(30a) A Graphical Approach to Assess Gasification: Novel Routes for F T Synthesis | AIChE

(30a) A Graphical Approach to Assess Gasification: Novel Routes for F T Synthesis


Ngubevana, L. - Presenter, University of the Witwatersrand
Hildebrandt, D. - Presenter, University of the Witwatersrand
Glasser, D. - Presenter, University of the Witwatersrand

The gasification of coal as a process is one of the most important in supporting our daily modern life, as it is used for electricity generation for our cities and in countries like South Africa; it is used for the production of CO and H2 (syngas) that is used as feedstock for the production of fuels through the Fischer Tropsch (FT) process.

It is of the utmost importance that we have a proper understanding of the gasification process; as optimising this process can help in the production of cheaper electricity, optimising the FT process and therefore lessens the dependence on natural gas and oil. There are a number of ways that one can interpret and optimise gasification and the purpose of this paper is to use graphical techniques to do this without the use of experiments. We are currently working with the Golden Nest Company to build a demonstration FT plant in China and we are using this approach in its design.

It is envisaged that we can use the fundamental principles of mass balance, energy balance, reaction equilibria and thermodynamics, to predict and therefore to optimise gasification under specific conditions. Employing these principles together with defined possible chemical reactions in the gasifier and arbitrarily chosen reaction extents, one is then able to achieve the objective and that is to optimise the feed composition to the process and the process conditions (temperature and pressure), so as to achieve a specified composition of syngas. There were eight possible reactions that were chosen and two of those were assigned reaction extents e1 and e2 and these were the syngas production from carbon and steam reaction (gasification reaction), and the other was the water gas shift reaction (WGS), respectively.

In solving the problem, the assumptions made were, the amount of carbon fed into the reactor was just enough for the production of syngas and for combustion with oxygen and that the gases in the system exhibit ideal gas behaviour. The feasible region of gasification was determined in three ways; firstly a mass balance was performed on the reaction components (hydrogen, steam, carbon monoxide, and carbon dioxide; and this can be viewed as an atom balance on hydrogen, carbon and oxygen), with the use of the defined reaction extents. Secondly, the reaction equilibrium constants were defined making use of the defined reaction extents and the ideal gas behaviour assumption and finally the energy balance was set up; also making use of the defined reaction extents and the resulting equations were solved numerically and their respective curves were plotted. The area within the abovementioned curves then gives us an idea where gasification is feasible. The feed composition and the process conditions were varied so as to investigate the effects this would have on the feasible gasification region. Also investigated were the H2:CO and the H2:CO2 ratios and the purpose of this was to determine the ?best' feed composition for the optimum production of hydrogen that is needed as a reactant in the FT process.

The results of varying the feed composition can be summarised as follows, increasing the amount of steam and increasing the amount of oxygen have the effect of increasing the feasible region but these have the consequence of high energy losses from the gasifier and this energy is lost as either temperature or large amounts of unreacted steam. It is also found that at relatively low temperatures, carbon deposition becomes a problem and therefore high temperatures are necessary. There are however in real world operations, limits to how high the temperatures can go depending on the nature of the coal used. An overall analysis of the reaction equilibria shows that high pressures are also necessary as this minimises carbon deposition and methane production, but this also has limitations such as downstream pressure requirements. As an overall result of the analysis it was concluded that graphical techniques can be employed to understand gasification.