Oxygen-enhanced combustion of unburned carbon in ash

Abstract

The unburned carbon content of ash is reduced by entraining a stream of the ash into a stream of oxidant and combusting the unburned carbon with oxygen in the oxidant.

Description

FIELD OF THE INVENTION[0001]The present invention relates to treating ash to reduce the amount of unburned carbon and carbon compounds in the ash.BACKGROUND OF THE INVENTION[0002]When solid fuels, such as coal or petroleum coke, are used to fire furnaces or boilers, combustion inefficiencies often lead to the ash (also termed “fly ash”) containing a small amount of unburned carbon (“UBC”). This problem is often exacerbated when conventional combustion-based technologies are employed to lessen the amount of nitrogen oxides (“NOx”) emitted. This unburned carbon, typically measured by the loss on ignition (LOI) test, can present several problems. First, since unburned carbon in the ash represents unrealized heating value of a portion of the fuel, this increment of heating value must be made up by firing more fuel. Second, and typically most critical, ash can be used to replace a portion of the cement used in concrete, but when the level of the unburned carbon in the ash exceeds some th...

AI Technical Summary

Benefits of technology

The invention relates to a method for reducing the carbon content of ash that contains unburned carbon. The method involves entraining particles of ash containing unburned carbon into a flowing stream of gaseous oxidant and passing them through a combustion chamber where the unburned carbon combusts to produce a flowing product stream of ash with reduced carbon content. The heat from the oxidant is sufficient to maintain the combustion but not high enough to melt the ash particles. Additional oxidant can be added to combust any additional unburned carbon. The reduced carbon content ash can then be separated from the gaseous product stream and used in other applications. The invention is useful in various combustion devices and can reduce carbon dioxide emissions.

Problems solved by technology

When solid fuels, such as coal or petroleum coke, are used to fire furnaces or boilers, combustion inefficiencies often lead to the ash (also termed “fly ash”) containing a small amount of unburned carbon (“UBC”).

This problem is often exacerbated when conventional combustion-based technologies are employed to lessen the amount of nitrogen oxides (“NOx”) emitted.

This unburned carbon, typically measured by the loss on ignition (LOI) test, can present several problems.

First, since unburned carbon in the ash represents unrealized heating value of a portion of the fuel, this increment of heating value must be made up by firing more fuel.

Second, and typically most critical, ash can be used to replace a portion of the cement used in concrete, but when the level of the unburned carbon in the ash exceeds some threshold (typically 3-6%) the ash is typically no longer acceptable for use as a cement replacement in concrete—causing the boiler operator to have to pay to dispose of the ash.

This is further compounded by the fact that as the amount of unburned carbon increases, the mass of ash that must be disposed of also increases.

Although several air-based processes have been proposed to burn the residual unburned carbon to allowable levels permitting the ash to be used for cement replacement, they are often expensive and extremely complicated.

For example, currently available processes include circulating fluidized bed arrangements that can be maintenance intensive.

Experience has also shown that these processes are limited in the ranges of unburned carbon content they can effectively beneficiate.

Although oxygen-based and oxygen enhanced systems have also been proposed for these processes, they are also very complicated systems and have met with limited commercial success.

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