Strategies for controlling solid biomass conversion in dual fluidized bed gasifiers

Licentiate thesis, 2016

In the chemical industry, Synthesis Gas (syngas) has been traditionally produced by steam reforming of natural gas and naphtha. Steam gasification of biomass offers an alternative route for the production of syngas from renewable sources. However, the product of biomass gasification cannot be used directly in most applications, as it contains impurities, such as tar species. Tar, which is the Achilles heel of the biomass gasification technology, is a complex mixture of hydrocarbons, which can condensate at temperatures as high as 300oC, causing clogging of pipes and coolers, deactivating downstream catalysts, and forcing unscheduled shut-downs of the whole process. Gas cleaning and upgrading techniques, as well as methods for controlling the formation of tar are required to limit the tar concentration to acceptable levels. Numerous investigations of this topic have led to the merging of the fields of steam reforming and gasification, whereby catalytic materials are placed inside the gasifier with the goal of steam reforming the undesirable tar species and converting them into valuable syngas. The feasibility of integrating gasification and catalytic steam reforming has been confirmed in several pilot and demonstration plants, with promising results. The preferred technology is indirect gasification in a dual fluidized bed (DFB) reactor, as it enables hosting of the catalyst in the steam gasifier. The levels of tar in biomass gasification remain high, and there is a lack of understanding as to how to control efficiently fuel conversion in general, and tar formation in particular. Efforts to reduce the load of tar have ranged from modifying the gasifier design to testing different catalytic materials, passing through optimization of operating conditions (e.g., temperature, steam-to-fuel ratio). However, previous investigations have mainly focused on assessing the marginal improvement in product gas quality that occurs after a modification in the operation. This work aims instead at identifying the most effective measures to control fuel conversion in DFB gasifiers, with the ambition of contributing to the rational operation and design of the gasifier. In this work, fluid-dynamics and chemical aspects of fuel conversion are explored experimentally under conditions relevant to industrial steam gasifiers. It is concluded that the activity of the catalyst is the primary tool to control both tar and char conversion, whereas the fluid dynamics of the bed plays a secondary role. Once the bed is well-fluidized, further optimization of the gasifier design results in relatively low improvement in terms of tar conversion, as compared to the benefits of using active bed materials. Accordingly, to improve the reliability of the biomass-to-syngas process, research efforts should be directed to understand catalyst activation, the functioning of the catalyst, and the mechanism underlying catalytic decomposition of tar.