Tar Destruction in a Coandă Tar Cracker

Increasing the utilisation of bioenergy systems has the potential to become a vital component in the struggle to maintain and fulfil global energy demands. In particular, biomass gasification can offer a solution to the ‘Energy Trilemma’, and provide an affordable, reliable and carbon neutral technology. The limiting factor hampering the progression of biomass gasification power plants is tar. Tars formed during the thermal breakdown of biomass, condense and foul downstream equipment, causing reliability issues and damaging energy conversion equipment, such as engines and turbines. Treating tar through partial oxidation offers tar destruction without waste and soot, as well as maintaining the heating value of the tar in the producer gas. Coandă burners which are fuelled by more conventional fuels have been proven to operate close to, and below, stoichiometric conditions; as such, these devices were prime for further investigation. The main objective of this research project was to develop a small-scale system which utilises a novel Coandă burner for tar destruction. An experimental rig consisting of a wood pellet pyrolyser, which produced a gas loaded with tar, and a Coandă tar cracker, was designed, constructed and operated in order to determine the effectiveness of the process, with respect to tar reduction. The principal experimental program was divided into two phases, so that comparisons of the tar composition, before and after treatment, could be formed. In the first experimental phase, wood pellets were pyrolysed at a range of temperatures between 500 and 800ºC. The pyrolysis products (gas, tar and char) were analysed. As the pyrolysis temperature increased from 500 to 800ºC there was a decrease in the yield of gravimetric tar in the sampled gas from 78.59 to 16.55 g/Nm3. In the second phase the tarry gas was treated by the Coandă tar cracker. The Coandă tar cracker was shown to be effective at significantly reducing the tar content in the product gas. The yield of key tar components in the treated gas was reduced for all tested pyrolysis temperatures. For example; when the pyrolysis temperature was 800ºC; the yields of benzene, toluene and naphthalene were reduced by over 90% and the gravimetric tar yield by 88%. The success of the tar cracker can be attributed to the high flame temperature (>1000ºC) and the addition of oxygen which leads to the production of a greater proportion of radicals in the flame which initiate tar destruction reactions.