Thermal Hydrogen: A Formula for Efficient, Emissions-Free Coal Use

Unfortunately, the pure oxygen from electrolysis alone is unlikely to produce sufficient oxygen for all hydrocarbon combustion.  Fortunately, however, the methods required to maximize the productivity of oxygen are the same methods that solve the hydrogen distribution problem.

For example, solid oxide fuel cells (SOFC) provide a second source of pure oxygen and a pragmatic hydrogen distribution pathway.  SOFCs work using the same principles as other fuel cells: an ion (H+) crosses an electrolyte to form a new chemical (H2O), and the reunion creates an electrical charge (and heat).

SOFCs are unique because oxygen ions (O2-), rather than hydrogen ions, cross the electrolyte.  This is significant because it allows syngas (a combination of H2 and CO) to be used as the fuel rather than hydrogen.  Syngas reforming requires about half as much oxygen as hydrogen reforming.  Second, the versatility of syngas allows the hydrogen to be distributed through the gasoline infrastructure.  A ratio of 2 mol of H­2 to 1 mol of CO can be reformed into methanol (CH3OH), a liquid at standard temperature and pressure.

Methanol can easily be reformed back into syngas using the fuel cell’s waste heat.  The fuel cell’s products are limited to CO2 and H2O, also known as carbonated (sparkling) water.  The carbonated water would be stored on board and returned to the chemical grid for sequestration.

To distribute the hydrogen intended for combustion, a second trick is required using air separation units (ASU).  ASUs create pure oxygen by cooling air until liquid oxygen forms.  They are also very productive at producing nitrogen (3.76 mol of N2 per mol of O2).  The nitrogen is intended to convert hydrogen into ammonia (NH3) via the Haber-Bosch process.  Ammonia can then be distributed for combustion using the natural gas infrastructure system.

So, while the ASU can be considered a “cheat” because it is nitrogen separation, only ~30 percent of oxygen is sourced from this process.  The ASU’s coolness is another asset: it helps to provide the cooling to allow liquid storage of both oxygen and ammonia at low temperature.  At scale, low-temperature liquid storage is cheaper than high-pressure liquid storage.

A schematic of the thermal hydrogen system is shown in the figure below.

Figure 1: Thermal hydrogen system.

 

So, what’s in this for coal?  Well, of all the energy sources, coal has perhaps the most to gain.  Coal is the only fossil fuel largely limited to one sector, and this would allow expansion into the heating and transportation sectors.