GB2114722A - Improvements in or relating to furnaces - Google Patents

Abstract

A method of combustion comprises monitoring a selected component of the effluent gases of the combustion process so as to produce a signal indicative of the oxidation level of said gases and utilising said signal to control the admission of air to the combustion process. The invention also relates to a cremator, incinerator or other furnace which comprises one or more combustion chambers for effecting combustion of charged material. Control means control the combustion process including means for adjusting the amount of air admitted to the or a selected combustion chamber for completing the combustion process. Monitoring means monitor a selected component of the effluent gases of the combustion process so as to produce a signal indicative of the oxidation level of said gases. <IMAGE>

Description

SPECIFICATION Improvements in or relating to furnaces This invention relates to furnaces and mainly, but not exclusively, to cremators.

An example of a well known cremator is the Dowson s Mason Single Cremator which is shown in longitudinal section of Fig. 1 and in cross section (on line Il-Il of Fig. 1) in Fig. 2 of the accompanying drawings. It comprises a primary combustion chamber 1 having a perforated hearth 2 with skids 3 for receiving a coffin 4 introduced into the chamber 1 through a charging door 5 at the front of the cremator.

Combustion products pass down through the hearth 2 into a secondary chamber 6 from which exhaust gases pass through lateral ports 7 and passages 8 in the rear half of the chamber 6 into a bottom or tertiary chamber 9. Vertical passages 10 in the side walls lead gases from lateral ports 11 of the tertiary chamber 9 up to longitudinal passages 12, 1 3 above the roof arch 14 of the primary combustion chamber 1. Venturi eductors 15, 1 6 in these passages 12, 13 exhaust the waste gases through a cremator outlet 1 7 to a flue connected to a chimney stack (not shown). A main burner 18 is mounted at the rear end of the primary combustion chamber 1 and the tertiary chamber 9 may be fitted with an after-burner 19 at the same end.Primary or top air is supplied through jets 20 in the roof arch 14 of the main chamber 1 for maintaining combustion in this chamber 1. The secondary chamber 6 is provided with front and rear air jets 21, 22 respectively for supplying secondary air to this chamber. The top air and the front and rear secondary air are independently controllable by the cremator operator. A draught sensor (not shown) senses the pressure in the combustion chamber 1. It is important that the cremator should operate at a negative pressure to prevent emission of fumes.

Suction is controlled manually or automatically by controlling the supply of air to the venturis. The flow of gases in the cremator is indicated by arrows in the drawings.

Efficient operation of the cremator is a skilled task requiring an experienced operator. Very briefly, three operating stages may be identified after a shut-down and with empty primary chamber 1: 1. Start-up and Preheating: the venturi eductors 12, 1 3 are operated to produce suction in the chamber 1 and the main burner 18 is started to heat the cremator to a suitable charging temperature of say 600700 C. The burner 18 is then switched off.

2. Charging: full suction is applied and secondary air is partially opened while all other air jets 20 are off. The charging door 5 is opened and the coffin 4 is pushed into the chamber 1. The door 5 is closed.

3. Cremation: the aim during this phase is to control the rate of combustion thus reducing the possibility of smoking, excessive temperature and pressurisation of the cremation chamber 1 and at the same time to complete the process in a reasonable time. If the temperature in the main chamber 1 is correct the coffin 4 should ignite readily but if it fails to do so or combustion is poor the burner 1 8 is switched on for a few minutes.

Top air is used with caution as it may encourage smoke formation. Also, needless use of air wastes heat, extends cremation times and causes loss of temperature.

Pollution control regulations and ethical considerations make smoke prevention an important requirement. Thus a smoke detector S is mounted in the flue offtake where products of combustion leave the cremator and enter the overhead flue. The detector (which is not further illustrated) comprises a light transmitter on one side of the cremator which focuses a beam of light onto a receiver at the other side of the cremator. Smoke will reduce the intensity of the light beam to an extent dependent on the density of the smoke. An indicator connected to the receiver gives a read-out of percentage obscuration. This indicator must be closely monitored by the operator particularly in the early stages of cremation soon after charging and late when the coffin breaks open.If smoke is detected corrective action by the operator is required to reduce the rate of combustion in the main chamber thereby in turn reducing the rate at which smoking volatiles are produced to the point where these volatiles are capable of complete burn-out by the secondary air. This is achieved by a combustion of one or more of the following operations: turn off burner, reduce or turn off primary air, increase secondary, air, the precise mode of control depending upon the operator's judgement of the combustion conditions at the time. Firstly, suction is adjusted and the amount of secondary air is increased to a maximum if necessary. If smoking persists the main burner is switched off (if it is in operation) and top air is shut off.

Smoke is produced by suspended carbon particles and indicates that combustion of carbonaceous material has not been completed.

However, the reverse is not necessarily true, i.e.

the absence of smoke does not necessarily mean that combustion is complete. The unburnt carbon may be present in small quantity insufficient to affect the smoke detector and some incompletely combusted gases, e.g. CO, will be invisible in any event.

It is an object of the present invention to improve the operation of cremators and other furnaces by ensuring substantially complete combustion of exhaust gases thereby obviating or mitigating the emission of incompletely combusted products including smoke.

According to a first aspect of the present invention there is provided a method of combustion comprising monitoring a selected component of the effluent gases of the combustion process so as to produce a signal indicative of the oxidation level of said gases and utilising said signal to control the admission of air to the combustion process.

According to a second aspect of the present invention there is provided a cermator, incinerator or other furnace comprising one or more combustion chambers for effecting combustion of charged material, control means for controlling the combustion process including means for adjusting the amount of air admitted to the or a selected combustion chamber for completing the combustion process, and monitoring means for monitoring a selected component of the effluent gases of the combustion process so as to produce a signal indicative of the oxidation level of said gases.

The invention will now be further described by way of example only with reference to Figs. 3 and 4 of the accompanying drawings which are views corresponding to Figs. 1 and 2 of the Dowson 8 Mason Single Cremator but modified in accordance with the invention. The description of the construction of the cremator will not be repeated. It suffices that the same parts bear the same reference numerals as in Figs. 1 and 2.

Hot-junction thermocouples T1 , T2, T3, T4 (e.g. chrome/alumel thermocouples) in protective sheaths (e.g. of ceramic or metal) are located in the main chamber 1 (T1), the tertiary chamber 9 (T2), the flue offtake box prior to the venturis (T3) and the cremator outlet after the venturis (T4), respectively. An oxygen sensing probe P is located in a vertical passage 10 in a side wall. The probe P may be a zirconium probe. The sheathed thermocouples T1-T4 and the oxygen probe P are commercially available items and need not be further described save to say that they are linked to suitable equipment for producing a computer or operator readable output signal depending on whether the cremator is intended to be automatically or manually controlled.

The basis of control in this embodiment is the measurement of oxygen concentration. The presence of surplus oxygen in the exhaust gases from the tertiary chamber is a reliable indication that combustion of these gases is complete provided that other operating conditions, particularly temperature, have been maintained.

The oxygen probe P senses the oxygen level continuously and when the level falls below a predetermined percentage figure additional air is introduced into the secondary chamber at the front and/or rear thereof by the secondary air jets 21,22. Our investigations have shown that about 7% oxygen by volume is a suitable percentage figure which provides adequate safeguards against sporadic high oxygen demands of the combustion process. It will be appreciated that a different percentage figure may be selected having regard to the fact that if, on the one hand, the figure is set too low incomplete combustion may occur at times of high oxygen demand and if, on the other hand, the figure is set too high excess secondary air will be introduced giving rise to the disadvantages already explained.

The use of gas analysis to indicate the completeness of combustion enables the smoke detector S to be eliminated. Preferably, however, this detector is retained as a safety measure to cope with situations in which smoke is generated as a result of equipment failure.

The oxygen probe P may be replaced or supplemented by a probe for sensing one or more other gases, e.g. carbon monoxide and carbon dioxide. Such probes are also available commercially and need not be further described.

While the detection of surplus 02 is an indirect indication that combustion is complete the presence of CO is a direct indication that combustion is incomplete because carbon in fully burnt-out exhaust gases will be present only as CO2. The measurement of CO2 also affords indirect confirmation that combustion has been completed. The 02, CO and CO2 gas sensing probes may be located in the waste gas stream at any convenient location or locations downstream of the tertiary chamber 9 and upstream of the venturi eductors 15, 1 6.

In a manually controlled cremator having the features described above, the thermocouples T1-T4 and the 02 and/or CO and/or CO2 probes may be connected to visual display gauges conveniently positioned for monitoring by the operator. By checking the gauges linked to thermocouples T1 and T2 the operator can monitor the temperatures of the main and tertiary chambers 1 and 9 and take appropriate corrective action if necessary by switching the main burner 18 and/or the afterburner 19 off or on. The thermocouples T3 and T4 will produce an alarm signal if an excessively high temperature is reached. In this event the operator must immediately attempt to reduce the level of combustion throughout the system by switching off both burners 18 and 1 9 and introducing maximum secondary air.If the gas probe P, through its associated gauge, indicates that the measured gas or gases are not at an appropriate level the inflow of secondary air is adjusted by the operator using the standard manual controls.

Preferably, however, the cremator is automatically controlled (with the possibility of manual override) by means of a micro-computer to which the output signals from the thermocouples and the gas probe(s) are supplied for comparison with preset values such that in appropriate circumstances the burner 18, 19 and the inflow of secondary air are controlled as necessary.

The available controls of the cremator are as follows: Suction or Draught-Regulation of air to the venturi eductors 15, 1 6 by operating a damper in the air supply pipe controlled by a reversing motor through a diaphragm type draught controller.

Top Air-Air jets 20 vertically downwards through the crown arch 14 of the main chamber 1, activated by power operated valves. Their purpose is to maintain combustion in the main chamber 1 and they are normally in three pairs, each pair controlled by one separately controllable valve.

Secondary Air-The secondary air supply through front and rear jets 21, 22 is controlled by respective power operated valves.

Main Chamber-The main burner 18 can be thermostatically controlled and switched on and off automatically as required. Control could be high/low or on/off.

Afterburner-The afterburner 1 9 may be of a similar size and pattern to the main burner and could be thermostatically controlled to maintain a temperature of say 8000C, in the tertiary chamber. It could be either on/off or high/low.

Smoke Indicator-The indicator controls smoke in an alarm condition by automatically increasing the secondary air and turning off the top air and burner if they are operative at the time.

The computer programme is adapted to control the operation in the following way: 1. Start-up from cold and pre-heat Main chamber is to be preheated to 5000C- 6000C by main burner 18 and tertiary chamber to a minimum of 6000C by afterburner 19. Switch to 'start' mode. Prerequisites are-flue damper must be open, charging door must be closed, air supply fan must be on and gas must be on. All are manual operations, door and damper positions safegarded by limit switches and gas and air supplies by pressure switches. 'Start' mode requires-suction on auto setting. Top air off, Secondary air off, Main burner on and controlling to 500--600"C, Afterburner on and controlling to a minimum of 6000C. On attaining required temperature indication is given 'preheat completed'.

2. Charge coffin On completion of prehating sequence, operator pushes 'charge coffin' button which requiressuction on maximum setting, Top air off, Secondary air on, and controlled by 02 sensor, Main burner off, Afterburner on and controller at a minimum of 6000C.

3. Operator charge coffin and switches to 'cremate' mode Cremate mode requires-suction on auto setting. Afterburner on and controlling to 8000C.

Main burner in 'rate of temperature change' mode so that it only turns up to full flame if temperature starts to fall or increases very slowly. Burner turns down if temperature increases rapidly (i.e. main chamber temperature).

Secondary air on and controlled by 02 sensor and temperature rate of change, top air on and controlled similarly but in opposed phase i.e. as secondary air increases, top air decreases and vice versa. Both top air and secondary air may be controlled by floating type controllers.

4. Smoke control Smoke, if it is going to be present in the flue gases leaving the cremator, will be registered during the 'cremate mode' and normally occurs very shortly after the coffin has been charged and perhaps 10-20 minutes after charging when the coffin breaks open. The following procedure is what one would expect to occur.

Is main burner on? If yes, shut off.

Is secondary air on? If yes, close top air, if no, open secondary air.

Is 02 reading high or low? Regulate the secondary air as necessary.

Is tertiary chamber temperature a mininmum of 6000C? If yes, no action. If no, turn up afterburner.

5. Special cremations Modifications and additions to the programme will be inserted to provide for special cases.

6. Fault finding Faults will be indicated visually on an annunciator panel showing areas in which faults have developed so they can be readily located and cured. This feature will electronically indicate a number of specific faults which at present can only be categorised rather than individualised without a fairly lengthy elimination process by the service engineer.

7. Flue gas temperature after the venturis In order to prevent burn off after or through the venturis (continuing of combustion process in or after venturis rather than its termination prior to the venturis) a temperature sensor will be located in the flue after the venturis. When a predetermined temperature limit is obtained irrespective of the state of other control parameters this unit will automatically switch off all burners and primary air and put on full secondary air.

If the temperature still continues to rise action will be automatically initiated to introduce a non combustible gas i.e. carbon dioxide into the primary combustion chamber, e.g. by way of the top air jets 20.

A flow-chart for the computer programme might read as follows:- a) Controlled set point temperature of main burner fixed at 800O with the upper and lower limits at 7000C and 8000C respectively. Between these two limits the burner would modulate according to positive and negative temperature gradients.

b) All top air inhibited during early stages say first 1 5 minutes of any cremation.

c) Top air (middle) on automatically at 1 5 minutes into cremation and staying on.

d) Top air (front end) on automatically at 25 minutes into cremation and staying on.

e) Top air (rear end) beyond 1 5 minutes into cremation cycle, on and off alternately with burner off and on.

f) Afterburner controlled at temperature of 8000C up to the 20th minute into the cremation cycle.

g) Afterburner inhibited beyond 20th minute into cremation cycle.

h) Main burner and all top air jets automatically turned to 'on' beyond 50 minutes into cremation cycle.

i) Front secondary air controlled proportionally with oxygen analyser, i.e. when oxygen level at 7% or below, then basically secondary air damper would open, when oxygen level rises, then the damper would close.

j) Rear secondary air in manual mode and left in a slightly open position.

k) Smoke controller set such that should the occasion arise where oxygen levels were high and yet smoke still occurring, then at 10% obscuration, the front secondary air damper would open.

I) Smoke controller set for an alarm condition, in that obscuration level reach 30% then the top air jets and burner, if on, would be turned off.

m) Suction controller was set such that if a low level of -3mm wg was reached then all top air and burner, if on, would be turned off.

It will be appreciated that numerous modifications may be made without departing from the scope of the invention as expressed in the appended claims. The description has been confined to a particular model of cremator in order to facilitate an understanding of the invention. A considerable degree of variety exists in the construction of cremators. For example, some cremators have only two rather than three combustion chambers; the afterburner may be omitted from or positioned differently in the tertiary chamber; likewise, the main burner may be positioned differently; also, the primary and secondary air inlets may be different in number and arrangement; operation may be fully manual to fully automatic. The invention is applicable mutatis mutandis to all such cremators.

The invention is not confined to cremators but may be extended to other furnaces, e.g.

incinerators, in order to ensure complete combustion of waste gases.

Claims (11)

Claims

1. A method of combustion comprising monitoring a selected component of the effluent gases of the combustion process so as to produce a signal indicative of the oxidation level of said gases and utilising said signal to control the admission of air to the combustion process.

2. A method as claimed in claim 1, wherein the selected component is oxygen, carbon dioxide or carbon monoxide or any combination of two or more of these gases.

3. A method as claimed in claim 1 or 2, wherein the admission of air to the combustion process is controlled automatically in dependence upon said signal so as to produce a slight excess of oxygen in the effluent gases.

4. A cremator, incinerator or other furnace comprising one or more combustion chambers for effecting combustion or charged material, control means for controlling the combustion process including means for adjusting the amount of air admitted to the or a selected combustion chamber for completing the combustion process, and monitoring means for monitoring a selected component of the effluent gases of the combustion process so as to produce a signal indicative of the oxidation level of said gases.

5. Afurnace as claimed in claim 4, wherein comparison means is provided for comparing said signal with a reference value and producing an output signal, and said adjusting means is adapted to automatically adjust the admission of air in dependence upon said output signal.

6. A furnace as claimed in claim 4 or 5, wherein the monitoring means comprises one or more gas sensors for sensing oxygen, carbon dioxide or carbon monoxide or any combination of two or more of these gases.

7. A furnace as claimed in any one of claims 4 to 6 constructed as a cremator of the kind having a primary combustion chamber for receiving a coffin, a secondary combustion chamber below said primary chamber, means for admitting primary air to said primary chamber, and independently controllable means for admitting secondary air to said secondary chamber, the cremator being adapted so that the admission of secondary air is controlled in dependence upon the signal derived from the monitoring means.

8. A furnace as claimed in claim 7, wherein the primary combustion chamber is equipped with a burner with thermostatic control including a temperature sensor in the primary chamber.

9. Afurnace as claimed in claim 7 or 8, having a tertiary combustion chamber fitted with an afterburner below the secondary chamber and the afterburner has a thermostatic control including a temperature sensor in the tertiary chambers

10. A furnace as claimed in any one of claims 7 to 9, wherein venturi eductors are provided for inducing a draught in the furnace and a temperature sensor is provided upstream or downstream of said eductors for detecting excessive temperatures at these locations.

11. A furnace as claimed in any one of claims 7 to 10, wherein means is provided for supplying carbon dioxide or other fire extinguishing gas to the primary combustion chamber by way of the primary air admitting means.

1 2. A method of combustion substantially as herein described with reference to the accompanying drawings.

1 3. A cremator substantially as herein described with reference to and as illustrated in the Figs. 3 and 4 of the accompanying drawings.