Latest Low-NO x Combustion Technology for Pulverized-coal-fired Boilers
Kenichi Ochi Kenji Kiyama Hidehisa Yoshizako, Dr. Eng. Hirofumi Okazaki Masayuki Taniguchi, Dr. Eng. OVERVIEW: Hitachi has long been working on the development of low NOx combustion technologies to satisfy the worldwide need for environmental conservation and has devised the world-first concept of “inflame NOx reduction” which has been applied in the field of pulverized coal combustion technology in the HT-NR burner series. Hitachi has developed new technology to lower NOx levels even further by improving the structural design and operation of a two-stage air port system which is used in combination with in-flame NOx reduction. This work has been progressed using a large-scale combustion test facility that simulates actual furnace conditions and through the use of a numerical analysis technique developed in-house. The new technology can lower boiler outlet NOx by 40% for the same level of CO.
A reduced NOx reaction model for pulverized coal combustion under fuel-rich conditions
Abstract A reduced NO x reaction model was developed for analysis of industrial pulverized coal firing boilers. The model was developed from experiments of laminar premixed combustion under a variety of stoichiometric ratios, burning temperatures, coal ranks (from sub-bituminous coal to anthracite) and particle diameters. Calculations agreed with experimental results for NO x and nitrogen species (NH 3 and HCN), if the model assumed that the hydrocarbon radicals were formed not only from pyrolysis of volatile matter, but also from char oxidation and gasification. The presence of hydrogen in char at the final burnout stage supported this assumption. NO x reduction by hydrocarbon radicals was the most important reaction in high temperature (>1500 K), fuel-rich, char combustion regions. NO x reduction from nitrogen species was sensitive to peak NO x concentration in volatile combustion regions, but NO x emission downstream had little influence from the peak NO x concentration. The heterogeneous reaction between char and NO x was important for fuel-lean or low-temperature conditions.
A study of gas composition profiles for low NOx pulverized coal combustion and burner scale-up
A turbulent diffusion flame of pulverized coal was investigated to clarify the relationship between flame structure and NOx emissions. NO was formed in the initial flame zone and was reduced in the fuel-rich, post-flame regions, where HCN and NH 3 were produced probably by the oxidation of coal by H 2 O and CO 2 . Burner scale-up also was examined by conducting parallel experiments in a small-scale combustor and a facility with a 25-fold larger coal feedrate. The performance and the profiles of gas species in the flame of the large-scale furnace were well simulated by the data obtained in a laboratory-scale burner where combustion air inlets were remotely located from the central fuel jet to surpress the transverse mixing of air and coal.