EP0156958B1 - Regulation method for the combustion air quantity of a burner apparatus - Google Patents

Description

The invention relates to a control method for the amount of combustion air in a furnace according to the type of claims 1, 2 and 4.

The purpose of the invention is to reduce fuel costs, reduce certain pollutants and reduce pollution in the combustion chamber as well as in the exhaust gas ducts. The invention relates to firing devices that are operated with solid, liquid or gaseous fuels.

In the article "Automatic control ups heater combustion efficiency" by D.H. Leonhard and others, in the magazine "Oil & Gas Journal", Volume 79, No. 38, September 1981, pages 134-138, Tulsa, Oklahoma, USA, describe a control method of the type mentioned at the beginning. For the amount of combustion air in a process heater in a refinery plant, the amount of air for the combustion process is controlled using two analyzers for measuring the carbon monoxide and oxygen content in the exhaust gas so that the concentration of both gases in the exhaust gas reaches a low value.

A continuous control method for combustion plants is known from the publication in the journal "Heiz-Kraft-Kraft" 35 (1983) No. 10, October, p. 447, in which the carbon monoxide content (using an inline measuring arrangement) CO) is determined in a flue gas duct and used to adjust the fuel-air ratio. Such arrangements work with CO values of 100 to 700 ppm in the exhaust gas. This way of working can cause boiler contamination and does not lead to optimal combustion control.

The basic consideration for firing optimization is based on the well-known fact that the CO content in the exhaust gas - plotted against the relative excess air λ - increases very steeply with decreasing air volume and that the knee point of the CO curve generally reaches the maximum for almost all fuels the efficiency coincides. If the excess air is too low, the efficiency drops because unburned substances get into the exhaust gas and ashes. If the excess air is too high, the thermal losses via the exhaust gas increase, which results in a decrease in efficiency. If the setpoint for CO is selected so that the maximum efficiency is adjusted, then significant fuel savings are possible.

However, depending on the type of firing, firing with the value for the CO concentration can lead to a collision with other requirements or targets (exceeding the approved CO value, soot formation). As is well known, the CO value in the area of its formation is not stable, but is subject to strong fluctuations. Another problem is the diagnosis and adjustment of the air / fuel ratio of the individual burners in multi-burner systems.

With an integral adjustment of the air / fuel ratio across all burners, errors can occur that reverse the goal of optimizing the furnace. The control method for firing optimization described below therefore takes into account the setting and diagnosis of each individual burning point, as well as the fact that the CO value fluctuates greatly in the area in which it is formed.

The object of the invention is to provide a method for regulating the amount of combustion air in a firing device which, using the known criteria for firing optimization, always works to a minimum of combustion losses even with changing firing conditions such as changes in load, fuel or combustion air. The control method should also be suitable for systems with several burning points within one furnace.

The invention is achieved with the method steps specified in the characterizing part of claims 1, 2 or 4. Due to the cyclically repetitive readjustment of the fuel / air mixture, a firing device operated in this way is able to react to a load change in the shortest possible time and to set the optimum operating point for the new load condition. The proposed control method is also suitable for firing devices with two or more burning points if it is ensured that the intended procedural steps are carried out separately for each burning point.

The invention is explained in more detail below with reference to FIGS. 1-3. 1 to 3 show the concentration of the exhaust gas component CO as a function of the percentage excess air λ in the burner chamber.

Finding the optimum working point for the air / fuel ratio is explained on a firing device with a burning point with reference to FIG. 1. According to FIG. 1, the optimum operating point of the firing device is approximately in the area of CO formation with an excess air λ in the range of B. To find this operating point, an increased excess air in the range of a λ value of A is first used. With the help of the currently customary methods, the amount of fuel and the burner supply air are measured and the fuel / air mixture is set so that the CO content in the exhaust air drops to a very low value. As a rule, such a concentration lies in the range of the detection limit of the CO sensor.

Starting from the combustion air volume Setting according to the X-value-A, the amount of air supplied to the burner is slowly throttled in a second process step; as the lack of air increases, the CO concentration increases. In the area of increasing CO formation, a fuel / air mixture is selected in which a CO concentration can be reliably determined in the exhaust gas with the transmitter. A CO concentration value of 50 ppm is assumed, which is denoted by the λ value C in FIG. The λ value C lies in an approximately linear curve region, the slope of which is given by the straight line G. The point of intersection of the straight line G with the X axis gives the operating point B, to which the excess air is adjusted in a third method step in order to achieve optimal combustion. In order to be able to determine the working point B from the CO concentration, the CO concentration has to be determined at two λ values during a first process step; in Figure 1 these are the λ values C and C ', with the CO concentration values of 50 ppm and 25 ppm in the exhaust air. The burner supply air quantities corresponding to the λ values C and C 'are stored and the slope of the straight line G is determined from them in conjunction with the CO values. Since the characteristic curve determined in this way depends on the type of fuel and the power, this procedure is particularly suitable for those combustion plants which use only one fuel and preferably work under a load.

The time sequences for finding the λ values C, C 'and B must be slow against the delay time for the CO measurement. As a guideline, the following can apply: To find the λ values, at least twice the time as for the 90% time of the CO measurement applies.

In some applications it is desirable not to adjust the excess air to the point of intersection B of the straight line G with the A-axis, but to work with a more or less excess air in a different manner. The slope of the CO concentration value against the burner air setting determined in the first method step makes it possible to find a working point corresponding to the desired excess air starting from the λ value B. The optimal λ value to be set for the fuel / air mixture is also an empirical value that depends on the type of firing and on the boiler and the burners.

In the described method for finding the optimum working point for a furnace with a burning point, the accuracy requirements for the measuring sensor for the CO measurement with regard to its zero point are quite high; the combustion takes place in operating point B in the range of a few ppm CO concentration with a measuring range of the sensor between 30 and 100 ppm CO. The cross sensitivity and the zero point drift of the transducer must therefore be small compared to the CO concentration present at the operating point. The zero point drift should be less than 1 ppm within a maintenance interval. In contrast, the demands on the sensitivity stability of the transducer for CO are lower, because the finding of the working point B is only slightly dependent on the measuring accuracy of the CO transmitter.

Because of the strong fluctuations in the CO concentration in the area of CO formation, the method according to FIG. 1 is difficult to implement for different types of burners and types of fuel. In such cases, the operating point B in the area of CO formation can be set by slowly reducing the amount of combustion air. Figure 2 illustrates this case. It is assumed that there is an excess of air with a λ value of A and an air deficiency at λ equal to B ^*. The lack of air is determined in a fuel / air ratio in which carbon monoxide is currently being formed. By adding and removing the air in the range of the predeterminable λ values A 'and B', the working point of the optimal burner supply air in range B is narrowed and fixed by averaging. With this iterative approach to deliberately narrow down work area B, the CO measurement is largely limited to a yes / no statement; the accuracy requirement for the CO measurement with regard to sensitivity is low.

The proposed method for regulating the amount of combustion air in a combustion device as a function of the CO concentration in the exhaust gas works at very low concentration values; this makes it possible to identify a single burner with a CO measurement in the flue gas and the method can also be transferred to firing systems with several combustion systems.

The method for the combustion air quantity of a combustion device with two burners is explained with reference to FIG. 3. Analogously, it can also be transferred to a firing device with any number of burners.

FIG. 3 shows the concentration of the exhaust gas component CO as a function of the percentage excess air λ in the burner chamber of a furnace with two burners. As can be seen in FIG. 3, both burners work with excess air in a region with the X value A. At a suitable point in the burner chamber with thorough mixing of the individual exhaust gases, a measuring transducer common to both burning points is provided for determining the CO concentration. The working points B1 and B2 of the two burners are determined cyclically in a selectable order in accordance with the method of claim 1 or claim 2. The working point of the burner that is not to be optimized is placed in the area of the low CO concentration.

If the two burners are optimized with the control method according to claim 2, starting from an excess of air for both burners with a λ value of A, the air volume for each individual burner is successively reduced and the CO concentration in the exhaust gas is measured after each air volume change. When CO is formed, the burner whose air volume change has led to the formation of the measured CO concentration is operated with so much excess air that CO is no longer formed in the exhaust gas. In the selected example according to FIG. 3, the one burner is fixed to the working point B2. The other burner is then checked and fixed at its working point B1 in the same way, with the CO concentration in the exhaust air being measured after each change in the air quantity on this burner.

If there are more than two burners in a firing device, the air volume is changed accordingly for all other burners by not changing the operating points of the burners that have already been set in a setting cycle. By resetting the burners with excess air in the areas of very low CO concentrations, each individual burner can be identified and thus adjusted by means of a CO measurement in the exhaust gases originating from all burners.

With the conclusion of an adjustment cycle for determining the optimal fuel-air ratios B1 and B2, a new adjustment process can in principle begin immediately, so that the firing optimization is continuously effective.

When recording the CO concentration in the exhaust gas, it is important to recognize the formation of CO quantitatively as early as possible. In firing systems with several burners, it can always be useful to also use several transducers if it is not guaranteed that the exhaust gases from the participating burners do not mix sufficiently. In order to obtain a representative measured value for the respective CO concentration in the entire exhaust air, various possibilities for evaluating the measured values are conceivable. For example, the mean value from the CO measured values of the individual sensors can serve as a measure of the CO concentration. Other evaluations of the CO measured values dependent on the structure of the firing device can also be carried out, e.g. B. by a transducer and several tapping points, the various sample gases are mixed and then fed to the transducer.

There are firing systems in which several burning points are grouped together (coal mills) and several such groups are supplied with different fuels. The exhaust gases are routed through a common exhaust. Because of the redundancy of the measured value acquisition, it is expedient here to work with several transducers and to control the excess air in such a way that the air supply is regulated jointly for one group of burners each.

Claims (7)

1. Method of controlling the combustion air volume (air volume λ) of a furnace plant with a burner as a function of the CO content (CO concentration) in the flue gas measured by a transducer, in which excess air is fed to the burner until the CO concentration in the flue gas reaches a low value, characterised by the following method stages:

a) when the air volume is deficient, various values of CO concentration are determined and stored with the associated air volumes (λ values C and C') and the gradient of a straight line (G) for the CO values versus the air volume (λ axis) is determined from the stored values;

b) starting from the air volume setting with excess air (λ value A), the excess air is reduced until a CO concentration which is presettable and reliably recordable with the transducer is obtained in the flue gas (for example 50 ppm CO, λ value C);

c) starting from the value for the CO concentration in the flue gas set according to method stage b) and the associated air volume, the working point of the burner is fixed at an air volume which corresponds to the point of intersection of the straight line (G) with the λ axis (λ value B) or deviates therefrom to a desired extent;

d) method stages b) and c) are repeated continuously or at presettable intervals of time in a cyclic sequence so that an optimal quantity of air and fuel for the combustion is obtained (Fig. 1

2. Method of controlling the combustion air volume (air volume λ) of a furnace plant with a burner as a function of the CO content (CO concentration) in the flue gas measured by a transducer, in which excess air is fed to the burner until the CO concentration in the flue gas reaches a low initial value (A), characterised by the following method stages:

a) starting from the initial value (A) set for the CO concentration, the excess air of the air volume is reduced until a reliably measurable second value (B^*) for the CO concentration in the flue gas is obtained (e.g. 10 - 50 ppm CO);

b) starting from the two values (A, B^*) for the CO concentration thus found, the working point of the burner is narrowed down iteratively in the range of presettable values for the air volume (λ values A' and B') by the addition or extraction of air and fixed by averaging until an air volume (λ value B) in the range of low CO concentration is obtained;

c) method stage b) is repeated continuously or at presettable intervals of time in a cyclic sequence so that an optimal quantity of air and fuel for the combustion is obtained (Fig. 2).

3. Method of control according to Claim 1 or Claim 2, characterised in that the working point (λ value B) of the burner is so fixed that a CO concentration which can still just be reliably determined is obtained.

4. Method of controlling the combustion air volume (λ) of a furnace plant with two or more burners, characterised in that

- a transducer common to all the burners for measuring the CO concentration is provided at a suitable point with good intermingling of the individual flue gases and

- that the working points (B1, B2) of the individual burners are determined cyclically in a selectable sequence according to the method in Claim 1 or Claim 2, the working points of all the other burners apart from the burner to be optimized being placed in the range of very low CO concentrations (Fig. 3).

5. Method of control according to Claim 4, characterised in that several transducers for measuring the CO concentration in the flue gas are provided and that a measured value for the CO concentration in the exhaust air is determined from the measured values of the individual transducers by averaging or another suitable evaluation of the CO concentration.

6. Method of control according to Claim 4, characterised in that a transducer and several gas sampling points are provided, the gas samples being first mixed and then supplied to the transducer.

7. Method of control according to Claim 4, characterised in that several burners are combined into one group and that in order to set the excess air, the air supply is controlled jointly for one group at a time.

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