ENERGY MANAGEMENT HANDBOOKS phần 2 pdf

ECONOMIC ANALYSIS 79 Time Value of Money Factors—Discrete Compounding i = 9% 80 ENERGY MANAGEMENT HANDBOOK Time Value of Money Factors—Discrete Compounding i = 10% ECONOMIC ANALYSIS 81 Time Value of Money Factors—Discrete Compounding i = 12% 82 ENERGY MANAGEMENT HANDBOOK Time Value of Money Factors—Discrete Compounding i = 15% ECONOMIC ANALYSIS 83 Time Value of Money Factors—Discrete Compounding i = 18% 84 ENERGY MANAGEMENT HANDBOOK Time Value of Money Factors—Discrete Compounding i = 20% ECONOMIC ANALYSIS 85 Time Value of Money Factors—Discrete Compounding i = 25% 86 ENERGY MANAGEMENT HANDBOOK Time Value of Money Factors—Discrete Compounding i = 30% 87 CHAPTER 5 BOILERS AND FIRED SYSTEMS S.A. PARKER Senior Research Engineer, Energy Division Pacifi c Northwest National Laboratory Richland, Washington R.B. SCOLLON Corporate Manager, Energy Conservation R.D. SMITH Manager, Energy Generation and Feedstocks Allied Corporation Morristown, New Jersey 5. 1 INTRODUCTION Boilers and other fi red systems are the most signifi cant energy consumers. Almost two-thirds of the fossil-fuel energy consumed in the United States involves the use of a boiler, furnace, or other fi red system. Even most electric energy is produced using fuel-fi red boilers. Over 68% of the electricity generated in the United States is produced through the combustion of coal, fuel oil, and natural gas. (The remainder is produced through nuclear, 22%; hydro- electric, 10%; and geothermal and others, <1%.) Unlike many electric systems, boilers and fi red systems are not inherently energy effi cient. This chapter and the following chapter on Steam and Condensate Systems examine how energy is consumed, how energy is wasted, and opportunities for reducing energy consumption and costs in the operation of boiler and steam plants. A list of energy and cost reduction measures is presented, categorized as: load reduction, waste heat recovery, effi ciency improvement, fuel cost reduction, and other opportunities. Several of the key opportunities for reducing operating costs are presented ranging from changes in operating procedures to capital improvement opportunities. The topics refl ect recurring opportunities identifi ed from numerous in-plant audits. Several examples are presented to demonstrate the methodology for estimating the potential energy savings associated with various opportunities. Many of these examples utilize easy to understand nomographs and charts in the solution techniques. In addition to energy saving opportunities, this chapter also describes some issues relevant to day-to-day operations, maintenance, and troubleshooting. Consider- ations relative to fuel comparison and selection are also discussed. Developing technologies relative to alterna- tive fuels and types of combustion equipment are also discussed. Some of the technologies discussed hold the potential for signifi cant cost reductions while alleviating environmental problems. The chapter concludes with a brief discussion of some of the major regulations impacting the operation of boilers and fi red systems. It is important to emphasize the need to carefully assess the potential impact of federal, state, and local regulations. 5.2 ANALYSIS OF BOILERS AND FIRED SYSTEMS 5.2.1 Boiler Energy Consumption Boiler and other fi red systems, such as furnaces and ovens, combust fuel with air for the purpose of releasing the chemical heat energy. The purpose of the heat energy may be to raise the temperature of an industrial product as part of a manufacturing process, it may be to generate high-temperature high-pressure steam in order to power a turbine, or it may simply be to heat a space so the oc- cupants will be comfortable. The energy consumption of boilers, furnaces, and other fi re systems can be determined simply as a function of load and effi ciency as ex- pressed in the equation: Energy consumption = ∫ (load) × (1/effi ciency) dt (5.1) Similarly, the cost of operating a boiler or fi red system can be determined as: Energy cost = ∫ (load) × (1/effi ciency) × (fuel cost) dt (5.2) As such, the opportunities for reducing the energy consumption or energy cost of a boiler or fi red system can be put into a few categories. In order to reduce boiler energy consumption, one can either reduce the load, in- crease the operating effi ciency, reduce the unit fuel energy cost, or combinations thereof. Of course equations 5.1 and 5.2 are not always that simple because the variables are not always constant. The 88 ENERGY MANAGEMENT HANDBOOK load varies as a function of the process being supported. The effi ciency varies as a function of the load and other functions, such as time or weather. In addition, the fuel cost may also vary as a function of time (such as in sea- sonal, time-of-use, or spot market rates) or as a function of load (such as declining block or spot market rates.) Therefore, solving the equation for the energy consumption or energy cost may not always be simplistic. 5.2.2 Balance Equations Balance equations are used in an analysis of a process which determines inputs and outputs to a system. There are several types of balance equations which may prove useful in the analysis of a boiler or fi red-system. These include a heat balance and mass balance. Heat Balance A heat balance is used to determine where all the heat energy enters and leaves a system. Assuming that energy can neither be created or destroyed, all energy can be accounted for in a system analysis. Energy in equals energy out. Whether through measurement or analysis, all energy entering or leaving a system can be determined. In a simple furnace system, energy enters through the combustion air, fuel, and mixed-air duct. Energy leaves the furnace system through the supply-air duct and the exhaust gases. In a boiler system, the analysis can become more complex. Energy input comes from the following: condensate return, make-up water, combustion air, fuel, and maybe a few others depending on the complexity of the system. Energy output departs as the following: steam, blowdown, exhaust gases, shell/surface losses, possibly ash, and other discharges depending on the complexity of the system. Mass Balance A mass balance is used to determine where all mass enters and leaves a system. There are several methods in which a mass balance can be performed that can be useful in the analysis of a boiler or other fi red system. In the case of a steam boiler, a mass balance can be used in the form of a water balance (steam, condensate return, make- up water, blowdown, and feedwater.) A mass balance can also be used for water quality or chemical balance (total dissolved solids, or other impurity.) The mass balance can also be used in the form of a combustion analysis (fi re- side mass balance consisting of air and fuel in and combustion gasses and excess air out.) This type of analysis is the foundation for determining combustion effi ciency and determining the optimum air-to-fuel ratio. For analyzing complex systems, the mass and energy balance equations may be used simultaneously such as in solving multiple equations with multiple unknowns. This type of analysis is particularly useful in determining blowdown losses, waste heat recovery potential, and other interdependent opportunities. 5.2.3 Effi ciency There are several different measures of effi ciency used in boilers and fi red systems. While this may lead to some confusion, the different measures are used to con- vey different information. Therefore, it is important to understand what is being implied by a given effi ciency measure. The basis for testing boilers is the American Soci- ety of Mechanical Engineers (ASME) Power Test Code 4.1 (PTC-4.1-1964.) This procedure defi nes and established two primary methods of determining effi ciency: the input-output method and the heat-loss method. Both of these methods result in what is commonly referred to as the gross thermal effi ciency. The effi ciencies determined by these methods are “gross” effi ciencies as apposed to “net” effi ciencies which would include the additional energy input of auxiliary equipment such as combustion air fans, fuel pumps, stoker drives, etc. For more information on these methods, see the ASME PTC-4.1-1964 or Taplin 1991. Another effi ciency term commonly used for boilers and other fi red systems is combustion effi ciency. Combus- tion effi ciency is similar to the heat loss method, but only the heat losses due to the exhaust gases are considered. Combustion effi ciency can be measured in the fi eld by analyzing the products of combustion the exhaust gases. Typically measuring either carbon dioxide (CO 2 ) or oxygen (O 2 ) in the exhaust gas can be used to determine the combustion effi ciency as long as there is excess air. Ex- cess air is defi ned as air in excess of the amount required for stoichiometric conditions. In other words, excess air is the amount of air above that which is theoretically required for complete combustion. In the real world, however, it is not possible to get perfect mixture of air and fuel to achieve complete combustion without some amount of excess air. As excess air is reduced toward the fuel rich side, incomplete combustion begins to occur resulting in the formation of carbon monoxide, carbon, smoke, and in extreme cases, raw unburned fuel. Incomplete combustion is ineffi cient, expensive, and frequently unsafe. Therefore, some amount of excess air is required to en- sure complete and safe combustion. However, excess air is also ineffi cient as it results in the excess air being heated from ambient air temperatures to exhaust gas temperatures resulting in a form of heat loss. Therefore while some excess air is required it is also [...]... Output (106 Btu/hr) Fuel Input (106 Btu/hr) 1 20 0 170 130 100 305 28 0 300 28 0 2 2 7 12 85.0 86.0 84.0 81.5 22 6 .2 1 92. 3 147.0 113.1 26 6.1 22 3.6 175.0 138.8 2 200 170 130 100 625 570 520 490 2 4 7 11 77.5 78.0 77.0 74.0 22 6 .2 1 92. 3 147.0 113.1 29 1.0 24 6.5 190.9 1 52. 8 3 100 85 65 50 600 570 540 500 2 2 7 11 78.0 78.5 76.5 73.5 113.1 96.1 73.5 56.6 145.0 122 .5 96.1 76.9 BOILERS AND FIRED SYSTEMS 3 Operation... 3500 6000 700 1000 300 25 0 125 301 to 450 3000 5000 600 900 25 0 20 0 90 451 to 600 25 00 4000 500 500 150 100 50 601 to 750 20 00 25 00 400 400 100 50 35 751 to 900 1500 — 300 300 60 — 20 901 to 1000 125 0 — 25 0 25 0 40 — 8 1001 to 1500 1000 — 20 0 20 0 20 — 2 110 ENERGY MANAGEMENT HANDBOOK Example: Determine the potential energy savings associated with reducing boiler blowdown from 12% to 10% using Figure... 5 .2 Typical Optimum Excess Aira Fuel Type ————— Natural gas Natural gas Natural gas Propane Coke oven gas No 2 oil No 2 oil No 2 oil No 6 oil Coal Coal Coal aTo Firing Method ——————— Natural draft Forced draft Low excess air — — Rotary cup Air-atomized Steam-atomized Steam-atomized Pulverized Stoker Cyclone Optimum Excess Air (%) ——————— 20 -30 5-10 04-0 .2 5-10 5-10 15 -20 10-15 10-15 10-15 15 -20 20 -30... 5.14 Automated continuous blowdown system For drum water impurity level of 6000 ppm: 1 12 ENERGY MANAGEMENT HANDBOOK Figure 5.15 Required percent blowdown Based on equation 5.5 % BD = A/(B - A) % BD = 26 2/(6000 - 26 2) % BD = 4.6% Graphical Solution Referring to Figure 5.15 Enter the graph at feedwater impurity level of 26 2 ppm and follow the line to the curves for 3000 ppm and 6000 ppm boiler drum water... experience potentially unstable operation with small changes in O2 (steep CO-O2 curve behavior) Figure 5 .2 Hypothetical CO-O2 characteristic curve for a gas-fired industrial boiler 92 ENERGY MANAGEMENT HANDBOOK Upper control limits for carbon monoxide vary depending on the referenced source Points referenced for gasfired systems are typically 400 ppm, 20 0 ppm, or 100 ppm Today, local environmental regulations... flue-gas temperature from 15 62 F to 28 5°F yields the following steam flow: (1.10)(x)(hf–h1) + (x)(h3–hf) = 11.8 × 106 Btu/hr Therefore, steam flow, x feedwater flow boiler blowdown = 11,388 lb/hr = 1.10(x)= 1.10(11,388)= 12, 527 lb/hr = 12, 527 – 11,388 = 1,139 lb/hr Figure 5.10a Heat in flue gases vs percent moisture by weight (Derived from Keenan and Kayes 1948.) 104 ENERGY MANAGEMENT HANDBOOK Determine... Condensate return (percent of steam flow) 25 % Assume condensate return free from impurities Calculation and Analysis Calculate the impurity concentration in the boiler feedwater (BFW): A = MU impurity × (1.00 - % CR) A = 350 ppm × (1.00 - 0 .25 ) A = 26 2 ppm Mathematical solution For drum water impurity level of 3000 ppm: % BD = A/(B - A) % BD = 26 2/(3000 - 26 2) % BD = 9.6% Figure 5.14 Automated continuous... 5. 12 (Unit input) = (unit output)/(efficiency) BOILERS AND FIRED SYSTEMS 105 Normal ——Measured—— Boiler Size Boiler Boiler Load Stack Temp O2 Unit Eff No (103 lb/hr) (103 lb/hr) (°F) (%) (%) ———————————————————————————————————————— 1 20 0 140 29 0 5 85.0 2 200 140 540 6 77.4 3 100 65 540 7 76.5 Plant steam demand 345 Figure 5.11 Unit efficiency vs steam load Figure 5. 12 Unit input vs steam load 106 ENERGY. .. a flash tank pressure of 20 psig, read the Steam percentage (Flash %) to be 12. 5% 114 ENERGY MANAGEMENT HANDBOOK Determine COND %: COND % = 100 - Flash % COND % = 100 - 12. 5 % COND % = 87.5% Determine htk using Chart A: Entering chart A with a flash tank pressure of 20 psig and following a line to the curve for saturated liquid, read the enthalpy of the drum water (htk) to be 22 6 Btu/lb Determine hex... the blowdown discharge water to be 68 Btu/lb Determine the % QC: % QC = [(htk - hex)/htk ] × COND % % QC = [ (22 6 Btu/lb - 68 Btu/lb) /22 6 Btu/lb] × 87.5% % QC = 61 .2% Determine the % Q: % Q = % QC + Flash % % Q = 61 .2% + 12. 5% % Q = 73.7% Conclusion Therefore, approximately 73.7% of the heat energy can be recovered using this blowdown heat recovery technique More on Flash Steam To determine the amount . 1 -2 Natural gas Low excess air .04-0 .2 0.1-0.5 Propane — 5-10 1 -2 Coke oven gas — 5-10 1 -2 No. 2 oil Rotary cup 15 -20 3-4 No. 2 oil Air-atomized 10-15 2- 3 No. 2 oil Steam-atomized 10-15 2- 3 . = 18% 84 ENERGY MANAGEMENT HANDBOOK Time Value of Money Factors—Discrete Compounding i = 20 % ECONOMIC ANALYSIS 85 Time Value of Money Factors—Discrete Compounding i = 25 % 86 ENERGY MANAGEMENT. = 9% 80 ENERGY MANAGEMENT HANDBOOK Time Value of Money Factors—Discrete Compounding i = 10% ECONOMIC ANALYSIS 81 Time Value of Money Factors—Discrete Compounding i = 12% 82 ENERGY MANAGEMENT