Power plant’s problem with soot deposit on heat exchanger
Boilers fired by fossil or biomass fuel in power plant always have problem caused by soot accumulation on tubes. After combustion of fuel, soot and ash in flue gas would deposit on the heating surface. The same problem also happens for waste heat boiler in paper mill, sugar mill, cement plant, metallurgy industry, etc. for WtE boiler, this problem is worse due to the solid derived fuel combustion produces higher ash content in flue gas. This fouling and slagging has lower heat transfer ratio and make boiler operate less efficiently. This result is indicated by higher flue gas temperature, lower boiler output, additional tube erosion and leakage, unplanned shutdown for soot cleaning.
Boiler soot cleaning methods
Over the past decades, power industry is fighting against soot buildup with boiler soot cleaning technology. Soot cleaning technology is utilized to manage the problem, but the theory behind the act of cleaning changes little. The methods include rapping system, manual shaking, explosion blasting, steam soot blowing, etc. New soot cleaning methods occur in following days. Water canon, which uses higher pressure water, clean the tube by washing. Sonic horn, an acoustic cleaning system, could remove soot by low frequency sonic wave.
Detonation cleaning system
In the past 8 years, a new technology was widely used in small biomass boiler, WtE boiler, waste heat boiler, etc. It is called detonation cleaning system. This new online cleaning method uses shock wave generated by the combustion of fuel gas mixed with air. The purging is done with the energy of sonic wave, air blast, detonation wave.
Characteristic of detonation cleaning system
Shock wave pulse is generated by the detonation of fuel gas mixed with air. The whole process is within 10 milliseconds. Combustion happens at 200-250m/s subsonic speed. Combustion pressure is up to 0.7Mpa. Only when fuel gas is mixed with air at certain ratio, the combustion could happen rapidly. Below is a table of fuel gas with flammable limits and standard combustion speed.
From the table, we can know that there are various fuel gases available. Due to wide flammable range, easy availability on market, cheap price, acetylene is chosen as fuel gas for detonation cleaning system.
Acetylene is unsaturated hydrocarbon. Under normal temperature, it is colorless gas. In standard state, it is slightly lower in density than air, spontaneous ignition temperature is 480℃. After mixing with air, spontaneous ignition temperature is 305℃. Mixture of 7-13% or 30% acetylene is most easily to burn. Flame propagation speed could reach 300m/s for mixture of 50% acetylene, combustion pressure could be 35Mpa. Chemical reaction equation is:
2C2H2 + 5O2 = 2H2O + 4CO2+ Q
From the equation, we can know that acetylene changes into carbon dioxide and water during combustion, and gives out heat. So acetylene is chose as the fuel gas for detonation cleaning system.
Working principle of detonation cleaning system
Shock wave, sonic wave and thermal cleaning effect generated by the combustion of acetylene could remove soot accumulated on the heating surface.
Decompressed acetylene gas goes through pipeline, solenoid valve, regulating valve, orifice plate and mixer, and to combustion chamber in the end. When acetylene gas proportion goes beyond the lower flammable limits, it is ignited and burns. Energy is released. After some interval (8seconds), combustion chamber is filled with gas, burning happens again. The cleaning process happens in a cycle like this. If the ignition fails, the fuel gas will flow out of chamber and get discharged together with flue gas.
Component and function of detonation cleaning system
Detonation cleaning system mainly consist of control part and mechanical part. Control part includes flow meter, control panel, ignitor, thermometer, cable, switch, etc. Mechanical part includes acetylene cylinder, pressure gauge, filter, mixer, damper, combustion chamber, pipeline, valve, etc.
Filter: it is installed on the acetylene pipeline to purify the fuel gas to avoid blocking in solenoid valve and mixer.
Combustion chamber: it keeps mixed air in turbulent status to achieve detonation.
Mixer: to mix fuel gas and air in proper proportion.
Flashback arrestor: it is installed at the outlet of acetylene cylinder for protection purpose.
Damper: to protect pressure gauge, flow meter, valve from instant high pressure airflow.
Installation of detonation cleaning system
Nozzle of soot cleaning system should keep a distance from heating surface of 300-350mm. Closer distance would cause smaller cleaning area, bigger distance would affect cleaning performance. In principle, the direction of shock wave from nozzle should be the same direction with flue gas, so the removed soot could be taken away by the flue gas.
Here is a case of detonation cleaning system installed for air preheater. After cleaning system is put into use, soot on heating surface is effectively removed, pressure drop at air preheater is reduced. Before overhaul, average pressure drop is 1.3Kpa. After installation, average pressure drop is down to 1.0Kpa. ID fan frequency is lower than before. Normally after overhual, the flue gas temperature would rise gradually, pressure drop at air preheater would increase. Before installation, temperature rise is 6-9℃ during 6 months. After installation, temperature rise is about 5℃ during 12 months. Under the same power load and season, flue gas from boiler with detonation system is 7-11℃ lower than that from boiler without detonation cleaning system.
As it is dry cleaning method, it could reduce the low temperature corrosion for air preheater. After half year operation, heating surface is found as clean as new during the minor overhaul. Flue gas temperature is stable around 154-165℃ after installation of detonation cleaning system, about 10℃ lower than before. Boiler efficiency is improved by 0.55%.
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