: Systems Integration Approaches to Monetizing CO2 via Integration of Shale Gas
Table of Contents:
It is no surprise that there is a growing concern over atmospheric conditions and how greenhouse gases are (GHGs) affecting our environment. Carbon dioxide (CO,) is one of the main components of growing GHG levels and accounts for nearly 77% of industrial GHG emissions (Rahman et al., 2017). In order to begin the process of restoring atmospheric GHG levels to acceptable conditions, sustainable practices should be implemented in order to reduce and minimize emissions created through industrial processes. This is easier said than done, as GHGs like CO, are typically considered as waste with no inherent chemical value due to their low-energy nature. Luckily, attention has been focused on finding ways of capturing, storing, or utilizing CO, in meaningful ways to help mitigate emissions and turn CO, into a usefi.il commodity.
Generally speaking, CO, uses can be separated into two categories: Sequestration and utilization. Carbon capture and sequestration (CCS) is the method of capturing carbon dioxide from sources like emissions or even the atmosphere in order to separate and store the gas in an environmentally beneficial manner. One of the most popular forms of CCS currently is that of enhanced oil recovery (EOR). With this method. CO, is injected at well sites in order to increase pressure and flush out oil that may have remained after initial pumping. Typically, at a new drill site, only about 20-40% of the oil is initially obtained. EOR can help recovery around 5-30% of oil that would otherwise be missed (Abidin et al.,
Carbon dioxide utilization (CCU) can circumvent these issues, in that the CO, is chemically converted and can be considered as a more permanent “molecular” sequestration. As mentioned earlier, one of the main issues with CCU is that CO, is a very stable molecule and requires either highly reactive co-reagents or large energy inputs in order to convert it into anything chemically useful. However, research has shown there are several viable routes for utilization, like polymer synthesis, fuel production, and biological conversion. While large scale implementations of these practices are not yet common, it has been argued that they have the potential to mitigate climate change and could help lead to a low-carbon economy (Barbato et al., 2014). With proper integration and optimization, more routes for CCU could become economically viable and turn CO, into a useful feedstock (Panu et al., 2019; Alsuhaibani et al., 2019; Tilak and El-Halwagi, 2018; Afzal" et al., 2018; Pokoo-Aikins et al.. 2010).
With the advancements in horizontal drilling and fracking techniques, the natural gas trapped inside shale formations has become much more accessible. Due to the rock’s low permeability, normal drilling techniques couldn't adequately release the gas. Shale gas is quickly becoming a dominant source of energy and feedstocks (Elbashir et al., 2019; Al-Douri et al., 2017). Like natural gas, shale gas consists primarily of methane along with other heavier hydrocarbons. Small fractions of the gas are also composed of CO,, nitrogen, and sometimes even hydrogen sulfide. The exact composition of the gas varies from well to well, which may also contain other trace chemicals. Before the gas can transported along pipelines, it must go through processing in order to remove impurities down to acceptable levels. One of the first steps of this process is the acid gas removal stage, where most of the CO, is separated. What is interesting about this stage is how pure the CO, stream exiting from this acid gas removal stage is. It is typical to have streams that are around 99% pure CO„ yet most often this stream is r ented directly into the atmosphere (Grobe, 2010). There is growing interest in using CO, for enhanced gas recovery (EGR), however this is nowhere near as prevalent or developed as EOR. While EGR and EOR are usually the main alternatives to venting the CO„ not much focus is present on utilizing the stream for purposes of CCU and creating value-added products.
Mineralization is a CCU method where CO, is reacted with calcium- and magnesium-containing minerals to produce carbonates. What is unique about this, compared to other CCU processes, is that the conversion of CO, into carbonate is thermodynamically favorable and exothermic. With this transformation into a more stable molecule, the CO, is permanently stored in a solid matrix. Two main routes exist for CO, mineralization: In situ and ex situ. In situ mineralization involves injecting pressurized CO, underground to react with minerals and form carbonates. Ex situ instead uses mined minerals or alkaline materials and reacts them with CO, in controlled conditions. While there are abundant geological minerals that are capable of mineralizing CO„ like olivine, serpentine, and wollastonite, the in situ method only serves as a permanent form of CO, storage and does not contribute to creating any valuable products (Gadikota and Park. 2015). One of the major downsides of mineralization as a form of CCU is that the reaction is kinetically limited. In situ mineralization is a slow process and one of the benefits of the ex situ method is the capability to carefully control and optimize the process to the point of reactions only taking hours rather than decades. However, in order to reduce reaction times, there is a large penalty to pay in terms of energy. One of the first steps in ex situ mineralization is to grind down alkaline minerals to increase surface area usually to particle sizes on the order of 10-100 pm (Gadikota and Park, 2015). This process, along with high reaction temperatures, is quite energy intensive and careful consideration must be taken to ensure this energy demand is not creating more CO, than it is utilizing. Table 1 summarizes key properties of common uon-carbouated minerals.
Additional steps may be taken in order to help facilitate the carbonation process. Acid dissolution is one way of chemically preparing the minerals for easier reactivity. In a two-step process, the dissolved mineral ions are then subjected to basic aqueous conditions where CO, is bubbled through. Catalysts and
Table 1. Mineralization properties of common non-carbonated minerals.
Adapted from sources Gadikota and Park, 2015; Zhao et al., 2013.
chelating agents have also shown potential in further accelerating the process. When operated under the right conditions, relatively pure products, like calcium carbonate, magnesium carbonate, and silica, are some of the possible value-added materials to be obtained. Applications for these products range from paper, plastic, and construction fillers to glass and ceramic materials.
An interesting approach to avoid some of these complications is to utilize alkaline industrial waste sources as a substitution to these natural minerals. Common wastes, like fly ash and waste cement, are high in calcium content and can undergo carbonation to produce calcium carbonate products. As some of these sources already exist as particulate matter, there is little to no need for comminution.
Problem Statement and Approach
The main objective of this chapter is to investigate the prospects of utilizing and taking advantage of the high purity CO, stream produced from shale gas processing and integr ating it with the CCU process of mineralization in order to create a sustainable and profitable system. Specifically, industrial waste will be investigated as a feedstock for mineralization due to its high carbonation potential and additional sustainability implications. Several waste sources will be considered and screened through an initial economic analysis in order to determine viability. Through literature analysis and reported experimental data, the integrated carbonation processes will be simulated using Aspen modelling as a method of detailed analysis. The optimized and finalized flow sheets will help evaluate mass and energy consumptions as well as estimations on equipment costs in order to fully evaluate the capital and operating costs of implementing the integrated processes. This will determine which processes are viable, profitable, and worth pursuing.
In order to get the most out of purchased and installed equipment for the carbonation process, it would be preferable to have a system that is adaptable to a variety of waste sources without the need for individualized reaction or separation vessels. Figure 1 shows this concept of a very simple and generalized process diagram which highlights the most important units. Here, the way in which the two processes of shale gas processing and waste mineralization are integr ated with the connection of a carbon dioxide exchange stream is clearly visible. This CO, stream is fed into a reaction vessel along with the chosen industrial waste particles and brine solution where carbonation occurs. Next, the mixture is transferred to a separations unit, where reacted carbonate products are collected and unreacted materials and solvent are recycled back to the reaction vessel.
Figure 1. Generalized flowsheet of integrated gas and mnieralization processes.
Waste source considerations
For the purposes of this work, industrial waste sources will be considered, based some of the following characteristics: Calcium content, availability, composition variability, and initial physical properties, like particle size. Obviously, the ideal waste source would have high calcium content, small particle sizes, and would be readily available in large quantities. However, industrial wastes vary greatly from one location to another and even within the same processing plant. In this section, some of the more viable waste sources will be considered and discussed.
2.1.1 Fly ash
Fly ash is generally produced as a byproduct of coal combustion but can be produced through other combustion processes, like municipal solid waste incineration (MWSI). The current production of fly ash is estimated to be around 500 million tonnes globally, with only around 16% of it being utilized in ways other than being disposed of in landfills (Ahmaruzzaman, 2010). Fly ash particles consist of toxic trace elements which can lead to environmental concerns when disposed of without treatment. While compositions vary, fly ash is broken into two classes: Class F and Class C. The main difference between the two is the calcium, silica, and iron content. Class F ash contains around 1-12% calcium while Class C contains around 30-40% calcium content. Fly ash exists as fine spherical particles, typically with sizes around 75 pm and surface areas as high as 1000 nr/kg. As a hazardous byproduct, the cost of purchasing fly ash mostly consists of transportation costs which approximate to around $15/ton (Ahmaruzzaman, 2010). Compared to the criteria for what is considered a desirable waste source, fly ash is a highly viable option for mineralization due to its abundance and high calcium content.
2.1.2 Waste ceinent/cement kiln dust
As buildings are demolished and waste concrete is pulverized, powder byproducts formed as aggr egates are recycled. This powder is known as waste cement powder, or simply waste cement. This waste cement can make up as high as a thud of total waste concrete and currently is mostly used as roadbed material or is disposed of. Waste cement averages around 30% calcium content and has a typical particle size distribution of around 10-200 pm (Katsuyama et al., 2005). The source of cement kiln dust (CKD), a byproduct of the cement manufacturing process, is similar. Cement manufacturing produces millions of tons of CKD annually, the majority of which is disposed of in landfills. Calcium oxide content can range from 20-60% with compositions varying depending on where the CKD was obtained (Huntzinger et al., 2009). With around 15-20 tons of CKD produced for every 100 tons of cement, it is also a highly abundant waste source (Bobicki et al., 2012). Both sources are also potentially hazardous, but due to their abundance and small particle sizes, it is likely mineralization could be used to viably transform them into safer, useful products.
2.1.3 Steehnakiiig slag
Steel slag is a byproduct of the steel manufacturing process. Initially a molten liquid, steel slag cools into a mixture of oxide and silicate materials. This can refer to multiple steps of the process and the waste may have corresponding names, like furnace slag or ladle slag. Typically, steelmaking slag contains around 25-55 wt% calcium oxide and has been proven to effectively produce calcium carbonate at relatively low pressures and moderate temperatures with proper solution conditions (Romanov et al.,
2015). Steehnakiiig slag is produced globally at about 200 Mt annually and is typically formed at a ratio of 0.2 tons of slag for every ton of steel (Said et al., 2013). Size distributions vary greatly from source to source, with ranges on the order of 1 mm'1 cm, and may require further comminution (Lekakh et al., 2008). As with the other sources, efficient CO: utilization will depend on the characteristics of the slag undergoing carbonation. but present work has shown promising results for a variety of steehnakiiig slag compositions and reaction conditions.
2.1.4 Other wastes
While the waste sources above are the most prevalent, there are a vast amount of other alkaline industrial waste sources, like red mud and other process waste. While these sources may also har e high alkalinity and small particle sizes, the main limitation will be the availability for industrial scale operations.
Brine water as a solution
Saline wastewater is a common byproduct of the production process of oil and gas. About 20-30 billion barrels of this wastewater is produced annually in the U.S., where around 65% is reinjected into well sites as a means of pressure control (Soong et al., 2006). The remaining wastewater is typically either treated or discharged. Discharging saline water into the environment has obvious negative implications and treatment costs can range anywhere from a few cents to a few dollars per barrel. Part of this discrepancy is due to differences in local regulation standards, but another reason stems from the fact that brine has varying concentration of Ca, Mg. and Fe along with the standard Na and Cl ions. While adjustments need to be made to the brine water, its utilization as a medium for mineral carbonatiou could lead to a higher productivity due to the presence of these ions, specifically calcium.
Carbonate formation occurs under pH conditions of 7.8 or higher. Because the typical pH range of brine is about 3 to 5, it requires modification before carbonation can happen. The addition of the industrial waste material will help to increase the pH of the brine and can reach reaction conditions with enough waste. However, the pH can be more readily adjusted with the addition of bases like NaOH. While additional reaction reagents like NaOH could help to decrease process tune, its environmental and economic impacts need to be carefully considered. Existing research on what conditions are optimal for carbonatiou in brine (pressure, temperature, etc.) is limited, but simulation optimization will hopefully help to supplement these missing parameters.