Boiler with circulating fluidal deposit with improved use of reacting substances

(12) PATENT DESCRIPTION (19) PL (11) 211124 (13) B1 (21) Application number: 384257 (51) Int.Cl.

(22) Filing date: November 9, 2006 F23C 10/00 (2006.01) (86) International filing date and number:

2006-11-09, PCT / US06 / 044016 (87) International application publication date and number:

31.05.2007, WO07 / 061668 (54) Circulating fluidized bed boiler with improved utilization of reagents

<td>(30) Priority: 2005-11-17, US, 11 / 281,915</td><td>(73) The right holder of the patent: MOBOTEC USA, INC., Walnut Creek, US</td>

<td>(43) Application was announced: 21.07.2008 BUP 15/08</td><td>(72) Inventor (s): BRIAN S. HIGGINS, Walnut Creek, US</td>

<td>(45) The grant of the patent was announced: April 30, 2012 WUP 04/12</td><td>(74) Representative: item. stalemate. Jan Dobrzański</td>

PL 211 124 B1

Description of the invention

The present invention relates generally to circulating fluidized bed boilers, and more particularly to circulating fluidized bed boilers with improved use of reactants designed to reduce the amount of undesirable combustion products.

The combustion of sulfur-containing carbon compounds, and in particular coal, produces gaseous combustion products containing an unacceptably high concentration of sulfur dioxide. Sulfur dioxide is a colorless gas, moderately soluble in water and aqueous liquids. It is mainly formed when sulfur-containing fuel or waste is burned. When sulfur dioxide is released into the atmosphere, it reacts slowly to form sulfuric acid (H2SO4), inorganic sulfates and organic sulfates. Occurring in the atmosphere SO2 or H2SO4 causes the occurrence of undesirable "acid rain."

According to the US Environmental Protection Agency, acid rain causes acidification of lakes and streams and damages trees growing in high altitudes and many sensitive forest soils. Additionally, acid rain accelerates the deterioration of building materials and paints, including irreplaceable buildings, monuments and sculptures. Before falling to the ground, gaseous SO2 and NOx, and their fine particle derivatives, sulphates and nitrates, also reduce visibility and harm public health.

Air pollution control systems for the removal of sulfur dioxide generally rely on the neutralization of the absorbed sulfur dioxide with an alkali to obtain inorganic salts to prevent sulfur emissions into the environment. The alkalis most commonly used in this reaction include calcite or dolomite limestone, limestone slurry and dry quicklime and slaked lime, and commercial products and by-products from the production of "Theodoric" lime and natural sodium carbonate and magnesium hydroxide. Once absorbed by the limestone, SO2 is captured in existing particle trap equipment such as an electrostatic precipitator or a bag filter station.

Circulating fluidized bed (CFB) boilers use a fluidized bed with coal ash and limestone or similar alkalis to reduce SO2 emissions. The bed may contain other fine particle additives such as sand or refractory materials. Circulating fluidized bed boilers are effective in reducing SO2 and NOx emissions. A 92% reduction in SO2 emissions is typical, but can be as high as 98%. The stoichiometric Ca / S ratio required to achieve this emission reduction has been determined to be approximately 2.2. However, due to inefficient mixing, the ratio often has to increase to 3.0 or above to ensure that the desired SO2 capture is achieved. A higher Ca / S ratio requires more limestone to be used in the process, thus increasing the cost of the process. Additionally, ineffective mixing creates the so-called "Hot spots in the combustion process that favor the formation of NOx.

Accordingly, there is a need for a circulating fluidized bed boiler with improved reagent utilization, designed to reduce the amount of undesirable combustion products, while at the same time reducing NOx formation.

The present invention relates to a circulating fluidized bed boiler with improved utilization of reactants, comprising:

(a) a circulating fluidized bed comprising: a dense bed; the lower part of the furnace adjacent to the dense part of the bed; and the top of the oven;

(b) a reagent for reducing emissions of at least one combustion product in the exhaust gas;

and (c) a plurality of nozzles for introducing secondary air downstream into the circulating fluidized bed to provide mixing of the reactant and exhaust gas in the furnace above the dense bed.

The boiler of the invention is characterized in that secondary air injection nozzles are placed in the furnace at a height where the gas and particle density is less than about 140% of the gas and particle density at the furnace exit and the amount of reagent required to reduce product emissions combustion is reduced.

In one preferred embodiment, the boiler according to the invention further comprises a recycle system for recycling transferred particles from the flue gas to the circulating fluidized bed. Preferably, the recycle system includes a separator to remove carry-over particles from the exhaust gas. More preferably, the separator is a cyclone separator. In another

In a preferred embodiment, the boiler according to the invention further comprises a fine particle collector downstream of the separator in the direction of flow of the exhaust gas.

A bag filter station is used in particular as a collector for very fine particles. In another preferred embodiment, the fine particle collector is an electrostatic precipitator.

In a further preferred embodiment of the boiler, according to the invention, the reactant is selected from the group consisting of caustic soda, lime, limestone, fly ash, magnesium oxide, ammonia soda, sodium bicarbonate, sodium carbonate, a mixture of sodium and potassium hydroxide, sodium hydroxide and the group of minerals. calcite, which includes calcite (CaCO3), gaspeite ({Ni, Mg, Fe} CO3), magnesite (MgCO3), otavite (CdCO3), rhodochrosite (MnCO3), siderite (FeCO3), smithsonite (ZnCO3), spherocobaltite (C) OCO3) and mixtures thereof, and in a particularly preferred case the reagent is limestone.

In a further preferred embodiment of the boiler according to the invention, in the dense bed of the circulating fluidized bed boiler the content is kept below the stoichiometric ratio (fuel abundant stage), and in the lower part of the furnace the content is kept above the stoichiometric ratio (fuel shortage stage). thereby reducing the formation of NOx. Preferably, the secondary air injection nozzles are arranged in the lower furnace of the circulating fluidized bed boiler. In another preferred case, the nozzles for forcing the secondary air are arranged asymmetrically to each other. In yet another preferred case, the secondary air injection nozzles are arranged mutually collinear or staggered or in any combination of these combinations. In a particularly preferred embodiment, the secondary air injection nozzles are located from about 3.048 m to about 9.144 m above the dense part of the bed.

In a further preferred embodiment of the boiler according to the invention, the secondary air injection nozzles are arranged at a height in the furnace where the ratio of the column exit density to the dense bed top density is greater than approximately 0.7. In another preferred embodiment of the boiler according to the invention, the jet penetration of each secondary air injection nozzle, unless impeded by obstacles, is greater than approximately 50% of the width of the furnace. In yet another preferred case, the jet penetration is greater than about 38.1 cm of water (pressure exerted by a column of water of a given height) above the furnace pressure. In a further preferred case, the jet penetration is between about 38.1 cm (15 inches) and 101.6 cm (40 inches) of water (pressure exerted by a column of water of a given height) above the furnace pressure.

In yet another preferred embodiment of the boiler of the invention, the secondary air injection nozzles supply from about 10% to 35% of the total air flow into the boiler.

These and other aspects of the present invention will be apparent to those skilled in the art from reading the following description of the preferred embodiment considered together with the drawings.

Brief description of the drawings

Figure 1 is an illustration of a prior art circulating fluidized bed (CFB) boiler;

FIG. 2 is an illustration of the circulating fluidized bed boiler constructed in accordance with the present invention with improved limestone utilization;

Figure 3 is a graphical representation of the dependence of gas and particle density versus furnace height in CFB.

Figure 4 is a graphical representation of the CO weight fraction versus the furnace height in CFB.

Figure 5 is a graphical representation of the volume average fraction of the particle fraction versus height for the baseline and the present invention; and

Figure 6 is a graphical representation of weighted turbulent flow kinetic energy versus height for baseline and for the present invention.

Description of the preferred embodiments of the invention

In the following description, like references designate like or corresponding parts in several variants. With respect to the following description, it should also be understood that terms such as "forward," backward, "front," back, "right," left, "leading," falling, and the like are auxiliary words and should not be interpreted as limiting terms. In the present invention, the term "reducible acid" refers to acids in which acidity can be reduced or eliminated by electrochemical reduction of the acid. In the description of an embodiment of the invention, the term "nozzle is used to define the channel for introducing the reagent, without any narrowing at the end. The term "injector" is used to denote

A reagent introduction channel having a venturi at the end. The orifice of the orifice may be in the form of a conduit or the mouth end of the nozzle. The reagent introduction device is a device that includes channels, nozzles, injectors, or combinations thereof.

Generally, with reference to the drawings, these serve to describe a preferred embodiment of the invention, but are not intended to limit the invention thereto. As best seen in Figure 1, there is shown a prior art embodiment of a circulating fluidized bed boiler, generally designated 1. The circulating fluidized bed boiler may include a furnace 2, a cyclone dust collector 3, a sealed chamber 4, and - optionally - external heat exchanger 6. The exhaust gases produced during combustion in the furnace 2 pass into the cyclone dust collector 3. In the cyclone dust collector 3, particles are also separated from the exhaust gases. The particles trapped in the cyclone dust collector 3 pass into the sealed chamber 4. The external heat exchanger 6 performs the heat exchange between the circulating particles and the pipes in the bed of this heat exchanger 6.

In a preferred embodiment, the furnace 2 comprises a water-cooled furnace wall 2a and air-distributing nozzles 7. Air-distributing nozzles 7 introduce fluidizing air A into the furnace 2 to create a fluidization state in the furnace 2, which are arranged in the lower part of the furnace 2. A cyclone dust collector 3 is connected to the upper part of the furnace 2. The upper part of the cyclone dust collector 3 is connected to the heat recovery area 8, which includes the flue gases produced during combustion in the furnace 2, and the lower part of the cyclone dust collector 3 is connected to a sealed chamber 4 into which the captured particles enter. There are superheater and preheater in the heat recovery area 8.

An air box 10 is placed at the bottom of sealed chamber 4 to discharge fluidizing air B upward through a distributor plate 9. Particles in sealed chamber 4 are introduced into an optional external heat exchanger 6 with tubes 5 in the bed under fluidization.

In a conventional CFB boiler, there may be good agitation or kinetic energy in the lower part of the furnace (i.e. in the dense part of the bed). However, the present invention is based on the unexpected finding that at the top of the furnace (i.e., above the dense bed), mixing may be insufficient to provide a more complete utilization of the reagents added to reduce emissions in the flue gas. As used herein, the top of the dense bed is generally at the point where the gas and particle density is greater than about twice the gas / particle density at the exit of the boiler.

At the bottom of the furnace, which is typically opposite to the carbon delivery point, the volatile matter (gas phase) from the coal quickly mixes and reacts with the available oxygen. This creates a hot, low-density gaseous cloud that tends to stay above the surrounding stream containing the particles strongly. This upward displacement cloud rises quickly, forming a channel, chimney or cloud from the bottom of the furnace to the roof. Limestone, which absorbs SO2 and thus reduces its concentration, is absent in this channel. It has surprisingly been found that upon impact on the roof of the furnace, these high SO2 flue gases can exit the furnace and escape through the cyclone before the SO2 is reacted sufficiently. Measurements at the furnace outlet duct showed more than 10 times higher SO2 concentration in the upper part of the furnace outlet duct than in the lower part of the duct.

In the furnace of a conventional circulating fluidized bed boiler, the bed material 11, which contains ash, sand and / or limestone, etc., is in the form of a slurry in the fluidized state. Most of the particles entrained by the flue gases escape from the furnace 2 and are trapped in the cyclone dust collector 3, whereupon they are introduced into the sealed chamber 4. The particles thus introduced into the sealed chamber 4 are aerated by the fluidization air B and, for cooling, are heat exchanged with pipes 5 of the optional external heat exchanger 6 in the bed. These particles are returned to the bottom of the furnace 2 via duct 12 for cooling. Recycle them through the stove 2.

In the present invention, a circulating fluidized bed boiler uses high velocity air injection above the dense portion of the bed to both reduce limestone consumption and reduce NOx emissions. Additionally, Hg and acid gas emissions can be reduced. The high rate of air injection above the dense part of the bed provides intensive mixing of the fluidized bed space, which translates into greater combustion and reaction efficiency, resulting in

The amount of limestone or other alkaline reagent required to neutralize the acids in the flue gas to an acceptable level is reduced.

In one embodiment of the present invention, generally designated 100 in FIG. 2, the circulating fluidized bed boiler of the present invention has a series of nozzles 20 for injecting secondary air through which secondary air is forced into the fluidized bed. Preferably, the nozzles are spaced predetermined to cause a swirling flow in the fluidized bed zone. More preferably, the secondary air injection nozzles 20 are arranged asymmetrically to cause swirling motion in the boiler. Since many boilers are larger in diameter than height, in one embodiment of the present invention, the user may position two sets of nozzles to swirl in opposite directions.

In one embodiment of the present invention, the secondary air injection nozzles 20 are located from about 3.048 m to about 9.144 m above the dense part of the bed. The air injection nozzles are preferably arranged to act in mutually separate levels or stages, with they are arranged on opposite walls of the reactor. This arrangement provides intensive mixing of the fluidized bed space, resulting in a higher reaction efficiency between SO2 and limestone, allowing less limestone to be used to achieve a given SO2 reduction level. The improved mixing allows the stoichiometric Ca / S ratio to be reduced to the same level of SO2 emission reduction.

The main elements for forcing air at high speed above the dense part of the bed are:

(1) placing the high velocity air injection nozzles well above the dense portion of the CFB bed, the dense portion of the bed being defined as the fraction having a density greater than twice that at the exit of the kiln (entry to the cyclone), (2) injection nozzles air at high velocity are preferably designed to cause swirling of the exhaust gas, thus further increasing the mixing in the direction of the gases, and (3) the high velocity air injection nozzles are high pressure air injection nozzles which produce a turbulent jet flow of high velocity, high momentum and high kinetic energy.

Likewise, the vigorous agitation induced by the present invention can also prevent the formation of channels or clouds and, consequently, a shorter residence time of the sulfur compounds in the furnace, thereby allowing them to react longer in the reactor and further increasing the reaction yield. Intensive mixing also ensures more even combustion of the fuel, thereby reducing the number of "hot spots in the boiler where NOx can form."

Preferably, the mass flow of air through the high velocity air injection nozzles should introduce between about 10% to 35% of the total air stream. More preferably, high velocity air nozzles should introduce between about 20% and 30% of the total air stream.

In a preferred embodiment of the present invention, the exit velocity from the nozzles should exceed approximately 50 m / s. More preferably, the muzzle velocity should exceed about 100 m / s.

The air stream may be hot (flowing from the air heater (air side) as it circulates), it may be at ambient temperature (coming from the air heater (air side) in the opposite direction to the circuit at the FD fan outlet (forced draft) or may be at ambient temperature (coming from the surroundings). Installing an uninsulated air duct that bypasses the air heater is much cheaper, but it adversely affects the overall efficiency of the boiler.

The prior art applications of high velocity air jets are limited to mixing combustion zones containing predominantly flue gas and thus do not increase limestone utilization efficiency. In the present invention, mixing is directed to the combustion zone of the furnace containing a large amount of inert particles, namely, particles of ash and limestone left after coal combustion. In addition, the prior art has used staging to reduce the amount of NOx formed or high rate blending for chemical addition. In the present invention, staging can be used in addition to mixing and is used to increase reaction time, control bed temperature, and reduce the effects of "chimney formation in the furnace."

The present invention may be best understood by reading the following examples:

PL 211 124 B1

Example 1

FLUENT, a computational program for fluid dynamics analysis available from Fluent Inc., Lebanon, NH, was used to model thermofluidic phenomena in a CFB plant in two phases. The FLUENT program is used to solve tasks related to the determination of parameters: velocity, temperature and concentration areas of substances for gas and particles in the furnace. Since the particle phase volume fraction in CFB is typically from about 0.1% to 0.3%, a granular model was used in this case to solve multiphase flow problems. Contrary to conventional dust fuel combustion models, in which the particle phase is characterized by a discontinuous phase model, in the granular model for both the gas phase and the particle phase, the behavior equations are solved in the Euler coordinate system.

The solved conservation equations included continuity, momentum, turbulence and enthalpy for each of the phases. In this multiphase model, the gas phase (&gt; 99.7% by volume) is the main phase, while the individual particle size and / or particle type phases are modeled as secondary phases. The volume fraction conservation equation has been solved between the main phase and the secondary phases. The granular temperature equation determined for the kinetic energy of the particle phase was solved, taking into account the loss of kinetic energy due to strong interactions between the particles in CFB. With the current model, it took five days to converge to a fixed solution running in parallel on six computer central units (CPUs).

While ash and limestone were processed in the particle phase, coal combustion was modeled as gas phase. Coal in the model was defined as gaseous volatile matter with an equivalent stoichiometric ratio and heat of combustion. For a CFB combustion system, consider the following two chemical reactions:

CHo, 85O0, i4N0.07So, 02 + 1.06 O2 - 0.2 CO + 0.8 CO2 + 0.43 H2O + 0.035 N2 + 0.02 SO2 CO + 0.5 CO2 —— CO2

The chemical-kinetic combustion model covered a number of gaseous substances, including the main products of combustion: CO, CO2 and H2O. The mass conservation equations were solved for each gaseous substance. These conservation laws have been extensively described and expressed in computational fluid dynamics (CFD) textbooks. In the simulation, the k-ε turbulence model was used, and the incompressible flow was adopted both for the baseline and for the cases according to the invention.

All differential equations were solved in the non-stationary state due to the non-stationary hydrodynamic characteristics of the CFB boiler. Each equation was solved to obtain a convergence criterion before starting the next cycle. After maintaining the run for the solution for several hundred cycles, and behaving in a steady-state manner, the cycle was increased to speed up convergence. Typically the model was solved for more than thirty seconds of real time to achieve realistic results.

The CFD calculation area used for modeling is 100 feet (30.48 m) high, 22 feet (6.71 m) deep, and 44 feet (13.41 m) wide. The furnace has a main air inlet through the grate and 14 nozzles for injecting primary air on all four walls. It also has 18 nozzles for injecting secondary air, 8 of which supply limestone, and 4 starter burners on the front and rear wall. Two feed systems at the front wall supply the pulverized coal to the furnace. Two other feed systems are connected to each of the cyclone tubing via a sealing loop. In two cyclones connected to the kiln by two pipes at the top of the kiln, they collect the solid material, mainly coal ash and limestone, and then it is recycled by introducing it into the kiln in its lower part. Flue gas containing mainly combustion products and fly ash and very fine particles of reacted (and / or unreacted) limestone leaves the top of the cyclone and continues downstream of the recycle stream. Water screens run down all four walls of the furnace. There were three stages of superheaters. The superheaters I and II are in the furnace, while the superheater III is in the recycle stream.

The cyclone was not included in the CFB calculation area because the hydrodynamics of the particle phase in the cyclone are too complex to be practically included in the calculations. Pendent superheaters are included in the model to explain heat absorption and flow stratification, which are accurately described by the actual number of superheaters hanging in the furnace with actual spacing between them. It should be noted that the geometry of the furnace in terms of width was symmetrical, so that the design area represents only half of the furnace. Consequently, the number of computational meshes is only half, which reduces computation time.

PL 211 124 B1

Table 1 shows the system operating conditions for the baseline including the main input parameters for the baseline simulation in the CFD furnace model.

Table 1

<td>Parameter</td><td>Unit</td><td>Value</td>

<td>System charge</td><td>^M.^ Vgross</td><td> 122</td>

<td>Net cargo</td><td>MWnet</td><td> 109</td>

<td>The rate of supply of the furnace in the system</td><td>MMBtu / h</td><td> 1226</td>

<td>Excess O₂ in the system</td><td>% -wet</td><td> 2,6</td>

<td>Excess air in the system</td><td> %</td><td> 14,9</td>

<td>Carbon flow in the system</td><td>kpph</td><td> 187</td>

<td>Total Air Flow (TAF)</td><td>kpph</td><td> 1114</td>

<td>Primary air flow velocity through the bed mesh</td><td>kpph</td><td> 476</td>

<td>Primary air velocity through 14 nozzles</td><td>kpph</td><td> 182</td>

<td>Primary air temperature</td><td>° F (° C)</td><td> 434 (223,33)</td>

<td>Secondary air flow velocity through 18 nozzles</td><td>kpph</td><td> 262</td>

<td>Secondary air flow rate through 4 starter burners</td><td>kpph</td><td> 104</td>

<td>Secondary air flow velocity through the 4 coal feed channels</td><td>kpph</td><td> 65</td>

<td>The speed of the air flow through the limestone supply channel</td><td>kpph</td><td> 11,5</td>

<td>Air velocity through the sealing loop</td><td>kpph</td><td> 12,8</td>

<td>Secondary air temperature</td><td>° F (° C)</td><td> 401 (205)</td>

<td>Limestone delivery speed</td><td>kpph</td><td> 40</td>

<td>Solids Recycle Rate</td><td>kpph</td><td> 8800</td>

Table 2 shows the carbon composition for the case of the baseline Table 2

<td>A sample</td><td></td><td></td>

<td>Time</td><td></td><td></td>

<td>Approximate technical analysis</td><td></td><td></td>

<td>Volatile material</td><td>[wt.% ar]</td><td> 15,09</td>

<td>Degassed coal</td><td>[wt.% ar]</td><td> 35,06</td>

<td>Ash</td><td>[wt.% ar]</td><td> 42,50</td>

<td>Moisture</td><td>[wt.% ar]</td><td> 7,07</td>

<td>HHV (Btu / lb)</td><td>[Btu / lb]</td><td> 6800,0</td>

<td>Elemental analysis</td><td></td><td></td>

<td>C.</td><td>[wt.% ar]</td><td> 41,0</td>

<td>H.</td><td>[wt.% ar]</td><td> 2,1</td>

<td>ABOUT</td><td>[wt.% ar]</td><td> 1,2</td>

<td>N</td><td>[wt.% ar]</td><td> 3,5</td>

<td>S.</td><td>[wt.% ar]</td><td> 2,63</td>

<td>Ash</td><td>[wt.% ar]</td><td> 42,5</td>

<td>H2O</td><td>[wt.% ar]</td><td> 7,07</td>

PL 211 124 B1

In the FLUENT software, coal is modeled as a stream of gaseous fuel and a stream of ash solids, with the flow rates calculated on the basis of the total coal flow velocity and the results of coal analysis. The gaseous fuel is modeled as CH0,85O0,14N0,07S0,02 and is assigned a combustion heat of - 3.47 x 107 J / kmol. This is equivalent to the elemental composition and heat values ​​of coal presented in the tables.

In the following, the results for the baseline case are compared with the results for the inventive case.

High velocity injection significantly improves mixing by distributing the air relatively uniformly in the furnace. Oven mixing can be quantified by the coefficient of variation (CoV), defined as the standard deviation of the O2 mole fraction averaged over the entire section, divided by the average O2 mole fraction. Values ​​of the variance coefficient (σ / x) for the O distribution₂ in the case of the baseline and the case of the invention, more than four horizontal planes are compared in Table 3. As can be seen, all four planes have a high CoV value for the baseline ranging from 66% to 100%, and in both cases according to the invention these values are significantly lower, indicating that mixing is significantly improved.

Table 3

<td>Furnace height [m]</td><td>Baseline</td><td>Invention</td>

<td> 10,0584</td><td> 66%</td><td> 43%</td>

<td> 14,9352</td><td> 84%</td><td> 40%</td>

<td> 20,1168</td><td> 100%</td><td> 47%</td>

<td> 24,3840</td><td> 80%</td><td> 46%</td>

As best seen in Fig. 4, the CO mass versus height was compared for the baseline case and the case of the invention. Due to the gradation in the case of the invention, the CO concentration in the lower part of the bed below the high velocity air injection nozzles is higher than that of the baseline. Above the nozzles for forcing air at high velocity, the concentration of CO decreases quickly and at the exit of the furnace it is even lower than at the baseline. A rapid decrease in CO concentration indicates better and more complete mixing.

The particle volume fraction distributions for the baseline case and the case of the invention are shown in Figure 5. The graph clearly shows that the bottom bed has a higher density than the sparse top bed. The solids volume fraction in the upper part of the furnace is between 0.001 and 0.003. The distribution also reveals clusters of particles in the bed which are one of the typical features of particle movement in CFB. Mixtures of air and exhaust gases travel upward through these clusters. Similar particle flow characteristics can be observed with the present invention; however, it is also observed that the lower bed below the high velocity air injection nozzles is somewhat denser than the baseline due to the low overall air flow in the lower bed. At the top of the bed according to the present invention, the particle volume fraction distribution is similar to that of the baseline.

The turbulent mixing of the jets from the air injection nozzles and bed particles is compared in the baseline case and the inventive case in Fig. 6. In the case of the baseline, the maximum turbulent kinetic energy occurs in the dense part of the bed at the bottom of the furnace due to the injection of secondary air. However, this greatest turbulence diminishes rapidly as these jets penetrate the bed and mix within the furnace. In the case of the present invention, the peak kinetic energy is well above the dense part of the bed, allowing significant penetration and agitation of the bed.

The turbulence of the flow dissipates into convective flow due to the vortex dispersion. This means that a large amount of kinetic energy results in a better mixing of the high velocity injection air and the exhaust gas. While in the case of the baseline high turbulence at the bottom of the bed is essential for mixing the dense part of the bed containing the particles, high turbulence at the top of the furnace as shown in the present invention significantly improves the mixing of solids and flue gases. This is one of the main causes of reduced CO levels,

The more uniform O2 distribution and improved heat transfer are observed with the present invention.

The mechanisms of SO2 and other chemical compounds reduction as a result of reaction with limestone due to mixing have been discussed above. However, the obtained results of the calculations were better than could be expected. The use of deep grading in the first stage reduces the size of the gas channels formed outside of it in the first stage. The use of nozzles to force air at high velocity above the dense part of the bed will destroy any channels that form and the channels will collapse below the dense part of the bed. Thus, the combination of staging and asymmetrically spaced high velocity air injection nozzles above the dense part of the bed resulted in unexpected results.

It is anticipated that the enhanced mixing achieved with the present invention reduces the stoichiometric Ca / S ratio in CFB from ~ 3.0 to ~ 2.4 while achieving the same level of SO2 reduction (92%). The reduction in the Ca / S ratio corresponds to the reduced amount of limestone required to keep the boiler in service while still meeting SO2 emission requirements. Since limestone for use in CFB units often costs more than fuel (coal or fine coal), this represents a significant reduction in operating costs for a CFB plant.

Some modifications and improvements will occur to those skilled in the art upon reading the above description. For example, the secondary air injection nozzles may be installed in a row or only a portion of the secondary air injection nozzles may be operative at any time. Alternatively, all secondary air injection nozzles are operable, with only a part of the nozzles operating to their full extent. It should be understood that all such modifications and improvements have been removed from this specification for the sake of brevity and readability but are within the scope of the claims.