GB2046415A

GB2046415A - Fluidised bed combustion method

Info

Publication number: GB2046415A
Application number: GB8005789A
Authority: GB
Original assignee: BOC Ltd
Current assignee: BOC Ltd
Priority date: 1979-02-23
Filing date: 1980-02-20
Publication date: 1980-11-12
Also published as: GB2046415B

Abstract

In order to improve the fluidised bed combustion of combustible solids, particularly those of low calorific value, oxygen is mixed with fluidising air supplied to a bed 10 of granular material via pipes 14. The temperature of the bed is monitored by a temperature sensor 38. The flow of oxygen is adjusted as necessary by valve 36 is response to changes in the sensed temperature so as to keep the temperature of the bed between chosen values. Addition of the oxygen to the fluidising air helps to keep down the amount of carbon carried out of the bed by the combustion gases. Solid to be burnt is fed into the bed from a screw feeder 18. Propane or methane is burnt by burners 40 and 42 to raise the temperature of the bed to a suitable operating value. Hot combustion gases ascend from the bed 10 into a flue 6 and boil water passed through boiler tubes 30. Ash is discharged from the bottom of the bed 10 by operation of a screw feeder 22. <IMAGE>

Description

SPECIFICATION Combustion method This invention relates to a method of combustion. It particularly relates to the combustion of solids in a fluidised bed.

The combustion of solids 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 solid to be burnt 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).

It is often necessary to have associated with the fluidised bed equipment for extraction of solid particles, such as fly ash, which become entrained in the hot combustion gases.

The bed may for example be raised to the chosen temperature, before the solid, liquid or gas to be burnt is supplied, by introducing an auxiliary fuel into the bed and burning it. Alternatively, burners of an auxiliary fuel may be mounted above the bed and hot combustion products directed from the burners into the bed. The auxiliary fuel may typically be a fuel gas such as propane or natural gas.

Once the desired temperature has been attained in the bed, the supply of the auxiliary fuel to the burners or the bed, as the case may be, can be discontinued provided the solid to be burnt can undergo a self-sustaining combustion reaction at that temperature.

There are however a number of problems associated with the operation of such fluidised bed combustion equipment. First, a number of materials, generally of low calorific value, such as oil shale, peat, refinery residue and coal tailings, are not autogenous at suitable operating temperatures for the fluidised bed combustion equipment, that is they are not capable of undergoing a self-sustaining combustion reaction with air at such temperatures. It is thus necessary to supply auxiliary fuel to the bed throughout most if not all the time the bed is operated. For many materials, this practice is uneconomic. Moreover, it is found impossible to burn some materials by this method as the auxiliary fuel reacts with oxygen from the air that is needed to support combustion of the material to be burnt.

Another problem that may be encountered is that combustible carbonaceous material tends to be carried out of the bed by the hot combustion gases and the fluisiding air.

In some instances it may be found that the chamber containing the bed needs to be of undesirably large size, or that the rate of combustion in the bed is undesirably low.

A further difficulty that may arise is for combustion of the main or auxiliary fuel to be burnt or of the products of combustion of such fuel to occur above the bed, particularly before the temperature of the bed has been raised to the chosen operating value, or before combustion is fully stabilised.

It has been proposed to use air or oxygen in a fluidised bed gasification method, in which a solid fuel is converted to a gas containing substantial proportions of methane and hydrogen. The process according to this invention is not concerned with gasification, but is instead concerned with combustion reaction in which the carbon is oxidised to oxides of carbon without substantial proportions of methane being formed as part of the gases formed as a result of the combustion reaction.

It is an aim of the present invention to provide a method of burning combustible solid which keeps down the amount of carbon that is carried out of the bed with the combustion gases.

According to the present invention there is provided a method of burning combustible solid comprising the steps of establishing a bed of noncombustible particulate or non-combustibie granular material, fluidising the bed with air, raising the bed to a combustion temperature, feeding the aforesaid solid to the bed, discharging ash from the bed allowing combustion gases to pass out of the bed without recycling them to the bed and without causing them to flow through any other fluidised bed in which combustion is taking place, enriching the air in oxygen so as to facilitate the combustion of the aforesaid solid and to keep down the amount of carbon carried out of the bed with the combustion gases, monitoring the temperature of the bed, and maintaining the temperature of the bed at a chosen value or between values by adjusting, as necessary, the flow rate of the gas or gases used to enrich the air in oxygen.

Increasing the concentration of oxygen in the air used to fluidise the bed of granular of particulate material helps to increase the rate at which thermal energy is generated during the burning of the liquid or solid material. It will thus be generally unnecessary to supply fuel gas (or any other combustible material than the chosen solid to be burned) of higher calorific value than the solid to be burned once the temperature of the bed has been raised to a suitable value. Thus, propane or other fuel gas (for example natural gas) is preferably burned to raise the temperature of the bed to a chosen value, but the supply of such fuel is preferably stopped once the chosen temperature has been achieved, and the solid is being supplied to the fluidised bed.

The method according to the invention makes possible the autogenous combustion of materials, typically of low calorific value, which it has not been possible to burn in a fluidised bed without the supply of another combustible material of high calorific value, for example, propane or other fuel gas throughout the combustion period. Examples of such combustible materials of low calorific value are low grade fuels, typically having a calorific value of less than 8 or 10 MJ/Kg. Examples of such solid fuels include oil shale peat, refinery residues and colliery tailings, coke breeze, solid waste from the timber, paper, rubber, food and textile industries, municipal waste, pitch, brown coals and lignites.It is also possible to use the method according to the invention to burn organic refuse and sludges containing combustible solids (eg. sewage sludge.) All these materials can be burned with the supply of additional fuel only during the period in which the bed is raised to its operating temperature. In some instances it will be desirable to continue to supply auxiliary fuel to the fluidised bed after the bed has reached its operating temperature so as to maximise the rate of combustion therein.

If desired, the concentration of oxygen in the air used to fluidise the bed may be increased (by the addition of commercially pure oxygen or oxygenenriched airtothefluidising air) during the period when the bed is raised to the chosen operating temperature. This will help to reduce the duration of such period: particularly if the operating temperature of the bed is established by introducing fuel gas such as propane into the bed and then burning it.

The solid to be burnt may be fed into the combustion apparatus above the bed and allowed to fall under gravity onto the top of the bed.

Typically, the air may be enriched in oxygen by mixing the air with commercially pure oxygen or oxygen-enriched air upstream of the fluidised bed.

Alternatively, commercially pure oxygen, or oxygenenriched air, may be introduced directly into the fluidised bed at one or more locations.

Initially, aggregates of stone or other noncombustible material may be used to form the bed.

In operation, ash will be formed as the solid burns.

The ash isfluidised by the air. It is typically continuously removed from the bed. Generally, the aggregates will be removed as well, so in time the ash takes the place of the aggregates and the non-combustible material. From time to time, additions of, for example, dolomite or limestone may be made to the bed so as to remove sulphur. Preferably the enrichment is performed so as to increase the volume of oxygen in the air up to 25% by volume.

The rate at which fluidising air is supplied may be so reduced that the total flow of oxygen into the fluidised bed remains unaltered from when the plant is operated without oxygen-enrichment of the fluidising air but the total flow of gas is reduced.

Preferably, the oxygen-enrichment of the air is controlled automatically. One or more temperature sensors are preferably located within the fluidised bed. The or each such temperature sensor is preferably associated with at least one automatic valve in a pipeline through which the oxygen for enrichment of the fluidising air is supplied, the arrangement being such that the operating temperature of the fluidised bed is maintained at a chosen value or within a chosen temperature range, typically at or above the temperature at which the combustion of the solid or liquid material, or both, is autogenous, but below the temperature at which fuses either ash generated by burning the solid or liquid material, or the particulate or granular non-combustible bed material. Typically, the bed may be operated at a temperature in the range 750-1250"C.

If desired, heat may be extracted from combustion gases leaving the bed, or from the bed itself or both.

Typically, such heat may be used to boil water and thus raise steam.

The method according to the invention makes it possible to keep to a minimum the amount of carbon that is carried out of the bed with the combustion gases. This is because the enrichment of the fluidising air in oxygen makes possible a faster rate of combustion and therefore completion of combustion within the bed itself. This can be done by reducing the velocity of the fluidising air. We therefore believe that enriching the fluidising bed in oxygen, particularly if this practice is started before the bed has been raised to its chosen operating temperature, makes possible nearly complete combustion of a solid fuel to be burnt.It is also possible to achieve a reduction in the amount of carbon carried out of the bed with the combustion gases by reducing the fluidising velocity but using the enrichment in oxygen to maintain the same stoichiometric ratio of oxygen to combustible material in the bed.

Reducing the fluidising flow rate reduces entrain mentoftheflyash and other extraneous matter in the gases leaving the bed.

The method according to the invention makes it possible to increase the amount of heat released per unit volume of fluidised bed over that released per unit volume of a conventional fluidised-bed combustion process. This makes it possible to keep to a minimum the volume of the fluidised bed combustion chamber of an apparatus custom-built to perform the method according to the invention. In consequence, the time required to warm-up the bed to the operating temperature may be reduced.

Further reductions in this time may be achieved, as aforementioned, if the oxygen-enrichment is initiated at the start of or during the warm-up period instead of at the end of the warm-up period when the supply of propane or other fuel gas is stopped.

It is not necessary to built to custom an incinerator or boiler or like plantforthe method according to the invention. It is possible to adapt existing fluidised bed combustion (or incineration) plant for this purpose. Adapting such plant to operate the method according to the invention makes it possible to reduce the rate of comsumption of fuel such as propane and to increase the rate at which solid or liquid material is fed into the fluidised bed for combustion. Moreover, by varying the degree of oxygen-enrichment of the fluidising air, which variation, if desired, may be effected automatically, differing rates of feeding combustible solid, liquid or gas into the bed may be catered for.

The method according to the present invention will now be described by way of example with reference to the accompanying drawing which shows schematically a fluidised bed combustion apparatus adapted to perform the method according to the invention.

Referring to the drawing, the combustion apparatus 2 has a combustion chamber4 and a flue 6 located above the chamber 4. The chamber 4 has a grate 8 on which a bed 10 of particles of noncombustible granular material is supported. The grate 8 is horizontal. Beneath it, is a chamber 12 in which terminate pipes 14 connected to a blower 16 operable to supply air to the chamber 12 and thus fluidise the bed 10 of granular material. Located above the left hand end of the grate 8 (as shown) is a screw feeder 18 whose outlet communicates with the 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 the right hand end (as shown) of the grate 8. The outlet of the screw feeder 22 communicates with a hopper 26. Located in the flue 6 are boiler tubes 30 through which water may be passed.

In communication with the pipes 14 downstream of the blower 16 is a source 32 of commercially pure oxygen. In the pipe 34 placing the source 32 of commercially pure oxygen in communication with the pipes 14 is an automatic valve 36. The valve 36 is operatively associated with a temperature sensor 38 in the fluidised bed.

Extending through the walls of the incinerator 2 into the combustion chamber 4 are propane burners 40and42.

In operation, the granular non-combustible material is fed into the combustion chamber 4 through screw feeder 18 and is fluidised by air drawn into the apparatus by operation of the blower 16. This air fluidisesthe granular material and thus establishes a fluidised bed. The propane burners 40 and 42 are then actuated so as to raise the temperature of the bed to a temperature above the ignition temperature of combustible solid to be burnt (typically above 750"C). When this temperature has been acquired, the temperature sensor 38 opens the valve 36 and thus allows oxygen to be mixed with the fluidising air to form oxygen-enriched air. This oxygenenriched air passes through the grate 8 and functions as the fluidising medium.At the same time as the valve 36 is opened solid combustible material with low calorific value (for burning) is fed into the fluidised bed from the screw feeder 18. At the same time as the supply of oxygen from the source 32 is initiated, so the propane burners 40 and 42 are automatically turned off. The rate at which oxygen is mixed with the fluidising air is predetermined so that the solid combustible material will burn spontaneously and in a self-sustaining or autogenous manner at the prevailing temperature. This combustion reaction is exothermic and thus the temperature of the bed is maintained. The flow of oxygen is proportioned in response to a signal from the temperature sensor either by a modulating control valve or an on-off control valve.For example, should the temperature of the bed rise to a chosen temperature, typically a temperature closed to that at which would fuse either the non-combustible bed material or the ash formed by the combustion of the solid material of low calorific value, the temperature sensor 38 will generate a signal which will cause the automatic valve 36 to close thereby shutting off the supply of oxygen. This will cause the bed tempera tureto drop until it reaches a chosen lower value at which the valve opens again. The programming of a valve to operate in such a manner is well known in the art and shall not be described in detail in this specification.

The solid 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 grate and will gradually move downwards under gravity to the inlet of the screw feeder 22 which is operated to convey the ash away into the hopper 26. Depending on the composition of the solid waste material to be burnt, the ash may have a composition which renders it suitable for use as a fertilizer, 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 the combustion will rise from the fluidised bed 4 and pass into the flue 6. These gases will heat the pipes 40 and boiling water passes therethrough thus raising steam. It desired, some boiler pipes or tubes may be located within the fluidised bed itself. Necessary heat for boiling water and thus raising steam is transferred to the tubes by means of the fluidised particles of non-combustible material.

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.

The enrichment ofthefluidising air in oxygen facilitates combustion of the solid to an extent that the amount of carbon carried out of the bed with the combustion gases is kept to a minimum.

Claims

1. A method of burning combustible solid comprising the steps of establishing a bed of noncombustible particulate or non-combustible granular material, fluidising the bed with air, raising the bed to a combustion temperature, feeding the aforesaid solid to the bed, discharging ash from the bed allowing combustion gases to pass out of the bed without recycling them to the bed and without causing them to flow through any other fluidised bed in which combustion is taking place, enriching the air in oxygen so as to facilitate the combustion of the aforesaid solid and to keep down the amount of carbon carried out of the bed with the combustion gases, monitoring the temperature of the bed and maintaining the temperature of the bed at a chosen value or between values by adjusting, as necessary, the flow rate of the gas or gases used to enrich the air in oxygen.

2. A method as claimed in claim 1, in which the solid has a calorific value below 10 MJ/kg.

3. A method as claimed in claim 1 or claim 2, in which methane or propane is burnt to raise the bed to a combustion temperature.

4. A method as claimed in claim 3, in which burning of the methane or propane is stopped once the chosen combustion temperature has been achieved, and the solid, liquid or gas is being supplied to the fluidised bed, the degree of oxygen enrichment of the air being sufficient to enable autogenous combustion of the aforesaid solid, liquid, or gas to take place.

5. A method as claimed in claim 3, in which the burning of the methane or propane is continued after the attainment of the chosen combustion temperature.

6. A method as claimed in any one of the preceding claims, in which the concentration of oxygen in the air used to fluidise the bed is increased during the period in which the bed is raised to the chosen operating temperature.

7. A method as claimed in any one of the preceding claims, in which the air is enriched in oxygen by mixing the air with commercially pure oxygen or oxygen-enriched air upstream of the fluidised bed.

8. A method as claimed in any one of claims 1 to 6, in which commercially pure oxygen, or oxygenenriched air, is introduced directly into the fluidised bed at one or more locations so as to enrich the air in oxygen.

9. A method as claimed in any one of the preceding claims, in which the enrichment is performed so as to increase the volume of oxygen in the fluidising air to up to 25% by volume.

10. A method as claimed in any one of the preceding claims in which the oxygen-enrichment is performed so as to increase the total flow of oxygen into the fluidised bed is increased, but the total flow of fluidising gas (excluding any gas to be burnt) is left unaltered or reduced.

11. A method as claimed in any one of the preceding claims, in which one or more temperature sensors are located in the fluidised bed and are operatively associated with at least one automatic valve in a pipeline through which the oxygen for enrichment of the fluidising air is supplied, the arrangement being such that the operating temperature of the fluidised bed is maintained at a chosen value or within a chosen temperature range.

12. A method as claimed in any one of the preceding claims, in which heat is extracted from combustion gases leaving the bed or from the bed itself, the heat being used to raise steam.

13. A method as claimed in claim 1, substantially as herein described with reference to the accompanying drawing.