Technical analysis of the hottest condensing waste

Technical analysis of condensation waste heat recovery furnace

1. Overview

generally speaking, although boilers with thermal efficiency of more than 100% are difficult to understand in common sense, if the latent heat of steam condensation in the flue gas is used, and the exhaust gas temperature is reduced sufficiently low, and the exhaust gas loss is very low, the thermal efficiency of the boiler will be increased to 100%, or even more than 100%

in the field of thermal energy engineering, the calculation of boiler thermal efficiency is based on the low calorific value of fuel, which is the same in foreign countries. If the boiler thermal efficiency is calculated according to the high calorific value of boiler, 100% thermal efficiency is impossible to achieve (energy conservation)

efficient condensation heat exchanger and air preheater are used to absorb the sensible heat in the exhaust gas at the tail of the boiler and the latent heat released by steam condensation, so as to improve the thermal efficiency of the boiler. This kind of boiler is condensing waste heat recovery boiler

condensing boilers originated in Europe. Germany, the Netherlands, Britain, Austria and other countries developed domestic condensing boilers in the 1970s. By the late 1980s and early 1990s, South Korea took the lead in applying condensing boilers to large and medium-sized industrial boilers. Condensing boilers are not only common to traditional boilers, but also a bold revolution and breakthrough in heating mechanism. In some European and American countries with high energy utilization rate, the thermal efficiency of the gas-fired condensing waste heat recovery hot water boiler is as high as 103%. In addition, the harmful components such as CO2 and NOx in the flue gas are also greatly reduced, which is very beneficial to environmental protection. In Europe and the United States and other countries, because the government encourages the use of condensing boilers, the demand is increasing. The utilization rate of condensing boilers is 60% in Switzerland, 50% in the Netherlands, 20% in Germany, 20% in Austria and 15% in Britain

condensing heat exchanger is a kind of low-temperature heat exchanger with large heat transfer area, and uses expensive corrosion-resistant stainless steel materials. Although the price is high, it is only a one-time investment, and its investment payback period is only a few months, and the saved fuel cost will soon be recovered

the condensing boiler can recover the latent heat of steam condensation in the exhaust gas and reduce the harmful gases in the flue gas, so it quickly established its position in the field of HVAC. The popularity of European countries' awareness of condensing boilers and the inclination of policies make the application of condensing boilers extremely extensive. In China, condensing boilers are still blank. One important reason is the lack of understanding of condensing boilers, and the other reason is the poor promotion and research and development of condensing boilers by manufacturers

Sri Lanka boiler, a Sino Korean joint venture boiler manufacturing enterprise, has been trying to combine the boiler technology of South Korea with the current situation of boilers in China from the beginning. It has successively developed condensing pressureless hot water boilers, condensing waste heat recovery boilers, condensing atmospheric hot water boilers, condensing pressure hot water boilers, and introduced the concept of energy utilization of South Korea into China, especially in the promotion of condensing boilers

2. Thermal efficiency analysis of condensing waste heat recovery boiler

the fuel contains a large amount of hydrogen, and a large amount of water vapor is produced by combustion. Each 1nm3 of natural gas can produce 1.55kg of steam after combustion, with considerable latent heat of vaporization, about 3700kj, accounting for about 10% of the low calorific value of natural gas. When the flue gas temperature is high, the water vapor cannot condense and release heat, which is wasted with the flue gas. At the same time, high-temperature flue gas also takes away a large amount of sensible heat, which together forms a large smoke loss

the waste heat recovery device of flue gas condensation uses water or air with low temperature to cool the flue gas, so as to reduce the flue gas temperature. Close to the heat exchange surface area, the water vapor in the flue gas condenses, and at the same time, it realizes the sensible heat release of flue gas and the latent heat release of water vapor condensation, while the water or air in the heat exchanger absorbs heat and is heated, so as to realize heat recovery and improve the thermal efficiency of the boiler

boiler thermal efficiency improvement: the theoretical flue gas volume of 1nm3 natural gas combustion production is about 10.3 Nm3 (about 12.5kg). Taking the excess air coefficient of 1.3 as an example, 14 Nm3 (about 16.6kg) of flue gas is generated. The flue gas temperature is reduced from 200 ℃ to 70 ℃, the physical sensible heat released is about 1600kj, the condensation rate of steam is 50%, the vaporization latent heat released is about 1850 kJ, and the total heat released is 3450 kJ, which is about 10% of the low-level calorific value of natural gas. If 80% of the flue gas is taken into the heat energy recovery device, the utilization rate of heat energy can be increased by more than 8% and the natural gas fuel can be saved by nearly 10%

in traditional boilers, the flue gas temperature is generally 160 ~ 250 ℃, and the steam in the flue gas is still in an overheated state, which is impossible to condense into liquid water and release the latent heat of vaporization. As we all know, the boiler thermal efficiency is calculated according to the low calorific value of fuel in the safety technical specification jgj88 (9) 2 for gantry and well frame material hoists, without considering the heat loss of vaporization potential heat in the high calorific value of fuel. Therefore, the thermal efficiency of traditional boilers can only reach 87% - 91%. The condensing waste heat recovery boiler reduces the exhaust gas temperature to 50 ~ 70 ℃, and fully recovers the sensible heat in the flue gas and the condensation latent heat of steam. 3. The models of this equipment can be roughly divided into digital display and microcomputer control

the volume composition of water vapor in the flue gas of condensing waste heat recovery boiler with natural gas as fuel is generally 15% - 19%, and the content of water vapor in the flue gas of oil fired boiler is 10% - 12%, which is much higher than the content of water vapor below 6% in the flue gas produced by coal-fired boiler. At present, the thermal efficiency of boilers is calculated by the low calorific value. Although the nominal thermal efficiency is high, the actual energy utilization rate needs to be improved because the difference between the high and low calorific value of natural gas is about 10%. In order to make full use of energy, reduce the exhaust temperature and recover the physical heat energy of the flue gas, when the wall temperature of the heat exchanger is lower than the dew point temperature of the flue gas, the water vapor in the flue gas will be condensed to release latent heat, and 10% of the high and low calorific value difference can be effectively used

3. Design idea and principle of condensing boiler:

exhaust gas temperature is one of the basic design parameters of the boiler. This parameter should be selected first when designing the boiler

the exhaust gas temperature of the boiler directly affects the economy of the boiler unit and the safety of the tail heating surface. Choosing a lower exhaust gas temperature can reduce the heat loss of boiler exhaust gas, which is conducive to improving the thermal efficiency of the boiler, saving energy and reducing the operating cost of the boiler. Therefore, how to effectively reduce the exhaust gas temperature of the boiler and make rational use of it is a major technical topic. Sri Lanka introduced the condensing waste heat recovery boiler developed by South Korea technology, which can reduce the exhaust gas temperature through the following methods:

(1) reduce the exhaust gas temperature by increasing the heat exchange area of the convective heating surface of the boiler body or by increasing the convective heat transfer coefficient

(2) add efficient fin condensation heat exchanger and heat pipe air preheater in the tail flue

the above methods can effectively recover the sensible heat and vaporization latent heat in smoke exhaust in practical application

4. Obvious advantages of condensing boiler:

(1) heat pipe air preheater and fin condensing heat exchanger are used, which effectively reduces the exhaust gas temperature

(2) a split burner is used to preheat the air required for fuel combustion, so as to fully burn the fuel and improve the furnace temperature

(3) in order to prevent the acid corrosion of exhaust condensate, the condensation energy-saving device uses the threaded pipe made of imported stainless steel, which has more than 2 times higher thermal conductivity than the straight pipe

(4) the harmful gas of flue gas is effectively controlled and flows into the neutralization tank with the condensate

to sum up, condensing boiler is the crystallization of heat transfer, physics, combustion, materials and other sciences. It takes pride in traditional boilers with absolute economy. The promotion of condensing boilers is a brilliant thought and a change of concept. At the same time, it will certainly promote the development of thermal engineering

(I) principle of condensing waste heat recovery boiler

1. Natural gas (LNG) and other gas fuels are mainly compounds formed by the combination of carbon (c) and hydrogen (H), which can maintain complete combustion. Because it does not contain sulfur, it does not produce sulfuric acid (H2SO4) and sulphurous acid (H2SO3) that corrode metals at low temperature. It is a clean fuel

main components of natural gas:

CH4 + C2H6 + C3H8 + C4H10 + N2

methane + ethane + propane + butane + nitrogen

(%) 90 + 6.8 + 2.5 + 1 + 0.3 =100

2. Hydrogen (H) contained in gas combines with oxygen to form water when it is burned in the boiler bench hammer furnace. The generated water absorbs about 10% of the gasification heat from the heat generated during combustion (high calorific value) and becomes steam, which is discharged together with the discharged flue gas. (in winter, water vapor meets with cold air and condenses. White smoke is observed from the chimney, which should be the reason.)

high calorific value of gas - heat of vaporization (latent heat) = low calorific value

9450kcal/nm3 - 950kcal/nm3 = 8500kcal/nm3

3. In high-temperature flue gas, water absorbs heat and evaporates into steam, which meets the heat transfer surface passing through cold air or cold water, recondenses into water and releases latent heat (heat of vaporization 539kcal/kg). The condensing waste heat recovery boiler is to set a heat exchanger through cold water and an air preheater for heating air on the smoke exhaust channel of the gas boiler. When the flue gas passes through the heat transfer surface of the heat exchanger, the steam recondenses into water, releases its vaporization heat (latent heat), and heats the medium (cold water or air) in the exchanger

4. Calculation of boiler thermal efficiency:

positive equilibrium efficiency of boiler:

η= (boiler output × Saturated steam enthalpy? Water supply × Feed water enthalpy) ÷ (fuel consumption × Lower calorific value of fuel)

5. Formulate and chart the principle of waste heat recovery furnace, as shown in figure-1. The saturation temperature of steam/warm water boiler water is relatively higher than that of low-temperature water boiler, and the exhaust gas temperature is also relatively high. If the sensible heat and latent heat are recovered by warm water, the instrument works reliably, and the amount of warm water that can be heated is too large to be treated, so an air preheater is set. It is used to heat the air required for fuel combustion, greatly improve the combustion state, increase the flame temperature of the furnace liner, and strengthen the radiation heat transfer in the furnace liner

(II) condensing waste heat recovery boiler and environmental protection

compared with burning heavy oil or light oil, gas pollution to the environment is relatively small, it is a kind of clean fuel. However, CO2, CO and NOx generated during combustion have an impact on the environment. In order to reduce this amount, developed and used low NOx burners and low NOx boilers in advanced countries. However, CO2 is still discharged, while NOx is controlled below 60ppm

generally, the content of CO2 in natural gas combustion smoke is about%; The content of NOx is about ppm. These harmful gases contribute to acid rain or greenhouse effect, induce the destruction of the atmospheric ozone layer or affect ozone generation. The use of condensing waste heat recovery boiler can greatly alleviate this environmental pollution

co2+h2o → h2co3

nox+h2o → hno2+hno3

in the above formula, it can be seen that CO2 and NOx combine with condensed H2O in the tail flue of the condensing waste heat recovery boiler to form the corresponding acid, which is discharged from the discharge pipe with the condensed water. However, the harmful components CO2 and NOx in the flue gas are much less salty, and the content of CO2 is reduced by about 40%, from the original 12% to%; NOx is reduced to below 20ppm

neutralization treatment is required when acidic condensate is discharged:

H2CO3 + ca2++oh- → CaCO3 + H2O

h2no3 + ca2++oh- → cano3 + H2O

a neutralization tank can be set to discharge condensate into the neutralization tank, and the pH value of the neutralization tank should be checked

(III) structure and appearance of condensing waste heat recovery boiler

(IV) structure and principle of heat pipe air preheater

heat pipe is in vacuum state