By Rob van den Berg
General Manager Gasification Technology, Shell (China) Projects & Technology Ltd
General Manager, China Technical Service Centre, Shell (China) Projects & Technology Ltd
Gasification Licensing Sales Manager, Shell Global Solutions
Embracing Clean Coal Technology
Environmentally responsible gasification technologies are helping to unlock the world’s coal reserves with increasing efficiency. The synthesis gas (syngas) produced is being used in chemical and integrated gasification combined-cycle (IGCC) plants worldwide. China, in particular, has embraced coal gasification technology as a way of using its abundant, indigenous coal reserves efficiently and with low environmental impact to manufacture methanol, ammonia, hydrogen, and synthetic liquid hydrocarbons. In other countries syngas, which burns as cleanly as natural gas, is generating power in IGCC plants.
The Shell Coal Gasification Process (SCGP), examples of its use, and a description of the continuous improvement culture that is pushing reliability and performance to new levels are the focus of this article.
Shell Coal Gasification Technologies for Different Needs
With 28 licenses sold worldwide, Shell is one of the main global suppliers of coal gasification technology and is unusual in being both the technology developer and a licensor with hands-on operational experience through an equity investment in a commercial gasification plant. This provides the company valuable operational insights, and a vested interest, in achieving ever-higher operational reliability and performance standards.
Shell began gasification research in the 1950s and built coal gasification demonstration plants in the 1970s and 1980s in the Netherlands, Germany, and the U.S. Two technologies have emerged from this pioneering work: The Shell Gasification Process (SGP) converts refinery residues to syngas and the SCGP uses solid feeds, including petroleum coke (petcoke), anthracite, bituminous coal, lignite (brown coal), and biomass, to produce syngas. There are two SCGP-related lineups for users with different needs.
Proven High-Efficiency Technology
SCGP syngas cooler technology (Figure 1) has been used commercially for more than 20 years. It can help achieve carbon conversion rates of over 99% and cold coal gas efficiency (the amount of energy in the coal converted to the energy in the combustible syngas) of 80–83%. A syngas cooler recovers most of the sensible heat in the syngas to produce high- or medium-pressure steam, which can reduce the operating costs within a facility.
An entrained-flow process uses an inert carrier gas to transport dry coal feedstock to the gasifier, where it contacts oxygen and steam. The gasifier has unique features, such as specially designed multiple burners and a membrane wall of high-pressure tubes designed to enable the safe and low-maintenance separation of syngas and slag.
The molten slag flows down into a water bath from where it can be extracted as a solid, thereby reducing wastewater pollution. Dry-filtered and wet-washed syngas quenches the syngas to about 900°C before it leaves the top of the gasifier. The syngas is cooled further in an external cooler to generate high- and medium-pressure steam as valuable by-products. The use of multiple burners provides the potential for easy scaling up and, more importantly, efficient slag removal with only a small amount of a fine fly ash, which is removed downstream to less than 1 ppmv.
In China alone, 23 SCGP gasifiers have come onstream since 2006 and six more are due to start up in the near future. Of these 23 gasifiers, 17 have dry pulverized coal intake capacities of over 2000 t/d. SCGP units are also running or planned in South Korea and Vietnam.
Proven Low-Capital-Costs Technology
For operators that need to lower their capital expenditure and are looking for wider feedstock flexibility while retaining good efficiency levels, Shell has developed a bottom-quench technology (Figure 2). This simplified lineup can reduce capital costs by up to 30% while satisfying the country’s basic, and some advanced, efficiency and environmental requirements.
The bottom-quench lineup retains the membrane wall and burner technology of the first lineup but has proven water-quench technology to replace the syngas cooler. This means less steel, less equipment, and a shorter manufacturing time, which substantially reduces capital costs. SCGP bottom-quench technology also helps to eliminate fouling risks to offer wider coal suitability.
In 2013, Shell and Wison Engineering successfully started a 1000-t/d SCGP bottom-quench technology demonstration plant in the Nanjing industrial park, which had 99% carbon conversion. In January 2014, Hulunbeier Jinxin, a subsidiary of the Yuntianhua Group, signed a licensing agreement for a bottom-quench gasifier to process lignite feedstock.
A Broad Range of Applications
In China, SCGP gasifiers are delivering syngas for methanol, ammonia, and hydrogen production. For example, the Yueyang Sinopec and Shell Coal Gasification Co. Ltd. (Dongting) joint venture has been successfully supplying syngas and steam to the associated Baling fertilizer plant since 2006. This has given Shell eight years of first-hand, local operational experience. The facility processes 2000 tonnes of pulverized coal a day and produces syngas for urea/fertilizer and caprolactam (nylon) manufacture.
In 2012, the first of two plants in Vietnam operated by Vietnam National Chemical Group began commercial operations.
Coal for IGCC Power
SCGP licenses for IGCC plants have been sold in Europe and Asia. Importantly for a power plant, the SCGP unit gives operational flexibility with the ability to follow load changes quickly.
In the Netherlands, the 2000 t/d Willem-Alexander (formerly Nuon) power plant operated from 1993–2013. Commissioned as a demonstration plant, it has proven SCGP technology’s reliability and low maintenance costs, which result from the robustness of the gasifier membrane wall and the long-life burners. The IGCC plant also demonstrated feedstock flexibility by processing more than 20 different coal types and blends, and running successfully with up to 30 wt% biomass.
South Korea is building its first IGCC plant, Taean IGCC No. 1. Here, Korea Western Power Co. Ltd will be using SCGP and gas-treating technologies for efficient generation of clean power for the country. SCGP technology was chosen for its good economic value, and high reliability and efficiency. Knowledge transfer and training were also key factors in the selection decision.
The 300-MW (net) IGCC plant aims to have a design target of over 42% efficiency (net).
The plant, which will process 2670 t/d bituminous and sub-bituminous coal, is designed to have emissions of less than 30-ppm nitrogen oxides and 15-ppm sulfur oxides. It also opens the possibility of carbon capture and storage for reduced greenhouse gas emissions through a readily available stream of concentrated high-pressure carbon dioxide.
In China, Shanxi Lu’an Coal Mine Group is constructing a coal-to-liquids plant with four SCGP gasifiers that will each have a 3200-t/d dry coal intake capacity.
Reaching New Performance Standards
Since its introduction in the early 1970s, SCGP syngas cooler technology has been continuously improved to enhance performance, extend equipment life, and widen the range of usable feedstocks.
Efficiency and Environment
SCGP syngas cooler technology is efficient and has a low environmental footprint. For example, independent assessments have shown that SCGP gasifiers have the highest exergetic efficiency of all coal gasification technologies.1 Typical ranges for oxygen and coal consumption are also low: 310–350 Nm3 oxygen per kNm3 syngas and 510–615 kg standard coal per kNm3 syngas, respectively. Indeed, SCGP gasifiers exceed China’s National Development and Reform Commission’s (NDRC) basic requirements for energy, coal, and water consumption, and meet many of its advanced requirements (Figure 3). Note that the requirements are for overall projects and therefore a plant’s performance depends partly on the downstream process employed.
Feed Flexibility, Selection, and Management
A wide variety of coal has been processed across the world, including feeds with 6–36% ash, up to 35% moisture content, and ash-melting points (fluid temperatures) from 1140°C to well over 1500°C. Low-quality coal types, including lignite, have been successfully gasified in commercial operations. Several SCGP technology users, including four in China, have implemented an effective strategy of blending high-ash coal with petcoke to promote stable gasification operations and high syngas output. These plants have shown the longest continuous runs and greatest uptime of all the SCGP plants. The ability to gasify low-quality coal with petcoke is likely to become increasingly advantageous as more coal-to-chemical facilities come online and coal quality generally deteriorates as high-grade reserves become exhausted.
Gasifying petcoke may be an attractive power-generation option in the Middle East, where petcoke could be a widely available, low-cost feed. Some recycled ash/slag or a locally sourced fresh ash would need to be added, as the SCGP design principle requires sufficient ash in the feed to form a protection layer in the gasifier.
Shell is capturing the extensive and growing knowledge of the behavior of different coal types in the SCGP in a database and has established a basic theory about the effects of coal quality on gasification, which it has verified using the experience database. The result is a powerful and practical modeling tool that is helping users to select and evaluate coal types without conducting expensive trial runs and to make day-to-day production decisions.
Substantial value can be captured early in coal gasification investments through good project definition and smart configuration (developing the best plan). A project’s value can be enhanced further by correct project execution and continuous improvement (optimizing the completed facilities).
Shell gives SCGP users intensive operational training and dedicated support for start-up and early runs. In addition, they help users to establish a plant management system to improve reliability, manage risk, and plan equipment maintenance. This structured process ensures that users learn about the plant and become familiar with it as quickly as possible, thereby increasing equipment reliability and establishing a solid foundation for high-load operation over extended cycles of use.
Shell also recognizes the importance of creating opportunities to learn from operational experiences in the pursuit of continuous improvement. To this end, all technical information relating to improvements is collected and used to build a database of lessons learned, to the benefit of its licensees. Shell has also facilitated eight global coal gasification technology users’ conferences in the past six years. These forums, hosted at different operating sites, provide a platform for licensees to discuss and improve their operations.
In addition, lead users strongly committed to improving the operation and design of this technology are working together on a reliability board to discuss specific experiences and capture the means for achieving good performance. The reliability board meets more frequently than the larger user conference to work on particular issues and implement the lessons learned.
Thus, the experience of solving problems and improving plant performance has become part of the process of continuous improvement. Any issues identified are fed to a team of people at Shell who develop solutions. This team includes process, mechanical, and control engineers. Using their understanding of the technology and their ability to communicate effectively and rapidly with clients, they can help to resolve the issues and, most importantly, feed improvements back to the master design to benefit future customers.
The benefits of this feedback process range from minor improvements and changes in the type of valve used to major developments such as dealing with blockage of the inlet of the syngas cooler or candle filter breakages. Some solutions apply to a range of situations, others just to a single plant. The change process is strictly managed; this combination of actions has resulted in positive feedback from clients.
Effect on Reliability
SCGP users are employing a structured improvement process to achieve high levels of reliability. In China, for example, users typically accumulate more than 300 days per year of normal operation. One chemical plant has achieved over 328 running days in each of the last five years, with a highest yearly total of 341 days (Figure 4). It is important to note that SCGP lineups do not require spare gasifiers, although multiple units are used for large-capacity projects. Some alternative lineups claim higher reliability levels, but use spare gasifiers, which add significantly to a project’s capital costs.
Burner lifetimes typically exceed 8000 hours, and the filter candles typically have lifetimes of two to three years.
To ensure that each SCGP lineup offers improved reliability and performance, Shell captures lessons learned from all the technology users in a database and applies them to improving the master design.
SCGP technology is proven to offer high-efficiency coal gasification, excellent environmental performance, and good feedstock flexibility. It has been applied worldwide to a broad range of successful chemical and IGCC applications, and a coal-to-liquids plant under construction. Due to the structured learning procedure and a culture of continuous improvement culture adopted by users worldwide, SCGP technology operates with ever-increasing levels of reliability and performance.
- Gräbner, M., & Meyer, B. (2014). Performance and exergy analysis of the current developments in coal gasification technology. Fuel, 116, 910–920.