Freeboard volatiles

Flue particles ia a fluidized bed are analogous to volatile molecules ia a Foiling solution. Therefore, the concentration of particles ia the gas above a fluidized bed is a function of the saturation capacity of the gas. To calculate the entrainment rate, it is first necessary to determine what particle sizes ia the bed can be entrained. These particles are the ones which have a terminal velocity less than the superficial gas velocity, assuming that iaterparticle forces ia a dilute zone of the freeboard are negligible. An average particle size of the entrainable particles is then calculated. If all particles ia the bed are entrainable, the entrained material has the same size distribution as the bed material. 

During activation the composition of the outlet gas is mainly determined by the quality of the charred material (volatile matter) and by the bed and freeboard temperature. At the operating temperature (800 °C—900 C) used in the experiments the water gas reaction essentially reaches equilibrium, which fixes the gas composition for a given air, fuel, and steam feedrate. For the total mass balance a closure from 95.6% to 102.6% was found. For elementary carbon the deviation was 86.5%—1065%. Further details on the production and influence of the process parameters on the adsorptive capacity are described elsewhere [ 5 ]. 

The flue gas exits from the bed through the freeboard, where oxidation (particularly of volatiles) is completed and the larger solid particles entrained in the flue gas fall back into the bed. Due to attrition or burnout, ash is usually reduced to a sufficiently small size that it exits the combustor with the flue gas. The bed material that accumulates is typically removed from the bed by an overflow drain, thus taking advantage of the bed s fluid characteristics. 

Indirect fired kilns are those heated externally. They are usually designed for applications where direct contact between the material and the gas providing the heat source is undesirable. In this case, the heat source is external to the kiln (Figure 1.9). Any internally flowing gas that is in the freeboard is used for purging any volatile or gas that arise from the bed as a result of chemical/physical reactions. Because of their low thermal efficiency, externally heated kilns are small, typically up to 1.3 m (50 in.) diameter and are used for niche applications such as calcining of specialty materials. 

Rapid mixing of hot circulating solids, fed above the pyrolysis bed, with the coal feed, is required, as well as good distribution of coal to the pyrolysis bed so that nonuniform volatile release to the freeboard does not occur. 

Bubbling bed combustors conduct almost all of their combustion and, simultaneously, considerable heat transfer within the relatively dense bubbling bed. The freeboard is primarily used to complete CO and volatiles combustion and to disengage solids from the raw combustion gas before the gas reaches the convective heat transfer surface. Elutriated particles that are captured by cyclones may be recycled, at relatively low temperature, to the fluidized bed combustor. 

At the top of the freeboard, the gas is accelerated (velocity as high as 5 m/s) and secondary air is injected to complete volatiles combustion, result-... 

The cooling of the bottom part of the first cyclone causes a flue gas temperature drop of about 50 °C in the first cyclone. The heated air from the cyclone cooling system, which has a temperature of about 200 °C was originally directly used as coal feed transportation air. Heat transfer from the freeboard and the fluidised bed to the air in the coal feed pipe increased the temperamre to 500 °C at the end of the coal feed pipe. To avoid burning of the coal in the feed pipe the coal feed air is cooled to 60 °C before it enters the coal feed pipe. The temperamre of the air leaving the end of the coal feed pipe is approximately 300 °C, which means that devolatilization of the high volatile coal starts already in the coal feed pipe. To avoid future problems caused by this premature devolatilization a new coal feed system, which injects the coal pneumatically upwards through the air distributor plate into the combustor will be installed in the third quarter of this year. 

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 ... 

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