Sorbent feeding

Coal is fed as a paste containing 25 wt % water, and sorbent is fed diy by a lock-hopper system with pneumatic conveying. The top size of each feedstock is 3 mm in). The latent heat lost evaporating the water fed with the paste is compensated by increased gas turbine power output resulting from the increased flue-gas mass flow rate. For the 80-MWe unit, there are six coal feed points (one per 4.5 m" [48 ft"]) and four sorbent feed points (one per 6.7 m" [72 ft"]), all entering beneath the tube bank along one wall. The bed depth is... 

An other important effect of bubbles is to cause the bed to expand. The bed expansion is a consequence of the bubble hold up in the bed and can be calculated from the bubble flow rate and the bubble rise velocity (29 34). For the considions here, a bubble void fraction of 0.3 is assumed, corresponding to an expanded bed height of about lm. From the expanded bed height, bed cross-sectional area, and sorbent feed rate, it can be readily shown that the mean residence time of the limestone in the bed is about 20 hours. 

The dry-sorbent injection system is shown schematically in Figure 2. This rather simple system provides reliable and accurate feeding of solid sorbent. The entire feed mechanism, including sorbent storage, was placed on a weigh scale with digital readout. The sorbent feed rate was constantly monitored from the slope of weight loss vs time. 

TABLE III. EFFECT OF INTERMITTENT SORBENT FEEDING ON S02 REMOVAL EFFICIENCY AND SORBENT UTILIZATION... 

In the first test, the Na2/S ratio was maintained at an average value of 1.30, and the sorbent feed schedule was 1 minute on (at Na2/S=2.6) and 1 minute off. The S02 removal efficiency obtained was only 58.6 percent, significantly less than the 77.6 percent value obtained with continuous feeding sorbent utilization was 45.1 percent versus 59.7 percent with continuous feeding. 

The second test was conducted with an average Na2/S ratio slightly less than 1, and the sorbent feed schedule was 3 minutes on, 1 minute off. Again the S02 removal efficiency was... 

Figure 12.16 Contact-sorption spray drying of lysine 1—wheat bran tank 2— pneumatic duct 3—spray dryer 4—sorbent feed 5—atomizer 6—lysine tank 7— cyclone 8—activator 9—blender. (From Tutova and Kuts, 1987.)...

In a combustor the release of sulfur is not uniformly throughout the bed it may vary with the radial and axial bed position caused by the fact that the devolatilization time of a coal batch added to a fluid bed is in general less than 20% of the total birmout time and consequently in the same order as the particle mixing time (10-20 sec.). This implies that the volatiles-sitlfur is released predominantly aroimd the coal feed points, while the char-sitlfur is released throughout the bed. This phenomenon has consequences for the combustor design and operation e g. the position of sorbent feed and removal, the pitch of the heat exchanger pipes etc. 

Salt transport in the process is entirely pneumatic. The dry sorbent is usually delivered by bulk carrier (truck or rail) and off-loaded pneumatically into covered storage silos. From the silos, it is pulverized to 10-20 microns mean mass diameter to optimize both the rate of decomposition and sorbent utilization (Bennett, 1992 Bennett and Nastri, 1990 Hooper, 1990). The pulverized sorbent is conveyed to a day silo, metered into the pneumatic conveying line, and distributed into the flue gas through a system of injection nozzles. The sorbent feed rate can be automatically controlled based on upstream and/or downstream sulfur dioxide gas stream concentrations (Bennett, 1992). The end product (a mixture of sodium sulfate, sodium chloride, sodium nitrate, fly ash, and unreacted soda) is a dry powder easily coUected by either a fabric filter or an ESP (Bennett and Nastri, 1990). 

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