When materials such as those referred to above are subjected to high temperatures for sufficient time in a gasification or combustion chamber they are broken down into constituents such as a synthetic producer gas and a solid residue such as a carbon char material. The producer das is usually of such a calorific value that it can be combusted in a separate secondary combustion process as described in EP 0444122. The gas can also be used as fuel for a gas turbine or a reciprocating engine or alternatively can be condensed into a liquid for industrial uses.

The disposal of waste material in this manner is an environmentally attractive proposition as energy can be recovered from the combustion gases. However, it is difficult to achieve reasonable efficiency whilst restricting the emission of pollutants to atmosphere to an acceptable level. Although gasification processes have been shown to reduce emissions heavy metals and nitrogen oxides it has proved difficult to control the process effectively.

Conventional apparatus for the thermal degradation of material comprises a combustion or gasification chamber in which the waste material is heated by an external heat source and an exhaust conduit through which the exhaust gases pass.

The degradation process is regulated by a control system that uses temperature sensors to detect the temperature at pre-selected locations in the apparatus and an oxygen sensor to determine the amount of oxygen present in the exhaust gases. If the temperature is too low the rate of combustion of the material may be too slow or the material may have been totally combusted and steps can then be taken to rectify the matter by injecting more air into the chamber or loading more fuel. This type of control system is complicated and difficult to use in that temperature has to be monitored at several locations and an analysis performed before remedial action is taken.

The energy recovered from such apparatus has to be balanced against the energy delivered to the process in the form of the external heat source.

It is an object of the present invention to obviate or mitigate the aforesaid disadvantages and to provide for a more efficient method and apparatus for the breakdown of material by thermal treatment.

The term"pyrolysis"is used in relation to the thermal degradation of waste material in the absence of oxygen. The term"gasification"is used to refer to the thermal degradation of material with an amount of oxygen insufficient for stoichiometric combustion (also known as starved air combustion). The term "combustion"is used in relation to the thermal degradation of a material with an amount of oxygen sufficient for stoichiometric combustion or with surplus oxygen.

According to a first aspect of the present invention there is provided thermal treatment apparatus for the thermal degradation of a material into at least a gas and a solid residue, the apparatus comprising a thermal chamber having a material inlet and a material outlet, means for transporting the material through the chamber at a predetermined rate from material inlet to material outlet, an air inlet to said chamber, a gas outlet and a pressure transducer for determining the pressure in the chamber, means for automatically controlling the volume of air introduced into the chamber through said air inlet in response to the determined pressure.

Thermal treatment is intended to include at least gasification and combustion processes. By using a pressure transducer to determine the pressure (absolute or relative to a reference pressure e. g. atmospheric) in the chamber and controlling the air introduced into the chamber in response there is no requirement for complex temperature measurement at different points in the chamber.

The term"air"is used herein for simplicity and is intended to include oxygen or any appropriate oxidising agent.

The means for automatically controlling the volume of air introduced into the chamber is preferably designed to change the pressure within the chamber so as to affect the amount of air that is drawn into the chamber. This may be achieved by controlling the flow of exhaust gas in the outlet. A flow control member is preferably disposed in the outlet to restrict or encourage the flow of gas in the outlet. The member may be a damper that may be rotatably disposed in said outlet.

The means for controlling the volume of air introduced into the chamber may alternatively comprise air injection apparatus in communication with said air inlet and a damper for controlling the volume of air injected into the chamber.

According to a second aspect of the present invention there is provided a thermal treatment method for the thermal degradation of a material into at least a gas and a solid residue using a thermal chamber with an air inlet and an exhaust gas outlet having a material inlet and a material outlet, the method comprising the steps of transporting the material through a heated chamber at a predetermined rate from material inlet to material outlet, determining the pressure in the chamber, and controlling the volume of air introduced into the chamber through said air inlet in response to the determined pressure.

Ideally the determined pressure is compared to a pre-selected pressure range and if it is outside the range the volume of air introduced into the chamber through said air inlet is increased or decreased accordingly.

The pressure within the chamber may be controlled to affect the amount of air that is drawn into the chamber. The pressure within the chamber is ideally controlled by controlling the flow of exhaust gas in the outlet. The flow of exhaust gas may be controlled by a movable restrictor such as a damper in the outlet.

Specific embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a diagrammatic representation of combustion apparatus being a first embodiment of the present invention; Figure 2 is a diagrammatic representation of combustion/gasification apparatus forming a second embodiment of the present invention; Figure 3 is a diagrammatic representation of combustion apparatus forming a third embodiment of the present invention; and Figure 4 is a diagrammatic representation of gasification apparatus being a fourth embodiment of the present invention; Referring to figure 1 of the drawings, the exemplary combustion apparatus comprises a combustion chamber 1 and an exhaust conduit 2 in the form of a stack.

Waste material 3 to be combusted (referred to herein as"fuel") is supplied though an inclined inlet 4 towards the top of the chamber. The inlet 4 has two air locks 5 that prevent the undesirable ingress of air. Inside the chamber 1 the incoming material is allowed to drop on to a moving perforated conveyor 6 such as a chain or slatted grate. The waste material 3 is transported during combustion to an outlet area 7 where the ash, char or other residue material 8 is dropped into a quench tank 9 for removal by an outlet conveyor 10. The quench tank is sealed from the surrounding atmosphere by an air lock 11.

Directly below the conveyor 6 there is a plurality of air inlet ports 11 that allow air to flow into the chamber 1. The air passes through the perforations in the conveyor 6 and mixes intimately with the fuel 3 supported on the conveyor.

A damper 12 is rotatably mounted in the exhaust conduit 2 and serves to control the flow of exhaust gases. Rotation of the damper 12 to the desired orientation is effected by a hydraulic or pneumatic ram 13 (although any other suitable actuator such as a stepper motor may be used) in response to signals issued by a controller 14.

The damper is cooled internally by a fluid such as air or water.

A pressure transducer 15 such as a photohelic cell or diaphragm is connected to the chamber by a conduit 16 and measures the pressure differential between the chamber interior and atmosphere (or another reference pressure). A signal representative of the differential valve is sent to the controller 14.

In operation, the hot combustion gases given off from the fuel 3 rise up the stack 2 leaving a draft and creating a suction or partial vacuum in the stack 2 and chamber 1 against which the damper 12 must operate. The negative pressure in the chamber 1 determines the volume of air that is drawn up from the inlet ports 11 and through the fuel 3 on the conveyor 6. This in turn has an impact on the temperature within the chamber and therefore the efficiency of combustion.

The orientation of the damper 12 in the stack 2 is used to regulate the negative pressure in the chamber 1 and therefore the volume of air that is allowed to enter the chamber. This in turn affects combustion and therefore the temperature within the chamber. If the damper 12 is moved to a position which partially closes the stack 2 the outlet flow of combusted gases is restricted and the negative pressure in the chamber is reduced in magnitude. This causes less air to be drawn up through the conveyor 6 and therefore the temperature within the chamber decreases. In the reverse situation, if the damper 12 is rotated to a position where the stack 2 is more open, more exhaust gas is able to pass up the stack thereby increasing the negative pressure and the amount of air drawn up through the inlet ports 11. This has the effect of increasing the temperature inside the chamber.

Optimum combustion conditions (dependent on the air/oxygen to fuel ratio) are initially set empirically by adjusting the feed rate of the fuel 3 entering the chamber 1 through the inlet 4, the travel speed of the conveyor 6 (and therefore the residence time of the fuel in the chamber) and the rotational position of the damper 12. When the desired combustion conditions are achieved a pressure reading is taken from the pressure transducer 15 to determine the negative pressure within the chamber. This value becomes the target pressure which is programmed into the control system. Should the negative pressure increase or decrease significantly from this target value the control system (i. e. the pressure transducer 15, the controller 14 and the damper 12) automatically operates to restore pressure to the target value. The target pressure may be changed during operation and a predetermined dead-band may be built into the control system to allow for a target pressure range rather than a specific value. Control of the target pressure range in this way thus regulates the temperature in the chamber and the air/oxygen to fuel ratio.

In the event that the rate of inflow of fuel 3 into the chamber decreases then a lower quantity of combustion gas is released into the chamber. This has the effect of increasing the magnitude of the negative pressure within the chamber. Under normal circumstances the volume of air drawn into the chamber increases and consequently the air/oxygen to fuel ratio is reduced resulting in unsatisfactory combustion.

However, in the apparatus described above the change in the pressure differential is detected by the pressure sensor 15 and a signal is sent to the controller 14 which in turn causes actuation of the ram 13 to rotate the damper 12 to restrict the outlet area in the stack 2 by such an amount as is necessary to restore the target pressure. This restores the'air to fuel ratio and maintains the desired combustion and temperature conditions.

In the reverse situation, if the volume of fuel entering the chamber through the inlet increases then a greater volume of combusted gas is given off and the negative pressure in the chamber has a tendency to decrease. Again this pressure change is sensed by the pressure transducer 15 and the controller 14 acts to rotate the damper 12 to open the stack 2 by an amount to restore the target pressure and therefore the air/oxygen to fuel ratio.

Operating the combustion apparatus in the way ensures that the excess air levels in the exhaust gases in the stack will remain substantially constant with variations in the rate of fuel flow. The only change is in the volume of exhaust gas passing up the stack.

In a modification the apparatus described above the negative pressure may be provided by means other than a natural draught caused by the rising combustion gases.

For example, a fan (not shown) may be used to induce the draught in the stack. In this embodiment an additional and independent control system is provided to set a greater target negative pressure in the stack than is required in the combustion chamber. This may be necessary to provide a sufficiently large pressure differential across the damper for it to operate effectively.

An alternative apparatus for gasification or combustion shown in figure 2 comprises a gasification chamber 50 with a fuel inlet 51, fuel conveyor 52 and residue quench tank 53 as before. An outlet conduit 54 is provided as before with a damper 55 for controlling the flow of the exhaust gases. However, in this embodiment a primary air inlet 56 is disposed at the top of the chamber 50 and the outlet conduit 54 has a secondary combustion zone 57 that is fed by secondary air injection apparatus 58. The outlet gases of the secondary combustion zone 57 pass upwardly to a heat recovery boiler 59 where a heat exchange process is used to generate steam. The opposite end of the boiler 59 has a duct 60 that is connected to an induced draught air fan 61 via a radial vane damper 62 which in turn is connected to an exhaust stack 63.

Pressure transducers 64, 65 such as, for example, photohelic sensors or diaphragms are used to monitor the pressure relative to atmosphere (or another reference pressure) in the gasification chamber 50 and the secondary combustion zone 57. They are connected to each by a respective conduit 66,67 and generate output signals in response. to the pressure differential measured. The signals are sent to respective controllers 68,69 that control the damper 55 in the outlet conduit 57 and the radial vane damper 62 adjacent to the induced draught air fan 61. The position of each of the dampers 55,62 is adjusted in response to signals from the controller 68, 69 by a pneumatic or hydraulic ram 70,71 although other actuators such as stepper motors may be used.

Fuel 72 is fed into the chamber 50, when required, through the inlet 51. As before, the inlet 51 has airlocks 73,74 to prevent the ingress of air into the chamber via this route.

Air is drawn into the chamber 50 during the gasification or combustion process via the air inlet 56 as a result of the pressure differential between the chamber interior and atmosphere. The inlet 56 has a plurality of baffles 75 to provide a labyrinth that restricts the rate at which air can ingress and a closure member 76 to prevent air ingress during an emergency shutdown. The labyrinth structure 75 prevents the excess air being induced into the chamber 50 in the event that that the induced draught fan 61 fails. More than one such inlet 56 can be provided across the chamber to ensure that air is efficiently mixed with the combustion gases so that even combustion conditions are achieved.

The gasification process is initiated by pre-heating the chamber with oil or gas burners El, E2 to raise the temperature within the chamber 50 to the required magnitude for breakdown (volatise) of the fuel 72 into a producer gas and solid carbon-based residue. When the desired temperature is attained fuel 72 is introduced into the chamber 50 through the inlet 51 and deposited on the conveyor 52 which transports the material at a constant rate through the chamber. The conveyor 52 is set to run at a rate than ensures sufficient residence time of the fuel in the gasification chamber for effective volatisation i. e. complete gasification of the fuel by heat to produce solid carbon char residue that is deposited into the quench tank 53 and a producer gas that passes into the outlet conduit 54. The heat and radiation generated by the burners E1, E2 ensure initial heating of the fuel and release of the producer gas.

Before the gasification process can be started the (negative) pressure at F1 above the secondary combustion zone 57 must be significantly greater than that in the gasification chamber 50. This ensures that there is a sufficient pressure differential across the damper 55 in order for it to operate effectively and to maintain the target negative pressure in the chamber 50. The desired pressure is induced by operation of the induced draught fan 61 and the radial vane damper 62. The pressure in at F1 is sensed by pressure transducer 64. The controller 69 is used to actuate the fan speed and/or the damper 62 position to achieve the desired pressure.

Above the fuel, in the vicinity of the burners El, E2, a proportion of the producer gas undergoes starved air combustion (i. e. at sub-stoichiometric conditions) and generates heat and radiation that is used to perpetuate the gasification of incoming fuel 72. The burners E1, E2 are turned off when the chamber temperature is sufficient to maintain combustion and radiation for gasification.

The producer gas passes into the outlet conduit 54 and into the secondary combustion zone 57 where turbulent air from air injectors 58 is mixed with the gas to ensure complete combustion. The hot products of combustion are then drawn into the heat recovery boiler 59, through the duct 60 and into the exhaust stack 63 by the induced draught fan 61.

The rate of gasification is dependent on four main factors: the temperature of within the gasification chamber 50; the radiated heat to which the fuel 72 is subjected from the combusting producer gas; the rate of heat exchange from the gas to the fuel; and the residence time of the fuel in the chamber. The rate of treatment of the fuel and the quality of gasification is dependent on accurate control of the temperature in the combustion chamber.

The temperature in the chamber is dependent on the quantity of air/oxygen being introduced into the chamber since the percentage of producer gas that is combusted is dependent on the amount of air present. The quantity of air allowed into the chamber through the air inlet 51 is dependent on the magnitude of the negative pressure in the chamber 50. This value can be controlled by operation of the damper 55 in the outlet conduit 54, the radial vane damper 62 and the induced draught fan 61.

Assuming that the inflow rate of fuel 72 is constant there is a direct correlation between the negative pressure in the chamber and temperature.

As in the embodiment described in relation to figure 1, the desired gasification conditions are initially set empirically by adjustment of the fuel inflow rate, the conveyor speed, the position of the damper 55 and the speed of the induced draught fan 61 (to adjust the magnitude of the negative pressure at F1. When the optimum conditions are realised a reading is taken from pressure sensor 65 associated with the gasification chamber to determine the negative pressure within the chamber 50. This value becomes the target pressure. The control system is programmed such that if the negative pressure increases or decreases significantly from this target value it automatically operates to restore pressure to the target value. As before, the target pressure may be changed during operation and a predetermined dead-band may be built into the control system to allow for a target pressure range rather than a specific value.

In the event that the pressure within the chamber moves out of the target range the pressure sensor 65 associated with the chamber 50 will issue a signal to the controller 68 to move the position of the damper 55 in the outlet conduit 54. This has the effect of changing the amount of air that is drawn into the chamber through the air inlet 56. The induced draught fan 61 and damper 62 may also be controlled by reference to the pressure reading from transducer 64 in order to control the negative pressure at Fl.

The control system is thus able to maintain the temperature in the gasification chamber and the air/oxygen to fuel ratio at desired levels even with variations in the rate of fuel inflow. This means that exhaust gas air levels will remain substantially constant and only the volume of exhaust gas will change.

The target pressure range can be changed if it is desired to increase the air/oxygen to producer gas ratio closer to stoichiometric conditions. This has the effect of increasing the temperature in the chamber. Likewise the target can be changed to reduce the ratio and decrease the temperature.

In the event that there is an increase in the fuel inflow rate there will be an increased volume of combusted gas given off into the chamber and the negative pressure in the chamber will tend to decrease resulting in the decrease in air intake and therefore a decrease in temperature. However, with the control system operating in accordance with the present invention the pressure sensor 65 detects the change in pressure and a signal is issued to rotate the damper 55 so as to allow open the outlet conduit 54 to induce an increase in the pressure thereby restoring the target pressure range. This ensures that the air/oxygen fuel ratio and the temperature are maintained for optimum gasification. Similarly, if the fuel inflow rate decreases the negative pressure tends to increase and the damper 55 is operated to close more of the outlet 54 so that the target pressure is restored.

In the embodiment of figure 3 the combustion apparatus of figure 1 has been modified slightly. Parts corresponding to those of figure 1 are indicated by the same reference numerals increased by 100 and are not further described except in so far as they differ from their counterparts. The combustion apparatus uses a forced air draught fan 180 to introduce air/oxygen into the space below the conveyor 106. The air inlet ports 111 used in the figure 1 method are closed and the damper 112 in the outlet stack is fully open. The flow of air emanating from the fan 180 is controlled by a damper 181 on an inlet 182 to the fan. The damper 181 is rotated by a ram 183 that is controlled by a controller 184 that is in communication with signals received from the pressure transducer 115. In an alternative configuration the air flow from the fan may be controlled by a speed regulator to which the control system is connected.

The same control system is used as described above. If the pressure within the chamber falls outside of the target range the differential pressure sensor issues a signal to the control system that will in turn issue a signal to move the damper ram 183 so as to open or close the inlet 182 to the fan 180 or to change the speed of the fan thereby restoring the target pressure.

It is important for the gasification or combustion process to be self-sustaining once the start-up burners have been switched off such that no external source of heat is required. This is achieved by combustion of a fraction of the producer gas above the fuel. The radiated heat from this combustion greatly enhances the rate of gasification and release of producer gas. In order to encourage this the walls of the chamber can be constructed to provide a"radiant"section to increase the amount of heat radiated to the fuel. An exemplary embodiment is shown in figure 4. In this embodiment the air is introduced through an inlet 289 in the chamber above the conveyor. Air may also be introduced with the fuel through the same inlet. The chamber walls 290 and ceiling 291 are lined with a reflective ceramic material 292 and are configured to be curve (e. g. concave) so as to radiate heat from the combusted gas 293 towards the fuel 203 on the conveyor 206. The radiation ensures that heat is such that the combustion process is self-sustaining. This may be used in combustion, gasification or pyrolysis.

The above described methods and apparatus provide a simple, accurate, robust, reliable and effective way to regulate the combustion or gasification process without significant manual involvement and without expensive or complex control systems.

It will be appreciated that numerous modifications to the above described design may be made without departing from the scope of the invention as defined in the appended claims. For example, the fuel conveyor in each case may be any type of structure by which the fuel can be moved through the combustion or gasification chamber at constant but adjustable rate. Examples include a travelling grate, a chain grate, an auger or a simple slope. Moreover, air (or any oxidising gas) may be drawn into the chamber via the fuel inlet instead of or in addition to through the air inlet. The pressure sensors described above may be used to detect the pressure differential between that in the chamber and any reference pressure.