EP0317731B1 - Combustion-controlling method of fuel with a highly variable calorific value - Google Patents

Abstract

In the operation of a refuse incinerator, the parameters affecting the combustion process are measured as a function of the water content of the refuse, the water content preferably being determined by means of a radiation receiver with downstream evaluation circuits responding to H2O bands in the spectrum. Evaluation of the spectra over a plurality of zones of the grate allows the air supply and the reduction of pollutants to be optimised. <IMAGE>

Description

The invention relates to a method according to the preamble of patent claim 1.

The control of the combustion in waste incineration plants, the combustion gases of which are usually passed through a heat exchanger in the form of a steam generator, is currently largely manual. The operator observes the combustion process on the grate and varies according to parameters that influence the combustion process, such as the distribution of the primary combustion air supplied under the grate to the zones, the conveying speed of the fuel in the zones, and other such parameters.

In contrast to steam generators, which are fired with traditional fossil fuel such as hard coal, heavy oil or natural gas, the problem with waste (but also with other fuels, such as peat) is that the calorific value of the fuel fluctuates greatly. While in a gas-fired boiler the steam flow at the outlet of the steam generator can serve as a reference variable for the control or regulation of the fuel and / or air throughput, there is no such direct connection in waste incineration; when using the fuel e.g. followed by a large amount of garden waste on a bale of waste paper, then plastic containers, glass, etc. These materials have a significantly different calorific value.

Attempts have already been made to ensure air permeability Detect fuel in the evaporation and degassing zone of the grate by indirectly measuring the differential pressure with respect to the primary air used from below in this zone, and to vary the volume of the refuse so that the average density of the fuel is kept reasonably constant, also the steam flow rate is specified as the reference variable. However, this type of control is associated with considerable dead time, since the passage of a discrete amount of fuel from the task to the ash discharge can take several ten minutes and the different calorific value, which mainly causes problems, only comes into play in the second and third zones; in the third zone insofar as there is a risk that glowing particles or too hot ash will be discharged.

Furthermore, the calorific value correlates only to a very limited extent with the density of the burning layer.

The primary object of the invention is to reduce the dead time in the control. Secondly, the combustion should also take place at different calorific values so that the garbage is really burned completely at the downstream end of the grate, which in turn allows a reduction in pollutants in the exhaust gases and a noticeable reduction in air consumption.

The solution according to the invention is defined in claim 1; the dependent claims relate to advantageous developments of this concept.

The known prior art does not provide any suggestions for this solution.

For example, the document FR-A786.908 discloses a device for regulating the combustion of a fossil fuel, in which a heat radiator in the exhaust stack is directed at a heat receiver transversely to the direction of flow. The less CO₂ or H₂O there is in the exhaust gas, the more heat there is, and the thermal expansion of the receiver is evaluated using a mechanical amplifier. Either the air supply or the fuel supply is regulated in order to optimize the air-fuel ratio. It is obvious that this device can only be used for fuels of at least approximately constant calorific value.

In contrast, the publication "Contrôle Centralisé et Régulation Des Usines de Traitment Industriel des Résidus Urbains" by J. Jullien, in AUTOMATISME, Volume 13, 1968 , pp. 213-219 deals with the problem of regulating waste incineration plants. Because of the relationship of garbage mass multiplied by the garbage calorific value and with the efficiency equal to steam mass times differential enthalpy, it is proposed to infer the calorific value from the latter two variables, while the garbage mass used is determined by weighing.

Patent Abstracts of Japan Volume 6 # 101 (M-135) (979) dated June 10, 1982 discloses that the waste density and the fall rate of the waste in a shaft in front of the combustion zone are used as measurement variables for the combustion control in waste incineration.

Patent Abstracts of Japan Volume 5 # 131 (M-84) (803) dated August 21, 1981 relates to the control of the combustion of dewatered sewage sludge, the temperature of the combustion, the fuel throughput, the air throughput and the fuel weight being evaluated as measured variables.

Patent Abstracts of Japan Volume 10 # 191 (M-495) (2247) dated July 4, 1986 discloses to monitor the burnout level during waste incineration by means of a video camera and to compare it with a predetermined size, after which the air supply is adjusted according to the deviations becomes.

The invention is based on the knowledge that in most cases the water content of the fuel is a characteristic indicator of its calorific value; for fuels with low water content and low calorific value, such as rubble, glass, metals, it is advisable to also record the CO₂ content of the combustion gases as early as possible; if necessary, the detection of only this CO₂ content is sufficient.

According to one aspect of the invention, which is defined in claim 2, a much better control of the heat output can be achieved if the product of the calorific value - determined preferably according to claim 1 - and the density of the fuel is evaluated as an input variable.

Which parameters are controlled individually will depend on the type of system.

For the invention it should also be noted that the timing chains can of course be suitably supplemented to closed control loops.

It is preferred to indirectly determine the water content by measuring the band spectrum of the radiation characteristic of H₂O or CO₂ in the evaporation and degassing zone, the intensity allowing conclusions to be drawn about the water content of the gas and thus of the fuel or the calorific value itself .

For the optimization of the combustion, one can then also record the radiation in the following zones, where the bands for H₂O and CO₂ allow characteristic values for the temperature of the combustion gases to be derived, while the acquisition of further bands, such as SO₂, NO _x , CO etc., make predictions with regard to enables the pollution of the gases and thus allows the flue gas cleaning systems usually provided today to be set to Optima.

• Fig. 1 zeigt, schematisiert, eine Müllverbrennungsanlage mit nachgeschaltetem Dampferzeuger,
• Fig. 2 ist ein gegenüber Fig. 1 vergrößerter Längsschnitt nur des Brennraums, und
• Fig. 3 ist ein entsprechender Schnitt in einer um 90° versetzten Ebene.

An embodiment of a plant in which the method according to the invention is used is shown in the accompanying drawing and is described below.

• 1 shows, schematically, a waste incineration plant with a downstream steam generator,
• FIG. 2 is a longitudinal section of the combustion chamber, and enlarged, compared to FIG. 1
• Fig. 3 is a corresponding section in a plane offset by 90 °.

1 shows the garbage feed chute 10, a feed device 12 in the form of a reciprocating slide, the grate 14 in the form of six rollers, which are arranged one behind the other, axially parallel, and the ash discharge 16. The steam generator 18 provides in connection with the invention no special features. Below the grate are primary air supply lines 20, each of which, as can be seen in FIG. 3, is subdivided into a "right" and a "left", separately controllable half.

A pressure measuring device 22 is arranged in each of the first two air supply lines 20a, the measured value of which fluctuates depending on the throttling of the flow through the fuel in the layer lying above the first two rollers and thus allows its average density to be determined. On the basis of this guide variable, the fuel volume used can be kept somewhat constant by accelerating or decelerating the entry by means of a slide 12.

Above the first two rollers, which define the evaporation and degassing zone, there are observation lances 26 in the walls 24 on both sides, at the outer end of which measuring sensors are provided with upstream filters and downstream electronic processing circuits. Depending on the spectrum that is of primary interest, the selection of sensors and / or filters is made. Here the band radiation is interested in H₂O, and / or CO₂, at a relatively low temperature in the range of e.g. 300..500 ° C, and the opto-electronic devices must be bent accordingly. (The same applies to the observation of the combustion and the ashes, as a matter of course).

The lances are directed to the "right" or "left" side of the grate, so that both sides are detected separately; this cannot be seen in the section according to FIG. 3, which lies approximately in the plane 3-3, but this applies analogously to the observation lances in the further zones. At least one pair of observation lances 28 is in the combustion zone above the middle two rollers of the Grate 14 attached, and at least one further pair of observation lances 30 is located above the burnout zone, that is, above the last two rollers.

The combustion process is observed via the lances 28, most advantageously in the flame front, while the lances 30 serve primarily to measure the temperature for the ash discharge.

It should be noted that the detection of the CO content in the combustion gases makes it possible to reduce their content to a minimum, although both an air deficit and an excess of air can result in excessively high CO contents; A differentiated evaluation is therefore necessary here.

In summary, and in addition, the method of the invention is such that the variations below are possible that the calorific value of the fuels in an area comprising the feed point and the evaporation and degassing zone is determined from properties of the combustion gases by means of a non-contact measurement method, that the density of the fuel on the grate is determined in the same area, and that Fuel feed is controlled depending on the product of the two quantities determined in the direction of constant heat output;
that a controlled parameter is the distribution of the primary air over the individual zones;
that a controlled parameter is the fuel mass distribution over the individual zones;
that a controlled parameter is the fuel feed volume per unit time;
that a controlled parameter is the distribution of secondary air supplied above the grate over the individual zones;
that one of the controlled parameters is the combustion air volume supplied per unit time;
that one of the controlled parameters is the temperature of the combustion air supplied;
that one of the controlled parameters is the distribution of combustion air across the conveying direction;
that one of the controlled parameters is the distribution of the fuel mass across the direction of delivery;
that one of the controlled parameters is the proportion of recirculated combustion gases that is added to the combustion air;
that the water content of the fuel is determined to determine the calorific value;
that the CO₂ content of the combustion gases is determined to determine the calorific value;
that the flame modulation of the radiation is also evaluated,
that the radiation is also detected in the combustion zone and evaluated for the purpose of fine control;
that the radiation is also detected in the burnout zone and evaluated for the purpose of fine control;
that additional bands of the radiation spectrum characteristic of the combustion process are recorded and evaluated;
that at least one of the bands is detected and evaluated which are characteristic of the compounds mentioned below: SO₂, CO, NO, NO₂, OH, CH, C₂, HCO;
in which the combustion gases are passed over heating surfaces of a steam generator, characterized in that the steam flow generated is used as an auxiliary variable in the control;
that the fuel density is determined by weighing;
and that the fuel density is determined from the pressure drop in the primary air.

Claims (19)

1. A method of controlling the combustion of fuels having a highly fluctuating calorific value, more particularly refuse, in which the fuel is fed via a feed station on to a grate to which primary air is supplied from below, and is conveyed along the grate over a vaporisation and degasification zone, a combustion zone and a burn-out zone, to an ash dump, and at least one parameter influencing the combustion is controlled in dependence on measured values, characterised in that the measured values are the water content of the fuel and/or the CO₂ content of the combustion gases in the region of the feed station and the vaporisation and degasification zone, the intensity of the radiation in this zone being measured by evaluating the H₂O and/or CO₂ band spectrum.
2. A method according to claim 1, characterised in that the calorific value of the fuels is determined from the measured values, in that the density of the fuel on the grate is determined in the same zone, and in that the fuel feed is controlled in dependence on the product of the two values found, the control being in the direction of constant heat output.
3. A method according to claim 1, characterised in that one controlled parameter is the distribution of the primary air over the individual zones.
4. A method according to claim 1, characterised in that one controlled parameter is the fuel mass distribution over the individual zones.
5. A method according to claim 1, characterised in that one controlled parameter is the fuel feed volume per unit of time.
6. A method according to claim 1, characterised in that one controlled parameter is the distribution of secondary air fed above the grate over the individual zones.
7. A method according to claim 1, characterised in that one of the controlled parameters is the volume of air for combustion supplied per unit of time.
8. A method according to claim 1, characterised in that one of the controlled parameters is the temperature of the air for combustion supplied.
9. A method according to claim 1, characterised in that one of the controlled parameters is the distribution of air for combustion transversely of the direction of conveyance.
10. A method according to claim 1, characterised in that one of the controlled parameters is the distribution of the fuel mass transversely of the direction of conveyance.
11. A method according to claim 1, characterised in that one of the controlled parameters is the proportion of recycled combustion gases mixed in with the air for combustion.
12. A method according to claim 1, characterised in that the flame modulation of the radiation is additionally evaluated.
13. A method according to claim 1, characterised in that the radiation is also detected in the combustion zone and evaluated for fine control purposes.
14. A method according to claim 1 or 13, characterised in that the radiation is also detected in the burn-out zone and is evaluated for fine control purposes.
15. A method according to any one of claims 1 or 12 to 14, characterised in that additional bands of the radiation spectrum characteristic of the combustion process are detected and evaluated.
16. A method according to claim 15, characterised in that at least one of the bands characteristic of the following compounds: SO₂, CO, NO, NO₂, OH, CH, C₂, HCO, is detected and evaluated.
17. A method according to any one of the preceding claims, in which the combustion gases are passed over heating surfaces of a steam generator, characterised in that the generated steam mass flow is utilised as an auxiliary quantity in the control.
18. A method according to claim 2, characterised in that the fuel density is determined by weighing.
19. A method according to claim 2, characterised in that the fuel density is determined from the pressure drop of the primary air.

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