(35f) Dry Catalytic Partial Oxidation of Diesel-Fuel Distillates Into Syngas

Dry partial oxidation of volatile compounds distilled from diesel fuel is being investigated as one means for producing synthesis gas for fuel cells and for pollution control devices requiring reductants for abatement of oxides of nitrogen. Use of volatile distillates eliminates potentially complex liquid-fuel injectors and fuel-air mixers, greatly simplifying reactors. The rationale is as follows: The longer-chain n-alkanes have the lowest autoignition temperatures of the hydrocarbons in diesel fuel. For example, cetane (n-hexadecane) has an autoignition temperature of 205 °C, well below its boiling point of 286.5 °C. If attempts are made to vaporize diesel fuel without appropriate precautions, the least stable molecules may crack before they vaporize, forming alkyl free radicals that may initiate polymerization chain reactions, leading to formation of tar on fuel nozzles and cores of liquid fuel droplets. To minimize tar and to allow rapid mixing with air, commercial diesel fuel is distilled below 200 °C. In one method, vapors are condensed in a reservoir, and the distillate, containing the most volatile components of the diesel fuel, is easily re-vaporized, mixed with air, and converted into syngas in a porous membrane reactor. In order to suppress formation of carbonaceous residues as fuel is heated above autoignition temperatures in the reactor heating zone, cooler air is passed through porous walls of zirconia-based ceramic to react with alkyl radicals as they form. Hot air (>900 °C) is passed into the reactor through porous cylindrical walls surrounding the reactor hot zone containing a bed of perovskite catalyst designed to contain oxygen-anion vacancies, enhancing diffusion of oxygen and providing a reservoir of mobile oxygen to react with adsorbed carbon from beneath. For distillates formed by collecting 20% by mass of the original diesel fuel, 75 mole% of the hydrogen in the distillate is converted into molecular hydrogen; 13% into CH₄ and 12% into H₂O (the latter formed by excess oxygen used to suppress carbon); 88 mole% of the carbon in the distillate hydrocarbons is bound in CO and only 5% in CO₂; the remainder chiefly in CH₄. In runs of an arbitrary 72 hours continuous operation, carbon deposition is negligible on the fuel nozzle, walls of the heating zone, in the catalyst bed, and in the exhaust, although some graphitic carbon appears on walls just above the hot zone. No soot was seen in exhaust traps. Gas chromatography of full diesel fuel and its distillates indicates that although n-alkanes >C₂₀ are largely eliminated, n-alkanes in the range C₁₆?C₁₉ (still likely to crack above 200 °C) are reduced by 70?75%, and C₁₃?C₁₅ reduced 50?65%, but were not eliminated due to significant vapor pressures below their boiling points and the simple means of distillation.