GB1586183A

GB1586183A - Fluidised bed system

Info

Publication number: GB1586183A
Application number: GB1134677A
Authority: GB
Original assignee: Coal Industry Patents Ltd
Current assignee: Coal Industry Patents Ltd
Priority date: 1978-03-03
Filing date: 1978-03-03
Publication date: 1981-03-18

Description

(54) FLUIDISED BED SYSTEM (71) We COAL INDUSTRY (PATENTS) LIMITED, a company organised in accordance with the laws of Great Britain of Hobart House, Grosvenor Place, London, S.W.1X 7AE England, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a fluidised bed system which is used to produce gases substantially free from contaminants.

It is envisaged that in a power plant solid fuel, e.g. coal, will be gasified to produce a combustible fuel gas, which will then be burned in a combustion stage, whence the gaseous products of combustion will be passed to a gas turbine for expansion.

The fuel gas may be produced either in a fixed, fluidised bed or entrained flow system, for instance using a water gas/producer gas-type treatment. The fuel gas normally contains proportions of carbon oxides, hydrogen, steam, methane and nitrogen and preferably contains substantial proportions of carbon monoxide and hydrogen. However the fuel gas also entrains during its production contaminants, which may be solids or liquids. The contaminants may be combustible, for instance solid fuel which has not been gasified, coke particles, tar, and hydrocarbons, or may be incombustible, for instance fly ash or alkali-metal compounds. The liquid contaminants are usually present as aerosols. Most of the solid contaminants may be removed by a cyclonetype separator. However a proportion of the contaminants reach the burning stage, wherein a further proportion is removed by being combusted.Nonetheless, a significant proportion of the contaminants remains in the gas stream and is introduced into the gas turbine along with the combustion product gases. This significant proportion of contaminants, especially if it contains ash particles or alkali-metal compounds, can cause great damage to the gas turbine by fouling, corrosion and erosion, particularly of the blading.

It is therefore an object of the present invention to provide a method whereby combustion product gases containing substantially no contaminants may be produced from a contaminated fuel gas.

According to the present invention a method of combusting a contaminated fuel gas to produce a substantially contaminantfree product gas comprises fluidising a bed of non-sticky, non-degradable particles with the fuel gas, introducing into the fluidised bed an oxygen-containing gas the bed being maintained by the combustion of the fuel gas, at a temperature whereat solid contaminants are sticky and adhere to the particles of the bed, and withdrawing a product gas from above the bed.'referred to in bed.

Any of the gases referred to in this specification may be either a single component gas or a mixture of at least two components.

Conveniently the fuel gas is obtained from a coal gasifier, which may for instance comprise one or a series of fluidised beds in which the coal is heated and gasified in the presence of steam and/or air or an oxygen containing gas, whereby a fuel gas comprising as combustible components predominantly a mixture of hydrogen and carbon monoxide is produced. Methods by which this may be carried out are well known in the art.

Preferably the fuel gas, before it is burned, is passed through one or a series of cyclone separators, wherein larger particulate contaminants are removed, the exhausting fuel gas containing incombustible particulate contaminants at a level of about lg/m3 The exhausting gas will in addition contain combustible particulate contaminants, aerosols of liquid contaminants and gaseous contaminants.

The bed of particles to be fluidised is preferably supported on a perforated plate of conventional type, and the fuel gas is introduced through the plate to fluidise the particles, the pressure of the gas preferably being superatmospheric. The fluidising system should be designed to prevent or minimise fouling which could thereby prevent satis factory bed fluidisation.

Most preferably the fluidised bed is of the rotating type, for instance as described in our co-pending application No. 21318/77 (Serial No. 1 581 672). In such a bed the gas can be burned efficiently, and particle carry over will be reduced compared to stationary beds. Also it is easier to remove oversize and agglomerated particles.

The oxygen-containing gas may be pure oxygen, or air, or a mixture of the two in any proportions. For a very inflammable fuel gas air is usually able to support combustion of the fuel gas. However for less inflammable fuel gases it is usually necessary to mix oxygen with the air. This may be accomplished before the mixture is introduced into the bed, or may take place in the bed.

When the bed is originally fluidised it will be necessary to ignite the mixture in the bed, but thereafter the combustion will be self-sustaining.

Conveniently the bed is maintained at a temperature of from 800 to 12000C, and preferably at a temperature of from 900 to 1 1000C. At this temperature substantiall all the solid contaminants in the fuel gas will have at least been partially softened, and will therefore be sticky. At best the combustible contaminants will be burned as soon as they enter the bed. Generally the gasified liquids and liquids in aerosol form such as light tars are sufficiently combustible that they do not have a chance to adhere to the particles in bed, since they are burned as soon as they enter the bed.

However some of the heavier aerosol liquids such as heavy tars are not burned immediately and become adhered to the bed particles. Similarly some of the fine particulate combustible contaminants, e.g. coal and coke particles, are burned immediately they enter the bed. However some of these coal and coke particles become adhered to the bed particles, through the agency of liquids already deposited on the bed particles. Substantially all the combustible contaminants are eventually burned, although a proportion of them is burned while adhered to the bed particles.

In the case of the incombustible contaminants, these are rendered sticky by the heat of the bed, either by being liquified or by being softened. Once liquified or softened they are able to adhere to the bed particles, thus rendering the bed particles sticky, and making them larger. The bed particles to which incombustible contaminants have adhered are themselves rendered sticky and so may agglomerate. To control the amount of agglomeration, and to remove the incombustible contaminants from the bed a proportion of the bed may be removed, either continuously or intermittently. The amount of the bed that is removed may be such that the size of the bed remains substantially constant, but preferably a weight of particles greater than the weight added to the bed by the contaminants is removed from the bed. The bed is then topped up with fresh uncontaminated particles.The fresh particles may be cleaned up particles which have already been through the bed, or may be entirely fresh particles.

In this way incombustible particulate and liquid contaminants and combustible contaminants are removed from the fuel gas.

The only contaminants which may still be retained in the product gas are gaseous incombustible contaminants, such as sulphur dioxide. However, gaseous contaminants may be substantially removed from the fuel gas by the bed if the particles of the bed are absorbent. For instance particles of or containing iron oxides, limestone or dolomite will absorb sulphur dioxide.

The non sticky, non degradable particles must all be of such a size that, under the fluidising conditions used, substantially none of them may leave the bed by entrainment in the product gas. They should therefore have a minimum particle size of about 100 jtm, but preferably about 500 clam, although this size will vary with the fluidising conditions used. The particles when not having contaminants adhered to them must have no substantial tendency to agglomerate. In addition the particles should not be of an easily abradable character. Preferred materials from which the particles may be made are various washed sands and crushed refractory.

Using the method of the present invention it is possible to remove from the fuel gas at least 90% of the contaminants, and in many cases up to 99% of the contaminants may be removed, to give a produce gas containing substantially only oxides of carbon, nitrogen and water vapour.

Although it is envisaged that the present invention will be particularly applicable to the combustion of coal-derived fuel gases, it is not limited to such applications, and may be used to burn for instance natural gas or a fuel gas derived from crude oil or peat, in which there are undesirable contaminants.

The invention will now be described, by way of example only, with reference to the accompanying drawing, in which there is shown schematically part of a coal processing plant, including an apparatus in which coal-derived fuel gas is burned using a method according to the present invention.

Referring now to the drawing, a part of a coal processing plant comprises a fluidised bed gasifier 1, a cyclone separator 3, a fuel gas burner 5 and a gas turbine 7. The fuel gas burner 5 comprises a housing 9 having a particle inlet 10 and outlet 12, a perforated plate 13, a gas inlet 14 and a gas outlet 15.

Located on the perforated plate 13 is a bed 16 of sand particles having a minimum size of 500 ,am. The particle outlet 12 leads to a particle treatment stage 19.

In use coal is fed to the gasifier 1 operating at a pressure of 15 bar and is fluidised by passing air and steam into the apparatus 1.

The apparatus 1 may comprise two or more fluidised beds operated in out-of-phase cycles. The apparatus produces a fuel gas having a calorific value of 4.0 MJ/m3 the combustible constituents of which comprise mainly carbon monoxide and hydrogen, and the manner of its production is well known in the art and needs no further explanation.

The fuel gas has entrained in it ash, carbon particles and tar and hydrocarbons as an aerosol. The fuel gas is then passed through the cyclone separator 3 wherein the larger of the entrained particulate contaminants are removed. The fuel gas that exits from the cyclone separator 3 still contains some particulate matter, but no more than about lg/m3 nin total.

The fuel gas then is passed into the burner 5 via gas inlet 14. At this stage oxygen or air is mixed with the fuel gas and passed into the burner 5. The fuel gas and air are introduced through the perforated plate 13 at a rate of 1.4 and 4.6 m,/s per square meter of the perforated plate 9 respectively. In this case the area of the plate 9 is one square meter. This fluidises the bed 16 of particles, imparting a fluidising velocity of 2.0 m/s to the bed. The bed has a depth of 0.25m and has a weight of 250 kg. In a continuous process the fuel gas is ignited by the heat of the bed, which is maintained at a temperature of 1 1000C by the combustion of the fuel gas.

While the fuel gas is being burned in burner 5 the following events occur. Combustible contaminants, such as coal and coke dust, tar and vapourised hydrocarbons are substantially completely burned, either while flowing freely in the bed or while adhered to the bed particles. The combustible contaminants adhere to the bed either due to their inherent stickiness, for instance, in the case of tars, or through the agency of a sticky material already adhered to the bed particles. The temperature at which the bed is maintained is a temperature at which the particulate incombustible contaminants e.g.

free ash, are either fused or at least softened, and these contaminants therefore become sticky and adhere to the bed particles. The coal or coke dust which is burned may also comprise a proportion of ash, and this will suffer the same fate as the free ash as soon as the carbonaceous material has been burned. Once the incombustible contaminants are adhered to or absorbed by the bed particles, the contaminants are substantially removed from the fuel gas, and are eventually removed from the bed as described hereinafter.

Product gas, comprising mainly steam, carbon dioxide and nitrogen is removed from above the bed 16 at a rate of 5.8 m3ds and is used to drive the gas turbine 7. The product contains less than 0.01 g/m3 n of contaminants, and therefore 96% of the contaminants have been removed by the burning process, which is operated according to the present invention.

Since approximately 1 g of contaminants has been removed, mainly by adherence to the particles of the bed 16, from every cubic meter (normal) of fuel gas passed into the burner, the weight of the bed will be increasing by approximately 5.1 kg. in every hour, (i.e. 2% of the bed per hour). Therefore a small proportion of the particles is removed from the bed through particle outlet 12.

However usually 5.0% of the bed is removed per hour and this portion is passed through the particle treatment stage 19, wherein the particles are processed to make them suitable to be recycled to the bed through particle inlet 10. By controlling the amounts of particles that are removed and replaced it is possible to maintain the bed at a constant size and having constant properties with regard to both the fluidisation and contaminant removal.

Thus by use of the present invention it is possible to burn contaminated fuel gas to give a substantially uncontaminated product gas which may be used to drive a gas turbine.

WHAT WE CLAIM IS:- 1. A method of combusting a contaminated fuel gas to produce a substantially contaminant free product gas, comprising fluidising a bed of non-sticky, nondegradable particles with the fuel gas, introducing into the fluidised bed an oxygen containing gas, the bed being maintained by the combustion of the fuel gas at a temperature whereat solid contaminants are sticky and adhere to the particles of the bed, and withdrawing a product gas from above the bed.

2. A method according to claim 1, in which the fuel gas is obtained from a coal gasifier.

3. A method according to claim 1 or 2, in which the fuel gas, before it is burned, is passed through one or a series of cyclone separators.

4. A method according to any one of the preceding claims, in which the pressure of the gas in the fluidised bed is superatmospheric.

5. A method according to any one of the preceding claims, in which the fluidised bed is of the rotating type.

6. A method according to any one of the preceding claims, in which the bed is maintained at a temperature of from 800 to

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. plate 13, a gas inlet 14 and a gas outlet 15. Located on the perforated plate 13 is a bed 16 of sand particles having a minimum size of 500 ,am. The particle outlet 12 leads to a particle treatment stage 19. In use coal is fed to the gasifier 1 operating at a pressure of 15 bar and is fluidised by passing air and steam into the apparatus 1. The apparatus 1 may comprise two or more fluidised beds operated in out-of-phase cycles. The apparatus produces a fuel gas having a calorific value of 4.0 MJ/m3 the combustible constituents of which comprise mainly carbon monoxide and hydrogen, and the manner of its production is well known in the art and needs no further explanation. The fuel gas has entrained in it ash, carbon particles and tar and hydrocarbons as an aerosol. The fuel gas is then passed through the cyclone separator 3 wherein the larger of the entrained particulate contaminants are removed. The fuel gas that exits from the cyclone separator 3 still contains some particulate matter, but no more than about lg/m3 nin total. The fuel gas then is passed into the burner 5 via gas inlet 14. At this stage oxygen or air is mixed with the fuel gas and passed into the burner 5. The fuel gas and air are introduced through the perforated plate 13 at a rate of 1.4 and 4.6 m,/s per square meter of the perforated plate 9 respectively. In this case the area of the plate 9 is one square meter. This fluidises the bed 16 of particles, imparting a fluidising velocity of 2.0 m/s to the bed. The bed has a depth of 0.25m and has a weight of 250 kg. In a continuous process the fuel gas is ignited by the heat of the bed, which is maintained at a temperature of 1 1000C by the combustion of the fuel gas. While the fuel gas is being burned in burner 5 the following events occur. Combustible contaminants, such as coal and coke dust, tar and vapourised hydrocarbons are substantially completely burned, either while flowing freely in the bed or while adhered to the bed particles. The combustible contaminants adhere to the bed either due to their inherent stickiness, for instance, in the case of tars, or through the agency of a sticky material already adhered to the bed particles. The temperature at which the bed is maintained is a temperature at which the particulate incombustible contaminants e.g. free ash, are either fused or at least softened, and these contaminants therefore become sticky and adhere to the bed particles. The coal or coke dust which is burned may also comprise a proportion of ash, and this will suffer the same fate as the free ash as soon as the carbonaceous material has been burned. Once the incombustible contaminants are adhered to or absorbed by the bed particles, the contaminants are substantially removed from the fuel gas, and are eventually removed from the bed as described hereinafter. Product gas, comprising mainly steam, carbon dioxide and nitrogen is removed from above the bed 16 at a rate of 5.8 m3ds and is used to drive the gas turbine 7. The product contains less than 0.01 g/m3 n of contaminants, and therefore 96% of the contaminants have been removed by the burning process, which is operated according to the present invention. Since approximately 1 g of contaminants has been removed, mainly by adherence to the particles of the bed 16, from every cubic meter (normal) of fuel gas passed into the burner, the weight of the bed will be increasing by approximately 5.1 kg. in every hour, (i.e. 2% of the bed per hour). Therefore a small proportion of the particles is removed from the bed through particle outlet 12. However usually 5.0% of the bed is removed per hour and this portion is passed through the particle treatment stage 19, wherein the particles are processed to make them suitable to be recycled to the bed through particle inlet 10. By controlling the amounts of particles that are removed and replaced it is possible to maintain the bed at a constant size and having constant properties with regard to both the fluidisation and contaminant removal. Thus by use of the present invention it is possible to burn contaminated fuel gas to give a substantially uncontaminated product gas which may be used to drive a gas turbine. WHAT WE CLAIM IS:-

1. A method of combusting a contaminated fuel gas to produce a substantially contaminant free product gas, comprising fluidising a bed of non-sticky, nondegradable particles with the fuel gas, introducing into the fluidised bed an oxygen containing gas, the bed being maintained by the combustion of the fuel gas at a temperature whereat solid contaminants are sticky and adhere to the particles of the bed, and withdrawing a product gas from above the bed.

1200"C.

7. A method according to claim 6, in which the bed is maintained at a temperature of from 900 to 11 000C.

8. A method according to any one of the preceding claims, in which a proportion of the bed is removed, either continuously or intermittently.

9. A method according to claim 8, in which the weight of particles removed is greater than the weight added to the bed by the contaminant, and the bed is replenished with uncontaminated particles.

10. A method according to any one of the preceding claims, in which the particles of the bed are absorbent for a gaseous contaminant.

11. A method according to claim 10, in which the bed particles comprise an iron oxide, limestone or dolomite.

12. A method according to any one of the preceding claims, in which the minimum bed particle size is 100 ,am.

13. A method according to claim 12, in which the minimum bed particle size is 500 ,am.

14. A method of combusting a contaminated fuel gas to produce a substantially contaminant free product gas, substantially as hereinbefore described with reference to the accompanying drawing.

15. A product gas when produced according to a method as claimed in any one of the preceding claims.