Dipl.-Ing. Wolfgang Maus, Emitec Gesellschaft für Emissionstechnologie mbH, Lohmar;

Dr.rer.nat. Eberhard Jacob, Emissionskonzepte Motoren UG (haftungsbeschränkt), Krailling;

Synthetische Kraftstoffe – OME1: Ein potenziell nachhaltig hergestellter Dieselkraftstoff

Synthetic Fuels – OME1: A Potentially Sustainable Diesel Fuel

Abstract

Future powertrain technologies will be measured against five criteria: I. CO2 neutrality, II. sustainability, III. exhaust emissions, IV. cost-effectiveness and V. functionality. Criteria I. to III. are statutory requirements that take priority and lay the foundations for sustainability. This paper assesses the potential of different powertrain technologies to maintain a clean environment. Methanol and DME can be synthesised directly from CO2 as a carbon source and from sustainably produced H2. They are valuable sources of energy but toxicity and high vapour pressure at ambient temperatures limit their use as fuels. Their conversion to non-toxic or liquid fuels removes these limitations. The resulting fuels are CO2-neutral and sustainable. Ether-based fuels with C1 building blocks that contain no C-C bonds are a particularly effective means of minimising emissions with less complex exhaust aftertreatment. These fuels have the potential to continue the success of the internal combustion engine for the next few centuries. The term sustainability as used below is based on these conditions. To demonstrate the potential of a non-toxic, liquid C1 diesel fuel this paper focuses on oxymethylene ether (OME1). OME1 is made from methanol on a commercial scale and has a cetane number of 38. It can be mixed with additives to produce OME1a diesel fuel (CN 48). This paper discusses the initial results of OME1a tests on using an unmodified single-cylinder diesel engine, which were carried out at the Institute of Internal Combustion Engines at the Technische Universität München. Under substoichiometric conditions, stationary tests revealed that particle number emissions were in the ambient air range at very low NOx levels without exhaust aftertreatment. There was no measurable PN/NOx trade-off. It should be possible to reduce NOx emissions through engine-based measures without loss of efficiency by adapting the combustion process to OME1 fuel up to the 400 mg/kWh range. In order to cut NOx tailpipe emissions to below 0.1 mg/kWh, a low-NOx SCR catalytic converter system with correspondingly high activity will have to be developed. If this were to be successful, OME engines would be able to meet the S-ZEV level (sub-zero emissions, below urban immission) for NOx as well.

1 Criteria for optimum powertrain technology after 2020 1.1 Boundary conditions for sustainable energy and future powertrain systems The era of oil-based mobility is going to come to an end at some stage. Fuel consumption and CO2 emissions are rising worldwide while resources are diminishing. This is the great challenge facing our society that governments and industry have to address by testing and establishing suitable alternatives. The transport sector needs a CO2-neutral and infinite supply of fuel to guarantee future mobility. These fuels will have to be produced synthetically. They will have a completely different molecular structure and different properties than mineral oil-based fuels and be specially designed to minimise emissions. Blending these pure fuels with fossil fuels would be a useful step towards the long-term transition to synthetic fuels in terms of meeting statutory emission standards and optimum mobility costs. Extensive trials still have to be carried out.

I. CO2-neutral II. Infinite supply III. Minimum WTW emissions (well to wheel)

Nitrogen compounds (NO2, NO, N2O, NH3)

Particles (particle mass PM, particle number PN)

Unburnt fuel components

CO and oxo compounds (CH2O, etc.)

I. – III. Legislation

IV. Cost-effectiveness: system costs, energy consumption V. Functionality: range, energy storage, comfort, etc.

I. – V. Social/environmental compatibility

Figure 1: Boundary conditions: sustainable energy and powertrain systems 2020+

Figure 1 lists the boundary conditions for sustainable energy and powertrain systems, which are described in more detail below: I. CO2 neutrality Developments, especially in the passenger car sector, are primarily driven by CO2 limits. By reducing fuel CO2 emissions, hugely expensive investments in consumption-reducing components, for instance, for vehicles with large engines, could be avoided. CO2 neutrality can be achieved if we use the CO2 produced by industrial processes (especially in steel and cement production [Sc14]) and during power generation [Ma10, 12, 13] as a raw material for fuels. CO2 extraction from the air (carbon-negative) is technically feasible with low temperature heat [Wu13] but it will become a viable prospect only when the combustion of fossil fuels comes to an end. II. There should be an infinite (900 million year) supply of energy for fuel production Energy can be sustainably produced from the sun, wind or water. Virtually continuous power sources, such as offshore wind farms and solar thermal power stations in desert

regions, are preferred options. Sustainably generated electricity can be used to produce hydrogen by electrolysis or for distinctly endothermic processes (e.g. methane and methanol cracking, dry reforming). The hydrogen can then be used to reduce CO2 to form methanol (CWtL process [Ma10, 12, 13]), [Ef13]. The storage of unused wind or solar power still represents a challenge. The amount of this excess power is going to increase substantially in the next decade. Electrolysis using PEM technology reacts within milliseconds to the fluctuating supply from sustainable energy sources. The hydrogen can be stored under pressure and used for the production of methanol from CO2 [Si14]. III. Minimal emissions In response to environmental considerations, future legislative developments will impose increasingly stringent limits. Locally emission free electric vehicles are to set a political example, however, it is important to take account of the WTW emissions of these vehicles, which are determined by the type of power generation. A ‘dark calm’ creates a ‘renewable gap’, that is, inability to meet daily demand. Nuclear energy or fusion cannot fill this gap so the only option for the foreseeable future is to rely on fossil fuels. If combustion engines could be designed to produce negative exhaust emissions, this would give them an advantage over “merely” emission-free electric vehicles. I. – III. Legislation Strategic products and systems for sustainable mobility should primarily satisfy statutory requirements. Cost-effectiveness and functionality are of secondary importance. IV. Cost-effectiveness The importance of mobility to prosperity and the associated costs are key factors provided that environmental requirements have been met. The development of a new supply chain will place a significant burden on system costs, which will be much higher for electric mobility and gaseous fuels than for market-compatible liquid fuels. Energy input, the amount of investment and operating costs for the production of oxygenated fuels depend on the complexity of the molecular structure. For instance, the production costs of C1 fuels increase in the following order: methanol/DME, OME1, OME3/5. C1 fuels are easier to synthesise than oxygenated C2, C3 and C4 fuels (see glossary). The synthesis of sustainable gasoline and diesel fuels during a transitional period should also be discussed. V. Functionality Functionality is determined by the specifications of the fuel, the engine, the component application and the specific exhaust aftertreatment system. Loss of functionality would be acceptable if it was due to statutory or environmental and/or economic requirements. I. – V. Social/environmental compatibility New fuels are expected to be environmentally compatible. A perfect fuel should be extremely safe to use and harmless to humans and the environment (emission prevention, rapid biodegradability). If we aim for general explosion safety we already have to exclude all gaseous fuels. The perfect fuel is a liquid with low flammability.

1.2 GHG emissions of different energy sources

Figure 2: GHG emissions of different energy sources as CO2 equivalents. The grey sections of the bars illustrate variations in bibliographic references. Greenhouse gas emissions, especially CO2, vary widely depending of the type of energy source. Figure 2 shows specific GHG emissions per kWh. Compared to crude oil, natural gas emits 25-30% less GHG. As a result, car makers such as Volkswagen are stepping up the production of gas-powered vehicles in an effort to reduce fleet CO2 emissions. Natural gas, which has a 25 times higher greenhouse effect with respect to CO2, is probably not a sustainable fuel in the long term because of leakages during extraction and transport (approx. 2%) and incomplete combustion in lean-burn engines (0.1-0.3%). Natural gas should preferably be processed at the point of extraction to form liquid fuels by CO2 recycling. Electric mobility in Germany is based on a fuel mix that included 45% coal in 2012. CO2

emissions from power generation continued to rise in 2013 despite a significant increase in the share of renewable energy. As mentioned in item III above, the future development of CO2 emissions depends on the continuing expansion of sustainable energy sources, such as photovoltaics, wind and water. A simultaneous increase in the share of renewables – as stipulated by government – and total electricity consumption would lead to more energy gaps that have to be plugged by fossil fuels with a corresponding rise in CO2 emissions. This would have a knock-on effect on the CO2 emissions of electric vehicles. Taking into account the upstream chain, electric vehicles could at best be described as low-carbon. For more detailed informations see [Sc14].

2 Powertrain technologies on the basis of electric energy and emission targets 2.1 Emission targets and the sub-zero emission potential of powertrains Figure 3 shows different types of powertrain based on sustainable electricity. Power can be stored in batteries or used to split water by electrolysis. After being compressed from 50 to 500 bar, H2 can be stored in high-pressure tanks ready to be used in fuel cells or in hydrogen engines.

Figure 3: Powertrains based on electric energy: the potential for sub-zero emission vehicles H2 should be used for the production of C1 fuels from CO2. The power-to-gas process produces methane, which can be compressed from 50 to 250 bar and power gas engines. We opted for the production of methanol as a storable primary source of energy and its conversion to liquid C1 fuels with a high oxygen content (CWtL process). Initially, only combustion engines that are powered by H2 or a liquid C1 fuel seem likely to have the potential to clean the air (negative emissions). In gas engines this process is complicated by low soot formation and methane’s resistance to catalytic oxidation. The combustion of complex molecules with C-C bonds is always associated with soot emissions. These fuels require significantly more complex exhaust aftertreatment to

achieve negative emissions. In the long term, only lean-burn engine designs with SCR exhaust aftertreatment have a future in minimising fuel consumption. 2.2 Emission targets

Species Concentration in clean air

ppb [La13] µg/Nm³ Immission limits

µg/Nm³ [Um12] EU VI limits

mg/Nm³

CH4 1,800 1,200

N2O 300 550

CO 200 230 10,000 (8h)

O3 30-50 60-100 120 (8h)

VOC 10-100

NO2 0.01-5 0.02-10

200/40 (1h/1a) approx. 75

NO 500 (1d)

SO2 0.1-2 0.3-5 125

PM10 50/40 (1d/1a) approx. 2

PM2.5 25 (1a)

NH4NO3/SO4 20-50

Particle number 1E+9-10 #/Nm³ 1E+11 #/Nm³ Table 1: Trace elements in the air (average values) and immission limits Table 1 contrasts concentrations of trace impurities in clean air with average values of immission limits and exhaust gas concentrations estimated on the basis of EU VI limits for commercial vehicles. A lean-burn engine with an oxidation catalytic converter would – if we initially disregard engine emissions – convert most of the oxidisable components of air (CO, volatile organic compounds (VOC), soot and ammonium nitrate) to CO2 and N2. Ozone, O3 and NO2 are decomposed to form O2 and NO. The prerequisite for minimum engine emissions is a durable injection system with small volume capability. Significant progress has already been made in this area [Sc13]. Exhaust gas filters effectively remove particles from engine emissions but also increase fuel consumption and intermittently exceed emission limits during regeneration phases. PN emissions can be reduced below clean air levels only with low-particle combustion. However, the reduction of NOx emissions of 50-70 mg of NOx/kWh by about 3 orders of magnitude represents a big challenge, which can primarily be solved by a highly active low-NOx SCR catalytic converter system. Any further reduction of NOx engine-out emissions through engine-based measures does not look very promising because it would reduce engine efficiency. Since combustion engines “process” the ambient air they are essentially the only engines that have the necessary sub-zero potential (S-ZEV) to meet this requirement.

3 Potentially sustainable C1 fuels 3.1 Overview C1 fuels do not contain C-C bonds and therefore produce only a very small amount of soot during combustion. The most basic oxygenated C1 fuels are methanol and dimethyl ether (DME), which are produced on a large scale mostly from natural gas via an intermediate process of synthesis gas production (CO/CO2/H2 mixture).

Figure 4: Flow diagram of the CWtL process Sustainable methanol can be made from CO2 and H2 by way of a CWtL process as shown in the flow diagram in figure 4. CO2 is produced by an oxyfuel coal-fired power station with

fluidised bed combustion. Alkaline electrolysers ( = 67%) produce hydrogen and oxygen. About 80% of the electrolysed oxygen is required for the operation of the power station and replaces an air separation plant. Methanol is generated by catalytic hydrogenation of

CO2 [Ma10, Ma12, Ma13]. The calorific value of the methanol is 1095 MW ( = 60.1%) and production costs are primarily a function of the electricity price and amount to €390/760 per tonne at an electricity price of 4 or 8 cents. [Jä14]. The market price of methanol fluctuated around €380 per tonne in 2013. Commercial Cu/ZnO-based catalytic converters that are able to synthesise methanol from CO2 and H2 on an industrial scale have been extensively tested in the field [Po11]. Sustainable methanol/DME is produced by four different methods from refuse, industrial waste, biomass and in a geothermic power station [Ti13].

Methanol and DME are intermediate products for the production of higher molecular designer fuels. C1 fuels can be produced from COx and H2 with high yields, which makes them more cost-effective than oxygenated fuels with C2, C3, C4 and C>4 components that are produced less selectively from synthesis gas. C1 fuels contain sulphur and other contaminants only at ultra-trace level. This results in noticeable improvements in the exhaust aftertreatment of C1 fuel-powered engines: the long-term activity and selectivity of the catalytic converters is subject to a substantially diminished chemical deactivation. This also brings a reduction in the concentration of platinum metals that are used in oxidation catalytic converters within reach. 3.2 Fuels for spark-ignition engines 3.2.1 Overview of properties Table 2 a provides an overview of fuels for spark-ignition engines and their properties: molecular formula, molecular weight, density, freezing point, boiling point, oxygen mass, kinematic viscosity and surface tension. The characteristic values of conventional gasoline (OK) are included for comparison.

Fuel Formula MolW Density Fp. Bp. O mass KinVis STension

Dalton. kg/l15 °C °C °C % cSt mN/m 20°

MeOH CH3O 32 0.80 -98 65 50 0.69 22.6

DMC C3H6O3 90.1 1.08 2-4 90 53.3 0.63 28.8

EtOH C2H6O 46.1 0.80 -114 78 34.7 1.52 22.6

OK ~CH2 100 0.75 -45 25-210 ≤3.7 0.53 21.6a

Table 2a: Physical properties of fuels for spark-ignition engines: the C1 fuels methanol and dimethyl carbonate (DMC), the C2 fuel ethanol and, for comparison, gasoline (OK) in accordance with DIN EN 228:2013 aoctane Table 2b lists other characteristic fuel values: octane rating, flash point, upper and lower explosion limits, energy density, the gasoline (OK) equivalence factor and the air demand. The GHS hazard pictograms are also shown.

Fuel RON Flash point

Low/high ex limit

Energy density

OK Equi

Air demand

Hazards GHS

^ °C % kWh/kg kWh/l l/l Nm³/kWh Pictograms

MeOH 114 9 14.7/ 5.5 4.4 2 0.95

DMC 108 14 4.2/12.9 4.4 4.7 1.9 0.84

EtOH 130 12 3.4/15 7.4 5.9 1.45 0.93

OK 95 -21 0.6/7.6 12 8.9 1 0.94

Table 2b: Characteristic values of fuels for spark-ignition engines: the C1 fuels methanol and dimethyl carbonate, the C2 fuel ethanol and, for comparison, gasoline (OK) in accordance with DIN EN 228:2013

3.2.2 C1 fuels 3.2.2.1 Methanol Methanol fuels have been commercially tested with worldwide success. At present, methanol as a fuel in the form of M100 or mixed with gasoline (M85, M15) is used only in China’s coal-producing regions. In the EU the supply and use of methanol is associated with high costs and effort because of the strict provisions of chemicals legislation relating to toxic substances that have to be fulfilled. This makes is highly unlikely that it will be available from filling stations that are open to the general public. In such cases, the chemicals legislation stipulates the use of non-toxic substitutes wherever possible. The combustion properties of methanol offer two significant advantages over gasoline. Its higher knock resistance permits higher compression. Methanol is 2.7% more efficient than gasoline because of its high burn rate and expansion during combustion caused by an injection-related increase in the amount of substance of 21.5% (gasoline: 8.2%) [Ch87]. The soot-free combustion of methanol simplifies the exhaust aftertreatment. Low production costs and favourable combustion properties are arguments in favour of methanol as an engine fuel while its highly toxic properties are arguments against. 3.2.2.2 Dimethyl carbonate (DMC) [Go12] DMC has a number of positive properties when used as a blend component for gasoline: higher knock resistance, greater efficiency, improved combustion stability and reduced emissions. This was demonstrated in engine tests using gasoline containing up to 20% DMC. DMC is recommended as a safe substitute for the cetane number improver MTBE. DMC is non-toxic and readily biodegradable and could therefore also be used as a substitute for methanol. DMC is produced from methanol by reaction with CO and O2 on a commercial scale. Market prices were between €800 and €1,000 per tonne in 2013, which means that it would be uneconomical at present to use it as a fuel for spark-ignition engines. Alternative syntheses from methanol and CO2 are currently being developed and, if successful, could dramatically reduce the cost of DMC [Gr13]. DMC with added ethanol to lower the freezing point should be tested for its suitability as a non-toxic and environmentally compatible fuel for small SI engines. There is a good chance that DMC will be widely used in SI engines as a sustainable and environmentally compatible fuel in the long term. 3.2.3 Synthetic C2-C4 alcohol-based fuels The knock resistant alcohols ethanol, 1-butanol and 2-butanol that produce very small amounts of soot during combustion are mentioned here only for the sake of completeness. These alcohols are produced from sugar and in the case of ethanol at very low cost. A mixture of higher alcohols can be extracted from synthesis gas on copper catalytic converters.

The production of ethanol by reaction of methanol with CO to form acetic acid and the reduction of the latter with H2 are carried out on an industrial scale [Ja12]. 3.2.4 Synthetic C4-C10 hydrocarbon-based fuels Synthetic gasoline is produced by the highly exothermic, catalytic dehydration of methanol via DME on H-ZSM-5 catalytic converters with a yield of up to 89%. The methanol-to-gasoline (MTG) process represents mature commercial technology. The system costs are lower than those of the Fischer-Tropsch process. MTG plants have been built in China to make gasoline from coal [Hi10]. The production of gasoline from residual biomass via synthesis gas/DME by way of a Bioliq process started on a trial basis in 2013. This gasoline contains 50% of highly branched C4-C9 alkanes. The remaining 50% is made up of aromatic compounds (approx. 28%), alkenes and cycloalkenes. Compared to gasoline, it contains a higher amount of >C9 components with a high boiling point, which evaporate at temperatures over 200 °C. These high boilers are spurious components that have to be removed by hydrotreating because they are precursors to soot formation and in some cases because of their high melting point [Ot13]. MTG gasoline is classified as a CMR substance in the GHS regulations (Tab. 2b) Recent research shows that the use of other zeolite topologies for catalysis can prevent the formation of the aromatic fraction [Ol12]. 3.3 C1 fuels for diesel engines 3.3.1 Overview Table 3a and b provides an overview of C1 fuels for diesel engines and their properties:

OME Formula MolW Density Fp. Bp. O mass KinVis STensb

n . kg/l15 °C °C °C % cSt mN/m

0 C2H6O 46.1 0.67 -140 -24.9 34.8 0.15 12.0a

1 C3H8O2 76.1 0.87 -105 42.3 42.0 0.33 22.8a

1a 260.5 0.88 -22c 44.0 41.6 0.58

3/4/5d C6H14O5 166.2 1.07 -19f 155-242 48.8 1.77e

DK CH1.86O0.05 250 0.83 -20c 160-380 0.6 2-4 26

Table 3a: Physical properties (molecular formula, molecular weight, density, freezing point, boiling point, oxygen mass, kinematic viscosity and surface tension) of C1 fuels for diesel engines: DME (n = 0), OME fuels and, for comparison, diesel fuel (DK) in accordance with DIN EN 590:2010. a[An13], b20 °C [Wa06], ccold filter plugging point (CFPP), dmixture OME3/4/5: 36/37/27% by weight [Pe12], e40 °C, fcloud point

OME CN Flp. Low/high exlimits

Energy density

DK equi

Air demand

Hazards GHS

n °C Vol.-% kWh/

kg kWh/l 15 °C

l/l Nm³/kWh Pictograms

0 60 -41 3.4/18.6 7.9 5.3 1.8 0.92

1 38a -18 2.2/19.9 6.5 5.6 1.73 0.90

1a 48a -10 2.2/19.9 6.6 5.7 1.70 0.90

3/4/5 72 69 - 5.4 5.8 1.67 0.87

DK >51 >55 0.6/6.5 11.7 9.7 1 0.96

Table 3b: Characteristic values of C1 fuels for diesel engines: DME (n = 0), OME fuels and, for comparison, diesel fuel (DK) in accordance with DIN EN 590:2010. a AFIDA [An13], b60 °C 3.3.2 Dimethyl ether (DME) Dimethyl ether (DME) can be very easily produced from methanol by catalytic dehydration or directly from synthesis gas. DME with the formula CH3-O-CH3 has been widely tested as a diesel fuel with good results [Ar07, We10]. A Hyundai 4-litre diesel engine with 800 bar injection pressure and low-pressure EGR showed a surprisingly large improvement in efficiency compared to diesel mode. However, the engine and the oxidation catalytic converter failed to meet EU VI limits. After the installation of a PM-Metalit® and an SCR catalytic converter the engine was able to comply with EU VI, including the PN limit. Fuel costs for the DME engine were around 31% lower than diesel mode [Le13]. Volvo/Mack plan to launch the series production of DME trucks in the US in January 2015. DME produces very small amounts of soot during combustion and so permits significant engine-based NOx reduction. The physical characteristics of DME are listed in Tables 3a and 3b. The oxygen content of 34.8% is too low for completely particle-free combustion. The volumetric energy content is 1.8 times lower than that of diesel. DME has a boiling point of -25 °C and must be handled as a liquid gas in pressure tanks. This represents a disadvantage in terms of supply chain and vehicle technology compared to conventional liquid fuels. The significantly lower viscosity and surface tension of DME when compared to diesel has a positive effect on spray break-up when the DME is injected into the engine’s combustion chamber. This increases the spray angle and reduces the jet penetration depth. The maximum injection pressure is limited by the easy compressibility of liquid DME [Ar07, Sa11, We10]. 3.3.3 Oxymethylene ether (OME) 3.3.3.1 General The insertion of an n number of oxymethylene groups (-O-CH2-) into the DME molecule produces oligomeric oxymethylene dimethyl ether (OME) with higher molecular weights and boiling points of 42, 156, 201 and 242 °C (at n = 1, 3, 4 and 5):

CH3-O-CH3 + n (-O-CH2-) CH3-(O-CH2)n-O-CH3 (1)

The physical properties, flash points and cetane numbers of DME [n = 0 in equation (1)] and selected OMEs (n = 1 and a mixture of n = 3, 4, 5) are listed in Tables 3a and 3b. The OMEs can be mixed with diesel fuel in any ratio and, with the exception of OME1, have high cetane numbers, good material compatibility, excellent low-temperature performance, high density and are toxicologically unproblematic. The disadvantage of these methanol derivatives is their relatively low volumetric energy density of 5.7-5.8 kWh/litre because of a high oxygen content of 42-50%. However, this density still exceeds that of methanol (4.4 kWh/l) and of DME (5.1 kWh/l). 3.3.3.2 Mono-oxymethylene ether (OME1) OME1 is so far the only member of the OME group that is produced in commercial quantities. Figure 5a shows a simplified diagram of the process employed by Ineos in Mainz.

Figure 5a: The principle of commercial OME1 production from methanol (Ineos, Mainz [Re13]). Methanol vapour is partly oxidised and partly dissociated catalytically on an Ag mesh to form CH2O by substoichiometric addition of air (methanol ballast process). The thermal balance of the overall reaction partly compensates for the highly endothermic methanol dissociation by exothermic oxidation of methanol. Excess methanol and the produced CH2O are condensed out of the exhaust gas and converted to OME1 on an ion exchange resin. The H2-containing exhaust gas is burned to generate electricity.

Figure 5b: The improvement potential of OME1 production The partial oxidation of methanol can be largely avoided during fuel production. Methanol can be endothermically dissociated by direct electric heating to form CH2O and H2. The resulting hydrogen is returned to the methanol synthesis (figure 5b). The fuel-specific properties of OME1 (table 4a and 4b) are characterised by a low boiling point that is similar to that of gasoline. Bibliographic references for cetane numbers of OME1 vary between 29 [Ve99] and 30 [Og00]. Recent measurement with an AFIDA [An13, Se13] showed a value of 37.6. At 0.33 cSt OME1’s viscosity is substantially lower than that of diesel. 3.3.3.4 Higher molecular OMEs OME2-5 is made by converting OME1 with trioxane at 80 °C in a reactive distillation system. Trioxane is produced commercially by trimerisation of CH2O [Bu10,12]. The EGR mode of a four-cylinder car diesel engine powered by OME2-6 mixtures generated low particulate emissions of 1-2 mg/kWh at 1.2-1.3 g/kWh NOx [Sa03]. Using a 20 volume-%-mixture of OME3/4 in diesel fuel as expected showed marked reductions of soot by up to 90% [Ja12]. A mixture of OME3/4/5 (in brief: OME) used as fuel for a Euro 2 car was also tested. Compared to diesel, OME had no effect on the number of nanoparticle emissions (PN). The PN emission level was within a range of 6-7 E6 #/cm³, probably because of contamination from long-term diesel operation [Pe13]. The use of OME fuel reduced the emission level of a Euro 4 car engine below the Euro 6 limit. We would like to stress at this point that that the combustion process will have to

undergo some further development before the engine could be recalibrated for OME operation. The substantial reduction of engine noise is another highly positive factor [Pe12]. The properties of OME come closest to what is required of the ideal fuel of the future. A CN of 72 is a sign of high ignitibility; and a flash point of 69 °C provides a high degree of safety. The stoichiometric air requirement is 10% lower than that of diesel. The GHS labelling obligation for flammable substances does not apply to substances with flash points below 60 °C (cf. Table 3b). 3.4. Toxicity and environmental compatibility of C1 fuels All C1 fuels are non-toxic and rapidly biodegradable (WGK1). This represents an important advantage over conventional fuels (compare hazard pictograms in Table 2b and 3b). 4 Engine tests with OME1/1a fuel [Hä13, Ja14] 4.1 Previous tests with OME1 Initial tests with OME1 using a 6-cylinder diesel engine (5.9 l) were not very successful.

‘The engine was unable to start on neat methylal and could only be operated at the low power modes. Even at low power conditions, engine operation was somewhat unstable, and extremely high HC and CO measurements indicated the occurrence of incomplete combustion and/or misfires’ [Ve99].

Tests on a single-cylinder engine revealed that “smoke-free” combustion of oxygenated fuels required an oxygen content of >38% by weight. Soot particle emissions were measured with a Bosch smoke meter. Smoke-free combustion was also shown to be possible with dimethoxymethane (OME1) but at the expense of incomplete OME1

combustion. There was a notable rise in HC and CO emissions at 30% EGR and =1, which necessitated the use of a three-way catalytic converter [Og00]. 4.2 Single-cylinder engines with measuring equipment Tests were carried out on a single-cylinder research engine. The capacity, crank geometry and cylinder head arrangement of this research engine was derived from the MAN D20 series. The engine setup included heated external charging, variable settings for the exhaust backpressure and cooled external exhaust gas recirculation (high-pressure EGR). The fuel was injected via a common rail system. The fuel delivery system, injectors and nozzles come from the MAN D20 series. [Pf10]. Table 4 contains a summary of the test parameters. The engine oil was a polypropylene glycol monobutyl ether with approx. 4% of ash-free additives made by Dow Chemical in Horgen. Gaseous components (CO, THC, CO2, O2, NOx) in the exhaust gas were detected by multi-component exhaust gas analysis (Horiba MEXA-7000 series) and with an AVL Sesam FTIR (CH2O, CH3OH, OME1). Soot was measured with an AVL MSS 483 (micro soot sensor). The measured soot concentrations were often close to the device’s detection limit of 0.05 mg/kWh. To measure the concentration of the particle number (PN) raw

exhaust gas was diluted with a direct sampling unit at a ratio of 1:10 and fed into a particle counting system (Horiba MEXA-2300 SPCS) to determine the particle number (PN) in a PMP-like process.

RPM 1,200 1/min

IMEP 13 bar

Boost pressure (abs.) 1.93 bar

Exhaust gas pressure (abs.) 1.58

Overall TC efficiency (sim.) 60%

Charge air temperatures 40 °C

Centre of combustion mass 8° crank angle after TDC

Injection pressure 1,800 bar

Injection Pilot and main injection

Table. 4: Parameters of the single-cylinder engine

4.3 Test fuels 4.3.1 OME1 test fuel with additives (OME1a) OME1 was improved with additives to align its properties closer to those of diesel. The purpose of this was to achieve the best possible results without changing the combustion process of an engine that had been calibrated for diesel fuel. OME1 was mixed with 6% by weight of higher polymer C2/C3 polyoxaalkanes to increase the CN and viscosity. We describe this additivised OME1 as OME1a. Its physical properties are shown in Table 3a. The average molecular weight rises from 76.1 to 260.5 and the viscosity of the OME1 can almost be doubled. The characteristic fuel values in Table 3b show that the CN (AFIDA) increases from 37.6 to 48.3. The additives also ensure adequate lubricity. 4.3.2 Diesel fuel and mixtures with OME1 Comparative measurements used FAME-free diesel in accordance with EN DIN 590:2011 and mixtures of OME1 (Ineos, Mainz) with 5% and 75% of diesel. 4.3.3 C3 fuel C3 fuel tests were based on dipropylene glycol dimethyl ether (DPGDME), (Clariant, Burgkirchen) as a representative sample. DPGDME is non-toxic, boils at 175 °C, has a very high CN of 86 and an oxygen content of 29.6%.

4.4 Results of test bench measurements [Hä13] 4.4.1 Measurements with OME1a fuel

EGR NOx Soot (MMS) Particle number (PN) CO HC

[%(m/m)] [g/kWh] [mg/kWh] [#/kWh] [#/cm³] [g/kWh] [g/kWh]

none 1.80 16 0.12 2.9E+11 6.8E+04 0.20 0.09

16.1 1.35 3.11 0.09 1.6E+11 4.5E+04 0.30 0.09

18.2 1.29 2.19 0.08 1.3E+11 3.8E+04 0.32 0.09

21.1 1.07 1.26 0.07 9.3E+10 2.9E+04 0.39 0.09

29.0 0.98 0.21 0.06 8.0E+10 2.8E+04 9.56 0.15

Table 5: Results of OME1a fuel tests using a single-cylinder engine The low particle number concentration, which is only about one magnitude above the level of intra-urban ambient air across a wide range, is remarkable. The usual particle number increase that occurs when the air ratio decreases as a result of higher EGR rates is no longer detectable with respect to this fuel. The soot values measured by the micro soot

sensor (MMS) are close to the device’s detection level. An EGR rate of 29% ( of 0.98) reduces NOx emissions to 0.21g/kWh. The rise in CO concentration is caused by the thermal decomposition of excess fuel. 4.4.2 Comparative measurement with diesel and OME1/diesel mixtures

Figure 6a: PN/ trade-off of OME1/diesel mixtures, and diesel compared to OME1a The measurement results of OME1a fuel detailed in Table 5 are charted in figure 6a and 6b. Figure 6a shows that the addition of approx. 5% of diesel to OME1a is sufficient to recreate

the PN/ trade-off behaviour of diesel.

a

Figure 6b: PN/NOx trade-off occurs only in OME1/diesel mixtures and diesel but not in OME1a Figure 6b shows the particle number increase associated with the EGR-based reduction of NOx emissions, which was observed when adding diesel to OME1.

Figure 7: PN/NOx trade-off of OME1a compared to DPGDME (C3 fuel) and diesel C3 fuel also burns virtually particle-free in the presence of an appropriate amount of excess air. However, it exhibits distinct trade-off behaviour (figure 7). This leads us to conclude that even molecules with three interconnected C atoms cause the formation of soot particles at higher EGR rates. Combined with their complex production method this is another argument against the use of ether-based C>1 fuels. 4.4.4 Catalytic exhaust aftertreatment Tests are currently being carried out on a downstream, turbulence generating oxidation catalytic converter (substrate D118 x L150 mm, 1.64 l: LS300/600 made by Emitec in

Lohmar; coating: 40g/ft³ platinum from Interkat, RG: 52,000 h-1). Concentrations of CO, CH2O, CH3OH and OME1 are expected to be below the detection limit of the measuring equipment. The use of a PM-Metalit® should reduce particle number concentrations by another order of magnitude as demonstrated by measurements taken during the exhaust aftertreatment of a DME-engine [Le13]. The system reduces particle concentrations below 104-105 #/cm³, the level occurring in the ambient air. At very low engine-out emissions, catalytic converters are subjected to correspondingly low loads during exhaust aftertreatment. As a result, the activity and selectivity of the catalytic converter can be significantly increased. There are plans for further research in this area. 4.5 Discussion of results With OME1a the formation of particles could be largely prevented when the O2 content in the

combustion air was reduced from 20.5 to 17% ( reduction from 1.4 to 0.98, see table 5) through EGR. This is a remarkable and important result because it ensures that the soot

formation limit during diffusive combustion in a diesel engine will be below = 0.98. Since the

combustion of OME1 at = 0.58 produces only minimal concentrations of the soot precursor C2H2 [Di10], it is likely that combustion will be practically soot-free even under transient engine operating conditions with local rich zones. As a result, engine-based measures will be an efficient method of minimising exhaust emissions. The almost particle-free combustion of OME1 is due to a high oxygen content of 42%. In the case of DME, an oxygen content of 34.8% already leads to a measurable increase in the PN concentration. The particle emissions of a DME diesel engine with low pressure EGR and an oxidation catalytic converter did not meet Euro VI limits for particle numbers; this could only be achieved after the addition of a PM-Metalit® with an SCR catalytic converter [Le13]. The combustion of C1 fuels in the substoichiometric range generally causes only a small amount of particle formation although differences in degree are to be expected. The particle number emission will increase with declining oxygen content (in %) in the following order: OME3/4/5 (48.8) < OME1 (42) < DME (34.8) < methane (0). This assumption is supported by the findings from flame chemistry (e.g. [Di10]). OME3/4/5 fuel creates excellent conditions for minimum emissions. It is closer than OME1 to diesel in terms of its physical properties and a high flash point of 90 °C provides additional safety. If we could fully exploit the potential for simplifying OME synthesis, it would be the ideal fuel of the future. Due to their flammability fuels made from of C2, C3 and higher C components will always produce greater particle emissions than C1 fuels. As EGR compatibility diminishes the complexity of the exhaust aftertreatment increases accordingly. Fuels with high oxygen content have positive combustion properties with higher combustion rates and higher engine efficiencies. Relevant tests on a complete engine using OME fuels have not been carried out yet. Compared to diesel, the stoichiometric air demand measured

in Nm³/kWh is around 7.3% lower for OME1 and around 9.4% lower for OME (table 3b). This reduces pumping losses and increases EGR compatibility, especially low-pressure EGR. 5 Conclusions and outlook It is commercially possible to replace fossil fuels with sustainable fuels. In terms of E fuel production from CO2 and electrolysis H2 by way of a CWtL process, C1 fuels require the relatively least effort. Preference should be given to liquid C1 fuels that could utilise the existing distribution and filling station network. The C1 diesel fuel OME1a burns without a significant amount of particle formation even when there is an insufficient amount of oxygen and so permits soot-free combustion in transient mode as well. This creates the right conditions for exhaust aftertreatment with negative emissions (S-ZEV). Further improvements have to be made to SCR technology. In order to achieve NOx conversion rates of >99,95% a highly active, low-NOx catalytic converter system that is supported by a heated catalytic converter will have to be developed. System designs and viability calculations for a large-scale plant have been completed. The plans show that, depending on renewable energy costs, CO2-neutral fuels can be produced at competitive prices when compared to second or third generation biofuels. The possibility in principle to store the excess energy generated by photovoltaic systems and wind farms in the form of fuel is an important aspect. Now is the time when it seems appropriate to stop automatically associating combustion engines with fossil fuels and think instead of sustainable engine fuels for the next few centuries. 6 Bibliography [An13] Analytik Service Gesellschaft, AFIDA, Advanced Fuel Ignition Delay Analyzer, afida.eu/afida.pdf 2013. [Ar07] C. Arcoumanis, C. Bae, R. Crookes, E. Kinosgita, ‘The potential of DME as an alternative fuel for CI engines: A review’, Fuel, 87, 1014-1030 (2007). [Bu10] J. Burger, M. Siegert, E. Ströfer, H. Hasse, ‘Poly(oxymethylene) dimethyl ethers as components of tailored diesel fuel: Properties, synthesis and purification concepts’, Fuel 89, 3315-3319 (2010). [Bu12] J. Burger, ‘A novel process for the production of diesel fuel additives by hierarchical design’, Dissertation, Technische Universität Kaiserslautern, 2012. [Ch87] F. Chmela, ‘Untersuchungen zur Vielstoffähigkeit eines den Kraftstoff direkteinspritzenden und wandanlagernden Verbrennungsverfahrens mit Fremdzündung’, Dissertation, Technische Hochschule Darmstadt 1987. [Di10] V. Dias, X. Lories, J. Vandooren, ‘Lean and Rich Premixed Dimethoxymethane/Oxygen/Argon Flames: Experimental and Modeling’, Combust. Sci. Tech. 182, 350-364 (2010).

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[We10] M. Werner, G. Wachtmeister, ‘Dimethylether- Dieselalternative der Zukunft?’, MTZ 71, 540/2 (2010). [Wu13] J. Wurzbacher, ‘Capturing CO2 from Air’, Empa Technology Briefing, 28.2.2013 www.empa.ch/plugin/template/empa/*/133210. 7. Glossary Ash-free engine oils: Polypropylene glycol alkyl ether base oils with ash-free additives. C1 fuels: Abbreviation for fuels containing C1 building blocks (isolated C atoms, without C-C bonds, e.g. methane, methanol, DME, OME1-5, DMC. C2 fuels: Abbreviation for fuels containing C2 building blocks (no more than two directly bonded C atoms), e.g. ethane, ethanol, diethyl ether. C3 fuels: Abbreviation for fuels containing C3 building blocks (no more than three directly bonded C atoms), e.g. propane, 1- and 2-propanol and dipropylene glycol dimethyl ether (DPGDME). CMR substances: Hazardous substances that are carcinogenic, mutagenic and toxic to reproduction. CN: Cetane number, unit measuring the ignitibility of diesel fuel [An13]. DK: Diesel in accordance with DIN EN 590:2010, may contain up to 7% by volume of FAME (fatty acid methyl ester). DME: Dimethyl ether, liquid gas, simplest oxygenated C1 fuel. E fuels: Fuels made from CO2 with an electricity-based energy content. GHS: Globally Harmonised System, international labelling of hazardous chemicals. OK: Gasoline in accordance with DN EN 228: Gasoline contains a mixture of C3-C10 hydrocarbons, including 30% of aromatic compounds. May contain up to 10% by volume of ethanol. Emits small soot particle concentrations during combustion in DI spark-ignition engines. Classified as a CMR substance in accordance with GHS. Oxyfuel combustion: The firing of power stations with oxygen instead of air. Oxygenate: Fuel additives, in particular:

C2 oxygenate: Fuel additives based on higher molecular polyethylene glycol ether.

C3 oxygenate: Fuel additives based on higher molecular polypropylene glycol ether.

C4 oxygenate: Butanols, octane boosters, e.g. MTBE with t-butoxy groups. OME: Abbreviation for polyoxymethylene dimethyl ether (POMDME). OME1: Abbreviation for monooxymethylene dimethyl ether, also known as dimethoxymethane (DMM) or methylal (common name).

OME3/4/5: C1 diesel fuel with the approximate molecular formula C6H14O5, mixture of tri-, tetra- and pentaoxymethylene dimethyl ether in the boiling range of diesel fuels. PAG engine oils: Base oils on the basis of polyalkylene glycol ether and particularly polypropylene glycol butyl ether. Used for ash-free engine oils. Acknowledgements We would like to thank Walter Jäger from Process Engineering in Engelskirchen for the design of CWtL plants. We would also like to thank Prof. Georg Wachtmeister and Martin Härtl from the Institute of Internal Combustion Engines at the Technische Universität München and Philipp Seidenspinner and Dr Thomas Wilharm from Analytik-Service-Gesellschaft, ASG mbH in Neusäß for their experimental support and useful discussions.