Modeling Mercury Capture by Powdered Activated Carbon in a Fluidized Bed Reactor

A steady state model of mercury capture on activated carbon in a bubbling fluidized bed of inert material is presented. The model takes into account the fluidized bed fluid-dynamics, the presence of both free and adhered carbon in the reactor as well as mass transfer limitations and mercury adsorption equilibrium. The activated carbon adsorption parameters and the relative amount of free versus adhered carbon in the reactor have been estimated with purposely designed experiments. Model results are compared with results from mercury capture experiments conducted with commercial powdered activated carbon at 100°C in a lab-scale pyrex fluidized bed of inert particles. The role of free versus adhered carbon in determining the overall mercury capture efficiency is discussed

A twin-bed test reactor for characterization of calcium looping sorbents

The reduction of sorbent CO2 capture capacity and the extent of particle attrition over iterated cycles are relevant to design of Calcium Looping processes (1-2). Thermogravimetric analyzers or fluidized bed reactors are generally used to evaluate the sorbent performance. One drawback of these reactors is that they do not reproduce the thermal history that is actually experienced by sorbent particles in real looping cycles. In this study, a novel experimental technique is proposed to overcome this limitation. The apparatus consists of two interconnected fluidized bed reactors operating as calciner and carbonator, respectively (Fig. 1). The two reactors are connected each other by a duct (whose openings can be located at adjustable level above the gas distributor) which permits pneumatic transport of the solids between the reactors. Silica sand is used as buffering inert material to prevent excessive temperature fluctuations due to solid transport and chemical reactions. The operating conditions (fluidization velocity and duct height) of the reactor have been tuned to maximize transfer of the sorbent at each cycle, while limiting the transport of sand (Fig. 2). Further tests were carried out to simulate multiple calcination/carbonation cycles (Fig. 3). Under the optimal experimental conditions more than 95% collection efficiency of the limestone was obtained while less than half of the sand was transferred. Additional tests were carried out at high temperature but under non-reacting conditions, so as to simulate the real thermal history of the particles. Please click Additional Files below to see the full abstract

THE INFLUENCE OF PARTICLE ATTRITION ON SORBENT INVENTORY AND PARTICLE SIZE DISTRIBUTION IN AIR-BLOWN CIRCULATING FLUIDIZED BED COMBUSTORS

This paper presents a population balance model aiming at the prediction of sorbent inventory and particle size distribution establishing at steady state in the bed of an air-blown circulating fluidized bed combustor fuelled with a sulphur-bearing solid fuel. The core of the model is represented by population balance equations on sorbent particles which embody terms expressing the extent/rate of sorbent attrition/fragmentation. The effect of the progress of sulphation on attrition and fragmentation is taken into account by selection of appropriate constitutive equations. Model results are presented and discussed with the aim of clarifying the influence of particle attrition/fragmentation on sorbent inventory and particle size distribution, partitioning of sorbent between fly and bottom ash, sulphur capture efficiency. A sensitivity analysis is carried out with reference to relevant operational parameters of the combustor

Sulfur Uptake by Limestone-Based Sorbent Particles in CFBC: The Influence of Attrition/Fragmentation on Sorbent Inventory and Particle Size Distribution

This paper presents a population balance model aiming at the prediction of sorbent inventory and particle size distribution establishing at steady state in the bed of an air-blown CFBC fuelled with a sulphur-bearing solid fuel. The core of the model is represented by population balance equations on sorbent particles which embody terms expressing the extent/rate of sorbent attrition/fragmentation. The effect of the progress of sulphation on attrition and fragmentation is taken into account by selection of appropriate constitutive equations. Model results are presented and discussed with the aim of clarifying the influence of particle attrition/fragmentation on sorbent inventory and particle size distribution, partitioning of sorbent between fly and bottom ash, sulphur capture efficiency. A sensitivity analysis is carried out with reference to relevant operational parameters of the combustor

Effect of steam on the performance of Ca-based sorbents in calcium looping processes

Calcium looping, a post-combustion “carbon capture and storage” process (see Figure 1), is usually carried out by means of a limestone-based sorbent in a dual interconnected fluidized bed reactor. The two stages of this process are limestone calcination and carbonation: in the former case, water vapor can be present as a product of the auxiliary fuel combustion needed to drive this endothermal step; in the latter case, water vapor is usually present in the combustion flue gas stream bearing the CO2 to be captured. This work pursues previous research concerning the hydration-induced reactivation of spent sorbents (1,2,3) further and aims at investigating the effect of the presence of water vapor on the performance of a limestone-based sorbent, with particular reference to the attrition/fragmentation tendency. To this end, experimental tests were carried out in a lab-scale apparatus, under typical operating conditions in terms of temperature and gas composition. The role of water vapor in changing the sorbent CO2 capture capacity (with respect to a base-case operation in which water vapor was absent) and the attrition/fragmentation tendency was examined (see, for example, Figure 2 up and down, respectively). Results from CO2 capture will be complemented with characterization of sorbent particles, by means of scanning electron microscopy, porosimetric and X-ray diffraction analyses. Please click Additional Files below to see the full abstract

ATTRITION OF BED MATERIALS AND FUEL PELLETS FOR FLUIDIZED BED GASIFICATION APPLICATION

This paper reports on a study of the attrition/fragmentation behavior of different bed materials and fuel pellets for application in fluidized bed gasification. Three different bed materials displaying catalytic activity, namely fresh and sintered dolomite and a Ni-alumina catalyst, were tested for their resistance to fragmentation and attrition in fluidized bed. The fresh dolomite displayed extensive particle breakage upon calcination and a large production of attrited fines during fluidized bed operation. The other two materials were much more resistant to attrition and appeared to be suitable for further long-term operation testing. The attrition/fragmentation resistance of three pelletized fuels, one based on wood and the other two on a mixture of wood and coal, was also characterized under both inert and gasification conditions. Pellet breakage by primary fragmentation upon devolatilization appeared to be rather limited for all fuels. On the contrary, attrition of carbon fines from the char particles during gasification was extensive, due to a gasification-assisted attrition mechanism