Carbon char combustion

Non-flammable plasticisers, such as tri-tolyl phosphate, tri-xylene phosphate, or a number of different brominated plasticisers, produce a dense hard brittle carbon char after initial combustion which then acts as a barrier to exclude oxygen. 

The oxygen diffuses through the boundary layer to the particle surface and countercurrent diffusion of char combustion products (carbon monoxide and carbon dioxide), see Figure 56. [73,77]... 

Step one is, oxygen diffusion in the porous system of the particle inwards to the char combustion front and the reaction site, (2) adsorption of oxygen to the active sites on the intraparticle char phase, (3) oxidation reaction with carbon, and (4) desorption of... 

Figure 8. The changes in the level of unbumt carbon within the char combustion product derived from the weathered coal series A and B.

Figure 10. The relationship between unbumt carbon within the char combustion product and the atomic O/C ratio of the parent coals.

The approach of combustion modeling used in the present work is similar to that of Chelliah et al. [6], In particular, the gaseous combustion is described by the detailed reaction mechanism, while the char combustion is described by the semi-global heterogeneous mechanism. The following reactions were originally taken into account for solid carbon ... 

The density of the wood particles entering the bed is 600 kg/m and will be reduced during devolatilisation. Since the average fixed carbon fraction is approximately 10 wt%, the density of a completely devolatilised particle is 60 kg/m . It is assumed that char combustion starts only after the end of devolatilisation. The terminal velocity at 850°C for different combinations of particle sizes and densities are given in Table 1. 

There are few questions concerning the experimental results (1) How is CO formed in the furnace (2) How does the secondary air supply affect the CO emissions As seen in Fig. 2 to Fig. 4, char combustion, as flying particles, in a fixed bed biomass furnace, has a little effect on the gas field in the lower part of the furnace. This can be explained by the fact that the mass of char emitted from the bed is only 5% of the carbon content in the wood, and a large amount of other fuels (tar, CO, UHC) are not yet completely oxidized. In the upper part of the fiimace, tar and other gas phase species are mostly consumed, leaving char bunvout the most contributing source for the CO distributions (note the... 

Char combustion kinetics have been previously reported for Antrim shale by Rostam-Abadi and Mickelson (9). In that study the authors reported that the rate was second order with respect to the char remaining and that there was noticeable chemisorption of (>2 Attempts to fit our data for the Antrim shale to a second order rate expression were unsuccessful and, in all cases, the data appeared to follow first order kinetics. Although we did not have the precision to measure O2 chemisorption, this phenomenon is consistent with our previous observations (6 ) of catalytic activity in those shales containing decomposed mineral carbonates. That is, the catalytic activity of CaO was attributed to its ability to chemisorb 02 As will be discussed in more detail below, the Antrim shale sample did not contain such carbonates and no catalytic behavior was observed. However, the magnitude of the rate constants reported by Rostam-Abadi and Mickelson (9) are very similar to those measured here. 

Experiments have shown that small amounts of certain metals can accelerate the rate of char combustion (4 9). A number of anions and cations have been shown to accelerate the combustion of carbons at concentrations of 10 to 1000 ppm. Table II shows the relative influence on the combustion rate of various salts added as solutions to purified graphite. Relatively small amounts of metals can accelerate the rate of combustion by many orders of magnitude. To effectively catalyze the combustion rate of coal, the metal which accelerates the rate must be distributed on nearly the molecular level, and be present in sufficient concentration to accelerate the rate. The range of relative acceleration of the combustion rate by different metals is shown in Figure 3. These estimates are made... 

The activity was also tested where char combustion was conducted in the regenerator. The average conditions for this run are given in Table VI. In this run 32 lbs. of char were burned, thereby exposing the acceptor to 0.73 lb. ash/lb. MgO CaO inventory. The combustion was trouble free, giving essentially flat temperature profiles and complete burnout of the carbon. No ash fusion difficulty was experienced. 

It has also been diseovered that potassium earbonate enhances the charring of polymers containing pentaerythritol-silica combinations as flame retardants. This has led to the discovery of base-catalysed intumescence of potassium bitartrate. Combinations of potassium bitaitrate and pentaerythritol show improved intumescence but carbon char oxidation by glowing combustion has remained a problem. Base catalysis is an attractive alternative to conventional acid-catalysed intumescent flame retardant systems as it could help to alleviate corrosion problems during polymer processing. Unfortunately, strongly basic residues also catalyse the oxidative destruction of the char-foam at high temperatures. 

In later work Baillet and Delfosse examined the effect of the flame retardant fillers on the formation of carbon oxides during pyrolysis of an EVA polymer and tested various additives for incandescence suppression effects [50]. Using the same quartz reactor described previously, they demonstrated that below the self-ignition temperature, the filled systems gave much more complete oxidation (higher C02 C0 levels) than the unfilled polymer. This supports the greater degree of char combustion referred to previously. 

Hsuen, H. K. D., and Sotirchos, S. V. Multiplicity analysis of char combustion with homogeneous carbon monoxide oxidation. Chem. Eng. Sci. 44(11), 2653-2665, 1989. 

---