Techno-economic analysis of olefin production based on Fischer-Tropsch synthesis

To reduce the dependence of olefin production on the limited petroleum, considerable efforts have been made in synthesizing olefins from alternative energy resources. Here, we evaluate the techno-economic feasibility of coal-to-olefin technology based on Fischer-Tropsch synthesis. To reduce the dependence of olefin production on the limited petroleum, considerable efforts have been made in synthesizing olefins from alternative energy resources. Here, we evaluate the techno-economic feasibility of coal-to-olefin technology based on Fischer-Tropsch synthesis. Olefins are important feedstocks in the modern chemical industry and are generally produced from petroleum. However, economic and environmental factors are currently spurring exploration of alternative routes for synthesizing olefins. There is increasing interest in the production of olefins from syngas (CO and H2), which can be derived from coal, biomass, and natural gas. In particular, China is rich in coal but deficient in oil and natural gas, so developing coal-to-olefin technology is important for ensuring energy security. One of the keys to coal-to-olefin technology is to design efficient catalysts for transforming coal-based syngas to olefins. In recent years, great progress has been made in selectively converting syngas to olefins. Bao and colleagues1Jiao F. Li J. Pan X. Xiao J. Li H. Ma H. Wei M. Pan Y. Zhou Z. Li M. et al.Selective conversion of syngas to light olefins.Science. 2016; 351: 1065-1068Crossref PubMed Scopus (846) Google Scholar proposed an oxide-zeolite (OX-ZEO) route, in which the oxide activates CO and H2 while the zeolite controls the subsequent C–C coupling. The OX-ZEO system exhibited 80% selectivity for light olefins with 17% CO conversion. Compared with the OX-ZEO route, another path called Fischer-Tropsch to olefin (FTO) synthesis possesses a higher CO conversion level. For example, de Jong and colleagues2Torres Galvis H.M. Bitter J.H. Khare C.B. Ruitenbeek M. Dugulan A.I. de Jong K.P. Supported iron nanoparticles as catalysts for sustainable production of lower olefins.Science. 2012; 335: 835-838Crossref PubMed Scopus (895) Google Scholar reported 53% selectivity for light olefins with 80% CO conversion over the Fe/α-Al2O3 catalyst. Zhong et al.3Zhong L. Yu F. An Y. Zhao Y. Sun Y. Li Z. Lin T. Lin Y. Qi X. Dai Y. et al.Cobalt carbide nanoprisms for direct production of lower olefins from syngas.Nature. 2016; 538: 84-87Crossref PubMed Scopus (525) Google Scholar prepared a CoMn catalyst with preferentially exposed facets, and it obtained 61% selectivity for light olefins with 32% CO conversion. It is important to declare here that the olefin selectivity mentioned above refers to the selectivity in hydrocarbons and does not include the CO2 by-product. Actually, during the conversion of syngas, the water-gas shift (WGS) reaction is inevitable because H2O and CO co-exist at high reaction temperatures, leading to a high CO2 selectivity of about 40%~50%. Moreover, about 10%~20% CH4 is generally produced in the hydrocarbons. As a result, during the syngas-to-olefin reaction, >45% of the converted CO is transformed into the undesirable C1 by-products (CO2 and CH4). Recently, some work has been done to suppress the formation of C1 by-products during syngas conversion. For example, de Jong and colleagues4Xie J. Paalanen P.P. van Deelen T.W. Weckhuysen B.M. Louwerse M.J. de Jong K.P. Promoted cobalt metal catalysts suitable for the production of lower olefins from natural gas.Nat. Commun. 2019; 10: 167Crossref PubMed Scopus (61) Google Scholar reduced the selectivity for CO2 and CH4 over the Co-based catalyst through the synergistic effect of Mn, Na, and S elements. Zhou et al.5Zhou W. Zhou C. Yin H. Shi J. Zhang G. Zheng X. Min X. Zhang Z. Cheng K. Kang J. et al.Direct conversion of syngas into aromatics over a bifunctional catalyst: inhibiting net CO2 release.Chem. Commun. 2020; 56: 5239-5242Crossref PubMed Google Scholar inhibited CO2 formation by co-feeding CO2 with syngas. Wang et al.6Wang P. Chen W. Chiang F.-K. Dugulan A.I. Song Y. Pestman R. Zhang K. Yao J. Feng B. Miao P. et al.Synthesis of stable and low-CO2 selective ε-iron carbide Fischer-Tropsch catalysts.Sci. Adv. 2018; 4: eaau2947Crossref PubMed Scopus (106) Google Scholar reported the suppression of CO2 by a phase-pure ε-Fe2C catalyst. However, these catalysts could run only at low CO conversion levels (<20%). Notably, we developed a hydrophobic [email protected] catalyst7Xu Y. Li X. Gao J. Wang J. Ma G. Wen X. Yang Y. Li Y. Ding M. A hydrophobic [email protected] catalyst increases olefins from syngas by suppressing C1 by-products.Science. 2021; 371: 610-613Crossref PubMed Scopus (88) Google Scholar that could suppress the total selectivity of CO2 and CH4 to less than 22.5% with an olefin yield of up to 36.6% at a relatively high CO conversion of 56.1%. Activity and selectivity, especially the latter, are two important criteria for evaluating the performance of a catalyst. Researchers usually do not consider the CO2 by-product and present the selectivity on a "CO2-free" basis, which paints an unreliably optimistic picture of the catalyst performance. The CO2 produced not only means useless consumption of CO but also contributes to the greenhouse effect. Thus, high selectivity in the total products is the real sign of good catalytic performance. Because coal-to-olefin plants usually operate continuously for months, catalyst stability is also crucial to potential industrial applications. Moreover, the raw-material consumption, CO2 emission, and product cost of FTO synthesis are important criteria for judging whether a new catalyst is promising for further commercialization in comparison with the existing industrial catalysts. Here, we conducted an economic and technical analysis comparing the hydrophobic [email protected] catalyst for syngas-to-olefin conversion with the traditional hydrophilic [email protected] catalyst. Assuming a plant capacity of 2.87 Mt/y coal (a typical capacity of a coal-to-olefin plant in China), we used Aspen Plus V11 to simulate the total coal-based FTO (CFTO) process. Because of the annual maintenance requirements, the working time of a chemical plant is generally designed to be 8,000 h/year. According to previous reports8Xiang D. Yang S. Qian Y. Techno-economic analysis and comparison of coal based olefins processes.Energy Convers. Manage. 2016; 110: 33-41Crossref Scopus (41) Google Scholar, 9Qin S. Chang S. Yao Q. Modeling, thermodynamic and techno-economic analysis of coal-to-liquids process with different entrained flow coal gasifiers.Appl. Energy. 2018; 229: 413-432Crossref Scopus (51) Google Scholar, 10Yang S. Xiao Z. Deng C. Liu Z. Zhou H. Ren J. Zhou T. Techno-economic analysis of coal-to-liquid processes with different gasifier alternatives.J. Clean. Prod. 2020; 253: 120006Crossref Scopus (28) Google Scholar and industrial pilot plants, the CFTO process mainly consists of a number of units (Figure 1), including the coal gasification (CG) unit, WGS unit, acid gas removal (AGR) unit, and Fischer-Tropsch synthesis (FTS) unit. In the CG unit, coal was ground with water for the production of coal slurry. Then, the coal slurry was fed together with oxygen into the gasifier for the production of crude syngas. After cooling, the crude syngas was fed into the WGS unit so the H2/CO ratio could be adjusted in the gas. During the AGR unit, H2S and CO2 were separated from the crude syngas. Then, the clean syngas was mixed with the recycled gas, fed into the FTS unit, and converted into high-value-added olefins. The reaction products were separated through a series of rectifying towers. A portion of the unreacted syngas was cycled back to the FTS reactor for further production of the targeted hydrocarbons and enhancement of the energy efficiency, and the rest was released as off-gas. On the basis of the simulation results, we calculated the raw material consumption, utility consumption, CO2 emission, energy efficiency, total capital investment, and product cost. Energy efficiency, defined as the ratio of product energy to input energy, is widely used for assessing the technical performance of a designed process. In our simulation, the input energy included the energy of coal, electricity, and steam. We calculated the energy of coal and hydrocarbon products on the basis of their lower heating values. Appropriate integration of the process units and optimization of the operating conditions allowed the energy efficiency of the CFTO process to reach 62.6%. Raw-material consumption and CO2 emissions are two important technical criteria for the CFTO process. The hydrophobic [email protected] catalyst could boost the production of olefins from syngas by suppressing the undesired CO2 and CH4, thereby increasing the utilization efficiency of the C atom and reducing the CO2 emission of the FTS reaction. Our calculation showed that producing 1.0 t olefins required about 4.0 t coal for the traditional [email protected] catalyst but only about 3.3 t coal for the hydrophobic [email protected] catalyst, implying much lower consumption for raw material. During the FTS unit, the CO2 emission over the traditional [email protected] catalyst was about 2.18 t/t, whereas it was reduced to 0.75 t/t over the hydrophobic [email protected] catalyst, implying that the CO2 emission of the FTS reaction was reduced by 65.6%. In this regard, the hydrophobic [email protected] catalyst was more economical and environmentally friendly than the traditional catalysts because of its lower raw-material consumption and CO2 emission. According to the reported methodology,8Xiang D. Yang S. Qian Y. Techno-economic analysis and comparison of coal based olefins processes.Energy Convers. Manage. 2016; 110: 33-41Crossref Scopus (41) Google Scholar we further calculated the product cost on the basis of the simulation results and several assumptions: (1) the product cost consisted of the cost for raw material, utilities, operation and maintenance, depreciation, plant overhead, administration, and distribution and sales; (2) the price of coal, water, electricity, and steam was US$60/t, US$0.3/t, US$0.105/kWh, and US$6.3/GJ, respectively; (3) there were 300 operators, and the salary was US$15,000/operator/year; (4) the capital investment was included in the product cost in the form of depreciation cost (assuming a lifespan of 20 years and a salvage value of 4%, we used the straight-line method to calculate the depreciation cost); and (5) in general, the prices of non-olefins products were approximately half as much as those of olefins. Thus, we assigned 100% weight to the mass of olefins and 50% weight to the mass of non-olefin products. In this case, the product cost of unit olefins was equal to the total product cost divided by the equivalent olefin production, which was the sum of olefin production and half of non-olefin production. The calculated product cost obtained over the traditional [email protected] and the hydrophobic [email protected] was US$839/t and US$667/t, respectively. Compared with the traditional [email protected] catalyst, the hydrophobic [email protected] catalyst cost 20.5% less, implying much better economic performance. As discussed above, the hydrophobic [email protected] catalyst possessed much lower CO2 emission than the traditional catalyst. Thus, the hydrophobic [email protected] catalyst will be more cost effective if a carbon tax is levied in the future. Moreover, the product cost of the [email protected] catalyst was lower than the market price of olefins (~US$950/t), suggesting that the hydrophobic [email protected] catalyst could display excellent potential in industrial applications. In the near future, with the development of more efficient catalysts and product-separation technologies, the CFTO route will become more feasible.