Evolutionary Changes in Furnace Combustion Conditions Which Affect Ash Deposition in Modern Boilers

Abstract

Extensive literature and research results are availible which are directed at analyzing or predicting the tendency of coal ash to deposit in radiant furnaces and onto convective surfaces. However, these results are generally based upon conventional pulverized coal (PC) furnace designs,combustion systems, and fuels utilized over the last 40 years. Such conventional systems are being supplanted on some new units by combustion systems designed to respond to recent changes in the market place.

Some recent changes in the market which ultimately affect the furnace combustion processes can be cited. Environmental laws have placed greater importance on reducing emissions of NO_x, CO, SO₂ and air toxics. Similarly, there is some political support to reduce global emissions of CO₂ and N₂O, and one response has been the introductory commercial usage of PC-based combined cycles. And finally, there is an increase in the practice of world sourcing for coal supplies for a single power station, leading to the need to design a single furnace to reliably combust as many as 20 disparate fuels. This trend may accelerate in Pacific Rim nations due to the reduction in imported fuel tariffs associated with recent open trade agreements.

This paper outlines some changes in operation and design of new pulverized coal fired radiant utility furnaces which affect coal ash deposition, including the following:

Deep staging, substochiometric combustion

Rotating dynamic classsifiers

Individual coal burner flame analyzers

World sourcing of coal supplies

Fully fired (hot windbox) combined cycles, with and without pyrolysis

Pulsating combustion

Preheating of coal-air mixture upstream of burner nozzle

Miscellaneous effects (waterwall tube orientation, indirect bin fired system, water deslaggers)