FREQUENTLY ASKED QUESTIONS ABOUT FAC^TM SYSTEM

Ash cooling & Post-combustion

 Q: How high is the temperature of the bottom ash at the FAC^TM (Futong Ash Cooler) discharging point?

A: The temperature at the discharging point depends on many parameters: such as the bottom ash rate, the ash grain size distribution, the length and speed of the FAC^TM extractor etc. Furthermore, each FAC^TM system is designed according to specific requirements. Therefore, it is difficult to define a general value.

Nevertheless, it can be said that in most cases the average ash temperature at the discharging point of the FAC^TM extractor is in the range of 80 - 150 °C in normal, continuous operating conditions of boiler.

Q: Does the post-combustion effect in the ash damage the Futong Super Steel Belt?

A: No. First of all, for most ashes the post-combustion in the FAC^TM system takes place mainly during the downfall of the ash through the boiler throat and hopper (if any) onto Futong Super Steel Belt, where it encounters the heated air flowing up from the FAC^TM into the boiler. Once deposited on the belt, the ash starts to cool down quickly.

If, on the other hand, the unburned carbon content in the ash is high or the post-combustion effect is slow, the ash can also continue to burn during conveying on Futong Super Steel Belt. Even if the burning ash is very hot and sometimes flames may hit the belt, this does not cause any damage, because the belt is kept relatively cool however, thanks to the cooling air flowing underneath it. The stainless steel from which Futong Super Steel Belt is made up, can operate without any problems in these conditions.

Q: What about cooling downstream of the FAC^TM?

A: By including Post-cooling Conveyors (PCC^TM) in the FAC^TM system, an effective further cooling after the FAC^TM extractor can be obtained. Just as the FAC^TM extractor, the Post-cooling Conveyor (PCC^TM) is operated with a slightly negative pressure inside the casing, so cooling air is drafted into the casing from the ambient. This cooling air is subsequently sent back to the FAC^TM via a connecting duct.

Also if a pneumatic conveying system is used, further cooling takes place. As a result, at the final discharge point of the FAC^TM system the ash temperature is generally close to  the temperature 80 - 150 °C.

Q: What is the actual temperature of the casing of the FAC^TM extractor?

A: The ambient air flowing into the FAC^TM extractor, not only cools the ash but also keeps the extractor’s casing at a low temperature. During normal operation, any part of the casing that is accessible to personnel, is at the temperature under 60°C. Any parts of the casing that may be at a higher temperature, are made inaccessible or adequately shielded by protection covers.

Q: How effective is the cooling of large clinkers?

A: Large clinkers are difficult to cool with traditional technology: even if the outer surface gets cooled rapidly, the core usually remains hot. However, with Futong patented PCBD^TM  (Pre-crushing Bottom Doors) technology, the cooling of large clinkers is more easy. Futong patented PCBD^TM is much different to the conventional flap bottom doors. Futong’s PCBD^TM can pre-crush the big ash clinkers less than 350mm, So as to improve cooling effect of the bottom ash on the steel belt dramatically. Meanwhile, it can greatly reduce the wear and load of the crusher, and effectively avoid the backflow the bottom ash to the bottom of the extractor.

Q: Ash clinkers falling from boilers can be as big as 1 x 1 x 2 m. Will the FAC^TM extractor not fail if such an impact occurs?

A: No. With Futong patented PCBD^TM (Pre-crushing Bottom Doors) technology, the big ash clinkers will fall on the impact bars firstly. The operator can monitor the real-time situation on the impact bars through video cameras in the control room. Then the big ash clinkers will be pre-cooled and pre-crushed into the size lower than 300 mm by the operators through the operation of horizontal-moved pre-crushing bottom doors and then

transported by Futong Super Steel Belt extractor.

Q: Is it necessary to avoid direct falling of large clinkers onto the Futong Super Steel Belt? If so, how is it done?

A: No. By its special design, the large ash clinkers will drop the impact bars firstly. Moreover, the Futong Super Steel Belt has a fine characteristic to absorb huge impact loads. Furthermore, under the boiler throat the supporting idlers are closely spaced in order to spread the impact over a large area.

Nevertheless, if the fall of large ash clinkers is very frequent, it may be preferable to avoid the direct impact of such lumps onto the Futong Super Steel Belt. This can be obtained by an asymmetric design of the bottom ash hopper, whereby the FAC^TM extractor and the boiler throat axes are offset. In that case, large clinkers would first fall on the sloped hopper walls.

Q: How can large ash clinkers be handled by the FAC^TM system?

A: The FAC^TM extractor is designed to ensure that anything that passes through the boiler throat, also passes through the FAC^TM extractor. Normally the width of Futong Super Steel Belt is 1200 mm [4 ft] whereas the height of the casing above the belt is typically 1000 mm [3’3”], whereas the boiler throat opening width is usually not more than 900 mm [3 ft]. This means that any big ash clinker falling from the boiler can be conveyed through the FAC^TM extractor to the primary crusher. The outlet size from the crusher is sufficiently low to enable further ash processing in downstream equipment.

Cooling Air and Effects on Boiler Performance

Q: Does the FAC^TM need fans/blowers to introduce the cooling air?

A: No. The FAC^TM system is applied in balanced draft boilers and uses the negative pressure inside the boiler. The FAC^TM extractor is in direct communication with the combustion chamber, so a negative pressure is obtained inside the FAC^TM system also. This negative pressure provides the motive force for drafting cooling air from the ambient into the FAC^TM extractor through special air inlet valves on the casing.

Q: Is the cooling air flow to the FAC^TM system controlled?

A: Yes. Depending on design requirements, the amount of air flowing into the FAC^TM system can either be kept constant or regulated. If the cooling air quantity has to be kept constant, the air inlet valves are fixed and the quantity of cooling air is equal to the design value (i.e. the maximum quantity that may be required). If the cooling air inflow is regulated, a control system is applied with dampers on the main air inlet valves, which are regulated as a function of the required cooling capacity (which depends mainly on the ash rate and temperature). In this way, the quantity of air flowing into the FAC^TM system is minimized continuously.

Q: Will the combustion in the furnace not be disturbed by the cooling air flowing from the FAC^TM into the boiler?

A: No. It has been amply verified with tests, that the cooling air coming from the FAC^TM does not affect the formation of NOx, the flue gas composition or the unburned carbon content of the fly ash. Firstly, this can be explained by the simple fact that the amount of cooling air required in the FAC^TM system is very low: it does not exceed 1.5% of the total combustion air flow to the boiler. Furthermore, the cooling air gets heated while cooling the ash in the FAC^TM system, and once it flows up into the boiler, it has reached a temperature of 300-400 °C [570-750 °F].Therefore, it acts exactly as combustion air for the furnace.

Q: Does the rating of the ID fans have to be increased because of the cooling air introduced into the boiler through the FAC^TM?

A: No. Cooling air needed for the FAC^TM system is not additional air, but part of the main combustion air. So, the amount of combustion air to the burners can actually be decreased. For example, if 1 % of the combustion air is the cooling air needed for the FAC^TM system, air to the burners can be reduced to 99 % of the total combustion air.

Q: If a part of the combustion air flows into the boiler through the FAC^TM system, and thus a lower quantity of air will flow through the air heater, will there be any negative effect on the boiler efficiency due to an increase of the flue gas temperature at the air heater outlet?

A: This applies only to the case of retrofit of a FAC^TM system in a boiler without any air heater bypass for flue gas temperature control. For new boilers, any flue gas temperature increase due to the FAC^TM system can be avoided by adequate air heater design. In the case of retrofit of a FAC^TM system in a boiler equipped with air heater bypass control, the flow rate through the bypass can be appropriately adjusted to ensure that there is no increase in flue gas temperature.

Even in the case of a slight increase of flue gas temperature after retrofitting a FAC^TM system, there will be a net boiler efficiency increase with the FAC^TM system, because the decrease in efficiency due to a higher flue gas temperature is more than offset by the recovery of the larger part of the energy contained in the bottom ash that is normally completely lost with a wet bottom ash system. In all cases, the balance is in favor of the FAC^TM system. For a more detailed explanation, see the paper "DRY BOTTOM ASH REMOVAL - ASH COOLING VS. BOILER EFFICIENCY EFFECTS".

Maintenance & Operation 

Q: Which are the regular maintenance activities required on the FAC^TM system?

A: Thanks to its sturdy design, the FAC^TM system is an extremely user friendly and low-maintenance system. Since the belt speed is very low, wear is negligible, and the only regular maintenance activity is the lubrication of the idler’s bearings (typically once every 3 months), which can be done with the system running. Any extraordinary maintenance can include the replacement of a roller’s bearing in case of failure (however unlikely); also this can be done with the system in operation.

All other maintenance activities can be done during scheduled outages; these include replacement of wear parts of spill chain and crusher and downstream equipment, if any (e.g. hammer mill, pneumatic system, ash conditioner etc.).

Q: How is the FAC^TM system operated?

A: In general, it is recommended to operate the FAC^TM system in a continuous mode during normal operation, but also a discontinuous operation is possible. The best way to operate the system depends on the operating conditions (ash rate, properties etc.) and on the preferences of the Customer.

Each FAC^TM system’s logic operating & control system is customized according to the Customer’s specific requirements. With the exclusive FICS (Futong Integrated Control System) logic & control system, usually both a fully automatic operation and a manual operating mode is possible. In either case, the system’s and operator’s safety during operation is guaranteed by a series of safety controls.

Q: Can the Super Steel Belt be repaired or replaced with the boiler in operation? What is the time period for replacement?

A: The Futong Super Steel Belt with its updated unique mesh and pan arrangement is very sturdy and extremely dependable. The mesh is grossly overdesigned in order to ensure that even if one or more wires break, the belt can keep on operating till the next planned stoppage of the plant. In more than hundreds of power plants which the FAC^TM systems installed, till date there has not been a single instance of a sudden need for replacement of the belt or even a piece of the belt. From the 19 years long experience with FAC^TM systems in operation now, it follows that the average expected life of the Futong Super Steel Belt is at least 10 years, and Futong gives a warranty on the belt of 5 years.

So, any repair or replacement of the belt can be executed during planned outages. Replacement of the entire belt can be done in 6-8 hours, replacement of a single piece would need maximum about 4 hours.

Q: What about refractory spalling?

A: Refractory spalling due to thermal shocks by water splashing is a very common phenomenon in water impounded hoppers. In the FAC^TM system, on the other hand, the hopper is dry, with fixing anchors, protection mesh and lined with the adequate quality of refractory, so no spalling at all occurs.

General

Q: Can the FAC^TM extractor be inclined? If so, how is the backslide of ash on the inclined section avoided?

A: Most FAC^TM extractors that are installed on the ground have an inclined section, in order to reach the required height at the discharge point without the need for excavation. If there is a pit where the crusher and the pick-up point for the other downstream equipment can be installed, the FAC^TM extractor can be horizontal.

Backslide of ash on the inclined section of the Futong Super Steel Belt depends on ash properties (e.g. size distribution and natural repose angle) and operating characteristics (e.g., belt speed), but it occurs very rarely. In general, for inclinations up to 30 ° this risk is not present. If for higher inclinations a risk of backslide is expected, transversal cleats may be installed on the Futong Super Steel Belt.

Q: The hopper in the FAC^TM system can be supplied with or without PCBD^TM (Pre-crushing Bottom Doors). What is the difference in these two options?

A: The application of PCBD^TM (Pre-crushing Bottom Doors) depends on several operational, geometrical and economical factors. Principally, the PCBD^TM (Pre-crushing Bottom Doors) provide a redundancy of the system, because they make it possible to separate the FAC^TM system from the internal boiler atmosphere and store ash inside the hopper. In this way, it is possible to operate the FAC^TM system in a discontinuous mode as a default even if the ash production is very high.

After the storage interval is finished, the ash stored inside the hopper can be extracted and pre-crushed in a staggered way by opening the doors in a sequence, pair-by-pair. In this way, the tension on the belt during extraction is minimized and the cooling optimized.

If on the other hand, no PCBD^TM (Pre-crushing Bottom Doors) are present, the ash is stored directly on the Futong Super Steel Belt during stoppage of the FAC^TM extractor, which does not present any problem. The main implication is that after restarting the system, the whole volume of accumulated ash is extracted at once, which leads to a higher tension on the belt and a less effective cooling of the ash. If these factors may be critical, which is more likely for large boilers or boilers with a large bottom ash production, it may be preferable to operate the FAC^TM system normally in a continuous mode if no bottom doors are present.

On the other hand, without bottom doors and hopper the FAC^TM system requires much less space, which means that it can fit even under boilers with a very low clearance without the need for excavation. This can be a major advantage especially for small boilers and boilers with a low bottom ash production, where a FAC^TM system without bottom doors can be operated discontinuously just as well.

Q: What can be the reason for a Customer to retrofit an existing bottom ash system with a FAC^TM system?

A: In any case the overall reason is that the FAC^TM system contributes to an improved boiler performance and to reduced costs related to the bottom ash handling. Specific reasons include the following:

• The elimination of water for ash cooling & transportation, which means savings on water consumption, no production of contaminated waste water, no need for ponds or dewatering bins, no risk of ice formation etc.

• The recovery of energy from the bottom ash to the boiler, and the significant burn-out of unburned carbon in the bottom ash, leading to an increased boiler efficiency

• The possibility for the re-use and sales of the dry bottom ash to other industries, e.g. the cement industry, instead of having to pay for disposal of the ash

• The possibility to simplify the overall ash handling by the integration of the bottom ash handling system with the fly ash handling system, thanks to the bottom ash pulverizers in the FAC^TM system

• The higher dependability of the system, which can lead to an improved boiler availability

• Lower operating & maintenance costs, thanks to the absence of high-wear components, the low power consumption (no high-power pumps, motors or fans), the fully automatic operation etc.