(258f) Upcycling Steelwork Residue Gas to High-Value Products

This study aims to develop and evaluate the techno-economic-environmental performance of residue gas-to-gasoline hydrocarbon processes (rGTL) via the dimethyl ether-to-gasoline (DTG) route using coke oven gas (COG), Linz–Donawitz gas (LDG), and blast furnace gas (BFG) as feedstocks. The study overcame significant challenges in process development to generate optimal syngas for fuel synthesis under the best operating conditions. The proposed rGTLs with the best co-feeding strategies of COG and LDG, COG standalone was compared to other gasoline platforms such as Fischer-Tropsch synthesis, methanol-to-gasoline, natural gas-to-gasoline, and power-to-liquid. As a result, the DTG route was identified as the best technological route for residue gas-based gasoline. It acts as a bridges between fossil and renewable gasoline, providing a market-competitive price and significant CO₂ equivalent (CO_2eq) reduction. The study also investigated the industrial and social impact of rGTL in various scenarios, revealing a significant opportunity for implementing rGTL in real commercial, industrial, and sustainable development societies. Furthermore, it provides stakeholders and governments with strategic planning and policymaking guidelines to effectively use a country's resources towards sustainable development goals.

1. Introduction

As the ever-increasing global demand for petroleum products and the depletion of petroleum resources, continues to rise globally, alternative feedstocks and alternative technological routes for producing transport fuel are being explored. Among these, gas-to-liquid (GTL) processes have been developed using abundant natural gas, unconventional gas, and industrial waste gas. Residue gases from the iron and steel-making industry, such as coke oven gas (COG), Linz–Donawitz gas (LDG), and blast furnace gas (BFG) can be upcycled for high-value products as they contain high-caloric species. Since the steel industry is responsible for 6% of anthropogenic CO₂ emissions (, and the growing efforts to reduce CO₂ emissions, upcycling residue gas into high-value chemicals and fuels instead of burning is gaining attention. GTL is a process that converts natural gas or gaseous hydrocarbons into liquid fuels. The conventional GTL route involves the use of a reformer to generate syngas, which is then hydrogenated into hydrocarbon products through Fischer-Tropsch (FT) synthesis. Another GTL route for gasoline-range products is the methanol-to-gasoline (MTG) process, which produces methanol from syngas and then converted into gasoline-range hydrocarbons via zeolite ZSM-5 catalyst. This reaction path starts from methanol dehydration to dimethyl ether (DME) and ultimately leads to the formation of C-C bonds for hydrocarbon production. To improve gasoline production, the DME-to-gasoline (DTG) approach has been introduced, which utilizes DME as an intermediate and improves syngas-to-DME conversion, reactor productivity, and gasoline yield compared to MTG. Furthermore, it is 25% less exothermic, reducing capital and operating costs for heat management. DTG is also 25% less exothermic, lowering heat management costs. DTG has better catalyst performance in the liquid phase, using DME synthesis from syngas over bi-functional Cu/ZnO/Al₂O₃ and γ-Al₂O₃ catalysts as a front-end for DTG over zeolite. DTG has significant potential to improve gasoline production efficiency and reduce costs.

This study examines the techno-economic-environmental performance of gasoline boiling-range hydrocarbon production processes using the DTG technological route, known as residual gas-to-liquid (rGTL), from either standalone or co-feed residue gases. Six rGTL processes with different feed gas were proposed, analyzed and compared with other gasoline platforms and technological routes, including FTS, MTG, natural gas-to-liquid (nGTL), and power-to-liquid (PTL). Furthermore, a sensitivity analysis of the impact of rGTL on industrial and social factors was examined to identify the energy- and cost-drivers and environmental friendliness. The study also offers specific guidelines for using various domestic resources, including residue gas, conventional/unconventional gas, petroleum, and renewable energy, for sustainable fuels.

2. Process development and analysis method

2.1. Process development

We developed six rGTL processes (i.e., four standalone and two co-feed strategies), including syngas preparation from various residue gases and fuel synthesis of DMES subsequent DTG, and simulated using Aspen Plus V11. While COG is a H₂-rich gas, any excess H₂ can be treated as a by-product (in rGTL1) or completely utilized for more gasoline production (rGTL2), both requiring additional CO₂. CH₄ in COG is reformed with outsourcing CO₂ and steam in the CSCR for the optimal H₂/CO ratio. CO₂ removal is required for the gas mixture before introducing to DMES to avoid the negative effects on the DME reactions. Finally, DTG converts DME into gasoline boiling-range hydrocarbon and light gas (C₂–C₄ hydrocarbons). From LDG and BFG, rGTL3 requires a shift of CO to H₂ with water-gas shift (WGS), while rGTL4 needs CO separation and WGS to adjust the H₂/CO ratio. rGTL5 and rGTL6 consider co-feeding COG with LDG and BFG, respectively, involving gas component separation and integration to generate syngas for fuel synthesis with DMES and DTG. rGTL5 separates H₂ from COG and combines it with CO₂ from LDG in reverse-water gas shift (RWGS) to create additional syngas for fuel synthesis alongside the CH₄ reforming in COG and H₂/CO mix from LDG. rGTL6 integrates gas components into syngas via separation processes, treating BFG with CO separation and CO₂ capture. The optimal H₂/CO is obtained from separated CO, syngas from reformer, and RWGS outlet. Fuel synthesis process configuration (DMES and DTG) is the same in all rGTLs, and CO₂ capture is implemented before releasing any gas stream.

2.3. Analysis method

The evaluation of the technology's economics and environmental impact considered several criteria, including carbon efficiency (CEF), energy efficiency (EEF), net CO_2eq emission (NCE), and unit production cost (UPC). Technical performance metrics such as CEF and EEF demonstrate the process efficiency in terms of carbon and energy conversion and storage in products. NCE measures CO_2eq emissions per gasoline gallon equivalent (GGE) produced. It comprised raw material inventory, direct emissions such as process vent-out gas, and indirect emissions as consuming conventional utilities. The UPC is the production cost per GGE, expressed in $/GGE. The total production cost (TPC) comprises the total capital investment cost (TCI) and the total operating cost (TOC). TCI is calculated using the purchased equipment cost and the Lang factor, and the annualizing capital cost (ACC) is estimated based on the TCI, interest rate, and plant-life. TOC includes the variable operating cost of raw materials and utilities and the fixed operating cost of labor, maintenance, and operating charges. rGTLs also generate light gas (C₂-C₄ hydrocarbons), CO₂, and H₂ as by-products, which contribute partial sale value and subtract from TPC for gasoline UPC estimation.

3. Results and discussion

The techno-economic-environmental results show that rGTL1 and rGTL2 are the most carbon-efficient processes but emit CO_2eq at 2.1 kg CO_2eq/GGE and no captured CO₂. Meanwhile, rGTL3 and rGTL4 outstandingly reduce emission, rGTL5 and rGTL6 have high energy efficiency due to the synergy of combining H₂- and CO-rich gases. rGTL1-5, which used both standalone and co-feed of COG and LDG, produced gasoline at lower prices (< 4.03 $/GGE) than rGTL4 and rGTL6. The BFG-related rGTL4 and rGTL6 had production costs of 8.67 and 5.77 $/GGE, respectively. The sensitivity analysis on the industrial and social impact of rGTLs indicates that the interest rate and the residue gas price have the most significant impact on gasoline UPC. Therefore, providing government incentives such as lower or no interest rates may be more effective in reducing UPC than incentives related to carbon taxation or cost-free residue feedstock.

Compared to other GTL technological routes, DTG is the most cost-effective route for converting residue gas into gasoline, with a UPC of 3.18-3.7 $/GGE and reducing CO_2eq emissions by up to 5.6 kg/GGE. In contrast, FTS and MTG routes have a higher UPC of 3.6-5.6 $/GGE and emit CO_2eq. Among various gasoline platforms, rGTL is more affordable than PTL (priced at 7.41 $/GGE) and more environmentally friendly than both fossil gasoline (8.78 kg CO_2eq/gallon) and nGTL (2.57-8.43 kg CO_2eq/GGE). Therefore, residue gas-based gasoline can bridge between fossil and renewable gasoline and is a promising solution for sustainable renewable energy from industrial waste gas. Additionally, considering countries’ petroleum, natural gas, industrial gas, and renewable energy resources, the rGTLs are encouraged for specific reasons. As the top steel-producing countries, China and India can use their abundant steelwork residue gas to expand their fuel markets. Brazil benefits from cheaper residue gas-based gasoline than natural gas-based or renewable gasoline, whereas France can achieve significant CO_2eq reduction by using rGTL.

4. Conclusion

In this study, we developed and evaluated rGTL processes from steelwork residue gases using DTG routes. The best feeding strategy for residue gas upcycling is the co-feed of COG and LDG, with competitive prices and significant CO_2eq reduction. The DTG route showed superior economic-environmental performance over other technological routes for gasoline from residue gas. Residue gas has potential for low-cost fuel with significant CO_2eq reduction, supporting sustainable alternative energy systems. Moreover, rGTL was identified as a promising sustainable gasoline option, less expensive than renewable gasoline and more environmentally friendly than crude oil and natural gas-based gasoline. Governments can encourage implementation of rGTL with specific guidelines on using domestic resources to reduce reliance on fossil fuels and CO_2eq emissions.