Ash handling: Why dry bottoms are better than wet bottoms

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Traditionally, bottom ash has been handled in a wet condition via established technologies such as impounded hoppers or submerged scrapper conveyors. A more modern approach has been to develop dry techniques that offer the following advantages, including increased thermal efficiency and reduction of unburned carbon, the removal of water systems and treatment and an improvement in ash quality.

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Alan Bullock, Clyde Bergemann, UK

Clyde Bergemann’s Drycon dry bottom ash handling system

Power plants are increasingly operating in a more competitive climate and in a world short of mineral resources, coal continues to be a raw material of choice and, correspondingly, by-products from coal combustion are increasing.

Stricter regulations and a heavier legislative burden coupled with increasing environmental awareness have made the operation of power plants more complicated and expensive. Resultant higher transport and disposal costs associated with waste and combustion by-products mean that the power industry must look for solutions to minimize the environmental impact. Reutilization of bottom ash from coal combustion is already showing positive results with its use in structural embankments and drainage systems. When mixed with fly ash it may also be used in the cement industry.

A new dry bottom ash handling system continues to burn the bottom ash during the extraction and cooling phase ” passing ambient air, instead of water, over the ash. Known as Drycon, the system not only minimizes emissions and non-recyclable waste products but also delivers increased boiler efficiency, due to the improved burning of the ash.

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Dry Versus Wet Bottoms

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Traditionally, bottom ash has been handled in a wet condition via established technologies such as impounded hoppers or submerged scraper conveyors (SSC). The use of water as opposed to air as a cooling agent can incur additional costs. Factors such as water treatment, corrosion damages, higher disposal costs and environmental problems, as well as the higher costs to maintain must all be considered.

Using a dry system means that no water is required in the process, therefore no water treatment is necessary. Reduced emissions and returning heat energy to the boiler resulting in lower coal usage and so with lower costs for emission trading are also highly beneficial to plant operators. The table below shows the main factors which compare between the two methods of conveying.

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Cost scenario ” Drycon versus SSC

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The following is an economic study of the relative costs of a Drycon bottom ash system against the more traditional SSC technology. The study is based on a typical European baseload pulverised coal fired power plant of 800 MW operating with imported coal. The economic factors assumed for the study assume depreciation over ten years and the interest on loan capital of 12 per cent.

Looking at the investment costs, it can been seen that although the Drycon is more expensive than the SSC on a unit basis and the cost of associated crushing equipment is similar, these are offset by simpler transport and storage equipment and the lack of requirement of water treatment equipment such as pumps, filters, heat exchangers etc.

Considering the consumptions on an annual basis, it can be seen that due to the Drycon roller design, the friction losses are significantly reduced and therefore have a positive effect on energy consumption and resultant wear. In addition, the SSC requires the provision of cooling water. At the associated costs indicated, it can be seen that the annual operating costs of the Drycon are approximately 47 per cent of those of the SSC.

As discussed earlier, the Drycon captures waste energy from the incomplete combustion of the bottom ash and introduces it into the boiler as preheated air at approximately 450 oC. This results in an overall increase in boiler efficiency of between 0.15 and 0.5 per cent.

The bottom ash resulting from the Drycon is a sellable product as it has good properties for the construction industry because it is low in carbon and it is dry and easily handled. In comparison the wet bottom ash from the SSC is generally disposed of and has the potential to impact the environment through water consumption and contamination.

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The Drycon System

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The Drycon system is mounted fully sealed to the combustion chamber. The negative pressure inside the boiler sucks air in a controlled manner mainly at the top end of the Drycon system into the bottom ash extractor system. The air moves counter flow direction along the surface of the ash which rests on the conveyor belt. This activates a re-burning process of the glowing ash, which reduces the unburned carbon level and frees up additional thermal energy. The air is heated up before it enters the combustion chamber and adds additional thermal energy to the steam generating process inside the boiler. Approximately one per cent of the total combustion air is required for the dry cooling system and can be already considered as a constant value in the boiler design. This ensures that the combustion process and the exhaust gas composition are not affected.

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The Arrangement

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Underneath the combustion chamber a transition chute or hopper is installed and supported from the floor by means of a steel structure. The transition chute or hopper is lined with special refractory and insulation at the inside to withstand the radiation from the combustion chamber of the boiler. As standard a heat resistant fabric compensator is installed between the boiler/hopper outlet and the Drycon intake, to compensate the movement between cold and hot condition of the boiler. The compensator is fabricated from elastic metal, resistant to high temperatures. To avoid damages due to high temperatures by the radiant heat from the combustion chamber, a special protecting skirt with lining can be provided at the inner side of the compensator.

The compensator will be delivered and mounted in one-piece only, to assure tightness and flexibility.

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Key Design Features

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Drycon presents many key features which contribute to the technology’s success in the market:

  • Chain and sprocket wheel ” the sprockets are manufactured from wear resistant materials and have exchangeable pockets for easy replacement. High breaking strength matched lengths of 25 x 95 mm case hardened chain are used with cast case hardened pins to connect to the steel carrying plates.
  • Tension and drive stations ” having a maximum stroke of 600 mm, the tension roller is mounted in lubricated long life heavy duty roller bearings. Tension is applied via manually or automatically operated hydraulic cylinders.
  • Supporting idler ” mounted at 600 mm centres and lubricated, manually or automatically, with hear resistant grease the 125 mm diameter rollers can be replaced from the outside whilst the boiler is still on-line.
  • Fines recirculation ” the system is self cleaning with the integration of a fines re-circulation plate on every sixth steel plate element to ensure clean operation.
  • Impact beams ” impact beams have been designed into the conveyor casing to withstand any loads imposed from large lumps ensuring that the drive elements are not damaged.
  • Jaw crushers ” large lumps can be broken down by jaw crushers in order to ensure they are an appropriate size to be cooled and conveyed.
  • Safe monitoring ” should a blockage occur, the drive motor stops immediately then enters a removal procedure before giving an alarm to the control room.

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Drycon Installed On SEC’S Heyuan Power Plant, China

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Heyuan Power Plant, owned by Shenzhen Energy Company (SEC) (65 per cent) and Hopewell Power (35 per cent), is situated in Heyuan City, Guangdong Province just 200km from Hong Kong. The first 600 MW unit of the brand new plant was put into operation in January 2009 followed by the second 600 MW unit in August 2009.

Drycon captures waste energy from the incomplete combustion of the bottom ash and introduces it into the boiler as preheated air at approximately 450 oC. This results in an overall increase in boiler efficiency of between 0.15 and 0.5 per cent.

In order to become one of the most efficient and cost effective coal-fired power plants in Guangdong today, SEC wanted to include modern technologies to ensure it meets these targets. One of the areas of consideration would be the removal of bottom ash.

The initial plan for the bottom ash removal project was for a wet removal system known as a SSC, but after serious consideration with the help and advice of Clyde Bergemann, SEC opted for a dry solution and awarded the contract to Clyde Bergemann for their Drycon technology on both 600 MW units.

Following a detailed review of available technologies, SEC chose Clyde Bergemann’s Drycon dry bottom ash technology to provide a high reliability, low maintenance solution whilst providing for the elimination of water commonly used in alternative solutions.

In addition to elimination of water, SEC noted the additional benefits provided by the Drycon system which include increased environmental protection; positive influence on the efficiency of the boiler; efficiency improvement by eliminating energy wastage on water recirculation and treatment; elimination of corrosion and limitation of erosion issues; and low lifecycle costs compared to wet systems.

From start-up in January until October 2009, Heyuan Unit 1’s required less than seven hours for maintenance with the boiler unit still on-load. Availability was recorded at 99.9 per cent. The client said it was very satisfied with Drycon technology and the services provided by Clyde Bergemann’s experienced engineers. After the PAC of Unit 2 in August 2009, SEC issued a commendatory letter to praise the performance of Clyde Bergemann staff.

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The Way Forward

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Many factors affect the choices plant operators make on implementing the right technologies to run their plants efficiently and cost effective. With the vast range of benefits and key design features of Drycon, this should certainly influence the decisions made on the future of plants around the world. In particular, the pressure on operators to ensure emission levels comply with legislation will certainly be eased with the environmental benefits of using dry ash conveying.

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