The energy-saving transformation technology of gas-fired boiler is developing continuously. One of the typical energy-saving transformation methods is flue gas condensation, namely flue gas waste heat recovery. In the era of traditional coal-fired boiler, due to less water vapor in the generated flue gas, and the dew point of acid substances in the flue gas is about 130 ℃, in order to prevent the boiler tail corrosion, the exhaust gas temperature generally needs to be designed above 150 ℃, so the flue gas condensation technology has not been paid attention to.

當鍋爐燃料改為以甲烷氣為主的天然氣時，燃料中硫化物含量極低，氫元素比例增高，燃燒產物中水蒸氣含量大幅升高，當水蒸氣溫度降到露點溫度以下時，水蒸氣凝結成液態水，放出大量汽化潛熱。因此，燃氣鍋爐排煙溫度降到足夠低，大量煙氣中水蒸氣的汽化潛熱放出並被合理收集再利用時，鍋爐效率會得到大大提高。

When the boiler fuel is changed to methane based natural gas, the sulfide content in the fuel is very low, the proportion of hydrogen element increases, and the water vapor content in the combustion product increases greatly. When the water vapor temperature drops below the dew point temperature, the water vapor condenses into liquid water, releasing a large amount of latent heat of vaporization. Therefore, when the exhaust gas temperature of the gas-fired boiler is low enough, the latent heat of vaporization of water vapor in a large amount of flue gas is released and reasonably collected for reuse, the boiler efficiency will be greatly improved.

根據高位發熱量和低位發熱量的定義，兩者的差值即為煙氣中水蒸汽全部凝結成水所放出的汽化潛熱。因現行標準下的鍋爐熱工效率測試中，是以燃料的低位發熱量進行計算的，因此，經常會出現排煙溫度低的燃氣冷凝鍋爐熱工效率超過 100%的情況，其原因就是在使用低位發熱量計算熱工效率時，忽略了 燃料燃燒後水蒸氣中汽化潛熱的利用。

According to the definition of high calorific value and low calorific value, the difference between them is the latent heat of vaporization released by the condensation of water vapor into water in flue gas. Because the boiler thermal efficiency test under the current standard is based on the low calorific value of the fuel, the thermal efficiency of the gas-fired condensing boiler with low exhaust temperature often exceeds 100%. The reason is that the use of the low calorific value to calculate the thermal efficiency ignores the use of the latent heat of vaporization in the steam after fuel combustion.

煙氣成分分析計算

Analysis and calculation of flue gas composition

煙氣成分是天然氣鍋爐完全燃燒後的產物，實際鍋爐運行中，為了保證燃料完全燃燒，實際空氣量是多於燃燒所需的理論空氣量的，他們之間的比值被稱為過量空氣係數，通過對近幾年鍋爐熱工測試結果的統計，鍋爐過量空氣係數通常在 1. 2 左右。從表 1 的天然氣成分可知，燃料完全燃燒後煙氣中主要成分為二氧化碳、過量空氣、燃料中剩餘氮氣和水，我們將其分為幹煙氣( 二氧化碳、氮氣、過量空氣) 和水蒸氣兩部分。

Flue gas composition is the product of natural gas boiler after complete combustion. In actual boiler operation, in order to ensure complete combustion of fuel, the actual amount of air is more than the theoretical amount of air required for combustion. The ratio between them is called excess air coefficient. Through the statistics of boiler thermal test results in recent years, the excess air coefficient of boiler is usually about 1.2. According to the composition of natural gas in Table 1, the main components of flue gas after complete combustion of fuel are carbon dioxide, excess air, residual nitrogen and water in the fuel, which are divided into dry flue gas (carbon dioxide, nitrogen, excess air) and water vapor.

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