Combustion near adiabatic

This is frequently the required mode of operation for fast oxidation reactions because the heat release is too fast to provide efficient heat exchange. Most combustion processes are nearly adiabatic (your home furnace and your automobile engine), and many catalytic oxidation processes such as NH3 oxidation in HNO3 synthesis are nearly adiabatic. 

Detailed comparisons for near adiabatic combustion of isooctane are shown in Figures 3 and 4. Figure 3 shows temperature and Figure 4 shows CO, CO2, O2 and unburned hydrocarbons as functions of equivalence ratio. The quality of these comparisons is very good and is similar to that obtained for the toluene experiments. Soot was not observed to form in measurable quantities for iso-octane mixtures which could be stably burned in the Jet-Stirred Combustor. 

Quasiglobal kinetics models, which have previously been shown to represent lean and stoichiometric combustion of a variety of hydrocarbon fuels, have been extended to represent lean and rich combustion of toluene and iso-octane. The model predicts the thermal state of the flow and emissions of CO, soot, and N0X. The thermal state of the flow and the stable species were shown to be accurately predicted for jet-stirred combustor experiments. For rich combustion, hydrocarbon intermediates and soot are additional combustion products. The global reactions and rates were developed to represent near-adiabatic jet-stirred combustor data and were then verified by comparison to the near iso-thermal jet-stirred combustor data. N0X emissions behavior was investigated with the quasiglobal kinetics model to represent rich combustion... 

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. 

Catalytic combustion is an environmentally-driven, materials-limited technology with the potential to lower nitrogen oxide emissions from natural gas fired turbines consistently to levels well below 10 ppm. Catalytic combustion also has the potential to lower flammability at the lean limit and achieve stable combustion under conditions where lean premixed homogeneous combustion is not possible. Materials limitations [1,2] have impeded the development of commercially successful combustion catalysts, because no catalytic materials can tolerate for long the nearly adiabatic temperatures needed for gas turbine engines and most industrial heating applications. 

Catalytic oxidation reactions on noble metal surfaces are sufficiently fast and exothermic that they can be operated at contact times on the order of one millisecond with nearly adiabatic temperatures of 1000°C. At short contact times and high temperatures complete reaction of the limiting feed is observed, and highly nonequilibrium products are obtained. We summarize experiments where these processes are used to produce syngas by partial oxidation of methane, olefins by partial oxidation of higher alkanes, and combustion products by total oxidation of alkanes. The former are used to produce chemicals, while the latter is used for high temperature catalytic incineration of volatile organic compounds. 

In many cases, fast-developing changes of state in a gas phase will be adiabatic or near-adiabatic, so that an approximate description of the process can be given with the adiabatic equations (3.37). For example, this applies to changes of pressure in sound waves, combustion and explosion processes in gases, and changes of pressure in fast-working gas compressors. 

If the reaction is conducted both adiabatically and witli stoichiometric air. tlie resulting temperature is defined as tlie theoretical adiabatic flame temperature (TAFT). It represents tlie nia.xinuuii temperature tliat tlie products of combustion (flue) can acliieve if the reaction is conducted both stoichionietrically and adiabatically. For tliis condition, all tlie energy liberated from combustion at or near standard conditions (AH°c and/or AH°29s) appears as sensible heat in raising tlie temperature of tlie flue products, AHp, tliat is ... 

The various methods have been applied at different temperatures isothermal calorimetric method usually at 77°, adiabatic method near room temperature, and heats of combustion at 25°C. Corrections to a common temperature would in most cases be smaller than the experimental error,... 

Or, more generally, to one-step reactions with a relatively weak dependency on T. This may occur in combusting flows where the adiabatic temperature rise is small for example, in a rich flame near extinction. 

As mentioned in the previous section, the condition for vapor phase combustion versus heterogeneous combustion may be influenced by pressure by its effect on the flame temperature (Tvol or Td) as well as by its effect on the vaporization temperature of the metal reactant (Th). For aluminum combustion in pure oxygen, combustion for all practical conditions occurs in the vapor phase. In air, this transition would be expected to occur near 200 atm as shown in Fig. 9.15 where for pressures greater than —200 atm, the vaporization temperature of pure aluminum exceeds the adiabatic flame temperature. This condition is only indicative of that which will occur in real particle combustion systems as some reactant vaporization will occur at temperatures below the boiling point... 

Temperatures estimated from the measured intensity distributions at each port location during an 02/Ar oxidizer run in the 24-inch long combustor are plotted in Fig. 8.4, along with the measured hemispherical emissive power in the wavelength range from 425 to 800 nm. (The hemispherical emissive power, E, is related to the radiant intensity, /, by = ttI. Radiant intensity is also referred to as radiance.) The stoichiometry, 0/F)/ 0/F)st, for this run was about f.fO. The measured combustion temperature was about 2900 K, as compared to an adiabatic flame temperature of about 3650 K. The intensity measurements indicate that ignition occurs about 12 in. downstream from the injector. The intensity is near its peak at the most downstream port location, which indicates that combustion is still underway at that location. 

The adiabatic surface temperature (for stagnation flow) and the adiabatic PSR temperature are shown in Fig. 26.4a as a function of the inlet fuel composition. The residence time in the PSR is simply taken as the inverse of the hydrodynamic strain rate. In both cases, the adiabatic temperature exhibits a maximum near the stoichiometric composition. The limits of the adiabatic operation are 8% and 70% inlet H2 in air for the stagnation reactor. For a PSR, the corresponding limits are 12% and 77% inlet H2 in air. Beyond these compositions, the heat generated from the chemical reactions is not sufficient to sustain combustion. 

A flame may be defined as a localized reaction zone which is able to propagate itself sub-sonically through the material supporting it. Most flames are concerned with exothermic reactions of this type, in which typically reactants at near ambient temperatures are converted more or less adiabatically to combustion products at 1000 K or above. Detailed kinetic studies have principally been confined to premixed flames, in which a well-defined reactant mixture at a known initial temperature is converted into combustion products in full chemical equilibrium at the final flame temperature. Assuming adiabatic combustion, the final conditions may be calculated thermodynamically. 

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