Modelling and efficiency improvement of a plasma-arc gasification reactor quench probe

Abstract

A steady state computational fluid dynamics (CFD) model was developed to describe the temperature and flow characteristics inside a laboratory scale plasma-arc gasification reactor quench probe with the aim of efficiency improvement. “Thermal efficiency” was mainly described according to the thermal and fluid profiles and characteristics inside the quench probe, while the chemical efficiency, which contributed to a minor part of the study, was based on an experimental approach. The CFD model was constructed using the STAR-CCM+ CFD simulation software in the 3-dimensional Eulerian-Lagrangian framework. The realisable k-ε turbulence two-layer model was applied to describe the physics properties of the simulation and water droplets were considered spherical and of constant density and size. An unreactive model was developed as temperatures were too low to deem the dominant reaction, i.e. the water gas shift reaction, active. Model data was validated through comparison thereof with experimental thermocouple measurements positioned inside the laboratory scale quench probe. U-tube measurements described the chemical gas composition downstream to the quench probe. Experimental data was categorised into phase 1 (non-reactive approach) and phase 2 (reactive approach), of which the feed of the latter included organic material as opposed to phase 1, which consisted of only a carrier gas feed. A variety of experimental cases were investigated by inducing changes to the experimental setup with regards to the water spray rate, gas feed and type, and supplied electrical current. Phase 1 was used to construct the base model, of which parameter analyses were conducted to refine the model for phase 2. The final model could describe quench probe conditions fairly accurate, with an average error of 9.87 % and a root mean square error of 6.96 °C. It was found that the temperature distribution inside the quench probe was strongly dependant on the velocity profile. The development of a recirculation zone inside the quench resulted in a longer residence time, increasing the cooling effect of the spray water. Furthermore, temperatures within range of full quenching were achieved relatively early after the first spray injection, indicating redundant water spray. Through use of the dimensionless temperature gradient and H2/CO ratio, the thermal and chemical efficiencies could respectively be investigated. Generally, the dimensionless temperature gradient averaged 0.8 towards the exit of the quench probe, indicating adequate quenching. Contrarywise, the H2/CO ratio ranged between 0.5 and 0.7 when ideally ratios of 1.0 to 2.5 are preferred for industrial application. It is therefore cardinal to improve chemical efficiency whilst not sacrificing the integrity of the thermal efficiency. Lastly, the model was used to investigate improvements to the current quench probe design with regards to the water flow rate, nozzle placement, number of nozzles and geometry. It was concluded that the water flow rate could be reduced in addition to lessening the number of nozzles to effectively achieve the same quenching results. Additionally, a larger diameter quench probe would achieve faster quenching rates, but due to the redundant water spray nozzles in the current application, similar results were achieved for smaller diameter cases.