GB2173885A - Combustion apparatus and process - Google Patents

Abstract

An incinerator comprises a combustion chamber 1 having a hearth comprising a grid 24, a vibrator 17 for vibrating the grid, and ash removal means 9, 18 for collecting ash that falls through the grid. The grid normally consists of a series of parallel bars and, when the hearth is elongated, these bars normally extend along the length of the hearth. The ash removal means comprises an inclined trough 9 and a hopper 18. Ash may be removed as a slurry. The incinerator comprises a substoichiometric chamber to which is supplied insufficient oxygen for complete combustion. Vibration of the grid allows ash to fall through the hearth and to be collected in the trough without ash causing abrasion to the incinerator components, to improve the rate of burning. <IMAGE>

Description

SPECIFICATION Combustion apparatus and process The combustion of combustible solids, such as refuse, for instance municipal, hospital, domestic or industrial refuse, inevitably produces ash. The ash may be relatively small in volume but it may be very unpleasant to handle since it may contain finely divided minerals, metal oxides and other abrasive materials.

Various ways are known for handling this ash. For instance when the combustion chamber has a solid or fluidised bed hearth the ash may be removed by digging or scraping it from the hearth, when the chamber is not in use. Various ways of removing the ash continuously have been proposed. For instance in BP 1,299,125 a fluidised bed is described which has a circular motion so as to pass the ash continuously to an outlet. In BP 2,026,146 a fluidised bed is provided with a base that can be pivoted downwardly so as to discharge ash from the base without having to cool the chamber. A slab hearth may be reciprocated longitudinally. In general the known methods are not suitable for small units and are not suitable for sealed chambers.

Particular problems arise where the chamber is to be operated sub-stoichiometrically and where the combustible gases produced in the sub-stoichiometric combustion chamber are led to a second chamber in which they may be burnt in the presence of additional oxygen, for instance as in BP 2,070,212. In a sub-stoichiometric combustion chamber it is essential to control the supply of air very accurately and yet continuous systems for the removal of ash from a combustion chamber are liable to permit uncontrolled entry of air. Accordingly it is generally necessary to remove the ash from such chambers batch-wise, when the chamber is not in operation.

An incinerator according to one aspect of the invention comprises a combustion chamber having a hearth comprising a grid, a vibrator for vibrating the grid, and ash removal means for collecting ash that falls through the grid. Removal of ash from the hearth is therefore achieved by vibrating the grid and so there is no need for continuous scrapers or other mechanical systems for dragging or digging the ash off the hearth. As a result the inevitable abrasion that is encountered by such systems, due to the abrasive nature of the ash, is avoided.

The construction of the grid, and the vibrator for the grid, may be based on the very well known quarry grizzlies and other grizzlies that have been conventionally used for very many years for classifying particulate materials into different size fractions.

The grid normally consists of a series of parallel bars and, when the hearth is elongated, these bars normally extend along the length of the hearth. Instead of or in addition to lengthwise bars there may also be transverse bars. The spacing between each pair of adjacent bars is normally from 2 to 10 mm or more, often about 5 mm. The top of each bar may have small riffles, preferably arranged herring-bone fashion.

The vibration should have vertical and horizontal components, with the horizontal component generally arranged suchthat solids on the hearth are moved by the vibration along the hearth towards a trash outlet for removing, e.g., cans, from the hearth.

The amplitude of vibration of the grid is generally less than 10 mm and most preferably less than 5 mm, typically 1 to 3 mm.

When the grid is being vibrated the frequency of vibration is generally above 500 vibrations per minute, typically 1000 to 3600 vibrations per minute. It is generally not necessary, and may be undesirable, to vibrate the grid continuously during use of the chamber and instead the grid is usually maintained static for most of the time and is vibrated only for short intervals, as and when required to clear the hearth of ash. Typically the grid is static for more than 80% of the time the chamber is in use and typically is vibrated for a period of 5 seconds to 1 minute or sometimes longer, e.g., up to 5 minutes, normally at intervals of 1 to 12 hours, usually 2 to 8 hours.

Ash will fall through the grid during vibration, and possibly also while it is static, and the apparatus includes means for collecting this ash and permitting its eventual removal from the incinerator. Removal could involve, for instance, collecting the ash by suction but this creates certain technical problems and preferably the ash removal means comprises an inclined trough that is beneath and at least substantially coextensive with the grid and a hopper into which the ash is discharged from the lowest edge of the inclined trough. The ash can be flushed along the trough by fluid jets, preferably of water, towards the hopper but instead of or in addition to this flushing mechanism it is preferred to cause the ash to travel along the trough by vibrating the trough, preferably as a result of mounting the trough for vibration with the grid.Thus when the grid is vibrated ash is shaken through the grid and into the trough and is shaken along the trough towards and into the hopper.

In order to minimise the escape of ash from the trough, except into the hopper, it is desirable for the trough to be substantially sealed relative to the grid except at the lower most edge, where the trough discharges into the hopper. The sealing may be caused by creating an air flow from around the edges of the trough towards and up through the grid, so that the upwardly and inwardly flowing air prevents ash from falling from the grid outside the trough, but preferably the sealing is provided by baffles that extend between the sides of the trough and the edges of the grid.

These baffles may be flexible but are preferably rigid. They may contain apertures for the inflow of air to the grid from outside the trough but preferably are continuous. In the preferred construction the trough has a base plate and side walls, and the side walls are connected to the edges of the grid.

In order to avoid the risk of ash escaping from the grid and the trough or other ash removal means, and in order to regulate the air flow through the grid, it is preferred to provide a housing that extends from the chamber, encloses the ash removal means, and is open to the grid, and a blower or other means for creating a higher air pressure in the housing than in the chamber. The pressure differential between the housing and the chamber will be quite small and will provide substantially no restriction on the freedom of ash to fall through the grid and yet ash particles that become entrained in the air in the housing are carried back towards the grid and up into the chamber.

This is particularly valuable since it permits the provision of a gap between the chamber and the grid so as to permit the grid to be vibrated whilst the chamber and the housing are stationary, since air will then flow through this gap and carry back into the chamber any ash or other particulate material that might otherwise escape from the housing. This gap will normally be no greater than the amplitude of vibration, i.e. generally below 5 mm. Thus it is possible to arrange the incinerator such that the only part that vibrates is the grid (and possibly the associated trough) with the result that the desired vibration can be achieved without the need of diaphragms or other solid means for preventing the passage of solids from the chamber through the gap and enables the desired vibration to be achieved using a relatively weak vibrator.

The housing extends from the chamber in order that the walls of the housing form, with the walls of the chamber, an enclosure that will substantially retain the elevated pressure within the housing relative to the pressure above the grid. The housing is normally provided with a discharge outlet to permit removal of ash from within the housing. As the housing is generally at a pressure above atmospheric pressure there will be a loss of pressure, and possibly escape of ash, if the outlet is merely a port that can be opened to the atmosphere. The outlet is therefore preferably provided with means permitting the removal of ash without exposing the interior of the housing to the atmosphere.For instance the outlet can be provided with an air lock having an inlet and an outlet whereby the ash can be collected through the inlet while the outlet of the air lock is closed and then, when the inlet of the air lock is closed, the outlet is opened to discharge the ash. Preferably however the ash removal means include a water supply and a water tight hopper into which ash that has fallen through the grid may be collected as a slurry and the outlet from the housing is located in this hopper such that, when the outlet is open, the slurry provides a liquid seal between the housing and the atmosphere.

Preferred systems according to the invention comprise an inclined trough that will discharge into a hopper at its lower most end and that is substantially sealed with respect to the hearth along its other edges and a housing that extends from the chamber as described above, and as a result it is possible to control very accurately the supply of air to the grid, by appropriate control of the blower or other means for creating elevated pressure within the housing. This is particular value when the combustion chamber is to be operated as a substoichiometric chamber. The combustion chamber may be an elongated chamber having a feed end provided with an inlet for combustible material and a closed end and the lower most end of the trough is preferably adjacent the closed end of the chamber whereby the majority of air entering the chamber enters it adjacent the closed end.Thus the majority of the air is available for combustion of feed that has travelled along the length of the chamber, and the amount of air available for combustion of freshly introduced feed is very low. This promotes controlled substoichiometric combustion. This controlled travel of air along an elongated chamber forms an essential feature of a second aspect of the invention. The total amount of air supplied to the chamber is generally below 30% of the stoichiometric amount.

In this second aspect of the invention an incinerator comprises a substoichiometric combustion chamber that is elongated and has a feed end including an openable inlet for baled combustible solids, a closed end, and means for supplying air substantially only into the closed end, a bale feed for charging combustible solids through the inlet as a bale that substantially fills the cross-section of the feed end, a secondary combustion chamber to which air can be supplied, and a duct leading the combustible gas produced in the substoichiometric chamber from that chamber, at a position adjacent the said inlet, to the second combustion chamber, whereby a bale introduced in the feed end is located between air entering the substoichiometric chamber and the duct to the second chamber.

The substoichiometric chamber of this aspect of the invention may be provided with a conventional, for instance solid, hearth that may be cleared of ash as and when necessary either manually or continuously, by known means. Preferably however part at least of the hearth of the substoichiometric chamber is a vibratable grid as described above.

It is preferred that the freshly introduced bale is, after initial pyrolysis in the feed end, caused to drop down onto the hearth of the chamber. This may be achieved by appropriate design of the walls of the chamber but conveniently the feed end and the closed end have separate hearths and the hearth of the closed end is lower than the hearth of the feed end and there is a step down between the hearths. Thus as the bale is pushed towards the closed end it will drop down this step onto the lower hearth. Preferably the hearth of the feed end is solid. Any ash on this hearth will be pushed inwards by an incoming bale.

Preferably the hearth of the closed end is a vibratable grid as described above. The feed end of the chamber may have a length such that it holds a single bale or two or more bales. The closed end will usually have a length such that it holds two bales, but may hold three or more.

Although small amounts of air may enter the substoichiometric chamber around, for instance, the inlet for the bale or along the length of the hearth of the chamber the majority of the air enters at or near the closed end so that the path of the majority of the gas flow from that end is over and around the most recently charged bale and then through the duct towards the second combustion chamber. In practice it is preferred that substantially all the oxygen that enters the chamber is used up in substoichiometric combustion of the bale or bales in the closed end with the result that the gas contacting the most recently introduced bale, in the inlet end, is substantially free of oxygen but consists mainly of N2, CO, CO2 and dissociated hydrocarbons. That bale is therefore subjected primarily to pyrolysis, for instance at a temperature of 800"C.

As the recently introduced bale substantially fills the cross-section of the feed end this restricts the flow of gas around it and the pyrolysis occurs mainly on the surface of the compressed bale. Closer to the closed end, however, the bales will have loosened, partly as a result of combustion or pyrolysis and partly because of, for instance, the drop down onto the lower hearth and as a result the gas can penetrate the mass that has already been partially pyrolised while it was at the feed end of the chamber.

The air supply to the closed end may be by, for instance, a conventional perforated pipe wherein the perforations are located within the combustion chamber to give the desired relative rates of flow of air at different places within the chamber, but generally the air supply is provided through the hearth and/or at an inlet in the extreme end of the closed end of the chamber. Preferably this is achieved by providing a housing that extends from the chamber and an inclined trough that is sealed to the grid but that discharges from and is open at an end adjacent the closed end of the chamber, as described above.

The invention is now described with reference to the accompanying drawings in which: Figure 1 is a vertical cross-section of one form of incinerator according to the invention.

Figure 2 is a section on the line ll-ll in Figure 1.

Figure 3 is a vertical cross-section through another form of incinerator according to the invention.

Figure 4 is a section on the line IV-IV in Figure 3.

Figure 5 is a section on the line V-V in Figure 3.

The incinerator shown in Figures 1 and 2 comprises a combustion chamber 1 having a hearth that is provided by a grid 2 of parallel bars 3 that extend along the length of the chamber and that have a narrow separation from one another. The chamber is provided with an inlet door 4 that can be raised substantially vertically in guides 5 to permit charging of combustible material into the chamber.

The chamber is also provided with a trash outlet 6 that, in normal operation, is closed by a plug door 7. A flue 8 leads from the top of the chamber to a stack (not shown).

An inclined trough 9 extends along the length of the grid and is formed of a base plate 10 and side walls 11. The side walls 11 are secured to the outer edges of the grid and at least one L piece 12 is secured to each wall. A flange 13 extends downwards from the chamber, its lower edge being below the upper edges of the side walls so that a narrow gap is defined between the walls and the flange 13.

The L members 12 are supported by resilient springs 14 that are mounted on members 15 that are secured to a rigid base 16. An eccentric motor 17 is connected to the base of the trough and, when operated, causes the unit consisting of the members 12, the grid 2 and the trough 9 to vibrate while the chamber 1 and the base 16 remain stationary.

The trough 9 is downwardly inclined at an angle of, typically, 5 to 15 , generally about 10 and so vibration of the trough and hearth will cause the ash that falls through the trough onto the base plate 10 to be conveyed down into a hopper 18.

The hearth 2 may also be inclined so as to shake coarse non-combustible refuse towards the outlet 6. The plugdoor 7 and the top 19 of the hopper may be opened when necessary to permit this refuse to be shaken into the hopper 18 or separate means of collecting the non-combustible refuse from the outlet 6 may be provided. Ash and refuse may be removed from the hopper 18 by opening its top 19.

During normal operation the top 19 of the hopper 18 is kept closed.

A water supply may be provided into the hopper 18 in order that the ash that is shaken into the hopper 18 is present as a slurry and an appropriate outlet is provided in the hopper for removal of this slurry.

Air is permitted to enter the chamber freely, for instance through the hearth 2, and secondary air may be admitted into the chamber 1.

The material that is charged through the door 4 may be any solid combustible material but is typically municipal, hospital, domestic or industrial refuse. It may be introduced loose, in bags, or, preferably, in bales.

If it is desired to operate the incinerator shown in Figures 1 and 2 as a substoichiometric combustion chamber then it is necessary to control carefully the amount of air that enters the chamber and in particular it is necessary to construct the trough 9 and the hopper 18 so as to permit only a regulated amount of air through the hearth 2. When the chamber 1 is operating as a substoichiometric chamber the outlet 8 will normally lead to a secondary combustion chamber to which additional air is supplied, for instance as described in GB 2,070,212.

An arrangement for the hearth in a substoichiometric chamber is shown in more detail in Figures 3 to 5, which also shows an alternative form of combustion chamber.

Referring to Figures 3 to 5, the combustion chamber 1 is formed of two parts, a feed end 21 at which the inlet door 4 is positioned and a closed end 22. Each end, or zone, has its own hearth, the hearth in the feed end or zone 21 being a solid hearth 23 and the hearth in the closed zone 22 being a vibratable grid 24.

The feed zone 21 has a cross-section only very slightly greater than the cross-section of the door opening 4, which cross-section is only slightly greater than the cross-section of bales formed by a bale feed (not shown). The bale feed may comprise a baling mechanism that forms refuse into bales of the desired size or may simply feed preformed bales into the chamber. Preferably the transverse width of the hearth 23 is substantially the same as the width of the door 4 and the walls of the chamber are inclined slightly outwards. The bales that enter through it in order that the bales thus are a relatively tight fit along the base of the closed zone and gases passing over the bale in the closed zone therefore tra vel primarily along the sides and over the top surface of the bales.The outlet 8 from the combustion chamber is located distant from the closed end so that air entering the cham ber at the closed end has to travel over a freshly introduced bale, in order to escape from the substoichiometric chamber 1. The outlet 8 leads to a secondary chamber, the entry to which is shown diagrammatically as 25. It may be supplied with a venturi or other air supply, for instance as described in GB 2,070,212.

In operation, the door 4 is opened, a new bale 26 is rammed into the inlet end 21 of the chamber, and the door 4 is immediately closed again. The act of ramming the new bale 26 into the chamber forces the previously introduced bale on hearth 23 towards the closed end. It is held above the level of the hearth 24 by the configuration of the walls of the closed end and by the hearth 23 but eventually falls down the step 27 to the lower hearth 24, of the closed end 22. This helps to shatter the bale and expose the inteiior of it to combustion gases. The movement of this previous bale into the closed zone 22 does, in turn, force towards the right hand end of that zone the residue of the bale introduced previously.Thus, in this preferred process of the invention, air is introduced primarily at one end of a substoichiometric combustion chamber and combustible gases are removed from the other and bales of combustible material are fed from the gas removal end to the air inlet end so that the air initially contacts the refuse that has been combusted to the greatest extent and the gases finally contact freshly introduced combustible material.

As the chamber is to be operated substoichiometrically it is necessary carefully to regulate the flow of air into the chamber and this is achieved by providing a housing 28 that extends with relatively air tight contact from the chamber, for instance at points 29. The housing 28 provides a relatively air tight zone but the housing is open to the grid 24 so that air can reach the inside of the chamber 1. It may be open over the entire length of the grid or only at the extreme end, distant from the inlet 4. A blower 30 forces air into the housing at controlled rate. An eccentric motor 17 and a trough 9 are provided as in the construction shown in Figures 1 and 2, the trough 9 including a base 10 has side walls 11 connected to the edges of the grid 24 and is mounted for vibration on springs 14 with a small gap between members 12 and 13, all as in Figures 1 and 2.The member 13 is provided by an internal shroud plate that extends downwardly from the chamber 22 along its side walls and the end adjacent the step 27 so that the only access for air into the cham ber 1 is through the gap along these two side walls and end between the member 12 and the shroud plate 13 and around the discharge end 31 of the trough, for instance following the dotted arrows shown in Figure 3. Air passing around the discharge end 31 partly flows up through the trough and the grid 24 along its length and partly over the discharge end 32 of the grid.

The discharge end 31 discharges ash into hopper 18 containing a water supply pipe 33 that will convert ash in the hopper into a slurry. This slurry is removed from the hopper by a sump pump 34 but a shroud plate 35 extends into the slurry throughout its removal so as to retain a water seal between the dis charge by the pump 34 and the interior of the housing.

A basket 36 is provided to collect non-combustible refuse that is shaken off the grid 24 and this is removed through a door 37 when necessary.

In the construction illustrated, the trough 9 may have an angle of around 10 whilst the grid 24 is shown as having two sections, one section substantially horizontal and the other having an inclination of about 8". Alternatively the entire grid can be given a gentle inclination, for instance about 2". Each of the bars forming the grid 24 may be typical wedge screen bars or may have riffle tops.

In a typical process the bale size will have each dimension above 500 mm, for instance 750 X 750 X 850 mm and a compacting ratio in the range 10:1 to 30:1, preferably around 20:1 and the bale will have a weight of 100 to 300 kg, typically about 200 kg.

The feed zone 21 has the same length as one bale and the closed zone 22 has the length of about two bales. The vibrator 17 causes the trough 9 and the grid 24 to vibrate at about 1500 vibrations per minute with an amplitude of about 3 mm. Air is forced into the housing 28 by blower 30 at whatever rate is theoretically required for the desired substoichiometric combustion, this rate being calculated having regard to the calorific value of the bales being charged.

In use, bales are charged gradually into the combustion chamber with an ignition burner (not shown) being provided to initiate ignition of the bales. After, typically, about one hour an approximate steady state is reached and thereafter a fresh bale is charged through the door 4 at the appropriate rate, as the previous bale is pyrolised and the entry of each bale forces the previous bales in the chamber into the closed end 22. As exfoliation of a bale in zone 21 proceeds that bale, which is initially supported by the side walls and the hearth of the zone 21, will eventually collapse down the step 27 onto the hearth 24. The vibrator is vibrated as and when necessary. The sump pump 34 may operate continuously but generally operates only when the sump becomes filled.

Claims (12)

1. An incinerator comprising a combustion chamber having a hearth comprising a grid, a vibrator for vibrating the grid, and ash removal means for collecting ash that falls through the grid.

2. An incinerator according to claim 1 in which the grid comprises a plurality of parallel bars.

3. An incinerator according to claim 1 or claim 2 in which the grid is inclined downwards along the length of the hearth.

4. An incinerator according to any preceding claim in which the vibrator is arranged to vibrate the grid with a horizontal and a vertical component of motion with the horizontal component arranged such that solids on the hearth are moved by the vibration along the hearth towards a trash outlet.

5. An incinerator according to any preceding claim in which the ash removal means comprises an inclined trough and a hopper arranged to receive ash from the trough.

6. An incinerator according to claim 5 in which the trough is mounted for vibration with the grid.

7. An incinerator according to claim 5 or claim 6 which comprises water distribution means for supplying jets of fluid for flushing ash along the trough towards the hopper.

8. An incinerator according to any preceding claim comprising a housing, that extends from the combustion chamber, encloses the ash removal means and is open to the lower side of the grid, and a means for creating a higher pressure within the housing than in the combustion chamber.

9. An incinerator according to claim 8 in which the housing is sealed from the atmosphere by a liquid seal in a hopper which is part of the ash removal means and which contains a slurry of ash and water in use.

10. An incinerator comprising a substoichiometric combustion chamber that is elongated and has a feed end including an openable inlet for baled combustible solids, a closed end, and means for supplying air substantially only into the closed end, a bale feed for charging combustible solids through the inlet as a bale that substantially fills the cross-section of the feed end, a secondary combustion chamber to which air can be supplied, and a duct leading the combustible gas produced in the substoichiometric chamber from the chamber, at a position adjacent the said inlet, to the second combustion chamber, whereby a bale introduced in the feed end is located between air entering the substoichiometric chamber and the duct to the second chamber.

11. An incinerator according to claim 10 having the features claimed in any of claims 1 to 9.

12. An incinerator substantially as described herein with reference to the drawings.