Tetsuya Hirata

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.

Physical model analysis of flame spreading along an electrical wire in microgravity

Microgravity experiments have shown that, when an electric wire (copper wire coated with ethylene-tetrafluoroethylene[ETFE]) is ignited at a point, two spherical diffusion flames move in opposite directions at a constant speed. In the present study, a mathematical model is presented to describe the combustion characteristics of ETFE-coated copper wire in microgravity. To explore the role of copper wire, radiation effects are intentionally neglected. The copper wire of very large thermal conductivity acts as either a heating or a cooling medium on the burning of wire coating. The uncoated wire part within the flame receives heat from the surrounding hot gas and transfers part of it to the coating for its gasification. On the other hand, at the station where the flame contacts with coating surface, the flame extinguishes locally due to the heat loss to the copper wire which plays a cooling medium. The gasification of the coating produces the outward Stefan flow, which tends to reduce the heat flux toward the coating through the gas phase. The numerical calculation results for the self-sustained burning case with variable wind speed and oxygen concentration have demonstrated that the various properties intrinsic to electric wire insulation burning, which are observed in microgravity experiments, can be characterized by such roles played by the copper wire and Stefan flow.