DME SYNTHESIS COULD UNLOCK REMOTE COAL RESERVES

Dimethyl Ether (DME) is a clean alternative fuel that can be produced from natural gas, coal or other organic resources through syngas. The properties of DME are suitable for the production of power generation fuel, transportation fuel, home fuel and chemicals. Y. Adachi, of the University of Tokyo, et al. discussed how the production of DME fuel from unutilized natural gas or coal resources in remote areas could contribute to solving the energy-environment dilemma, in Fuel, 79 (2000) pages 229-234.

The coal department of Japan’s Agency of Natural Resources and Energy of the Ministry of International Trade and Industry has decided to promote DME-producing technology and conduct research and development to confirm its usefulness at a pilot plant. The 5-year project started in fiscal year 1997 is being implemented as a technology development program in the Center for Coal Utilization, Japan.

DME is synthesized from syngas H2,CO) in three steps:

• Methanol synthesis reaction
• Dehydration reaction
• CO shift reaction

The basic features of the NKK process are the utilization of a slurry-bed reactor and the addition of a shift function to the dehydration catalyst. The slurry-bed reactor is an apparatus where the reaction gas forms bubbles, which react as they rise in the slurry--a solvent containing fine catalyst particles. The reactor structure is simple and the heat of reaction is quickly absorbed by the solvent that has a large heat capacity, and owing to the high-heat conductivity of the solvent, the temperature within the reaction vessel is equalized so that it is easy to control its temperature. Also, there are fewer restrictions on the shape and strength of the catalyst in the slurry bed than in the fixed bed.

Figures 1 and 2 show the CO conversion and the selectivity of one-pass mode reaction experiments obtained by bench-scale tests at NKK (reaction vessel interior diameter is 90 millimeters, slurry bed is 2 meters in height). The maximum production capacity is 50 kilograms per day of DME where the total selectivity of DME, MeOH and CH4 is considered 100 percent. The CO conversion rises as the temperature increases, but it shows a maximum. This is understood as the effect of the equilibrium restrictions that appear as the conversion approaches equilibrium. At 5.0 MPa and 300ºC, CO conversion of over 50 percent and DME selectivity of over 90 percent are obtained.

Figure 1

Figure 2

Coal provides 17 percent of Japan’s primary energy. Almost all the firing coal is imported. It is transported across land and sea, received and stored, and the necessary handling equipment and dust prevention measures push up the costs, and its use lowers the quality of the environment.

Figure 3 shows the conceptual flowchart of the overall system which would be used to produce DME from coal or natural gas, then transport it and provide it for the thermal powerplant. Of course, the cost of DME which is synthesized from coal, is much higher than that of coal. However, it is expected that the use of DME made from coal would result in the following improvements over coal-fired thermal power production:

• Coal cleaning would no longer be necessary and all the produced coal could be used effectively.
• It would be possible to use low-quality coal, lignite coal, etc.
• Coal-bed methane, a coal development byproduct, is often an unexploited energy source.
• At the transportation stage the DME could be handled in the same way as liquefied petroleum gas, hence the shipping and receiving base equipment could be simple and dust free.
• This ash-free clean fuel would eliminate the need for desulfurization and ash disposal treatment.
• As a gaseous fuel, DME would achieve high electricity generation efficiency because of the use of combined-cycle power production.
• The problems of providing a coal yard and dealing with ash would be solved.

Figure 3

Australian Blair Athol coal (calorific value of higher heating value: 6,520 kilocalories per kilogram, converted to lower heating value: 6,240 kcal/kg, mine site coal price: US$20.1 per ton) was selected as a typical example of steam coal. The powerplant scale was assumed to be 500,000 kilowatts (operating rate of 75 percent). The quantity of fuel consumed to produce electric power was set as 1.294 million tons per year of pulverized coal for the coal thermal plant. The net quantity of coal consumed for DME production and power generation is 1.328 million tons per year.

When the price of coal is 0.322 yen per megacalorie (Mcal), which is the average price of Blair Athol coal at the mine site, the production cost of DME at the production site (same as that of mine site) is 2.18 yen/Mcal. Also, using natural gas with a price of 0.50 yen/Mcal gives 1.83 yen/Mcal of DME at the production site. When the DME (with the price of 2.18 yen/Mcal) is transferred to power generation site (for example Japan) and converted to electricity it gives electricity at a cost of 8.16 yen/Mcal. By using conventional power generation plant with environmental protection apparatus, electricity is produced with the same coal at a cost of 9.4 yen/Mcal. It is clear that the DME process can produce electricity at a cheaper price.

The total environmental load for DME flow from coal production at mine sites to power production and waste material treatment in Japan was compared with the equivalent environmental load for coal-fired power production. The switch-over to DME will reduce the environmental load as follows:

• As the net coal consumptions are equal, the quantities of CO2 emission are almost identical. The effective use in the gasification furnace of released coal-bed methane will reduce it by 20 percent equivalent to CO2.
• No sulfur compounds will be emitted into the atmosphere after the switch-over to DME. The sulfur in coal will be completely removed after the coal gasification.
• After the switch-over to DME, the quantity of NOx generated will be reduced sharply from coal-burning powerplant levels, as NOx will not be produced.
• Ash will be discharged as molten slag from the coal gasification furnace, which is easier to use than fly ash and elutes no heavy metals.

The most appropriate reaction conditions (catalyst, temperature, pressure, retention time) have been clarified by 50-kilogram per day bench-scale testing, but to develop a practical system, 5-ton per day pilot plant testing will be carried out. The challenges through this testing are as follows:

• Determination of the construction of the DME synthesis slurry reactor vessel and establishment of a method of controlling its internal temperature distribution.
• Creation of a highly efficient and highly selective DME synthesis process.
• Development of the technology needed to scale up a commercial-scale reactor vessel (several hundreds of tons per day).
• Economic and environmental assessment of the total system encompassing the syngas production, DME synthesis, DME transportation and DME utilization stages.

The International Development Engineering Society of Japan is continuing to perform more detailed case studies of DME fuel introduction, and based on the results of this work, an international workshop in the year 2000 has been planned.