Combustion fuels, heating values

Combustion effieieney is a measure of eombustion eompleteness. Combustion eompleteness affeets fuel eonsumption direetly, sinee the heating value of any unburned fuel is not used to inerease the turbine inlet temperature. To ealeulate eombustion effieieney, the aetual heat inerease of the gas is ratioed to the theoretieal heat input of the fuel (heating value). 

Net Heat of Combustion (High Heat Value) for Various Liquid and Gaseous Fuels [4]... 

A typical heat balance for Run LSF 34 on No. 6 oil is given in Table V. The calculated efficiencies are also given in the table. Heat input terms consist of the input heat from the fuel, the fuel sensible heat, and the makeup water sensible heat. The heat available from combustion of the fuel is calculated from the measured volumetric flow rate, the measured fuel heating value, and the measured fuel density at the nozzle temperature. The fuel sensible heat contains the fuel mass flow rate, the measured temperature at the nozzle, a reference temperature, and an estimated specific heat for the oil of 0.480 Btu/lb°F. The specific heat was taken from graphical information in the ASME Power Test Code. Similarly, the water sensible heat calculation contains a tabular value... 

The fuel heating value is the total amount of energy that can be liberated by combustion of a fuel. The heating value may be normalized by mass, moles, or volume accordingly, it will have units of kj/kg, J/mol, or J/Nm. Customary units include Btu/lbm, Btu/lbmol, and Btu/scf. In the case of volume, some standard temperature and pressure must be chosen, such as 25°C and 1 atm. Two examples of unit volumes referenced to such conditions are the normal meter cubed (Nm ) and the standard cubic foot (scf). 

The calorific value or heat of combustion or heating value of a sample of fuel is defined as the amount of heat evolved when a unit weight (or volume in the case of a sample of gaseous fuels) of the fuel is completely burnt and the products of combustion cooled to a standard temperature of 298 °K. Net calorific value assumes the water leaves with the combustion products without fully being condensed. Fuels should be compared based on the net calorific value (Table 2.55). The calorific value of coal varies considerably depending on the ash, moisture content and the type of coal, while calorific values of fuel oils are much more consistent. Table 2.55 shows that oil and gas have about 30% less specific CO2 emission than coal, based either on carbon content or energy output the only reason for this difference is the water content of the fuel and the energy loss due to water evaporation. Water condensation and heat recovery seems a way to increase the net efficiency and hence reduce emissions. 

Schematically, CO2 capture can be achieved following three main strategies (Figure 39.1) [12] (1) oxy-combustion (or oxy-fuel combustion) where the fuel combustion is performed with pure or enriched O2 instead of air, so that a CO2/ H2O mixture is produced (2) pre-combustion, where the carbon from the fuel is removed prior to combustion (decarbonization) either as CO2, as coke, or in other forms, and whereby the primary fuel heating value is transformed into H2 through partial oxidation, steam reforming, or autothermal reforming with subsequent water-gas shift (WGS) reaction and (3) post-combustion, where CO2 recovery is performed at the end of pipe from a wet exhaust flue gas, usually at 10-30% (v/v) CO2 concentration. The target separations to achieve in these processes to make them feasible are O2/N2 for oxy-combustion, CO2/H2 for precombustion, and CO2/N2 for post-combustion CO2 capture.

The mass or volume heating value represents the quantity of energy released by a unit mass or volume of fuel during the chemical reaction for complete combustion producing CO2 and H2O. The fuel is taken to be, unless mentioned otherwise, at the liquid state and at a reference temperature, generally 25°C. The air and the combustion products are considered to be at this same temperature. 

Measuring the gross heating value (mass) is done in the laboratory using the ASTM D 240 procedure by combustion of the fuel sample under an oxygen atmosphere, in a bomb calorimeter surrounded by water. The thermal effects are calculated from the rise in temperature of the surrounding medium and the thermal characteristics of the apparatus. 

In the expression for heating value, it is useful to define the physical state of the motor fuel for conventional motor fuels such as gasoline, diesei fuel, and jet fuels, the liquid state is chosen most often as the reference. Nevertheless, if the material is already in its vapor state before entering the combustion system because of mechanical action like atomization or thermal effects such as preheating by exhaust gases, an increase of usefui energy resufts that is not previously taken into consideration. 

It is useful to examine the combustion process appHed to soHd wastes as fuels and sources of energy. AH soHd wastes are quite variable in composition, moisture content, and heating value. Consequendy, they typically are burned in systems such as grate-fired furnaces or duidized-bed boilers where significant fuel variabiUty can be tolerated. 

The first gas producer making low heat-value gas was built in 1832. (The product was a combustible carbon monoxide—hydrogen mixture containing ca 50 vol % nitrogen). The open-hearth or Siemens-Martin process, built in 1861 for pig iron refining, increased low heat-value gas use (see Iron). The use of producer gas as a fuel for heating furnaces continued to increase until the turn of the century when natural gas began to supplant manufactured fuel gas (see Furnaces, fuel-fired). 

The volatiles contents of product chars decreased from ca 25—16% with temperature. Char (lower) heating values, on the other hand, increased from ca 26.75 MJ /kg (11,500 Btu/lb) to 29.5 MJ /kg (12,700 Btu/lb) with temperature. Chars in this range of heating values are suitable for boiler fuel apphcation and the low sulfur content (about equal to that of the starting coal) permits direct combustion. These char products, however, are pyrophoric and require special handling in storage and transportation systems. 

Aniline can be safely incinerated in properly designed faciHties. It should be mixed with other combustibles such as No. 2 fuel oil to ensure that sufficient heating values are available for complete combustion of aniline to carbon dioxide, water, and various oxides of nitrogen. Abatement of nitrogen oxides may be required to comply with air poUution standards of the region. 

The vortex burner maintains stable combustion temperature when the organic concentration ia the waste is sufficiently high and has a heating value of ca 10.5—12.6 MJ/kg (4500—5400 Btu/lb). AuxiUary fuel may be required when the chloride concentration ia the waste exceeds 70% (30). 

Figure 27-11 gives the theoretical air requirements for a variety of combustible materials on the basis of fuel higher heating value (HHV). If only the fuel lower heating value is known, the HHVean be calculated from Eq. (27-6). If the ultimate analysis is known, Eq. (27-7) can be used to determine HHV. 

FIG. 27-13 Available heats for some typical fuels. The fuels are identified by their gross (or higher) heating values. All available heat figures are based upon complete combustion and fuel and air initial temperature of 288 K (60 F). To convert from MJ/Nm to Btii/ft, multiply by 26.84. To convert from MJ/dm to Btii/gal, multiply by. 1588. 

In both cases heat is taken from the exhaust gases to feed the reaction process, enhancing the heating value of the resulting modified fuel, which is then fed to the combustion chamber. But the main thermodynamic feature is that the exergy loss in the final exhaust gas is thus reduced and the efficiency increased. 

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