As we all know, thermal power plants have two major heat losses, namely, the condensing heat loss of steam turbine exhaust system at the cold end and exhaust heat loss of the tail of the boiler system. The main factor that affects the heat loss of thermal power plant boiler exhaust heat is the exhaust gas temperature. At present, the exhaust temperature of coal-fired power plant boilers in China is mostly between 120 and 140 ℃, and the boiler efficiency is about 90% -94%. Among various heat losses, the heat loss from exhaust heat emission accounts for more than half of the heat loss of the boiler. If the exhaust temperature of the utility boiler can be effectively reduced to 70-90 ℃, the boiler efficiency will increase by 2% -5% and the coal consumption will decrease by 2 ~ 5g / kWh, carbon dioxide emissions also have a substantial reduction. Therefore, with the rising energy prices in recent years and the increasingly stringent energy-saving emission reduction requirements, recycling of waste heat from the tail of flue gas from power station booms has drawn great attention.

There are a variety of design options to reduce the boiler exhaust gas temperature: one is to reduce the exhaust gas temperature through the optimization of combustion; the other is to increase the heating surface of the boiler to reduce the exhaust gas temperature; the other is to increase the heating surface of the boiler air preheater to reduce exhaust smoke Temperature; fourth is to add low-temperature economizer in the boiler tail flue, condensate or other media to absorb exhaust heat to reduce exhaust gas temperature. However, after several experimental studies and field demonstrations, it is the most direct and easiest way to recover the waste heat from the flue gas using a low-temperature economizer, and it is also the most effective and feasible method for heat recovery.

The use of low-temperature economizer can effectively recover the waste heat of flue gas, improve the utilization efficiency of high temperature flue gas and reduce the emission loss. Its use mainly in the following areas:

1, the use of recycled flue gas heat through the air heater heating

In order to prevent or reduce the low-temperature air preheater corrosion and blocking ash, the need to install a heater to improve air preheater inlet air temperature.

This gas-water heat exchanger to replace the conventional gas heater hot air heater, circulating water as a heat medium, the heat in the flue gas side of the heat release to one or two cold, will enter the air pre-heater before the cold Pre-heating, so as to achieve the recovery of flue gas heat, and reduce the conventional steam heater auxiliary steam consumption.

2 , the use of low-temperature economizer heat recovery system condensate

The use of low-temperature economizer to heat the condensate in two ways: First, let the flue gas and condensate water directly heat exchange, this method has the advantage of a heat exchange, high heat exchange efficiency, the disadvantage is that if the heat exchanger leak once , Will directly contaminate the condensate, affecting the safe operation of the unit; the second is to set the water heat exchanger, flue gas and condensate indirect heat exchange, this method has the advantage of two heat exchange, heat exchange efficiency than a heat exchanger Low, the advantage is that the system is safe, easy to adjust.

The connection of the low-temperature economizer in the thermal system has a direct impact on the economic effect, the method of analysis and calculation as well as the safety and reliability of the operation. By its very nature, there are only two kinds of connection systems for the low-temperature economizer into the thermal system: one is the low-temperature economizer connected in series with the thermal system, referred to as the series system for short, as shown in FIG. 1; the other is the low- In the thermal system, referred to as the parallel system, shown in Figure 2.

Low-temperature economizer - low-pressure heater system design should take into account the two aspects of economy and safety. The choice of entry point for the low-temperature economizer, that is, which one or which low-pressure heaters are connected in series or in parallel by the low-temperature economizer can be determined by a specific economic analysis.

3, the use of recycled heat for heating or for the production process

The use of low-temperature economizer waste heat recovery flue gas for heating or for the production system is the most direct and is the most simple and feasible method of heat recovery. By setting the low-temperature economizer, the use of circulating medium (water or other organic media) for long-distance heat transfer, to provide users with heat or steam production, to achieve the heat recovery. Low-temperature economizer - heating / production system is also an independent operation of the circulatory system, do not need to change other equipment, systems can be transformed to complete, to waste heat boiler exhaust heat recovery, lower exhaust gas temperature, improve boiler efficiency. Even if the low-temperature economizer itself during the use of the fault, just stop the fault equipment can be, does not affect the normal operation of the unit.

There are two ways to heat condensate using flue gas waste heat:

First, set the low-temperature economizer, flue gas and condensate water directly for heat exchange, this method has the advantage of a heat exchange, high heat transfer efficiency, the disadvantage is that once the heat pipe leak, condensation will leak to The flue gas side affects the operation of the unit while the heat exchanger tube side is subject to the higher pressure of the condensate system (about 4 MPa), which increases the manufacturing cost of the heat exchange equipment.

The second is to set the low-temperature economizer and water heat exchanger, so that indirect flue gas and condensate heat exchange, this method has the advantage of system security, easy to adjust, because the low temperature economizer water side do not need to withstand high pressure, Greatly reducing the manufacturing cost of the heater itself. The disadvantage is the secondary heat exchange system is complex, increasing the corresponding closed-type water system, while the heat transfer efficiency is lower than the level of heat transfer, and this way to increase a water heat exchanger and the corresponding circulating pump, expansion tank and other equipment Higher investment costs.

Depending on their location, the low-temperature economizer can be divided into the following three situations:

1, low-temperature economizer set at the air preheater exit, the flue on the front of the precipitator.

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