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Exhaust gas boiler flue gas temperature is reduced prolifera
Author:admin  Time:2015-08-06 14:05:18
Boiler flue gas contains a large amount of sensible and latent heat, make full use of flue gas heat can reduce energy consumption and thus reduce emissions of pollutants. The higher the moisture content of the flue gas, gas-fired, steam condensation process will release a lot of latent heat of vaporization, and produce large amounts of water, and less impurities in natural gas, a relatively clean condensate, natural gas, flue gas heat recovery and therefore become a hot research In the heating system, gas-fired boiler flue gas heat recovery can take different technical route. The most common is in the tail of a conventional gas boiler additional condensing heat exchanger, research in this area includes theoretical and experimental study of heat transfer [1-4], enhanced heat transfer and corrosion studies [5-7], condensation heat transfer device applications development and demonstration projects, etc. [8-9].

Gas-fired boiler flue gas dew point at about 55 ℃ (excess air ratio at 1.15), only the heating medium temperature is below 55 ℃ to recover flue gas condensation heat at 30 ℃ or less in order to achieve better thermal recovery effect In the field of central heating, heat supply network backwater temperature above 50 ℃, and therefore can not fully recover flue gas condensing heat. This additional gas directly in the rear of the boiler condensing heat exchanger method often only recover part of the latent heat of flue gas, we can not realize the depth of recovering heat of condensation.

In recent years, with the absorption of heat exchanger technology [10-11] matures, the use of absorption heat exchanger technology can achieve depth of the use of waste heat, the system using an absorption heat pumps to produce a chilling medium, so that the flue gas discharge smoke cooler, heat recovery more thoroughly, a lot of water vapor is condensed, energy saving and environmental effects are more significant, this technology route has been gradually recognized by the industry and concern. [12] This technique is explained, and in relation to waste heat recovery systems and heat transfer theory and experiment carried out the research, design and equipment development condensation heat transfer device, and they are being successfully applied in several boiler room. As the amount of application of new technologies, increasing water vapor is condensed, the flue gas carbon oxides, nitrogen oxides and other pollutants dissolved in the condensate, thereby reducing the direct emissions into the atmosphere each the amount of pollutants, the total emissions be? exhaust gas temperature of the technology to make the system more and more low temperature below 30 ℃ emissions can be done, how it affects the temperature of the exhaust gas to reduce the spread of pollutants? Aimingresearch depth environmental emissions flue gas heat utilization technology, research findings will provide important reference for the promotion and application of technology.

2 based on the absorption heat of the flue gas waste heat utilization system

Will absorb the heat of the flue gas heat recovery concept is applied to a gas boiler, which process shown in Figure 1. In a gas absorption heat pump boiler room and additional flue gas condensing heat exchanger, absorption heat pump with natural gas as the driving energy to produce cold medium, the cooling medium and the flue gas in the flue gas condensing heat exchanger in heat transfer, heat transfer process can direct or indirect contact heat exchanger, the system exhaust gas temperature drops below the dew point, condensation of water vapor in the flue gas heat, the purpose of the recovery of waste heat and moisture. First heat supply network backwater into the absorption heat pump is heated, and then into the gas boiler heating to the design temperature confessed to complete the heating process water heating network. Gas-fired boiler flue into the bottom of the chimney, induced draft fan is placed at the top of the flue gas condensing heat exchanger withdrawn after mixing with the smoke into the flue gas absorption heat pump condensing heat exchanger, the system reduces the exhaust gas temperature to 30 ℃ After the following into the chimney emissions to the atmosphere. Extracting additional spacer between the chimney flue and the flue gas outlet back. Our experimental system using direct contact flue gas condensing heat exchanger.

 


This technology allows the system to heating efficiency (the ratio of the heat system and low heat input in the system of gas supply) increased by more than l0%. Currently the technology has been used in Beijing GLD boiler room heat recovery projects, Beijing bamboo plant waste heat recovery boiler project, achieved good energy saving effect. Incremental investment of this technology (including investment absorption heat pump, flue gas condensing heat exchangers and ancillary pumps, valves and other equipment) generally within 3a can be recycled.

3 smoke condensate purification mechanism of smoke components

The purification of exhaust smoke condensate component is complex heat and mass transfer. Flue gas condensation is water vapor in the flue gas is condensed into a liquid film or fine water droplets on the heat exchanger wall, then gathered into large droplets or small streams. In this process, the flue gas condensation of different components will be dissolved in aqueous solution or react with the condensed water, smoke harmful substances can be removed, so that the flue gas to reduce harmful gas content.

Flue gas condensation process in NOx. Purification: nitrogen oxides NO, NO2, N2O, N2O5, collectively referred to NOx. Constitute air pollution and photochemical smog substance mainly NO, NO2, other negligible. Boiler flue gas nitrogen oxides mainly is NO, NO2 and less content. NO slightly soluble in water, dissolved amount is negligible. NO2 soluble in water to form nitrous and nitric acid solution. NO2 water-soluble reaction:

2NO2 + H2O9HNO3 + HNO2.

4 direct-contact flue gas condensing heat exchanger cleaning effect test

Flue gas condensation can reduce the harmful substances in the exhaust, but its purifying effect is affected by many factors. These factors include the amount of condensate, fuel type, temperature heat exchange medium. Studies have found that [13]: NOx purification of flue gas condensing flue gas SO2 effects related to the presence or absence. Without the presence of SO2 in the flue gas, the flue gas condensation, NOx purification better, generally NOx absorption rate can reach 10% to 20%; the presence of SO2 in the flue gas, flue gas condensation, NOx purification effect significantly lower NOx absorption rate generally 10% or less. Since SO2 hydrophilic stronger than NOx, SO2 in the flue gas is present, the role of SO2 and NOx water inhibits the action of water, making the smoke condensate to reduce NOx purification effect. For the system shown, the system herein than conventional gas boiler flue gas temperature is lower, the greater the amount of condensate, which requires purification effect experimental study. Experimental system using direct contact flue gas condensing heat exchanger, the advantages of this approach are: greatly increases the contact area between the flue gas-water two-phase flow, instantaneous heat and mass transfer, to achieve enhanced heat transfer effect .

4.1 Test program

Two conditions of the system were tested, case 1 is not open flue gas heat recovery systems, case 2 is on flue gas heat recovery systems. Analysis based on two sets of waste heat recovery effect quantitative test results. Located in the flue gas heat recovery systems sampling hole, the inner diameter of the effective sampling aperture 100mm, hole position 1 A, B, as shown, A three point C is located at the boiler exhaust port is shown, B point is located absorption heat pump at the exhaust port, C point is located at the outlet of the chimney total. Data A, B point for data analysis and verification routine, this paper analyzes C points

4.2 Test Data Analysis

System conditions 1: waste heat recovery system is shut down, only the gas boiler. The conditions, the fuel consumption of 1572m3 / h, the total for the heat of 14.0MW.

System conditions 2: waste heat recovery system is turned on, the gas boiler and absorption heat pump running. The condition, gas boiler fuel consumption of 1572m3 / h, absorption heat pump the fuel consumption is 358.5m3 / h, the total system for the heat of 19.01MW, where the boiler for the heat of 14MW, waste heat recovery system for heat the 5.01MW (which contains 1.9MW Waste Heat).

Condition 2 and Condition 1 as compared to the system for the unit of heat (1MW · h) of natural gas to reduce fuel consumption by the 112.3m3 to 101.6m3, energy-saving heating rate of 9.5%. Energy-saving heating rate equal working conditions 2 gas consumption and a poor condition and more than 1 gas consumption of the gas consumption of the working conditions.

In Table 1, the flue gas sampling and analysis is based on results of the two conditions at the outlet of the chimney of the total (C point) of data were compared. Table 1 * for the conversion value is the condition NOx emission rate of 1 converted to NOx emission rate under the condition 2 for the heat supply of the same conditions. Table 1 NOx emission factor is defined as the quality of after each 1m3 of natural gas combustion emissions. As can be seen, the same gas consumption 1m3, condition 2 to condition 1 emission factor decreased 5.73 percent.

5 flue gas waste heat utilization system depth analysis and evaluation of emissions

Total 5.1 System Emissions Analysis

Set two conditions the same heat supply (1MW · h), total emissions condition 1 = 112.3m3 × 0.75g / m3 = 84.225g; total discharge condition 2 = 101.6m3 × 0.707g / m3 = 71.831g, case 1 total emissions reduction of 14.7%. Analyze the contribution rate consists of two parts, in part because of reduced fuel saving emissions, this part of the contribution to energy saving contribution rate that is about 9.5%, and the other part is the use of flue gas heat recovery absorption so that the concentration of pollutants decreases,Contribution to total emissions of about 5.2%.

5.2 exhaust gas temperature to reduce the impact of the diffusion of flue gas analysis

The impact of the flue gas waste heat recovery technology diffusion of pollutants can be analyzed in two ways. With lower exhaust gas temperature, on the one hand, flue gas heat recovery amount increases, for the same heat saving fuel, saving fuel will reduce NOx emissions rate, the maximum ground concentration of NOx is reduced; on the other hand , with the decrease of the flue gas temperature, pollutants easy to spread and increase the body's maximum flue gas NOx concentration on the ground. The role of two factors a positive and negative, so the need for two separately analyzed to obtain its overall effect.

5.2.1 The exhaust gas temperature and fuel-saving impact on the spread of smoke

In order to clearly analyze the impact of two factors, first under the assumption that the exhaust gas temperature constant, analysis affects only save fuel emissions; and then assume the same fuel consumption, exhaust gas temperature analysis only reduce the impact on emissions. Using HJ "Environmental Impact Assessment Guidelines atmosphere" mode recommended 2.2-2008 calculated using Screen3 software simulation, the software is calculated using the Gaussian model.

 


Comprehensive analysis of both positive and negative factors, the exhaust gas temperature is reduced to the maximum ground concentration of NOx impact than fuel-saving effects on NOx maximum ground concentration is more significant, indicating lower exhaust gas temperature are the main factors, together NOx maximum ground As the concentration is lower flue gas temperature is gradually increased.

5.2.2 The actual two conditions pollutant dispersion analysis

Table 2 indicates the conversion value of wood, the case 1 is converted to the data in the emissions of heat conditions are the same as for case 2. Standard state is 101325Pa, 0 ℃, case 1 corresponds to the status of flue gas emissions 101360Pa, 54.1 ℃, Case 2 corresponds to the status of flue gas emissions 101310Pa, 22.3 ℃. NOx maximum concentration on the ground floor and maximum distances in Table 2. Ground concentration varies with landing distance (landing place and chimney) of


According to the model calculation results, waste heat recovery condition (case 2) NOx maximum ground concentration increased slightly, increasing the 2.88mp. g / m3. After the visible, waste heat recovery ground conditions NOx concentration reaches a peak from Figure 4, with the landing distance decay rate is greater than the gas boiler heating mode (case 1), the waste heat recovery condition, although a slight increase in NOx maximum ground concentration, but rapid decay, after the 300m distance chimney, NOx concentration on the ground two conditions on the basic corresponded. In either condition, the NOx concentration on the ground are much lower than GB 3095-2012 "ambient air quality standards" in the second-class district of 1h mean NO2 (200mg / m3). Waste heat recovery condition ground maximum distance is reduced 17.6%, reducing the scope of pollution.

6. Conclusions

① system total reduction: through the use of flue gas heat recovery systems measured depth concluded that the test conditions, the total system emissions by 14.7 percent. Analyze the contribution rate consists of two parts, one to save fuel to reduce the amount of emissions, this part of the contribution to energy saving contribution rate that is about 9.5%, and the other part is the use of flue gas heat recovery absorption so that the concentration of pollutants decreases, Contribution to total emissions of about 5.2%.

Factors ②NOx maximum ground concentration of: fuel savings and reduce flue gas temperature average maximum ground impact on the NOx concentration. With the fuel savings, NOx maximum ground concentration decreases; With reduce exhaust gas temperature, NOx maximum ground concentration. NOx exhaust gas temperature to reduce the impact on the maximum ground concentration is more significant, so that the overall trend is with decreasing exhaust gas temperature of, NOx maximum ground concentration increases.

③ heat recovery condition NOx maximum ground concentration increased slightly, but decays rapidly. In either condition, its emissions are much lower than the GB 3095-2012 "ambient air quality standards" in the second-class district of 1h mean NO2 (200mg / m3); and a heat recovery condition maximum landing distance decreases 17.6%, reducing the scope of pollution.

④ With lower exhaust gas temperature, the overall impact of the proliferation of small smoke in the promotion and application of waste heat recovery technology process, using the technology of gas boiler rooms are not necessary to re-evaluate the environmental impact.
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