+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. the flue gas cools and the sodium condenses. alkali compound (Na2O + K2O) sublimes at ash fusion temperature about 1.Additional information in Ash Agglomeration Forming Mechanism in CFB: In general. particle surfaces are “conditioned” for bonding and the resulting deposit forms rapidly fouling the HRA. Sodium is frequently present in the active form making it the primary catalyst bonding (fluxing) agent. . and loop seal. and increase the furnace exit gas temperature (FEGT). At actual CFB furnace operating condition. In addition. However. unit load reductions. when low-rank coal burns inside the furnace. Alkalies which vaporize during combustion are often classified as Active Alkalies and they are free to react or condense subsequently in the boiler. 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. around 880 degC for Sodium and 760 degC for Potassium. these affects the boiler efficiency and steam temperature control and leads to higher heat rate.270 degC. 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. during CFB furnace calcination process Sodium is released adding more problems to the boiler. blocking the bed inventory circulation system or ash removal system. high Alkali content of fuel tend to lower the ash melting temperature that leads to volatilization of Alkali compound. 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. reduced the fluidity of the bed and ash. while on the other hand. Thus. limestone contains sodium chloride (inorganic form of Active Sodium) in in low to high quantity. 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. Over-all. and hot loop. Post EPC: • With boiler design in place and fixed. hence. Potassium usually in stable form of silicates which do not breakdown when heated making the Potassium less active. 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. While the bed and ash are circulating within the boiler. starts to pressurize the furnace. cyclones. and low plant reliability due to forced shutdowns. - Stripper cooler and Intrex nozzle resizing. - Superheater tube leaks due to high temperature operation. • Supplied by Foster Wheeler (FW). • Fuel Delivery System: Pet-Coke. - Supplied by Foster Wheeler. • 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. and Limestone. - Superheater tube bundle redesign and replacement. - Ash Build-up in the furnace. - Extended outages due to cyclone plugging. - Chronic problem of bottom ash removal system. Ash Avalanche in primary SH Agglomerated Ash Collected in Intrex . JEA went to capital improvement and modifications with little success including: - Furnace division wall removal. resulting in unit load reduction and unit trips.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. Coal. - Excessive operating temperatures. 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. - Furnace grid nozzle replacement. to determine the mechanism of ash agglomeration in FW CFB’s. a kaolin-based product for 30 days. unit derating. The intermediate compound then reacts with Alkalies to form Alkali Alumino-Silicates.• 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. • NGS looked for suitable inert additives that would disrupt the bonding chemical reaction. • Result: - NGS CFB Boilers have not experience any significant ash buildup related incidents such as cyclone plugs. and forced shutdowns since the use of additive began in June 2008. DMCI Plant Manager stated before that they use fuel additive and improved the operation of the CFB boiler including heat rate and output. and reduce agglomeration. Oct 2012. - The loop seals and portions of the furnace grid that required up to 10 days of jackhammering to remove agglomerated ash are now cleaned with a vacuum truck in few hours. has not been cleaned in any form in almost 3 years. • JEA tapped Microbeam Technologies Inc. Other web references . improve ash chemistry. - Unit forced outage rate was improved from over 13% to 1% by 2011. - Convection tubes are found to be cleaned with drastically reduced scale buildup. Note: • To be confirmed. prior to 2008 used explosive blast cleaning every 3 – 6 months. - CFB’s are operating for 3 years without HRA cleaning and zero cyclone plugs. - The HRA or back-pass of the boiler. which exhibit much higher melting point temperatures and lack the stickiness of the alkali compounds at CFB temperatures. • Aurora fuel additive chemically captures the Alkalies before they form low-melting point compounds. • NGS tested Imerys Aurora fuel additive. - Key plant performance metrics improved with Aurora additive. Aurora forms an intermediate compound at boiler operating temperatures that is highly porous and reacts with high efficiency. Sources: AECOM Report Alstom Combustion Engineering (CE) B&W Steam 41sth edition BV Power Plant Handbook PowerMag: “Reducing Ash Agglomeration in JEA’s CFB Boilers”.
Report "Memo Additional Information on CFB Fouling Tendency.pdf"