Designs for CO2 Capture
To reduce greenhouse gas emissions, a concerted effort is in place to develop and deploy technologies capable of limiting the production and release of carbon dioxide (CO2) into the atmosphere. This may take the form of economic incentives, but a regulatory limit on CO2 release may be applied in the future, and would require an adjustment throughout the energy industry and beyond. As 85% of greenhouse gas emissions are energy related, and 95% of those gas emissions are CO2, researchers are heavily invested in looking for ways to prevent CO2 release into the atmosphere. Gasification provides opportunities to limit and capture CO2, and therefore is an integral part of a carbon cap scenario. Gasification research and development is focused on solutions to the problem of CO2 release that will not negatively impact energy use or economic growth.
Gasification has inherent advantages over conventional combustion due to the ability of the technology to achieve extremely low emissions of sulfur and nitrous oxides (SOX and NOX) in addition to low levels of particulate matter and other contaminants such as heavy metals. The higher temperature and pressure process streams involved in gasification also allow for easier removal of CO2 for geological storage or for sale as a byproduct.
In combustion, air and fuel are mixed, combusted and then exhausted at near atmospheric pressure. In gasification, on the other hand, oxygen is normally supplied to the gasifiers and just enough fuel is combusted to provide the heat to gasify the rest. Since air contains a large amount of nitrogen along with trace amounts of other gases, which are not necessary in the combustion reaction, combustion gases are much less dense than synthesis gas (syngas) produced from the same fuel. Pollutants in the combustion exhaust are at much lower concentrations than that in syngas, making their removal more difficult and expensive.
CO2 Capture Technologies
This chart shows all of the current and potential technologies that are capable of removing CO2 from process gas streams. Gasification and integrated gasification combined cycle (IGCC) allow for the unique ability to remove CO2 gas from the syngas produced by gasification, prior to the combustion of the syngas in a gas turbine to produce electricity. This is what is known as Pre-Combustion CO2 capture. In a pulverized coal combustion power plant, the CO2 must be removed following combustion in what is known as Post-Combustion CO2 capture.
How Plant Design Changes
For gasification applications, or IGCC, the plant modifications required to add the ability to capture CO2 are minimal. The syngas produced by the gasifiers needs to be treated through various processes for the removal of impurities already in the gas stream, so all that is required to remove CO2 is to add the necessary equipment, an absorber and regenerator, to this process train. In combustion applications, modifications must be done to the exhaust stack and because of the lower concentrations of CO2 present in the exhaust, much larger volumes of total gas require processing, necessitating larger equipment.
Cost and Complexity of CO2 Capture Compared to Other Technologies
The reduced level of complexity for gasification CO2 capture is due to the fact that the gas stream from which the CO2 must be removed is more concentrated, and of a much smaller volume than competing technologies. This allows for CO2 capture systems in gasification applications to be much smaller and more efficient than for other technologies, as mentioned in the previous section.