Sulfur containing organic compounds in the raw producer gas of wood and grass gasification

The detailed description of gas streams in biomass gasification plants is necessary for the correct design and operation of these units. Sulfur containing compounds are usually present in biomass gasification gas, since sulfur is typically found in the feedstock. Sulfur compounds are important contaminants present in the gas streams, since even at concentrations as low as a few ppm they poison catalysts causing significant technical challenges to the production process. The determination of contaminants is often challenging due to their low concentration and the presence of steam and tars in the gas streams. Here a method is presented, which allows the qualitative and quantitative analysis of an extensive number of organic sulfur compounds found in low concentration in biomass gasification gas. The method is a combination of an adequate sampling technique (based on the liquid quench of the sampled gas) and a sensitive analytical equipment (gas chromatograph coupled with a sulfur chemiluminescence detector, CG/SCD). This work shows that several organic sulfur species are found in biomass gasification gas, which are usually not reported, but have to be considered for the design of biomass-based gasification plants. The presence of these compounds is discussed considering the feedstock used, gasification conditions and the sampling technique. Moreover, the results presented here evidence that only measuring thiophene, benzo[b]thiophene and dibenzothiophene in the producer gas can be misleading, since the sum of concentrations of all other organic sulfur compounds could be above the tolerable limits for total sulfur in gasification-based processes. (C) 2014 Elsevier Ltd. All rights reserved.

Investigations into the Effects of Biofuel Contaminants on Solid Oxide Fuel Cells

Hossein Madi

Solid Oxide Fuel Cells (SOFCs) have gathered considerable attention as a clean, highly efficient conversion device for the production of both electricity and heat from fuels. Due to their high operating temperatures, SOFCs do not require pure hydrogen as fuel, but exhibit a high fuel flexibility, a major advantage concerning the high cost of hydrogen production. Nonetheless, certain requirements apply in terms of fuel purity. The presence of contaminants such as sulfur compounds, siloxanes, halogens, volatile organic compounds and tars within the fuel stream, even at trace levels can significantly reduce the lifetime of key components such as the fuel cell stack and reformer. Cleaning technologies can remove harmful impurities from the biofuels so as to meet the cleanliness requirements of SOFC stacks, though these come at higher cost. The purpose of this study is to provide guidelines regarding the maximum impurity concentrations in the biofuels which SOFCs can tolerate. This work has two principal aims: (i) to identify any threshold tolerance limits of different contaminants, if they exist and (ii) to investigate degradation mechanisms caused by contaminants. Siloxane D4, hydrogen chloride and sulfur were selected as biogas impurities. In addition, the effect of toluene (a common impurity in wood gasifier product gas) was also investigated on the performance of Ni-YSZ SOFCs. Short tests lasting 100-500 h were carried out to study the influence of the selected impurities on state-of-the-art Ni anode-supported SOFCs. Experiments were performed on single cells provided by Topsoe Fuel Cell (TOFC) and SOLIDpower and in some cases on 11-cell short stacks, provided by TOFC. Contamination tests were performed on simple hydrogen feed and more complex feeds such as simulated biogas or reformed biogas. Furthermore, in the cases of contamination with siloxane D4 and hydrogen chloride, single cell results were compared with those of short stacks. In-situ (electrochemical impedance spectroscopy- EIS) and ex-situ (electron microscopy and chemical analysis) characterization methods were used to identify the prevailing degradation mechanisms. Suggestions were made, which Ni-YSZ anode processes are affected by each contaminant. Siloxane deposit as SiO2 and block electrode pores and TPB (triple-phase-boundary) electrochemical reaction zone; HCl adsorbs on Ni particles to hinder the electrochemical reaction; sulfur, both from H2S and thiophene, adsorbs reversibly on Ni and leads to an immediate sharp cell voltage drop, which remains small at low S-concentrations. It was found that the complete removal of siloxanes is necessary, that S-compounds are acceptable to a level of 0.5 ppm and that there is no need to remove HCl and tar compounds from the biogas, for the concentration levels in which they are expected to be found in anaerobic digester gas.

EPFL2016

Gasification of Hay in a Bench Scale Fluidised Bed Reactor with Emphasis on the Suitability for Gas Turbines

Christian Ludwig, Marco Wellinger

Thermal gasification of pasture plants (hay) is so far considered as difficult mainly due to the high amount of inorganics (ash, alkalis). Under certain circumstances it could be an additional resource for power production. There has been some experiments to test hay in gasification plants but most lack inorganic analysis. A bubbling fluidized bed was set up and gasification experiments with extensive analysis of the gas composition was conducted. Dolomite, silica and alumina particles were used as bed material. Silica proved to work successfully at 700 degrees C. Ash experiments showed that at higher temperatures silica forms compounds that melt and lead to unstable gasification conditions. Dolomite proved to be not stable enough for fluidised bed conditions, since it was ground down to smaller particles and subsequently entrained. Alumina was the best bed material tested, which was used up to 750 degrees C with good results. The longest run was 10 h at 750 degrees C without defluidisation. Despite the low temperature in the process the bulk tar concentration was low. The heating value of the gas was good (4-6 MJ m(-3)). The concentration of sodium and potassium in the syngas after the warm gas filter at 400 C is low enough towards a cofiring IGCC application with 10% energy input from hay. (C) 2012 Elsevier Ltd. All rights reserved.

Elsevier2012

Liquid-Quench Sampling System for the Analysis of Gas Streams from Biomass Gasification Processes. Part 2: Sampling Condensable Compounds

Christian Ludwig, Tilman J. Schildhauer

The performance of a continuous liquid-quench-based sampling system is presented, with regard to the sampling of condensable compounds. Toluene is used as a model molecule representing tars commonly found in the gas streams of biomass gasification plants and related research units. It is shown that the operational parameters of the sampling system can be adjusted for a correct sampling of condensable compounds. Moreover, the systematic measurement uncertainty caused by the use of the sampling system is evaluated in the case of the characterization of a raw producer gas from a bubbling fluidized-bed gasifier considering both condensable and noncondensable compounds. It is concluded that the systematic uncertainty is small for H-2, CO, CH4, C7H8, and C10H8, comparable to the random measurement uncertainty for C2H4, C2H6, N-2, H2O, and C6H6 and larger than the random measurement uncertainty for CO2, C2H2, C3H6, and COS.

Amer Chemical Soc2012

Investigations into the Effects of Biofuel Contaminants on Solid Oxide Fuel Cells

Hossein Madi

EPFL2016