Combustion calculation

For combustion calculations, it is acceptable to take the average molecular weight of dry air to be 28.97, and to assume a simplified composition of 79.0 niole% nitrogen and 21.0 mole% oxygen (or equivalently, 76.8 wt% nitrogen and 23.2 wt% oxygen). 

Consider the combustion of ethane (C H ) in pure oxygen. If 100 lb of ethane are available and 10% excess oxygen is supplied to ensure complete combustion, calculate (1) the amount of oxygen supplied, and (2) compositions of the reactants and products on mass and molal bases. 

This technique is particularly useful in handling combustion calculations where the nitrogen in the combustion air passes through unreacted and is used as the tie component. This is illustrated in Example 2.8. 

Assume complete combustion. Calculate the mass of air required to bum a unit mass of combustible. 

As-received is applicable for combustion calculations moisture-free and mineral-matter-free, for classification purposes. 

Burcat [ Thermochemical Data for Combustion Calculations, in Combustion Chemistry. (W. C. Gardiner, Jr., ed.), Chapter 8. John Wiley Sons, New York, 1984] discusses in detail the various sources of thermochemical data and their adaptation for computer usage. Examples of thermochemical data tit to polynomials for use in computer calculations are reported by McBride, B. J Gordon, S., and Reno, M. A., Coefficients for Calculating Thermodynamic and Transport Properties of Individual Species, NASA, NASA Langley, VA, NASA Technical Memorandum 4513, 1993, and by Kee, R. J., Rupley, F. M and Miller, J. A., The Chemkin Thermodynamic Data Base, Sandia National Laboratories, Livermore, CA, Sandia Technical Report SAND87-8215B, 1987. 

What the three-step model really points out is that it is theoretically correct to carry out basic combustion calculations for a PBC system based on the mass flow and stoichiometry of the conversion gas from the conversion system and not based on the mass flow of solid fuel entering the conversion system. The two-step model approach applied on a PBC system, which is equivalent to assuming that the conversion efficiency is 100 %, is a functional engineering approach, because the conversion efficiency is in many cases very close to unity. However, there are cases where the two-step model approach results in a physical conflict, for example the mass flows in PBC sysfem of batch type cannot be theoretically analysed with a two-step model. 

Several fundamema physical law s apply id combustion calculations. These arc reviewed briefly as follows ... 

Conservation of Matter. This is the familiar statement that "mailer is neither destroyed nor created." There must be a weight balance between the sum of the weights entering a process and the sum leaving. In other words. A pounds of fuel combined with B pounds of air will always result in A + B pounds of products. (It should be noted that when a pound of a typical coal is burned, releasing 13.500 Btu, the quantity of mass converted to energy amounts to only 3.5 x 10 1(1 pound, a loss too small to be measured or considered in conventional combustion calculations. Obviously, this conversion is of significance to nuclear reactions.)... 

Table 2 summarizes the molecular and weight relationships between fuel and oxygen and lists the heal of combustion for the substances commonly involved in combustion. Most of the weight and volume relationships in combustion calculations can be determined by using the information in this table and the seven fundamental laws. 

In the United States the practice is to use the high heat of combustion in boiler combustion calculations. In Europe the low heal value is used. 

The combustion calculations arc Ihe starting point for all design and performance determinations for boilers and their related component parts. They establish (a) Ihe quantities of the constituents involved in the chemistry of combustion, heat released, and (c) the efficiency of the combustion process under both ideal and actual conditions. 

Hydrocarbon AH° of combustion, calculated from bond energies, kcal mole-1 AH0 of combustion, experimental values," kcal mole-1 Discrepancy, kcal mole-1... 

Related Calculations. Use the method given here when making combustion calculations for any type of coal—bituminous, semibituminous, lignite, anthracite, cannel, or coking—from any coal field in the world used in any type of furnace—boiler, heater, process, or waste-heat. When the abused for combustion contains moisture, as is usually true, this moisture is added to the combustion-formed moisture appearing in the products of combustion. Thus, for 80°F (300 K) air of 60 percent relative humidity, the moisture content is 0.013 lb per pound of dry air. This amount appears in the products of combustion for each pound of air used and is a commonly assumed standard in combustion calculations. 

The composition of the gas is given on a volumetric basis, which is the usual way of expressing a fuel-gas analysis. To use the volumetric-analysis data in combustion calculations, they must be converted to a weight basis. This is done by dividing the weight of each component by the total weight of the gas. A volume of 1 ft3 of the gas is used for this computation. Find the weight of each... 

Combustion calculations show that an oil-fired watertube boiler requires 200,000 lb/h (25.2 kg/s) for air of combustion at maximum load. Select forced- and induced-draft fans for this boiler if the average temperature of the inlet air is 75°F (297 K) and the average temperature of the combustion gas leaving the air heater is 350°F (450 K) with an ambient barometric pressure of 29.9 inHg. Pressure losses on the air-inlet side are, in inFLO air heater, 1.5 air supply ducts, 0.75 boiler windbox, 1.75 burners, 1.25. Draft losses in the boiler and related equipment are, in inH20 furnace pressure, 0.20 boiler, 3.0 superheater, 1.0 economizer, 1.50 air heater, 2.00 uptake ducts and dampers, 1.25. Determine the fan discharge pressure and horsepower input. The boiler burns 18,000 lb/h (2.27 kg/s) of oil at full load. 

Compute the quantity of air required for combustion. The combustion calculations show that... 

The following combustion calculations were carried out under isobaric conditions, based on the assumption that the combustion of fuel proceeds without any heat loss to the surroundings (i.e. adiabatically) and that the state of chemical... 

Given the following enthalpies of combustion, calculate the enthalpy change for the reaction of 25.0 g of C2H2 according to the equation... 

Enthalpy of reaction and sttuidard entlialpy of reaction are not always employed in engineering retiction/combustion calculations. The two other terms tliat liavc been used are tlie gross (or liiglier) heating value and tlie net (or lower) healing value. These arc discussed later in this Section. 

S. R. Brinkley, Computational Methods in Combustion Calculations, in Combustion Processes, vol. 2 of High Speed Aerodynamics and Jet Propulsion, B. Lewis. R. N. Pease and H. S. Taylor, eds., Princeton Princeton University Press. 1956. 64-98. 

In combustion calculations, one primarily wants to know the variation of the temperature with the ratio of oxidizer to fuel. Therefore, in solving flame temperature problems, it is normal to take the number of moles of fuel as 1 and the number of moles of oxidizer as that given by the oxidizer/fuel ratio. In this manner the reactant coefficients are 1 and a number normally larger than 1. Plots of flame... 

In most combustion calculations, it is acceptable to simplify this composition to 79% N2,21% O2 79 moles N2/2I moles O2 = 3.76 moles N2/mole O2. 

Possible reasons are the limited knowledge of the uncertainty of parameters, and the fact that global sensitivity methods are computationally very intensive. In the future, it is expected that both these limitations will be lifted and detailed uncertainty analyses will appear for combustion calculations. On the other hand, one of the main applications of sensitivity analysis has been to form a qualitative picture about which parameters should be known precisely in order to reproduce accurately a set of experimental observations. 

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