OCR Text |
Show 10, which had a temperature drop from 710 °C to 470 °C. In the final simulations the predicted temperatures were within 5 °C of the reported values. 1.3 Reaction Model A comprehensive reaction model was developed and incorporated into the CFD software that describes all the important processes occurring as the sorbent particles pass through the boiler. W h e n limestone is heated to a temperature greater than 767 °C, it decomposes into C a O and CO2. For small particles, the decomposition rate is kinetically controlled with C O 2 gas emanating from the pores in the solid. The calcination model (Flament, 1993) accounted for the kinetic rate and the development of the internal pore structure. Calcination increases the surface area and pore volume of particles due to the release of CO2. The increase in surface area and porosity is related to the extent of calcination and the relative molar volumes of CaC03 and CaO. The change in particle surface during calcination, Sexp was experimentally derived, reaching a nominal value of approximately 10,000 - 30,000 m2/kg of C a O (Flament, 1993). Once the particle has calcined, it can react with SO2 to form gypsum (sulphation). Sulphation occurs on the surface of internal pores in the calcined particle. The SO2 at the outer surface of the particle is transported to the internal pores by diffusion. If the pores are substantially larger than the mean free path of the SO2, then a bulk diffusion mechanism is controlling. When the pore size is similar to the mean free path then Knudsen diffusion is controlling. To account for these effects, the particle was treated as a porous sphere. This model takes no account of the tortuous reaction path. Instead, the particle was modelled using a simplified pore tree model, described by (Flament, 1987), and a constant pore diffusion rate. The pore size distribution was assumed to be such that the integrated pore volume between a minimum pore radius, rmin, and a given pore radius, rp, increased with the natural logarithm of rp. The minimum and maximum pore trunk radii were calculated from the initial porosity, surface area and radius of the calcined particle. During sulphation, the structure of the particle changes due to the build up of CaS04 on the pore walls. The CaS04 has a larger specific volume than the C a O that it is reacting with, causing plugging of the pore structure and increasing the minimum pore radius, rmin. The CaS04 layer that formed on the particle was described in terms of a shape factor and a layer thickness. The shape factor accounted for the portion of CaS04 that replaced the reacted C a O and the portion that fills up pores. The change in sulphate layer thickness is related to the reaction rate, k^ thus the sulphation chemical reaction rate, ksui, is corrected for diffusion of SO2 through the sulphate layer. With the change in pore shape factor and sulphate layer thickness known, the change in pore radius and surface area can be calculated. The porosity is also affected by plugging due to simple volume filling and due to the blockage of bottle type pores with filling at the neck. Sintering may also reduce pore surface area. The sintering rate increases with temperature and is proportional to the square of the available surface area. Both the plugging and sintering effects control the change in surface area and porosity of a |