Adiabatic processes flame temperature

The flame temperature increases significantly when air is replaced with oxygen because N2 acts as a diluent that reduces the flame temperature. Figure 1.13 is a plot of the adiabatic equilibrium flame temperature for CH4 combustion, as a function of the oxidizer composition, for a stoichiometric methane combustion process. The flame temperature varies from 3600 to 5000°F (2300 to 3000 K) for air and pure... 

Figure 1.14 is a similar plot of the adiabatic equilibrium flame temperature for CH4 flames as a function of the stoichiometry, for four different oxidizer compositions ranging from air to pure 02. The peak flame temperatures occur at stoichiometric conditions. The lower the 02 concentration in the oxidizer, the more the flame temperature is reduced by operating at nonstoichiometric conditions (either fuel rich or fuel lean). This is due to the higher concentration of N2, which absorbs heat and lowers the overall temperature. Actual flame temperatures will be less than those given in Figures 1.13 and 1.14 because of heat losses from the flame, which is not an adiabatic process. 

There were several important results from those experiments. The first is that the NOx emissions for low-level oxygen enrichment were nearly an order of magnitude higher than for high-level enrichment. The second is that the experimental NOx trends were the same as those predicted by theory. However, the experimental measurements were about an order of magnitude lower than the theoretical predictions. This is due to the fact that actual flames are not adiabatic processes, since a large amount of heat is radiated from the flames. The actual flame temperature is usually much lower than the adiabatic equilibrium flame temperature. 

Because this reaction is highly exothermic, the equiUbrium flame temperature for the adiabatic reaction with stoichiometric proportions of hydrogen and chlorine can reach temperatures up to 2490°C where the equiUbrium mixture contains 4.2% free chlorine by volume. This free hydrogen and chlorine is completely converted by rapidly cooling the reaction mixture to 200°C. Thus, by properly controlling the feed gas mixture, a burner gas containing over 99% HCl can be produced. The gas formed in the combustion chamber then flows through an absorber/cooler to produce 30—32% acid. The HCl produced by this process is known as burner acid. 

Adiabatic flame temperatures agree with values measured by optical techniques, when the combustion is essentially complete and when losses are known to be relatively small. Calculated temperatures and gas compositions are thus extremely useful and essential for assessing the combustion process and predicting the effects of variations in process parameters (4). Advances in computational techniques have made flame temperature and equifibrium gas composition calculations, and the prediction of thermodynamic properties, routine for any fuel-oxidizer system for which the enthalpies and heats of formation are available or can be estimated. 

Adiabatic Reaction Temperature (T ). The concept of adiabatic or theoretical reaction temperature (T j) plays an important role in the design of chemical reactors, gas furnaces, and other process equipment to handle highly exothermic reactions such as combustion. T is defined as the final temperature attained by the reaction mixture at the completion of a chemical reaction carried out under adiabatic conditions in a closed system at constant pressure. Theoretically, this is the maximum temperature achieved by the products when stoichiometric quantities of reactants are completely converted into products in an adiabatic reactor. In general, T is a function of the initial temperature (T) of the reactants and their relative amounts as well as the presence of any nonreactive (inert) materials. T is also dependent on the extent of completion of the reaction. In actual experiments, it is very unlikely that the theoretical maximum values of T can be realized, but the calculated results do provide an idealized basis for comparison of the thermal effects resulting from exothermic reactions. Lower feed temperatures (T), presence of inerts and excess reactants, and incomplete conversion tend to reduce the value of T. The term theoretical or adiabatic flame temperature (T,, ) is preferred over T in dealing exclusively with the combustion of fuels. 

The adiabatic flame temperature is defined as the maximum possible temperature achieved by the reaction in a constant pressure process. It is usually based on the reactants initially at the standard state of 25 °C and 1 atm. From Equation (2.20), the adiabatic temperature (7 i[Pg.30]

Flammability limits can also be estimated by using calculated adiabatic flame temperatures and a chemical equilibrium program [Mashuga and Crowl, Flammability Zone Prediction Using Calculated Adiabatic Flame Temperatures, Process Safety Progress, 18 (3) (1999)]. 

Adiabatic cracking reactor, 10 617-618 Adiabatic decomposition, of hydrogen peroxide, 14 61-62 Adiabatic dehydrogenation, 23 337 Adiabatic dehydrogenation unit, 23 339 Adiabatic evaporation, general separation heuristics for, 22 319 Adiabatic flame temperature, 12 322 Adiabatic flash calculation, 24 681 Adiabatic nitration process, 17 253—255 Adiabatic pressure-reducing valve,... 

The computational process may indicate, for example, that Qj decreases monotoni-cally with T and that Q2 increases monotonically with T. The adiabatic flame temperature is determined by interpolation of the computed results. Practical computations are carried out with the aid of computer programs.I The results of this example are as follows ... 

This book is divided into four parts. The first part (Chapters 1-3) provides brief reviews of the fundamental aspects relevant to the conversion from chemical energy to aerothermal energy. References listed in each chapter should prove useful to the reader for better understanding of the physical bases of the energy conversion process energy formation, supersonic flow, shock wave, detonation, and defl agration. The second part (Chapter 4) deals with the energetics of chemical compounds used as propellants and explosives, such as heat of formation, heat of explosion, adiabatic flame temperature, and specific impulse. 

In the simple two-component system of PVC binder and oxidizer, the important propellant properties of specific impulse, density, adiabatic flame temperature, and burning rate increase with an increase in solids loading. This is shown in Figure 8, where theoretical calculated values of specific impulse, adiabatic flame temperature, and density are given for a range of oxidizer content for PVC plastisol propellants comprised of only binder and oxidizer. [Calculated values of specific impulse reported throughout this paper are for adiabatic combustion at a rocket chamber pressure of 1000 p.s.i.a. followed by isentropic expansion to 1 atm. pressure with the assumptions that during the expansion process chemical compo-... 

To determine the need for recombination or dissociation processes in a flame, one must first consider the mole number of the final equilibrium composition. A constrained enthalpy and pressure equilibrium calculation will determine the adiabatic flame temperature and the species distribution at that temperature. If the mean molecular weight (IT = Ylk WkXk) is larger than that of the reactants, then recombination must occur. If the W is smaller for the products, then dissociation must take place. Note that the mole number (moles per mass of gas) is the reciprocal of the mean molecular weight. At the adiabatic flame conditions there will be the expected stable products as well as a distribution of other species, including free radicals. 

A reaction at the initial temperature changes the characteristics of an explosive mixture before the flame front and introduces an element of nonsteadiness into the process of propagation of the combustion wave. The method proposed in [1] to describe this effect consists in replacing the original non-steady problem by a quasi-steady one with adiabatically increasing initial temperature Ta(f) and an effective source of heat release which takes this increase into account. We test this method below by comparing it directly with the results of a numerical solution of the original non-steady problem. 

Pure hydrogen gas at room pressure and temperature is adiabatically combusted with air. The combustion takes place with an amount of air that is 30% in excess of what is stoichiometrically required. Calculate the adiabatic flame temperature of the process, the work lost, and the thermo-dynamic efficiency of the process. Assume air to consist of a mixture of 79 mol% of N2 and 21 mol% of 02. 

Assume that the hydrogen feed stream is 1 mol/s. The reactants are fed to the combustor at 298.15 K and 0.1 MPa, and the heat of reaction will be used to raise the temperature of the product mixture to its final value, the adiabatic flame temperature. The first law for this adiabatic process can be written as... 

Biomass differs from conventional fossil fuels in the variability of fuel characteristics, higher moisture contents, and low nitrogen and sulfur contents of biomass fuels. The moisture content of biomass has a large influence on the combustion process and on the resulting efficiencies due to the lower combustion temperatures. It has been estimated that the adiabatic flame temperature of green wood is approximately 1000°C, while it is 1350°C for dry wood [41]. The chemical exergies for wood depend heavily on the type of wood used, but certain estimates can be obtained in the literature [42]. The thermodynamic efficiency of wood combustors can then be computed using the methods described in Chapter 9. 

The equation is correct because the total enthalpy is conserved and the enthalpy is a state function so that the enthalpy change is independent of the path the process takes as long as the initial and final states are kept unchanged. The adiabatic flame temperature 7 can be found by solving the equation for Tf. 

It is unlikely that the reaction can be carried out under such conditions that the process is adiabatic (i.e., no heat is lost to the surroundings). However, when there is adequate premixing of fuel and air so that a short non-luminous flame is obtained the combustion is expected to be very nearly adiabatic and the flame temperature is high. 

Most combustion reactions do not operate at stoichiometric or zero percent excess air. Incomplete combustion and higli carbon monoxide levels would result mider Uiis condition. If all Uie heat liberated by the reaction goes into heating up tlie products of combustion (tlie flue gas) die temperature acliieved is defined as Uie flame temperature. If die reacUon process is conducted adiabatically, with no heat transfer loss to die surroundings, the final temperature achieved in die flue gas is defined as die adiabatic flame temperature. If die combustion process is conducted widi theoretical or stoichiometric air (0% e.xcess air), die resulting temperature is defined as die theoretical flame temperature. (Theoretical or stoichiometric air is defined as diat exact amount of air required die completely react widi the compound to produce o. idized end products. Any air in excess of this stoichiometric amount is defined as excess air.)... 

A thermodynamic quantity of considerable importance in many combustion problems is the adiabatic flame temperature. If a given combustible mixture (a closed system) at a specified initial T and p is allowed to approach chemical equilibrium by means of an isobaric, adiabatic process, then the final temperature attained by the system is the adiabatic flame temperature T. Clearly depends on the pressure, the initial temperature and the initial composition of the system. The equations governing the process are p = constant (isobaric), H = constant (adiabatic, isobaric) and the atom-conservation equations combining these with the chemical-equilibrium equations (at p, T ) determines all final conditions (and therefore, in particular, Tj). Detailed procedures for solving the governing equations to obtain Tj> are described in [17], [19], [27], and [30], for example. Essentially, a value of Tf is assumed, the atom-conservation equations and equilibrium equations are solved as indicated at the end of Section A.3, the final enthalpy is computed and compared with the initial enthalpy, and the entire process is repeated for other values of until the initial and final enthalpies agree. 

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