Process Simulation and Environmental Assessment of the Production of Dimethyl Ether from Digestate

World energy demand is strongly increasing. The continue grow of the world’s population, the enhancement of welfare, industrialization and fast-growing emerging economies development (e.g. China, India) cause a continue grow in the demand of energy [1]. In this concern, the transport sector accounts for almost two-thirds of the total liquid fuels demand. The demand of liquid fuel for transport application is strongly increasing because the sharp increasing in vehicle ownership number in emerging economies by passing by around one billion of vehicles to around 2.5 billion in just twenty years in the world due the fast economy growing mainly of China and India. This high growth rate of energy demand is in contrast with the necessity to reduce carbon emission, aimed to minimize the impact of energy utilization on the environment. For these reasons, the production of transport fuels from renewable and eco-friendly sources (solar, wind, water, tides, biomass or waste) is quickly increasing during the next twenty-thirty years, although fossil sources are still the most used energy sources (more than 80%), mainly due to economic reasons [2].

In this concern, the production of dimethyl ether (DME) from renewables or waste permits to obtain an alternative fuel for Diesel engines, with its high cetane number (>55) and a high well-to-wheel efficiency compared with other fuel (i.e. methane, ethanol, and Fisher-Tropsch fuel). Furthermore, DME represents a reliable environmentally friendly fuel due to the significant reduction of NOx emissions and total absence of SOx and particulate matter in engines exhaust gases respect to conventional diesel fuel [3, 4].

Dimethyl ether (DME) is the simplest of ether with molecular formula C₂H₆O. DME is a colorless, non-toxic, non-corrosive and non-carcinogenic molecule with a normal boiling point of -25°C that can be liquefied above 0.5 MPa at ambient temperature. Physical properties of DME are similar to those of LPG, and the technologies developed for storage and transport of LPG can be easily converted to accommodate DME with similar safety guidelines and codes [5]. DME is also an important chemical intermediate to produce a wide variety of chemicals, such as diethyl sulphate, methyl acetate and light olefins [6,7]. Nowadays, DME is mainly used as an aerosol propellant in several spray cans, replacing the banned ozone-destroying CFC compounds but in the last decades, it is receiving a growing attention as an alternative eco-friendly fuel. In 1995, an extensive collaborative research effort among Amoco (actual BP), Haldor Topsoe and Navistar International Corporation, demonstrated that DME could be a reliable alternative fuel for diesel engines with low-emission of NOx, SOx and particular matter, manufacturable at large-scale from methanol by a simple dehydration technology [8]. These studies remarked the outstanding performances of DME as a diesel alternative fuel and showed total compliance with the most stringent California ULEV (ultra-low emission vehicle) emission regulations for medium-duty vehicles. Because the necessity to change the fuel distribution infrastructures and the modifications to engine devices, DME market as diesel alternative fuel was challenging. Indeed, the primary DME market was the blending of with LPG cut because of their very similar chemical-physical properties. For instance, Amoco patented a DME/LPG blend for automotive application [9]. The current key markets for DME as fuel are (1) blending with LPG, (2) alternative fuel for diesel engines, (3) fuel for power generation by gas turbines plant, and (4) chemical intermediate for olefins and synthetic-gasoline production.

DME can be produced from a variety of feed-stock including natural gas, coal, crus-oil residual oil and biomass. First, raw-materials are converted into synthesis gas (syngas), a mixture of hydrogen, carbon monoxide, and carbon dioxide. The fossil-derived hydrocarbons (e.g. natural gas, oil or coal) can be converted to syngas by well-known technologies. Steam reforming is the most used way to produce syngas by adopting reaction temperature and reaction pressure in the range of 700 – 900°C and 30 – 50 bar, respectively, by using a nickel-based catalyst [10]. The production cost of syngas strongly depends on the price of raw material and its market economy. Syngas can be produced also by non-fossil sources. For instance, gasification of biomass or waste (thermal or catalytic) is a sustainable way to produce syngas [11-15]. After syngas production, DME synthesis can be performed in two different ways. One way is a conventional two-step process (indirect synthesis) which consists in the methanol synthesis from syngas followed by methanol dehydration in two distinct reactors; the other way is a one-step process (direct synthesis) which directly produces DME from syngas in a single reaction unit [16]. Indirect synthesis way is the most consolidated process for dimethyl ether production at industrial scale. 

Indirect synthesis is the traditional way to produce DME. This route consists on methanol dehydration reaction reported below:

2CH₃OH=CH₃OCH₃+H₂O.

Methanol dehydration is an exothermic reversible reaction (-∆H°_298K=23.5 kJ/mol) that proceeds without mole number variation. For this reason, operation pressure does not affect equilibrium conversion while low reaction temperatures have a thermodynamic benefit toward DME production. Methanol dehydration is a reaction catalyzed by acid catalysts and several investigations have been published in order to individuate an active, selective and stable catalyst at relative low temperature for the above-mentioned thermodynamic advantages. Depending on catalyst characteristics, methanol dehydration can be carried out in both vapor and liquid phase, with reaction temperature in the range 100-300°C and pressure up to 20 bar. γ-Al₂O₃ is the traditional catalyst for vapor phase methanol dehydration.

In the direct synthesis way methanol synthesis (typically from a CO/CO₂/H₂ mixture) and methanol dehydration step to produce dimethyl ether are carried out in one-pot process over a redox/acid catalyst.

Table 1 reports the reactions set concerning direct synthesis process with the related reaction heats.

Table 1. Reactions set of direct synthesis of DME

The main steps occurring during the direct synthesis process are usually the synthesis of methanol by hydrogenation of CO₂ (a) or CO (b) over redox catalytic function, methanol dehydration to DME over acid function (c) and eventual water gas shift reaction (WGSR) catalysed by the redox function (d). The traditional catalyst for the direct synthesis of DME is a redox/acid bifunctional catalyst operating at reaction condition similar to those adopted for methanol synthesis (250-280 °C and 5 – 10 MPa). A redox function is necessary to promote methanol formation and acid function necessary to dehydrate the alcohol to the ether. The traditional catalyst for methanol synthesis Cu/Zn/Al₂O₃ (CZA) is used as redox function for methanol synthesis, combined withγ-Al₂O₃ or zeolites to produce DME [17-22].

It is important to remark that the market of dimethyl ether is strongly affected by production costs as well as by price, availability, accessibility and safety of the starting raw material. In this concern the utilization of waste as carbon source to produce syngas for DME synthesis is a scenario to merit at least a techno-environmental assessment. The global solid waste generation by urban residents is expected to increase from 3.5 Mtons/day in 2010 to 6.1 Mtons/day in 2025, with a total management cost of $375 billion in 2025 [23]. Therefore, the management of MSW is a very challenging issue to be addressed. Municipal solid waste may be considered as a valuable source of carbon usable for both energy and chemicals production. Incineration/thermo-valorization of MSW is the major solution adopted for unsorted MSW. On the contrary, an efficient MSW collection and sorting of MSW is the first step for recovering or recycling high-added value products pushing towards a circular economy system. In fact, the organic fraction of municipal solid waste (OFMSW) may be used as feedstock for anaerobic digestion.  Anaerobic digestion is a process which not only reduces the amount of organic waste, i.e. organic fraction of MSW (OFMSW), sewage sludge, livestock manure and so on, but also produces energy in the form of biogas [24]. Biogas, the primary end-product of the anaerobic digestion process is a mixture of methane and carbon dioxide (CH₄/CO₂ molar ratio ranges from 1.1 to about 2.5) with minor quantities of nitrogen, hydrogen sulfide and volatile organic compounds. Biogas is usually used as fuel in combined heat and power plants [25] or upgraded to produce bio-methane for automotive sector. In addition to biogas, digestate containing a high amount of non-biodegradable components is also produced from an anaerobic digestion process. Currently, digestate is mainly used as nitrogen-rich organic fertilizer, avoiding the use of mineral fertilizers for agricultural soil. Nevertheless, in the last decade, the number of anaerobic digestion plants is growing significantly, causing also a significant growth of digestate production which need to be valorized in alternative way. In this regard, gasification may be used to produce syngas from digestate [26].

In this work, a process simulation of the production of dimethyl ether via indirect synthesis way is presented by adopting a syngas composition equals to that obtained from digestate gasification. The aim of the study is to assess the environmental impact in terms of equivalent carbon dioxide emission of the investigated case, pushing towards a waste-to-chemicals scenario.

A plant size equal to 100’000 t/y of digestate was considered as collective digestate treatment plant.

Table 4 shows the process parameter values obtained by simulations. Global CO conversion on WGS section was equal to 56 %mol, that is sufficient to remove 35.5 kt/y of CO₂ by Selexol^®, which corresponds to removing level of 90 % of CO₂ from the H₂-rich stream. The carbon capture makes possible obtaining a (H₂-CO₂)/(CO+CO₂) molar ratio equal to 2, suitable for the methanol synthesis. The global conversion of CO+CO₂ to MeOH was about 54% (thanks to the syngas recycle) with a methanol production of 4.8 kt/y.

A purge split ratio equal to 10 % was set in order to keep equal to 2 the (H₂-CO₂)/(CO+CO₂) molar ratio in the reactor inlet.

The reaction conversion of MeOH to DME was equal to 88% (equilibrium value) with a production of about 3.2 kt/y of dimethyl ether.

Also, the electricity production was considerable. Purge gas was used in an engine to produce electricity and thermal energy for the heat integration. Electricity consumption of compressors and chiller to cool final DME condenser decreased the gross electricity from 7.4 MWe to 2 MWe.

The environmental assessment results shown in Table 5, highlights the environmental benefit of the proposed process scheme. The emissions of the plant derived by char disposal and direct CO₂ in the flue gases, essentially. This thanks to the captured CO₂ by Selexol®, after increasing of CO₂ partial pressure by WGS section. The biggest CO_2eq saving value corresponds to avoided digestate disposal for a total of 182 kt/y of CO_2eq. The second biggest saving was for the green electricity production (8.6 kt_CO2eq/y), due to the high specific saving of 600 kg_CO2eq/MWhe.

Table 4. Process simulation main results

Table 5. Annual CO₂ equivalent emissions. Negative values are CO₂ savings

Finally, specific CO₂ saving is equal to 0.97 kgCO_2eq/MJ by considering the indirect synthesis of DME from digestate-derived syngas. Considering the emissions of diesel are equal to 0.099 kgCO2/MJ [32], a total saving of 1.07 kgCO_{2 eq} /MJ was obtained.

Without considering the environmental benefit of digestate utilization, a positive global emission derived by DME production is obtained, i.e. +0.99 kgCO_{2 eq}/MJ. The syngas quality is not optimal for an industrial synthesis without considering a waste raw material. Low global yield to DME (3.2 kgDME/kgdigestate) and high compression power make the process less convenient. In general, the DME production by the indirect synthesis can be considered when the raw material is a waste and energy for the compression of syngas-to-methanol has a low cost.