1.1 Introduction
Carbon emission in the environment is a major societal issue because it is closely linked to the global warming and resulting in negative impacts on weather patterns and sea level rise. Two major carbon compounds emitted into the atmosphere by human activities and other natural causes are carbon dioxide and methane [1]. While methane can be more problematic greenhouse gas (24–130 times more harmful than carbon dioxide), it is emitted in a lesser volume and it eventually gets converted to carbon dioxide by atmospheric reactions. Once captured, the utilization of methane is much less challenging because methane is very reactive and can be easily used for power or the productions of other chemicals and fuels. Sources of methane are not all human made; a significant amount of methane is produced from animal manure, agricultural sources, leakage of methane hydrates, etc. Methane leakage from oil and gas industry (commonly called associated gas or gas flares), from landfill gas, or from other biomass waste treatment facilities is being captured and put to useful utilization. Since the major issue with methane emission is its capture, this book will not address the treatment of methane emission.
The emission of carbon dioxide is more human made. As demands for energy have increased globally, CO2 levels have risen sharply, from pre-industrial levels of 280 ppm a century ago to over 400 ppm since 2013. These levels also include emissions generated from other, nonenergy sources, including steel, aluminum, and cement production; fermentation; chemical production; and other industrial sources. Continued economic and population growth drives the increase in CO2 emissions, and the levels of emissions, if left unchecked, are projected to exceed 530 ppm by 2100. Goals for stabilizing CO2 levels at ≤450 ppm were set in 2007 to avoid serious impacts on the environment and health [2]. To meet these goals, however, requires substantial improvements in energy efficiency, increased deployment of renewable and nuclear energy, and the development of new technologies for mitigating CO2 emissions arising from the use of fossil fuels and other anthropogenic sources [3].
In 2014, the global carbon emissions from fossil-derived fuel combustion were estimated to be 9,855 million metric tons of carbon, or nearly 36 gigatons (Gt) of CO2 [4]. About a quarter of these emissions come from mobile sources, such as automobiles and other forms of transportation. Much of the remainder comes from point sources—plants used for generation of electricity and heat for industry, business, and home use, as well as industrial production processes. It is these so-called point sources that are the focus of technologies for mitigating global CO2 emissions. Efforts are being made to reduce carbon emission from vehicles by replacing internal combustion engine-driven vehicles with hybrid and electric vehicles.
Carbon dioxide is a very stable molecule, and it is emitted at a significantly large scale. According to a national academy report [1], approximately 10,000 teragrams (Tg) of waste gas carbon is emitted globally each year (see Table 1.1), representing a large volume of potential inputs for carbon treatment technologies. However, these gaseous waste streams are heterogeneous in their composition, are emitted from a wide range of geographically distributed sources, and are not always easily transported from their sources to locations where they can be processed. This heterogeneity poses challenges for carbon dioxide treatment. As shown in Figure 1.1, sources of GHG include fossil fuel combustion, natural gas systems, coal mining, waste incineration, etc. Energy-related activities are the primary sources of U.S. anthropogenic greenhouse gas emissions, accounting for 83.8% of total greenhouse gas emissions on a carbon dioxide (CO2) equivalent basis in 2016. Energy-related CO2 emissions alone constituted 78.9% of national emissions from all sources on a CO2 equivalent basis, while the non-CO2 emissions from energy-related activities represent a much smaller portion of total national emissions (4.9% collectively). Emissions from fossil fuel combustion comprise the vast majority of energy-related emissions, with CO2 being the primary gas emitted (see Figure 1.1). Globally, approximately 32,294 million metric tons (MMT) of CO2 were added to the atmosphere through the combustion of fossil fuels in 2015, of which the United States accounted for approximately 15%.
TABLE 1.1 Chronological Increase in Carbon Emission [1] | Year | Carbon Emission (Tg) |
| 1900 | About 600 |
| 1942 | About 1,200 |
| 1956 | About 2,000 |
| 1966 | About ... |
