Abstract
The main product of biomass gasification process is a gas, called producer gas, that is a mixture of CO, H2, CH4, CO2, and N2. The producer gas can be used for combined heat and power production, or for production of synthetic liquid fuels and chemicals. During gasification, by-products are also produced, such as char, the solid residue, which is currently treated as a waste and whose disposal represents a significant cost for plant owners.
Char is a highly carbonaceous material with a well-developed porosity and a high amount of ash and mineral species. These characteristics suggest many similarities with activated carbon, which is usually derived from charcoal and used in adsorption and catalytic applications. Therefore, many studies have been focusing on char properties for using it as a substitute for activated carbon. In the framework of a circular economy, the re-purpose of char and its valorization have multiple benefits, since char is currently an expense in the management of a gasification plant and its use could avoid the continuous purchase of expensive adsorbents and catalysts. The aim of this research project is to explore ways to turn gasification char from waste to value-added product. Both low temperature applications (as adsorbent) and high temperature applications (as catalyst) are investigated.
In particular, char from local commercial gasification plants has been tested for CO2 and H2S adsorption, and for catalytic tar cracking. As adsorbent, char proved to be very effective for adsorption of H2S, which is a common pollutant in producer gas and in biogas from anaerobic digestion; but also for adsorption of CO2 for carbon capture and storage. Its performance was compared with that of commercial activated carbon, proving to be extremely competitive in terms of adsorption capacity.
Char showed good activity as catalyst too, giving high values of tar conversion. Tar, which is the liquid by-product of gasification, composed of organic contaminants, by condensing in the system can block and damage turbine blades, heat exchangers, pipes, or any other processing equipment downstream of the gasifier. It is thus of paramount importance to reduce the concentration of tar, if not remove it completely. Many methods are currently available for the removal of tar from producer gas, but catalytic methods are widely reported to be the most effective ones, and char, for its peculiar characteristics, has raised a lot of interest for this type of application. Furthermore, char has the advantage of being inexpensive and readily available in the gasifier. The gasification char used for this study was tested for catalytic cracking of toluene, which was chosen as model tar compound.
Different operating conditions were tested, varying the test temperature and the feed-gas composition. Eventually, a detailed characterization of the material was carried out for understanding the changes occurred in its structure and properties.
Overall, the results obtained for the different pathways investigating char repurposing are very promising. Char collected from existing commercial gasification plants, destined to be discarded as waste, proved to be suitable for both adsorption and tar catalytic cracking applications. Further tests involving application in real-scale systems would be necessary. However, it is of great interest that what is currently a waste product and a cost to gasification plants could be efficiently used, either as it is or after minor activation processes, and be considered as a valuable product instead.