Approximately 10,000 teragrams (Tg) of waste gas carbon is emitted globally each year (see Figure 2-1), representing a large volume of potential inputs for carbon utilization 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 utilization. As shown in Table 2-1, sources of carbon dioxide range from highly concentrated by-products of chemical manufacturing to relatively dilute flue gas streams from power plants with contaminants that can be problematic for carbon utilization technologies. Methane in waste gas ranges from widespread biogas from waste treatment and landfill facilities, containing challenging contaminants, to sparse coal mine venting of dilute methane in air. This chapter describes the sources, compositions, and geographical distributions of carbon dioxide and methane in gaseous waste streams and explores how those characteristics relate to the specifications for carbon utilization feedstocks. This chapter will also attempt to distinguish between those sources of methane in waste gas that are more likely to be directly reused as fuel rather than chemically transformed through carbon utilization technologies.
Carbon utilization technologies focus on conversion of two major carbon resources—carbon dioxide and methane—into useful products. Although some gaseous waste streams contain both of these carbon resources, in general carbon dioxide and methane waste gases differ in terms of their physical and chemical properties and in the locations, magnitudes, and other properties of the waste gas streams. The following sections describe key characteristics of carbon dioxide– and methane-containing waste streams.
Carbon Dioxide Waste Streams
Global emissions of carbon dioxide waste caused by human activities, primarily fossil fuel combustion, have been increasing rapidly. Counter to the global trend, carbon dioxide emissions have been decreasing in the United States, from an estimated peak of 5.38 billion metric tons of carbon dioxide in 2004 (after accounting for uptake by forestry and land use change) to an estimated 4.56 billion metric tons of carbon dioxide in 2016 (approximately 14 percent of the world total of 35 billion metric tons of carbon dioxide, or approximately 10,000 Tg of carbon) (EPA, 2018). Trends in carbon dioxide emissions result from long- and short-term drivers, including population and economic growth, market trends, technological changes, and fuel choices. Carbon dioxide emissions in the United States come almost exclusively from the combustion of carbonaceous fuels in five major sectors: electricity generation, transportation, industrial processes and fuel use, residential fuel use, and commercial fuel use (see Figure 2-2).
TABLE 2-1 Gaseous waste streams containing carbon dioxide and methane in the United States in 2016. SOURCE: EPA, 2018.
|Waste Gas||Approximate Magnitude in the United States||Composition||Chemical Species Also Found in Waste Gas|
|Carbon dioxide from ammonia manufacture||12.2 Tg (MMT)||> 98% CO2||Hydrogen, water, carbon monoxide, nitrogen|
|Carbon dioxide from fossil fuel combustion||4,966 Tg (MMT)||12-15% CO2||Nitrogen, nitrogen oxides, particulates, sulfur oxides|
|Carbon dioxide from cement, iron/steel, and glass production||82.9 Tg (MMT)||20-35% CO2||Carbon monoxide, particulates, nitrogen and nitrogen oxides, sulfur oxides|
|Carbon dioxide from natural gas production||25.5 Tg (MMT)||3-4% CO2||Water vapor|
|Methane from oil and gas||1.54 Tg (38.6 MMT CO2e)||50-95% CH4||Ethane and other light alkanes as the other dominant species|
|Methane from landfill gas||4.3 Tg (107.7 MMT CO2e)||50% CH4
|Methane from digestor gas||0.36 Tg (8.9 MMT CO2e)||70% CH4
|Methane from coal mine vents||2.2 Tg (53.8 MMT CO2e)||1-10% CH4|
NOTE: CO2e: carbon dioxide equivalent.
About one-third of U.S. carbon dioxide emissions come from electric power plants that burn fossil fuels and generate waste gases at high rates. For example, a single 1,000-megawatt coal-fired power plant, operating at full capacity, generates approximately 1,000 tons of carbon dioxide per hour or roughly 9 Tg (million metric tons [MMT]) per year. U.S. power plant emissions totaled 1,800 Tg (MMT) in 2016 (EPA, 2018). Concentrations of carbon dioxide in the flue gases from electric power plants are typically in the range of 12 to 15 mol% for coal-fired plants and 3 to 4 mol% for natural gas–powered plants (Songolzadeh et al., 2014). As shown in Figure 2-3, waste streams from power plants are geographically distributed throughout the United States and are disproportionately concentrated near major population centers. In addition to carbon dioxide, these waste gases can contain a variety of contaminants, including nitrogen oxides, sulfur oxides, heavy metals, fly ash, and other species.
In contrast to the waste streams from electric power plants, which are widely distributed, available in large volumes, and have variable levels of contaminants, manufacturing facilities can emit waste streams that are smaller in volume but contain higher concentrations of carbon dioxide and fewer contaminants. The chemical industry is the largest source of industrial CO2 emissions in the United States (Figure 2-4). These include waste streams from hydrogen and ammonia manufacturing facilities, which produce particularly concentrated sources of CO2. Another source of highly concentrated industrial CO2 is from biofuel processing facilities, particularly ethanol fermentation plants. Cement, steel, and glass manufacturing facilities also emit significant volumes of carbon dioxide; in 2016 such facilities emitted 39.4, 1.2, and 6.9 Tg (MMT) of carbon dioxide, respectively. Cement production represents the second largest source of industrial carbon dioxide emissions in the United States. While emissions from glass production have stayed relatively constant over the past 15 years, emissions from steel production have decreased due to technological improvements and increased scrap streel utilization.
Methane Waste Streams
Carbon dioxide is fundamentally a low-value, low-energy waste gas that is often emitted in large flows from individual sources. Methane, by contrast, is a high-value, high-energy molecule that is emitted as waste in far lower quantities, often in intermittent flows. Chemical
uses of methane-containing waste gases compete against the value of methane as a fuel, limiting possible utilization pathways.
Global annual emissions of methane to the atmosphere total approximately 558 Tg/yr (MMT/yr). Approximately 60 percent of these emissions are due to anthropogenic activities, dominated by fossil fuel production and use (105±28 Tg/yr) and agriculture and waste processing (188±54 Tg/yr) (NASEM, 2018). In the United States, which emitted approximately 26.3 Tg (MMT) of methane in 2016 (EPA, 2018), the dominant sources of methane as a waste gas are petroleum and natural gas systems, enteric fermentation (emissions by ruminant animals), manure emissions, landfills, wastewater treatment, and coal mining (Figure 2-5). Waste gas from landfills and wastewater treatment are referred to as biogas, since the methane is generated by microorganisms under anaerobic conditions.
Among the major categories of methane emission sources, a recent National Academies report characterized the degree of confidence in emission rates as high for coal-mining vent gases, as medium for enteric emissions and petroleum and natural gas systems, and as low for landfills and manure management (NASEM, 2018).
In contrast to carbon dioxide, where large point-source emissions are distributed throughout the United States, chemical uses of methane-containing waste gases may be much more limited by the dispersed and intermittent nature of methane waste gas sources. Even if captured, the value of methane as a fuel may limit possible nonfuel utilization pathways for methane. In order to identify which portions of the 26.3 Tg (MMT) of methane in waste gas in the United States might be available for nonfuel carbon utilization, a sector-by-sector analysis of emissions was performed by the committee. As described below, a large fraction of methane emissions in waste gas will face significant barriers to use as a feedstock for carbon
utilization due to difficulties in capture, intermittency in emissions, or competition from direct reuse as fuel.
Oil and Natural Gas Production
Methane emissions from the natural gas supply chain have been estimated to be approximately 1.7 percent of total natural gas production, both globally (IEA, 2017) and in the United States (Littlefield et al., 2017). Alvarez et al. (2018) places the total at a higher level, approximately 2.3 percent of natural gas production, but there is general agreement on the nature of the major sources of emissions. Table 2-2 lists estimates of the emissions from the top five sources in the natural gas sector, which collectively account for approximately 4 Tg (MMT) of methane per year or about half of the estimated total emissions from the natural gas sector that appear in the methane inventory assembled by the U.S. Environmental Protection Agency.
A variety of assessments have estimated the extent to which these emissions might be captured or reduced. For example, the International Energy Agency, in its special section on natural gas in the 2017 World Energy Outlook (IEA, 2017), estimated that approximately half of methane emissions associated with natural gas production could be reduced at a cost that could be justified based on the fuel value of the natural gas recovered or saved. Other analyses have made similar conclusions (ICF International, 2014, 2016). Many proposed strategies for methane emission reduction involve preventing, rather than utilizing, emissions. For example, among the largest sources of emissions in the natural gas supply chain are pneumatic controllers, which use gas pressure to open and close control valves, releasing methane as the controller operates. Natural gas can be replaced by compressed air to power pneumatic devices, eliminating this source of emissions. Methane emitted as a result of leaks and methane released as unburned fuel from compressors could also be prevented by sealing the
TABLE 2-2 Total emissions and basis of emission estimation for the top five emission sources within the natural gas systems. SOURCE: EPA, 2018.
|GHGI Emission Source||Sector||2016 Emissions (kiloton)|
|Gathering and boosting stations||Production||1955.1|
|Transmission station total fugitive emissions||Transmission and storage||580.1|
|Gas engines (compressor exhaust vent)||Production||245.6|
|Engines (compressor exhaust in transmission)||Transmission and storage||253.6|
leaks and improving the compressor engine performance. While such measures would prevent methane emissions, they do not fall within the scope of chemical utilization as defined in this report. Other sources of methane emissions in natural gas production may be less preventable but nonetheless have limited potential for carbon utilization processes, largely due to their intermittent nature. For example, emissions from compressor and pipeline blowdowns, as well as venting of wells to remove accumulated liquid, can lead to very large flow rates of methane (reaching approximately 1,000 kilograms per hour; Allen et al., 2015), but the events may be brief and infrequent, making utilization challenging.
Overall, the most consistent major source of methane emissions that is potentially available for utilization processes is vented or flared associated gas. Associated gas is natural gas that is co-produced with oil. In some production regions (e.g., the Bakken field in North Dakota), early development was focused on petroleum production, and the infrastructure necessary for delivering natural gas to markets was not initially available. In the absence of natural gas gathering and transmission infrastructure, some of the natural gas produced along with oil (associated gas) was vented or flared. Venting of associated gas has declined rapidly in the United States as natural gas gathering and transmission infrastructure is built into new production regions (EPA, 2017). Currently, approximately 0.2 percent of the 30 trillion cubic feet of natural gas production in the United States is flared. Allen et al. (2016) report a detailed analysis of flaring in upstream oil and gas operations in the United States and conclude that a majority of the flared gas is concentrated at a small number of locations. Associated gas flaring therefore represents the most consistent source of methane waste gas in the oil and gas sector that is available in relatively large volumes. This gas is typically in the range of 50 to 90 percent methane, with the remainder being ethane, propane, butanes, and other light hydrocarbons. Depending on the formation, the gas may contain also hydrogen sulfide or carbon dioxide.
Underground coal mines are another source of methane emissions. In active mines, various types of forced ventilation shafts are used to prevent dangerous accumulation of methane, which can lead to explosions. In 2016 approximately 251 underground mines across the United States collectively emitted a total of 1.64 Tg (MMT) of methane, comparable to the amount flared in the oil and natural gas sector (EPA, 2018). This methane is available at ventilation shaft discharge points, where methane makes up about 1-10 percent of emitted gas, with mine shaft air comprising the rest. This source of methane remains active while underground mines are active; however, underground mining of coal has been decreasing in the United States, and, once a mine becomes inactive, emissions from venting decrease over time (NASEM, 2018). Therefore, while currently a relatively large-scale source of methane, this source of methane is generally projected to decrease going forward.
Enteric Fermentation and Manure
Another large category of methane emissions is enteric fermentation. Enteric fermentation emissions in the United States come primarily from beef and dairy cattle, ruminant animals that emit methane mainly via exhaled gases and eructation. Only about 2 percent is estimated to be due to flatulence (NASEM, 2018). Unless the animals are confined in an enclosure, it is difficult to collect these emissions for utilization. When confined to enclosures such as dairy barns, methane from enteric fermentation will be mixed with emissions from manure. Emissions from manure will depend on manure management practices, which vary widely. The most extensive methane production from manure management occurs when manure slurries are sent to anaerobic digesters. If the digester is enclosed, a gas stream consisting of approximately 30-60 percent methane can be recovered (EESI, 2017; Fulhage et al., 1993). The remainder of the gas stream can include ammonia, hydrogen sulfide, and a variety of organic compounds including organic sulfides, disulfides, C4 to C7 aldehydes, amines, quinoline, dimethylpyrazine, and organic acids, along with lesser amounts of C4 to C7 alcohols, ketones, aliphatic hydrocarbons, and aromatic compounds (NRC, 2003).
In landfills, microorganisms digest buried wastes in anaerobic conditions, generating gaseous waste in relatively large quantities. Because this landfill gas consists primarily of methane and carbon dioxide, in roughly equal quantities, it has significant energy content. Energy recovery from landfill gases has been practiced for decades in the United States; however, the economic value of this waste gas utilization has been marginal, with landfill gas energy recovery becoming more or less prevalent as the prices of other fuels, most notably natural gas, vary over time. A challenge to the utilization of landfill gases as fuel is the potential for these waste streams to include trace contaminants such as various halogenated organic compounds, hydrogen sulfide, mercury-containing compounds, siloxanes, ketones, and sulfur-containing species (EPA, 1998, 2008).
Overall, roughly two-thirds of the 26.3 Tg/yr of methane in waste gas in the United States will face serious barriers to carbon utilization. The barriers are primarily related to the ability to capture the methane or the intermittency of the sources; however, the fuel value of methane will also potentially drive some emissions to be prevented rather than find use as a chemical feedstock. While some sources of carbon dioxide waste gases (e.g., transportation source) also face these challenges, fixed point sources of carbon dioxide emissions from power generation or other industrial applications provide volumes of waste carbon dioxide that are likely to remain greater than the potential uses in carbon utilization for many decades.
Finding 2-1 Waste gas streams containing carbon dioxide or methane are extensive but also heterogeneous. The chemical composition of contaminants of waste gases, difficulties in gas capture, and intermittency in waste gas flow can limit utilization opportunities.
The carbon sources contained in waste streams are rarely available in pure form, and the composition of waste gas streams varies widely, depending on the source of the waste gas. Which chemical species found in gaseous carbon waste streams pose challenges to carbon utilization processes also depends on the utilization technologies to be used. Some carbon utilization processes require purified streams. For example, carbon dioxide used in food products and food processing has high purity requirements. On the other hand, some carbon utilization technologies can valorize species other than carbon dioxide and methane in waste gases, making the impurity of the waste gas stream a benefit. For example, LanzaTech1 uses microbes that convert carbon-rich waste gases containing carbon monoxide, hydrogen, carbon dioxide, methane, and other species into a variety of products. The microbes use carbon monoxide as an energy source; this carbon monoxide would otherwise need to be treated as an air pollutant, and this avoided cost increases the economic benefit of the carbon utilization technology.
Because detailed characterizations of waste stream compositions are not widely available and the tolerance of carbon utilization processes for impure carbon dioxide and methane feeds is not generally known, it can be challenging to determine separation and purification requirements. In addition, most current carbon utilization activities take an opportunistic approach to accessing waste streams, rather than a systematic approach to match waste streams with the utilization processes for which they are best suited. Psarras et al. (2017) mapped potential industrial CO2 sources with potential utilization locations to calculate transportation and other costs of utilization processes at different locations and with different concentration needs. A coordinated approach requires better integration between separation and purification targets and information about the waste stream and processes to be used.
Priority Research Area
Gaseous Carbon Waste Characterization
Research is needed to map the detailed compositions and magnitudes of gaseous carbon waste streams, with particular attention to co-emitted species that could either hinder or enhance carbon utilization processes. This could increase opportunities for matching waste streams with appropriate utilization processes.
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