Introduction

The present invention relates to an ash-fouling prevention system suitable for use in an energy conversion system comprising a fluidised bed unit, wherein the energy conversion system comprises a furnace and a heat exchanger having a number of spaced-apart tubes.

Fluidised bed units are used in fluidised bed combustion processes and are capable of thermally treating substances that are normally difficult to combust using other technologies such as poultry litter, peat, bone meal, spent mushroom compost and the like. The use of fluidised bed units with such fuels results in very efficient combustion. Fluidised bed units also have low emission rates of nitric oxides, which is beneficial to the environment. The use of fluidised bed units is well known and in particular, fluidised bed units can be used with an associated heat exchanger to provide a heated liquid, typically water, which can be stored for later use.

Ash fouling is a serious problem in energy conversion systems and heat exchangers which can result in reduced efficiency and, in some case, failure of the devices in question. This is particularly important when dealing with high ash content fuels such as poultry litter or spent mushroom compost. In such cases, the ash and other particulates in the exhaust materials would quickly build-up to block the passage of gases therein.

Fluidised bed units require regular cleaning to prevent build-up of ash. A wide variety of techniques have been developed to cope with this issue. However, many of these techniques, such as steam pressure cleaning, are complex and expensive to operate and therefore unsuitable for use in small scale fluidised bed units. Similarly, many other known ash removal systems are extremely expensive to operate and complex to install and implement.

It is an object therefore of the present invention to provide an ash-fouling prevention system suitable for use in an energy conversion system comprising a fluidised bed unit that overcomes at least some of the above-mentioned problems. Statements of Invention

According to the invention there is provided an ash-fouling prevention system suitable in an energy conversion system, the energy conversion system comprising a fluidised bed unit and a heat exchanger having a number of spaced-apart tubes, characterised in that the ash-fouling prevention system comprises a heat exchanger ash blower which in turn comprises a heat exchanger pressurised air supply coupled to a rotatably mounted elongate hollow shaft inserted between the tubes, the shaft having a plurality of venting apertures dispersed along its length.

In this way, the heat exchanger ash blower will prevent the build-up of ash on the tubes of the heat exchanger in a convenient and efficient manner. The temperature in the heat exchanger can reach 900 -1200 ⁰ C. Under these conditions, any ash that was allowed to build up on the heat exchanger tubes, would be transformed from its originally powdery form to a hard clinker. The presence of clinkers on the heat exchanger tubes would greatly reduce their efficiency and would also reduce the flow of exhaust gases around the heat exchanger, such that the efficiency of the other heat exchanger tubes would also be reduced. Additionally, once clinkers form, more ash and soot will build up thereon, and may eventually totally block the area surrounding the heat exchanger tubes in question. Furthermore, the use of pressurised air in the ash blower is particularly suitable for use with fluidised bed units. Air is very useful is this situation as it is relatively simple to supply and the equipment required is neither overly complex nor expensive.

In an embodiment of the invention there is provided an ash-fouling prevention system in which in which the venting apertures are aligned with the spaces between the spaced- apart tubes of the heat exchanger. In this way, thorough and efficient soot and ash removal is facilitated. The heat exchanger tubes will not interfere with the airflow from the heat exchanger ash blower and the air will be able to circulate throughout the heat exchanger to provide thorough ash removal. Furthermore, this location of the venting apertures minimised the amount of air required to perform a thorough ash removal.

In one embodiment of the invention there is provided an ash-fouling prevention system in which the heat exchanger comprises tubes arranged in rows wherein the rotating hollow shaft is located in an interstice between rows. This location for the heat exchanger ash blower is particularly efficient as it does not interfere with the flow of exhaust gases around the heat exchanger and therefore does not compromise the efficiency thereof.

In another embodiment of the invention there is provided an ash-fouling prevention system in which the venting apertures of the heat exchanger ash blower are aligned with the spaces between the tubes in a row. In this way, it is ensured that the air flow from the heat exchanger ash blower is distributed throughout the heat exchanger.

In a further embodiment of the invention there is provided an ash-fouling prevention system in which the heat exchanger comprises a plurality of rows of U-shaped tubes, each tube having a pair of substantially parallel legs bridged by an arch section; and the venting apertures of the heat exchanger ash blower are aligned with the spaces between the legs of the tubes. In this way, the air flow from the ash blower will not be impeded by the heat exchanger tubes and thus can circulate about the heat exchanger, providing efficient ash removal.

In an alternative embodiment of the invention there is provided an ash-fouling prevention system in which the heat exchanger ash blower is operational for approximately 1 minute every 30 minutes. This is a particularly efficient interval for ash fouling prevention, keeping the amount of air used to a minimum while also ensuring sufficient ash removal.

In an embodiment of the invention there is provided an ash-fouling prevention system in which the shaft completes substantially one rotation in one minute. Again, this allows for thorough and efficient ash removal using as little air from the air supply as possible.

In an alternative embodiment of the invention there is provided an energy conversion system in which the heat exchanger comprises a heat exchanger assembly which in turn comprises a pair of heat exchanger units disposed one above the other to form an upper heat exchanger unit and a lower heat exchanger unit such that the heat exchanger ash blower is located substantially centrally in the upper heat exchanger unit. In this way the heat exchanger ash blower will have maximum effect in the upper portion of the heat exchanger assembly wherein the ash fouling is more prevalent than the lower portion. - A -

In a further embodiment of the invention there is provided an ash-fouling prevention system in which the in which the heat exchanger pressurised air supply supplies pressurised air at approximately 6 bar. This is particularly efficient for ash and ash removal, and further is a convenient air pressure to supply.

In one embodiment of the invention there is provided an ash-fouling prevention system in which the furnace comprises a furnace freeboard; the heat exchanger comprises a heat exchanger freeboard such that the furnace freeboard is in communication with the heat exchanger by way of a freeboard interconnector wherein the freeboard interconnector is provided with at least one freeboard ash blower. In this way, the ash- fouling prevention system can also prevent ash fouling in the freeboard areas of the energy conversion system. Similarly to the heat exchanger, if the soot and ash were allowed to build up in the freeboard interconnector, the high temperatures would cause clinkers to form, which would hinder the flow of exhaust gases through the freeboard interconnector and would eventually cause the freeboard interconnector to become blocked.

In another embodiment of the invention there is provided an ash-fouling prevention system in which the freeboard ash blower comprises an air nozzle projecting into the freeboard interconnector. In this way, the ash in the freeboard interconnector will be accelerated away from the freeboard interconnector and into the heat exchanger from where the heat exchanger ash blower will further process the ash. In the absence of the freeboard ash blower, ash would settle in the freeboard interconnector. The freeboard ash blower imparts sufficient acceleration to the ash to ensure that it passes through the freeboard interconnector.

In a further embodiment of the invention there is provided an ash-fouling prevention system in which the freeboard ash blower is connected to a freeboard pressurised air supply. This is a particularly efficient manner of providing air to the freeboard ash blower.

In an embodiment of the invention there is provided an ash-fouling prevention system comprising a plurality of freeboard ash blowers. Preferably there are three freeboard ash blowers. In this way, the effect of the freeboard ash blowers will extend across the full freeboard interconnector area, ensuring that the exhaust within the freeboard interconnector may be accelerated substantially uniformly.

In an alternative embodiment of the invention there is provided an ash-fouling prevention system in which the freeboard interconnector comprises a base and the freeboard ash blowers are each equally spaced from the base. This also provides for uniform acceleration of the exhaust flow in the freeboard interconnector.

According to the invention there is provided an energy conversion system comprising a fluidised bed unit and a heat exchanger wherein the furnace comprises a furnace freeboard and the heat exchanger comprises a heat exchanger freeboard such that the furnace freeboard is in communication with the heat exchanger by way of a freeboard interconnector wherein the energy conversion system further comprises an ash-fouling prevention system having a heat exchanger ash blower and a freeboard ash blower.

In this way, an energy conversion system may be provided that does not suffer a significant reduction in efficiency due to ash fouling.

In one embodiment of the invention there is provided an energy conversion system in which the heat exchanger ash blower comprises a rotating ash blower and the freeboard ash blower comprises a pulsed ash blower. In this way, the heat exchanger ash blower can effect ash-fouling prevention in a wide area, while the freeboard ash blower can effect the necessary acceleration of the ash in the freeboard interconnector.

In another embodiment of the invention there is provided an energy conversion system further comprising a pressurised air supply connected to the ash blowers. The use of air in the ash blowers is particularly useful as it is convenient and inexpensive to provide.

In a further embodiment of the invention there is provided an energy conversion system wherein the freeboard interconnector comprises a base which is inclined from the furnace freeboard to the heat exchanger freeboard and the freeboard ash blower comprises a plurality of nozzles operable to project an airflow into the freeboard interconnector. In this way, ash is prevented from settling in the freeboard interconnector. The ash will either be directed back to the fluidising bed unit or the into the heat exchangers, depending on the direction of the incline. Alternatively, the ash will rejoin the exhaust gas flow.

In one embodiment of the invention there is provided an energy conversion system in which the nozzles being aligned in a row parallel to the base of the freeboard interconnector. In this way, the air from the freeboard ash blower is directed at any ash that may settle thereon and will blow the ash off the base of the freeboard interconnector.

In an alternative embodiment of the invention there is provided an energy conversion system in which the nozzles are located in the base of the freeboard interconnector. In this way, the nozzles blow upwardly from the base of the freeboard interconnector and so remove any ash that has settled on the base.

In one embodiment of the invention there is provided an energy conversion system in which the nozzles are substantially flush with the base. This is a particularly efficient way of ensuring the free-board ash blowers disturb the ash settled on the base.

Detailed Description of the Invention

The invention will now be more clearly understood from the following description of an embodiment thereof given by way of example only with reference to the accompanying drawings in which:-

Fig. 1 is a diagrammatic representation of the system according to the invention;

Figs. 2(a), (b), (c) and (d) are top, front, side and perspective views of the heat exchanger incorporating the heat exchanger ash blower;

Fig. 3 is a diagrammatic representation of the hollow shaft of the heat exchanger ash blower;

Fig. 4 is a diagrammatic representation of portion of the freeboard interconnector; Fig. 5 is a diagrammatic representation of the energy conversion system of the invention showing the air supply to the heat exchanger ash blower and freeboard ash blower; and

Fig. 6 is a diagrammatic representation of an embodiment of the freeboard ash blower.

Referring to the drawings and initially to Figure 1 thereof, there is shown an energy conversion system, indicated generally by the reference numeral 1 , comprising a fluidised bed unit 3, a by-product fuel feed system 5 feeding the fluidised bed unit 3, a heat exchanger 7 operatively coupled to the fluidised bed unit 3, an exhaust filter 9 operatively coupled to the heat exchanger 7 and an internal draught system. The internal draught system comprises a forced draught fan 11 and an induction draught fan 13, which are operable to maintain a flow of exhaust gases in the direction from the fluidised bed unit 3 through the heat exchanger 7.

The fluidised bed unit 3 further comprises a charging inlet 15 for fuel delivered by the byproduct fuel feed system 5, a diesel burner (not shown) connected to a burner inlet 17 and a furnace sump 19 containing fluidised bed media. The furnace sump 19 tapers inwardly towards the bottom of the furnace sump where there is a clinker extraction unit, in this case comprising a furnace ash removal auger 21 and a furnace ash removal auger outlet, located at the bottom of the furnace sump 19. The fluidised bed unit 3 further comprises an air introducer assembly most of which is mounted substantially in the furnace sump 19 for delivering air up through the fluidised bed media in the furnace sump 19. The air introducer further comprises the forced draught fan 11 from the internal draught system. The forced draught fan 11 is connected to an air intake 22 which is in turn connected to an air inlet (not shown) located in the furnace sump 19 above the furnace ash removal auger outlet 21. Above the furnace sump 19 is the furnace freeboard 23, comprising the upper furnace freeboard 23a and the lower furnace freeboard 23b.

The by-product fuel feed system 5 comprises a hopper 25, a variable speed auger 27 and a fuel conveyor 29 to deliver fuel from the hopper to the charging inlet 15 of the fluidised bed unit. The variable speed auger 27 is operated to deliver a desired amount of fuel from the hopper 25 onto the fuel conveyor 29.

The heat exchanger 7 comprises a pair of heat exchanger units, an upper heat exchanger unit 31 and a lower heat exchanger unit 33. The lower heat exchanger unit 33 is provided with a cold water flow pipe 35 and the upper heat exchanger unit 31 is provided with a hot water return pipe 37. Below the lower heat exchanger unit 33 is a heat exchanger sump 39 which is provided with a heat exchanger ash removal auger 41 to remove ash from the heat exchanger sump. The heat exchanger 7 further comprises a heat exchanger freeboard 42, which is operatively coupled to the fluidised bed unit by way of a freeboard interconnector 34. In this way, the upper furnace freeboard 23a, the freeboard interconnector 34 and the heat exchanger freeboard 42 provide a path for the flue gases from the fluidised bed unit 3 to the heat exchanger 7. A heat exchanger exhaust conduit 43 operatively couples the heat exchanger 7 to the exhaust filter 9.

The exhaust filter 9 is a bag filter having a plurality of bags to catch the fly ash from the exhaust gases. The exhaust filter 9 comprises an ash extractor auger 45 located at the bottom of the exhaust filter 9. The induction draught fan 13 is between the exhaust filter 9 and an exhaust outlet 47, and draws exhaust gases through the energy conversion system from the fluidised bed unit 3, through the heat exchanger 7 and through the exhaust filter 9.

In use, a by-product fuel is delivered from the hopper 25 along the fuel conveyor 29 and is delivered into the fluidised bed unit 3 where it is burnt at a temperature of at least 850 ^° C for at least two seconds. The temperature of the fluidised bed is between 610 ^° C and 750 ^° C, preferably approximately 670 ^° C. Just above the fluidised bed, in the lower furnace freeboard 23b, the temperature is approximately 850 ^° C and at the top of the upper furnace freeboard adjacent the freeboard interconnector 34, the temperature is in the region of between 900 ^° C and 1200 ^° C. The height of the furnace freeboard and the negative pressure is such that the fuel remains in the region at or above 850 ^° C for a minimum of 2 seconds and this ensures that all pathogens are killed.

A plurality of temperature sensors are arranged in the fluidised bed unit 3. There are four temperature sensors in the fluidised bed itself, one temperature sensor in the lower furnace freeboard 23b just above the fluidised bed and another temperature sensor in the upper furnace freeboard. These temperature sensors closely monitor the temperature of the fluidised bed unit and if the temperature should deviate from the desired values or ranges, corrective action may be taken. If the temperature of the fluidised bed lowers, the variable speed augers are operated to increase the amount of fuel that is delivered to the fluidised bed unit 3. If the fuel has a relatively high moisture content, the fuel may not immediately cause the temperature to rise in the fluidised bed and other action must be taken. In such an instance, further fuel may be added or alternatively, the diesel burner is started and provides a boost to the fluidised bed.

The hot exhaust gases rise up through the furnace through the lower and upper furnace freeboards, through the interconnecting freeboard 34 and down through the heat exchanger 7. The heat exchanger 7 comprises a plurality of tubes (not shown) filled with water and the water in the tubes is heated by the hot exhaust gases passing over the tubes. The hot exhaust gases are then passed out of the heat exchanger to the exhaust filter 9 where fly ash is removed from the exhaust gases and the filtered exhaust gases are released into the atmosphere. The exhaust gases released into the atmosphere are still at approximately 150°C to 200°C. An exhaust filter has an ash extractor auger 45 which removes ash out from the filter. The ash taken from the filter typically has a phosphorous content of 18% by weight of the ash and 8% potassium by weight of the ash and may be sold on as a useful by-product for fertilizers and the like.

The heat exchanger 7 may also be coupled to a heating system (not shown) which comprises a radiator bank and at least one fan for circulating hot air surrounding the fan. In order to couple the heat exchanger to the heating system, the hot water return pipe 37 is connected to the radiator bank and the cold water flow pipe 35 is connected to a water source such as a water buffer tank or a return from the radiator bank. If a water buffer tank is used the water filling the water buffer tank may come from the radiator bank. The heating system is preferably for an animal housing such as a poultry housing however the present invention could be used as a heating system with other types of animals, agricultural processes such as mushroom growing or domestic heating systems.

The fluidised bed unit 3 and heat exchanger 7 are comprised within metal casings providing a substantially rectangular cross section. The upper boundary of the casing of the upper furnace freeboard 23a, freeboard interconnector 34 and heat exchanger freeboard 42 are all at the same level however the upper furnace freeboard 23a opens into the freeboard interconnector 34 at a lower level than the heat exchanger freeboard 42 such that the base of the freeboard interconnector 34 is inclined upwardly from the fluidised bed unit 3 to the heat exchanger 7. In this way, any soot or ash that settles on the base of the freeboard interconnector 34 will be guided to fall back into the fluidised bed unit for combustion or may be guided simply back into the steam of exhaust gases and will be carried onward through the energy conversion system.

The energy conversion system 1 further comprises an ash-fouling prevention system which in turn comprises the heat exchanger ash blower 32 and three freeboard ash blowers comprising the blower nozzles 36. The freeboard ash blowers 36 are mounted in side of the freeboard interconnector 34, with the freeboard ash blowers 36 being substantially in line with the floor of the freeboard interconnector 34. Pressurised air is periodically passed through the blower nozzles 36 to dislodge any settled ash from the floor of the freeboard interconnector 34. The freeboard ash-blowers 36 preferably operate in a pulsed manner.

The heat exchanger ash blower 32 is mounted in the upper heat exchanger unit 31 such that it extends across the heat exchanger and between a plurality of tubes (not shown) of the upper heat exchanger unit 31. The heat exchanger ash blower 32 is rotatably mounted on the upper heat exchanger unit 31.

Referring now to Figs. 2 (a), (b), (c), (d) and Fig. 3 in which like parts have been given the same reference numerals as before, there is shown the upper heat exchanger unit indicated generally by the reference numeral 31 fitted with the heat exchanger ash blower 32. Each heat exchanger unit 31 , 33 comprises a tube mounting plate 50 mounting an array of U-shaped heat exchanger tubes 52, such that the open end of the U-shape engages the tube mounting plate 50 with the tubes 52 projecting perpendicularly from the tube mounting plate 50. Each heat exchanger tube 52 comprises a pair of parallel legs bridged by an arched portion. The tubes 52 are arranged in parallel horizontal rows, wherein the tubes within each row are oriented at substantially 45° the horizontal such that one leg of each U-shaped tube 52 is higher than the other. Furthermore each tube 52 partially overlaps its neighbouring tubes 52 within a row.

The heat exchanger ash blower 32 comprises a hollow elongate cylindrical shaft 54 having a number of venting apertures 56 along its length and fitted with a mounting block 57 at each end. The heat exchanger ash blower 32 is connected to an air supply (not shown) and a motor (not shown) that causes the shaft 54 to rotate as air in blown through it. The heat exchanger ash blower 32 is rotatably mounted within the heat exchanger 7 such that the shaft extends through the upper heat exchanger unit 31 from the side thereof into the interstice between two rows of U-shaped tubes 52. The shaft 54 is positioned within the upper heat exchanger unit 31 such that the venting apertures 56 therein are aligned with the gaps between the legs of the U-shaped tubes. The mounting block 57 of at least one end of the heat exchanger ash blower 32 is secured to the outer casing of the heat exchanger (not shown).

Referring now to Fig. 4, in which like parts have been given the same reference numerals as before, there is shown the freeboard casing, indicated generally by the reference numeral 58, which forms the freeboard section of the energy conversion system 1 The freeboard casing 58 forms the upper furnace freeboard 23a, the freeboard interconnector 34 and the heat exchanger freeboard 42. The freeboard casing 58 comprises a first casing assembly 60 defining the upper furnace freeboard 23a and the freeboard interconnector 34; and a second casing assembly 62 defining the heat exchanger freeboard 42. The first casing assembly 60 and second casing assembly 62 are connected together at a welded joint 64. The base of the freeboard interconnector 34 slopes upwardly from the upper furnace freeboard 23a to the heat exchanger freeboard 42.

Referring now to Fig. 5, in which like parts have been given the same reference numerals as before, there is shown an embodiment of energy conversion system indicated generally by the reference numeral 70 comprising a fluidising bed unit 72 connected by way of a second freeboard interconnector 74 to a pair of heat exchangers 76. The energy conversion system 70 further comprises the ash-fouling prevention system which in turn comprises a pair of heat exchanger ash blowers 78 which have a shaft (not shown) projecting into the side of the heat exchangers 76 and a heat exchanger pressurised air supply. The heat exchanger pressurised air supply comprises a compressed air tank 80 connected to a pressurised air pipe 82, which is in turn connected to each of the heat exchanger ash blower shafts. The pressurised air pipe 82 is further connected to a plurality of freeboard ash blowers 84 which project into the base of the second freeboard interconnector 74, and so the compressed air tank 80 forms the freeboard pressurised air supply.

Referring now to Fig. 6, in which like parts have been given the same reference numerals as before, there is shown the arrangement of freeboard ash blowers 84 on the base of the second freeboard interconnector 74. The freeboard ash blowers 84 comprise 12 nozzles projecting into the base of the second freeboard interconnector 74, being arranged evenly across the base in four rows of three. Each row comprises an air conduit 86 linking the freeboard ash blowers 84 with the pressurised air pipe 82.

In use, the furnace 3 is supplied with fuel via the fuel conveyor 9 and that fuel is burned in the fluidised bed unit generating heat, exhaust gases, ash and other exhaust materials. An internal draught system (not shown) causes the exhaust materials to flow from the upper furnace freeboard 23a through the freeboard interconnector 34 and down through the heat exchanger 7. As the exhaust materials travel, ash will accumulate on the surfaces it contacts, for example inside the freeboard areas and on the tubes 52 of the heat exchanger 7. Ash that gathers on the tubes 52 will hinder the performance and efficiency of the heat exchanger 7; therefore it is necessary to remove it from the tubes 52 on a regular basis.

The ash-fouling prevention system comprising the heat exchanger ash blower 32 and the freeboard ash blowers 36 operate to prevent the accumulation of ash in unwanted areas. The heat exchanger ash blower 32 operates, in general, one minute out of every thirty minutes, during which time the pressurised air travels down the rotating shaft and out the venting apertures 56 in the shaft 54. This air is provided at a pressure of 6 bar. This causes the high-speed air to circulate around the pipes 33 of the heat exchanger 7 and removes the ash that has gathered on them. The ash is blown off the tubes 52 and falls down into the heat exchanger sump 39, from where it is removed and may be used as fertiliser. As the exhaust materials travel though the energy conversion system from the upper furnace freeboard 23a through the freeboard interconnector 34 to the heat exchanger freeboard 42, it must travel up the slight incline in the freeboard interconnector 34. This will cause the exhaust material to decelerate in the freeboard interconnector 34, thus causing the ash to begin to fall and settle in the freeboard interconnector 34. However, the freeboard ash blowers 36 operate to provide acceleration to the exhaust materials ensuring that the ash remains suspended in the exhaust gases until the exhaust materials reach the heat exchanger 7. The freeboard ash blowers 36 operate in a pulsed manner. The freeboard ash blowers are pulsed every approximately 30 minutes for a duration of 60 seconds.

The use of pressurised air to prevent ash-fouling is particularly useful as it not complex to ensure a supply of pressurised air. Typically, the compressed air tank 80 comprises a 150 litre tank of compressed air. The compressed air, while convenient to provide is a limited resource and must be used in a controlled manner. The ash-fouling prevention system of the invention has been designed to make effective use of this limited resource while still providing the require protection from ash fouling. The manner of operation of the ash-fouling prevention system is a balance between the prevention of ash-fouling and the management of a limited resource.

It will be understood that the term exhaust materials can refer to the products of combustions of the fluidised bed unit including exhaust gases and particulates including ash and the like. Furthermore, it will be understood that the term ash has been used to refer to the particulates in the exhaust materials, and may include fly ash, soot, and other particulates. The particulates typically range in size from 5 to 15 microns.

By thermal treatment or thermally treating the by-product, what is meant is that the byproduct is burnt or incinerated in the fluidized bed. Reference has been made to the incineration of waste and/or by-products and the terms have been used largely interchangeably throughout the specification. For example, in some jurisdictions, poultry litter or mushroom compost is considered to be a by-product whereas in other jurisdictions it is considered to be a waste. In the specification the terms 'comprise', 'comprises', 'comprised' and 'comprising' or any variation thereof and the terms 'include', 'includes', 'included' or 'including' or any variation thereof are considered to be totally interchangeable and they should all be afforded the widest possible interpretation.

The invention is not limited to the embodiment herein described, but may be varied in both construction and detail within the terms of the claims.