Staged-coal injection for boiler reliability and emissions reduction

Abstract

A staged-coal injection procedure for coal-fired boilers used in power generation. The procedure includes the steps of combusting a first type of coal in a first zone of a furnace; and combusting a second type of coal in a second zone of the furnace. The second zone is at a position separate from the first zone.

Description

TECHNICAL FIELD AND BACKGROUND OF THE INVENTION[0001]The present invention relates to the field of power generation. In particular, the invention relates to a staged-coal injection procedure for coal-fired boilers used for power generation.[0002]As economical performance and compliance with stringent environmental regulations becomes increasingly important, energy companies must find cheaper ways to create power while reducing nitrogen oxides (NOx) and unburned carbon (UBC) emissions. One method currently being employed by energy companies is to use Powder River Basin (PRB) coal in boilers instead of eastern bituminous coal.[0003]PRB coal is relatively low in cost, has a low sulfur content, and is in reliable supply. It also has a higher reactivity than eastern bituminous coal due to a lower fixed carbon-to-volatile ratio. This results in lower NOx and UBC emissions. The volatile portion of PRB coal carries a significant amount of nitrogen, and under staged combustion, the volatiles...

AI Technical Summary

Benefits of technology

[0010]Therefore it is an object of the invention to provide a staged-coal injection procedure that reduces fireside corrosion.

[0011]It is another object of the invention to provide a staged-coal injection procedure that minimizes mercury emissions by (1) enhancing the ability to form mercuric chloride, and (2) maximizing the formation of PRB based carbon in ash.

[0012]It is another object of the invention to provide a staged-coal injection procedure that reduces the production of NOx.

[0018]According to another preferred embodiment of the invention, a coal combustion method includes the steps of providing a first type of coal including a selected first compound; providing a second type of coal including a selected second compound; combusting the first type of coal in a first zone of a furnace; and combusting the second type of coal in a second zone of the furnace. The first and second zones are positioned so as to substantially prevent the first compound from combining with the second compound during combustion of the first and second types of coals.

[0022]According to another preferred embodiment of the invention, the method includes the step of providing overfire air in a third zone of the furnace to aid in complete combustion of the first type of coal and second type of coal.

[0032]According to another preferred embodiment of the invention, the overfire port introduces combustion air into a third zone of the furnace to aid in complete combustion of the first and second types of coal.

Problems solved by technology

Despite the lower cost and reduction in emissions associated with PRB coal, the use of PRB coal in boilers has created problems that can reduce the boiler's efficiency and reliability.

While all coal-fired boilers experience some ash deposits, the ash composition of PRB coal can create special problems.

Ash deposits formed from the combustion of PRB coal are difficult to remove, and may cause severe slagging and fouling on heating surfaces.

As a result, deposits from the PRB coal can have a significant effect on the amount of heat absorbed by the boiler.

Because the exit gas temperatures are higher, ash entering the convection passes is often above its fusion temperature, causing ash to deposit on the superheater and reheater surfaces and further reduce the ability of the boiler to make steam, thereby reducing the boiler's efficiency.

Additionally, these higher temperatures may result in the production of NOx, thereby increasing emissions.

Although the benefits of blending are numerous, boiler tube fireside corrosion can be a significant problem due to the formation of alkali chloride compounds which attach to the boiler tube surface, especially in a reducing environment such as that caused by staged combustion.

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