PREDICTING BOILER EFFICIENCY DETERIORATION USING ENERGY BALANCE METHOD: CASE STUDY IN 660 MW POWER PLANT JEPARA, CENTRAL JAVA, INDONESIA

This research aims to determine the deterioration of boiler efficiency in Tanjung Jati B Unit 3 and 4 coalfired power plant with capacity 2x660 MW in Jepara Central Java Indonesia using energy balance (indirect method) based on ASME PTC 4-2018. The deterioration of boiler efficiency per year estimated using linear regression. From the results of the research, it is found that the deterioration in boiler efficiency of unit 3 is 0.19% per year and unit 4 is 0.44% per year. Large heat losses that vary for each performance test are greatly influenced by the use of various coal properties.


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The auxiliary steam required for the burner atomizing steam and pulverizer inert steam is supplied from the boiler tertiary SH inlet header after necessary pressure reduction. Soot blowing steam is also supplied from the tertiary SH inlet header.
A blowdown tank is provided to receive the various drains from the boiler pressure parts like economizer, water walls, steam drum, soot blower and auxiliary steam lines etc. The hot drains from blow down tank are led to the boiler drain pit after suitable attemperation.
Testing Conditions (Performance test) is carried out at 660 MW load (100% load). Testing is carried out twice in a year and in two different periods. That are the rainy and dry periods. This is due to the weather condition, which may affect the level of water content in coal, ambient temperature, and moisture content.
In order to obtain a reliable and comparable result of the test consecutively, internal testing conditions must be carried out according to the standard. The following conditions must be done as a minimum standard when testing: boilers must be operated in automatic control mode, the turbine generator must be kept at a constant load of 660 MW net, continuous blowdown (CBD) must not be operated during the testing process, all soot blowing process must be carried out and completed before testing and must be stopped during the test, coal silos must be filled with sufficient quantities for testing with the same coal, coal filling (coal unloading) may not be carried out during testing, each ash hopper must be emptied at least 2 hours before the test is carried out, all drain line valves must be closed.

ANALYSIS
There are two methods to calculate boiler efficiency, namely by the direct (input-output) method and indirect (heat losses/energy balance) method. In the input-output method, the addition of total heat to the working fluid in the economizer section, the evaporator, heat and reheating are calculated and the results are divided by the fuel input energy. The uncertainty of the direct method is quite large between 3% -6% because it is very difficult to measure the flow rate of fuel and working fluid with an accuracy of more than about 5%, so this direct method is not too accurate and is not used in the calculation of boiler efficiency(EF), as defined in eq.1 [9]: where QrO is output, MrF is the measured mass flow rate of fuel, HHVF is higher heating value of fuel The basic equation of EF on the energy balance method is presented in eq.2. This method has a small uncertainty between 0.4% -0.8%, so this method is widely used in the practice of calculating boiler efficiency. In this method, it is considered that the total fuel input energy is transferred to the working fluid or lost in various ways, but this loss can be known. There are 6 kinds of heat losses in the boiler and all is calculated in the form of energy losses per unit mass of fuel (kJ/kg) or (Btu/lb).
where and are the sums of losses and credits calculated on percent input from fuel basis, and are the sum of the losses and credits calculated on Btu/hr (W) basis, or boiler efficiency ( ) can be expressed in eq.3 [10]: where 1 is heat loss due to heat in dry gas, 2 is heat loss due to moisture in fuel, 3 is heat loss due to moisture from burning hydrogen in fuel, 4 heat loss due to moisture in air, 5 is combustible in refuse, 6 is heat loss due to surface radiation and convection (according to ABMA chart), is fuel higher heating value, 1 is heat credit (entering dry air, sensible heat in fuel, moisture entering with inlet air), 2 is pulverizers, boiler circulation pump, air preheater drive power consumption, is boiler heat output.

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Other losses, such as losses due to unburned combustibles (unburned hydrogen and hydrocarbon, carbon monoxide), sensible heat of residue, Nox formation and radiation to bottom ash hopper and sensible heat in slag, etc. are not considered for the boiler efficiency calculation because the magnitude of the losses is negligibly small.
Heat loss due to heat in dry gas ( 1 ) is presented in eq.4.
where M D is dry gas, HD is enthalpy of dry gas at air heater (AH) outlet gas. Heat loss due to moisture in fuel ( 2 ) is presented in eq.5.
where M WF is moisture from water in fuel, H is enthalpy of water vapor at AH outlet gas Heat loss due to moisture from burning hydrogen in fuel ( 3 ) is presented in eq.6.
where M WH 2 F is moisture in the combustion of hydrogen in fuel, × H is enthalpy of water vapor at AH outlet gas (hydrogen and water in the fuel are all defined to water vapor) Heat loss due to moisture in air ( 4 ) is presented in eq.7.
where M WDA is absolute humidity, M DA is dry air. Combustible in refuse ( 5 ) is presented in eq.8.

= C × 33700
where C is unburned carbon in fuel Heat credit ( 1 ) is presented in eq.9.
where BDA is entering dry air, BWA is moisture entering with inlet air, BF is sensible heat in fuel. Pulverizers, boiler circulation pump, air preheater drive power consumption ( 2 ) is presented in eq.10.
where Q is power consumption of pulverizer, Q is power consumption of BCP, Q ℎ is power consumption of AH.
Boiler heat output is presented in eq.11.
where is Super Heater (SH) outlet steam enthalpy, is main steam flow, is eco inlet water enthalpy, is eco inlet water flow, is reheat steam flow, is Reheater (RH) outlet steam enthalpy, is RH inlet steam enthalpy, is SH spray water flow, SH spray water enthalpy, is RH spray water flow, is RH spray water enthalpy.

RESULTS AND DISCUSSION
Calculation of losses and boiler efficiency at 2x660 MW TJB #3,4 Power Plant was carried out from Commercial Operation Day (COD) to the last Performance Test (PT). Table 1 and Table 2 show the parameter data for calculation. In Table 3 and Table 4, we can see losses and boiler efficiency for unit 3 and unit 4.
Boiler efficiency values vary for each performance test for both unit 3 and unit 4 boilers due to the coal properties used also vary that indicated in Table 3 and Table 4. One of the coal properties that causes boiler efficiency difference is the different calorific value of the used coal. The results of the 1 st performance test of boiler unit 3 with coal calorie 5900 kcal/kg, producing boiler efficiency of 89.71%, and the 10 th performance test with lower coal calorie of 5542 kcal/kg giving the efficiency of 88.38% indicated in Tables 1 and 3. Tables 2 and Table 4 present the result of the 13 th boiler performance test obtained efficiency value of 89.08% with the calorific value of 5814 kcal/kg and lower efficiency value of 87.48% obtained from the use of coal with the calorific value 5456 kcal/kg. This is consistent with the results of previous research where the calorific value of coal will have a significant effect on boiler efficiency. The higher calorific value of coal will increase as the value of boiler efficiency. The use of lignite coal with the calorific value 4300 kcal/kg will give the efficiency of 77.51% compared to semibituminous coal with the calorific value 5800 kcal/kg producing 80.20% boiler efficiency [4]. Moisture in coal is also the main parameter that gives effect to the efficiency of the pulverized coal-fired power plants [1]. From Tables 1 and  Tables 3, moisture in fuel at boiler unit 3      254 due to moisture in air ( 4 ), and combustible in refuse ( 5 ), but on 8 th , 9 th , and 10 th performance test unit 4 it looks heat loss due to moisture from burning hydrogen in fuel ( 3 ) greater than the heat loss due to heat in dry gas ( 1 ), this is due to the hydrogen content (H) in coal is quite large. This is in line with the previous research that the value of boiler efficiency is affected by the hydrogen content in the fuel. The higher of heat loss due to moisture from burning hydrogen in fuel ( 3 ) will reduce boiler efficiency [8].
The deterioration of boiler efficiency is calculated using linear regression method [8]. Figure 4 shows the linear regression method is only used in 4 th ~ 13 th performance tests for boiler unit 3, due to the 1 st ~ 3 rd performance test have significant deviation. Whereas in Figure 5, is only used for 3 rd ~ 10 th performance tests due to 1 st ,2 nd ,11 th ,12 th and 13 th performance also have a significant deviation. From the calculation results, it is found that the deterioration of boiler efficiency for boiler unit 3 is 0.09% per 6 months or 0.19% per year and the deterioration of boiler efficiency for boiler unit 4 is 0.22% per 6 months or 0.44% per year.