By Harry Morehead
Director, Gasification and IGCC Sales and Marketing, Siemens Energy, Inc.
Manager, Business Development, Siemens Fuel Gasification Technology GmbH & Co. KG
Gasification is a technology with a long and checkered history. It was widely used to produce “town gas” for lighting and cooking in the 1800s before it was replaced by electricity and natural gas. Yet its commercial deployment for industrial applications and power generation has been limited, despite several attempts to kick-start the industry.
Historically, interest in coal gasification has tended to peak when access to other fossil fuels was limited or their prices were high. For example, gasification received a great deal of attention in the 1970s during the oil crisis, and at various periods in recent years as a response to high natural gas prices.
A major factor behind gasification’s stuttering commercialization has been the upfront cost. Coal gasification plants typically require capital investments of hundreds of millions of dollars, and in some cases billions. With the effects of the recent global financial crisis still being felt, bringing down the capital cost is essential if coal gasification is ever to truly take off.
Companies such as Siemens have been able to make progress through technology advances as well as a growing number of references, which in itself will reduce costs and build confidence in the feasibility of the technology.
The development of the Siemens gasification process—a pulverized fuel, pressurized, entrained-flow gasification technology—was begun in 1975 by Deutsches Brennstoffinstitut Freiberg/Sa. (DBI, German Fuel Research Institute). The main objective was to create a conversion technology that would allow the use of locally abundant lower-rank coals, including lignite, to partially replace the demand for crude oil and natural gas. Many countries are now taking advantage of their local energy resources and converting those resources into low-carbon electricity, chemical feedstocks, and clean transportation fuels. Heading into China’s 13th Five-Year Plan, the China National Coal Association recommends to “shift from viewing coal as a fuel to considering it a raw material to produce a wide array of products. Based on the initial results of coal conversion demonstration projects in China, such as coal-to-liquids, coal-to-olefins, and coal-to gas, China’s coal industry should accelerate the construction of large-scale, clean, and efficient coal-conversion projects, which could effectively replace some oil and gas.”1
The government of the former German Democratic Republic intended to build several gasification plants around central Germany to supply major chemical companies through long-distance pipelines with syngas produced from coal. Due to the low rank of this lignite and its high salt content, the gasification process developed had to address special requirements for the feeding system and the gasifier itself. The first test facility, built in 1979 with a thermal capacity of 3 MW, was used to examine the technical concept and to test the targeted saliferous lignite for the construction of a large-scale demonstration facility in 1984 at the Gaskombinat Schwarze Pumpe site. Between 1994 and 1998 further test facilities were erected at a Siemens site in Freiberg, among them a 5-MWth cooling screen reactor. Up to now these facilities have been used to gasify more than 90 candidate gasification feedstocks—including different ranks of coal, municipal- or industrial-provenance sewage sludge, petroleum coke, waste oils, bio-oils, bio-slurries, and several liquid residues—in order to investigate their gasification behavior and to analyze the quality and characteristics of the gasification products.
Through this systematic research and development, the range of application of the Siemens Fuel Gasification (SFG®) technology was extended from conventional fuels, such as coals and oils, to also include residual and waste materials and biomass. Over the years since its privatization in 1991, the technology has been owned by several companies. Since its purchase by Siemens in 2006, the Siemens gasification group has been organized under Siemens Fuel Gasification Technology (SFGT) GmbH & Co. KG and has extended its footprint to China, South Korea, and the Americas.
Essentially there are several basic gasifier designs, differentiated by whether they use pure oxygen or air, wet or dry coal feed, the reactor’s flow direction, and the syngas cooling process. Oxygen-blown and entrained-flow gasifiers, such as those designed by Siemens (see Figure 1), are likely to be the most popular going forward.
Oxygen-blown gasifiers have the advantage of being a compact, cost-effective design and they produce a very clean syngas that can be directly processed after dust removal. These gasifiers operate under high pressure in the range of 40–46 bar, which allows a high syngas output per single gasifier, resulting in fewer trains and subsequently lower CAPEX per ton of final product.
Entrained-flow gasifiers operate at temperatures higher than the ash-melting temperature. Typical operation temperatures are in the 1300–1800°C range. At these high temperatures, the gasifier produces only the components hydrogen, carbon monoxide, and carbon dioxide—no hydrocarbons such as phenols or tar, as is the case for fixed-bed gasifiers. The fuel flexibility ranges from biomass, petroleum coke, oils, tar, and liquid chemical residues to all kinds of coals such as lignite, sub-bituminous and bituminous coal, or even anthracite. For most feedstock, the carbon conversion rate is in the range of 96–99.5%.
Today, gasification processes around the world must limit the production of gas, solid, and liquid wastes. Siemens believes the oxygen-blown, entrained-flow gasifiers represent the environmentally best available technology due to a lack of waste production. There are no gaseous emissions. Solids emissions are in the form of vitrified slag, which is inorganic and nonleaching, and can be sold as construction material. Solids entrained in the gas, process fines, are extracted as filter cake. Liquid-phase waste is becoming an increasingly important issue for gasification plants because water consumption must be reduced; almost certainly, zero liquid discharge systems will be a future requirement in many parts of the world. A quench system can be supplied with a variety of process waters, such as gas condensates from the CO shift or condensate from a methanation unit. The combination of an entrained-flow gasifier and a dry-feed system has the lowest freshwater consumption of all available industrial-scale gasification technologies, typically in the range of 0.35–0.45 ton freshwater/ton coal despite having a full water quench, which is usually fed by recirculating the gas condensate. Eventually, water is discharged from the quench system to limit the salt concentration based on the material and fouling constraints. The typical entrained flow gasifier blow-down rate is lower than other industrial-scope gasification technologies, in the range of 0.1–0.15 ton water/ton coal.
The performance of such entrained-flow gasifiers has already surpassed the older fixed-bed gasifiers, such as those used in the past in South Africa.
Prior to 2007-08, the number of Siemens gasification references was very limited and some had been built 30–40 years ago or were no longer in operation. Today, however, there are seven projects operating, in construction or under development with Siemens gasifiers, mostly in China where there are extensive coal reserves. Siemens’ current focus is on “design-to-cost” for the gasification island, taking into account the associated subsystems, in order to simplify the entire process and thus reduce costs.
Generally, larger gasifiers are more efficient and require less pipework and other components. Work has therefore centered on developing a gasifier that offers the optimum size in terms of efficiency and cost.
Much of the SFG development work has been carried out at the Siemens Fuel Gasification Test Center in Freiberg, Germany, which is one of the most comprehensive gasification test facilities in the world (see Figure 2). The centerpiece of the test center is a 5-MWth gasification reactor equipped with Siemens’ innovative cooling screen design. This design allows the reactor to gasify a broad range of coals with ash contents up to 30–35% and high ash-melting temperatures. This reduces start-up and shutdown times at the commercial scale from two to three days (compared to refractory-lined gasifiers) to approximately two hours. The cooling screen has a lifetime of at least 10 years and eliminates the need for the annual or bi-annual shutdowns customary with refractory-lined reactors, resulting in a significantly higher availability.
This Siemens-owned test center has been instrumental in developing and testing the SFG-200 and later the larger SFG-500 (2000 t/day coal capacity). Six of these units are now successfully operating at plants around the world.
As part of the continuing effort to reduce costs, Siemens has now developed the SFG-850 gasifier, introduced to the market at the end of March 2014 (see Figure 3). The reactor with this system is sized for larger gasification plants producing chemical feedstocks, synthetic natural gas, or clean transportation fuels, as well as IGCC applications using the most advanced gas turbines.
The SFG-850 is designed to enhance the profitability of future gasification plants by reducing specific plant costs, along with the associated production costs of synthesis gas. An SFG-850 gasifier can convert around 3000 t/day of coal into more than five million standard cubic meters (Nm3) of high-quality synthesis gas.
The SFG-850 gasifier is based on the same technical design as the SFG-200 and SFG-500; however, the proven central burner design and dry coal feed system have now been optimized further in the SFG-850. As with its predecessors, the new gasifier has a high degree of fuel flexibility. All of the proven advantages of Siemens gasifier technology, including its short start-up and shutdown times and the tried-and-tested serviceability of its water-cooled design, contribute to its ability to maintain a high level of availability.
The SFG-850, however, is bigger than its predecessor: Its outer diameter is 0.5 m larger than the SFG-500. Although not a huge increase, this offers a 33% increase in coal throughput capacity. With a length of 22 m, an outer diameter of 4.8 m, and weighing 380 tonnes, the SFG-850 gasifier is one the world’s largest. The new model can be completely fabricated and tested at the factory. Despite its size and weight, the unit can be transported to its installation location in one piece, eliminating the time and expense of field fabrication.
Gasification Has a Bright Future Based on Today’s Hard Work
As noted before, the cost of gasification-based plants is the major challenge for the gasification industry. Companies across the industry are working to reduce the cost of gasification and all of the upstream and downstream processes that will allow gasification plants to operate economically. At Siemens, we believe that the introduction of the SFG-850 makes a significant step forward in reducing the cost of the gasification island. In addition to the economy of scale compared to a standard Siemens SGF-500 reference plant in China, Siemens has achieved further reduction in capital investment cost by optimizing the selection of equipment, valves, and instruments—incorporating lessons learned from today’s operating plants to select better construction materials and developing a more compact gasification island layout.
Beyond the gasification island, Siemens is partnering with industry leaders in coal milling and drying and CO shift catalysts; improvements in these areas will further increase the efficiency of tomorrow’s gasification plants. For example, Clariant, a world leader in specialty chemicals, and Siemens have introduced a new jointly developed sour gas CO shift technology specifically designed for coal gasification. This advanced “low steam” shift technology with Clariant’s ShiftMax® 821 catalyst reduces capital expenditure for the shift unit by up to 20% and optimizes operating costs with up to 30% lower catalyst volume and significantly less steam consumption. This will make gasification plants more economically appealing and, hence, more competitive than what is currently offered.
Ongoing cost reductions will enable greater use of gasification, especially in the industrial sector where countries worldwide are looking to leverage their domestic, low-cost energy resources, such as coal, to produce high-value products including low-carbon power, chemical feedstocks, and clean transportation fuels. The ability to use low-rank coals will make the gasifier particularly attractive to markets such as Indonesia, which currently has no real use for such coal. Gasification would unlock this resource. The same is also true for countries such as Australia, Mongolia, Vietnam, and even Thailand where gasification projects are being considered. There could even be possibilities in the U.S. as natural gas prices rise.
Gasification may also be used as a strategic tool in some countries to reduce dependence on imported fossil fuels. Turkey, which has coal but is heavily dependent on gas imports, is one such country. Ukraine is another prime example; the recent conflict with Russia highlights why gasification might be an attractive option to the purchase of Russian natural gas. As gasification technologies continue to demonstrate better performance, better reliability, and lower costs, more countries will be able to economically justify the use of this innovative clean energy conversion technology to produce the power, chemical feedstocks, and clean transportation fuels they need.
- Wang, X. Z. (2014). Advancing China’s coal industry. Cornerstone, 2(1), 15–18.