Study on the thermodynamics performance of industrial boiler

Transcription

1 Study on the thermodynamics performance of industrial boiler M. Faizal a,b, M. H. Hamzah b, A. Navaretsnasinggam b a ADP, Taylor's University Lakeside Campus, Selangor, Malaysia b Department of Mechanical Engineering, University of Malaya, Kuala Lumpur, Malaysia Abstract Boilers are prime source of thermal energy in most industrial units. The combustion efficiency of any boiler is an important factor as it directly controls the fuel consumption. In industrial boilers, a thermal energy facility is selected to meet process requirements under full load and the worst weather conditions, through the boiler itself seldom operates at full production capacity.seasonal variations in weather conditions greatly affects the requirement of thermal energy. The boiler thermal efficiency is weakly effected by excess air ratio, unit load and fuel lower heating value varying from 90.3% % for wide range of above variables. Thus, in this study factors that are critical in assessing energy efficiency in industrial boilers are studied and discussed. 1.0 Introduction High pressure and high temperature steam for industrial needs are supplied from boilers. The job of a boiler is to apply heat to water to create steam. There are 2 approaches of a boiler. Firstly, the Fire Tube Boiler (Figure 1.0) which are considered as an outdated model Figure 1.0 Fire tube boilers It consists of a tank of water perforated with pipes. The hot gasses from the coal of wood fire runs through the pipes to heat the water in the tank. The entire water tank is under

2 pressure. Major explosion could happen if the tank burst. The common boilers today are the Water Tube Boilers (Figure 1.1). The water runs through a rack of tubes that are positioned in the hot gases from the fire source. Figure 1.1 Water tube boilers In real industrial boilers, things are more technical and complicated because the goal of a boiler is to extract every possible energy from the burning fuel to improve its efficiency. In Malaysia, under the Factories and Machinery Act 1967, each steam boiler to be manufactured or brought into this country needs a valid certificate of fitness (CF) before it may be used. Therefore, approval of the steam boiler design needs to be obtained in advance from the Department of Occupational Safety and Health (DOSH) before it can be manufactured, installed or used. Application may be made by the manufacturing company, installer, supplier or owner of the steam boiler. Local boiler manufacturers need to register as a manufacturer with the department. 1.1 Basic concept This paper is to study the thermodynamics performance of industrial boiler. But first, the basic concept needs to be studied. The working principle of industrial power plant consist of a boiler, where the heat originating from combustion of gases, fuels, or other sources (Figure 1.2) is transferred to the water. (Kuck, 1995)

3 Figure 1.2 A h/c-o/c diagram of fuels (free of water and inert substances) with lines of constant exhaust-gas dew point (Ptot = 1 bar) and lines of constant relative quantity of dry air, calculated for complete stoichiometric combustion (Adapted from Kuck, 1995)

4 The water then become a superheated vapor after receiving the heat energy from the combustion process and enters the turbine and produces work by rotating shaft connected to an electric generator. As necessary part of the process cycle, the steam then enters the condenser and condensed by rejecting heat to a cooling medium or reservoir such as a lake, a river, or the atmosphere. In areas where water is precious, the power plants are cooled by air instead of water. This method of cooling, which is also used in car engines, is called dry cooling. Several plants in the world, including some in the United States, use dry cooling to conserve water. (Çengal, Boles, 2007) The saturated liquid then enters the pump and compressed to the operating pressure of the boiler to complete the cycle. (Amsworth, 2007) Figure 1.3 Power Plant Cycle 1.2 Efficiency Steam power plants are responsible for the production of most electric power in the world, and even small increase in thermal efficiency can mean large savings from the fuel requirements. Therefore, every effort is made to improve the efficiency of the cycle on which power plants operate. The basic idea behind all the modifications to increase the thermal efficiency of a power cycle is the same: Increase the average temperature at which heat is transferred to the working fluid in the boiler, or decrease the average temperature at which heat is rejected from the working fluid in the condenser. (Çengal, Boles, 2007) It means that, to improve the boiler efficiency, achieved working fluid temperature should be as high as possible.

5 1.2.1 Burner The efficiency of a boiler is a measure of the ability of it to generate the steam demand from a given fuel supply. (T.West, 2002) A boiler should always be supplied with more combustion air than is theoretically required, in order to ensure complete combustion and safe operation. The function of a burner is to fix the fuel and air in proportions that are within the limits of flammability, as well as to provide condition for steady and continuous combustion. (Ozdemir, 2004) A well-designed burner will mix the fuel and air so that a minimum amount of excess air is needed to achieve complete combustion, provided that the burn are clean and well maintained. Operation of boilers and furnaces at optimum combustion efficiency involves control of excess air supply to the burners. Ideally excess air and stack temperature should always be maintained at the optimum levels established through the tune-up procedures. As a rule of thumb, boiler efficiency can be increased 1% for every 15% reduction in excess air; or 1.3% reduction in oxygen. (Enercon, 1989) In practice, however, excess oxygen and temperature will vary in response to variations in load on boiler, and some deviation from the optimum operating levels cannot be avoided. (Ozdemir, 2004) In order to complete combustion, the desired air flow in a fan can be achieved by the employment of a variable speed control. By installing variable speed drive controls the motor will be allowed to operates at variable speeds that are based on load requirements. Energy savings are due to the decreased power required to operate the motor at reduced speeds. (Ozdemir, 2004) Variable speed control is a way of the most efficient control method. It provides only the power necessary to overcome system resistance at given condition. Currently, variable speed drives are commonly used in modern industrial and commercial boilers. It is particularly effective when operating conditions call for frequent low load periods. (Zeitz, 1997) Condenser Higher temperature of working fluids from the boiler will make the gas leaves the turbine at very high temperature. The diffusion of condensing boilers has been hampered by the fact that their efficiency advantages become significant only when reaching a low return water temperature level supplied by the heating plant. (Lazzarin, Schibuola, 1986) It means that the high temperature entering the condenser will erase any potential gains in the thermal efficiency. The situation can be improved somewhat by using regeneration to decrease the temperature at which heat is rejected from the working fluid in the condenser, which will increase the efficiency.

6 2.0 MATHEMATICAL MODEL: 2.1 Direct Method: The energy gain of the working fluid (water and steam) is compared with the energy content of the boiler fuel. It is known as 'input-output method' due to the fact that it needs only the useful output (steam) and the heat input (i.e. fuel) for evaluating the efficiency. This efficiency can be evaluated using the formula. Boiler Efficiency = Heat Output / Heat Input If you want a quick, reasonably precise method then do the I/O (input-output) method. This efficiency is as follows: Boiler Efficiency= W1 (H1-h1) + W2 (H2-h2)/C W1= main steam flow (lb/hr) W2 = reheat steam flow (lb/hr) H1 = Enthalpy main steam (btu/lb) H2 = Enthalpy reheat steam (btu/lb) h1 = Enthalpy feed water (btu/lb) h2 = Enthalpy steam enter re-heater (btu/lb) C = Total heat input from fuel (btu/lb) Another way to calculate this is by using the parameters for the calculation of boiler efficiency by direct method are: 1. Quantity of steam generated per hour (Q) in Kg/hr. 2. Quantity of fuel used per hour (q) in Kg/hr. 3. The working pressure (in Kg / cm2) and superheat temperature if any 4. The temperature of feed water ( C) 5. Type of fuel and gross calorific value of the fuel (GCV in Kcal/kg of fuel) Where, Boiler efficiency (η) = [Q x (hg hf) / q x GCV] x 100 hg -Enthalpy of saturated steam in kcal/kg of steam hf - Enthalpy of feed water in kcal/kg of water Due to very scarce information in calculating the efficiency based on the Direct Method, we are assuming average Boiler Efficiency is 93%.

7 2.2 The Indirect Method: The Efficiency is the Difference between the Losses and the Energy Input. Three main losses to be considered: 1) Dry flue gas loss, 2) Loss due to moisture from the combustion of hydrogen, and 3) Radiation and convection loss. These values can be determined by using equipment that should be readily available to every steam plant that should provide accurate data to guide the operator towards optimizing the efficiency of the boiler. 2.3 Dry Flue Gas Loss Dry flue gas loss accounts for the heat lost up the stack in the dry products of combustion that is CO2, O2, N2, CO and SO2. Calculation: LDG, % = [DG x Cp x (FGT - CAT)] x 100 HHV (lb, F and Btu/lb) DG is the weight of dry flue gas, lb/lb of fuel, Cp is the specific heat of flue gas, usually assumed to be 0.24, FGT is the flue gas temperature, F, CAT is the combustion air temperature, F, HHV is the higher heating value of the fuel, Btu/lb.

8 2.4 Loss Due to Moisture from the Combustion of Hydrogen The hydrogen component of fuel leaves the boiler as water vapor, taking with it the enthalpy, or heat content, corresponding to its conditions of temperature and pressure. It is steam at very low pressure but fairly high temperature, the stack temperature, and most of its enthalpy is in the heat of vaporization. This makes it a significant loss, commonly about 11% for natural gas and 7% for fuel oil. The difference reflects the relative hydrogen content of these two fuels. Calculation: LH, % = [900 x H2 x (hg hf)] HHV H2 is the weight fraction of hydrogen in the ultimate analysis of the fuel, HHV is the higher heating value, hg is the enthalpy in Btu/lb of water vapor at 1 psi and the flue gas temperature (FGT) in F, and hf is the enthalpy of water at the combustion air temperature (CAT) in F. hg can be determined from steam tables or from the equation hg, Btu/lb = (0.467 x FGT). hf can also be determined from steam tables, or from the simple relationship hf, Btu/lb = CAT - 32.

9 2.5 Loss to Radiation and Convection LR is not normally measured; instead it is estimated using a chart prepared by the American Boiler Manufacturers Association (ABMA). Values applicable to modern boilers having all walls water-cooled have been selected from the ABMA chart and are presented in Figure App 1-1. Figure App 1-1 Radiation and Convection Losses for Various Boiler Sizes Unaccounted-For Losses Should take reasonable assumptions concerning these losses are 0.1% for natural gas, 0.2% for light oil. For heavy oil, a value between 0.3 and 0.5% may be appropriate, to account for fuel heating and, perhaps, atomizing steam. Using Boiler Efficiency Data Efficiency (h) = 100 (LDG + LH + LR + LUA) Then an efficiency curve can be drawn for each boiler. Usually boiler efficiency is highest somewhere between 50% and 80% of maximum capacity rating (MCR). It is generally lower at low load due to higher excess air and higher radiation and convection losses, and lower at high load due to high stack temperature. With the boiler efficiency curves as a guide the operator can shift loads between boilers to maintain each at close to its highest efficiency. In designing or refurbishing a plant, the profile of demand versus time should be carefully considered, and boiler sizes should be selected so that they operate in the most efficient range most of the time. If existing boilers have a high stack temperature at full load, and operate there much of the time, it might be cost effective to add heat recovery equipment such as economizers or air heaters.

10 3.0 SAMPLE CALCULATION 1: 3.1 Data Collection The various parameters that serve as the prerequisites for carrying out performance evaluation of a boiler are listed as follows 1. Steam temperature 2. Steam pressure 3. Flue gas temperature 4. Fuel water temperature 5. Fuel consumption 6. Steam flow rate 7. % Contents of gases present in flue gas (Flue gas analysis) 8. % contents of constituent elements in fuel (Ultimate analysis of fuel) 9. Air required burning the required quantity of fuel. (Stoichiometric /theoretical and Actual) 3.2 Average values: Steam temp. (C) = Steam Pressure = 6.07 Flue gas temp (C) = 170 Feed water temp. (C) = Fuel Consumption = 24 litres / hr = 24 x (Sp gravity of Furnace Oil = 0.943) = Kg/hr. 3.3 Determination of Steam Flow Rate Avg. Water pumped in 30 sec Water pumped in 60 sec (1 min) Water pumped in 1 hour = 7.82 lit. = Lit = X 60 = lit.

11 3.4 Determining Operating Time of Pump per Hour Operating Time of Pump per hour = operating time of burner / hour + 10(No. of times burner got ON per hour = [ (50)] sec. = 2499 sec. = min = hr. Total Operating time of pump = hr. Water consumption/hr capacity of pump = Total operating time of pump/hr x Actual Pumping = x938.4 = lit. /hour Steam flow rate = Water consumption = Kg/hr. The flue gas was collected using Air sampling apparatus and was analyzed using GAS CHROMATOGRAPHY. The analysis revealed the following results: CO2 = 9.77% CO = % 3.5 Furnace Oil Analysis (Ultimate Analysis) The standard values of various constituent elements present in furnace oil are as follows: Carbon (C) = 84 % Hydrogen (H2) = 12% Oxygen (O2) = 1.5 % Sulphur (S) = 1.5% Nitrogen (N2) = 0.5 % Moisture (M) = 0.5 Gross colorific value (GCV) = 10,000 Kcal/Kg.

12 3.6 Calculation of Requirement of Theoretical Amount of Air Consider a sample of 100 kg of furnace oil. The chemical reactions are: Element Molecular Weight C = 12; O2 = 32; H2= 2; S = 32; N2= 28; CO2 = 44 SO2 = 64; H2O = 18; C + O2 H2 + 1/2 O2 S + O2 = CO2 = H2O = SO2 3.7 Constituents of fuel C + O2 = CO = kg of carbon requires 32 kg of oxygen to form 44 kg of carbon dioxide therefore 1 kg of carbon requires 32/12 kg. I.e kg of oxygen. (84) C + (84 x 2.67) O2 = CO2 2H2+ O2 = 2H = 36 4 kg of hydrogen requires 32 kg of oxygen to form 36 kg of water, therefore 1 kg of hydrogen requires 32/4 kg i.e. 8 kg of oxygen (12) H2 + (12 x 8) O2 = (12 x 9) H2O S + O2 = SO = kg of sulphur requires 32 kg of oxygen to form 64 kg of sulphur dioxide, therefore 1 kg of sulphur requires 32/32 kg i.e. 1 kg of oxygen 1.5(S) + (1.5 x 1)O2 = 3SO2 Theoretical CO2 % by volume = Moles of CO2 / Total Moles (Dry)

13 Performance Evaluation of an Oil Fired Boiler A Case Study In Dairy Industry 355 = 7.006x100 / ( ) = 15.45% 3.8 Calculation of Constituents of Flue Gas with Excess Air % CO2 measured in flue gas = 9.77(measured) % Excess Air = [ (Theoretical CO2 % / Actual CO2 %) 1 ] x 100 (7) = [ (15.45 / 9.77) 1 ] x 100 = % Theoretical air required for 100 kg of fuel burnt = Total qty. of air supply required. = x = kg with excess air Excess air quantity = = Kg O2 = X 0.23 = Kg N2 = = Kg Flue gas with % excess air for every 100 kg fuel. CO2 H2O SO2 O2 = Kg = Kg = 3 Kg = Kg N2 = =

14 3.9 Calculation of Theoretical CO2 % in Dry Flue Gas by Volume Moles of CO2 in flue gas = /44 = Moles of SO2 in flue gas = 3/64 = Moles of O2 in flue gas = /32 = 5.82 Moles of N2 in flue gas = /28 = Theoretical CO2 % by volume = Moles of CO2 / Total Moles (Dry) = / ( ) = 9.77 % Theoretical O2 % by volume = 5.82 / = 7.93 % Total mass Of Flue gas(m) = = kg 3.10 Efficiency Calculation (Indirect Method) The direct method of efficiency calculation could not be applied due to the wetness of the steam (i.e. steam was not saturated), therefore the efficiency is calculated by using indirect method as shown: i) Dry gas Loss = m x Cp x (Tf Ta ) x 100 / Where m = total mass of flue gas Dry gas loss = 23.02x 0.23 x (170 34) x 100 / = 7.2 % ii) Wet gas loss = 9 x H2 [584 + Cp (Tf Ta)] / GCV where H2 percentage of H2 in fuel = 9 x 12 [ (170 34)] / = 7.04 % iii) Radiation loss = 1% (approx from ABMA) Total losses = = 15.24% Efficiency (η) = 100 ( ) = % 2 nd Law Efficiency = (Actual / Ideal) x 100

15 = (84.76 % / 93%) x 100 = 91.14% 4.0 SAMPLE CALCULATION 2: Let us calculate Boiler efficiency of coal fired boiler. Ambient temp is 80 F and Back End Temperature (Exhaust gas temp) is 302 F. The percent composition of Coal is as under: Carbon, C ; Hydrogen, H ; Nitrogen, N ; Oxygen, O ; Suphur, S ; Moisture, H 2 O ; Ash - 7.0; The Combustion calculations of the above fuel are already explained in detail in the other article. From the above calculations, Unit Wet Gas, Kg/Kg of fuel = Unit Wet Air + (1-Ash) = ( ) = Unit Dry Gas, Kg/Kg of fuel = Unit Wet Gas (Moisture in Air + Water produced = Higher Heating Value, HHV or Gross Calorific Value, GCV in BTU/Lb = C (H 2 -O2/8) S during combustion) Lower Heating Value, LHV or Lower Calorific Value, LCV or Net Calorific Value, NCV, BTU/lb Let us use HHV and LHV notation. = HHV 1030(9. H 2 + Moisture) HHV = (14600 x ( /8) x 1.3)/100

16 = BTU/lb ( Kcal/kg) LHV = (9*4.1+3)/100 = BTU/lb (7050 Kcal/kg) 4.1 Calculations of the Losses based on Higher Heating Value: a) Dry gas losses: Exhaust gases always leave the boiler at a higher temp than ambient. Heat thus carried away by hot exhaust gases is called Dry gas losses Heat Losses, La = Unit Dry Gas x Cp x (Tg-Ta) x 100/HHV b) Loss due to Moisture in fuel: = x 0.24 x (302-80) x 100 / = 5.48 % The moisture present in the fuel absorbs heat to evaporate and get superheated to exit gas temperature. Lb = Moisture in Fuel x (1089-Ta+0.46xTg) x 100/HHV = 0.03 x ( x 302) x100 / = % c) Loss due to Moisture Produced during combustion: Lc = Moisture Produced x (1089-Ta+0.46xTg) x100/hhv d) Loss due to Moisture in air: = x ( x 302) x100 / = 3.23 % Ld = Moisture in Air x Cp of Steam x (Tg-Ta) x 100/HHV = x x 0.46 x (302-80) x100 / = % Here, Moisture in Air = lb/ lb of dry air at 60% Relative Humidity Cp of steam = 0.46 e) Un-burnt fuel loss:

17 This is purely based on experience. Un-burnt fuel loss depends up on type of Boiler, grate, grate loading and type of fuel. For Bio-Mass fuels, it ranges from 1.5 to 3 %, for oils from and almost nil for gaseous fuels. Let us consider Un-burnt fuel loss, Le = 2.5 % for Coal. f) Radiation Loss: Radiation Loss is because of hot boiler casing loosing heat to atmosphere. ABMA chart gives approximate radiation losses for fired boilers. Let us take a radiation Loss, Lf = 0.4 % in this case. g) Manufacturer s margin: This is for all unaccounted losses and for margin. Unaccounted losses are because of incomplete combustion carbon to CO, heat loss in ash, etc. This can be 0.5 to 1.5 % depending up on fuel and type of boiler. In this case, let us take, Manufacturer s margin Lg = 1.5%. Total Losses = La + Lb + Lc + Ld + Le + Lf + Lg = = % Therefore, Efficiency of the boiler on HHV basis = 100 Total Losses = = % 4.2 Efficiency based on LHV: Efficiency based on LHV = Efficiency on HHV x HHV/LHV = x / = % 2 nd Law Efficiency = (Actual / Ideal) x 100 = (89.29 % / 93%) x 100 = 96.01%

18 5.0 Results and Discussion The boiler efficiency itself is affected by 3 main factors, that are: a) Fuel qualities / fuel lower heating value (LHV) b) Boiler unit loads c) Excess air ratio 5.1 Fuel Qualities / Fuel Lower Heating Value (LHV) Based on the study conducted on steam power plant using coal as the combustion fuel in the boiler, the fuel quality significantly affects the heat losses due to waste gases and heat loss due to unburned carbon. With better LHV(lower heating value) the two heat loses above decreases. Therefore, more energy will be transferred to the boiling water to generate steam. Accordingly, the boiler thermal efficiency was found to improve. With the improvement in the thermal efficiency and increase of LHV, there is substantial reduction in fuel consumption. Boiler thermal efficiency was found to improve about 0.6% when the LHV value increased from 8.08 MJ/kg to MJ/kg. (Tanatsakunvatana, 2006). However, with higher LHV value, the temperature of the waste gases slightly increases. This is due to higher flame temperature. The higher flame temperature apparently increases the NOx emission due to reaction of oxygen gases (O2) with nitrogen gas (N2) at high temperature.therefore, with the increase fuel LHV, the NOx formation in the boiler furnace increases too but there is significant improvement in the boiler thermal efficiency.

19 Table 1.0: Heat losses, thermal efficiency and fuel consumption for 300MW boiler tested at different fuel qualities. (Tanatsakunvatana, 2006). Boiler test run with increasing fuel quality (1 to 5) Variable Unit Excess air ratio 1.19 % 1.19% 1.19% 1.19% 1.19% Heat loss due to waste gas,q 2 Heat loss due to incomplete combustion,q 3 Heat loss due to unburned carbon,q 4 Heat loss due to radiation & convection,q 5 Heat loss with bottom ash/slag,q 6 Thermal efficiency of the boiler (ŋ) Fuel consumption by the boiler,m (kg/s) Specific fuel consumption, m(kg/s) % % ~0 ~0 ~0 ~0 ~0 % % % % Kg/s Kg/s From the data above, it is clearly shown with the increase in fuel quality, the thermal efficiency of the boiler, ŋ increases while the fuel consumption by the boiler, m decreases. (Tanatsakunvatana, 2006).

20 5.2 Boiler Unit Loads The required thermal output of the boiler was secured by the flow rates of superheated and reheated steam, while the steam pressure and temperature are controlled and maintained by the control system. The boiler unit load refers to the power output of the boiler itself. For example a 300MW boiler is operated with power output of 287MW, 249 MW and 200 MW. The corresponding reduced unit loads are 98%, 83% and 67% respectively. For the reduced boiler loads, the efficiency of the boiler improves due to reduced flame temperature and the temperatures of the waste gas are slightly reduced.the reduction in the flame temperature corresponds to the reduced boiler loads. When the temperature of waste gas is reduced, less heat is loss to the ambient air or more heat can be transferred to the boiling water to generate steam. However, due to diminishing of thermal efficiency of the power cycle, the specific fuel consumption increases with the reducing unit loads. Table 1.2: Heat losses, thermal efficiency and fuel consumption for 300MW boiler tested at different unit loads. (Tanatsakunvatana, 2006) Boiler test run with decreasing boiler unit loads (1 to 3) Variable Unit Boiler unit loads 98% 83% 67% Heat loss due to waste gas,q 2 Heat loss due to incomplete combustion,q 3 Heat loss due to unburned carbon,q 4 Heat loss due to radiation & convection,q 5 Heat loss with bottom ash/slag,q 6 Thermal efficiency of the boiler (ŋ) Fuel consumption by the boiler,m (kg/s) Specific fuel consumption, m(kg/s) % % ~0 ~0 ~0 % % % % Kg/s Kg/s From the study done, it can be concluded with the reduced boiler unit loads, the thermal efficiency of the boiler increases.

21 5.3 Excess Air Ratio With the increasing oxygen level (O2) ( in excess air condition), the NOx emission were found to increase from 245 ppm to 395ppm, indicating the fuel reacts with oxygen (O2) to form nitrogen oxides during the fuel combustion. The carbon monoxide was only detected during the combustion with low values of oxygen level (lean air or insufficient air for stoichiometric combustion to take place). The excess air ration of 1.12% to 1.14% would provide the most effective and reliable operation of the boiler. The thermal efficiency of the boiler is weakly affected by the excess air ratio ranging between 0.5%. Table 3: Heat losses, thermal efficiency and fuel consumption for 300MW boiler tested at various excess air ratios. (Tanatsakunvatana, 2006) Boiler test run with increasing excess air ratio (1 to 5) Variable Unit Excess air ratio 1.05 % 1.08% 1.11% 1.13% 1.16% Heat loss due to waste gas,q 2 Heat loss due to incomplete combustion,q 3 Heat loss due to unburned carbon,q 4 Heat loss due to radiation & convection,q 5 Heat loss with bottom ash/slag,q 6 Thermal efficiency of the boiler (ŋ) Fuel consumption by the boiler,m (kg/s) Specific fuel consumption, m(kg/s) % % ~0 ~0 ~0 % % % % Kg/s Kg/s From the study above, it has been proven that the thermal efficiency of the boiler are weakly affected by the excess air ratio, ranging in the boiler test runs within 0.5% band.. (Tanatsakunvatana, 2006)

22 However, the overall efficiency of a cycle that uses boiler as a heating element to generate work through mechanical movements like a turbine in a Rankine cycle, the efficiency is effected by: a) Super heating the steam to higher temperatures b) Increasing the boiler pressure 5.4 Superheating the steam to higher temperature By superheating the steam to high temperature, the average temperature at which heat is added to the steam can be increased. Thus, both the net work and heat input increases. At the same time, by superheating the steam to higher temperature the moisture content of the steam at the turbine exit decreases. However, the maximum temperature that the steam can be superheated is limited due to metallurgical considerations, where high temperature of the steam can melt or deform the turbine blades. (Cengel et al, 2002) Wnet increases with the superheating the steam to higher temperature at the same time there is reduction in the moisture content Figure 4.0: T-s diagram on normal condition (cycle ) and the cycle condition when the steam is superheated to higher temperature (cycle ) (Cengel et al, 2002)

23 5.5 Increasing Boilder Pressure By increasing the boiler operating pressure, the temperature at which boiling takes place raises. This is in turn raises the average temperature at which heat is added to the steam and thus raises the thermal efficiency The undesirable effect of increasing the boiler pressure is the moisture content at the steam exit increases. This undesirable affect can be corrected by reheating the steam. The steam reheating is done by flowing the steam that exits the first turbine into the boiler for reheating the steam before the reheated steam is flowed into the second turbine. The average temperature during reheat process can be increased by increasing the number of expansion and reheat stages. However, the use of more than two reheat stages is not practical. The third reheat stage would increase the cycle efficiency by about half of the improvement attained by the second reheat. This gain is too small to justify the added cost and complexity. (Cengel et al, 2002) There is increase Wnet, but at the same time there is increase in moisture content Figure 5: T-s diagram on normal condition (cycle ) and the cycle condition when boiler pressure is increased (cycle ) (Cengel et al, 2002)

24 5.6 Irriversibilities There are a lot of sources of irriversibilities that affected the efficiency of a boiler. Fluid friction causes pressure drops in the boiler resulting to a lower pressure of a steam to drive the turbine. Also, the pressure at the turbine inlet is somewhat lower than that at the boiler exit due to the pressure drop in the connecting pipes. The other major source of irreversibility is the heat loss from the steam to the surroundings as the steam flows through various components. (Çengal, Boles, 2007) The biggest energy losses in a conventional oil fired boiler occur through the chimney. (Ozdemir, 2004) The size of the heat loss depends on the temperature and volume of gas leaving the boiler; therefore reducing either of these will reduce the heat loss. Some stack gas heat losses are unavoidable, but to eliminate these losses, the stack gas temperature would have to be reduced to the air temperature around the boiler. (Ozdemir, 2004) Additional losses occur at the bearings between moving parts as a result of friction. Steam that leaks out during the cycle and air that leaks into the condenser represent two other source of loss. (Çengal, Boles, 2007) 6.0 Conclusions From the study, we know that the 1 st Law efficiency or the ideal efficiency is 93%. This is the highest possible efficiency that we can get. However, after defining all the losses or irreversibilities, the actual efficiency is 89.29%. So, the 2 nd law efficiency can be calculated as 96.01%. This 4% of loses, looks rather small but It can save us a lot of energy and fuel consumption if it can be recovered. This losses is due to heat loss of flue gasses, moisture due to combustion of hydrogen that produce water vapour, heat loss due to radiation, uncounted losses like unburned carbon and incomplete combustion, fluid friction, pressure losses due to pipe joints/ connections and heat loses due to leakage of steam from the boiler. These losses however can be reduced by increasing the steam to higher temperature, increasing the boiler operation temperature, increasing the lower heating value or the fuel quality, and reducing the boiler load to reduce the heat losses due to decline in waste gas. The excess air ratio does not much affected the boiler efficiency but important for the environment. The effective excess air ratio is between 1.12% and 1.14%.

25 7.0 References Refrigeration & Air conditioning, Ahmadul Ameen, 2006, Prentice Hall Thermodynamics: An Engineering Approach, SI Version, Yunus Cengel, Michael Boles, Mc Graw Hill Website: (howstuff works.com) "Optimal control of energy loses in multi boiler steam system" Janusz Hujak, 2nd May 2009 "Potential for energy savings in heating systems through improving boiler controls" A.Liao & A.L Dexter, Oct 2203 "Experimental study on effects of operating conditions and fuel quality on thermal efficiency and emission performance of a 300MW boiler unit firing Thai lignite" V.Tanetsakunvatana & V.I Kuprianov, Apr 2006 Andrew Amsworth, English Wikipedia project, 29 November 2007 Andriuchenko A. Thermal cycle and process optimization in the electric power station. Mocow: MIR; 1974, in Russian R.M Lazzarin and l.schibuola, Performance analysis of heating plants equipped with condensing boilers. Heat Recovery System & CHP 6, c1986) Institut fiir Thermodynamik der Technischen Universitfit Braunschweig, Hans-Sommer- Str. 5, D Braunschweig, Germany. 30 april 1995 T. West, From mechanical to electronic control in industrial burners, Technical Bulletin energy Technology and Control Ltd.,2002 Engin Ozdemir. Technical Education Faculty, Electrical Education department, kocaeli University, Izmit, Turkey Ronald A. Zeitz, Energy Efficiency Handbook. Council of Industrial Boiler Owner (CIBU), 1997 Burner Control System, Enercon Technical Information Series, The National Energy Conservation Centre, 1989 School of Manufacturing System and Mechanical Engineering, Sirindhorn International Institute of Technology, Thammasat University