Synthetic natural gas

Figure 1. A coal gasifier used in the production of synthesis gas that is then used to make synthetic natural gas.

Synthetic natural gas is a gaseous fuel that can be created from coal and other carbon-containing feedstocks and serves as a substitute for natural gas. Synthetic natural gas is suitable for transmission in natural gas pipelines because its chemical composition is practically identical (although its history is different). This natural gas substitute typically contains at least 95% methane in it. An intermediate step in the process of creating synthetic natural gas is the production of synthesis gas, also known as syngas.

Natural gas is a major component of the world's energy supply, as it is used widely in residential, commercial, and industrial applications. However, synthetic natural gas can be desireable in regions where natural gas supplies are limited or unavailable. This type of natural gas is desireable because it has combustion characteristics similar to natural gas, and because of this, minimal changes need to be made to use synthetic natural gas.

Production

Synthetic natural gas is created through a thermo-chemical conversion. The first step in this conversion is the gasification of the solid carbon source, whether it is coal or biomass (which would create bio synthetic natural gas called Bio-SNG), with steam or oxygen. Here the coal is heated with a limited supply of oxygen or air to produce synthetic gas (syngas), which consists primarily of carbon dioxide:

$2\mathrm{C}+\mathrm{O}{\phantom{A}}_2\to 2\mathrm{C}\mathrm{O}$

To reduce the amount of nitrogen in the synthetic gas, modern gasification plants often use pure oxygen instead of air. If steam is added along with this oxygen, the water-gas reaction occurs as part of the gasification process:

$\mathrm{C}+\mathrm{H}{\phantom{A}}_2\mathrm{O}\rightleftharpoons \mathrm{C}\mathrm{O}+\mathrm{H}{\phantom{A}}_2$

These processes drive off some of the volatile material, and the product is known as synthetic gas (syngas). Generally, this occurs in a gasifier, which feeds coal into a high pressure, high temperature vessel and distributes steam or oxygen evenly while removing ash to gasify the coal. This synthesis gas is a mixture containing H₂, CO, CO₂, H₂O, and CH₄ along with other hydrocarbons and some impurities. The exact composition of this gas depends on the type of reactor used, operating conditions, and other aspects of the gasification process. This gas also contains impurities such as oils, tars, hydrogen sulfide, and ammonia which must be removed before further processing.

To purify this gas, it must undergo gas cleaning and gas conditioning. During gas cleaning, impurities such as ammonia and sulfur compounds are removed from the synthesis gas, whereas gas conditioning is the process by which components of the gas are converted so that its final composition is suitable for its intended use. What is left after this cleaning and conditioning is a purified synthesis gas consisting primarily of carbon monoxide and hydrogen, along with smaller amounts of methane and carbon dioxide.

To finish making synthetic natural gas, the syngas undergoes water-gas shift reactions and methanation. The water-gas shift reaction combines carbon monoxide with steam to produce carbon dioxide and hydrogen in the following reaction:

$\mathrm{C}\mathrm{O}+\mathrm{H}{\phantom{A}}_2\mathrm{O}\rightleftharpoons \mathrm{C}\mathrm{O}{\phantom{A}}_2+\mathrm{H}{\phantom{A}}_2$

This reaction requires a metal catalyst to occur. Since the end goal of this gas is to obtain methane (the main component of natural gas), some of the carbon monoxide is retained to achieve to receive the desired gas composition before methanation. At this point, the carbon dioxide is separated out using a gas scrubbing process, and only the final methanation step remains. During methanation, carbon monoxide reacts with the hydrogen that was created, with the help of a nickel catalyst, to produce methane and steam in the following reaction:

$\mathrm{C}\mathrm{O}+3\mathrm{H}{\phantom{A}}_2\to \mathrm{C}\mathrm{H}{\phantom{A}}_4{\phantom{A}}^{+}\mathrm{H}{\phantom{A}}_2\mathrm{O}$

At this point, the production of synthetic natural gas is complete after approximately half of the original carbon in the coal has been converted into methane. Much of the carbon is converted into carbon dioxide.

Bio-SNG

Bio-SNG is produced similarly to conventional synthetic natural gas, but is instead produced through the gasification of biomass. Biomass such as forestry residues or energy crops is used. To make Bio-SNG, biomass is initially dried and undergoes gasification. After this, it undergoes gas conditioning, SNG synthesis, and finally gas upgrading.