The Ignition and Combustion of Cerium Doped Diesel Soot

Particulates (soot) were sampled from the regenerative trap of an automotive diesel engine run under three speed/load conditions. The fuel was doped with a cerium-based catalyst to promote oxidation of the soot bed. The soots were subjected to combustion testing in a DTG under both temperature ramping and isothermal conditions, and under temperature ramping in a small fixed bed. The combustion gas was 10% oxygen in nitrogen, which was supplemented in the case of the fixed bed with other gases found in diesel exhausts. The temperatures of initiation of combustion Tin were measured in the DTG and in the fixed bed. The kinetic rates of oxidation were calculated from the DTG results taking into account the influence of oxygen transport. The effect of gas composition on ignition and burnout in the fixed bed was determined.

Ignition did not occur in the DTG, but rather burning progressed steadily. There was a large increase in the kinetic rate constants with cerium present, but the activation energy was unchanged. There was no difference in rates between the two cerium additives. No consistent differences were observed between the combustion kinetics of soots generated under different engine conditions. At temperatures above 600°C a decline in the catalytic effect of the doped samples appeared.

In the fixed bed the catalysed samples tended to partly burn and then ignite and be totally consumed. The appearance of ignition was erratic with the addition of minor gases to the combustion gas. A simple combustion and heat transfer model was able to predict ignition temperatures. Loss of adsorbed hydrocarbons takes place before bed ignition.