High Temperature Superheated Steam Generation Technology and Its Application in Pyrolysis and Gasification Processes

Abstract

With the increase in plastic wastes every year, the chemicals used to make plastics and the microplastics generated during disposal are making an increasingly significant negative impact on the environment. Consequently, it is necessary to recycle plastic wastes; various methods have been considered such as pyrolysis and gasification. However, these methods require applying high thermal energy to the plastic wastes. The energy required can be supplied using electrical energy as well as high-temperature superheated steam generated from waste heat recovery. This study performs preliminary research on a combination of a pyrolysis pretreatment device, gasification reactor, and high-temperature superheated steam production system to construct a composite system as part of plastic waste recycling technology. During recycling, the temperature of the superheated steam must exceed 500 ℃ to facilitate pyrolysis. Considering heat loss during transport, steam should be produced at 700 ℃ by the superheated steam production system, which is installed after the gasifier, for waste heat recovery. The pollution by fine dust and corrosive gases generated from the superheater during the gasification of plastic wastes significantly affects its heat transfer performance and lifespan. Therefore, commercial software was used to perform thermal and flow characteristic experiments and computational analysis on the cylindrical heat exchanger of the superheated steam production system, which also serves as the waste heat recovery system. Variations in flow and heat transfer characteristics were observed when fins were applied to the inner walls of the superheater for improved heat transfer performance. In the cylindrical superheated steam generator (CSSG), the steam flows downward in a swirling motion from the top, creating a secondary flow. It lacks internal fins or baffles to resist the downward flow; hence, the steam descends rapidly and is either expelled or reascends along the perimeter of the exhaust pipe. Consequently, the superheated steam is discharged at 705.5 ℃. In CSSG, the superheated steam entering from the top creates a swirl flow and secondary vortex. While moving downward, the steam follows the bottom shape, rises again, and either gets discharged or follows a rising flow around the discharge pipe. Both circular and helical fins act as flow resistors against the swirling downward flow along the wall; moreover, the secondary flow occurs in between the fins, enhancing heat transfer. Circular fins block the flow of steam, resulting in a shorter contact time between the steam and wall. In contrast, helical fins guide the flow around the inner walls of the CSSG, which significantly increases the contact time between the steam and wall. Therefore, it is essential to employ helical fins to extend the contact time between the steam and inner wall surfaces and thereby enhance the internal temperature of the CSSG. To observe the changes in the composition of plastic wastes, an electrical heating method was applied to the pyrolysis pre-treatment device,. and the pyrolysis residues and volatile gases supplied to the gasification reactor were analyzed. To analyze the impact of applying each fin shape within the CSSG using quantitative indicators, both the Nusselt number and swirling strength were calculated. The Nusselt number is a dimensionless number that represents the wall heat transfer characteristics of heat exchangers. The finless shape had the smallest Nusselt number at 4,132. Applying the circular fins increased it to approximately 128%; however, the impact of the fin's angle on the Nusselt number was minimal. For helical fin shapes, the Nusselt number increased by at least 386% compared to the finless shape, except that for the downward fin. However, at the highest steam discharge temperature, the Nusselt number was the smallest for the 15° angle fins among the upward fins. For circular fins, the impact of the first fin is significant. However, the remaining fins have a minor influence on the Nusselt number, leading to negligible differences owing to the angle of the fins. For helical fins, the Nusselt number increases with the increase in fin angle owing to the decrease in steam discharge. However, correlating the Nusselt number and steam discharge temperature in the CSSG is challenging as most of the flow inside is swirling. The increase in the value of swirling strength, which represents the intensity of the vortex at a point, can enhance the heat transfer performance. The finless shape has the lowest swirling strength of 85.2 1/s. When circular and helical fins are applied, it increases up to 11.3% and 44.1%, respectively. Furthermore, the swirling strength trend corresponds with the steam discharge temperature trend. When fins are applied inside the CSSG, the first fin impedes the downward flow and creates a swirl flow, thereby increasing the swirling strength. Furthermore, the swirling strength at the bottom is enhanced by the gathering steam in the pipe from the bottom. To conclude, helical fins create more active swirl flows in the space between the fins, resulting in the highest calculated swirling strength. The following research was performed on the characteristics of the pyrolysis pretreatment device and gasification reactor for plastic wastes. After heating the system, raw materials were loaded in the pyrolysis pretreatment device to conduct pyrolysis experiments. For the gasification reactor, simulations of chemical reactions and fluid dynamics were performed using a commercial computational analysis program based on the results obtained from the pretreatment device experiments. The thermal decomposition pretreatment device for plastic wastes employed an electrical heating method to a dual-screw transport system. A 12 kW power was supplied, and the device temperature was maintained above 350 ℃ because it is the temperature range where the pyrolysis is active, and it is considered to account for the thermal loss during the transport of superheated steam generated in the CSSG. This approach aims to reduce the pyrolysis reactor’s consumption of electrical energy by supplying it with superheated steam. The experimental results indicated that 60.6% of the plastic wastes were converted into thermal decomposition gas during transportation within the pretreatment device. After thermal decomposition, the remaining residue exhibited reduced moisture content and increased fixed carbon content. The thermal decomposition gas primarily consisted of carbon dioxide (31.7%), methane (13.6%), hydrogen (10.4%), and carbon monoxide (10.3%). The residue and volatile gases generated in the pretreatment device after thermal decomposition were intended to be converted into syn-gas for a water gas shift (WGS) reactor to produce hydrogen. To achieve this goal, the fluid characteristics and substance transformations in the internal fluid area of the gasification furnace were examined using commercial software for numerical analysis. The plastic waste gasification furnace utilizes both the plasma heat and heat generated during partial oxidation as heat sources. The material input employed experimental data from the thermal decomposition pretreatment device. The fluid flow within the gasification furnace follows a swirling pattern along the wall owing to the supply of oxygen–nitrogen mixed gas for partial oxidation. The internal temperature of the furnace changes due to chemical reactions. The outlet temperature of the furnace calculated at the standard oxygen supply was 708.1 ℃. As the flow rate of the oxygen–nitrogen mixed gas from the standard oxygen supply increased by 60%, 80%, and 120%, the outlet temperature was 529.5 ℃ at the lowest gas flow rate, while at 120%, it reached 708.0 ℃. When the flow rate of the oxygen –nitrogen mixed gas was increased, carbon monoxide was found to be the major component in the syn-gas produced.