas cleaning show that aluminosilicates are suitable for alkali sorption [5], [6], [7]. Investigations on the sorption mechanism show the necessity of water [8], [9], [10], [11]. The gas purity downstream the sorbent, which is the actual important figure, was rarely determined in a direct way. It was determined either indirectly by analyzing the sorbent capacity and back calculation of the gas purity or by condensing the gas stream [12], [13], [14].
H2S is the most released sulphur species [15]. The ability of metal oxide sorbents for H2S removal and their working temperatures are well known [16], [17]. The achievable H2S concentrations in gasifier derived gases range from about 1 ppmv for CuO to 150 ppmv for CaO [18]. Even though the sorption reaction already shows its sensitivity to water many syngases used for those investigations were balanced with nitrogen or helium [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30]. Investigations conducted in non balanced syngases with steam contents of 5 vol.% are subjected to significantly different conditions [31], [32]. Thus, the used “syngas” compositions differ from those in the Güssing gasifier, so that the achievable gas purity under Güssing gasifier conditions still has to be determined.
The removal of HCl was mainly investigated at temperatures up to 650 °C using Ca-based, Na-based, and mixed Ca–Na sorbents [33], [34], [35], [36], [37], [38], [39]. Fujita et al. [40] investigated hydrogrossular at temperatures up to 950 °C. While this sorbent can reduce the HCl level near zero in dry atmosphere, the HCl fixation is suppressed by water at higher temperatures, e.g. above 767 °C in presence of 10% water in the gas. Thus, this type of sorbent is not suitable under Güssing gasifier conditions.
Chemical equilibrium calculations done so far showed good agreement with experimental data, so that these are suitable tools for feasibility studies for biomass gasification [41].
Aim of the present work was to proof the feasibility of chemical hot gas cleaning under Güssing conditions. The main focus of this work was laid on KCl, H2S, and HCl removal.
Section snippets
Process model
In order to determine the achievable KCl, H2S, and HCl concentrations of derived gases in presence of sorbents, a thermodynamic process model of the Güssing plant (Fig. 2) was designed using SimuSageTM (GTT-Technologies). The results of the thermodynamic process model, which is a set of interlinked reactors with local equilibrium, are obtained by Gibbs free energy minimization. The Fact 5.3 pure substances database (GTT-Technologies) was used as thermodynamic data base for the equilibrium
Results of thermodynamic calculations
The calculated alkali and chlorine concentrations in biomass derived and cleaned syngas at 850 °C are shown in Fig. 4. Potassium species have higher concentrations than sodium species in biomass derived syngases. Therefore, the potassium reduction from up to 24 ppmv seems to be the most important requirement for the decrease of condensation temperature downstream the gasifier. However, due to the higher partial pressure over the formed Na-containing phase albite (NaAlSi3O8) in comparison to the
Conclusions
The UNIQUE project aims at integrating the fluidized bed steam gasification of biomass and the hot gas cleaning and conditioning system into one reactor vessel by placing a bundle of catalytic ceramic candle filters in the gasifier freeboard, by using a catalytically active mineral substance for primary tar reforming, and by addition of sorbents into the bed for removal of detrimental trace elements.
In order to develop a chemical hot gas cleaning integrated into fluidised-bed steam gasification
Acknowledgement
The work described in this paper has been done in the framework of the UNIQUE project, funded by the EC in the 7th framework program (Contract number 211517).
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