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Coal Gasification in a Global Context

2.1 Applications of Coal Gasification

Any carbonaceous feedstock, may it be gaseous, liquid, or solid, can undergo a partial oxidation. As soon as oxidation heat is released, high-temperature conditions evolve permitting other gases, such as steam or carbon dioxide, to react with the carbonaceous feedstock. The result is the autothermal breakdown of the feedstock to the smallest stable chemical units that can still carry some energy. These units are the gases hydrogen, carbon monoxide, and sometimes methane. The breakdown process is called gasification and the gaseous product is called synthesis gas or syngas. Although the term “syngas” traces to gases produced for the sole purpose of downstream syntheses, it established itself as a term for any product gas from gasification independent of application. (Further details and thermodynamic definitions are provided in Chapter 4.)

The composition of the syngas varies and is essentially linked to the quality of the feedstock and the conditions of the gasification process, such as temperature and pressure. Furthermore, each kind of gasification process is specialized in a certain feedstock spectrum. Finally, the usage of the gas produced specifies varying parameters, such as heating value, pressure level, H2/CO ratio, and maximum concentration of sulfur compounds.

Hence, the closing of the gap between carbonaceous feedstock and a selected final product, which is intended to be sold from the plant, is a technical and economical optimization problem with usually more than one solution. In this framework, the different gasification technologies are the basis for competition on the market.

The conversion chain of gasification plants as shown in Figure 2.1 can be generalized in three steps moving from feedstock to product: gasification, gas treatment, and conversion to product.

Figure 2.1 Gasification conversion chain from feedstock to product. (The numbers in gigawatt (GW) refer to global syngas capacity of currently operating units and plants under construction [1].)

In Figure 2.1, the sum of installed and under-construction capacity in GW syngas is distinguished for feedstock and products. On the feedstock side, it can be seen that coal with 126.9 GW represents more than 75% of the global feed for gasification plants. And coal is expected to grow by another 74 GW until 2018 [1]. The gasification step is currently accomplished in nearly 300 plants employing approximately 800 gasifiers. The intermediate product of syngas undergoes a gas treatment step including usually a cleaning stage (e.g., particulate matter removal, acid gas removal) and a preparation stage (e.g., water gas shift). Subsequently, the syngas is subjected downstream to the final conversion step. If the syngas is combusted in a combined cycle, the generated product is electricity from a so-called integrated gasification combined cycle (IGCC) process. This is true for only 14 GW or 8.4% of the total syngas produced. Of more importance are the products that preserve the chemical energy in form of gaseous or liquid fuels (e.g., town gas, substitute natural gas, gasoline, and diesel fuel) as well as chemicals such as ammonia, methanol, or hydrogen. The chemicals are dominant, representing nearly 50% of the syngas capacity.

Another aspect with regard to Figure 2.1 is that mixtures of different solid feedstock are frequently fed to gasifiers. They are called blends and consist of different coals or mixtures of coal and petroleum coke (petcoke). But also on the product side, single plants are not limited to one specific output. Syntheses in parallel or in conjunction with a combined cycle can be feasible and the plants produce several products (e.g., Schwarze Pumpe, Germany: methanol and power), which is referred to as “polygeneration.”

2.2 The Three Generations of Coal Gasifiers

There have been many surveys summarizing the early history of gasification that should not be repeated here [2,3]. But it is, in general, reasonable to distinguish three main generations of gasifiers serving larger-scale industrial applications.

2.2.1 First Generation of Coal Gasifiers

The first generation of industrial coal gasifiers arose from the idea of supplying a chemical synthesis with gas produced from coal. A typical example is the Winkler fluid-bed gasifier, which found its first commercial application in 1926 at the Leuna site close to Leipzig, Germany. From this framework emerged the term syngas. Because the process operated – as all gasifiers at that time – at atmospheric conditions, the advantages of a pressurized process quickly became clear. It was Professor Rudolf Drawe (1877–1967) who first saw the high potential in replacing the commonly used air with pressurized oxygen and steam mixtures as gasifying agents, which was possible after the invention of the Linde-Fränkel air separation process. In 1927, the German engineering company Lurgi patented the first pressurized oxygen-blown fixed-bed gasifier, which was commercially applied in 1936 in Hirschfelde, close to Dresden, Germany.

Besides the upcoming NH3 and methanol market, the fast development in Germany was mainly driven by the need to produce liquid fuels from domestic sources such as lignite, which was induced by both World Wars. And a technology for coal dust gasification – the Koppers-Totzek atmospheric entrained-flow process – was developed in Germany in the 1940s.

Among this first generation of gasifiers, the Lurgi fixed-bed dry bottom (FBDB) technology remained the most successful one because it was the only pressurized technology available for years. However, the FBDB process left quite some leeway to improve single-unit capacity, gas quality (high CO2 and CH4 content, tar production), and steam consumption.

2.2.2 Second Generation of Coal Gasifiers

Besides minor individual factors, the oil crises relaunched interest in coal gasification again leading to the development of a second generation of coal gasification processes. The global targets of the development, which took place from the 1970s until early 1990s, can be summarized as follows [4–6]:

• For fluid-bed and entrained-flow processes, the gasification pressure should be raised from atmospheric to 20 to 60 bar to reach higher single-unit capacities (up to 500 MW) and lower capital investment.
• For fluid-bed and fixed-bed processes, performance should be enhanced as regards carbon conversion rates, cold gas efficiencies, and consumables.
• Another focus was on the integration of heat recovery from syngas by steam generation if gasification is employed for power generation.
• The upcoming environmental concerns, especially the emission of sulfur species from coal gasification was reviewed in detail.
• The development of higher single unit capacities requires high on-stream time (plant availability) and higher operational flexibility regarding feedstock and load variation compared to the former approach of running many gasifiers in parallel.

These development efforts brought the second generation of coal gasifiers to the market comprising several technologies such as the British Gas/Lurgi (BGL) [7], high-temperature Winkler (HTW) [8], U-Gas [9], Kellogg Rust & Westinghouse (KRW) [10], Texaco [11], Gaskombinat Schwarze Pumpe (GSP) [12], E-Gas (Dow Chemical) [13], Shell [14], and Prenflo (Uhde) [15] processes. Most of them have become proven, mature technologies that have been successfully implemented in IGCC plants as well as for methanol, ammonia, and acetic acid anhydride syntheses. These are considered to be the traditional technologies sold from the shelf contributing to the substantial growth in China within the last 10 years.

2.2.3 Third Generation of Coal Gasifiers

However, after a decade of relative silence surrounding coal gasification, beginning around 2000, there have been four general trends that renewed interest in the technology:

• Substitution of crude oil by other energy carriers, such as biomass or coal, targeting supply security and local energy price stabilization (e.g., China)
• Increasing interest in the use of low-grade coals with high ash or moisture contents in emerging nations (e.g., India, Indonesia)
• Sustained efforts to reduce CO2 emissions because of the potential of CO2 separation from pressurized syngas (e.g., the United States)
• Stabilization potential of polygeneration plants for high electricity generation fluctuations caused by the increasing share of renewable energy sources (e.g., Europe)

Numerous design variations of second-generation gasification processes were suggested for the state-of-the-art processes from Shell [16], Uhde [17], Siemens (formerly GSP) [18], GE (formerly Texaco) [19], Lurgi FBDB [20], and CB&I (E-Gas) [21].

But also a third generation of newly developed gasification processes emerged, such as those developed by Kellogg Brown & Root (KBR) [22], Pratt & Whitney Rocketdyne (PWR) [23], and Mitsubishi Heavy Industries (MHI) [24]. In parallel, five new Chinese processes were brought to commercial reality inside China: Hangtian Lu (HT-L) technology, the two-stage-oxygen gasifier developed at Tsinghua University Beijing, the gasifiers from the East China University of Science and Technology (ECUST), the two-stage-coal gasifier from the Thermal Power Research Institute (TPRI), and the multicomponent slurry gasification (MCSG) technology...