Thermal treatment device

Abstract

A thermal treatment device 300 comprises a horizontal primary chamber 301 with a floor with a hearth 304 which is heated to generate fumes 308 from waste material received in the primary chamber. A ve

Description

Field of the Invention The present invention relates to improvements in certain high temperature waste combustion systems such as incinerators, waste gasification devices and other similar equipment. The improvements predominantly apply to devices that operate as continuous feed systems. The primary applications for this technology are the disposal of biomass wastes that are shredded, or wastes such as human bio-solids, animal manures or other forms of biomass sludge. These improvements can also be employed for certain waste to energy or combined heat and power systems. Background of the Invention As human populations increase, certain biomass waste streams generated by domestic, agricultural, commercial and industrial activities have become excessive and often quite problematic. Many of these wastes are hazardous at one level or another. Some of them are accumulating in greater and greater quantities with no suitable destination other than burial in landfills or storage in ever growing waste piles. Domestic wastes include typical household garbage, as well as human bio-solids (sewage). Agricultural wastes include large quantities of animal manures generated at large production farms, along with the offal wastes generated at packing plants. Commercial wastes include tremendous volumes of packaging materials, especially plastics. And industrial wastes contain an endless list of dangerous pollutants. Efforts to recycle at least some of these wastes have proven to be very limited in scope and often too costly to be sustainable. One example of the growing problems with human bio-solids and factory farm manures is run-off of the nutrients contained in the fecal matter. These wastes are often spread on land as both a method of disposal and as fertilizers. This allows rain to then wash the material into nearby watersheds. Algae blooms fed by these wastes are showing up in lakes and oceans around the globe. When these blooms die they deplete oxygen levels in the water, thus killing fish that get trapped in the de-oxygenated dead zones. Added to this problem is the issue of antibiotic residues contained in many of these wastes. As a result, these residues are then being dispersed into the environment. This unwitting process gives rise to increasing antibiotic resistance through the creation of bacterial super-bugs that can cause dangerous infections which are sometimes untreatable with existing antibiotics. There is a growing concern in the global medical community that we may soon run out of useful antibiotic therapies for many common, but serious infections. Beyond the above concerns, everyone should now be aware of the concerns regarding discarded plastics. This waste stream has invaded every part of the biosphere including rivers, lakes, oceans, and much of its land-mass. Many people throw away plastic bottles, etc., as if they believe the stuff will harmlessly disappear. This is definitely not the case. Plastic wastes are causing serious harm to the environment and its inhabitants. As long as these waste streams continue to be produced it is imperative that better methods for the management and disposal of them is required. Certain of these wastes need to be destroyed because they are harmful, while others are just voluminous, with little or no ability to be recycled, and with no intrinsic value otherwise. The technology described herein has the ability to extract energy from almost any biomass waste, and at the same time, it can destroy any dangerous pathogens contained therein, producing a resultant ash residual that is inert and harmless. Waste to energy technologies such as combined heat and power (CHP) systems will be one the most important solutions to this ever growing biomass waste problem. In many situations the lowest cost approach with the highest return in value is the conversion of the waste directly into energy in the form of heat and / or electricity. A prime example is the disposal of animal manures and human bio-solids. With both of these waste streams, energy can be extracted while at the same time converting the material into harmless ash. There are many other examples, such as refuse derived fuel (RDF) which is the final remnants left after separating municipal waste; as well as the direct conversion to energy of plastic waste that can't, or won't, be recycled. The foundation prior art patents that underpin these improvements to the existing technologies are the inventor’s earlier work described in patent publication numbers US 5,611,289A and US 6,116,168A which describe Brookes batch feed gasifiers, and European patent EP 2 063 965 Bl which describes Brookes continuous feed gasifiers that use a similar hot hearth thermodynamic and fluid flow processes to that described in the inventor’s original batch feed designs. The Brookes continuous feed process lends itself very well to the waste to energy approach for dealing with many of these wastes. Two important advantages are achieved with this technology, firstly, the waste mass is reduced to a small amount of inert ash, and secondly, useful energy is extracted from the waste in the form of heat and / or electricity. The present application relates to improvements to thermal treatment devices and gasifiers and, particularly to improvements to Brookes gasifiers. The improvements are applicable to both batch feed and continuous feed gasifiers. Summary of the Invention The present invention provides a thermal treatment device comprising: a horizontally disposed primary chamber for receiving waste material, the primary chamber having a floor comprising a hearth which can be heated to generate fumes from the waste material; a vertically disposed mixing chamber having an air input for mixing air with the fumes; a horizontally disposed secondary chamber extending below the primary chamber and configured to allow combustion of the fume from the mixing chamber to heat the hearth of the primary chamber; and a burner mounted in a wall of the mixing chamber no higher than the bottom of the hearth and directed to fire into the secondary chamber, wherein: the burner is angled downwards at between 25 and 40 degrees to the horizontal. The mixing chamber air input may be positioned in the top of the mixing chamber and may be positioned directly above the burner flame. The thermal treatment device may further comprise a hearth shield in the secondary chamber to protect sections of the hearth. The thermal treatment device may further comprise a floor shield to protect sections of a floor of the secondary chamber. The burner may be purged with a flow of air through the burner when the burner is off or on standby. The flow of air may be provided by a different system from the air for combustion provided to the burner when it is on. The flow of air may be controlled by a solenoid valve which is in an open position when unpowered. The present invention also provides a thermal treatment device comprising: a horizontally disposed primary chamber for receiving waste material, the primary chamber having a floor comprising a hearth which can be heated to generate fumes from the waste material; a vertically disposed mixing chamber having an air input for mixing air with the fumes; a horizontally disposed secondary chamber extending below the primary chamber and configured to allow combustion of the fume from the mixing chamber to heat the hearth of the primary chamber; and a deceleration chamber for receiving the fume from the combustion chamber, wherein the cross sectional area of the deceleration chamber is greater than the cross-sectional area of the secondary chamber. The deceleration chamber may be vertically disposed to provide an upward turn in the direction of flow of the fume as it flows through the deceleration chamber. The deceleration chamber may further comprise a depression in the floor for collecting fly ash drop out. The thermal treatment device may further comprise an exit duct in fluid communication with the deceleration chamber and extending above the deceleration chamber. Brief Description of the Drawings Figure 1 is a cross sectional side elevation view of a Brookes batch feed device. Figure 2 is a cross sectional plan view of the batch feed device of Figure 1. Figure 3 is a cross sectional side elevation view of a Brookes continuous feed device. Figure 4 is a cross sectional plan view of a Brookes continuous feed device. Figure 5 is a cross sectional front elevation view of a Brookes continuous feed device showing additional features. Figure 6 is a cross sectional ash side elevation view of the continuous feed device of Figure 5. Figure 7 is a cross sectional ash side elevation view showing improvements to a continuous feed device according to the present invention. Figure 8 is a cross sectional plan view of the ash side showing improvements to a continuous feed device according to the present invention. Figure 9 is a cross sectional side elevation view of the feed side showing improvements to a continuous feed device according to the present invention. Figure 10 is a cross sectional plan view showing improvements to a continuous feed device according to the present invention. Figure 11 is a side elevation view of a continuous feed device showing an alternative gasifier exhaust exit location. Figure 12 is a flow diagram of a typical heat recovery system. Description of the Brookes gasification device. Figures 1 &2 show the basic configuration of a Brookes batch feed gasification device 20. The special geometry of this device is a unique approach to dealing with the broad spectrum of bio-mass wastes. Prior to loading the waste mass 22 into the primary chamber 30, the warm-up burner 48 is started and the heating of the secondary chamber 46, 52 is initiated. When the secondary chamber 46, 52 reaches the appropriate operating temperature the waste mass 22 is loaded into the primary chamber 30 through the loading door 32. As the secondary chamber 46, 52 heats up a hearth 36, which forms the floor of the primary chamber 30, also begins to heat up because of the geometric configuration that makes the hearth 36 the ceiling of the secondary chamber 46, 52. The hearth 36 is designed to transfer heat rapidly into the waste mass 22 causing the volatiles, predominantly hydrocarbons and moisture contained in the waste mass 22, to evaporate and form a fume 54. This mostly combustible fume 54 travels from the primary chamber 30 into the mixing chamber 40 through an opening 38 in the separation wall 50, 51. The loading door 32 has small air openings 34 to allow a minimal amount of air into the primary chamber 30 to help escalate the temperature in the primary chamber 30 by causing a very limited amount of oxidation of the combustible fume 54. The primary chamber air openings 34 are controlled manually or automatically to ensure that approximately 95% of the oxidation of the combustible fume 54 will occur in the secondary chamber 46, 52. As the fume 54 generated in the primary chamber 30 enters the mixing chamber 40 through the opening 38 described above, the fume 54 mixes with a combustion air supply 49 and, in conjunction with the heat from the burner 48, the combustibles contained in the fume 54 begin to oxidize. This is a highly exothermic reaction that predominantly takes place in the secondary chamber 46, 52 under the hearth 36. In turn, the hearth 36 heats up and an energy circle is established wherein the heat of the hearth 36 evaporates the volatile components of the waste mass 22 forming the fume 54 that inevitably oxidizes under the hearth 36 continuing the cycle of waste evaporation until the waste mass 22 is reduced to an inert ash residual. The hot exhaust created by this process travels down the burner 48 side of the secondary chamber 46, 52 making a 180 degree turn around a division wall beneath the hearth 36 (see Fig. 2) and then travels back under the other side of the hearth 36, in a continuation of the secondary chamber 46, 52, before exiting the gasification unit as hot exhaust consisting mostly of hot air, carbon dioxide and steam. A thermocouple 58 is positioned so that it detects the hot exhaust temperature at a predetermined location in the secondary chamber 46, 52 that guarantees the minimum dwell time of the hot exhaust flow at the required temperature (by regulation) before the hot exhaust temperature is allowed to begin to cool. The thermocouple 58 feeds a signal to a temperature controller 56 which in turn feeds a signal 57 which then controls the burner 48 so that it holds the secondary chamber 46, 52 temperature at the proper operating set point temperature. Figures 3 &4 show prior art improvements to the Brookes batch feed gasification device to convert it to a continuous feed device. The thermodynamic and fluid flow aspects of the batch feed unit are the same for the continuous feed device as indicated by numbering in the drawings. New equipment that has been added to make the conversion to continuous feed are shown by the numbers above 100. The full details of this improvement are explained in EP 2 063 965 B1. A major advancement in this device is the use of augers 106 to continuously move shredded or sludge-like bio-waste 152 across the top surface of the hot hearth 36, in the primary chamber 102. The augers 106 are driven by variable speed gear motors 150 that are controlled either manually or automatically. The volatiles in the bio-waste 152 evaporate as it travels across the hearth 36. The bio-waste starts out as raw, sometimes wet, untreated material and ends up as inert ash. The waste is moved out from the feed hopper by the augers 106, travels across the hot hearth 36, and the final ash residual is pushed into an ash hopper on the opposite side of the hearth 36 by the same augers 106. As the bio-waste traverses the hearth 36 from the feed side to the ash side it is exposed to increasingly higher temperatures which purge the bio-waste 152 of the volatiles contained therein, including moisture, hydrocarbons, and other such volatiles, including most, if not all of the carbon. The pathway of the flow of the combustible fume 54 and the hot exhaust is the same for both the batch feed and the continuous feed gasification device. The fume 54 enters the mixing zone 40 through the opening 38 in the separation wall 50, 51. The fume 54 begins to mix with the secondary air 49 and the heat supplied by the burner 48, 118. This initiates the oxidation of the combustibles contained in the fume 54. The oxidized hot exhaust travels through the secondary chamber 46, 52, turning 180 degrees through the opening 104 in the division wall. The hot exhaust continues under the hearth 36 and exits on the opposite side of the gasification device 20 from where the burner 118 and secondary air openings 49 are located. Figures 5 &6 show certain additional features of the continuous feed gasification device 200. Many of the features indicated in the drawings are common to the previous devices and are referred to herein for completion purposes. An important change from the previously described device shown in Figures 1 through 4 is the positioning of the warm up burner 213. In the earlier Figures 1 &3 the burner 48, 118 is shown firing directly downward from the top of the mixing chamber 40. The positioning of the burner 48, 118 at the top of the mixing chamber 40 and the vertically downward firing direction of the burner 48, 118 can be problematic because heat generated in the secondary chamber 46, 52 can rise back into the burner 48, 118 when it is switched to the off or standby position. This will occur when the gasification device reaches a steady-state, auto-thermic condition at the operating temperature, which generally occurs with the continuous feed process. When this happens, it can cause damage to the burner 48, 118. To prevent damage, the burner's air supply fan may be set up to remain operating even when the burner's fuel supply is shut off. In Figures 5 &6 the warm-up burner 213 location is shown firing directly into the secondary chamber 208 from the back and side of the mixing chamber 220 of the thermal treatment or gasification device 200. The burner 213 is described as firing in a horizontal direction into the secondary chamber 208 somewhat below the bottom of the hearth 207. The purpose of the change in burner 213 positioning is two-fold; firstly, it is a less vulnerable location with respect to heat damage when it is off or on standby, and secondly, it is a more direct arrangement for warming up the secondary chamber 208 to the proper operating temperature. It should be noted that the warm up burner 213 must have the energy output capacity to bring the temperature of the secondary chamber 208 up to the proper operating range, a minimum of 850"C for most bio-wastes, before the waste 230 is introduced into the primary chamber 204. Another feature shown in Figure 6 is a burner isolation valve 218. The purpose of this slide plate valve is to help protect the warm up burner 213 from heat from the secondary chamber 208 backing up into the burner when it is off or on standby, that is, when the temperature of the secondary chamber 208 is at the proper operating set point. The problem with this burner isolation valve 218 configuration is that the warm up burner 213 may need to switch on and off occasionally during the operation of the thermal treatment / gasification device 200. The use of the burner isolation valve 218 would not be allowed in most jurisdictions because it presents a serious hazard. If the burner isolation valve 218 fails to open for any reason when the temperature controller 233 signals the burner to switch on, serious injury could occur to anyone nearby the burner 213 due do backlash flames or a backfire when the burner 213 attempts to ignite. Good and safe combustion practice says that no burner firing zone should be physically impeded or interfered with when a burner is operational or on standby. Figure 5 shows a waste transport method 206 that moves the waste 230 across the hearth 207. To date, after years of researching this process, it appears that the best method to move the waste 230 across a hearth 207 is with augers 106 as shown in Figures 3 &4. The other features that are shown in Figures 5 &6 are either common to the prior art shown in Figures 3 &4, or are added to the system for improved operation. The thermal treatment / gasification device of Figures 5 &6 has a primary chamber 204 that communicates with a mixing chamber 220 through an opening 205 in a separation wall at the back of the primary chamber 204. A warm up burner 213 heats the secondary chamber 208 to its operating temperature after which the bio-waste is introduced into the primary chamber 204. The fume 216 generated in the primary chamber 204 as a result of the heating of the hearth 207 travels into the mixing chamber 220 and begins to oxidize in the secondary chamber 208 which creates significant heat, some of which is conducted through the hearth 207 into the waste 230 situated in the primary chamber 204. The waste 230 is moved continuously across the hearth 207 from the feed hopper 203 to the ash hopper 211. As the waste 230 traverses the hot hearth 207 the combustible hydrocarbon structures, moisture and other volatiles, form the fume 216 that enters the mixing chamber 220 through the opening 205. A secondary chamber 208 temperature feedback loop 233 controls the burner 213 to switch on and off as needed to maintain the temperature of the Secondary Chamber 208 at the required set point. This design is common to the batch feed and continuous feed devices described herein. Figure 5 shows an ash removal auger that may be used in any continuous feed device. The residual ash 231 must be continuously taken away from the ash hopper 211 or the ash 231 will back up into the primary chamber 204 which would obviously become problematic very quickly requiring an emergency shut down of the system. Figures 7 through 11 show views of a Brookes continuous feed gasifier which incorporates the present invention. The main structural features of the thermal treatment device are the same as described above. In operation, biomass sludge waste or shredded biomass waste is moved across the hot hearth 304 of the gasifier 300 from the feed side 332 to the ash side 333 using slowly turning augers 305. The hearth 304 is heated from below by the hot gases 325 travelling through the secondary chamber 302. These hot gases 325 are generated when the combustible fume 308 is evaporated out of the waste mass, then drawn into the mixing chamber 303 and oxidized (an exothermic reaction) in the secondary chamber 302. A secondary chamber 302 heat-up and support burner 311 is installed to ensure that the secondary chamber 302 operates at or above the required minimum temperature (which may be set by legal regulation) throughout the process. The burner 311 is designed to be able to switch on and off as necessary to maintain the proper operating temperature without interruption and for the proper dwell time (which may also be set by legal regulation). The temperature may be monitored by a thermocouple 335 placed in the secondary chamber 302. A temperature control device 336 controls the operation of the burner 311 so that the secondary chamber 302 temperature is maintained at the required minimum temperature. The position and direction of the burner 311 along with other features described below protect the hearth 304, the floor of the secondary chamber 302 and the burner 311 itself from excessive heat damage which can occur, in particular, at the corner where the fume moves from the mixing chamber 303 where it is moving in a substantially vertical direction into the secondary chamber 302 where it is moving in a substantially horizontal direction. This is where the highly exothermic process of oxidation of the combustible fume 308 is the most intense. From that point the hot gases continue through the secondary chamber 302 channel, past the thermocouple 335 and toward the exit from the secondary chamber 302. Prior to the hot gases 325 exiting from the secondary chamber 302 they enter a deceleration chamber 326. This section of the flow path has a larger cross-sectional area than the second chamber which slows down the fume. This causes some of the particulates that may be entrained in the hot gas flow 325 to drop out. They can be collected in a fly ash collection zone 328 which may be in the form of a depression in the flow of the deceleration chamber 326. The fly ash 327 can be removed, for example periodically by a vacuum system that may be inserted through the vacuum access port(s) 329 as needed. Alternatively, the fly ash may be continuously removed by an auger system. The hot gases or hot exhaust 330 exit the gasifier 300 through an exhaust duct 338. In a waste to energy system, such as that shown in Figure 12, the hot exhaust 330 will travel to some form of heat exchanger 352 that will recover useful energy of some type, such as hot water, steam or electricity. A CHP (combined heat and power) system is a very suitable application for this technology. In the embodiments shown in Figure 7-11, the burner 311, sometimes called a warm-up burner, but also described as the secondary chamber heat-up and support burner, is an integral part of the functioning of the gasifier 300. Besides being the heat source to bring the secondary chamber 302 up to the required operating temperature, the burner 311 must also be able to switch on and off as needed to maintain the required minimum operating temperature in the secondary chamber 302. In the prior art (e.g. Figure 6), the burner 213 is treated as a secondary device and includes an isolation gate 218 to separate or close off the burner from the secondary chamber 208. This is a dangerous design and may be illegal in some jurisdictions. More importantly, if the gate 218 fails to open when the burner 213 is required to switch on it would be prevented from supplying heat to the secondary chamber 208 when the temperature falls below the required operating temperature. The following improvements make the burner 311 an integral part of the proper and safe functioning of the gasifier. An important factor is the sizing of the burner 311. Upon the initial start-up when the gasifier 300 is cold, the burner 311 must be large enough to raise the temperature of the secondary chamber 302 to the correct operating temperature before any bio-mass waste (230 in Figure 5) is introduced into the primary chamber 301. The correct temperature of the secondary chamber 302 is often specified by the appropriate Environmental Protection Authority. There is a need to protect the hearth 304 and certain other refractory zones from heat damage when the burner 311 is firing, particularly when a suitably large burner 311 is employed. Figure 7 shows the ash side or secondary chamber heat-up and support burner 311 side of the secondary chamber 302. In this embodiment, the burner 311 is mounted on the back wall of the gasifier 300 such that it fires into the ash side of the secondary chamber 302. The burner 311 is situated horizontally in the centre of the ash side channel of the secondary chamber 302. Vertically, the burner 311 is mounted no higher than the bottom of the hearth 304. It may be immediately below the bottom of the hearth. Importantly, the burner 311 is directed to fire downward at an angle “x” 312 that is between 25 degrees and 40 degrees to the horizontal. The actual angle may be optimised based on the width of the mixing chamber303 and height of the secondary chamber 302. This arrangement prevents the hottest part of the burner flame envelope from impinging on the hearth 304 which can cause premature failure of the hearth 304. An angle less than 25 degrees downwards from the horizontal results in too much heat reaching the hearth. An angle of more than 40 degrees downwards from the horizontal results in too much heat being directed to the floor of the secondary chamber 302 and excessive localised heating near the junction between the mixing chamber 303 and the secondary chamber 302. The hearth is usually made from a high density, high strength, castable, refractory material. Typically, the hearth 304 refractory, usually a high density, high strength castable material, has an upper temperature limit of approximately 1600°C. The maximum temperature generated by a typical air fired fossil fuel burner is in excess of 1900°C in its hottest zone which will damage the hearth 304 refractory over time. The purpose of positioning the burner 311 as indicated in Figures 7 and 8 is to reduce the risk of premature hearth 304 failure from excessive heat by firing the flame envelop of the burner 311m such a way that its flame does not directly impinge on the hearth 304 until the flame has cooled as a result of flame front dispersion. It can be seen in prior art Figure 6 that the warm-up burner 213 has a horizontally directed output orifice. This configuration would quickly cause damage to the hearth 207 refractory. As well as firing the burner 311 at a downward angle as described, an additional feature is the placement of the secondary air input 321. A secondary air supply duct 320 delivers air from an air fan 309 to the secondary air input 321 in the top of the mixing chamber 303. This air is supplied to support the oxidation of the combustible fumes 308 and the supply is on at all times during the operation of the gasifier 300. The secondary air input 321 is preferably positioned in the top of the mixing chamber 303 in line with the burner 311 flame envelope and is directed downwards. The secondary air input 321 is preferably in line with the burner 311. The air flow may be directed approximately orthogonally across the flame to further urge the flame downwards. The effect of locating the secondary air input 321 in this way is to assist in dissipating the heat of the burner 311 flame envelope. The air flow may divert the burner flame down and away from the underside of the hearth 304, thus reducing its direct heat impact on the hearth 304. This special positioning of the combustion air input 321 helps to disperse the burner heat when it is on and firing which helps reduce potential heat damage to the hearth 304. A further improvement for protecting the integrity of the hearth from the highest heat zones generated by the burner 311 is the addition of a hearth shield 322. This hearth shield may be a sacrificial or replaceable component. The hearth shield 322 may be made from a high strength, high temperature castable refractory with a ceramic fibre insulating refractory on the surface adjacent to the hearth 304. The hearth heat shield 322 may be located only where the hottest temperatures occur, for example under the hearth where the mixing chamber 303 meets the secondary chamber 302. The hearth shield 322 may be cast from a high temperature refractory and may have a ceramic fibre inner lining adjacent to the bottom of the hearth 304 and the side of the hearth 304 that faces the mixing chamber 303. The shield 322 insulates the most vulnerable part of the hearth 304 from both the excessive heat generated when the burner 311 is on, and also from the heat generated by the initial oxidation of the combustible fume 308 as it mixes with the flow of secondary air 320 in the mixing chamber 303. The directional change at the corner where the mixing chamber 303 meets the secondary chamber 302 causes a spike in the heat in that zone. A fire ball forms in this zone due to the change in direction and velocity of the flow. The excessive heat due to this flame front event dissipates quickly as the hot gases 325, shown in Figure 9, travel along the secondary chamber 302. The hearth heat shield is not needed on the whole underside of the hearth as it may reduce the thermal conductivity of the hearth in areas where the device geometry and component positioning makes the heat less intense. The secondary chamber may include a floor shield 323. The floor shield 323 may be positioned on the floor at the start of the secondary chamber 302, as shown in Figure 7. This shield 323 protects the floor of the secondary chamber 302 in the zone where the burner 311 flame might impinge on the secondary chamber 302 floor and cause damage to the floor refractory. The shield 323 may be cast from a high temperature, high strength castable refractory. The second chamber floor heat shield 323 may be a replaceable or sacrificial part that helps protect the floor of the secondary chamber 302 from damage in the area where flame impingement from the burner 311 occurs. The floor heat shield may cover the whole floor or may cover only part of the floor of the secondary chamber to reduce material costs. An ash side access door 319 may be provided so that the hearth shield 322 can be inspected from time to time and repaired or replaced as needed. The access door 319 may allow maintenance personnel to enter the ash side of the gasifier 300 to inspect, repair and / or replace the hearth heat shield 322 or the secondary chamber floor heat shield 323. The gasifier 300 may include an improved burner 311 which has unique features to allow it to remain on standby while the gasifier 300 is operating. When the burner 311 is not firing (i.e. it is off or on standby), it must be protected from the heat in the zone where the mixing chamber 303 and the entrance to the secondary chamber 302 meet. In this zone the oxidation of the combustible fume 308 is the most intense and the process is highly exothermic. If the burner 311 is not protected from the excessive heat it will be damaged. The solution to this problem is to purge the burner 311 with a small flow of cooling air from a guaranteed source when the burner is off or on standby. This provides a means for protecting the burner 311 from heat backlash from the secondary chamber 302 without having to keep the burner combustion air fan operating when the burner is not running. The ideal guaranteed source for such air is an air compressor 313, preferably with a relatively large compressed air reservoir to allow cooling air 318 to flow for a period of time even if the compressor is not running. The period of time is preferably 4 to 6 hours. Only a small amount (for example 4% of the maximum burner air input) of air flow 318 is needed to protect the burner 311 from being damaged. An alternative approach may be to set up the burner 311 so that the burner combustion air is always on. This approach is not feasible with many burner types as burner air inputs are often not easily adjusted and often the air input goes off when the burner is off. However, a problem with that approach is that control over the secondary air or oxygen supply to the mixing chamber is compromised by the excess air supplied from the burner 311 when it isn't firing. Only a small percentage of the burner air needed for the combustion of the burner fuel is needed for cooling the burner 311 when it is on standby. It is desirable to have the secondary air supply 320 be the primary air or oxygen source for oxidizing the fume 308. The secondary air supply 320 can then be controlled using an oxygen sensor without having a significant, uncontrollable, secondary air input being introduced from the burner 311 combustion air fan when the burner 311 is off or on standby. Note that the air supplied by the burner 311 air fan when it is operating is normally set at the stoichiometric fuel to air ratio. This amount of air is not wanted or needed when the gasifier 300 is operating in the steady-state or auto-thermic condition. A further advantage of the independent cooling air system is that two separate systems supply air to the burner, allowing for air to be supplied to the burner to prevent damage should either source fail, as a further safety feature. The unique compressed air system that cools the burner 311 has the certain components. There is an air compressor unit 313 including a reservoir for compressed air, a solenoid valve 316, and a feed tube 318 that feeds the air into the appropriate part of the burner 311. The system may include a pressure regulator to reduce the pressure of the cooling air from that stored in the compressed air reservoir and manage the air flow. The pressure regulator may include a pressure gauge 314. The system may also include an air flow indicator 315 and a throttling valve 317 to allow manually or automatic control the flow to ensure an appropriate flow of cooling air. With these components a suitable volume (preferably approximately 4% of the burner air maximum) of fresh air can be fed into the burner 311 housing to keep the burner cool when it is off or in the standby mode. In one example, the cooling air travels from the air compressor 313 through the pressure regulator 314 and the air flow indicator 315, then to the solenoid valve 316, then to a throttling valve 317 and then out of an exit pipe 318 that feeds the cooling air into the burner housing to keep the burner 311 protected. The standard operation of one example of a burner 311 cooling system is that the solenoid valve 316 is signalled electrically to close when the burner 311 needs to come on to heat the secondary chamber 302. The solenoid valve opens when the secondary chamber 302 temperature is at or above the required temperature at which point the burner 311 switches to standby. By design, the electrical signal to the solenoid valve 316 will switch off so that it opens the solenoid to keep the burner 311 cool. This safety feature ensures that the burner is still cooled even when power to the system, or specifically power to the solenoid valve 316, fails. A control system operates the cooling air flow 318. The air compressor 313 provides high pressure air and the pressure regulator 314 reduces this pressure significantly for the purpose of cooling the burner 311. The air flow indicator 315 along with the throttling valve 317 allows the operator to set a suitable flow rate through the exit pipe 318 to cool the burner 311 and prevent heat damage. The solenoid valve 316 is normally open when un-powered and closes to shut off the cooling air flow 318 when the burner 311 is on and firing. When the burner 311 switches off or to the standby mode the solenoid valve 316 is controlled to become un-powered. Figure 10 shows a secondary chamber thermocouple 335 which feeds a controller 336 which in turn sends a signal 337 to the burner 311 when the secondary chamber temperature falls below or goes above the required secondary chamber 302 operating temperature, in one example by more than 0.3%. When the burner is signalled to start up the solenoid valve 316 will close and shut off the cooling air from the compressor 313. The burner 311 will shut down when the secondary chamber temperature has increased by a pre-set amount above the required secondary chamber 302 operating temperature. The solenoid valve 316 will then open to allow cooling air to flow from the compressor 313 to protect the burner 311 from over heating while it is on standby. The control sequence for the compressed air burner 311 cooling system is designed to function even in the event of an electrical power failure. The air compressor 313 should have a relatively large reservoir or compressed air tank. Except for the solenoid valve 316, all of the other features that control or indicate the air flow to the burner 311 will function without electrical power. Since the solenoid valve 316 supplied is to be normally open and closed under electric power; in the event of a power failure the solenoid valve 316 will automatically open and allow compressed air from the air compressor 313 reservoir to flow through the burner 311 and keep it cool. Since the amount of air needed is very small and at relatively low pressure, the reservoir will provide cooling air 318 long enough (in one embodiment, 5 hours) for the gasifier 300 to cool down adequately while the hot gases are being directed through the emergency bypass stack 350 and as the system goes into the shut-down mode. The burner 311 cooling system described protects the burner 311 from heat damage using the minimum flow of cooling air required. This allows the operator much better control over the excess air, and thus oxygen levels in the secondary chamber 302 because the primary source of air for oxidation will be the secondary air fan 309 which can have a variable speed drive motor. Reducing the excess oxygen in the secondary chamber 302 helps reduce the thermal production of oxides of nitrogen which is a regulated air emission that must be abated if the regulation limits are exceeded. Note that reducing the excess air in the secondary chamber 302 reduces the amount of nitrogen as well as the amount of oxygen that is being input into the mixing chamber 303. This limits the potential for the formation of oxides of nitrogen in the secondary chamber 302. This same compressed air cooling system could be applied to the top mounted, down fired burner design shown in Figure 3 (prior art) or in any similar system that uses a secondary chamber heat-up and support burner that must be able to switch from on to standby as part of the secondary chamber temperature control sequence. In an embodiment, which may or may not include the burner related innovations described above, the gasifier 300 has a deceleration chamber 326 located at the end of the secondary chamber 302 through which the hot gases 325 in the secondary chamber 302 must travel. The hot gases 325 will have entrained some particulate or fly ash 327 as the augers 305 move the waste across the hearth 304 in the primary chamber 301. As the hot gases 325 enter the deceleration chamber 326 they immediately begin to slow down due to the increase in cross-sectional area of the deceleration chamber 326. The cross-sectional area of the deceleration chamber 326 channel should be at least 60% greater, and up to or greater than 100% greater, than the cross-sectional area of the secondary chamber 302 channel. The hot gases 325 have usually completed fully oxidizing while they travel along the ash side 333 channel of the secondary chamber 302. The flow of the hot gases 325 turn 180 degrees through the opening 331 in the secondary chamber 302 central dividing wall 324. The hot gases then travel along the feed side 332 of the secondary chamber 302 and enter the deceleration chamber 326. It should be noted that even though most of the ash and particulate produced in the primary chamber 301 is pushed into the ash hopper by the augers 305, some particulate is entrained in the fume 308 and travels with the hot gas flow 325. The purpose of the deceleration chamber 326 is to remove some, if not most, of this particulate or fly ash 327. The deceleration chamber 326 has a channel cross-sectional area significantly larger than the cross-sectional area of the secondary chamber 302. This increase in cross-sectional area will cause the hot gases 325 to slow down. As the hot gas flow 325 enters the deceleration chamber 326 its flow velocity reduces significantly due to the increase in cross-sectional area of the deceleration chamber 326. In the embodiment shown in Figure 9, at the point where the cross-sectional area changes, the channel also turns vertically upward forcing the hot gasses 325 flowing through the channel to rise. The reduction in velocity of the hot gases 325 in conjunction with the upward turn in the direction of the hot gases 325 will cause some of the entrained particulate or fly ash 327 to drop to the floor of the deceleration chamber 326 and into the fly ash drop out zone 328. Figure 10 shows a cross-sectional plan view, and Figure 9 shows a cross-sectional elevation view, of the deceleration chamber 326. The cross-sectional area of the deceleration chamber 326 should be a minimum of 60% up to more than 100% greater than the cross-sectional area of the secondary chamber 302 channel. This increase in the cross-sectional area of the hot gas 325 flow channel causes the velocity of the hot exhaust gases 325 to abruptly decelerate, or slow down, just as the gases turn 90 degrees upward toward the exit duct 338. The hot exhaust gases 325 will have entrained a certain amount of particulate, or fly ash 327, generated during the gasification of the waste mass (230, Figure 5) as the waste is being moved across the hearth 304 of the primary chamber 301 by the augers 305. A significant amount of the entrained particulate or fly ash 327 will drop out on to the bottom of the deceleration chamber 326 as a result of this decrease in velocity of the hot gases 325. The larger and / or heavier particles drop out due to the pull of gravity when the velocity of the flow can no longer carry them upward. Figure 9 shows a depression in the floor of the fly ash drop out zone 328 to reduce or eliminate potential re-entrainment of the dropped out fly ash 327. The amount of fly ash 327 that will drop out will depend on the composition of the waste mass. There may be vacuum access doors 329 in the side wall of the deceleration chamber 326 to allow for intermittent removal of the fly ash 327 build up in the fly ash drop out zone 328. A vacuum system with a stainless steel suction tube that can be inserted through the vacuum access doors 329 can be used as necessary to reduce the build up of fly ash 327 in the fly ash drop out zone 328. Removing the fly ash 327 occasionally with the vacuum system will prevent excessive build up in the fly ash drop out zone 328 and that will help eliminate the re-entrainment of the dropped out fly ash 327. Alternatively, fly ash may be continuously removed from the drop out zone 328 by augers. Figure 10 shows the possible design modifications that can be utilized for expanding the deceleration chamber 324. To create a significant reduction in the velocity of the hot exhaust gases 325 the additional possible expansion 334 can be incorporated in the design of the gasifier. This expansion 334 would be beneficial for wastes that produce very small particles of fly ash 327 and wherein the hot gas flow 325 velocity will need to be very slow for such particles to drop out. As shown in Figure 9, the hot exhaust gas 325 exit duct 338 should be located above the level of the hearth 304 to allow for an effective drop out of the fly ash 327. Figure 11 shows an alternate, higher, hot exhaust exit duct 338 placement that may be used to improve the reduction in any remaining fly ash 327 that enters the exhaust exit duct 338. With this placement more of the fly ash 327 will fall into the fly ash drop out zone 328. This happens because the upward momentum of the particulate will become reduced as the particles climb against gravity. The further the particles have to travel upward, the more likely that some of them will fall back before reaching the exhaust exit duct 338. There are important reasons why it is preferable to limit the amount of particulate or fly ash 327 that enters the exit duct 338 and thus, the downstream section of the total system. If the system is being used for energy recovery as shown in Figure 12, any particulate or fly ash 327 that has continued flowing with the hot exhaust 325 through the exit duct 338 will, over time, reduce the efficiency of the heat exchanger 352 by forming a coating of fly ash 327 on the heat exchange surfaces, particularly on the insides of the heat exchanger tubes. The heat exchanger is usually of a shell and tube type, in which the hot exhaust 325 travels through multiple tubes that are surrounded by water or some other fluid to be heated. The more fly ash 327 that is carried into the downstream equipment the more often the equipment, such as the heat exchanger 352 will require cleaning. This need for cleaning the build up of fly ash 327 also applies to all of the duct work leading up to the abatement unit 353. The abatement unit 353 generally comprises a bag-house filter. The Brookes hot hearth gasifier designs are uniquely suited for utilizing the deceleration chamber 326 method to reduce the fly ash 327 content that is entrained during the gasification process. The fly ash 327 is generated in the primary chamber 301 and is carried into the secondary chamber 302 as part of the combustible fume 308. Both the Brookes batch feed and continuous feed gasifiers have the same basic geometry which allows for the inclusion of a deceleration chamber 326 located just before the hot exhaust gases 325 exit the gasifier through the exit duct 338. It is to be understood that any feature described in relation to one embodiment may be used alone or in combination with other features described and may also be used in combination with one or more features of one or more other embodiments, so long as technically feasible. Furthermore, equivalents and modifications not described may also be employed without departing from the scope of the invention, which is defined by the following claims.

Claims

1. A thermal treatment device (300) comprising:a horizontally disposed primary chamber (301) for receiving waste material, the primary chamber having a floor comprising a hearth (304) which can be heated to generate fumes from the waste material;a vertically disposed mixing chamber (303) having an air input (321) for mixing air with the fumes;a horizontally disposed secondary chamber (302) extending below the primary chamber (301) and configured to allow combustion of the fume from the mixing chamber (303) to heat the hearth (304) of the primary chamber (301); anda burner (311) mounted in a wall of the mixing chamber (303) no higher than the bottom of the hearth (304) and directed to fire into the secondary chamber (302), wherein:the burner is angled downwards at between 25 and 40 degrees to the horizontal.

2. The thermal treatment device (300) of claim 1, wherein the mixing chamber air input (321) is positioned in the top of the mixing chamber (303) and is positioned directly above the burner (311) flame.

3. The thermal treatment device (300) of either claim 1 to 2, further comprising a hearth shield (322) in the secondary chamber (302) to protect sections of the hearth (304).

4. The thermal treatment device (300) of any one of claims 1 to 3, further comprising a floor shield (323) to protect sections of a floor of the secondary chamber (303).

5. The thermal treatment device (300) of any one of claims 1 to 5, wherein the burner (311) is purged with a flow of air through the burner (311) when the burner is off or on standby.

6. The thermal treatment device (300) of claim 6, wherein the flow of air is provided by a different system from the air for combustion provided to the burner (311) when it is on.

7. The thermal treatment device (300) of either claim 5 or 6, wherein the flow of airis controlled by a solenoid valve (316) which is in an open position when unpowered.

8. A thermal treatment device (300) comprising:a horizontally disposed primary chamber (301) for receiving waste material, the primary chamber having a floor comprising a hearth (304) which can be heated to generate fumes from the waste material;a vertically disposed mixing chamber (303) having an air input (321) for mixing air with the fumes;a horizontally disposed secondary chamber (302) extending below the primary chamber (301) and configured to allow combustion of the fume from the mixing chamber (303) to heat the hearth (304) of the primary chamber (301); anda deceleration chamber (326) for receiving the fume from the combustion chamber (302), wherein the cross-sectional area of the deceleration chamber (326) is greater than the cross-sectional area of the secondary chamber (302).

9. The thermal treatment device (300) of claim 8, wherein the deceleration chamber (326) is vertically disposed to provide an upward turn in the direction of flow of the fume as it flows through the deceleration chamber (326).

10. The thermal treatment device (300) of claim 8 or 9, wherein the deceleration chamber (326) further comprises a depression in the floor for collecting fly ash drop out.

11. The thermal treatment device (300) of any one of claims 8 to 10, further comprising an exit duct (338) in fluid communication with the deceleration chamber (326) and extending above the deceleration chamber.

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