WO2007063386A1

WO2007063386A1 - METHODS AND SYSTEMS FOR REDUCED NOx COMBUSTION OF COAL WITH INJECTION OF HEATED NITROGEN-CONTAINING GAS

Info

Publication number: WO2007063386A1
Application number: PCT/IB2006/003366
Authority: WO
Inventors: Rajani K. Varagani
Original assignee: L'air Liquide, Societe Anonyme Pour L'etude Et L'exploitation Des Procedes Georges Claude
Priority date: 2005-12-02
Filing date: 2006-11-27
Publication date: 2007-06-07
Also published as: CA2631898A1; US20070125282A1; EP1957868A1; KR20080084998A; JP2009517626A; AU2006321344A1

Abstract

A stream of nitrogen-containing gas is heated and injected into a stream of coal and conveying gas to produce a stream of mixed nitrogen-containing gas, coal, and conveying gas. Oxygen is injected into the stream of mixed nitrogen-containing gas, coal, and conveying gas to produce a stream of mixed nitrogen-containing gas, coal, conveying gas, and oxygen. The mixed nitrogen-containing gas, coal, conveying gas, and oxygen are combusted in a combustion chamber.

Description

METHODS AND SYSTEMS FOR REDUCED NOx COMBUSTION OF COAL

WITH INJECTION OF HEATED NITROGEN-CONTAINING GAS

Background

NO_x generally refers to nitrogen monoxide NO and nitrogen dioxide NO₂. Both are major contributors to acid rain and smog (ground level ozone) issues. The NO_x partition in the flue gases of pulverized coal boilers is typically more than 95% NO and the remainder NO₂ (Mitchell S.C., NO_x in Pulverized Coal Combustion, IEA Clean Coal Center Report CCC/05, 1998). During coal- combustion, the NO_x production originates from three different mechanisms: • Fuel-NOχ mechanism,

• Thermal-NO_x mechanism, and

• Prompt-NO_x mechanism.

In pulverized coal boilers, 70% to 80% of NO_x is formed from the fuel- bound nitrogen species (fuel-N) via the fuel-NO_x mechanism, and the remaining NO_x is formed from atmospheric nitrogen (N₂), via the thermal-NO_x mechanism (5-25%) and via the prompt-NO_x mechanism (less than 5%) (Wu Z., NO_x control for pulverized coal-fired power stations, IEA Clean Coal Center Report CCC/69, 2002). Understanding and limiting the NO_x formation in pulverized coal combustion is therefore strongly related to the fuel-N conversion mechanism. A complex series of reactions explains the transformation of coal bound fuel- nitrogen into NO_x or N₂, including more than 50 intermediate species and hundreds of reactions.

The two main parameters affecting the fuel-NO_x formation process are the volatile matter content of the fuel and the stoichiometry (air/fuel ratio). Coal nitrogen content (bound nitrogen only), also strongly impacts NO_x emission levels. Coal typically contains 0.5% to 3% nitrogen by weight on a dry basis. For comparison, natural gas also contains some nitrogen (0.5 to 20%); however it is molecular nitrogen N₂, and thus is not affected by the fuel-NO_x mechanism. Figure 1 summarizes the main reactions affecting fuel-nitrogen in the combustion process (Zevenhoven R., Kilpinen P., Control of pollutants in flue gases and fuel gases, Picaset Oy, Espoo, ISBN 951-22-5527-8, 2001 ). Four main steps can be identified: 1- Devolatilization releasing coal nitrogen compounds (coal-N) in a gaseous phase (Volatile-N), mainly as HCN, some as NHj. The remaining coal-nitrogen compounds stay in the solid phase (char), and are referred to as char-N.

2- HCN evolution to NHi species. 3- NHj oxidation to NO or reduction to N₂ depending on local conditions.

4- Rebuming, as some NO is recirculated back to the hot reducing zone of the flame and converted back to N₂ while contacting CHi radicals

Both voIatile-N and char-N can be evolved as NO or as N₂. Fuel-NO_x formation is minimized by implementing specific conditions leading to N₂ rather than NO (see Van Der Lans R. P., Glarborg P. and Dam-Johansen K., Influence of process parameters on nitrogen oxide formation in pulverized coal burners, Prog. Energy Combust. Sci. Vol. 23, p. 349-377, 1997; Bowman C.T., Kinetics of Pollutant Formation and Destruction on Combustion, Prog Energy Combust Sci 1 33-45, 1975; and Proceedings of the 6^th International Conference on Technologies and Combustion for a Cleaner Environment, Oporto, Portugal, 2001 )

For a given coal and particle size, three main conditions will independently or in combination promote fuel-bound nitrogen conversion into molecular nitrogen N₂ rather than NO: - Fuel rich (reducing) conditions at the burner level: by arranging fuel-rich "zones" in the furnace during the devolatilization stage, the nitrogen species in gas phase (volatiles) are more likely to be reduced to molecular nitrogen (N₂) rather than oxidized to NO.

High temperature in the early stages of combustion increases the volatiles yield. As volatiles burn close to the burner exit, controlling the volatile-N (gas) to N₂ conversion is much easier than the char-N (solid) to N₂ conversion. High temperature at the burner exit also increases both the rebuming rate of recirculated NO and the conversion rate of volatile-N into N₂ (see Sarofim A. F., Pohl J. H., Taylor B. R., Strategies for Controlling Nitrogen Oxide Emissions during Combustion of Nitrogen-bearing fuels, 69th Annual Meeting of the AlChe, Chicago, IL, 1976; and Bose A.C., Dannecker K.M. and Wendt J.O.L., Energ. Fuel, Vol. 2, p.301 , 1988. ) - Long residence times in the high temperature and reducing zones in the boiler lead to higher fuel-N to N₂ and NO to N₂ conversion.

Prior research indicates that oxygen was used alone to decrease the NOx formation. Due to safety and other concerns, oxygen was injected at a relatively low temperature and also in the burner just before the combustion.

Summary

A method is provided and a system for performing the method. A stream of nitrogen-containing gas is heated and injected into a stream of coal and conveying gas to produce a stream of mixed nitrogen-containing gas, coal, and conveying gas. The mixed nitrogen-containing gas, coal, and conveying gas are combusted with oxygen in a combustion chamber.

Brief Description of the Drawings

For a further understanding of the nature and objects of the present system and method, reference should be made to the following detailed description, taken in conjunction with the accompanying drawings, in which like elements are given the same or analogous reference numbers and wherein: Figure 1 is a schematic summarizing the main reactions affecting fuel- nitrogen in the combustion process; Figure 2 is a schematic view of the system with oxygen injection upstream of the burner.

Figure 3 is a schematic view of the system with oxygen injection at the burner.

Figure 4 is a perspective view of a tubular injection element having rectangular apertures;

Figure 5A is a schematic of a circular aperture for use in a tubular injection element; Figure 5B is a schematic of a rectangular aperture for use in a tubular injection element;

Figure 5C is a schematic of a triangular aperture for use in a tubular injection element; Figure 5D is a schematic of an elliptical aperture for use in a tubular injection element;

Figure 6 is a perspective view of a tubular injection element having three sets of rectangular apertures;

Figure 7 is a perspective view of a tubular injection element having three sets of decreasingly shorter rectangular apertures;

Figure 8 is a perspective view of a tubular injection element having rectangular apertures arranged in a staggered pattern;

Figure 9 is a perspective view of a tubular injection element having a vertically non-uniform distribution of rectangular apertures; Figure 10 is a perspective view of a tubular injection element having an aerodynamic pointed tip with rectangular apertures;

Figure 11 is a perspective view of a tubular injection element having an aerodynamic rounded tip with rectangular apertures;

Figure 12 is a perspective view of a tubular injection element having an aerodynamic rounded tip with elliptical apertures;

Figure 13 is a perspective view of a tubular injection element having an aerodynamic pointed tip with elliptical apertures;

Figure 14 is a cross-sectional view of two concentric injections with swirler- type injection elements; Figure 15A is a perspective view of two injections with a swirler disposed on the nitrogen lance and a tangentially injecting injection element disposed on an inner wall of the fuel duct wherein the swirl and tangential injections are generally in the same direction;

Figure 15B is a perspective view of two injections with a swirler disposed on the nitrogen lance and a tangentially injecting injection element disposed on an inner wall of the fuel duct wherein the swirl and tangential injections are generally in the opposite direction;

Figure 16 is a side elevation view of a swirler showing opening and wall widths; Figure 17 is a perspective view (with the aerodynamic tip not illustrated) of four injection elements radially spaced from one another having a leg with at least one aperture at an end thereof;

Figure 18 is a side elevation view (with the aerodynamic tip illustrated) of the injection element configuration of Figure 17;

Figure 19 is a front elevation view (with the aerodynamic tip illustrated) of the injection element configuration of Figure 17;

Figure 20 is a front elevation view of a two-injection element configuration having a fin configuration; Figure 21 is a side elevation view of the two-injection element configuration of Figure 20;

Figure 22A is a side elevation view of an axial injection element with a vertically oriented, elliptical cross-sectional shape;

Figure 22B is a side elevation view of an axial injection element with a horizontally oriented, elliptical cross-sectional shape;

Figure 23 is a perspective view of a tubular injection element having three radially spaced apertures at an end, thereof for injecting oxygen at an angle to the axis;

Figure 24A is a side elevation view of a tubular injection element with apertures configured as circles arranged in a circle with one aperture in the middle;

Figure 24B is a side elevation view of a tubular injection element with a saw tooth-shape pattern of apertures at a peripheral portion thereof;

Figure 24C is a side elevation view of a tubular injection element with a four-wedge type pattern of apertures;

Figure 24D is a side elevation view of a tubular injection element with a star-shaped aperture;

Figure 24E is a side elevation view of a tubular injection element with a curved, cross-shaped aperture disposed at a center thereof; Figure 24F is a side elevation view of a tubular injection element with a curved, cross-shaped aperture similar to that of Figure 24E but having a greater thickness and extending to a peripheral portion thereof; and Description of Preferred Embodiments

Increasing the temperature of the coal to safely release volatiles in an oxygen deficit environment prior to combustion can hinder the NOx formation mechanism. This is achieved in the proposed method and system by injecting high temperature nitrogen-containing gas upstream of the burner and optionally by injecting oxygen-containing gas in the burner or just upstream of the burner. Injection of hot nitrogen-containing gas in an O₂-deficit environment causes devolatilization of the coal to release volatiles and can also decompose them to N₂. The fuel-bound nitrogen in char can also be decomposed to N₂. This process also increases the residence time of the volatiles-N and char-N in the main combustion zone thereby favoring decomposition to N₂. Further NOx reductions are obtained by the optional injection of oxygen-containing gas in the burner or just upstream of the burner to compensate the injected nitrogen and to increase the temperature of the flame in fuel rich condition. Both the processes promote formation of N₂ instead of NOx.

The system for burning coal with reduced NOx emissions includes the following: a source of a mixture of coal and conveying gas; a source of oxygen- containing gas; a source of nitrogen-containing gas; a heating device adapted and configured to heat nitrogen from the nitrogen source; a combustion chamber; a burner disposed at a wall of the combustion chamber; a burner operatively associated with a combustion chamber; a fuel duct in fluid communication with the source of a mixture of coal and conveying gas, the fuel duct extending towards the burner; and a nitrogen-containing gas injection element in fluid communication with the heating device and the fuel duct, the nitrogen injection element being adapted and configured to inject heated nitrogen-containing gas from the heating device into a stream of a mixture of coal and conveying gas and mix therewith inside the fuel duct.

A method of combusting coal with reduced NOx emissions includes the following steps. A stream of nitrogen-containing gas is heated. The heated stream of nitrogen-containing gas is injected into a stream of coal and conveying gas to produce a stream of mixed nitrogen-containing gas, coal, and conveying gas. The mixed nitrogen-containing gas, coal, and conveying gas are introduced at a burner disposed at a wall of a combustion chamber. The coal is combusted with oxygen in the combustion chamber. The system or method can include any one or more of the following aspects:

- an oxygen-containing gas injection element is in fluid communication with the source of oxygen and the fuel duct, the oxygen-containing gas injection element fluidly communicating with the fuel duct downstream of where the nitrogen-containing gas injection element fluidly communicates with the fuel duct and upstream of or at the burner, the oxygen-containing gas injection element being adapted and configured to inject oxygen-containing gas from the oxygen- containing gas source into the stream of mixed heated nitrogen-containing gas, coal, and conveying gas

- the source of oxygen-containing gas and the source of nitrogen- containing gas comprise an Air Separation Unit (ASU)

- the conveying gas comprises flue gas from the combustion chamber mixed with oxygen-containing gas from the oxygen-containing gas source - the heating device is adapted and configured to directly impart heat to nitrogen from the nitrogen-containing gas source from a flame

- the heating device is a heat exchanger adapted and configured to exchange heat between nitrogen-containing gas from the nitrogen-containing gas source and heat from combustion of the coal and oxygen in the combustion chamber

- the conveying gas is flue gas from the combustion chamber mixed with oxygen

- the conveying gas is air

- the oxygen-containing gas injection element fluidly communicates with the fuel duct at the burner - the nitrogen-containing gas injection element fluidly communicates with the fuel duct at a peripheral portion of the fuel duct

- the nitrogen-containing gas injection element fluidly communicates with the fuel duct along a central axis of the fuel duct - the oxygen-containing gas injection element fluidly communicates with the fuel duct at a peripheral portion of the fuel duct

- the oxygen-containing gas injection element fluidly communicates with the fuel duct along a central axis of the fuel duct

- the nitrogen-containing gas and oxygen are obtained from an ASU - the step of heating a nitrogen-containing gas stream comprises directly imparting heat from a flame to the nitrogen stream

- the step of heating a nitrogen-containing gas stream comprises indirectly imparting heat from a flame to the nitrogen stream via a heat exchanger.

- the step of heating a nitrogen-containing gas stream comprises indirectly imparting heat from the step of combusting to the nitrogen stream via a heat exchanger.

- the stream of nitrogen-containing gas is heated to a temperature such that a desired level of devolatilization occurs

- the stream of nitrogen-containing gas is heated to a temperature in the range of from about 1 ,000 ⁰F to about 1 ,800 ⁰F.

- injection of the heated stream of nitrogen-containing gas causes devolatilization of most of a volatile species content in the coal.

- collecting at least some of any flue gas produced from the step of combusting, injecting oxygen-containing gas into the collected flue gas to mix therewith, and introducing the mixed oxygen and flue gas to the burner

- oxygen-containing gas is injected into the collected flue gas in an amount such that an oxygen concentration in the mixed oxygen-containing gas and flue gas is from about 3% to about 20%.

The proposed method and system also reduces the fuel-NOx formation in a coal combustion process. As described in the above section, the fuel bound N can be transformed into either molecular N (N₂) or NO depending on the local conditions where the devolatization took place. Injecting hot nitrogen-containing gas into the coal stream releases volatiles and fuel-bound N compounds in a reducing environment. The reducing environment drives the coal derived N compounds to convert to N₂. The temperature and quantity of nitrogen-containing gas to be injected depends on the type of coal and the NOx reduction targets. The temperature of the nitrogen-containing gas is chosen to be above the devolatilization temperature of the volatile species in the coal. The volatilization characteristics of various general types of coals are well known. In the case of a specific type of coal, the volatilization characteristics may be determined experimentally in a known manner. Generally speaking, the temperature should be selected such that a desired degree of devolatilization occurs for the particular type of coal being combusted. A suitable temperature is in the range of from about 1 ,000 ⁰F to about 1 ,800 ⁰F. The location of nitrogen-containing gas injection should be strategically placed so that just enough residence time is available for the devolatilization and the conversion to nitrogen-containing gas to occur. Injecting hot nitrogen-containing gas more than this distance can pose safety issues as volatiles are very flammable and unfavorable combustion could occur.

The nitrogen-containing gas need not be pure nitrogen. Indeed, gaseous mixtures having a majority of nitrogen with minor amounts of other gases are suitable for use with the process and system. Such minor constituents include O₂ and inert gases such as Ar and CO₂. A preferred source for both the nitrogen- containing gas to be heated and the 02 is from an air separation unit (ASU). Suitable ASU's include those operated via pressure swing adsorption (PSA), vacuum swing adsorption (VSA), cryogenic distillation, and membrane permeation. Typical N₂ and O₂ concentrations in nitrogen-enriched and oxygen- enriched streams from these types of ASU's are well known and need not be repeated here. Other sources of the nitrogen can include a gaseous mixture comprising nitrogen and flue gas The oxygen-containing gas to be optionally injected into the mixed nitrogen-containing gas, conveying gas, and coal also need not be pure. Suitable gases include those having an oxygen concentration greater than that of air. up to 100% pure oxygen. In the case of N2 from an ASU, the nitrogen-containing gas can be heated in "direct fired mode" or "indirect fired mode". In a direct fired mode the incoming nitrogen-containing gas is heated by direct contact with a small flame. In indirect fired mode, the nitrogen-containing gas is heated at a heat exchanger taking heat from a small flame or from a combustion process.

In fuel-rich flame conditions, injection of oxygen in the main combustion zone increases the temperature. This higher temperature reducing environment promotes formation of N₂ from the remaining volatiles released from the coal. If optional oxygen-containing gas injection is selected, the oxygen-containing gas is injected at a location which achieves both safety goals and good mixing with the stream of coal/conveying gas/nitrogen-containing gas. The location is desirably upstream of the burner throat in order to reduce the risk of incurring partial combustion of coal particles in local pockets that are oxygen-enriched. At the same time, the location is not so close to the burner that little mixing of the oxygen-containing gas and coal/conveying gas/nitrogen-containing gas is achieved.

The conveying gas comprises any gas to convey fuel particles from a particle storage or generation location, e.g., mills, to the burner level and the combustion chamber. For example, this gas can comprise the primary air used to convey pulverized or micronized coal in a coal-fired boiler. Preferred conveying gases are air and mixtures of recirculated flue gas and oxygen. Typically, these mixtures of recirculated flue gas and oxygen include about 60-90% CO₂, 5-20% N₂, and 3-20% O₂. An especially preferred mixture of recirculated flue gas and oxygen contains about 80% CO₂ and about 20% O₂. Several different types of injection elements may be employed. It should be noted that each of the nitrogen-containing gas and oxygen-containing gas injection elements may be the same as one another or different. Several examples of injection elements follow.

A system for performing the method is best illustrated in Figures 2-9. As best illustrated in Figure 2, in one embodiment a stream of coal and conveying gas 1 enters fuel duct 8. A heated stream of nitrogen-containing gas from first injection element 3 is mixed with the coal and conveying gas downstream of element 3. Oxygen-containing gas is optionally injected into the mixed coal, conveying gas, and nitrogen-containing gas by injection element 4 upstream of burner 9. The mixed nitrogen-containing gas, oxygen-containing gas (if optionally injected), coal, and conveying gas is introduced to combustion chamber 6 via a burner where combustion 7 takes place.

As best illustrated in Figure 3, in another embodiment a stream of coal and conveying gas 1 enters fuel duct 8. A heated stream of nitrogen-containing gas from first injection element 3 is mixed with the coal and conveying gas downstream of element 3. Oxygen-containing gases optionally injected into the mixed coal, conveying gas, and nitrogen-containing gas by injection element 5 at burner 9. The mixed nitrogen-containing gas, oxygen-containing gas (if optionally injected), coal, and conveying gas is introduced to combustion chamber 6 via burner 9 where combustion 7 takes place.

It should be noted that injection elements 3, 5 need not be disposed centrally along an axis of the fuel duct 8. Rather, they may be disposed along a peripheral portion of the fuel duct 8. Some of these various configurations are best illustrated in some of the following injection element designs.

Radially Injecting Injection Elements Designs:

As illustrated in Figure 4, one injection element 10 is a tube having a closed end 16 and plurality of rectangular apertures 13. This design provides radial injection from the circumferential face of the injection element 10. The length, Di, and width, D₂, of these apertures, as well as the circumferential arc distance, D₀, between two adjacent apertures may be varied to control the momentum ratio J (ratio of the oxygen-containing gas or nitrogen- containing gas jet momentum to the momentum of the stream of non-gaseous fuel/conveying gas). D-i, D₂, and D₀ also control the penetration of the injection gas into the primary stream or primary stream mixed with nitrogen-containing gas as appropriate. A small D₂ZD₁ ratio (streamlined rectangular apertures) will minimize the perturbation to solid fuel particles, such as coal. A big D₂ZD₁ ratio (bluff-body slots) will have a greater influence on the solid phase and will push solid fuel particles, such as pulverized coal, away from the centerline of the burner primary air duct. Those two different aspect ratios will lead to different distribution of particles and nitrogen or oxygen at the duct outlet. Those three parameters, S-i, D_{1 ,} and D₂, in turn, control the penetration of the injection gas into the primary stream or primary stream mixed with nitrogen- containing gas as appropriate. A small D₂ZD₁ ratio (streamlined slots) will minimize the perturbation to the solid phase. A big D₂ZD₁ ratio (bluff-body slots) will have a greater influence on the solid phase and will push the coal particles away from the centerline of the burner primary air duct. Those two different aspect ratios will lead to different distribution of particles and nitrogen or oxygen at the duct outlet. As shown in Figures 5A-5D, the slot shape itself could be circular, rectangular, triangular, or elliptical, respectively. As depicted in Figure 6, the injection element 20 includes apertures 23 arranged in axially extending rows along the axis of the injection element 20. This pattern performs a better mixing if the axial distance D₃ between two adjacent apertures 23 in a same row is sufficiently large. The dimension D₃ between the apertures 23 could be the same or could vary in the axial direction towards the closed end 26.

As best illustrated in Figure 7, the length dimensions Di, D₄, and D₅ of the apertures 33 in injection element 30 may vary from short to long going in the direction of the closed end 36. Alternatively, these length dimensions could vary in any order from short to long, long to short, long to short and then back to long, short to long and then back to short, and other permutations. In addition, the dimensions Di or D₂ could also vary in the azimuthal (radial) direction. This offers more precise control over the penetration of the injection gas into the primary stream. Finally, D₃ can be tailored to the conditions of each process to optimize mixing and minimal redistributions of particles. As shown in Figure 8, the apertures 43 in injection element 40 need not extend in the axial direction. Rather, they may be staggeredly disposed at different angles Θ with respect to one another. Θ can vary from less than 180° (streamlined slotsZaxial slots) to 90° (bluff-body slotsZradial slots).

As depicted in Figure 9, the injection element 50 need not have a uniform distribution of apertures 53 in the azimuthal direction. As discussed previously, in coal-fired boilers, the coal particle loading is not always uniform throughout the cross-section (sometimes due to the so-called "roping phenomenon"). In the case of coal, the particle concentration in the stream of coalZconveying gas 56 (or coal/conveying gas/ nitrogen-containing gas) at the bottom of the injection element 50 may be higher than the same in the stream of coal/conveying gas (or coal/conveying gas/ nitrogen-containing gas) 57 at the top of the injection element 50. In this figure, the thickness of arrows represents the loading of particles in the gas stream. The advantage offered by this is that more nitrogen- containing gas or oxygen-containing gas could be introduced in the locations where particle loading is higher 58 than locations where particle loading is lower 59. This will reduce the likelihood of creating local pockets with less devolatilization potential (in the case of nitrogen-containing gas injection) or local pockets that are fuel-lean (in the case of oxygen-containing gas injection) each of which could lead to higher levels of NO_x. With respect to this problem and solution, the particle loading distribution could easily be determined by experimental or modeling studies.

Similar to the injection element designs 10, 20, 30, 40, the apertures 53 may be staggered and vary in size in the axial and azimuthal directions. The distance between apertures 53, the number of rows of apertures 53, or the surface area of apertures 53 could also be varied.

This injection element 50 has a particularly beneficial application to coal- fired boilers whose burner geometry include coal concentrators or splitters (identified technique in the prior art for reducing NO_x emissions from pulverized coal burners). Varying levels of nitrogen-containing gas or oxygen-containing gas injection may be located to achieve higher concentration of N₂ or O₂ in coal richer zones. As a result, the equivalence ratio between coal and N₂ (in the case of nitrogen-containing gas injection) coal and O₂ (in the case of oxygen- containing gas injection) can be controlled in the coal richer zone (concentrated zone) as well as in the coal leaner zones.

Aerodynamic Injection Element Designs:

As depicted in Figures 10-14, the injection element 100, 110, 120, 130, 140 may have an aerodynamic closed end 106, 116, 126, 136, 146. An aerodynamic shape tends to reduce re-circulation of the stream of coal/ conveying gas (in the case of nitrogen-containing gas injection) or of the stream of coal/conveying gas/nitrogen-containing gas (in the case of oxygen-containing gas injection), and creation of a particle deficient and low/reverse velocity zone in the wake of the injection element 100, 110, 120, 130, 140.

Referring to the injection element 100 of Figure 10, rectangular apertures 103 could be added to closed end 106 in all the permutations described in Figures 1-7. The closed end 106 could be pointed, and terminate at point P-|. The distances Ds and D₉ and the angle a defined by lines Li and L₂ could be varied in order to optimize the mixing in a shortest distance and to cause least disturbance to the non-gaseous fuel.

Referring to the injection element 110 of Figure 11 , rectangular apertures 113 could be added to closed end 116 in all the permutations described in

Figures 1-7. The closed end 116 could be rounded, instead of extending to point P₂ at the intersection of lines L₄ and L₅. The distances Dio and Dn, and the angle δ defined by lines L₄ and Le could be varied in order to optimize the mixing in a shortest distance and to cause least disturbance to the non-gaseous fuel. As illustrated in Figure 12, elliptical (or circular) apertures 123A, 123B,

123C may be present on injection element 120. The injection element 120 extends to a rounded tip 126. Each of apertures 123A, 123B, and 123C is configured to inject streams of nitrogen-containing gas or oxygen-containing gas PA. PB, P_C into the mixed stream of coal/conveying gas (in the case of nitrogen- containing gas injection) or coal/conveying gas/nitrogen-containing gas (in the case of oxygen-containing gas injection) at an angle to the axis of the lance.

As shown in Figure 13, elliptical (or circular) apertures 133A, 133B, 133C may be present on injection element 130. The injection element 130 extends to a pointed tip 136. Each of apertures 133A, 133B, and 133C is configured to inject a stream of nitrogen-containing gas or oxygen-containing gas P₀, PE, PF into the mixed stream of coal/conveying gas (in the case of nitrogen-containing gas injection) or coal/conveying gas/nitrogen-containing gas (in the case of oxygen- containing gas injection) at an angle to the axis of the oxygen lance.

Swirl-Type Injection Element Designs:

The designs presented in this section are based upon the patented Oxynator® (US 5,356,213) concept. It is designed to minimize mixing distance and to prevent high nitrogen or oxygen concentrations near the pipe walls. With respect to the first configuration and as illustrated in Figure 14, the arrangement of the fuel duct 231 with respect to the conduit 239 defined by walls 232A, 232B is a tube within a tube. Nitrogen-containing gas is fed to the central injection element 235 from oxygen lance 236. It is injected with a swirl S2. Oxygen-containing gasis fed from conduit 239 to the single peripheral injection element 234, which is disposed flush with the inner wall of fuel duct 231. Oxygen-containing gasis injected from the inner wall of fuel duct 231 with a swirl Si by injection element 234. The directions of swirls Si, S₂ may the same or different. The flow passage leading to and from the peripheral injection element 234 could be aerodynamically (like a venturi) designed to cause minimum disturbance to the flow. In other words, shoulders before and after the injection element 234 could be used. It should also be understood that fuel duct 238 need not extend beyond injection element 231 A, 231 B.

With respect to the second configuration, the conduit 239 may actually be a plurality of conduits surrounding the fuel duct 231 , any or all of which feeds injection element 234.

As shown in Figure 15A, another Oxynator®-based design includes fuel duct 241 surrounded by a conduit 249 (known by those ordinarily skilled in the art as a secondary or transition stream zone) defined by walls 242A, 242B. Disposed in a central axis of fuel duct 241 is nitrogen-containing gas lance 244 at the end of which is an injection element 244 (based upon Oxynator®. Disposed along the inner wall of the fuel duct 241 is a plurality of tangentially injecting injection elements 245A, 245B, 245C, 245D. In operation, nitrogen-containing gas fed by lance 244 to injection element 244 is injected into fuel duct 241 with a swirl S₃. Oxygen-containing gasfed by conduit 249 to injection elements 245A, 245B, 245C, 245D is tangentially injected with respect to fuel duct 241 into fuel duct 241 with a swirl S₄ that is in the same direction as swirl S₃.

As shown in Figure 15B, another Oxynator®-based design includes fuel duct 251 surrounded by a conduit 259 (known by those ordinarily skilled in the art as a secondary or transition stream zone) defined by walls 252A, 252B.

Disposed in a central axis of fuel duct 251 is nitrogen-containing gas lance 254 at the end of which is an injection element 254 (based upon Oxynator©. Disposed along the inner wall of the fuel duct 251 is a plurality of tangentially injecting injection elements 255A, 255B, 255C, 255D. In operation, nitrogen-containing gas fed by lance 254 to injection element 254 is injected into fuel duct 251 with a swirl S₅. Oxygen-containing gas fed by conduit 259 to injection elements 255A, 255B, 255C, 255D is tangentially injected with respect to fuel duct 251 into fuel duct 251 with a swirl S_δ whose direction is opposite that of swirl S₅. All of the Oxynator®-based designs of Figures 14, 15A, and 15B may be varied as follows. As depicted in Figure 16, injection element Arc 222 along the circumferential border of open space 221 between two adjacent vanes 223 has a dimension A-i. On the other hand, the circumferential edge of vane 223 has a dimension A₂. The number of vanes 223 and the dimensions A1 , and A1 may be varied in order to optimize the mixing and particle loading. The ratio of dimensions A-i, A₂ may be chosen to optimize the injection velocity and thus the penetration of the jet. A small ratio A₂/Ai is preferred to minimize the disturbance to the solid phase.

Bluff Body Injection Element Designs:

Oxygen-containing gas may be injected at several locations at roughly a single axial position by several different injection elements.

As shown by Figures 17-19, extending from a lance portion 301 is an injection element comprising a leg member having first and second portions 302A, 303A and at least one aperture 304A at the end of second portion 303A. Other injection elements similarly comprise a leg member having first and second portions (302B, 303B; 302C, 303C, 302D, 303D) and at least one aperture 304B, 304C, 304D at the end of the second portions 303B, 303C, 303D. While not depicted in Figure 17 for clarity's sake, an aerodynamic tip 306 is included at the end of lance portion 301 just after the junction between lance portion 301 and the first portions 302A, 302B, 302C, 302D.

As illustrated by Figure 19, each injection element has height and length dimensions D-₁₃, Du. The injection elements inject nitrogen-containing gas or oxygen-containing gas into the fuel duct at an angle β with respect to an axis of the fuel duct and defined by lines l_io, and Ln. By strategically placing the injection elements of at various locations, mixing of the oxygen and the coal/conveying gas is enhanced by controlling the jet momentum. The cumulative projection area of all these injection elements perpendicular to the flow area is much smaller than the flow area of the primary stream. Thus, these injection elements do not offer any significant obstruction to the flow of the particle-laden stream. In this design, the dimensions D1₃, and Di₄, injection angle β, and a diameter of each aperture could be independently adjusted to precisely control the nitrogen-containing gas penetration or oxygen-containing gas penetration and local mixing.

As depicted in Figures 20-21 , the first and second portions are replaced with shapes that are more streamlined. Extending from a lance portion 401 are radially spaced fins 402. The side elevation of Figure 19 depicts a plurality of apertures 403 on surfaces of at least two fins that face in a direction perpendicular to that of the flow of the coal/conveying gas. However, this type of surface, an opposed surface on the other side of the fin or a surface of the fin facing downstream could have apertures 403 to introduce injection gas with precise control over the jet momentum and local penetration of the injection gas. The lance 402 portion terminates in an aerodynamic body 405 having an aerodynamic tip 406. Each of the fins 402 is aerodynamically streamlined in shape. The apertures 403 are configured as circular holes, slots, slits, and other shaped openings such as those depicted in Figures 3A-3D.

In all the bluff body designs of Figures 16-21 , the shape of any tip at the end of the lance has an aerodynamic design with or without one or more openings. The openings on the tip could be of any design previously described above.

Axially Injecting Injection Element Designs:

Another type of injection element is configured to inject nitrogen-containing gas or oxygen-containing gas axially into the flow of coal/conveying gas from a surface that faces downstream. This surface could have any number of apertures of any shape. Some exemplary shapes 701 A-F are best shown in Figures 24A-F. The number of apertures, size, shape and angle of injection could be adjusted in order to optimize mixing and solid fuel loading. Baffles arranged near the outlet end can facilitate a uniform mixing of nitrogen-containing gas and/or oxygen-containing gas (the use of baffles is an improvement over prior art designs as it accomplishes more efficient mixing by increasing the turbulence at the outlet end). Various baffles number, shape and size may be utilized. As the velocity control of the jet outgoing from the pipe is a crucial parameter governing burner aerodynamics, the cross-sectional area of those baffles will be chosen carefully.

Similar types of axially injecting injection elements have a modified cross- section. As gravity has an influence on motion of the particles, a vertical elliptical cross-section, for example, will cause fewer disturbances to the particle trajectories and at the same time could provide improved mixing. Modifications of the cross-section of the pipe allow decreasing or increasing the velocity of the axial nitrogen-containing gas or oxygen-containing gas jet. As best illustrated in Figure 22A, nitrogen or oxygen lance 503 terminates in a horizontally oriented elliptical end 502. Similarly, Figure 22B depicts a vertically oriented elliptical end 505.

As depicted in Figure 23, another axial injecting-type of injection element includes member 601 having radially spaced apertures 602A, 602B, 602C on a downstream surface. Each of apertures 602A, 602B, 602C is configured to inject flows of nitrogen-containing gas or oxygen-containing gas F₄, F₅, Fe at an angle with respect to an axis of the fuel duct.

Preferred processes and apparatus for practicing the present invention have been described. It will be understood and readily apparent to the skilled artisan that many changes and modifications may be made to the above- described embodiments without departing from the spirit and the scope of the invention. The foregoing is illustrative only and that other embodiments of the method and system may be employed without departing from the true scope of the invention whose aspects are described in the following claims.

Claims

CLAIMS:

1. A system for burning coal with reduced NOx emissions, comprising: a) a source of a mixture of coal and conveying gas; b) a source of oxygen-containing gas; c) a source of nitrogen-containing gas; d) a heating device adapted and configured to heat nitrogen from said nitrogen-containing gas source; e) a combustion chamber; _r f) a burner disposed at a wall of said combustion chamber; a burner operatively associated with a combustion chamber; g) a fuel duct in fluid communication with said source of a mixture of coal and conveying gas, said fuel duct extending towards said burner; and h) a nitrogen-containing gas injection element in fluid communication with said heating device and said fuel duct, said first injection element being adapted and configured to inject heated nitrogen-containing gas from said heating device into a stream of a mixture of coal and conveying gas and mix therewith inside said fuel duct.

2. The system of claim 1 , further comprising an oxygen-containing gas injection element in fluid communication with said source of oxygen-containing gas and said fuel duct, said oxygen-containing gas injection element fluidly communicating with said fuel duct downstream of where said nitrogen-containing gas injection element fluidly communicates with said fuel duct and upstream of or at said burner, said oxygen-containing gas injection element being adapted and configured to inject oxygen-containing gas from said oxygen-containing gas source into the stream of mixed heated nitrogen-containing gas, coal, and conveying gas.

3. The system of claim 1 , wherein said source of oxygen-containing gas and said source of nitrogen comprises an ASU.

4. The system of claim 1 , wherein said conveying gas comprises flue gas from said combustion chamber mixed with oxygen-containing gas from said oxygen source.

5. The system of claim 1 , wherein said heating device is adapted and configured to directly impart heat to nitrogen-containing gas from said nitrogen- containing gas source from a flame.

6. The system of claim 1 , wherein said heating device is a heat exchanger adapted and configured to exchange heat between nitrogen- containing gas from said nitrogen-containing gas source and heat from combustion of said coal and oxygen in said combustion chamber.

7. The system of claim 2, wherein said conveying gas is flue gas from said combustion chamber mixed with oxygen-containing gas.

8. The system of claim 2, wherein said heating device is a heat exchanger adapted and configured to exchange heat between nitrogen- containing gas from said nitrogen-containing gas source and heat from combustion of said coal and oxygen in said combustion chamber.

9. The system of claim 6, wherein said heating device is a heat exchanger adapted and configured to exchange heat between nitrogen- containing gas from said nitrogen-containing gas source and heat from combustion of said coal and oxygen in said combustion chamber.

10. The system of claim 1 , wherein said conveying gas is air.

11. The system of claim 2, wherein said oxygen-containing gas injection element fluidly communicates with said fuel duct at said burner.

12. The system of claim 1 , wherein said nitrogen-containing gas injection element fluidly communicates with said fuel duct at a peripheral portion of said fuel duct.

13. The system of claim 1 , wherein said nitrogen-containing gas injection element fluidly communicates with said fuel duct along a central axis of said fuel duct.

14. The system of claim 1 , wherein said oxygen element fluidly communicates with said fuel duct at a peripheral portion of said fuel duct.

15. The system of claim 1 , wherein said oxygen element fluidly communicates with said fuel duct along a central axis of said fuel duct.

16. A method of combusting coal with reduced NOx emissions, comprising the steps of: heating a stream of nitrogen-containing gas; injecting the heated stream of nitrogen-containing gas into a stream of coal and conveying gas to produce a stream of mixed nitrogen-containing gas, coal, and conveying gas; and combusting the coal from the stream of mixed nitrogen-containing gas, coal, and conveying gas with oxygen-containing gas at a burner in the combustion chamber.

17. The method of 16, further comprising the step of injecting oxygen- containing gas into the stream of mixed nitrogen-containing gas, coal, and conveying gas upstream or at the burner.

18. The method of claim 16, wherein the nitrogen-containing gas and oxygen are obtained from an ASU.

19. The method of claim 16, wherein said step of heating a nitrogen- containing gas stream comprises directly imparting heat from a flame to the nitrogen stream.

20. The method of claim 16, wherein said step of heating a nitrogen- containing gas stream comprises indirectly imparting heat from a flame to the nitrogen-containing gas stream via a heat exchanger.

21. The method of claim 16, wherein said step of heating a nitrogen- containing gas stream comprises indirectly imparting heat from said step of combusting to the nitrogen-containing gas stream via a heat exchanger.

22. The method of claim 16, wherein the stream of nitrogen-containing gas is heated to a temperature in the range of from about 1 ,000 ⁰F to about 1 ,800 ⁰F.

23. The method of claim 16, wherein injection of the heated stream of nitrogen-containing gas causes devolatilization of most of a volatile species content in the coal.

24. The method 16, wherein the conveying gas is air.

25. The method of claim 16, further comprising the steps of: collecting at least some of any flue gas produced from said step of combusting; injection oxygen-containing gas into the collected flue gas to mix therewith; and introducing the oxygen-containing gas oxygen and flue gas to the burner.

26. The method of claim 25 wherein oxygen-containing gas is injected into the collected flue gas in an amount such that an oxygen concentration in the mixed oxygen-containing gas and flue gas is from about 3% to about 20%.

27. The method of claim 17, wherein said step of heating a nitrogen- containing gas stream comprises indirectly imparting heat from said step of combusting to the nitrogen-containing gas stream via a heat exchanger.

28. The method of claim 17, wherein the stream of nitrogen-containing gas is heated to a temperature in the range of from about 1 ,000 ⁰F to about 1 ,800 ⁰F.

29. The method claim 17, wherein the conveying gas is air.

30. The method of claim 17, further comprising the steps of: collecting at least some of any flue gas produced from said step of combusting; injection oxygen-containing gas into the collected flue gas to mix therewith; and introducing the mixed oxygen and flue gas to the burner.

31. The method of claim 27, wherein the stream of nitrogen-containing gas is heated to a temperature in the range of from about 1 ,000 ⁰F to about 1 ,800 ⁰F.

32. The method of claim 31 , further comprising the steps of: collecting at least some of any flue gas produced from said step of combusting; injection oxygen-containing gas into the collected flue gas to mix therewith; and introducing the mixed oxygen-containing gas and flue gas to the burner.