GB2286345A

GB2286345A - Feeding a fluidised bed

Info

Publication number: GB2286345A
Application number: GB9402526A
Authority: GB
Inventors: Mark Frederick Wickham
Original assignee: Individual
Current assignee: Individual
Priority date: 1994-02-09
Filing date: 1994-02-09
Publication date: 1995-08-16
Also published as: GB9402526D0

Abstract

A fluidised bed combustion device (2) uses a fuel feed duct (13) within a sleeve (12) of pressurised gas to achieve a gas curtain (11) for an emergent fuel stream to the bed, the bed itself is fluidised by gas supplied through sparge tubes (4) supported on clearance fit ring mountings (5), with an extraction hopper (8) beneath the bed having a collection and discharge axis aligned with the fuel feed to the bed. <IMAGE>

Description

Fluldised Bed Combustion This invention relates to fluidised bed combustion and is particularly concemed with apparatus fuelled by materials that have traditionally been classified as difficult fuels to burn in a fluidised bed, having either one or more of the iollowing characteristics:- low bulk density, large amount of fines, low calorific value, high volatile content and a proportion of inert material, which is either larger and/or more dense than the bed material.

The invention embraces the feeding of fuel to a fluidised bed combustion apparatus, the restraint and support of fluidising gas bed supply and distribution or sparge tubes of a fluidised bed combustion apparatus, and the removal of extraneous bed material from a fluidised bed combustion apparatus.

A fluidised bed combustion apparatus generally comprises a bed of inert particulate material (for example sand) which in operation is fluidised by air or a relatively inert gas passing through the bed material.

UK patents nos 1540378 and 2030689 teach two configurations of such fluidised bed combustion apparatus.

More specifically, the air or inert gas passed to the bed, to fluidise the bed, supports combustion of fuel also passed to the bed (for so-called 'primary phase combustion') and may also support combustion of volatile gases generated within the bed and which may continue to burn in the volume above the bed (for so-called 'second phase combustion').

Fluidised bed combustion apparatus may be used as primary heat generators (a hot gas generator or boiler, for example), as gas producer plant (in which the bed is operated endothermically as a gas producer), or as incinerator plant.

When used as a hot gas generator, gas producer or incinerator, a fluidised bed combustion unit is normally enclosed within a reaction vessel completely lined with refractory material, and usually has its solid fuel charged onto the bed through one or more substantially horizontal apertures in the side wall of the vessel. UK 2030689 is concerned with such an arrangement.

When charging fuels having characteristics as previously described, there is a tendency for a large proportion of the fuel, especially the light fine particles, to be blown off the end of the feeding aperture, and be caught up in the upward moving combustion gas flow and be displaced or'elutriated' from the fluidised bed combustion apparatus before its combustion has been completed.

The larger heavier particles that remain fall down onto the bed, where they burn.

However, the loss in quantity of fuel reaching the bed can cause insufficient heat to be re!eased to the bed, resulting in too low a combustion temperature.

In addition, the fuel which does reach the bed falls in a small area of the bed, and burns without sufficient combustion air (sub-stoichemetrically), causing inefficient combustion and localised overheating.

Other areas of the bed may receive insufficient fuel or none at all, causing low bed temperatures to occur. Generally there can be insufficient fuel delivered to the bed for good primary combustion conditions.

One aspect of the present invention concerns a fuel feeding device which directs nearly all of the fuel to the bed and provides the optimum amount of combustion air with the fuel where it is required, creating improved combustion conditions in the bed, and more even temperatures within and across the bed.

According to that aspect, a fuel feeding device, for a fluidised bed combustion chamber, the device comprises an inclined duct, with a fuel feed end and a fuel delivery end, a fuel transit passage within the duct between the feed and delivery ends, a sleeve around at least a (downstream) portion of the duct, means for feeding fluid, such as gas, under pressure between the sleeve and duct walls, to form a collar or curtain of fluid, eg, gas, around a fuel stream emergent at the delivery end.

In a fluidised bed combustion apparatus the fluidising gas must be distributed evenly below the bed, to ensure that the bed itself fluidises evenly.

There are several different types of distributors used to achieve this, however this aspect of the present invention relates to a horizontal sparge tube type of fluidising gas distributor system. UK patent no 1540378 is concerned with such a sparge tube construction.

This type of distributor system usually comprises a multitude of sparge tubes, which are laid in parallel to one another, on the same horizontal centre line, at a pitch that ensures there is a significant gap between adjacent sparge tubes.

Usually each sparge tube is cantilevered, having one end attached to a mounting plate by welding, whilst the free end projects horizontally into the bed itself. The sparge tube, which may be up to 3 metres in length, requires supporting along its length, to ensure that it remains straight, and does not bend or twist in the harsh conditions found within the fluidised bed (high temperatures, oxidising or reducing atmosphere, abrasion and corrosion, pressure).

The sparge tubes, which are usually manufactured from heat resisting stainless steels, operate at temperatures in the order of 650 - 950 degrees centigrade. At these temperatures the sparge tubes have expanded considerably from cold and any supporting or restraining devices must allow the sparge tubes freedom to expand.

Known arrangements such as in UK patent no 1540378 for sparge tube support to accommodate expansion utilise a lug welded to the underside of the sparge tube, and a lug welded to the base plate or other supporting structure and a link attached to both lugs by a pin at either end.

This arrangement permits the sparge tube to expand and prevents the tube from moving upwards or downwards from its fixed horizontal position in the bed.

However the arrangement does very little to prevent the sparge tube from bending or moving sideways.

Previous industrial applications of this supporting arrangement have proved to be unsatisfactory, with examples of sparge tubes distorting in the horizontal plane resulting in sparge tubes moving together and other sparge tubes moving apart.

Distortion causes the fluidising gas to be distributed unevenly across the bed, and causes premature erosion of the sparge tube walls.

Where sparge tubes have moved closely together, the fluidising gas, jetting substantially horizontally from the sparge tube nozzles, impinges on adjacent tubes at high velocity, causing wear.

A second aspect of the current invention concerns sparge tubes support in such a manner that the problems mentioned above are largely overcome.

According to that aspect, a fluidising gas distribution or sparge tube mounting, for a fluidised bed combustion chamber, comprises one or more ring supports located along and restraining the tubes from lateral deformation when subject to combustion chamber heat.

A fluidised bed combustion apparatus has the potential to burn a wide range of solid fuels and waste products. However, one of the liming factors in the reliable operation with these fuels has been the quantity, size and density of inert material within the fuel (eg ash, sand, stones, pebbles, metals, glass etc).

With fuels such as wood, chicken litter, and bark, for example, the quantity of inert material represents a relatively small percentage. However, if the inerts are larger and/or more dense than the bed material itself, they will accumulate at the point where they are fed onto the bed, until a time is reached when the accumulation of inerts noticeably affects the performance of the bed itself.

The larger and/or more dense material does not fluidise as easily as the finely graded bed material and as a result, fluidising gas distribution across and within the bed becomes uneven. Additionally, as the fuel continues to be fed onto the poorly fluidised area of the bed, the fuel becomes starved of combustion air and does not burn efficiently.

With the lack of fluidisation in the area of fuel feed, movement of fuel to other parts of the bed where it could burn more readily is restricted. The resultant poor combustion conditions deteriorate progressively, until output from the fluidised bed combustion apparatus has reduced below acceptable levels, and there is danger that the apparatus itself may become damaged or even unsafe.

As combustion conditions deteriorate, it can become difficult to control bed temperatures and localised clinkering of the fuel and bed material may occur. Once this happens, it usually becomes necessary to take the apparatus out of production, cool it down and remove the bed material and replace it with a clean batch. This maintenance operation, which typically can occur every three weeks, can take up to three days to complete - during which time the apparatus is out of production, usually at great cost.

An alternative preventative measure for the problem of a contaminated bed is to install fuel pre-treatment apparatus, which removes the oversize and/or denser inert material, or pulverises it to the size of the bed material. However, such pretreatment apparatus has high capital and maintenance costs.

A third aspect of the current invention concerns an extraction arrangement whereby the accumulated oversize, dense, inert material may be removed from the fluidising bed strata, into the lower unfluidised bed strata, where it does not cause any operational problems. The arrangement also permits the oversize, dense, inert material to be removed completely from the fluidised bed combustion apparatus while the apparatus remains in operation.

According to that aspect, an extraction hopper, for a fluidised bed combustion chamber, comprises a funnel shaped floor for the combustion chamber underneath the bed support, with a controllable discharge aperture.

The arrangement completely eliminates the problems associated with bed contamination, without the need for costly pretreatment, and helps to ensure continuous reliable operation of the apparatus.

One aspect of the invention provides a fuel feeding device which directs nearly all of the fuel onto the fluidising bed surface at a predetermined point, according to the dimensions of the fluidised bed combustion apparatus in which it is being used.

The device ensures a downward momentum for the fuel, and sometimes this is assisted with combustion air, recycled flue gas or a proportion of each. The velocity of the fuel, or fuel and air/gas mix is sufficiently high that the majority reaches the fluidised bed surface, against the opposing force of the fluidising gas - leaving the fluidised bed surface in an upward direction.

The operation of the fuel feeding device is advantageously improved by the inclusion within the arrangement of a feature which provides a high velocity collar or curtain of air, recycled flue gas or a proportion of each, which completely surrounds the fuel, or fuel air/gas mix.

The purpose of the gas curtain, which is usually of higher velocity than the fuel, or fuel air/gas mix, is to keep the fuel within the envelope of the curtain.

A second advantage of the curtain is its ability to cool the fuel feeding device, where it projects through the refractory wall and into the fluidised bed combustion chamber. This helps to keep the walls of the fuel feeding device at a low enough temperature so that the wall material does not distort or buckle, and more importantly at a low enough temperature to ensure that the fuel does not stick to the walls of the fuel feeding device, through sweating or burning.

A second aspect of the invention provides a support for a tube (sometimes termed 'sparge' tube) type of fluidising gas bed supply and distributor system, which ensures freedom for the sparge tubes to expand as the temperature of the bed increases and provides support and restraint to prevent the sparge tubes moving from their horizontally set position.

The arrangement comprises one or more 'ring supports per (sparge) tube, which on assembly slip over the sparge tubes and are fixed in their design position by their attachment to a supporting structure located below the sparge tubes, and away from the 'aggressive' fluidising bed strata.

The 'rings' are not attached to the sparge tubes in any way, and are designed with an internal diameter slightly larger than the external diameter of the sparge tubes, in order to provide sufficient clearance for expansion and to prevent the sparge tube binding on the 'ring' as the sparge tube expands, dragging the 'ring' with it.

The 'rings' desirably have a lug attached, by welding, which may be fixed to its supporting structure by way of a bolted or welded connection, for example. The supporting structure is designed in such a way that the 'rings' are maintained substantially in their fixed position, even when the fluidised bed is working at its operational temperature. It is convenient to attach the 'rings' to supports fixed to the base plate of the vessel containing the fluidised bed.

As the base plate remains relatively cool, compared to the fluidised bed temperature there is usually no requirement for any expansion facility to be built into the support.

Where the support structure cannot simply be fixed to the base plate of the vessel, for example where there is an extraction point for extraneous bed material, as described in the third aspect of the invention, it has been found advantageous to attach the 'rings' to a supporting structure fixed horizontally across the bed extraction point.

Due to the high temperature of the bed materials being extracted, this supporting structure has to be designed in such a way as to provide for its own expansion. It has been found advantageous to design this horizontal supporting structure from a tubular member. However, it could be of any cross section profile, with one or more expansion joints designed at suitable positions along its length.

The expansion facility is usually made by the insertion of a solid piece of material having the same or similar composition as the tubular member, into the tubular members themselves, and is fixed usually by welding to one of the tubular members.

The other end of the solid material is left free to expand within the other tubular member. The 'rings' are then fixed to similar supports as those fixed to the base plate, which are usually attached to the tubular supports by welding.

A third aspect of the invention provides an arrangement whereby bed material, usually contaminated with oversize and/or dense inert material, may be removed from the fluidising bed strata, into the unfluidised bed strata or outside of the fluidised bed vessel altogether.

The arrangement comprises a hopper formed to the underside of the fluidised bed containment vessel, positioned below the point on the fluidised bed surface where the fuel lands.

At the bottom of the hopper there is provided a valve, which may be opened to extract bed material and contaminants, which flow freely from the vessel under the influence of pressure within the fluidised bed and gravity. The valve is of a large enough size to ensure that any contaminants which pass down through the gap between adjacent sparge tubes may be removed.

The hopper is usually designed with an internal angle of a similar or greater size than the angle of repose of the bed material itself. This is advantageous, as it permits bed material to be drawn down relatively evenly between the gaps of adjacent sparge tubes, over a wide area of the fluidised bed, under the fuel feeder.

A specific embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: Description of the Drawings Figure 1, diagrammatically illustrates an arrangement embodying the invention in which a fluidised bed combustion unit is a hot gas generator supplying heat to a heat recovery boiler or process plant.

This Figure shows a fluidised bed combustion unit 1 operating as a hot gas generator, being fed with a fuel and air/gas mixture 19, through a feeding device which comprises an inner duct 13 and an outer duct 12 both of which are usually manufactured from heat resisting stainless steel tubing, however these could be virtually any cross sectional shape.

The fuel 18, which can be delivered to the feeding device by any one of a number of different speed controlled feeders, such as a walking floor, moving floor, belt floor, drag link, live bottom screw etc, is usually mixed with combustion air or recycled flue gas 17, which assists the downward movement of the fuel and increases its velocity and momentum, until it leaves the inner duct 13 and continues in a downward direction in the combustion chamber, until it meets with the fluidised bed surface.

The outer tube 12 is of a larger diameter than the inner tube 13, creating an annulus between the two ducts, through which high velocity air 15 is passed causing an air curtain 11 to be formed, which shrouds the fuel and air/gas mixture keeping it within the curtain 11 until the fluidised bed surface is reached.

The quantities of air and gas are controlled by valves. The annulus between the inner and outer ducts is maintained at the same dimension around its circumference and along the full length of the duct, by spacers 16 usually attached to one of the ducts by welding. It is also possible to use a design of spacer which creates a spiral motion to the air curtain.

The fluidising air, recycled flue gas or combination of both 10 is fed from a plenum chamber 22 to an array of horizontally mounted sparge tubes, which are usually welded at one end to a tube plate, and are supported by one or more 'ring' supports 5 which are fixed to either the base plate 6 of the fluidised bed combustion unit or the stainless steel supports 7 spanning the bed extraction hopper 8.

Fluidising bed material and/or inert contaminants 23 that are of a similar or greater density than the bed material, and usually larger, may be removed from the fluidised bed apparatus by opening valve 9. Bed material is drawn down between the gaps of adjacent sparge tubes under pressure and gravity, and flows from the conical region directly above the valve 9 according to the angle of repose of the bed material, shown on the diagram as a chain dotted line.

Figure 2, diagrammatically shows one arrangement embodying the 'ring' support aspect of the invention and shows one way in which it may be connected to a stainless steel support, spanning a bed extraction hopper.

This figure shows a section through a part of a fluidised bed combustion apparatus, and details a typical sparge tube type of 'ring' support system, where the sparge tube 4 is supported by a 'ring' support 5. The 'ring' support 5 usually has a lug 24 attached to it by welding. The 'ring' support 5 may be attached by its lug 24 to an additional lug 26 by a bolted or welded connection 25. Lug 26 is connected usually by welding to the base plate 6 or a supporting structure 7, comprising tubular members 27 and 28, and solid section 29.

The supporting structure is attached to the base plate 6 and bed extraction hopper 8 usually by welding. The solid section 29 is inserted into the tubular sections 27 and 28 and is fixed by welding to one of the tubes, while the other end is left free to expand into the opposing tube.

Claims

1.

A fuel feeding device, for a fluidised bed combustion chamber, the device comprising an inclined duct (13), with a fuel feed end and a fuel delivery end, a fuel (18) transit passage within the duct between the feed and delivery ends, a sleeve (12) around at least a (downstream) portion of the duct, means for feeding fluid, such as gas, under pressure between the sleeve and duct walls, to form a collar or curtain (11) of fluid, such as gas, around a fuel stream emergent at the delivery end.

2.

A fuel feeding device, as claimed in Claim 1, including an intake in the duct for the introduction of a supplementary fluid, such as air or recirculated combustion gas (17), for admixture with fuel within the duct passage.

3.

A fuel feeding device, as claimed in Claim 1 or Claim 2, incorporating one or more spacers between the sleeve and duct, to promote a desired motion in the gas curtain around the fuel relative to the fuel path.

4.

A fuel feeding device, for a fluidised bed combustion chamber, substantially as hereinbefore described, with reference to and as shown in, the accompanying drawings.

5.

An extraction hopper (8), for a fluidised bed combustion chamber, comprising a funnel shaped floor for the combustion chamber undemeath the bed support, with a controllable discharge aperture.

6.

A hopper, as claimed in Claim 5, with a collection and discharge axis generally aligned with the delivery region of a fuel feeding device for delivering fuel to the bed from above.

7.

An extraction hopper, for a fluidised bed combustion chamber1 substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.

8.

A fluidising gas distribution or sparge tube mounting, for a fluidised bed combustion chamber, comprising one or more ring supports (15) located along and restraining the tubes (4) from lateral deformation when subject to combustion chamber heat.

9.

A sparge tube mounting, as claimed in Claim 6, wherein the sparge tubes are cantilevered from one end in a combustion chamber wall (2) and are located remotely from that end by one or more spaced ring supports.

10.

A sparge tube mounting, for a fluidised bed combustion chamber, substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.

11.

A fluidised bed combustion device comprising a fuel feed incorporating an inclined duct for feeding fuel, a sleeve around the fuel duct, means for feeding pressurised gas into the sleeve, to achieve a delivery of fuel to the combustion chamber within a collar or curtain of pressurised gas; a fluidised bed combustion chamber incorporating a plurality of combustion fluidising gas distribution or sparge tubes, for delivering pressurised combustion bed support gas, and upon or to which combustion material is deposited and above which fluidised bed combustion occurs; a fluidising distribution or sparge tube mounting incorporating one or more ring supports located around the tube and spaced longitudinally thereof and restraining the associated tube from lateral deformation when subject to combustion chamber heat; an extraction hopper incorporating a funnel shaped combustion chamber floor underneath the fluidised bed and sparge tube support and with a discharge axis generally aligned with the fuel delivery region to the bed of the fuel feed device.

12.

A fluidised bed combustion device substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.

13.

A fluidised bed combustion device, as claimed in any of the preceding claims, incorporating multiple fuel fed devices and corresponding aligned extraction hoppers in a common combustion chamber.