Soot precursor

If this step occurs late in the pyrolysis process, the hydroxyl radicals that form could attack the soot precursors. Thermal diffusivity may also have an effect. In premixed flames the S03 must dissociate into S02, which removes H atoms by... 

One of the emerging technologies that is showing great promise is the use of hydrated mineral fillers such as aluminium and magnesium hydroxides, as such materials can provide high levels of flame retardancy without the formation of smoke or corrosive and potentially toxic fumes. The use of fillers as flame retardants has recently been reviewed by Rothon [23]. Essentially the key features are an endothermic decomposition to reduce the temperature, the release of an inert gas to dilute the combustion gases and the formation of an oxide layer to insulate the polymer and to trap and oxidise soot precursors. 

The global reactions considered include the conversion by pure pyrolysis of toluene to acetylene and the conversion of isooctane to ethylene, oxidative pyrolysis of the acetylene and ethylene, and partial oxidation of the parent fuels and these hydrocarbon intermediates to CO, H2, and H2O. The specific reactions and rates for this system are given in Table II. Soot formation is assumed to be a function of temperature and oxygen and precursor concentrations. In the present study the soot precursors are taken to be acetylene and toluene, expressed as C2 hydrocarbons. 

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. 

All the experimental results for alkali and alkaline-earth metals can be interpreted by considering that soot precursors P (e.g., polyacetylenes in the combustion of aliphatic hydrocarbons (10)) are both oxidized by OH radicals with the rate Vi and are turned into small soot particles with the rate V2. Depending on the kind of metal and the experimental conditions, one or the other of these reactions will be promoted, with the rates becoming Vi and V2, respectively. 

High-temperature pyrolysis reactions of hydrocarbons are responsible for the production of PAH and solid carbon black particles, soot. This phenomenon is common in diffusion flames where, at high temperatures and without oxygen, hydrocarbon fuel aggregates follow pyrolysis and condensation paths with the formation of heavy aromatic structures. Many PAH s identified in aerosols have been found to be mutagenic and are certainly important soot precursors. This formation of carbonaceous particles has recently become one of the main topics in chemical reaction engineering, especially in the field of pyrolysis and combustion of hydrocarbon fuels. This interest rises from environmental concerns about PAH and soot particle emissions because of their dangerous impact on the human health (Oberdorster et al., 2004). 

This mechanism involves the formation of ring-ring aromatics, interconnected by aliphatic chains, which after successive dehydrogenation reactions progressively organize ordered graphite structures. Vlasov and Warnatz (2002) propose a competitive role for the polyyne molecules as important soot precursors. The polyyne pathway is derived from the works of Krestinin (2000) and assumes that these species grow rapidly with successive additions of C2H radicals... 

Blanquart G, Pepiot-Desjardins P, Pitsch H. Chemical mechanism for high temperature combustion of engine relevant fuels with emphasis on soot precursors. Combust Hame... 

The sub-stoichiometric combustion involves the risk of soot formation as a result of pyrolysis reactions with acetylene and polycyclic aromatic hydrocarbons as soot precursors [107] [465]. The soot formation will start below a certain steam-to-carbon ratio depending on pressure and other operating parameters [111]. However, the data in Table 1.8 shows results from a soot-free pilot test (100 Nm NG/h) at a low steam-to-carbon ratio of 0.21 [111]. 

Soot precursors and residual methane are converted by steam reforming and shift reactions in the catalyst bed. These reactions are in equilibrium in the gas leaving the catalyst bed and the ATR reactor. The catalyst size and shape is optimised to have sufficient activity and low pressure drop to achieve a compact reactor design. The catalyst must be able to withstand the high temperature without excessive sintering or weakening and it should not contain components volatile at the extreme conditions. A catalyst with nickel on magnesium alumina spinel has proven to fulfil these requirements [107] [111]. 

Sakaguchi S., Behavior of closed pores formed in consolidation process for silica soot precursor. J. Non-Cryst. Solids 1995 189 43-49... 

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