Staged combustion aromaticity

However, substantial amounts of the unsubstituted aromatics were found, as shown in Figure 13. At 900°C more than 60 percent of each liquid product was unsubstituted aromatics. The amounts of one, two, and three ring molecules (i.e., benzene, naphthalene, and phenenthrene) varied with the molecular weight of the starting material. For example, the liquid product of MD-3 at 900°C was more than 50 percent benzene while naphthalene was more than 30 percent of the liquid product from MD-4 at 900°C. These unsubstituted aromatics are more thermally stable than substituted aromatic molecules and can be considered soot precursors in staged combustion processes. 

Thus, the most likely survivors of fuel-rich, first-stage pyrolysis are unsubstituted and oxygen substituted aromatics and nonbasic nitrogen compounds. These are the precursors to soot and N0x formation during oxygen-rich, second-stage combustion. 

N0X emissions tend to be higher due to the higher fuel nitrogen levels of coal-derived fuel oils. However, it appears, based on small scale lab tests (2) and limited commercial tests (3), that staged combustion should allow N0X emissions standards for coal-derived fuel oils to be met. One environmental concern that had not been addressed in these tests is the emissions of PNA. This is a potential concern due to the highly aromatic nature of coal-derived fuel oils. 

Further support to the hypothesis that the two families of aromatics originate from different chemical reactions comes from the effect of metal oxides. It is well-known that the addition of metal oxides to PVC produces a noticeable suppression of the amoxmt of aromatic hydrocarbons evolved during the pyrolysis with a consequential increase of fhe char residue, allowing the explanation of the smoke suppressant action of these additives in the combustion of PVC. In the case of PA pyrolysis, mefal oxides were nof foxmd able to suppress the evolution of aromatics, whereas metal chloride produced the selective suppression of benzene, naphthalene, and anthracene. These results show that the aromatic suppression agents are the metal chlorides and not the metal oxides. In fact the metal oxides were trasformed into the corresponding chlorides by the HCl evolved during the early stages of the PVC pyrolysis. 

The thermal decomposition process, while written as an orderly process, may be quite disordered. Nevertheless, it is proposed that as the coal particle temperature rises during thermal decomposition (which may also be the initial stages of the combustion process) (Chapters 14 and 15), the bonds between the aromatic clusters in the coal macromolecule break and create lower-molecular-weight fragments that are detached from the macromolecule—the larger fragments of this decomposition process are often (collectively) referred to as the metaplast. 

Ballistreri et a/. found that the principal mechanism of smoke inhibition was a cross-linking process catalyzed by metallic species. In the mechanism of PVC decomposition, as shown in Figure 5.8, mainly the production of non-substituted aromatic hydrocarbons (benzene, naphthalene, anthracene) was suppressed by the catalytic effect of metal oxides on the intermolecular cross-linking reaction, at the expense of the intramolecular rearrangement of the polyene. Smoke production was reduced due to the formation of less fuel. In the presence of certain metal oxides, a diminished char yield was also observed. It was concluded, however, that oxidation occurred at a later stage of the combustion, i.e. after the evolution of smoke in the pyrolytic process. 

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