Use of plasma reforming of fuel for reduction of the toxicity of diesel engines

Abstract

621.436Problems in the use of plasma reforming of Diesel fuel for reduction of the toxicity of exhaust gases are considered. Results of experiments on the decomposition of Diesel fuel in nonequilibrium plasma of a spark discharge are given. It is shown that under these conditions the content of hydrogen in the obtained synthesis gas can reach 10% or more.The widespread use of internal-combustion engines in different spheres of human activity has had serious ecological impacts, related mainly to pollution of the environment by toxic components of the exhaust gases (EG) of automotive engineering. In parallel with the known methods for reduction of the toxicity of exhaust gases (catalytic cleaning, electric-discharge plasma systems) [1], a method using a hydrogen-enriched gas mixture obtained by decomposition of a part of the Diesel fuel in the discharge [2] is promising.A compact plasmatron that runs off an on-board power source can be used to convert a part of the fuel to a hydrogen-containing gas mixture for both direct injection to the combustion chamber and treatment of EG in the plasmachemical reactor or catalyst. The presence of hydrogen in the fuel mixture can lead to more complete burning of fuel at a smaller effective temperature and the corresponding decrease of soot and carbon and nitrogen oxides in EG. Plasma increases the partial oxidation reaction that converts hydrocarbons of the fuel to the hydrogen-enriched gas mixture (the oxygen/carbon ratio is unity, exothermal reaction):For Diesel fuel the reaction occurs with liberation of about 10-15% of the energy:C 14 H 26 + 7O 2 → 14CO + 13H 2 . Figure 1 gives a basic diagram of the use of an on-board plasmatron-reformer of fuel with direct injection of a hydrogen-enriched synthesis gas to the engine. The effect of the operation according to this scheme is a reduction of the concentrations of NO x , hydrocarbons, and soot particles, and improvement of the thermal efficiency of the engine.We estimate the amount of hydrogen necessary for the case where partial replacement of the Diesel fuel by hydrogen does not considerably change the operating conditions of the engine but decreases the emission of toxic components. On the basis of experiments [3], we admit substitution of 10% of the Diesel fuel by hydrogen. Fuel consumption for a typical engine is ,100 g/min. Since the heat power of hydrogen is almost threefold higher than the heat power of the Diesel fuel, 3 g of hydrogen are enough to substitute 10 g of the Diesel fuel. In this case, the molar concentration of hydrogen will be more than 50%, which guarantees stable operation of the engine with very lean mixtures. The maximum pressure of combustion of these mixtures increases slightly.Modern plasmatrons have a high efficiency (higher than 80%). A specific efficiency of a typical plasmatron is 1 g/h of conversion products at an electric power consumption of 1 W. The power consumption necessary for production of 3 g/min of hydrogen is ,200 W.Introduction of hydrocarbon (ethylene, propane, methane) additives ...