Fast Fluidization

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A portion of the granular bed material, unburnt fuel, gaseous volatile matter, solid carbon and ash is carried upward i. A majority of the combustion taking place in combustion chamber 10 occurs in upper region In sharp contrast to prior art fast fluidized bed reactors, the fast fluidized bed reactor in accordance with the present invention does not require that the flue gases and the entrained granular bed material, including unburnt fuel, solid carbon, ash solids , be directed into a solids-gas separator e.

Rather, as noted above, the entrained solids and gases are carried upward into the upper region 18 of combustion chamber 10, where further combustion takes place. Flue gases eventually exit from upper region 18 through cylindrical exit throat 5 which has a smaller cross-sectional area than that of upper region 18, as will be discussed below.

Exit throat 5 facilitates separation of the granular bed material from the flue gases exiting therethrough, as well as facilitating circulation of the solids inside combus - tion chamber When operating as an adiabatic combustor, the flue gases are then typically fed to a process heat supply or boiler as well. For example, they may be fed to kilns, veneer dryers, etc. It is generally known that the quantity of particles transported by an ascending gas is a function of the gas flow velocity to the third to fourth power.

Thus, greater solids reaction surface can be achieved by: a maintaining maximum solids' saturation in the ascending gas flow, and b increasing the vertical velocity of the fluidizing gas to a desired level sufficient to provide the desired carry-over from the fluidized bed into upper region For any fuel having a given specific ash particle size distribution, this vertical gas velocity must be sufficiently high, as noted above, but must not be so high as to cause intensive erosion of the refractory liner, which is preferably provided on the interior surface of upper region 18, due to very nigh ash concentration in this region, as will be discussed below.

The interior surface of upper region 18 is cylindrically shaped in order to achieve swirling flow in the upper region, as discussed more fully below.

Fast Fluidization, Volume 20

In accordance with the invention, means are provided for tangentially supplying a second stream of pressurized air referred to herein as "secondary" air to the upper region 18 of combustor chamber 10 through at least one opening 19, and preferably at least two oppositely disposed openings Still more preferably, a plurality of pairs of openings 19 are provided at several aggregate points in upper region As shown in Fig.

The cross-sectional view shown in FIG. As embodied herein, a source of pressurized air, e. In accordance with the invention, it is critical that the secondary air be supplied at a sufficient velocity, and that the geometric characteristics of the interior surface of upper region 18 be adapted, to provide a Swirl number S of at least about 0. Preferably, the reactor of the present invention is constructed and operated in a manner adapted to yield these minimum values of Swirl number and Reynolds number when operating at minimum reactor capacity i.

On the other hand, the Swirl number and Reynolds number must not exceed those values which would result in an unacceptable pressure drop through combustion chamber It is this cyclone of turbulence which enables the reactor of the present invention to achieve specific heat release values higher than about 1.

As a result, the size of the combustor of the present invention can be significantly reduced, compared to prior art combustors which have a specific heat release of only about 0. Exit throat 5 and the interior of the upper region 18 of combustion Chamber 10 must exhibit certain geometric characteristics, together with the applicable gas velocities, in order to provide the above-noted requisite Swirl number and Reynolds number. These features are explained below and are discussed generally in "Combustion in Swirling Flows: A Review," supra, and the references noted therein, which publications are hereby specifically incorporated herein by reference.

Fuel combustion is substantially completed in the cyclone of turbulence in upper region 18 at a temperature below the fusion point, which provides a friable ash condition. Although the fluidized bed reactor of the present invention is fluidized in the "circulating" or "fast" fluidization regime, it differs fundamentally from prior art fast fluidized bed reactors, in that it does not require the use of a cyclone particle separator to effect separation of the flue gases from the solids carried thereby, e. Rather, the cyclone of turbulence in upper region 18, and the accompanying large internal reverse flow zones created therein when the cylindrical exit throat 5 is appropriately sized see below , effectively prevent all but the smallest solids e.

The elimination of such cyclone separators will significantly reduce the size and the cost of reactor systems constructed in accordance with the present invention. In the embodiment shown in FIG. As a result, it will be necessary to frequently bleed off these solids through conduit If combustion chamber 10 is designed and operated so as to achieve a Swirl number of at least about 0. Such recirculation zones are known generally in the field of conventional cyclone combustors i.

Such recirculation zones in upper region 18 act to return the solids to the lower region This, coupled with the high level of turbulence in upper region 18, results in significantly improved solids-gas heat exchange and, therefore, a relatively uniform temperature throughout combustion chamber As mentioned, the combusion chamber 10 should be constructed such that the value of the ratio X lies within the range of from about 0.

The greater the value of X, the lesser the pressure drop through combustion chamber 10 and the greater the Swirl number; so that, generally, higher values of X are preferred. However, for values of X in excess of about 0. By way of illustrative hypothetical example, for a non-adiabatic combustor having a capacity Q com of 7.

From the above analysis, and particularly equation No. As is also apparent from the equations set forth above, construction of combustion chamber 10 in a manner such that the cross-sectional area of the fluidized bed bottom is smaller than that of upper region 18 is preferred, since this will facilitate the obtaining of the requisite Swirl number. This is especially important when high moisture content fuel is used and when low pressure drops are desired. Moreover, the use of a smaller bottom cross-sectional area permits the use of higher bottom gas velocities, which, in turn, permits combustion of a fuel having larger particle sizes, while insuring that such particles can be fluidized in the bed.

In constructing a combustor in accordance with the present invention, it is clear from the above analysis that many parameters may be varied in order to achieve the requisite Swirl number and Reynolds number. For example, the values of the parameters X, Y, and Z can generally be adjusted as necessary, within the constraints imposed by the need to obtain an acceptably low pressure drop through the entire system, and within the constraints on the value of X discussed above.

In this regard, it should be noted that the maximum acceptable pressure drop through a combustor is generally on the order of mm w. However, as a result of the improved heat transfer exhibited by the overall system of the present invention, a pressure drop of mm w. We have found, based on comparative analytical analysis of hypothetical constructions of the invention, that:.

The present invention can be applied to most nonuniform combustible particulate solid materials, such as, for example, wood wastes, municipal refuse, carbonaceous matter e. However, it also can be used for liquid and gaseous fuel. The method of the present invention can also be used for boiler applications which, from an economical standpoint, require low excess air for combustion and, therefore, heat absorption in the fluidized bed lower region In the boiler embodiments of the invention, the cross section of lower region 11 is preferably of quadrangular shape and of a larger size, in order to accommodate a heat exchange surface of reasonable size 'in the fluidized bed volume.

As shown in the dashed lines in FIG. The tube arrangement may be of any suitable size, shape and alignment including vertical tubes , as is well known in the art. Preferably, heat exchanger tube arrangement 29 will be operatively connected to a process heat supply or to a conventional boiler drum, not shown. The heat exchanger cooling media may comprise any suitable conventional liquid or gaseous nedia, such as, for example, air. In boiler applications, the exhaust gases exiting from reactor exit throat 5 are preferably fed to a boiler convective tube bank not shown in a conventional manner.

Turning now to FIG. Like reference numerals have been used in FIG. In particular, the embodiment shown in FIG. Cooling fluidized bed 40 comprises an ordinary i. The fluid entering tube arrangement 42 is preferably supplied from a conventional boiler steam drum not shown.

Fast Fluidization, Volume 20 (Advances in Chemical Engineering)

Purchase Fast Fluidization, Volume 20 - 1st Edition. Print Book & E-Book. ISBN , Fast fluidization is a technique for bringing gas at high velocity into intimate contact with a fine solid in an en- trained dense suspension characterized by.

The bed is fluidized by tertiary pressurized air supplied from a plenum 43 through openings 44 in a support surface. These openings may take the form of nozzles. Fluidized bed 40 is comprised of the granular material and other solids overflowing from lower region 11 into bed 40 through opening 41, as will be explained below. Heat exchanger tube arrangement 42 functions as a cooling coil to cool fluidized bed The cooled solids leave bed 40 through an orifice 75 extending through the bottom part of the partition 76, which separates bed 40 from the fast fluidized bed contained in lower region 11, and re-enter lower region 11 of reactor chamber 10 to be again fluidized therein.

The fluid passing through tube arrangement 42 is consequently heated and preferably fed, for example, to a conventional boiler drum not shown. The movement of solids from fluidized bed 40 to the fast fluidized bed in lower region 11 of combustion chamber 10 is motivated by specially oriented tertiary air jet nozzles 44 and monitoring air jet nozzles Monitoring nozzles 77 may be supplied from a separate plenum Preferably, nozzles 44 and 77 will be constructed as shown in FIG.

Fast Fluidization

For a better understanding of how this boiler embodiment functions to improve the turndown ratio, a preferred procedure for initially placing it into operation from the cold condition to a full load and then turn it down to a desired level will be explained. The ignition burner not shown , preferably located above the lower region 11, is turned on, while primary, secondary, tertiary and monitoring air are shut cff.

When the combustor's refractory and its internal volume temperature exceed the solid fuel ignition temperature, the primary air and secondary air are partially turned on, while the tertiary and monitoring air remain shut off. From this moment, an adiabatic fluidized bed combustor scheme is in operation in reactor chamber 10, and when the temperature again exceeds the solid fuel ignition temperature, solid fuel is fed into combustion chamber After the solid fuel is ignited and, consequently, the combustor's exit gas temperature has risen, the monitoring air is then turned on.

To keep the combustion temperature on the rise, at this time the secondary air flow is gradually increased, with a simultaneous increase in the solid fuel feed rate, and the ignition burner is shut off. If the gas exit temperature continues to rise, a further increase in the secondary air flow and fuel feed rate should be pursued. At the point when the gas exit temperature achieves its highest designed level, the tertiary air flow is turned on and continuously increased until it reaches its full rate.

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Simultaneously, the fuel feed rate and the secondary air flow are ilso continuously increased. At this moment, if the gas exit temperature is at the desired, i. The minimum capacity of the reactor, i.

Namely, while maintaining the desired fuel-air ratio, the secondary air flow is reduced until the lowest acceptable temperature level in combustion chamber 10 i. Further reduction in the unit's capacity is achieved by gradually decreasing the monitoring and tertiary air flows, in that order.

1st Edition

As a result, the solids' circulation through cooling fluidized bed 40 would be reduced to a minimum, and likewise the heat exchange process between bed 40 and heat exchanger tubes In brief review, the key feature, in terms of obtaining a high turndown ratio according to the embodiment depicted in FIG. Furthermore, the above-desired boiler turndown ratio improvement has an additional advantage over known circulating fluidized bed boilers.

The latter fact results, in part, from the fact that it is possible, by using a separate fluidized bed 40, to utilize the optimum fluidization velocity therein, and the fact that fluidized bed 40 is comprised of small particles, e. This embodiment preferably can be used for applications requiring a higher capacity.

The higher capacity results from the construction of the FIG.

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In the preferred embodiment shown, boiler drum is situated above combustion chamber Like reference numerals are used in FIG. Moreover, only those structural and operational features which serve to distinguish the embodiment shown in FIG. In particular, in the embodiment shown in FIG. Preferably, heat exchange surfaces are provided in duct to recover heat from the flue gases. For example, a bank of superheater steam tubes may be positioned in duct as shown, and steam from boiler drum may be fed thereto.

Fluidization XV | Proceedings | Engineering Conferences International

Alternatively, or conjunctively, tubes might comprise a conventional evaporative convective surface. The output of the economizer would preferably be supplied to boiler drum , while the output of the air heater would, for example, be utilized as the secondary air supply.

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  3. Computational Fluid Dynamics Modeling of Palm Fruit Pyrolysis in a Fast Fluidized Bed Reactor.
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  6. The Dynamics of Fast Fluidization;

After passing tube banks and , the flue gases exit from duct via opening Fly ash is collected at the end of duct as shown, and is removed via port In accordance with the embodiment of the invention shown in FIG. As embodied herein, such means for separating the solids from the combustion gases includes a suitable conventional cyclone separator 24 or a plurality thereof operatively connected between inlet port 23 and exit port Flue gases exit from cyclone separator 24 through port 35, and are then typically fed to the process heat supply or boiler, as the case may be.