OCR Text |
Show jolid Phase In utilities, coals arc pulverized typically to 75%<75 micron with the largest particles being around 150 micron. W h e n injected into the fiamc, the particles are rapidly heated at a rate up to 10-^- 10^ K/s depending on the particle size. The volatiles are given off after heating up the coal to around 500*C. The dcvolatilizalion process is fast and in typical swirling large-scale flames is completed within 30 ms. Combustion of char particles may take several seconds. Particle Trajectories. Assuming that the drag is the main force acting on particles, the Lagrangian equations of motion of a particle arc: 2*--(u„-u,) 18/i Q Re a = -*-=• - PPD1 24 (7) (8) In these equations, U, is the instantaneous velocity component in i-dircction while the subscript p refers to the coal particle while C<j and R e stand for the drag coefficient and particle Reynolds number, respectively. Details of the particle tracking procedure together with a description of its turbulent dispersion part are given in Boysan et al., (1983) and Weber et al., (1984); see also Sommcrfcld et aL(1992).. Particle Heating. Heating and cooling of coal particles in a flame occurs by convection, radiation and combustion at the particle surface and inside the particle. Since the particles are small, it m a y be assumed that the temperature is uniform over the particle. The particle temperature may be calculated from the following energy conservation equation: d(cpmpTp) Jt _ Qcmw +Qnd + Qcomb (9) Details on calculating the three sources Qcony. Qrad *"<* Qcomb given for example in Visser (1991). DevolatlHzatlon. It has been recognized that coal dcvolatilizalion has a major impact on the flame properties in the burner vicinity (see Brewster et al., 1988; Truelove and Jamalludin, 1986; Gorner, 1991). The dcvolatilizalion rates of similar coals range over several orders of magnitude (Wall, 1987), depending on the heating rate and final temperature of particles. Under rapid healing conditions (>104 K/s), substantially more volatilcs arc given off than under low heating rates (1 K/s). Thus, the proximate volatile matter content cannot be used in the modeling, but instead a yield factor defined below is applied. Q = VM, VM (10) *STM Studies on coal pyrolysis are in general agreement on the asymptotic nature of high temperature volatile yield ( V M m a x) with increasing temperature for all but lignite coals. The ultimate yield appears to reach an asymptote around 1000'C. The extent £ 60 - • / • KXX)°C 1 V T 7 1 / / / « VMmax. = 0.65 / • % • 40 60 Residence Time (ms) FIGURE 3: DEVOLATILIZATION DATA FOR HVB COAL VALLEY COAL; • - 1400°C," - 1200°C, A - 1000°C, -Eq.(ll) that the volatile matter exceeds the proximate volatile matter is dependent on the coal type and pyrolysis conditions (Solomon and Colket, 1979). In experiments of Knill et al. (1989), panicles were heated in reducing gas at 10^ K/s. The maximum volatile matter yield exceeded the proximate volatile matter content by a factor of 1.56-1.74 for seven high-volatile coals. The yield did not increase above 1000*C and could be approximated within +/- 2 % by multiplying the proximate volatile matter by 1.64. W h e n another eight bituminous coals of the proximate volatile matter content in the range 19 to 4 0 % were tested in the same drop-tube furnace (Knill et al., 1990), the yield factor varied over the range 1.1 to 1.8. Sayre et al. (1992), have compared these devolalilization data with the experimental results of Howard (1981) and Smith et al. (1989) and observed the lack of correlation between the proximate volatile matter content and high-temperature volatiles. Although, for most of high-volatile coals examined, the yield factor is in the range 1.6-1.7, there are few coals of the same rank which do not follow the trend. For low-and medium-volatile coals the yield factor is widely scattered. Many devolalilization models have been developed. In the simplest model of Badzioch and Hawskley (1970) the volatile yield is assumed to be independent of the particle temperature. The devolalilization rate is modeled using an Arrhenius expression for a single chemical reaction. The two model parameters; a pre-exponential factor and an activation energy are adjusted to represent the drop-tube experiments. A two competing reaction scheme has been proposed by Kobayashi el al. (1977), as a simplification to the multiple parallel reaction model of Anthony et al. (1974). In this model the high-tempcraturc volatile yield is dependent on the particle temperature. More complex models which consider evolution of different volatile soecics arc also available, see for example Niksa el al. (1988). For high-volatile bituminous coals, Knill et al. (1989), have proposed the following formulae to calculate the particle temperature dependent volatile yield V M : T p < 773 K 773 < T p < 1273 K 1273 < T n VM=0 VM = (0.5+(Tp-773)/1000) V M m a x VM = VM max (H) 11-11 |