Capturing Carbon Dioxide – An Overview

Global warming and climate change are the critical issues that mankind is facing today. CO2 is the main culprit in this game. Fossil fuel powered power plants are the main producers of this excess CO2. With restrictions on the emissions coming into effect, installing Carbon Capture and Storage Systems in power plants is of prime importance.

The formation of CO2 is the essential part of releasing energy from Carbon. The burning of the fossil fuel produces flue gases that contain CO2. In the current scenario, all fossil fuel power plants emit this CO2 to the atmosphere.

Oxygen for the combustion comes from air. Air contains 23.2% by weight of Oxygen with the balance being mainly Nitrogen. During Combustion, the Carbon combines with Oxygen to form CO2. Therefore, the Flue gases that leave the combustion chamber contain CO2, Nitrogen and some unused Oxygen from the air. In a coal fired unit, typical values of flue gas will be CO2 12%, O2 around 3%, and the balance Nitrogen at 85%. (A typical natural gas fired gas turbine will have CO2 4%, O2 15% and N2 81%).

Now the first task of the CCSS is to separate this 12% of CO2 from the flue gas so that it can go for storage. A 1000 MW coal fired unit produces 2500 tons of gas per hour. This is almost 300 tons per hour of CO2 separation. The task is enormous.

There are two ways to capture the CO2.

The first method is to capture CO2 after the combustion or post combustion capture by

• absorption by using liquid or solid chemicals.
• adsorption by physical or chemical methods.
• filtering using membrane filters.

In the current scenario with more than two million MW of thermal power plants in service, the power industry looks forward to a system that can retrofit to existing power plants. Many of the post-combustion capture systems have the advantage that they can become add-ons in existing plants. The priority is thus to implement post combustion capture of CO2.

Another method is to capture the CO2 before actual combustion. Carbon in coal converts to an intermediary gas containing Hydrogen and Carbon Monoxide that burns to produce heat. CO2 releases during the formation of the intermediary gas. Unlike in normal combustion, CO2 concentrations are much higher so the effort to capture it is much less.

An Integrated Gas Combined Cycle (IGCC) where coal is converted to a gas and then fired in a combined cycle, which is already in an advanced stage of development, will be the most cost effective way of power generation. Precombustion capture of CO2 has to be part of the IGCC. This will be the future of fossil fuel power.

Whatever the method, the cost of implementing a CSSS could increase the cost of power generation by around 30 to 40%. Three-fourths of this increase will be for the capture of CO2. Costs are expected to come down when the technology is stabilised and is included as part of the original equipment supply.