Performance of Regenerative Air Preheater of Pulverized Coal Fired Boilers

Abstract

: The air preheater (APH) is the final heat trap in the boiler\'s flue gas path. Effective pre-combustion coal drying and efficient combustion in the boiler are required for APH to operate efficiently. The effectiveness of heat transfer in APH is significantly influenced by the characteristics of relative humidity and ambient air temperature (AAT). Relative humidity (RH) and air density are also impacted by variations in AAT, which in turn alters the heat capacity of the air. If there is insufficient reserve heat exchange capacity in APH, an increase in AAT will diminish the possibility of waste heat recovery. Additionally, a change in AAT will affect the power consumption of the fans and the leakage through various seals.

Introduction

I. INTRODUCTION

In pulverised coal fired power generation, moisture in coal is a major problem. Due to its hygroscopic nature, coal acquires a lot of surface moisture with seasonal changes. The performance of the pulverizer is impacted by coal moisture. Effective coal pulverisation and pneumatic transport depend on efficient coal drying. Traditional large pulverised coal-fired boilers use a coal drying mechanism that incorporates waste heat recovery from hot flue gas before discharge through stack by producing hot air stream (regenerative or recuperative APH) for coal drying purposes. Additionally, the need for cost- and energy-effective coal drying processes is critical because, in some cases, the combined weight of coal ash and moisture exceeds 50% of the coal "as received." Therefore, an appropriate and effective drying system on flue gas waste heat recovery will increase profitability while lowering the cost and emissions of power generation. The capacity for drying, however, is constrained by the hot primary air (PA) temperature and available hot airflow. The air preheater (APH) inlet flue gas temperature determines the PA temperature, which in turn limits the mill drying capacity. An increase in this temperature will reduce the boiler's overall efficiency, and too much hot PA may start a fire in the mill when the amount of coal fed is decreased to match a decrease in the demand for steam. Once more, any indirect heat exchange mechanism for coal drying within boiler combustion PA intake - preheating - combustion and exhaust circuit has an inherent limit on coal moisture removal capacity in the pulverising process that compromises overall boiler efficiency as heat of evaporating moisture is consumed from fuel energy.

The heat retained in APH is consumed by the moisture from evaporated coal, and extra accessible heat from secondary air aids in achieving the furnace's combustion conditions. AAT also influences how much heat is available for steaming, which has an impact on how well APH works. The performance of the APH is impacted by the relative humidity (RH) and AAT, which determine heat loss resulting from moisture in the air.

II. AIR PREHEATER

The last heat trap in the boiler's heat transfer route, known as the APH, raises the ambient air temperature (AAT) by transferring heat from flue gas that would otherwise be squandered by being released through the stack. It functions as a heat exchanger, heating the air exiting the APH. APH typically provides 10% to 12% of the boiler's thermal efficiency and 10% to 15% of the total fuel heat trapping. In a boiler, air is warmed since it is utilised to dry coal and expedite the achievement of the pulverised coal's ignition temperature for effective combustion. While the heat content of hot primary air is crucial for determining the pulverised coal milling system's drying capacity, the heat content of hot secondary air dictates how quickly ignition temperature is reached and how stable the flame front velocity is at the burner mouth. Insufficient hot PA heat content will restrict raw coal fed to the pulverizer's ability to dry, which will lower the pulverised coal throughput capacity. Similar to this, inadequate hot secondary air temperature and heat content may cause the flame front to hunt, occasionally losing ignition and increasing carbon in ash loss from unburned coal particles. The amount of heat rejected to the chimney may be greatly decreased by using APH, which can successfully recover heat from flue gas at lower temperature levels than the economiser, enhancing the boiler's efficiency.

The boiler efficiency rises by around 1% for every 22°C decrease in the temperature of the flue gas discharge. By using hot air, the APH increases stability, intensifies, and enhances combustion. Additionally, it improves the boiler's efficiency by recovering waste heat, burning inferior fuel effectively, and speeding up heat transmission in the boiler, which minimises the need for heat transfer space. Complete combustion is achieved as a consequence, there are less unburned fuel particles in the flue gas, and enhanced combustion allows for quicker load variation. Hot air may burn coals of lower quality effectively. As a result, APH is essential in lowering fuel and auxiliary power use.

The two primary categories of APHs are recuperative and regenerative. The early power boilers had recuperative shell and tube APHs with air flowing horizontally through the tubes' baffle-plated outer surfaces and flue gas flowing vertically through them. Ash fouling, especially at the hot end of the flue gas passage via tubes, has a significant negative impact on the efficiency of this type of tubular recuperative APH.

In a radially split cylindrical shell known as the rotor, hundreds of high efficiency heat-exchanging metal components are compactly and densely distributed within sector-shaped compartments. Heat transfer is facilitated by the corrugated design of the heating components and the tight packing of the baskets. Sector plates and a sealing system in the APH separate the gas side from the air side. The housing surrounding the rotor has connections for ducts on both ends and is sufficiently sealed by radial and circumferential sealing members that create a secondary air passage through one (bi-sector) or primary and secondary air passage through two (trisector) sectors of the APH and a gas passage through a different sector. Between these two sides, the rotor revolves. Heat is collected by the elements as they go through the hot flue gas stream and released as they move through the air flowing passage(s) as the rotor gently spins the elements through the air and gas passageways, raising the temperature of the air utilised in combustion. Flue gas and air flow in typically opposing directions.

Despite having a surface area per unit volume and volume per unit load (m3/kW) that are superior to tubular APHs (350 m2/m3 and m3/kW, respectively), rotary APHs nevertheless have substantial air leaks from the air side (pressure: +6.5 to 7.5 kPa) to the flue gas side (-0.3 to -0.8 kPa). This is due to tip sealing. Due to inefficient or delayed combustion in the furnace, air leakage to the flue gas side of the APH may result in a fire danger. Despite the fact that a lot of fan power is expended on it, the tramp air leakage cannot be used for combustion. In a regenerative rotary APH, a minimum leakage of 5%–7% is obviously inevitable. In this study, the tri-sector regeneration type APH is the only one whose performance is evaluated in relation to the AAT variation.

For high moisture coal drying, the regenerative APH's usual rotational direction matches the direction of heat transfer from flue gas to PA preheating, followed by secondary air preheating. For high volatile, low moisture coal, the rotational direction must be reversed to avoid a fire danger in the APH caused by unburned carbon in the ash and an unwelcomely high, hot PA temperature at the mill intake.

III. AIR PREHEATER PERFORMANCE

The performance of auxiliary equipment is becoming more crucial as the efficiency of pulverised coal burned thermal power production is progressively stressed. Due to inadequate waste heat recovery and beneficial combustion air leakage into exhaust flue gas, the regenerative APH is a cause of lost thermal efficiency. Furthermore, it is challenging to gauge the temperature of the APH exit flue gas and the effectiveness of the air heater due to tramp air intrusion, APH bottom ash hopper leaks, and intake air duct leaks.

Various routes between the rotor and stator may be the source of the leakage in regenerative APH. Due to damage to the circumferential seal, air and flue gas may bypass the rotor through the APH. Due to no appreciable change in total air or flue gas flow, they may reduce APH heat transfer efficiency but have no impact on induced draught (ID), forced draught (FD), or PA fan power consumption. Larger diameter regenerative rotary APHs with greater hot end and cold end temperature differences (on average, 185–200°C) and with provisions for thermal expansion have a higher prevalence of circumferential seal leakages. The radial seal leakage may happen at either the hot end or the cold end of the regenerative APH, causing, respectively, hot air entrance and cold air intrusion in the flue gas route. The power consumption of the ID and FD fans is increased by the hot and cold air leakage through the radial seals, but the boiler's thermal efficiency is not improved. Insufficient total combustion air flow from excessive radial seal leaking may result in lack of ignition. When expressing APH air ingress as a percentage of O2 in APH exit flue gas flow, radial seal leakages are expressed as a fluctuation in O2 concentration in flue gas before and after the regeneration APH. However, as was already said, neither the American Society of Mechanical Engineers' (1991) standard nor any performance guarantee test describe the flue gas or air traversing the APH rotor owing to circumferential seal leakage properly, nor have they characterised it adequately.

Always monitor APH air ingress in the flue gas route based on changes in boiler exit flue gas flow rather than changes in overall air flow through the APH. For the purpose of adjusting the flue gas exit temperature, which must be ensured during the performance guarantee test and is necessary for assessing the overall boiler efficiency, it is crucial to evaluate changes in the total flue gas flow via the APH. Although it shows a guaranteed pressure drop over the APH that would have otherwise been significantly greater than the test result, it ignores the heat transfer loss associated with bypassing the APH and the impact of air and flue gas bypass. More air and flue gas flow will be observed with less of system resistance and the pressure drop across the APH will be lessened with more APH bypass.

The weight of air moving from the air side to the gas side of the air heater is known as the APH leakage percentage. This index serves as a gauge for the health of the rotor post and radial seals on the air heater. Air heater leakage rises when air heater seals deteriorate. The increase in air heater leakage raises the PA, FD, and ID fans' station service power needs, raising unit net heat rate and potentially restricting unit capacity. An indicator of the internal health of the air heater is the gas side efficiency index. The air heater's gas side efficiency declines when internal problems like ash pluggage and basket wear get worse. Typically, there is also an increase in exit gas temperature and a decrease in air heater air outlet temperature, resulting in an increase in unit heat rate.

The amount of heat energy (qg) received by APH from flue gas may be specified as:

V. FIELD STUDY TO DETERMINE THE EFFECT OF AAT IN APH EFFICIENCY

A. Plant Configuration Choosen Foe Field Study

A pulverised coal fired thermal power plant with a modified Rankine steam cycle, operating in the subcritical regime with a gross power generation capacity of 135 MWe, and fitted with a regenerative, reheat boiler having a corner firing arrangement was selected in order to assess the performance variation with reference to design criteria and running plant performance of a regenerative APH with variation in AAT. During a field investigation of the plant in central India, which has a significant variation in AAT throughout the year, a collection of operational metrics pertinent to the assessment of APH performance was gathered. To eliminate bias, all data were collected during a 24-hour span to avoid variation in RH factor on the same day. at virtually identical plant generating load conditions (with a variance in plant load of less than 0.5%).

On the field research day in question, the RH in the area was close to 40%. Table 1 lists the information gathered on the mass of air flow (ma), flue gas flow (mg), and flow of fly ash (mA) in flue gas at the APH's input and outlet temperatures as determined by the plant data acquisition system. As indicated in Table 1, the mass flow rate is expressed in tonnes per hour (TPH), whereas the temperatures of the air leaving the APH, the AAT, the flue gas entering the APH, and the flue gas exiting the APH are expressed in degrees Celsius (°C).

Table 1 Operating parameters of APH at 135 MWe gross power generation with different AAT

The variation of mass flow rate of air when plotted against AAT is shown in Figure 2. The graph in Figure 2 plotted against given data matches very close to the relation as shown for ma and AAT (Tae). Therefore, it is seen that due to 8°C increase in temperature there is about 35 TPH decrease in air flow which is almost 4.37 TPH/°C change in AAT. Table 2 provides the enthalpy of air at various AAT along the calculated energy loss to evaporate air – moisture at RH of 40%. The specific humidity is expressed as moisture content in kg per kg of wet air. The results of Table 2 are used to represent graphically the relationship between AAT with fuel energy lost to evaporate air – moisture per kg of intake air in APH as shown in Figure 3.

Conclusion

Deterioration in APH performance inevitably results in a loss of unit capacity that causes commercial losses, a reduction in boiler efficiency because of incomplete combustion, increased dry flue gas loss, and finally a rise in energy use in the draught system. The loss of ID fan margin and increase in ID fan power consumption due to air leakage in the flue gas path within the APH eventually limits the amount of extra O2 levels available in the furnace to ensure combustion completeness, limiting unit capacity. As a result, it is determined that using regenerative APH in tropical regions with greater AAT is not an energy-efficient alternative since significant fuel energy is wasted to remove moisture from the air in addition to the high moisture levels low-rank coal used for pulverised coal fired power generation system. In reality, hot PA drying will result in a greater loss of fuel energy since it dries out low-rank, high moisture coal, which leaves less heat available for steam production. As a result, atmospheric fluidized bed dryers that use waste heat from flue gas downstream of ID fans and before exhaust through stacks can more economically accomplish high moisture coal drying (Bhattacharya and Banerjee, 2011). The partial flue gas recirculation through pulverizer (PFGR) system can lower the APH heat transfer loading because the capacity of coal drying can occasionally be limited by an inadequate hot PA temperature, which in turn affects the capacity of power production. Additionally, a decrease in the demand for coal drying results in a rise in secondary air temperature at APH, which ensures improved combustion with a decreased risk of NOx formation and less extra air.

References

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Copyright

Copyright © 2023 M. Ravi Kumar, D. Kulandaivel, K. Ramesh . This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.