(135g) On the Behaviour of Highly Volatile Feedstocks in Fluidised Bed Reactors during Advanced Thermochemical Conversions

For the last 200 years fossil fuels (e.g., coal, oil and natural gas) have provided us with cheap and convenient sources of energy. However, their massive utilization has also caused many problems to the environment, such as ozone depletion and global warming, associated with various emissions to land, water and air. Since the world energy consumption is destined to grow, finding effective, sustainable solutions to prevent the effects of anthropogenic global warming, will be the greatest challenge that our worldwide community face in the 21^st century. Further to struggling with the effects of global warming, our society needs to face another great challenge: to develop economic and environmentally acceptable solutions for managing the ever-increasing volumes of municipal solid waste that are generated worldwide. Although recycling clearly has a critical role to play in reducing the amount of waste, there is further opportunity and environmental benefit in recovering energy from what might previously have been seen as materials destined for landfill. In particular, advanced thermochemical technologies (ATTs), like pyrolysis and gasification, have an important role to play in converting waste into clean energy or fuels, hence promoting the green energy transition. ATTs can be easily coupled with intermittent renewable energy, such as solar or wind, in order to enhance the performance of operation and further reduce the carbon footprint. In the last years, great interest has been showed towards biohydrogen as an alternative source of energy and ATTs in combination with renewable energies represent an attractive and sustainable alternative for the production of low-carbon hydrogen. All these aspects are of prime importance to carry forward the concept of the decarbonisation and meet the goal of ‘net-zero’ for 2050.

Fluidized bed reactors are among the most promising technologies for the thermochemical treatment of solid feedstocks for sustainable applications, due to their enhanced heat and mass transfer, and operating flexibility. Nevertheless, there are still unsolved challenges when operating with highly volatile solid feedstocks, such as biomass and waste. Because of their relatively low density, these materials experience axial segregation, tending to segregate along the bed height causing complications in the hydrodynamics of the bed. This behaviour is typically observed when operating with biomass. Moreover, volatiles released within the bed evolve in form of endogenous bubbles, which further enhance segregation of the feedstock. In addition, further issues arise because of the presence of synthetic polymers in waste, which tend to form agglomerates that sink to the bottom of the reactor. This phenomenon is detrimental as it causes de-fluidization of the bed and early shutdown of the operation. In both cases, solid and gas phases do not take advantage of the bed-to-fuel transfer phenomena, which are essential for high product yields and quality. All these aspects are essential part of the design of industrial fluidized bed units and relevant to all thermochemical conversions, since the evolution of volatiles is the first and common stage in pyrolysis, gasification and combustion operations.

The aim of this work is to provide a better understanding of the devolatilization behaviour and bed-feedstock interaction in fluidized bed reactors. This is done by investigating reacting biomass and plastic particles in a lab-scale fluidized bed reactor at high temperatures and different fluidization regimes relevant to industrial operations. The experiments were conducted in a fluidized bed operated at different temperatures ranging from 500 to 650 °C, which fall in the typical range of advanced thermochemical treatments conditions, and under either inert (pyrolysis) or semi-oxidizing (gasification/combustion) conditions. Beech wood and polypropylene spherical particles in the range of 8-12 mm were chosen to resemble the biomass and plastic components of waste, respectively. A novel non-invasive X-ray imaging technique, coupled with gas analysis measurements, has been used to investigate the thermal decomposition behaviour of a single particle after under-bed feeding via a purposely designed single particle injector. A small tracer of lead ranging from 1.5 to 2 mm was inserted into the half-drilled biomass and plastic particles in order to make them visible upon X-ray exposure. X-ray images have been collected with a time resolution of 36 frame-per-second and spatial resolution of about 1.6 mm/pixel. Algorithms of images post-processing and particle tracking have been developed in MATLAB environment in order to investigate the dynamic behaviour of the feedstock particle within the bed. A comprehensive assessment at minimum fluidization condition provided deeper insight about the devolatilization behaviour within the bed, which was found to be independent of the oxidizing nature of the fluidization medium. Size and void fraction distribution of endogenous bubbles were measured via X-ray imaging. Results showed that a fully developed endogenous bubble carries with it a volume of volatiles considerably larger that the volume predicted by a pseudo-first order rate law, which is usually used to describe the devolatilization behaviour of solid feedstocks. The evolution of void fraction of the endogenous bubbles released showed the presence of three main regions, namely cloud, wake and centre of the bubble, which mainly consists of gas produced form the devolatilization of the particle. In particular, the presence of the wake phase is known to be responsible of the upward movement of solids in fluidized beds. This finding confirms the existence of a lift effect acting on the feedstock, which can be assumed to occur when the bubble is fully formed and completely detached from the fuel particle. Gained knowledge provided by direct visualization from the collected X-ray images allowed a quantitative assessment of the lift effect acting on the reacting particle, which can then be used to develop a one-dimensional model to validate experimental results. Interestingly, the in-bed release of volatiles appeared to be independent of the presence of oxygen in the fluidizing medium. This finding was supported by visual evidence provided by X-ray visualization, which showed that endogenous bubbles size and frequency are independent of fluidizing gas properties. This behaviour was attributed to the controlling oxidation mechanisms of the volatile matter released, as demonstrated by the interchange phase theory of oxygen between emulsion phase and endogenous bubbles. The model showed accurate predictive capabilities for biomass particles at both minimum fluidization and bubbling regimes, while it failed in predicting the behaviour of plastic particles. The peculiar behaviour observed for this material was associated with its completely different mechanism of degradation, where the melting step was believed to have a great impact on both volatiles release and particle motion within the bed. The change in particle physical properties and formation of dense plastic-sand agglomerates might be responsible for the behaviour of this material.

Further experimental work will be conducted in order to gain a better understanding of the thermal decomposition behaviour of plastic materials in fluidized beds. The main focus will be on the mechanism of agglomeration of polypropylene particles at high temperatures. Experiments will be carried out by inserting a thermocouple inside the half-drilled sample and recording the temperature profile during the particle decomposition. This information will be essential to understand the thermal history of polypropylene under the thermal degradation conditions of fluidized beds, which details, unlike traditional thermogravimetric reactors, are still uncertain. Moreover, thermal information of the reacting particle might be crucial to understand the characteristic time of formation of plastic-sand agglomerates, and therefore explain their interaction with fluidized beds described previously. The collected data might be useful to build a mathematical model of plastic degradation in fluidized beds, in order to implement and improve existing numerical and CFD models.

Results obtained from this study highlighted the importance of developing systematic methodologies for the investigation of single particle devolatilization which, at present, appears to be the most effective approach to gain deeper insight on the interaction between feedstock and fluidized bed during thermochemical conversions. This is a central aspect for future development and exploitation of novel technologies, as well as improvement of existing ones, for thermal treatments of solid waste feedstock.