Optimization of the Automotive Ammonia Fuel Cycle Using P-Graphs

Ammonia is a potential low-carbon alternative automotive fuel. However, it is produced commercially via the energy- and greenhouse gas-intensive Haber-Bosch process, and its nitrogen footprint may also detract from its environmental benefits. Thus, whether its use as an automotive fuel is sustainable from a life-cycle standpoint remains in question. In this study, a P-graph model is developed to determine the best well-to-wheel pathway for the use of ammonia as an automotive fuel, using carbon and nitrogen footprints as dual environmental criteria. Multiple fossil fuel-based and biomass-based ammonia production processes are considered, as well as different drivetrain configurations that include internal combustion engine vehicles (ICEV) and fuel cell vehicles (FCV). In the case of ICEV, the model also considers the secondary fuels needed to allow ammonia use in existing engines. Solving the P-graph model identifies the optimal pathway as cyanobacteria-based ammonia production coupled with FCV. This pathway has a carbon footprint of 4.96 g CO2 equiv/km and a nitrogen footprint of 0.325 g reactive N/km. The model also identifies a cluster of near-optimal solutions, for which possible technology improvements are discussed.