Hydrogen and Combined Heat and Power Production from the Steam Gasification of Plastic Wastes. An Experimental and Modelling Analysis Including Techno-Economic and Environmental Assessment

Abstract

In this work a modular high temperature (850°C to 1150°C) steam gasification process (DMG®) for production of hydrogen, to be used mainly as a clean fuel in the transport system, and combined heat and power production from mixed unrecyclable plastic wastes is studied which is being developed for commercial production by Powerhouse Energy group (PHE). This includes experiments, process modelling, techno-economic and environmental assessments. Three main scenarios were defined for the process operating conditions which include production of 2 tonnes per day (TPD) hydrogen as the first scenario, the second scenario for maximum conversion of the carbon content of the feedstock to gas products, and the third scenario for the maximum hydrogen production. It should be noted that in the second scenario the maximum conversion of the carbon content of the feedstock may not be necessarily 100% conversion, or it may not be possible in a self-sustained process (in terms of energy). Therefore, this scenario is investigated to identify the possible process conditions and outputs for this scenario. An Aspen Plus model was developed for the process and was further calibrated and validated using experimental data obtained from a pilot-scale unit. The model was used for sensitivity analysis for a commercial unit with 35 TPD feed capacity based on the design capacity of the first commercial unit. The modelling results are used to define the optimum process conditions in each defined scenario. Then, each scenario was defined in the developed model for four process temperatures of 850°C, 950°C, 1050°C, and 1150°C. The results from process modelling including mass and energy balance are used for techno-economic analysis and environmental assessments. In techno-economic analysis, three approaches are considered, including fixed sale prices for all products and by-products, considering electricity sale price to domestic, and non-domestic customers, and performing economic evaluation of the process for all scenarios. In the environmental assessment two alternatives of landfilling and incineration to DMG® are defined and the greenhouse gas (GHG) emissions from each alternative are assessed for the same capacity and a comparison between the alternatives are made. Finally, the optimum process condition and sale strategy for the main products are suggested. Based on the results of the performed analyses, it was concluded that using the DMG® technology for production of hydrogen, electricity and heat can be economically viable and that hydrogen with a competitive sale price in compare with hydrogen produced from the available alternative technologies in the market can be produced. Also, it was concluded that DMG® has less environmental impacts in terms of GHG emissions than those from incineration, and considering the plans for banning landfilling of plastic wastes, DMG® can be a good alternative. The specific results are discussed in the result and discussion section and summarised in the conclusion chapter.