1

Dry Bottom Ash Technology Improves Coal

Fired Boiler Operations

Authors:

Vincenzo Quattrucci

Romolo Carrieri

Magaldi Power S.p.A.

Juan Antonio Herrera Giraldez

Endesa Generaciòn S.A.

Correspondence address:

Magaldi Power S.p.A.

Via Irno 219

84100 Salerno, Italy

tel. +39-089-688111

romolo.carrieri@magaldi.com

Abstract

Environmental regulations demand ever greater efforts from energy utilities to reduce the polluting

load from power generation. Till now, many efforts have been dedicated to reduce particularly gaseous

emissions. However, liquid effluents as well are under increasing scrutiny to be controlled and possibly

reduced, especially in areas where natural water resources are limited or environmentally vulnerable.

In the same time, the privatised energy markets are demanding ever higher cost-effectiveness in power

generation. In order to conciliate these often conflicting interests, truly innovative technologies are

necessary.

In the Spanish coal fired power plant of Los Barrios, owned and operated by Endesa Generaciòn, an

innovative dry technology has been retrofitted reducing strongly the operating and maintenance costs

for the bottom ash and pyrites handling, and in the same time providing a reduction of the

environmental impact of the plant. In particular, the use of water in bottom ash handling and the

production of waste water has been eliminated completely, and due to several effects the boiler’s heat

losses have been reduced, leading to a saving of coal consumption and a reduction of gaseous

emissions including CO2. Additional benefits of the project include a higher dependability of the

system, preventing forced boiler outages, and a better saleability of the bottom ash.

In this paper, an analysis of the performance of the power plant’s previous wet bottom ash and pyrites

handling is provided, leading to the reasons for replacing it by the dry system. Initial operating results

of the new dry system are also presented to evaluate the improvements reached.
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Introduction

ENDESA is one of the largest private electrical groups in the world, with a total installed capacity of

42,000 MW, 133,600 GWh distributed capacity and more than 20.5 million customers in 12 countries.

It is the principal electricity company in Spain, Chile, Argentina, Colombia and Peru and has strong

presence in Brazil, Italy, France, Portugal and the Dominican Republic. Endesa’s headquarters are in

Madrid, Spain.

Endesa Generaciòn’s Los Barrios Coal Fired Power Plant is located in the Algeciras Bay, close to

Gibraltar’s Rock in a beautiful landscape between Spain, Africa and the Atlantic Ocean. The power

plant was delivered in 1985 and comprises one sub-critical forced circulation boiler rated at 1831

MTPH of steam @ 168 bar / 540 °C for a net electricity production of 552 MWe. The coal is imported

mostly from South Africa and Colombia. The unit is equipped with electrostatic precipitators and a

desulphurisation system for flue gas cleaning.

For the bottom ash and pyrites handling, the unit was originally equipped with a wet sluice system. In

order to reduce its environmental impact, the system used a ‘closed loop’ for water treatment and

recirculation. This way of operation, however, led to high operating & maintenance costs due to the

high power consumption, the need for manual operation and the rapid wear and corrosion of the

equipment. Furthermore, the wet process resulted in increased boiler heat losses, a relatively poor

saleability of the ash and at one occasion even to a forced boiler outage due to system failure.

In a recent project carried out by Magaldi Power S.p.A., the wet bottom ash and pyrites handling

system in Los Barrios #1 was replaced by a dry MAC (Magaldi Ash Cooler) system. Magaldi Power is

part of the Magaldi Group, a leading bulk materials handling company based in Salerno, Italy. The

patented MAC technology was developed by Magaldi as an alternative for the conventional wet bottom

ash handling systems. Since its first application at the end of the 1980s, the MAC system has been

installed in 64 coal-fired boilers worldwide, in boilers with capacities ranging from 20 to 700 MWe and

a total installed boiler capacity of 16,500 MWe.

In this paper, the retrofit project of the MAC dry bottom ash and pyrites handling system in Los Barrios

unit #1 is discussed, through the following steps:

- Description of the original wet system

- Decisional parameters for the dry retrofit project

- Description of the dry system

- Discussion of the results obtained by the project
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The Original Wet Bottom Ash and Pyrites Handling System

Sluice system description

The originally installed bottom ash and pyrites handling system used ejectors, fed by high-pressure

water to sluice, the bottom ash and the pyrites from the water-impounded hopper and the pyrites tanks,

respectively, to the downstream system for separation

of ash from the water, and the treatment of the water

for recirculation into the sluice system.

The bottom ash was quenched and collected in the

water impounded hopper connected to the boiler

throat. An hydraulic seal trough was provided to allow

for boiler expansion. The lower part of the hopper was

divided into two separate outlets. From each hopper

outlet, the ash was discharged periodically through a

crusher into the hydraulic conveying line. The

conveying line was fed with high-pressure water

through ejectors to sluice the ash to the decanting silo.

In the decanting silo, most of the water was drained

from the ash, which was unloaded from the silo into

trucks. The drained water was discharged into the

settling ponds. The water coming from the settling

ponds was passed through a distributor tank where a

flocculation agent was added, before flowing into the

filter tanks. The filtered water was recirculated to the

jet pumps, whereas the filters were cleaned

periodically, producing a slurry that was pumped back

to the decanting silos.

Pyrites discharged from the 6 nos. coal mills were

accumulated in transition tanks (one for each mill),

from which they were sluiced to a pyrites transfer

tank. From the transfer tank, the material was pumped

into the decanting silo for the bottom ash.

Pumps for water recirculation

Settling ponds
4

The overflow water coming from the hopper, the pyrites system and the decanting silos was pumped to

the distributor tank where it joined the water from the ponds.

For redundancy, the whole sluice system was provided with two independent conveying lines, with a

design capacity of 40 t/h each. Normally, one line was in operation while the other one was in stand-by

to allow maintenance. Each line included the complete array of equipment, including piping, pumps,

ejectors, crushers, settling tank, settling pond and decanting silo.

A flow diagram of the original wet bottom ash and pyrites handling system is given in Figure 1.

Figure 1: Flow Diagram of the original wet bottom ash and pyrites handling system

Sluice system performance

In the following paragraphs, the system’s performance is evaluated according to the division into the

following aspects:

1. Workforce required for system operation

2. Maintenance activities

3. System availability

4. Power consumption

5. Water use and processing

6. Ash disposal / re-use
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7. Effects on boiler performance

8. Environmental and safety aspects

Workforce for system operation

The unloading of the hoppers required the supervision by a dedicated operator. The operation consisted

of starting and stopping of relevant equipment (high pressure water pumps, crushers, slide gates), and

the visual monitoring of the ash discharge. In order to remove all ash, it was often necessary to execute

several cycles of filling up and emptying of the hoppers. In case of jamming of the hoppers or crushers,

manual intervention through the poke-holes or access doors was required to remove the jammed ash.

When an access door was opened for this activity, the operator was directly exposed to the internal

boiler ambient, involving a significant safety hazard.

Maintenance Activities

All parts of the sluice system exposed to the water were subject to corrosion and wear, and ash carried

along with the re-circulated water caused accelerated wear and risk of plugging of piping and fittings in

the water re-circulation system. This lead to a frequent need for replacement of piping, pumps, fittings,

filters etc.

The boiler throat pressure parts and the part of the refractory not submerged in the water suffered from

water splashing, leading to accelerated corrosion and spalling due to thermal stress. Furthermore, the

lower part of the hopper had to be cleaned out frequently to remove the ash deposits that slowly grew

to obstruct the hopper and even the boiler throat.

In addition, the settling ponds required cleaning out approximately once every two years, involving

their emptying and removal of the accumulated sluice. All these activities involved a significant

number of manhours and materials.

System Availability

The permanent circulation of bottom ash caused the deterioration of the equipment constituting the

system. If a failure of the bottom ash system would cause the excessive accumulation of ash inside the

water impounded hopper, up to the point where the ash would start to invade the lower part of the

boiler, the shut-down of the boiler was required. In one such instance, a boiler outage was caused

which lasted 11 days.
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Power consumption

Due to the need to recirculate large quantities of water for cooling and conveying of ash and pyrites,

the sluice system had a large number of pumps in operation. Furthermore, crushers were needed to

reduce the ash size during every hopper unloading cycle. For the pumps alone, there was an installed

power of more than 1000 kWe. The effective annual power consumption for the bottom ash and pyrites

system was 1,136 MWh.

Water use and processing

The sluice system’s water use was huge: for each kg of ash produced, approximately 20 kg of water

were required to convey the ash to the decanting silos. Furthermore, the water impounded hoppers had

to be fed continuously with water in order to keep the water level constant, and the seal trough was

continuously flushed with water in order to avoid plugging with ash.

Although most of the water was recirculated inside the closed loop system, there was still a net

consumption of fresh water and a modest purge of waste water into the sea. The total waste water

production was reported to be app. 20,000 m

3

per year. The consumption of fresh water resulted from

the various water losses in the system, consisting of:

• evaporation of water from the hoppers into the boilers

• the water content in the ash discharged from the dewatering bins

• waste water losses

According to Endesa’s data, total water losses summed a 30,000 m

3

per year.

Although the effective costs of the fresh water consumption and the waste water production were

relatively low, the indirect costs related to the water use were high: firstly, the high operating &

maintenance costs (corrosion and wear, power consumption etc.), and furthermore the increased ash

disposal and transport costs due to the ash having app. 30% residual water content.

Ash and pyrites disposal / re-use

Due to the high unburned carbon content of the ash, the relatively coarse size distribution and the

contamination with pyrites, the saleability of the wet bottom ash was poor. In practice, the best method

was to deliver the mix of bottom ash and pyrites, at zero cost, to a local factory for deposit.
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Effects on boiler performance

In solid-fuel fired boilers, a significant quantity of energy crosses the boiler throat, in the form of

radiation from the furnace flames, the enthalpy (sensible heat) of the bottom ash and the chemical

energy contained in the unburned carbon in the bottom ash.

In a water impounded hopper system, this heat is completely lost: burn-out of unburned carbon in the

ash is stopped abruptly when the ash is quenched in the water bath, and the absorption of radiation and

sensible heat causes evaporation of water. Water vapour flowing into the boiler throat influences

negatively on the boiler’s heat balance. In order to limit the evaporation, the water bath temperature has

to be kept low by a continuous addition of water (which impacts on the water recirculation costs).

In addition, the discharge of pyrites is accompanied by a loss of coal. In the wet handling system, this

coal was dispersed into the sluicing water. The total estimated quantity of coal lost in this way was

1000 t/year.

The total heat losses due to the sluice system in Los Barrios #1 was thus the sum of the following

factors:

• Loss of radiation through boiler throat

• Loss of bottom ash enthalpy (sensible heat)

• Loss of bottom ash chemical energy (unburned carbon)

• Loss of coal powder with pyrites

It was estimated that these combined heat losses amounted to app. 6800 kWt in terms of boiler output.

Environmental and safety aspects

Safety issues were caused mainly by the manual operation while environmental problems were related

to the water required for the process.

A serious safety hazard was posed to the operators when they needed to work through an open access

door in order to remove jamming of ash in the hoppers or crushers. Another safety hazard was posed by

the presence of water and mud on the working floor due to leakages from the sluice system.

The direct environmental impact of the sluice system was mainly due to the discharge of residual waste

water into the sea. Other, indirect effects include the loss of heat from the boiler, causing increased fuel

consumption and gaseous pollutants emissions, including CO2.
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Decisional Parameters for the Dry System Retrofit

The deterioration of the equipment constituting the sluice system caused by the wear and corrosion, led

to ever growing operating and maintenance problems. The frequent problems with ash accumulation in

the hopper to a level that put the boiler operation at risk, culminated at one instance into the total

unavailability of the power station for 11 days. Furthermore, during all scheduled stops of the power

station, a complete cleaning out of the hopper had to be carried out to remove the deposits of ash on the

hopper walls. In some occasions in which the power station had stopped because of problems in

different equipment, the cleaning of the water impounded hopper caused a lengthening of the

unavailability time.

As these problems could only worsen in the future, a serious overhaul of the bottom ash handling

system was thus considered necessary. The options that Endesa had, were either to carry out a major

repair and maintenance project on the existing sluice system, or to replace it by a different system.

The repair project would have consisted of the substitution of the critical elements of the system,

including valves, ejectors, crushers, hydraulic seal etc. and in the realisation of adjustments and cleanups. This would require a relatively low capital investment, but would only reduce and not eliminate

the pre-existing problems. Furthermore, it would not deal with the growing pressure from

environmental regulation to minimise fresh water use and waste water effluents.

The choice for an alternative to the sluice system therefore pointed towards a dry system, eliminating

all water-related problems at the source. Specifically, the MAC system was considered as the only

viable alternative for application in Los Barrios, for the following reasons:

• The MAC system had a much better reference record than any other dry system. In particular, a

MAC system had already been installed in Endesa’s Almeria power plant in 1997, in a boiler

identical to the one in Los Barrios, providing Endesa with valuable experience

• The operating experience of the MAC system in Almeria had shown much lower operating and

maintenance needs than that of the sluice system in Los Barrios #1

• Additional benefits would be obtained with the MAC system, especially a reduction of boiler

heat losses due to the burn-out of unburned carbon in the ash, and the recovery of heat to the

boiler. This is an issue of ever growing importance due to the need to improve coal-fired power

stations’ efficiency in order to reduce emissions of gaseous pollutants, in particular CO2. With

the trading of CO2 likely to be implemented within the near future, any reduction of CO2-

emissions will mean an economical benefit.
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With the MAC system, it was also possible to minimise the capital investment for the retrofit by

transforming the water-impounded hopper into a dry hopper and the decanting silos into dry storage

silos, fitted for the MAC system.

Based on this knowledge, a detailed feasibility study was then made that convinced Endesa that the

best return on investment would be obtained by the removal of the existing sluice system and the

retrofit of a MAC system for the dry handling of both bottom ash and pyrites.

The MAC Dry Bottom Ash and Pyrites Handling System

At the end of 2002, Endesa and Magaldi signed the contract for the turn-key supply of a MAC dry

bottom ash and pyrites handling system for Los Barrios #1. Demolition and erection activities started in

May, 2003, and the retrofit project was completed in a record 35-days scheduled boiler outage. The

MAC system was put into operation at the beginning of July, 2003.

MAC system description

With the MAC system in Los Barrios # 1, the bottom ash and pyrites are extracted, cooled and

conveyed from the boiler to the storage silo in a completely dry way. In the attached Figure 2, the

MAC system is represented schematically. The system’s main components are described in the

following section.

Mechanical expansion joint

The mechanical expansion joint provides the sealed

connection between the boiler throat and the bottom

ash hopper, whilst allowing for boiler expansion. The

multi-layer joint is high-temperature resistant and is

installed with a special steel frame. The mechanical

expansion joint was preferred over the hydraulic seal

because of the following advantages:

• No use of water, so no corrosion or blockages

due to ash depositing

• Higher safety, because no risk of outward

water splashing during positive pressure excursions in the boiler

• Zero maintenance

Mechanical expansion joint
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Ash hopper with bottom doors

The water impounded hopper of the sluice system was turned into a dry ash hopper to fit in the MAC

system. To this purpose, the following modifications were carried out:

• The hydraulic seal trough was substituted by the

mechanical expansion joint

• New insulation panels and refractory lining were

installed

• The two conical outlets of the hopper were

restructured and provided with bottom doors

With the bottom doors, the hopper outlets can be

closed completely in order to store ash inside the

hopper. In this way, the MAC system can be operated

discontinuously. The hopper’s storage volume of app. 100 m

3

allows a MAC system stoppage time of

more than 8 hours with the boiler at full load. The bottom doors have refractory lined ash contact

surfaces and are actuated by hydraulically operated cylinders.

The MAC extractor

The MAC extractor is the system’s key component,

extracting and cooling the ash to a sufficiently low

temperature to allow the discharge into the

downstream processing equipment. The extractor has

a conveying capacity of 20 t/h of dry ash. The heart of

the MAC extractor is the Magaldi Superbelt, which

receives and extracts the ash falling from the hopper.

The Superbelt is a unique stainless steel belt conveyor

with a plate-and-mesh design that provides a great

mechanical strength with a free thermal expansion.

Because there is no relative motion between the belt

and the ash transported on it, the belt’s wear is

negligible. The Superbelt has a length of 42 m and an

Internal view of dry ash hopper

MAC extractor under hopper, seen from tail side
11

effective width of 1200 mm, and is enclosed inside a steel casing.

Outside the boiler hopper area, the MAC extractor is inclined at an angle of 30° and finally discharges

the ash into a crusher. The extractor’s dimensions allow the passage of any ash lump that can pass

through the boiler throat.

Under the Magaldi Superbelt, a Spill Chain is installed to remove any fines that may settle on the

extractor’s bottom. The Spill Chain consists of two lateral chains connected by scraper flights that

sweep the accumulated dust to the MAC head section, where it is discharged into the crusher.

Cooling and postcombustion

The bottom ash falling into the MAC system typically has a temperature of app. 800 °C. The ash is

cooled by a small, controlled amount of ambient air during the downfall through the hopper and during

conveying on the Magaldi Superbelt. At the Magaldi Superbelt discharge, the bottom ash temperature

is less than 100 °C during normal operation.

The cooling air is drafted into the casing through inlet valves due to the negative pressure in the boiler,

which is app. –70 mm w.g. at the hopper level. The quantity of the cooling air drafted into the MAC

system is less than 1 % of the total combustion air required for the boiler.

The cooling air picks up the heat from the ash and the belt, and thus gets heated up to app. 300 °C

before flowing into the boiler. The oxygen provided by the air allows a significant burn-out

(postcombustion) of unburned carbon in the ash to take place in the MAC extractor.

In total, the cooling air flowing from the MAC into the boiler recovers up to 90% of the total heat

passing through the boiler throat back to the furnace, reducing boiler heat losses.

Crusher

The 54” single-roller crusher receives the ash

discharged from the MAC extractor and reduces the

size of big ash lumps to a maximum outlet size of app.

80 mm. This size is appropriate for the further

handling by the downstream equipment. A second,

stand-by crusher is provided on a trolley to allow the

quick exchange in case of necessity.

Single-roller crusher in operation
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Bucket elevators and storage silos

The bottom ash discharged from the crusher is fed

into one of two bucket elevators through a two-way

diverter valve. With the low speed bucket elevators,

driven by chains, the ash is conveyed up into the

storage silos. There is one bucket elevator for each

silo, with one working and the other in stand-by. Each

elevator is provided with an electric drive.

The storage silos actually are the previous decanting

silos which have been modified to enable the storage

of dry ash. Each silo has a capacity of app. 400 m

3

.

The silos are equipped with an air cannon system,

blowing air into the lower conical part, to prevent

material clogging. The silos are also equipped with

level probes and a safety pressure valve.

A vibrating feeder and a mixer-unloader under each

silo allow a controlled and dust-free discharge of the

bottom ash into open trucks.

Dry pyrites handling system

The pyrites handling system has the function to collect

and convey the pyrites from the mill to a storage silo

in a completely dry way. The pyrites discharged from

the 6 nos. coal mills are collected in transfer tanks

(one for each mill). The tanks are equipped with

double knife gate valves at the inlet and outlet. A fire

detection and protection system has also been

provided. From the transfer tanks, the material is

extracted by vibrating feeders and for a controlled

feeding of the belt conveyor. The belt feeding rate is

adjustable in order to optimise the system

performance and wear. With the dry pyrites conveyor,

Bucket elevators and storage silos

Pyrites transfer tank
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the material is conveyed from the tanks to the downstream bucket elevator. The conveyor is a hightemperature resistant belt conveyor with a steel cover to prevent the release of coal dust into the

atmosphere. The bucket elevator delivers the material into the storage silo, from which it is finally

removed by trucks in order to be carried again to the coal yard, in this way recovering the coal powder

that was wasted with the previous system.

MAC system operation

Normally, the MAC system is operated in a continuous mode. The hopper’s bottom doors are open and

the ash falling down from the boiler is extracted and cooled continuously by the Magaldi Superbelt,

which runs at the normal operating speed. The ash discharged from the crusher is conveyed into one of

the silos by the bucket elevator. The other bucket elevator and silo are standby.

The system can also be operated discontinuously by closing the hopper bottom doors and storing the

ash inside the hopper. At full boiler load, a storage time of more than 8 hours can be reached. This

discontinuous operating mode can be used to carry out maintenance on the equipment, if required.

After restarting the MAC system, the stored ash is extracted gradually from the hopper by opening the

bottom doors sequentially (pair by pair).

In both the continuous and the discontinuous operating mode, the system can be operated fully

automatically, or manually if preferred.

In Table 1, some design and operating data of the MAC dry bottom ash handling system are

summarized:

Table 1: Design and operating data for MAC dry bottom ash handling system

Normal bottom ash production 5 t/h

Maximum bottom ash production 10 t/h

Bottom ash density 0.76 t/m

3

Bottom ash temperature in boiler throat 800 °C

Pressure in boiler throat −70 mm w.g.

MAC extractor design capacity 20 t/h

Superbelt speed 0.4−5 cm/s

Normal bottom ash temperature at Superbelt discharge < 100 °C

Maximum cooling air flow into MAC system 24 t/h
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Figures 2.a and 2.b present P&I Diagrams for the MAC system in Los Barrios.

Figure 2.a: Flow Diagram of MAC system until diverter valve

Figure 2.a: Flow Diagram of MAC system downstream diverter valve
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The pyrites system is operated discontinuously, either fully automatically or manually. At any time,

maximum 5 of the 6 coal mills are in operation. The gate valves at the inlets and outlets of the transfer

tanks are alternatively operated in order to separate the mill environment (at positive pressure) from the

downstream equipment (at atmospheric pressure). Pyrites are stored in the transfer tanks for up to 5

hours at normal rate (0.15 t/h). The transfer tanks are discharged consecutively, on the base of a high

level signal or according to a preset time. Although normally one tank is discharged at a time, the belt

conveyor and bucket elevator are designed to be able to handle the total quantity of material discharged

from all tanks at the same time.

In Table 2, some design and operating data of the pyrites handling system are summarized.

Table 2: Design and operating data for pyrites handling system

Normal pyrites production per mill 0.15 t/h

Maximum pyrites production per mill 0.75 t/h

Pyrites density 0.76 t/m

3

Pyrites maximum temperature 120 °C

Pyrites storage time 5 h

Pyrites conveyor design capacity 5 t/h

Transfer tank working volume 1 m

3

Tank Loading time 300 min

Tank Unloading time (including belt cleaning interval) 12 min

Tank Cycle duration 312 min
16

Figure 3 shows the P&I Diagram for Pyrites handling system.

Figure 3: Flow Diagram of Mill Rejects handling system

Results of the Retrofit Project

At present, the boiler has been in operation with the MAC system since July 2003. During the first 10

months of operation, the expectations that convinced Endesa to retrofit the wet bottom ash and pyrites

handling system, have been mostly confirmed or exceeded. Following are some of the main results that

have been observed so far by the power plant. For secrecy reasons some sensible data can not be

shown.

Operating & Maintenance costs

The operating costs of the MAC system are negligible compared to the previous sluice system, as the

system is in automatic operation, running autonomously without the need for supervision or

intervention by operators. Till now, no significant maintenance activities have been necessary, and the

expectations are according to the schedules.

Based on present data, the total annual operating & maintenance costs for the bottom ash and pyrites

handling are forecasted to be 78% lower than the previous system.
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Consumption of water

Apart from the negligible amount of water required for the mixer-unloaders below the silos, the water

consumption of the MAC system is zero. This means, that a total saving of 30,000 m

3

per year of fresh

water is obtained.

Treatment and discharge of waste water

No waste water is produced by the MAC system, which means that no water has to be treated anymore

for recirculation into the system, and neither is there any discharge of waste water into the sea. This

compares to the significant costs for chemical treatment of the recirculation water with the previous

sluice system, and the fees to be paid on the 20,000 m

3

per year of waste water discharged into the sea.

System availability

Until now, no forced boiler outages have been caused by the MAC system, completely in agreement

with the expectations based on the MAC system’s unchallenged dependability record. This compares to

the outage of 11 days that was caused by the previous system.

Power consumption of the system

Due to the absence of pumps for the recirculation of large amounts of water, the power consumption of

the dry bottom ash and pyrites handling system is much lower than that of the previous wet system.

The actual power consumption of the dry system during normal operation is 75% lower than that of the

sluice system.

Coal saving and CO2 emission reduction

With the MAC system, the boiler’s heat losses have been reduced by the following effects:

1. Thanks to the burn-out of the bottom ash, its unburned carbon content has been reduced from app.

12% in the previous system to less than 4% measured presently. The heat released by this process

ends up in the boiler due to the cooling air that flows from the MAC system into the boiler.

2. The sensible heat contained in the bottom ash and the flame radiation through the boiler throat are

largely recovered and sent back into the boiler by the cooling air used in the MAC system.

Compared to the previous sluice system, this results in a heat recovery of some 4200 kWt. This would

correspond to a boiler efficiency increase of 0.25%. In practice, this boiler efficiency improvement will

translate into a reduction of coal consumption and thus of the emission of CO2 and other pollutants.
18

The above estimated boiler efficiency improvement is expected to lead to a coal saving of app. 4500

tons per year.

In addition, with the dry pyrites handling system the coal discharged with the pyrites from the mills is

now delivered back to the coal yard. In this way, app. 1000 tons per year of coal are saved compared to

the previous sluice system.

It follows that the total saving of coal that is obtained with the dry bottom ash and pyrites handling

system can amount up to 5500 tons per year. Apart from the direct saving on fuel costs, this also

corresponds to a reduction of the CO2-emission. Reckoning that by the combustion of one ton of coal

app. 2.2 tons of CO2 are produced, the coal saving of 5500 tons per year leads to a saving of app.

12,000 tons of CO2 per year.

The re-use of the ash in the cement industry leads to a further reduction of CO2-emissions. For every

ton of ash used in the cement production, app. 1.5 tons of CO2 are saved. With a total bottom ash

production of app. 40,000 tons per year, this means a further annual saving of 60,000 tons of CO2.

Ash saleability

Due to the low unburned carbon content of the ash and the absence of pyrites, the sales potential of the

dry bottom ash is higher than that of the previously produced wet ash . At present, the dry bottom ash is

supplied to a local cement factory for use as inert material for the cement production. If further

pulverized, the bottom ash could even by mixed with the fly ash and so applied for the concrete and

cement production according to the European norm EN450. This would further increase the ash sales

revenues and also reduce total ash handling costs by simplifying the overall ash management.

Therefore, a feasibility study for the extension of the MAC system with a pulverization station and a

pneumatic conveying system to the fly ash silos, is currently under preparation.
19

Conclusions and Further Work

The sluice system originally installed in Los Barrios #1 for the wet handling of bottom ash and pyrites

has effectively been replaced by the completely dry MAC system. The MAC system for dry bottom ash

and pyrites handling has been in operation since the beginning of July, 2003.

The first results of the project have been in line with the expectations: the plant’s performance has been

improved and its environmental impact has been reduced. In particular, the following benefits have

been obtained:

• The complete water recirculation and treatment system for both the bottom ash and pyrites handling

has been eliminated, as well as fresh water use and waste water production

• Operating and maintenance costs have been reduced drastically, thanks to the saving on operational

workforce, materials and labour for maintenance and on electrical power consumption. Total O&M

costs of the MAC system are forecasted to amount to only 22% of those of the previous sluice

system.

• A significant reduction of the boiler’s heat losses has been obtained. Unburned carbon content in

the bottom ash has dropped from an average 12% to less than 4%, and around 1000 tons of coal are

recovered from the dry pyrites system. The total heat recovery corresponds to a coal saving of app.

5500 tons per year, equivalent to a reduction of the CO2-emissions by 12,000 tons per year.

• The saleability of the dry bottom ash has been improved thanks to the low unburned carbon content

and the dry state. The re-use of the bottom ash in the local cement industry leads to economical

benefits as well as a further reduction of CO2-emissions of app. 60,000 tons per year.

To these figures, additional benefits are added:

o A significant increase in space availability in the plant thanks to the elimination of the ash

settling ponds, settling tanks and related water recirculation equipment

o A cleaner and safer working environment thanks to the inherent safety of the MAC system and

the disappearance of water, mud and puddles on the working floor

At present, a feasibility study is under way for the extension of the MAC system with a pulverization

station and a pneumatic conveying system to the fly ash silos. This would further increase the ash sales

revenues and also reduce total ash handling costs by simplifying the overall ash management.
20

Bibliography

• Austin D.R., Milana G. & Carrea A., “Bottom ash dry extraction helps to achieve ‘Zero water

discharge’ and gives additional benefits”, Power-Gen Europe ’94, Koeln, Germany, Vol. 8, May

1994

• Tarli R. & Carrea A., “An efficient and non-polluting bottom ash extraction system”, Energy for

the transition age, Florence, Italy, June 1992

• Carrea E., Scavizzi G.C. & Barsin J., “Dry bottom ash removal – Ash cooling versus boiler

efficiency effects”, Proceedings of the Int. Joint Power Generation Conference, FACT -–Vol. 22,

Book No. H01162, 1998

• Vlachos S. & Carrea A., “Dry ash system retrofit improves efficiency of unit 4 at Ptolemais power

station”, Power Engineering International, Ed. May-June 1996

• Carrea E. & Graziadio M., “Bottom ash carbon content reduction by means of dry extraction

system, a Thermie project” Fifth international conference on technologies and combustion for a

clean environment, Lisbon, July 1999