| OCR Text |
Show analysed to confirm the carbon bum-out efficiency although, for convenience, this was more usually determined from an oxygen balance. Temperatures were measured using R-type shielded thermocouple which had been previously calibrated against a standard suction pyrometer to evaluate errors in measurement. The radial variation in gas temperature was usually within 15°C of the mean axial value at the same position. Stainless steel water-cooled probes were used to extract gas for the continuous measurements of N O , N O x , 0 2 and C O using on-line analysers. N 0 2 was only 5 to lOppm of the total N O x and has not been included in the data presented. All NO reduction data are reported relative to a baseline NO emission at the furnace exit, measured prior to the start of coal feeding, and including both carrier gas and bum-out air flow. The baseline N O concentration at the exit, normally around 600ppm, was set by adjusting the flow of ammonia into the propane supply. Prior to dilution by carrier and bum-out air, the actual N O concentration at the rebum coal injection point would be around 20 -30 % higher than the baseline value exit value. An ammonia balance confirmed that all the N H 3 was quantitatively converted to N O before reaching the rebum zone for all burner operating conditions. 3. Model Description A three-dimensional CFD model was developed and validated against the experimental data. In order to reduce processing times, computations were started from the inlet of the rebum zone since experimental measurements had shown that the combustion products from the burner were essentially well mixed by this point. The code uses a k-e turbulence model and a eulerian method to formulate the conservation equations of the gaseous and particulate phases. Radiative heat transfer is incorporated using a six flux model [12] where scattering is assumed to be isotropic. The computational domain extends to the exit of the furnace and the mesh size chosen (26x26x100) was sufficiently fine to provide acceptable grid independent solutions. Coal devolatilisation is simulated using a first-order, single reaction model [13]. The subsequent combustion of the volatile gases (assumed as hydrocarbons, the C/H ratio being derived from the elemental analysis of the coal) is modelled using a two-step reaction mechanism [14] where CxH^x/2 02 ->xCO+y/2H2 (I) C O + 1/2 0 2 -> C 0 2 and H 2 + 1/2 0 2 -> H 2 0 (H) Reaction (I) is controlled by the turbulence-decay model of Magnussen et al [15], as is the hydrogen burning rate in the second step. The rate of C O combustion is modelled assuming turbulent diffusion and chemical control [16]. A standard diffusion and kinetically limited char particle oxidation model is used and the C O / C 0 2 product ratio is obtained from the literature [17]. The emphasis of the modelling studies was on the rebum zone N O formation and reduction mechanisms. NO chemistry Several possible routes exist for NO reduction under rebum conditions. The model considers NO-reduction by fuel-N species, hydrocarbon fragments [8] and char particles [10,18-20]. The possible effects of other minor reducing agents such as C O and soot formed by the coal |
