Combustion heat release calculations

Duiser (1989) calculates emissive power from rate of combustion and released heat. As a conservative estimate, he uses a radiation fraction (/) of 0.35. He proposed the following equation for calculating the emissive power of a pool fire ... 

In this activity, you will calculate the heat of combustion of the fuel in a candle. The burning candle will heat a measured quantity of water. Using the specific heat of water, the mass of the water, and the increase in temperature, you can calculate the amount of heat released by the burning candle using the following relationship ... 

How can you measure the heat released by a burning candle and calculate the heat of combustion of candle wax ... 

Calculate the amount of heat released during a combustion reaction. 

To calculate the heat release from combustion and the temperature of the products of combustion, the thermodynamic path shown in Figure 15.20 can be followed18. The actual combustion process goes from reactants at temperature T to products at temperature T2. However, it is more convenient to follow the alternative path from reactants at temperature T that are initially cooled (or heated) to standard temperature of 298 K. The combustion reactions are then carried out at a constant temperature of 298 K. Standard heats of combustion are available for this. The products of combustion are then heated from 298 K to the final temperature of 7i. The actual heat of combustion is given by18 ... 

At higher temperatures e.g. 230°C, the hardboards at Infinite time have lost less than half their total combustion heat while semi-hardboards will have lost all, if the extrapolation used is valid. Table II includes the total calculated heat release at 230 and 160°C respectively. 

Net heat of combustion was calculated based on the gross heat of combustion and the stoichiometry of combustion of the fuel mixture and a typical composition of the fuel mixture. The rate of heat release from burning fuel gas (<3ruei) is... 

The heat of combustion amounts to 4272 cal/g of aluminum fuel and the adiabatic flame temperature calculated by the chemical equilibrium program is 3036 K. Thus, the heat released when aluminum is burned with steam is about 57% of the amount released when aluminum is burned with O2. Many experimental investigations have been carried out on the combustion of aluminum in atmospheres where the primary oxidizer was O2 [7-16], and also in atmospheres where the primary oxidizer was H2O and/or CO2 [16-19]. There is general... 

Chemical kinetics and thermochemistry are important components in reacting flow simulations. Reaction mechanisms for combustion systems typically involve scores of chemical species and hundreds of reactions. The reaction rates (kinetics) govern how fast the combustion proceeds, while the thermochemistry governs heat release. In many cases the analyst can use a reaction mechanism that has been developed and tested by others. In other situations a particular chemical system may not have been studied before, and through coordinated experiments and simulation the goal is to determine the key reaction pathways and mechanism. Spanning this spectrum in reactive flow modeling is the need for some familiarity with topics from physical chemistry to understand the inputs to the simulation, as well as the calculated results. 

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. 

From the technology of combustion we move to the molecular mechanism of flame propagation. We shall give a molecular-kinetic expression for the heat release rate by calculating the frequency v of collisions of fuel molecules with other molecules (v is proportional to the molecular velocity and inversely proportional to the mean free path), further taking into account that only a small (1/j/) part of all collisions are effective. The quantity 1/v—the probability of reaction taken with respect to a single collision— depends on the activation heat of an elementary reaction event, as well as on the fraction of all molecules comprised of those radicals or atoms by means of which the reaction occurs. The molecular-kinetic expression for the coefficient of thermal conductivity follows from formulas (1.2.4) and (1.2.3). 

A simple model of the chemical processes governing the rate of heat release during methane oxidation will be presented below. There are simple models for the induction period of methane oxidation (1,2.>.3) and the partial equilibrium hypothesis (4) is applicable as the reaction approaches thermodynamic equilibrium. However, there are apparently no previous successful models for the portion of the reaction where fuel is consumed rapidly and heat is released. There are empirical rate constants which, due to experimental limitations, are generally determined in a range of pressures or concentrations which are far removed from those of practical combustion devices. To calculate a practical device these must be recalibrated to experiments at the appropriate conditions, so they have little predictive value and give little insight into the controlling physical and chemical processes. 

FIGURE 15.15 Combustion efficiency (%) of various PC/ABS materials calculated using the THE/ML measured in the cone calorimeter, and the heat release per ML for the complete combustion of the volatiles monitored in the PCFC. Systems that do not show flame inhibition show combustion efficiencies of around 1, according to the well-ventilated fire scenario of the cone calorimeter. Systems, in which adding aryl phosphates result in flame inhibition, show combustion efficiencies of around 0.8. When the release of phosphorus is reduced by competing reactions in the solid state, combustion efficiencies of between 0.8 and 1 are observed. 

Candidate polymers were screened for flammability using microscale MCC according to a standard method [18], In the test, a 3-5 mg sample is heated at a rate of 1 K/s from ambient temperature to 850°C. The pyrolysis gases are purged from the sample chamber with nitrogen, mixed with excess oxygen, and combusted at 900°C. Heat released by combustion of the pyrolysis gases is calculated... 

A second measure of flammability is the amount of heat liberated by combustion of the fuel gases. The same molar groups that were assigned additive charring contributions were assigned additive heat release contributions based on the HR measured in the MCC. Figure 16.3 shows the results of these calculations as the additive model of HR versus the measured HR for 84 polymers. A heat release, HR < 12kJ/g (enclosed by the dashed circle) was considered as a second criterion for low flammability. 

Figure 19.18 presents a comparison of the effective heat of combustion (EHC), calculated as the ratio of the total heat release (THR) to the total mass lost (TML), for all the formulations. An average value of... 

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