and some other US coal ashes revealed
that solids content can be as high as 60
percent. The actual value of percent solids depends on the rheology of the mixture, which should be tested before the
final design. Slurry mixing is accomplished in vendor proprietary equipment, and then pumped to landfill.
Mass balances are prepared for different ranges of coal composition, different loads (100 percent, 75 percent,
50 percent, minimum percent boiler
MCR), and ranges of coal and limestone compositions.
SLURRY COMPOSITION &
To provide a more detailed analysis of
the mixer design, it is helpful to have water quality data, plant slurry composition
data, and particle size distribution data.
Information from a hydrocyclone supplier (or settling tank, or thickener) is useful
to determine the range of split between
components entering dewatering and
leaving as gypsum waste.
WET FGD, HEAT AND
PROCESS FLOW DIAGRAM
As shown in Figure 2, details to the
process design may be added as available. The basic process is to feed limestone to a silo and ball mill and produce a limestone slurry for feed to the
absorber. Recirculating absorber slurry
contacts the flue gas containing sulfur
dioxide in a counter current process.
The gypsum slurry product is processed in hydrocyclone. Hydrocyclone
bottoms are combined with a chloride purge stream, and sent to a mixer,
where the slurry stream is mixed with
flyash, and is then pumped to landfill.
The size of the mixer unit is set by
the volumetric flow rate of the slurry.
The final prooduct is calcium sulfate
dydroxide (gypsum). Using plant data
for SO2 removal required, provides the
amount of calcium sulfite (hemihy-
drate) and calcium sulfate (dihydrate).
The plant data stoichiometry (amount
of limestone used), provides the excess
calcium carbonate and limestone in
the waste. The suspended solids of the
gypsum slurry leaving the absorber,
after primary dewatering, are assumed
to be about 45 percent by weight. This
gypsum slurry is mixed with dry fly
ash from an ash silo (collected from an
electrostatic precipitator and / or fabric
filter). The final mixture of 50 percent
solids and 50 percent water is a typical
value. A recent test with PRB coal ash
1 Plant layout drawings, plot plan / elevation drawings of Wet FGD ash / gypsum areas, landfll area
Basic engineering data for plant, operating manuals, feld checked
piping & instrument diagrams / piping plans, process fow diagrams
(chiefy of the ash and FGD by-product handling systems), heat
and material balances.
3 Process data for loads: maximum, rated and minimum load point data
4 How does the load of the plant vary and plant load variation v. time?
Objective of the project is to safely dispose FGD by products and FGD
waste water or disposing fy ash, and any other requirements, such as
covering an impoundment.
6 Flyash silo capacity available data and the drawing of the fy ash silo(s); if it possible to locate the mixers under the silos (preferred)
7 Amount of buffer capacity (this impacts the real need for backup redundancies)
8 Combine the ash disposal and the FGD by-product disposal (without forced oxidation, no gypsum is formed)?
9 Does the plant sell some of its fy ash, e.g.to the cement industry?
10 What is the market for gypsum or other FGD byproducts?
Analysis of samples for the makeup water, waste water, economizer ash,
bottom ash, and fy ash? Particle size analysis of the dry materials are
12 What is the boiler heat input with the fuel for Units 4 and 5, at 110% 100%, 75%, 50%, and 25% loads.
13 Will there be future blends of different coals?
14 Are boiler data sheets available? Original and latest?
15 What is the split of fy ash / bottom ash?
16 What are the operating effciencies of the boiler, FGD and ash handling systems?
17 Is there an up to date single line diagram, and electrical load / motor list?
18 Identify the type of CCRs being considered: bottom ash? Economizer ash? Air preheater ash?
19 Distance to the ash disposal site and elevation difference between the plant and the ash disposal site.
Data Collection for Plant—Some Major Documents Required TABLE 1