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What are the four types of pulverized coal plants?
- Wet bottom (slagging T>Tmelt ash)
- Dry bottom ( >85% fly ash)
- Front Fired
- Tangental Fired
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Give the steps in pulverized coal firing.
- Coal -> Crushed -> Screen -> Pulverized -> Burner -> Ash
- 0.3-0.4m <10mm <74micron air^
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Rank fuels on volumetric heat release rate (High to Low)
Gasoline, Oil, Anthracite, Bituminous, Lignite, Biomass
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Rank fuels on capacity required (high to low)
Lignite, High VOC Bituminous, Medium Bituminous
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Rank fuels on Height to pipes distance.
Anthracite, Bituminous, Oil
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What effect does capacity and temperature have on grate heat rate?
As both increase heat rate increases.
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How do you convert to steam boiler capacity?
t/h(steam) = 0.75 MWth heat input
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Give three characteristics of Fluidized Bed Boilers
- 1) Fuel burns in fluidized suspension of inert particles.
- 2) Combustion zone is kept low by extracting heat directly from combustion zone.
- 3) Exceptionally high degree of heating.
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What is the difference between FB and other boilers?
Difference lies in how the gas and solids move around in the furnace.
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What are the main advantages of FB boilers?
- 1) Low emission of SO2 and NOx
- 2) Very fuel flexible
- 3) Easy to operate and stable
- 4) Lower maintenance
- 5) Cost less than PC for lignite fuels
- 6) Cost less than PC if FGD and SCR is required?
**Note FB has a higher N20 (GHG) emssion than PC.
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What are the two main types of FB?
- 1) Bubbling - Low Speed (1-2m/s), 10mm dp
- 2) Circulating - High speed (4.5-5.5m/s), 6mm dp
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What kind of fuel can FB IDEALLY burn?
FB can burn any fuel that has energy sufficient to raise itself and combustion gas above its ignition temperature.
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Why is FB insensitive to quality of fuel (2)?
- 1) Large thermal inertia allows most fuels to be heated to ignition without losing too much heat from the boiler.
- 2) Mixing makes it possible to dissipate the released heat and fresh fuel particles around the bed for easy ignition.
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Why is FB low acid rain emissions (2)?
- 1) Low combustion temp allows limestone to capture Sulfur in combustion zone.
- 2) Low temp and stagged air does not allow nitrogen to form NOx.
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Rank fuels on volumetric heat release rate (High to Low)
Gasoline, Oil, Anthracite, Bituminous, Lignite, Biomass
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Rank fuels on capacity required (high to low)
Lignite, High VOC Bituminous, Medium Bituminous
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Rank fuels on Height to pipes distance.
Anthracite, Bituminous, Oil
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What effect does capacity and temperature have on grate heat rate?
As both increase heat rate increases.
-
How do you convert to steam boiler capacity?
t/h(steam) = 0.75 MWth heat input
-
Give three characteristics of Fluidized Bed Boilers
- 1) Fuel burns in fluidized suspension of inert particles.
- 2) Combustion zone is kept low by extracting heat directly from combustion zone.
- 3) Exceptionally high degree of heating.
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How does circulating fluidized bed boiler work (2)?
- 1) CFD Loop of solids
- 2) Convection loop of gas
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Give the steps to operate a FB boiler (4).
- 1) Preheat bed solids
- 2) Let T increase until > Tign
- 3) Extract heat
- Water tubes in bed
- Regulating air passing through bed
- 4) Ash either stays in bed or entrained as fly ash.
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Give 3 advantages to keeping FB temperature low.
- 1) Avoid melting or agglomerating bed solids
- 2) Reduction of NOx
- 3) Allows for Sulfur removal.
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What are the FB rules of thumb (6)?
- 1) A = Vgas/Uf
- 2) Hb = 0.4 - 1.5m (Hb = 0.7Uf - Gives residence time of 0.7s)
- 3) Dp ~ 0.75mm
- 4) Uf = 2.5Umf
- 5) Qgrate = 3.3*Uf|300k/excess air (1.2 = 20% excess) (Mw/m2)
- 6) About 2.908 MJ/kg heat is generated when 1kg of dry air is consumed.
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