Investigation of single particle devolatilization in fluidized bed reactors by X-ray imaging techniques

A non-intrusive X-ray imaging technique has been used to investigate the behaviour of solid feedstock particles in a lab-scale fluidized bed reactor operated at temperatures up to 650 °C. Beech wood and polypropylene particles of different sizes have been chosen to represent the main constituents of typical thermochemical processes feedstock. The experiments were conducted under either oxidizing or inert conditions. The presence of oxygen showed a strong effect on the overall devolatilization time, which was found to be in the range of 30-112 seconds and 40-174 seconds for beech wood and polypropylene, respectively. Surprisingly, the oxidizing nature of the fluidizing medium appears to have no influence on the volatiles release within the bed in form of the so-called endogenous bubbles. These volatiles bubbles are responsible for a lift force acting on the feedstock particle itself, which ultimately encourages the segregation towards the bed surface. A one-dimensional physical model has been developed to predict particle axial location over time, taking into account both dynamic and thermal conversion behaviour of a single feedstock particle. A revised version of the model has been proposed due to new knowledge of endogenous bubbles size provided by a novel X-ray imaging approach. Results showed very accurate predictions of the 1D model for biomass particles, which segregate towards the bed surface according to the multiple bubble segregation pattern. However, the model fails in describing plastics behaviour, possibly due to different mechanisms of reactions. The observations reported in this work show the importance of investigation at single particle level and may serve to promote new methods to gain a better understanding of plastics thermal decomposition in fluidized beds, whose mechanism is still uncertain

Steam - oxygen gasification of refuse derived fuel in fluidized beds: Modelling and pilot plant testing

A one-dimensional kinetic model for steam‑oxygen gasification of refuse derived fuel in a bubbling fluidized bed reactor has been developed. The model incorporates the reaction network of steam‑oxygen gasification within the fluid dynamics of a fluidized bed to predict waste and tars conversion, gas composition and overall gasification performance. The model was validated by comparing outlet products composition and temperature profile with experimental data from a pilot-scale fluidized bed gasifier, operated at different conditions. The model showed accurate predictive capability and ease of computation. The effects of the operating conditions on gas yield and process efficiency were evaluated and the most appropriate fuel feeding height, equivalent ratio and the relative amount of steam to inject were identified

Application of acoustic techniques to fluid-particle systems – A review

Acoustic methods applied to opaque systems have attracted the attention of researchers in fluid mechanics. In particular, owing to their ability to characterise in real-time, non-transparent and highly concentrated fluid-particle systems, they have been applied to the study of complex multiphase flows such as fluidised beds. This paper gives an overview of the physical principles and typical challenges of ultrasound and acoustic emission AE methods when applied to fluid-particle systems. The principles of ultrasound imaging are explained first. The measurement techniques and signal processing methodologies for obtaining velocity profiles, size distribution of the dispersed phases, and solid volume fraction are then discussed. The techniques are based on the measurement of attenuation, sound speed, frequency shift, and transit time of the propagated sound wave. A description of the acoustic emission technique and applications to fluid-particle systems are then discussed. Finally, extensions and future opportunities of the acoustic techniques are presented

Axial segregation behaviour of a reacting biomass particle in fluidized bed reactors: experimental results and model validation

Axial segregation behaviour of a single biomass particle in a lab-scale bubbling fluidized bed has been investigated from both experimental and modelling perspectives. Experiments were conducted using beech wood particles of different sizes, ranging from 8 to 12 mm under either oxidizing or inert conditions. The fluidized bed reactor was operated at temperatures and fluidization velocity ratios, U/Umf, in the range of 500–650 °C and 1–2, respectively. A one-dimensional model has been developed to predict the axial location of the particle over time, taking into account both dynamic and thermal conversion mechanisms. X-ray imaging techniques allowed to identify endogenous bubbles released during devolatilization and carry out direct measurements of their size. This information was used to propose an expression for the lift force acting on the fuel particle. The model showed very accurate predictions and the segregation behaviour of the fuel particle appeared to be independent of the nature of the fluidizing medium

Combining multi-typologies landslide susceptibility maps: a case study for the Visso area (central Italy)

The research proposes a simple but geomorphologically adequate method to produce a combined landslide susceptibility map. In fact, in a logic of real use, offering type-specific landslide susceptibility maps to land use planners and administration could be not a successful solution. On the other hand, the simple grouping of more types of landslides could be misleading for model calibration considering that the relationships between slope failures and geo-environmental predictors should be conveyed by the abundance of each type of landslide resulting not specific and diagnostic for each typology. In this test, after having produced independent models for flow, slide and complex landslide by exploiting MARS (Multivariate Adaptive Regression Splines) and a set of type-specific geo-environmental variables, a combined landslide susceptibility map was obtained by combining the scores of the three source maps. The combined map was finally validated with a new unknown archive, showing very good performances

Thermodynamic modelling and evaluation of a two-stage thermal process for waste gasification

Tar generation and ash disposal represent the strongest barrier for use of fluid bed gasification for waste treatment, whereas sufficing for both is only possible with expensive cleaning systems and further processing. The use of plasma within an advanced two-stage thermal process is able to achieve efficient cracking of the complex organics to the primary syngas constituents whilst limiting the electric power demand. This study focused on the thermodynamic assets of using a two-stage thermal process over the conventional single-stage approach. These include, for example, the fact that the primary thermal waste decomposition is performed in conditions of optimal stoichiometric ratio for the gasification reactants. Furthermore, staging the oxidant injection in two separate intakes significantly improves the efficiency of the system, reducing the plasma power consumption. A flexible model capable of providing reliable quantitative predictions of product yield and composition after the two-stage process has been developed. The method has a systematic structure that embraces atom conservation principles and equilibrium calculation routines, considering all the conversion stages that lead from the initial waste feed to final products. The model was also validated with experimental data from a demonstration plant. The study effectively demonstrated that the two-stage gasification system significantly improves the gas yield of the system and the carbon conversion efficiency, which are crucial in other single stage systems, whilst maintaining high energy performances

Preliminary Assessment of Sorption Capacity on Solid CO2-Sorbents at Conditions for Sorption-Enhanced Processes

This work aims to assess solid sorbent capacity to operate CO2 capture under industrial conditions relevant to biogas/bio-syngas upgrading systems to green H2 and food-grade CO2 through Sorption-Enhanced Water Gas Shift (SEWGS) technologies. The pursued degree of innovation is the process intensification to remove CO2 in a more sustainable industrial practice reducing the CO2 footprint of a potential H2 production process. A lab-scale apparatus is appropriately designed and built to operate at relevant industrial scale conditions. The core of the system is a fixed-bed reactor equipped with mass flow meters/controllers and online gas analyzers. CO2 capture experiments were carried out to investigate the effect of pressure (1.0-1.4 MPa) on different commercial and synthesized solid sorbent materials (hydrotalcite-like compounds). The best sorbent is a commercial hydrotalcite impregnated with 20 wt% of K2CO3, with an average sorption capacity of 0.85 mmolCO2/gad at 1.4 MPa and 623 K. The explored conditions are compatible with an industrial operation where syngas is available at low-to-moderate pressure

Minimally Invasive Video-Assisted Parathyroidectomy: Lesson Learned from 137 cases

Abstract: Background: Since February 1997, a technique of minimally invasive video-assisted parathyroidectomy (MIVAP) was developed at our institution for the treatment of sporadic primary hyperparathyroidism (sPHPT). In this study we analyzed the entire series of patients who underwent MIVAP during the last 3 years. Study Design: One hundred thirty-seven patients with sPHPT were selected for MIVAP. Selection criteria were: diagnosis of single adenoma based on preoperative localization studies (ultrasonography, sestamibi scintigraphy, or both), and no previous neck surgery or concomitant large multinodular goiter. The procedure, already described, is performed by a gasless video-assisted technique through a single 1.5-cm central skin incision above the sternal notch. Quick, intraoperative parathyroid hormone assay was used in 134 cases (97.8%) to confirm the complete removal of all hyperfunctioning parathyroid tissue. Results: Mean operative time was 54.3 +/- 22.6 minutes. The conversion rate was 8.8%. One laryngeal nerve palsy was registered (0.7%), as was one case of persistent hyperparathyroidism. In six patients (4.4%) a transient symptomatic postoperative hypocalcemia was observed. Two thyroid lobectomies were associated using the same minimally invasive access. At a mean followup of 15.4 +/- 10.6 months, all but two patients were normocalcemic. The cosmetic result was considered excellent by most of the patients (92.8%). Conclusions: Although not all patients with sPHPT are eligible for MIVAP, this approach can now be proposed in a bigger proportion (67% of patients). As already demonstrated in a previous study, also in a large series of patients, after greater experience has been achieved, the results and the operative time are the same as in traditional surgery, with better cosmetic result and a less painful course. (J Am Coll Surg 2000; 191:613-618. (C) 2000 by the American College of Surgeons)