Modifying transport air to control NOX

Abstract

Using oxygen-lean gas as the transport medium in which pulverized fuel solids are conveyed to the burner of a combustion system permits combustion at levels of combustion rate and NOx production under circumstances under which those levels would not be attainable if the transport medium were air.

Description

[0001] This application claims priority from U.S. provisional application Ser. No. 60 / 852,904, filed Oct. 19, 2006, the entire content of which is hereby incorporated herein by reference.

[0002] This invention was made with United States Government support under Cooperative Agreement No. DE-FC26-00NT40756 awarded by the Department of Energy. The United States Government has certain rights in the invention.FIELD OF THE INVENTION

[0003] This invention relates to combustion of fuel solids that are fed to a combustion chamber suspended in a stream of transport air. Such a stream is typically produced in a pulverizer prior to being fed to the combustion chamber.BACKGROUND OF THE INVENTION

[0004] Many industrial combustion systems operate within constraints imposed by the equipment and by legal regulatory requirements. Equipment constraints include minimum and maximum feed rates of fuel and oxidant, and legal regulatory requirements include limitations on production of emissions such as nitrogen...

Examples

example 1

[0046]In this example, a bituminous coal is assumed to be pulverized in a conventional pulverizer and combined with air at a mass ratio of transport air to fuel (TaF) of 1.8. The coal is assumed to have a moisture content of 8.5 wt. %. The moisture in the coal is reduced to 2.7 wt. % by combining it with air at a temperature of 500° F. in the pulverizer. The resulting stream of air and pulverized coal leaves the pulverizer at a temperature of 180° F.

[0047]If the same coal is then assumed to have an initial moisture content of 15 wt. %, then to reach the same final moisture content of 2.7 wt. % the amount of air at 500° F. fed to the pulverizer and combined with the fuel must be increased so that the TaF ratio of air to fuel increases to 2.0. This increase in the amount of air and, thus, the amount of oxygen being fed to the burner would be expected to cause an increase in the production of NOx upon combustion of this fuel.

[0048]In accordance with this invention, adding preheated nit...

example 2

[0049]This example shows the effect of switching from a bituminous coal to a subbituminous coal, having a lower specific energy value. The increased moisture content as well as the lower specific energy content of the subbituminous coal requires that a higher amount of coal be pulverized and fed to the burner to achieve the same rate of energy production in the combustion chamber. If this increased flow rate of fuel to the combustion chamber is achieved without increasing the TaF ratio, then much more oxygen will be fed into the combustion chamber because of the increased air flow, leading to increased production of NOx. If instead it is assumed that the same degree of moisture removal is achieved, even then the TaF must increase again leading to production of increased amounts of NOx. The calculations presented in Table 2 again illustrate that adding nitrogen to the transport air, rather than increasing the amount of transport air, would be expected to produce less NOx.

TABLE 2Examp...

[0001] This application claims priority from U.S. provisional application Ser. No. 60 / 852,904, filed Oct. 19, 2006, the entire content of which is hereby incorporated herein by reference.

[0002] This invention was made with United States Government support under Cooperative Agreement No. DE-FC26-00NT40756 awarded by the Department of Energy. The United States Government has certain rights in the invention.FIELD OF THE INVENTION

[0003] This invention relates to combustion of fuel solids that are fed to a combustion chamber suspended in a stream of transport air. Such a stream is typically produced in a pulverizer prior to being fed to the combustion chamber.BACKGROUND OF THE INVENTION

[0004] Many industrial combustion systems operate within constraints imposed by the equipment and by legal regulatory requirements. Equipment constraints include minimum and maximum feed rates of fuel and oxidant, and legal regulatory requirements include limitations on production of emissions such as nitrogen...

Examples

example 1

[0046]In this example, a bituminous coal is assumed to be pulverized in a conventional pulverizer and combined with air at a mass ratio of transport air to fuel (TaF) of 1.8. The coal is assumed to have a moisture content of 8.5 wt. %. The moisture in the coal is reduced to 2.7 wt. % by combining it with air at a temperature of 500° F. in the pulverizer. The resulting stream of air and pulverized coal leaves the pulverizer at a temperature of 180° F.

[0047]If the same coal is then assumed to have an initial moisture content of 15 wt. %, then to reach the same final moisture content of 2.7 wt. % the amount of air at 500° F. fed to the pulverizer and combined with the fuel must be increased so that the TaF ratio of air to fuel increases to 2.0. This increase in the amount of air and, thus, the amount of oxygen being fed to the burner would be expected to cause an increase in the production of NOx upon combustion of this fuel.

[0048]In accordance with this invention, adding preheated nit...

example 2

[0049]This example shows the effect of switching from a bituminous coal to a subbituminous coal, having a lower specific energy value. The increased moisture content as well as the lower specific energy content of the subbituminous coal requires that a higher amount of coal be pulverized and fed to the burner to achieve the same rate of energy production in the combustion chamber. If this increased flow rate of fuel to the combustion chamber is achieved without increasing the TaF ratio, then much more oxygen will be fed into the combustion chamber because of the increased air flow, leading to increased production of NOx. If instead it is assumed that the same degree of moisture removal is achieved, even then the TaF must increase again leading to production of increased amounts of NOx. The calculations presented in Table 2 again illustrate that adding nitrogen to the transport air, rather than increasing the amount of transport air, would be expected to produce less NOx.

TABLE 2Examp...

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