(655b) Chemical Recuperation of Low-Grade Exhaust Heat by Steam Reforming of Dimethyl Ether

Energy consumption that was causing several fatal environmental problems all over the world, have been continued to grow consistently. In addition, for most of energy supply, Japan depends on the foreign country. Problems such as soaring of fuel price have been actualized. Under those circumstances, the securement of stability supply for energy, the prevention of the global warming, or the establishment of the new energy supply system which have aspects of high energy efficiency and furthermore, environmental friendly is required. In our country, the cascading system of energy use has been developed after oil crisis, and the highly effective use for energy has been promoted. A lot of attempts to use of exhaust heat have been executed through the process of a variety of economic activities. For instance, the exhaust heat has been used regularly as the steam for the heating of the fuel in all the chemical plants.

However many trial has been done, the case that energy could not be used effectively has come out. That is because three disproportions in this reason exist, that the demand of exhaust heat does not suit the supply of exhaust heat fully. Firstly, the geographic disproportion that the supply ground and the demand ground are in long distance, secondly, the time disproportion that supply is not suitable for demand changed with time. Thirdly, qualitative disproportion that supply is not satisfied with the demand needed for higher exergic temperature or electricity. That is because that, with sensible heat, being transported, stored, or transformed to electric energy is difficult. According to a quantitative investigation of the exhaust heat, the amount of the exhaust heat discharged in the atmosphere as unused is about three times of energy consumption. Especially, utilization of low-grade exhaust heat of bellow 573 [K] is difficult. Energy is wasted to the ambient.

If the exhaust heat energy could be recuperated as the fuel that it is possible to be transported and stored, those problems would be dissolved.

In this study, the process that chemically recuperates the exhaust heat as a fuel with by reforming of dimethyl ether (DME) was proposed. The empirical examination in regard to the feasibility of that process was discussed. Nowadays, DME have attracted much attention as a petroleum substitute fuel, which is synthesized from even low-grade gas fields or biomass, and also as one of hydrogen resources having high energy density. In especial, reforming of DME is performed in relatively low temperature between 523 [K] and 673 [K] and the reforming of DME is an endothermic reaction as CH₃OCH₃ + 3H₂O + 122 [kJ/mol] = 6H₂ + 2CO₂. The lower heating value of effluents is 1453 [kJ/mol] that is 9% higher than that of reactant DME (1332 [kJ/mol]). If the exhaust heat energy would be used for the reforming process, the exhaust heat energy could be recuperated as hydrogen, fuels of high quality. The image and the merits of this process are shown in Fig.1 and Fig.2 respectively.

Some studies on steam reforming of DME have been investigated from the view point of the development of catalyst, and advanced catalyst with high conversion rate in lower temperature of 600 [K] have been developed. The development of the reformer, which can obtain highly conversion of DME at around the temperature of 773 [K], has been in progress now to apply for the fuel cell or the chemically recuperated gas turbine system. Compared with those investigations, this study has some different aspect. In terms of energy recuperation, the system, which can collect much heat as much as possible at low temperature, is better. In case the temperature of exhaust heat is low or the quantity of exhaust heat is small, the improvement design of reactor, which can obtain sufficient conversion ratio, must be necessary. The feasibility study and development of the system, which it can correspond to those requests in utilization, are necessary. The required feature of device for energy recuperation process is simple structure, cheap, and easy maintenance. For reformer, fixed bed type, fluidized bed type or reaction and heat transfer monolithic type is considered. When use with practical process is assumed, the fixed bed type, whose maintenance is easy with simple structure, has big advantage. In regard to the optimization, quantization of rate equation becomes necessary. Some investigations concerning the individual catalyst for the purpose of reforming in low temperature with high reaction ratio is done, however the kinetics of reaction has not been investigated yet. Although the reactor of fixed bed type have been used for the measurement of the catalyst reaction, those measurement are done with the identical W/F as a conditional setting of experiment, or under the dilution conditions with inert gas to measure the instinctive character of each catalyst.

In this study, fixed bed type was adopted experimentally. Catalytic activity for steam reforming of DME was estimated in the atmospheric pressure, the S/C=1.55 and reaction temperature of 523 [K] to 573 [K] with practical condition for the purpose of chemical recuperation of low-grade heat. Furthermore by changing the W/F [g-cat h/mol], in other words catalyst bed thickness, fundamental experiment was investigated in regard to the design of the fixed bed reactor.

Catalytic activity was estimated using a fixed-bed flow quartz reactor at temperature of 523 [K] to 573 [K]. The catalyst of 1 [mg] or 4 [mg] was placed inside the reactor on silica wool. The catalysts are contributed by JFE Co. Major component of that catalyst are cupper and alumina. The catalysts heated under a H₂ flow (20 [ml/min]) for 120 [min] at 573 [K] prior to the catalytic measurement. The feed mixture with steam/DME = 3.1 was allowed to pass through the reactor at a flow rate of 82 [ml/min] (W/F = 10 to 80 [g-cat h/mol]) in the maximum. Each catalyst can be pre-treated and heated for the same treatment time with this experimental setup. The effluents were analyzed by two sets of gas chromatography equipped with thermal conductivity detectors and capillary columns with a He and N₂ carrier. H₂, CO₂, CO, DME, and traces of CH₄ were the only products detected under the present experimental conditions. Conversion of DME and the selectivity of hydrogen were calculated with quantity and composition of effluents. The schematic diagram of equipment is shown in Fig.3. The percentage of methanol was calculated with hydrogen selectivity.

From the results of experiments at the temperature of 573 [K] shown in Fig.4, increase in conversion ratio of DME and selectivity ratio of hydrogen with increase of W/F were confirmed. Approximately 100% conversion ratio of DME and selectivity ratio of hydrogen were achieved at W/F of 40 [g-cat h/mol]. The composition of effluent gas was consisted of mixture of 74.6% hydrogen, 19.8% carbon dioxide, and 5.6% carbon monoxide, lower than 1% DME, methanol and methane shown in Fig.5.

In lower W/F range, formation of methanol was confirmed. Steam reforming of DME is composed of two consecutive reactions. The first step is the hydration of DME to form methanol over solid acid catalysts such as alumina: CH₃OCH₃ + H₂O = 2CH₃OH, and the next step is the steam reforming of methanol: CH₃OH + H₂O = 3H₂ + CO₂. On the other hand, in higher W/F range, formation of CO or CH₄ was confirmed. The reverse water gas shift reaction; CO₂ + H₂ = CO + H₂O, and the methanation reaction; CO + 3H₂ = CH₄ + H₂O, were seemed to occur simultaneously. For the purpose of observation for catalytic activity at lower temperature, steam reforming of DME also examined at the temperature of 523 [K] shown in Fig.6. From results of experiments, high conversion ratio of DME and selectivity of hydrogen was confirmed. The composition of effluent gas was consisted mixture of 74.3% hydrogen, 22.7% carbon dioxide, 2% methanol, 1% DME. Carbon oxide and methane was lower than 1% shown in Fig.7. The increase of lower heating vale is 8.9% compared to reactant DME.

From those investigations, the process of chemical recuperation of low-grade exhaust heat can be achieved enough at the temperature of 523 [K] to 573 [K] if reformer or heat exchanger is produced optimally.