Elsevier

Chemical Engineering Journal

Volume 334, 15 February 2018, Pages 1754-1765
Chemical Engineering Journal

A chemical looping scheme of co-feeding of coke-oven gas and pulverized coke toward polygeneration of olefins and ammonia

https://doi.org/10.1016/j.cej.2017.11.156Get rights and content

Highlights

  • Proposed a new process of pulverized coke CLC-assisted COG-to-olefins & ammonia.

  • Improved olefins capacity by 25% with additional 0.4 Mt/y ammonia for the new process.

  • The co-feeding process resulted in exergy saving by 13% with an IRR increase of 5.6%.

Abstract

The coke-oven gas (COG)-to-olefins (CGTO) process is an important technical path of coal chemical industry. However, it is far more effective and friendly in terms of energy efficiency and environment impact. A new scheme of pulverized coke (PC) chemical looping combustion (CLC)-assisted COG-to-olefins and ammonia (PCCLC-CGTOA) is proposed in this paper. The intensive energy and material couplings in the new process are realized by applying an energy-efficient and environmental-friendly CLC technology to the current CGTO. The PCCLC-derived CO2 is an effective carbon source to adjust ideal composition of syngas toward olefins production, while the PCCLC-derived N2 with the excessive H2 in COG is used for producing ammonia. The process configuration is flexibly adapted to market demand of olefins and ammonia in terms of economic optimization. Process modelling helps its process integration and intensification, and confirms its competitiveness and feasibility. The proposed PCCLC-CGTOA acclaims 58% of hydrogen utilization and 65% of exergy efficiency, contrasting to only 29% and 52% in the CGTO. There is a 25% increase of olefins production with additional 0.4 Mt/y ammonia output at the cost of economic PC. A higher internal rate of return is also expected (18.5% for the PCCLC-CGTOA versus 12.9% for the CGTO).

Introduction

Olefins are regarded as the most important petrochemicals and the primary building blocks for various chemical intermediates, polymers, and rubbers. Their production capacities are critical to petrochemical industry as well as the sustained economic development of a country. Ammonia is another critical chemical and second-largest synthetic commodity worldwide. About 80% of ammonia is used to produce nitrogen fertilizers [1]. Both olefins and ammonia play pivotal roles in the broad industries, and their production heavily source back to economic supplies and efficient utilization of feedstock. Traditional olefins production heavily depends on oil and natural gas worldwide, while oil and coal in China [2]. Two thirds of ammonia is produced from natural gas worldwide, while 97% of ammonia is produced from coal and coke in China [3]. However, those feedstock are not equally distributed in geological sites. Coal can be easily accessed and is also economic choice of feedstock, but heavy emitter of CO2. Therefore, the selection of coal as an economic feedstock and technological advancement in olefins and ammonia production under circumstance of the higher energy efficiency and less carbon emissions has been a long-term highly-demanding strategy.

On the other hand, the metallurgical industry continuously depends on coal coking process, in which coke-oven gas (COG) is largely by-produced. The COG is an alternative coal-derived fuel gas, containing enriched H2 (58–60 vol%) and CH4 (23–27 vol%), as well as CO (5–8 vol%) and CO2 (<3 vol%) [4]. The most COG is currently burnt, which leads to considerable wastes in view of energy, resource, and relevant environmental problems. Considering the valuable hydrogen resource in the COG, the implementation of the COG for high-efficiently producing olefins and ammonia will be favored. In the coking plants, considerable pulverized coke (PC) is currently excluded by the metallurgical industry because of its improper size. It is necessary to find a value-added approach to access its economic carbon resource. Besides, the current COG-to-olefins (CGTO) is less competitive in terms of hydrogen utilization and energy efficiency. However, the co-feeding of PC and co-production of ammonia has not been addressed in the process.

Several references addressed on concept designs of coal and COG co-feeding systems. Lin et al. [5] proposed the adjustment of syngas composition via steam reforming of coal gasified gas and CH4-rich gas separated from the COG for methanol production, which achieving a fuel saving more than 5% and a 70% reduction of CO2 emissions. Man et al. [6] proposed a novel co-feeding process of COG-assist coal-to-olefins, in which COG reacted with CO2 and steam to adjust syngas composition. Its results showed that energy efficiency of the co-feeding process increased 10% and life cycle carbon footprint was reduced by 85% in comparison to conventional coal-to-olefins process. Above researches were focused on improving techno-economic performance of the coal derived chemicals processes under the ignorance of the long-distance transportation difficulty of COG. The molar ratio of hydrogen to carbon in syngas was defined as an R value (R = (H2-CO2)/(CO + CO2)). The ideal R is about 2 for methanol and olefins synthesis, while the R of the COG feedstock is theoretically 5.2 [7]. The syngas produced by currently partial oxidation of the COG has an R value between 2.5 and 3, which is improperly involved hydrogen oxidation. This approach is also unavoidably involved a highly power-consuming air separation unit. A simple consideration of element balance in the addressed two feedstock (COG and PC) derives from the potential solution of a proper approach to adjust the R value of the COG reforming syngas. However, there will be a consequent process optimization and intensification to finally answer the technical feasibility in hydrogen utilization, energy efficiency, cost reduction, and minimization of environmental impact. Bermudez et al. [8] proposed a CO2 dry reforming process of the COG for methanol production. Yi et al. [7] and Gong et al. [9] proposed the dry reforming of the COG by using CO2 separated from exhaust gas and oxygen-combustion for methanol production, achieving an energy conservation by 5–16 percentage point and a CO2 reduction by 50–70%. However, the aforementioned processes incurred the supply of costly pure CO2 separated from tail gas or oxygen-enriched combustion, in which significant energy penalty was inevitable.

Chemical looping combustion (CLC) is originally a carbon-abatement combustion process, excluding costly separation of oxygen in either air or CO2 in the flue gas, thus it can provide the pure CO2 [10], [11]. CLC uses solid oxygen carrier (OC), rather than oxygen in air, to oxidize fuels in a fuel reactor (FR), and solid OC is later oxidized by air in an air reactor (AR) by means of recirculation of OC between the FR and AR [12], [13]. The gases from the fuel reactor are primarily CO2 and H2O, in which the H2O can be easily separated by condensation and thus a high concentrated CO2 can be achieved [12]. There are also additional benefits of the CLC, such as non-flammable combustion toward the eliminated NOX emissions and efficient control of other air pollutants based on decreased gas production.

There are some new promising technical developments of the CLC concept since the inception, such as the cost-effective and energy-efficient chemical looping hydrogen production, the chemical looping reforming, and the chemical looping gasification [14], [15], [16], [17], [18], [19], [20], [21], [22]. Its initial research focused on the gas fueled CLC, followed by solid fuels CLC in recent years, especially for coal. This is because coal plays an important role in power generation and is also an important source of greenhouse emissions [11], [23]. Several concept designs integrating coal fueled CLC and power consumption indicated that approximately 10% of net electricity efficiency benefit can be earned in the CLC power plant using anthracite as feedstock with CO2 emissions as low as 96 kg/MWh [24]. Furthermore, Edrisi et al. [25] proposed a new plant configuration for ammonia synthesis by using N2 and H2 from three chemical looping reactors, in which ammonia production capacity can be theoretically increased by about 30%. These studies clearly exhibited potentials of the CLC-based process on the improvement of energy efficiency and the realization of the CO2 reduction. But so far there is no report on the integration of CLC in the joint production of both olefins and ammonia.

This study proposed a novel process of integrating PC-fueled CLC and dry-steam reforming of the COG for olefins production (PCCLC-CGTO). To increase flexibility of this new scheme, the purified N2 from the CLC was further proposed for ammonia production in the PCCLC-CGTOA. These two new configurations will be contrasted to the conventional CGTO. The involvement of the CLC not only realized the cost-effective and energy-efficient CO2 and N2 supply, but also brought in several other opportunities, such as the exclusion of the high-cost oxygen production via the conventional air separation, the potential couplings in energy transfers between exothermic and endothermic reactions, the productivity increase of olefins and ammonia, and thus finally significant increase economic and environmental benefits. This study addressed a thorough analysis on key technical and operational parameters of the proposed processes via process modeling. It laydown a sound theoretical basis on the elucidation of the process feasibility, the process optimization and intensification in the co-feeding of both the COG and PC for producing olefins and ammonia.

Section snippets

Olefins and ammonia production configuration

The process of the conventional CGTO includes major components of acid gas removal unit (AGRU), air separate unit (ASU), methane partial oxidation unit (MPOU), methanol synthesis unit (MSU), and methanol-to-olefins unit (MTOU). While the process of the PCCLC-CGTO includes major components of the AGRU, MSU, MTOU, and two new units of dry-steam reforming unit (DSRU) and PC-fueled CLC unit (CLCU), as shown in Fig. 1. Sulfur compounds of the generated CO2 from the CLCU and the crude COG are

Assessment criteria

This study used Aspen Plus to conduct a thorough analysis on key technical and operational parameters of the proposed systems toward the realization of the optimized utilization of the COG and PC for olefins and ammonia production. In order to elucidate the process feasibility in view of the technical and economic benefit, several key indices of hydrogen utilization efficiency (φ), exergy efficiency (η), fixed capital investment (FCI), total product cost (TPC), and internal rate of return (IRR)

The analysis of key operational parameters

There are three key operational parameters for this proposed novel process: (1) the molar ratio of steam/COG in the DSRU (S/COG); (2) the molar ratio of CO2/COG in the DSRU; (3) the split ratio of the COG to the APU. The S/COG and CO2/COG affect the R ratio, the flow rate of olefins, and further the whole plant performance. Besides, the split ratio will determine the flow rate between olefins and ammonia (the distribution of productivity).

The effect of molar ratios of the S/COG and CO2/COG on

Conclusions

A novel process of PCCLC-CGTOA, coupling PC-fueled CLC and COG reforming toward the co-production of olefins and ammonia, was proposed in this study. The supply of the energy-efficient CO2 was realized via a PC-fueled CLC, so that the ideal molar ratio of hydrogen and carbon (R = 2.0) for methanol synthesis first and further olefins can be reached. Ammonia was also introduced as its production line to take full advantage of the use of the enriched H2 in COG and the purified N2 from PC-fueled

Acknowledgements

This work was supported by the U.S. Department of Agriculture (No: 5040-12630-004-00D), the Key Project on National Key Research and Development Plan of Intergovernmental International Scientific and Technological Innovation Cooperation (No: 2016YFE0108400), the National Natural Science Foundation of China (No: 21676001; No: 21706001), the Special Project on Major Science and Technology of Anhui Province (No: 15czz02045), the Anhui University Talent Startup fund (No: J01006120), and the

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