Ignition front

These zones are (1) the ignition front zone at the propellant surface and in the subsurface layers, and (2) the luminous combustion zone in the gas phase. 

The conversion process occurs both on macro- and micro-scale, that is, on single particle level and on bed level. In other words, the conversion process has both a macroscopic and microscopic propagation front. One example of the macroscopic process structure is shown in Figure 10. The conversion front is defined by the process front closest to the preheat zone, whereas the ignition front is synonymous with the char combustion front. 

The subsection, theory, concentrates on the theory of the method(s) of relevance for this work, that is, the methods aiming at the ignition front rate, the conversion gas rate (by many authors referred to as burning rate or combustion rate), the conversion gas... 

The subsection, results and conclusions, focuses on the results and conclusions with respect to sought quantities, that is, the ignition front rate, conversion rate, and conversion stoichiometry. Each work is rounded off with comments by this author, with respect to the content of the physical model and the mathematical model making up the theory of the measurement method, as well as the application of verification methods. 

Below are the theories behind the measurement methods of ignition front rate and combustion rate. 

The physical and mathematical model of the determination of ignition rate are not explicitly formulated. However, it is understood from the displayed graphs that the ten thermocouples inserted in the fuel bed are used to detect the speed of the ignition front. The thermocouples are placed with a known distance between them. Furthermore, Rogers states The ignition rates were computed as the product of the rate of travel of the ignition front and the initial bed density . No verification method was used. 

No explicit mathematical model of the method was presented. However, a short descriptive model was outlined For each run, the average ignition and combustion rate (expressed as weight of fuel ignited or burnt per unit bed area and unit time) were calculated by determining the time taken for the ignition front to pass down through the bed and the completion of burn-out, respectively. No discussion is presented about limitations and assumptions of the method. 

For each run, the pot was loaded with a bed 125-130 mm deep, with an ignition source (char coal soaked in kerosine) on top of the bed. A low air flow was used during ignition. The ignition front propagating down towards the grate was monitored by thermocouples connected to chart recorders (Figure 7). 

Stubington et al [6] presented a short descriptive model of how the ignition front rate and the combustion rate are determined. No mathematical models to calculate the ignition rate and the combustion rate are shown. However. As far as this author can understand, the calculation results are time average values, that is, no time instant values are obtained by the method used by Stubington. No uncertainty analysis was presented and no verification method was used. The methods used are unclearly defined. Consequently, the results would be difficult to reproduce. Nevertheless, the study includes interesting result. 

The primary objective of Gort s experimental work [10] was to study the effect of volume flow of primary air on ignition front rate and combustion rate. Another aim was to study the dependency of ignition front rate and combustion rates on moisture content, volatile content, and particle size. 

The ignition front rate is defined as the multiplication of bed density and speed of ignition front... 

The speed of ignition front is evaluated between the thermocouples at the position 300 and 150 mm above the grate. The distance between the thermocouples is... 

The ignition front rate for the underfired case is defined as the initial weight of the bed divided by time of travel through the whole bed and the cross section of the bed. 

The time of travel is determined as the time difference between the start of the run and when the ignition front has reached the top of the bed. This evaluation of ignition front rate results in a overall time average value. 

Nine studies regarding the thermochemical conversion of packed-beds of biomass have been reviewed. The review is summarized in Table 7 below. The focus of the survey has been on the theories of the methods applied to measure ignition front rate, conversion rate (combustion rate, burning rate), conversion gas stoichiometry, and air factors. 

To approach the analysis of, and to be able to comprehend, the complex phenomena of thermochemical conversion of solid fuels some idealization has to be made. For a simplified one-dimensional analysis, there is an analogy between gas-phase combustion and thermochemichal conversion of solid fuels, which is illustrated in Figure 41. Both the gas-phase combustion and the thermochemical conversion is governed by a exothermic reaction which causes a propagating reaction front to move towards the gas fuel and solid fuel, respectively. However, there are also some major differences between the conversion zone and the combustion zone. The conversion front is defined by the thermochemical process closest to the preheat zone, which is not necessarily the char combustion zone, whereas for the flame front is defined by the ignition front. In practice, many times the conversion zone is so thin that the ignition front and the conversion front can not be separated. 

Figure 45 The macroscopic ignition front rate as function of air velocity through an overfired batch bed and the three conversion regimes separated by the lines (a) and (b).[33]...

Regime I prevails at low volume fluxes of primary air. The overall conversion rate is controlled by the diffusion rate of oxygen to the oxidation of the solid char. Regime I is characterized by the fact that the overall conversion rate is slower than the macroscopic ignition rate, which implies that solid fuel convertibles are accumulated behind the macroscopic ignition front. In other words, the conversion zone is growing thicker. [9,33,40]... 

Figure 58A-C shows that the heat source promoting the drying process changes during the course of the batch combustion. At times to and ti there is an artificial heat source located in the over-bed section. At time t2 the flaming combustion has started. The flames feed back heat to the bed by means of radiation. At time t3 the basic heat flow comes from the char combustion and the ignition front, by means of conduction and radiation. [12,24]...

Pyrolysis commences at bed surface temperatures in the range of 150-300°C [22,23]. Almost simultaneously, flaming combustion takes place in the combustion system above the fuel bed (see Figure 58C). At t2 the pyrolysis is sustained by heat from over-bed flames. The heat is transported by radiation. At times ts to t4 the dominant heat source has changed to the char combustion zone (ignition front) instead. The heat from the ignition front is also transported by means of conduction and radiation. 

The heat accumulation in the bed surface layer causes the ignition of the char combustion process. The heat is supplied from the over-fire process (see Figure 58C). When the char combustion process commenced, the macroscopic ignition front sustains itself with heat from the exothemic oxidation reactions. Large amounts of the heat released by the char combustion zone are also conducted and radiated away both upwards and downwards in the bed. The downward propagation rate of the macroscopic ignition front is controlled by several factors, such as biofuel moisture content, primary air rate and air temperature [33]. The temperature of the macroscopic propagating char combustion zone is around 1000-1200°C in batch-bed combustion of solid biofuels [38,41]. 

The process of deformation of the explosive in the zone subjected to wave compression produces reaction centers similar to those formed when an explosive is subjected to mechanical stresses. The formation of centers in the compression wave is also the condition that determines the possibility of further development of the explosion. The development of centers leads to the formation of an ignition front which, in turn, becomes a source of additional reinforcement of the compression wave, compensat-... 

Figure 1 Ignition front moving against the airflow in the small experimental rig.

Figure 3 shows the temperature at 150 mm and at 300 mm above the grate. The short peak in temperature is the front temperature, most likely influenced by the surface temperature of the nearby particles. In this example the air mass flow is low, and all fuel is not converted but accumulates upstream of the ignition front. The temperature in the partly converted layer is about 100 C lower than in the front. As the bed shrinks the upper thermocouple finds itself above the bed. Because of radiative cooling by the walls the uncorrected temperature is about 200°C lower than the bed temperature. 

The temperature curves show how the width of the accumulated layer broadens as the front moves downwards. When the ignition front reaches the grate, there is a peak in temperature (at about 43 minutes) caused by combustion of the accumulated char. 

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