Hirofumi Okazaki

Pyrolysis and Ignition Characteristics of Pulverized Coal Particles

Pyrolysis and ignition characteristics of pulverized coals were examined under similar burning conditions to those of industrial burners. In the early stage, fine particles (less than 37 μm) were mainly pyrolyzed by convective heat transfer from surrounding gas. The coals ignited when pyrolyzed volatile matter mixed with surrounding air and formed a combustible mixture. Pyrolysis of large particles was delayed, but accelerated after ignition by radiant heat transfer from coal flames. The effects of radiant heat transfer were strong for intermediate-size particles (37-74 μm). Ignition temperature was examined analytically by using a modified distributed activation energy model for pyrolysis. The calculated results agreed with experimental ones obtained from both laboratory-scale and semi-industrial-scale burners.

Reducing the minimum load and NOx emissions for lignite-fired boiler by applying a stable-flame concept

For the purpose of improving the load range and NOx emission level of lignite-fired power plants, a new combustion technology, called NR-LE burners (NOx Reduction-Load Extension), has been developed in co-operation between Babcock-Hitachi and Fortum. A single-burner combustion test was performed in Japan with this new NR-LE type burner using Czech lignite. Adapting the flame-stabilization ring and a special additional air-nozzle resulted in achieving a stable flame, which enables: - The burner minimum load to be less than 50% (Boiler load: 30-40%) - Low NOx emissions of less than 200 mg/m3 (6% O2, dry base) The first commercial full-scale application of the NR-LE burner was by the IPP power producer in the Czech Republic (Sokolovska Uhelna, a.s. at Vresova Unit2 boiler with steam parameters 325 t/h, 535 °C, 13.5 MPa). The commissioning test runs of the new burners were carried out during September to October 2001. The boiler is now in commercial operation, with (i) a 30% minimum load without supplementary fuel, and (ii) lower NOx emission levels.

LES of Pulverized Coal Combustion Furnaces

LES (large eddy simulation) is applied to combustion simulations of two large scale pulverized coal-fired furnaces. One application is a boiler furnace with the coal feed rate of 3,000 kg/h. The results of LES show good agreement in not only distributions of temperature, NO concentration, and CO concentration on the vertical center line but also NO and CO emissions and UBC (unburned carbon in ash). The calculation error of NO emission is 10%. The other application is a horizontal furnace with a low NOx burner with the coal feed rate of 560 kg/h. LES predicts temperatures and oxygen concentrations accurately; but the standard k-e model does not. The flame width calculated by the standard k-e model is narrower than that by LES. These calculated results indicate that the drawback of the standard k-e model is its low calculation accuracy for the coal jet flame decay and lift-off height.Copyright

Modeling of NOx and CO Reactions for Commercial Scale Pulverized Coal Firing Boilers

The NOx-CO reaction model was developed for commercial scale pulverized coal firing boilers. The model was used to decide optimize combustion performance and it featured simplified sub models of NOx reduction, pyrolysis and CO oxidation. To improve combustion performance of commercial scale pulverized boilers, it is most important to predict the distribution of NOx and CO concentrations and search for the best conditions in a short time. The NOx reduction sub model was developed for both volatile and char combustion. The pyrolysis sub model was developed from a comparison of Distributed Activation Energy Model results and experimental data. The CO oxidation sub model was characterized by its consideration of the effect of hydroxide radicals. The simulation gave a lower CO oxidation rate for a long reaction time, the same as the actual result. The NOx-CO reaction model successfully analyzed NOx and CO concentration distributions of commercial scale boilers for power plants (260–1000MW).Copyright