Understanding the Mechanism of CO2 Scrubbing

We are living in critical times. According to the Intergovernmental Panel on Climate Change, global greenhouse gas emissions must stabilize by 2015 to prevent catastrophic temperature rises. However, the International Energy Agency forecasts a 50 percent surge in energy consumption by 2030, and historical trends suggest this prediction is likely accurate [source: The Guardian].

Since the industrial revolution began, approximately 551 billion tons of carbon have been burned, leading to a 100 parts per million increase in atmospheric carbon levels compared to just over two centuries ago [source: The Guardian]. Given these alarming statistics, humanity faces critical choices about energy use: trading SUVs for hybrids, for instance.

What if we could enjoy the best of both worlds? In the context of carbon sequestration and CO₂ scrubbing, this means keeping our coke. As explained in What is Clean Coal Technology? and How Carbon Capture Works, coke is a solid form of carbon. Carbon sequestration encompasses various methods to capture carbon dioxide for long-term storage or reuse. CO₂ scrubbing is a specific carbon capture technique applied after fossil fuel combustion but before emissions are released into the atmosphere.

Despite its current high cost, CO₂ scrubbing is widely regarded as one of the simplest methods to cut carbon dioxide emissions, as it doesn’t demand any lifestyle adjustments. There’s no need to install solar panels or connect to wind farms, nor any guilt about leaving the lights on all day. Simply continue burning fossil fuels, and let the scrubbers take care of the rest. But what exactly does "the rest" involve? Read on to discover more.

The CO2 Scrubbing Process

No matter where you live, unless you’re in a yurt, the electricity in your home likely comes from a power plant. In the United States, over 85 percent of energy is derived from burning fossil fuels, while China is constructing power plants at a rate of two per week [source: Herzog/ CCS].

Most power plants generate energy by burning coal or other fossil fuels to produce steam, which drives turbines to create electricity. However, alongside steam, a mix of flue gases is also released into the air. Many of these emissions are greenhouse gases that exacerbate the greenhouse effect.

Not all greenhouse gases have the same impact. Although carbon dioxide typically accounts for less than 15 percent of a power plant's emissions by volume, it contributes to 60 percent of the greenhouse gas effect [source: U.S. Department Of Energy/Marion].

Post-combustion carbon capture, as the name suggests, prevents CO₂ from entering the atmosphere by separating it from other flue gases after combustion. Once the gases are scrubbed, they are released into the air. While some scrubbing methods also target other greenhouse gases like sulfur and mercury, CO₂ remains the primary focus due to its significant impact.

The most common method for isolating CO₂ involves using a liquid solvent that binds with the gas and separates it from other components. Popular solvents include aqueous ammonia and monoethanolamine (MEA).

Regardless of the specific solvent used, the process follows a similar pattern:

After fossil fuel combustion, the resulting gases are cooled and collected. A solvent is introduced, absorbing the CO₂ and forming a new compound through a reversible chemical reaction. This compound separates into a more solid state, is pumped to another chamber, and reheated. The heat releases the CO₂, which is then diverted for storage. The solvent is recycled back into the system, and the cleaned flue gas is emitted into the atmosphere.

In addition to solvent-based methods, other techniques for CO₂ scrubbing include:

While each of these methods has shown effectiveness, the complexities of isolating CO₂ from flue gases have spurred ongoing research for more efficient solutions. The quest to discover the most cost-effective and efficient air-cleaning method continues. Next, explore the challenges associated with CO₂ scrubbing.

Challenges of CO2 Scrubbing

Like many emerging technologies, CO₂ scrubbing encounters several challenges. These hurdles vary depending on the method used and may include solvent degradation due to other flue gases, membrane corrosion, reduced adsorption caused by impurities, and higher energy demands and costs.

Currently, the most pressing issues are energy costs and requirements. Some studies suggest that existing capture technologies cost approximately $150 per ton of carbon captured, increasing electricity bills by 2.5 to 4 cents/kWh [source: U.S. Department of Energy]. Other estimates indicate prices as high as 9 cents/kWh, marking an 84 percent increase compared to electricity from plants without carbon capture [source: U.S. Department of Energy].

Costs fluctuate significantly due to factors such as scrubbing system design, plant location, operational efficiency, and whether the technology was integrated during construction or retrofitted. For example, electricity from a new coal plant with CO₂ scrubbing may cost 57 percent more, while retrofitting an existing plant can increase costs by up to 290 percent [source: Williams].

Beyond high costs, CO₂ scrubbing is energy-intensive. It requires processing large volumes of gas (since CO₂ makes up only a small portion of flue gas) and compressing the captured CO₂ into a storable form, which consumes substantial energy. As a result, carbon capture itself consumes significant fuel. New coal plants with carbon capture may use 24 to 40 percent more fuel, while retrofitted plants may use up to 77 percent more [source: Williams].

Despite its energy consumption and emissions, CO₂ scrubbing still removes 80 to 90 percent of carbon dioxide from flue gas [source: GreenFacts]. This is crucial, as scientists estimate that reducing CO₂ emissions by 60 to 80 percent may be necessary to prevent severe climate impacts [source: Marion]. Next, explore the applications of CO₂ scrubbing.

Applications for CO2 Scrubbing

While the ability to scrub CO₂ from any location may become a reality in the future, current technology limits its use to stationary sources like power plants that burn fossil fuels. This might seem restrictive, but fossil fuel combustion is the largest emitter of CO₂, with power plants accounting for over one-third of global emissions [source: Herzog].

Despite efforts to reduce energy consumption and shift to renewable sources, fossil fuels remain a dominant energy source due to human reluctance to change. Even with advancements in cleaner and more efficient power plants, their widespread adoption will take decades, given the 40-year lifespan of existing plants [source: RWE]. By 2030, two-thirds of CO₂ emissions are expected to come from current plants [source: ScienceDaily].

Clearly, a method to mitigate emissions is essential. CO₂ scrubbers, the only carbon capture technology applicable to existing plants, offer a temporary solution until cleaner energy sources become mainstream. Regulations like U.S. EPA mandates and the Kyoto Protocol, which caps CO₂ emissions for participating countries, further drive the adoption of this technology.

Several nations have implemented emissions trading systems to assign a cost to carbon emissions. For example, the European Commission has warned that failing to adopt carbon capture technology could result in an $80 billion higher expense compared to its implementation [source: The Guardian]. This suggests that while carbon scrubbing is costly, inaction could prove equally expensive over time.