Clearly, one of the major requirements for a hydrogen-based transportation system is the production of the hydrogen itself.
At the recent Hydrogen Expo USA in Washington, ChevronTexaco Technology Ventures (CTTV) and Modine Manufacturing presented a paper outlining their development of an innovative, distributed, natural-gas-fed, smaller scale Steam Methane reformer for hydrogen production.
In principle, ChevronTexaco believes that development of an economic distributed hydrogen infrastructure is fundamental to the success of a hydrogen economy and the commercialization of fuel cells in both transport and stationary power applications.
The new compact reformer design features mechanical as well as thermal integration of the steam reforming, catalytic oxidizer, and water-gas shift reactions in a single vessel. The design is thermally neutral and requires no external cooling and no control loops, and improves the energy balance of an SMR system.
The production design target for the system is approximately 40kg of hydrogen per day—enough to service a neighborhood’s worth of hydrogen-fueled cars.
The company concludes that its design has the potential to surpass the near-term targets set by the DOE for hydrogen cost: $13.94/GJ or $1.98/kg by this year. CTTV calculations yield a total reforming cost of $12.95/GJ or $1.84/kg.
However, despite the energy and economic improvements delivered through this design, the compact reformer still operates at a negative energy balance (i.e., more energy is used in producing the hydrogen than is obtained from the hydrogen), and hydrogen costs linked to the cost of natural gas seem bound to rise steeply.
Natural gas is only a short-term option.—Jack Johnston, ExxonMobil, GCEP presentation
CTTV used a hypothetical cost of $4.00/MMBTU of natural gas in its calculations. The spot price for natural gas at Henry Hub for the week of 13–20 April 2005 was, by contrast, $7.10/MMBTU. The industrial market hasn’t seen $4.00 natural gas since 2002.
This doesn’t reflect a problem with the CTTV design—as noted above (and as we’ll explore a bit more below), it is energy- and cost-efficient within its class. It does, however, highlight the larger problem of Steam Methane Reforming as a process with natural gas as its feedstock for the production of hydrogen.
Some quick background first.
Hydrogen is prevalent on earth, but is usually bonded to carbon or oxygen—e.g., “hydrocarbon” fuels, biomass, or water. It takes energy to break those bonds. There are a variety of processes for this, with more under development.
The US already produces 9 million tons of hydrogen per year primarily for use in ammonia production, petroleum refining, and methanol production, with steam methane reforming accounting for 95% of that production. Globally, the figure is closer to 48%.
The DOE notes that 9 million tons of hydrogen would power 20–30 million cars.
The basic Steam Methane Reforming process uses steam to heat natural gas to approximately 850ºC over a nickel catalyst bed, yielding a mixture of CO and H2O. The mixture is then cooled and catalyzed with steam again to yield pure hydrogen and CO2 (lots of CO2).
At this point, SMR is the most cost-effective process for hydrogen production. That will change, as the projection of the cost curves of select processes and feedstocks plotted to the right from Dr. Yogi Goswami at the University of Florida shows. (Click to enlarge.)
Shown on this graph is the DOE target price for hydrogen produced from natural gas in 2010: $10.56/GJ or $1.50/kg. The DOE has higher-priced targets for other processes.
Given the probable supply constraints with natural gas in the future, the cost outlook for natural gas may even shift further to the left—i.e., cost more, sooner.
There are a variety of process approaches that are being explored to reduce the energy imbalance in the production of hydrogen from natural gas, and to sequester the CO2 thereby generated. Little of that will be able to affect the cost aspect, however. That alone—should the price of natural gas continue to rise—may be enough to make this approach a non-starter in the medium- to long-term.
That also begs the question of where the 9 million tons (and rising) already spoken for in the US will come from, and how cost-effectively.
Gloom about the feedstock aside, the CTTV approach seems pretty interesting. The company’s key attributes for the reformer are:
Safe, robust and reliable
Low operating cost through improved fuel efficiency
Low capital costs through reduced system components and controls complexity
Manufacturable in high volumes
The process chemistry for small scale SMR is the same as in a large scale refinery, but the authors of the paper point out that there are severe economy-of-scale penalties.
Scaling the process down from larger systems results in greater heat losses that contribute directly to lower production efficiency, higher operating costs, and ultimately higher cost of hydrogen. To address these challenges, the project approach aims at developing a small scale SMR that is: (1) thermally and mechanically integrated to maximize heat recovery, minimize heat loss, and minimize balance of plant components, (2) able operate at pressure required for purification step to minimize electrical power consumption, and (3) thermally balance to achieve passive temperature control and to minimize the number of process control loops.
The CTTV team combined all process reactions and necessary heat transfer steps into a single, unitized vessel assembly. Concentric fin-type heat exchangers coated with catalysts allow the heat generated by the endothermic oxidizing reaction to be directly transferred to the endothermic steam reforming reaction.
The paper asserts that the maximum reformer hydrogen production rate is 55 kg/day, or 7,810 GJ of energy. The maximum natural gas consumption rate to achieve that production is 145 kg/day, or 8,051.85 GJ. In other words—you’re still running a net negative energy ratio—more energy goes in than comes out. Better than typical SMR systems, but still at a deficit.
NREL Technical Report 570-27637: Life Cycle Assessment of Hydrogen Production via Natural Gas Steam Reforming
Hydrogen Supply Technologies, Goswami