1. Background and process overview
Dimethyl ether (DME) is a clean energy source and as it generates no sulfur oxide or soot during combustion its environmental impact is low. Owing to its non-toxicity and easy liquefaction properties, DME is easy to handle and therefore can be used as a domestic-sector fuel (substitute for LPG), transportation fuel (diesel vehicles, fuel cell vehicles), power plant fuel (thermal plants, cogeneration plants, stationary fuel cells), and as a raw material for chemical products. Currently DME is produced by dehydrating methanol. Approximately ten thousand tons per year are produced in Japan, and 150 thousand tons per year worldwide. DME’s main use is as a spray propellant. Given the above-described superior properties, if DME were to become widely available in large volumes at a reasonable price, DME could be used as a fuel in a wide variety of fields.
2. Development objectives and technology to be developed
The objective of this study is the development of a process to directly synthesize DME from syngas (mixture of H2 and CO), (Chemical reaction (1)).
Existing technology produces DME through the dehydration of methanol, (Chemical reaction (3)). The scale of the plant in actual operation, however, is rather small, and scaling-up is an issue to produce DME as a commercial fuel. In addition, the equilibrium conversion of the methanol synthesis reaction (Chemical reaction (2)) is relatively small, as show in Figure 1. On the other hand, because the methanol synthesis equilibrium limitation can be avoided in the direct DME synthesis reaction (1), a higher conversion rate can be achieved.
The reaction formulae relating to the direct synthesis of DME are provided below. The equilibrium conversions for methanol synthesis and direct DME synthesis are given in Figure 1. Since direct synthesis produces the maximum conversion with a syngas composition of H2/CO=1, the process is suitable for the syngas produced from coal gasification (H2/CO=0.5-1). The targets of the development study include the following:
- DME production rate: 100 tons/day or greater, syngas total conversion: 95% or greater, DME selectivity: 90% or greater, DME purity: 99% or greater
- Establishing scale-up technology
- Optimizing entire system
- Establishing stable plant operation
Chemical reaction:
- 3CO+3H2 → CH3OCH3+CO2
- CO+2H2 → CH3OH
- 2CH3OH → CH3OCH3+H2O
3. Progress and development results
The development of the direct synthesis process has been led by JFE. The first step was the search for a direct synthesis catalyst. The second step was a small bench plant (5 kg/day). The third step was a 5 ton/day pilot plant (Photo 1) constructed with the aid of the Ministry of Economy, Trade and Industry (METI). The fourth step was a 100 ton/day demonstration plant (Photo 2) also constructed with the aid of METI. This demonstration plant was completed in November 2003 and is scheduled to have six long- term continuous operation runs over 2 to 5 months from the end of 2003 to 2006. The total production of the demonstration plant has reached approximately 19,000 tons.
4. Issues and feasibility of practical application
DME is already utilized as a fuel on a small-scale in an inland area of China. In Japan, activities toward the practical application of DME as a fuel are being undertaken by: DME International Co., Ltd., which was established by the same ten companies that established the DME Development Co., Ltd., aiming to study the commercialization of DME; Japan DME Co., Ltd., which was established by Mitsubishi Gas Chemical Co., Inc., Mitsubishi Heavy Industries, Ltd., JGC, and Itochu Corp.; and Mitsui & Co., Ltd. and Toyo Engineering Corp., which adopted the methanol dehydration process. These activities aim to supply DME product in 2006. All of them plan to use natural gas as the feedstock, because natural gas requires only a small initial investment. In the future, however, they are expected to switch to coal, which has larger reserves than natural gas, as the feedstock. Using natural gas, the CO2 obtained from the CO2 separation column is returned to the syngas generation furnace; if coal were to be used, however, the CO2 would be released into the atmosphere. When long-term CO2 storage technology is established in the future, the coal-based process will be able to provide purified CO2 without any additional equipment. In the West, DME has drawn attention in a wide range of fields as a new fuel. There, however, it is expected to be mainly used in diesel vehicles to fully leverage the fact that DME combustion generates no soot.
References
1) Tsunao Kamijo et al., Lecture papers of The 8th Coal Utilization Technology Congress, Tokyo, pp.194-205, 1998.
2) Yotaro Ohno et al., Preprint for the lectures of the 30th Petroleum and Petrochemical Discussion Meeting, Tokyo, pp. 26-29, 2000.
3) Yotaro Ohno: Japan DME Forum Workshop, Tokyo, pp.113-122, 2002.
Source: New Energy and Industrial Technology Development Organization (NEDO), Japan Coal Energy Center (JCOAL)
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