Bubbling fluidisation

Mixed plastics waste appears to be well suited for use in energy recovery, either as a co-eombustion fuel in a power plant designed for solid fuels, or as the sole fuel in speeially designed plants. This paper reports test results on the co-combustion of mixed household plasties with eoal. The tests were performed in a bubbling fluidised bed low-pressure steam boiler. The results show that both inorganic and organic total specific emissions were lower for mixed household plasties than for coal. Tabulated data are presented. 3 refs. 

Bokkers, G. A., Laverman, J. A., Van Sint Annaland, M., and Kuipers, J. A. M., Modelling of large-scale dense gas-solid bubbling fluidised beds using a novel discrete bubble model, Chem. Eng. Sci. 61, 5590-5602 (2006). 

In gas-solid systems, as the gas velocity is further increased, gas bubbles form within the bed to give a bubbling fluidised bed. The velocity at which the first bubble forms is called the minimum bubbling velocity, mb. An empirical correlation typically used to predict b at ambient conditions has been given by Abrahamsen and Geldart 14... 

The pressure drop and bed expansion profiles shown in Figure 9 represent the different stages from fixed to bubbling fluidisation for a typical gas-solid system of fine particles. Knowing the bed height, the average bed voidage can be obtained ... 

Ding, J., and Gidaspow, D., A bubble fluidisation model using kinetic theory of granular flow. AIChE. J. 36(4), 523 (1990). 

The Circulating Fluidtsed-Bed gasification process was not as sensitive to ash sintering as the small-scale bubbling fluidised-bed reactor. The amount of potassium chloride increased from bottom ash to hot filter dust. 

Static heating of coated mullite (AI2SI05) bed material obtained from a bubbling fluidised bed gasification experiment with straw to study coating-induced agglomeration. 

The gasification experiments have been conducted in a lab-scale bubbling fluidised bed gasifia with an internal diamaa of 74 mm and a bed hei t of 200-250 mm, A screw conveyor continuously feeds the feedstock. The bed mataial used in the expaiments is silica with a diamaa of 0.27 mm and the particle size of the feedstock is 0.7-2 mm. 

In the Netherlands only few stand-alone bubbling fluidised bed combustion applications are in operation to process specific wood, sludge and industrial waste streams. The wood- fired bio-energy-plant at Cuyk represents the cunent state of the art of BFBC, handling fresh wood, saw residues, park-/garden residues with steam parameters of 100 bar and 520°C. Stand-alone FBC, like the one at Cuyk, offers approximately 30% (net) electrical efficiency. This value does not offer the energetic advantage (40% electrical efficiency) co-combustion does,... 

The pyrolysis oils chosen for the Oilon combustion tests were produced from various hardwoods and softwood using either a bubbling fluidised bed or circulating fluidised bed. The edacity of production ranged tq> to 1 t/h. Since the oils are not yet commercial and producers still try to improve them, the oik are marked as in Table 1. 

Despite the obvious advantages of fluidised-beds for temperature control, there is uncertainty as to their suitability for carrying out the OXCO reactions since these are a combination of gas phase reactions and reactions on the catalyst surface. CSIRO has begun investigating the OXCO reactions in a small-scale bubbling fluidised-bed reactor and initial results have been encouraging in that they are similar to those reported here for a fixed-bed reactor. 

A SER based on low-pressure interconnected bubbling fluidised beds applied to power plants with CO2 capture is proposed by Ref. [54], In this case, the H2-rich gas produced is converted in a high-temperature SOFC and regeneration is carried out by recovering the waste heat from the fuel cell through an internal heat transfer loop. 

Gas stream containing a reacting chemical species (A) is passed through a bubbling fluidised bed catalytic reactor, at a rate of 350 m /h. The reactor has fine catalysf parficles present in the dense phase, which occupies 74% of the bed volume. The density of fhe catalyst in the dense phase is 89 kg cat/m. The reaction is first order with rate constant k = 5 X 10 m /(kg cat)(s). The bubble side mass transfer coefficient is reported as (k i) = 1.1 s. The diameter of the reactor tube is 50 cm. Calculate the height of fhe fluidised bed reactor required to achieve 90% conversion. 

This paper presents some of the opportunities CFD offers when applied to analyse different combustion systems. Practical examples presented are ash deposition predictions on heat exchanger surfaces and walls in a bubbling fluidised bed furnace and detailed nitrogen oxide emission predictions for the same furnace type. Furthermore, the extension of a standard model using process specific data is presented for the fuel conversion process in a black liquor recovery furnace. 

Hereafter three examples are presented for the application of CID to analyse advanced combustion systems. Each of the examples covers a specific technical problem and shows how standard CFD models need to be adjusted to address individual questions. The first example deals with the increase of boiler availability due to reduced ash deposition on furnace walls and superheater surfaces. The second one addresses the question of reduced nitrogen oxide emissions from a bubbling fluidised bed combustor and the last example presents a novel model for black liquor droplet combustion. 

From the advanced fuel analysis the ash forming elements of the fuel are identified, their melting behaviour is calculated under furnace conditions and a stickiness criterion as function of ash particle temperature is defined for each individual fuel. In the CFD calculations this stickiness criterion is utilised by checking the particle temperature at its impaction on a wall or superheater surface. If the particle temperature is above the stickiness criterion, the ash particle sticks at the wall and the location is recorded as location for possible deposition. On the other hand, if the particle temperature is below the criterion, the particle rebounds back to the furnace and continues its flight. Figure 1 shows a deposition map for the back wall of a bubbling fluidised bed freeboard. The coloured dots show the locations for particle hits at the specified temperature on the wall and clearly indicate the areas of possible deposition in this furnace. The picture on the left of the figure shows the deposit situation in the real furnace and serves as validation for the applicability of the tool. 

Figure 1. Visual validation of ash deposit prediction in the freeboard of a bubbling fluidised bed furnace.

Yorquez-Ramirez MJ, Duursma GR. Insights into the instantaneous freeboard flow above a bubbling fluidised bed. Powder Technology 116 76-84, 2001. 

Walker BV. Gas-solid contacting in bubbling fluidised beds. PhD dissertation, Cambridge University, 1970. 

The concept of two fluid phases in a bubbling fluidised bed and its application to a reactor model originates in the work of Johnstone and colleagues at the University of Illinois (1, 2 ) Since then numerous and different modifications have been made to the basic model ( ) but practically all these variants assumethat the interstitial flow (the interstitial or dense phase) is the minimum fluidisation value. Flow in the form of bubbles (the bubble or cloud phase) is the difference between interstitial and total flow. This is simply formulated... 

Gas in a bubbling fluidised bed can be thought of as divided into three phases that flowing interstitially (the dense phase), that associated with the bubble wakes and that in the essentially empty space of the bubbles. If f is the fraction of bed volxime... 

In a bubbling fluidised bed of a group B powder, such as a sand or other inerts, the microscale hydrodynamic environment is known to vary between dilute phase (bubbles), containing little or no solid, and which have thermal properties approximating those of a gas, and dense phase, which is very close in properties to a loosely packed bed. Both have thermal properties which can be described by existing correlations, ensuring a reasonable degree of confidence in this part of the model. 

Hence, while the macroscale fluid mechanics can likely be better described in a circulating fluidised bed than in a bubbling fluidised bed, the opposite may be said of the microscale heat transfer. 

This plant is situated about 120 km south of Stockholm. The plant consists of a bubbling fluidised bed (BFB) steam boiler (Boiler 3) for combined heat and power (CHP) operation, two circulating fluidised bed (CFB) boilers for hot water production and a hot water accumulator. Tests were performed in Boiler 3, for CHP production. 

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