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Christopher Blozek and John Rnch, Benetech, Inc., US, Andy Miller, Benetech, South Africa, Dandle Vorster, New Vaal Colliery, South Africa, and Asghar Eskandani, Benetech GmbH, Germany, introduce a new concept developed for the control of coal dust emissions.

Many countries have adopted environmental standards for fugitive dust emissions, and health and safety regulations pertaining to allowable respirable dust emissions.

High levels of coal dust can present health, safety and environmental concerns, and can dramatically affect facility operations in the event of a fire or explosion. Even moderate levels of coal dust can add to facility operating and maintenance costs due to increased housekeeping expenses, reduced equipment reliability, lost coal, greater wear on components and decreased worker productivity. Violation of local/national health, safety and environmental laws can also result in fines and jeopardise plant operations. Depending on the coal type, highly friable coals tend to generate large amounts of dust during mining, transportation, storage and handling processes.

In order to address the problem of coal dust emissions from various types of coals and operating equipment, Benetech has formulated a unique approach called Total Dust Management (TDM). This process is formed from the selective integration of a number of different technologies combined with custom engineering and service, thus producing a cost-effective solution to controlling dust emissions. TDM encompasses the following technologies:

- Fog and chemical dust suppression application systems and foam, wetters and residual surfactant chemicals. Fabric filters, including pulsejet, high-volume low-pressure reverse pulse, insertable and bin-vent dust collectors.

- Containment systems, including hoods, scrapers, skirting, baffles and curtains.

- Custom-modelled and engineered transfer chutes and dust tight inspection doors that minimise dust generation.

- Agglomeration, pneumatic and sluicing systems that safely and effectively remove collected dust.

- Washdown systems designed to minimise housekeeping expenses and provide a clean and safe work environment.

A typical TDM programme begins with an audit. This audit represents a needs/condition assessment that identifies dust issues, recommends and prioritises solutions, and provides budgetary estimates for system modifications. If the plant includes dust collectors, the audit also encompasses flow measurements, a design review and balancing of the duct systems. The foundation of the audit is an in-depth understanding of the plant's needs and goals. Each audit is customised to address these needs, which can include safety and environmental compliance; productivity enhancement; reliability; labour cost reduction and deployment; risk management; efficiency; or corporate objectives such as profitability.

Black Dog generating station

This year, Benetech, Inc. was contracted by Xcel Energy to perform a TDM audit at its Black Dog generating station, located on the Minnesota River just south of Minneapolis & St Paul, Minnesota. The original coal-fired 1 and 2 units, installed in the 1950s, were recently replaced by natural gas combined cycle units. Units 3 and 4 are dual-fuel boilers with steam turbines that continue to utilise coal as the primary fuel. Combined power production capability is approximately 600 MWe. The fuel source for these two units is extremely dusty low-sulphur Powder River Basin (PRB) coal from Wyoming, US. Approximately 900,000 tpa of PRB coal is received by unit trains equipped with air-actuated rapid discharge cars.

Benetech became involved at Black Dog to assist with fugitive dust control in the early 1980s. A dust suppression system was designed and installed by Benetech to control dust from the track hopper feeder location to the 50 t stack-out hopper and reclaim system. This automated application system utilised a combination of Benetech's proprietary foam, wet and atomised 'fog' technologies. This combination of technologies has successfully controlled fugitive dust throughout the coal conveying system, coal pile stackout and into the bunker cascade system.

When the station switched to 100% PRB coal in 1991, dust collectors were added to handle the increased amount of dust expected at the coal breaker building and newly upgraded tripper system. Coal dust is pneumatically conveyed from the two breaker building collectors to a storage silo. From here, dust is processed by a Benetech DustMaler agglomeration system for dust-free delivery either to stackout or the bunkers. Two of the three collectors that service the tripper gallery also process their collected dust using a smaller DustMaler prior to discharging back onto the tripper belt for a dust-free delivery to the bunkers.

The remaining tripper collector discharges directly into its dedicated coal bunker.

The scope of the TDM audit for Black Dog included an assessment of the six existing dust collectors, including the reduction or elimination of condensate-related problems. Black Dog also wanted the audit to address dusting in the tripper area and to develop a preventative maintenance programme for the dust collector systems. A number of issues were identified early in the audit process including tail load point coal spillage, worn transfer chutes and dust accumulation in the tripper area, which required constant housekeeping.

The dust collector section of the audit documented the current condition of the system in terms of performance, safety, design and serviceability. An important element of the effort was the ductwork flow audit, which included the determination of flow volumes, velocities and pressure drops across each element of the existing ductwork. Figure 1 shows the audit crew performing collector measurements. This information was critical in identifying any necessary dust/hood modifications and system-balancing issues. The audit also identified capacity deficiencies associated with the dust collectors, and made recommendations for increasing capacity. In addition to the written recommendations, it also provided budgetary costs to correct the identified deficiencies.

The audit indicated that existing volumetric flows through the collectors averaged at 69% of original design flow requirements. A close inspection revealed excessive leakage at the foam explosion doors, and plant personnel reported that several doors had been replaced after being sucked into the collectors, as presented in Figure 2. This condition not only robbed the ductwork of available fan capacity but also allowed cold air to be drawn into the collector, which resulted in the problem of condensation. A recommendation was submitted to use an existing metal retrofit explosion door to replace the existing foam doors. It was estimated that collector efficiency could be improved by up to 20% by implementing the suggested changes. Additional investigation revealed several plugged and partially-plugged ductwork sections. When plant maintenance personnel had removed the plugs, cleaned the ductwork and replaced ductwork sections, the system was ready to be balanced.

Based on Benetech's design calculations, it was determined that the original collector had been designed for duel velocities of 4000 ft/min. Current LS National Fire Protection Association guidelines recommend a minimum of 4500 ft/min. To achieve current NFPA guidelines would, have meant replacing the current dust collector fans with high volume units. The higher volume would have also raised the air to cloth ratio in the dust collector, thereby shortening bag life and lowering overall system static pressure available to capture dust at the hoods.

An analysis of the existing collected dust particles determined that the dust could be conveyed, without dropping out, at the original design flow after proper balancing. Each dust collection system fan damper was placed in the full open position to achieve maximum flow. Each dust collection hood was then balanced, by use of an existing blast gate, to the 4000 ft/min velocity. Subsequent testing using plant 'worse case' operating scenarios confirmed that the hoods were capturing the dust, and the transfer points were remaining dust-free without dust accumulation in the ductwork.

A number of the other recommendations that resulted from the audit included:

o The installation of new head scrapers in proper positions to minimise dust carryover (Figure 3).

o The installation of proper side skirting to match belt hardness, chute design and service requirements.

o The extension of select loading hoods.

o Changing the angle of select trough idlers to reduce spillage and provide for more stable belt alignment.

The implementation of the TDM programme is still in progress. However, the combination of Benetech's dust suppression and DustMaler agglomeration systems, as well as the recent dust collection system balancing and planned collector explosion door upgrades, have provided significant improvement in overall dust management at Black Dog. Over the years. Black Dog and Benetech have demonstrated that a well-planned approach to Total Dust Management can be efficient and effective, yet still satisfy

Schleenhain brown coal mine

Following the German reunification, Benetech GmbH was contracted to assist in the refurbishment of Mibrag's (Mitteldeutsche Braunkohlengesellschaft mbH) Profen and Schleenhain mines, located near Leipzig, in the former East Germany. An important element of the mine's refurbishment programme was environmental protection, which included fugitive brown coal dust control.

Benetech first surveyed Profen and installed three dust suppression systems in 1998. The success of this project led to the award of a second contract at the Schleenhain mine.

The Schleenhain mine had been closed since 1995 for refurbishment with modern conveyor belt operation. To achieve this goal, Schleenhain was equipped with 38 km of conveyors, the longest one (No. 71) having a length of 4 km and a width of 2 m. From the mine to the sieve and crusher, the coal is transported at a maximum capacity of 4200 tph.

Lippendorf power plant, consisting of two units of 900 MW each and jointly owned by Vattenfall, E.ON and EnBW, demands high quality coal with respect to heating value, ash content and sulphur. To comply with these criteria, Mibrag established a pile management blending system with a maximum capacity of 400,000 t. In 1999, the mine began supplying approximately 33,000 tpd (11 million tpa) of brown coal to the nearby Lippendorf power plant (Figure 4). With a reserve of over 400 million t of coal, Schleenhain is expected to operate through until 2040.

Essential criteria established by Benetech GmbH with Mibrag for the design of dust suppression systems were:

o Coal quantity.

o Coal size and portion of coal fines.

o Amount of potential fugitive dust at dumpers, handling, stackout, etc.

o Amount of potential fugitive dust from conveyors, transfer points, storage piles, etc.

o Fire and/or explosion potential at all site locations.

o German and European environmental regulations for fugitive dust emission.

o Quality of the coal containment system to be planned (processing time of the coal, covered/uncovered conveyors, chute design and sealings, etc.)

o Operational costs of the dust suppression equipment, chemicals and maintenance service.

o Ease of equipment modifications for potential changes in mining operations and coal properties.

o System reliability, maintenance and current workforce load.

o Housekeeping and worker productivity.

Benetech carried out a detailed project analysis to determine how these criteria could best be met to effectively control dust emissions at the Schleenhain mine.

The results of the analysis clearly showed a need for both residual and wet dust suppression systems. The residual systems ensure dust suppression during the long conveyor runs from the mine via the sieve and crusher station to the pile and during pile storage. The wet system is used to reactivate the residual effect following coal reclaim from the pile, for the conveying run to the transfer and corner tower prior to the bunker of the power plant.

During the engineering of both systems, special attention was paid to manifolds placement, entrained air velocities, spray penetration into the coal flow, spray patterns and chemical selection. During stackout, Benetech's BT-415-based residual dust suppression system was deployed with four suppression spray manifolds at conveyors No. 2, No. 12, No. 32 and No. 42 (Figure 5). A second dust suppression application system was placed at the sieve and crusher. The latter is designed to apply a suppression spray at the discharge of the sieve, at the discharge of the crusher, and at the head of conveyor No. 73 following the sieve and crusher chutes.

Spray locations are organised to maximise penetration into the moving coal stream. To accommodate German weather conditions, all water, chemical, solution piping and chemical storage tanks were heat traced and insulated to prevent freezing down to -25 °C. To prevent freezing at the spray tip assembly and flexible hose connections, compressed air is injected into these components at shutdown during cold weather operation. Figures 6 and 7 show the considerable difference between untreated and treated coal at the stackout under comparable circumstances.

After stackout, a Benetech BT-205W wet suppression system was installed at the transfer tower, which is designed to apply a suppression spray at the transfer chute from conveyor No. 75 to No. 80 and from No. 76 to No. 90. BT-205 is applied to cover new coal surfaces that are inevitably generated during the reclaim process and to reactivate the residual effect of BT-415 (Figure 8).

Monitoring of the dust suppression process is essential. The suppression systems are controlled by a Benetech custom-designed system based on Siemens series programmable controllers with Profibus communication network capabilities. Through this network, Benetech can retrieve data concerning conveyor belt operations and report data concerning the functions of the dust suppression system.

In addition to the installation of the Benetech systems, the Schleenhain project analysis resulted in detailed recommendations for the mechanical upgrade of the coal containment systems in the corner tower. These were accepted and carried out by Mibrag, achieving considerable reductions in dust.

To address build-up and spillage issues during periods of high moisture coal conveyance, in 2000 Benetech recommended and installed a PACS system at the belt conveyor chute of the stacker. The system has greatly reduced time spent clearing build-ups and conducting housekeeping duties.

Working with Mibrag, Benetech GmbH has shown the effectiveness of the consistent and continuous application of the TDM concept in treating brown coal at the Schleenhain and Profen mines. Major factors in this success are the unique systems designed for site-specific conditions.

New Vaal Colliery

In 1999, Benetech SA was contracted to address dust emission issues and coal pluggage problems at New Vaal Colliery. This colliery is situated south of Vereeniging, on the Orange Free State bank of the Vaal River, approximately 70 km south of Johannesburg. It is an Anglo Coal mine, a division of Anglo Operations Ltd, established to supply coal to Lethabo power station. The colliery is an opencast operation that produces approximately 17 million tpa at full output. There are two factors that make this colliery different from other mines supplying coal to power stations:

o It produces the lowest quality coal used by an Eskom power station.

o It is the first major colliery developed to mine coal by opencast methods from reserves mined previously by underground means.

This method of opencast mining has resulted in unique challenges concerning the bulk material process. The two main challenges include:

o The 'sticky' nature of the coal arising from the composition and sludge deposits present in the old underground workings.

o Dust generation, due to the high percentage of fines and spontaneous combustion.

The use of dry screening further exacerbates bulk material process, resulting in extremely high dust levels and chute blockages that cause production loss.

Benetech SA Pty Ltd is based in the coal mining town of Witbank, Mpumalanga. In 1999, Benetech SA was invited by New Vaal to carry out a TDM audit that would address the challenges discussed above. During the audit, the key areas identified as a major source of dust generation were the primary tip and crusher, the primary and secondary screening plants, and the ROM stockpiles during stacking and reclamation. To address fugitive dust emissions in these areas, Benetech SA installed two chemical dust suppression systems: one at the primary tip and crusher and one at the first transfer point after the tip (TH03).

For these application systems, Benetech SA chose a 'wet' suppression system at the primary tip and crusher and a 'residual' system at TH03. Wet dust suppression combines the use of water with an effective surfactant wetting agent. Dust suppression systems use strategically located spray manifolds to introduce a suppressant solution that controls airborne dust levels. After being wetted, gravity forces the dust particles downward into the coal flow. Comparative studies have shown that surfactant systems significantly reduce the water required to control dust emissions from 4% to less than 1% moisture addition. 'Residual' dust suppressants work in a similar manner, but combine wetting agents, binders and humectants to provide long-term dust control. With this technology, coal can be treated just before stockpiling, to control dust during the stackout procedure as well as fugitive dust emissions from the pile during windy conditions. A secondary benefit is that residual dust suppressors also enhance pile compaction, which in turn reduces the occurrence of spontaneous combustion.

After the installation of the two systems, the downstream dust problems in the primary and secondary dry screwing plants and the ROM stockpiles were reduced considerably. Previously installed dust extraction and/or suppression systems proved to be both unreliably and maintenance-intensive, which led to the decommissioning of these systems, and overall cost saving and performance improvement by utilising the Bcnetech systems.

To address the problem of 'sticky' coal causing blockages, Benetech SA was given permission to install a PACS'- system into one of the underpans at the primary screens for testing and evaluation (Figure 9). Benetech's PACS technology uses compressed air to prevent the build-up of accumulation on chute walls before pluggage can occur. Each nozzle directs a blast of air 360° radially for a distance of 0.6 -1m along the inside surface of the chute, via a spring-loaded nozzle head mounted to the side of the chute or hopper. Multiple nozzles are typically mounted on the chute. They are automatically controlled to fire in a sequence and duration such that material build-up is prevented.

Coal flow throughout was maintained and blockages were dramatically minimised on the test stream. This resulted in further installations in critical areas in the plant and stockyard, with the later systems being integrated into the SCADA system. The dust suppression system and the PACS have proven themselves \o be effective in terms of cost and operation. This is demonstrated by the recent milestone of achieving a production record, and possibly a world record, of 1.94 million ROM t in August 2002.

Conclusion

Total Dust Management represents a new approach to addressing fugitive dust emissions from bulk material handling systems. Often, suppliers compete with a single technology to address all of the different dust issues that may arise at a facility. A more cost-effective method is to optimally select the best technologies to address the different conditions that exist. Furthermore, by selecting a single source supplier, responsibility to meet required performance levels can be focused on one organisation that is also responsible for making these various technologies work in harmony with each other.

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