Understanding Clean Coal Technology

Among fossil fuels, coal is the most environmentally damaging. Its combustion releases pollutants that drive global warming, cause acid rain, and contaminate water sources. Despite the growing focus on nuclear energy, hydropower, and biofuels, many assume that coal's days are numbered.

However, coal remains a cornerstone of energy production, supplying half of the electricity in the United States. Its affordability and abundance ensure its continued use [source: Energy Information Administration]. Clean coal initiatives aim to mitigate environmental damage by employing advanced methods to purify coal and control its emissions.

What Is Coal?

Coal, a fossil fuel rich in carbons and hydrocarbons, serves as a key component in manufacturing plastics, tar, and fertilizers. A byproduct known as coke is essential for melting iron ore, facilitating the production of steel. However, the majority of coal — 92 percent of the U.S. supply — is utilized for generating electricity [source: Energy Information Administration]. Power plants operated by electric companies and businesses burn coal to produce steam, which drives turbines and generates electrical power.

Burning Coal Releases Carbon Dioxide Emissions

The combustion of coal emits carbon dioxide and other pollutants through flue gas, visible as the dense clouds exiting smokestacks. Certain clean coal technologies focus on purifying coal prior to combustion. For instance, coal washing, a preparation method, eliminates undesirable minerals by combining crushed coal with a liquid, enabling impurities to separate and settle.

Additional systems optimize coal combustion to reduce emissions of sulfur dioxide, nitrogen oxides, and particulates. Wet scrubbers, or flue gas desulfurization systems, tackle sulfur dioxide — a significant contributor to acid rain — by treating flue gas with a mixture of limestone and water. This reaction produces synthetic gypsum, a material used in drywall.

Low-NOx (nitrogen oxide) burners decrease nitrogen oxide emissions, which contribute to ground-level ozone, by limiting oxygen and refining the combustion process. Electrostatic precipitators capture particulates, which can worsen asthma and respiratory conditions, by electrically charging particles and collecting them on plates.

Gasification eliminates the need to burn coal directly. In integrated gasification combined cycle (IGCC) systems, coal interacts with steam and hot pressurized air or oxygen, breaking carbon molecules apart. The resulting syngas, composed of carbon monoxide and hydrogen, is purified and used in a gas turbine to generate electricity. Additionally, the heat from the gas turbine drives a steam turbine, further enhancing energy production.

Next, we'll explore the most advanced clean coal technologies and the steps required to make clean coal commercially viable.

Reducing Greenhouse Gas Emissions Effectively

Carbon capture and storage (CCS), one of the most promising clean coal technologies, captures and stores carbon dioxide (CO2) emissions from stationary sources such as power plants. Given CO2's role in global warming, minimizing its release is a critical global priority. Researchers are developing various methods to identify the most effective and cost-efficient approaches to carbon capture.

Flue-gas separation extracts CO2 using a solvent, isolates it with steam, and condenses the steam into a concentrated stream. This process produces commercially viable CO2, which helps offset its cost. Another method, oxy-fuel combustion, burns fuel in pure or enriched oxygen, producing flue gas mainly composed of CO2 and water, thereby avoiding the energy-intensive separation of CO2 from other gases. A third approach, pre-combustion capture, removes CO2 before combustion as part of a gasification process.

Storing Carbon Dioxide

Once captured, CO2 is stored in secure containers to prevent its release into the atmosphere. The two primary storage methods, geologic and oceanic, must safely contain CO2 for hundreds of years until emissions decrease. Geologic storage involves injecting CO2 into the earth, using depleted oil or gas fields, deep saline aquifers, or unminable coal seams that absorb CO2. Enhanced oil recovery already utilizes CO2 to maintain pressure and improve oil extraction.

Ocean storage, still in its early stages, involves injecting liquid CO2 into deep waters (500 to 3,000 meters), where it dissolves under pressure. However, this method may slightly lower pH levels and potentially harm marine ecosystems. All CO2 storage methods require meticulous planning and monitoring to prevent environmental issues that could outweigh the benefits of CO2 containment.

Make Cutting Greenhouse Gas Emissions More Affordable

As alternative energy sources are not yet capable of replacing coal, which is both affordable and abundant, clean coal technology aims to reduce the severe environmental impacts of coal emissions. However, utility companies and businesses prioritize economic viability over environmental benefits, meaning the technology must be financially feasible to gain widespread adoption.

The process of cleaning coal and capturing its emissions substantially increases the cost per BTU of what is otherwise a low-cost fuel. While selling byproducts such as gypsum or commercial CO2 for use in sodas and dry ice can help offset these costs, implementing a carbon charge could make emission reduction economically viable.