Parameter study of waste shiitake substrate/waste polyethylene Co-gasification using Aspen Plus kinetic modeling

Abstract

Waste Shiitake Substrate, a major agricultural waste in Taiwan, is typically packaged in plastic bags (Waste Polyethylene, PE) during mushroom cultivation process. Consequently, these plastic bags become additional waste that requires proper disposal. In recent years, Taiwan has produced over 150,000 tons of waste shiitake substrate annually, with approximately 1580 tons of plastic bags used for mushroom cultivation. Up to now, there has not been an effective method for handling this waste. This study aims to develop a co-gasification process using the complementary characteristics of WSS and PE. Unlike traditional direct combustion, the co-gasification approach can convert these wastes into cleaner bioenergy. This study aims to develop an Aspen Plus model for a laboratory-scale 1 kWth bubbling fluidized bed gasifier. This model incorporates kinetic reactions and tar formation during co-gasification. The model accurately predicted the experimental results. Different variables were investigated, including gasifier temperature, blending ratio (BR) of the two fuels, mixing ratio (MR) of the gasifying agent, and equivalence ratio (ER). The results showed that hydrogen production correlated positively with the steam-to-carbon ratio in the gasification medium, whereas carbon monoxide (CO) was more affected by the feedstock type and equivalence ratio, with CO reaching its peak at an equivalence ratio of 0.2. When gasifying using only waste shiitake substrate (BR = 0%), at an ER of 0.2, a gasification furnace temperature of 950 °C, and an MR of 100% (pure CO2), the maximum CO mass flow rate in the syngas is obtained. Waste plastics contain significant amounts of volatile matter and no oxygen, leading to increased production of CH4, CO, and H2 during co-gasification with the addition of waste plastic. For plastics, high temperature and low ER conditions promote the production of CO, H2, and CH4 because of the significant tar content, thereby enhancing the Cold gas efficiency (CGE). When the gasification temperature is 950 °C, with 80% polyethylene (BR = 80%) and an ER of 0.1, the highest HHV is achieved. These results can serve as a reference for determining the operational conditions for the scale-up of gasification systems.