Homogeneous ignition analysis

For quantitative analysis, the amount of sample taken is usually measured by mass or volume. Where a homogeneous sample already exists, it may be subdivided without further treatment. With many solids such as ores, however, crushing and mixing are prior requirements. The sample often needs additional preparation for analysis, such as drying, ignition and dissolution. 

Over 160 analyses were conducted using 58 thin sections. Twelve elements were mapped on a minimum of three areas per thin section. The windows of analysis correspond to areas of 64 x 50 pixels (1.7 X 1.3 mm) and were considered sufficiently homogeneous to be representative of the sample. The composition was determined using a time analysis of 800 ms per pixel and an average was calculated using the area analyzed. The method provides a nearly complete chemical analysis, with the exception of loss on ignition. 

A more elaborated approach is that of Frank-Kamenetski who relaxed the assumption of homogeneous solid temperature allowing conduction within the solid. A similar eigenvalue analysis will lead again to a critical ignition temperature (Tc) and the ambient temperature required for ignition to occur (T0). Nevertheless, the ambient temperature, given that conduction of heat from the core to ambient is allowed, becomes a function of the volume of the solid, and hence, for each 7 0 a critical volume, Vc is obtained. 

Unfortunately, later the development of two areas— homogeneous kinetics and heterogeneous catalysis—occurred almost independently, which caused serious intrinsic discrepancies. For instance, the traditional chain theory implies the participation of surfaces also in chain termination, which determines the existence of the low-pressure ignition limit. In the framework of this approach, two regimes—diffusional and kinetic—are distinguished. In the latter case the parameter that describes the process is the probability of surface decay of chain carriers per one collision. It is worth noticing, however, that this assumes only a disappearance of active species from the gas phase, without any analysis of its mechanism and even stoichiometry. This is why the heterogeneous termination reactions are usually represented in kinetic models as a formal reaction ... 

H. Suzuki, N. Koike, H. Ishii, M. Odaka "Exhaust Purification of Diesel Engines by Homogenous Qiarge with Compression Ignition Part 2 Analysis of Combustion Phenomena and NOx Formation by Numerical Simulation with Experiment." SAE Paper 970315. 

Ignition temperatures determined by differential scanning calorimetry (DSC) usually correspond reasonably well to those obtained by a Henkin-McGill study. DSC values tend to be more reproducible from laboratory to laboratory, and from sample to sample. Differences in heating rate, sample size, homogeneity, etc., can cause some variation in values obtained with thermal analysis technique. For any direct comparison of ignition temperatures, it is best to run all of the mixtures of interest under identical experimental conditions, thereby minimizing the number of variables. 

---