GB2093367A - Combustion method and apparatus - Google Patents

Abstract

A fluidised bed 10 of non- combustible granular material is supported on a grate 8, fluidising air being supplied from the chamber 12 through the grate 8. Combustible substance to be burned is fed typically by a screw-feeder 22 into the bed 10 which is first raised to a combustion temperature. Oxygen is supplied to one or more portions of the bed 10 through nozzles 32 so as to combat localised incomplete combustion, a localised undesirably low thermal output per unit volume of bed, or a localised undesirably low rate of steam generation (where the bed is used to raise steam). In an alternative construction, chamber 12 is subdivided and the oxygen is supplied via the fluidising air inlets. <IMAGE>

Description

SPECIFICATION Combustion method and apparatus This invention relates to a method of and apparatus for combustion. It particularly relates to combustion of substances in a fluidised bed.

The term 'substance' as used herein encompasses solids, liquids and gases.

The combustion of combustible substances in a fluidised bed has been practised as follows: a bed of inert particulate or granular material such as sand, ash or limestone is established. The bed is fluidised by air. The temperature in the bed is raised to a chosen value by heating the bed. When the bed has attained the chosen temperature, the substance to be burned is introduced into the bed.

If desired, heat may be extracted from the hot gases or the bed itself and, for example, used to raise steam.

Difficulties arise in achieving entirely satisfactory control over such a fluidised bed combustion process. The flow rate of the air needs to be above a certain minimum so as to obtain adequate fluidisation of the bed. It also needs to be below a certain maximum so as to keep to acceptable levels the carry-over or elutriation of unoxidised carbon from the bed. In addition, the rate of flow of air has to be above a minimum so as to ensure that there is sufficient oxygen for all the carbon to be adequately oxidised. As the quality of the fuel being burned varies so this minimum will change. In practice, it is often found that in some parts of the bed particularly near a wall there is inadequate combustion and therefore the bed does not operate as efficiently as it should in theory.In some fluidised bed combustion apparatus the fluidising air is supplied in a number of separate channels to a fluidising plate on which the bed is supported. This enables one part of the bed, in effect, to be shut down if, for example, an excessive temperature is being attained therein or if, for example, it is required to perform routine maintenance.

It is an aim of the present invention to provide methods and apparatus for the combustion of substances in fluidised beds whereby it is possible to ameloriate at least one of the above-mentioned disadvantages.

According to the present invention there is provided a method of burning a combustible substance comprising the steps of establishing a bed of non-conbustible particulate or noncombustible granular material, fluidising the bed with air, raising the bed to a combustion temperature, discharging ash from the bed, and locally enriching in oxygen the air supplied to any part of the bed in which there is inadequate combustion of the said substance so as to obtain improved combustion.

The invention also provides apparatus for burning a combustible substance in a fluidised bed comprising a combustion chamber bounded at its lower end by a distributor plate able to support a bed of non-combustible particulate or noncombustible granular material, means for supplying an upward flow of air through the distributor plate so as to fluidise the bed, means for heating the bed, means for feeding the combustible substance into the bed, means for discharging ash from the bed, and a plurality of spaced-apart gas supply pipes, each being adapted to supply to a chosen region of the bed oxygen or a gas mixture whose concentration of oxygen is greater than that in air so as to enrich locally in oxygen the air supplied to any part of the bed in which there is inadequate combustion of the said substance.

The expression 'inadequate combustion' is used herein to refer to any one (or combination) of the following occurrences: (a) incomplate oxidation of combustible material; (b) a relatively low thermal output per unit volume of bed (or relatively low bed temperature) as compared with other regions of the bed, with the average thermal output per unit volume of bed (or average bed temperature), or with the specification for average (or local) thermal output per unit volume of the bed (or for average (or local) bed temperature) that the apparatus is intended to meet; and (c) where the bed is used to raise steam, a relatively low rate of formation of steam per unit volume of bed as compared with other regions of the bed, with the average rate of formation of steam per unit volume of the bed, or with the specification for average (or local) rate of formation of steam per unit volume of the bed that the apparatus is intended to meet.

The gas for locally enriching the air in oxygen may be introduced below the distributor plate but at a position in which it does not become generally mixed with all the air or above the distributor plate in the bed itself. Thus, the said gas supply pipes may terminate either just below the distributor plate or in the bed itself. However, if the air supply to the distributor plate is routed through several separate channels which do not communicate with one another the gas for locally enriching the air in oxygen may be supplied to any chosen one of the said channels, if desired, at a region sufficiently upstream of the distributor plate for said gas to become fully mixed with the air in the said channel upstream of the distributor plate.

If the gas for locally enriching the air in oxygen is introduced directly into the bed (i.e. without first passing through the distributor plate) it may be introduced parallel to the general flow of air through the bed or perpendicuiar thereto.

Alternatively, it may have a parallel component of velocity and a perpendicular (or normal) component of velocity. Normally, for the purposes of combatting inadequate (or inefficient) combustion in any localised part of the bed parallel addition may be employed. However, on some occasions it will be desirable to create a lateral flow of gas in the bed so as to combat uneven heat release in different parts of the bed. If desired, the gas supply pipes may have outlets whose angle of inclination is able to be varied according to the flow regimen it is desired to create within the bed or within any part of the bed.

Preferably, the temperature within chosen regions of the bed is monitored by means of thermocouples or optical pyrometers or other temperature sensing devices. Each such temperature sensing device may be operatively associated with a flow control valve in the pipe for supplying gas to enrich in oxygen the air supplied to that region. The arrangement may be such that oxygen is supplied to the region from the said pipe only when the sensed temperature is below a chosen value. Instead of temperature sensing devices, there may be a plurality of observation ports built into the combustion chamber, and if desired, closed-circuit television may be used to monitor the bed. In another alternative, hot gases may be led out from chosen regions of the bed and their temperature measured outside the combustion chamber.If such an arrangement is employed there may be automatic control of oxygen introduction through the said gas supply pipes in the same manner as there is if the temperature is monitored at different locations within the bed itself.

If desired, the air may be generally enriched in oxygen during the period in which the bed is raised to a combustion temperature. This helps to shorten the period required to raise the bed from cold to a temperature at which combustion of the combustible substance can take place. Once the temperature of the bed has reached a chosen value the gas used for the purposes of general enrichment of the fluidising air in oxygen may no longer be supplied. If it is required at any time to boost the general thermal output of the bed, the air may be generally enriched in oxygen and the rate of supplying combustible substance to the bed increased. Alternatively, local enrichment of the air in oxygen may be employed to boost the thermal output of the bed.

General oxygen enrichment of the air, or local enrichment at chosen regions, may also be employed to maintain the termal output of the bed if the calorific value of the combustible substance supplied to the bed varies. Thus, if there is a decrease in the calorific value of the combustible substance additional oxygen may be supplied in one of the above-mentioned methods, and such supply stopped, once the calorific value of the substance increases again to a suitable level.

If the off-gas from the bed includes combustible fluid, oxygen or a gas whose oxygen concentration is greater than 22% by volume (the gas typically being pure oxygen) may be introduced into the gas leaving the bed so as to assist in the oxidation of such volatile material (i.e. by supporting combustion of combustible constituents in the gas). In addition, the use of oxygen to support combustion above the bed will tend to reduce the amount of carbon elutriating from the bed. In a sectionalised bed, that is one in which the air supply is through separate channels which do not communicate with one another, if it is desired to "shut-down" one or more sections while keeping down the adverse effects of this on the combustion in other sections of the bed, a supply of nitrogen may be substituted for the air. It is thus possible to maintain the bed fully fluidised.Since the general flow of gas through the bed is laminar, any mixing of the nitrogen with the air that does take place will not have a substantial effect on combustion in other sections of the bed. Such use of nitrogen also helps to prevent caking of sections that are shut down. When the sections of the bed that are shut down are required again, fuel (i.e. propane) is supplied to those sections and the rate of flow of nitrogen gradually decreased with a concomittant increase in the rate of supply of air such that the oxygen concentration in the gas supplied to such sections is increased gradually from 0 to 21% by volume.

If, in an emergency, it is desired to shut down the bed, the supply of nitrogen may be substituted for the air and any gas used to enrich the air in oxygen. Thus, combustion can be stopped without allowing the bed to settle. The nitrogen supply may be continued until the bed is cooled to the required temperature and will also help purge combustible gases from the apparatus.

The method and apparatus according to the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a schematic representation of fluidised bed combustion apparatus embodying localised means for adding oxygen to the fluidising air; Figure 2 illustrates an oxygen supply system for use with the apparatus shown in Figure 1; Figure 3 is a schematic representation of an alternative form of fluidised bed combustion apparatus embodying means for the localised addition of oxygen to the fluidising air, and Figure 4 illustrates oxygen supply equipment for use with the apparatus shown in Figure 3.

Referring to Figures 1 and 2, the combustion apparatus 2 has a combustion chamber 4, and a flue 6 located above the chamber 4. The chamber 4 has a distributor plate 8 on which a bed of particles of non-combustible granular material is supported. The grate 8 is typically but not necessarily horizonal. Beneath it is a chamber 12 in which terminate pipes 1 4 connected to a blower 1 6 operable to supply air to the chamber 12 and thus fluidise the bed 10 of granular material. The distributor plate 8 has a plurality of apertures evenly distributed over the surface area of the plate so as to facilitate uniform fluidisation of the bed 10.

Located above the left-hand end of the plate 8 (as shown) is a screw feeder 1 8 whose outlet communicates with a chamber 4 and whose inlet communicates with a hopper 20. Another screw feeder 22 has an inlet communicating with the bottom of the chamber 4 at its right hand end (as shown). The outlet of the screw feeder 22 communicates with a hopper 26. If the combustible substance is liquid or gaseous it may be fed into the bed through suitable pipes or tuyeres (not shown).

Located in the flue 6 are boiler tubes 30 through which water may be passed. Located within the space occupied by the bed 10 are rows of equally spaced-apart nozzles 32. The nozzles face vertically upwards. As shown in Figure 2, each nozzle 32 is connected by a pipe 34 with a valve 36 in it to an oxygen main 38 connected to a source of oxygen (not shown) which may typically be a vacuum insulated vessel containing liquid oxygen fitted with a vaporiser for vaporising the oxygen. Located near to each nozzle 32 is the inlet of a pipe 42 through which, in operation of the fluidised bed combustion apparatus, gas can be conducted out of the bed 10 to enable its temperature to be sensed and, if desired, recorded and its composition analysed.If desired, the temperature sensor (not shown) associated with each such pipe 42 may be used to generate signals to control the operation of the valve 36 associated with the adjacent nozzle 32, the arrangement being such that the valve 36 is open and hence oxygen supplied only when the sensed temperature is below a chosen value.

Extending through the walls of the combustion apparatus 2 into the combustion chamber 4 are propane burners 50 and 52.

In operation, the granular non-combustible material is fed into the combustion chamber 4 through screw feeder 1 8 and is fluidised by air drawn into the apparatus by operation of the blower 1 6. This air uniformly fluidises the granular material and thus establishes a fluidised bed. The propane burners 50 and 52 are then actuated so as to raise the temperature of the bed to a temperature above the ignition temperature of the combustible solid to be burned (typically above 7500). In acquiring this temperature, the valves 36 will be open and oxygen supplied to the nozzles 32.The additional oxygen so supplied will accelerate the achievement of the ignition temperature. (Although only one row of nozzles 32 is shown in Figure 1 of the drawings, there may in fact be several such rows, each nozzle being associated with its own oxygen-control valve 36.) The spacing between the nozzles is such that conditions similar to those that would obtain if the air were enriched in oxygen immediately downstream of the 6 lower 1 6 are achieved. Once the ignition temperature of above 7500C has been acquired the temperature sensors (not shown) associated with the pipes 42 will generate signals closing the valves 36. At the same time solid combustible material is fed into the fluidised bed from the screw feeder 1 8.The propane burners 50 and 52 may then be turned off if the fuel undergoes a self-sustaining or autogenous combustion reaction at the prevailing temperature. Since the combustion of the fuel is exothermic the temperature of the bed will typically be maintained at above the ignition temperature without any addition of oxygen through the nozzles 32. It may be, however, that in one vicinity of the bed there is a localised reduction in temperature. If this does happen, it would be sensed by a temperature sensor associated with the pipe 42 in that vicinity and oxygen introduced into the bed through the nozzle 32 associated with that pipe. In consequence the temperature will be raised in the section of the bed above that nozzle 32. Once the temperature has reached a chosen value the valve 36 will close.

Material for burning may be fed continuously into the chamber 4. Ash formed as a result of the combustion tends to fall to the bottom of the bed onto the distributor pipe 10 and will gradually move downwards under gravity to the inlet of the screwfeeder 22 which is operated to convey the ash away into the hopper 26. Depending on the composition of the solid waste material to be burned, the ash may have a composition which renders it suitable for use as fertiliser, road aggregate or other industrial or agricultural product. The ash is typically not recycled to the bed.

The hot gases formed as a result of combustion will rise from the fluidised bed 4 and pass into the flue 6. These gases will heat the pipes 30 and boiling water passes therethrough and thus raising steam. If desired, some boiler pipes or tubes may be located within the bed itself. The necessary heat for boiling water and thus raising steam is thus transferred to the tubes by means of fluidised particles of non-combustible material as well as by the hot gas.

Typically, the gases exhausted from the flue 6 may be passed through a cyclone to remove any solids contained therein and a scrubber to remove undesirable acidic gases such as sulphur dioxide.

The cyclone and the scrubber are not shown in Figures 1 and 2.

Referring to Figures 3 and 4 of the drawing, a combustion apparatus 102 has a combustion chamber 104 and a flue 106 located above the chamber 104. The chamber 104 has a distributor plate 108 on which a bed 110 of particles of noncombustible granular material is supported. The plate 108 is typically (but not necessarily) horizontal. Beneath the plate 108 is a chamber 11 2 divided into four cuboid "sections" 11 5 by three equally spaced apart vertical partitions 11 3.

(If desired a smaller or larger number of such sections may be employed). In each section of the chamber 112 terminates a pipe 114. The pipes 114 are connected to a main 117 in which a blower 1 16 is disposed. The blower 1 16 is operable to supply air to each section of the chamber 112 and thus fluidise the bed 110 of granular material.

Located above the left-hand end of the plate 108 is a screw-feeder 11 8 whose outlet communicates with a chamber 104 and whose inlet communicates with a hopper 120. Another screw feeder 122 has an inlet communicating with a bottom of the chamber 104 at the right hand end (as shown) of the plate 108. The outlet of the screw feeder 1 22 communicates with a hopper 126. Located in the flue 106 are boiler tubes 1 30 through which water may be passed.

As shown in Figure 4, in communication with each pipe 114 is a gas supply pipe 132. The gas supply pipeline 132 is connected by a conduit 134 to an oxygen supply pipeline 136 and a nitrogen supply pipeline 1 38. In the oxygen supply pipeline 136 are disposed an automatically operable on-off valve 140, a flow meter 142 downstream of the valve 140, and a manually operable flow control valve 144 intermediate the on-off valve 140 and flow meter 142. In the nitrogen supply pipeline 138 are disposed an on-off valve 146, a flow meter 148 downstream of the valve 146 and a manually operable flow control valve 1 50 between the valve 146 and the flow meter 148.

The oxygen pipeline 136 is connectible to a source (not shown) of oxygen and the nitrogen pipeline 138 is connectible to a source (not shown) of nitrogen. In addition to the pipeline 1 32 and its associated oxygen and nitrogen pipelines, there is also provided a conduit 1 52 which has its inlet and outlet communicating with the pipeline 114 and a flow meter 1 54 disposed therein for the purpose of measuring the airflow rate. Although not shown in Figures 3 and 4, each of the four pipes 114 has associated therewith such means for supplying oxygen and nitrogen thereto as well as a flow meter for the air passing therethrough.

Extending through the walls of the combustion apparatus 102 into the combustion chamber 104 are propane burners 170 and 172.

In operation, the granular non-combustible material is fed into the combustion chamber 104 through screw feeder 11 8 and is fluidised by air drawn into the apparatus by operation of the blower 11 6. This air fluidises the granular material and thus establishes a fluidised bed. The propane burners 1 70 and 1 72 are then actuated so as to raise the temperature of the bed to a temperature above the ignition temperature of the combustible solid to be burned (typically above 7500C). In order to accelerate the achievement of the ignition temperature the air passing through each one of the sections 113 is enriched in oxygen supplied from the oxygen pipeline 1 36. Typically, the air may be enriched in oxygen by up to 5% by volume or more.Once the ignition temperature has been achieved, the supply of oxygen through the pipeline 136 may be stopped and solid combustible material for burning is fed into the fluidised bed from the screw feeder 11 8. The propane burners 170 and 1 72 are also turned off if the solid material is capable of undergoing selfsustaining or autogenous combustion reaction at the prevailing bed temperature. In general, it will be possible to maintain the bed at the required temperature by virtue of the heat evolved during the exothermic reaction without having to burn auxiliary fuel such as propane.

If, for example, it is found that the temperature in any part of the bed has fallen below a desired value (e.g. the temperature may be monitored at selected locations for example just above the distributor plate in each section of the bed) then the air supplied to that section (through the appropriate section 11 3) may be enriched in oxgyen without enriching the air supplied to the other sections 113. This helps to raise the temperature within the chosen section 113.

If it is desired to reduce the thermal output of the combustion apparatus the rate of feeding solid combustible material to the bed may be reduced and the rate of supplying fluidising air correspondingly reduced by closing one of the pipes 114 to the blower 11 6. In order to prevent the fluidisation of the bed from being disrupted by virtue of this closure of one of the pipes 114, an equivalent amount of nitrogen may be supplied to the pipe from the nitrogen supply pipeline 138.

Once it is desired again to increase the rate of output of heat from the combustion apparatus the rate of feeding of combustible material to the bed may be increased and air substituted again for the nitrogen supply.

If, for example, in the operation of the apparatus it is required to shut it down in an emergency, this may be done by stopping operation of the blower 11 6 and supplying nitrogen to all four pipelines 114. This therefore enables the bed to remain fluidised while it cools down.

In normal operation of the combustion apparatus 102, the solid material for burning may be fed continuously into the chamber 104. Ash formed as a result of the combustion tends to fall to the bottom of the bed onto the plate 108 and will gradually be removed by the screw feeder 1 22 which is operated to convey the ash into the hopper 126. Depending on the composition of the solid waste material to be burned, the ash may have a composition which renders it suitable for use as a fertiliser, road aggregate or other industrial or agricultural product. The ash is typically not recycled to the bed.

The hot gas is formed as a result of the combustion will rise from the fluidised bed 104 and pass into the flue 106. These gases will heat the pipes 1 30 and boiling water passes therethrough thus raising steam. If desired, some of the boiler pipes or tubes may be located within the fluidised bed itself. Heat is thus transferred from the particles within the bed to the tubes as well as from the hot gases.

Typically, the gases exhausted from the flue 106 may be passed through a cyclone to remove any solids contained therein and a scrubber to remove undesirable acidic gases such as sulphur dioxide. This cyclone and the scrubber are not shown in Figures 3 and 4.

Claims (14)

1. A method of burning a combustible substance comprising the steps of establishing a bed of non-combustible particulate or noncombustible granular material, fluidising the bed with air, raising the bed to a combustion temperature, discharging ash from the bed and locally enriching in oxygen the air supplied to any part of the bed in which there is inadequate combustion (as hereinbefore defined) of the said substance so as to obtain improved combustion.

2. A method as claimed in claim 1, in which the gas for locally enriching the air in oxygen is introduced below a distributor plate (on which the bed is supported when not fluidised) but at a position in which it does not become generally mixed with the fluidising air.

3. A method as claimed in claim 1, in which the gas for locally enriching the air in oxygen is introduced directly into the bed.

4. A method as claimed in claim 3, in which said gas has a component of velocity parallel to the general flow of air through the bed.

5. A method as claimed in claim 3 or claim 4, in which said gas has a component of velocity normal to the general flow of air through the bed.

6. A method as claimed in any one of the preceding claims, in which the gas for enriching the air in oxygen is introduced to a chosen location when a temperature sensor indicates that the temperature is below a chosen value.

7. A method as claimed in any one of the preceding claims, in which the air is generally enriched in oxygen during the period in which the bed is raised to a combustion temperature.

8. A method as claimed in claim 1, in which the air is generally enriched in oxygen and the rate of supplying fuel to the bed is increased.

9. A method as claimed in any one of the preceding claims, in which oxygen or gas whose oxygen concentration is greater than 22% by volume is introduced into the gas leaving the top of the bed, and the so formed gas mixture burned.

10. A method as claimed in any one of the preceding claims, in which part or all of the bed is 'shut-down' (i.e. combustion stopped) by substituting nitrogen for air.

11. Apparatus for burning a combustible substance in a fluidised bed comprising a combustion chamber bounded at its lower end by a distributor plate able to support a bed of noncombustible particulate or non-combustible granular material, means for supplying an upward flow of air through the distributor plate or as to fluidise the bed, and a plurality of spaced-apart gas supply pipes, each being adapted to supply to a chosen region of the bed oxygen or gas mixture whose concentration of oxygen is greater than that in air so as to enrich locally in oxygen the air supplied to any part of the bed in which there is inadequate combustion (as hereinbefore defined) of the said substance.

1 2. A method of burning a combustible substance substantially as herein described with reference to Figures 1 and 2 or Figures 3 and 4 of the accompanying drawings.

1 3. Apparatus for burning a combustible substance substantially as herein described with reference to, and as shown in, Figures 1 and 2, of Figures 3 and 4 of the accompanying drawings.

14. A method burning a combustible substance including any one of or any combination of the steps (or techniques) described herein.

1 5. Apparatus for burning a combustible substance including any one of or any combination of the features disclosed herein.