Fluidised bed incinerators

Abstract

To raise or lower the operating temperature of a fluidised bed to a desired level, the operating temperature of a second fluidised bed is raised or lowered to a temperature above or below that of the desired level and bed material is transferred between the two beds to bring the operating temperature of the first bed to the desired level. The first bed may be an incinerator and to lower the operating temperature the bed material of the second bed is cooled by a fluid in a heat exchanger and material is transferred between the two beds. The transfer of bed material may be brought about by a pump or one bed may be caused to overflow into the other with a corresponding counter-flow of bed material through a duct connecting the two beds.

Description

This invention relates to a method of and apparatus for raising or lowering the operating temperature of a fluidised bed.

Fluidised beds are well known and may be employed to raise or lower the temperature of articles supported in the bed and they may also be used as incinerators by burning combustible material in the bed. When a fluidised bed is used to heat treat material such as metal castings it is necessary to keep the operating temperature of the bed at a substantially constant value. Alternatively if the bed is being used to cool articles supported in it then again it is necessary to control the operating temperature of the bed. Furthermore if the fluidised bed is used as an incinerator and the calorific value of the material being burnt in the bed is high then the operating temperature of the bed may rise to an unacceptable level in which case the temperature of the bed has to be reduced. If a heat exchanger is located in the bed so that by passing fluid of the appropriate temperature through the heat exchanger the temperature of the bed can be controlled, it has been found that the tubes of the heat exchanger may become partially covered with bed material thereby reducing the heat transfer from the bed material to the fluid in the tubes and this situation can also lead to pockets of defluidisation of the bed. When the bed is used as an incinerator the tubes of the heat exchanger are subject to a high corrosion risk due to the hot wet products of combustion and also to erosion effects due to the fluidised bed particles.

It is an object of the present invention to provide a method of raising or lowering the operating temperature of a fluidised bed in which these difficulties are overcome.

It is a further object of the present invention to provide fluidised bed apparatus in which the above-mentioned difficulties are overcome.

According to a first aspect of the present invention in a method of raising or lowering the operating temperature of a first fluidised bed of refractory particulate material to a desired level, the operating temperature of a second fluidised bed of refractory particulate material is raised or lowered to a temperature above or below that of the desired level of the first bed respectively and sufficient bed material is transferred between the two beds to raise or lower the operating temperature of the material of the first bed to said desired level.

According to a second aspect of the invention fluidised bed apparatus comprises first and second beds of refractory particulate material, separate means for fluidising the two beds, means associated with the second bed for raising or lowering the operating temperature of the bed and means for transferring bed material between the two beds.

In order that the invention may be more readily understood it will now be described, by way of example only, with reference to the accompanying drawings in which:

FIG. 1 is a diagrammatic side elevation of fluidised bed apparatus in accordance with one embodiment of the invention,

FIG. 2 is a diagrammatic side elevation of a fluidised bed incinerator in accordance with one embodiment of the invention,

FIG. 3 is a diagrammatic side elevation of a fluidised bed incinerator in accordance with the present invention, the bed having internal circulation of the bed material, and

FIG. 4 is a diagrammatic side elevation of a fluidised bed incinerator in accordance with a still further embodiment of the invention.

Referring to FIG. 1, fluidised bed apparatus consists of two vessels 1, 2 each containing a bed 3 of refractory particulate material such as sand. In each vessel the bed material is supported on a gas distributor 4. The distributors may take the form of apertured plates or porous material or any other form of distributor suitable for use with a fluidised bed. Beneath each distributor there is a wind box 5 and gas, conveniently air, is supplied under pressure to each of the boxes 5 through a pipe 6. The two vessels are interconnected by a duct 7 extending between the vessels at a level above the distributors 4 and the duct contains pumping means 8 which may take the form of a vane or other suitable type of pump. When the apparatus is in use and the beds are fluidised the normal surfaces of the beds are indicated by reference numeral 9. Above the level of the surfaces 9 the two vessels 1, 2 are interconnected by a further duct 10. Located in the vessel 2 is a heat exchanger 11 which is in heat transfer relation with the bed material 3 when the material in the vessel 2 is fluidised.

If the fluidised bed in vessel 1 is to be used to heat articles 12 such as metal castings to a desired level then fluid is circulated through the heat exchanger 11 to raise the temperature of the material in the bed 2 to a higher level than the desired temperature of the material in the vessel 1. Sufficient bed material is transferred from vessel 2 into the vessel 1 to raise the temperature of the bed material in the vessel 1 to the desired level. At the same time an equal quantity of bed material from the vessel 1 is transferred to vessel 2. In this way the operating temperature of the bed material in vessel 1 can be raised to the desired level. The bed material is transferred for example by operating the pumping means 8 to pump the fluidised bed material from vessel 1 into vessel 2 thereby raising the level of the surface of the material in vessel 2 so that bed material flows from vessel 2 along the duct 10 and into the vessel 1. Obviously the pumping means could be operated in the opposite direction so that bed material is pumped from vessel 2 to vessel 1 through the duct 7 causing the level of the surface in vessel 1 to rise and for bed material to flow through the duct 10 into vessel 2.

If it is desired to cool the articles 12 suspended in the vessel 1 then the operating temperature of the first bed has to be reduced so a coolant fluid is passed through the heat exchanger 11 in vessel 2 thereby reducing the temperature of the bed material in that vessel to below the desired operating temperature of the bed in vessel 1. Hot bed material from vessel 1 is then transferred to vessel 2 and cooler bed material from vessel 2 is transferred to vessel 1 thus lowering the operating temperature of the bed material in vessel 1.

Referring now to the embodiments of the invention shown in FIGS. 2, 3 and 4 where the invention is applied to a fluidised bed incinerator, a vessel 21 is lined with solid refractory material 22 and has an internal partition 23 dividing the space within the vessel into two side-by-side compartments 24 and 25 respectively. The base of each compartment comprises an apertured plate 26 and a bed of refractory particulate material 27 is contained in each compartment and supported on the base 26. A flue 28 extends out of the vessel from the compartment 24 and waste material such as town refuse is introduced into the compartment 24 by way of a hopper 29 and a screw feeder 30. At the base of the compartment 24 there is an outlet 31 through which the ashes of combustion and non-combustible content of the waste material can be periodically removed. A heat exchanger in the form of a bundle of boiler tubes 32 is positioned in the compartment 25 so as to be in heat transfer relation with the bed material in that compartment when the material is fluidised.

In use, air under pressure is supplied below the apertured plates 26 in order to fluidise the beds to particulate material in the compartments 24 and 25 to a state beyond that of teter. A quantity of waste material and gas or fuel oil if necessary are introduced into compartment 24 to start combustion of the waste material. As soon as the bed has reached a temperature high enough to promote spontaneous combustion of the waste material supplied to it the gas or fuel oil is cut off.

Referring now to FIG. 2, the partition 23 extends from a position above the bases of the beds and above the normal operating surfaces of the fluidised beds. A small amount of bed material may pass between the two compartments but the bed material in compartment 24 will be considerably hotter than that in compartment 25. When the temperature of the bed material in compartment 24 approaches its desired operating level, the transfer of bed material between the two compartments is increased by increasing the degree of fluidisation of the bed in compartment 25 by increasing the air supplied to it thereby causing the bed to expand and for some of the bed material to spill over from compartment 25 into compartment 24. At the same time hot bed material from compartment 24 flows beneath the partition 23 into compartment 25 where it gives up most of its excess heat to cooling fluid circulated in the heat exchanger 32. In this way, hot bed material flows from compartment 24 to compartment 25 and cooler bed material flows from compartment 25 to 24 thus reducing the temperature of the bed material in compartment 24. The transfer of bed material may be carried out continuously or it may be operated intermittently so as to reduce the bed temperature in compartment 24 to a level below the desired operating temperature and then allowing the operating temperature to rise to approaching the maximum desired level before bed transfer takes place again.

Referring to the embodiment of the invention shown in FIG. 3 the apertured plate 26 in the compartment 24 is inclined downwardly towards the plate 26 in the compartment 25 and the space beneath the inclined plate 26 is divided into a plurality of sections 33, 34 and 35 arranged side-by-side and each having its own air supply. In use, air at different mass flows is supplied to the sections 33, 34, 35 with the supply to section 35 being at a higher mass flow than that supplied to section 34 which in turn is at a higher mass flow than that supplied to section 33. This differential air mass flow causes the bed material in compartment 24 to be in a state of agitation and to circulate continuously as indicated by the arrow 36. Waste material introduced into the compartment by the screw feeder 30 is quickly enveloped by the circulating bed material and is drawn down to the base of the bed. The combustible content of the material is burnt in the bed and the non-combustible content is displaced to the outlet 31. The circulation of the bed material in compartment 24 is in a vertical plane and in a direction downwardly towards the base of the partition 23. This circulation of the bed material influences the bed material in compartment 25 and hot bed material is continuously transferred into compartment 25 and cooler material spills over the top of the partition 23 into the compartment 24. A deflector plate 37 is positioned at the base of the flue 28 to prevent bed material from being drawn into the flue as it is being transferred from one compartment to the other.

Referring to the embodiment of the invention illustrated in FIG. 4, a high pressure air jet is employed to assist and control transfer of the bed material from compartment 24 to 25. The apertured plate 26 in the compartment 25 is located at a level above the base of the partition 23 and a duct 38 extends from compartment 24 beneath the partition 23 and upwardly to a level above the plate 26 in compartment 25. A cover is fitted above the upper end of the duct 38 to reduce the flow of bed material into the duct from compartment 25 and an injector pipe 39 extends upwardly through the duct 38 and terminates close to the base 26 in the compartment 25.

In use, the bed material in compartment 24 is caused to circulate in a vertical direction due to the differential air mass flow supplied to the compartment. Some of the bed material will enter into the duct 38. When it is desired to transfer bed material between the two compartments, air at a high pressure is supplied to the injector pipe 39 and the air entrains hot bed material in the duct 38 and transfers it to above the plate 26 in the compartment 25. A corresponding amount of cooler bed material is forced out of the compartment 25 over the partition 23 and into compartment 24. The pressure of air supplied to the injector pipe 39 and hence the quantity of bed material transferred between the two compartments is controlled according to the temperature of the bed material in compartment 24.

In the embodiments of the invention where the beds of refractory particulate material are shown supported on apertured plates 26, the plates serving as distributors for the fluidising medium, an alternative to an apertured plate is a porous plate or any other form of distributor. Where the plate 26 of the compartment 24 is shown with three air boxes beneath it, it is to be understood that two or more air boxes may be employed or a single box may be employed if the air distributor is so arranged that the permeability/porosity of the distributor varies along its length to allow differing mass flows through it along its length.

In other embodiments of the invention various types of pumping devices may be used for pumping bed material from one compartment to another. For example mechanical or electro magnetic pumps may be used and the pumps may be cooled by positioning them in a duct through which air under pressure is passed.

In a still further embodiment of the invention the two compartments are separated by a partition which is porous and by vibrating the partition, bed material can be caused to pass from one compartment to the other.

In all the embodiments of the invention where a boiler tube type heat exchanger is shown it is to be understood that other types of heat exchanger may be employed.

In the embodiments of the invention which relate to a fluidised bed incinerator, the transfer of bed material between the two compartments serves two useful purposes. The replacement of hot bed material by cooler bed material in the incinerator reduces the operating temperature of the incinerator and thus avoids the necessity of introducing quenching water or air attemperation in the incinerator and much of the heat contained in the hot bed material is given up to the fluid in the heat exchanger, the fluid being used for power generation or other process requirements. As the heat exchanger is not contained in the part of the fluidised bed in which combustion takes place, corrosion of the heat exchanger due to hot products of combuation is eliminated. Furthermore, erosion of the heat exchanger is minimised by the use of fluid mass flow rates lower than those obtaining in the main combustion fluidised bed.

Claims

We claim:

1. Apparatus comprising first and second beds of refractory particulate material, said beds being located in separate compartments arranged side by side in a vessel, a partition forming a common wall between said compartments and extending from a position above the lowest level of said particulate material to a position above the normal upper surfaces of said particulate material, separate means for fluidising the two beds, the means for fluidising the first bed being arranged to fluidise the bed in a non-uniform manner to cause a greater degree of agitation of the bed adjacent said partition than away from said partition, thereby promoting circulation of the material in said first bed from the upper surface of said first bed which is away from said partition downwardly toward the bottom of the partition, and means associated with the second bed for raising or lowering the operating temperature of the second bed and for transferring material between the two beds.

2. Apparatus comprising first and second beds of refractory particulate material, said beds being located in separate compartments arranged side by side in a vessel, separate means for fluidizing the two beds, a partition forming a common wall between said compartments and extending from a position above the lowest level of said particulate material to a position above the normal upper surfaces of said particulate material, means associated with the second bed for raising or lowering the operating temperature of the second bed, and means for transferring bed material between the two beds which comprises a duct extending beneath the partition from a position above the base of the first bed to a position above the base of the second bed, and pumping means in the duct arranged to transfer bed material through the duct from the first bed to the second bed.

3. Apparatus as claimed in claim 2 in which said pumping means comprises a pneumatic injector.

4. Apparatus comprising first and second beds of refractory particulate material, said beds being located in separate compartments arranged side by side in a vessel, separate means for fluidizing the two beds, a partition forming a common wall between said compartments and extending from a position above the lowest level of said particulate material to a position above the normal upper surfaces of said particulate material, means associated with the second bed for raising or lowering the operating temperature of the second bed, and means for transferring bed material between the beds, said vessel defining a duct extending beneath the partition from a position above the base of the first bed to a position above the base of the second bed and pumping means in the duct arranged to transfer bed material through the duct from the first bed to the second bed.