GB2064355A - Fluidised bed combustor - Google Patents

Abstract

A method of operating a fluidised bed combustor 1 comprises the steps of passing a pressurised oxygen- containing gas, for example air, through two vertically aligned beds 7 and 8 containing fuel material to promote fluidisation of the beds 7 and 8 and combustion of the fuel in the beds. The air also maintains the beds at a pre-determined temperature. The beds 7 and 8 are operated without a free-board between each bed. It is thus possible to achieve a rapid turn- down by slumping the lower bed by stopping the air supply thereto. A conventional fuel may be fired on one bed to help, with the aid of the deep beds which retain heat, the complete combustion of, for example, a particularly difficult waste fuel which is to be fired on the other bed. Heat and waste fuel can be transferred from one bed to the other bed. <IMAGE>

Description

SPECIFICATION Fluidised bed combustor This invention relates to fluidised bed combustors, and in particular to techniques for burning a wide range of solid, liquid and gaseous untreated or otherwise prepared waste material and conventional fuels, in a fluid bed combustor.

The combustor can be used for the purpose of heat recovery, incineration or hot gas generation.

Wastes can react endothermically or exothermically.

Conventional firing equipment for oil, gas and coal has been developed over many years.

However, future increases in the cost of conventional fossil fuels will ensure a growing interest in the heat value of residual wastes derived from industrial and commerciagresidential sources. Conventional firing equipment cannot handle wastes such as sludge without involving design compromises in the equipment. Research work in fluid bed combustion technology has shown for example, that this technique can successfully burn coal with a wide range of inert burdens and therefore may be applied to more general waste residues.

The benefits of fluid bed combustion are well documented in the literature. These include improved heat transfer rates, reduction in corrosion, reduction in corrosion, reduction in combustor/boiler fouling, and the capability of low temperature combustion. Low combustion temperatures of, for example, 600-1 0000C provide low thermal NOX formation and ash clinkering tendency. Also, sulphur may be retained in the inert phase of a fluidised bed. A particular advantage of the fluid bed combustor in the context of waste burning is that the combustion zone is resistant to quenching by thermal shock on the admission of low calorific value fuels owing to the large heat capacity of the inert phase of the bed.This factor accommodates intermittency in waste fuel supply since the combustion process will be sustained on change-over from a high to a low calorific value fuel during normal running conditions.

There are many companies who dump residual waste and would consider the transport costs associated with moving waste to a central combustion plant as excessive. On-site burning for space heating or for process stream may become economically attractive e.g. using units in the range 0.5-3 MW output, or larger as fossil fuel costs rise.

Sincle fluid bed combustion units are limited in turn-down ratio, because of the need to maintain air flow rates firstly below the condition of excessive particle elutriation and secondly above the minimum fluidisation velocity. Arguably, a maximum turn-down ratio of 3:1 can be achieved in practice with a single bed. Fluid bed combustors are essentially steady state devices and means should be provided to allow for turn-down without the need to alter air flow rates in individual beds.

According to a first aspect of the invention, there is provided a method of operating a fluidised bed combustor including the steps of passing oxygen containing gas through at least two vertically aligned beds, the pressurised oxygencontaining gas promoting fluidisation of the beds and combustion in the beds, the beds being operated without a free-board between the beds.

A heat exchange medium may be passed through heat exchanger devices in heat exchange relationship to the beds and to a free-board above the beds.

According to a second aspect of the invention, there is provided a method of firing a fluidised bed combustor having at least two vertically aligned beds, which comprises vertically slumping at least one first said bed, whilst maintaining the fluidisation and combustion of a fuel in a second said bed which is above the slumped bed or beds.

Slumping is the de-activation of a fluidised bed, i.e. all the bed particles become settled when the flow of pressurised oxygen-containing gas through the bed is stopped.

According to a third aspect of the invention, there is provided a method of firing a fluidised bed combustor having at least two vertically aligned beds, which comprises firing a first fuel in a first said bed, and firing a second fuel in a second said bed, which is above the first said bed, and circulating the second fuel downwards from the second said bed to the first bed.

It is preferred in this method that the first fuel is a clean fuel, for example, gas, and the second fuel is a waste fuel, for example, an involatile liquid.

Preferably each bed depth is matched to the fuels and wastes to be fired. For example, the depth of the top bed can be such as to be suitable for coal firing whilst a waste or conventional fuel is burnt on the lower bed.

The combustor is preferably operated so as to provide low NOx emissions. It is known that staged combustion can reduce significantly the NOx production. The present combustor provides a facility for air staging by means of lower and upper air distributors provided for the respective beds. For example, fuel would be introduced say through the lower fuel injectors such that the lower bed would be run sub-stoichiometric, and the second bed would provide additional air for complete combustion. Staged combustion is a known method of reducing NOx resulting from chemical bound N2.

Modular construction of the combustor allows the production of a single bed or multi-bed combustor with matched heat absorption surfaces for the fuels envisaged. Thus a single shallow bed combustor can be built for example for coal firing, and other fuels and wastes as well as combinations of beds.

Heat exchange surfaces in the bed(s) or freeboard may take many forms according to the application. For example, vertical or horizontal tubes and coils containing heat absorbing fluids for the purpose of heat exchange could be used.

Alternatively, heat exchange surfaces arranged to pass both through the bed(s) and free-board could be employed. The combustor can be arranged to include shell cooling as necessary in conjunction with any of the above heat exchanger arrangements noted above as required.

The oxygen-containing gas will in general be air, but other O2-containing gases can be used.

The invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a partial sectional-partial schematic view of the lower part of a fluid bed combustor utilized in the method of the present invention; Figure 2 is an enlarged view in section of the combustor shown in Figure 1, and Figure 3 is a schematic diagram of an experimental fluidized bed combustor air system utilized for testing the method of the present invention.

Figure 1 shows a fluidised bed combustor of modular construction having a housing 1 comprising four substantially identical standard sections 2, 3, 4 and 5. The walls of each standard section define a right cylinder, though other shapes can be used. Two air distributors 6 and 1 1A are disposed in a spaced relation in the housing 1 to divide the housing into two vertically stacked compartments which define two beds 7 and 8.

An intermediate section 6a is disposed between the standard sections 2 and 3, into which are incorporated air and fuel injector(s) which will be described later.

An air injection pressure chamber 9 is located below the lower bed 7. Air enters the pressure chamber through an inlet 10. Air is injected through distributors 1 A into the lower bed 7. It is to be understood that the airflow rate through the air distributors 1 1A is above the minimum fluidisation velocity and low enough to avoid excessive particle elutriation.

Fuel is injected into the lower bed 7 through injector(s) 11 positioned adjacent to the air distributors 11 A.

Fuel is fed between the beds 7 and 8 via fuel injector(s) 12. The fuel injector(s) 12 may for some fuels (e.g. wastes) face down the housing, so that the second fuel enters the lower bed 7 first.

Air is passed through a second air inlet 13 to air distributors 6 extending across the housing 1 between the lower bed 7 and the upper bed 8. The air distributors 6 are in the vicinity of the fuel injector(s) 12. The air flow rate through the air distributors 6 is maintained above the minimum fluidisation velocity and low enough to avoid excessive particle elutriation.

The housing 1 has a cover 1 5 and an outlet 1 6 through which hot gases from the fluidised beds 7 and 8 leave. The housing 1 is supported on an entablature 17.

Figures 1 and 2 show tubes 30 with flanges affixed to the standard sections 2-5 which allow access into the interior of the combustor, for purposes of insertion and withdrawal of materials, additional air injection location of instrumentation and ash removal.

Figure 1 shows the approximate upper surface, given by the reference numeral 18, of the upper bed 8. The space 19 between the surface 18 of the fluidised bed and the gas outlet 16 is called the free-board.

As shown in Figures 1 and 2, a modular bed construction is employed which utilises a number of circular standard sections (2, 3, 4 and 5), into which are incorporated, both in the beds 7 and 8 and in the free-board 19, heat absorption devices 20 for steam/hot water generation. Within each bed 7 and 8, the heat absorption devices can be readily selected/designed to match the heat release pattern both in the beds 7 and 8 and in the free-board 19. The heat absorption devices 20 in the bed(s) and free-board may take many forms according to the application. For example, vertical and/or horizontal tubes can be used. These may be of different configuration, i.e. straight or coiled.

Alternatively, heat exchange surfaces arranged to pass both through the bed(s) and free-board can be employed. The combustor can incorporate standard heat absorption devices to accommodate different heat release rate patterns in the bed 7 and 8 or in the free-board 19 as a result of using, for example, different types of fuels and wastes.

For example, the proportion of total heat release within the beds and free-board may be expected to vary with the volatile matter and fixed carbon content of the fuel. If more heat absorption is required, it is possible to add another standard section with heat absorption devices at the appropriate location in the combustor. The positioning of the heat absorption devices 20 is particularly important when burning wastes.

Flame after-buming in the free-board region 19 is common with high volatile matter fuels and can also be caused by gas by-passing in the fluidised bed when gaseous fuels are burnt.

The use of combustors having a modular construction incorporating heat absorption devices allows considerable flexibility in accommodating these effects with a relatively small manufacturing range of standard sections (2, 3, 4 and 5). With such flexibility of heat absorption, design and execution, high operating thermal efficiencies can be obtained for a wide range of conventional and waste fuels. The modular construction of the combustor allows the load-following characteristics (for example, steam generation for a process plant where the demand fluctuates) to be improved as a result of having heat absorption areas accurately positioned and designed for the fuel(s) intended, in the lower bed, upper bed and free-board regions. It is also possible to obtain efficient tum-down characteristics, where the turn-down is the fraction of full-throughput of the combustor to which the actual throughput of the combustor can be reduced without loss of continuity.

In normal practice, the bed material will be sand, though it is possible to use other materials such as ground limestone to absorb sulphur present in the fuels.

Figure 3 is a simplified schematic diagram of an experimental fluidised bed combustor air system.

A grit and dust extraction unit 50 is situated at the outlet of the combustor 51, so that grit and dust particles can be removed from the exit flue gases.

The flue gases are then passed to an air heat via duct 52. The air heater is a heater having a temperature control and a light-up burner 55.

Having passed through the air heater 53, the waste hot gases are released through the stack 56. Cold air from a fan 57 may be passed through the air heater 53 in a heat-exchange relationship, so that air fed to the beds in the combustor 51 is pre-heated to a desired temperature. The hot air from the air heater 53 enters the combustor via an upper flow control valve 58 and a lower flow control valve 59. Air passing through the upper flow control valve 58 is injected into the upper bed 8 via air inlet 13 and distributors 6 (see Figures 1 and 2). Air passing through the lower flow control valve 59 is injected into the lower bed 7 via air inlet 10 and distributors 11 A (see Figures 1 and 2).

It should be recognised that the location of the injection points of the wastes and fuels depends upon the type of waste and fuel and the quantity available. If the waste, such as waste lubricating oil, is available in quantities such that there is always sufficient waste for 100% combustor use, and the waste is of sufficiently high calorific value as to be self-sustaining in combustion, the use of an additional fuel may be unnecessary.

The use of fuels such as coal and municipal refuse (e.g. shredded or pelletised) may require the deployment of a fuel feed onto the top of the bed(s). This may or may not be in addition to the use of the fuel feed arrangements already described.

The method of operating the combustor comprises the steps of passing pressurised air through the two vertically aligned beds 7 and 8 containing fuel material, the pressurised air promoting fluidisation of the beds 7 and 8 and combustion of the fuel in the beds. The air also maintains the beds at a predetermined temperature. The beds 7 and 8 are operated without a free-board between each bed. It is thus possible to achieve a rapid turn-down by slumping the lower bed (the upper bed cannot be slumped for this purpose).

The combustor 1 can use fluidised beds 7 and 8 of various depths, in order to accommodate a wide range of liquid, gaseous and solid fuels. The beds ensure that the fuels have sufficient residence time in the beds for complete combustion. The beds are vertically aligned with each other and are operated without a free-board between the beds.

A conventional fuel may be fired on one bed to help, with the aid of the deep beds which retain heat, the complete combustion of, for example, a particularly difficult waste fuel which is to be fired on the other bed. Heat and waste fuel can be transferred from one bed to the other bed. For waste fuel firing and multi-fuel firing of, for example, a conventional fuel and a waste fuel, a high residence time for the waste fuel may be required to ensure complete combustion. The multi-fuel firing capability allows the efficient burning of poor quality waste fuel, with the assistance of clean support fuels if necessary. The modular construction permits the combustor to be constructed relatively simply for the employment of beds of various heights for the firing of a wide range of fuels. As noted above, difficult to burn wastes may require a high combustion residence time in deep bed(s).However, the use of reduced depth beds can be employed with the plurality of vertical stacked beds, plurality of vertical stacked air and fuel feeds. In addition, the combustor can be operated with a shallow bed for the firing of say coal. The modular construction permits many variations and permutations of combustor bed depth, number of beds, fuel feed, air feeds etc., to be built to suit the waste and/or fuel being burnt.

Single bed heat generation systems are very limited on turn-down, because a fluidised bed is only fluidised above a minimum fluidising velocity.

Also a higher air flow rate can cause excessive particle elutriation. Within these constraints, a maximum turn-down of 3:1 can be achieved in practice with a single bed. With a two-bed system, it is possible to have a 6:1 turn-down ratio, although in practice this may be limited to a 4:1 or 5:1 to achieve efficient combustion. A single combustor can provide turn-down ratios similar to those in modern Shell Boiler practice. If the lower bed 7 is stumped by switching off the pressurised air flow through the lower bed, the upper bed 8 will still be at working temperature. The depth of the lower bed, and transfer of heat from the working top bed 8, will help to keep the lower bed 7 warm. This means that the beds will keep warmer for greater periods and a rapid increase in response can be achieved for good load-following after start-up.It is possible to connect together single combustor units either in series or in parallel, in order to provide an increased thermal output and close load-following performance.

It is intended to achieve conventional load following behaviour by modulating air and fuel flow rates to either one or both of the beds. In the case of the former one bed takes the base load and the other the modulating load.

It is possible to obtain the efficient combustion of a difficult waste fuel using the twin air and fuel injection system. For example a clean fuel may be injected into the upper bed 8 to provide a high temperature combustion zone. A poor quality waste fuel is then injected into the lower bed 7 and this fuel must progress through the high temperature upper bed in passage to the freeboard region. Some re-circulation of fuel and heat is likely to occur between the upper and lower beds thus assisting burn out and increasing combustion efficiency. It should be recognised that in the case of multi-fuel firing there is no constraint on the phase of the fuel injected into either the lower or upper bed regions. A separate burner, firing a conventional fuel, either gas or oil, can be used to start up the combustion.

An example of a basic modulation sequence of fuel and air burning is as follows. After bringing the bed up to starting temperature the combustion air level is modulated to the required level into the lower bed 7 and fuel or waste would be initially introduced to the upper bed 8. The upper bed 8 reaches its operating temperature, such that a second fuel or waste can be introduced and burnt.

With twin beds, it should be possible to achieve a 2:1 tumdown on each bed, and possibly a 3:1 turndown on a bed if the fuel and excess air specifications permit. For higher turn-downs (greater than 5:1 or 6 :1), two or more combustors can be employed on one site, in the manner that conventional boilers are now used.

The combustor in a form substantially as described above can be used as a hot gas generator, for example, for drying applications. In this form, the combustor will not have any heat absorption devices.

There are a wide range of fuels which can be fired in the combustor, including for example: Gaseous (1) Waste gases.

(2) Contaminated air.

Liquid (1) Waste lubricating oils.

(2) Waste cutting oils.

(3) Fuel oil sludges.

(4) Petro-chemical wastes, e.g.

(Flotation Clarifier Skimmings) API separator sludge.

Tank cleanings 8 misc. oils.

Biological treatment sludge.

Spent caustic.

Solid (1) Low volatile matter solid fuel.

(2) Variable quality graded solid waste.

(3) Woodwaste.

The following examples of conventional fuels may be used in conjunction with the waste streams, either to ensure complete burn-out of the waste and/or to provide base load if the supply of waste is variable.

Gaseous: (1) Coke Oven Gase.

(2) North Seat Nat. Gas.

(3) L.P.G.

Liquid: Petroleum Fuels.

Solid: Coal.

By means of the present invention, efficient combustion of a wide class of solid, liquid and gaseous waste fuels can be obtained by matching heat absorption and heat release zones within a combustor. It is also possible to have efficient operation of a single combustor by obviating the need for part load operation of a bed. Flexibility in thermal output in the range 0.1-10 MW can be obtained with the use of different diameters of standard heat absorption sections. High turndown ratios can be obtained without the need for load following, by alterations in fluidizing airflow rate and waste fuel flow rate to a single combustor. It is also possible to manufacture single and multiple fluid bed systems economically using production line techniques.

By using the method of the invention, a turndown ratio of approximately 2 :1 can be obtained without deterioration in uniform fluidization conditions, which can occur with waste fuels when air flow rates are reduced throughout a single bed. A close load following performance can be obtained when used in single modular combustors as components of a multiple bed system.

It is necessary to match the extent of heat absorption in different zones of the bed and freeboard to the prevailing heat release pattem which itself depends upon the nature of the waste fuel.

For example, the proportion of total heat release within the bed and free-board may be expected to vary with a volatile matter and fixed carbon contents. The fiame after burning in the free-board region is common with high volatile matter fuels and can also be caused by gas by-passing in the fluid bed when gaseous fuels are burnt. The use of different heat absorption devices can allow considerable flexibility in accommodating these effects with the aim of increasing thermal efficiency.

Claims (25)

1. A method of operating a fluidised bed combustor including the steps of passing oxygencontaining gas through at least two vertically aligned beds, the pressurised oxygen-containing gas promoting fluidisation of the beds and combustion in the beds, the beds being operated without a free-board between the beds.

2. A method as claimed in claim 1, wherein a heat exchange medium is passed through heat exchange devices in heat exchange relationship to the beds and to a free-board above the beds.

3. A method as claimed in claim 2, wherein the heat exchange devices comprise vertical or horizontal tubes and coils containing heat absorbing fluids for the purpose of heat exchange.

4. A method as claimed in claim 2 or 3, wherein the heat exchange devices are arranged to pass through both the bed(s) and free-board.

5. A method as claimed in claim 2, 3 or 4, wherein the combustor is further cooled by shell cooling.

6. A method as claimed in any one of claims 1 to 5, wherein the oxygen-containing gas is air.

7. A method of operating a fluidised bed combustor as claimed in claim 1 and substantially as herein described with reference to and as illustrated in the accompanying drawings.

8. A method of firing a fluidised bed combustor having at least two vertically aligned beds, which comprises vertically slumping at least one first said bed, whilst maintaining the fluidisation and combustion of a fuel in a second said bed which is above the slumped bed or beds.

9. A method as claimed in any one of claims 1 to 6, wherein at least one first said bed is vertically slumped whilst the fluidisation and combustion of a fuel in a second said bed is maintained, the second bed being above the slumped bed or beds.

10. A method of firing a fluidised bed combustor having at least two vertically aligned beds, which comprises firing a first fuel in a first said bed, and firing a second fuel in a second said bed, which is above the first said bed, and circulating the second fuel downwards from the second said bed to the first said bed.

11. A method as claimed in any one of claims 1 to 6, wherein a first fuel is fired in a first said bed, and a second fuel is fired in a second said bed, which is above the first said bed, and the second fuel is circulated downwards from the second said bed to the first said bed.

12. A method as claimed in claim 10 or 11, wherein the first fuel is a clean fuel, and the second fuel is a waste fuel.

13. A method as claimed in claim 12, wherein the first fuel is a gas, and the second fuel is an involatile liquid.

14. A method as claimed in claim 10,11, or 13, wherein each bed depth is matched to the fuels and wastes to be fired.

1 5. A method as claimed in claim 14, wherein the top bed is coal fired, the depth of the bed being suitable for coal firing, whilst a waste or conventional fuel is burnt on the lower bed.

16. A method as claimed in claim 15, wherein heat and waste fuel is transferred from one bed to the other bed.

17. A method as claimed in any one of claims 10 to 16, wherein residence time for the waste fuel is large enough to ensure complete combustion.

18. A method as claimed in any one of claims 8 to 17, wherein the combustor has at least a 2:1 turn-down ratio on each bed, obtained by altering the fluidising air flow rate and the waste fuel flow rate.

19. A method as claimed in any one of claims 8 to 18, wherein the combustor is operated so as to provide low NOx emissions.

20. A method as claimed in claim 19, wherein there is staged combustion by means of lower and upper air distributors provided for the respective beds.

21. A method as claimed in claim 19 or 20, wherein a gaseous fuel or a fuel which can be vapourised is introduced into a lower bed through lower fuel injectors, such that the lower bed is run sub-stoichiometric, and the second bed provides additional air for complete combustion.

22. A method of firing a fluidised bed combustor as claimed in claim 8 and substantially as herein described with reference to and as illustrated in the accompanying drawings.

23. A method of firing a fluidised bed combustor as claimed in claim 10 and substantially as herein described with reference to and as illustrated in the accompanying drawings.

24. A fluidised bed combustor of modular construction having a housing which is divided into at least two vertically stacked compartments by at least two air distributors, the at least two vertically stacked compartments defining a't least two vertically aligned beds, at least two fuel injectors positioned adjacent to the at least two air distributors, and means for vertically slumping at least one first said bed.

25. A fluidised bed combustor suitable for operation in accordance with any one of the preceding method claims and substantially as herein -described with reference to and as illustrated in the accompanying drawings.