Thermal Hydrogen: A Formula for Efficient, Emissions-Free Coal Use
By Jared Moore, Ph.D.
With abundant natural gas supply available for the foreseeable future, coal must find new ways to compete. Though coal is still much cheaper, natural gas has fundamental advantages that enhance its competitiveness in the electricity sector: capital costs, efficiency and emissions. For coal to remain competitive under increasing emissions constraints, a bold new strategy may be necessary to create new energy carriers.
The economic motivation for new energy carriers is a compelling combination of supply-side economics (energy dominance), and energy and capital efficiency (“small is beautiful”). Energy dominance because cheaper local energy suppliers displace oil. “Small is beautiful” because of improved energy efficiency and reduced excess electricity capacity.
The thermal hydrogen system enables these advantages to be pollution free using hydrocarbons without carbon capture. All hydrogen is distributed using the existing infrastructure system. Here’s how:
In our modern economy, hydrocarbons are almost always combusted with air. Since air is ~80 percent nitrogen, the products of combustion (exhaust) are similarly diluted with nitrogen. Carbon capture is the technical term for separating the nitrogen and it is required to isolate CO2 for sequestration. The capture process makes up the vast majority of the costs for carbon capture and sequestration (CCS). In fact, the commodity value of CO2 might be greater than the sequestration costs (~$5/t CO2).
Thermal hydrogen is an energy system engineered to enable emissions-free hydrocarbons without carbon capture. This is accomplished by taking advantage of the chemical separation inherent to electrolysers and fuel cells. In both instances, chemicals are isolated when they cross an electrolyte, and with some clever engineering, these opportunities negate the need for carbon capture.
Electrolysis is the separation of H2O (or CO2) and is powered by excess electricity (and heat). The pure oxygen created from electrolysis is typically dumped. However, the pure oxygen is an opportunity to enable hydrocarbon oxidation in the absence of nitrogen – thus pre-empting the need for carbon capture.
The pure oxygen enables the simplest and most efficient thermodynamic cycles possible: the Allam cycle for electricity generation and auto-thermal reforming for hydrogen/syngas generation. Therefore, a source of pure oxygen can make hydrocarbons more competitive with decarbonization – not less.
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.
A move to the chemical sector can level the playing field for coal in terms of both efficiency and emissions. The reason for the diminished efficiency advantage between coal and gas is the endothermic nature of reforming. Regardless of hydrocarbons, similar chemicals (oxygen and steam) are required for reforming. These chemicals reform the char (carbon) and locked-in moisture of coal into syngas the same way they would reform natural gas. Coal reforming is only slightly less efficient than gas reforming because of the need to cool the gas to filter out the impurities. Coal gasification is becoming increasingly competitive. China is already using it to create methanol to be blended with gasoline (sometimes illegally). Therefore, it’s reasonable to believe that methanol would be strictly dominant if it could be used in a SOFC, which is twice as efficient as an engine.
The figure below quantifies the supply-side and efficiency advantages of an economy-wide implementation of thermal hydrogen. More technical details on this model are provided in the publication which has been peer-reviewed.[i]
Figure 2: Energy supply curves of the U.S. energy economy: a) 2014, b) thermal hydrogen.
Since the discovery of the value of coal, gas and oil, each has taken its turn facing the beginning of the end. Again and again, innovation finds ways to do it better – to increase supply and efficiency while reducing emissions. What is required is an open mind to view carbon for its energy density and versatility rather than as something inherently dirty. Thermal hydrogen shows that the only thing inherently dirty about fossil fuels, including coal, is how cheap they are. Therefore, it is quite possible that this is just the end of the beginning rather than the beginning of the end.
Moore, J, “Thermal Hydrogen: An Emissions Free Hydrocarbon Economy,” International Journal of Hydrogen Energy, 42 (2017) 12047- 12063.
Jared Moore, Ph.D. is an independent energy technology/policy adviser and president of Meridian Energy Policy.