Case study: Importance of “after cooler” arrangement in the fly ash system of thermal power plants

December 3, 2024

P. Phani Kumar, Divisional Engineer, Telangana Power Generation Corporation Limited, TGGENCO, 600MW, KTPP, Kakatiya Thermal power project

In thermal power plants, fly ash is a byproduct of coal combustion that must be continuously and effectively conveyed from Electrostatic Precipitators (ESP) to storage silos. During the rainy season, high humidity and moisture condensation inside fly ash pipelines can lead to blockages. This disrupts the flow of ash, causing overloading of ESP hoppers and forcing plants to reduce their power generation to avoid system failure. In severe cases, ash is dumped onto floors and manually cleared later, affecting efficiency and safety. This article presents a case study on this issue and proposes solutions to mitigate the problem.

Problem Overview: Moisture Condensation and System Blockages

During the monsoon or in high-humidity conditions, moisture can accumulate inside the fly ash conveying system, leading to condensation within the pipelines. This results in blockages that hinder the smooth transport of ash from ESP hoppers to storage silos. As the ash accumulates:

  • ESP hoppers overload, forcing a reduction in unit load to avoid damage.
  • Manual removal of ash becomes necessary, leading to operational inefficiencies and safety risks.
  • Reduced plant output results in decreased power generation and overall plant performance.

Fly ash is a byproduct of coal combustion in thermal power plants, and its effective transportation is crucial for smooth plant operations.

In thermal plants, fly ash is conveyed from buffer hoppers to silos using a Transport Air Compressor (TAC) operating at pressures of 2 to 3.5 kg/cm². Ideally, the outlet air from the TAC should have a temperature between 45 to 60 degrees Celsius and be free of moisture to prevent condensation within the fly ash pipelines. However, if TAC outlet air temperature exceeds 150 to 180 degrees Celsius, the moisture stays in a gaseous state. Upon exposure to external rain and cooler conditions, the moisture condenses inside the fly ash pipes, leading to blockages that disrupt fly ash transport.

Case Study: 600 MW, TGGENCO Power Plant at Jay Shankar, Bhoopalpally, Telangana

At the 600 MW thermal power plant in Jay Shankar, Bhoopalpally, Telangana, severe fly ash pipeline blockages were encountered during the rainy season. These issues were caused by condensation of moisture inside the fly ash conveying lines, which directly impacted the plant’s ability to efficiently transport fly ash from the ESP hoppers to the silos.

The power plant utilized Atlas Copco ZA 5 single-stage air compressors, 3000cfm capacity without intercoolers or after-coolers. These compressors had an outlet air temperature of around 175 degrees Celsius, which was significantly higher than the recommended range of 45 to 60 degrees Celsius. During the monsoon season, the high-temperature air, combined with external cooling due to continuous rain exposure, led to moisture condensation inside the pipelines. The result was frequent line chokes that caused fly ash transport interruptions and, in some instances, forced the plant to reduce its unit load to prevent overloading the ESP hoppers.

Conversely, during the summer season, when external temperatures were high and there was no rain, the fly ash conveying system operated without significant issues, as there was minimal condensation inside the pipelines.

Analysis of the Problem

The root cause of the blockage in the fly ash pipelines was the condensation of moisture, which is primarily influenced by:

  1. High outlet air temperature from the TAC (around 175°C), causing moisture in the air to remain in a gaseous state.
  2. Exposure to external rain and lower temperatures during the monsoon season, causing the moisture to condense inside the fly ash pipelines.
  3. Lack of air cooling mechanisms (such as intercoolers or after-coolers) in the installed air compressor units, resulting in elevated air temperatures that exacerbate condensation problems.

Solutions of the problem

To address these issues, the following solutions were implemented:

  1. Installation of After-coolers: After-coolers with “Water cooled type” having following specifications were installed and commissioned to the existing air compressors (ZA5Atlas Copco make) to reduce the outlet air temperature to the recommended range (45°C) duly removing the moisture content. This would ensure that the air remains dry and cool, preventing moisture from remaining in a gaseous state and condensing within the fly ash pipelines.After-cooler= Water cooled type
    Flow rate= 3000cfm
    Inlet pressure= 2 to 3.5kg/cm2(g) max
    Inlet/Outlet Temp =180-200/45 degree
    Water inlet/outlet temp 35/40 Deg,C
    Pressure drop 0.1 Kg/Cm2(g)
    Tube=Copper

Water requirement= 685LPM

  1. Insulation of Fly Ash Pipelines: Insulating the fly ash pipelines, especially during the rainy season, would reduce the effect of external cooling and prevent condensation caused by exposure to rain.
  2. Improved Air Flow Management: Monitoring and regulating the air temperature and pressure more closely during seasonal changes would help optimize the fly ash conveying process. Automated control systems could be installed to adjust the TAC air output based on real-time weather conditions and operational needs.
  3. Maintenance and Inspection: Increasing the frequency of maintenance and inspection during the rainy season would help detect early signs of condensation and blockages, allowing for timely corrective action before the system is severely impacted.

The case study highlights the challenges posed by moisture condensation in fly ash pipelines during the rainy season. The absence of cooling systems in the air compressors, combined with external exposure to rain, led to frequent blockages and operational inefficiencies. Implementing solutions such as installing after-cooler and proper insulation helped and prevent moisture condensation, ensuring smoother fly ash transport and preventing reductions in unit load during the monsoon season. These corrective measures brought significance improvement and efficient plant operations in the fly ash system.

Reasons for Fly Ash Pipeline Choking During the Monsoon Season

Overview

Fly ash transportation is a crucial part of thermal power plant operations, where fly ash is pneumatically conveyed through pipelines using air from Transport Air Compressors (TAC). During the monsoon season, the high relative humidity (ranging from 70% to 90%) causes significant moisture-related challenges that result in fly ash pipeline blockages. The following are main reasons:

Causes of Fly Ash Pipeline Choking in Monsoon

  1. High Humidity and Moisture Content in the Air: During the monsoon season, relative humidity levels rise significantly, ranging from 70% to 100%. This results in an increased moisture content in the air, which directly impacts the fly ash conveying system. The moisture content in the air varies depending on ambient temperature and humidity:
    • At 30°C and 100% humidity, the moisture content in the air can reach 21.3 grams per cubic meter (g/m³).
    • At 70% humidity, the moisture content would be 14.91 g/m³.
  1. High Temperature of TAC Air Outlet: The air from the TAC, operating at pressures of 2–3 kg/cm², is typically at high temperatures (about 175°C). At such temperatures, any moisture in the air remains in a gaseous or steam state. The fly ash is lifted from the Air Lock Vessel (ALV) and travels through pipelines at a velocity of approximately 10 meters per second.
  2. Condensation in Fly Ash Pipes: During monsoon, when these pipelines are exposed to rain, the external cooling effect from the rain causes the temperature of the pipeline to drop significantly. As a result, the high-temperature air inside the pipes cools down rapidly, causing the moisture in the air to condense. The amount of condensation formed is proportional to the temperature drop. As the temperature of the conveying air drops, moisture changes from a gaseous to a liquid state inside the pipes, leading to the formation of water droplets.
  3. Decrease in Fly Ash Velocity: The condensation of moisture inside the pipes increases the weight of the fly ash being conveyed, leading to a decrease in fly ash velocity. The self-weight of the wet fly ash results in lower transportation efficiency, and at times, this decrease in velocity leads to partial blockages.
  4. Formation of Blockages: When fly ash pipelines are blocked by wet ash, the pressure from the TAC continues to push more ash into the system. This results in the fly ash jamming the entire section behind the blockage. The rising TAC pressure is an indicator of complete blockage in the pipeline, as the pressurized air fails to move the ash forward.
  5. Challenges in Clearing Blockages: While initial or terminal blockages can sometimes be cleared by using ESP duct valves and the vent fan at the silo, blockages due to moisture condensation often occur in the middle of the fly ash pipeline. These blockages are more difficult and time-consuming to clear, often leading to prolonged downtimes and the potential need to reduce the unit load to protect the ESP hoppers.

Analysis of Water Content in Air and preventive methods

The water content in the air during the monsoon significantly contributes to the pipeline issues. Below is an analysis based on the rated air quantity of a typical TAC and approximation moisture:

  • Rated Air Quantity of TAC:
    • 3000 cubic feet per minute (cfm) = 5090 cubic meters per hour (m³/hr).
  • Total Water Content in Air:
    • At 100% humidity(Approx. Value)
      Water content = 5090 m3/hr×21.3 g/m3/1000=108 kg (liters of water)
    • At 70% humidity(Approx. Value)
      Water content = 5090 m3/hr×21.3 g/m3×0.7/1000=76 kg (liters of water)
    • This high amount of moisture present in the air during the monsoon season condenses inside the fly ash pipelines, causing wet fly ash to accumulate and choke the system.

Most of the Thermal power plant air compressors in ash plant may not have inter cooler, after coolers or driers. For immediate remedy from monsoons the following may be adopted to prevent fly ash line blockages

  1. Silos filter bags to be cleaned and vent fan shall be in service
  2. Provide duct valve for each pass in the ESP duct
  3. Provide canopy/cover on the fly ash pipe line if possible throughout the length to avoid rain.
  4. Provide auto drains at each receiver tank and on pipe line before ALV (Air lock vessel)
  5. Arrange external water spray on the existing Air receiver tanks for removing moisture from the TAC outlet air, so that this arrangement may act like after-cooler and will have dry air outlet with minimum moisture. This type of arrangement will help in removal of moisture up to 50 to 60% approx. This is a temporary arrangement in case of emergency only
  6. The spray water on the receiver tank may be reused by providing water container and a re-circulation pump

Conclusion

For an efficient fly ash system operation, an after-cooler and drier arrangement at the TAC outlet is crucial. Many thermal power stations might have not installed during erection time, the aforementioned solutions provide a cost-effective and result-oriented method to ensure uninterrupted fly ash system operation during the monsoon season. These measures can prevent unit load reductions, protect ESP hoppers from high ash levels, and save significant costs.

At our plant, implementing these measures has resulted in zero fly ash line chokes even during the continuous downpours. Consequently, the fly ash system runs smoothly without any blockages, avoiding load reductions and saving crores of rupees.