Patent Application
20090007827
The present invention relates generally to a combustion system equipped with, e.g., a slagging Cyclone™ combustor, which is adapted to minimize nitrogen oxide (NOx) formation during staged combustion operation by selective use of oxygen, and a method of operating the combustion system of the invention with decreased NOx emissions.
Cyclone boilers are among the most efficient commercially-operated coal combustion systems, currently representing about 8% of the coal-fired boiler capacity in the United States. As the name implies, the Cyclone combustor operates typically at high speeds (˜200+mph) with cyclonic flow characteristics.
Coals for Cyclone firing must be selected carefully to ensure that the molten ash can flow and tap steadily out of the furnace. The use of Cyclone fuels with slag viscosity factors or T250 values of 2450° F. (1616° K) and 2300° F. (1533° K) for burning bituminous and sub-bituminous coals, respectively is recommended. T250 denotes the temperature at which the slag can flow at a viscosity of 250 poise. Occasionally, the slag may solidify in the Cyclone or furnace and require special operating practices or mechanical intervention to achieve acceptable slagging conditions. Refractory lining of the Cyclone is necessary to maintain high combustion temperatures and proper heat transfer performance.
Burning of fossil fuels in air generates NOx (NO+NO2) from the oxidation of fuel-nitrogen content and/or oxidation of atmospheric nitrogen in the combustion air. Air staging is a commercially practiced method for NOx reduction wherein the main combustion zone is operated fuel-rich (sub-stoichiometric) by diverting a part of the total combustion air and reintroducing it downstream through overfire air (OFA) ports. Typical cyclone combustion stoichiometries range from 0.9 to 1.0 and with the addition of OFA, the overall stoichiometry is raised to a range of 1.10 to 1.25. Air-staged combustion in Cyclone-fired units generates typically 40-70% less NOx relative to unstaged combustion.
Combustion stoichiometry or stoichiometric ratio (SR) is defined as the actual oxidizer-to-fuel mass ratio divided by the stoichiometric (theoretical) oxidizer-to-fuel mass ratio as expressed below:
For a known fuel, the stoichiometric oxidizer-to-fuel mass ratio can be calculated directly from the chemical composition of the oxidizer and the fuel. The actual oxidizer-to-fuel mass ratio is calculated from a desired operating condition. Based on this definition, the stoichiometric, fuel-lean, and fuel-rich operations correspond to SR=1.0, SR>1.0, and SR<1.0, respectively. Stoichiometric operation corresponds to a theoretical condition where there is just sufficient oxidant to completely oxidize the fuel. In practical combustors, fuel/oxidant mixing imperfections result in requiring excess oxidant levels to burnout the combustibles. The excess oxidant is either added directly into the combustion zone or injected through furnace openings downstream of it.
Flue gas recirculation (FGR) into a fuel-rich, sub-stoichiometric combustion zone can destroy the NOx content of the recycled stream and convert it to N2, thereby reducing the net NOx emissions at the stack. However, the FGR flow into the Cyclone furnace can also quench the combustion reactions and reduce the temperature below the recommended values for melting the coal ash.
Fuel reburning is another proven commercial technology in which a supplementary fuel (e.g., natural gas, fuel oil, or pulverized coal) and air are added at an elevation above the generally fuel-lean (stoichiometric ratio, SR≧1.0) main flame zone to create a locally oxygen-deficient, reburn zone (SR<1.0). In the reburn zone, the supplementary fuel generates hydrocarbon radicals, amines, and cyanic species that react with the incoming main combustion zone elevation products to convert NOx to N2. Additional air is introduced through the OFA ports above the reburn zone to burn out the combustible matter at overall stoichiometries of 1.10 to 1.25. Up to 70% NOx reduction with 30% fuel reburn has been demonstrated in coal-fired boilers.
Unfortunately, neither the air staging process nor the fuel reburning method alone can reduce the NOx emissions in coal-fired units sufficiently to environmentally compliant levels. Although post-combustion flue gas treatment (i.e., SCR and SNCR) processes could be installed to achieve the desired emissions target, the NOx removal cost would increase substantially.
Thus, there exists a need for a system and process which do not have the above-mentioned shortcomings and can achieve maximum in-furnace NOx reduction via a more cost-effective approach involving partial use of oxygen in the Cyclone combustor and/or fuel reburning.
The present invention discloses a system and method for minimizing nitrogen oxide (NOx) emissions resulting from the combustion of a carbonaceous fuel.
For the purposes of the present invention, the term air shall have its common meaning, a gas comprising about 21 percent oxygen and about 78 percent nitrogen. Accordingly, as would be appreciated by the skilled artisan, the terms oxygen and air are not synonymous in name, composition, or purpose with regard to the method and system of the present invention. In regard to gaseous streams, the term oxygen stream as used in the claims shall mean a gaseous stream comprising at least 85 percent oxygen and preferably at least 90 percent oxygen.
A preferred system of the present invention comprises a boiler having a combustion zone; a slagging Cyclone combustor arranged at a lower region of the combustion zone; an injector for supplying a carbonaceous fuel and an oxygen stream into the combustor, the oxygen stream providing about 2-15% of the total oxygen flowing into the boiler via all recycled flue gas, air, and oxygen streams, wherein the fuel and the oxidant are utilized by the combustor at a combustion stoichiometry of less than 1.0 to generate a combustion product; and overfire air ports for supplying overfire air into an upper region of the combustion zone to contact the overfire air with the combustion product produced by the combustor at about the upper region of the combustion zone and increase overall stoichiometry above 1.0, thereby substantially completing the combustion process and reducing oxidation of nitrogen-carrying species in the combustion product to nitrogen oxide.
A preferred method of the present invention comprises the steps of providing a boiler having a combustion zone; providing a combustor at a lower region of the combustion zone; introducing a carbonaceous fuel and an oxygen stream into the combustor, the oxygen stream providing about 2-15% of the total oxygen flowing into the boiler via all recycled flue gas, air, and oxygen streams; introducing overfire air into an upper region of the combustion zone; combusting the fuel and the oxidant at a combustion stoichiometry of less than 1.0 to generate a combustion product; and contacting overfire air with the combustion product about the upper region of the combustion zone to increase overall stoichiometry above 1.0, thereby substantially completing the combustion process and reducing oxidation of nitrogen-carrying species in the combustion product to nitrogen oxide.
As an option, oxygen may be supplied through the secondary air entrance of the Cyclone combustor, preferably with a multi-hole oxygen lance, and the overfire air may be supplied through a plurality of overfire air ports disposed on at least one elevation. Preferably, the overfire air is distributed equally among the plurality of overfire air ports, but in alternative embodiment the overfire air can also be distributed unequally amongst the plurality of overfire air ports.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which preferred embodiments of the invention are illustrated.
In the drawings:
a shows a frontal view of a oxygen enrichment of a Cyclone furnace for staged sub-stoichiometric operation;
b shows a frontal view of a oxygen enrichment of a Cyclone furnace for staged sub-stoichiometric operation
The present invention relates, among other aspects, to a method of minimizing NOx emissions in boiler units equipped with coal-burning Cyclone combustors by selective use of oxygen during staged combustion operation. In an embodiment, a portion of the oxidizer/air flow to the Cyclone combustor is replaced with oxygen to create a hot sub-stoichiometric combustion zone via reducing the diluent effect of nitrogen and other inert gases present in oxidizer/air. Preferably, oxygen enrichment of the Cyclone is equivalent to 2-15% of the total oxygen flow via the air, recycled flue gas and oxygen stream to the boiler. This allows the Cyclone to operate at lower stoichiometries while maintaining the required high combustion temperatures for proper slag tapping. High-temperature combustion at low stoichiometries accelerates the fuel pyrolysis, enhances the production of NOx reduction precursors, and improves the char burnout.
Referring to the drawings,
In a preferred method of injecting oxygen, the oxygen is injected via a multi hole secant lance injector. The lance is preferably placed in the secondary air conduit of the Cyclone combustor, the holes of the injector being positioned in a manner allowing co-current flow of the injected oxygen with any gaseous stream flowing within the secondary air conduit and into the Cyclone barrel. The lance can be of any design, preferably a cylindrical construction is used, wherein the length of the injector extends across the entire or a substantial portion of the secondary air conduits width. The lance generally is design to pass through one wall of the secondary air conduit at a median elevation, and fitted to the opposite wall to maintain injector elevation across the width of the secondary air conduit. Alternatively in embodiments utilizing rigid injector structures, opposite wall fittings may not be necessary.
An injector according to the present invention generally comprises a plurality of openings along the injector length. The openings may vary in shape and size such that a large shape may only require a single opening to permit optimal oxygen stream flow whereas a smaller shape may require multiple openings to permit optimal oxygen stream flow. Preferably the openings are of a circular shape, equally spaced along the injector length. Alternatively the opening(s) can be spaced non-equally or irregularly along the injector length, and may be of any non-circular shapes, such as but not limited to elliptical, rectangular, triangular shapes, and any combination thereof for example.
An injector according to the present invention is preferably located sufficiently within the secondary air conduit, allowing the oxygen stream injected though the lance to be adequately mixed with the gaseous mass flowing through the secondary air conduit prior to the combined stream being introduced in the Cyclone barrel. Adequate mixing provides the benefit of a uniform temperature distribution within the Cyclone barrel, enhancing the Cyclones ability to function and melt ash at a combustion stoichiometry below 1.0 and as low as about 0.5.
More NOx reduction can be achieved by extracting a small amount of flue gas 7 from the convection pass section and downstream of the furnace exit 9 of the boiler and recirculating it into the boiler through wall penetrations between the Cyclone combustion zone and the OFA ports. Alternatively, the flue gas recirculation (FGR) 8 can flow through a set of small burners 11 (for the optional firing of a mixture 12 of fuel and oxidizer) equipped with swirl blades to achieve desired flow and mixing patterns. In any case, the FGR flow is expected to be less than 25% of the total flue gas exiting the boiler. Typical Cyclone furnace stoichiometry will range from 0.5 to 1.0. In the event of firing coal through reburn burners, the reburn burner stoichiometric ratio is determined from the coal feed rate, transport air flow rate, and the flow rate and composition of the recycled flue gas. In one embodiment, the reburn burners may include an oxygen injector, such as a centerline oxygen lance 13. With or without reburning, the combined stoichiometry of all fuel and gaseous streams entering the boiler prior to the introduction of OFA should be about 0.5 to about 1.0 for maximum NOx reduction. With the addition of overfire air flow, the overall combustion stoichiometry is raised to 1.10 or higher to burn out the combustibles such as chars, hydrocarbons, and CO.
Possible applications include Cyclone furnaces and other slagging combustors arranged as single wall, opposed wall, one-level, or multi-level.
The present invention offers the following demonstrated advantages:
Extension of the lower combustion stoichiometry operability limit for Cyclone combustors down to 0.6 or lower;
Minimum oxygen requirement at deeply staged Cyclone furnace operating conditions;
NOx emissions below levels achieved with conventional staged combustion or reburning operation;
Higher turndown (ability to operate with good slag tapping at very low firing rates); and
Improved slag tapping, while minimizing mechanical intervention and changes in operation for removing solidified slag from the furnace.
The invention will be further illustrated by the following examples, but should not be considered as limited by those examples. Computer modeling and pilot-scale testing performed demonstrated the above-mentioned benefits.
Computer simulations of coal combustion in an air-staged Cyclone furnace were performed under part load operation (70% firing rate) with and without oxygen injection, and at the baseline full load (100% firing rate) air-blown operation without oxygen addition. Two methods for oxygen injection into the cyclone combustor were simulated by computer modeling. One method involved a single-hole centerline lance and the other was a multi-hole secant injector at the secondary air entrance to the cyclone barrel. Oxygen injection at the secondary air entrance to the Cyclone barrel demonstrated a greater potential for adequate slag tapping than another arrangement involving a single-hole O2 lance positioned along the Cyclone burner centerline.
Using the NASA Computer Program for Calculation of Complex Chemical Equilibrium Compositions and Applications (by McBride, B. J., and Gordon, S., NASA Reference Publication 1311, June 1996), the adiabatic flame temperature was computed over a 0.6 to 1.0 range of stoichiometric ratios for the premixed combustion of a high-volatile eastern bituminous coal with air as well as oxygen-enriched air. Pure oxygen was assumed to flow into a cyclone combustor at levels equivalent to 5% and 10% of the total oxygen entering the boiler (including the pure oxygen and various air streams) to burn the fuel and to generate a flue gas with 3.2% residual O2 on a dry basis at the boiler exit. Since the coal feed rate and the oxygen flow rate into the cyclone combustor were held constant at a fixed oxygen enrichment level, the cyclone stoichiometric ratio was changed by varying the air flow to the cyclone and the overfire air ports.
Proof-of-concept tests were performed at 5 million Btu/hr in a pilot-scale facility equipped with a Cyclone combustor. Oxygen lances were installed separately in the Cyclone and reburn burners for evaluation. Pure oxygen gas flow to the Cyclone combustor was varied from 0 to 10% of the total equivalent oxygen that entered the boiler via the air, recycled gas stream, and oxygen. In one series of tests, a high-volatile eastern bituminous Pittsburgh #8 seam coal was fired in both the Cyclone furnace and reburn burners. Best performance results of 112 ppmv NOx (0.146 lb/million Btu), 59 ppmv CO, and good slag tapping from the bottom of the primary furnace were achieved at 0.7 Cyclone stoichiometry with two levels of OFA ports and 1.17 overall stoichiometry, 10% coal reburning with air and 21% FGR, and 7% oxygen enrichment in the Cyclone furnace via a 5-hole oxygen lance as shown in
In a different series of tests, a sub-bituminous Powder River Basin Black Thunder coal was fired in the Cyclone furnace with two levels of OFA ports but without coal reburning or FGR. At 1.18 overall combustion stoichiometry, the average NOx concentration was 95 ppmv (0.126 lb/million Btu), and the average CO was 17 ppmv when the Cyclone furnace was staged close to 0.7 stoichiometry and the pure oxygen gas flow to the Cyclone furnace via the multi-hole lance was equivalent to 5% of the total oxidizer flowing into the furnace. Without the pure oxygen gas flow to the Cyclone furnace, the lowest stoichiometry for continuous slag tapping was 0.7. Under this condition and 1.17 overall stoichiometry, the NOx concentration was 108 ppmv (0.148 lb/million Btu), and the CO level was 24 ppmv. Un-staged NOx and CO emissions levels were 759 ppmv (1.04 lb/million Btu) and 27 ppmv, respectively. Oxygen enrichment at the 5% equivalent level extended the lower stoichiometry limit of the Cyclone furnace to 0.6 while maintaining good slag tapping. At 0.6 Cyclone furnace stoichiometry, 5% oxygen enrichment and 1.11 overall boiler stoichiometry, the NOx and CO emission levels were 96 ppmv (0.120 lb/million Btu) and 66 ppmv, respectively.
U.S. Pat. No. 6,910,432 B2 discusses embodiments where oxygen is introduced at various points either within or adjacent to the secondary air stream for selective oxygen enrichment in localized regions of the cyclone barrel. Unlike the prior art, in the present invention a uniquely designed multi discharge-hole oxygen lance was used to promote uniform dispersion and mixing of oxygen with the secondary air stream, and to elevate the flame temperature in the vicinity of the interior cyclone walls while achieving good slag tapping and low NOx emissions under sub-stoichiometric conditions. Other oxygen injection lances with non-uniform dispersion and mixing patterns that created locally oxygen-rich zones were also tested, but proved substantially less effective in minimizing NOx emissions.
This Invention was made with Government support under Contract No. DE-FC26-05NT42301 awarded by the Department of Energy. The Government may retain certain rights in this invention.
| Number | Date | Country | |
|---|---|---|---|
| 60942028 | Jun 2007 | US |
