The first full-scale “clean coal” plant opened in Canada this October. The Owned by SaskPower, the Boundary Dam Carbon Capture and Storage Project in the Canadian province of Saskatchewan is “the world’s first post-combustion carbon capture and storage (CCS) facility attached to a coal-powered plant.”1While many applaud this step towards more environmentally sustainable energy, others question the decision to further develop coal energy. The Sierra Club in Canada views it as a waste of resources that could be invested in conservation and renewable energy endeavors instead . This is one of two big, groundbreaking CCS power-plant projects expected to begin operating this year, along with the U.S.-backed Kemper County Carbon Dioxide Capture and Storage Project in Mississippi .
But what exactly does it mean to be a clean coal plant?
“Clean coal” describes coal producers’ efforts to reduce the negative environmental impact of coal in three major ways. Clean coal practices seek to 1) reduce pollutants released during coal combustion, 2) to increase energy efficiency, and 3) to capture and store CO2 in more sustainable ways to reduce greenhouse gas emissions.There has been tremendous progress in the first of these goals, reducing pollutants, ever since the passage of the Clean Air Act. While the SaskPower project’s primary contribution is a carbon dioxide capture and storage system, a more recent approach to clean coal, it’s important to review the fundamentals first.
The first goal of clean coal is to eliminate harmful pollutants like sulfur dioxide, nitrogen oxides, and trace metals. SO , which causes acid rain detrimental to the environment and can be absorbed in our bloodstream through our lungs, has historically been removed using smokestack scrubbers, which dissolve harmful gases in a slurry of sorbent materials to remove it from the output gas . This method has efficiencies close to 99% . However the process is costly and produces wet scrubbing waste . Using lime spray on this waste yields a solid form, making the waste easier to handle. Other alternatives for dealing with SO2 emissions include limestone sorbents with high reactivity . Removal of SO2 can occur prior to, during, or after combustion, but generally has higher efficiency post-combustion .
Nitrogen oxides, which cause tropospheric ozone and smog that exacerbates lung tissue and contributes to asthma2, can be removed using selective catalytic reduction, which achieves a 90 percent reduction  or using low NOx burners. Toxic trace metals like mercury and arsenic best removed by calcium based sorbent3. Utilizing another method called pressurized fluidized combustion burns the coal between 760 and 930 C, below the level needed to generate NOx, permitting strong reduction of NOx emissions and SO2 by limestone injection . The resulting flue gas increases efficiency as it is recycled to turn turbines within the plant.
Turning to the matter of increasing efficiency, coal gasification scheme can yield higher energy conversions . Convention coal combustion fully oxidize the carbon-rich fuels to generate heat while clean coal energy may use gasifiers that instead use partial oxidation conversions, requiring oxygen and steam at elevated pressures. Air usually consists of N2 and O2, but with these high pressures there is no nitrogen, and therefore a lower volumetric flow rate, increasing the partial pressure of contaminants
Now we turn to CO2 emission mitigation and storage. The Carbon Capture and Storage system the Canada plant is utilizing has three components: capturing the carbon dioxide, transporting it, and securely storing the CO2 either in underground depleted oil and gas fields or in deep saline aquifer formations .
The first goal, removing carbon, can be done before or after combustion. Pre-combustion processes convert fuel into gaseous mixtures of hydrogen and carbon dioxide, but these methods are less used in large-scale industry partially because they require larger changes to existing plant technology . Post-combustion processes for separating CO2 are similar to the ones for separating other pollutants and use a liquid solvent to absorb and separate the CO2. Oxyfuel combustion, the third technique used here, consists of the gasifier method of using pure oxygen rather than air for combustion mentioned above.
Transportation of carbon dioxide is well understood because countries like the U.S. have four decades of experience transporting carbon dioxide in pipelines for commercial purposes .
In the final storage stage, carbon dioxide is stored under high pressure in rock formations with many pores to hold the carbon dioxide, a high degree of permeability and connectivity between the pores, and an extensive barrier at the top to contain the carbon dioxide for hundreds or thousands of years . The CO2 is first compressed to a super-critical state where its almost as dense as water. The CO2 will react with reservoir rocks to form a mineral, transforming it into a solid mineral. It can also be injected into oil reservoirs to help with oil recovery. In general, carbon dioxide is stored at depths exceeding 800 meters to ensure it’s kept safely in its compressed, dense state .