Matter loss, higher volatile

Two Possible Mechanisms. We tentatively propose two causal mechanisms to account for the observed correlation of higher volatile matter loss with heating rate. Let us assume first that a coal sample is taken to reaction temperature infinitely fast (i.e., exceeding about 104oC/sec). The material then decomposing is essentially the raw, unaltered coal which is only partially ordered and therefore more prone to pyrolyze extensively than a more ordered structure. By comparison, coal heated slowly (less than 103oC/sec) to reaction temperature reaches that temperature as a partially pyrolyzed but more ordered structure than the raw coal. The subsequent pyrolysis should therefore be less extensive. 

The standard test method for proximate analysis (ASTM D-3172) covers the methods of analysis associated with the proximate analysis of coal and coke and is, in fact, a combination of the determination of each of three of the properties and calculation of a fourth. Moisture, volatile matter, and ash are all determined by subjecting the coal to prescribed temperature levels for prescribed time intervals. The losses of weight are, by stipulation, due to loss of moisture and, at the higher temperature, loss of volatile matter. The residue remaining after ignition at the final temperature is called ash. Fixed carbon is the difference of these three values summed and subtracted from 100. In low-volatile materials such as coke and anthracite coal, the fixed-carbon value equates approximately to the elemental carbon content of the sample. 

Pyrolysis of biomass materials occurs under normal conditions at relatively low temperatures (300 to 500 C), producing volatile matter and char. Very rapid heating causes pyrolytic weight loss to occur at somewhat higher temperatures. In general, the volatile matter content of cellulosic materials approximates 90% of the dry weight of the initial feedstock. Woody materials contain between 70% and 80% volatile matter, and manures contain 60% volatile matter. However, it is known (3) that cellulosic materials can be completely volatilized when subject to very rapid heating (>10,000 C/sec). Several relatively complete reviews of the mechanisms and kinetics of cellulose pyrolysis are available in the literature (4-7). 

For example, as the carbon content of the coal increases, the active thermal decomposition occurs in successively higher temperature ranges and the maximum weight loss decreases quite substantially (Figure 13.10). In addition, different macerals in any one particular coal also generate different amounts of volatile matter, and, thus, similar trends are noted when exinites, vitrinites, and inertinites of the same rank are thermally decomposed (Figure 13.11). 

In a laboratory simulation of volatilization artifacts (Poster et al., 1995), urban air particulate matter was exposed to PAH-free air for 20 hours in a Berner impactor. Losses of 4-ring PAHs were 21-33% at a pressure of 0.09 atm and <15% at higher pressures. Volatilization of perdeuterated PAHs which were surface-coated onto the particles was greater, amounting to 50% at 0.09 atm. These volatilization losses were expected to be upper limits, since the clean air used for the experiments would have tended to strip PAHs from the particles. 

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