Design and Analysis of Zero CO2 Emission Powerplants for the Transportation Sector

Hydrogen fuel cell powered vehicles provide a feasible pathway to elimination of CO₂ emissions from the transportation sector if the hydrogen is produced from renewable energy sources, or the CO₂ from hydrogen production is sequestered on a large scale. The lack of a hydrogen distribution infrastructure and the lack of dense hydrogen storage technology are fundamental roadblocks along this path. One alternative approach is to use a high energy-density liquid fuel (natural or synthetic, such as methanol) as an intermediate hydrogen carrier, and generate the hydrogen on demand in an onboard fuel processor. This demands, however, development of technologies for on-board CO₂ capture, storage, and recycling to eliminate direct emission into the atmosphere. This paper presents a thermodynamic analysis of feasibility of on-board carbon dioxide sequestration as well as various process/design schemes for the hybrid power generation-CO₂ sequestration system. The primary difficulty in capturing CO₂ from small-scale power plants (such as the internal combustion engine) is the extremely diluted state of CO₂ in the exhaust gases. In contrast, onboard fuel processors have the potential to provide a highly concentrated CO₂ exhaust stream, which could be separated, liquefied, and stored onboard at ambient temperatures with a minimal energy penalty. Current research efforts in small scale fuel processing are focused on producing a hydrogen-rich (or pure) stream from liquid hydrocarbon fuel with high yield and at a sufficient rate to provide the necessary vehicle power. Very few efforts reported in the open literature also address the need to capture the byproduct CO₂ that is produced. The additional requirement of CO₂ capture calls for fundamental change in the fuel processing strategy and reformer design. Several process or design schemes for fuel processing are identified, which produce hydrogen while allowing for CO₂ capture. For example, in autothermal reforming of hydrocarbon or alcohol fuels, catalytic reactions of the fuel with air yield a product stream (hydrogen and CO₂) that is diluted with nitrogen. Under the added constraint of CO₂ capture, advanced oxygen membranes could be used to supply pure oxygen rather than air to the reaction, resulting in a more concentrated, nitrogen-free product stream which is favorable for CO₂ capture. Simultaneously, this improves the efficiency of downstream hydrogen purification and utilization processes; thus, the penalties associated with CO₂ capture are partially offset. In a similar manner, steam reforming of liquid fuels may not be the most attractive fuel processing option for automotive applications without consideration of CO₂ capture. However, because the product stream is never diluted with air, it becomes a very attractive option for integrated fuel processing/CO₂ sequestration systems. Consideration of CO₂ capture early in the design stages of the fuel processing system allows a portion of the energetic penalty for CO₂ sequestration to be recovered. While the design, analysis, and demonstration of an integrated onboard fuel processor with CO₂ capture and storage is the ultimate goal, this technology is relevant to all small-scale, distributed power generation applications and should be an integral part of future CO₂ abatement strategies.