Experimentelle und simulative Untersuchung der Wasserdampfvergasung von Klärschlamm und weiteren biogenen Brennstoffen

Thesis of Daniel Schweitzer
Universität Stuttgart, 2018

The allothermal dual fluidised bed steam gasification process is a thermochemical conversion process for converting a solid fuel into a hydrogen-rich and high quality product gas. By using experimental and simulative methods, the effect of different low-cost biogenic fuels such as sewage sludge, cattle- and pig manure as well as wood pellets (as reference fuel) on the gasification process was investigated.
The experimental investigations have shown that steam gasification of these biogenic fuels is possible: In the relevant temperature range of 750 - 950 °C, no bed agglomerations were created; however, when using ash-rich fuels such as sewage sludge, the ash sintered into mechanically stable ash clusters, which can cause hydrodynamical problems in the process. Investigations have shown that the fuel preparation can influence the particle size distribution of these ash clusters. It was shown that the hydrodynamic problems of the used experimental plants can be minimised by grinding the fuel to a maximal particle size of 2 mm.
Gasification experiments of the biogenic fuels have shown that a high gas yield of 0.7 - 0.8 m3/kgfuel,daf can be achieved. Hence the gas yield of the biogenic fuel is just about 20 % below the gas yield of wood pellets. Likewise the product gas composition varies only slightly between the fuels. Just for sewage sludge higher H2-concentrations and slightly lower CO-concentrations in the product gas were measured.
But the significantly higher concentration of trace gases in the product has to be considered, when gasifying biogenic fuels. Especially for sewage sludge very high (gravimetric) tar yields of up to 90 gtar/kgfuel,daf were measured. Furthermore, when gasifying biogenic fuels, high NH3 concentrations of up to 6 vol.-%dry, H2S-concentrations of up to 7000 ppmvdry and Cl-concentrations of up to 1300 ppmvdry were measured. Likewise the concentration of trace gases (NOx, SOx, HCl) in the flue gas of the combustor increased significantly. However, it was shown that by selecting suitable bed materials, the concentration of these trace gases in the product gas of the gasifier can be reduced significantly. Especially limestone is suitable for reducing the tar concentration in the product gas. Even at a low limestone content of 20 wt.-% in the bed material, the tar concentrations can be reduced by about 80 %. Furthermore limestone showed a positive effect on reducing the NH3 and H2S concentrations in the product gas.
Additional experiments have shown that the accumulating ash can cause undesired oxidation and reduction reactions in the process: reduction reactions of the fuel ash in the gasifier can lead to an oxygen transport from the combustor into the gasifier and hence to a decrease of the H2- and CO-concentration in the product gas. Such a redox-system can decrease the gas yield and shift the heat balance.

Based on the experimental data, a process model was created for the steam gasification of sewage sludge. By using this model a energy balance of the process was derived. The results of the process simulations showed that, in comparison to the gasification of wood pellets, due to the lower fuel conversion, no additional energy is necessary in the combustion reactor at gasification temperatures of 800 °C. By using process simulations cold gas efficiencies close to 70 % were calculated. Furthermore it was shown that the side reactions caused by the ash can have a significant effect on the overall efficiency of the process.

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