WASTE INCINERATOR, METHOD OF COMBUSTION AND WASTE-TO-ENERGY FACILITY

Abstract: This invention relates to a waste incinerator, a method of controlling the combustion in a waste incinerator for power generation, and a waste-to-energy facility. The incinerator provides for ultra high temperature combustion, with the average temperature of gases maintained at or above 1100 DEG C for at least five seconds. The construction of the hearth and its underfire air holes, the provision of a mechanism for the controlled removal of ash with minimum disruption of the combustion bed, the inclusion of an overfire air supply through nozzles producing spaced-apart contra-rotating air flows, and control means to monitor the combustion bed and conditions in the upper zone of the incinerator, enable variation of the energy output from the incinerator by plus or minus at least 20 %. The retention of the combustion gases at a high temperature for a long period in the incinerator enables sulphur scrubbing to be carried out, at least partially, in the incinerator, which in turn reduces corrosion problems downstream. A waste-to-energy facility of the invention includes a plurality of modules, each module including an incinerator, a labyrinth, a boiler, an economiser, an acid gas scrubber and a bag house, at least some of these being themselves modular and replaceable on a rail system. In a preferred form the waste-to-energy facility may include two plants, each of six waste-to-energy modules. An integrated control system for the facility enables the maintenance of a reliable energy output as well as a secure capacity for handling a particular volume of waste material.

Description: WASTE INCINERATOR, METHOD OF COMBUSTION AND WASTE-TO-ENERGY FACILITY TECHNICAL FIELD This invention relates to the incineration of waste material for power generation, and to a method of controlling combustion for power generation. BACKGROUND ART The use of landfill sites for disposing of bulk refuge is becoming less acceptable internationally, as the volume of waste material grows and the identification of suitable sites become more difficult. In addition, substantial costs are associated with ongoing management of landfill sites to minimise environmental risk. Many landfill sites worldwide have become and are recognised as hazardous sites through soil and ground water contamination and methane emissions, for example. Landfill gas is approximately 52%CH4 and 46%C0 2 . While both are "greenhouse effect" gasses, CH is up to 73 times as "effective" as CH 2 as a global warming potential gas. In addition to the methane environmental contamination, landfill sites release substantial volumes of hydrogen sulphide, poly-aromatic hydrocarbons, halogenated compounds and other toxic gases into the atmosphere. The landfill management programmes for leachate containing heavy metals are extremely expensive and frequently unlikely to represent a viable solution. Moreover, in many parts of the world, many landfill sites have inadequate or no management. The eventual destination of heavy metals and dioxin from such landfill sites is the water table or harbour. Waste material offers a huge potential source of fuel for power generation which, currently, is largely wasted, and an alternative to landfill is incineration. In this specification the term "waste fuel" is used to describe any form of material that can be incinerated, but particularly garbage or waste materials. Two zones are important in terms of incineration: the combustion bed zone where there is a high flame temperature; and the combustion gas zone. The temperature in the combustion gas zone may vary considerably between lower and upper regions of conventional incinerators, but an average temperature can be determined from temperature probes positioned at specified locations within the incinerator. Waste incineration systems may be broadly categorised into two types in respect of the combustion gas zone, low temperature and high temperature. Most conventional waste incineration systems are of the low temperature type in which incineration of the fully mixed gases occurs at an average temperature up to about 538 °C, which is too low for decomposition of all toxic materials. Conventional higher temperature incineration systems employ temperatures in the range 538 °C to 900°C, but this temperature may also be insufficient for decomposition of all toxic materials, and is still not high enough for efficient commercial power generation. Incinerators have been developed which enable primary combustion at temperatures of 1000°C, to improve the breakdown and decomposition of toxic chemicals and materials, e.g. as described in United States Patent Specification No. US 4,852,504. That specification describes a waste fuel incinerator which provides for primary combustion in the combustion bed zone under oxidising conditions (i.e. with excess underfire air). The oxidising conditions enable the high temperature combustion. However, the system requires the heating to high temperature of a large mass of material in the combustion bed itself. Furthermore, the speed of primary combustion may be too rapid, and the time during which the combustion gases are retained at or about 1000°C too short, to adequately reduce or eliminate some toxic materials. US 4,852,504 also discloses the use of one or more overfire air inlet passageways. This overfire air is provided to cool and dilute the flue gases. The combustion chamber of US 4,852,504 is formed using moulded refractory olivine panels, which enable the incinerator to operate at the required higher temperatures. Known higher temperature waste incinerators, such as that described in US 4,852,504, have a limited capacity, a limited ability to control the combustion temperature and residence time in the primary combustion area, and do not allow any substantial regulation of the energy output. Therefore, they do not allow for optimum control of combustion to remove toxic materials and gases, and they do not facilitate the control of energy output to enable a constant supply of energy to a grid system, i.e. they do not provide for any security of power supply. Thus, conventional high temperature waste incinerators used for power generation offer an all or nothing approach to power generation. There is little, if any, ability to adjust the power output from individual incinerators. Furthermore, little consideration is given to systems which facilitate maintenance to minimise downtime. As a result, power generation from such systems is unpredictable. Whilst that may be acceptable where the power generation of the incinerator is only a small contribution to a major grid, it is not acceptable where the system is a major contributor to a particular grid, for example in small island communities or regions with relatively small populations and electricity requirements. Thus, it is an object of the present invention to provide a waste incinerator, method of combustion and/or waste-to-energy facility which reduces or overcomes the above-mentioned problems, or which at least provides the public with a useful alternative. Other objects of the invention may become apparent from the following description which is given by way of example only. In this specification the term "average temperature" is used to refer to the average temperature of fully mixed gases in the combustion gas zone. The term "splayed U-shaped hearth" is used to describe a hearth having a substantially flat base portion and steep sides. SUMMARY OF THE INVENTION According to one aspect of the present invention, there is provided a waste incineration unit, including: an incinerator with a hearth adapted to enable combustion under reducing conditions; - a waste fuel inlet for the supply of waste material to the hearth; ash removal means to remove ash from the hearth; and overfire air supply means to mix combustion gases and provide oxidising conditions in an upper zone of the incinerator; and wherein the combustion gases are retained at an average temperature > 1 1 00°C for at least five seconds. Preferably, the combustion gases are retained at an average temperature of substantially 1 1 25°C for at least five seconds. Preferably, the incinerator is adapted to provide a substantially uniform temperature across the column of combustion gases. In one preferred form, the hearth may include two or more splayed U-shaped hearths. Preferably, the or each hearth may have sides at angles of substantially 70° to 75° from the horizontal. Preferably, each hearth side may include a plurality of underfire air nozzles. Preferably, the nozzles are adapted to penetrate air through waste fuel in the or each hearth to at least a mid-region of the hearth. Preferably, the airflow through each nozzle, or groups of nozzles, may be separately controllable. In a further preferred form of the invention, the overfire air supply means may include a first set of overfire air holes at a first height in the incinerator. Preferably, the first set of overfire air holes may be angled to cause the rotation of air and combustion gases, in a first rotating direction. Preferably, the overfire air supply means may further include a second set of overfire air holes at a second height in the incinerator, above and spaced apart from the first set. Preferably, the second set of overfire air holes may be angled to direct the flow of air and combustion gases to rotate in the opposite direction to the rotation induced by the first set of overfire air holes. Preferably, a waste incineration unit of the invention may further include means for introducing lime for sulphur scrubbing, through the overfire air holes. In a further preferred form, the ash removal means may include tiered pushers, each pusher adapted to push material from a different longitudinal portion of the or each hearth. Preferably, there may be a set of tiered pushers associated with the or each hearth. Preferably, in the or each hearth, there may be a lower pusher adapted to remove ash from a portion of the hearth closest to an ash outlet, one or more mid-pushers adapted to move material through a mid-portion of the hearth, and an upper pusher adapted to move material towards the mid-portion from a region of the hearth adjacent the waste fuel inlet. In a further preferred form, a waste incineration unit of the present invention may further include waste fuel supply adjustment means, adapted to enable adjustment of the calorific value of waste fuel supplied to the incinerator. Preferably, a waste incineration unit of the present invention may further include control means adapted to control combustion in the incinerator and provide a required energy output for a power generator driven by the energy produced by the combustion process. In a further preferred form, a waste incineration unit of the invention may further include slope monitoring means to monitor a slope on the combustion bed during operation of the incineration unit. Preferably, the waste incineration unit may further include heat monitoring means adapted to monitor the temperature across the combustion bed. Preferably the waste incineration unit may further include combustion gas monitoring means in an upper zone of the incinerator. Preferably, the control means may control the operation of the tiered pushers to maintain a preferred slope on the combustion bed, as monitored by the slope monitoring means. Preferably, the control means may also control the overall supply, and/or regional supply, of air through the underfire air nozzles to control the temperature across the combustion bed as monitored by the temperature control means. Preferably, the control means may also control the air supply through the overfire air holes and/or the supply of lime to the overfire air holes, in response to the combustion gas monitoring means. Preferably, the control means may enable variation of the energy output by plus or minus at least 20%. In a further preferred form, a waste incineration unit of the present invention may further include a secondary combustion chamber into which combustion gases from the incinerator are directed, and including a high temperature zone and a cooling zone. Preferably, in a waste incineration unit of the invention, the combustion gases are retained in the incinerator and/or the high temperature zone of the secondary chamber at an average temperature > 1 100°C for at least 7 seconds. Preferably, the gases are retained at this average temperature for about 8 seconds. According to a further aspect of the present invention, there is provided a waste incineration unit, including an incinerator with a hearth, a waste fuel inlet for the supply of waste fuel to the hearth, ash removal means for removing ash from the hearth and slope control means adapted, in use, to enable maintenance of a pre-selected slope on a combustion surface in the hearth from the waste fuel inlet at one end to an ash outlet at the opposite end. Preferably, the slope control means may include slope monitoring means to monitor the height of the combustion surface along a length of the hearth, and wherein the means of ash removal is adapted to enable movement of material along different portions of the hearth length independently. According to a further aspect of the present invention, there is provided a waste incineration unit, including an incinerator with a hearth adapted to enable combustion under reducing conditions, a waste fuel inlet for supplying waste fuel to the hearth, means for removing ash from the hearth and overfire air supply means to mix the combustion gases and provide oxidising conditions in an upper zone of the incinerator, said overfire air supply means including a first set of overfire air holes adapted to produce a first rotating region of air and gases above the combustion bed and a second set of overfire air holes adapted to produce a counter-rotating region of air and gases above the first rotating region of air and gases. According to a further aspect of the invention there is provided a waste incineration unit including an incinerator with a hearth adapted to enable combustion under reducing conditions and overfire air supply means to mix combustion gases and provide oxidising conditions in an upper zone of the incinerator, and wherein the combustion gases are retained at a sufficiently high temperature and for a sufficient period to enable sulphur scrubbing, or partial sulphur scrubbing in the incinerator. According to a further aspect of the present invention, there is provided a method of waste fuel incineration, including: supplying waste fuel to a hearth in an incinerator under incineration conditions, to form a combustion bed; - supplying air into the combustion bed at a level to control primary combustion under reducing conditions; supplying turbulent air above the combustion bed in a manner which mixes the combustion gases, provides excess oxygen for substantially complete combustion in an upper zone of the incinerator and controls the flow of the combustion gases through the incinerator, such that the combustion gases are retained at an average temperature ≥ 1 1 00°C for at least five seconds. Preferably, the method may further include removing ash from beneath the combustion bed during combustion. In one preferred method of waste fuel incineration, the supply of air into the combustion bed may be at a level of substantially 80% or less of the stoichiometric requirement for complete combustion. Preferably, the oxygen level in the upper zone of the incinerator may be maintained at substantially 1 0% above the level required for complete combustion. Preferably, the method may further include maintaining a preferred slope on the combustion bed. Preferably, the method may further include controlling the calorific value of waste fuel supplied to the combustion bed. Preferably, the method may further include transferring the combustion gases from the upper zone of the incinerator to a secondary combustion chamber, including a high temperature zone and a cooling zone, and whereby the combustion gases are retained in the incinerator and/or the high temperature zone of the secondary chamber at an average temperature > 1 100°C for at least seven seconds. Preferably, the method of waste fuel incineration may further include sulphur scrubbing in the incinerator. According to a further aspect of the present invention, there is provided a method of waste fuel incineration wherein waste fuel is supplied to a hearth in a lower zone of an incinerator under incineration conditions, whilst conditions in an upper zone of the incinerator enable acid scrubbing in the incinerator. Preferably, the method may include supplying turbulent air above the combustion bed in a manner which substantially separates the lower zone from the upper zone. Preferably, the supply of turbulent air may include the supply of a lower layer of rotating air and the supply of an upper layer of contra- rotating air. Preferably, the method may further include the controlled supply of lime in the upper layer of contra-rotating air. Preferably, the method may further include maintaining the combustion gases in the upper zone at an average temperature > 1 100°C for at least five seconds. According to a further aspect of the present invention, there is provided a waste-to-energy module adapted to generate power from waste incineration, including a waste incineration unit as hereinbefore described, a boiler for generating steam to drive a power generator, combustion gas emission control means and control means adapted to enable control of the energy output from the waste incineration unit by at least plus or minus 20%. According to a further aspect of the present invention, there is provided a waste-to-energy module adapted to generate power from waste incineration, including: an incinerator adapted to enable combustion under reducing conditions, - a labyrinth adapted to capture flyash-borne material and cool combustion gases, a boiler for steam generation to drive a turbine for power generation, an economiser for pre-heating feed water for the boiler, an acid gas scrubber, - and a bag house for collecting particulate material, and wherein at least the boiler, economiser and bag house are modular and replaceable. According to a further aspect of the present invention, there is provided a waste-to-energy facility adapted to maintain a reliable energy output, including at least one plant of a plurality of waste-to-energy modules, each module as hereinbefore described, at least one power generator and integrated control means adapted to enable control of the energy output from the waste-to-energy modules to accommodate fluctuations in power demand from the facility. Preferably, the waste-to-energy facility may include six waste-to- energy modules in the or each plant. Preferably, the waste-to-energy facility may include two plants of waste-to-energy modules. Preferably, the integrated control means may be adapted to enable control of the energy output from each waste-to-energy module by at least plus or minus 20%. Preferably, at least the boiler and associated economiser and the bag house of each waste-to-energy module are supported on a rail system and the waste-to-energy facility includes a cross-rail system onto which a boiler, economiser and/or bag house maybe transferred for removal and replacement. Other aspects of the present invention may become apparent from the following description, which is given by way of example only, and with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 : shows a schematic view through the side of a waste incineration unit of the present invention, in one preferred form; FIGURE 2: shows a schematic end view of the waste incineration unit of Figure 1 , from A; FIGURE 3: shows a transverse section through a hearth of the incinerator of Figure 1 ; FIGURE 4: shows a plan view of the hearth of Figure 3; FIGURE 5: shows a side view of the waste fuel supply mechanism and ash pushers, of a waste incineration system of the invention in one preferred form; FIGURE 6: shows a plan view of the waste fuel supply mechanism for an incinerator system of the invention, in one preferred form; FIGURE 7: shows a schematic representation of an incineration unit of the invention within its furnace cover and showing some of the servicing facilities; FIGURE 8 shows a ducting and fan system which may form a part of an incineration system of the invention; FIGURE 9: shows a schematic representation of a labyrinth which may form part of the waste incineration system of the invention; FIGURE 10: shows a plan view of the labyrinth of Figure 7 and incinerator unit of the invention; FIGURE 1 1 : shows a schematic representation of a waste-to- energy unit or module of the invention, in one preferred form; FIGURE 12: shows a plan view of the waste-to-energy module of Figure 9; FIGURE 13 shows: A. side, B. plan and C perspective views of a boiler and economiser of the waste-to-energy module of Figures 9 and 1 0; FIGURE 14 shows: A. side, B. plan and C perspective views of a lime scrubber of the module of Figures 9 and 1 0; FIGURE 15 shows: A. side, B. plan, C perspective and D. end views of a bag house of the module of Figures 9 and 10; FIGURE 16: shows a schematic representation of a waste-to- energy power facility of the present invention, in one preferred form. DETAILED DESCRIPTION OF THE INVENTION Waste Incineration Unit With reference to Figures 1 -4, an incinerator 1 of the present invention consists of a cylinder 2 manufactured from panels of olivine refractory material (as is apparent, for example, in Figure 6). The cylinder 2 has a domed roof 3 with a central exhaust 4. At the lower zone 5 of the cylinder 2 are two splayed U-shaped hearths 6. At the bottom of each hearth 6 is a tiered system of ash pushers 7. Underfire air holes 8 are provided in lower portions of the hearths 6. A first set 9 of overfire air holes are provided towards an upper portion of the hearths 6 and a second set of overfire air holes 10 are provided above and spaced-apart from the first set 9. Each hearth 6 has a side angle of 70° or more (preferably about 73°) and narrows to a base of about 0.55m. The angle of the sides is selected to ensure that ash and aggregate formed during combustion slide back down the hearth sides into the combustion bed. In addition, the steeper sides to a hearth means that the surface area of the combustion bed is less dependent on the depth of fuel in the hearth. The underfire air holes 8 are relatively small but numerous, and are in both sides of each hearth. They are also self-cleaning and air- limiting so that they do not get blocked by aggregate, and so that the flow of air into the fuel can be controlled to maintain combustion under reducing conditions and optimise combustion across the combustion bed. Thus, there is an important relationship between the ability to control airflow through the underfire air holes 8, and the distance across each hearth. The inclusion of two parallel hearths enables a combustion bed of substantially twice the area to be infused effectively and controllably with air from the underfire air holes 8. It is anticipated that incinerators of the present invention of greater capacity may be produced by the inclusion of three or more hearths. The capacity of an incinerator with two hearths is about 20 tonnes of waste in various oxidising stages. The incinerator in one preferred form may have a diameter of about 1 3.2m and a height of about 1 3m. With reference to Figure 4, each hearth side 1 1 may be divided on its rear side by segmenters 1 2, which separate the airflow to the underfire air holes 8, so that airflow may be increased or decreased to different regions of the combustion bed, from either side of the hearth. It will be appreciated that in an alternative arrangement air could be supplied to individual underfire air holes independently. Thus, the multiple hearths, numerous, small underfire air holes on each side of each hearth, and the ability to control air flow into different regions of each hearth, enables control of combustion in the bed to achieve substantially homogeneous combustion across the bed. Infrared monitors (not shown) are provided in the roof 3 of the incinerator 1 . These monitors enable the temperature across the whole surface of the combustion bed 14 to be monitored and the under-fire air supply to be adjusted in different regions to optimise combustion in all parts of the bed. Selected underfire air holes may include auxiliary fuel lines. An auxiliary fuel supply is used initially to bring the incinerator up to minimum operating temperature before the main waste fuel supply is introduced, and to ensure complete oxidation of the final charge at any shutdown. In addition, it may be used in selected parts of the combustion bed where combustion is inadequate. Auxiliary fuel ports 1 3 may be of particular importance to ensure complete combustion of material prior to ash removal. The auxiliary fuel burners are a part of an integrated control system for the ultra high temperature combustion system of the invention. Thus, when an increased energy output is required, or if the calorific value of the waste fuel drops for a long period of time, then these burners may be activated and/or other high calorific value material may be introduced to the fuel (hot waste oil in the short term and rubber in the longer term). The auxiliary fuel supply may be H2 and CO, which may be derived from a vitrifier used to process flyash to form a physical matrix which immobilises heavy metals in glass. The first set of overfire air holes 9 are oriented to produce circulation of air in one direction around the incinerator 1 , and at a sufficiently high velocity to penetrate into the centre of the chamber. The second set of overfire air holes 1 0 are oriented to form circulation of air and gases in the opposite direction. Thus, the first and second sets of overfire air holes produce contra-rotating airflows. They may be spaced-apart by 1 to 2m, but preferably about 1 .5m. Because primary combustion in the combustion bed of an incinerator of the present invention is carried out under reducing conditions, oxygen is required through the overfire air holes to ensure continuing and complete combustion in the upper zone 1 5 of the incinerator 1 . The rotational airflow from the overfire air holes, and particularly, the contra-rotating airflows, provides for improved gas mixing, and facilitates control over the residence time of the gases through the incinerator 1 . It also provides effective separation of the incinerator 1 into two combustion zones: a lower zone having reducing conditions immediately adjacent the combustion bed, and an upper zone having oxidising conditions. During operation of an incinerator of the present invention, air supply through the underfire air holes to the fuel is maintained at about 80% of the stoichiometric requirement for complete combustion. Under these reducing conditions, a larger proportion of volatile gases are released from the fuel at the site of primary combustion. The temperature in the combustion bed is about 1 700°C, and the residence time of fuel across the hearth is a minimum of one hour. Under such reducing conditions, there is a direct and linear relationship between temperature in the bed and air supply to the bed. Thus, the combustion temperature in the bed can be readily controlled by the regulation of underfire air supply, according to the nature of the fuel. This is important given the variable calorific value of waste material. The average temperature is maintained in the upper zone of the incinerator 1 at greater than or equal to 1 100°C by the use of refractory panels and the over supply of oxygen through the overfire air holes. The incinerator operates at a 5-20% excess oxygen level in this region to facilitate complete combustion. The nature of the construction of the incinerator, with its refractory panels, ensures that the temperature is substantially uniform across the diameter of the gas column. This contrasts with the situation in conventional incinerators with water-cooled walls which have a peripheral ring of lower temperature adjacent the walls, permitting "short-cutting" of some gases through the incinerator. The domed surface of the roof 3 of the incinerator improves the structural integrity of this region of the incinerator, and the central positioning of the exhaust 4 ensures that there can be no "short cut" of combustion gases from the incinerator, as would be the case if the exhaust were from the side of the chamber. A negative air pressure is maintained in the incinerator 1 to ensure that combustion gases are drawn through the central exhaust 4. With reference to Figures 9 and 10, gases leaving the incinerator 1 enter a secondary chamber, the labyrinth 20. This chamber consists of an array of refractory panels which have sufficient thermal mass to smooth temperature variations in the system caused by variations in fuel or surges at ash push. The labyrinth 20 also gives a further residence time of at least two seconds at high temperature, allowing any flyash- borne material to reduce and allowing coarse flyash to drop out of the gas stream. The primary region 21 of the labyrinth also facilitates the removal of subsequent CaS0 4 and residual calcined lime and silica from the gases, which results from sulphur scrubbing in the incinerator 1 . In addition, a secondary region 22 of the labyrinth includes two banks of sacrificial pre-boiler screen tubes 23 which enable the temperature of the gases to be reduced rapidly below the eutectic slagging temperature of sodium and potassium. The sacrificial tubes can be cleaned by mechanical wrapping without shutting down the incinerator. Furthermore, since they are in a high erosion and corrosion situation, they are designed for rapid replacement. Thus, the sacrificial tubes, which are manufactured from high carbon steel are easily disconnected from the boiler for replacement. The primary region 21 and secondary region 22 of the labyrinth 20 are separated by a water-cooled wall 24. This water cooled wall 24 is sacrificial or replaceable like the sacrificial tubes 23. It is only practical to use this type of structure in the labyrinth because of the ability to have sulphur scrubbing in the incinerator, reducing the rate of acid damage to this water-cooled wall and the sacrificial tubes. An advantage of using this water-cooled wall 24, as opposed to the design of conventional labyrinths which include a substantially u-shaped configuration of refractory panels with a horizontal chamber between a primary region and a secondary region, is that there is an acute change in direction and velocity of combustion gases from the primary region 21 to the secondary region 22, beneath the water-cooled wall. This facilitates the dropping of fly ash into the collection areas 25. Furthermore, the labyrinth can be produced in a more compact form. Incinerators require some means for removal of ash from beneath the combustion bed. This is generally achieved by the use of an ash pusher or ram positioned in the base of the hearth which periodically during combustion pushes ash from beneath the combustion bed to an ash outlet. This operation can cause substantial disruption to the combustion bed, both through the physical movement and an air surge during operation of the pusher. In order to minimise this disruption, and further improve the ability to control combustion, the incinerator of the present invention includes multiple tiered hearth pushers 30, 31 , 32 in each hearth, and a monitoring system to monitor the slope of the combustion bed. This monitoring system may use, for example, an ultrasound or infra-red system. With reference to Figures 3-5, with the twin hearth incinerator shown in the figures, a 3-tier pusher system is preferred. The pushers 30, 31 , 32 may sit one on top of the other, or alternatively, there may be a "floor" 33 separating and/or supporting the second pusher 31 above the first 30 for the length of the second pusher 31 , and the third pusher 32 above the second 31 , for the length of the third pusher 32. The first pusher 30 pushes ash to the ash outlet. On the other hand the material moved by the second 31 and third 32 pushers is only partially or incompletely combusted. Movement of the pushers 30, 31 , 32 in relation to one another tends to cause a rolling movement of material in the combustion bed, rather than a straight push. This rolling movement aids drying and gasification, and is more effectively achieved in the embodiment excluding the separating floors 33. The tiered pusher system enables material to be moved progressively across the hearth by the sequential operation of the pushers 30, 31 , 32 with minimal disruption to the combustion bed 14. It also enables an optimal slope to be maintained on the combustion bed. This slope, in conjunction with the angle of the hearth sides, is important in determining the combustion area. It is also important in controlling the rate of progress of fuel across the combustion bed. The angle of the slope, as monitored by the monitoring apparatus, can be adjusted by appropriate movement of one or more pushers 30, 31 , 32. It will be appreciated that division of the combustion bed into two areas (i.e. in each hearth), and the inclusion of three separate pushers in each hearth, means that only a small portion of the overall combustion bed is disrupted during the operation of any given pusher. For an incinerator of greater capacity, which may include more than two hearths, each hearth of greater length, there may be a need to include a fourth or more tiers of pusher. With reference to Figure 3, it can be seen that each pusher 30, 31 , 32 has a trapezoidal cross-section. Triangular-shaped fillers (not shown) may be included between the hearth wall and the edge of each pusher. This configuration of pusher reduces the risk of a pusher jamming due to aggregate/ash becoming trapped between the pusher and the hearth wall. Each pusher is controlled by a hydraulic ram, as shown in Figure 5, and the pusher control area 35 may be pressurised to enable control over the amount of air going into and out of the incinerator 1 during operation of the pushers, and to avoid ash and gases coming back into the pusher control area 35. Ash removed from the incinerator 1 is pushed through an ash outlet 36, having an ash door 37, into the ash chamber 38 which includes a hopper 39 for ash disposal. Because the temperature in the upper zone 1 5 of the incinerator 1 is substantially higher than in conventional waste incinerators, and because the residence time of gases in the incinerator is longer, it is possible to introduce high temperature lime scrubbing in the incinerator 1 itself. Thus, lime can be sprayed into the incinerator through the over fire air holes, preferably the second set 10. Smaller particles of lime are preferred since they are more easily carried in the airflow and are less susceptible to sintering. At the elevated temperatures existing in the incinerator the CaCθ3 is calcined and the newly formed CaO reacts with the acid gases to form CaCI, CaS0 4 and residual CaO. By reducing the acid gas level in the flue gases, corrosion problems in the boiler are reduced and the particular configuration of labyrinth described above can be used. Furthermore, the CaS0 4 , CaO and silica are commercially useful residues, for example for cements, in the absence of carbon and heavy metals. Laser spectroscopy is employed to monitor the constituents of the gases in the primary incinerator 1 , and also in the primary region 21 of the labyrinth 20, so that lime addition can be controlled by monitoring the sulphur content. This is important in a waste incineration unit given the high variability of sulphur content. With reference to Figure 6, fuel is supplied into the incinerator 1 via inlet 40 . This fuel is supplied in a manner which avoids any uncontrolled release of air into the incinerator 1 , and provides for some degree of control over the nature of the fuel. Thus, the primary fuel (garbage or waste material), is supplied by a walking floor 41 to an entry chamber 42. This chamber 42 has a hydraulic ram 43 operating at 90° to the walking floor. Once the chamber 42 is full, the ram 43 pushes the fuel into a second lock chamber 44. This chamber 44 is essentially sealed to the outside by the front face 45 of the first ram 43, an upper surface 46 of an extended lower ram 47 from a lower chamber 48 and the lower surface 49 of a vertical ram 50 above. The lower ram 47 is retracted and the vertical ram 50 then forces the material into the lower chamber 48. The lower ram 47 then pushes the waste material into the furnace through inlet 40. At that stage, the upper surface 46 of the lower ram 47 again seals the floor of the second lock chamber 44, the vertical ram 50 can be raised and the first ram 43 retracted for the next feed of waste material. The waste fuel supplied to an incinerator of the invention may be in any form, for example, directly from collection contractor vehicles or after compaction from transfer stations. The calorific value of the waste material entering the incinerator may be adjusted by the addition of high calorific material (e.g. tyres, oils, hospital waste and the like), or low calorific material (e.g. sewage sludge and other wetter material. High calorific material such as tyres may be added to the entry chamber 42 by an additional ram mechanism 51 . Low calorific material may be added to the lower chamber 48, for example by a screw conveyor-type system (not shown). Thus, the incinerator 1 is always sealed from the main waste fuel loading floor area 53 , preventing disturbance of the oxidising conditions and the release of smoke and hot gases from the furnace to the loading floor area 53. Figure 7 shows an incinerator 1 in context in an operational waste incineration unit or waste-to-energy module. The primary oxidising chamber 1 is housed within a furnace cover 60 which separates it from the loading floor area 53. A containerised system for waste material is shown as a rack 61 of containers 62. Beneath the loading floor area 53 and furnace cover 60 there may be service tunnels 63. These service tunnels 63 provide access to regions beneath the incinerator 1 , the pusher control area 35, and indeed other components of the waste-to-energy module described hereafter. The service tunnels 63 may also provide ducting for cabling and air supply duct 70 to the incinerator 1 . The supply of air may be controlled through fans 64. As shown in Figure 8 the fans 64 may draw hot air 68, via ducting, from an upper region of the space between the furnace cover 60 and the incinerator 1 . Cool air 66 may be drawn in from the loading floor area 53 or elsewhere. Air mixing units 67 provide for the controlled mixing of hot and cold air through the two sets of fans; one set 64a, 64b, supplying hot air 68 via ducting to the underfire air holes, and the second set 64c, 64d providing air 69, via ducting, at a regulated temperature to the over-fire air holes. The ability to regulate the air temperature through both the underfire and overfire air holes provides a further mechanism for controlling the combustion conditions within the incinerator 1 . Method Of Combustion The method of combustion employing an incinerator of the present invention will now be described. During start up, an ancillary fuel or accelerant is provided through the underfire air holes in the hearth. Once combustion is underway, waste fuel is supplied via the controlled waste supply system. A predetermined height and slope of combustion surface is maintained through a combination of three factors: (a) continuing supply of new fuel; (b) the removal of ash at a controlled rate by pushers; and (c) control over the rate of combustion across the combustion bed by monitoring the temperature across the combustion bed and adjusting airflow through the under fire air holes. The fuel and combustion gases are retained in the primary combustion area, under reducing conditions, at substantially 1 700°C for about one-and-a-half seconds. Excess oxygen is then introduced in a region of high turbulence. The contra-rotating over-fire air flows provide the required excess oxygen, high turbulence, control over the residence time of gases in the incinerator, and a degree of separation between the primary combustion bed and the upper combustion zone of the incinerator. Depending on the level of the sulphur content in the combustion gases, high temperature sulphur scrubbing may be initiated in the upper zone by the inclusion of lime particles through the overfire air holes. Through the control of combustion in the combustion bed and airflow through the over-fire air holes, the combustion gases are retained at an average temperature of substantially 1 100°C or greater for five to eight seconds. Secondary combustion continues in the primary area of the labyrinth into which the combustion gases are drawn from the central exhaust of the incinerator. The gases are then rapidly cooled in the labyrinth before entering the boiler for steam generation for energy production. A computer-based fuzzy logic control system is employed in the combustion system of the invention to control fuel feed rate, fuel calorific value adjustment, the temperature, rate and distribution of underfire air supply, the temperature and rate of overfire air supply, sulphur scrubbing in the incinerator and ash removal. The system monitors and responds to changes in O2 and temperature measurements at the outlet of the incinerator, CO and S content in the incinerator and/or labyrinth, and temperature across the combustion bed slope. The O2 signal controls the flow of primary air. Increasing O2 levels reduce the flow of primary air through the underfire air holes. The temperature signal and a measurement of the actual primary airflow then controls the waste fuel feed rate. CO spikes trigger a signal to temporarily increase the amount of overfire air. CaCθ3 is introduced to the upper zone of the incinerator, as required, to reduce the level of SOx by at least 50% and up to 90% in the exhaust gases before they enter the boiler, and to moderate heat spikes. This reduces erosion and corrosion in the boiler, and also of the water wall and sacrificial tubes of the labyrinth. With the lower flame temperature used in a reducing bed, Nθx is less of a problem than with an oxidising bed. However, in addition to the sulphur scrubbing, NOx scrubbing may be used to reduce NOx by spraying a solution of urea into the incinerator above the flame zone. The system may include two injection levels for the urea to avoid urea injection into the flames. The height of the flames will control which level to use. The design of the incinerator of the present invention, and its integrated fuzzy logic control system, provides for ultra high temperature combustion, with the average temperature of the gases maintained at or above 1 100° C for more than five seconds, and preferably about eight seconds, to optimise combustion and minimise toxic exhausts. It further provides a waste incineration system whereby the combustion process can be tailored to provide a required energy output. The level of control provided over the combustion process enables a 40% variation in energy output (plus or minus 20% capacity) . This is a highly significant advantage over conventional waste incineration units. The combustion gases are retained at about 1 700°C in the area of the combustion bed for approximately one and a half seconds. They are then retained in the upper zone of the incinerator for at least four seconds at greater than or equal to an average temperature of 1 1 00°C. Thus, the total residence time in the incinerator is in the range five to eight seconds. This long residence time, at ultra high temperature, with good turbulence and high radiated heat from a huge thermal mass gives the incinerator of the invention excellent performance characteristics, especially as there are no cold surfaces collecting radiant heat. The ultra high temperature combustion process destroys dioxin and is a near total dioxin sink. In a fully operational system employing an incinerator of the invention, heavy metals will be collected in a bag house filtering system. The fly ash resulting from the process, may be managed to minimise environmental risk. For example, the material may be employed in a vitrification process which immobilises the heavy metals not recovered and facilitates the recycling of chorine as HCl, elemental mercury, sulphur and other metals. The resulting glass may be used to make bi-products. In contrast with a landfill system for dealing with waste, the present ultra high temperature combustion system requires much less space, does not have the potential for ground water to contamination, controls dust within the system, has no CH 4 emissions, retains complete control over heavy metals and dioxin, and optimises energy recovery. The exhaust from the incinerator includes virtually no detectable hydrocarbons and very little carbon monoxide. Waste-To-Energy Facility Waste incineration units of the invention may be used in a waste- to-energy facility which generates power from waste incineration in a manner which provides for a secure minimum power output, adjustable to accommodate peak power demands, and a secure capacity for handling a minimum volume of waste material. Figure 1 1 represents a single unit 1 00. A preferred waste-to-energy facility includes two plants of six power generation units or modules. Each unit 100 is a stand-alone unit, integrated into a system through integrated control means to control the overall power output, and through the use of a common fuel source, common bi-product processing and common maintenance systems. It will be appreciated that facilities providing a greater or lesser capacity may be provided by having more than two or only one plant of power generation units, respectively. A facility with twelve units, with an electrical output as shown in Table 1 , is designed to have a refuse throughput of about 1 .35 million tonnes per year, whilst only requiring storage capacity for a maximum three days of refuse. No. Of Furnaces 1 2 Nominal Contracted Throughput ~ Tonnes/pa 193,482 386,963 580,445 870,668 1 , 160,890 Maximum Contracted Throughput Tonnes/pa 193,482 386,963 604,630 967,409 1 ,330,187 Tonnes/hr Min 16.6 33.1 49.7 74.5 99.4 Max (1 chain down) 13.8 41 .4 69.0 1 10.4 1 51 .8 Nominal rating 22.1 44.2 66.3 99.4 132.5 Max contracted 22.1 44.2 69.0 1 10.4 151 .8 Max 27.6 55.2 82.8 1 24.2 165.7 Catch-up (hours) one furnace down 3 days 108 1 8 0 0 for Catch-up (hours) one furnace down 6 days 216 36 0 0 0 for Min Net Electrical Output MW 9.4 18.8 28.2 42.2 56.3 Mom Net Electrical Output MW 12.9 25.8 38.7 58.1 77.5 Max Net Electrical Output MW 16.4 32.9 49.3 74.0 98.6 Electrical Output Modulation (Max- 7.1 14.1 21.2 31 .8 42.3 Min) One or more turbines is associated with each plant of a waste-to- energy facility, with steam from the individual modules piped collectively to the or each turbine. With reference to Figures 1 1 and 1 2, it can be seen that each waste-to-energy module 100 of a waste-to-energy facility includes a waste feed system 101 an incinerator 1 , with its own ash removal system, a labyrinth 20, a boiler 1 02, an economiser 103, an acid gas scrubbing unit 104 and a bag house 105. A stack ( 1 06 in Figure 1 6) for exhaustion of gases may be associated with each module or common to a number of modules. Waste is supplied to each plant of modules through a common refuse tipping hall or waste fuel loading floor area 53 (see Figures 7 and 1 6). The refuse may be compacted or direct from refuse collection. Each waste feed mechanism 101 is adapted to supply waste to its incinerator 1 , and to enable modulation of the calorific value of the waste by the controlled feed of high calorific material, such as oil or tyres, or low calorific material, such as sewage or other wet waste, to the fuel supply immediately prior to entry into the incinerator, as previously described. Ash from an incinerator exits to an outlet chamber 38, from which the dry ash is deposited in rail carriages, in a controlled manner, for transportation away from the incinerator via bed ash rail tunnels 1 07, for further processing at a bed ash retrieval plant 1 31 . A common rail system may service all incinerators, or at least those in a single plant. Combustion gases exiting the top of the incinerator 1 are ducted to a first hot zone of the labyrinth 20. The gases pass down and around the water-cooled wall into a cooling zone of the labyrinth where rapid cooling occurs as the gases pass banks of sacrificial tubes. Fly ash is deposited and collected in the base of the labyrinth. The gases then pass to the boiler 102 where they are used to generate super heated steam used to drive a turbine for power generation. The gases then pass into the economiser 103 where feed water for the boiler is pre-heated. Details of the boiler 102 and economiser 103 are shown in Figure 1 3. Ash hoppers 108 are included in the base of each of the boiler 1 02 and economiser 103 for collecting fly ash. Both the boiler 102 and the economiser 103 are mounted on rail carriages 109. Details of the acid gas scrubber 104 are shown in Figure 14. The combustion gases are directed through ducts 1 10 to a lower region of the scrubber 1 04. Ash hoppers 1 1 1 are positioned in the base of the scrubber 104 and two turbulence manifolds 1 1 2 are positioned above the entry of the gases into the scrubber 104. Fans 1 1 3 are provided to produce two contra-rotating air flows through the turbulence manifolds. Between the turbulence manifolds 1 1 2 there are provided lime slurry nozzles 1 14. Thus, hydrated lime in a fine atomised slurry form is added to the gas stream, in the midst of a region of high turbulence. The lime feed is controlled by the SO2 and HCl levels in the exhaust gases. This wet-dry scrubbing system is designed to remove more than 80% of SOx remaining after the high temperature scrub and 95% of HCl. As the gases pass from the acid gas scrubber 104 unit to the bag house 105, an activated carbon scrub is implemented in the duct 1 1 5 to remove heavy metals and any newly formed dioxin. Thus, there is turbulence manifold 1 1 6 positioned in the duct 1 1 5, with active carbox injection nozzles 1 1 7. The gases exiting the acid gas scrubber 104, containing lime particles, activated carbon particles and the absorbed acid gases, pass to the bag house 105. The bag house 105 consists of ten separate compartments 1 20, although it will be appreciated that the number of compartments is not critical. Each compartment 1 20 can be separately shut off for cake removal without interfering with the continuing operation of the bag house. Gases enter along entry duct 1 21 , pass through the compartments 1 20 and into exit duct 1 22. Ash is collected in ash hoppers 1 23 and transported to an ash hopper 1 24 which discharges into a fly ash rail car 1 25. The clean gases exit the bag house to the flue gas duct 1 26 and thence to the stack. A main draft fan 1 27 provides a negative pressure throughout the system to draw gases from the incinerator 1 through the bag house 105 to the stack. The bag house 105 is supported on a rail carriage 1 28. The control system associated with each incinerator 1 enables fine control of combustion in the combustion bed and the temperature and residence time of gases through to the boiler, which enables in turn a 40% variation (plus or minus 20% from an average output) in power generation from each waste-to-energy module. With this degree of control, if one of six incinerators needs to be shut down for maintenance, the remaining five can still retain the required minimum power generation. Thus, the control systems across all modules may be integrated to provide overall control of the power output from the facility. It will be appreciated that where greater or less control of energy output from each unit is possible, fewer or more units, respectively, will be required in a plant. The integrated capacity for waste incineration and energy output of a waste-to-energy facility forming a part of the present invention, is summarised in Table I. It can be seen that for a twelve-unit facility, there is a maximum waste incineration capacity of about 1 66 tonnes per hour, with a maximum net electrical output of 98.6MW. The electrical output modulation is about 42MW. A facility of this aspect of the invention has been designed so that all plant functions are duplicated, including the ash railways, ash processing plant, turbines and plant control systems. In addition, individual units or modules have duplication of all critical components, including fans, instruments, hydraulic components, bags for the bag house, etc. Most of these can be replaced without shutting down the individual unit. Any component involved in an unplanned outage that is not duplicated can be replaced within three days, inclusive of the hearth panels of incinerator. In a fully operational facility, a spare boiler, economiser and bag house will be available, and will be replaceable on the cross rail system 1 30 within six days, to allow scheduled maintenance with minimal impact. The dry ash removal system from the incinerator includes a double rail system so that ash can continue to be removed if maintenance is required on one or other of the rail systems. A rail system also avoids high maintenance requirements associated with alternative conveyor-type systems, and the fact that the ash is treated dry reduces corrosion and maintenance problems associated with wet ash removal commonly used in waste incineration systems. Maintenance requirements for the boilers are reduced by the partial acid scrubbing in the high temperature incinerators, and by the inclusion of the sacrificial tubes in the second zone of the labyrinth. Waste heat from a power generation facility may be employed in a desalination unit to produce potable water. It is the nature of incinerators of the present invention, and the means for controlling combustion and gas composition, which enables waste incineration units of the invention to be used in a commercially viable manner for the generation of power. The incinerators themselves are relatively low cost since the material from which they are constructed forms the dual function of providing structural integrity and refractory characteristics. The ability to maintain the combustion gases at an average temperature of greater than or equal to 1 100° for at least 5 seconds enables acid scrubbing to be carried out in the incinerator, which in turn enables the use of a labyrinth system which facilitates the collection of flyash-borne material. Together these features enable the boiler to be constructed from a much lower grade of material than would conventionally be the case where corrosion and erosion are a major factor. The boilers of waste incineration units of the invention can therefore be smaller and considerably lighter than conventional boilers and this enables them to be manoeuvrable and readily replaceable. Where in the foregoing description, reference has been made to specific components or integers of the invention having known equivalents then such equivalents are herein incorporated as if individually set forth. Although this invention has been described by way of example and with reference to possible embodiments thereof, it is to be understood that modifications or improvements may be made thereto without departing from the scope or spirit of the invention.

Claims: CLAIMS: 1 . A waste incineration unit including: an incinerator with a hearth adapted to enable combustion under reducing conditions; a waste fuel inlet for the supply of waste material to the hearth; ash removal means to remove ash from the hearth; and overfire air supply means to mix combustion gases and provide oxidising conditions in an upper zone of the incinerator; and wherein said waste incinerator unit is adapted to retain combustion gases at an average temperature greater than or equal to 1 100°C for at least 5 second. 2. A waste incineration unit according to claim 1 adapted to retain combustion gases at an average temperature of substantially 1 1 25 °C for at least 5 seconds. 3. A waste incineration unit according to either claim 1 or claim 2 wherein the incinerator is adapted to provide a substantially uniform temperature across the column of combustion gases. 4. A waste incineration unit according to claim 1 including two or more splayed U-shaped hearths. 5. A waste incineration unit according to either claim 4 wherein the or each hearth has sides at angles of substantially 70° to 75° from the horizontal. 6. A waste incineration unit according to claim 5 wherein each hearth side includes a plurality of underfire air nozzles. 7. A waste incineration unit according to claim 6 wherein the nozzles are adapted to penetrate air through waste fuel in the or each hearth to at least a mid-region. 8. A waste incineration unit according to claim 7 wherein the air flow through each nozzle, or groups of nozzles, is separately controllable. 9. A waste incineration unit according to any one of the preceding claims wherein the overfire air supply means includes a first set of overfire air holes at a first height in the incinerator. 10. A waste incineration unit according to claim 9 wherein the first set of overfire air holes are angled to cause rotation of air and combustion gases in a first rotating direction. 1 1 . A waste incineration unit according to either claim 9 or claim 1 0 wherein the overfire air supply means further include a second set of over fire air holes at a second height in the incinerator, above and spaced-apart from the first set. 1 2. A waste incineration unit according to claim 1 1 wherein the second set of overfire air holes are angled to cause the rotation of air and combustion gases in the opposite direction to the rotation induced by the first set of overfire air holes. 1 3. A waste incineration unit according to any one of the preceding claims further including means for introducing lime for sulphur scrubbing through the overfire air holes. 14. A waste incineration unit according to any one of the preceding claims wherein the ash removal means include tiered pushers, each pusher adapted to push material from a different longitudinal portion of the or each hearth. 1 5. A waste incineration unit according to claim 1 4 including a set of tiered pushers associated with the or each hearth. 1 6. A waste incineration unit according to claim 1 5 wherein the or each set of tiered pushers includes a lower pusher adapted to remove ash from a portion of the hearth closest to an ash outlet, one or more mid-pushers adapted to move material through a mid-portion of the hearth, and an upper pusher adapted to move material towards the mid-portion from a region of the hearth adjacent to the waste fuel inlet. 1 7. A waste incineration unit according to any one of the preceding claims further including waste fuel supply adjustment means, adapted to enable adjustment of the calorific value of waste fuel supplied to the incinerator. 1 8. A waste incineration unit according to any one of the preceding claims further including control means adapted to control combustion in the incinerator and provide a required energy output for a power generator driven by the energy produced by the combustion process. 1 9. A waste incineration unit according to claim 1 8 wherein the control means includes slope monitoring means to monitor the slope on the combustion bed during operation of the incineration unit. 20. A waste incineration unit according to claim 19 wherein the control means controls the operation of the tiered pushers to maintain a preferred slope on the combustion bed, as monitored by the slope monitoring means. 21 . A waste incineration unit according to claim 1 8 wherein the control means further includes heat monitoring means adapted to monitor the temperature across the combustion bed. 22. A waste incineration unit according to claim 21 wherein the control means controls the overall supply and/or regional supply of air through the underfire air nozzles to control the temperature across the combustion bed, as monitored by the heat monitoring means. 23. A waste incineration unit according to claim 1 8 wherein the control means further includes combustion gas monitoring means in an upper zone of the incinerator. 24. A waste incineration unit according to claim 23 wherein the control means further controls air supply through the overfire air holes and/or the supply of lime to the overfire air holes, in response to the combustion gas monitoring means. 25. A waste incineration unit according to any one of claims 1 8-24 wherein the control means enables variation of the energy output from the waste incineration unit by plus or minus at least 20%. 26. A waste incineration unit according to any one of the preceding claims further including a secondary combustion chamber into which combustion gases from the incinerator are directed, said secondary combustion chamber including a high temperature zone and a cooling zone. 27. A waste incineration unit according to claim 26 wherein the combustion gases are retained in the incinerator and/or high temperature zone of the secondary chamber at an average temperature greater than or equal to 1 100°C for at least 7 seconds. 28. A waste incineration unit according to claim 27 wherein the combustion gases are retained at an average temperature greater than or equal to 1 100°C for substantially 8 seconds. 29. A waste incineration unit including an incinerator with a hearth, a waste fuel inlet for the supply of waste fuel to the hearth, ash removal means for removing ash from the hearth and slope control means adapted, in use, to enable maintenance of a pre- selected slope on a combustion surface along the length of the hearth, from the waste fuel inlet at one end to an ash outlet at the opposite end. 30. A waste incineration unit according to claim 29 wherein the slope control means includes slope monitoring means to monitor the height of the combustion surface along the length of the hearth. 31 . A waste incineration unit according to claim 30 wherein the ash removal means is adapted to enable movement of material along different portions of the hearth length independently. 32. A waste incineration unit according to claim 31 wherein the ash removal means includes tiered pushers, each pusher adapted to push material from a different longitudinal portion of the hearth. 33. A waste incineration unit according to claim 32 including at least two parallel splayed U-shaped hearths. 34. A waste incineration unit according to claim 33 wherein the ash removal means includes a set of tiered pushers in each hearth. 35. A waste incineration unit according to claim 34 wherein a lower pusher in the or each splayed U-shaped hearth is adapted to remove ash from a portion of the hearth closest to an ash outlet, one or more mid-pushers is adapted to remove material through a mid-portion of the hearth, and an upper pusher is adapted to move material towards the mid-portion from a region of the hearth adjacent the waste fuel inlet. 36. A waste incineration unit according to claim 35 wherein each pusher has a substantially trapezoidal cross-section, wider at its base than its top. 37. A waste incineration unit including an incinerator with a hearth adapted to enable combustion under reducing conditions and overfire air supply means to mix combustion gases and provide oxidising conditions in an upper zone of the incinerator, and wherein the combustion gases are retained at sufficiently high temperature and for a sufficient period to enable sulphur scrubbing, or partial sulphur scrubbing, in the incinerator. 38. A waste incinerator unit according to claim 37 wherein the combustion gases are retained at an average temperature greater than or equal to 1 100°C for at least 5 seconds. 39. A waste incineration unit including an incinerator with a hearth adapted to enable combustion under reducing conditions, a waste fuel inlet for supplying waste fuel to the hearth, means for removing ash from the hearth, and overfire air supply means to mix combustion gases and provide oxidising conditions in an upper zone of the incinerator, said over fire air supply ' means including a first set of over fire air holes adapted to produce a first rotating region of air and gases above the combustion bed and a second set of over fire air holes adapted to produce a counter-rotating region of air and gases above the first rotating region of air and gases. 40. A waste incineration unit according to claim 39 in which the combustion gases are retained at an average temperature greater or equal to 1 100°C for at least 5 seconds. 41 . A waste incineration unit according to either claim 39 or claim 40 further including means for introducing lime for sulphur scrubbing through the overfire air holes. 42. A method of waste fuel incineration, including the steps of: supplying waste fuel to a hearth in an incinerator under incineration conditions to form a combustion bed; - supplying air into the combustion bed at a level to control primary combustion under reducing conditions; supplying turbulent air above the combustion bed in a manner which mixes the combustion gases, provides excess oxygen for substantially complete combustion in an upper zone of the incinerator and control of the flow of combustion gases through the incinerator such that the combustion gases are retained at an average temperature greater than or equal to 1 100°C for at least 5 seconds. 43. A method according to claim 42 further including removing ash from beneath the combustion bed during combustion. 44. A method according to either claim 42 or claim 43 wherein the supply of air to the combustion bed is at a level of substantially 80% or less of the stoichiometric requirement for complete combustion. 45. A method according to any one of claims 42-44 wherein the oxygen level in the upper zone of the incinerator is maintained at substantially 10% above the level required for complete combustion. 46. A method according to any one of claims 42-45 further including the step of maintaining a preferred slope on the combustion bed. 47. A method according to any one of claims 42-46 further including controlling the calorific value of waste fuel supplied to the combustion bed. 48. A method according to any one of claims 42-47 wherein the supply of turbulent air includes the provision of spaced-apart counter-rotating air flows. 49. A method according to any one of claims 42-48 further including transferring the combustion gases from the upper zone of the incinerator to a secondary combustion chamber, including a high temperature zone and a cooling zone, and whereby the combustion gases are retained in the incinerator and/or the high temperature zone of the secondary chamber at an average temperature greater than or equal to 1 100°C for at least 7 seconds. 50. A method according to any one of claims 42-49 further including sulphur scrubbing of the combustion gases in the incinerator. 51 . A method according to claim 42 further including controlling combustion in the incinerator to provide a required energy output for a power generator driven by the energy produced by the combustion process. 52. A method according to claim 51 wherein combustion can be controlled to enable variation of energy output by plus or minus at least 20%. 53. A method of waste fuel incineration wherein waste fuel is supplied to a hearth in a lower zone of an incinerator under incineration conditions whilst conditions in an upper zone of the incinerator enable acid scrubbing in the incinerator. 54. A method according to claim 53 further including supplying turbulent air above a combustion bed in the hearth in a manner which substantially separates the lower zone from the upper zone. 55. A method according to claim 54 wherein the supply of turbulent air includes the provision of spaced-apart, counter-rotating air flows. 56. A method according to claim 55 further including the controlled supply of lime in an upper region of the contra-rotating air flow. 57. A method according to any one of claims 53-56 further including the step of maintaining the combustion gases in the upper zone at an average temperature greater than or equal to 1 1 00°C for at least 5 seconds. 58. A waste-to-energy module adapted to generate power from • waste incineration, including a waste incineration unit according to any one of claims 1 -41 , a boiler for generating steam to drive a power generator, combustion gas emission control means and energy output control means adapted to enable control of the energy output from the waste incineration unit by at least plus or minus 20%. 59. A waste-to-energy module adapted to generate power from waste incineration, said module including: an incinerator adapted to enable combustion under reducing conditions; - a labyrinth adapted to capture fly ash-borne material and cool combustion gases; a boiler for steam generation to drive a turbine for power generation; an economiser for pre-heating feed water for the boiler; - an acid gas scrubber; and a bag house for collecting particulate material; and wherein at least the boiler, economiser and bag house are modular and replaceable. 60. A waste-to-energy facility adapted to maintain a reliable energy output, said facility including at least one plant of a plurality of waste-to-energy modules according to either claim 58 or claim 59, at least one power generator and integrated control means adapted to enable control of the energy output from the facility to accommodate fluctuations in power demand from the facility. 61 . A waste-to-energy facility according to claim 60 including 6 waste-to-energy modules in the or each plant. 62. A waste-to-energy facility according to claim 61 including 2 plants of waste-to-energy modules. 63. A waste-to-energy facility according to claim 60 wherein the integrated control means enables control of the energy output from each waste-to-energy module by at least plus or minus 20%. 64. A waste-to-energy facility according to claim 60 wherein the integrated control means includes energy output control means associated with each waste-to-energy module adapted to control combustion in the incinerator of that module to enable variation of the energy output by plus or minus at least 20%. 65. A waste-to-energy facility according to claim 64 wherein at least the boiler, economiser and bag house of each waste-to-energy module are supported on a rail system and the waste-to-energy facility includes a cross-rail system on to which boilers, economisers and/or bag houses may be transferred for removal and replacement. 66. A waste incineration unit substantially as herein described and with reference to the accompanying drawings. 67. A method of waste fuel incineration substantially as herein described and with reference to the accompanying drawings. 68. A waste-to-energy module substantially as herein described and with reference to the accompanying drawings. 69. A waste-to-energy facility substantially as herein described and with reference to the accompanying drawings.

Inventors: ALBER FRITZ, DAVIES WARWICK JOHN, GARDNER JOHN OWEN, MAYSON MICHAEL ROBERT, NIELSEN ERIK, ALBER, FRITZ, DAVIES, WARWICK, JOHN, GARDNER, JOHN, OWEN, MAYSON, MICHAEL, ROBERT, NIELSEN, ERIK
Applicants: ALBER FRITZ, DAVIES WARWICK JOHN, GARDNER JOHN OWEN, MAYSON MICHAEL ROBERT, NIELSEN ERIK, OLIVINE NZ LTD, ALBER, FRITZ, DAVIES, WARWICK, JOHN, GARDNER, JOHN, OWEN, MAYSON, MICHAEL, ROBERT, NIELSEN, ERIK, OLIVINE (NZ) LIMITED
Application year: 2000
Main classification: F23G5
IPC codes: F23G5, F23L1, F23L9