Char particle size distribution

Despite the absence of char noted above, it would be surprising if there were not some reflection of the method of manufacture in the particle size distribution. An oil lamp has a steady flame of fairly constant size, while a wood fire has a variable, flickering flame. A naive, intuitive view would be that the wood fire can be expected to produce a broader range of particle sizes, whatever the mean and mode of the distribution might be. The constantly changing geometry of the flame will tend to allow some particles to escape before they have combusted very far, others perhaps before they are fully formed. 

Acridine char nitrogen, retention as function of burnoff, 307/, 308/ Advection fluxes, calculation, 41-43 Aerosol particle size distribution, molecular clusters, 317 Aerosol scavenging pathway, acetic and formic acid formation, 223 Aerosol species, transformation over the western Atlantic, 52 Aerosol sulfate airborne determination, 298 See also Sulfate... 

It (1 ) deals with the derivation of relations giving the particle size distribution in the bed, overflow, and carryover streams and their respective weights. This theory will be extended to include the effects of particle growth or shrinkage (Z>1 or Z<1). For typical combustion of char containing sulfur followed by sulfur dioxide absorption by limestone, relations will be derived to determine the extent of sulfur retention. The reaction, carryover, and overflow rates will be evaluated with particular attention to their dependence on Z. 

Figure 4, Particle size distribution of char, and limestone, Pift/), feeds ...

Figure 4.36 Comparison of the particle size distribution of a coal and a tyre pyrolysis char 127 (a) char, (b) coal.

Features Fine particle size tight particle size distribution jagged angular shape helps scatter light efficiently good optical chars. low oil absorp. high hydrophilicity acids ease of disp. low sp.gr. cost effective Properties 1.2 p avg. particle size fineness (Hegman) 2 sp.gr. 2 g/ml oil absorp. 60 g/100 g pH 11.2 (5% aq.) hardness (Mohs) 2-3 90% anhyd. solids... 

Elizalde O, Leal P, Leiza JR. Particle size distribution measurements of polymeric dispersions a comparative study. Part Syst Char 2000 17 236-242. 

Wen et al. (llO) developed a fluid-bed reactor model for the hydrogasification of char using the bubble assemblage concept. Solids were assumed to be completely mixed in each compartment with exchange of gas between the biibble phase and emulsion phase. Particle size distribution was not considered in the model and this m y affect the predicted conversions of char. 

It is a principal conclusion that in large wood chip pyrolysis, experimental product distribution versus time behavior cannot be predicted with simple first order kinetics for any components. This deficiency is pronounced as particle size increases and the proposed secondary reactions (of tar) add to the primary products. It is speculative but interesting to suppose that cracking reactions occur in the char which is consistent with the greater and delayed appearance of unsaturated hydrocarbon peaks for experiments on longer pellets. 

Adopting the approach developed above for the char particles combustion, the size distribution function of limestone particles as a result of sulfation reaction in the overflow stream which is the same as in the bed is given by. 

The mathematical model for char combustion described in the previous two sections is applicable to a bed of constant volume, i.e., to a fluidized bed of fixed height, Hq, and having a constant cross-sectional area, Aq. The constant bed height is maintained by an overflow pipe. For this type of combustor operating for a given feed rate of char and limestone particles of known size distributions, the model presented here can predict the following ... 

The above calculation is quite tedious and gets complicated by the fact that the properties which ultimately control the magnitude of these fourteen unknown quantities further depend on the physical and chemical parameters of the system such as reaction rate constants, initial size distribution of the feed, bed temperature, elutriation constants, heat and mass transfer coefficients, particle growth factors for char and limestone particles, flow rates of solid and gaseous reactants. In a complete analysis of a fluidized bed combustor with sulfur absorption by limestone, the influence of all the above parameters must be evaluated to enable us to optimize the system. In the present report we have limited the scope of our calculations by considering only the initial size of the limestone particles and the reaction rate constant for the sulfation reaction. 

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. 

Adventitious mineral matter is transformed directly to ash in the combustion zone. Depending on the temperature-time history, the ash particles will be spherical or semirounded. The size distribution of this ash depends on the size distribution of the adventitious mineral matter. Intrinsic mineral matter forms ash nodules in the pores of the char, and as char burnout proceeds, the ash nodules coalesce on the surface of the char. 

Most CFD providers track particles in the reactive flow field by solving the pertinent equations for the trajectory of a sfafisfically significant sample of individual particles that represents a number of the real particles with the same properties. For example, following the Rosin-Rammler size distribution (Figure 6.6), coal particles are tracked using a statistical trajectory model followed by the modeling of the kinetics of devolatilization and subsequent volatile and char combustion as discussed previously in this chapter (Figure 6.9). Models similar to the law presented earlier are used for droplet combustion of atomized fuel oil. 

The critical issues associated with manipulating this mechanism include volatile yield in the combustor (the distribution between volatile matter and char), devolatilization kinetics, and char oxidation kinetics. In the management of emissions formation (e.g., NO emissions), manipulation of specific mechanisms becomes important. Fuel particle size, heating rate, and combustor temperature influence the proportional distribution between volatile matter and char. The chemical structure of the fuel—various coals, coal waste, petroleum coke, wood waste. 

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. 

---