Makoto Takafuji

A study of combustion behavior of pulverized coal in high-temperature air

High-temperature air combustion is a promising technology to increase the usage of combustion energy and to improve combustion efficiency. This technology has been mainly developed for gaseous fuels, and recently application of this technology to solid fuels like pulverized coal has also become of interest. For the development of high-temperature air combustion technology for pulverized coal, it is important to experimentally investigate the combustion behavior of pulverized coal in high-temperature air. In this study, high-temperature air is applied to a pulverized coal burner to investigate the effect of air temperature on ignition, coal burnout, and NOx emission. Pulverized coal is introduced into a cylindrical furnace of 1 m diameter and 3 m height using a water-cooled stainless nozzle of 15 mm diameter. Combustion air is preheated using a heat exchanger with a gas burner and electrical furnace. The temperatures of the combustion air are set to 623 or 1073 K in order to compare the effect of air temperature. It is observed that ignition delay decreases as the air temperature increases, which is due to the more rapid devolatilization caused by higher particle heating rates. It is possible to form a stable flame even for low-volatile coals like anthracite. The difference in measured peak flame temperatures between 623 and 1073 K air is about 100 K, which is smaller than expected. Coal burnout is improved in the 1073 K air condition, which seems to be due to the increase of porosity of the particle. NOx concentration decreases for higher temperature due to enhancement of the reduction zone by rapid devolatilization of coal, as the volatile and fuel nitrogen release is enhanced in high-temperature air.

Fundamental Study of the Pulverized Coal Char Combustion in Oxyfuel Mode with Drop Tube Furnace

The combustion characteristics of coal char particles in either O2/N2 or O2/CO2 conditions were experimentally investigated. Especially, the char burnout, the char particle temperature and the shrinkage of the char particles were discussed. A Drop Tube Furnace (DTF: whose wall temperature was set at 873, 923 and 973 K) was used as the experimental apparatus. The experimental results revealed that, in equivalent oxygen concentration, the char burnout and the char particle temperature were higher in O2/N2 conditions than those in O2/CO2 conditions. The shrinkage of the char particle did not show the large difference in either O2/N2 or O2/CO2 conditions. Up to 15% of char burnout, the char particle diameters were reduced gradually. Up to 80% of char burnout, the char particle diameters were not changed. This is supposed that the chemical reaction is mainly occurred not on the external surface but on the internal surface of the char particle. Over 80% of char burnout, sudden shrinkage could be seen. Finally, an empirical equation for the prediction of the char particle shrinkage was introduced. Further investigation is required in high operating temperature, where CO2 gasification may have a large influence on the char burnout.

Prediction of Heat Recovery Characteristics of Oxyfuel Combustion Boiler Using CFD

Oxyfuel combustion is one of the promising technologies to reduce CO2 emission from pulverized coal fired power plant. In order to apply this technology to the commercial boiler, it is important to predict the boiler performance (especially heat recovery characteristics) in Oxyfuel combustion condition. In this study, prediction of heat recovery characteristics of Oxyfuel combustion boiler using CFD was conducted. As a result, it was shown that the same boiler performance can be achieved in Oxyfuel combustion mode as that in Air combustion mode.