Increasing Importance of Carbon Dioxide Emissions Control
Carbon dioxide (CO2) capture, utilization, and sequestration (or storage) (CCUS) is increasingly becoming a core supporting technology component of clean coal projects, such as coal gasification facilities, to reduce the overall environmental impact of coal utilization.
Historically, the focus of emissions control was on sulfur dioxide and particulates, where a typical pulverized coal (PC) or coal gasification power plant was required to incorporate supporting technology to remove these pollutants to meet specific emissions regulations. Along with stricter limits on these as well as a broad range of pollutants including mercury, attention has also fallen on the issue of CO2 emissions from fossil fuel-based power production, given that 85% of greenhouse gas emissions are energy related, and 95% of those gas emissions are CO2. CO2 is becoming the focus of attention as its relation to global climate change becomes clearer.
Rule-making or regulatory control being put in place for controlling CO2 is likely to keep changing and evolving over time. Future possibilities range from economic incentives to reduce CO2 emissions, to firm government regulations that could severely limit or impose cap and trade scenarios on emissions of CO2, any of which would require an adjustment throughout the energy industry and beyond. Gasification research and development is focused on solutions to the problem of CO2 release that will not negatively impact energy use or economic growth. Regulations or other effects of carbon cap scenarios are likely to serve as drivers for gasification because of its inherent advantages for CO2 capture, given that its overall process scheme can be easily designed (or modified) to allow for economic CO2 capture.
Gasification vs. Combustion in CO2 Emissions
In a conventional pulverized coal combustion-based power plant, air and fuel are mixed, combusted and then exhausted at near atmospheric pressure. Since air contains a large amount of nitrogen which substantially dilutes the combustion exhaust gases, CO2 is considerably diluted in the exhaust, and being near ambient pressure, the exhaust gases are of low density. Removing the CO2 following combustion is known as "post-combustion" CO2 capture, which is made relatively difficult, energy-intensive, and expensive because of the dilution and low pressure of the exhaust gases.
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; moreover, gasification is often performed at elevated pressure. The resulting syngas is typically at higher pressure and not diluted by nitrogen, allowing for much easier, efficient, and less costly removal of CO2 (and other pollutants as well, demonstrated by the extremely low emissions of sulfur and nitrous oxides in addition to low levels of particulate matter and other contaminants such as heavy metals characteristic of gasification processes). Gasification and integrated gasification combined cycle (IGCC)'s unique ability to easily remove CO2 from the syngas prior to its combustion in a gas turbine (called "Pre-Combustion" CO2 capture) is one of its great advantages over conventional combustion.
The following figure demonstrates that even without taking into account the advantages IGCC has in ease of carbon capture it still emits less carbon dioxide emissions than the coal-fired PC plants.
If capturing and storing CO2 is incentivized or mandated, then the benefits of IGCC over PC plants become even more pronounced. The ease and effectiveness with which carbon capture technology is added to IGCC systems is shown in the following graph. The economic advantages that PC has over IGCC without carbon capture are made up for and even reversed slightly, with IGCC being slightly less expensive on a mills/kWh basis. In addition, the capture technology used with IGCC is able to remove approximately 90 percent of CO2, giving it a slight edge against both subcritical and supercritical coal-fired PC power plants.