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. ... Read more