Removal of atmospheric Carbon Dioxide produced by burning fossil fuels is becoming a popular issue among environmentalists and government environmental agencies. But the issue needs much more study of possible side-effects.
'Anthropogenic Global Warming' is used to justify need for reducing atmospheric Carbon Dioxide. Note the word 'Anthropogenic', meaning 'caused by human activity', principally by burning fossil fuels such as coal and natural gas that contain the element Carbon. Natural significant sources of Carbon Dioxide emissions such as volcanoes, rock weathering, and green plants that make Carbon Dioxide at night, are not part of the sequestration project.
One area of study involves capturing gas resulting from combustion at industrial sources such as electrical generating plants and 'sequestering' Carbon Dioxide by pumping the spent combustion gas underground into an aquifer that contains 'brine', or salty water, in which the salt content is too high for normal uses such as drinking and crop irrigation.
Hot combustion gas will rise up a smokestack and disperse all by itself due to the difference in temperature and therefore difference in density of the spent gas compared with surrounding air. No additional energy input is necessary. Not so with gas going down a hole into a buried aquifer. That gas must be pressurized and pumped down the hole. Pumping requires energy to run the pump, which requires that additional fuel be burned to drive the generator that provides energy. So right away, disposal of fossil fuel combustion products underground creates more combustion products and burns more fuel, creating more Carbon Dioxide.
All of the combustion gas could be forced into the aquifer, or Carbon Dioxide could first be separated and only Carbon Dioxide pumped down.
Separating Carbon Dioxide from combustion gas requires even more energy to run the separator, requiring that even more fuel be burned to produce power to operate the separation process, and creating additional Carbon Dioxide. Already, less energy is available for customers, or a bigger electrical generating station must be built and operated to provide the original amount of electrical energy to customers plus additional energy to operate the Carbon sequestration process.
Carbon fuel comes out of the earth as coal or natural gas, and the sequestration process would return that Carbon to the earth through burial in an aquifer. But note -- although there is no Carbon released to the atmosphere, Carbon becomes Carbon Dioxide by burning. Each atom of Carbon that burns is attached to two atoms of Oxygen. Normally, plants would absorb atmospheric Carbon Dioxide, use the Carbon to make plant tissues, and release Oxygen back into the air. But this regeneration cycle is stopped when Carbon Dioxide is buried underground and out of reach of plants. Carbon sequestration buries both the Carbon and attached Oxygen. The net result is depletion of atmospheric Oxygen.
Could Carbon Dioxide in combustion gas be stripped of Oxygen by industrial methods and the Carbon reused? Yes, but the reduction process consumes more energy than is available by burning recovered Carbon in the electrical generating station. If this worked, it would be a Perpetual Motion machine and violate the concept of entropy.
How serious is depletion of atmospheric Oxygen? The sequestration process will likely be tried at one or two electrical generating stations for several years. If sequestration proves successful, and becomes popular, more and more electrical generating stations may begin using the process. How much will one large electrical station running on fossil fuel deplete the atmosphere of Oxygen during thirty years of operation? How about a hundred generating stations, or a thousand, all sucking Oxygen out of the air in the name of Global Warming prevention? Would the Carbon sequestration process become Breathing Prevention? Would Oxygen depletion in the upper atmosphere result in Ozone Layer depletion, letting more ultraviolet radiation from the sun reach the ground and upping the rate of skin cancer? How much Oxygen will we remove if Carbon sequestration spreads to other industrial operations?
What happens in the aquifer?
An immense amount of pressurized Carbon Dioxide will be forced into the aquifer's brine. Just as with any container, when there is pressure, leaks are likely to develop.
Brine is more dense than fresh water and tends to stay in the lowest part of the aquifer. But put pressure on the brine and the brine will seek to remove that pressure by flowing to regions where pressure is lower. Brine under pressure may begin moving upward, contaminating overlying freshwater aquifers.
If pressurized brine acquires access to a geological fault, brine may reach the surface as a salt spring or even a blowout of fizzy salt water that renders surrounding land useless for growing crops or erecting buildings.
Carbon Dioxide plus water makes Carbonic Acid. Carbonic acid is able to dissolve Calcite cement holding sand grains together in aquifer rock. Calcite cement becomes Calcium Bicarbonate and dissolves in water. During the process of dissolving Calcite, other chemical elements such as Lead may be released from aquifer rock into the water and move with migrating brine into overlying freshwater aquifers and surface springs and blowouts. Overlying limestone beds are soluble in Carbonic Acid. Caves may be created or enlarged. In extreme cases, the surface may settle or collapse as bedrock is removed by dissolving in Carbonic Acid.
What if Carbon Dioxide is not separated and all of the combustion gas is pumped down into the aquifer? That would raise the energy cost of pumping additional gas, but eliminate the cost of gas separation.
Unprocessed combustion gas contains a large fraction of Nitrogen, as does air. Molecular Nitrogen is rare in natural aquifer water.
At the time of combustion, gas temperature is very high. Nitrogen reacts with Oxygen to form Nitrogen Oxides in small concentration, although a small concentration becomes a big amount during thirty years of electrical generating station operation. Nitrogen and Nitrogen Oxides may form Nitrate, a health hazard if sufficiently concentrated. In addition, in the case of coal, there are other chemical elements and compounds such as trace amounts of Mercury present during combustion. Some of these materials can be harmful to water quality or may act as catalysts for combination of other materials to form undesirable and harmful chemical compounds.
The Lamotte sandstone aquifer in northern Missouri, chosen for initial testing of Carbon sequestration, can be considered as a huge reactor vessel in which chemical processes may occur. Chemical elements and compounds pumped into the Lamotte aquifer may react with dissolved materials in the brine and with the rock itself to form harmful substances. Combustion gas contains Carbon and Nitrogen, the essential components of Cyanide. Other natural and combustion materials may combine underground with Carbon and Nitrogen to create Methyl Isocyanate, a deadly gas at atmospheric pressure and temperature. Trace amounts of Mercury in combustion gas or in rock could convert to Methyl Mercury, a highly poisonous substance. These and other nastys would flow with migrating pressurized brine venting into overlying freshwater aquifers or to surface springs and blowouts.
The biggest question is whether Anthropogenic Global Warming is real and harmful. Earth's atmosphere and climate warm and cool all the time and have done so forever. Temperature stopped rising during the past several years. During geologic time there have been episodes when earth's climate was much warmer than now and times when there were Ice Ages. Life survived. We survived.
Are people influencing climate? Is a warmer climate good or bad? Will Global Warming hysteria continue but reverse direction if another cooling trend sets in during the next couple of decades? Do we really want to spend our fuel and energy resources, and money, removing Carbon Dioxide from air when plants do that for free? Can we afford to deplete Oxygen from our atmosphere in the name of Carbon sequestration to 'Fight Global Warming'?
We can pump combustion gas into the ground. But a great deal more thought and study are needed before we decide to do this.
William Jud, Geologist