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Viewing Fuel Flow in a Fluidized Bed

Thursday, July 15, 2021 | Derek Stuart
Categories : Industry

Fluidized bed boilers are used in many applications where fuels are difficult to burn. In these boilers, fuel particles are suspended in a hot, bubbling fluidized bed of ash and other particulate materials (sand, limestone etc.) through which jets of air are blown to provide the oxygen required for combustion or gasification. The resultant fast and intimate mixing of gas and solids promotes rapid heat transfer and chemical reactions within the bed. FBC plants are capable of burning a variety of low-grade solid fuels, including most types of coal, coal waste and woody biomass, at high efficiency and without the necessity for expensive fuel preparation (1).

Fluidized Bed Blog

An important factor for the boiler efficiency is the condition of the steam tubes in the heat exchanger. Fouling with ash and slag will insulate the tubes from the hot combustion gases and reduce the effectiveness of heat transfer.

Many types of biomass have low ash fusion temperature - the temperature at which the fly ash softens and becomes sticky -  increasing the risks of fouling the heat transfer surfaces. Wet wood, straw and palm waste are among the more difficult fuels in this respect.

Valuable insights into the fuel flow can be gained by viewing the surface of the bed. Unfortunately, smoke and flames make it difficult for an ordinary camera to produce a clear image of the fluidized bed.

Fluidized Bed BlogAMETEK Land’s MWIR-Borescope-640 imager operates at a wavelength of 3.9 µm. Typical furnace gases are transparent at this wavelength, so the imager can see through flames which would otherwise obscure the view. The long wavelength makes it much less sensitive to dust particles than a camera using visible light. The MWIR-B-640 uses borescope optics to view a large part of the furnace with only a small penetration through the boiler wall. The borescope configuration also allows us to keep the sensitive electronics outside the hot and dirty process environment. An air purge and water cooling protect the delicate objective lens from the hot and corrosive process environment.

Fluidized Bed Blog

The images below show the inside of a fluidized bed boiler that burns waste biomass. In this case, the fuel is partially oxidized and gasified in the bed, and the resulting gases are fully burned in the upper part of the boiler. The images clearly show the fluidized bed, the heat transfer surfaces and the supplementary burners. They allow the operator to assess the condition of the heat transfer surfaces and to extend the time the boiler can operate before it is taken offline to clean the heat transfer surfaces.

In conclusion, we have seen how a borescope imager operating in the mid-infrared region at 3.9 µm can give us valuable insights into the operation of a fluidized bed boiler. The imager was able to view a large portion of each boiler interior with very little interference from smoke and flames. It could see the fuel flow and measure temperatures in different parts of the furnace. And all this was done with minimal interference to the boiler’s operation.



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