Analysis of CO₂ utilization for methanol synthesis integrated with enhanced gas recovery

Highlights

• Integrated sequestration and utilization concepts for optimum CO₂ abatement.

• Analyzed and compared four different process integration scenarios.

• Developed performance metrics based on resource uptake, energy and CO₂ emission.

• Integrating methanol synthesis with EGR plants showed best performance metrics.

• Natural gas with high CO₂ content from EGR is tolerable in the methanol plant.

Abstract

This paper investigates a methanol (MeOH) synthesis route based on CO₂ utilization integrated with enhanced gas recovery (EGR) and geo-sequestration (e.g. depleted gas wells near EGR unit). A key driver behind this work is the need to understand the proposed MeOH synthesis route in relation to the dynamics of the CO₂ breakthrough phenomenon associated with EGR. The performance of the proposed MeOH synthesis is evaluated via model analysis of various process flowsheet configurations and reported in terms of CH₄ intensity, CO₂ intensity, thermal energy intensity, methanol productivity and CO₂ uptake flexibility. It is found that the proposed MeOH scheme can effectively consume natural gas (NG) with relatively high CO₂ content (up to 23.2% mole). Simulation results show that the proposed MeOH synthesis process configuration, utilizing EGR and using CO₂ geo-sequestration and utilization components, results in the highest CO₂ abatement intensity (45.5%) among other comparable scenarios investigated here.

Introduction

In light of recent reports related to the threats of climate change, mitigating anthropogenic CO₂ is an imperative task in the 21st century (IPCC, 2013). To date, the most widely employed CO₂ mitigation mechanism is by capturing the CO₂ produced from stationary power generation sources (e.g. coal-fired power plants (PP)) due to their large contribution in total CO₂ emission (about 78%) (EIA, 2013). The captured CO₂ can then be sequestered in geological storage (i.e. carbon capture and sequestration (CCS)) or/and utilized for production of valuable intermediate chemicals and products (i.e. carbon capture and utilization (CCU)) (Styring et al., 2011). Fig. 1 shows some existing practices for CO₂ utilization (CCU) and sequestration (CCS). For example, captured CO₂ from power plants can be either utilized to make methanol or sequestered in depleted gas/oil fields.

The CCS approach offers an attractive means to abate a substantial amount of CO₂ (estimation for worldwide geological storage capacity is about 1700 Gt_CO2, an equivalent of 80 years of the global net CO₂ emission (Hendriks et al., 2004)). However, the high cost associated with carbon capture most likely to be burdened by the CO₂ emitting plants is one of the major obstacles (Styring et al., 2011). As a result, the most efficient way to incentivize the capture of CO₂ is by familiarizing the emitting companies with the potential markets for the captured CO₂. This perspective supports the CCU approach in which the captured CO₂ is commoditized as a co/primary-reactant for the productions of valuable products (Styring et al., 2011). Although CCU remains as an attractive approach, the market for the utilization of captured CO₂ is relatively small (11%–17%) compared to the total production of CO₂ (Aresta, 2010, Styring et al., 2011). Considering the dynamic nature of electricity demand, there might be mismatch between fluctuation in carbon capture rate and the demand for CO₂ from utilization units. Moreover, the core concept of CCU is the recycling of CO₂ (i.e. not mitigating) (Styring et al., 2011). Thus, without an appropriate scale of atmospheric CO₂ capture to close the carbon loop, the recycled carbon will eventually be released back into the atmosphere in the form of CO₂ (e.g. the lifetime for urea and methanol is about 6 months) (IPCC, 2005).

While the standalone approaches (i.e. either sequestration or utilization) have associated strength and weakness, an integration of CCS and CCU – known as CCUS – might be able to effectively utilize both advantages of the respective approaches, i.e. enhancing both environment and economic incentives (Styring et al., 2011). In other words, the economic incentive provided through CCU might be able to drive large-scale CCS projects, while large scale abatement capability of CCS helps to ensure CO₂ mitigation objective is achieved. The integrated framework of carbon capture sequestration and utilization suggests merging of different entities within the framework in order to explore potential improvements in both economic and environmental aspects. There are several known CCUS concepts in the literature (Huijgen and Comans, 2003, Li et al., 2015, Mazzotti et al., 2009, Melzer, 2012, Zevenhoven et al., 2014). Li et al. (2015) proposed a geoengineering based CCUS approach which is an integration of CO₂ geological storage and deep saline water/brine recovery. Other applications of CCUS are enhanced oil recovery (EOR) and enhanced coal-bed methane recovery (ECBMR) which can increase the economic lifetime of oil and coal-bed methane reservoirs as well as providing excellent grave ‘geological bottles’ for CO₂ storage (Mazzotti et al., 2009, Melzer, 2012). Another breakthrough improvement (in terms of cost reduction) resulted from integration between entities within CCUS framework is a mineral sequestration process integrated within a power plant, in which the CO₂ capture and sequestration units promisingly can be combined into one unit (Huijgen and Comans, 2003, Zevenhoven et al., 2014). From the aforementioned applications, the integration between entities within the CCUS framework is a vital perspective in light of CO₂ emission mitigations.

Although, enhanced gas recovery (EGR) might be viewed as an impediment to the long-term strategy for converting to a cleaner energy model, one must account for the role and magnitude of EGR in increasing NG recovery which helps to facilitate this transition. As the name implies, EGR might be able to enhance the economic life of NG reservoirs (e.g. conventional NG, shale gas, coal-bed methane and tight gas). As a result, EGR provides a geological storage option for the captured CO₂ as well as transforming the captured CO₂ into a value-adding chemical (Câmara et al., 2013). Unlike other reservoirs, conventional NG reservoirs (considered in this study) is relatively easy to be explored due to their favor physical characteristics (i.e. porosity and permeability). However, due to the decline in reservoir pressure as a result of NG exploration, on average approximately 25% of the NG remains inaccessible by using conventional extraction methodologies (IEA, 2009). The fundamental principle of EGR is to utilize the remaining inaccessible NG by injecting a cushion gas, e.g. CO₂, N₂, with suitable physical characteristics (density, viscosity etc.) into the lower portion of the reservoirs. Oldenburg (2003) showed that in the interested pressure range of EGR (60–130 bars); CO₂ has a density higher than CH₄ by 2–6 folds. In addition, CO₂ is also more viscous than CH₄ which gives CO₂ a lower mobility ratio. Due to those two favorable physical characteristics of CO_2, EGR can be carried out in gas fields by injecting CO₂ into the lower portion of the reservoir in order to re-pressurize the NG reservoir and ‘pushing up’ the native gas (Meer, 2005). Using CO₂ as a cushion gas for EGR has received attention in the literature due to the additional benefit from substantial CO₂ long-term abatement which might generate economic incentive through carbon pricing scheme (Oldenburg, 2003). A comprehensive study of CO₂ storage in depleted gas fields conducted by International Environment Agency (IEA), confirmed that in the year of 2050, about 13 years of total CO₂ production (based on 2015 CO₂ emission rate) can be stored permanently in available depleted gas fields (IEA, 2009). The magnitude of the NG enhancement reported in the literature based on simulation and experimental studies, varies from 0% to 25% depend on various factors, e.g. geological characteristics, CO₂ injection strategies, etc (Hussen et al., 2012, IEA, 2009, Khan et al., 2013, Meer, 2006, Oldenburg et al., 2003, Turta, 2003).

Despite the potential noticeable enhancement in NG recovery, application of EGR with CO₂ injection are limited because of the potential CO₂ breakthrough due to extensive mixing which will deteriorate the NG integrity (Meer, 2005). Two test fields for EGR with CO₂ injection were carried out in Budafa-Szinfeletti, Hungary (reported about 11.6% NG recovery enhancement) and Gaz-de-France-K12-B, Netherlands (reported no clear evidence of positive effect of EGR) (Meer, 2006, Turta, 2003). The large-scale EGR with CO₂ injection project named ‘CLEAN’ from 2008 to 2011 in Altmark Natural gas field provided a valuable site specific knowledge for future EGR projects (SubSealQ, 2013). Carbon dioxide breakthrough is a well-recognized phenomenon in EGR with CO₂ injection in which the injected CO₂ starts blending extensively with the native gas through diffusion and dispersion mechanisms (Kamalipour et al., 2014). This associated phenomenon can increase the CO₂ content in NG significantly (reported up to 60.5% (Hussen et al., 2012)) and there are high uncertainties about when this breakthrough may occur. Khan et al. (2013) simulated EGR with CO₂ injection and reported CO₂ mole fraction up to 30% as a result of CO₂ breakthrough. Oldenburg et al. (2003) simulated the Rio Vista gas field in Central Valley of California and reported a rapid acceleration of CO₂ breakthrough within 5 years that increases the CO₂ mass fraction in the raw NG up to 50%.

Methanol (methyl alcohol, CH₃OH or MeOH) is an important chemical that can be regarded as fuel, a hydrogen carrier or a feedstock to produce more complex chemical compounds. Moreover, the role of MeOH synthesis in delivering a cleaner energy model is reflected through the so-called ‘Methanol economy’ – a novel conceptual framework proposed by Olah et al. (2009). Carbon dioxide can be utilized in methanol production process via reforming reactions, e.g. steam methane reforming (SMR), dry methane reforming (DMR), bireforming, and trireforming (Olah et al., 2009). A NG feed composition of up to 50% mole of CO₂ and 50% mole of CH₄ can be accommodated in DMR-MeOH reaction (MeOH production coupled with DMR) (a).

$$C{H}_4+C{O}_2+2H_2\leftrightarrow 2C{H}_{3}OH\left(a\right)$$

While the SMR-MeOH route utilizes NG mixture with relatively lower CO₂ content (25% and 75% mole of CO₂ and CH₄, respectively) as shown in reaction (b).

$$\frac{3}{2}C{H}_4+\frac{1}{2}C{O}_2+H_{2}O\leftrightarrow 2C{H}_{3}OH\left(b\right)$$

Therefore, although CO₂ breakthrough might occur in EGR, the produced NG mixture can still be used in methanol production process via reforming reactions (a and b) depending on the extent of the breakthrough phenomenon.

In the Business-As-Usual (BAU) operation of NG production plant, native CO₂ in NG stream must be removed to enhance the integrity of NG and maintain high methane composition. However, in MeOH production plant, the presence of native CO₂ in NG can be tolerated without a major CO₂ separation process as demonstrated by reactions (a) and (b). This flexibility is a large advantage point in methanol synthesis process. Once the ‘culture’ of the two plants is merged (i.e. NG and MeOH plants are integrated) CO₂ removal step in NG production plant potentially can be eliminated as the MeOH plant can accommodate (up to a certain limit) a mixture of CH₄ and CO₂ in the reforming reactions (Olah et al., 2009). The benefit of such integration is more evident in the sour gases (native CO₂ and H₂S) removal step, where a solvent with lower energy demand for regeneration (e.g. tertiary amine (DMEA) compared to primary amine MEA) can be used to selectively remove mainly H₂S (Alonso, 2015). Potentially, this might facilitate the Sulphur recovery step as the removed acid gas stream is mainly H₂S. The utilization of native CO₂ for MeOH production might bring significant benefits for NG reservoirs that have high native CO₂ content. One typical example is the giant Natuna gas field in Indonesia which was discovered in 1970, however, commercial activities did not take place due to the high level of CO₂ (up to 70%) contained in the gas (IEA, 2009, SubSealQ, 2013). By integrating the NG production chain with MeOH production, most of the native CO₂ can be utilized for MeOH production, thus eliminating the need for extensive CO₂ removal to meet the BAU gas quality standards.

From all the above, by coupling EGR (as a mean for CO₂ sequestration and enhancement of NG production), with MeOH synthesis plant (as a mean for CO₂ utilization) as shown in Fig. 1, the potential benefits are as follows:

• The enhancement in NG recovery (reported in the range of 0%–25%) (Hussen et al., 2012, IEA, 2009, Khan et al., 2013, Meer, 2006, Oldenburg et al., 2003, Turta, 2003), thus subsequently increasing the MeOH productivity.

• Providing a possible solution for the grand challenge of CO₂ breakthrough associated with EGR process by directly utilizing the native CO₂ in NG within the MeOH synthesis route.

• Potential cost reduction in NG purification due to the elimination of CO₂ removal step because the mixture of CH₄ and CO₂ is tolerable for reforming reactions, especially via DMR, SMR and bireforming routes.

• Substantial permanent CO₂ abatement by storing CO₂ in EGR process.

• Increasing the economic value of captured CO₂ as a cushion gas in EGR process.

Despite those attractive potential benefits, to the best of our knowledge, there are few studies that have addressed the integration aspects of NG production coupled-with-EGR and MeOH synthesis. Al-Megeren and Xiao (2012) only briefly mentioned the opportunity for utilization of native CO₂ in raw NG in bireforming to produce syngas with a suitable composition for Fischer-Tropsch synthesis. Most publications have only focused on CO₂ sequestration within EGR domain or CO₂ utilization in methanol synthesis. Hussen et al. (2012) conducted model-based analyses for EGR injection strategy and concluded that high CO₂ injection rate into late stage of the gas field can provide desirable outcomes. Khan et al. (2013) performed simulations and analysis on EGR and concluded that it is uneconomical to continue NG exploitation when the fraction of the associated CO₂ reaches 30% mole (around 15 years after EGR commencement). However, this mixture of NG (30% mole CO₂) might be suitable for SMR-MeOH route (b). Olah et al. (2009) proposed MeOH production via syngas that can utilize a mixture of up to 50% mole in CH₄ and CO₂. Milani et al. (2015) analyzed a novel CCU-MeOH (CO₂ utilization in MeOH synthesis) process configuration and reported 25.6% reduction in CH₄ uptake compared to a conventional NG-based MeOH process. Taghdisian et al. (2015) also investigated the CCU-MeOH integration scheme and further proposed a multi-objective optimization methodology for minimizing CO₂ emission while maximizing MeOH productivity. This paper presents and analysis an application of CCUS by integrating the EGR with MeOH production based on the CO₂ utilization route as shown in Fig. 1. This integrated scheme will allow for the CO₂ in the raw NG and from power plant to be utilized in MeOH synthesis. For this purpose, the captured CO₂ is sent respectively to the NG production plant to be used in EGR and also to MeOH plant to produce methanol at lower methane intensity. The NG mixture (CO₂ and CH₄) from the NG production wells is sent to the MeOH plant to react with the captured CO₂ in the reformer (based on bireforming reaction) and produce syngas mixture suitable for MeOH synthesis. For comparison purposes, as shown in Fig. 2, four scenarios (including the aforementioned CCUS scheme) are considered:

• BAU (Business-As-Usual) scenario where power plant (PP) and MeOH production chain (including NG plant with no EGR) operate as standalone units.

• CCS scenario where PP is coupled with a post-combustion carbon capture (PCC) plant (PP + PCC) and the captured CO₂ is sent to geo-sequestration while MeOH production chain (including NG plant with no EGR) operate independently.

• CCU scenario where PP + PCC and MeOH production chain (including NG plant with no EGR) are integrated. Captured CO₂ is only used to synthesize methanol.

• CCUS scenario (this study) where PP + PCC and MeOH chain (including NG plant with EGR) are integrated. Captured CO₂ in this scenario is diverted into three places: EGR, MeOH plant, and geo-sequestration. Geo-sequestration (e.g. depleted gas wells near EGR wells to avoid the need for long-distance CO₂ transportation (Diamante et al., 2014)) is deployed only to stabilize the operation of MeOH and EGR due to possible fluctuations in PP + PCC load.

In the present study, brief descriptions for each process: PP + PCC, MeOH plant, and NG plant with and without EGR are presented. A representative and rigorous case study for EGR reported by Hussen et al. (2012) is adopted in this study. Own elaborations are performed, based on the reported results for the selected case study, to determine the production rate profile for CH₄ and CO₂ with and without EGR. These variables are used as an input into MeOH synthesis simulations. A comprehensive model of CO₂ utilization in a reformer coupled with MeOH synthesis reactor is developed using Aspen Plus V8.6 (AspenTech, USA). The objectives of this study are to seek answers for the following questions: 1) how and at what extent that the CO₂ breakthrough phenomenon associated with EGR can be effectively digested within the proposed CCUS scheme? 2) what are the proportions of each operation fragment: geo-sequestration, EGR and CO₂ utilization that makes CCUS scenario more attractive approach compared to the other three scenarios? To address the above objectives, simulation results are analyzed to access five key metrics of each scenario, namely CO₂ emission intensity, CH₄ intensity, energy intensity, MeOH productivity and CO₂ uptake flexibility. The methodology of calculating CO₂ emission is equivalent to that adopted by Milani et al. (2015). Comprehensive comparisons between scenarios based on those aspects are performed by using the spider-web analytical technique which is a graphical analysis tools suitable for multiple criteria assessments and evaluations for associated strength and weakness of each scenario.

The outcomes from this study may enhance the integration between carbon capture, sequestration and utilization to be more attractive approach compared to polarized applications (i.e. CCS or CCU). In addition, this study is expected to contribute to the research progress for enhancing the feasibility of EGR by bridging the gap between current view about EGR (CO₂ breakthrough might not be economically tolerable) and the proposed view of EGR in this study (CO₂ breakthrough might be accommodated in MeOH production).