Gasification-Based Conversion of Mixed Polyolefin Waste to Methanol in Ghana

Abstract

Ghana generates 26.7 kg of plastic waste per capita annually, of which 12 kg is released into the environment, while only 9.5% is recycled. Gasification converts mixed and contaminated streams into syngas, from which methanol can be produced as a platform chemical for a range of fuels and polymers.

This work evaluates the process design, heat integration, and techno-economic performance of an integrated plastic-waste gasification-to-methanol process within the Ghanaian context. A mixed polyolefin feedstock containing 50 wt% PP, 30 wt% HDPE, and 20 wt% LDPE was considered as the base case at a plastic capacity of 75 t/day and compared with a co-gasification case containing 20 wt% palm kernel shell (PKS) biochar. Both cases were designed for the same final-product configuration and methanol production rate. The process was modelled in Aspen Plus using a pressurised oxygen–steam bubbling fluidised-bed gasifier, followed by syngas cleaning and conditioning, methanol synthesis, and purification. The gasifier operated at 30 bar with an overall equivalence ratio of about 0.30 and a steam-to-oxygen ratio of 2.45–2.78 molmol−1, with staged freeboard oxygen injection maintaining the temperature above approximately 980 ◦C along the freeboard.

Co-gasification displaced part of the plastic feed, so that 15% less plastic waste is treated for the same product output. Modelled carbon conversion was approximately 98% in both cases (98.4% and 98.2%), with cold gas efficiencies of 83.5% and 84.1%, respectively. Co-gasification produced a more CO-rich syngas and reduced the H2/CO ratio from 2.22 to 1.77. The heavy-tar concentration was limited to approximately 2 gNm−3 in both cases, while staged freeboard oxygen injection reduced the modelled naphthalene concentration by 86% and 82%, respectively. The lower H2/CO ratio in the co-gasification case required partial water–gas shift before CO2 removal to obtain the required methanol-synthesis stoichiometry. Both cases produced approximately 4.2 th−1 of Grade A methanol. The overall carbon retained in the final methanol was 60.9% for the base case and 57.4% for co-gasification, while the net energy efficiencies were 56.9% and 57.3%. Heat integration recovered approximately 19 MW of process heat in both cases, reduced the cold-utility demand by about 10%, and enabled approximately 1.2 MW of power generation.

At the base scale, the minimum selling prices for methanol were 761USD/t for mixed plastic and 792USD/t for co-gasification. At the base market, with weighted realised selling prices of 626 and 697USD/t respectively, the latter lifted by the compliance-driven willingness-to-pay for its biogenic share sold as EU bio-methanol, the NPVs were −32.0MUSD and −23.5MUSD, showing that neither case was economically viable under the reference assumptions. Increasing plant scale improved the economic performance, with co-gasification reaching a positive NPV at three to four gasifier trains, corresponding to 192–256 t/day of polyolefin feed and 100–134 kt/y of methanol.