The Mitsubishi Heavy Industries (MHI) gasification technology was developed with the goal to maximize the gasifier efficiency (e.g., dry feed, and a unique two-stage reactor design) and reduce costs (e.g., air-blown to eliminate power consumption and capital cost of an oxygen plant). The development commenced in the early 1980s under a joint venture formed between MHI, the Japanese government, and several power/utility companies. This joint venture, called Clean Coal Power R&D Co., built a 2-ton-per-day (tpd) process development unit (PDU), and later, a 200-tpd pilot plant unit at Nakoso (about 200 km north of Tokyo) in Japan to test the technology. In 2004, a 1,700-tpd, 250-MW, IGCC demonstration plant was built, also at Nakoso.  The plant started operation in 2007 and is currently undergoing various demonstration testing, using different types of coal.

Figure 1 shows a simplified drawing of the MHI gasifier.  The reactor consists of two sections (or stages): a lower combustor and an upper reductor. Dry milled coal is fed at two separate points into the gasifier with a portion being fed into the combustor together with air (or enriched air) where it is burned to produce carbon monoxide (CO) and carbon dioxide (CO₂), plus water vapor. The temperature generated at the combustor is sufficiently high to melt the coal ash. The molten slag falls to the bottom of the gasifier where it a quenched in a water bath and then removed using a lock hopper system. The gas produced in the combustor rises to the reductor where the remaining coal is added, without any additional air. At the reductor stage, heat provided by the hot combustor gas is used to drive the endothermic gasification reactions. The reductor is operated at a lower temperature than the combustor. Any molten ash carried over by the upward gas is solidified.  The syngas produced exits the reductor through a syngas cooler generating steam.  A cyclone is used downstream of the sygnas cooler to collect the chars and recycle them to the combustor section to increase the overall carbon conversion efficiency. The raw syngas leaving the reductor section of the gasifier is typically at at 2,200°F, high enough in temperature that very little hydrocarbon gases and liquids are formed.

Figure 1: A Simplified Drawing of the MHI Gasifier
(source: Mitsubishi Heavy Industries, Ltd.)

Demonstration and Early Commercialization
The MHI gasifier has been tested in both a 2-tpd PDU, and later, a 200-tpd pilot plant unit.  Experience gained was used to design, build, and operate a 250-MW IGCC demonstration plant at Nakoso, Japan. Figure 2 and 3 show a simplified block flow diagram and a picture, respectively, of the demonstration plant. The plant started operation in September 2007, and has the following key features:

• MHI’s dry-feed, two-stage gasifier.
• Oxidant for gasification is extracted from the gas turbine with additional compression as required.
• A relatively small air separation plant (ASU) is included to supply nitrogen as pressurized gas for coal transport, and the byproduct oxygen produced is added to the extracted air to enrich it. This ASU is typically 20-25% of the size that would require for an equivalent oxygen-blown IGCC plant.
• Raw hot gas leaving the gasifier is cooled by the use of a high temperature syngas cooler integrated with a ‘membrane water wall’ which produces a saturated steam to be sent onto the heat recovery steam generation (HRSG) unit for superheating.
• Cyclones and porous filters are used to remove the chars and recycle them to the gasifier.
• Downstream low temperature syngas cooling, acid gas removal and sulfur recovery are conventional low-temperature technologies.
• The power block consists of a modified MHI M7010DA gas turbine with air extraction for the gasifier, a HRSG and steam turbine.

The plant gasifies approximately 1,700 tpd of coal, and produces 250 MW of power with an efficiency of 42% (LHV net).  Demonstration of operation is expected to continue until the end of 2009. 

Figure 2: A Simplified BFD of Nakoso’s 250 MW IGCC Demo
(source: Mitsubishi Heavy Industries, Ltd.)

Figure 3: View of the 250 MW IGCC Demo Plant at Nakoso, Japan
(source: Mitsubishi Heavy Industries, Ltd.)

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