Dry-ash system retrofit improves efficiency of unit at Ptolemais power station
Greek utility reduces waste-water and ash disposal costs
By Stauros Vlachos, Public Power Corp., and Alberto Carrea, Magaldi Ricerche & Brevetti s.r.l.
Public Power Corp. (PPC), the largest electric utility in Greece, supplies electricity to the Greek mainland and its islands. Currently, PPC not only dominates the production of electricity, but it also has a monopoly in its transmission and distribution.
The installed capacity of PPC`s generation system–thermal, hydroelectric, and renewable–is 9,200 MW, with 8,250 MW to mainland Greece and 950 MW to the Greek islands. PPC has 98 percent of the total installed capacity in Greece.
In 1995 PPC supplied 35 TWh to mainland Greece, and it is estimated that over the next 10 years this will grow at 3 percent to 4 percent annually. Fossil fuels supply 88 percent; hydro, 10 percent; and imports account for 2 percent of the electric supply. Lignite-fired power plants account for 80 percent of the power generated by fossil-fuel facilities.
Lignite from Western Macedonia is fired at Kardia (4 x 300 MW), Ptolemais (1 x 300 MW + 2 x 125 MW + 1 x 70 MW), Aios Dimitrios (2 x 300 MW + 2 x 310 MW operating + 1 x 360 under construction), and the Amindaion power plant (2 x 300 MW operating). Lignite mines in Peloponnese supply the fuel used at Megalopolis A (2 x 125 MW + 1 x 300 MW) and Megalopolis B (1 x 300 MW) power plants.
Magaldi, a 60-year-old Italian company, designs and manufactures bulk-material handling systems. The company has also developed a bottom-dry-ash extraction system for pulverized-coal boilers, which has eliminated the need to have water for cooling and handling the ash.
Dry, bottom ash at Ptolemais
In order to improve the removal of bottom ash on Unit 4 at the Ptolemais power station, PPC made the decision to replace the existing ash-removal system, as shown in Figure 1, with a dry-bottom-ash system manufactured by the Italian company Magaldi. This new system is able to remove the unit`s bottom ash without using water for cooling the ash or for cooling or conveying.
A proposal for this project was submitted to the European Community (EC) under their Thermie program in October 1993. The EC approved the proposal in July 1994, and the contract was signed in September 1994. Startup of the system commenced in July 1995.
The original bottom-ash system used a submerged chain conveyor which removed the ash from the boiler. A rubber belt then conveyed the bottom ash to the final silo.
In the old ash-removal system, more than 140 tons per hour (t/hr) of service water was required to cool the ash. The ash-cooling water accounted for more than 90 percent of the total waste-water produced from Unit 4. Tables 1 (a), (b) and (c) show details of the lignite, bottom ash and fly ash for Unit 4 for the period 1993 through 1994. These figures are based on an output from the unit of 300 MW. The end result is that for a total input of 970 MWt to the boiler, 37.2 MWt (3.8 percent) was lost to the fly ash and bottom ash.
In addition to removing the bottom ash from Ptolemais Unit 4, the system is also able to recover unburned carbon in ash and eliminate any contaminated water-waste.
The dry-ash-removal system, at the time of the Thermie proposal, had only been applied to pulverized-coal-fired boilers. As a result, the installation of a dry-bottom-ash-removal system at a Greek lignite-fired boiler was unique because, until installed at Ptolemais, dry-ash-cooling systems had been limited to boilers with no more than 3 t/hr of bottom ash. At Ptolemais, much higher rates of bottom ash, 6 to 8 t/hr, were expected.
Another concern of the design engineers was the application of a dry-bottom-ash system to a boiler-producing ash with very high unburned material content which could pose a risk of the ash igniting on the extraction belt. However, the potential benefits and the steps taken to limit the risk with adequate countermeasures during the design phase led to the decision to replace the existing water-cooled, bottom-ash-removal system with a dry-bottom-ash-removal system.
New bottom-ash system
A schematic of the dry-bottom-ash extraction system installed on Unit 4 at Ptolemais is shown in Figure 2. Bottom ash from the boiler falls onto a slow-speed steel belt. The belt is continuous and is cooled by a flow of air. The amount of cooling air is controlled with variable inlet dampers using the furnace`s negative pressure. Ash cooling takes place in the sloped section of the dry-cooling system`s extractor, in the primary crusher and in the postcooler. Depending upon the boiler`s operating conditions, the bottom ash is discharged at a temperature ranging between 40 and 100 C into the intermediate bin.
At this point, two options are available: Ash can be recycled to the lignite silo, or it can be conveyed to the ash silo. The reason for the dual pathway is that when the system was originally proposed, it was not possible to forecast the amount of unburned material in the ash.
Recycling of the ash to the lignite silo is by a bucket elevator, chain conveyor and a rubber belt. In the event the storage path rather than the recycle path is chosen, it is conveyed by a positive-pressure, pneumatic handling system.
After startup of the retrofitted unit, it was found that unburned material in bottom ash was greater than expected. As a result, management decided to recycle the bottom ash as much as possible to the lignite silo. Details of the lignite, bottom ash, and fly ash for the period July through December 1995 are shown in Tables 2(a), (b) and (c). During this period, the bottom ash was 100-percent recycled to the lignite silo.
Before comparing the different performances of Unit 4 before and after retrofit of the dry-bottom-ash system, it is important to review the first six months of the unit`s operation, with the dry-bottom-ash-removal system.
Before startup, one major concern was the risk of increased bottom ash because of recycling of the bottom ash. If the bottom ash to be recycled contained material difficult to pulverize, then recycling this material to the boiler could have a negative effect on the amount of bottom ash produced.
After six months of operation it was possible to say that there was no negative impact from recycling the bottom ash. It was found that the pulverizing efficiency of the bottom ash and the lignite were comparable, with no difference in the percentage of fly ash and bottom ash produced.
Another serious concern was the risk of increased unburned material in the fly ash from recycling bottom ash to the boiler. However, it was found that unburned material in fly ash was lower in the first six months of operation than when the unit was operated with the wet-bottom-ash system.
The tables included with this article illustrate the comparison of the heat and material balances of Unit 4 before and after retrofitting the unit with a dry-bottom-ash system. In both instances it has been assumed that the unit operated at 300 MW, for a total of 6,000 hours per year.
A reduction in unburned material in Unit 4`s fly ash and bottom ash was observed when the unit was operated with the dry-bottom-ash-removal system. As a result, there was a saving in lignite and a subsequent increase in the boiler`s efficiency. Prior to the retrofit, the unit consumed 3.23 million tons per year. However, it has been calculated that due to increased unit efficiency, there will be a reduction of 76,000 tons of lignite per year.
Because the bottom ash is now being 100-percent recycled to the boiler, the amount of bottom ash produced by Unit 4 has been reduced to zero and the station`s cost of ash disposal has also been lowered.
Total fly-ash production, as a consequence of the improved boiler efficiency, has also not been increased. Because the fly ash is now recycled in the construction industry, there is no solid waste to be disposed of from Ptolemais Unit 4.
With the bottom-ash retrofit complete, the only water requirement for the dry-ash system is water for the hydraulic seal–about 3 t/hr, a big difference from the 140 t/hr of water required prior to the retrofit. Likewise, waste-water from the unit has now been reduced by more than 90 percent. Figure 3 shows the difference in waste-water between the original wet-ash-removal system and the dry-ash-removal system.
In the past, the pulverizers have limited the output of Unit 4. However, the plant`s output has been increased by 1.6 percent since the installation of the dry-ash-removal system. This has occurred even though the fuel feed remained the same. Figure 4 shows the reason for this increase in capacity is because the recycled bottom ash has a heating value which is more than double that of lignite, so boiler efficiency is increased. EN
Ash postcooler and ash intermediate storage at Ptolemais Power plant