Reaction freeboard

Several models have been proposed to account for reaction in the freeboard. Yates and Rowe (1977) developed a simple model based upon complete mixing of particles in the freeboard, coupled with either BMF or PF of the freeboard gas. Two model parameters are the rate of particle ejection from the bed, and the fraction of wake particles ejected. Kunii and Levenspiel (1990 1991, pp. 305-307) proposed a model of freeboard reaction which accounts for the contact efficiency of the gas with the solid, and the fraction of solid in the freeboard. A comprehensive freeboard entrainment model is... 

The model equations were solved for various combinations of the parameters, and it was reported that in many cases freeboard reactions lead to greater conversion than that achieved by the fluid bed itself. The details of the results are also reviewed by Yates (1983). 

Most of the fluidized bed coal combustion and gasification models ignore freeboard reactions of volatiles and char For shallow beds, volatiles bum predominantly in the freeboard In addition the char particles splashing from the bed surface can also react with the oxygen in the freeboard Yates and Rowe (ll8) have proposed a model for the reactions occurring in the freeboard Such an approach can be adopted for modelling the freeboard reactions in the FBC ... 

At least two ac tions can take place in the freeboard classification of solids and reaction of solids and gases. 

Overpressure of Store drum at proper temperature material in drum, Keep drum away from heat source due to external heat input or self reaction is complete before drumming heating. Allow adequate freeboard for material Provide adequate sprinkler protection Thermally initiated venting (e.g., melt-out bungs) CCPS G-3 CCPS G-15 CCPS G-22 CCPS G-29... 

If this is the case, the design and configuration of the gas outlet in the freeboard region can have an important effect on the fines residence time distribution and holdup above the bed. The importance of the freeboard region for further reaction of fines merits greater emphasis in both experimental and theoretical studies. 

In the following sections, we discuss reactor models for fine, intermediate, and large particles, based upon the Kunii-Levenspiel (KL) bubbling-bed model, restricting ourselves primarily to first-order kinetics. Performance for both simple and complex reactions is considered. Although the primary focus is on reactions within the bed, we conclude with a brief discussion of the consequences of reaction in the freeboard region and near the distributor. 

Generally, it is advantageous to avoid reaction in the freeboard, as much as possible, since the temperature control and near-isothermal conditions observed in the fluidized bed are nearly impossible to achieve in the freeboard region. This is particularly problematic for a complex reaction, since the selectivity is often temperature-dependent. Experiments have shown that the following design features influence the extent of particle entrainment, and, by extension, the likelihood of reaction in the freeboard region ... 

The solid concentration in the freeboard is sufficiently high to contribute significantly to the reactions occuring in AFBC s. Reactions of particular importance are those between NO and char, the burnout of carbon monoxide, and the combustion of the recycled fines. Space is not available to cover the subject adequately and the reader is referred to selected literature on the subject (101-104). It is of interest to note, however, that the elutria-tion rate of larger particles has been found to be greatly augmented by the concentration of fines (103) in a bed, so that the recycle of fines may influence bed behavior in an unobvious manner. 

The freeboard space above the bubbling fluidized bed must be considered in the reactor model if the entrainment rate is high and the reactions in the freeboard are not quenched, for example, by cooling. 

Once formed, the PAH can be emitted as gas or as generating by themselves the smallest particulate matter. These competitive reactions, among the present compounds in the freeboard, will be the ones which determinate the total PAH emissions and will depend on the feed and the bed nature. 

---