Design and Operation of a Chemical-Looping Reformer for Catalytic Upgrading of Biomass-Derived Gas
Doctoral thesis, 2013
The removal of tar from the raw gas produced during biomass gasification is necessary before the gas can be used as a feedstock for synthetic fuels. The currently commercially available technologies for gas cleaning, i.e., scrubber systems, have various disadvantages, such as heat penalties and loss of the energy that is chemically stored within the tars. In addressing these problems in this work, a novel method for secondary tar cleaning is proposed and experimentally validated. This method combines tar cleaning with catalyst regeneration in a system of dual fluidized beds, which is experimentally explored in two reactors systems of identical size: a cold Perspex unit; and a high-temperature steel unit. Fluid dynamics and operation controllability were evaluated in the cold system; the results indicate that solid circulation is controllable and that adequate gas tightness of the loop-seals can be achieved. The knowledge gained from operating the cold system was subsequently used to demonstrate the tar-cleaning method in the hot reactor system. This was done using the raw gas from the Chalmers biomass gasifier, which contained 20–30 gtar/Nm3tar. The tar-reforming activity, catalyst regeneration, and potential for catalytically adjusting the raw gas were investigated. The results of these experiments indicate that almost all tars can be reformed using a catalyst that consists of nickel oxide supported on an α-Al2O3 matrix (NiO/Al2O3). Promising tar-reforming properties were also observed when the naturally occurring ore ilmenite (FeTiO3, in its most reduced form) was used; in this set-up, approximately 60% of the tar fraction was reduced. A clear trend towards enhanced hydrogen production was seen in the reactor system, and this was enhanced with increases in the operating temperature. The accumulation of coke on the catalyst was minimized, as the transported carbon was continuously oxidized to CO2 in the regenerator, even though as much as 30 gtar/Nm3gas were feed into the reformer reactor. Deactivation of the catalysts was not detected during the experiments.
A method for the online determination of the moisture content of gas, which was originally developed for measuring the moisture content of the raw gas from a gasifier, was experimentally evaluated using flue gases from biomass combustion in a grate furnace and in a CFB boiler. In the CFB experiments, the moisture-measuring unit was operated in series with an FT-IR cell, to enable comparison with a well-established technique. The proposed method gave highly accurate measurements and a response time that was in seconds, in the absence of significant hysteresis.
Possibilities to increase the oxygen distribution and even out both the temperature and the profile of unburned species in a CFB furnace were investigated by the incorporation of a metal oxide (ilmenite) into the bed material. The investigation was conducted in the Chalmers 12-MWth boiler/gasifier system, and up to 40 wt.% of the ordinary used silica-sand bed material was replaced with ilmenite. Experiments, which initially involved the boiler only and that subsequently also included the gasifier, showed that the amount of unburned spices was lowered when ilmenite was mixed in with the bed material. In addition, the study revealed that more of the combustion reactions were allocated to the furnace when ilmenite was used, in comparison with using silica-sand only, in which case part of the combustion occurred in the cyclone. Given the similarities in process layout between the boiler/gasifier system and the proposed tar-cleaning reactor system, operation with metal oxides in boiler/gasifier system can provide important information related to the scale-up of the tar-cleaning method.
Catalytic gas cleaning