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(From the archive of ”bulk solids handling", article published in Vol. 32 (2012) No. 4 , ©2012 bulk-online.com)The selection of suitable equipment is one of the most important issues related to unloading of fossil fuels, handling and transporting systems adopted in Indian thermal power stations which fire fossil fuels such as coal and lignite. The fuel unloading and transportation system adopted depends on the quantity to be handled, size, type of the fuel, the fuel source, other project requirements etc. These aspects have profound influence on the selection of unloading and handling systems.

Introduction

[caption id="attachment_81440" align="alignright" width="225"] Fig. 1: Source, size, quantity, and quality of fuel are the key factors to be considered while designing a suitable unloading and transport system for fossil fuel.[/caption]Thermal power stations in India use different fossil fuels based on various considerations. Sub bituminous coal and lignite are the principle fossil fuels used. The theme of this article is unloading at ports and transportation to plant sites. This implies that transportation of the fuel from the source of supply is over long distances. In this scenario, only coal is to be considered for a study of this type since transportation of lignite over long distances is not in vogue. Therefore, in this article, essentially unloading of coal at ports which is in common practice and transportation to project sites is taken up for study and hence the term fossil fuel refers to coal only. In a way discussion of mine mouth stations is also not relevant to this theme. Size, quantity, quality, of the fuel and source of fuel supply are the key factors to be considered while designing a suitable unloading and transporting system.

Project Requirements in India

In the current situation of the power sector in India, source of coal and therefore the selection of various handling systems play a very important role. With the implementation of the 5 × 800 MW Mundra Ultra Mega Power Plant project for the first time in India, the capacity of power stations for public utility has been upgraded from the erstwhile, 210 MWs/250 MWs and 500 MWs although these units still exist and projects with such capacities are still being planned and executed. However, the main thrust is for implementation of large sized power projects. With the new trend of enhancement of the capacity, the attendant requirement of receipt and handling of larger quantities of coal as compared to that required for smaller units have to be addressed.The required quantity of coal for operation of the boilers and the quantity of coal to be handled can be reduced if higher calorific value coal is used such as the imported coal. Importing of coal has its own inherent problems and therefore there is a need to plan projects using indigenous coal only if the problems associated with import of coal are difficult to obviate. Hence, in this article unloading and transportation of both imported and indigenous coal are discussed. Specific differences, if any, are discussed under respective section.

Sea or Rail Transportation

Receipt and unloading of imported coal does not necessarily mean unloading at a captive port or at a captive coal terminal near the plant. At times imported coal is received through rail also depending upon the contract with the coal supplier. Similarly, indigenous coal does not mean only rail or road transportation. Indigenous coal can also be received through sea and unloaded at a port or at a terminal. Thus it can be seen that there are varieties of solutions to meet the project requirements, some of which could be purely non technology related and some of which can be purely technology related.Setting aside all such issues discussed in the above paragraphs, which are project specific and therefore outside the scope of this article, we can consider and discuss cases where the coal is received and unloaded either at a captive port or at a captive terminal or at a unloading berth inside an existing port and transported to a power plant. In this context it may also be noted that transportation on sea for long distance transportation has been the most reliable and dependent method and will continue to remain so in future as well.The whole system of unloading and transportation of coal can be split in to two parts: (i) receipt, unloading and storage of coal at the port or at the terminal which can generally be called seaborne coal handling system, and (ii) transportation of coal from the port or the unloading terminal to the power plant site.

Seaborne Cargo Handling

Seaborne coal handling system in the parlance of power plant engineers and consultants means transportation of coal using ships or using ships and barges, unloading at a port or a coal terminal and storage near the port or the terminal for further transportation to the power plant site.[caption id="attachment_81439" align="alignright" width="225"] Fig. 2: Unloading of a large coal carrier with a grab type unloader. (Picture: Tata Power)[/caption]Ships can be unloaded using mechanized systems or non mechanized systems. One example of non mechanized unloading is at Mithapur where coal is received by 40,000 dwt ships and coal is unloaded on to the berth using cranes travelling on the berth. The unloaded coal is transported by trucks to a storage area for further distribution to various customers. It is said that about 10,000 tonnes of coal can be unloaded in a 24 hour day this way. Imported or indigenous coal is transported by ships from the port of dispatch on the seas on a prefixed voyage route to the port of receipt for unloading or to a coal terminal meant for unloading for the project. Depending upon the draft at the unloading location, large, medium, or small ships can directly call at the unloading berth or can be lightered mid stream from where coal is further transported through barges up to the unloading berth.The decision to adopt direct unloading of large or medium sized ships or to have the mother ships lightered and then transport the coal up to the unloading berth is based on the quantity required to be unloaded and available draft, ability for movement during high tide etc. If draft available can allow large sized ships to berth at the port or at the terminal there is no need for adopting barges. Examples for this are the New Tuticoirn Port, Mangalore Port, Adani Port, Ennore satellite Port etc. If however adequate draft is not available as in the case of the Trombay terminal, large ships will have to be lightered mid stream and then coal will have to be transported using barges.Sizes of ships are classified as Handimax (up to 50,000 dwt capacity), Panamax (over 50,000 dwt up to 75,000 dwt capacity) and Cape Size vessels (over 75,000 dwt capacity). This is a general classification and a more detailed classification is available which is not very relevant for coal transportation as discussed in this article. There are self unloading type and ships which are unloaded using berth mounted unloaders. Self unloading vessels are also generally called geared vessels. Geared vessels can be fitted with simple grab bucket type unloading cranes or can be fitted with sophisticated belt conveying systems for continuous unloading. Barges do not have unloading systems fitted in to them and are unloaded using berth mounted machinery only.

Types of Ship and Barge Unloaders

[caption id="attachment_81438" align="alignright" width="225"] Fig. 3: The grab unloader is the most widely used coal unloader. (Picture: Tata Power)[/caption]There are essentially two types of unloaders: Viz intermittent unloaders and continuous unloaders. In case of intermittent unloaders, the unloading operation is cyclic involving many elements in the operation. An example of this is the grab bucket type unloader. The working cycle of such an unloader involves lowering of the bucket in to the ship’s hatches, opening and closing of the grab buckets for collection of coal, filling up of buckets, lifting the buckets back from the ship’s hatches and leading the bucket to the receiving hopper for unloading the coal, opening the grab bucket, discharging the material and resetting the bucket for the next operation. Because of the cyclic nature of operations, the throughput gets fixed by the cycle time.In case of continuous unloaders the number of such operations is far less and therefore the cycle time is less and so the throughput is expected to be higher than intermittent unloaders. Throughput also gets affected by the unloading of coal owing to the shape of the pile. The unloading rate is not constant throughout the height of the pile. Because the amount of material available for unloading is less at the top of the pile, the unloading rate that can be achieved at this location is smaller.As unloading operation continues, at the middle portions of the pile more coal would be available, and therefore the rate proportionally increases and peaks in some areas. As the amount of material reduces with progressive unloading from the top of the pile, the unloading rate reduces again and at the end, clean up of material is required to be carried out using dozers. In this way of unloading operations, there is a certain average rate and a peak rate possible. Peak rate is the free digging rate.The ratio of average rate (also called the through the ship capacity) and the peak capacity varies with the type of unloader as it is governed not only by the pile configuration but also by the nature of operation of the unloader viz cyclic in the case of intermittent unloaders and almost continuous type of operations in case of continuous unloaders. For an intermittent unloader the ratio can be 50 to 60 percent and for a continuous unloader it can be about 75 percent. In case self unloading vessels fitted with belt conveying systems for unloading, this ratio can be as high as 90 percent.

Unloading Rate

The required unloading rate is central to adoption of different systems. Required unloading rate is calculated by the total annual requirement for the plant, number of ship loads, number of barge loads and other logistics, dead time hours or the time not available for unloading at the port or the terminal each time a ship or a barge calls at the port or the terminal, berth occupancy, number of hours of operation in a year leaving about 30 days for overhauling of ships. This rate is the average unloading rate required. There is also another method of fixing the unloading rate.Based on the actual quantity of coal required per day, chartering rate, berth occupancy, turnaround time etc, and a prefixed daily unloading rate is fixed and the unloading system has to achieve this.As discussed above, available draft gives a strong guide line for basic selection of the transportation system. The unloading rate, as fixed above gives a very good idea about the quantum and therefore the practicality of adopting ship or ship and barge system. For smaller unloading rates such as up to 3 million tonnes per year ship and barge system could be practical provided draft available in the selected area allows barge movement at least during the high tides.An example of this are the Trombay ship and barge unloading systems. For higher unloading rates, ship and barge system may not be practical and cost effective. In such cases, it is better to adopt direct unlading of large, medium, or small sized ships as required depending upon the required unloading rate.

Selection of Ship or Barge Unloader

[caption id="attachment_81435" align="alignright" width="225"] Fig. 4: A continuous barge unloader with chain mounted buckets used for free flowing materials.[/caption]For selection of the capacity of the ship unloader, the required unloading rate is again central to selection of the unloader. The number of unloaders can also be fixed based on the required unloading rate. Next step is to decide on the type of unloader which is governed by the cost and the lump size of coal to be handled viz whether the unloader is berth mounted or the ships are geared and therefore are of self unloading type. This is to be decided based on various factors such as chartering rate of ships, availability of geared ships capable of giving the desired unloading rate, etc.In India, self unloading vessels with conveying systems for unloading are not very popular. In India, when geared ships are to be adopted, generally these are ships fitted with grab bucket cranes, the number of such cranes depending upon the required unloading rate. Such geared vessels are adopted in the TNEB’s Tuticorin coal unloading terminals and in the Ennore satellite port. When it is not possible to charter such ships for varying reasons, berth mounted unloaders are to be adopted and generally it is so.An intermittent unloader (either berth mounted or mounted inside the ship for self unloading), can handle higher size lumps of up to 500 millimetres as in the case of Tuticorin project but has a lower efficiency and lower average to peak ratio compared to continuous unloaders which will result in higher peak evacuation rate compared to the continuous unloaders and this peak capacity will be the rated capacity of the ship unloader.[caption id="attachment_81434" align="alignright" width="225"] Fig. 5: Continuous ship unloaders at the Tuticorin bulk terminal. (Picture: Tata Consulting Engineers)[/caption]A continuous unloader can be fitted with a screw unloader or with bucket unloader with facility for vertical and horizontal travel. These types of unloaders can handle coal of generally 50 to 75 millimetres and higher size lumps are difficult to be handled. Continuous unloaders have higher efficiency and higher average to peak ratio compared to grab bucket type unloaders which will result in lower peak evacuation rate compared to the grab bucket unloaders and this peak capacity will be the rated capacity of the ship unloader.

Unloading Berth Details

An unloading berth would have rail tracks for movement of the unloaders as in case of berth mounted unloaders or travelling hoppers as in the case of Tuticorin terminals where geared ships are adopted, evacuating berth conveyor, water supply and dust suppression systems, fire fighting system facilities etc. The berth would also be fitted with mooring dolphins on either side for mooring and anchoring of ships, fenders and other miscellaneous facilities.[caption id="attachment_81433" align="alignright" width="225"] Fig. 6: Coal piles at a bulk handling terminal in China. (Picture: Siyuwj, Wikimedia Commons)[/caption]The length of the berth is decided by the maximum length of highest capacity ships to be received, single banking or double banking in case of an unloading of barges, length of highest capacity barges to be received etc. The width of the berth would be decided considering the rail centers for the unloader or the unloading hoppers, evacuating conveyor requirement etc. Typically, the length of an unloading berth for receiving 50,000 dwt ships would be of order of about 250 metres and the width would be suitable for the selected unloading arrangements.The evacuation system would comprise of feeders at the outlet of the unloading hoppers with facility for speed variation and evacuating conveying system. The feeders can of belt type, reciprocating type etc. The capacity of the feeders and the evacuating conveyors would be equal to the peak unloading rate of the unloaders.

Storage and Onward Transportation

Storage of coal within the port premises is necessitated some time as a back up in the event ships do not arrive or shipping operations are suspended for a certain period during which time coal supply to the power plant would not be available. Also, if there are off take from the plant due to some plant operation problems storage will become necessary.It also becomes necessary in cases where coal has to be supplied to different plants from the same port. In such a case, all necessary systems such as stacking and reclaiming systems etc are required to be installed. Adequate storage area also has to be planned near the port area. In the Udupi power project, Mundra UMPP and North Chennai project, large storage in the port area is provided.Onward transportation to the plant would be either by belt conveyors or pipe conveyors or through a Merry Go Round System (MGR).[caption id="attachment_81432" align="alignright" width="225"] Fig. 7: Belt conveyors are the best means of transportation of coal from port to plant. (Picture: Olivér Svéd ©123RF.com)[/caption]Although belt conveyors are the best means of transportation of coal from the port to the plant in view of their high reliability, they were not very popular earlier except in cases where the transportation distance was about 4 to 5 kilometres mainly due to cost. Designers were adopting lower speeds and therefore the cost would be much higher than other modes of transportation. But with the development of high speed conveyors and advanced design techniques, the cost has been optimised, and long distance conveying through conventional troughed belt conveyors are gaining popularity now. Developments in design have also taken into account the reliability requirements.Another recent trend has been the adoption of long distance pipe conveyors which are environmentally friendly, safe against sabotage etc. In the earlier eras pipe conveyors especially for long distance conveying were still under development and therefore were not very popular and not attractive cost wise.Again, as in the case of conventional troughed belt conveyors, developments in design techniques have optimised the cost. Reliability also has been given due considerations in these recent trends.These concepts arising from the recent trends and developments have been implemented in many projects and the results can be established only after observations over time. In case of pipe conveyors, although lot of developments are taking place in design methods, lot of impetus is still required to be infused to go further in design developments and also to set international design standards.

Merry Go Round Systems

In this scenario, Merry Go Round (MGR) systems were the most popular mode of transportation in the past. With the development of high speed conveyors and improvements in conveyor design techniques, the cost of transportation using conventional troughed belt conveyors and pipe conveyors have become very competitive. All the same even now MGR systems are also in vogue in some projects.In this method of transportation, a rake loading system such as flood loading system is normally installed at the port end which can load the rakes in motion at a high loading rates using storage and live hoppers, gates weighing systems etc. At the plant end the rakes arriving from the port or the terminal are unloaded into a track hopper of about 250 metres length with paddle feeders for unloading the coal.The coal wagons are Bottom Opening Broad Gauge wagons (BOBR) only which can unload coal into an underground track hopper and not other type of wagons. The key to economic success of this system is the distance of transportation as shorter the distance, shorter can be the cycle time and there by achieving higher throughputs for the system. 25 kilometres transportation is a very good distance from cost point of view.The track and the rolling stock can be captive or by railways. Of late, Indian railways do not encourage usage of BOBR wagons due to limited availability of such wagons. Alternatively, the rolling stock also has to be captive. Although Indian Railways have introduced “Own you wagon scheme” where in the railways operate the wagons for some time and return the wagons to the plant developer adoption of the MGR system has not been adopted on a large scale.

Challenges in System Design

Coal size plays a very important role in the design of transportation, unloading and handling systems. In the earlier decades all the utilities were burning only Indian coal. When indigenous coal is used, the coal size can be as large as 400 millimetres to 1 metre. Because of this, there are lot of challenges to be faced by the utilities and the designers.Typically, the large size lumps block free flow of smaller size particles at the track hopper/receiving travelling hoppers in ship unloading systems and damage the discharge chutes and their liners, conveyor belts and the other equipment as well. The problems are compounded by the presence of large sized boulders. The difficulties faced are mainly in receiving, screening, crushing, design of chutes, etc. Coal handling systems have to be provided with lot of features to avoid all these problems which increases the cost significantly. Hence when the import regulations were liberalized, utilities started opting for importing of coal.Imported coal is sized with the top size varying between 50 to 100 millimetres. This has reduced the preparation and handling problems to a large extent. 50-millimetre size coal is acceptable in coal pulverizing mills supplied by overseas suppliers and the need for crushing has been eliminated. In the on going Ultra Mega Power Project (UMPPs) at Mundra, this concept has been adopted. If the size of the “as received coal” can be restricted to 50 millimetres, raw coal itself can be stacked instead of the crushed coal, even if coal has to be reduced to -25 millimetres.India has large reserves of coal which have not yet been fully explored and hence the usage of domestic coal has to continue. Therefore many utilities including some of the future UMPPs will still have to use Indian coal. In such cases, to obviate the problems associated with handling large size coal, the size of the “as received” coal has to be restricted to say 100 millimetres at the mine itself.This has already been adopted in some cases. In mine mouth stations, the size of the as received coal has been reduced to almost 100 millimetres with mining activity being contracted. In the 2 × 600 MW project of Essar at Mahan (now under engineering) this concept has been adopted since the first stage, crushing is expected to be at the mines itself.

Techno Economic Studies

In power projects the main consideration is the tariff or the rate at which electricity is supplied to the public. Generally the tariff is fixed. One of the main inputs to this cost is the landed cost of coal. Landed cost of coal comprises the ex-mines cost and the supply cost. The two together give the landed cost at site. Supply cost should take into account the cost of all the infrastructural facilities, operating and other project related costs. Since the landed cost plays an extremely crucial role, a detailed techno economic feasibility study is most important.In a feasibility study, all technically feasible alternatives have to be evaluated along with the cost for each of the alternatives and the technically best alternative with the least landed cost has to be selected. The landed cost with such a technically best alternative has to be considered for calculation of the tariff and required corrections if any in the selected alternative to meet the tariff requirements, have to be carried out.

Conclusions

• Project requirements are to be considered before planning any unloading and transportation system.
• Various solutions are available for transportation and unloading systems.
• Sea transport for transportation over long distances will continue to be the main stay for future also
• New trends are emerging. However for these to get implemented in future lot of efforts is required in the on going projects considering the cost and efficacy of such trends.
• Developments in design procedures and setting international standards have to get further impetus for pipe conveyors.

Techno economic feasibility studies have to be carried out before adopting a suitable alternative with the least landed cost of coal to ensure the electricity tariff is within the limits set for it. ■

A Note from the Editor

For all statements in this article that refer – directly or indirectly – to the time of publication (for example “new”, “now”, “present”, but also expressions such as “patent pending”), please keep in mind that this article was originally published in 2012.

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