The condensing heat exchanger is a waste heat recovery device added at the tail of the natural gas boiler. When the flue gas passes through the heat transfer surface in the channel, the temperature drops below the dew point temperature, so that the water vapor in the exhaust gas condenses and releases latent heat to be transferred to the recovery working medium. , You can recycle a lot of energy in the smoke, so as to achieve the effect of energy saving and environmental protection. With the continuous development of the manufacturing industry, a variety of new and efficient condensation heat exchange devices have emerged endlessly, and there have been very large improvements in terms of structure and actual waste heat recovery effect.
1 Characteristic analysis of flue gas
The majority of natural gas is hydrocarbons, and the content of water vapor in the flue gas of gas boilers is relatively high. Analysis shows that, among the available thermal energy in flue gas, the latent heat of vaporization of water vapor accounts for a considerable share. Every 1m3 of natural gas can produce 1. 55 kg of water vapor after combustion, with considerable latent heat of vaporization, about 3 700 kJ / Nm3, accounting for more than 10% of the low calorific value of natural gas. In traditional boilers, the flue gas temperature is generally 160 ~ 250 ℃, the water vapor in the flue gas is still in a superheated state, it is impossible to condense into liquid water and release the latent heat of vaporization. Therefore, the theoretical thermal efficiency of traditional natural gas boilers can generally only reach about 95%. Using condensing heat exchangers, as long as the flue gas temperature is reduced below the flue gas dew point temperature, the sensible heat in the flue gas and the latent heat of condensation of water vapor can be recovered. Calculated on the basis of low calorific value, the thermal efficiency of natural gas boilers can reach and exceed 110%.
1.1 Dew point calculation
It can be seen from the observation that the flue gas dew point temperature changes with the change of excess air coefficient. Because according to Dalton's law of partial pressure, the temperature of the dew point is proportional to the partial pressure of water vapor in the flue (that is, the content of water vapor). As the excess air coefficient increases, the relative volume of water vapor in the flue decreases. The volume fraction of water vapor will decrease, and its dew point temperature will also decrease. In fact, although the content of natural gas varies from place to place, because its main components are methane and account for the vast majority, other components have little effect, and the calculated dew point temperature error does not exceed 0.3% (in line with the actual requirements) ), And due to the fact that there are many factors influencing the actual combustion, the calculation is impossible to be very accurate, usually fluctuating within a range around the theoretical value, in actual application, it needs to be corrected and analyzed according to different situations.
Under the condition that the partial pressure of water vapor is constant, when the air is cooled to the state of saturated wet steam, water droplets will precipitate out, and the temperature at this time is the dew point temperature. Natural gas combustion characteristics analysis (calculated with 1 m3 natural gas) The volume fraction of water vapor in the flue gas reaches 17.4%. If the combustion is carried out at atmospheric pressure, when the air excess coefficient α is 1.1 (the calculations in this article use this Calculation basis), the corresponding flue gas dew point temperature is 57 ℃.
1.2 Thermal efficiency analysis
The heat in the flue gas exists in two forms: sensible heat and latent heat, so the heat loss of the boiler is also composed of the sensible heat loss and latent heat loss of the flue gas. The sensible heat loss depends on the temperature of the flue gas and the heat capacity of the flue gas components; the latent heat loss depends on the amount of water in the flue gas in the form of water vapor. When water vapor condenses, there is a complex phenomenon in the flue gas: due to the low partial pressure of the water vapor, and the main non-condensable gas near the condensate film, such as N2, CO2, O2, etc., the water vapor in the flue gas needs to pass through The non-condensable gas layer can reach the surface of the liquid film to condense. The water vapor condensation rate in the flue gas is equal to the ratio of the amount of condensed water produced by the combustion of unit volume of natural gas to produce flue gas and the amount of water vapor produced by combustion, where the water vapor produced by combustion includes water vapor and air produced by natural gas combustion Water vapor brought by gas. Calculation of heat exchange efficiency according to energy conservation:
(1) Among them: Q is the low calorific value of gas; Ha is the enthalpy of air at the inlet; Hg is the enthalpy of gas at the inlet; Hf is the enthalpy of exhaust gas; Φ is the condensation rate of steam; Density; r is the latent heat of vaporization; Vh is the volume of water vapor in the flue under standard conditions.
Only the latent heat in the flue gas has such a huge influence on the thermal efficiency of the boiler. If the exhaust gas temperature can be reduced below the dew point to recover the latent heat, the thermal efficiency calculated on the basis of the low heat value can be increased to at least 10%. And as the flue gas temperature decreases, the sensible heat loss of flue gas will also be relatively reduced, then the increase in thermal efficiency will be more obvious, further proving the importance of reducing the flue gas temperature for boiler efficiency.
The latent heat of water vapor in the flue gas can only be recovered below 57 ℃. The heat that can be recovered depends on the required utilization temperature and utilization rate. If the utilization temperature is close to the dew point temperature of the exhaust gas, only less heat can be recovered. The lower the utilization temperature, the more heat is recovered. Therefore, at low temperatures, the residual hot cold water can obtain a high recovery rate, while at higher temperatures, the output heat energy will be reduced to the amount of energy that can be recovered.
2 Other influencing factors of waste heat recovery
2.1 The problem of wear on the heated surface of the waste heat recovery unit
The waste heat recovery tube row is designed as a membrane tube row (or H-shaped tube row). This structure forces the flue gas flow to be laminar, and there is no flue gas disturbance between the tube rows. Compared with the light pipe type, it is the most difficult arrangement of the heating surface. Moreover, due to the friction between the boundary pipe row of each flue and the flue gas, a distribution mode with a high intermediate velocity and low velocity on both sides is formed. Therefore, the flue gas velocity near the pipe wall is lower than the average value, and the flue gas disturbance is relatively weak, which alleviates the wear of the fly ash on the economizer. In addition, the flue gas flow rate has the greatest influence on the wear of the heated surface. The flue gas flow rate should not be too large when the heated surface is arranged. During the design, the flue velocity of the heated surface can be changed by adjusting the lateral intercept of the tube row, which can effectively avoid the waste heat recovery device Wear problem.
2.2 The problem of scaling on the inner wall of the waste heat recovery tube
The scaling of the inner wall of the heated surface pipe mainly occurs in the evaporation section because the salt-dissolving capacity of steam is very different from that of water. And the highest point temperature in the waste heat recovery system will not exceed 120 ℃, the entire system is still in the liquid phase, and the problem of scale formation on the inner wall of the pipe is less.
