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ADDITIONAL INFORMATION ON CFB FOULING TENDENCY

To : John Quirke

Cc : Archie Go

From : Elmer Garcia Perello

Date : August 5, 2015

Re : Fouling Issue on CFB Boilers

Information Review Action Approval

This memo is additional information from the report submitted by Henry Wong of AECOM titled

Boiler Type Selection for Semirara Coal dated June 5th

of 2015 for internal use only.

It is clear and accepted that sodium (Na2O) content of coal more than 5% has a significant effect

on boiler heat recovery area (HRA) or also known as convection back-pass if the boiler unit is not

properly designed affecting boiler efficiency, steam temperature control, and reliability.

Quote From page 7 of 35:

A circulating fluidized bed boiler would not necessarily immune to HRA fouling

For the same amount of alkalis a CFB may be more problematic. Because of the fact that there is

a constant recirculation of bed material and ash from the hot cyclone back to the furnace the bed

alkalis tend to concentrate. This is exacerbated by the fact that the Semirara coal is low in sulfur

and a lesser amount of fresh limestone is needed to be injected per pound of coal. Therefore, to

keep the alkali % low inert material such as PC ash would need to be fed into the CFB to flush the

bed of alkalis. Increased inert material reduces the fouling potential by keeping the alkali % in the

bed from building up.

Unquote:

No boiler is immune to inherent quality of coal ash that will result either to low fouling or severe

fouling tendencies, there will always be a gas-side localized fouling and, worst case, plugging the

entire superheater tube bundle blocking the flue gas as flow.

Having the CFB unit with same amount of fuel specification and mass flow with same 3% - 5% or

greater Alkali content, same boiler design for both furnace and HRA as with the PC units, CFB

units will have fouling problem more than that of the PC units. This is primarily due to ash

agglomeration forming at CFB operating temperatures.

Additional information in Ash Agglomeration Forming Mechanism in CFB:

In general, when low-rank coal burns inside the furnace, alkali compound (Na2O + K2O) sublimes

at ash fusion temperature about 1,270 degC. Alkalies which vaporize during combustion are

often classified as Active Alkalies and they are free to react or condense subsequently in the

boiler. However, Potassium usually in stable form of silicates which do not breakdown when

heated making the Potassium less active, while on the other hand, Sodium is frequently present

in the active form making it the primary catalyst bonding (fluxing) agent.

At actual CFB furnace operating condition, high Alkali content of fuel tend to lower the ash

melting temperature that leads to volatilization of Alkali compound, around 880 degC for Sodium

and 760 degC for Potassium. While the bed and ash are circulating within the boiler, Alkali

compounds acted as sticky glue and start fusing the ash to the lower metal temperature of

furnace tubes then builds up the particles to a larger mass of agglomerated ash formation in a

short operating span. This resulted to slag formation in the furnace, cyclones, and hot loop, and

loop seal. The inherent cleaning ability of fluidized bed has no match to the sticky ash

agglomeration rapidly building up within the boiler.

This phenomenon disrupts the CFB operating condition, blocking the bed inventory circulation

system or ash removal system, reduced the fluidity of the bed and ash, starts to pressurize the

furnace, and increase the furnace exit gas temperature (FEGT).

Once there is an increase of FEGT, a widespread distribution of sodium vapor in the HRA and

combustion gases assures contact with ash particles of about less than 100 microns, then pass

through superheater and reheater metal surfaces, the flue gas cools and the sodium condenses.

Thus, particle surfaces are conditioned for bonding and the resulting deposit forms rapidly

fouling the HRA.

In addition, limestone contains sodium chloride (inorganic form of Active Sodium) in in low to

high quantity, hence, during CFB furnace calcination process Sodium is released adding more

problems to the boiler.

Over-all, these affects the boiler efficiency and steam temperature control and leads to higher

heat rate, unit load reductions, and low plant reliability due to forced shutdowns.

Counter-Ash Agglomeration:

During EPC:

Design the CFB boiler considering fuel and limestone with high Alkali content to sustain steady

operation before planned shutdown maintenance.

Post EPC:

With boiler design in place and fixed, introduce an inert material to be fed with the bed

material to counter or dilute the concentrated Alkalies and disrupt the bonding or catalytic

reaction between ash particles and reduce ash agglomeration.

Reference Experience:

Jacksonville Electric Authority (JEA) Northside Generating Station (NGS) 2 x 300 MW CFB Boiler

built in 2002 and the biggest CFB boilers at that time.

Supplied by Foster Wheeler (FW).

Fuel Delivery System: Pet-Coke, Coal, and Limestone.

Riddled with problems for the first 6 years of operation due to combustion quality and ash

agglomeration. Problems include:

- Frequent Unit Shutdowns due to ash build-up in the superheaters.

- Chronic problem of bottom ash removal system, resulting in unit load reduction and unit

trips.

- Extended outages due to cyclone plugging.

- Ash Build-up in the furnace.

- Excessive operating temperatures.

- Superheater tube leaks due to high temperature operation.

- Supplied by Foster Wheeler.

JEA went to capital improvement and modifications with little success including:

- Furnace division wall removal.

- Superheater tube bundle redesign and replacement.

- Furnace grid nozzle replacement.

- Stripper cooler and Intrex nozzle resizing.

Problem still exist when Intrex final superheater and back-pass reheater assessment had a

significant ash buildups in only 2 3 months of operation after shutdown maintenance.

Ash Avalanche in primary SH Agglomerated Ash Collected in Intrex

JEA turned their direction into Six-Sigma based process optimization method and found the

root cause is due to rapid ash agglomeration due to high levels of Alkalies.

JEA tapped Microbeam Technologies Inc. to determine the mechanism of ash agglomeration in

FW CFBs.

NGS looked for suitable inert additives that would disrupt the bonding chemical reaction,

improve ash chemistry, and reduce agglomeration.

NGS tested Imerys Aurora fuel additive, a kaolin-based product for 30 days.

Aurora fuel additive chemically captures the Alkalies before they form low-melting point

compounds. Aurora forms an intermediate compound at boiler operating temperatures that is

highly porous and reacts with high efficiency. The intermediate compound then reacts with

Alkalies to form Alkali Alumino-Silicates, which exhibit much higher melting point

temperatures and lack the stickiness of the alkali compounds at CFB temperatures.

Result:

- NGS CFB Boilers have not experience any significant ash buildup related incidents such as

cyclone plugs, unit derating, and forced shutdowns since the use of additive began in June

2008.

- The HRA or back-pass of the boiler, prior to 2008 used explosive blast cleaning every 3 6

months, has not been cleaned in any form in almost 3 years.

- Unit forced outage rate was improved from over 13% to 1% by 2011.

- Key plant performance metrics improved with Aurora additive.

- The loop seals and portions of the furnace grid that required up to 10 days of jack-

hammering to remove agglomerated ash are now cleaned with a vacuum truck in few

hours.

- Convection tubes are found to be cleaned with drastically reduced scale buildup.

- CFBs are operating for 3 years without HRA cleaning and zero cyclone plugs.

Note:

To be confirmed, DMCI Plant Manager stated before that they use fuel additive and improved

the operation of the CFB boiler including heat rate and output.

Sources:

AECOM Report

Alstom Combustion Engineering (CE)

B&W Steam 41sth edition

BV Power Plant Handbook

PowerMag: Reducing Ash Agglomeration in JEAs CFB Boilers, Oct 2012.

Other web references