(436b) Biomass Based Sustainable Ammonia Production
In this work, a systematic methodology to optimize the biomass to ammonia process and to evaluate its economics and environmental performance is presented. Switchgrass is selected as raw material due to its large potential as a bioenergy crop. The biomass is treated following two paths: gasification and anaerobic digestion. Three gasification alternatives have been considered: indirect gasification, direct gasification with oxygen and steam and direct gasification with air and steam. Two different reforming alternatives have been studied: autothermal reforming and steam methane reforming. Finally, two different ammonia configurations have been presented: direct cooling and indirect cooling multibed reactors. Different approaches to the modelling issue have been employed: first principles, equilibrium conditions, surrogate models, industrial data, etc. The objective function consists of a simplified evaluation of the profit. The entire problem is decomposed and formulated in GAMS as a set of NLPâs for each major alternative where the decision variables are the operating conditions of the gasifiers, the synthesis reactors, etc.
The results show that the most promising alternative to produce ammonia from biomass consists of the combination of indirect gasification with steam methane reforming and direct cooling ammonia reactor. This technology presents an investment of about 316 MMâ¬ and a production cost of 380â¬ per ton of ammonia. The gasifier processes present better economic results than the digestion ones due to the larger yield to product from the biomass while the digestion generates a large amount of unconverted digestate that it is not transformed into ammonia. The environmental index shows a release of the carbon dioxide associated with the process to be about 1 kg CO2 per kg of ammonia, approximately one fourth of the current values. Finally, a sensitivity analysis based on the switchgrass price and a scaling study for the plant is also presented.
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