Table of Contents:
: Alternative Pathways for CO2 Utilization via Dry Reforming of Methane
The reforming of methane is an important step in the production of a valuable chemical precursor “Syngas” or synthesis gas. Syngas is a mixture of carbon monoxide and hydrogen and serves as an important intermediate to produce a large range of value-added chemicals (i.e., methanol, acetic acid, dimethyl ether, etc.) and ultra clean fuels via Fischer Tropscli (FT) technology (Abusrafa et al., 2019; Alsuhaibaui et al., 2019; Choudhury et al., 2019). Commercially, the reforming of methane is done via three well-known technologies: Steam Reforming of Methane (SRM). Partial Oxidation of Methane (POX) and Autotliermal Reforming (ATR). All the aforementioned reforming technologies utilize oxidants to chemically convert methane to syngas. For instance, SRM utilizes steam at high pressure, while partial oxidation utilizes oxygen to produce syngas. ATR utilizes a combination of both steam and oxygen in a specific ratio that yields a syngas ratio compatible with a downstream FT reaction, while allowing the reaction to occur under auto-thermal condition. Hie auto-thermal condition signifies a point in temperature wherein no external heat is required to drive the reaction, in other words, the reaction produces enough energy in situ to drive itself spontaneously. Equations (l)-(5) summarize the stoichiometric reactions and associated energy requirements for SRM. POX and ATR reactions:
First introduced in the 1930s (Van Hook, 1980), SRM technology was implemented at a very small scale and was limited to a few locations in the United States with easy access to natural gas. The major development of this technology took place in the 1960s, when ICI started two reformer plants using tubular reactors at high pressure condition using naphtha as the primary feedstock (Rostrup-Nielsen, 2004). However, almost a decade before ICI setup then facility, Haidar Topsoe had designed then first reformer and hydrogen plant based on the SRM technology at 40 bar pressure (Rostrup-Nielsen. 2004). The primary utilization of syngas from SRM was for hydrogen production for the Ammonia plants, which is a starting material for urea fertilizer. The implementation of Topsoe technology had significantly reduced the energy demands of the ammonia plants and resulted in significant cost savings (Dybkjaer, 1995). Further energy reductions were realized when M.W. Kelloggs built integrated reformer plants combining SRM with steam turbines. As naphtha was the primary feedstock in Europe, the steam reforming of naphtha became a key technology in building up the town gas industry of the United Kingdom, which consequently replaced erstwhile low-pressure gasification processes. Apart from the technologies of ICI and Topsoe (Rostrup-Nielsen, 2004), SRM at low temperatures under adiabatic conditions, which was pioneered by British Gas, served as an attractive option to produce methane rich gas for heating and other utility purposes (Appl and Gossling, 1972). Later, the focus of SRM was also shifted to methanol, acetic acid synthesis and other oxo-alcoliols apart from Ammonia production (Rostrup-Nielsen, 1984). The low-temperature reforming process developed by British Gas had served to be an important pathway for conversion of naphtha to methane duiing the energy crisis of the 1970s. The application of SRM was not only limited to the production of hydrogen, methanol and other petrochemicals, but could also be used in the n on and steel industry for direct reduction of non-ore (Appl and Gossling, 1972; Mondal Kartick et al., 2016). The trend in 1990s was mostly towards the hydrocracking of heavy hydrocarbons for production of gasoline fuels in refineries (Rostrup-Nielsen and Rostrup-Nielsen. 2002). The later part of the 1990s and the early 2000s saw a great demand for hydrogen production from SRM mainly due to two reasons: (a) the availability of cheap natural/shale gas. and (b) the boom in fuel cell technology and refineries around the globe. Although renewable technologies, such as wind, solar, biofuels and other alternative technologies, drew great attention in 2000s, the dependency on SRM for hydrogen production has never declined (Rostrup-Nielsen and Rostrup-Nielsen, 2002).
The mam challenge associated with SRM from the tune of its inception was its inherent ability to produce solid coke at low steam to carbon ratios. This challenge was addressed by operation at extremely high temperatures (> 1000 °C), and in the presence of large quantities of steam. The presence of enormous quantities of steam however lead to lower conversions and resulted in a significant rise in energy costs. Nevertheless, the utilization of SRM was never less popular throughout its age. Another less commonly adopted method of carbon reduction was to passivate with Sulphur (Rostrup-Nielsen, 1984), however, this resulted in the contamination of products, leading to issues in downstream FT and other synthesis processes. Noble metals, such as Pt and Rli, also provided an alternative catalytic system to Ni, but economically less viable options for reduction of coking tendency of the SRM reaction (Trimm, 1997).
In later 1970s, CO, utilization as a part of SRM provided an attractive pathway of stabilizing the syngas ratio of the SRM product (Rostrup-Nielsen et al., 1988; Rostrup-Nielsen, 2002). However, like SRM. the possibility of coke formation in the combined reaction was extremely high. Formation of coke from this reaction had always been a crucial factor limiting the application of this reaction on standalone basis. However, the presence of CO, in SRM was still uecessaiy for adjusting the quality of syngas for downstream applications such as FT and methanol synthesis reactions that require a H,:CO ratio of 2:1. The introduction of ATR technology also provided an attractive, albeit expensive, pathway to alleviate the coking tendency in SRM since its inception in the early 1980s (Rostrup-Nielsen et al., 1988: Rostrup- Nielsen. 2002). Apart from the SRM process, POX has also been a well-known and established technology since the 1940s (Prettre et al.. 1946). POX is relatively simple reaction compared to SRM and DRM. as it typically represents the partial combustion of methane. The challenge with this reaction is the cost associated with the production of pure oxygen. However, the benefit is that it could be conducted under homogenous conditions at high temperature without the need for a catalyst. Low temperature operation requires a catalyst, which would result in lower energy requirements than high temperature homogeneous reactions. Additionally, this reaction could also be beneficial in generating high pressure steam as the combustion process is highly exothermic. In 2007, Shell built the world’s largest gas-to-liquids (GTL) plant, the Pearl GTL Plant. The plant uses POX reforming technology to produce syngas and has the world's largest air separation units (ASU) for oxygen generation. Due to the benefit of economy of scale, the syngas production by POX at a large scale (140.000 bbl/day syncrude capacity plant) is much cheaper than smaller scale plants (Wood et al., 2012). The combination of POX and SRM as ATR has also gained significant traction recently. A joint venture between SASOL and Qatar Petroleum (QP) built a state-of- the-art GTL facility, the ORYX GTL plant, in Ras Laffau, Qatar using ATR technology. The capacity of this plant is about 35,000 bbl/day of syncrude products, mainly diesel, and naphtha. In this backdrop, the Dry Reforming of Methane (DRM) process becomes important to consider since it can produce syngas by utilizing CO, as the oxidant, as shown in equation (6). Since CO, is an abundantly available in the flue gas streams (albeit at lower concentrations), it is anticipated that using it will reduce the operating costs and carbon footprint of syngas production.
The major challenges associated with DRM are the high coking tendency, high endothermic nature and low quality of syngas ratio. Syngas ratio is particularly important as only a specific quality of syngas is desirable for downstream application. For instance, in FT reaction and methanol synthesis, a syngas ratio of 2:1 is acceptable. However, as evident from equation (6) below for DRM. the H,:CO ratio is 1:1 only. Also, due to the reverse Water Gas Shift reaction activity at high temperatures, the H,/CO ratio is lowered further.
The energy demand to drive the reaction is also very high, it requires about 1.2 tunes more energy than the endothermic SRM process. The CO, that results from combusting methane or other fossil fuels to di n e this reaction can significantly increase the carbon footprint of the DRM process. Nevertheless, if the reaction is coupled with other renewable sources of energy like solar or from excess heat from other parts of the plant, the CO, fixation from this process would be meaningful.
Carbon formation in the DRM process generally takes place via following pathways:
The aforementioned reactions (Methane decomposition and Boudouard reaction) are highly active at low temperature conditions (400 °C to 650 °C), while at high temperatures (beyond 800 °C), the rate of primary reforming reactions is much higher, resulting in significantly lower coke formation. The other problem with coke formation is that it physically damages the reactor tube that would either require serious maintenance or complete replacement of the tube. Due to these three limitations, DRM is still considered to be a grey area that requires further investigations.
Based on extensive modelling and experimental work, it was observed that DRM can be implemented either in combination with other reforming technologies (Clialliwala et al.. 2017; Challiwala et al., 2017; Elbashir et al., 2018), or by implementing a separate scheme that could either handle the carbon formed or adjust the syngas ratio as per downstream requirements. The two new pathways developed for DRM reaction based on the concept of reaction segr egation are as follows:
i) CARGEN (Or CARbou GENerator) Technology—a combination of low temperature carbon forming process and high temperature syngas reformer using a two-reactor setup (Elbashir et al., 2018).
Figure 1. Strategy to address issues relating to DRM.
ii) DRM+COSORB Technology—a combination of DRM and a separate CO absoiptiou unit (COSORB—CO AbSORBtion) that removes CO to adjust the syngas ratio that meets downstream specifications. Figure 1 below pror ides a flowchart describing the options available to manage and implement DRM technology on a larger scale (Afzal et al., 2018).
In this chapter, these two process configurations are discussed in terms of the concept development, the merits and the challenges for implementation of these technologies.