(165e) Optimal Production of DME from Switchgrass Based Syngas Via Direct Synthesis

Biofuels have become one of the major alternatives to reduce our dependency on fossil fuels. Typically, researchers have focused their efforts on the production of bioethanol, Fischer Tropsch liquids (FT) and biodiesel from biomass because of their easy implementation in the transportation system. However, there are a number of alternative fuels that can be produced out of the same raw materials such 2,5-dimethylfuran (DMF) or dimethyl ether (DME)  from lignocelluloses and/or algae . In particular, DME is a multi-purpose fuel. It has similar properties as propane and butane, so that it can replace Liquefied Petroleum Gas, LPG. It can also be used as diesel substitute, reducing NO_x, SO_x, and particulate emissions. Furthermore, DME can also replace chlorofluorocarbons as aerosol propellant. The advantage of producing DME is that the cetane number is closer to that of crude based oil and in fact, most of the production process is similar to the production of bioethanol or FT fuelsm since DME production is based on catalytic synthesis from syngas.

In this paper, a conceptual optimal design for the production of DME and/or power from switchgrass is proposed. It is possible to produce DME using a two step reaction process comprising methanol synthesis followed by its dehydration. In the literature, a number of simulation studies on the production of DME from coal and natural gas using process simulators, ASPEN Plus, are available.^1-6 Lately biomass has also been included as raw material ^7-8 Alternatively, a single step reaction to synthesize DME directly can be used. In this case, a hybrid catalyst is used avoiding the thermodynamic limitations of the methanol synthesis. The single step reaction leads to high CO conversion.Besides that, the investment is relatively lower than the two step reaction process.⁹ To systematically design such a process, a superstructure is formulated embedding two different gasification technologies, direct Renugas gasification using oxygen and steam or indirect Ferco gasification, using two chambers, a gasifier fed with olivine, steam and biomass and a combustor, where the char is burned to reheat up the sand providing the energy for the gasification. Next, two reforming modes are considered, partial oxidation, which is exothermic but with a lower yield to hydrogen or steam reforming, endothermic but with a higher yield to hydrogen. Subsequently, the raw syngas is cleaned from solids, ammonia and traces of hydrocarbons. Sour gases are removed next and the gas composition is adjusted in terms of the H₂ to CO ratio. Next, DME is produced following direct synthesis using a one step technology. The process is governed by a series of equilibria. The unreacted gas can be either recycled or used within a Brayton cycle for the simultaneous production of DME and power. The liquid phase is separated using a sequence of distillation columns where CO₂is also desorbed and send back to the sour gaes removal system. The problem is formulated as a mixed integer nonlinear programming problem (MINLP) to determine the topology of the system, the use of the syngas to power or DME production and the operating conditions of the different units. Next, we design a heat exchanger network. Subsequently, a sensitivity analysis is performed to determine the limiting prices for DME and power to produce either or both of them. Finally, a detailed economic evaluation is carried out to determine the production and investment costs.

The optimal topology involves indirect gasification followed by steam reforming. DME production is favored over power for current electricity prices. The optimal H₂to CO ratio to be fed to the reactor is 1, an interesting result since it is similar to the one needed for second generation bioethanol production. Thus, the investment cost for the production of 197 kt/yr  of DME adds up to 133 M$, at a price of $0.31 /kg. Apart from DME, hydrogen is also obtained (9.6 kt/yr of hydrogen) providing an interesting asset. The production of electricity is favored for electricity prices above 0.09 $/kWh. In case of producing electricity alone, up to 105 MW are produced with an investment cost 40% higher than that required for chemicals production and no hydrogen is produced. Even though, the electricity is generated at a competitive price of 0.07$/kWh.

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