Tar Reforming under a Microwave Plasma Torch

Because of the scarcity of nonrenewable natural resources, such as petroleum and natural gas, the use of biofuel is needed. Gasification is a major process used to obtain renewable fuels from biomass; however, the gas cleaning system is a constraint for its broad utilization. During the pyrolysis process, a mixture of organic compounds in the gas phase is produced and must be removed from the gases before it is used in the most practical applications. In order to remove such organic compounds, which are known as tar, large, sophisticated, problematic, and expensive gas cleaning systems are added to the gasifier gas exit. Previous papers have shown that the plasma torch has the potential to destroy produced tar, being a simpler and less-expensive system than traditional gas cleaners. This work presents a qualitative and quantitative evaluation of a microwave plasma system running on tar destruction and its reforming. In order to evaluate a 1 kW microwave plasma system performance, an apparatus was developed and installed at ITA Laboratory of Plasmas and Processes (LPP-ITA). The system runs at atmospheric pressure with nitrogen and argon as carrier gas under a large range of flow rates. Experiments were performed using a gas mixture of N-2, H2O, ethanol, and tar at controlled concentration in order to simulate the gases produced by a gasifier. The injected tar was obtained from pine pyrolysis and characterized for energy purposes. In order to reduce tar viscosity, it was diluted in commercial ethanol (92.5% ethanol and 7.5% water) and its concentration varied from 0.8 g(tar)/Nm(gas)(3) to 4.2 g(tar)/Nm(gas)(3) Species formed in the microwave plasma torch were identified using an optical spectrometer. The reactor exit gases had their composition evaluated on tar content as well as for noncondensable gases. As a result, this paper shows that no tar content was detected at the reactor outlet, indicating that all supplied tar was destroyed in the plasma reactor. The main detected products were CO and solid carbon (C-(s)). Furthermore, neither NO nor CO2 were detected, and an indication of H-2 formation was obtained. This paper concludes that the microwave plasma system is capable of destroying and reforming tar efficiently and produces mainly H-2, CO, O-2, and C-(s) as byproducts.