Alternative Routes for Ethanol Production from Renewable Resources

Alternative Routes for Ethanol Production from Renewable Resources: A Thermodynamical Approach

Mulder, Jan-Maarten

de Swaan Arons, J. (mentor)
van der Kooi, H.J. (mentor)
Dewulf, J. (mentor)

Applied Sciences

Applied Thermodynamics and Phase Equilibria

Chemical Engineering

2000-03

In a sustainable technological context it is expected that organic chemicals have to be produced from biomass. However, the question can be raised if enough biomass can be produced for this purpose, and even more so if the world's population doubles in the next forty years and with a living standard of that of today in western Europe for all people. Furthermore, an important bottleneck is the low conversion efficiency of solar exergy into biomass of 0.71 %. However, in a sustainable world all processes must be driven by solar exergy that is converted as efficiently as possible into electricity, other forms of work or into chemical compounds. At the moment the conversion efficiency of multicrystalline silicon solar panels is 12.6 %, which is more than an order of magnitude larger than that of photosynthesis. Based on this, two different production routes were compared on the basis of exergetic efficiency, sustainability, cost price of the products and environmental pollution expressed in the Eco-indicator 99. The first route is the so-called biomass route where wheat crop is grown and harvested yielding straw and grain. Wheat grain is hydrolysed and by fermentation partly converted into ethanol. Other products are cake and gluten. In the second route electricity from multicrystalline silicon photo voltaic cells is used in the electrolysis process of water to produce hydrogen and oxygen. Hydrogen and carbon dioxide react over a catalyst to produce ethanol but also methanol, methane and carbon monoxide are produced. Carbon dioxide is obtained by monoethanolamine absorption from flue gases of a power station. Both routes are not completely renewable, it has been tried to increase the renewable resources content considerably while keeping in mind present available technology. The results of the exergy analysis show that it is (roughly 10 times) better to produce ethanol via the electrolysis route than via the biomass route. Next, a sustainability coefficient has been based on three aspects: First, the fraction renewables of all (exergetic) inputs. Secondly, the exergetic efficiency of the process. The last aspect concerns the closing of material and has now only been calculated from the amount of carbon dioxide that results from non-renewable inputs in the process. Furthermore it is assumed that carbon dioxide is stored in empty natural gas fields or in sandstone formations below the North Sea. Thus the carbon cycle is not closed, and this has been done to be able to perform the calculations and to be in line with present available technology. The biomass route and the electrolysis route have respectively 99.8% and 90.7% renewable inputs. However, the electrolysis rout has a higher production efficiency. The economic evaluation shows that ethanol produced from the electrolysis route is the most expensive mainly due to the production costs of current solar cells. According to the Eco-indicator 99, the electrolysis route scores as the most environmental unfriendly, in contrast to the sustainability parameter. Electricity produced by solar cells is not necessarily completely renewable, but the material cycles have to be closed using renewable exergy and solar radiation is the only renewable exergy source on Earth. The results of this comparison can serve as a guide for industry in deciding which technology should be followed in the future and what kind of research and development work has to be done. It also shows that thermodynamics and especially the concept of exergy can play an important role in the determining sustainable development in a technological sense.

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Student theses

master thesis

(c) 2000 Jan-Maarten Mulder