(Image credit: Pexels)
Coal ranks number one on the dirty scale when it comes to burning fossil fuels for power generation. When coal burns, it releases carbon dioxide and methane, known emissions that contribute significantly to global warming. It also creates sulfur dioxide (SO2), nitrogen oxides (NOx), mercury along with particulate matter or soot, all of which have a negative impact on both the environment and humans. As a result, more plants have shifted to natural gas and renewables to generate electricity, which (as of 2016) powers 34% of the US electrical grid, with coal accounting for only 30% and falling.
That being said, the EIA (US Energy Information Administration estimates the US sits on approximately 3.9-trillion short tons (2,000 pounds) of coal, or enough to power the country for 283 years and as a result, national utilities have been developing clean coal technology to utilize that abundance of fossil fuel that would lessen or outright prevent those harmful emissions. Much of the challenge behind clean coal technology is its commercialization, which is both costly and energy demanding.
Integrated gasification combined-cycled power plant outlining Pre-combustion capture. (Image credit: National Energy Technology Laboratory)
The definition of ‘clean coal’ often refers to the carbon capture and storage process (CCS)- collecting waste carbon dioxide from power plants and storing it so that it doesn’t enter the atmosphere, which is usually deposited a mile or so underground. The most prominent form of this type of technology is broken down into three methods with the first known as Pre-combustion capture- this process involves removing CO2 from coal before combustion is completed. Coal is saturated with a fuel and in some cases steam to produce a gas comprised mainly of carbon monoxide and hydrogen.
The CO2 is then separated from the hydrogen during cryogenic distillation or a chemical absorption process, leaving a hydrogen-rich fuel source that can be utilized for any number of applications. The advantages here are higher CO2 capture along with steam pressure for increased electrical output. Unfortunately, this platform is still undergoing development for large-scale operations and is challenging to adapt to existing or retrofitted power plants.
The Oxy-fuel combustion process. (Image credit: National Energy Technology Laboratory)
The second method is termed Oxy-fuel combustion- this technique involves using highly pure oxygen for combustion resulting in a flue gas that’s primarily CO2 and H2O, which is separated by condensing water. The advantages of utilizing this type of combustion are high flame temperatures (producing more steam for efficient power generation) and the easy separation of gases. The downside is more considerable costs and more substantial electric power requirements to maintain the separation process.
The third process is designated as Post-combustion capture- the principle application here involves the capture of CO2 emissions from flue gas after combustion. This scrubbing process consists in diluting the CO2 in the flue gas using inert gases such as nitrogen and/or argon and then passing the emissions through an amine solution to remove the CO2 and other pollutants. The advantages here are the system’s ability to be retrofitted on nearly every existing plant without costly overhauls as well as the flexibility of not needing to change the combustion cycle. The disadvantages are the significant energy costs for the CO2 scrubbing itself as well as the unfavorable condition of the flue gases when the process is complete.
Example of CO2 capture and storage via geological and saline aquifer. (Image credit: Jarl Arntzen via Wikipedia)
After capturing the CO2, it still needs to be transported and stored (unless it’s commercially viable) with the most common being geological. What this means in the CCS chain is sending the captured emissions through a pipeline and into oil or natural gas depleted drill sites several miles below ground and stored there instead of being released into the atmosphere. Another method would be to store the carbon dioxide in saline aquifers with water that is unfit for human consumption or agriculture applications as well as injecting it into cavernous rock under the ocean and after millions of years, combines chemically with the sediment.
It’s important to note that most of these CCS technologies are still being developed and all forms of CCS must be carefully instituted and monitored to avoid contaminating the environment and ultimately outweigh the benefits of containment. The technology presented here isn’t perfect and in most cases, still under development for implementation and widespread deployment. It isn’t designed to be a stop-gap application for aging and retiring coal fleet but rather an ambitious endeavor for a viable solution on how to take advantage of the most abundant source of power on the planet.
Renewables are promising and are already being mitigated most industrial nations with the hopes of phasing out coal altogether, but it can’t yet replace coal or natural gas on a scale as large as the US electrical grid. As long as technology continues to advance, engineers will find ways to utilize it to develop efficient systems to render coal a clean and efficient power source for future generations.
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