The main aim of this process is to overlapp chemical absortion of CO2 with the gasification reactions in a dual fluidised bed steam gasifier. The work is performed under the EC-projects "AER-Gas I" and "AER-Gas II". The coordinator of the 2 projects is Center for Solar Energy and Hydrogen Research. More information can be found at http://www.aer-gas.de.
The main characteristic of the proposed process for efficient and low cost conversion of biomass is the CO2 removal in the reaction zone of the gasifier. Due to the shifting of the reaction equilibrium the hydrogen concentration increases significantly. Therefore, the single step generation of a product gas with a high hydrogen content for fuel cell application or synthesis gas production is achievable. As the CO2 absorption is a highly exothermic reaction, the realised heat is integrated directly into the endothermic gasification/reforming process. The spent absorbent has to be regenerated in a subsequent process step.
For the realisation of the proposed technology, a dual fluidised bed reactor is employed containing a CO2 absorbent, e. g. dolomite. The development of a catalytic absorbent material with high tar cracking efficiency is also a key aspect of the AER process. Therefore, the ongoing project work is focused on the investigation of different natural and synthetic absorbent materials in regard to their CO2 absorption capacity, chemical and mechanical stability under real process conditions with repeated absorption - regeneration steps. The process parameters defined in a fixed bed and in a fluidised bed reactor are applied to a circulating fluidised bed system (Fast Internally Circulating Fluidised Bed, FICFB reactor), which allows a continuous production of hydrogen parallel to absorbent regeneration.
Based on the results of the laboratory work, the process will also be implemented at the biomass CHP Güssing, to test the process also under industrial scale and conditions.
The first step was to find a suitable adsorbens. Here different partners in the project tested almost 100 possible adsorbens on their CO2 capacity, their attrition resistance and their catalytic activity for tar reforming. The adsorbents were classified according to their properties and the best adsorbens were tested in the 100kW gasifier.
An example for the classification for some materials is given in the following figure:
After the selection of the best materials, these were tested first at Stuttgart in a singel fluidised bed and afterwards at Vienna, University of Technology in the dual fluidised bed, with contineous absorbtion and regeneration. During contineous operation a hydrogen content of almost 70% could be achieved in most of the experiments. A typical gas composition of the 100kW gasifier during AER-mode is shown in the following figure:
In future work, more absorbens will be tested and also the type of biomass (wood pellets, straw pellets, etc.) will be varied.
After the proof of concept was done very succesful during the first AER-Gas project, it was decided to test the concept also at the biomass CHP Güssing at industrial scale and operating conditions. Two test runs are foreseen in this project, the first was done during summer 2007 and the second is planned for 2008. The main aim of the first run was to check, if in principle it is possible to operate the biomass CHP under AER-conditions and during the second run it will be tried to operate the biomass CHP for about 2 weeks under AER-conditions.
The first step to realise the AER-conept at the biomass CHP Güssing, were cold flow model experiments. Here the optimal flows, bed material particle size, etc. for the dual fluidised bed were determined and according to these results the biomass CHP was operated during the test run.
The experimental test run can be divied into 3 time sections:
The experiment run very well and at gasification temperatures below 690°C a noticable absorption of CO2 could be observed. The gas composition during normal operation, AER-Gas mode and in comparison from the 100kW gasifier at TUV is given in the following figure:
The experiments at the 100kW dual fluidised bed gasifier showed, that direkt in the gasifier hydrogen contents above 70vol% can be achieved. For other applications like synthesis, also the H2:CO ratio can be adjusted.
The experimental run at the biomass CHP Güssing showed, that the AER-gas concept can also be realised in an existing dual fluidised bed steam gasification plant without major modifications.
The main results were: