Coal-to-Synthetic Natural Gas and Hydrogen

In addition to liquid fuels, gasification can be used to produce gaseous fuels like synthetic natural gas (SNG) and hydrogen (H₂). SNG is equivalent to natural gas, which is mostly methane, and can be substituted for it in natural gas applications. Hydrogen is predicted by some to be the energy carrier of the future because it is extremely clean when reacted with oxygen (producing water) and has a high energy density by mass. Hydrogen can be used to feed fuel cells or combusted in a hydrogen turbine to generate electricity. Hydrogen could also power fuel cell vehicles. Although there are technical challenges to overcome, a clean coal gasifier to produce H₂ would be a key component of a hydrogen economy.

	Great Plains Synfuels Plant, Beulah, ND. Photo courtesy of Dakota Gasification Company.

Market
SNG has a large potential market: essentially any application that currently uses natural gas could use SNG. In particular, gasification can be used on-site for industrial applications to produce SNG (and electricity, if necessary), allowing continued operation of natural gas equipment but from a coal source. The Department of Energy‘s Energy Information Administration (DOE/EIA) reports in the Annual Energy Outlook 2010 that in 2008, United States industrial use of natural gas was about 34% of the total domestic consumption . A 2007 NETL study looked at the feasibility of on-site gasifiers in industrial facilities for the production of SNG and found that many industrial sites could benefit from the use of relatively small gasifier systems to either produce SNG, power, H₂, or synthesis gas (syngas).

The Great Plains Synfuels Plant (GPSP), a coal-to-SNG gasification plant located in Beulah, North Dakota, has been operating since 1984 and produces more than 54 billion scf of SNG per year. The GPSP also produces ammonia for use as fertilizer and pipes captured CO₂ to two Canadian oil fields for Enhanced Oil Recovery (EOR).

Hydrogen production is already a large industry, with most of the produced H₂ used in refineries for hydrocracking—where H₂ is used to break heavy petroleum products down into lighter, more useable hydrocarbons—and in the production of ammonia (for use as a fertilizer). In 2004, 50 million metric tons of H₂ was produced globally, with the United States producing and consuming around 19% of that. 95% of U.S. production was done at the site of use. In the United States, for the year 2003, 53% of H₂ was used by refineries, 34% for ammonia synthesis, and the remainder for methanol and other industrial applications. Worldwide, H₂ is used more for ammonia production (57%), than refining (27%), with an additional 10% used for methanol production. As refining applications, such as the hydrocracking of heavy tar sands, increase, the near-term market for H₂ is expected to grow. Hydrogen for use as a transportation fuel is still a long way off, with some models predicting slow growth starting in 2015, but H₂ for other fuel cell applications, particularly electricity generation, may be more near-term. For more information on current and future hydrogen markets, please consult the report DOE’s Argonne National Laboratory, located in the References/Further Reading section below.

Advantages
SNG and H₂ can be produced by gasification of coal or other carbonaceous-fuel sources. Coal is abundant domestically, with current estimates predicting over two centuries worth of consumption at present rates. SNG is able to substitute for natural gas, a much scarcer and more volatile commodity than coal. In this way, gasification to SNG helps increase fuel diversity, protecting against an over-reliance on a single energy source. As SNG in use is identical to natural gas, the existing natural gas infrastructure can still be utilized.

	The combustion of hydrogen and oxygen produces a nearly invisible flame as seen in this photo of the Space Shuttle's main engine. The clean reaction generates only water and heat. Photo courtesy of NASA.

When fuel cell technology matures for transportation or electricity, these same advantages in fuel diversity will apply towards gasification of coal-to-H₂. A hydrogen-based transportation system would be extremely clean, with most emissions stemming from the production of the hydrogen itself. The actual generation of electricity using hydrogen fuel cells produces only water as a byproduct. Gasification has the potential to cleanly produce H₂, using pre-combustion capture of harmful emissions.

Additionally, production of SNG and H₂ is well-suited to co-generation within an integrated gasification combined cycle (IGCC) plant or other syngas application. Gasification plants could divert syngas from electricity generating turbines during off-peak periods to produce SNG or H₂. The Great Plains Synfuels Plant, for example, diverts some syngas away from SNG production to produce ammonia during periods when the fertilizer is in high demand.

Challenges
In a 2007 NETL study, potential industrial customers of coal-to-SNG gasification for onsite use in natural gas applications indicated that reliability (and hence, availability) is important and needs to be near 100%, either through increased performance or redundancy. Some applications are able to also fire oil, allowing for onsite storage of backup fuel. Availability is still a challenge for gasification, although some sites have achieved very high availability. The Great Plains Synfuels Plant, for example, has consistently produced 90 to 92% of its rated output capacity.

Producing SNG from coal is still more expensive than the natural gas it would replace. For this reason, the aforementioned 2007 NETL study focuses on locations and applications where the gasifier could be integrated with an industrial process  that uses natural gas. This would help economics and would allow the facility to guard against fluctuating natural gas prices. The coal transport infrastructure is well-developed, and coal is both abundant and relatively inexpensive.

Another challenge to coal-to-SNG or H₂ gasification is in transporting a gaseous fuel, which can be difficult because of the gases’ low densities. SNG must be cooled and then compressed for transport through a close-to-capacity pipeline infrastructure. In addition, pipelines are restricted by geographical features like oceans, for example. It can be liquefied (called Liquefied Natural Gas or LNG) for transport by ships or tanker trucks.

Hydrogen is even more difficult to transport. In fact, even liquefied H₂ is four times less dense than liquid gasoline and just to reach liquid state it must be cryogenically cooled and greatly compressed. Current hydrogen-powered test vehicles store compressed hydrogen at 700 bar (over 10,000 psi; almost 700 times atmospheric pressure ). Transporting H₂ through pipelines is also difficult because of its low density and high flammability. Finding a way to economically store and transport hydrogen is a major challenge to deployment as H₂ is a gaseous fuel.

More general challenges to gasification are discussed in the Challenges section.

Research and Development
In order to improve capital and operating costs, availability, decrease plant complexity, and, in general, address the challenges described above, there are several avenues of research and development (R&D) underway. These are discussed in depth in the section Gasification Research and Development.

The NETL has a Hydrogen and Clean Fuels Program dedicated to R&D towards the production of H₂ from coal with near-zero environmental emissions. A brief summary of the program, which includes public and private research coordination, is included below, but please visit the program’s site for full details.

The Hydrogen and Clean Fuels Program is split into four areas:

• Central Hydrogen Production
• Alternative Hydrogen Production
• Utilization
• Systems Studies

Central Hydrogen Production focuses on producing H₂ directly from coal gasification while Alternative Hydrogen Production looks at converting coal to a high-hydrogen-content liquid hydrocarbon, alcohol, or SNG (as discussed previously) for delivery—see Challenges above for a note on the difficulties transporting hydrogen. Both areas conduct research and development on lowering the cost and raising the efficiency of unit operations. This includes improving gas cleanup, heat exchange, water-gas-shift reactors (to increase hydrogen content), separation processes, and integration. Additionally, the Alternative Hydrogen Production area conducts computational analyses to identify optimal intermediate products, reaction pathways, and catalysts.

The Utilization group oversees research on the use of H₂, in fuel cells for example. Fuel cell technology has several challenges to overcome before widespread adoption, discussed on the Fuel Cells section. In the interim, the group is evaluating the use of hydrogen/natural gas mixes in special reciprocating engine designs (the homogenously charged compression ignition engine, for example). Lastly, Systems Studies conducts overall analyses related to the effect of H₂ from coal. This includes studies like:

• Comparison of Current and Future Technology for Hydrogen from Coal
• Comparison of Hydrogen Production Costs from Coal and Natural Gas
• Costs of Producing Hydrogen from Liquid Fuels from Coal

More on each of these areas of research and development and the Hydrogen and Clean Fuels Program, in general, can be found on NETL’s site.

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