Study on Technology Clusters for Direct Utilization of CO2-Rich Natural Gas and

Wu Qing

(Department of Science and Technology Development, China National Offshore Oil Corporation, Beijing 100010)

Abstract: Based on industrial production with an annual capacity of million tons of methanol, ammonia/urea, etc., a platform technology is developed for direct, green, efficient, and high-value mega-size utilization of the CO2-rich nature gas, which is the technology of CO2-rich natural gas dry reforming and hydrogen reaction. The following technologies are discussed, such as CO2-rich natural gas dry reforming integrated with the Fischer-Tropsch synthesis to ole fins (FTO)technology for producing high value-added linear alpha ole fins (LAO); CO2-rich natural gas dry reforming integrated with low carbon ole fin linear hydroformylation technology to produce higher carbon alcohols; direct methanol production from CO2 and hydrogen; and the new cutting edge technology of photo-catalytic process. In addition, simple techno-economic evaluations of two technologies mentioned above are discussed. The CO2-rich natural gas dry reforming integrated with FTO technology can achieve about 30% of internal rate return (IRR), while the low carbon ole fin linear hydroformylation technology could have a static payback period of 2.57 years when the capacity of 2-propylhexanol (2-PH ) reaches 100 kt/a.Based on the mega-size green and high-efficient CO2-rich natural gas direct utilization technology, a hybrid energy and chemical production system framework with good prospects is preliminarily designed. A modern industry zone with an annual capacity of more than 10 Mt of CO2 converted to high value-added products is underway.

Key words: CO2-riched natural gas; dry reforming; Fischer-Tropsch to olefins; Fischer-Tropsch to aromatics; linear hydroformylation; techno-economic evaluation

1 Introduction

Natural gas is the most important low-carbon fossil fuel. China’s natural gas consumption is increasing rapidly these years, while up to 45.3% of gas supply were imported from abroad in 2018, which increased by 6.3 percentage points compared with the gas imports in 2017[1]. Thus, it is an urgent task for China to accelerate the natural gas exploitation and utilization.

The South China Sea holds the most oil and gas resources in China, accounting for about one-third of China’s total oil and gas resources, which are equivalent to 12% of the world’s total[2-3]. More than 83% of the oil and gas resources in this region are natural gas, which generally contains 20-80% or even more than 92% of carbon dioxide (CO2) that is referred to as “CO2-rich natural gas”in this paper. According to the commercial natural gas transmission regulations, the CO2content in natural gas should not be higher than 2% (0.2% for LNG)[4]. Thus, to utilize these CO2-rich natural gas resources in traditional industrial and residential sectors, a decarbonization process is necessary.

Currently, the technology of separating CO2from natural gas will cause a large amount of energy consumption and 2.5%—7% of methane loss[5], while the GHG effect of methane is 29 times as much as CO2. As a result, the mega-size utilization of CO2-rich natural gas in the South China Sea must solve the CO2issue to avoid the waste of resource and energy in an attempt to grapple with environmental emission problems.

This paper discusses the mega-size green and highefficient CO2-rich natural gas direct utilization technology and the construction of a hybrid energy and chemical production system framework, based on the current status of South China Sea’s gas exploitation, the gas composition characteristics, and CNOOC’s industrial practices.

2 Status of Direct Utilization of CO2-Rich Natural Gas

2.1 Hainan base of China BlueChemical

The China BlueChemical Co., Ltd. (China BlueChem),a subsidiary company of China National Offshore Oil Corporation (CNOOC), is a large-scale and modernized enterprise engaging in the development, production,and sales of mineral fertilizers and chemical products.Up to now, China BlueChem has developed into one of the largest listed companies in terms of the production volume of fertilizers and methanol in China.

The Hainan base has four sets of production plants, viz.:2 sets of ammonia & urea plants and 2 sets of methanol plants. The annual utilization of CO2-rich natural gas reaches 3.6 billion m3, with CO2content equating to about 25%. Compared with the natural gas power generation utilization, 3.367 Mt of CO2emissions could be reduced annually.

2.2 Direct utilization of CO2-rich natural gas(containing about 25% of CO2)

(1) Ammonia synthesis unit

In the Hainan base, there are two sets of Fudao synthetic ammonia plants, Phase I and II, both of which are equipped with urea plants.

The designed capacity of the Fudao Phase I synthetic ammonia plant is equal to 1.0 kt/d, and the originally designed feedstocks are being supplied from the Yacheng 13-1 natural gas field with low CO2content. In order to adapt to a CO2-rich natural gas as the feedstock (CO2≦25%), the chemical unit simulation calculation is first applied to calculate the overall material and heat balance of the system before and after the synthetic ammonia unit, in order to further balance the existing equipment capacity to achieve effective process integration and equipment matching.

Upon considering the characteristics of high CO2and nitrogen content in the CO2-rich natural gas (Dongfang 1-1 natural gas, in which the CO2content is 18%, the N2content is 20%), a combination of natural gas predecarbonization and the second stage furnace plus oxygen enrichment (less nitrogen supplementation) is adopted.That is, no denitrification of the feed gas is carried out.Due to the introduction of excess nitrogen, a supplemental part of oxygen enrichment (with the oxygen-enriched air flow rate reaching 2400 m3/h, and the oxygen content after mixing being equal to 26.1%) is used in the secondstage furnace, and as a result, a cryogenic air separation unit is needed to produce enriched oxygen. The predecarbonization of raw materials adopts the staged decarbonization method, in which the CO2content in fuel gas is controlled under 1%, and the CO2content in the process gas is 5.4% according to the ammonia carbon balance, to meet the total CO2requirement of the urea plant.

The designed capacity of the Fudao Phase II synthetic ammonia plant is 1 500 t/d, and is directly based on using the CO2-rich natural gas.

The urea units in Phase I and Phase II are rated at 520 kt/a and 800 kt/a respectively. After the balance of the whole plant, the CO2was slightly surplus. Thus, a 30 kt/a CO2recovery unit (dry ice) was set up.

At present, the annual consumption of CO2-rich natural gas is about 1.5 billion m3in the two sets of synthetic ammonia units and two sets of urea plants at the Hainan Base of CNOOC. With the increase of CO2content in natural gas, the amount of CO2removed from the synthetic ammonia-urea series is also increasing. Due to the limited market of dry ice,CNOOC has used this surplus CO2for EOR (enhanced oil recovery).

(2) Methanol unit

Table 1 shows the basic information of the two methanol units in the Hainan base, which directly consumes 2.09 billion m3of CO2-rich natural gas annually (with a CO2content of 25%). Table 2 shows the analysis data of natural gas consumed in the Hainan base.

Table 1 Basic data of methanol plant

Table 2 The composition of CO2-rich natural gas feedstock

(3) Dry ice and CO2delivered to plastic unit

Based on the demand of the above-mentioned urea and methanol plants, the Hainan base can consume 3.5 billion m3of CO2-rich natural gas (with a CO2content of 20%—25%), and the surplus CO2can be recycled to produce dry ice with a production scale of 30 kt/a. A demonstration plant of CO2-derived plastic production had been built earlier, but it did not realize commercial application yet due to product quality issues.

3 Technology Development and Hybrid Energy &Chemicals Production System Construction

To further enhance the direct utilization of CO2-rich natural gas resources, especially for the higher CO2content resources (≧25%, preferably 30%—50%),CNOOC is cooperating with the Shanghai Advanced Research Institute of Chinese Academy of Sciences to promote the commercialization process of the following technologies:

3.1 Light alkane reforming to syngas technology

In an attempt to utilize the South China Sea natural gas with high CO2content (30%-50%) coupled with a small amount of C2and C3alkanes (≤3%), CNOOC and the Shanghai Advanced Research Institute of the Chinese Academy of Sciences have developed the high-efficiency Ni-based nano-mesoporous composite catalysts for CO2-light alkane dry reforming to produce syngas, for exploring the possibility to get rid of the dependence on traditional steam reforming, and realize the key technical breakthrough.

The main chemical reactions in the process of CO2-light alkanes dry reforming for syngas production are as follows (taking methane as the light alkane):

A schematic diagram of the process flow of CO2-rich natural gas dry reforming to syngas (H/C=1.0) is shown in Figure 1, and the desulfurization unit may not be provided. The jointly developed technology focuses on solving the problem of carbon deposition during the thermal decomposition of hydrocarbons at high temperature and in the disproportionated carbon at the high concentration of carbon monoxide (CO) in the product gas. Under the reaction conditions covering a temperature of 850 °C and a pressure of 2.0—3.0 MPa,the single-pass conversion rate of light alkanes (C1—C3)can be more than 98%, and the H2/CO ratio of syngas can be flexibly adjusted in the range of 0.7—2.0.

Figure 1 Process flow diagram of CO2-light alkanes dry reforming to syngas process

3.2 CO2 hydrogenation to methanol technology

The annual consumption of four sets of chemical plants in the Hainan base has reached 3.5 billion m3(CO2content:20%—25%), which plays an important role for the development of CNOOC’s upstream gas fields. However,the increased carbon dioxide content in natural gas brings about the problems of low catalyst activity and decreased methanol production capacity. Some foreign companies and research institutions, such as Topsoe, are researching on the high-efficiency CO2hydrogenation catalyst and related technologies. CNOOC and the Shanghai Advanced Institute of Chinese Academy of Sciences are conducting industrial side-line tests for CO2-rich hydrogenation to methanol technology, and are preparing to construct an 10 kt/a industrial demonstration unit to provide the basic data as the support for commercial applications.

The main chemical reactions of CO2hydrogenation process are as follows:

Table 3 shows some data of the single tube experiments for the pilot and industrial demonstration catalyst.

The catalysts currently used in the pilot and the industrial demonstration units have completed more than 4 000 hours of stability testing, and the hydrogen is sourced from the surplus industrial hydrogen of the Hainan Chemical Base.

The CO2-rich natural gas is difficult to use directly due to its high CO2content, but the CO2contained therein also compensates for the “less carbon and more hydrogen”characteristics inherent in common natural gas, which can especially comply with the requirements of adding carbon in natural gas chemical processes. By taking a 500 kt/a methanol plant using CO2-rich natural gas of the South China Sea as an example, the carbon efficiency is 82.5%,and the energy efficiency is 80.4%.

3.3 Syngas to ole fins technology coupled with linear α-ole fin technology

Olefins, including low-carbon olefins (ethylene,propylene, butene) and high-carbon α-ole fins (ole fins),are very important and high value-added chemical raw materials. Compared with the low-carbon olefins, the high-carbon α-ole fins have higher value, and are widely used in fine chemicals production, such as surfactants,plasticizers, and high-grade lubricating oils. At present,the production of high-carbon α-olefins in the world is mainly achieved via ethylene oligomerization. The market for high-carbon α-olefins in China is huge, but greatly relies on imports[6-7].

Table 3 Single tube experiment results of CO2 hydrogenation

The technology of direct production of olefins by using syngas, developed by CNOOC and the Shanghai Advanced Research Institute of Chinese Academy of Sciences, has made progress in lab-scale tests. The already developed FTO catalyst with low-methane and high-ole fin selectivity can regulate the reaction network by designing the active sites and mesostructures. Under mild reaction conditions, the catalyst possesses high olefins selectivity in the syngas-to-olefins process, with the methane selectivity equating to less than 5%, the total ole fins selectivity being higher than 80%, and the ole fin to alkane ratio being higher than 30. Meanwhile, the carbon number of products shows a significant narrow interval and high selectivity distribution, with the C2-C15selectivity being higher than 90%, while the product distribution completely does not obey the ASF law[8-10].Figure 2 is a schematic flow chart showing the principle of dry reforming coupled with F-T synthesis to ole fins.

By taking a total hydrocarbon production scale of 600 kt/a as an example based on the syngas with a H2/CO ratio of 1 produced by dry reforming of natural gas, the technoeconomic analysis shows that the production cost of α-ole fin is 8 178 RMB/t. The IRR is higher than 30% based on the market price of year 2018, which shows a good risk mitigation capability. The data are shown in Table 4.

Figure 2 Schematic diagram of the principle flow diagram of CO2-rich natural gas dry reforming coupled with F-T synthesis of ole fins

Figure 3 Main reactions of hydroformylation process

Table 4 Techno-economic analysis of F-T ole fin synthesis coupled with dry reforming

3.4 Dry reforming coupled with ole fin hydroformylation technology

Coupling the dry reforming technology with the olefin hydroformylation reaction can further increase the added value of the product, thereby improving the technoeconomic indicators of the project. The light olefin hydroformylation technology jointly developed by the CNOOC Oil & Petrochemicals Co., Ltd. and the CNOOC Tianjin Chemical Design and Research Institute Co., Ltd.,has a light ole fin conversion rate of higher than 90%, and the aldehyde yield is higher than 90%. The main reactions of hydroformylation is shown in Figure 3.

The technology of dry reforming technology of carbon dioxide and light alkane coupled with the mixed C4hydro-formylation to produce valeraldehyde and 2-propylhexanol (2-PH), is applied in the process using C4provded by the DCC unit of the CNOOC Hainan Dongfang refinery coupled with the syngas produced by dry reforming of carbon dioxide and light alkanes.For comparison, Table 5 shows the properties of the C4resource and the properties of residual C4hydrocarbons in the coal-derived MTO plant of the CNOOC Dongfang refinery. According to the n-butene flow rate of the two kinds of C4materials in Table 5 (11.28 t/h and 9.32 t/h, respectively), the theoretical production of 2-propylhexanol (2-PH) is about 87 kt/a and 72 kt/a,respectively. Both of them show the economy-of-scale.Upon considering the purchase of additional C4resources(such as the post-MTBE C4hydrocarbons from the CNOOC Huizhou refinery), a 100 kt/a 2-propylhexanol plant could be built up.

Table 5 Properties of C4 in CNOOC Hainan Dongfang Re finery

If all the post-MTBE C4hydrocarbons of Dongfang refinery are used to produce high-carbon alcohols, the theoretical demand for syngas is about 6 200 m3/h,and the hydrogen demand is about 4 000 m3/h. The CO2-rich natural gas in the South China Sea is used as the raw material to produce 10 200 m3/h of syngas with a hydrogen/carbon ratio of 2.3 achieved by steam reforming technology, followed by PSA for hydrogen purification and separation to get 4 000 m3/h of pure hydrogen production and 6 200 m3/h of syngas with a hydrogen-to-carbon ratio of 1.0, which are used to produce high-carbon alcohols from post-MTBE C4hydrocarbons.

Upon considering the purchase of post-MTBE C4from the CNOOC Huizhou refinery, it is possible to build a 100 kt/a industrial plant to produce 2-propyl hexanol (2-PH). The investment in an 100 kt/a 2-PH plant is about 900 million RMB (including about 50 million RMB for C4raw material processing equipment). The technoeconomic analysis shows that its annual revenue is about 350.2 million RMB, and the static payback period is about 2.57 years.

In fact, CNOOC’s refineries produce annually about 2.0 Mt of C4hydrocarbons, including 500 kt/a of n-butene(1-butene and 2-butene). As a result, it can produce 487 kt/a of 2-PH. If the 2-PH production process is coupled with South China Sea’s CO2-rich natural gas conversion technology, it needs about 70800 m3/h of synthesis gas with a hydrogen to carbon ratio of 2.3. After being dehydrogenated by the PSA unit, 27 800 m3/h of pure hydrogen and 43 000 m3/h of synthesis gas (with a hydrogen/carbon ratio of 1.0) are separated, indicating a promising future for the technology.

3.5 New technologies for natural gas direct conversion and utilization

The chemical utilization of methane can be divided into two methods: one is the indirect method, which first converts natural gas into syngas (a mixture of CO and H2), which is then subject to synthesis reaction to form high value-added chemicals or intermediate chemicals through the Fischer-Tropsch synthesis and other routes.In this domain, the processes such as F-T synthetic oil and methanol to olefins have been industrialized. The other method is the direct conversion of methane into high value-added chemicals, which has been a research direction parallel to the indirect method. The main catalytic conversion methods include: Oxidative Coupling of Methane (OCM), Methane Dehydroaromatization(MDA), and anaerobic methane to olefins. They all belong to high-temperature (873—1373 K) processes, and the yield of C2products can hardly been increased greatly,since the reaction itself has a potential safety issue,which has brought forth not only great challenges to the reactor design, but also stringent requirements for catalyst stability.

Judging from the perspective of resources and energy development strategies, the method of high temperature thermal catalytic methane conversion is not ideal.It is very urgent to develop new technologies for direct conversion and utilization of methane, such as photocatalysis or electrocatalytic CO2conversion, using solar energy or renewable wind power or surplus nuclear power as the energy resource to directly convert CO2into fuels and chemicals such as carbon monoxide,formic acid, methanol, and hydrocarbons under normal temperature and pressure conditions, and to realize the effective utilization of CO2and storage of clean renewable energy, which shows a promising future.

CNOOC has been cooperating with the Shanghai Advanced Research Institute, Chinese Academy of Sciences, to develop methanol and C2+chemicals production technology by direct catalytic conversion of methane. The catalysts performance is as follows: reaction temperature ≤300 ℃, selectivity of methanol and other chemicals ≥90%, yield ≥1 mmol/g/h, electrochemical CO2reduction current density ＞ 20 mA/cm2, selectivity of oxygenate product ＞ 80%, and an over potential ＜0.2 V.The synthesis oxygenate product selectivity is more than 90% and the light energy utilization efficiency is more than 1% under the irradiation condition of visible light(≥420 nm) or AM1.5 sunlight.

3.6 Hybrid energy chemical system construction

The Hainan Island is rich in renewable resources. It is located in the tropical region and has abundant solar energy resources. The sunlight hour is 1 750—2 750 h per year, and the total solar radiation is large, with an average of 4 600—5 800 MJ/m2annually. Among them,the total amount of irradiation in Dongfang city is the largest, about 5 800 MJ/m2in 2 750 h per year, which is equivalent to 0.16—0.2 t of standard coal per year on one m2. The total reserves of wind resources on land is 8 283.8 MW, and the technologically feasible capacity is 1,284 MW. The theoretical reserves of hydropower resources in Hainan are 995 MW, the total installed capacity of hydropower can be 772 MW, and the annual power generation is 2.625 billion kWh, which is equivalent to the annual consumption of 967 kt of standard coal in thermal power plants. Meanwhile,the geothermal resources and tidal energy of Hainan Province are also abundant. The tidal energy resources are 377 MW, which is equivalent to an 110 MW power generation capacity.

Therefore, due to Hainan’s renewable resources,especially the abundant solar and wind energy resources of the Dongfang city, and also considering national nuclear power program in the Hainan coastal areas,we can build a low carbon hybrid energy chemical system[11-12], to efficiently use the CO2-rich natural gas coupled with the existing industries and planning projects of the China Blue Chem Company and the CNOOC Refinery Corporation, to produce high valueadded green chemicals, which are in line with the Hainan Green Development Strategy and its low carbon,clean and ecological development pathways. Figure 4 is a schematic diagram of the construction of a hybrid energy chemical system planned by CNOOC in Hainan,which is currently being promoted step by step. After the completion of this system, the CNOOC Hainan Base will realize the direct conversion of 10 Mt of CO2a year into the high-value energy and chemical products,achieving a win-win situation between the pro fit and the environment.

4 Conclusions

Besides CNOOC’s practice of direct utilization of South China Sea’s CO2-rich natural gas with a CO2content of 20%—25%, CNOOC and the Shanghai Advanced Institute of Chinese Academy of Sciences have developed the mega-scale green and efficient direct utilization technologies to utilize the CO2-rich natural gas with a CO2content of over 30% and have started to build a hybrid energy chemical system to adapt to the total direct utilization of CO2-rich natural gas with a CO2content of 20%—80%. After the completion of the Hainan base, the annual utilization of CO2will reach 10 Mt, which is of great value.

Figure 4 Schematic diagram of a hybrid energy chemical system based on the South China Sea CO2-rich natural gas resources