We all know about the common usage of plastics in our day to day lives. Therefore, we do generate a significant amount of plastic waste at the end of a day. Unaddressed waste plastic is pushing our lives in potential danger causing a significant environmental pollution and damaging land and water bodies. Plastic majorly contains carbon and hydrogen with little amount of oxygen, chlorine, nitrogen etc., depending up on the types of plastic. Number of hydrogen atoms in a single molecule of plastic (called monomer) is significantly high (roughly 2-4 times) compared to closest major numbers of atoms of carbon. Thus, harvesting energy from the waste plastic is a smart choice. Moreover, hydrogen energy is our current global option to reduce the carbon footprint into the environment which has been abruptly contributing towards the global warming!
Technically, breaking the chemical bonding between carbon and hydrogen present in the polymeric structure of a plastic is very difficult and can be achieved by spending a lot of energy (electricity). Therefore, generating hydrogen from the plastic was not economically competitive by conventional thermal cracking. On the other hand, plasma being a forth state of matter contains very high energy free electrons which has tremendous potential to break such a very strong chemical bond between carbon and hydrogen. If the carbon is captured in its solid forms, thus hydrogen can be produced from plastic (waste).
With this aim, a development of an integrated technology has been attempted in this project PROSPER. The major focus was in designing a suitable reaction chamber wherein plastic would be melted and subsequently pyrolyzed to produce smaller molecules including monomers. As it is known that the molten plastic is a very sticky in nature and thus it has been a challenge to dynamically run such a reaction chamber wherein solid plastic would get melted (liquid phase) and subsequently pyrolyzed to produce gaseous volatiles containing such smaller molecules. A spouted bed reaction chamber (reactor) which is a special type of fluidized bed reactor was found suitable to overcome such a challenge of handling solid, liquid and gaseous phases enabling continuous production of gaseous products. A certain type of modifications inside such a spouted bed reactor was incorporated for a better hydrodynamic and efficient reactions. The types of internals were a porous draft tube and a fountain confiner.
Computational fluid dynamic simulations were performed to get more inside of the mechanism of functioning of such a spouted bed reactor. Simulation results could guide us to suitably chose the dimension of the reaction chamber, internals and typical operating conditions at a temperature range of 500 to 750°C. A set of experiments were also performed in a spouted bed of typical diameter of 0.5 m and height of 1.5 m to study the hydrodynamics of poly-ethylene plastic at room temperature. An amount of 5 kg plastic has been tested to suitably process in such a spouted bed reactor having previously mentioned dimensions.
The smaller molecules obtained from the plastic pyrolysis are typically (>95% by volume) methane (C1, molecule containing 1 atom of carbon), ethane, ethene, acetylene (C2, molecule containing 2 atoms of carbon), propane, propene, propyne (C3, molecule containing 3 atoms of carbon), butane, butene, butyne (C4, molecule containing 4 atoms of carbon) with small amount of carbon monoxide and dioxide. These may be called organic volatiles which can also be used as a feedstock for further plastic production. However, our further aim was to produce hydrogen and methane which are much lighter molecules than the major constituents of the volatiles. The hydrogen and methane has much more thermal energy densities per unit mass than the mixture of volatiles. On the other hand, hydrogen itself is a green energy option for our current global trends to attain “net zero” policy.
Thus, it was necessary to break the chemical bonds of carbon and hydrogen to obtain more amount of gaseous hydrogen while trapping the solid carbon atoms. For this reason, a direct current-based plasma torch and plasma reaction chamber was designed using computational fluid dynamic simulation with suitable multi-physics. Based on the desired design parameters, a plasma torch along with plasma reaction chamber were developed and commissioned in this project PROSPER. The used plasma torch could run with up to 15 kW power. The plasma flame was developed using argon gas. The electrodes of the plasma torch were made from copper and hafnium. In order to provide necessary power to the torch, a 20 kW power supply was also commissioned.
One of the higher molecular weight containing mixture of C3 and C4 (major constituents of organic volatiles from plastic) was thus allowed to pass through a plasma flame operated in the range of 5.5 to 12.5 kW power. Systematic experiments were performed to obtain hydrogen and methane from the C3 and C4 gas mixture. It is found that, nearly 55 % (v/v) hydrogen and 40 % (v/v) methane were produced at a rated power of the torch at 12.5 kW. So far, no literature has reported such a huge fraction of conversion of C3& C4 to produce 55% hydrogen and 40% methane. Further studies are being planned to achieve a production of more than 90 % hydrogen with a more efficient and higher power plasma torch.
