US20070034126A1

US20070034126A1 - In-Furnace Reduction Of Nitrogen Oxide By Mixed Fuels Involving A Biomass Derivative

Info

Publication number: US20070034126A1
Application number: US11/426,906
Authority: US
Inventors: Wei-Yin Chen; Hamid Sarv
Original assignee: Individual
Current assignee: Babcock and Wilcox Co; University of Mississippi
Priority date: 2005-06-27
Filing date: 2006-06-27
Publication date: 2007-02-15

Abstract

A method of reducing nitrogen oxide emissions formed during fuel combustion by introducing biomass ash into a combustion chamber.

Description

PRIORITY INFORMATION

This application claims priority to U.S. Patent Application No. 60/694,181, filed Jun. 27, 2005, the contents of which are incorporated herein by reference in their entirety.

GOVERNMENT SUPPORT

This invention was made with support from United States Department of Energy Grant Nos. DE-FG26-02NT41552 and DE-FG-04-NT42183. The United States government has certain rights to this invention

FIELD OF THE INVENTION

The present invention relates generally to the field of reduction of nitrogen oxide emissions. More specifically one embodiment of the present invention relates to the reduction of nitrogen oxide emissions by introducing a reburn fuel with biomass-derived products, such as biomass char and biomass ashes.

SUMMARY AND BACKGROUND OF THE INVENTION

Reburning is a three-stage, in-furnace combustion technology designed for the reduction of nitrogen oxide (NO) by introducing a supplemental fuel above the primary combustion zone, where the majority of NO is chemically reduced to nitrogen in this fuel rich environment. Wendt et al., introduced the reburning concept and their experimental results in 1973. Reburning is attractive because it can retrofit old boilers; it operates at a relatively lower operation cost than the post combustion NO_xcontrol technologies such as selective catalytic reduction (SCR). Pilot-scale and full-scale tests of reburning in the past several decades, however, have demonstrated a 60% NO reduction floor. Thus, reburning technology on its own, has not been sufficient to meet the stringent regulations established by laws.
It is known that the use of a dual-function fuel for reburning can effectively reduce NO in the coal-fired utility furnaces. One of the components natural gas, reduces NO, and the other, a small amount of lignite ash reduces one of its major reaction intermediates hydrogen cyanide (HCN). HCN is a precursor to NO formation under oxidizing conditions.
Ashes from lignite-fired power plants are geographically limited to the Northern Great Plains (North and South Dakota and Montana), and Southern United States (Texas, Louisiana, and Mississippi). Transportation of lignite ashes to boilers in other parts of the United States poses a cost constraint.
One embodiment of the present invention substitutes biomass ash for lignite ash as the secondary component in reburning fuel. Biomass ashes have the desirable mineral components for HCN reduction, including high content of carbon and low content of nitrogen. Embodiments of the present invention include biomass ash having a carbon content of about 10% to more than about 50%.
Additionally, biomass is renewable, and the Renewable Energy Credits (RECs) and renewable energy portfolio standards make the technology under development more economically attractive. Furthermore, pulp mills that produce biomass ashes as waste are distributed in areas where the lignite is scarce.
Embodiments of the present invention revealed that a mixed fuel containing a biomass ash is likely to achieve about a 85% NO reduction. Additionally, the cost of embodiments of the present invention are expected to be low
In another embodiment of the present invention, bioash per se cain be used as the reburn fuel. In other embodiments of the present invention, biomass ash can be used as a reburn fuel secondary component.
The mixed-fuel concept for NO reduction can also be extended to other fuel-rich zones of the coal combustion process. Reduction of NO in most wall-fired, coal-burning boilers is achieved via the installation of low-NO_xburrs. Low-NO_xburners operate by controlled separation and mixing of fuel and oxidizer to create locally fuel-rich and fuel-lean zones.
Introduction of ignite ash or biomass derivatives into the fuel-rich zones can catalyze the reduction of NO and decomposition of HCN (a NO formation precursor). This can be accomplished with at least two embodiments of the present invention. In staged operations, where part of the combustion air is supplied through the overfire air (OFA) ports above the substoichiometric (fuel-rich) combustion zone for NO reduction, the lignite ash or biomass derivatives can be injected anywhere between the burner level and OFA ports. To maximize the NO removal efficiency, it's preferable to introduce the compounds closer to the burner level. In the absence of OFA ports (unstaged combustion), the lignite ash or biomass derivatives can be strategically introduced into locally fuel-rich zones of the low-NO_xburner flame via especially, designed nozzles for effective NO reduction.
Without being bound by theory or mechanism, it is believed that about 90% of NO can be effectively reduced in the reburning zone by natural gas, but the resulting nitrogen-containing intermediates (eg., HCN, NH₃, and char-nitrogen) oxidize farther downstream in the burnout zone, which ultimately attribute to the observed 60% net NO reduction floor. During natural gas reburning, hydrocarbon free radicals including C, CH— and CH₂, chemically reduce NO to HCN, a major reaction product in the reburning zone, A significant portion of HCN oxidizes to NO in the burnout zone that limits the overall NO reduction efficiency. During coal reburning, a significant portion of the char nitrogen oxidizes to form NO in the burnout zone. To break these reduction barriers, advanced reburning must involve means for simultaneously minimizing NO, and its reaction intermediates, i.e. HCN and char-nitrogen.
One of the present inventors has described variables, kinetics and mechanisms of heterogeneous reburning, i.e., reburning involving a coal-derived char (Chen and Ma, 1996; Chen and Tang, 2001). Young chars derived from low rank coals, lignite ant sub-bituminous coal, contain catalysts that effectively reduce not only NO, but also HCN. Moreover, minerals in the young chars effectively catalyze the gasification of carbon in the chars by CO₂and O₂for production of CO. Gaseous CO, a rich product of incomplete combustion in a reburning stage, effectively scavenges surface oxides and regenerates the carbon active sites on the char surface. The mechanistic information renders it possible to design a highly efficient reburning fuel.
Mixed fuels containing these multiple functions have been designed and tested in a simulated, bench-scale reburning apparatus a 1100° C. with a 0.2 s residence time. Reburning experiments were carried out in a ceramic flow reactor with a simulated flue gas comprising about 16.8% CO₂, about 1.95% O₂, and about 0.1% NO in a helium base. These concentrations of CO₂, O₂, and NO were chosen to be consistent with those of a coal primary flame operated at a stoichiometric ratio of 1.1. Methane is usually used as one of the reburning, fuel components for its capability of converting NO to HCN and for the speedy production of CO for scavenging surface oxides, if char is present in the system. A second component is chosen mainly for the effective reduction of HCN; they include lignite ashes, ashes of sub-bituminous coals ashes from utility boilers, ashes from Bunsen burners, and a biomass fly ash produced from a paper mill.
In general, HCN reduction always increases with higher amounts of these secondary, fuels. Ashes of sub-bituminous coals are much less reactive than those of lignite. Ashes collected from utilities are less effective than those from laboratory by a Bunsen burner probably due to the differences in their temperature history and level of sintering. Ash collected from a bag-house seems to be more effective in HCN reduction than ashes from electrostatic precipitators, moreover, the ash from a bag-house is capable of reducing another reaction intermediate, NH₂. High efficiencies of NO and HCN reductions have been observed. For instance when methane and ashes from lignite-fired power plants are used, about 90% of NO is converted to species other than NO and HCN.
Embodiments of the present invention use biomass fly ashes in reburning. Biomass is renewable, and its power plants are spread in areas where lignite is remote. Experiments indicate that biomass fly ash has a comparable HCN reduction effectiveness to that of lignite ash from power plants. Unlike coal chars, biomass' high carbon content (including biomass with a content of about 50%) and low nitrogen content (including less than 0.1%) maker it possible to effectively reduce NO in reburning stage without being concerned about the production of additional NO from char in the burnout stage. Assuming NO is stable and 60% of HCN will convert to NO in the burnout zone, the projected NO production from a full three-staged reburning process is found to be a function of stoichiometric ratio SR2 and the ratio of ash to methane. The projected NO has an optimal conversion of 85% at SR2=0.945, a moderately rich condition. Results also suggest the formation of HCN and NH₃due to the reactions among NO, CH₄and biomass ash at such high SR2.
In a time when the price of natural gas experiences large fluctuations, embodiments of the present invention provide cost savings by the use of a combination of biomass fly ash and natural gas for reburning for even greater NO_xreduction efficiency than gas reburning alone. Mixed-fuel reburning involving biomass ash also appears to be more coast effective than SCR white achieving the same level to NO_xreduction efficiency. The minerals in biomass have high volatility and they form deposits in the downstream segment of our stimulated reburning reactor where the temperature starts to drop. Nevertheless, due to its relatively small quantity in the practical boilers, calculations suggest the ash-fouling tendency, index remains to be low to medium. Moreover, calculations suggested that T_250poise, the temperature that causes the viscosity of softened ash to reach the 250 poises threshold, decreases from 1232 to 1149° C. after the heterogeneous reburning with biomass ash is introduced into a bituminous-coal fired boiler.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a shows a representative re-burning process. Embodiments of the present invention would replace the pulverized coal with a natural gas/ash mixture.
FIG. 1 b shows the general arrangement of a staged combustion process with low-NO_xburners an overfire air ports. It is understood that the present invention may even be employed in non-staged combustion approaches as described herein.
FIGS. 2 and 3 are graphs showing the effects of lignite and sub-bituminous coal ashes on total fixed nitrogen species.
FIG. 4 is a graph showing the effects of the feeding rate of ground biomass fly ash on the yields of HCN and NH₃.
FIG. 5 is a graph showing effects feed rate of biomass fly ash (not ground) on the yields of HCN, NH₃and NO.
FIG. 6 is a graph showing effects of the feeding rate of ground biomass fly ash on the yields to HCN and NH₃.

DESCRIPTION OF THE INVENTION

Needs for Breaking the 60% NO Reduction Floor
Nitrogen oxide, NO, is one of the six criteria air pollutants regulated by the National Ambient Air Quality Standard (NAAQS) in 1991 due to its detrimental effects on public health and environment, Specifically, it reacts with oxygen and water to forms nitric acid that can destroy trees, kill aquatic life in streams and lakes, and damage buildings and statues. It also reacts with hydrocarbons and forms ozone and smog through photochemical reactions that reduce visibility and pose risks to public health.
Stationary combustion processes contribute to about 50% of the global nitrogen oxide production. The stringent Clean Air Act and its Amendments (CAAA) have effectively reduced their emissions in the last three decades. One of the (CAAA philosophies is to establish emission standards based on the “best technologies,” or the “new source performance standards, ” or NSPS in short, that are updated periodically. The CAAA of 1990 set NO_xemissions at 0.5 or 0.6 lb/MBtu (in its oxidized form, NH₂), depending on the rank of coal, which has been achieved largely by installing low-NO_xburners (LNB).
As the second philosophy, the CAAA of 1990 took into accounts the air quality standards in establishing its regulations. For ozone non-attainment areas, the emission standards are especially stringent. Regulatory actions resulting from the revised NAAQS for ozone and particulate matters smaller than 2.5 microns (PM_2.5) require source emission reductions of NO_xto or 0.15 lb/MBtu for much of the nation. Selective catalytic reduction (SCR) is capable of achieving low NO_xemissions, but there are additional costs associated with this technology. Consequently, less expensive, advanced NO_xreduction technologies would be welcomed by the industry.
Reburning, see FIG. 1 a, is an emerging three-state, in-furnace combustion technology designed for the reduction of NO by introducing a small amount of reburning fuel above the primary flame where the majority of NO is chemically reduced to nitrogen in this fuel rich environment. Wendt et al., introduced the reburning concept and their experimental results in 1973. Tests on a full-scale, three-staged boiler at Mitsubishi Heavy Industries (Takahashi et al., 1983) resulted in over 50% reduction of the NO produced in the primary flame. Reburning is attractive because old boilers can be retrofitted at a relatively low operation cost than the post combustion technologies, such as SCR.
Pilot and full-scale research in tire last three decades, however, has demonstrated a floor, about 60% of NO produced in the primary flames, in either gas or coal reburning, below that floor NO cannot be reduced further. The equilibrium NO concentrations are about two orders of magnitudes higher than those observed experimentally suggesting the reburning reaction is kinetically controlled in addition, evidence suggests that the nitrogen-containing reaction intermediates, including HCN and char, are likely the major causes of the observed NO reduction floor exiting from a three-staged process. During natural gas reburning, hydrocarbon free radicals, including C—, CH— and CH₂—, effectively convert NO to HCN, a minor reaction product in the reburning zone (Miller and Bowman, 1989; Burch et al., 1991). However, a significant portion of this HCN oxidizes to form NO in the burnout zone and this limits the overall NO reduction efficiency. During coal reburning a significant portion of the char nitrogen oxidizes to form NO in the burnout zone (see, e.g., , Molina et al., 2000), which also places a cap on the overall NO reduction efficiency.
Embodiments of the present invention break these limitations in either gas or coal reburning by simultaneously minimizing the productions of NO, and its reaction intermediates, HCN and char-nitrogen in the reburning zone.
With respect to FIG. 1 a, an embodiment of the invention is the use of bioash in the reburn process. A commercial boiler (10) that has three burn zones, the main combustion zone (15) the reburn zone (20) and the burnout zone (25) is shown. With the present invention bioash is added as reburn fuel (35) or as a reburn fuel additive.
Heterogeneous Reaction Mechanisms
Early research suggests that lignite from Mississippi and North Dakota have reburning efficiencies comparable to that of methane (Burch et al., 1991; 1994). Moreover, it has been demonstrated that, in contrast to the chars derived from the bituminous coals, heterogeneous reactions on the lignite char surface contribute to high NO reductions that are comparable to gas phase NO reactions (Chen and Ma, 1996). The effectiveness of lignite during reburning has also been demonstrated in a 1.0 and a 0.1 MBtu/hr pilot scale combustion facilities (Payne et al., 1995; Pershing, 1995). Recently, Chen and Tang (2001) extensively studied the variables, kinetics and mechanisms of reburning with chars. They found that the efficiency of heterogeneous reburning depends on the origin of the char, char preparation history, and the presence of oxidants, CO₂and O₂, and the reducing agent, CO. In addition to its large internal surface area, evidence suggest that the effectiveness of lignite char is attributable to its ability to promote two consecutive reactions: 1) the catalytic gasification of char by CO₂and O₂for production of CO, and, 2) the scavenge of surface oxygen complexes, C(O), including those formed after adsorption of NO, by gaseous CO, for the regeneration of reactive sites. More interestingly, lignite ash also catalyzes the decomposition of HCN, a major intermediate of NO conversion and a major contributor of the 60% reduction floor during gas reburn. These reaction mechanisms can be viewed from three major aspects: char gasification, catalytic roles of minerals, and role of gaseous CO, which are discussed in more detail below.
Char Gasification by NO, O₂and CO₂
It is generally believed that (e.g., Mims, 1991; Illan-Gomez et al., 1996) char gasification by NO, similar to the catalytic gasification of carbon by other oxidizing agents, involves the following kinetically-controlled elementary steps: a) chemisorption of NO on the catalyst b) transfer of oxygen from the catalytically active sites to the carbon reactive sites, and c) desorption of oxygen from the carbon surface. It is known that alkali and alkaline earth metals, including K, Na, Ca and Mg (Allen, 1991; Hansen et al., 1992; Hansen and Dam-Johansen, 1993; Shimizu et al., 1993; Lin et al., 1993; Illan-Gomez et al., 1995a-e; Illan-Gomez et al., 1996; Guo and Hecker, 1996; Garcia-Garcia et al., 1997; Jensen et al., 1997; Bueno-Lopez et al., 2002), catalyze reductions of NO in fuel-rich, oxidizing environments. Chars with high content of catalytic constituents are expected to promote reactions steps a) and b) above, rendering the desorption step, c), a rate controlling step, or the slowest step in the overall char oxidation mechanism.
Catalytic Roles of Minerals in Chars
In addition to the catalytic reduction of NO, the minerals in lignite also participate in two types of reactions that indirectly, enhance NO reduction in the reburning process (i) catalytic conversion of HCN, and, (ii) production of gaseous CO through catalytic gasification of char.
HCN and NH₃are the two major stable reaction intermediates before NO is chemically reduced to N₂in fuel-rich environments (Miller and Bowman 1989, Burch et al., 1991, 1994). Burch et al., (1991) and (Chen and Tang (2001) demonstrated that lignite char catalytically converts HCN by lignite ash to amine radicals, NH_i, and other species. Over 60% of NO at temperatures greater than 1100° C. is converted to HCN in the reburning stage when natural gas is used (Burch et al., 1991) About 60% of HCN recycles back to NO with a smaller fraction of the HCN is converted to N₂in the burnout zone. Recycle of amines to NO in the burnout zone, however, is much lower. By introducing a small amount of lignite ash produced by a Bunsen burner, the yield of total fixed nitrogen (or TFN in short, i.e., the total yields of NO, HCN and NH₃) reduces from 900 to 200 ppm in methane returning at stoichiometry 0.9. Increasing the amount of lignite ash is expected to further reduce HCN in the reburning zone, and therefore the final NO yield from the burnout zone. Since HCN recycling to NO is the major contributor of the NO reduction floor, the addition of catalytic minerals represents a corner stone in our design of a multi-functional returning fuel in the present project.
Radovic et at. (1983a) Hengel and Walker (1984), and Lizzio et al., (1990) found that mineral matters in lignite catalyze the char oxidation and thus the yields of their oxidation products, CO and CO₂, are much higher than those from the gasification of high rank coals. Chen and Tang (2001) reported a higher CO concentration, 2 to 3%, in simulated lignite-reburning zone than that in a bituminous-coal reburning zone. As it is discussed below, gaseous CO increases the oxygen turnover rate on the char surface and therefore, enhances NO reduction.
Role of CO
Chen and Tang (2001) found that the remarkable effectiveness of lignite char in reburning appears to be attributable to its ability to promote two consecutive reactions: 1) the (gasification of char by CO₂and O₂for production of CO, and, 2) the removal of surface oxygen complexes, including those formed after adsorption of NO, by gaseous CO, for the regeneration of reactive sites, i.e., C(O)+CO_(g)
CO₂+C₁. The observed high CO productions and CO scavenging surface oxides, C(O), in reburning are consistent with the published work on char gasification. The role of CO in scavenging surface oxides have been discussed by Reif (1952), Blackwood and Ingeme (1960), Laurendeau (1978), Levy et al., (1981). Chan et al., (1983), De Soete (1990). Calo and Hall (1991), Chen et al., (1993), and Molina et al., (2000),
Design of Mixed Fuels for Effective Reburning
The mechanistic discoveries discussed in the last section suggested a new route for effective reduction of NO in reburning. This is accomplished by using a mixed fuel containing the following two functions: (i) a gaseous hydrocarbon component that produces free radicals in the reburning flame for effective reduction of NO, and (ii) a catalyst for effective conversion of HCN to NH₃.
Fly ashes have been considered industrial wastes that have little commercial value. Therefore., our focus of the second component has been placed on the ashes produced from utilities and other power plants; the cost of such an approach involves transportation expenses only. Moreover carbon in the ashes is likely to enhance the NO conversion
The present inventors have demonstrated that over 90% of NO from the primary flame can be reduced at a stoichiometric ratio (SR) 0.9 at 11100° C. by methane reburning (Burch et al., 1991). Moreover, the NO reduction efficiency is higher at higher temperatures implying the same NO reduction efficiency may be achievable with less reburning fuel at normal flame temperatures, 1100° C. to 1700° C. The inventors have also demonstrated that the additional NO reduction due to higher temperature is accompanied with higher HCN production; thus catalytic HCN conversion in reburning stage is desirable.
Based on the HCN reduction efficiency reported by Chen and Tang (2001), 130 tons of lignite fly ash is needed everyday for a 172 MWe bituminous-fired power plant. Lignite productions in the US, however; are limited to the Northern Great Plains (North and South Dakota and Montana), and Southern United States(Texas, Louisiana and Mississippi). Although it is not a major cost factor, transporting lignite ash from the lignite-fired power plants to the utilities and industries in eastern states still presents a cost concern.
To reduce the transportation and operating costs, and further enhance the reburning, efficiency, attention of the choice of the secondary fuel during this study was extended to sources other than the coal-derived fuels.
In an embodiment of the present invention, biomass ash is an ideal candidate for several reasons:
It is rich in the minerals that catalyze HCN reduction,
It is rich in the minerals that catalyze NO reduction,
It usually contains 50% or more of unburnt carbon that is potentially effective in reducing NO,
It usually has very low content of nitrogen and therefore conversion of fuel nitrogen to NO in the burnout stage is not a major concern,
The geographical distribution of biomass power plants (Smith et al., 2003), complements that of lignite-fired power plants (Bonskowski, 1999), and,
Biomass is renewable, and the Renewable Energy Credits (RECs) and renewable energy portfolio standards make the technology under development more economically attractive.
With respect to the present invention, biomass comprises plant matter and other biologically derived solid wastes. Examples of common biomass include crop residues (such as, for example, wheat straw, corn stover, nut shells, orchard prunings, vineyard stakes, sugar cane, etc.), municipal solid waste (such as, for example, trash, rubbish, refuse, etc.), forest residues (such as, for example, pine bark, slash, forest thinning, etc.), and urban wood waste (such as, for example, construction resides, wood chips, saw dust grass clippings, and backyard prunings). Their effectiveness in reburning will vary, but depend on the contents of alkali (such as potassium and sodium), alkaline earth materials (such as calcium and magnesium) and transition metals (such as iron).
The renewable biomass is abundant and readily available in the areas where the lignite is scarce. Pulp mills that utilize biomass for power generation are also spread in areas where the lignite is remote. Ashes from a biomass power plant are usually considered a waste and require cost in their disposal. Therefore, the biomass fly ash was selected for reburning based on both technical and economical reasons. Minerals in plants usually have low concentrations of the relatively inert elements silicon and aluminum. Unlike the coals, biomasses usually have low contents of silica oxides and aluminum oxides) which are inert in reburning. Minerals in many plants are rich in alkai (such as potassium and sodium), alkaline earth metals (such as calcium and magnesium) and transition metals (such as iron), which are potential catalysts for many oxidation and hydrogenation reactions (Mims, 1990; Allen, 1991; Hansen et al., 1992; Hansen and Dam-Johansen, 1993; Shimizu et al., 1993; Lin et al., 1993; Illan-Gomez et al., 1995a-e; Illan-Gomez et al., 1996; Guo and Hecker, 1996 Garcia-Garcia et al., 1997; Jensen et al., 1997 Bueno-Lopez et al., 2002). For instance the ash derived from burning a pine bark has 29.05% Ca, 16.24% K and 7.03% Mg and 0.58% Fe (e.g., Misra et al., 1993). Grate boilers used for burning wood often produce a fly ash with 50% or more of unburnt carbon, which makes it particularly attractive for reburning under the pulverized coal combustion conditions. The nitrogen content in wood ash is normally insignificant due to conversion to N₂during combustion.
In addition to ashes derived from biomass, those derived from sub-bituminous coals have also been considered candidates as the secondary component. It is known that sub-bituminous coals often possess properties closer to lignite than bituminous coals. Although the contents of catalytic components in sub-bituminous coals are lower than those in lignite, the catalytic activities of the products of sub-bituminous coals reburning so that technology can be geographically available to a larger number of power plants in the United States.
Experimental/Examples
The following describes examples or embodiments of the present invention. As such, the following should in no way be construed as limiting thereof.
Apparatus for Simulated Reburning
Reburning experiments were carried out in a ceramic flow reactor (Burch et al., 1991; Burch et al., 1994; Chen and Ma, 1996; Chen and Tang, 2001) with a simulated flue gas comprising about 16.8% CO₂, about 1.95% O₂, and about 0.1% NO in a helium base. However, in other embodiments of the present invention the concentration of the ingredients of the flue gas can vary widely. The above concentrations of CO₂, O₂, and NO were chosen to be consistent with those of a coal primary flame operated at a stoichiometric ratio of 1.1. Helium, instead of nitrogen, was used as the base gas to minimize the heat-up time after the gas enters the reactor at room temperature. The flow reactor was an alumina tube (Bolt Technical Ceramics) with an inside diameter of 1.91 cm and an overall length of 64 cm. The central portion of the reactor tube was enclosed in a 30-cm long, electrically heated furnace (Lindberg Model 55035), which provided tube temperatures up to about 1150° C. A particle feeder, which utilized controlled aerodynamic stripping, of the powder surface by passing, a carrier gas through a narrow, concentric space between a cylindrical piston and the inner wall of particle reservoir to affect the feed rate, was designed, fabricated, utilized (Burch et al., 1991) and modified (Tang and Chen, 2001) for delivering coal, char and catalyst particles at low and steady rates. For ash powders with strong cohesive forces, particles tend to agglomerate and smooth feeding becomes difficult. Nevertheless, this problem can be avoided by diluting the ash samples with chromatographic silica gel particles. Silica gel particles have been experimentally demonstrated to be chemically inert to the reactions discussed herein.
Most of the experiments were conducted at about 1100° C. Ashes were usually fed at about 0.01 to about 0.05 g/min; the lowest value corresponds to feeding North Dakota lignite in reburning at a stoichiometric ratio (SR2) of about 0.9. The gas residence time in isothermal zone of the reactor is about 0.2 s. The SR2 was selected because the NO reduction is optimal, 90% (Burch et al., 1991).
Fixed gas species of interest were monitored by an on-line instrument package. The analyses included NO_x(Chemiluminescence based, Thermo Environmental Instruments Model 42C), CO and CO₂(infrared based, California Analytical Instruments Model ZRH), and N₂O (infrared based, Horiba Model VIA-510).
Diverting the reactor effluent through a straight tube impinger filled with 0.5 L of 0.1N HNO₃aqueous solution or a specified time interval collected the reaction intermediates, HCN and NH₃. The captured solutions were pH adjusted with NaOH and analyzed for CN⁻ and dissolved ammonia with specific ion electrodes (Orion Research). Poisoning of the cyanide electrode by sulfur ions was prevented by adding an aqueous solution of Pb(NO₃)₂prior to adding the NaOH. Sulfide ions were precipitated as PbS. Due to the acidic nature of the impinger solution recovery of HCN and NH₃by this method was tested using known standards and found to be near quantitative for NH₃but only 70% for HCN in the range of 100 to 700 ppm of HCN. Thus NH₃values have been presented as measured while HCN values reported have been corrected for collection efficiency.
BET surface area analysis of the samples used was done by a Quantachrome Corporation, NOVA 2200 Multi-Station High Speed Gas Sorption Analyzer Version 7.11. The surface areas of selected ashes were analyzed by BET method with nitrogen at 77.4 Kelvin.
Samples
The reburning in this embodiment was conducted with a mixture of methane and selected ashes. A suite of ash samples of the present invention (5-8) and comparative examples (1-4) were either produced in our laboratory by a Bunsen burner or collected from power plants in the United States. They include the following:
(1) North Dakota lignite lab-produced ash—the raw lignite was collected at the Beulah mine, and a Bunsen burner produced its ash in a ceramic crucible at about 650° C.
(2) Stanton Station Unit #1—this fly ash was collected from an electrostatic precipitator of a lignite-fired boiler.
(3) Stanton Station Baghouse—this fly, ash sample was collected from a dust bag of a lignite-fired boiler.
(4) Coal Creek Station—this fly ash was collected from a lignite-fired boiler; the means of collection was not specified.
(5) Columbig U#2—this fly ash was collected from a sub-bituminous coal-fired boiler, the means of collection was not specified.
(6) Chinese subbituminous lab produced ash—this is a mixture of a high-ranks coal from Zhunger, China, and a low-rank coal from Shenhua, China. It is blended to increase the overall fusion point and avoid slagging by a power plaint in China. Its ash was produced in a ceramic crucible by a Bunsen burner at about 650° C.
(7) Bowater Newsprint's Paper Mill biomass fly ash—this ash was collected from a grate boiler that burns 85 to 90% of pine bark and a small fraction of sludge from the thermal-mechanical pulping process of the paper mill at about 815° C. The dried ash was ground and sieved; particles that were less than 150 microns were used for experimentation.
(8) Bowater Newsprint's Paper Mill biomass fly ash—this is the same sample as the one above but it was sieved without grinding and the fraction that was between 150 and 106 microns was used for reburning experiments. The composition of this sample is less representative to the raw sample from the pulp mill than sample #7, mentioned above. This small-particle fraction has lower carbon and hydrogen contents than the raw ash because it was burnt closer to completion. Utilizing this small particles fraction of raw ash may avoid an expensive unit of grinding.
The analytical results, and other information related to the samples used are shown in Tables 1-5. It is interesting to note that the biomass ash has a low content of nitrogen (0.02 to 0.07 wt %), and high contents of Ca (35 wt %) and carbon (13.6 to 29 wt %), these characteristics are representative of biomass ash and are beneficial to NO reduction in the reburning environment.

Biomass ash has a lower fusion temperature than those of the coal-derived ashes, and it forms a hard deposit of the inner wall of the reactor. This deposit cannot be removed by brushing with organic solvents. Thus, an acid (3 M HNO₃) digestion procedure was developed to remove the deposits and the inert nature of the reactor wall is regenerated.

TABLE 1


Analysis of the Samples (Weight %, Dry Basis)

Sample	Na₂O	MgO	Al₂O3	SiO₂	P₂O₅	SO₃	K₂O	CaO	TiO	MnO₂	Fe₂O₃	LOI

Coal Creek Station Lignite Fly Ash	3.07	4.17	17.04	50.1	0.23	1.43	2.32	16.72	0.86	0.05	7.26	0.08
Stanton Station Baghouse Lignite Fly Ash	3.2	5.83	16.43	35.51	0.18	5.99	1.25	21.17	0.86	0.07	9.53	2.28
Stanton Station Unit#1 Lignite Fly Ash	1.73	3.54	14.21	22.46	<0.02	20.64	0.95	31.69	0.83	0.05	4.66	0.46
North Dakota Lignite Coal	4.6	7.03	9.87	20.3	0.44	21.08	0.39	22.46	—	0.12	9.09	—
										(Mn₂O₄)

	Na₂O	MgO	Al₂O₃	SiO₂	K₂O	CaO	Fe₂O₃	TiO₂

Chinese Blended Coal - Ash Analysis	0.68	1.47	36.14	35.65	0.43	10.16	10.75	1.90

	Fixed Carbon	Volatile Matter	Ash	Moisture

Chinese Blended Coal - Proximate Analysis	45.55	23.53	12.71	18.21

	Carbon	Hydrogen	Oxygen	Nitrogen	Sulfur	NCV (kJ/kg)

Chinese Blended Coal - Elemental Analysis	54.87	3.43	9.62	0.8	0.36	20993

TABLE 2


Proximate, Ultimate and Mineral Ash Analysis of the Biomass Fly Ash Samples (Weight %)

As-Received Biomass Flyash Sample from the Paper Mill

	Fixed Carbon	Volatile Matter	Ash	Moisture

Bowater Biomass Flyash - Proximate Analysis	20.66	10.92	68.42	19.73

	Carbon	Hydrogen	Direct Oxygen	Oxygen by difference	Nitrogen	Sulfur

Bowater Biomass Flyash - Elemental Analysis (dry basis)	29.15	0.95	9.38	1.25	0.07	0.16

Fine-Particle Fraction (between 150 and 106 micron) of the As-Received Biomass Fly Ash Sample

	Fixed Carbon	Volatile Matter	Ash	Moisture

Bowater Biomass Flyash - Proximate Analysis	7.66	9.53	82.8	15.57

	Carbon	Hydrogen	Direct Oxygen	Oxygen by difference	Nitrogen	Sulfur

Bowater Biomass Flyash - Elemental Analysis (dry basis)	13.59	<0.05	5.27	3.37	0.02	0.17

Fine-Particle Fraction (less than 150 micron) of the As-Received Biomass Fly Ash Sample after Grinding

	Fixed Carbon	Volatile Matter	Ash	Moisture

Bowater Biomass Flyash - Proximate Analysis	18.91	11.11	69.98	13.22

	Carbon	Hydrogen	Direct Oxygen	Oxygen by difference	Nitrogen	Sulfur

Bowater Biomass Flyash - Elemental Analysis (dry basis)	24.74	0.71	9.45	4.35	0.07	0.15

Mineral Ash Analysis from the Paper Mill

			Calcium	Organic
	Sodium	Magnesium	Carbonate	Nitrogen	Phosphorus	Sulfur	Potassium	Calcium	Volatiles (LOI)

Bowater Biomass Flyash (dry basis)	0.40	0.82	34.85	0.11	0.62	0.41	1.74	15.27	45.77

TABLE 3


BET Surface Areas of the Samples

	BET Surface Area
Sample	(m²/gm)

Stanton Station Unit 1 Lignite Fly Ash	0.8038
Stanton Station Baghouse Lignite Fly Ash	4.776
Columbig U#2 Sub-bituminous Fly Ash	1.201
North Dakota Lignite Bunsen Burner Ash	3.36
Bowater Biomass Fly Ash (sieved between 106 and	79.49
150 microns)
Bowater Biomass Fly Ash (ground and sieved to	159.4
less than 150 microns)

TABLE 4


Sources and Transportation Cost of Biomass Fly Ash

	Size	Location of		Location of	Distance	Ash Requirement	Cost of Biomass Ash
Name of Power Plant	(MWe)	Power Plant	Name of Pulp Mill	Pulp Mill	(miles)	(tonnes/day)	(dollars/ton NOx)

Hennepin Unit 1	80	Hennepin, IL	Madison Paper Company	Alsip, IL	103	41	78
Lakeside Unit 7	40	Springfield, IL	Madison Paper Company	Alsip, IL	189	19	103
Kodak Park	62	Rochester, NY	Norboard Industries Inc.	Deposit, NY	187	32	78
Niles Station	108	Niles, OH	Mead Corporation	Chillicothe, OH	224	55	115

TABLE 5


Cost Benefits of Mixed Fuel Reburn

								Gas Reburn
		Gas Reburn	Baseline		%	Mixed Fuel	%	Cost	Mixed Fuel
Power	Size	Heat Input	NOx	Gas Reburn	Reduc-	Reburn	Reduc-	(dollers/	Reburn Cost	Gas	Overall
Plant	(MWe)	(%)	(lb/MMBtu)	(lb/MMBtu)	tion	(lb/MMBtu)	tion	ton NOx)	(dollers/ton NOx)	Savings	Savings

Hennepin	80	18	0.75	0.25	67%	0.113	85%	3600	2409	17%	15%
Unit
1
Lakeside	40	23	0.97	0.39	60%	0.146	85%	3966	1796	39%	36%
Unit 7
Kodak	62	18	1.35	0.6	56%	0.203	85%	2400	1374	17%	12%
Park
Niles	108	8 to 18	1.1	0.55	50%	0.165	85%	3273	1705	17%	11%
Station

Effects of Coat- and Biomass-Derived Fly Ashes

FIG. 2 illustrates the effects of coal-derived ashes during reburning with methane at stoichiometric ratio (SR2)=0.9. This SR2 is selected because maximum NO reduction was observed at SR2=0.9 in our earlier study (Burch et al., 1991). In most cases, addition of an ash at 0.01 g/min feeding rate enhances the reduction of HCN, signifying its likely effect on the final yield of NO from a burnout state. This ash feeding rate corresponds to ash loading found in lignite returning at SR2=0.9. Among these ashes, the ash derived from North Dakota's lignite and prepared in our laboratory in a Bunsen burner has the highest catalytic reactivity. Lignite ashes from utility boilers show lower reactivity probably due to ash sintering at high temperatures in utility boilers. The laboratory ash is typically prepared at 650° C. while the utility ash has usually experienced a much higher temperature history, 1100 to 1700° C. Results in FIG. 2 also suggest that ash derived from a sub-bituminous coal has a lower catalytic activity than those of lignite. In fact, at this particular ash feed rate, the effects of ash from the sub-bituminous coal are within the statistical errors of the experimental data from methane reburning.
Increasing the ash loading enhances the HCN conversion, as illustrated in FIG. 3. Remarkably high HCN reductions are achieved when the feeding rate is increased to 0.05 g/min. For instance, about 90% of NO is converted to species other than NO and HCN when methane and lignite ashes from lignite-fired power plants are used. The bag-house ash seems to be particularly effective; it catalytically reduces both HCN and NH₂. This bag-house ash has high content of sodium, magnesium, and iron, (Table 1) and a higher surface area than the other power plant ashes (Table 3). Charges left on the ash particles collected in electrostatic precipitators may have negative influences on the catalytic activities. Nevertheless, further investigations will be needed to elucidate the exact chemistry.
A ground biomass fly ash sample of the present invention, Sample #7, discussed above, was tested for its reburning at SR2=0.9. The carbon, hydrogen and oxygen (by difference) contents were taken into account in the calculation of SR2. Thus, higher feeding rates of ash correspond to lower feeding rates for methane. As illustrated in FIG. 4, the biomass fly ash does reduce the HCN. Its HCN reduction efficiency at 0.01 g/min feed rate and at the same overall stoichiometry is equivalent to lignite fly ashes from power plants. It appears that the decreases in HCN yields have caused increases in the NH₃yields, which was not observed in reburning with lignite ash. Thus, it implies that lignite ash contains richer catalysts than biomass ash in converting NH₃to N₂. Unlike coal chars, the high carbon and low nitrogen content (less than 0.1 wt %, dry basis) of biomass fly ash render it possible to effectively reduce NO in reburning stage without being concerned about the production of additional NO from char in the burnout stage.
For the data presented in FIGS. 2 through 4, the mixed-fuel experiments were conducted at SR2=0.9. This SR2 was chosen because the NO production from reburning with methane is at its minimal value (Burch et al., 1991), and addition of lignite ash (that contains little carbon) does not change the value of SR2. The strategy of choosing SR2 and ash, feeding rate for reburning involving biomass ash, however, is a more complex issue. The carbon oxygen and hydrogen contents of biomass affect the estimation of SR2; therefore, the number of variables in reburning increases dramatically. If the carbon in the ash is more effective than methane, it is likely that there is an optimal ratio of methane to ash at a specific SR2. Second, it is speculated that the TFN yields discussed above depend on SR2, and an optimal SR2 may exist. Thirdly since the particles of different sizes have different elemental compositions (see Table 2) their reactivity in reburning is likely to be different. To better understand the effects of these variables two sets of experiments were conducted for the identification of the optimal fuel ratio (methane to ash) and SR2. We assume that the carbon in biomass ash is more effective than methane in reducing NO and the search for the possible optimal point is conducted in the region from fuel rich to slight lean, i.e., 0.9<SR2<1.02.
In the first set of experiments, runs were conducted at different SR2 and methane-to-ash ratios. The small particles of the as received ash sample (with low carbon content) were used in these experiments. FIG. 5 shows the effects of both SR2 and fuel stoichiometric ratio of carbon-in-methane to carbon-in-ash, or “fuel ratio” in short, on the distribution of nitrogen species during reburning at SR between 0.9 and 1.02. Biomass ash has an elemental composition CH_0.342O_0.132. Our attempt of these experiments is to determine, with minimal test effects, if there is an optimal point in the three-dimensional figure representing concentration as a function of SR2 and fuel ratio. In other words, the two-dimensional curves presented in FIG. 5 represent the intersections of a vertical plane with the surface functions of concentrations.
In addition to the concentrations of individual species FIG. 5 also includes the concentrations of TFN and projected NO concentrations from a full, three-staged reburning process. The projected concentration is estimated based on the assumptions that NO will be stable in the burnout zone. Moreover, we assume that 60% of HCN and 0% of NH₃will convert to NO in the burnout zone. As showing on the left end of the figure, these assumptions project 60% NO reduction floor that has been demonstrated by many researchers.
The most striking observation of these results is that there indeed exists an optimal point at about SR2=0.945. The projected NO from a full, three-staged reburning process is about 160 ppm, or 84% reduction, which is very close to the cap proposed under the Clear Sky Initiative requiring some plants to reduce NO_xemissions to 0.15 lb/MillionBtu.
In addition to the high NO reduction efficiency, the SR2 is higher than those commonly used. The optimal ash rate from this set of experiments is about 0.02 gm/min, which corresponds to a ratio of stoichiometric coefficients of methane to ash=0.094:0.007. Since the carbon in biomass ash is effective in reducing NO, the methane consumption rate at SR2=0.945 is 0.094 versus 0.133 at SR2=0.9; see the insert in FIG. 5. This implies a 30% savings in methane consumption.
The second set of experiments has conducted at SR2=0.945 with varying fuel ratio, see FIG. 6. The as-received ash sample was ground and sieved before the experiments. As shown in Table 2, this sample has higher hydrogen and carbon content than that used for FIG. 5. The projected optimal overall NO concentration from a three-staged reburning process is about 170 ppm, or 83% reduction. It is interesting to note that the HCN and NH₃yields from reburning stare continue to increase with increasing feeding rate of ash. When we studied reburning with coal-derived chars (without methane) in the past (Chen and Ma, 1996; Chen and Tang, 2001), these reaction intermediates were not significant due to the lack of hydrogen in the chars.
As shown on the left-handed-side of FIG. 6, gradual replacement of methane by carbon in the ash at low ash feed rate results in additional reduction of NO; the majority of NO converts to HCN and NH₃. Gas phase mechanisms postulated by Miller and Brown (1989) and the heterogeneous NO reduction mechanisms postulated by Chen and Tang (2001) cannot fully explain these productions of HCN and NH₃. Since the nitrogen in the ash can contribute no more than 17 ppm of nitrogenous species, it is anticipated that the production of these species involves heterogeneous reactions among NO, CH₄, biomass ash and other components; catalytic reactions are likely involved. Better understanding, of these mechanism will undoubtedly assist the development of control strategies in the future the production of HCN and NH₃continues to increase even beyond the optimal ash feed rate at 0.015 gm/min. The optimal point appears to be caused by the reduced efficiency of NO reduction by the ash and production of HCN beyond the optimal feeding rate.
The data collected here do not suggest if there are differences in the efficiency of the small particle fraction of the raw ash and that of the ground ash. The optimal NO reductions seem to appear at approximately the same fuel ratio and SR2.
The minerals in biomass have high volatility and they from deposits in the downstream segment of our simulated reburning reactor where the temperature starts to drop. Thus, the behavior of biomass ash, in terms of slagging of furnace walls and fouling of heat exchanger tubes at pulverized coal combustion temperatures have been investigated. Slagging reforms to the formation molten and resolidified deposits on furnace walls and other areas exposed to radiant heat while fouling refers to the formation of high temperature bonded deposits on convection heat absorbing surfaces such as the tubes that constitute the superheater and reheater in a furnace.
STEAM its generation and use, 40^thEdition, Stultz and Kitto, Eds., Copyright © 1992. The Babcock & Wilcox Company, describes a composition-based algorithm for estimating the temperature that causes the viscosity of the ash to pass the 250 poise threshold, T_250poise. Low T_250poisetemperatures indicate low fusion temperatures and consequently increased slagging tendency, and provides a fouling index in four categories: low, medium, high and severe. Based on the biomass compositions shown in Table 2, calculations suggested that T_250poisedecreases from 1232 to 1149° C. after the heterogeneous reburning with biomass ash is introduced into a bituminous-coal fired boiler. Calculations also suggest the ash-fouling index for a bituminous-coal fired boiler remains in the categories of low to medium after biomass ash is introduced. Thus, the effects of introducing biomass into the boiler on slagging, and fouling are not expected to be significant.
Quantifications of total fixed nitrogen (NO, HCN and NH₃) in a simulated reburning apparatus achieved up to 90% NO_xreduction with a mixed fuel based on recently elucidated reaction mechanisms, a single reburning technology is likely to significantly break the 60% reduction floor observed in the last three decades. It is expected to meet the Environmental Protection Agency's regulation of removing 85%, or 0.15 lb/million Btu, of NO_xin boilers. The technological and economical impacts of the present invention are significant.
In addition to biomass injection for NO_xreduction implementation in the reburn arrangement, the method can be applied in other configurations where high-temperature, fuel-rich environments exist. Reduction of NO_xin most wall-fired, coal-burning, boilers is achieved via the installation of low-NO_xburners. As stated earlier, low-NO_xburners operate by controlled separation and mixing of fuel and oxidizer to create locally fuel-rich and fuel-lean zones. Introduction of lignite ash or biomass derivatives into the fuel-rich zones can catalyze the reduction of NO_xand decomposition of HCN (a NO_xformation precursor). This can be accomplished in two ways. In staged operations, where part of the combustion air is supplied through the overfire air (OFA) ports above the into the substoichiometric (fuel-rich) combustion zone for NO_xreduction, the lignite ash or biomass derivatives can be injected anywhere between the burner level and OFA ports (as shown in FIG. 1 b). To maximize the NO_xremoval efficiency, it is preferable to introduce the compounds closer the burner level. In the absence of OFA ports (unstaged combustion), the lignite ash or biomass derivatives can be strategically introduced into locally fuel-rich zones of the low-NO_xburner flame via especially designed nozzles for effective NO_xreduction.

References

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The invention thus being described, it will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope or spirit of the invention. Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the Specification and Examples considered as exemplary only and not intended to limit the scope and spirit of the invention,
Unless otherwise indicated, all numbers expressing quantities of ingredients, properties such as reaction conditions, and so forth used in the specification and sample claims are to be understood as being modified in all instances by the term “about.” Accordingly unless indicated to the contrary, the numerical parameters set forth in the specification and sample claims are approximations that may vary depending upon the desired properties sought to be determined by the present invention.
Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the invention are approximations, the numerical values set forth in the experimental or example sections are reported as precisely as possible. Any numerical value however, inherently contain certain errors necessarily resulting from the standard deviation found in their respective testing measurements.
While specific embodiments of the present invention have been shown and described in detail to illustrate the application of the principles of the invention, those skilled in the art will appreciate that changes may be made in the form of the invention covered by the following claims without departing from such principles. For example, the present invention may be applied to new steam generator construction, or to the replacement, repair or modification of existing steam generators. In some embodiments of the invention, certain features of the invention may sometimes be used to advantage Without a corresponding use of the other features. Accordingly all such changes and embodiments properly all within the scope of the following claims

Claims

1. A method of reducing nitrogen oxide emissions formed during combustion of a primary fuel, comprising the steps of:

providing a combustion chamber with a main combustion zone where the primary fuel and an oxidizer are introduced, and a reburn zone;

introducing a supplemental reburning fuel into the reburn zone to create an oxygen-deficient, fuel-rich combustion zone; and

injecting biomass ash into the reburn zone to reduce nitrogen oxide emissions.

2. The method of claim 1, further comprising a diverting step, wherein a portion of the oxidizer is introduced into the combustion chamber downstream of the reburn zone.

3. The method of claim 1 wherein the reburning fuel comprises at least one of gaseous, liquid and solid fuels.

4. The method of claim 1, wherein flue gas formed during combustion is recirculated and introduced with the reburning fuel.

5. The method of claim 3, wherein the gaseous fuel comprises at least one of natural gas, CH₄, and C₂H₆, and other gaseous hydrocarbon fuels.

6. The method of claim 3, wherein the liquid fuel comprises at least one of fuel oil, and other liquid hydrocarbon fuels.

7. The method of claim 3, wherein the solid fuel comprises coal.

8. The method of claim 1, wherein the biomass ash is derived from at least one of plant matter, biologically derived solid waste, crop residues, municipal solid waste, forest residues, urban wood waste, pine bark, sludge from a pulping process, and pulp mill waste.

9. The method of claim 1, wherein a temperature in the reburn zone is about 1,100° C.

10. The method of claim 1, wherein a stoichiometric ratio in the reburn zone is about 0.95.

11. The method of claim 3, wherein the reburning fuel and the biomass ash are introduced together into the combustion chamber.

12. The method of claim 1, wherein the primary fuel is coal.

13. A method of reducing nitrogen oxide emissions formed during combustion of a primary fuel, comprising the steps of:

providing a combustion chamber with a main combustion zone where fuel and an oxidizer are introduced; and

injecting biomass ash into a substoichiometric region of the combustion chamber to reduce nitrogen oxide emissions.

14. The method of claim 13, wherein the substoichiometric (fuel-rich) region is created by at least one of providing a portion of the oxidizer into the combustion chamber at a location downstream of the main combustion zone, and controlled separation and mixing of the oxidizer and the primary fuel introduced by a burner into the combustion chamber.

15. The method of claim 13, further comprising the step of providing a reburn zone within the combustion chamber, and introducing a supplemental reburning fuel into the reburn zone, the reburning fuel comprising at least one of gaseous, liquid and solid fuels.

16. The method of claim 13, wherein flue gas formed during combustion is recirculated and introduced with the reburning fuel.

17. The method of claim 16, wherein the gaseous fuel comprises at least one of natural gas, CH₄, and C₂H₆, and other gaseous hydrocarbon fuels.

18. The method of claim 16, wherein the liquid fuel comprises at least one of fuel oil, and other liquid hydrocarbon fuels.

19. The method of claim 16, wherein the solid fuel comprises coal.

20. The method of claim 13, wherein the biomass ash is derived from at least one of plant matter, biologically derived solid waste, crop residues, municipal solid waste, forest residues, urban wood waste, pine bark, sludge from a pulping process, and pulp mill waste.

21. The method of claim 13, wherein a temperature in the reburn zone is about 1,100° C.

22. The method of claim 13, wherein a stoichiometric ratio in the reburn zone is about 0.95.

23. The method of claim 16, wherein the reburning fuel and the biomass ash are introduced together into the combustion chamber.

24. The method of claim 13 wherein the primary fuel is coal.