Sulfur capture

By the late 1980s six principal commercial CEBC technologies were available (42). In 1993 the largest CEBC ia operation is expected to be the Pyropower Corporation s 165 MWe reheat coal-fired unit, under constmction siace 1991 at the Poiat Aconi Station of Nova Scotia Power Corp. (43). Combustion and SO2 control ia this unit is to be carried out ia the water-cooled riser. The unit is expected to operate at 870°C to optimize sulfur capture. The cyclone separators are refractory-lined and are supported approximately 30 m above grade. 

Habib (4) has emphasized the importance of the sulfur-release step in the mechanism for SOx reduction. If a catalyst captures SOx but cannot release it, it soon becomes saturated and ineffective. For example, if CaO captured SOx until it was transformed to CaSO, it would capture 57% sulfur, based on the weight of the CaO. For the FCCU under consideration, 50 tons of CaO added to the 500-ton unit (10% additive) would capture 28.6 tons of sulfur. At a sulfur capture rate of 10 tons a day, the CaO would be effective for only 2.9 days. Since the average catalyst residence time in the unit is 100 days, use of such a material would not be practical. 

If sulfur capture must be greater than 95 percent, then treatment of Claus or Selectox tail gas will be required. 

Lime was added to the bed to reduce sulfur emissions.16 Calcium to sulfur ratio was about 1.7 to 2.0, and resulted in 90 percent sulfur capture.5 Emissions of the pilot test are summarized in Table 2-1.5... 

Bonn, B. Mtinzner, H. Sulfur Capture with Limestones in Fluidized Bed Combustion, Inst. Energy Symp. Ser., No. 4 (1980). 

Sulfur Capture (Selection of Bed Temperature and Particle Size)... 

In order to predict emissions from AFBC s it is necessary to couple a model of the sulfur capture of individual particles into a system s model which takes into account the SO2 formation, removal, and transport. Because the single particle behavior is so complex, most such models (10, 20, 21, 22) use simplified, usually empirical, fits of single particle behavior determined, for example, from thermogravimetric analysis. 

The optimum conditions needed to achieve the desired sulfur capture efficiency (90 percent) is dependent on sorbent type, coal type, and fluidized bed design. Representative conditions are a bed temperature of 1116 K, a sorbent particle of 1 mm, and a Ca/S ratio of about 3. The bed temperature selection is based on the admittedly shaky evidence that this is where the maximum sulfation peaks. The particle size selection is a compromise between a desire to use high gas velocities and the observed decrease in sulfation with increasing size (Figure 2). The Ca/S ratio corresponds to a maximum sulfation of about 0.3. For these conditions the mass feed rate of a limestone sorbent is 0.345 that of the coal feed and the flow rate of the partially sulfated spent sorbent is 0.285 the coal feed rate. 

A Pore Diffusion Model of Char Gasification with Simultaneous Sulfur Capture... 

A model Is presented for char gasification with simultaneous capture of sulfur In the ash minerals as CaS. This model encompasses the physicochemical rate processes In the boundary layer, In the porous char, and around the mineral matter. A description of the widening of the pores and the eventual collapse of the char structure Is Included. The modeling equations are solved analytically for two limiting cases. The results demonstrate that pore diffusion effects make It possible to capture sulfur as CaS In the pores of the char even when CaS formation Is not feasible at bulk gas conditions. The model predictions show good agreement with experimentally determined sulfur capture levels and reaction times necessary to complete gasification. 

Sulfur Capture During the Initial Stages of Char Gasification... 

The model and the results presented here illustrate the physicochemical processes involved in char gasification with simultaneous sulfur capture. In particular, they demonstrate that diffusion limitations in the gasification reactions enable the conversion of CaO to CaS within the char even though CaS formation is not feasible at bulk gas conditions. Furthermore, this first version of the model correctly predicts the trends observed experimentally. Future effort in this area will focus on quantitative comparisons of model predictions with results from carefully designed gasification experiments. 

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