CN115121101B - Method for adjusting temperature of smoke inlet end of SCR (selective catalytic reduction) system in thermal power unit - Google Patents

Abstract

This application proposes a method for adjusting the temperature of the smoke inlet end of the SCR system in a thermal power unit, which includes: setting a heating unit between the smoke exhaust end of the thermal power unit and the smoke inlet end of the SCR system; obtaining the required power of the power grid; and setting the power of the power grid. threshold; compare the required power of the power grid with the power threshold; if the required power is less than the power threshold, increase the power of the heating unit to increase the flue gas temperature at the smoke inlet end of the SCR system. In this application method for adjusting the temperature of the smoke inlet end of the SCR system in a thermal power unit, by increasing the power of the heating unit, the smoke at the smoke inlet end of the SCR system is heated, thereby ultimately increasing the temperature of the smoke at the smoke inlet end of the SCR system. Ensure the stable denitration of the SCR system. As a result, the deep peak shaving of thermal power units is not restricted by the SCR system, ensuring the realization of flexible operation modes of thermal power units.

Description

A method for adjusting the temperature of the smoke inlet end of the SCR system in thermal power units

Technical field

The present application relates to the technical field of thermal power units, and in particular to a method for adjusting the temperature of the smoke inlet end of the SCR system in the thermal power unit.

Background technique

Thermal power units usually use SCR (Selective Catalytic Reduction, Selective Catalytic Reduction) systems for denitration to prevent excessive nitrogen oxides from being produced in the boiler and polluting the environment.

At the same time, in order to meet different power demands, the power generation of thermal power units needs to be peak-shaving. However, since the SCR system cannot denitrify when the temperature of the smoke inlet end is low, the thermal power units need to maintain a high operating load, which results in Thermal power units cannot be adjusted deeply during the peak shaving process, which affects the realization of flexible operation of thermal power units.

Contents of the invention

The present application aims to solve, at least to a certain extent, one of the technical problems in the related art.

To this end, the purpose of this application is to provide a method for adjusting the temperature of the smoke inlet end of the SCR system in a thermal power unit.

In order to achieve the above purpose, this application provides a method for adjusting the temperature of the smoke inlet end of the SCR system in a thermal power unit, which includes: setting a heating unit between the smoke exhaust end of the thermal power unit and the smoke inlet end of the SCR system; obtaining the temperature of the power grid Require power; set the power threshold of the power grid; compare the power requirement of the power grid with the power threshold; if the required power is less than the power threshold, increase the power of the heating unit to increase the The flue gas temperature at the inlet end of the SCR system.

Optionally, the adjustment method further includes: after increasing the power of the heating unit, setting a flue gas temperature threshold at the smoke inlet end of the SCR system; and obtaining the flue gas temperature at the smoke inlet end of the SCR system. ;Compare the flue gas temperature at the smoke inlet end of the SCR system with the flue gas temperature threshold; adjust the power of the heating unit according to the result of the comparison, so that the flue gas temperature at the smoke inlet end of the SCR system is equal to the Flue gas temperature threshold.

Optionally, adjusting the power of the heating unit according to the comparison result includes: if the flue gas temperature at the smoke inlet end of the SCR system is greater than the flue gas temperature threshold, reducing the power of the heating unit; If the flue gas temperature at the smoke inlet end of the SCR system is less than the flue gas temperature threshold, the power of the heating unit is increased.

Optionally, obtaining the smoke temperature at the smoke inlet end of the SCR system includes: setting a first temperature detection unit at the smoke inlet end of the SCR system; obtaining the smoke temperature at the smoke inlet end of the SCR system based on the first temperature detection unit. air temperature.

Optionally, the adjustment method further includes: setting a second temperature detection unit at the smoke exhaust end of the thermal power unit; and obtaining the flue gas temperature at the smoke exhaust end of the thermal power unit according to the second temperature detection unit.

Optionally, the flue gas temperature threshold is 300 degrees Celsius.

Optionally, the power threshold is 30% of the rated power of the power grid.

Optionally, the adjustment method further includes: electrically connecting the power end of the heating unit to the power supply end of the thermal power unit, and connecting the heating unit in parallel with the power grid; according to the The power and the required power of the power grid are used to adjust the power generation of the thermal power unit.

Optionally, electrically connecting the power end of the heating unit to the power supply end of the thermal power unit, and connecting the heating unit in parallel with the power grid includes: connecting the power supply end of the thermal power unit to the third power grid. The power end of a transformer is electrically connected, and the first transformer is connected in parallel with the power grid; the power supply end of the first transformer is electrically connected to the power end of the second transformer; the second transformer is electrically connected. The power supply terminal is electrically connected to the power terminal of the heating unit.

Optionally, the adjustment method further includes: setting a first switch between the power supply end of the first transformer and the power consumption end of the second transformer; A second switch is provided between the electrical terminals of the heating unit.

The technical solution provided by this application can include the following beneficial effects:

By increasing the power of the heating unit, the flue gas at the smoke inlet end of the SCR system is heated, thereby ultimately increasing the temperature of the flue gas at the smoke inlet end of the SCR system and ensuring stable denitration of the SCR system. As a result, the deep peak shaving of thermal power units is not restricted by the SCR system, ensuring the realization of flexible operation modes of thermal power units.

Additional aspects and advantages of the application will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the application.

Description of the drawings

The above and/or additional aspects and advantages of the present application will become apparent and readily understood from the following description of the embodiments in conjunction with the accompanying drawings, in which:

Figure 1 is a schematic flow chart of a method for adjusting the temperature of the smoke inlet end of the SCR system in a thermal power unit proposed by an embodiment of the present application;

Figure 2 is a schematic structural diagram of a method for adjusting the temperature of the smoke inlet end of the SCR system in a thermal power unit proposed by an embodiment of the present application;

As shown in the figure: 1. Thermal power unit, 2. SCR system, 3. Heating unit, 4. Power grid, 5. First temperature detection unit, 6. Second temperature detection unit, 7. First transformer, 8. Second Transformer, 9. First switch, 10. Second switch, 11. Boiler, 12. Steam turbine, 13. Generator.

Detailed ways

The embodiments of the present application are described in detail below. Examples of the embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements with the same or similar functions. The embodiments described below with reference to the drawings are exemplary and are only used to explain the present application and cannot be understood as limiting the present application. On the contrary, the embodiments of the present application include all changes, modifications and equivalents falling within the spirit and scope of the appended claims.

In related embodiments, the smoke exhaust end of the thermal power unit 1 is connected to the smoke inlet end of the SCR (Selective Catalytic Reduction) system 2 , and the power supply end of the thermal power unit 1 is connected to the power consumption end of the power grid 4 .

It can be understood that the thermal power unit 1 supplies power to the power grid 4 , and the flue gas discharged from the thermal power unit 1 is denitrated through the SCR system 2 .

Under different power requirements of power grid 4, the power generation of thermal power unit 1 also changes accordingly. That is, when the power requirement of power grid 4 is higher, the power generation of thermal power unit 1 needs to be increased. When the power requirement of power grid 4 is low, the power generation of thermal power unit 1 needs to be increased. Then the power generation of thermal power unit 1 needs to be reduced. Thus, the peak load regulation of the thermal power unit 1 is achieved.

However, since the denitration of SCR system 2 requires a higher temperature of the flue gas at the smoke inlet end, and because the power generation power of thermal power unit 1 is low, the temperature of the flue gas at the exhaust end is also low. Therefore, in SCR system 2 Under the restriction, the reduction of the generated power of thermal power unit 1 is restricted, and the demand for deep peak regulation of thermal power unit 1 cannot be met.

In order to solve the above technical problems, as shown in Figure 1, the embodiment of the present application proposes a method for adjusting the temperature of the smoke inlet end of the SCR system 2 in the thermal power unit 1, including:

S1: Set the heating unit 3 between the smoke exhaust end of the thermal power unit 1 and the smoke inlet end of the SCR system 2;

S2: Obtain the required power of power grid 4;

S3: Set the power threshold of power grid 4;

S4: Compare the required power of power grid 4 with the power threshold;

S5: If the required power is less than the power threshold, increase the power of the heating unit 3 to increase the flue gas temperature at the smoke inlet end of the SCR system 2.

It can be understood that when the required power of the power grid 4 is less than the power threshold, after the thermal power unit 1 undergoes peak regulation, the flue gas temperature at the exhaust end of the thermal power unit 1 is correspondingly lower. Correspondingly, the flue gas temperature at the inlet end of the SCR system 2 By increasing the power of the heating unit 3, the flue gas at the smoke inlet end of the SCR system 2 is heated, thereby ultimately increasing the temperature of the flue gas at the smoke inlet end of the SCR system 2 and ensuring the stable denitration of the SCR system 2. As a result, the deep peak shaving of the thermal power unit 1 is not restricted by the SCR system 2, ensuring the realization of a flexible operation mode of the thermal power unit 1.

It should be noted that the required power of the power grid 4 refers to the power required by the power grid 4. When the power consumption is large, the power required by the power grid 4 is greater. When the power consumption is small, the power required by the power grid 4 is greater. Small.

The power supply end refers to the power output end, and the power consumption end refers to the power input end.

In some embodiments, as shown in Figure 2, the heating unit 3 may include a heating chamber and a heating wire. The heating chamber has a smoke inlet end and a smoke exhaust end. The smoke inlet end of the heating chamber is connected to the smoke exhaust end of the thermal power unit 1. The smoke exhaust end of the heating chamber is connected to the smoke inlet end of the SCR system 2, and the heating wire is arranged in the heating chamber. As a result, by increasing the power of the heating wire, the flue gas is heated when passing through the heating chamber, thereby increasing the flue gas temperature at the smoke inlet end of the SCR system 2.

In some embodiments, S4 and S5 can be executed by a controller. As shown in Figure 2, the power supply end of the controller is connected to the power end of the heating wire, and the power of the heating wire is controlled by the controller.

In some embodiments, as shown in Figure 2, the thermal power unit 1 may include a boiler 11, a steam turbine 12 and a generator 13. The steam outlet end of the boiler 11 is connected to the steam inlet end of the steam turbine 12, and the smoke exhaust end of the boiler 11 is connected to the SCR. The smoke inlet end of system 2 is connected, the output shaft of steam turbine 12 is connected to the input shaft of generator 13, and the power supply end of generator 13 is connected to the power consumption end of power grid 4.

In some embodiments, the adjustment method further includes:

After increasing the power of heating unit 3, S6: Set the flue gas temperature threshold at the smoke inlet end of SCR system 2;

S7: Obtain the flue gas temperature at the smoke inlet end of SCR system 2;

S8: Compare the flue gas temperature at the smoke inlet end of SCR system 2 with the flue gas temperature threshold;

S9: Adjust the power of the heating unit 3 according to the comparison result so that the flue gas temperature at the smoke inlet end of the SCR system 2 is equal to the flue gas temperature threshold.

It can be understood that after increasing the flue gas temperature at the smoke inlet end of the SCR system 2, the power adjustment of the heating unit 3 is used to maintain the flue gas temperature at the smoke inlet end of the SCR system 2 at the set flue gas temperature threshold, thereby avoiding the problem of continuous heating. The problem of excessive energy loss occurs in the smoke at the smoke inlet end of SCR System 2. This effectively reduces the heating cost of the smoke at the smoke inlet end of SCR System 2. It also avoids the problem of excessive energy loss at the smoke inlet end of SCR System 2. The problem of outage ensures that the deep peak shaving of thermal power unit 1 is not restricted.

It should be noted that the power of the heating unit 3 is measured as E _d , the flue gas temperature at the smoke inlet end of the SCR system 2 is measured as _To , and the flue gas temperature at the smoke exhaust end of the thermal power unit 1 is measured as _Ti , then the SCR system 2 enters The flue gas enthalpy H _yo at the smoke end is:

H _yo =h(T _o );

That is, the enthalpy value of the flue gas at the smoke inlet end of SCR system 2 is a function of the flue gas temperature at the smoke inlet end of SCR system 2;

The flue gas enthalpy value H _yi at the exhaust end of thermal power unit 1 is:

H _yi =h(T _i );

That is, the enthalpy value of the flue gas at the exhaust end of thermal power unit 1 is a function of the flue gas temperature at the exhaust end of thermal power unit 1;

The power E _d of heating unit 3 is:

E _d =Q _y (H _yi -H _yo );

That is, the power of heating unit 3 is a function of the difference between the flue gas enthalpy value at the exhaust end of thermal power unit 1 and the flue gas enthalpy value at the smoke inlet end of SCR system 2;

At the same time, the power of thermal power unit 1 is calculated as E _g , the thermal load of thermal power unit 1 is calculated as Q _b , and the thermal efficiency of thermal power unit 1 is calculated as eta _cp , then the power E _g of thermal power unit 1 is:

E _g =Q _b × η _cp ;

T _i =g(Q _b );

That is, the flue gas temperature at the exhaust end of thermal power unit 1 is a function of the power of thermal power unit 1;

Therefore, when the power of the thermal power unit 1 is lower, the flue gas temperature at the exhaust end of the thermal power unit 1 is also lower. At the same time, by increasing the power of the heating unit 3, the flue gas temperature at the smoke inlet end of the SCR system 2 can be increased. By reducing the power of the heating unit 3, the flue gas temperature at the smoke inlet end of the SCR system 2 can be reduced.

In some embodiments, S7 to S9 can be executed by a controller, and the controller adjusts the power of the heating unit 3 according to the comparison result between the flue gas temperature at the smoke inlet end of the SCR system 2 and the flue gas temperature threshold.

In some embodiments, in S9, adjusting the power of the heating unit 3 according to the comparison result includes:

If the flue gas temperature at the smoke inlet end of the SCR system 2 is greater than the flue gas temperature threshold, the power of the heating unit 3 is reduced;

If the flue gas temperature at the smoke inlet end of the SCR system 2 is less than the flue gas temperature threshold, the power of the heating unit 3 is increased.

It can be understood that the power of the heating unit 3 is positively correlated with the flue gas temperature at the smoke inlet end of the SCR system 2, and the power of the heating unit 3 is reduced or increased to maintain the flue gas temperature at the smoke inlet end of the SCR system 2. at the flue gas temperature threshold, thereby avoiding the problem of excessive energy loss due to continuous heating of the flue gas at the smoke inlet end of SCR system 2, effectively reducing the heating cost of the flue gas at the smoke inlet end of SCR system 2, and also avoiding the problem of SCR system 2 If the flue gas at the smoke inlet end is too low, the SCR system 2 will shut down, ensuring that the deep peak regulation of the thermal power unit 1 is not restricted.

In some embodiments, in S7, obtaining the flue gas temperature at the smoke inlet end of the SCR system 2 includes:

A first temperature detection unit 5 is provided at the smoke inlet end of the SCR system 2;

The smoke temperature at the smoke inlet end of the SCR system 2 is obtained according to the first temperature detection unit 5 .

It can be understood that the first temperature detection unit 5 is used to obtain the flue gas temperature at the smoke inlet end of the SCR system 2, so that the power of the heating unit 3 is adjusted by using the flue gas temperature at the smoke inlet end of the SCR system 2.

In some embodiments, as shown in FIG. 2 , the first temperature detection unit 5 may be a temperature sensor, and the signal output end of the temperature sensor is electrically connected to the signal input end of the controller. Thus, the temperature sensor converts the flue gas temperature at the smoke inlet end of the SCR system 2 into an electrical signal and sends it to the controller, and the controller adjusts the power of the heating unit 3 according to the electrical signal.

In some embodiments, the adjustment method further includes:

A second temperature detection unit 6 is provided at the smoke exhaust end of the thermal power unit 1;

The flue gas temperature at the exhaust end of the thermal power unit 1 is obtained according to the second temperature detection unit 6 .

It can be understood that the acquisition of the flue gas temperature at the exhaust end of the thermal power unit 1 through the second temperature detection unit 6 not only facilitates monitoring of the flue gas temperature at the exhaust end of the thermal power unit 1, but also makes use of the flue gas at the exhaust end of the thermal power unit 1 temperature, and more accurately adjust the power of the heating unit 3.

In some embodiments, as shown in FIG. 2 , the second temperature detection unit 6 may be a temperature sensor, and the signal output end of the temperature sensor is electrically connected to the signal input end of the controller.

In some embodiments, the flue gas temperature threshold is 300 degrees Celsius.

It can be understood that after setting the flue gas temperature threshold to 300 degrees Celsius and continuing to increase the power of the heating unit 3, the adjustment will begin until the flue gas temperature at the smoke inlet end of the SCR system 2 exceeds 300 degrees Celsius, thereby causing the SCR system 2 to The flue gas temperature at the end is maintained at 300 degrees Celsius to ensure stable denitration of SCR system 2.

In some embodiments, the power threshold is 30% of the power grid 4 rated power.

It can be understood that when the required power of the power grid 4 is lower than 30% of the rated power, the thermal load of the thermal power unit 1 is also lower than 30% through peak shaving. At this time, the temperature of the flue gas at the exhaust end of the thermal power unit 1 decreases, so that The flue gas temperature at the smoke inlet end of SCR system 2 cannot meet the denitration requirements. Therefore, after the power of heating unit 3 is increased, SCR system 2 can still achieve stable denitration while achieving deep peak shaving of thermal power unit 1.

In some embodiments, the adjustment method further includes:

As shown in Figure 2, the power consumption end of the heating unit 3 is electrically connected to the power supply end of the thermal power unit 1, and the heating unit 3 is connected in parallel with the power grid 4;

The generated power of the thermal power unit 1 is adjusted according to the power of the heating unit 3 and the required power of the power grid 4 .

It can be understood that the thermal power unit 1 supplies power to the heating unit 3, avoiding the use of additional power supply components, effectively reducing the heating cost of the flue gas at the smoke inlet end of the CR system, and the heating unit 3 is connected in parallel with the power grid 4, so that the thermal power unit 1 The power is equal to the sum of the required power of the power grid 4 and the power of the heating unit 3, thereby facilitating the control of the power generation of the thermal power unit 1 and realizing the flexible operation of the thermal power unit 1.

In some embodiments, as shown in Figure 2, electrically connecting the power end of the heating unit 3 to the power supply end of the thermal power unit 1, and connecting the heating unit 3 in parallel with the power grid 4 includes:

Electrically connect the power supply end of the thermal power unit 1 to the power consumption end of the first transformer 7, and connect the first transformer 7 in parallel with the power grid 4;

Electrically connect the power supply end of the first transformer 7 to the power consumption end of the second transformer 8;

The power supply terminal of the second transformer 8 is electrically connected to the power consumption terminal of the heating unit 3 .

It can be understood that through the arrangement of the first transformer 7 and the second transformer 8, the power supply of the thermal power unit 1 can be adapted to the power consumption of the heating unit 3, thereby ensuring stable power supply of the thermal power unit 1 to the heating unit 3.

In some embodiments, the first transformer 7 may be a high voltage transformer.

It should be noted that the high-voltage transformer is a special transformer used to convert the high-voltage system into a low-voltage system to supply power to the factory where thermal power unit 1 is located.

The power of the first transformer 7 is calculated as E _c and the required power of the power grid 4 is calculated as E _w . Then the power E _g of the thermal power unit 1 is:

E _g =E _w +E _c ;

And E _c includes E _d . Therefore, when setting the power E _g of the thermal power unit 1, the power E c of the first transformer 7 should be added based on the power E _w required by the power grid 4, and after adding the heating unit 3, the power E _c of the first transformer 7 should be added. The power E _d of heating unit 3 should be increased.

In some embodiments, as shown in Figure 2, the adjustment method further includes:

A first switch 9 is provided between the power supply end of the first transformer 7 and the power consumption end of the second transformer 8;

A second switch 10 is provided between the power supply end of the second transformer 8 and the power consumption end of the heating unit 3 .

It can be understood that the setting of the first switch 9 and the second switch 10 facilitates the control of the switches of the second transformer 8 and the heating unit 3, making the heating of the smoke at the smoke inlet end of the SCR system 2 safer.

It should be noted that in the description of the present application, the terms "first", "second", etc. are only used for descriptive purposes and cannot be understood as indicating or implying relative importance. Furthermore, in the description of the present application, unless otherwise stated, the meaning of “plurality” is two or more.

Any process or method descriptions in flowcharts or otherwise described herein may be understood to represent modules, segments, or portions of code that include one or more executable instructions for implementing the specified logical functions or steps of the process. , and the scope of the preferred embodiments of the present application includes additional implementations in which functions may be performed out of the order shown or discussed, including in a substantially simultaneous manner or in the reverse order, depending on the functionality involved, which shall It should be understood by those skilled in the technical field to which the embodiments of this application belong.

In the description of this specification, reference to the terms "one embodiment," "some embodiments," "an example," "specific examples," or "some examples" or the like means that specific features are described in connection with the embodiment or example. , structures, materials or features are included in at least one embodiment or example of the present application. In this specification, schematic representations of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials or characteristics described may be combined in any suitable manner in any one or more embodiments or examples.

Although the embodiments of the present application have been shown and described above, it can be understood that the above-mentioned embodiments are illustrative and cannot be understood as limitations of the present application. Those of ordinary skill in the art can make modifications to the above-mentioned embodiments within the scope of the present application. The embodiments are subject to changes, modifications, substitutions and variations.

Claims (9)

1. A method for adjusting the temperature of the smoke inlet end of the SCR system in a thermal power unit, which is characterized by including: A heating unit is provided between the smoke exhaust end of the thermal power unit and the smoke inlet end of the SCR system; Obtain the required power of the power grid; setting a power threshold for the power grid; Comparing the required power of the power grid with the power threshold; If the required power is less than the power threshold, increase the power of the heating unit to increase the flue gas temperature at the smoke inlet end of the SCR system; Wherein, the adjustment method also includes: Electrically connect the power end of the heating unit to the power supply end of the thermal power unit, and connect the heating unit in parallel with the power grid; The generated power of the thermal power unit is adjusted according to the power of the heating unit and the required power of the power grid. 2. The method for adjusting the temperature of the smoke inlet end of the SCR system in the thermal power unit according to claim 1, characterized in that the adjustment method further includes: After increasing the power of the heating unit, setting a flue gas temperature threshold at the smoke inlet end of the SCR system; Obtain the flue gas temperature at the smoke inlet end of the SCR system; Compare the flue gas temperature at the smoke inlet end of the SCR system with the flue gas temperature threshold; The power of the heating unit is adjusted according to the comparison result so that the flue gas temperature at the smoke inlet end of the SCR system is equal to the flue gas temperature threshold. 3. The method for adjusting the temperature of the smoke inlet end of the SCR system in the thermal power unit according to claim 2, wherein the adjusting the power of the heating unit according to the comparison result includes: If the flue gas temperature at the smoke inlet end of the SCR system is greater than the flue gas temperature threshold, the power of the heating unit is reduced; If the flue gas temperature at the smoke inlet end of the SCR system is less than the flue gas temperature threshold, the power of the heating unit is increased. 4. The method for adjusting the temperature of the smoke inlet end of the SCR system in the thermal power unit according to claim 2, characterized in that obtaining the flue gas temperature at the smoke inlet end of the SCR system includes: A first temperature detection unit is provided at the smoke inlet end of the SCR system; The smoke temperature at the smoke inlet end of the SCR system is obtained according to the first temperature detection unit. 5. The method for adjusting the temperature of the smoke inlet end of the SCR system in the thermal power unit according to claim 2, characterized in that the adjustment method further includes: A second temperature detection unit is provided at the smoke exhaust end of the thermal power unit; The flue gas temperature at the exhaust end of the thermal power unit is obtained according to the second temperature detection unit. 6. The method for adjusting the temperature of the smoke inlet end of the SCR system in the thermal power unit according to claim 2, characterized in that the flue gas temperature threshold is 300 degrees Celsius. 7. The method for adjusting the temperature of the smoke inlet end of the SCR system in the thermal power unit according to claim 1, wherein the power threshold is 30% of the rated power of the power grid. 8. The method for adjusting the temperature of the smoke inlet end of the SCR system in the thermal power unit according to claim 1, characterized in that the power end of the heating unit is electrically connected to the power supply end of the thermal power unit, and the The heating unit connected in parallel with the power grid includes: Electrically connect the power supply end of the thermal power unit to the power end of the first transformer, and connect the first transformer in parallel with the power grid; Electrically connect the power supply end of the first transformer to the power consumption end of the second transformer; The power supply terminal of the second transformer is electrically connected to the power consumption terminal of the heating unit. 9. The method for adjusting the temperature of the smoke inlet end of the SCR system in the thermal power unit according to claim 8, characterized in that the adjustment method further includes: A first switch is provided between the power supply end of the first transformer and the power consumption end of the second transformer; A second switch is provided between the power supply end of the second transformer and the power consumption end of the heating unit.