GB2140587A - Improvements in and relating to combustion processes - Google Patents

Abstract

A sensor (1) e.g. for O2 produces a proportionate signal used by a controller (2) to cause any necessary change (3) in supply (5,6) of fuel gas and/or combustion promoting gas in order to bring the component to a preset level. The sensor (1) may be connected to sample main flue and/or pilot chamber (9) gases. <IMAGE>

Description

SPECIFICATION Improvements in and relating to combustion processes This invention relates to operation of combustion apparatus and has particular application to monitoring and controlling ratios of fuel gas and combustion promoting gas, normally air, for furnace, boiler and other combustion systems.

Thermal efficiency of such systems obviously depends upon how closely the gas-air ratio can be kept to stoichiometric conditions, i.e. where all fuel combustibles of the gas are consumed and no oxygen of the air is left in the exhaust from the system. The usual practice is to utilise pressure sensitive valves for control purposes aimed at maintaining a reasonable constancy in the gas-air ratio. However, variations in temperature and/or barometric pressure and/or energy content per standard volume of fuel gas militate against the objective of reasonable constancy of gas-air ratio, certainly leading to variations from stoichiometric conditions that substantially lower thermal efficiency if not, in some instances, give rise to dangerous conditions.

It is an object of this invention to improve upon such usual practices, i.e. obtain better control of gas-air ratios, and to do so by specifically monitoring and controlling that ratio.

According to one aspect of this invention there is provided a method of controlling mixtures of fuel gas and combustion-promoting gas for or in a combustion process, wherein a component ofthe mixture, whose content in relation to the combustion process is indicative of thermal efficiency of that process, is sensed as to its said content and corresponding signals produced, those signals are compared with signals representative of reference information for said content in orderto produce control signals, and the control signals are used to adjust the supply rate of at least one of the fuel gas and the combustion-promoting gas.

Another aspect of this invention concerns apparatus to carry out such method comprising sensing means for said component content, comparator means for output signals from the sensing means and reference signals, and means for varying at least one gas supply means, i.e. either or both according to the comparator output.

Said sensing is conveniently in relation to exhaust gas after combustion, say via samples from main exhaust and/or from any pilot exhaust; and oxygen represents a suitable component for sensing, say by electro-chemical means.

Comparison and control signal generation functions are readily achieved by a microprocessor, which could serve to multiplex or select sampling signals from pilot amd/or main exhaust.

Systems hereof can control mixing of gases for combustion purposes in a manner that does not require the usual reducing valves and proportioning valves etc.

Supply means for the gases will be relatively controllable, and will usually comprise a pump for each gas. The pumps may have their own controllers/motors with at least one relatively controllable by said control signals, or appropriately adjusted control signals for each.

Alternatively, a common motor, of either fixed speed or variable speed type, can be used to drive two pumps. One pump will deliver fuel gas and the other pump may deliver air, both into a common manifold from whence the mixture will be delivered to a burner or burners. The common motor which drives both pumps can be directly connected to one pump but indirectly connected to the other pump via a variable ratio gearbox or similar device. By changing the ratio of this gearbox it will be possible to alter the speed at which one pump operates in relation to the other and thus change the gas-air ratio.

Overall, the control system will serve, via its said control signals, to reverse any variation of the gas-air ratio from a preset value or range therefor. Of course, under stoichiometric burning conditions there would be no oxygen left in the exhaust gases but this ideal condition is virtually impossible to achieve. Therefore, the system would be set to allow for a small percentage of oxygen to be acceptable in the exhaust gases, but variations above or below this pre-set level would start up the process whereby corrections to the gas-air ratio were made to bring the mixture back to the pre-determined level.

Generally, an oxygen sensor will be fed with a supply of exhaust gas from a flue stack or furnace discharge, but sometimes it is, as above, proposed to take a sample ofthe gas directly from the main feed manifold into a small pilot jet which would be continuously burning and to sample this exhaust gas as to oxygen content.

Specific implementation of the invention will now be described, by way of example, with reference to the accompanying schematic block drawing.

In that drawing, a gas-mixture supply manifold system 10 serves to feed one or more burner nozzles (not shown). Branches 10A and 10B of the manifold system are connected to outputs of two pumps 5,6 for fuel gas and combustion-promoting gas, normally air. Those pumps may be of the sliding vane type, scroll type or similar having suitable characteristics and capacities for handling gas and air under normal conditions.

The pumps 5 and 6 are shown with a common drive coupling 7 from a suitable prime mover or motor 4, but with a variable ratio gearbox in the drive line to the pump 6. The variable ratio gearbox 3 is adjustable by a control signal from a controller 2 that may conveniently be a microprocessor suitably programmed to perform the required functions hereof.

Athrottle valve 8, say of butterfly-type, is shown in the manifold system 10 for making adjustments to the volume of mixture passed to the burner or burners. A similar result is achievable by control of the speed of the motor by a variable speed device.

In order to implement this invention, oxygen sensing means 1 samples exhaust gases, i.e. the results of combustion of the gas mixture. A suitable oxygen sensor may be of a type available from City Technology Limited which is, in effect, an electrochemical cell that generates an electrical signal directly proportionate to the amount of oxygen present.

The sensor 1 is shown sampling alternatively, via a selection valve 11, from the main exhaust flue or stack via s sampling tube, or from a pilot burner chamber 9, and will, of course, normally have associated therewith a small pump 12 to assure supply of the exhaust gas.

Which one or both of the sensor supply systems are provided is a matter for sensible design choice in relation to the combustion process to be controlled, normally, of course, for a furnace or boiler.

Provision of both is assumed and electrical signals from the controller 2 will be used to control the state of the valve 11, controller 2, which could serve by control of the valve 11 to multiplex the supplies to the sensor 1 and thus inputs to the controller 2.

Appropriate use of the controller output or outputs if desired or required, whether via the indicated variable ratio gearbox or via ratio controllers for either or both of the pumps 5 and 6, will clearly serve always to bring the system towards a condition where no more than a preset level of oxygen is present in gaseous products of combustion.

It will be evident that the controller could alternatively comprise a comparator operative relative to a preset or presettable reference representative of desired exhaust oxygen level and inputs from oxygen sensing means. Its output will then be conditioned to serve as input to the variable ratio gearbox or directly to the controller of a pump, and timing circuitry will serve to control the comparator and/or presentation of sample signals thereto.

Benefits to be derived from embodiments of the invention are as follows: 1. By arranging forthe gas-air ratio to be close to the ideal mixture the burner efficiency will be at its best. As stated, due to barometic pressure changes coupled to temperature variations and the energy content of the gas having variations it is very rare for burners to be operating near to their ideal level. A system embodying this invention, because it can be arranged to sample on a regular basis, will hold the gas-air ratio close to whatever level it has beem preset to without an operator having to sample, analyse and make necessary adjustments (a lengthy process).

2. As the system is continuously aware of the oxygen level in the flue or pilot jet it can readily be arranged for this information to be displayed on meters in the control room or elsewhere as required.

This also applies to the gas temperature in the flue.

3. The system is simple to instal and operate, either as original equipment or retrofitted, and the savings in fuel are considerable.

4. The system is not restricted to pipe gas and can be used forvapourisedfluids on condition that they can be haldled by the pump. It is therefore, possible to use this invention to burn waste products.

5. In the case of oil-fired or coal-fired boilers or furnances a sensor such as at 1 can be used to operate, with suitable circuitry, the damper control or fuel level control to bring back the oxygen level in the flue to the preset level, the link between the controller and the damper or fuel control then, if desired, being a simple mechanical device such as a motor driven actuator. Experiments already carried out by the inventors indicate that savings in the region of 2% to 5% may be expected compared with burners working with conventional control systems.

In its broadest aspect, the invention may be viewed as method and/or apparatus for controlling a combustion process wherein gaseous products of combustion are applied to means for sensing a component thereof, say oxygen, and for producing a proportionate signal, and utilising that signal to cause a change in or relating to the combustion process to bring said component to a preset level.

Claims (17)

1. A method of controlling a combustion process wherein gaseous products of combustion are applied to means for sensing a component thereof and, for producing a proportionate signal, and utiliing that signal to cause a change in or relating to the combustion process to bring said component to a preset level.

2. A method of controlling mixtures of fuel gas and combsution-promoting gas for or in a combustion process, wherein a component of the mixture, whose content in relation to the combustion process is indicative of thermal efficiency of that process, is sensed as to its said content and corresponding signals produced, those signals are compared with signals representative of reference information for said content in order to produce control signals, and the control signals are used to adjust the supply rate of at least one of the fuel gas and the combustionpromoting gas.

3. Apparatus for carrying out the method of claim 2, comprising sensing means for said component content, comparator means for output signals from the sensing means and reference signals, and means for varying at least one gas supply means according to the comparator output.

4. Method or apparatus according to claim 2 or claim 3, wherein the sensing of the said component is in exhaust gas after combustion.

5. Method or apparatus according to claim 4, wherein said sensing is via samples from main exhaust gas.

6. Method or apparatus according to claim 4, wherein said sensing is via samples from pilot exhaust gas.

7. Method or apparatus according to claim 5 and claim 6, wherein said sensing is selectively from main exhaust gas and pilot exhaust gas.

8. Method or apparatus according to any preceding claim, wherein said component is oxygen and sensing is via electro-chemical means.

9. Method or apparatus according to any preceding claim, wherein the comparing and control signal generation is via a micro-processor.

10. Method or apparatus according to claim 9 with claim 7, wherein selection of sensing is available via the microprocessor in a multiplexed mode.

11. Apparatus according to any one of claims 3 to 10, comprising relatively controllable supply meansforfuel gas and combustion promoting gas and means for controlling their relative rates of operation in accordance with said control signals.

12. Apparatus according to claim 11,wherein each supply means comprises a pump.

13. Apparatus according to claim 12, wherein each pump has its own rate controller and at least one of the latter is variable by said control signals.

14. Apparatus according to claim 12, wherein the pumps have a common drive means and a variable ratio drive transmission means for one of the pumps.

15. Apparatus for monitoring and controlling gas-air mixtures substantially as herein described with reference to and as shown in the drawing.

16. A furnace or boiler equipped with apparatus according to any one of claims 3 to 15.

17. A method of controlling mixtures of fuel gas and air for a furnace boiler or other combustion apparatus, substantially as herein described with reference to the drawings.