Study on exhaust gas waste heat recovery technology of gas fired boiler

Date:2016-05-27 17:11:37




Industrial boilers with coal as the main fuel still occupy the leading position. With the rapid development of natural gas industry, this kind of clean energy as the fuel of the boiler will gradually increase. Compared with coal, burning natural gas while emissions of sulfur dioxide and nitrogen oxides content rarely, reducing the pressure on the environment, but after combustion produces a large number of water vapor with high temperature flue gas emissions into the environment, resulting in a serious waste of energy. The condensing type boiler can be used to recover the sensible heat and latent heat in the high-temperature flue gas, so as to make full use of the energy to reduce the operating cost.

Condenser for heat exchanger is added in the waste heat recovery device of gas at the tail of the boiler, when the flue gas in the channel through the heat transfer surface, the temperature dropped below the dew point temperature, so that water vapor in the flue gas condensation releases latent heat transfer to the recovery of refrigerant, can smoke a lot of energy to be recycled, so as to achieve the effect of energy saving and environmental protection. With the continuous development of the manufacturing industry, a variety of new efficient condensation heat transfer devices are emerging, regardless of the structure or the actual waste heat recovery effect has been a very big improvement.

1 Characteristics of flue gas

The vast majority of natural gas components for hydrocarbons, gas boiler flue gas water vapor content is higher, the analysis shows that the use of heat energy in the exhaust, the latent heat of vaporization of water accounted for a considerable share. After 1m3 natural gas combustion can produce 1.55 kg water vapor, with a significant latent heat of vaporization, about 3700 kJ/Nm3, accounting for 10% of the low calorific value of natural gas. In the traditional boiler, the flue gas temperature is generally in 160 to 250 degrees Celsius, the flue gas water vapor is still in an overheated state, it is not possible to condense into liquid water and release latent heat of vaporization. Therefore generally only the traditional natural gas boiler theoretical thermal efficiency reached 95%, by using condensing heat exchanger as long as the flue gas temperature drops to below the dew point temperature of the flue gas, flue gas recovery of sensible heat and water vapor condensation, according to net calorific value as a basis for calculating, natural gas boiler thermal efficiency can meets and exceeds 110%. YISHION natural gas as an example of flue gas dew point temperature and boiler thermal efficiency calculation and analysis, table 1 for the composition of pure natural gas.

1.1 dew point calculation

When the water vapor pressure is constant, the air is cooled to the state of saturated wet steam, and the water drop is separated out, and the temperature is the dew point temperature. Natural gas combustion characteristics analysis (in terms of 1 m3 natural gas) of water vapor in the flue gas volume fractions up to 17, 4%, if burning at atmospheric pressure, when the excess air coefficient is 1.1 (calculation in this paper take this as calculated on the basis of), the corresponding flue gas dew point temperature is 57 degrees. The dew point temperature with the change of the excess air coefficient is shown in figure 1.

Through observation, the dew point temperature of flue gas varies with the change of excess air coefficient. Because according to Dalton's law of partial pressure, high and low dew point temperature and the flue water vapor partial pressure volume (i.e., water vapor content) is proportional to the. With the increase of excess air coefficient, reduce the relative volume of water vapor in the flue, the water vapor volume fraction will decline, the dew point temperature decreases. Actually, although the local natural gas composition and content of different, but because the main ingredient is methane and accounted for the vast majority, other components is very small, the calculation of dew point temperature error is less than 0.3% (in accordance with the actual requirements of the range) and due to the actual combustion is influenced by many factors, but also makes the calculation may not achieve very accurate, usually near the theoretical value of a range of fluctuations in practical applications still need to carry on analysis and correction according to different situations.

1.2 thermal efficiency analysis

Heat in flue gas exists in 2 forms of sensible heat and latent heat, so the heat loss of the boiler is composed of significant heat loss and latent heat loss of flue gas. However, the apparent heat loss depends on the temperature of the flue gas and the heat capacity of the flue gas composition, the latent heat loss is determined by the amount of water in the flue gas. When the condensation of water vapor, in the flue gas are complex phenomena: due to the water vapor partial pressure is low, and in the vicinity of the condensate film is mainly non condensable gases, such as N2, CO2, O2, and water vapor in the flue gas needs through without gas layer in order to reach condensation of the film surface. Water vapor in flue gas condensation rate is equal to the ratio of by unit volume of natural gas combustion generated flue gas generated by the condensation of water and the burning of the production amount of water vapor. Among them, produced by the combustion of water vapor including natural gas generated by the combustion of water vapor and air and gas into the water vapor.

Only the latent heat in flue gas on the boiler thermal efficiency of such a huge, if can make the exhaust gas temperature decreased below the dew point of the latent heat to be recycled, low quantity of heat as a benchmark for calculating the thermal efficiency of at least increased to more than 10%. And with the reduction of smoke temperature, the significant heat loss of flue gas will be relatively reduced, so the thermal efficiency will be more obvious, and further prove that the efficiency of the boiler to reduce the exhaust gas temperature is of great significance.

The change trend of the actual thermal efficiency of the boiler under different flue gas temperature can be obtained. Boiler efficiency with the change of smoke temperature is divided into 2 distinct areas: 60 to 180 degrees Celsius change is slow, and in the 20 ~ 60 C. This is mainly because the proportion of latent heat loss due to water vapor loss is larger than that of flue gas. When the boiler flue gas temperature is reduced to 20, the boiler efficiency is up to 107.4%.

The latent heat of water vapor in the flue gas can be recovered at 57 degrees C, and the heat recovery depends on the required temperature.