MEDIA BACKGROUNDER:

Fuel Cell Auxiliary Power Unit (APU) to Reduce Heavy Duty Truck Engine Idling

CONTEXT

Idling of heavy-duty diesel engines is a common practice in the trucking industry. Long distance truck runs typically include rest periods of between 6-8 hours per day. During these periods, the engine remains running at idle to provide heating and cooling for the cab and sleeper compartment, heat to keep the engine oil warm in cold weather, and electricity to power on-board equipment and appliances. Idling of the truck’s large-displacement diesel engine is an extremely inefficient way to generate heat and electricity. Although some manufacturers offer small-displacement diesel auxiliary power units to provide heat and electricity and reduce the need for the large-engine idling, non-polluting, high efficiency fuel cell units may provide a better solution.

PROBLEM

The EPA estimates the up to 500,000 trucks will consume a total of 1.2 billion gallons of gasoline during rest period idle conditions. Engine idling is very inefficient (< 10%), which contributes to the problem. This results in a cost of $1.8 billion (at $1.50 gallon/diesel) and produces 11 million tons of carbon dioxide (C02) and approximately 150,000 tons of nitrogen oxides (NOx) per year.
Fuel cells offer a high-efficiency, low emission, and low-noise alternative that would supplant the need for truck engine idle. At efficiencies approaching 50 percent, these fuel cell auxiliary power units (APU) could dramatically reduce fuel consumption, cost, and pollutant emissions. However, fuel cells require hydrogen or a hydrogen-rich fuel gas that also must be free of impurities that can "poison" the cell. These impurities, particularly sulfur, are present in common diesel (DF-2) fuel.
In order to process diesel fuel into a suitable gaseous fuel for the fuel cell, the following commercially available processing steps are typically employed:

1. Hydrogenation – Diesel fuel is mixed with hydrogen and vaporized. This mixture is fed to a hydrogenation reactor to convert the organic sulfides found in the liquid petroleum fuel to hydrogen sulfide. This is done to more easily remove the sulfur compounds. Hydrogen is obtained by separating it from the reformate gas by utilizing pressure swing adsorption (PSA).
2. Desulfurization – Once the organic sulfides have been converted to hydrogen sulfide, a guard-bed of zinc oxide can be used to absorb them. The sulfur needs to be reduced down to very low part per million concentrations because of the poisoning effects on both the reforming catalyst and fuel cell components.
3. Reforming - Once the diesel fuel is desulfurized, it is reformed into a hydrogen-rich synthesis gas that contains H₂, CO, CO₂, H₂O, and possibly some methane if desired.

Although effective, the equipment cost, energy requirement, and required footprint are greater than desired. This is especially true for applications like truck APU’s where onboard processing/conversion of the fuel is required. Converting DF-2 to a clean, fuel cell ready gas requires a new fuel processing technology that can meet the cost and space requirements presented by a heavy-duty truck APU’s.

RESEARCH/SOLUTION

The National Energy Technology Laboratory (NETL) is supporting the development of new technology to provide suitable fuel gas to the fuel cell for a variety of hydrocarbon fuels with an emphasis on diesel. This development is being accomplished through sponsored research and development (R&D) of DOE’s Solid State Energy Conversion Alliance (SECA). The SECA program is a focused development effort to produce low-cost, mass-manufactured fuel cell technology. By making technology applicable to multiple markets/applications, costs can be driven down through high volume production. Fuel processing is a key component to this program.

NETL is also conducting a variety of onsite R&D projects to support the effort. System or technology integration studies allow for identification of novel compact, high-efficiency, and low-cost systems. Fundamental studies to better understand and model hydrocarbon reforming are being conducted. Desulfurization technologies are being evaluated and developed to remove sulfur from the fuel to avoid "poisoning" of the reforming and fuel cell catalysts. Identification of sulfur-tolerant reforming and fuel cell anode catalysts is also being conducted to mitigate the need for extensive sulfur cleanup.

According to NETL senior scientist David Berry, "fuel processing of diesel fuel for fuel cell applications is an extensive challenge and a key to commercialization of the technology. Through a highly coordinated effort among the private sector, national laboratories, universities, and other federal agencies, it is envisioned that successful technology development will be achieved."