Due to years of industrial development in China, industrial boilers using coal as the main fuel still occupy a dominant position. In order to cater to the national conditions of the rapid development of industry at the same time also caused the haze and other air pollution is becoming increasingly serious, the state realized this after the publication of a number of policies related to industrial development to protect the environment.

The rapid development of the natural gas industry has also brought the dawn of environmental protection. Boilers fueled by this clean energy will gradually increase. Compared with coal, combustion of natural gas emissions of sulfur dioxide and nitrogen oxides content is very small, reducing the pressure on the environment, but the combustion of a large number of water vapor with high-temperature flue gas emissions into the environment, resulting in a serious waste of energy. The use of condensing boilers will be high temperature flue gas in the sensible and latent heat to be recovered, you can achieve the full use of energy to reduce operating costs.

First, the secret of environmental protection boiler - condensing heat exchanger

Condensing heat exchanger is added to the tail of the natural gas boiler waste heat recovery device, when the flue gas in the channel through the heat transfer surface, the temperature is reduced to below the dew point temperature, so that the exhaust smoke in the water vapor condensation to release latent heat transfer to the recovery of the workpiece, you can be a large amount of energy in the exhaust to be recycled, so as to achieve energy-saving and environmentally friendly results. With the continuous development of the manufacturing industry, a variety of new high-efficiency condensing heat exchanger devices are emerging, whether from the structure or the actual waste heat recovery effect has been a very big improvement.

Second, flue gas analysis

Natural gas composition of the vast majority of hydrocarbons, gas boiler exhaust smoke in the higher content of water vapor, analysis shows that the exhaust smoke can be used in the thermal energy, the latent heat of vaporization of water vapor accounted for a large share. Every 1 m3 of natural gas combustion can produce 1. 55 kg of water vapor, with considerable latent heat of vaporization, about 3,700 kJ/Nm3, accounting for more than 10% of the low-level heat of natural gas. In conventional boilers, the exhaust temperature is generally 160 to 250 ℃, the flue gas in the water vapor is still in a superheated state, it is impossible to condense into liquid water and release latent heat of vaporization. Therefore, the theoretical thermal efficiency of traditional natural gas boilers generally can only reach about 95%, the use of condensing heat exchanger as long as the flue gas temperature down to the flue gas dew point temperature below the flue gas can be recovered in the flue gas sensible heat and latent heat of condensation of water vapor, calculated on the basis of the low level of heat generation, natural gas boiler thermal efficiency can be reached and exceeded 109%.

1, dew point calculation

     In the case of water vapor partial pressure is unchanged, so that the air cooled to saturated wet steam state, there will be water droplets precipitation, the temperature at this time is the dew point temperature. Natural gas combustion characteristics (calculated as 1 m3 of natural gas) flue gas water vapor volume fraction of 17˙ 4%, if the combustion is carried out at atmospheric pressure, when the air excess coefficient α is 1.1 (in this paper, the calculations are used as a basis for calculating the corresponding flue gas dew point temperature is 57 ℃.

It can be observed that the flue gas dew point temperature varies with the excess air coefficient. Because according to Dalton's law of partial pressure, the dew point temperature is directly proportional to the partial pressure of water vapor in the flue (i.e., the content of water vapor), with the increase of the excess air coefficient, the relative volume of water vapor in the flue decreases, and the volumetric share of water vapor decreases, and its dew point temperature decreases as well. In fact, although the composition content of natural gas in various places is different, but due to its main components are methane and account for the vast majority of the other components have a small impact, the calculated dew point temperature error is not more than 0.3% (in line with the range of practical requirements), and due to the actual combustion of the influence of a number of factors, but also makes the calculation is not possible to achieve a very accurate, is usually in the theoretical value of the fluctuations in the vicinity of a range of values in the practical application. Application also needs to be corrected according to the different circumstances of the analysis.

2, thermal efficiency analysis

      Heat in the flue gas in the form of sensible heat and latent heat in two forms, so the heat loss of the boiler also consists of sensible heat loss and latent heat loss of the flue gas. The sensible heat loss depends on the temperature of the flue gas and flue gas components of the heat capacity; latent heat loss depends on the flue gas in the form of water vapor in the amount of water. When the water vapor condensation, there are complex phenomena in the flue gas: due to the low partial pressure of water vapor, and in the vicinity of the condensate film is mainly non-condensable gases, such as N2, CO2, O2, etc., the water vapor in the flue gas needs to pass through the layer of non-condensable gases in order to reach the surface of the liquid film to condense. The condensation rate of water vapor in the flue gas is equal to the ratio of the amount of condensed water produced by the combustion of natural gas per unit volume of flue gas to the amount of water vapor produced by combustion, which includes the water vapor produced by combustion of natural gas and the water vapor brought in by air and gas.

Only the latent heat in the flue gas has such a huge impact on the thermal efficiency of the boiler, if the exhaust temperature can be reduced to below the dew point of the latent heat to be recovered and utilized, to the low level of heat as the basis for calculating the thermal efficiency can be increased to at least 10% or more. And with the reduction of the exhaust temperature, the flue gas sensible heat loss will be relatively reduced, then the thermal efficiency will be more obvious, further proving the importance of reducing the exhaust temperature to improve the efficiency of the boiler.

     Further calculations can be derived from the actual thermal efficiency of the boiler at different exhaust temperature trends. The boiler efficiency with the change in exhaust temperature is divided into 2 more obvious areas: in the 60 ~ 180 ℃ change slowly, while in the 20 ~ 60 ℃ change is larger. This is mainly due to the exhaust loss of latent heat loss of water vapor accounted for a larger proportion than the result of flue gas sensible heat. When the boiler exhaust temperature down to 20 ℃, the boiler efficiency theoretically up to 107.4%.

       Exhaust smoke in the latent heat of water vapor below 57 ℃ to be able to recover, can be recovered heat depends on the required utilization temperature and utilization rate. If the utilization temperature is close to the dew point temperature of the exhaust smoke, only a small amount of heat can be recovered. The lower the utilization temperature, the more heat is recovered. Therefore, a high recovery rate can be obtained for waste heat chilled water at low temperatures, while at higher temperatures the heat output drops to the amount of energy that can be recovered.

Waste heat recovery

 1. The whole process of avoiding the wear and tear of the heating surface of the waste heat recovery device

      Waste heat recovery device tube row designed as a membrane type tube row (or H-type tube row), this structure forces the flue gas flow tends to laminar flow, there is no flue gas perturbation between the tube rows, in the same smoke speed, compared with the spiral ribbed and light tube type is the most difficult to wear the heated surface of the arrangement form. And due to the friction between the pipe rows and the flue gas at the boundary of each flue, the formation of the middle of the high flow rate, the distribution of the two sides of the low flow rate. Therefore, the flue gas flow rate near the pipe wall is lower than the average value, and the flue gas disturbance is relatively weak, which alleviates the wear of fly ash on the economizer. In addition, the flue gas flow rate has the greatest influence on the wear of the heating surface, the flue gas flow rate should not be too large when arranging the heating surface, and the flue gas flow rate of the heating surface can be changed by adjusting the transverse cutoff distance of the pipe rows in the design, which can effectively avoid the wear of the pipe rows of the waste heat recovery device.

 2、 Flue resistance

      The whole flue resistance of the boiler is mainly overcome by the induced draft fan and chimney pull-out force, of which the induced draft fan is the main factor. The overall flue gas resistance of the boiler is bound to increase after the installation of waste heat recovery device. For example, the heat calculation result of the No.3 furnace of a power plant shows that the flue resistance increases by about 70 Pa. In the installation of waste heat recovery at the same time whether the induced draft fan to further improve the output to ensure the normal operation of the boiler body after the installation of waste heat recovery, depending on the site conditions to determine.

3, has been worried about a small problem - waste heat recovery tube wall scaling

      Heating surface tube wall scaling mainly occurs in the evaporation section, because the salt dissolving capacity of steam compared with water is very different. In the waste heat recovery system in the highest point of the temperature will not exceed 120 ℃, the entire system is still in the liquid phase, the tube wall scaling problem is small.

Summarize

1, compared with coal and oil, natural gas is a very ideal clean energy, the emission of flue gas on the environmental pressure is small, and is very suitable for its conversion into condensing waste heat recovery boiler, improve boiler utilization efficiency.

2, natural gas boiler emissions flue gas contains a certain amount of water vapor, if the exhaust temperature is lowered to below the dew point temperature to recover the latent heat of vaporization released by the water vapor, the boiler efficiency can be increased by more than 10%.

3, reasonable setting of key technical parameters, can realize the long-term stable operation of the waste heat recovery system, some domestic power plants have successfully designed and installed the waste heat recovery system, for the power plant has brought good economic benefits.