Techno-enviro-economic evaluations of biomass gasification for the production of ammonia from synthesis gas utilizing multi-scale modeling

The world is steadily moving toward greener alternatives of established chemical production processes. Process modeling and simulation play a vital role in conceiving, designing and optimizing these greener processes. In many cases, these simulation tools are limited to a few project design stages, thus limiting their true potential. This thesis presents the benefits of process modeling and simulation at different levels of process development. The work focuses on ammonia production, which sustains global agricultural and mining activities. Ammonia production has traditionally been based on large-scale plants. The thrust toward large-scale production in order to gain economic advantages has overshadowed the benefits that could be derived from small-scale production plants. Additionally, the ammonia industry is a major consumer of global fossil fuels, and in the process of providing ammonia, also burdens the planet with greenhouse gases. In order to ensure sustainable ammonia production, this thesis highlights the techno-economic advantages that result from small-scale ammonia plants based on biomass feedstock. The use of biomass as a potential feedstock can help neutralize the carbon footprint of ammonia production. To establish the feasibility of such a process, this thesis presents the application of process simulations at different modeling levels.

A Computational Fluid Dynamics (CFD) model was used to understand the flow patterns inside a dual fluidized bed gasifier. This elevated the understanding of the hydrodynamics of the gasifier freeboard, which is neglected by the conventional two-phase methodology. The CFD simulation was utilized to perform a Residence Time Distribution (RTD) analysis of the reactor. Four different tracer approaches namely the frozen velocity approach, the snapshot approach, the data sampling approach and the transient approach, were compared. The RTD analysis formed the basis of a steady-state compartment model that was developed in ASPEN Plus simulation software. The ASPEN Plus gasifier model decoupled the pyrolysis, gasification, and combustion sections of the gasifier to affect a better comprehension of the process and the results. The model predicated satisfactory results upon validation. Additionally, the model could also be used to predict the output for different biomass feedstocks.

ASPEN Plus is used to simulate the biomass gasification process as well as downstream gas conditioning that leads to ammonia production. Apart from major constituent gases, the model accounted for tar production in the gasifier process as well as for its subsequent removal. Three different configurations of the biomass-to-ammonia process, namely, Autothermal Reforming (ATR), Steam Methane Reforming (SMR), and CO₂ Reforming (CR) have been modeled and compared. The configurations differ on the basis of the reforming of the methane and tars, which are present in the syngas. The output of the ASPEN Plus simulation is fed into an MS Excel-based framework, which performs Life Cycle Costing (LCC) and Life Cycle Assessment (LCA) for the simulated flowsheet. Finally, these outputs are utilized by an MS Excel-/VB-based Multi-Objective Optimization (MOO) framework in order to optimize both LCC and LCA for changing process parameters.

Three biomass feedstocks, namely, wood, straw pellets, and bagasse, which are available in Australia, India, and Brazil, respectively, have been compared in order to understand the effect of biomass composition, the supply chain, and the national economic and environmental scenario on the viability of the biomass-to-ammonia process. The MOO results that were derived for changing flowsheet configurations as well as for different biomass feedstocks that were grown at different locations worldwide are expected to highlight a more rigorous analysis of the biomass-to-ammonia process. Such an analysis will provide decision makers with consistent and comparable data to objectively judge the viability of the proposed process. This analysis distinguishes this thesis from past studies that focused on ammonia production from biomass, and presented their LCC and LCA results for a single process configuration and for a single feedstock. The results predicted that different biomass feedstocks, process configurations, and geographical regions have their own advantages. The results provide a benchmark for comparison of other biomass-to-ammonia processes on a techno-enviro-economic scale. The methodology can be employed as an example in the development of different sustainable chemical production processes.