System Analysis of Thermochemical-Based Biorefineries for Co-Production of Hydrogen and Electricity

Fuels derived from biomass feedstocks are a particularly attractive energy resource pathway given their inherent advantages of energy security via domestic fuel crop production and their renewable status. However, there are numerous questions regarding how to optimally produce, distribute, and utilize biofuels such that they are economically, energetically, and environmentally sustainable. Comparative analyses of two conceptual 2000 tonne/day thermochemical-based biorefineries are performed to explore the effects of emerging technologies on process efficiencies. System models of the biorefineries, created using ASPEN Plus®, include all primary process steps required to convert a biomass feedstock into hydrogen, including gasification, gas cleanup and conditioning, hydrogen purification, and thermal integration. The biorefinery concepts studied herein are representative of ‘near-term’ (ca. 2015) and ‘future’ (ca. 2025) plants. The ‘near-term’ plant design serves as a baseline concept and incorporates currently available commercial technologies for all non-gasifier processes. The ‘future’ plant design employs emerging gas cleaning and conditioning technologies for both tar and sulfur removal unit operations. Gasifier technology employed in these analyses is centered on directly-heated, oxygen-blown, fluidized-bed systems. Selection of the gasifier pressurizing agent (CO₂ v. N₂) is found to be a key factor in achieving high hydrogen production efficiency. Efficiency gains of 8-percentage points appear possible with CO₂ capture using Selexol or Rectisol-type processes. A 25% increase in electric power production is observed for the ‘future’ case over the baseline configuration due to improved thermal integration while realizing an overall plant efficiency improvement of 2 percentage points. Exergy analysis reveals the largest inefficiencies are associated with the (i) gasification, (ii) steam and power production, and (iii) gas cleanup and purification processes.