Industrial boiler low-load operation is common and long-standing. Low-load operation has a great impact on boiler thermal efficiency, combustion efficiency, and pollutant emissions, which has attracted boiler user's attention.

Industrial Boilers Low-Load Operation Hazards

1. Affect normal production

The combustion is unstable, the bed temperature is easy to fluctuate, and it is difficult to adjust, the bed temperature will be low, and the return adjustment is not easy to control, which may cause flame-out;

2. Affect the life of the heat exchange pipes inside the boiler

In low-load combustion, the amount of coal and ash returned will be unbalanced, resulting in uneven heating of the heat exchange pipes, which may easily lead to local overheating of the pipes, causing overheating of the pipe wall, and long-term operation will increase the probability of pipe bursting;

3. Increase the corrosion of the rear heat exchange pipes

During long-term low-load operation, the exhaust gas temperature will decrease, and the probability of low-temperature corrosion of the heat exchange pipes arranged at the rear will increase;

4.Affect the boiler thermal efficiency

The low furnace temperature will cause incomplete combustion, and the boiler efficiency will definitely decrease, which will affect the operating cost of the steam boiler.

The above phenomena have resulted in industrial boiler operation economy and safety reduction  and the aggravation of environmental pollution.

Therefore, it is necessary to analyze the causes of industrial boiler low-load operation from the perspective of industrial boiler capacity selection and heating load changes, and to propose solutions.

Industrial Boilers Low-Load Operation Causes

1 Unreasonable selection of industrial boiler capacity

The industrial boiler capacity is an important indicator in the design of the heating source (ie the boiler room), and the rational selection of the boiler capacity is the primary factor to avoid low-load operation.

Boiler selection is the most important core issue in the process of heating and heat source, and it must meet the requirements of heating load and heat medium parameters. Taking a heating project in an industrial park as an example, the designed heating load of the park is 120 tons of steam. Considering the later development, two 75t/h circulating fluidized bed steam boilers are selected for the boiler room, with a total capacity of 150 t/h. Table 1 is the heat load of the boilers in different periods after the two boilers are put into use.

It can be seen from Table that the boiler capacity of the park is in the design and selection. The boiler capacity is too large during equipment procurement and boiler room construction, and the boiler has been in a low-load operation state for a long time. The main causes are as follows:

(1) The heating load fluctuates greatly

When designing the heating source, the selection of boiler capacity and quantity shall be based on the maximum load in the terminal heat load. However, the maximum heat load of the terminal production process of each factory or workshop cannot occur at the same time. Therefore, there is a large deviation between the heat supply heat load and the maximum heat load, which can only be matched by adjusting the operating load of the boiler, so that it is often at a low load operating status.

In the design concept of heating and heat source, the quality of heating is emphasized and the load rate is neglected. The total capacity of the general industrial boiler room is 10%~20% higher than the heat required for the planning of hot fish load, resulting in a larger capacity configuration. The "car" phenomenon is serious.

(2) The expected target of heat supply and heat load has not been achieved

When the final heat load is constructed in stages, the heat source unit selects the model according to the final heat load capacity when selecting the boiler, and also considers some potential heat users when selecting the model. Due to the lack of more accurate heat load data, only through Estimated method to determine heat loads for various heat users. However, the terminal heat negative limit is slow under the big name flow. In the end, the requirement of the total heat load capacity cannot be met, so that the operation of the boiler room and the heating system cannot meet the design load, resulting in unreasonable capacity allocation, which is also an important factor that causes industrial boiler equipment to operate at low load all year round.

2 The diversification of heating load brings instability

Heating heat load is divided into industrial heat load and civil heat load. The industrial heat load includes the heat load of the production process, the domestic heat load of the factory area, and the heating heat load of the industrial building. Civil heat load includes heating heat load of residential and public buildings, domestic heat heat load, etc.

The use of the heating system in the park is divided into production process, heating, domestic hot water heat load. Under normal circumstances, the heat load of the production process changes with the process cycle of the production process; the heating heat load changes with the outdoor air temperature; the domestic heat load changes with the time of water use in the morning and evening.

1 Changes in heat load of production process

The heat load of the production process is the heat load of the heat-using equipment in the production process. Due to the variety of heating equipment and heating methods, the heat load of the production process is generally estimated according to the measured data or the estimated heat consumption index per unit output.

The heat load of the production process belongs to the annual heat load. The main changes are: large changes in day and night load, large continuous load fluctuations, excessively concentrated load peaks, and periodic changes in load, etc. 

The specifics are:

(1) Day and night load changes

There are many heat users who smoke during the day and stop working at night:

(2) The continuous load fluctuates greatly

The production process load is continuous, but the steam consumption required by the load varies with the production process, and the heat load also fluctuates. The magnitude of the fluctuation is determined by the change in the production process;

(3) The load decreases sharply

The equipment uses little or no steam in a certain process cycle;

(4) Periodic change of load

The high and low cycle changes of the heat load used in the production process cause passive changes in the heat supply load.

As the heat load of the production process changes, the operating load of the industrial boiler also changes. If the heat load of the production process is at a low level for a long time, the industrial boiler will be in a low-load operation state for a long time.

2 Heating heat load changes

Heating design heat load refers to when the outdoor temperature is the heating calculation temperature.

In order to achieve the required indoor temperature, the heat supplied by the heating system to the building per unit time.

The heating heat load changes continuously with the outdoor weather conditions, and the supply and return water temperature and flow rate of the heating source are determined according to the outdoor temperature during the heating period. In order to ensure the quality of heating, meet the requirements of use, and make the preparation and transmission of thermal energy economical and reasonable, the heating system will be regulated for heating. The change of heating heat load is mainly in two aspects: daily load change and heating seasonal load change. 

(1) Daily load change

Heating heat load changes vary with outdoor air temperature. The peak of heat load is from 5:00 to 6:00, 17:00 to 18:00, and the low peak is from 14:00 to 15:00.

(2) Changes in heat load during the heating season

The heating period in my country is generally concentrated from November of the current year to March of the second year, and the peak is in December of the year and January of the second year. When the building envelope and pipe network are laid to a certain extent, the temperature of the water supply in the early stage of heating It will not be too high to leave a certain margin. When the heating heat load increases, increase the temperature of the hot water to expand the heating capacity. At the end of the heating period, with the increase of the outdoor temperature, the heating heat load decreases.

3 Changes in living heat load

In daily life, the amount of hot water consumed for washing face, bathing, washing clothes, and scrubbing dishes. In factories, hospitals, and schools, there are also hot water supplies, steamed rice, etc.

The change of domestic heat load depends on the change in the amount of hot water. The amount of hot water in residential buildings depends on the perfection of the sanitary equipment in the house and people's living habits. Water consumption is also related to its production nature and working system.

The domestic heat load belongs to the annual heat load, and the amount of hot water has a cycle of day and night. As the domestic heat load decreases, the operating load of the industrial boiler also decreases.

Industrial Boilers Low-Load Operation Solutions

The selection of industrial boiler capacity and the change of heating load are the main reasons for the low-load operation of industrial boilers. The optimal combination of capacity and number of boilers should be comprehensively considered in the boiler selection, and the number and capacity should be switched according to the change of heat demand during the operation summary, so that the boiler can maintain a high load rate during most of the operation cycle.

1 Reasonable selection

(1) Determine the heat supply heat load according to the terminal heat load, the simultaneous use coefficient and the heat loss coefficient of the heat network;

(2) Determine the boiler type according to the type and parameters of the heating medium;

(3) According to the heating load and its changes, select the capacity and number of T industrial boilers, including the number of operating units in heating season and non-heating season;

(4) The selection of the capacity and number of industrial boilers is also related to factors such as whether to set up a backup boiler, the safety and reliability of heating, the possibility of maintenance, whether to expand, and the economy of boiler room investment, which should be comprehensively selected when selecting models. consider.

In addition, some heat source units consider the safety point of view. When choosing a boiler, it is often considered that it is safer to choose a boiler with high parameters. For industrial boilers with a higher rated pressure and rated outlet water temperature, the low-load operation should be performed according to the actual demand during operation. In fact, from the point of view of the safety of industrial boiler design, this is completely unnecessary, which not only causes waste of heat source investment, but also is a factor that leads to low-load operation of industrial boilers.

2 Optimize the operation mode of industrial boilers

(1) Optimize the combustion of industrial boilers. The optimized combustion data of boilers with different furnace types and different parameters are different. The optimized combustion curves under different loads can be obtained through experimental tests, and the operation can be adjusted with reference to the optimized curves.

(2) When multiple boilers are running, the load of adjacent boilers should be reasonably distributed, and the output of industrial boilers should be adjusted in time.

Conclusion

Improper capacity, less number of boilers, and changes in heating load during boiler selection are the main reasons for industrial boiler low-load operation. Therefore, rationally disposing the heating source, optimizing the capacity and number of industrial boilers, and optimizing the operation adjustment mode are effective measures to solve the problem of boiler low-load operation.