Process flow sheet synthesis: Reaching targets for idealized coal gasification

The gasification of coal is a process that has been commonly used to produce a mixture of gases containing primarily carbon dioxide and hydrogen, called syngas. This syngas is used as an intermediate in the production of many chemicals such as ammonia, synthetic hydrocarbons, and methanol (to name a few). Coal gasification has a reputation for being “dirty” in terms of its emissions in comparison with other syngas creation technologies, such as methane reforming. However, there is remarkably little information on what the “best case” for coal gasification could actually be and how existing process perform relative to that “best case.” The goal of this article is to formulate a preliminary and conceptual flow sheet for the gasification process; this flow sheet is not intended to be a finalized design or a definitive solution. It is intended to illustrate the method of setting and achieving design objectives and provide a basis of comparison for either new or existing processes. Thermodynamics can be used to describe any process, or system of processes. Of particular interest are the properties of enthalpy and Gibbs free energy. Using these two thermodynamic properties together as vectors on a diagram of free energy (ΔG) against enthalpy (ΔH), it becomes possible to develop better process flow sheets that combine the thermodynamics of chemical reactions and the dynamics of physical operations on a single diagram. This article will discuss the selection of the independent mass balances that best describe the process as a whole, then the choosing of design objectives, how these objectives might be achieved, and their implications for the process as a whole. Using these ideas one is able to show how to improve the carbon and operating efficiency of a gasification process, making the process more reversible. It was found that there will always be a price to pay for using coal as the feedstock for creating synthesis gas but there is room for improvement, most notably in how combustion is carried out and how energy is used internally within a process. © 2014 American Institute of Chemical Engineers AIChE J, 60: 3258–3266, 2014