Char particle diameter

Figure 3. Experimental and calculated conversion temperature and char particle diameter for Experiment 1...

Figure 10. Variation of X with char particle diameter during combustion in a turbulent fluidized bed [2].

Burning times for coal particles are obtained from integrated reaction rates. For larger particles (>100 fim) and at practical combustion temperatures, there is a good correlation between theory and experiment for char burnout. Experimental data are found to obey the Nusselt "square law" which states that the burning time varies with the square of the initial particle diameter (t ). However, for particle sizes smaller than 100 p.m, the Nusselt... 

Material Lignite Char Gas Nitrogen Column Diameter 15 cm Velocity 0.46 m/s Bed Height 30 cm Particle Diameter ... 

A final question that needs to be answered is the number of particles drawn into the gas jets during bubble eruptions. Based on the literature and experimental observations undertaken during the course of this study, it is assumed that a layer with a thickness equal to the mean particle diameter in the bed is involved in the ejection process. From the surface exposed to a particular gas jet the total mass of involved particles is then calculated. Size classes for char and sand particles are allocated the same percentage of the ejected particle mass as in the bed as a whole. 

The bed material is sand, which is mixed with char produced by the pyrolysis of wood. Three methods were used for size analysis sieving, microscopic examination and laser diffraction. All the methods gave similar values for the panicle diameters of sand and char, though microscopic examination showed that many char particles had a needle form and therefore one significantly longer dimension that could not be captured by the other two methods. The different particles are illustrated in Figure 2 and the initial size distributions for sand and char in Figures 3 and 4. 

Many additional factors may influence the final design pressure drop (especially with small particle diameters) solids flow (for some feedstocks and at high specific capacities) cinder or slag formation entrainment of fine char particles heat recovery of the fuel gas, etc. 

Measnrements on coal particles of different sizes indicate that the burning times of both the volatiles and residue vary as the square of the initial particle diameter, which is in accord with the surface area proportionally. The porous structure of the char also exerts an effect on the burning operation as does particle temperature up to several hundred degrees above the gas temperature. 

The final apparent density, p, was obtained by assuming < > = < >gand using measured values of the final mass and the diameter of the char particles in Equations 14 and 15. The initial shape factor varied between 0.9 and 1. 

Figure 9. Variation of burning rate with the diameter of char particies in a turbulent fluidized bed [2]. 1) Kinetic limit [2] 2) Diffusion limit 3) Model of Haider et al. for burning rate of char particles in turbulent fluidized beds [2] o Haider et al. for char particles burning in a turbulent fluidized bed with air as fluidizing medium [2] c La Nauze for petroleum coke particles burning in air fluidized bubbling bed [17] a Chakraborty and Howard for char particles burning in air fluidized bubbling bed [38].

Initial diameter of char particle Diffusion coefficient of Oj in k, k N,... 

A lignite char obtained by carbonisation in a stainless-steel, swept, fixed-bed reactor[9] was used to produce the activated carbons in the presence of CO2. This solid was produced in several batch reactions of 60 g of lignite particles (diameter 0.2 - 0.5 mm) with a heating rate of 8 C/min until 900+3 C and this temperature was held for 3 hours to ensure effective devolatilisation. A stream of 21/min of N2 was introduced to sweep the volatiles. 

The pore structure can markedly affect char reactivity. Most coals in general, and coal chars in particular, are highly porous and contain a polymodal pore size distribution. Pores normally are classified into macropores (>500 A in diameter), transitional pores (20-500 A in diameter), and micropores (< 20 A in diameter). Upon pyrolysis, pores in coals open up but still contain microporosity. Coal chars, in general, and lignitic chars, in particular, retain the polymodal pore distribution. The surface areas of coal chars can range between 100 and 800 m /g. Most of this surface area and therefore the active surface area resides inside the char particles so the accessibility of the reactive gases to the active sites is very important. 

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