Patent Application
20100275825
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.
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 oxides. While in many cases the combustion systems achieve satisfactory levels of operation within those constraints, there remains a need for methodology that would enable a combustion system to achieve those satisfactory levels of operation when faced with conditions that might be thought to prevent the system from satisfying all its applicable equipment constraints and legal regulatory constraints.
One aspect of the present invention is a combustion method comprising pulverizing solid carbonaceous fuel containing bound nitrogen, feeding the pulverized fuel to a burner, and combusting the pulverized fuel at said burner under conditions under which said combustion would produce a given amount of NOx if said fuel was mixed with air in said pulverizer and fed mixed with said air to said burner, but
wherein said fuel is mixed in said pulverizer with a gaseous oxygen-lean stream that has an oxygen content sufficiently less than that of air that combustion of said fuel mixed with said gaseous stream produces less NOx than said given amount, and said fuel mixed with said gaseous stream is fed to said burner and is combusted at said burner.
A preferred aspect of this combustion method comprises providing a combustion system wherein solid carbonaceous fuel containing bound nitrogen can be pulverized and mixed with air, the mixture of fuel and air can be fed to a burner, and the mixture of fuel and air can be combusted at said burner under conditions under which said combustion produces a given amount of NOx and a given combustion rate,
pulverizing in said pulverizer solid carbonaceous fuel containing bound nitrogen combustion of which at said burner to attain at least said given combustion rate when said fuel is pulverized, mixed with air, fed to said burner, and combusted at said burner, requires increasing the feed rate to said burner of air or fuel in said mixture thereby producing an amount of NOx higher than said given amount,
mixing said pulverized fuel with, instead of air, a gaseous oxygen-lean stream that has an oxygen content sufficiently less than that of air that combustion at said burner of said fuel mixed with said gaseous stream produces NOx in an amount equal to or less than said given amount, and combusting said fuel mixed with said gaseous stream at said burner.
Further aspects of the combustion method of the present invention comprise providing a combustion system wherein solid carbonaceous fuel containing bound nitrogen can be pulverized and mixed with air, the mixture of fuel and air can be fed to a burner, and the mixture of fuel and air can be combusted at said burner under conditions under which said combustion produces a given amount of NOx and a given combustion rate, the conditions including the fuel having a moisture content below a predetermined value, a specific energy content above a predetermined value, or both,
pulverizing in said pulverizer solid carbonaceous fuel containing bound nitrogen which has a moisture content sufficiently higher than said predetermined moisture content value or a specific energy content sufficiently lower than said predetermined specific energy content value, as the case may be, that attaining at least said given combustion rate when said fuel is pulverized, mixed with air, fed to said burner, and combusted at said burner, requires increasing the feed rate to said burner of the air in said mixture thereby producing an amount of NOx higher than said given amount,
mixing said pulverized fuel with, instead of air, a gaseous oxygen-lean stream that has an oxygen content sufficiently less than that of air that combustion at said burner of said fuel mixed with said gaseous stream produces NOx in an amount equal to or less than said given amount, and combusting said fuel mixed with said gaseous stream at said burner.
As used herein, “oxygen-lean stream” means a gaseous stream containing oxygen at an oxygen content less than 21 vol. %.
As used herein, “specific energy content” means the amount of energy per unit of mass of fuel that can be produced upon complete combustion of the fuel, in BTU or equivalent unit of energy per pound or equivalent unit of mass.
As used herein, “combustion rate” is the rate of energy generation per unit of time.
As used herein, references to amounts of NOx produced and to NOx production mean the aggregate amount of gaseous oxides of nitrogen, regardless of chemical formula and including mixtures thereof, produced per quantity of energy generated, typically expressed in units such as pounds of NOx per million BTU.
As used herein, the term “bound nitrogen” means nitrogen present in a molecule other than as N2.
The present invention can be carried out employing equipment which is generally known to those familiar with this technological subject. Referring to
Air stream 7 and oxidant stream 9 represent oxygen-containing streams, one or both of which are typically fed to burners employed in this sort of combustion operation. Thus, combustion of the fuel can be carried out using air as the only source of combustion oxygen, or using one or more gaseous streams having an oxygen content higher than that of air as the source of combustion oxygen, or using both air and one or more gaseous streams having an oxygen content higher than that of air as the source of combustion oxygen. The air fed as stream 7 or the oxidant fed as stream 9 is typically fed out of the burner from one or more openings very close to the opening out of which the fuel stream is fed, so as to directly enter the combustion zone; such stream or streams are typically referred to as “primary air” or “primary oxidant”. In a preferred embodiment the primary air or primary oxidant is fed concentrically around the fuel stream. Depending on the burner design, air or oxidant having an oxygen content higher than that of air may also be fed out of the burner from openings further from the fuel stream than the primary streams are; such streams are typically referred to respectively as “secondary air” or “secondary oxidant”. Some burner designs include openings for feeding secondary air or secondary oxidant, and also include openings which are further from the fuel stream than the openings for the secondary air or secondary oxidant and which are used to feed additional air or oxidant which is termed respectively “tertiary air” or “tertiary oxidant”.
In some combustion apparatus, a stream 17 of air, or of oxidant having an oxygen content higher (or in some embodiments lower) than that of air, is fed into combustion chamber 1 downstream of burner 3. The stream 17, known as an “overtire” stream, is used to stage the addition of oxygen into the combustion chamber so as to control the formation of NOx.
Pulverizer 13 can be of any conventional type which is capable of receiving fuel solids, represented by stream 14, and a gaseous stream 15, pulverizing the fuel solids, and producing a feed stream 11 of pulverized fuel solids carried in the flowing gaseous stream. Typically, as is well known, the gaseous stream 15 is air, such that the pulverizer produces a flowing stream 11 of pulverized fuel solids carried in what is termed transport air.
The fuel can be any solid carbonaceous material which is capable of being combusted in air or oxygen-enriched air to produce heat. The most preferred example of fuel useful in this invention is coal, which as is well known embraces a variety of types of coals having a variety of specific energy contents (expressed as, for instance, BTU per unit mass of the coal), a variety of moisture contents (either as mined or as it is taken from the facility in which it is stored at the site where it is to be combusted), and a variety of combustible content as a percentage of the total amount of solids fed to the pulverizer.
In conventional modes of operation of combustion apparatus of the type described in
Combustion of the fuel solids at the burner, because the fuel contains bound nitrogen, generally produces some amount of nitrogen oxides (referred to herein as “NOx”, which term is used to refer to any gaseous compound consisting of nitrogen and oxygen in any atomic ratio, and any mixture of any such compounds). Operators can employ any of a number of techniques to lessen the amount of NOx produced by the combustion. Such techniques include, without limitation, various methods for staging the combustion such that the fuel emerging from the burner into the combustion chamber combusts in a combustion zone which contains less oxygen than the total amount of oxygen necessary to combust all the fuel solids present, thereby forming a relatively fuel-rich atmosphere, and then providing additional oxygen thereafter to complete the combustion with as yet unburned fuel and with products of incomplete combustion of the fuel from the first stage. Various ways enabling staging of the combustion are known, including burners adapted for staging the flow of oxidants into the combustion chamber, as well as methodology in which first portion of staged oxidant is fed at the burner and additional oxygen is supplied, such as by streams 17 of overtire air, at a point downstream of the combustion zone thereby to ensure completion of the combustion.
A particularly preferred technique for limiting or reducing the amount of NOx formed upon combustion of fuel containing bound nitrogen, such as coal, that can be employed together with the method of the present invention described herein, is that disclosed in U.S. Pat. No. 6,957,955, involving injection of a small amount of oxygen into the fuel-rich region of the combustion zone near the burner.
Achieving the particular desired level of NOx formation by any of these established techniques generally also requires that the rate of air flow to the burner is within a characteristic range. If a situation is present which would require altering the rate of air flow to the burner to a rate outside this range, it may not be possible to control of the amount of NOx formation at or below the desired levels.
Thus, so long as a number of operation conditions are satisfied, including the mass ratio of transport air to fuel solids passing through the pulverizer and fed to the burner, the air flow rate through the pulverizer and associated feed pipes to the burner, and the particular air flow rates required for the given combustion apparatus to stage the combustion so as to attain a given level of NOx production, the apparatus of the type depicted in
However, even in combustion systems that achieve the desired combustion rate (or to avoid decreasing the combustion rate) while maintaining (or not exceeding) a given level of NOx production, whether that production rate and the NOx production level are considered to be optimized or not, there may be circumstances under which the combustion system may not be able to combust fuel at satisfactory levels of combustion rate and NOx production without having to modify the system extensively. There may be circumstances under which adjustments to the flow of transport air would risk changing the mass ratio of transport air to fuel, and/or changing the transport air flow rate, so much that operation of the pulverizer would be compromised, or solids would drop out of the stream 11.
The present invention enables the combustion system to be employed under such circumstances, without an increase in NOx production, and without compromise of the proper functioning of the pulverizer or of the feeds to the burner.
In accordance with this discovery, the gaseous stream 15 that is fed to pulverizer 13 is, instead of air as in conventional operation, a gaseous stream that contains oxygen but that has an oxygen content lower than that of air, preferably lower than 21 vol. % oxygen. The particular amount of oxygen that should be present in the stream that is thus fed to the pulverizer as stream 15, and the flow rate thereof, will depend on the particular circumstances of the combustion operation and of the conditions necessitating reduction of the oxidant content of the stream, as will be described below. Typically, the oxygen content would be on the order of 5 vol. % to 20 vol. %.
This oxygen-lean stream can be formed in many different ways. It can be a stream produced or obtained from another industrial process as is, without further modification of its chemical composition. Examples of such streams include flue gas (recycled from combustion chamber 1, or obtained from another combustion system) or other gas formed by removing some oxygen from air, and byproduct or offgas streams from other chemical processing units. The oxygen-lean stream can be composed by combining oxygen with one or more gases such as nitrogen, carbon dioxide, water vapor, argon, and the like, or with a mixture of any of them. In addition, the oxygen-lean stream can be formed by combining air with another oxygen-lean stream such as any of the foregoing or mixtures thereof.
Operation of the combustion system described herein with fuel transported to the burner in the oxygen-lean stream permits operation under a number of types of circumstances that might otherwise have been considered to prevent compliance with the constraints described herein including maintaining a satisfactory combustion rate, keeping NOx production below an acceptable maximum level, and satisfying the operational requirements of the pulverizer including maintaining the mass ratio of transport medium to fuel solids, and maintaining the flow rate of the transport medium, at workable levels.
For instance, if the type of coal to be fed to the burner has a lower specific energy content, compared to coal that can be fed to the burner in transport air and combusted at a satisfactory combustion rate and a satisfactory NOx production rate, then in order to attain the same or better levels of combustion rate and of NOx production, one would have to increase the feed rate of the air-coal mixture to the burner. However, increasing the feed rate of the air-coal mixture would increase the rate at which oxygen was being fed to the burner and to the combustion chamber (because of the amount of oxygen contained in the increased amount of air) with the result that production of NOx would be increased.
Now, in accordance with the practice of the present invention, the amount of NOx that would be produced by combustion of such coal having a lower specific energy content can be reduced below what it would be if the transport medium was air, by feeding to the burner a feed stream of such lower-energy fuel transported in an oxygen-lean stream (which, as defined herein, contains less oxygen than air) rather than in air.
In addition, in some types of pulverizer the flow rate of the transport medium can be increased when the coal has a higher moisture content than the moisture content for which the optimum combustion operation conditions have been determined. The increased flow rate of the transport air facilitates drying of this wetter coal. However, the resulting increase in the flow rate of the transport air, which causes an increase in the mass ratio of transport air to fuel, also causes an increase in the amount of oxygen being fed with the fuel into the burner and the combustion chamber, which would be expected to cause an increase in the amount of NOx produced upon combustion of this fuel.
Now, in accordance with the present invention, the fuel having this increased moisture content is combined in the pulverizer with an oxygen-lean transport gas stream containing less oxygen than air, so that when an increased amount of this transport gas is combined with the fuel to produce the feed stream which is fed to the burner, combustion of the fuel produces less NOx than would be produced if that fuel had been fed to the burner with an increased amount of transport air and its accordingly increased amounts of oxygen.
Thus, it can be seen that the present invention is useful in any circumstances under which one would have thought it necessary to increase the flow rate of transport air into the combustion chamber, but where increasing the flow rate of transport air would be expected to lead to an increase in the production of NOx in the combustion chamber.
The amount by which the oxygen content of the oxygen-lean transport stream should be reduced below the oxygen content of air to establish the composition of the fuel feed stream that when combusted produces less NOx than would have been produced by its combustion if it was fed in a stream with transport air can readily be determined experimentally or by calculation.
Moreover, it has been determined in accordance with the present invention that the effect of reducing the oxygen content of the transport medium used to convey the fuel into the burner and into the combustion chamber may be characterized by reference to the “primary stoichiometric ratio”, by which is meant the ratio of oxygen in the mixture of fuel and transport medium to the amount of oxygen that would be required for complete combustion of the combustible matter in that mixture. Referring to the primary stoichiometric ratio can provide a way to control the implementation of the method of the present invention.
Thus, in a given combustion system wherein air is the transport medium with which the fuel is mixed as it is conveyed from the pulverizer to the burner, there will be a value or a range of values for the primary stoichiometric ratio at which the combustion can be carried out to achieve a given combustion rate while attaining satisfactorily low levels of NOx production. Then, in order to carry out combustion under circumstances such as those mentioned herein which might be expected to cause an increase the production of NOx because of an increase in the amount of air as the transport medium, one would reduce the oxygen content of the transport medium (while increasing the mass ratio of transport medium to fuel or increasing the transport medium flow rate, as the case may be) down to a level such that the primary stoichiometric ratio is brought to what its value was when the NOx production levels were satisfactory before the circumstances changed that necessitated increasing the flow rate of the transport air. Typically, a satisfactory primary stoichiometric ratio that achieves this objective is 0.18-0.22, and preferably 0.19-0.21.
In an optional but preferred embodiment of this invention, the oxygen-lean gas stream 15 fed to the pulverizer is at a temperature of at least 100° F., preferably at least 200° F., up to 1000° F., preferably up to 500° F., when it is contacted with the fuel in the pulverizer. This embodiment enables the operator to supply to the combustion chamber some heat energy to make up for a deficit in the heat of combustion that is incurred because of the lower amount of oxygen fed to the combustion chamber. This temperature can be achieved by heating the entire stream in an appropriate heater before it is fed to the pulverizer, or by mixing a heated stream with another stream that has not yet been heated. For example, an oxygen-lean stream from another processing unit, or flue gas, that is already at a temperature above ambient, is mixed with air at ambient temperature.
Also, as indicated above, another preferred embodiment is to combine the fuel in the pulverizer with a gaseous stream having an oxygen content lower than that of air, and to feed this stream to the burner and combust it at the burner in a combustion zone having a fuel rich zone while injecting into the fuel rich zone a small amount of oxygen to promote reduction in the formation of NOx in the combustion zone. The injected oxygen (preferably injected as one or more streams of at least 90 vol. % purity oxygen) comprises less than 20% of the stoichiometric amount required for complete combustion of the fuel. A stream of air (separate from any air mixed with the fuel) is fed through the burner in an amount adjusted so that the stoichiometric ratio in the fuel rich zone (i.e. the total amount of oxygen present from any source divided by the total amount of oxygen necessary for complete combustion of the fuel present) is up to 0.99 and at least 0.1, and preferably up to 0.85 and more preferably at least 0.6.
Of course, in any of the embodiments of the present invention, the aggregate amount of oxygen fed in the transport gas, in streams of primary, secondary and tertiary air and oxidant that are employed, in any overfire streams, and in any stream of additional injected oxygen according to the embodiment as described in the preceding paragraph, should be sufficient to enable combustion of the fuel fed to the combustion chamber, and preferably comprises a slight excess of the amount necessary to completely combust the fuel fed to the combustion chamber.
The present invention is illustrated in the following examples, which are computer-generated simulations of the operation of a coal-fired combustion chamber.
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.
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.
In accordance with this invention, adding preheated nitrogen to the transport air fed to the pulverizer reduces this increased flow of oxygen that would otherwise flow to the burner, even as the total mass of transport gas fed to the burner increases, and would lead to production of less NOx than would be produced if only air were used as the transport medium for this fuel. These results are illustrated in the following Table 1.
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.
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.
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.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US07/81800 | 10/18/2007 | WO | 00 | 9/18/2009 |
| Number | Date | Country | |
|---|---|---|---|
| 60852904 | Oct 2006 | US |
