Statistical Analysis of Regenerative Air Preheater of Pulverized Coal Fired Boilers

Abstract

The data gathered from the field study and different parameter recorders connected to the data acquisition system of the distributed control system panel for measuring flow, pressure, and temperature are subject to typical systematic instrumentation measurement uncertainties with regard to the primary measuring elements, sensors, transducers, amplifiers, digital converters, and recorders, as well as the effects of the environment. The acceptability of data is statistically confirmed by taking into account the unpredictability of the inaccuracy in recorded parameters for temperature, pressure, and flow measurement. The statistical output on the measured and expected air flow through the APH (ma) in relation to different AAT (Tae) is summarized.

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. REGENERATIVE AIR PREHEATER

In regenerative type APH thousands of high efficiency heat exchanging metal elements are closely spaced and compactly arranged within sector shaped compartments of a radial divided cylindrical shell called rotor. Heating elements are having corrugated profile and placed in closely packed baskets, which provide a medium for heat transfer. The gas side and air side in APH are separated by Sector plates and sealing arrangement. The housing surrounding the rotor is provided with duct connections at both ends and is adequately sealed by radial and circumferential sealing members forming 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 another sector. Rotor rotates between these two sides. As the rotor slowly revolves the elements through air and gas passages, heat is absorbed by elements while passing through the hot flue gas stream and the same heat is released while passing through the air flowing passage(s) thus increasing the temperature of air used in combustion. The direction of flue gas and air are generally opposite.

Though the rotary APHs are superior to tubular APHs in terms of heat exchange effectiveness and surface area per unit volume [350 m2/m3 and volume per unit load (m3/kW)], tip sealing is the source of severe air leaks from the air side (pressure: +6.5 to 7.5 kPa) to flue gas side (–0.3 to –0.8 kPa) as reported by Bhatt (2007). The air leakage to flue gas side of APH may cause fire hazard in presence of combustible in flue gas due to poor or delayed combustion in furnace. The leakage tramp air is unavailable for combustion though significant fan power is spent over it. Obviously, a minimum leakage of 5%–7% is unavoidable in regenerative rotary APH. In this paper, evaluation of performance is done only on the tri-sector regenerative type APH with respect to the variation of AAT.

The normal direction of rotation of the regenerative APH follows the direction of heat exchange from flue gas to PA pre-heating followed by secondary air preheating and this is suitable for high moisture coal drying. The direction of rotation need to be reversed for high volatile low moisture coal to prevent fire hazard in APH resulting from unburnt carbon in ash and undesirably high hot PA temperature at mill inlet.

III. STATISTICAL ANALYSIS

The data collected from the field study and various parameter recorders associated with data acquisition system of the distributed control system panel for measurement of flow, pressure and temperature are having usual systematic instrumentation measurement uncertainties with reference to primary measuring elements, sensors, transducers, amplifiers, digital converters and recorders including the environmental impacts.

Considering the randomness of the error in recorded parameters in temperature, pressure and flow measurement, the acceptability of data is statistically validated. The summary of statistical output on measured and predicted air flow (ma) through APH with respect to various AAT (Tae) is as given in Table 1 and graphically represented in Figure 1.

Conclusion

Deterioration in APH performance invariably leads to loss of unit capability leading to commercial losses, loss of boiler efficiency due to incomplete combustion, along more of dry flue gas loss and ultimately higher energy consumption in the draft system. The leakage of air in the flue gas passage within APH causes loss of ID fan margin and rise in ID fan power consumption ultimately reduces availability of excess O2 levels in the furnace for ensuring combustion completeness; which finally restricts unit capacity. Therefore, it is concluded that the use of regenerative APH in tropical countries with higher AAT is not an energy efficient option where RH of air is substantially high and considerable fuel energy is wasted to dry up air-moisture beside the high moisture low-rank coal used for pulverised coal fired power generation system. In fact, hot PA drying will increase the loss of available fuel energy as to dry up the low-rank high moisture content coal and thereby less of heat is available for steam generation. As such, high moisture coal drying can be more economically achieved through atmospheric fluidised bed drier using waste heat of flue gas downstream of ID fan and before exhaust through stack (Bhattacharya and Banerjee, 2011). The partial flue gas recirculation through pulveriser (PFGR) system can reduce the APH heat transfer loading, since coal drying capacity sometimes get restricted due to insufficient hot PA temperature and thereby causes power generation capacity restriction. Besides, a reduction in coal drying need in turn increases more secondary air temperature at APH ensuring better combustion with lower NOx generation potential with less excess air.

References

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Copyright

Copyright © 2023 D. Kulandaivel, M. Ravi Kumar, 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.