3rd BSME-ASME International Conference on Thermal Engineering 20-22 December 2006, Dhaka, Bangladesh
ASSESSMENT OF THE THERMAL PERFORMANCE OF A COAL-FIRED POWER PLANT BOILER UNIT M.G. Rasul and D.M. Tappenden School of Advanced Technologies and Processes, Faculty of Sciences, Engineering and Health Central Queensland University, Rockhampton, Queensland 4702, Australia (Email:
[email protected])
ABSTRACT Assessment of the thermal performance of a coal-fired power plant boiler unit is presented with an aim to increase the boiler efficiency. Thermal efficiency of the boiler unit was determined and analysed according to the American Society of Mechanical Engineers (ASME) test code and using energy balance method of calculation. Performance test of the boiler unit was conducted for two different mill patterns, Pattern 1: ABCE and Pattern 2: BDEF. These patterns had different burning zones within the boiler unit. The data obtained from the plant show that the boiler efficiency is 90.35% for Pattern 1 and 90.75% for Pattern 2. The areas of high energy losses which require further investigation for efficient operation and maintenance are identified and recommended to the plant authority. Energy conservation measures should be introduced in the areas of high energy losses for improving the boiler efficiency. Keywords: Boiler efficiency, thermal performance, coal-fired power plant, energy balance. 1. INTRODUCTION Power producers are always under ever increasing pressure to maintain production stability, meet strict emissions regulations, increase asset life of equipment, increase profits and adapt to changing economic conditions in real-time. The need to meet these demands on time and within tight budgets can prove to be a real challenge for an organization. For a power industry every little percentage increase in plant’s thermal efficiency can make millions of dollars revenue for the industry. Energy conservation measures should be introduced and implemented in any existing process industries including power industries in order to achieve the highest level of energy efficiency and reduce energy consumption per unit product output [1-3]. Many areas of production in a coal-fired power plant, such as boiler unit, condenser, cooling tower, etc, can reduce the overall efficiency of the plant, if appropriate energy conservation measures are not introduced. Rasul et al. [2] reported that a cement industry in Indonesia can save about 1.264 x 105 US dollars per annum by implementing energy conservation measures in that existing industry. The power plant in this study operates on the basis of the principles of Rankine cycle [4]. The plant has a production capacity of 1400 MW. This is achieved by 4 Babcock-Hitachi boilers each rated at 350 MW. Although the Babcock-Hitachi boiler systems are highly efficient, there have been few major outages in the past due to boiler failure. Each boiler burns coal at 35 kg/s and consists of primary, secondary and tertiary platen super-heaters; also primary and secondary re-heaters and an economizer set-up. The boilers are opposing wall-fired boilers with three rows of burners on each side of the boiler as shown in Figure 1 (A, B, C, D, E and F represents row of burners) and each row having 5 individual burners i.e. the total of 30 burners in 6 rows. The superheater tubes act as a heat exchanger to increase the steam temperature to the desired entropy for the production of mechanical energy further on in the system at the high pressure (HP) steam turbine. The boilers re-heaters are very useful as they regenerate steam to the desired temperature and pressure which then taken again through another set of super-heater tubes that than move onto the interstitial pressure (IP) turbine where the cycle starts again via a set of super-heater tubes. The boiler is fed with
pre-heated air from the rotating hood air heater. This reheated air reduces the energy required to heat the coal (fuel) to a combustible level and also lessens the percentage of moisture content in the fuel supply.
Figure 1: Arrangement of boiler mill Patterns Although the reference plant has its own pride for the effective use of its resources and the skills of its employees, there was a need for an appropriate and simple system of performance testing to determine the boiler efficiency accurately and identify the major areas of energy losses and energy conservations opportunities. This study determined and analyzed the efficiency of a coal-fired power plant boiler unit using the energy balance method of calculation and following the guidelines of the American Society of Mechanical Engineers test code [5]. Performance test of the boiler unit is conducted for two different mill patterns; Pattern 1: ABCE and Pattern 2: BDEF in Figure 1 i.e. fuel burns in two different burning zones in the boiler unit. The areas that require further investigation for improving the energy efficiency is identified and recommended to the plant authority for implementing boiler retrofitting and/or energy conservation measures. 2. THEORETICAL CONSIDERATION The foundation of this study is largely based on basic thermodynamics. In theory, the calculation of boiler efficiency (η B ) is quite straightforward and can be calculated by usable energy outputs divided by energy inputs if both inputs and outputs are known. There are two main methods of calculating thermal efficiency, the Direct Method and the Energy Balance Method [5]. The energy balance method is used in this study as it provides a detailed account of where the fall in efficiency may occur. It also generates a very low level of uncertainty in the results due to the large amount of measurements required. The large number of test points is really its only downfall, where in some cases estimation is required as the results are physically difficult to measure. The direct or input/output method is the simplest technique for calculating efficiency. With its simplicity though comes a larger degree of error and uncertainty. This method does not help to identify the sources of possible inefficiencies within the plant. Methods of boiler efficiency calculation and their relevant concerns are discussed below [5, 6]. 2.1 Direct Method of Calculation The Eq. (1) can be used to calculate the efficiency of the boiler in direct method,
ηB =
mw (h1 − h2 ) × 100 m f Qcal
(1)
where, mw is the feed water flow to the steam generator (kg/s), h1 is the enthalpy of steam directly upstream of high pressure (HP) turbine stop valve (MJ/kg), h2 is the enthalpy at outlet of the final feed heater (MJ/kg), mf is the burning rate of fuel (kg/s) and Qcal is the calorific value of the coal at constant pressure as-fired, as-sampled or as- received (MJ/kg). 2.2 Energy Balance Method of Calculation A simplified equation to calculate the efficiency of the boiler in energy balance method can be given by,
⎧⎪ Lg + Lmf + Lh + La + Lc + Lr ⎫⎪ ⎬ × 100 Hf ⎪⎩ ⎪⎭
η B = 100 − ⎨
(2)
where, Lg is the heat loss due to dry flue gas (kJ/kg), Lmf is the heat lost to moisture being in the fuel (kJ/kg), Lh is the heat lost due to moisture in burning of hydrogen (kJ/kg), La is the heat loss due to moisture in the combustion air (kJ/kg), Lc is the heat loss due to incomplete combustion (kJ/kg) and Lr is the % heat loss due to radiation to surroundings (kJ/kg). In energy balance method, it is possible to gain insight where the loss in efficiency can be occurred and then be able to reduce these losses to increase the boiler efficiency. The items for major losses are discussed as follows; (i) Moisture in the fuel; greater amounts of energy are required to combust the fuel if there is large percentage of moisture; Fuel (coal) that is stored in stockpiles in the outdoor environment will be subject to this attribute. This loss is relatively uncontrollable; (ii) Combustion of Hydrogen; from the combustion of fuel, gases and moisture are taken up the stack; this relies heavily on the calorific value of the coal; (iii) Moisture in air; relative humidity of air can greatly effect the performance of a boiler, due to the fact that the fuel combustion is effected by the moisture content of the air. In large power plant such as the plant in study, the air is pre-heated by a rotating-hood air heater to prevent the input air for combustion being high in humidity; (iv) Heat loss (stack) flue gas; there are many factors that influence this loss; (a) High velocity draft in boiler – not allowing full heat transfer; (b) Water scale in boiler tubing push more heat up stack; (c) Excess air, air heater leakage, inlet air temperature and final gas temperatures can also affect this loss. A problem also with having the gas temperature too low is the acid dew point of the flue gas can be reached and acid deposits can be produced. The acid dew point is the temperature at which sulfuric acid begins to form on the cold end of the air heater and precipitator. The moisture absorbs the sulfur from the gas and starts to degrade the metal surroundings producing large rust deposits in the final stages of the air heater and ducting work to the precipitator; (v) Heat in Pulveriser Rejects; the heat loss due to the rejection from the mills is deemed to be insignificant due to the model of pulverisers that are installed at the plant; (vi) Soot-blower loss; a variable that can either cause loss due to the system steam being used or to ineffective heat transfer due to the amount of soot encrusted on the boiler tubing; (vii) Blowdown; Water within the system is replaced with de-mineralised water and thus dilutes any chemicals in the drum water; (viii) Incomplete combustion; this can occur if; ineffective pulverisation of fuel, low loads (ramping down for night consumption), insufficient air supply (air-fuel ratio), % ash in fuel (supplier) is occurred; (ix) Combustibles in ash; this can occur if the fuel feed rate into the boiler is too high. It can also occur if the percentage of carbon in ash from the economiser hopper is too high. Further factors that can affect the amount of carbon in ash are [7, 8]: • • •
Fineness of grind from the pulverisers: Finer the powdered fuel, better is the burning (combustion) rate. Coal properties (uncontrollable): Higher the volatile matter in the coal, better is the burning rate. Nitrous Oxides (NOX): Some burner designs can produce less NOX. Burner designed for lower NOX levels can reduce amount of carbon in ash.
Miscellaneous factors; these losses include radiation, leaks etc; they can occur if there is insufficient cladding/insulation around header drums and piping, etc.
3. PERFORMANCE TEST AND ON-SITE DATA COLLECTION PROCEDURE Two tests were conducted with full load on the boiler (~350MW) and varying mill patterns (giving different burn zones) to determine the boiler efficiency. Mill patterns were ABCE for the Test 1 from 9am-12pm, and pattern BDEF for the Test 2 from 2:30pm-5:30pm. The readings were taken over a 3 hour test period for each test-load-pattern and at a steady state for the accuracy of the data. The chemical analysis of coal, ash and flue gas samples was done. The grid measurement for the temperature and CO2, O2 and CO at the air heater outlet and inlet were done. The velocity traverse at each air heater inlet and outlet duct was done. Temperature and humidity measurements at the forced draft fan inlet (ambient conditions) were also done. The sampling procedures for coal, ash and flue gas are detailed below. 3.1 Coal and Ash Sampling The coal was sampled and measured from the operational gravimetric coal feeders. There were four feeders online as there are only four mills in operation for the supply of fuel to the boiler. Ash was taken from the bottom of the boiler unit. The sample was taken for one hour’s worth of ash from the boiler. Furnace ash was sampled and collected over an hour period. This involved discharging the furnace ash conveyor onto the ground for mixing and test sampling ease. Fly-ash sampling was collected from the air heater outlets. 3.2 Flue Gas Sampling Flue gas sampling is extremely important to keep as accurate as possible. Incorrect sampling may critically affect the boiler overall efficiency as flue gas composition are major indicators of combustion efficiency. Better the combustion, lower is the CO in the flue gas. Correct sampling determines the exact amount of unburned carbon in the flue gas which is one of the indications of the quality of combustion. Ash heater inlet measurements were taken straight after the gas had come from the boiler. This is known as “Economizer Outlet”. There were five sampling points across each duct. Each duct is 4 meters deep and of a square cross-section. Velocity, O2, CO2, CO and temperature were measured from this economizer outlet. Air heater outlet measurements were taken in the four sampling ports across the width of the duct. The duct was 4 meters deep and of circular cross-section. 4. RESULTS AND DISCUSSIONS The on-site measured data and analysis for different properties and energy losses are given in Table 1. It can be seen from Table 1 that the amount of heat loss in dry flue gas is the main cause of inefficiency of the boiler. There are many factors that influence this loss, which could be from high velocity draft in boiler that may not allow full heat transfer between the energy produced by the combustion process and the boiler tubing. Water scale in boiler tubing affects heat transfer. Excess air, air heater leakage, inlet air temperature and final gas temperatures can also affect this loss. Table 1: Onsite measured results and analysis Properties
Ultimate Analysis of Coal
Coal Consumption
Description Carbon Hydrogen Sulphur Oxygen Nitrogen Moisture Ash Gross calorific value of coal
Test 1 65.76 03.48 0.49 3.87 1.38 8.38 16.64 26201 33.527
Test 2 66.94 3.52 0.45 4.68 1.44 9.53 13.44 26627 33.518
Units % % % % % % % kJ/kg kg/s
Atmospheric Conditions
Absolute Humidity
Air Heater outlet flue gas Analysis
Air heater temperature
Unburned carbon Ash
Enthalpy
Atmospheric pressure Ambient air temperature (dry bulb) Wet-bulb temperature Saturation vapour pressure @ DBT Saturation pressure of water @WBT Partial pressure of vapour @ atms Partial pressure of vapour @ test temp Relative humidity Absolute humidity O2 in dry gas CO2 in dry gas CO in dry gas Nitrogen Dry gas Nitrogen in flue gas Dry air Moisture in flue gas Wet gas AH inlet gas temperature AH outlet gas temperature AH inlet air temperature AH outlet air temperature Furnace hopper ash pit Economizer hopper Dust from AH outlet gas Unburned carbon in dry refuse Dry refuse Unburned carbon Actual unburned carbon Enthalpy of vapour in gas, hg @ air heater air outlet temperature Enthalpy of saturated liquid, hf @ ambient temperature Enthalpy of saturated vapour, hg @ ambient temperature
Mean specific heat
Heat losses
Equivalent losses
Heat in dry flue gas Moisture in air Moisture in fuel Moisture from burning of hydrogen Unburned carbon Surface radiation Total heat losses Dry flue gas loss Moisture in fuel loss Moisture from fuel heat loss Moisture in air loss Unburned carbon loss Surface radiation loss Total equivalent heat losses
101 31.43 22.25 4.61 2.69 2.06 2.062
101 31.43 22.25 4.61 2.69 2.06 2.062
kPa 0 C 0 C kPa kPa kPa kPa
44.72 0.013 5.2 13.82 0.04 80.94 11.97 8.93 11.6 0.55 12.51 360.15 139.43 46.68 307.44 0.021 0.053 0.024 0.025 0.0171 0.004 0.653 2733
44.72 0.013 5.21 14.08 0.02 80.69 12.0 8.92 11.59 0.56 12.56 343.3 134.9 48.46 309.26 0.013 0.089 0.019 0.021 0.137 0.003 0.667 2729
% kg/kg Vol% Vol% Vol% Vol% kg/kg-fuel kg/kg-fuel kg/kg-fuel kg/kg-fuel kg/kg-fuel 0 C 0 C 0 C 0 C kg/kg-fuel kg/kg-fuel kg/kg-fuel kg/kg-fuel kg/kg-fuel kg/kg-fuel kg/kg-fuel kJ/kg
131.68
13.68
kJ/kg
2558.3
2558.3
kJ/kg
1.0001 1292.25 26.26 217.99 808.94 141.99 47.16 2534.51 4.92 0.83 3.08 0.10 0.54 0.18 9.65
0.9965 1237.47 25.64 247.53 816.98 95.73 47.93 2471.28 4.64 0.93 3.06 0.1 0.36 0.18 9.25
kJ/kgK kJ/kg kJ/kg kJ/kg kJ/kg kJ/kg kJ/kg kJ/kg % % % % % % %
In Test 1 there was much more heat loss due to high exit gas temperatures. Having a 360 0C gas temperature coming from the boiler (air heater inlet), with an exit gas temperature of 139 0C after passing through the air heater producing a heat transfer to change the air heater inlet air temperature from 46 0C to 307 0C. Whereas with Test 2 the inlet and outlet gas temperatures were 343 0C and 135 0 C respectively, with inlet and outlet air temperatures of 48 0C and 135 0C respectively. It can be observed from Table 1 that in Test 1 there is a large amount of heat energy not being used during the combustion process and thus producing a higher gas exit temperature from the boiler. It can also be seen that from an energy loss point of view, Test 1 produced 1292 kJ of energy loss due to heat in dry flue gas and Test 2 produced 1237 kJ of energy loss. From the data obtained and calculation of the heat losses shown in Table 1, the boiler efficiency was found to be 90.35% for Test 1 and 90.75% for Test 2. These values are comparable with a typical efficiency of an industrial boiler at full load operating conditions [9]. In fact these values lies at the higher end within the range of efficiency an industrial boiler can have. The difference is related to the amount of heat loss due to moisture present in the coal, with Test 1 producing 218 kJ of energy loss and Test 2 producing 248 kJ of energy loss. This relates directly to the amount of heat lost due to combustion. In Test 1 there was less energy loss due to moisture content of fuel, therefore a higher gas exit temperature would be produced, which can be seen from the results. Conversely higher moisture content produced lower gas exit temperatures as the combustion process was further complete. A relatively large amount of heat loss was also occurred in the amount of unburnt carbon produced. In Test 1 approximately 141 kJ of energy was lost due to unburnt carbon and in Test 2 approximately 96 kJ of energy was lost due to unburnt carbon. This also relates to the amount of complete combustion of the fuel in both tests, where the higher the complete combustion, the lower the amount of unburnt carbon. 5. CONCLUSIONS The boiler efficiency of 90.35% and 90.75% were found in Test 1 and Test 2 respectively. Incomplete combustion in Test 1 produced slightly more heat losses throughout the boiler system compared to Test 2. In Test 1 there was much more heat losses due to high exit gas temperatures. The amount of heat loss in dry flue gas is the main cause of the inefficiency of the boiler. A higher efficiency could be achieved if loss in dry flue gas can be reduced. Several factors that influenced this loss could be high velocity draft in boiler, water scale in boiler tubing, etc. Excess air, air heater leakage, inlet air temperature and final gas temperatures can also affect this loss. There could be various factors that can influence the amount of losses a boiler will effectively encounter. Those are: • • • • • • •
Poor maintenance, inefficient insulations, leaks, etc. Failure in maintenance of air-fuel ratio to guarantee complete combustion of fuel. Water-side fouling/scaling of boiler piping, water treatment system not functioning. Fire-side fouling due to improper soot blowing techniques, ash accumulation on boiler piping. Unacceptable pulverization of fuel from mills – causing incomplete combustion. Incorrect flow rate of air into and away from the system (if not kept at negative pressure there could be a high chance of boiler excursion). High fuel burning rate - results more unburned carbon in ash.
6. RECOMMENDATIONS Heat losses in dry flue gas should be reduced in order to achieve higher overall efficiency of boiler. This can be done by: reducing draft velocity in boiler, controlling water scale in boiler tubing, reducing air heater leakage, reducing exit gas temperature, etc. From a combustion point of view, the mill pattern may have influenced the drop in efficiency seen between Test 1 and Test 2. To understand the combustion difference in mill patterns, an exact reproduction of either Test 1 or Test 2 would provide the knowledge of changing mill patterns and the amount of complete combustion. Repeatability of testing is a requirement for precisely calculating boiler efficiency in order to
recommend any energy conservation measures or any boiler retro-fitting. Further testing to reproduce the results and addressing the above mentioned factors can be recommended to the plant authority in order to accurately determine the boiler performance. REFERENCES [1]
[2]
[3] [4] [5] [6] [7] [8] [9]
Rasul, M.G. , Tanty, B.S. and Khan, M.M.K, Energy Savings Opportunities in Iron and Steel Industry, Proceedings 2nd BSME-ASME International Conference on Thermal Engineering, 2-4 January 2004, Dhaka, Bangladesh, Vol 2, pp.1116-1122. Rasul, M.G., Widianto, W. and Mohanty, B., Assessment of the Thermal Performance and Energy Conservation Opportunities in a Cement Industry in Indonesia, Applied Thermal Engineering, 25 (2005), pp.2950-2965. Rasul, M.G., Tanty, B.S. and Mohanty, B., Modelling and Analysis of Blast Furnace Performance for Efficient Utilization of Energy, Applied Thermal Engineering (in press), 2006. Moran, M.J. and Shapiro, H.N., Fundamentals of Thermodynamics, Chapter 8, 5th Edition, John Wiley and Sons, Inc, 2004. American Society of Mechanical Engineers (ASME) Code for Fired Steam Generators – PTC – 4-1998; Three Park Avenue, New York, NY 10016-5990, p.20. Australian Greenhouse Standards (2001)– Generator Efficiency Standards – V 1.2; (Pages A.2). Covey, D., Boiler Process and Combustion, Stanwell Corporation Limited, Rockhampton, Queensland, Australia. 2001, pp. 41-50. Covey, D., Production Plant Training, Stanwell Corporation Limited, Rockhampton, Queensland, Australia, 2001, pp.15-18. Council of Industrial Boiler Owners (CIBO), March 2003, http://www.cibo.org
