EP1923634A1 - Adjustment of fuel gas/air mixture via the burner or flame temperature of a heating device - Google Patents

Abstract

The method involves calculating a run of burner or flame temperature from an output temperature of a burner or a flame temperature. Fuel gas and combustion air stream are adjusted according to determined fuel gas volume or mass flow and combustion air volume or mass flow. Another burner or flame temperature is measured and is compared with the calculated burner or flame temperature, where the measured temperature at a preset point of time is larger or smaller than the calculated temperature at the point of time at a dynamic process, so that fuel gas flow or air mass is reduced or increased.

Description

The invention relates to a method for controlling a fuel gas-air mixture via the measured at the burner, the burner flame or in the vicinity of the burner flame of a heater temperature.

To control a fuel gas-air mixture of heaters, especially condensing appliances, the measurement of the burner or flame temperature can be used. Basis of such a regulation is the setting of a fuel gas-air mixture to a target temperature, the z. B. is measured at the burner. It should be noted that larger deviations in a temperature difference (between a target and actual temperature) should be avoided, otherwise suffers by the increase in the resulting CO emissions combustion quality.
The WO 2006/000366 A1 discloses a method for controlling a fuel gas-air mixture of a burner, in which the flame temperature is detected and regulated in dependence on the desired burner load and air ratio to a target temperature in the steady state. For this purpose, characteristic curves are used which assign the burner load to a specific setpoint temperature. Depending on the air mass flow, a target temperature to be controlled is determined. If, for example, the load is increased, on the one hand the current temperature is measured and the setpoint temperature is determined. If the setpoint temperature is greater than the outlet temperature, the fuel quantity is enriched until the deviation of the actual value from the setpoint value no longer exists. Since a certain inertia is to be expected during a load change due to the heat capacity of the burner system, the measured temperature changes even in the dynamic state, if the fuel quantity is not changed. So while at Load change, the temperature in the dynamic state of a starting temperature to a higher set temperature increases continuously, teaches the WO 2006/000366 A1 to additionally grease the fuel quantity. As a result, the fuel gas-air mixture is first over-enriched; the temperature rises above the setpoint temperature, which is why the amount of fuel gas is emaciated, whereby it falls below the setpoint temperature. Ultimately, the temperature swings to the setpoint temperature.

The invention has for its object to provide a method for controlling a fuel gas-air mixture on the burner or flame temperature available, in which to avoid pollutant emissions, the heating or cooling of the burner components, especially during the starting phase and the modulation phase in dynamic state is taken into account.

• mit dem Sensor wird eine Ausgangstemperatur gemessen,
• bei Vorgabe einer Brennerbelastung werden der notwendige Brenngasvolumen- oder -massenstrom, unter Berücksichtigung der Verbrennungsluftzahl, des Verbrennungsluftvolumen- oder -massenstroms sowie aus einem Kennfeld oder einer Funktion die belastungsabhängige Brenner- oder Flammensolltemperatur bestimmt,
• aus der Ausgangstemperatur und der Brenner- oder Flammensolltemperatur wird ein zeitlicher Verlauf der Brenner- oder Flammentemperatur errechnet,
• gemäß des ermittelten Brenngasvolumen- oder -massenstroms und Verbrennungsluftvolumen- oder -massenstroms werden der Brenngas- und Verbrennungsluftstrom eingestellt,
• die Brenner- oder Flammentemperatur wird gemessen und mit dem errechneten Verlauf der Brenner- oder Flammentemperatur verglichen,
• ist die gemessene Brenner- oder Flammentemperatur zu einem bestimmten Zeitpunkt im dynamischen Verlauf ungleich der errechnete Brenner- oder Flammentemperatur zu diesem Zeitpunkt, so wird der Brenngasstrom oder die Luftmenge angepasst.

According to the invention this is realized according to the features of claim 1 with a method for controlling a fuel gas-air mixture of a fuel gas burner preferably a heater with the aid of a sensor for detecting the burner or flame temperature T and a control with the following method steps:

• the sensor is used to measure a starting temperature
• when a burner load is specified, the required fuel gas volume or mass flow, taking into account the combustion air number, the combustion air volume or mass flow as well as from a characteristic field or a function, determines the load-dependent burner or flame setpoint temperature,
• a chronological progression of the burner or flame temperature is calculated from the starting temperature and the burner or flame set temperature,
• in accordance with the determined fuel gas volume or mass flow and combustion air volume or mass flow, the fuel gas and combustion air flow are adjusted,
• the burner or flame temperature is measured and compared with the calculated course of the burner or flame temperature,
• If the measured burner or flame temperature at a certain time in the dynamic course is not equal to the calculated burner or flame temperature at this time, the fuel gas flow or the air quantity is adjusted.

This considerably accelerates the adjustment.

For smaller deviations applies: If the measured burner or flame temperature at a certain time in the dynamic course smaller than the calculated burner or flame temperature at this time, the fuel gas flow is increased or the amount of air reduced. If the measured burner or flame temperature at a certain time in the dynamic course greater than the calculated burner or flame temperature at this time, the fuel gas flow is reduced or increased the amount of air. In the case of larger deviations, changing the mixture may result in a shift of the flame position, so that the measures mentioned above do not necessarily contribute to the regulation to the setpoint. In the majority of cases, however, due to the continuous control and the therefore relatively small expected deviations, it can be assumed that enrichment with too cold a flame leads to an increase in temperature and a decrease to a temperature decrease.

For the regulation of the fuel gas-air mixture, a PI controller is preferred. A control value is used to determine a control value from a control deviation (difference between setpoint and actual temperature). For a PI controller, it is normal for the P-controller part to quickly compensate for an occurring system deviation, with the I-controller component subsequently eliminating the remaining system deviation. Thus, a PI controller operates quickly and accurately with the appropriate setting.

The choice of an I-controller or P-controller, however, could be disadvantageous. Depending on the system, the control can regulate very quickly with a very large selected integral component (I component), but there is a large jump in the temperature profile or a strong CO emission. When choosing a very small I component, the jump is very small, but the regulation time is very long.

For a reliable measurement result, the inertia of a temperature measurement system must be considered. This can be traced back both to the sensors used and to the system behavior itself.

Figur 1

einen Verlauf der Brennertemperatur T in Abhängigkeit der Belastung Q̇ nach dem Stand der Technik,

Figur 2

einen Verlauf der Brennertemperatur T in Abhängigkeit der Zeit t während eines Modulationssprungs von 20 kW auf 10 kW,

Figur 3

einen Verlauf der CO-Emissionswerte in Abhängigkeit der Zeit t während eines Modulationssprungs von 20 kW auf 10 kW,

Figur 4

einen Verlauf der Brennertemperatur T in Abhängigkeit der Zeit t während eines Aufheizvorgangs über lineare Kennlinien,

Figuren 5 bis 7

eine Regelung der Modulation auf einer höheren Leistung nach dem erfindungsgemäßen Verfahren und

Figuren 8 bis 10

eine Regelung der Modulation auf einer niedrigen Leistung nach dem erfindungsgemäßen Verfahren.

The invention will now be explained in detail with reference to FIGS. Show here

FIG. 1

a profile of the burner temperature T as a function of the load Q̇ according to the prior art,

FIG. 2

a curve of the burner temperature T as a function of the time t during a modulation jump from 20 kW to 10 kW,

FIG. 3

a course of the CO emission values as a function of the time t during a modulation jump from 20 kW to 10 kW,

FIG. 4

a profile of the burner temperature T as a function of time t during a heating process via linear characteristics,

FIGS. 5 to 7

a modulation of the modulation at a higher power according to the inventive method and

FIGS. 8 to 10

a regulation of the modulation at a low power according to the inventive method.

As can be seen from FIG. 1, the burner surface temperature T is low at a higher power and high at a low load since the flame lifts Q̇ from the burner surface with increasing load.

FIG. 2 shows the system behavior during a load change (modulation jump) of the heater from 20 kW to 10 kW. The diagram illustrates that during operation, the difference between a burner or flame target temperature T ₂ , to which it is to be controlled, and an output temperature T _{0 can be} relatively large. Similarly, the system behaves during startup, because the heater (or the burner temperature) passes from the cold state in a modulation-dependent hot state.

Curve 3 in FIG. 2 shows the behavior of a heater with a modulation jump from 20 kW to 10 kW on the condition that the air ratio lambda is kept constant.

In this case, the burner or flame temperature of the curve 3 follows as a function of the time t, which can be reproduced by means of an exponential function, up to a stationary final value.

The heating or cooling processes taking place during modulation jumps during the modulation can be described by the following function: $$T_1\left(t\right)={\mathrm{<figure-callout\; id="0"\; label="T"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">T</figure-callout>}}_0+\left(T_2-{\mathrm{<figure-callout\; id="0"\; label="T"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">T</figure-callout>}}_0\right)\cdot e^{-\frac{\mathit{\tau }}{t}}$$

• T₀- die Ausgangsbrenner- oder -flammentemperatur,
• T₂- die Brenner- oder Flammensolltemperatur,
• T₁ (t) - der zeitliche Verlauf der Brenner- oder Flammensolltemperatur, t - die Zeit und
• τ - ein Regelungsparameter, der vorgegeben wird und Einfluss auf den Gradienten hat.

In the equation 1 mean

• T ₀ - the output burner or flame temperature,
• T ₂ - the burner or flame set temperature,
• T ₁ (t) - the time course of the burner or flame set temperature, t - the time and
• τ - a control parameter that is specified and has an influence on the gradient.

The method according to the invention makes it possible to modify a stationary setpoint temperature value T _{2 of} the control into a setpoint value T ₁ (t) as a function of time, the time profile of the burner or setpoint flame temperature T _{1 of} an e-function (such as curve 3, FIG. Figure 2) follows and depending on the output and burner or flame set temperature of the modulation or the load change is.

Indicators of a well-functioning control system are, in addition to the CO ₂ emissions that result from the excess air, especially under safety aspects, the CO emissions. FIG. 3 shows values of the CO emissions of the system with an exemplary modulation jump from 20 kW to 10 kW.

The curve 1 shows a CO curve in the event that a control would dose the amount of gas in such a way that it would be regulated to the temperature target value immediately after the burner start. In this case, due to the large temperature difference, the gas valve would open so much that the combustion would no longer be standard or "clean". One recognizes at the beginning a clearly excessive CO-emission, which decreases with increasing time. Curve 2 shows a CO trend, taking into account the procedure according to the invention at the same modulation jump (from 20 kW to 10 kW) sets.

The CO emission according to the inventive control, shown as curve 2 in Figure 3, shows that the CO emissions can be kept permanently at a low level. Thus, the excessive pollutant emissions which occur during combustion, in particular during the heating phase at startup or during modulation jumps, which would occur if the procedure according to the invention were regulated directly to the target temperature, are prevented.

During the starting process itself, incomplete combustion usually occurs, in particular due to system-induced mixture enrichment. This effect results in a high CO emission characteristic for the start of the device. This increase in CO is assumed to be systemic in the control according to the invention and is to be differentiated from the illustrated CO courses.

Another embodiment provides an approximation of the setpoint temperature T ₁ the heating behavior of the system (curve 6, Figure 4) via linear sections within a characteristic before, z. B. via an approach with two (curve 5, Figure 4) or with several sections (curve 4, Figure 4) such that always a sufficient combustion quality is guaranteed.

First, the method for controlling the modulation on a higher burner power will be described. It is assumed that the burner is switched off and the heater is in a standby state. With a temperature sensor, preferably with a PTC sensor, an output temperature T ₀ at the burner, z. B. T ₀ = 25 ° C measured.

For a heat demand, eg Q̇ = 20 kW, a corresponding air mass flow (eg for lambda = 1.3 and Q̇ = 20 kW) is set on the blower via a mass flow sensor . From a stored gas valve characteristic, as shown in Figure 5, the heat demand of 20 kW corresponding step number (eg 280) is determined and set. From a stored temperature characteristic is the heat demand of 20 kW corresponding load-dependent burner or flame target temperature T ₂ determined, eg T ₂ = 350 ° C (see Figure 6).

The moment of a modulation jump or a heat load change is kept fixed by setting the time t to the value 0. At each subsequent time t ₁ to t _x , the time profile of the burner or flame temperature T _{1 is determined} according to equation 1 (Figure 7). The burner or flame temperature is measured and compared with the calculated burner or flame temperature T ₁ (t).

The control only intervenes when a deviation of the measured burner or flame temperature (actual temperature) from the calculated burner or flame temperature T ₁ (t) (setpoint temperature) occurs. This deviation between the actual and desired temperature is controlled by the stepper motor of the gas valve, so that when the measured burner or flame temperature is greater than the calculated burner or flame temperature T ₁ (t), the fuel gas flow is reduced or the amount of air is increased or if the measured burner or flame temperature is greater than the calculated burner or flame temperature T ₁ (t), the fuel gas flow is reduced or the amount of air is increased.

The inventive method is terminated as soon as the measured burner or flame temperature of the burner or flame setpoint T ₂ corresponds.

A regulation of the modulation on a low load is similar to the regulation described above and can be seen in Figures 8 to 10. It is assumed that the above-mentioned current load, eg Q̇ = 20 kW and from the measured at the burner output temperature T ₀ , eg T ₀ = 350 ° C. After the modulation change from Q̇ = 20 kW to Q̇ = 10 kW, the fan controls the corresponding air volume and the gas valve is set to the expected number of steps (in this case 150 steps) depending on the measured air mass flow. The necessary number of steps is determined from the stored gas valve characteristic.

The temperature increases with reduced load and the rise follows the curve shown. From the stored temperature characteristic, the corresponding for Q̇ = 10 kW Burner or flame set temperature T ₂ , eg T ₂ = 550 ° C determined. The timing of the modulation jump is held fixed by setting the time t to the value 0. At each time t ₁ to t _{x of} the current time, the time profile of the burner or flame temperature T _{1 is determined} according to equation 1 and compared with the current, measured burner or flame temperature. Deviations between the measured and calculated temperature are also compensated by an adjustment of the stepper motor of the gas fitting.

The control method according to the invention is intended to prevent the pollutant emissions which occur during combustion, in particular during the start or during modulation jumps during the modulation. It is not regulated directly to a predetermined end-desired temperature value, but by the natural heating or cooling behavior of the system is integrated into the scheme. Thus, larger jumps in the temperature difference between the target and actual temperature are avoided and achieved a good combustion quality.

With the aid of the method according to the invention, it is possible to adapt a regulation oriented to the measurement of the burner or flame temperature for modulation jumps.

An unclean system behavior, eg. B. when heating the system after the burner start is avoided, in which the control after the burner start is always in operation and the quality of combustion is permanently tested and regulated.

Claims (5)

Method for controlling a fuel gas-air mixture of a combustion gas-operated burner, preferably a heater, with the aid of a sensor for detecting the burner or flame temperature (T) and a control with the following method steps: with the sensor, an initial temperature T _{0 is} measured, - Given a burner load ( Q̇ ) are the necessary Brenngasvolumen- ( V̇ _B ) or mass flow ( ṁ _B ), taking into account the combustion air number (λ) of the combustion air volume ( V̇ _L ) or mass flow ( ṁ _L ) and from a map or a function determines the load-dependent burner or flame setpoint temperature T ₂ = f ( Q̇ ), from the outlet temperature T ₀ and the burner or flame setpoint temperature T ₂ , a time characteristic of the burner or flame temperature T ₁ ( t ) = f ( T ₀ , T ₂ , t ) is calculated, - According to the determined fuel gas volume ( V̇ _B ) or mass flow ( ṁ _B ) and combustion air volume ( V̇ _L ) or mass flow ( ṁ _L ), the fuel gas and combustion air flow are adjusted, the burner or flame temperature is measured and compared with the calculated burner or flame temperature T ₁ (t), if the measured burner or flame temperature is greater or less than the calculated burner or flame temperature T ₁ (t) at a given time t in the dynamic curve, then the fuel gas flow or the air quantity is reduced or increased. Method for controlling a fuel gas-air mixture according to claim 1, characterized in that the fuel gas stream is reduced or the amount of air is increased when the measured burner or flame temperature at a certain time t in the dynamic curve greater than the calculated burner or flame temperature T. ₁ (t) at this time t is, or the fuel gas flow is increased or the amount of air is reduced if the measured burner or flame temperature at a certain time t in the dynamic curve smaller than the calculated burner or flame temperature T ₁ (t) to this Time t is. Method for controlling a fuel gas-air mixture according to claim 1 or 2, characterized in that the determination of the time profile of the burner or flame temperature according to a formula $$T_1\left(t\right)=T_0+\left(T_2-T_0\right)\cdot e^{-\frac{\mathit{\tau }}{t}}$$

where τ is a control parameter. Method for controlling a fuel gas-air mixture according to one of claims 1 to 3, characterized in that the method is terminated when the measured burner or flame temperature of the burner or flame set temperature T ₂ corresponds. Method for controlling a fuel gas-air mixture according to claim 4, characterized in that the regulation of the fuel gas-air mixture after reaching the desired flame temperature T _{2 is carried out} according to a conventional PI control.

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