Destruction of Tar in a Novel Coandă Tar Cracking System

Abstract

The main objective of this research program was to develop and test a small-scale system that used a novel Coanda burner for tar destruction through partial oxidation. An experimental rig consisting of a tar injector, in which wood pellets were pyrolyzed, and a Coanda tar cracking unit was designed, constructed, and operated to determine the effectiveness of the unit. The experimental program was divided into two phases, so that comparisons of the tar composition with and without treatment could be made. In the first phase, wood pellets were pyrolyzed at a range of temperatures between 500 and 800 °C and the pyrolysis products (gas, tar, and char) were analyzed. Increasing the temperature from 500 to 700 °C caused an increase in the production of hydrogen, methane, and carbon dioxide. As the pyrolysis temperature increased from 500 to 800 °C, there was a decrease in the yield of gravimetric tar in the sampled gas from 67.2 to 15.7 g/nm3. This reduction can be attributed to higher pyrolysis temperature, causing an increase in thermal cracking and depolymerization reactions, which, in turn, promotes production of permanent gas species. In the second phase, the gas produced in the first phase was treated in sub-stoichiometric conditions in the Coanda tar cracker. When the yield of tar species found in the treated and untreated gases is compared when the pyrolysis temperature of the tar injector was set at 800 °C, benzene was reduced by 95%, toluene was reduced by 96%, naphthalene was reduced by 97.7%, and the gravimetric tar yield was reduced by 86.7%. The Coanda tar cracker was shown to be effective at significantly reducing the tar content in the product gas. The reduction can be attributed to the high flame temperature (>1000 °C) and the addition of oxygen, which leads to the formation of free radicals, causing tar destruction.