EGEE 101
Energy and the Environment

Fluidized Bed Utilities

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There are other approaches to the combustion of coal (not just pulverized coal combustion). In Pennsylvania, we have a coal mining legacy of abandoned culm/gob piles (a mixture of rock and rock containing coal that not of high enough quality for use — thus was rejected and piled at the mine site). One such man-made mountain is shown below. The term culm is used in the anthracite region of PA in the east, while the term gob pile is used in the bituminous regions that occupy the west of the state.

Photograph of a sizable hill of rejected rock and coal (culm/gob pile)
A man-made Culm/gob "mountain" from the waste from coal mining.
Source: Western PA Coalition for Abandoned Mine Reclamation

Watch the following 2:52 minute video "Circulating Fluidized Bed - CFB Boiler Process".

Click here for a transcript of the Circulating Fluidized Bed video.

Sumitomo SHI FW is advancing steam generation technology with its state of the art circulating fluidized bed technology. Due to its fuel flexible, low-temperature burning process, CFB technology can utilize all types of coals, lignites, petroleum cokes, and carbon-neutral fuels like biomass and recycled waste to produce clean and economical power generation. It offers a major advantage over the limitations of conventional combustion-- the ability to burn the widest range of fuels.

Conventional coal technology requires the fuel to be finely ground and dried before entering the furnace. These steps are not needed for the CFB. Instead, the fuel is coarsely crushed and dropped into fuel chutes, which lead to ports in the lower section of the CFB's furnace. Unlike conventional boilers that burn the fuel in a massive high-temperature flame, CFB technology utilizes circulating hot solids to cleanly and efficiently burn the fuel in a flameless combustion process.

Its low uniform combustion temperature minimizes the formation of nitrogen oxides and allows the injection of limestone to capture acid gases as the fuel burns giving this CFB the lowest furnace emissions. Since the fuel's ash doesn't melt, heat transfer surfaces stay clean, allowing the hot solids to conduct their heat efficiently throughout the entire boiler, while fouling and corrosion are minimized. The payoff-- low plant emissions, low maintenance, and high plant reliability.

To achieve very high combustion efficiency for all fuels, the solid particles in the furnace are collected by steam-cooled, solid separators which recycle most of them back to the furnace. Before re-entering the furnace, the particles pass through a high-performance INTREX heat exchanger where steam coils are submerged in a bubbling bed of hot solids to efficiently produce high temperature superheat steam.

To further reduce plant operating costs and emissions, our CFBs can utilize vertical tube supercritical steam technology to achieve the highest plant efficiency. And for full capture of greenhouse gas emissions, we offer Flexi-Burn CFB technology, which can produce a CO2 rich flue gas for beneficial use by the oil, agricultural, chemical, and construction sectors. circulating fluidized bed technology is an important part of the solution to meet the world's energy needs while conserving natural resources and preserving our environment. Visit us at shi-fw.com for more information on our exciting technology.

Credit: Sumitomo SHI FW

After the coal has been separated from the rock the coal can be combusted but we use a different approach for the combustion process. This video shows the process of coal combustion via a circulating fluidized bed, showing the heat exchange to produce high-temperature steam, the circulation process for the coal char, with SO2 capture, and electricity production. In some locations, the hot water from the turbine is also used by local municipalities (such as for prisons, etc.) When this occurs it is called co-generation.

Circulating Fluidized Bed combustion is another method of burning coal that uses existing technology to produce electricity with very low emissions of both Nitrogen Oxide (NOx) and Sulphur Oxide (SO2) both of which contribute to the acid deposition challenges. Note: the fuel need not be coal but we will use coal for this discussion since we are in a lesson about electricity generation from fossil fuels. Fuel flexibility is one of the reasons that this boiler configuration is attractive, but most PA fluidized beds tend to be in the anthracite region where there is abundant and FREE fuel: culm piles (or so-called "gob" piles in Western PA). Culm piles are the waste material left behind from the mining process (mostly rock but some coal too). Fluidized Bed combustion is a way to reprocess this waste. They burn a much larger particle size than pulverized coal combustion (mm in size or larger), at a much lower temperature, over a longer period of time. They also have an in situ (in place) mechanism to capture the SO2 that would otherwise contribute to acid deposition. So here is how it works:

These "man-made mountains" of reject coal actually contain a significant quantity of coal. By removing the culm pile, producing electricity, there are employment, taxation, and cleaning open-pit operations (more on that when coal mining is discussed) opportunities.

The various stages are shown in this interactive figure (click on an asterisk for more information). Start with coal and work your way through the diagram in a counterclockwise direction.

Click here for a text description of the figure above.

Diagram of an Atmospheric Circulating Fluidized Bed Boiler.

Coal and Limestone are fed into the combustion chamber. The combustion chamber has air intake feeds and a center partition. Ash waste exit the combustion chamber at the bottom and are disposed of. Products enter the two attached cyclones at the top and then head to the heat exchanger. From the heat exchanger, the products enter fabric filters. Fly ash heads to disposal and flue gas exits via the stack.

Steam is a byproduct of both the combustion chamber and the heat exchanger. This is used to power a steam turbine which powers a generator and produces electricity. Excess steam is returned to the heat exchanger or to the boiler as feedwater.

Additional information:

  • Coal: This can include cleaned reject coal from culm piles or other fuels (biomass, etc.)
  • Limestone: A sorbent (absorber) for the Sulphur dioxide
  • Cyclones: Separates particles from the fluid using centrifugal forces
  • Fabric Filters: Prevents the larger particles from escaping with the flue gases. It is similar to a vacuum cleaning filter bag but at much higher temperatures.
  • Disposal/Truck: Transports the waste to old, abandoned coal mines to fill in the open pits.
  • Steam Turbine: Spins quickly so the generator spins quickly (1,000’s of revolutions per minute)
  • Generator: Spins quickly so it can generate lots of electricity

Credit: DOE

Once the coal is cleaned to reject the associated rock, it is crushed and the relatively large pieces of coal are combusted in the presence of limestone. By suspending the particles with air flowing from underneath, enables the coal to be combusted slowly, the ash that forms can be rejected. The lime is formed from the added limestone (CaCO3), the heat causes calcination which is the formation of lime (CaO) + carbon dioxide gas (CO2). This combustion approach gives the coal a long residence time in the bed, allowing lower temperature combustion (thus less NOx), and the S is not emitted into the atmosphere because it is captured in situ (in place) by the lime (the gas forms a sold coating on the lime particle).

CaO + SO2 + 1/2 O2 --> CaSO4 (gypsum)

The lime/gypsum is a solid that can be used to fill in the old strip mining holes to restore the damage from old abandoned mines. Unlike the output from the scrubbers - described later, this gypsum is not pure enough to be used as a wallboard material (it is just a coating over the lime particle). More on this when we discuss acid deposition solutions in unit 3.