Modified enthalpy of combustion

The modified enthalpy of combustion corresponds to a hypothetical biomass that would not incorporate nitrogen, or it can be interpreted as assuming NH3 instead of N2 as final product of nitrogenous compounds combustion. 

The ratio between the enthalpy of combustion and the degree of reduction is the oxycaloric quotient, Qq. Similarly, the modified oxycaloric quotient is defined as  

The interest of this notion is that several authors [27,28,30,45-47] have reported an almost constant value of the oxycaloric quotient for CHON compounds. Since enthalpy of combustion is defined with respect to a reference state [48], this state must be the same as for the degree of reduction, i.e. for any species /  

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The correct choice of the state of the compounds is crucial to obtain a correct enthalpy balance [6]. For the energy balance around the reactor depicted in Figure 5, the following state of the carrier flow of each species must be as listed in Table 2 and the modified enthalpy of combustion is listed for some compounds in Table 3. 

The enthalpy of combustion of the strains analyzed for CHN content is given in Table 16 [37,38]. Each determination was performed 3 to 5 times. The average values obtained for each group are summarized in Table 17. The enthalpy of combustion of all the microorganisms are rather close one to another and there is no clear difference between the 4 groups (Table 17). The differences are clearer for the modified enthalpy of combustion due to different nitrogen eon-tents (see Table 11). 

The average oxycaloric quotient is -112.4 kJ e -moP for the 30 strains. Thus, a good estimate of the enthalpy of combustion is obtained with Thornton s rule. The low variability in y among the different strains (Table 11) is reflected by the similar modified enthalpy of combustion of bacteria (- 460.3 kJ CmoP ) and yeast (-481.1 kJ C-moP ). Thus, an average value can be used to calculate the energy balance in a first approximation. 

Table 16 Enthalpy of combustion, modified enthalpy of combustion and oxy-caloric quotient of biomass [37,38].

In Appendix A, the author did not calculate, using a modified equilibrium diagram, the fuel chemical exergy of methanol. An assault on the calculation would involve the JANAF thermochemical tables (Chase etal., 1998). In view of the immature status of the DMFC, the author deferred the latter significant task. Note, however, that the net enthalpy of combustion is given in Kotas (1995), Table Bl, as 726.6 kJmol which is of course substantially smaller than the fuel chemical exergy in kW smol ... 

In combustion problems it is usually more convenient to work with the enthalpy h = u + p/p instead of the internal energy u. A modified form of the energy equation may be derived from equation (21) by making the... 

The physics and modeling of turbulent flows are affected by combustion through the production of density variations, buoyancy effects, dilation due to heat release, molecular transport, and instabiUty (1,2,3,5,8). Consequently, the conservation equations need to be modified to take these effects into account. This modification is achieved by the use of statistical quantities in the conservation equations. For example, because of the variations and fluctuations in the density that occur in turbulent combustion flows, density weighted mean values, or Favre mean values, are used for velocity components, mass fractions, enthalpy, and temperature. The turbulent diffusion flame can also be treated in terms of a probabiUty distribution function (pdf), the shape of which is assumed to be known a priori (1). 

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