With the development of industry and the application of steam, boilers appear in almost every industry. At the same time, due to the country's environmental protection requirements, coal-fired boilers have gradually withdrawn from the stage of boiler history, and natural gas boilers have become the most used steam generation equipment. For gas-fired boilers, although its design is compared with traditional coal-fired boilers, it already has a relatively high thermal efficiency. We can still use other measures to further improve its thermal efficiency. The author will briefly talk about the waste heat recovery of flue gas from natural gas boilers.
The heat released after the combustion of natural gas is used for boiler evaporation. Due to the design of the boiler itself, the heat in the flue gas cannot be fully used, and the flue gas containing high-value heat is directly discharged into the atmosphere, causing a lot of waste.
According to the way of exothermic heat in exhaust smoke, we can divide it into two categories: physical sensible heat and vaporization latent heat.
Physical sensible heat：
It is achieved by reducing the flue gas temperature, and the boiler flue gas temperature drops by 15-20℃, which can increase the thermal efficiency by 1%;
Vaporization latent heat：
It is realized by the phase change of the water vapor in the flue gas that condenses into water, and the latent heat of the flue gas is recovered, and a large amount of heat can be recycled.
If we can recover all the physical sensible heat and vaporization latent heat in the flue gas, it will bring huge energy savings and reduce fuel costs.
Let us understand the simple potential analysis as follows：
1、Horizontal natural gas boiler
(1) Evaporation capacity=4000t/h, working pressure=10barg, cold make-up water temperature=10℃
(2) The original exhaust gas temperature=240℃, the exhaust gas temperature after installing the economizer=60℃
(3) The thermal efficiency of the original boiler = 88%, and the thermal efficiency after installing the economizer = 97%
(4) Natural gas low calorific value=36360Kg/Nm3, natural gas unit price=3.8RMB/Nm3
(5) One year working hours = 2000h (5 days a week, 8h per day)
2、Original cost
(1) Natural gas consumption = 4000x(2781.7-10x4.187)/36360/0.88=342.5Nm3/h
(2) Natural gas cost=342.5x3.8=1300RMB/h
Current cost
(1) Natural gas consumption = 4000x(2781.7-10x4.187)/36360/0.97=310.7Nm3/h
(2) Natural gas cost=310.7x3.8=1181RMB/h
Annual recovery cost
Annual recovery cost = (1300-1181) x 2000 = 240,000RMB
From the simple example above, we can find that only through simple flue gas recycling, the fuel cost can be saved. In the boiler room, the simple way is to directly use the heat to heat the boiler feed water, so as to reduce the consumption of natural gas.
The following is a reference for some successful cases：
1. Boiler flue gas recovery in Tianjin branch of a top 500 daily chemical company
Project Description：
With two 8t/h boilers (one used and one standby), the original economizer was corroded and the heat exchange efficiency was low.
Before transformation：
Boiler exhaust gas 190℃ (the original economizer outlet 120℃)
After transformation：
Design system exhaust gas <60℃ (actual operation exhaust smoke 47℃), and the water temperature is heated from 8℃ to 98℃
2. Boiler flue gas recovery of a feed company
Project Description：
1 t/h boiler 1 flue gas recovery design, natural gas operating pressure is 5~7barg
Before transformation：
The exhaust gas temperature of the boiler is 230℃, no economizer, and the flue gas is directly discharged to the atmosphere through the chimney
After transformation：
Use flue gas to heat the cold water supply of the water tank, cyclic heating, exhaust gas temperature 120℃, water inlet 30℃ and water outlet 55℃
3. A household paper waste heat recycling project
Project Description：
The temperature of the humid air discharged from the production equipment is 320℃, the flow rate is 7.61m3/s, and the mass water content is 30.5%
Before transformation：
The original process recovers part of the heat in the waste hot air, that is, the hot and humid air is fed into the air through the primary heat exchanger heating system, and the final discharge temperature after heating the air into the air is 264°C.
After transformation：
Produce 15barg saturated steam 860kg/h, and increase the combustion air intake temperature by 10℃ on the original basis, and the final exhaust gas temperature will be 135℃-145℃.
In summary, if you start to use natural gas boilers, please do not ignore the potential energy-saving opportunities around you. Please feel free to contact your YUJI application engineer for more details!
