Fuel Combustion PDF

FME 702 ENERGY AND ENVIRONMENT 2. FUEL COMBUSTION 2.1 Introduction Fuel combustion is defined as the intentional oxidation of materials within an apparatus that is designed to provide heat or mechanical work to a process, or for use away from the apparatus. This definition aims to separate the com...

FME 702 ENERGY AND ENVIRONMENT 2. FUEL COMBUSTION 2.1 Introduction Fuel combustion is defined as the intentional oxidation of materials within an apparatus that is designed to provide heat or mechanical work to a process, or for use away from the apparatus. This definition aims to separate the combustion of fuels for distinct and productive energy use from (i) the heat released from the use of hydrocarbons in chemical reactions in industrial processes, or (ii) The use of hydrocarbons as industrial products. In most engineering applications, the process is controlled, mostly by controlling the amount of fuel into the combustion chamber. The process is initiated by a spark (e.g. in Spark-ignition engines), spontaneous (e.g. in compression-ignition engines), or by a torch for burners, furnaces, etc. To ensure complete combustion, air is usually supplied in excess of the stoichiometric requirement, with the excess being passed out with the combustion products. It should be noted that only oxygen in air is involved in the combustion process. Fuel use in various applications can be classified as shown in Table 2.1 below. Table 2.1 Fuel use Applications Fuel Applications Solid (coal, wood, bagasse etc) Furnaces, boilers, kilns Liquid (Gasoline, Diesel oil) I-C engines, boilers, Liquid (Fuel oil) Boilers, furnaces, kilns, etc Gas (natural gas) + Kerosene Gas turbines Gaseous fuels Heat and power plants The choice of the fuel used for the many industrial processes depend on the particular application, practical circumstances, economic and environmental considerations. Fossil fuels can also used for non-energy purposes. Three types of non-energy use are considered. (i) Feedstock Page 1 of 31 They are used as raw materials in chemical conversion processes in order to produce primarily organic chemicals and, to a lesser extent, inorganic chemicals (especially ammonia) and their derivatives (OECD/IEA/Eurostat, 2004). (ii) Reductant They are used directly as a reducing agent for production of metal and inorganic chemicals, or used indirectly via the intermediate production of electrodes used for electrolysis for metal production. (iii) Non-energy product They are used directly (i.e., without chemical conversion) for their physical or diluent properties or which are sold to the chemical industry as chemical intermediate. Table 1.2 shows examples of fuels used in some non-energy applications. Table 1.2 Types of use and examples of fuels used for non-energy applications Type of use Example of fuel types Product/process Feedstock Natural gas, oils, coal Ammonia Naptha, natural gas, ethane, Methanol, olefins (ethylene, propane, butane, gas oil, fuel oils propylene), carbon black Reductant Petroleum coke Carbides Coal, petroleum coke Titanium dioxide Metallurgical cokes, pulverised Iron and steel (primary) coal, natural gas Metallurgical cokes Ferroalloys Petroleum coke, pitch (anodes) Aluminium1 Metallurgical coke, coal Lead Metallurgical coke, coal Zinc Non-energy Product Lubricants Lubricating properties Paraffin waxes Misc. (e.g. candles, coating etc. Bitumen (asphalt) Road paving and roofing White spirit2, some aromatics As solvent (paint, dry cleaning) 1 Also used in secondary steel production (in electric arc furnaces) 2 Also known as mineral turpentine, petroleum spirits, industrial spirit (or Special Boing Point (SBP) solvents) 2.2 Introduction to Combustion chemistry Page 2 of 31 (i) Combustion Reactants Combustion reactants (reacting constituents) are usually fuel and an oxidant. Atmospheric air is the common oxidant. In some advanced applications such as rocket propulsion, oxygen alone or such alternatives as hydrogen peroxide, nitrogen oxides etc may be used as the oxidant. In their simplest form, fuels are represented by the great variety of either natural gas or manufactured hydrocarbons identified by CxHy. (ii) Combustion products Since the most common reactants in fuel-air combustion consist of the elements carbon, hydrogen, oxygen and nitrogen in various combinations, the simpler products resulting from oxidation are their complete oxides, CO2, and H2O together with inert nitrogen, and sometimes incomplete oxides, CO, and or unburned hydrogen plus traces unburnt oil oxides of nitrogen NOx. Routine product analysis can be done directly by selective chemical adsorption of stable cooled products as in Orsat apparatus or in modern online combustion-products analysis kits. More accurate volumetric results are obtained from instruments based on pressure measurements at constant volume using mercury as the displacing fluid. In chromatographic methods, a mixture sample is introduced into a carrier fluid and the components of the mixture are separated physically owing to their different rates of adsorption and desorption with a stationary bed material. As each component material leaves the bed, the relative proportions are determined by some detecting instrument such as flame ionization detector. It senses substantial increase in ion concentrations in a hydrogen-oxygen flame owing to presence of carbon compounds. The components can be identified by comparison with known samples. In summary, combustion product analysis can be done using: i) Orsat apparatus ii) Chromatography iii) Portable electronic instruments iv) Continuous emission monitors. In a complete combustion process, the main combustion products are CO2 and H2O. Figure 2.1 shows the effect of amount of combustion air to combustion gas concentrations. 𝐶𝑥 𝐻𝑦 + 𝑛4 𝑂2 → 𝑛1 𝐶𝑂2 + 𝑛2 𝐻2 𝑂 Page 3 of 31 Fig. 2.1 Combustion gas concentrations at % of the theoretical combustion air (iii) Proportions of reactants and cooled products For a hydrocarbon fuel CxHy, the stoichiometric combustion equation on molar basis is; 𝐶𝑥 𝐻𝑦 + 𝑚𝑠 (𝑂2 + 3.76𝑁2 ) → 𝑛1 𝐶𝑂2 + 𝑛2 𝐻2 𝑂 + 𝑛6 𝑁2 2.1 Where ms is stoichiometric molar Molar balance gives; Carbon: C 𝑛1 = 𝑥 𝑦 Hydrogen: H 𝑛2 = 2 Oxygen: O 𝑛1 + 0.5𝑛2 = 𝑚𝑠 or 2ms = 2n1 +n2 𝑦 ∴ 𝑚𝑠 = 𝑥 + 4 Nitrogen: 𝑛6 = 3.76 𝑚𝑠 Volumetric air-fuel ratio (A/F) = 4.76 𝑚𝑠 𝑦 = 4.76 (𝑥 + 4) 2.2 Page 4 of 31  28.96 𝑦 Gravimetric air-fuel ratio (A/F) = 12𝑥+𝑦 × 4.76 (𝑥 + 4) 𝑦 137.9(𝑥+ ) 4 = 2.3 12𝑥+𝑦 For non-stoichiometric mixtures of a hydrocarbon in air, the general combustion equation on molar basis is; 𝐶𝑥 𝐻𝑦 + 𝑚𝑠 (𝑂2 + 3.76𝑁2 ) → 𝑛1 𝐶𝑂2 + 𝑛2 𝐻2 𝑂 + 𝑛3 𝐶𝑂 + 𝑛4 𝐻2 + 𝑛5 𝑂2 + 𝑛6 𝑁2 2.4 Molar balance gives; Carbon: C 𝑛1 + 𝑛3 = 𝑥 𝑦 Hydrogen: H2 𝑛2 + 𝑛4 = 2 Oxygen: O2 𝑛1 + 0.5𝑛2 + 0.5𝑛3 + 𝑛5 = 𝑚𝑠 Nitrogen: 𝑛6 = 3.76 𝑚𝑠 Note: 𝑛5 𝑂2 is frequently in the lean (excess air) case and (𝑛3 𝐶𝑂) in rich (excess fuel) case. 𝑛4 𝐻2 usually combine with O2 to form water. Example 3.1 For a gaseous fuel methane, CH4, determine; (a) The stoichiometric A/F ratio by volume and mass (gravimetric) (b) Wet and dry products of combustion (c) Mass of dry products per unit mass of fuel. Example 3.2 Determine the stoichiometric A/F ratio for petrol (C6H14) hence deduce the chemical equation if the petrol is burnt in 20% excess air and the wet volumetric analysis of the products if; (a) All the water vapour is present (b) The products are cooled at an atmospheric pressure and temperature of 1 bar, 15°C. Determine also the dry volumetric analysis. Estimate the chemical equation if only 80% of the air required for stoichiometric combustion is provided. Page 5 of 31  iv) Deduction of A/F ratio from the exhaust gas analysis An analysis of the products affords a method for calculating the amount of air supplied in a combustion process. The underlying principle having to do with the stoichiometry under the conservation of matter. In some cases, the composition of the fuel may be determined from the analysis of the products, as done using an Orsat apparatus. Example 3.3 Methane, CH4 is burnt with atmospheric air and by means of an Orsat apparatus the following percentage composition of the products of combustion were recorded: CO2 = 10%, O2 = 2.37%, CO = 0.53%, N2 = 87.10%. Determine; (i) The A/F ratio. (ii) The combustion equation Note: the Orsat analysis uses the composition of dry gases only. The solution process involves writing the composition equation for 100 mols of the dry products, introducing later coefficients for unknown quantities and solving them. Example 3.4 The products of a hydrocarbon fuel of unknown composition have the following composition as measured by an Orsat apparatus. CO2 = 8%, O2 = 8.8%, CO = 0.9%, N2 = 82.3%. Calculate; (i) The composition of the fuel on mass basis. (ii) The A/F ratio. 2.3 Fuel Fired Equipment (a) Liquid Fuel Combustion To burn oil, particularly the heavier grades efficiently, it is necessary to breakdown the fuel into small droplets (atomization) which can be quickly heated and mixed with air. Fuel droplets of lighter oils are vaporized by heat from the downstream flame and produces gases, which readily react with oxygen. Fuel droplets of residue oils partially vaporize and the gas burns leaving behind a shell of liquid. The shell crack with further heating, forming ash. An oil burner is required to deliver the fuel into the combustion chamber in a form suitable for combustion. This is achieved by vaporizing or atomizing the fuel. Page 6 of 31 To atomize oil satisfactorily, it is necessary to control the viscosity of the oil. If it is too thick, large droplets will form, and will not burn fully. If the oil is too thin, the droplets will be too small and evaporate too quickly, causing lift off from the burner. Oil burners can be categorized in a myriad of ways. Table 2.2 gives classification of burners. Table 2.2: Classification of burners Criteria (with respect to…) Burner types (a) Control - Manual (hand), - Semi-automatic - Fully automatic (b) Service - domestic use - commercial use - industrial use (c) Fuel type - Gasoline - Kerosene - Oil – light/medium/heavy domestic – light/medium/heavy industrial (d) Preparation process - Vaporizers - Sprayers (atomizers) - Combined vaporizers and sprayers (e) Spraying/atomizing agent - Air - Steam (f) Method of projecting the fuel - Gravity - Pressure on oil - Induction - Centrifugal force (g) Method of mixing - Outside mixing (drooling, atomizer, projector, centrifugal) - Inside mixing (chamber, injector, centrifugal) (h) Force used to project - High pressure - Low pressure - Centrifugal force (i) Draught (Draft) - Natural - Forced (j) Ignition - Electric (intermittent, continuous) - Gas - Gas-electric - Oil pilot - Manual (k) Location of burner - Inside the boiler - Outside the boiler (l) Operation - Continuous - High/low - Intermittent (m) Burners introducing - Centrifugal vanes (inside/outside) centrifugal force - Rotary (vertical shaft, horizontal shaft, motor driven, fan driven) Page 7 of 31  (n) Miscellaneous types - Pot - Gun - Multiple spray - Venturi, etc. Note: No one criterial will suffice the specification of a burner, normally a combination of criterion is used. In the industry, mostly most burners are atomizing burners. They can be classified into three, depending on the source of energy used to disintegrate (atomize) the fuel. They include: (a) Pressure atomizer – in which pressure energy is employed. (b) Rotary atomizer – in which centrifugal energy is imparted into the fuel. (c) Twin-Fluid – in which a gas is made to impinge on the liquid. (i) Pressure Atomizers The fuel is preheated to reduce its viscosity to 0.15-0.24Cst (Centi-Stokes). The oil is then pumped through a nozzle at pressures between 7 and 35 bar to produce the fuel spray. The nozzle converts the pressure energy of the fuel to kinetic energy so that the oil is disbursed from the nozzle as a spray of very fine droplets. A pressure atomizer burner is shown in Fig. 2.2. The oil through the swirl chamber through the tangential ports, rotates in the chamber and is issued through the orifice in the form of a hollow conical film. The core expands due to the centrifugal force, gets thinner and finally disintegrates into droplets. The size and distribution of the droplets is governed by the design and mechanical finish of the nozzle, the physical properties of the oil, and the pressure applied. The air for combustion is admitted through an air register that surrounds the burner. The register controls the amount of air, the mixing of oil and air, and also imparts a rotary motion to the air which creates a zone of low pressure along the axis, and thus brings about recirculation of the burning gases to stabilize the flame. The main characteristics of pressure atomizers are: i) Cheap to install ii) Oil ways are fine and must be cleaned regularly iii) Very sensitive to oil viscosity iv) Sensitive to draught variations. Page 8 of 31  Fig. 2.2 Pressure atomizer (ii) Rotary Atomizers Atomization of the oil is achieved by centrifugal force. The oil flows through a central pipe to the inner surface of a revolving hollow tapered cup. (see Fig. 2.3) Page 9 of 31  Fig. 2.3 Rotary atomizer The cup is rotated at high speed (6000 rpm) by means of an electric motor or by an air turbine driven by proportion of the atomizing air. The friction between the oil and the wall of the cup causes oil to rotate with the cup, and centrifugal force together with the taper of the cup cause the oil to be discharged from the rim at high velocity. Good atomization is obtained and the size of the droplets is finer than that obtained with other atomizers. About 15% of the air required for combustion is supplied as primary air around the outside of the cup. The main characteristics of the rotary atomizer are: i) Not too sensitive to oil viscosity ii) Easy to clean iii) Moderate cost iv) Ideal for fluctuating loads (iii) Twin-fluid or Blast Atomizers Three main types; (a) Low pressure with air at 0.03 – 0.06 bar as the atomizing medium. (b) Medium pressure with air at 0.35 – 1.1 bar. (c) High pressure type. Can use steam or air at pressure greater than 1 bar. (a) Low pressure burners A large proportion or sometimes all the combustion air is passed through the burner. The air is supplied by means of a single stage centrifugal fan, which is simple, cheap and easily obtainable. Secondary air is adjusted to give effective control of the furnace temperature. The main characteristics are: i) Easy to maintain. ii) Controllable flame shape iii) Insensitive to draught. (b) Medium Pressure burners The secondary air is controlled separately and supplied by a rotary compressor. They are used for boilers and furnaces where good control is required. (c) High Pressure burners These are similar to medium pressure burners but require a more expensive compressor. Steam is frequently used as an atomizing fluid when plentiful supply of steam exists, e.g. as in a steam Page 10 of 31  furnace. The disadvantage is that a separate equipment has to be installed to generate the atomizing steam. Fig. 2.4 shows a twin-fluid atomizer burner. Fig. 2.4 Twin fluid atomizer Table 2.3 Common problems in burners Condition Cause Remedy (Action) Sparky flame Atomization Check and clean nozzles Flame impingement Incorrect air supply Check air control adjustments Flame pulsates Too high oil temperature Adjust preheat Too high air velocity Smoke Too little air Adjust air/oil Seal air leaks High particulate Atomization Check nozzle preheat Fuel input too high Reduce fuel Check design (b) Solid Fuel Combustion In a bed of burning solid fuel (wood, coal, peat, etc.) under-grate air combines with the carbon to produce CO2 and CO. These hot gases rise through the bed and drive off volatiles of the fuel (HC’s Page 11 of 31 such as Methane). Above the bed, secondary air is admitted which burns off the CO and the volatiles. In order to burn the fuels efficiently, it is necessary to introduce optimum quantities of air for combustion. Too little air will cause smoking with consequent loss due to unburnt fuel. Because of visible smoke, this problem is usually corrected quickly. The introduction of too much combustion air is more common in boilers, furnaces and vehicles, but less apparent, and therefore can continue undetected for long periods. The use of excessive quantities of air leads to substantial energy losses and can also cause operation problems, e.g. scaling in furnaces. Control of air/fuel ratio is very important particularly in high temperature exhausts, i.e. furnaces and kilns, where stack losses can be up to 60% of the fuel input. A simple oxygen analyzer and high temperature thermometer can detect high excess air quantities which can often be rectified by simple adjustment of the fuel control or the air fans. Control of thermal Input (i) Over-firing Losses can occur due to use of excessive amounts of fuel input into the furnaces or boilers, i.e. over-firing. This leads to high stack temperatures and avoidable energy losses. Over-firing is generally associated with incorrectly adjusted burners and/or fouled heat transfer surfaces. (ii) Under-firing Low thermal inputs are easily detected because the boiler or furnace outputs are low. However, over-firing and therefore excessive losses, are not apparent. A regular check of stack temperatures can ensure that the burner outputs are optimized. (iii) Fuel-air ratio Experience has shown that many burners are incorrectly adjusted, particularly under low load conditions. Wear on cams, linkages, fuel pump adjustments affect the performance of energy conversion equipment. Regular combustion checks can identify any shortcomings in maintenance, cleanliness, etc. (iv) Flue gas temperature High flue gas temperatures are associated with the following conditions: - Too high firing rate, usually due to incorrect setting of controls. Page 12 of 31  - Fouled heating surfaces – in boilers it could be fouling of surfaces on fireside or scaling on surfaces on the water side or both. Fouled heating surfaces impede the heat transfer resulting in more heat being rejected to the stack in form of higher flue gas temperature. 2.4 Fuel Fired Equipment and Applications (a) Furnaces The purpose of a process furnace is to supply heat to the contents in controlled manner. The furnace may be used for heating metals to a precisely controlled temperature for heat treatment (annealing, quenching, case hardening, precipitation hardening, tempering, normalizing) or for melting. Furnaces are manufactured in many different types and sizes, and may be either batch or continuous type. Furnaces may categorized into two: (i) that generate heat by burning fuels- may be of the direct or indirect types, and (ii) those that use electricity- may be resistance or induction types. Batch Furnaces The batch furnaces process the product in batches, i.e. the furnace doors must be opened and closed at the beginning and end of each batch cycle. Since this is a significant source of energy loss, the loading and unloading times should be minimized. It is also important to load the furnace completely to minimize the energy loss per unit product. Continuous Furnaces Continuous furnaces process the product continually by moving it through the heating zones on chains or conveyers. Since the loading and unloading doors are open most or all of the operating time, there is significant loss of energy through these openings. A significant loss of energy may be due to the conveying mechanism, which is heated to the operating temperature of the product, and cools off outside the furnace before re-entering the loading area. A re-design of the conveyer system so that it remains in the heated area may be better. Direct fired furnaces The products of combustion are in direct contact with the product being heated. The heat transfer process from the flame to the product is more effective compared to the indirect heated Page 13 of 31 furnace. The higher rate of heat transfer in this type of furnaces can lead to a local surface overheating of the product, unless the furnace temperature is properly controlled. Indirect Heated Furnaces In this type of furnaces, the heat is transferred through some form of heat exchanger. This type of furnace may be used to provide a controlled environment for oxidizing or reducing, by introducing an artificial atmosphere independent of the combustion process. Since the heat transfer from the flame to the product is not as effective as with direct fired furnace, it is expected that the flue gas temperature will be higher, resulting to higher heat losses unless heat recovery is used. (b) Dryers Dryers use heat to evaporate water or solvents from materials such as lumber, grains, ceramics, paints and carbon electrodes. The same principles of energy management described for furnaces apply, and much of the equipment is similar in concept. A major difference is in operating temperature, which is generally much lower than furnaces, as this avoids damage of the product. As a result, the direct fired heaters (dryers) must operate with very high percentage of excess air. This means that the excess air cannot be reduced to achieve the energy savings. Indirect fired dryers can operate at normal values of excess air within the combustion chamber. With direct and indirect fired heaters there is a large amount of heat in the exhausted air in form of evaporated water vapor or solvent. Often the solvents must be incinerated before discharge to the atmosphere by burning additional fuel I the dryer discharge and raising the temperature to about 900°C. Recovery of the heat in the dryer exhaust can be achieved by a heat exchanger which is used to preheat the incoming air for drying with indirect fired dryers or the combustion air for firing in the direct dryers. (c) Kilns There is no fundamental difference between furnaces and kilns from the energy management point of view. The ceramic and brick industries use stationary kilns to sinter their products. On the other hand, rotary kilns are used by cement and pulp industries. Some rotary kilns burn pulverized coal or refuse-derived fuel. The large heat input to the rotary kiln provides opportunities for the insulation of heat exchangers to recover flue gas heat. 2.5 Air Pollution Control – Process and Equipment Page 14 of 31 The combustion processes for heat generation, transportation and chemical processes emit pollutants that are harmful to the environment. The four most effects of air pollution are: i) Greenhouse effect ii) Acid rain iii) Ground level ozone iv) Smog Greenhouse Gas Effect Sun’s short-wave radiation penetrates the atmosphere and heats up the earth. The warmed earth reradiates back the excess heat in from of long wavelengths radiation because its at lower surface temperatures. Water vapour and greenhouse gases such as carbon dioxide, nitrous oxides, and methane absorb the infrared radiation, thus heating the atmosphere and the earth’s surface. The heating of the atmosphere by blocking the escape of infrared radiation is known as greenhouse gas effect, and is responsible for global warming. Acid Rain Acid rain results from combining of nitrogen and Sulphur oxides with atmospheric water vapour. These pollutants originate from coal burning, metal smelting, vehicles and all other fuel burning activities. Nitric oxide and Sulphur oxides, when combined with water vapour, form nitric and sulphuric acid that return to earth as acid rain, snow or fog that leads to acidification of lakes and other surface waters. Ground Level Ozone Ground level ozone is produced by the chemical reaction between nitrogen oxides and volatile organic compounds (VOCs) and is the key NOx and VOC related air quality problem. NOx is formed by burning fossil fuels. VOCs are formed mainly from evaporation of liquid fuels, solvents, and organic chemicals. Ozone damage to crops and vegetation can be significant. Ozone sensitive crops include beans, tomatoes, potatoes, soybeans and wheat. Smog Originally the term ‘smog’ meant a combination of smoke and fog, but it has recently come to refer to a combination of fine particulate matter and ground level ozone. Smog can also contain other harmful components such as nitrogen oxides, volatile organic compounds, sulphur dioxide Page 15 of 31 and carbon monoxide. Human-made smog is derived from coal emissions, vehicular emissions, industrial emissions, forest and agricultural fires and photochemical reactions of these emissions. The colour of smog is determined by these suspended particles and is often brown or deep grey but can also be white. Smog has adverse respiratory effects. 2.6 Energy Management Opportunities Energy Management Opportunities is a term used to represent the way that energy can be used wisely to save money. It is intended to provide management, operating and maintenance personnel with ideas to identify the opportunities. Energy management opportunities are sub-divided into Housekeeping, Low-Cost and Retrofit categories. Housekeeping Opportunities i) Maintain proper burner adjustment It is good practice to have an experienced burner manufacturer’s representative set up burner adjustments. Furnace operators can then identify the appearance of a proper burner flame for future reference. The flame should be checked frequently and always after a significant change in operating conditions affecting the fuel, combustion air flow, or furnace pressure. ii) Check excess air and combustibles in the flue gas A continuous oxygen and combustibles analyser is the best arrangement, but cost is high. Sampling tests with an Orsat or by other chemical means can be a reliable guide for proper combustion conditions. Re-adjustment of the fuel/air ratio control should be done promptly if required. iii) Keep heat exchange surfaces clean This is required more frequently with oil fired furnaces and for these applications, the use of permanently installed steam or air soot blower may be justified. iv) Replace/Repair missing and damaged insulation Heat radiation from a furnace with inadequate insulation can be easily detected during a plant survey. v) Check furnace pressure regularly Air leakage into or gas leaking out of a furnace can be controlled by maintaining a slightly positive furnace pressure. The control dumpers in the furnace flue gas ducting or related controls should be readjusted if the furnace pressure is not at a correct value. Page 16 of 31 vi) Schedule production to operate furnaces at or near maximum output It may be possible to operate the furnace at maximum load every other day, instead of at 50% load continuously. Alternatively, the work may be switched to a smaller furnace which can operate near full load continuously. Low Cost Opportunities i) Replace damaged furnace doors or covers Furnace doors or covers which are warped or damaged can be source of considerable leakage of air into or out of the furnace. These should be replaced with doors or covers with tight fitting seals. ii) Install adequate monitoring instrumentation The minimum requirement is to have the ability to determine the energy used per unit output, so that significant deviations from this can be identified and corrective action taken. The fuel or Watt meter may be a portable instrument which can be then used on several furnaces. Additional instrumentation will be required to identify individual losses. Measurements of flue gas temperature and oxygen content can be used to indicate the flue gas loss. If a heat exchanger is used to recover heat from the flue gas, the temperature of the gas and air in and out of the heat exchanger can be used to check the performance. iii) Recover heat from equipment cooling water It is often possible to use the warm water discharge from equipment coolers for the purposes such as process washing. In some systems the water discharge may be too cool to be useful. In these instances, the installation of a water flow control valve and temperature controller may be helpful. The water flow is controlled automatically from the water temperature at the cooler outlet so that the water temperature is high enough to be useful, while maintaining proper cooling. The control system will also reduce water use. Retrofit Opportunities (i) Install a heat exchanger in the flue gas outlet The cost of a heat exchanger is significantly affected by the temperature of the gas entering the unit. Careful consideration should be given to introducing cold air into the gas stream, if required, to lower the gas temperature enough to use economic materials. Stainless steels or alloys cannot be used for temperatures above 900°C. If the recovered heat is to be used to preheat combustion air, the burner manufacturer should be consulted to determine the maximum allowable temperature. Frequently it will be as low as 250°C. It is unlikely that it will be higher than 400°C since it would require alloy steels instead of carbon steel. If it is not possible to preheat the combustion air, it may be possible to heat the process water or to install a waste heat boiler to utilize the heat energy in the flue gases. Page 17 of 31  Table 2.4 gives an example of an evaluation of Energy management analysis in a building. Table 2.4 Energy Managements Opportunities in a building Combustion Analysis Using Exhaust Gas Data From the exhaust gas analysis, several combustion parameters can be determined to help evaluate the operating performance of the furnace or boiler. They include: i) Excess air ii) Carbon dioxide iii) Combustion efficiency iv) O2 reference v) Emissions conversions. i) Excess Air As discussed earlier, for complete combustion of a fuel, more air than the stoichiometric air is required, and was termed as excess air. The excess air is expressed as the percentage air above the amount theoretically needed for complete combustion. The more refined the fuel, the less excess air is needed. Typical excess air values for fuels are shown in Table 2.5 below. Table 2.5 Excess air values for common fuels Fuel Excess air values Natural gas 5 – 10% Page 18 of 31  Industrial Diesel Oil (IDO) No.2 oil 10 – 20% Residual Fuel Oil (RFO) No. 6 oil 10 – 25% Coal 20 – 40% Biomass (bagasse) 30 – 50% In combustion of oils, the effect of excess air on CO2 and O2 levels in the exhaust on dry basis are shown in Table 2.6. Table 2.6 Effect of excess air on CO2 emissions % excess air %CO2 %O2 Nil (stoichiometric) 16% 0% 30% 12% 5% 50% 11% 7% 75% 9% 9.5% 120% 7% 12% It should be noted that though excess air reduces the amount of CO2 emissions, it lowers the thermal efficiency of the combustion as shown in Fig. 2.6. Typical target values of level of emissions are shown in Table 2.7 below. Table 2.7 Emission target values for common fuels Fuel Max CO2 Target CO2 Target O2 Coal 19% 14% 6% Fuel oils 16% 13% 4% Natural gas 12% 11% 2% The amount of excess air can be estimated using the formula: %𝑂2 𝑚𝑒𝑎𝑠𝑢𝑟𝑒𝑑 % 𝐸𝑥𝑐𝑒𝑠𝑠 𝑎𝑖𝑟 = × 100 20.9 − %𝑂2 𝑚𝑒𝑎𝑠𝑢𝑟𝑒𝑑 If the amount of CO is high, the excess air can be computed using the formula: %𝐶𝑂 %𝑂2 − % 𝐸𝑥𝑐𝑒𝑠𝑠 𝑎𝑖𝑟 = 2 × 100 %𝐶𝑂 20.9 − (%𝑂2 − 2 ) An expression of excess air referred to as labda (λ) is also used. It is expressed as: Page 19 of 31  % 𝑒𝑥𝑐𝑒𝑠𝑠 𝑎𝑖𝑟 𝜆= +1 100 Excess air for various fuels can be expressed in graphical or tabular forms. Fig.2.5 Effect of Excess air on Combustion efficiency (Fuel oil) ii) Carbon Dioxide CO2 The CO2 concentration can be determined using the equation: 20.9 − %𝑂2 𝑚𝑒𝑎𝑠𝑢𝑟𝑒𝑑 %𝐶𝑂2 = 𝐶𝑂2 (max) × 20.9 Where CO2 max is the theoretical maximum concentration produced by the fuel used. iii) Combustion Efficiency Combustion efficiency is expressed as a percent and determined by subtracting individual stack heat losses, as percent of the fuel’s heating value, from the total heating value of the fuel (100%). Page 20 of 31 Dry gas loss and latent heat loss due to H2 in the fuel are typically the largest sources of stack loss. Others can be included, such as heat loss from moisture in the air and fuel and losses from the formation of CO rather than CO2. This basic form for calculating efficiency is described in the ASME Power test code 4.1 and is applicable for losses other than flue losses when determining total system efficiency by the Heat-Loss method. 𝑓𝑙𝑢𝑒 ℎ𝑒𝑎𝑡 𝑙𝑜𝑠𝑠𝑒𝑠 𝑝𝑒𝑟 𝑙𝑏 𝑜𝑓 𝑓𝑢𝑒𝑙 % 𝑛𝑒𝑡 𝑐𝑜𝑚𝑏𝑢𝑠𝑡𝑖𝑜𝑛 𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 = 100 − × 100 𝑓𝑢𝑒𝑙 ℎ𝑒𝑎𝑡𝑖𝑛𝑔 𝑣𝑎𝑙𝑢𝑒 𝑝𝑒𝑟 𝑙𝑏 𝑜𝑓 𝑓𝑢𝑒𝑙 Flue heat losses = Lg + Lh + Lm + Lco (Individual heat losses are described below) Where: Lg = heat loss due to dry gas Lh = heat loss due to moisture from burning hydrogen Lm = heat loss due to moisture in fuel Lco = heat loss from the formation of CO Heat Losses due to dry gas (Lg) Lg = 𝑊𝑔 × 𝐶𝑝 × (𝑇𝑓𝑙𝑢𝑒 − 𝑇𝑠𝑢𝑝𝑝𝑙𝑦 ) Where; Wg = the weight of the flue gases per pound of as-fired fuel. Cp = specific heat of the exhaust gas mix. Tflue = flue temperature Tsupply = combustion supply air temperature (44𝐶𝑂2 +32𝑂2 +28𝑁2 +28𝐶𝑂) 12×𝑆 And Wg = × (𝐶𝑏 + ) 12×(𝐶𝑂2 +𝐶𝑂) 32 Where: Cb = carbon content, specific to the fuel. Cp = specific heat capacity, varies with temperature. A good estimate is given by 𝐶𝑝 = 0.24 + 0.00038 × (𝑇𝑓𝑙𝑢𝑒 − 200), regardless of the fuel. S = sulphur content of the fuel Heat loss due to water from combustion of hydrogen (Lh) Page 21 of 31 Where the fuel has a high hydrogen content, latent heat loss from the water formation can be very significant. 𝐿ℎ = 8.936 × 𝐻 × (ℎ𝑙 − ℎ𝑟𝑤 ) Where: 8.936 = weight of water formed for each hydrogen atom H = fractional hydrogen content of the fuel hl = enthalpy of water at the exhaust temperature and pressure hrw = enthalpy of water as a saturated liquid at fuel supply temperature Heat loss due to moisture in fuel (Lm) Moisture in the fuel is determined from lab analysis of the fuel and can be obtained from the fuel supplier. 𝐿𝑚 = 𝑓𝑟𝑎𝑐𝑡𝑖𝑜𝑛𝑎𝑙 𝑚𝑜𝑖𝑠𝑡𝑢𝑟𝑒 × (ℎ𝑙 − ℎ𝑟𝑤 ) Where: hl = enthalpy of water at exit gas temperature and pressure hrw = enthalpy of water as a saturated liquid at fuel supply temperature Heat loss due from the formation of carbon monoxide (LCO) Carbon in the fuel reacts with oxygen to form CO first, then CO2, generating a total of 14,540 Btus of heat per pound of carbon. If the reaction stops at CO because of insufficient O2 or poor mixing of fuel and air, 10,160 Btus of energy are lost. %𝐶𝑂 𝐿𝐶𝑂 = %𝐶𝑂 × 10,160 × 𝐶𝑏 2 +%𝐶𝑂 Where: Cb = fractional carbon content The Siegert formula is also widely used to determine flue gas losses and efficiency. 2𝐴 i.e. 𝑄𝑓𝑙𝑢𝑒 = (𝑇𝑠 − 𝑇𝑎 ) × (21−𝑂 + 𝐵) 2 and 𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 = 100 − 𝑄𝑓𝑙𝑢𝑒 Where: Qflue = flue loss Ts = flue temperature Page 22 of 31  Ta = supply air temperature O2 = measured volumetric oxygen concentration expressed as a percentage A2, B = fuel dependent constants. Typical values for constants are shown in Table 2.8 below. Table 2.8 Siegert Constants Fuel A2 B Natural gas 0.66 0.009 Fuel oil 0.68 0.007 Town gas 0.63 0.011 Coking oven gas 0.60 0.011 LPG (propane) 0.63 0.008 Another expression of the Siegert formula gives; 𝐾 ×∆𝑇 %𝑙𝑜𝑠𝑠 = +𝐶 %𝐶𝑂2 Where: %loss = total flue gas loss as a percentage of the fuel’s gross energy (HHV) K, C = constants for the fuel type, as given in table below %CO2 = CO2 as percent of dry gas in flue gas ΔT = temperature difference in °C between the flue gas and combustion air. Table 2.9 Siegert Constants Fuel K C Fuel oil 0.56 6.5 Coal 0.63 5.0 Natural gas 0.38 11.0 In boiler and furnaces, the temperatures are kept fairly constant, and radiation losses can be approximated as: Boilers: 2-5% Furnaces and kilns = 10% iv) Oxygen reference Page 23 of 31  20.9 − 𝑂2,𝑟𝑒𝑓𝑒𝑒𝑛𝑐𝑒 Corrected gas concentration (ppm) = gas conc. measured × 20.9 − 𝑂2,𝑚𝑒𝑎𝑠𝑢𝑟𝑒𝑑 v) Emission rate calculations using dry gas factors The emission rate calculation presented below is described in EPA Method 19. This uses the dry gas factor Fd. Dry factors are incorporated into the values found in Table 2.9 below. The table values (Ft), convert the measured concentrations of emission gases CO, NOx, and SO2 from PPM to pounds per million Btu of fuel. 20.9 𝐸 = 𝐶𝑔 × 𝐹𝑡 × (20.9−𝑂 ) 2,𝑚𝑒𝑎𝑠𝑢𝑟𝑒𝑑 Where: E = Emission rate (pounds/MBtu of fuel) * Cg = Gas concentration (PPM) Ft = Emission rate conversion factor from Table 2.10 (below) O2 measured = Oxygen concentration from flue measurement (%) *To convert emission rate to metric equivalent units, kg/kJ, multiply E in the equation above by 2.236. Table 2.10 Emission rate conversion factors to convert from ppm to pounds per million btu of fuel for selected gases Ft** Nat. gas propane Oil #2 Oil #6 Coal Wood Bagasse Coke (dry) SO2 0.00145 0.00145 0.00153 0.00153 0.00164 0.00153 0.0016 0.00164 NOx 0.00104 0.00104 0.00110 0.00110 0.00118 0.00110 0.001 0.00118 CO 0.00063 0.00063 0.00067 0.00067 0.00072 0.00067 0.00067 0.00072 ** Ft units are lb/MBtu PPM Energy units Conversions 1 BTU = 1.05 kJ 1 lb (pound) = 0.45 kg 2.5 Fuel Switching Fuel switching is the substitution of one energy source for another in order to meet requirements for heat, power, and/or electrical generation. In industrial applications, it may also refer to the practice of substituting among natural fuel, electricity, coal, and LPG within 30 days without modifying the fuel consuming equipment, without affecting the rate of production. The major reason for fuel switching is fuel cost. Other factors may include; Page 24 of 31  i) Environmental regulations (climate change, health hazards) ii) Agreement with energy or fuel supplier iii) Equipment capabilities Fuel switching can play a major role in decarbonization in power generation, and when the end- user sectors such as transport, industry and buildings (which are energy intensive) move to fuels that are lower in emissions intensity. e.g. - Switching from coal to gas in power generation. - Replacing gasoline/diesel use in transportation with biofuels, hydrogen and electric- powered vehicles. - Replacing kerosene stoves with LPG burners etc. Case study – Cement Industry Industry routinely switches fuel in response to relative crisis or shortages and in attempt to reduce costs and risks. Fuel switching is appreciated through case studies, and fuel switches in the cement industry is an ideal case. In the 1950’s coal was the fuel for the cement industries. When oil and gas were discovered, coal was abandoned and for two decades oil and gas dominated the cement industry. The oil embargo of 1973 and subsequent rise in the price of oil prompted the reversion to use of coal by most plants Embargo – an official ban on trade or other commercial activity with a particular country. In the 1973 oil embargo, OPEC declared an embargo on oil to countries that supported Israel in the Israel- Arab war which lasted for six months. The Cement Manufacturing Process The cement manufacturing process has seven stages: i) Quarrying and transport of raw materials including limestone, canker, iron ore, bauxite, pozzolana, gypsum, etc. ii) Storage of raw materials. iii) Preparation of raw materials (reclaiming, metering, grinding) iv) Fuel transportation, storage, preparation, charging. v) Clinker production. vi) Cement grinding and storage vii) Packaging and dispatch. Page 25 of 31 Energy use in the Cement Industry The cement manufacturing process is energy intensive. The major energy consumers are shown in Table 2.11. Table 2.11 Energy use in Cement industry Form of energy Major consumers Electrical Quarry, raw material preparation, cement milling and packaging Thermal (coal) Pre-calcining and Clinkerization Thermal (oil) Transport of raw materials, kiln firing (a) Fuel oil The transport, storage and use of fuel oil is relatively a standard industrial process, which entails preheating, pumping and injection. Fundamental considerations of the fuel burner are: (i) Carry or transport fuel to the burner tip and release with desired velocity. (ii) Provide adequate oxygen-rich air at the tip to ensure complete burning of the fuel. (iii)Atomize the fuel at the tip so that the burning is fast and results into quick heat release. (iv) Provide mechanism to adjust flame shape and type. (v) Provide mechanism for adjusting direction of burner and central direction of the flame. (vi) Provide mechanism to connect air supply from the fan to burner (vii) Provide means for initial firing (gas-firing) Figures 2.6 – 2.9 show typical construction/assembly of a cement kiln burner. Page 26 of 31 Fig. 2.6 Fig. 2.7 Page 27 of 31 Fig. 2.8 Fig. 2.9 (b) Coal Coal transport, storage, preparation and charging Many coal mines are far from the user and it is necessary to have appropriate transport and storage systems. Trucks can be used for local transport, but trains and ships are preferred for bulk transport of coal. Storage systems include; Page 28 of 31  a) Coal silo – usually made of steel or concrete with a capacity to last 3-4 days. The bins should be self-cleaning. b) Stock-piles – could be open or covered. Stock piles must be located away from heat sources, well drained, and separated from foreign materials with no ignition temperature. Firing Equipment The manner and devices for burning coal depend on the size of the particles. Coal can be fired via three main equipment: a) Fuel bed – large particles combustion is done in fuel beds whereby mechanical devices provide continuous or intermittent feed for ignition and proper distribution of combustion air. The effectiveness of the combustion depends on the particle size. b) Fluidized bed combustion – the fuel is combusted in a bed of solid particles, which is fluidized by injection of air at the bottom of the bed. The burning rates are higher and the temperature range is 850°C. c) Suspension firing – the coal is pulverized and burned in a similar fashion as oil or gas. Pulverized coal is blown into the combustion chamber by air and burns in a manner similar to that of droplets of liquid. Burning rates are high and the temperatures are greater than 1500°C. Coal Grinding (Pulverization) This process uses tumbling or roller mills. Grinding elements consist of roller and balls, which are cheap and insensitive to foreign bodies in the field of the materials. They can be operated economically at a constant throughput. Environmental Issues & Impacts of Coal Use i) Air pollution compared to other hydrocarbons ii) High potential hazard of spontaneous coal fires and coal dust explosion. iii) Potential contamination of soil and ground water from coal storage. iv) Noise pollution from processing equipment. Sources of emissions i) Kiln system combustion chambers ii) Coal pulverizing mill iii) Fugitive emissions from storage and handling facilities iv) Fugitive emissions from transport of coal. Page 29 of 31 The pollutants (coal) i) Particulate matter Sources of particulate coal emanate from a) Coal transport – trucks used to transport coal from quarry to the cement plat are generally not covered. Particulate matter emissions take place due to vibrations and wind. b) Storage and handling within the plant – coal is generally store in the open in the form of ‘dumps’. The emissions of such storage depend on the wind speed and the moisture content of the coal. Dust emissions also take place during the dumping of the coal. c) Coal feed and conveyance - fugitive particulate matter emitted through feeding the coal to crushers by loaders, and transport of coal for further processing through conveyer belts. Because these emissions are released at ground level, they are minimally diffused and may cause a lot of public health hazard. ii) Oxides of sulphur (SOX) SOx emissions from cement kilns primarily depend on the content of the volatile sulphur in the raw materials. Coal generally contains more sulphur than fuel oil. iii) Oxides of nitrogen (NOX) NO and NO2 are the dominant oxides of nitrogen in cement and kiln exhaust gases. No substantial difference of NOx emission occurs in shifts of fuel from oil to coal. iv) Carbon monoxide (CO) One of the serious impacts of the use of solid fuel is the potential increase in the frequencies of short-term carbon monoxide releases. Consequent to unsteady state operation of the combustion system resulting from surges of solid fuel feeding due to poor feeding controls. It is anticipated that no substantial difference in overall CO2 emissions will occur. v) Volatile Organic Compounds (VOCs) In combustion processes in general, the occurrence of VOCs is often associated with incomplete combustion. In the cement kiln, these emissions will be low under normal steady state operations due to large residence time of gases in the kiln, and high temperatures. Excess concentrations may increase during start-ups or upset conditions. There’s no significant difference in the emissions of VOCs due to fuel switch over. vi) Polychlorinated Dibenzodioxins (PCDD) & Polychlorinated Dibenzofuran (PCDF) Page 30 of 31 These are extremely toxic chemical compounds. Any chlorine input the presence of organic material may cause the formation of PCDD and PCDF in combustion processes. The chlorine content of coal is generally higher than that of fuel oil. Introduction of coal therefore, in place of oil will in most cases increase the chlorine input into the kiln system. vii) Stack emissions from coal pulverisation mills The emissions originating from the kiln are divided into parallel streams; one from the kiln stack (e.g. chimney) and the other from the coal pulverization stack. Continued use of electrostatic precipitators (EP) and bag/fabric filters are necessary to control the particulate matter emissions from the stacks. viii) Spontaneous dust fires and coal dust explosions One potential hazard of coal, depending on its characteristics and ambient conditions, is the property to spontaneously ignite and combust in rather low temperatures which lead to outbreak of fires in stored coal. The spontaneous combustion may occur naturally or the ignition may be triggered by other causes e.g. lightning, electrical or mechanical sparks. ix) Other related impacts Open storage of coal on unpaved soil surface with no proper drainage system may lead to contamination of soil and ground water, particularly consequent to rainfall. The rain water passing through the stacks of coal may reach certain dissolved organic constituents and carry them to soil and ground water. Page 31 of 31