The temperature profile at the discharge depends on many parameters; such as the ash rate, the ash grain size distribution, the length and speed of the MAC extractor etc. Therefore, each MAC system is designed according to specific temperature requirements of the downstream process. According to a normal design, the temperature of the ash at the primary crusher discharge is in the range 100-300° C , while the external temperature of the MAC casing is kept at 60° C.

Large lumps are difficult to cool: even if the outer surface gets cooled rapidly, the core usually remains hot. However, the crusher (after the MAC extractor discharge) reduces the size of any lump to less than 75 mm [3 inch], resulting in a quick cooling during the further processing downstream the primary crusher.

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

Yes. Depending on design requirements, the amount of air flowing into the MAC 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 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 MAC system is minimized continuously.

No. It has been amply verified with tests, that the cooling air coming from the MAC 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 MAC system is very low: normally, 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 MAC 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.

No. Cooling air needed for the MAC 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 MAC system , air to the burners can be reduced to 99 % of the total combustion air.

This applies only to the case of retrofit of a MAC 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 MAC system can be avoided by adequate air heater design. In the case of retrofit of a MAC 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 MAC system, there will be a net boiler efficiency increase with the MAC 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 favour of the MAC system. For a more detailed explanation, see the paper "DRY BOTTOM ASH REMOVAL - ASH COOLING VS. BOILER EFFICIENCY EFFECTS".

No. By its special design, the Superbelt has a fine characteristic to absorb huge impact loads. Furthermore, under the boiler throat the supporting rollers are closely spaced in order to spread the impact over a large area.

No, as is explained in the answer to the previous question. Nevertheless, if the fall of large ash lumps is very frequent, it may be preferable to avoid the direct impact of such lumps onto the Superbelt. This can be obtained by an asymmetric design of the bottom ash hopper, whereby the MAC extractor and the boiler throat axes are offset. In that case, large lumps would first fall on the sloped hopper walls.

The MAC extractor is designed to ensure that anything that passes through the boiler throat, also passes through the MAC extractor. The width of the Superbelt 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 about 1 meter [3,3 ft]. This means that any big ash lump falling from the boiler can be conveyed through the MAC extractor to the primary crusher. The outlet size from the crusher is sufficiently small to enable further ash processing in downstream equipment.

Thanks to its sturdy design, the MAC system is an extremely user-friendly and low-maintenance system compared to the wet conventional bottom ash handling systems. Since there is no friction between the belt and the ash handled, wear is negligible, and the only regular maintenance activity is the lubrication of the roller’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.).

The Magaldi Superbelt with its unique mesh and pan arrangement is very sturdy and extremely dependable compared to the wet conventional bottom ash handling systems. The mesh is based on a damage tolerant design 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 2,8 million hours of cumulative operation of all the MAC 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 20-year long experience with almost a hundred of MAC systems in operation now, it follows that the average expected life of the Superbelt is at least 10 years, and Magaldi 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 8-10 hours, replacement of a single piece would need about 4 hours.

Refractory spalling due to thermal shocks by water splashing is a very common phenomenon in water impounded hoppers. In the MAC system, on the other hand, the hopper is dry and lined with the adequate quality of refractory, so no spalling at all occurs.

Most MAC 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 MAC extractor can be horizontal.
Backslide of ash on the inclined section of the Superbelt 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 Superbelt.

The application of bottom doors depends on several operational, geometrical and economical factors. Principally, the bottom doors provide a redundancy of the system, because they make it possible to separate the MAC system from the internal boiler atmosphere and store ash inside the hopper. In this way, it is possible to operate the MAC 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 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 bottom doors are present, the ash is stored directly on the Superbelt during stoppage of the MAC 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 MAC system normally in a continuous mode if no bottom doors are present.
On the other hand, without bottom doors and hopper the MAC 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 MAC system without bottom doors can be operated discontinuously just as well.

In any case the overall reason is that the MAC 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 MAC 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.

There are many advantages in a direct comparison between a MAC and a SSC. The first is the obvious elimination in the MAC system of the water in the ash cooling and process, differently from SSC, which is requiring a certain quantity of water. Also to be considered is the complete elimination of water related equipment, like pumps, pipelines, filters, heat exchangers etc.
Reliability is also a main advantage of the MAC versus the SSC : the availability factor of the MAC is much higher than the SSC, due to the intrinsic MAC belt damage tolerant design, while the SSC chain could break without any warning.
Another advantage of the MAC is the recovery of energy which in a wet system is lost. Almost 90% of the energy which is lost in the water of a wet system can be recovered by the MAC, leading to an increase of the boiler efficiency.
Maintenance costs are also in favour of the MAC, if compared with an SSC, and, last but not the least, the dry ash obtained from the MAC system is a material that can be more easily sold (e.g. in the cement industry), other than the wet ash obtained from a SSC system.