The Use of Natural Gas in Coal-Fired Boilers
AUTHOR: Alison Doig, Geoffrey F. Morrison
DATE: September 1997
Improved supply and availability, combined with demonstrated operational and environmental benefits of using natural gas over oil and coal, have resulted in considerable interest by utilities and industry in the use of natural gas in existing coal-fired boilers.
There are three main options for integrating natural gas into coal-fired boilers:
- fuel staging (reburn) for NOx control;
- cofiring of coal with gas in existing boilers; and
- integration of a natural gas turbine with an existing coal-fired boiler to form a high efficiency combined cycle.
Natural gas reburn can be competitive with alternative NOx reduction technologies for up to 70% NOx reduction and gas reburn combined with selective non-catalytic reduction is the most cost effective means of achieving higher levels of NOx reduction. Cofiring of coal with gas gives poorer results than reburn for NOx reduction but can provide a number of operational benefits. Slagging and fouling can be reduced. Plants with limited coal milling capacity can operate at higher capacity possibly with a mill out of service. Particulate emissions and stack gas opacity may be reduced.
The third option of integrating a natural gas turbine with an existing coal-fired boiler provides a means of increasing efficiency when repowering an old coal-fired boiler. All of the above advantages must be weighed against the increased cost of gas as a fuel over the life of a plant.