Development and study of different numerical plasma jet models and experimental study of plasma gasification of waste

摘要

Thermal plasmas have been widely used in a large number of high-technological industrial applications. The conversion of organic matter to a high-quality syngas by using plasma torches is called plasma gasification. The original goal for this plasma treatment was to either melt or immobilize solid materials such as ash and metals, making them safe for disposal. The recent emphasis in the waste industry on a circular economy requires more advanced conversion technologies which yield improved resource recovery. This has triggered the recent development of the plasma gasification technology for waste treatment.In this work, two subjects associated with plasma gasification have been studied, i.e. plasma jet modelling and plasma gasification of refuse-derived fuel (RDF). The first part of the research related to plasma jet modelling is situated by outlining the details of thermal plasma systems and the challenges of modeling the complex physical phenomena involved. An important issue arising from the mixing of high-temperature plasma gas(es) with surrounding gas(es) is the correct estimation of thermodynamic and transport properties of the resulting gas mixture. The use of mixing rules for the calculation of thermophysical properties of a gas mixture is common practice. However, it was recognized that the influence of these approximations on the accuracy of the simulated flow field has not yet been quantitatively investigated.A model of the plasma jet from a direct current (DC) hybrid water/gas-stabilized torch, particularly suited for plasma gasification, issuing in nitrogen atmosphere has been developed. With this model case, three computational fluid dynamics (CFD) simulations were performed, which differ in the extent to which mixing rules are used for the calculation of the thermophysical properties of the ternary (Ar/H2O/N2) gas mixture. The first model approach (model 1) estimates the properties of the gas mixture by using mixing rules with the temperature-dependent properties of each individual gas. The second model approach (model 2) calculates the properties of the plasma gas (Ar/H2O) rigorously and combines them with those of nitrogen by using mixing rules to estimate the properties of the ternary gas mixture. Model 3 represents the full multicomponent approach in which no mixing rules are used and the thermophysical properties of the gas mixture are calculated rigorously.The effect of the mixing rules for the calculation of gas mixture properties is evaluated through comparison of calculated temperature, velocity and gas concentration fields of the plasma jet flow. The results revealed that the use of approximate mixing rules can greatly influence the calculated flow of a plasma jet. It was demonstrated that the effect caused by deviations from the exact molecular transport properties by using low-accuracy mixing rules such as the one of Mason and Saxena for thermal conductivity is non-negligible. It is proven that the assumption of a negligible contribution of the laminar transport properties in relation to their turbulent counterparts (frequently postulated in literature), is not self-evident. In plasma jet modelling, the level of turbulence in the high-temperature region close to the torch exit is often low and the interaction of the thermophysical properties at the boundary of the jet in this quasi-laminar region determines to a great extent the onset of turbulence and hence the entrainment of surrounding gas.The second subject studied in the PhD work is the plasma gasification of waste. The thermal plasma application in solid waste treatment is first put in relation to conventional thermochemical waste conversion methods. The advantages and challenges of plasma gasification are explained and the different possibilities for end-use of the products (syngas and slag) are listed. The state of the art of plasma gasification is illustrated by summarizing all the plasma gasification facilities currently operating in the world. The plasma gasification system at the Institute of Plasma Physics (IPP) in Prague (Czech Republic) is one of only a limited number of academic installations and has delivered significant contributions to this field of research. The configuration of this reactor system and details of its extensive diagnostics system are described.Refuse-derived fuel (RDF), a processed mixture of excavated municipal and industrial solid waste was selected as the feedstock to evaluate the performance of the in-flight plasma gasification process for materials with a high inorganic content. During the experimental run, different combinations of gasifying agents (CO2, O2 and H2O) were added to the reactor volume and the material was supplied at different mass flow rates (i.e. 21.3 and 28.9 kg h−1). The sets of experimental data consist of syngas composition and flow rate, energy losses from the torch and the reactor walls, and temperature distribution in the reactor volume. Nine experimental cases with different operating parameters were identified during steady-state operation. The production of high-quality syngas with low levels of tar (132-543mg/Nm3) was demonstrated for all cases. The measured syngas composition was in good accordance with the calculated syngas composition in thermodynamic equilibrium. The effects of equivalence ratio, material feed rate and type of gasifying agent were investigated by comparing the performance criteria (carbon conversion efficiency, CO yield and H2 yield) and energy efficiencies of the different cases. The carbon conversion efficiency of plasma gasification with a RDF feeding rate of 28.9 kg/h ranges between 82 and 87%. The highest registered cold gas efficiency and mechanical gasification efficiency are 57% and 97%, respectively.It was found that the oxy-steam plasma gasification showed the highest material conversion efficiency. Furthermore, it was found that for the same equivalence ratio, the H2/CO ratio in the syngas can be inverted by interchanging CO2 with H2O as gasifying agent without affecting the performance of the process. The lower performance of the RDF experiments compared to biomass experiments on the same plasma gasification system were attributed to the coarser particle size, higher moisture content and higher ash fraction of RDF. The comparative analysis between this single-stage plasma gasification experiment and two-stage plasma gasification of a similar waste material revealed advantageous characteristics (higher CO and H2 content and higher CO/CO2 ratio) for the syngas produced by the former system. The tar destruction efficiency is considered similar for both systems. The inorganic content is collected as a vitrified slag in the two-stage plasma gasification system, whereas a large portion of the residual material in the single-stage in-flight gasification system was recovered as particulates and only a small fraction of the inorganic fraction was vitrified in the wake of the plasma jet.It was concluded that the good control of the characteristics of the high-quality syngas and the overall flexibility of the system make plasma gasification a promising technology for the treatment of refuse-derived fuel.