Rotatable fluidized bed incinerator

Details Rotatable fluidized bed incinerator Rotatable fluidized bed incinerator

1999-06-14

2001-04-24

Ferensic, Denise L. (Department: 3749)

Furnaces

Refuse incinerator

Refuse suspended in or supported by a fluid medium

C110S243000, C110S244000, C110S219000, C110S258000, C110S264000

Reexamination Certificate

active

06220189

ABSTRACT:

BACKGROUND
This invention relates to a rotatable fluidised bed incinerator for incineration of combustible material.
Towns and cities are faced with an increasingly difficult problem in dealing with the disposal of wastes such as sewage. Traditionally, sewage sludge has been disposed of by open dumping, landfill or dumping in the sea. These practices either require large land acreage and are therefore becoming less attractive as the areas available for disposal near metropolitan areas rapidly disappear, or, in the case of sea dumping, are now or are shortly to be prohibited by new legislation.
Incineration of sewage sludge is recognised as an alternative possibility. Lurgi fluidised bed incinerators are widely used at present to burn sewage sludge. In this process the sludge must be de-watered to about 30% solids before combustion can be maintained. At this concentration of water, the sludge has the consistency of a thick paste and combustion takes place with no significant release of heat. If a filter press is used for de-watering to 35% solids, then the sludge has the consistency of cardboard and a significant heat output can be achieved. Lurgi fluidised bed incinerators are used to burn sewage sludge at a typical consumption rate of about 0.02 kg(dry)/s/m
2
and, accordingly over 300 Lurgi style incinerators of 3 meters bed diameter would be required to cope with the expected future UK disposal requirements of about 5 million tonnes for the year.
Another possible approach is to dry the sludge to a dry powder and feed it into pf coal fired power station boilers. To feed paste sludge at, say, 30% solids into power station boilers would, however, require massive transport of an undesirable material which is difficult to handle.
A further problem with rotatable fluidised bed incinerators is the elutriation of matter either before or after combustion. This results in either the undesirable distribution of ash or less effective or incomplete combustion of waste material. Furthermore, the bed depth is typically relatively shallow which imposes limits upon the volume of material which can be processed by a fluidised bed incinerator at any given instant in time.
SUMMARY
It is an object of the present invention to at least mitigate some of the problems of prior art fluidised bed incinerators.
Accordingly, the present invention provides a rotatable fluidised bed incinerator comprising a rotatable combustion chamber, means for rotating the combustion chamber, means for introducing combustible material into the chamber, means for introducing a gas into the chamber to create a fluidised bed within the chamber, wherein the rotatable combustible chamber is arranged to maintain a substantially constant gas velocity across the fluidised bed or with decreasing radius of the chamber.
There is further provided a rotatable fluidised bed incinerator comprising a rotatable combustion chamber, means for rotating the combustion chamber, means for introducing combustible material into the chamber, means for introducing a gas into the chamber to create a fluidised bed within the chamber, wherein the cross-sectional area of the chamber increases with decreasing chamber radius.
Advantageously, as the velocity of the gas is radially substantially constant, elutriation of either ash or combustible material is substantially reduced. Furthermore, the depth of the fluidised bed can be substantially increased as compared to prior art fluidised beds.
Preferably, the bed of the fluidised bed incinerator is caused to rotate rapidly about its longitudinal axis. In this way, the effective weight of the bed can be increased dramatically. Fluidisation occurs when the pressure drop across the bed is equal to the weight of the bed, in the case of a bed of near monosized particles, that is particles having substantially the same cross-section. Normally, the weight of the bed is determined by gravity. However, with the bed under rotation, the air flow passing through the bed can be increased proportionally to the “G” level produced by the rotation and the process is therefore intensified. In tests, a rotating fluidised bed has been operated at accelerations of up to 200 G.
In the case of sewage sludge incineration, the rotating bed may be operated at a relatively low “G” level, for instance, from about 5 G to about 20 G, for example about 10 G, giving a much more modest pressure drop. The relatively small amount of heat released, can be removed by the flue gases.
A typical rotating fluidised bed sewage sludge incinerator according to the invention suitable for burning all the sludge from a medium size town of about 100,000 people is estimated to be only about 500 mm long×600 mm diameter. The incinerator of the invention preferably has a short aspect ratio and is desirably provided with a cylindrical chamber having walls which taper in a radial direction in order to influence or reduce the radial gas velocity required for the fluidised bed.
Preferably the rotating fluidising bed is operated together with a de-watering unit, more preferably of the centrifugal type. As a result, there can be achieved further process integration and intensification for example by combining the rotatable fluidised bed with the de-watering unit drive.
In the process of the invention, the sewage sludge is preferably de-watered to a solids concentration of from 20 to 50% , more preferably 28 to 35% by weight.
The fluidised bed preferably comprises particles of an average diameter of from 0.1 to 3 mm, and, for example, sand particles of about 1 mm diameter have been found to be very suitable.
The combustible gas is preferably air, which can, if desired, be oxygen enriched or mixed with a liquid or gaseous fuel, such as, for example, propane. The combustible gas may be at ambient temperature, or pre-heated as desired, for example, to a temperature of 200 to 400° C.
The rotatable fluidised bed is also operable at a speed such that tumbling of the upper layer of bed material takes place, and, for example, speeds producing accelerations of from 0.5 to 2 radial g at the bed, have been found to be very suitable. The tumbling action of the particles from the top of the chamber allows waste to be introduced which is immediately engulfed by red hot particles. Light materials such as paper are immediately consumed before they are elutriated (as happens in a conventional municipal or fluidised bed incinerator). The intense longitudinal and circumferential mixing which can be achieved in the apparatus according to the present invention solves one of the major problems of known fluidised bed apparatus in which the transverse mixing is limited to a distance comparable to the bed depth.
The rotatable fluidised bed provides an intensively turbulent intimate mixture of sewage sludge particles and air. Very high combustion efficiency can be achieved and this is important in improving the quality of the ash and reducing the amount of pollutants emitted.
The turbulent mixing characteristics common to all fluidised bed combustors are further enhanced in the rotating fluidised bed design of the present invention because the density of the recirculating bed material is optimised in various zones in the furnace using the extra degrees of freedom resulting from this design. For example, varying the rotation speed can be used to vary the turndown ratio of the device, as indicated above. This parameter is relatively difficult to vary in a conventional static fluidised bed.
A considerable amount of heat is absorbed and retained by the large mass of particles making up the fluidised bed, thereby creating a large thermal “fly wheel”. The large thermal mass and the extreme turbulence can greatly reduce the potential for cold and hot spots to occur within the incinerator, in turn reducing the potential for stratified pockets of poor combustion to occur. In the case of the tumbling bed operation, the high turbulence can lead to a very high axial mixing.
In other, preferred, aspects of the invention, absorbent materials, such as limestone or dolomite, can be used to captu

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