A tale of underground alchemy
Hydrogen can be used to power gas turbines, heat boilers and for a synthetic fuel. But its production in the depths of the coal pits is, at this experimental stage, complex and risky. Firstly, a tank delivers liquid oxygen to the site of the mine. The liquid oxygen is allowed to flow into a secure pool, where it expands and evaporates into a gas. Control valves guide the gas through pipes into the mine, direct to the coal deposits, where the gasification process begins.
The experiment is carefully monitored with sensors and underground cameras, on the search for irregularities. Monitoring methods are a vital part of the research; while the coal undergoes gasification various dangerous and explosive gases are produced. So measures must be taken to ensure that there are no explosions or leaking of dangerous gases.
Monitoring is also done from the surface by geologists and chemists, 25 metres above where the gasification occurs. The researchers want to be sure that gas does not leak through porous soil layers. They use a georadar to see if there are any structural changes in the underground cavity. They also check for possible gas leakages.
Another city with a long history of coal mining is Liege, in Belgium. It was here that coal gasification was investigated in a laboratory before beginning the full-scale experiment in Poland. The coal in a mine is not fully exposed, so to recreate the inside of a coal mine realistically, pieces of Polish coal were mixed up with a neutral material. The mix was then placed within the laboratory reactor, where different gases were introduced at different temperatures and pressures. These parameters control just what happens during the gasification process: for example, a higher pressure yields more methane, while a higher temperature yields more hydrogen and carbon monoxide.
The technique of chromatography was used to analyse the resulting gases. There are three types of coal gasification: gasification with carbon dioxide gives a gas high in carbon monoxide, gasification with steam gives a gas high in hydrogen, and gasification with hydrogen will produce a gas with a high methane content – forming the base of synthetic natural gas found in energy networks.
In Poland the experiment has been running successfully during a three week period. About 50 kg of coal has been gasified per hour without any traces of a dangerous leak. The researchers have been sampling and analysing the resulting hydrogen and other gases. Chromatography is used to identify the various gas components: carbon dioxide, carbon monoxide, hydrogen, nitrogen, oxygen, and some pollutants, like sulphur compounds, for example.
Further research will most likely focus on improvements to increase the production rate of hydrogen and decrease the levels of dangerous gases. Gasification has the great advantage of also utilizing the smaller deposits of coal that are traditionally overlooked. This automatically increases the economic potential within a mine and could give new life to struggling coal regions within Europe. Furthermore, with less waste of coal, more energy will be produced in a more environmentally-friendly manner.