Utilization of waste heat from flue gas in power plants to promote energy saving

There are many design options for reducing the exhaust temperature of the boiler: one is to reduce the exhaust temperature by optimizing the combustion; the second is to increase the heating surface of the boiler to reduce the exhaust temperature; the third is to increase the heating surface of the boiler air preheater to reduce the exhaust Temperature; Fourth, add a low-temperature economizer in the flue at the end of the boiler, and use condensate or other media to absorb the exhaust heat of the exhaust to reduce the exhaust temperature. However, after many experimental studies and on-site demonstrations, the use of low-temperature economizers to recover the waste heat of flue gas is the most direct, simple, and most effective and feasible method of waste heat recovery.

The use of low-temperature economizers can effectively recover flue gas waste heat, improve the utilization efficiency of high-temperature flue gas, and reduce emission losses. Its uses are mainly in the following aspects:

1. Use low-temperature economizer to heat the condensate of the reheat system

There are two ways to use the low-temperature economizer to heat condensate: First, let the flue gas and condensate directly exchange heat. This method has the advantages of first-level heat exchange and high heat exchange efficiency. The disadvantage is that if the heat exchange tube leaks , Will directly pollute condensate and affect the safe operation of the unit; the second is to set up a water-to-water heat exchanger to allow indirect heat exchange between flue gas and condensate. This method has the advantage of two-stage heat exchange, and the heat exchange efficiency is higher than the first-stage heat exchange Low, the advantage is that the system is safe and easy to adjust.

2. Use the recovered flue gas heat to heat the air through the heater

In order to prevent or reduce the low-temperature corrosion and ash blocking of the air preheater, a heater is needed to increase the air temperature at the inlet of the air preheater.

This air-water heat exchange heater replaces the conventional steam heater, uses circulating water as the heat medium, releases the heat absorbed on the flue gas side to the primary and secondary cold air, and enters the cold air before the air preheater Pre-heating, so as to realize the recycling of flue gas heat, and reduce the auxiliary steam consumption of conventional steam heaters.

The connection method of the low-temperature economizer in the thermal system directly affects its economic effect, the method of analysis and calculation, and the safety and reliability of operation. In essence, there are only two connection systems for the low-temperature economizer connected to the thermal system: one is that the low-temperature economizer is connected in series to the thermal system, referred to as the series system, as shown in Figure 1; the other is that the low-temperature economizer is connected in parallel In the thermal system, referred to as parallel system, as shown in Figure 2.

The design of the low-temperature economizer-low-pressure heater system must take into account both economics and safety. The choice of the entry point of the low-temperature economizer, that is, which stage or several stages of low-pressure heaters the low-temperature economizer is connected in series or in parallel, can be determined by specific economic analysis.

3. Use the recovered heat for heating or for the production process

Using the low-temperature economizer to recover the waste heat of flue gas for heating or for the production system is the most direct and the most simple and feasible method of waste heat recovery. By installing a low-temperature economizer and using a circulating medium (water or other organic medium) for long-distance transmission of heat, heat or steam for production is provided to the user, and heat recovery is realized. The low-temperature economizer-heating / production system is also an independently operated circulation system, which can be transformed without changing other equipment and systems, so as to achieve the purpose of recovering and recovering waste heat from the flue gas of the boiler, reducing the flue gas temperature, and improving the efficiency of the boiler. Even if the low-temperature economizer itself fails during use, it is sufficient to simply stop repairing the faulty equipment, and it will not affect the normal operation of the unit.

 

There are two ways to use the waste heat of flue gas to heat condensate:

One is to set a low-temperature economizer and a water-to-water heat exchanger to allow indirect heat exchange between flue gas and condensate. This method has the advantages of system safety and easy adjustment. Since the water side of the low-temperature economizer does not need to withstand high pressure , Greatly reducing the manufacturing cost of the heater itself. The disadvantage is that the two-stage heat exchange system is complicated, and the corresponding closed water system is added. At the same time, the heat exchange efficiency is lower than the first-stage heat exchange, and this method adds a water-water heat exchanger and corresponding circulating water pump, expansion water tank and other equipment Investment costs are higher.

Second, it is to set a low-temperature economizer to allow flue gas and condensate to directly exchange heat. The advantage of this method is first-level heat exchange and high heat exchange efficiency. The disadvantage is that once the heat exchange tube leaks, the condensate will leak To the flue gas side affects the operation of the unit, and the heat exchanger tube side needs to be subjected to a higher pressure of the condensate system (about 4MPa), which increases the manufacturing cost of the heat exchange equipment; the low-temperature economizer can be divided according to its location The following three situations:

1. The low-temperature economizer is installed on the flue in front of the air preheater outlet and the dust collector inlet.

While reducing the exhaust temperature of the boiler, the specific resistance of fly ash is reduced, the dust removal efficiency is improved, and the discharge of pollutants is reduced. However, because the smoke temperature is controlled above the acid dew point, the waste heat of the flue gas cannot be fully utilized. At the same time, the decrease of the flue gas temperature increases the difficulty of anti-corrosion of the dust collector, and increases the possibility of dust blocking in the dust collector.

2. The low-temperature economizer is installed before the outlet of the induced draft fan and the inlet of the desulfurization tower.

After the low-temperature flue gas is cooled to a suitable temperature, it directly enters the desulfurization tower. There is no harm to low-temperature corrosion caused by the induced draft fan and other equipment, and the waste heat of the flue gas can be used to the maximum extent. The low-temperature economizer is installed in front of the desulfurization tower, which reduces the consumption of flue gas evaporation water and plays a certain water-saving effect. At the same time, the problems of heat exchange tube wear and ash plugging are also relatively light. However, because the flue gas entering the low-temperature economizer has not been dedusted, the dust concentration is high, the working environment of the low-temperature economizer is harsh, the wear is large, and the life is short. In addition, it will also cause acid corrosion of electric dust removal, induced draft fan, flue, etc., which increases the cost of equipment corrosion protection.

 

Although the installation of low-temperature economizer in the unit improves the power generation efficiency of the unit and reduces the energy consumption of power generation, it also has certain disadvantages. It is generally believed that there are the following aspects:

1. After installing a low-temperature economizer, the resistance of the flue gas has increased, the power consumption of the induced draft fan has increased, and a certain investment in equipment has been added to increase the power consumption of the plant.

2. The installation of flue gas recuperator will increase the maintenance and maintenance of the equipment and the accident rate of the unit.

3. After the flue gas temperature is reduced by the low-temperature economizer, the flue gas temperature is close to the dew point temperature, and there is a risk of low-temperature corrosion.

The advantages and disadvantages of the low-temperature economizer are as follows:

The main point of the low-temperature economizer is to comprehensively utilize the flue gas heat, improve the quality of the flue gas waste heat recovery as much as possible, and improve the utilization value, thereby effectively reducing the coal consumption of the power supply of the unit. The main advantages are as follows.

1. Absorb waste heat, and use the waste heat for heating heating network water, domestic hot water, raw water, condensed water and other media to improve boiler efficiency and bring certain economic benefits;

2. Reduce the exhaust temperature, so that the flue gas reaches the optimal desulfurization efficiency state when it enters the desulfurization tower, greatly reducing the cooling water consumption in the desulfurization tower and saving precious water resources.

3. For the low-temperature economizer arranged in front of the dust collector, the specific resistance of the dust in the flue gas and the volume flow of the flue gas can be reduced by reducing the exhaust temperature, thereby improving the efficiency of electric dust removal and reducing the dust. emission.

4. If the waste heat of the exhaust gas heats the condensate in the steam turbine thermal system, the condensate absorbs the heat of the exhaust gas in the low-temperature economizer system to reduce the exhaust gas temperature, and it is heated to increase the temperature before returning to the turbine low pressure The heater system replaces part of the low-pressure heater. Part of the steam turbine's recuperation steam extraction will be saved, and the steam extraction will continue to expand and work under the condition that the steam inlet of the steam turbine remains unchanged. Therefore, the energy consumption of the unit can be saved under the condition that the amount of power generation remains unchanged.

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