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Viewing Fuel Flow in a Stoker-fired Boiler

Wednesday, August 4, 2021 | Derek Stuart
Categories : Industry
Stoker-fired boilers are used in many applications with thermal capacities below 50 MW. In these boilers, the fuel is fed by a stoker onto the grate, where it is ignited by heat from the refractory above. As the fuel travels down the grate, it continues to burn, with primary air coming from below. Once it reaches the end of the grate, the non-combustible residues fall into the ash pit. Meanwhile, the flue gases travel into a mixing chamber where the combustion process is completed, finally passing through the heat exchanger where they give up their heat to generate steam.

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.

Some fuels such as coal will feed readily and cause few problems. Others, such as many types of biomass, can be fibrous and sticky, making them hard to feed into the furnace. They often 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 fuel as it moves through the boiler. Unfortunately, smoke and flames make it difficult for an ordinary camera to produce a clear image of the grate.

AMETEK 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.

The image below shows the fuel feed in a 30 MW sloping grate boiler that burns straw. It has a 9 m by 5 m grate with fuel fed by four feed chutes. The issue here is that the straw does not feed evenly and causes blockages in the feed chute and on the grate. The fuel is feeding from the top right and moves down towards the entrance to the ash hopper at the bottom left. You can see the heat transfer surface at the top left, and we have a clear view of the process, which allows us to measure the temperature of the heat transfer surfaces and view the fuel flow through the furnace. 

Viewing Fuel Flow

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 stoker-fired, biomass-fueled 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 on the grate 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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