Modelling combustion systems

In this section collections of kinetic data for modelling combustion systems are listed and described. The emphasis is on critically evaluated data because of its greater reliability but other sources are also given since only a proportion of the data necessary for modelling have been thoroughly assessed. 

Warnatz [71] has produced an extensive compilation of data for modelling combustion systems. Although there is little direct evaluation of the material, recommendations are made, usually based on previous evaluations, where those are available. Reactions of small alkanes are covered. 

Availability of empirical data on the behavior of coals to be characterized from working plants or model combustion systems similar to those used in practical operation Elimination of the Si02 share of the ash which does not participate in the slagging process. 

In Chapfer 7.2, J.H. Frank and R.S. Barlow describe the basic characteristics of non-premixed flames wifh an emphasis on fundamenfal phenomena relevant to predictive modeling. They show how the development of predictive models for complex combustion systems can be accelerated by combining closely coupled experiments and numerical simulations. 

I. Friberg R. and Blasiak W., Part I - A Mass Flow Analysis of Packed-Bed Combustion Systems in the Context of the Three-step Model - A New System Theory , accepted for publication in the journal Archivum Combustionis... 

The objectives of this project are consistent with the objectives (1) and (4) above. The general objective of this project has been to verify a new measurement method to analyse the thermochemical conversion of biofuels in the context of PBC, which is based on the three-step model mentioned above. The sought quantities of the method are the mass flow and stoichiometry of conversion gas, as well as air factors of conversion and combustion system. One of the specific aims of this project is to find a physical explanation why it is more difficult to obtain acceptable emissions from combustion of fuel wood than from for example wood pellets for the same conditions in a given PBC system. This project includes the following stages ... 

The physical model serves as the platform for the mathematical model used to indirectly measure the mass flow and stoichiometry of the conversion gas, as well as the air excess numbers of the conversion and combustion system, respectively. 

The system consists of the conversion and combustion system, according to the three-step model. A system boundary is put around the combustion system. In other words, a mass-balance is carried out over the combustion system. 

For the sake of brevity the reader is referred to Paper II for the details regarding the constitutive mathematical models of the method applied to measure the mass flow and stoichiometry of conversion gas as well as air factors for conversion and combustion system. Below is a condensed formulation of the mathematical models applied. Here a distinction is made between measurands and sought physical quantities of the method. 

The experimental system consists of a PBC system and a measurement system. The PBC system consists of a conversion system and a combustion system, according to the three-step model, and primary and secondary air lines. A boiler system is not required to realise the measurement method. The measurement system consists of twelve measuring devices (sensors) and a data acquisition system. 

The three-step model, which is a new system theory, identifies the least common functions (unit operations) of a packed-bed combustion system. The three steps are referred to as the conversion system, the combustion system, and the boiler system. Previously, PBC has been modelled in two steps [3,15]. The novel approach with the three-step model is the splitting of the combustion chamber (furnace) into a combustion system and a conversion system. The simple three-step model is a steady-state approach together with some other simplifications applied to the general three-step model. The simple three-step model implies a simplified approach to the mathematical analysis of the extremely differentiated and complex PBC process. 

Some new concepts have been deduced in the context of the three-step model, for example, the conversion system, the conversion gas, the conversion efficiency, and the combustion efficiency. Two new physical quantities have been associated with the conversion gas. The physical quantities are referred to as the mass flow and the stoichiometry of the conversion gas. The conversion efficiency is a measure of how well the conversion system performs, that is, the degree of solid-fuel convertibles that are converted from the conversion system to the combustion system. The combustion efficiency is defined as the degree of carbon atoms being oxidised to carbon dioxide in the combustion system. In other words, the combustion efficiency is a measure of the combustion system performance. 

A new mesurement method, based on the three-step model, has been verified. It is based on a mass-balance approach similar to the work carried out by Rogers (see Appendix A) [18]. Besides information about the mass flow and the stoichiometry of the conversion gas, this method also determines the air factors of the conversion system and the combustion system, which Rogers method did not. 

The combustion system and the boiler system of a PBC system, according to the three-step model, can be modelled by means of CFD codes [4,5]. However, an allround bed model [6,7] to simulate the thermochemical conversion of the solid fuel inside the conversion system does not yet exist. Bed models of the conversion system will herein also be referred to as CFSD code computational fluid-solid dynamics), analogue to the CFD code. From the three-step model point of view, it is clear that without a CFSD code simulating the thermochemical conversion of the packed bed in the conversion system, simulation of the whole PBC system can never be completely successful. 

Very often in the literature, the physical model used to describe a PBC system is a two-step model [18,19] that is, the conversion system and the combustion system are regarded as one unit referred to as the combustion system (combustion chamber, furnace), see Figure 14. The two-step model is based on the assumption that the conversion system is ideal that is, the conversion efficiency [3] is 100%, which is not the case in real solid-fuel fired systems. However, the two-step model is a functional engineering approach. 

According to the three-step model, proposed by the authors, a PBCS can be divided into three subsystems, namely a conversion system, combustion system, and boiler system. It is in the conversion system that the thermochemical conversion of the solid fuel takes place. The conversion system can be designed according to several conversion concepts. The conversion concept can be classified with respect to fuel-bed mode (batch and continuous), fuel-bed configuration (countercurrent, cocurrent and crosscurrent), fuel-bed composition (homogeneous and heterogeneous), and fuel-bed movement (fixed, moving and mixed). 

Mashayek, F. 1999. Simulation and modeling of two-phase turbulent flows for prediction and control of combustion systems. 12th ONR Propulsion Meeting Proceedings. Eds. G. D. Roy and S. L. Anderson. Salt Lake City, UT. 88-95. 

In practical combustion systems, such as CO boilers, the flue gas experiences spatial and temporal variations. Constituent concentration, streamline residence time, and temperature are critical to determining an efficient process design. Computational fluid dynamics (CFD) modeling and chemical kinetic modeling are used to achieve accurate design assessments and NO, reduction predictions based on these parameters. The critical parameters affecting SNCR and eSNCR design are listed in Table 17.4. 

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. 

In combustion systems it is generally desirable to minimize the concentration of intermediates, since it is important to obtain complete oxidation of the fuel. Figure 13.5 shows modeling predictions for oxidation of methane in a batch reactor maintained at constant temperature and pressure. After an induction time the rate of CH4 consumption increases as a radical pool develops. The formaldehyde intermediate builds up at reaction times below 100 ms, but then reaches a pseudo-steady state, where CH2O formed is rapidly oxidized further to CO. Carbon monoxide oxidation is slow as long as CH4 is still present in the reaction system once CH4 is depleted, CO (and the remaining CH2O) is rapidly oxidized to CO2. 

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