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.
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.
Changes in Quartz During Heating and the Possible Effects on Si Production
In Si and FeSi production, the main Si source is SiO2, in the form of quartz. Reactions with SiO2 generate SiO gas that further reacts with SiC to Si. During heating, quartz will transform to other SiO2 modifications with cristobalite as the stable high-temperature phase. Transformation to cristobalite is a slow process. Its rate has been investigated for several industrial quartz sources and has been shown to vary considerably among the different quartz types. Other differences in behavior during heating between these quartz sources, such as softening temperature and volume expansion, have also been studied. The quartz-cristobalite ratio will affect the rate of reactions involving SiO2. The industrial consequences and other implications of the observed difference between quartz types are discussed. Initial studies of industrial quartz were published by Ringdalen et al. In the current work, a new experimental method has been developed, and an investigation of several new quartz sources has confirmed the earlier observed large variation between different sources. The repeatability of the data has been studied and the effect of gas atmosphere investigated. The results from the earlier work are included as a basis for the discussion.