By Frank Zeman
Much has been said about the potential benefits and consequences of the industrial process known as fracking, discussed previously in this section. Although highly controversial, fracking is already widespread and is having significant effects on energy production in the United States. By not only opening up shale gas reserves but increasing production in the oil sector, fracking has dramatically increased the availability of natural gas and reversed the decades-long decline in U.S. oil production. The consequences are significant—for example, the need to effectively treat produced water could mean significant costs for upgrading capacity and capabilities at the water treatment facilities handling the water. Still, for the first time since record keeping began, natural gas generated as much electricity as coal (April 2012). A mild spring and expanding wind power production (natural gas is its backup) also have had an effect, but the charts presented by the Energy Information Agency are shocking. This result is linked to the precipitous drop in natural gas prices, which then sat at 10- year lows of less than $2 per million BTU. This price level was also seen in the fall of 2005 and winter of 2000. One unintended consequence of this price drop is a seismic shift in the world of hydrogen.
Hydrogen is often thought of as the miracle fuel at the heart of the hydrogen economy—a vision of ultra efficient cars emitting only water vapor, with solar panels blanketing the built environment and wind turbines turning silently on the horizon. Electrolysis is the preferred means of hydrogen production because it is a closed loop, with water being split to produce the hydrogen that is recombined in the engine to produce energy. A critical factor in the benefits obtained from this system is the source of the electricity. Unlike fossil fuels, which are found and then refined, hydrogen must be manufactured. If the cycle is closed (water to water), then at best you get out as much as you put in. If we consider a 70% efficient electrolyzer and a 70% efficient engine, then we’d have to put in twice what we get out. Although this is more efficient than gasoline in a car, we have to consider the source of the electricity to fully identify the benefits. If it’s coal, then only one-third of the energy in the fuel fed to the plant leaves as electricity, reducing the efficiency to 15% (comparable to gasoline) and only marginally reducing emissions because coal has a higher carbon signature. So the fracking revolution’s increase in natural gas production appears to be reducing our coal consumption (even as coal exports increase), thereby improving the footprint of hydrogen (based on grid averages) but not the cost. The high cost is the main reason almost no hydrogen is produced via electrolysis in the industrial sector.
Hydrogen is produced today through a process called steam methane reforming. As the name implies, the energy required to produce the hydrogen is taken from the stored chemical energy in the methane. Methane itself consists of a carbon molecule with four hydrogen atoms attached. The hydrogen of the hydrogen economy is often a gas consisting of two hydrogen atoms bound together (H2). So if you could simply disassociate the methane, you could produce solid carbon and two molecules of hydrogen gas, known as thermal cracking. An alternative (and the most common method) is to selectively convert the carbon in methane to CO2 by reacting it with water (H2O). By allowing the carbon to strip the water of its oxygen, the process also produces two additional molecules of hydrogen, bringing the total produced to four. In essence, this is the same mechanism as the hydrogen economy with the substitution of chemical energy from the carbon in place of electricity in the electrolyzer. Both options require catalysts to assist the reactions.
Thermal cracking is not as widespread as steam methane reforming because it requires energy and high temperatures. The advantage of the process is the absence of CO2 in the product stream. The process is likely not emissions-free, though, because the most obvious source of heat for raising the process temperature up to 2000 degrees Fahrenheit is methane. So there are so-called fugitive emissions associated with the process, but no CO2 from the reaction. The produced carbon could be used industrially or added to the soil layer as a means of carbon sequestration or fertilization. There are ways to provide the heat without creating CO2, such as solar thermal or resistive heating. Solar thermal heat, via concentrators, will limit the location of the facility and will likely be very expensive (a combination of the cost of heliostats and intermittency of the sun). Alternatively, electricity can simply be dumped into the reactor, much like a toaster, to maintain temperatures. Here again the fugitive emissions associated with the electricity source are important as well as the cost. Ironically, such a system might provide economic support for wind energy because it could consume the excess electricity produced during periods of high wind or low demand.
Steam methane reforming was first described back in 1868 and is the industry standard today. It consists of two steps: the reforming reaction and the water gas shift (WGS) reaction. The reformer reacts steam with methane and also operates at high temperatures (1500 degrees Fahrenheit), although this is slightly lower than needed for thermal cracking. The WGS reactor operates at much lower temperatures (600 degrees Fahrenheit) and often in two stages (high- and lowtemperature shift). The resultant gas stream contains H2 and CO2 along with small amounts of unreacted methane and carbon monoxide. Either pressure swing adsorption or amine scrubbing can separate the main components of the gas stream. In pressure swing adsorption, the gas stream is pressurized and then passed through a bed of material that selectively adsorbs the target gas (H2). Once the bed is saturated, the gas stream is shifted to a second bed and the first bed is de-pressurized, thereby releasing the gas in pure form. The name is derived from the fact that each bed swings between high and low pressures, alternatively adsorbing and releasing the H2. Amine scrubbing is a chemical system most often associated with capturing CO2 from power plants. It is a liquid-based system in which CO2 is dissolved in one column and then released, via steam heating, in another. Either way, the H2 can be purified to levels suitable for use in a fuel cell vehicle or other device.
So now that H2 is readily available and at a lower cost, why would we want fossil H2? The reason is that there are health benefits to making the switch that depend on the source of hydrogen. The acute health benefits stem from the change in technology, from internal combustion to fuel cell. The combustion of gasoline and diesel in air leads to a whole host of pollutants (sulfates, NOx, particulates, volatile organics) that in turn lead to a host of ailments (headaches, sore throat, asthma, chronic illness). One economic estimate suggests that the benefits of reducing these category pollutants could range between $10 and $200 billion annually1. This is aside from any climate concerns. Adding in these climate costs— those for emissions of CO2, CH4, black carbon, and particulates—sees the savings jump to between $30 billion and $250 billion per year. This is the cost equivalent of a gas tax ranging from $0.20 to $1.60 per gallon. Overall, the switch to fuel cell vehicles running on hydrogen from natural gas could reduce mortality in the United States by almost 5%. Hydrogen via electrolysis from wind power is estimated to be more beneficial but also more costly.
Fuel cell vehicles are also more costly, as is the infrastructure needed to produce hydrogen from wind and distribute it to the refueling stations. In a sense, this is the quandary of renewable or clean technologies. The newer products (generally more expensive owing to immaturity) require additional infrastructure (more cost) and have a hard time competing with existing technologies (capital is retired). Now that the million-dollar Honda Clarity has come and gone, hopefully newer versions offered by Mercedes and Toyota, among others, will reach the market at more competitive prices. It will make a big difference if the vehicle can be fitted with an on-board reformer and charged with natural gas (even from home). Lowering the overall cost could help market penetration, and cheap natural gas will be a big part of that. The current price of natural gas is $2.737 per million BTU and gasoline is $3.411 per gallon, which can be converted to $2.59 per gigajoule for natural gas and $25.84 per gigajoule for gasoline. So natural gas is onetenth the cost of gasoline on a per unit energy basis, and fuel cells are expected to be more than twice as efficient as the internal combustion engine. The elephant in the room is the cost of externalities, currently borne by the taxpayer, not the car owner. When societal health costs are even rudimentarily factored in, natural gas in fuel cell vehicles is the future today.
Posted on: August 6th, 2012