Replacement of Diesel Fuel by DME in Compression Ignition Engines: Case for India

Abstract

Decarbonising of transport, industrial and all sectors of economy is a necessity to stop or reverse global warming. Use of batteries, fuel-cells, hybrid topographies with smaller IC engines and use of alternative fuels like methanol, ethanol, DME in the IC engines are some of the ways through which emission of green-house gases can reduced/eliminated. Diesel engines are highly efficient due to higher compression ratios and are used in the heavy-duty transportation vehicles. DME is a single molecule fuel having high cetane number and which can be used as a drop-in fuel on the diesel engines albeit with retro-fitment of these engines with a new pressurized fuel system. DME with a chemical formula CH3-O-CH3 can be produced by different feedstocks such as coal, natural gas, biomass and bio-waste and municipal solid waste. India has a large reserve of high ash coal and generates high quantities of biomass and MSW, all of which can be converted to DME by use of clean production technologies. India’s transport and industrial sectors consume about 100 billion liters of diesel fuel per year produced entirely from imported petroleum. This amount of diesel can be replaced by indigenously produced DME from locally available coal, biomass and MSW.

1. Introduction

Decarbonizing the transportation sector is the highest priority to reduce global warming, air pollution and associated health hazards. There is a need to develop ultra-low carbon or carbon-negative fuels which can be produced from renewable sources like waste, organic substances, plants and trees etc. DME is a low cost, low carbon and zero soot and PM chemical stock that can be used on CI engines. DME can be produced from various feedstocks such as wood, methanol, wastes, biomass etc. Its use as a substitute for diesel fuel is due to high cetane number of 55–60 and superior combustion characteristics. It is a colorless, non-toxic, mildly narcotic and easily inflammable gas at normal temperature and pressure similar to LPG.

Many CI engine manufacturers like Isuzu, Nissan, Mitsubishi, Volvo have developed DME fueled CI engine powered vehicles. DME vehicle’s reliability have been validated in the field with running tests of 100,000 Km or more. DME fueled CI engines exhibit higher combustion efficiency and soot free combustion in comparison to corresponding diesel engines.

Diesel engines are used widely in power generation industry. Expansion of the power generation sector is fueling the growth of diesel engine production. Global diesel power engine market is estimated to grow at a CAGR of more than 3.5% during the period from 2021 to 2026. Demand for reliable electricity due to the industrial expansion, development of commercial infrastructure, electrification of human habitats, and uninterrupted power supply is the major cause for the growth of diesel engine power market [1]. Therefore, there is a valid case for developing these engines to operate on DME.

2. Energy consumption trends and future of diesel engines and DME

Figure 1 presents the global energy consumption from 1971 to 2019 in Exajoules (10¹⁸ J). World energy consumption has been increasing steadily from 1971 (176.8 EJ) to 2019 (418 EJ), an increase of 136% [2]. Largest increase has been in oil and electricity consumption. It is expected that the increase in these two sources will continue in the future. With the development of new battery technologies and solar PVs, increase in rate of electricity consumption including that generated from renewables will increase further.

Figure 1.

World total energy consumption by source (1971–2019) [2].

Figure 2 gives the world energy consumption history and projections. Figure 2(a) shows that increase in industrial energy consumption is expected to be the largest followed by the transportation sector although growth in residential and commercial sectors is also expected at a relatively moderate rate. Figure 2(b) gives the world energy consumption on the basis of fuel. Use of petroleum and other liquids are anticipated to increase till 2050 and will be about 250 quadrillion Btu. Renewables, during the same period, are expected to rise to 40 quadrillion Btu. Therefore, the share of petroleum and other liquids will be around six times that of renewables. In the heavy-duty sector diesel-based compression ignition engines are the preferred power source. Therefore, there is a sound future for the CI engines in all sectors, i.e., power generation, transportation, industry etc. Conversion of CI engines to use DME will lead to reduction in fossil fuel consumption and prevent degradation of environment.

Figure 2.

World energy consumption, history and projections [3]. a) World end-use consumption by sector. b) World end-use energy consumption by fuel.

2.1 Consumption of diesel by end use

Among the petroleum fuels, diesel is a critical fuel for transportation especially heavy-duty vehicles, agricultural vehicles, construction and earth moving equipment. Power generation is another important area where diesel is used. Figure 3 shows the end-use of diesel fuel in the USA, Japan and India, countries which are the one of the largest consumers of diesel fuel. In all three countries transport sector is the major consumer of mineral diesel. Conversion of trucks and busses to DME will lead to major reduction of fossil fuel and associated emissions.

Figure 3.

End-use of diesel fuel in USA, Japan and India [4, 5, 6]. (a) USA – 241 billion liters per year. (b) Japan – 60 billion liters per year. (c) India – 100 billion liters per year.

2.2 Categorization of diesel engine market

IMARC group has categorized the global diesel engine market as shown in Figure 4.

Figure 4.

Categorization of the diesel engine market based on power rating and end-user [7].

Power rating of the diesel engines varies from 0.5 MW to more than 5 MW used in automotive and non-automotive applications all over the world. Figure 4 illustrates that development of engine technology to utilize DME in diesel engines will have a positive effect in all sectors and across continents. Thus, a concentrated effort by different OEMs to come out with DME conversion kits or even new DME engine designs will not only reduce fossil fuel consumption and environmental deterioration but also has a firm economic viability. All sectors are expected to exhibit increase in use of diesel engines, however highest growth is expected from the power generation sector, trucks and busses and industrial sector. The global diesel engine market reached a value of US$ 212.4 billion in 2021 [7]. It has been predicted that the demand for diesel engines including all sectors and sizes should increase to US$ 240 billion in the year 2025 registering a compound annual growth rate of over 3% from 2020 to 2025 [8].

3. DME fuel production

Top Key producers of Dimethyl Ether (DME) include: a) Akzo Nobel, b) Shell, c) The Chemours Company, d) China Energy, e) Mitsubishi Corporation, f) Ferrostal GmbH, g) Grillo Werke, h) Jiutai Energy Group, i) Oberon fuels and j) Zagros. In Japan several large scale DME plants have been set-up [9]. China is the bulk producer of DME from Chinese coal and production plants have also been set-up in Trinidad and Tabago, North America, Indonesia and Uzbekistan. First bio DME production plant was constructed in Sweden. Global production of DME at present is roughly 9 million tons per year [10]. Different feedstocks can be used in the production of DME. These are natural gas, coal, waste from pulp and paper mills, forest products, agricultural by-products, municipal waste and dedicated fuel crops e.g., switch grass. Methanol dehydration is the main process for production of DME currently. Synthetic gas can however be produced by gasification of coal, biomass, natural gas reforming [11].

There are three pathways to produce DME: a. Two-step process, b. One-step process, c. Liquid-one-step process called as bio reforming. Typically, DME is produced through a two-step process with syngas the feedstock (Table 1). Methanol is first produced from Syngas, followed by dehydration of methanol into DME (Eqs. (1)–(4)) (Table 1, Figure 5) [12].

Equation	Process Desc	Eq.
CO + 2H2 → CH3OH	Methanol formation	(1)
2CH3OH → CH3OCH3 + H2O	Methanol dehydration with solid-acid catalyst	(2)
H2O + CO → CO2 + H2	Water gas shift reaction	(3)
3H2 + 3CO → CH3OCH3 + CO2	Net reaction	(4)

Table 1.

Two-step synthesis process of DME [12].

Figure 5.

Two-step process for synthesis of DME [13].

In Japan, Korea and China single-step process is used most commonly (Eqs. (5)–(6)) (Table 2 and Figure 6). In this process, the methanol formation, methanol dehydration and water-gas shift reaction are merged. Different feedstocks can be used in this process such as methane, scrubbed bio-gas, syngas, etc. In case of methane feedstock methane-dry-reforming is done before the single-step process. Japan Steel Company has reached production capacity of 100 t DME/day by this process (Olah, et al. 2009). Mass balance shows that for every gm DME produced 1.43 gm of CO2 is used in the process. In overall reaction, CO2 sequestration is 0.48 gm per gram of DME.

Eq. (2)	Process Desc	Eq.
3CH4 + 3CO2 → 6CO + 6H2	Methane-dry-reforming	(5)
6CO + 6H2 → 2CH3OCH3 + 2CO2	DME synthesis	(6)
3CH4 + CO2 → 2CH3OCH3	Net	(7)

Table 2.

Single-step synthesis of DME [12].

Figure 6.

Single-step process for DME synthesis [13].

Third process for DME production is bio-reforming (Table 3, Eqs. (8)–(12)). For optimized process to produce methanol Metgas is used, which is 2:1 H2:CO ratio syngas. Two processes to produce Metgas are methane-steam-reforming and methane-dry-reforming step. Next steps are methanol formation and DME synthesis by dehydration of Methanol. Water produced during the dehydration of methanol is used in the methane-steam-reforming process. CO2 consumed during per gm formation of DME is 0.48 gm. There are no emissions of CO2 during bio-reforming process, however there will be CO2 emissions regarding process’s energy requirements.

Equation	Process Desc	Eq.
2CH4+ 2H2O → 2CO + 6H2	Methane-steam reforming	(8)
CH4 + CO2 → 2CO + 2H2	Methane-dry-reforming step	(9)
4CO + 8H2 → 4CH3OH	Methanol formation	(10)
4CH3OH → 2CH3OCH3 + 2H2	Methanol dehydration	(11)
3CH4 + CO2 → 2CH3OCH3	Net	(12)

Table 3.

Bio-reforming process for production of DME [12].

Both one-step and two-step DME production process are mature technologies. There are many companies which have developed single-step process for producing DME. Notable among these are Haldor-Topsoe A/S Denmark, JFE Holdings, Japan, Korea Gas Company, S. Korea, Air Products USA, NKK Japan, Oberon Fuels USA etc. Two-step DME production process has been developed by companies like Toyo Japan, Mitsubishi Gas Company Japan, Lurgi Germany, Udhe Germany. Several companies have developed novel processes and technologies for production of DME.

World’s first bio-DME demonstration plant in Sweden (started in 2010) uses black liquor (waste from paper and pulp industry) to produce high-quality syngas which is used for synthesis of DME (Figure 7).

Figure 7.

Bio-DME production plant in Sweden [13].

3.1 Production process developed by Oberon fuels

Oberon Fuels has developed proprietary skid-mounted, small-scale production units that convert methane and carbon dioxide to DME from various feedstocks, such as biogas from dairy manure and food waste. These small-scale plants are affordable as compared to a large plant, do not require large infrastructure and permits etc. for operation. These small-scale production units can produce 10,000 gallons (37854.12 liters) of DME per day to cater to the regional fuel markets [14].

Schematic of the plant is shown in Figure 8, consists of SMR, make-up syngas compressors, methanol synthesis reactors, pre-cut column and DME column and DME storage tanks.

Figure 8.

Schematic of a small size DME production plant developed by Oberon fuels [14].

The Oberon Fuels methane-gas-to-DME process has the following three major steps:

1. Syngas production

2. Methanol synthesis

3. Simultaneous DME synthesis and separation via catalytic distillation.

First two steps are common in large scale industrial application. Catalytic synthesis of DME with purification has been investigated in detail and has been demonstrated industrially by Oberon Fuels at Brawley California. In this plant, all production, storage, piping, tanks and valves are overground. DME produced from un-scrubbed (60% methane) HSAD (High Solid Anaerobic Digestion) of food waste, yard waste, bio-waste is called as Bio-DME. Chemically Bio-DME and DME are chemically same, Bio-DME uses biogas (typically produced from anaerobic digester), whereas DME is produced from pipeline natural gas.

4. DME handling, storage and distribution

Table 4 shows the boiling points of propane, butane and DME.

Table 4 illustrates that the boiling point of DME is in between that of propane and butane and all three are gas at room temperature of 15°C. The storage and handling facilities for LPG can therefore be repurposed for DME at a low cost. LPG storage and handling infrastructure is widespread in many countries including India. In addition to the use of existing LPG facilities new installations need to be created for DME. In the existing LPG installations changes will be needed for seals, valves, pressure regulators, gaskets, pumps to handle DME.

4.1 Suitability of materials and storage and handling equipment

Storage, handling and transport of DME is as a liquid under pressure similar to LPG. DME is compressed to 5 bar pressure for liquefaction at room temperature. Material compatibility of DME with the seals and gaskets in stationary and moving parts has to be established. ASTM specification D7901 gives directions on safety and handling of DME, which includes elastomer selection for gaskets and seals (to avoid their failure). Also, all equipment for DME storage and handling have necessarily to be overground. Table 5 provides compatibility of different elastomers with DME as per ASTM D7901. Rubber and plastics swell and deteriorate easily on contact with DME as compared to LPG. Nitrile rubber seals (NBR) and fluoro rubber seals (FKM) are used with LPG but swell in contact with DME and therefore cannot be used with DME.

	Propane (C3H8)	DME(CH3OCH3)	Butane(C2H6)
Boiling point (°C)	−42°C	−24°C	−1°C

Table 4.

Boiling points of propane, DME and butane [15].

Elastomer	Compatibility rating for use of DME
Natural Rubber (NR), Isoprene, Butadiene Styrene (SBR, Buna S), Butadiene (BR), Butyl (HR), Ethylene Propylene (EPR, EPDM, EP), Polyacrylate (ACM), Fluor elastomer – Di polymer, Fluor elastomer – Terpolymer	4
Polychloroprene (Neop rene, CR), Chlorosulfonated Polyethylene (CSM)	3
Polysulfide (T)	2
Silicone (VMQ), Fluor silicone (FVMQ), Perfluoro elastomer (FFKM), Polytetrafluoroethylene (PTFE), Nitrile (NBR, BUNA-N), Hydrogenated Nitrile Butadiene Rubber (HNBR)	1
Rating Legend (at room temperature); 1 = Little or minor effect, 0 to 5% volume swell, 2 = Minor to moderate effect, 5 to 10% volume swell,; 3 = Moderate to severe effect, 10 to 20% volume swell, 4 = Not recommended for DME use

Table 5.

Compatibility of DME with elastomers as per ASTM D7901 [12].

Cylinder tanks for DME fuel will be similar to LPG, will be fabricated structure consisting of cylindrical shell and panel with filling valve, outlet valve, return valve, safety valve, overfill prevention device, quick coupling and fluid level gauge.

5. Use of DME in diesel engines

5.1 Fuel properties and special fuel injection system

A pressurized fuel injection system for DME is an essential requirement. Thus, the tank, fuel pumps, fuel piping and the fuel injector have to be kept under suitable pressure. Fuel pumps have to pressurize the fuel circuit to a pressure which is higher than the saturation vapor pressure of DME at the operating temperature. This will prevent the DME fuel to vaporize and cause cavitation in the fuel circuit before the fuel injection into the cylinder. Temperature of the fuel inside the fuel injector reaches to 80°C and the pressure of the fuel circuit after the fuel pumps has to be increased accordingly. A pressure higher than 30 bar is considered adequate for keeping the DME in liquified form even at higher temperatures encountered during operation of the engine. Feed-pumps will be able to pressurize the fuel to the required pressure.

ASTM standard range for viscosity of liquid fuels has a range of 1.39 to 4.2 cSt at 40°C whereas viscosity of DME is within 0.185 cSt and 0.23 cSt. Low viscosity of DME will result in leakages past clearances used for sealing like plungers and barrels, seals and gaskets and pump gears etc. Low lubricity will result in high wear and seizure of the moving parts in fuel injection system. Viscosity and lubricity enhancing additives are added to the DME to overcome these problems.

Bulk modulus of DME is less than diesel by an order of magnitude. This implies that DME is much more compressible than diesel. High compressibility of DME will result in delay in the injection timings. Injection lag will be higher as compared to diesel and therefore the ECU has to be programmed accordingly. The compression work of DME in the fuel pumps and injectors is much higher than diesel fuel and the parasitic power for pumping of fuel is higher. Large compressibility of DME also results in injection instability and this problem can be overcome by modifying the nozzle design and control of fuel temperature.

Table 6 compares the chemical and physical properties of DME, diesel fuel and LPG (propane, butane). Diesel properties are compared to DME to understand the similarity between the two in compression-based ignition, whereas comparison of DME to LPG is required to know about the similarity in fuel handling of the two fuels.

Property	DME	Diesel	Propane	Property	DME	Diesel	Propane
Chemical structure	CH3-O-CH3	—	C3H8	Explosion limit ((% by volume of air))	3.4–18.6	0.6–7.5	2.1–10.1
Molar mass (g/mol)	46	170	44.097	% wt. Carbon	52.2	86.0	82
Lower heating value (MJ/Kg)	27.6	42.5	46.3	% wt. Hydrogen	13.0	14.0	18
Liquid density (Kg/m3)	667	831	500	% wt. Oxygen	34.8	0	0
Cetane number	> 55	40–55	—	Critical temperature (K)	400	708	369.9
Stoichiometric Air-Fuel ratio	9.0	14.6	15.8	Critical pressure (MPa)	5.37	3.00	4.301
Auto -Ignition temperature (°C)	235	250	470	Critical density(Kg/m3)	259	—	220
Boiling point (°C)	−20	180/ 370	−42	Kinematic Viscosity of liquid (cSt)	<1	3	4.29
Latent heat of vaporization (kJ/Kg)	460 (−20°C)	250	372	Surface tension (at 298 K) N/m	0.012	0.027	0.007
Modulus of elasticity	100–1000 MPa Depend on Temp. & Press.	1400 Mpa	220 MPa	Vapor pressure (at 298 K) kPa	530	≪10	580

Table 6.

Comparison of properties of DME and diesel fuel [12, 15].

Table 6 shows that auto-ignition temperature of DME is lower than diesel at pressure higher than atmospheric. Also, the cetane number of DME is higher than that of diesel. Thus, DME fuel is suitable in CI engines and has high potential to replace diesel fuel. At the same time, boiling point of DME is close to propane below Zero °C and the handling, storage and distribution of DME is similar to LPG. World over, LPG is used as cooking fuel and also for transport, therefore there is adequate experience in handling and storage of LPG. Use of LPG storage and handling facilities for DME with modifications to the seals, gaskets and certain metallic parts can be done with lower efforts and cost.

DME is an oxygenated fuel with an oxygen percentage of nearly 35%. Higher Cetane number results in lower ignition delays and smaller pre-mixed combustion phase, lower peak cylinder pressures and lower NOx formation. Absence of C-C bonds leads to sootless combustion. In the DME molecule each carbon atom is bound to three hydrogen atoms on one side and oxygen atom on the other. Bond energy of C-H is 414 kJ/mol and that of C-O bond is 359.0 kJ/mol. Higher C-H bond energy is responsible for shorter ignition delays and higher cetane number of DME.

LHV of DME is almost half that of diesel, therefore to obtain the same horsepower, flow rate of DME is about 1.7 times that of diesel. This means larger storage tanks for DME, higher diameter pipes and tubes for fuel flow, higher flow capacity of the DME pumps and the fuel injectors. Duration of Injection (DOI) of DME will be longer than diesel and the Start of Injection (SOI) has to be advanced accordingly. Lower boiling point of DME translates into faster vaporization of injected fuel in the combustion chamber. This along with lower critical temperature of DME results in superheated vapor in the combustion chamber, adequate air-fuel mixing is ensured. Large heat of vaporization also lowers the in-cylinder temperatures and lower NOx emissions.

Chain combustion reaction is possibly through one of the following competing pathways [12]:

1. C–O bond fission (pyrolysis mechanism):

   $${\text{CH}}_3{\text{OCH}}_3={\text{CH}}_3\text{O}+{\text{CH}}_3.$$E13

2. Hydrogen abstraction (oxidation mechanism):

   $${4\text{CH}}_3{\text{OCH}}_3+{\text{O}}_2={4\text{CH}}_3{\text{OCH}}_2+{\text{2H}}_2\text{O}.$$E14
   $${\text{CH}}_3{\text{OCH}}_2={\text{CH}}_2\text{O}+{\text{CH}}_3$$E15

As the C-O bond energy is smaller than C-H bond, distortion of the C-O bonds in the DME molecule weakens the bonding strength and the breakage of the C-O bonds earlier. Pyrolysis is more likely to start the chain reaction at relatively low temperatures, showing as lower auto-ignition temperature.

Change of piston and cylinder heads in the existing diesel engines may not be required other than design of the fuel injection nozzle to suit the volumetric flow rate and existing piston profile. However, in order to have an optimum design of combustion chamber to suit the spray characteristics of DME changes to the piston bowl and re-location of piston rings may be needed. For best performance of the engine valve timings may also need to be modified resulting in design and development of new camshafts. For modifying existing diesel engines, it becomes necessary to retrofit a new fuel injection system right from the fuel tank, feed pumps, pressure pumps, common rails and fuel injectors.

DME is similar to LPG in terms of safety. Vapor of DME is heavier than air and settles to the ground similar to LPG. Sufficient ventilation is necessary in locations where DME is being used whether for stationary installations or transport. Ignition limit of DME is 3.4% - 18.6% by volume and therefore necessary precautions have to be implemented which will be similar to LPG. Global Warming Potential (GWP) of a molecule is its adverse effect on climate change. GWP includes both the molecules lifetime and ability to absorb radiation. DME is atmospherically not dangerous and does not contribute to global warming.

Common rail fuel injection systems have been developed for DME fueled CI engines and these engines have demonstrated good engine performances and efficiency along with significant reduction in harmful exhaust emissions. This has been made possible by having a good control of the fuel injection characteristics and temperature. The common rail concept for DME fuel have also proven effective in simple and safe fuel handling. Figure 9(a) illustrate a comparison of the concepts use for DME fueled diesel engines, figure is self-explanatory. Figure 9(b) shows a schematic of the DME fuel storage and distribution system on engine.

Figure 9.

Fuel injection system and injection characteristics of a DME fueled engine [16]. (a) Comparison of the fuel injection concepts for DME fueled engines. (b) DME storage, handling and injection system for a DME fueled diesel engine. (c) Comparison of simulated injection characteristics of Diesel and DME DHOS injector.

Figure 9(c) presents a comparison of the simulated injection characteristics of diesel and DME digital hydraulic operating system (DHOS) injectors. Piston lifts in both the cases are similar, whereas the fuel injection pressure for diesel injector is about 1200 bar and for the DME injector it is around 800 bar. Duration of injection (DOI) for the DME injector is higher as compared to the diesel injector and similar trend is reflected in the period of needle lift of the injector. The injection rate of fuel for the DME case is higher than diesel all along the injection and the DOI is higher as can be seen in the bottom-most figure. Higher volume of fuel flow for DME vis-a-vis diesel can be seen in the figure due to lower density and LHV of DME compared to diesel.

5.2 Emission characteristics

Stoichiometric combustion of DME in air yields 1.91 m of CO₂. This is equivalent to 66 gm of CO₂ per MJ (LHV) of combusted DME.

CH3OCH3 + 3(O2 + 3.78N2) → 2CO2 + 3H2O + 11.34N2

DME combustion in air

(16)

Stoichiometric combustion of one gram of diesel C₁₂H₂₃ yields 3.16 grams of CO₂.

C12H23 + 17.75(O2 + 3.78N2) → 12CO2 + 11.5H2O + 67.095N2

Diesel combustion in air

(17)

Combustion of one gm of DME in air emits less CO₂ than combustion of one gm of Diesel, difference being 1.25 gm less CO₂. Experimental investigations have brought out that there is substantial reduction in particulate matter (PM), NOx and combustion noise when DME is used as a fuel in CI engines. Combustion efficiency (BSFC) of DME fuel in a CI engine is similar to diesel (Figure 10) and so fuel consumption can be similar on an energy basis. Also shown in Figure 10 are comparison of the road load emissions of NOx, CO, HC and Smoke (PM) on a DME and diesel fueled engine. Smoke is undetectable and NOx, CO, HC are lower in DME engine fitted with an oxidation catalyst.

Figure 10.

Comparative analysis of emission data from neat DME & mineral fueled CI engine [17].

Figure 11 gives the NOx emission data vs. engine efficiency for a 1.15 MW diesel power generation unit engine fueled with DME and with different levels of EGR. As the DME fueled engine does not produce any soot and ultra-low PM emissions, higher EGR levels can be used on the engine to reduce the NOx emissions. Against a 950 ppm NOx regulation, as the EGR is increased, very low NOx levels of 30 ppm can be achieved albeit with an engine efficiency penalty of 3%.

Figure 11.

NOx emission data from a DME-fueled 1.15 MW diesel power generation unit [18].

Figure 12 shows emission results on 6-cylinder 7 liter turbocharged/intercooled heavy-duty diesel engine operating in Japanese D13 mode driving cycle. NOx and CO2 emissions can be reduced with DME fueled engine vis-a-vis diesel engine at comparable fuel economy. In addition, combustion noise of DME fueled engines are lesser than their diesel counterparts.

Figure 12.

Comparison of fuel consumption (BSFC), NOx and CO2 emissions on a 6-cylinder 7 liter turbocharge/intercooled heavy-duty diesel engine operating in Japanese D13 mode [19].

In many countries research and development of DME fueled CI engines has been carried out and commercial trails done successfully. Japan, Europe, North America, China and South Korea are the leaders in development of DME fueled engines. A brief summary of the development of DME engines is presented in Table 7.

Region/Country	Details
Europe	Denmark, Haldor Topsoe developed the first DME fueled vehicles in 1996, Euro 4 compliant in 1998. In Sweden Volvo developed the first DME-fueled bus in 1999, 2nd gen bus in 2005, 3rd gen DME truck in 2015
North America	Consortium was formed between Pennsylvania State University, Air Products and Chemicals Inc., the Department of Energy (DOE), Navistar International and Caterpillar between 1999 and 2001 to work on a project to convert diesel busses to DME-fueled busses
Japan	National Traffic Safety & Environmental Laboratory (NTSEL) of Japan formed a consortium with Nissan diesel motors and Bosch Japan (1998–2001) to develop a heavy-duty DME fueled bus engine with mechanical fuel injection. DME-fueled vehicles have been taken up by the National Institute of Advanced Industrial Science and Technology (AIST) in collaboration with motor and oil-supply companies since 2003. In 2005, a medium-duty DME fueled truck with 7.1 liter engine was developed by AIST, Japan Oil and Gas & Metals Corporation (JOGMEC). An in-line 6-cylinder truck equipped with a diesel engine (turbocharged, intercooled, EGR, fitted with oxidation catalyst and NOx storage catalyst) has been converted to DME by Nissan diesel motors along with NTSEL. Isuzu Motors Japan has developed light and medium-duty common rail FIE DME engines.
China	National Clean Vehicle Action Program was started in 2005 in which ten DME-fueled busses fitted with CI engines and mechanical fuel injection system were developed by a consortium consisting of Shanghai Motor Company, Shanghai Jiao Tong University (SJTU) and Shanghai Coking & Chemical Corporation. 2nd & 3rd generation DME vehicles fitted with common-rail fuel injection system and after-treatment devices have been developed to comply with Euro-5 emission norms. Chinese government has funded a “863 project” in which SJTU is researching new technologies for DME fueled vehicles. DME stations have been developed by ENN. In particular Shaghai city has been considering acceptance of DME as a standard fuel for trucks, taxis and busses to reduce PM2.5 pollution.
S. Korea	In 2000 Korea Institute of Energy Research (KIER) took up DME engine research project. Took up a project to convert a diesel bus fitted with a 8.071 liter displacement engine to DME in 2005. A DME engine with 1.582 liter displacement and common rail fuel injection system was developed for passenger car by Hanyang University (HYU). Korea Automotive Technology Institute (KATEC) took up a project to modify a sports utility vehicle (SUV) powered by a 1.991 liter displacement engine to use DME fuel in 2009.

Table 7.

Global development of DME engines [12, 18].

6. Life cycle analysis of energy, emissions and GHG for DME fuel

Life cycle studies are used to compare the effect of different fuels/energy sources/ transport technologies. Life-cycle analysis consists of Well-to-Pump (WTP) and Pump-to-Wheel (PTW) estimation of energy consumption and emission of pollutants and GHG impact of these pollutions. WTP path consists of a) recovery and transport of raw material, b) production of the fuel, c) transportation of fuel, and d) distribution. PTW is the vehicle operation part of the pathway. Figure 13 shows a schematic to illustrate the Well-to-Wheel cycle for different fuel/transport technology combinations.

Figure 13.

Well-to-wheel cycle for transportation fuels [17].

Many studies on life cycle energy consumption and emissions for DME fuel have been carried out globally. Lee et al. have investigated Well-to-Wheels emissions of greenhouse gases and air pollutants of di-methyl ether from natural gas and renewable feedstocks in comparison with petroleum gasoline and diesel in the United States and Europe. For this purpose they have used Greenhouse gases, Regulated Emissions and Energy use in Transportation (GREET) model developed by Argonne National Laboratory (ANL). They have used five pathways to calculate the WTW by use of DME as fuel, these are 1) fossil NG with large-scale DME plants, 2) methanol from fossil NG with large-scale plants for both methanol and DME (separately), 3) land-fill-gas (LFG) with small-scale DME plants, 4) manure-based biogas with small-scale DME plants, 5) methanol from black liquor gasification with small-scale DME plants. They have studied DME production and use in the US and Europe in two class of vehicles (light-duty (LDV) and heavy-duty vehicles (HDV). Their studies show that WTW consumption of fossil energy and emission of GHG emissions in production and use of DME fuel is very low as compared to diesel and gasoline vehicles. Five pathways used in the production of DME are shown in Figure 14.

Figure 14.

Five pathways for production of DME [20].

In this study a small DME production plant is assumed to have a capacity of 25 MTPD (metric ton per day) and a large-scale plant is one with a capacity of 3600 MTPD. Case NG uses the fossil NG to produce DME directly in a large scale DME production plant. NG is supplied to the DME production plant through a pipeline. In case MeOH fossil NG is first converted to methanol and thence to DME. In this case the methanol production plant is close to the source of NG and methanol is transported to the DME production units by rail. Biogas from two sources has been considered, i.e., a) landfill gas (LFG) and b) during production and treatment of manure (MANR). Both are taken as renewable alternatives. Biogas is made up of CH4 and CO2 and is generated by anaerobic digestion (AD) of organic wastes. In both cases DME plant are small scale plants co-located with the source of biogas. Before the biogas can be fed into the DME plant it has to be cleaned in separate reactors where impurities like Sulfur compounds etc. are removed and the biogas is upgraded to the required composition.

Figure 15(a) depicts the WTW energy consumption for DME production through different pathways. DME production seems to consume more energy per MJ of fuel produced as compared to gasoline and diesel in both US and EU. This is because the conversion efficiency of raw material to DME is significantly lesser than gasoline and diesel. Although, conversion of fossil NG to DME directly or through MeOH is lesser than gasoline and diesel, however, this may also be due to scale of operation and size of plants which are much bigger and established for gasoline and diesel. Also, production of DME from renewable sources will result in zero or negligible consumption of fossil fuel.

Figure 15.

WTW energy consumption and emissions for DME produced through different pathways. (a) Comparison of WTW energy consumption for DME production vis-a-vis gasoline and diesel production in US and EU [20]. (b) WTW GHG emissions from DME production as compared to diesel and gasoline for US and EU [20].

Figure 15(b) shows the GHG emissions from DME production gasoline and diesel consumption and MeOH production. The emissions consist of the following components, a) For preparation and transportation of feedstock, b) Production of fuel and its transport, c) Avoided combustion and non-combustion emissions, d) biogenic CO2 in fuel and e) Fuel combustion. In the case of DME produced from bio-gas, avoided combustion and non-combustion are a major portion of the emissions inventory and in reducing the WTW emissions to very low/negative values. Thus, WTG GHG for LFG and manure based bio-gas are 6 and − 1 gCO2 e/MJ respectively and are 93% and 101% lower than US diesel. In the EU LFG and manure based bio-gas to DME process shows 6 and 12 gCO2 e/MJ of GHG emissions respectively which are 92% and 87% lower than EU diesel. If, however, regional electricity is used for production of DME then the WTW GHG emissions in the US will increase to 25 and 1 gCO2 e/MJ for LFG and manure bio-gas respectively. In the EU, the corresponding figures for DME production are 19 and 13 gCO2e/MJ from LFG and manure biogas respectively. Thus, it can be seen that the energy mix of the process has a strong impact on the WTW emissions as well.

In Figure 16, WTW GHG emissions vs. WTW vehicle energy consumption are plotted for some alternative fuels and petroleum-based gasoline and diesel. Figure 17 plots the results for synthetic diesel from farmed wood, synthetic diesel from waste wood and black liquor, ethanol from sugarcane (Brazil) and DME from waste wood and black liquor. WTW GHG emissions of DME produced from waste wood and black liquor are the lowest of all the fuels studied. WTW energy consumption (MJ/100 Km) for DME is second lowest after petroleum fuels. This may be due to the established production process of petroleum fuels and well-optimized engine and vehicle technologies for petroleum-based fuels. It is possible the life cycle energy consumption of DME fuel will reduce as the production technology for DME is matured and CIDI engines are designed specifically for DME fuel.

Figure 16.

Well-to-wheel GHG emissions & vehicle energy consumption for some alternate fuels [18].

7. Strategy for India

7.1 Indigenous production

High Ash Coal to DME - Coal reserves in India (350 billion tons) [22] predominantly consist of high ash content (20–30% and sometimes >40%) and therefore not considered techno-economically suitable for power generation. This high ash content coal can however be converted to syngas through suitable process and catalyst and the syngas can be converted to DME through a two-step or single-step synthesis. NITI Ayog of Government of India has taken up a Methanol Mission for conversion if high-ash coal of India into methanol/DME. Under this mission Bharat Heavy Electricals Limited (BHEL), a public sector unit of government has developed an indigenous process to convert Indian high-ash coal to methanol and has set-up a pilot coal-to-methanol plant, 0.25 TPD methanol with feedstock of 1.2 TPD coal using fluidized bed gasifier. Methanol with a purity of 98–99.5% has been produced in the pilot plant. As the process matures and costs come down higher commercial scale plant of coal-to-methanol/DME can be set-up with clean coal technology to produce DME for transportation and for blending with LPG. Clean-coal technologies like carbon capture and storage and use can be used during gasification of coal to syngas. Moving bed gasifiers, fluidized bed gasifiers and entrained flow gasifiers are some of the reactors to produce syngas from coal (Figure 18).

Figure 17.

Different type of coal gasifiers [21]. (a) Moving Bed Gasifier concept. (b) Fluidized Bed Gasifier concept. (c) Entrained Flow Gasifier concept.

Biomass and municipal solid waste to DME - India generates 62 million tons of municipal solid waste per year [24]. India also generates about 230 million metric tons of surplus biomass (28 GW) and about 115 million metric tons bagasse (14 GW) annually which includes agricultural residues. Organic content of the MSW (50%) [25] and the biomass/bagasse is suitable for gasification to syngas. National Chemical Laboratory has developed indigenous catalysts for conversion of methanol to DME and are in the process of developing catalysts for converting syngas to methanol/DME. This route will have the advantage of very low WTW life cycle energy and emissions footprint.

7.2 Conversion of diesel engines to DME

Annual consumption of diesel fuel in India is more than 100 billion liters [23]. Relative consumption of diesel fuel by different type of vehicles in the transport sector in India is shown in Figure 17. In the transport sector, Trucks (HCV/LCV) are the major consumers of diesel fuel (40%) followed by private cars/ SUVs (19%) and Busses/State Transport Undertakings (14%).

Figure 18.

Transport sector, use of diesel sector (India) [23].

Thus, a concentrated effort to convert diesel trucks and busses to use DME as fuel will have a major effect on reduction of GHG in India. Conversion of private diesel vehicles will be difficult as these vehicles are powered by different types and make of diesel engines. Figure 19 displays the details of the Industrial sector regarding consumption of diesel fuel. In this sector, equipment such as construction, boring, drilling, earth-moving etc. are the largest users of diesel fuel (38%) followed by Industry (29%) and Gensets (24%). It may not be possible to take up conversion of the construction and earth moving equipment diesel engines to operate on DME due to the nature of the work involved or diesel use in furnaces etc., however conversion of gensets to DME can be taken up and will have a positive effect on the GHG emissions and the reduction of use of fossil diesel.

Figure 19.

Use of diesel fuel by industrial sector in India [23].

Conversion of diesel engines in the agricultural sector (13% consumption of diesel fuel) to DME will also be beneficial for India in terms of reduction of GHG and diesel fuel. Agriculture equipment like tractors, diesel pumps, agricultural implements, tillers, harvesters, thrashers etc. operate in limited areas and the provision of DME dispensation facilities to cater to the demands of a particular area may be economically feasible.

8. Conclusion

Decarbonizing of the transport and other engineering sectors has become an urgent necessity to stop further global warming. Electrification of power trains by using batteries, fuel cells, electric transmission etc. promise to drastically reduce and even reverse the global warming trend. Battery and fuel cell technologies are however still in the evolution phase and for them to match the power density, energy density, low-cost and the reliability of liquid fuels-based IC engines is expected to take at least another two to three decades. Meanwhile the IC engines technology has reached a high level of maturity, sophistication, knowledge and economy of scale. IC engines-based power trains will continue to dominate the transportation, industrial, power generation and other sectors for next three decades. Therefore, it is imperative that solutions are found to make IC engines-based power trains clean and energy efficient. Compression Ignition (CI) engines with diesel fuel are used in many heavy-duty applications like long-distance trucks, busses, agricultural equipment, earth moving and construction equipment. Conversion of these diesel engines to use an alternative clean fuel like DME will aid in protecting the environment and conservation of energy. Primarily this will involve development of a pressurized fuel injection system and matching the fuel injection parameters to the physical and chemical properties of DME. Sufficient developments have been done in the past in development of fuel injection systems for DME fueled CI engines, therefore with the collaboration of the engine OEMs and the fuel equipment manufacturers diesel engines conversion to DME can be taken up in an economical way and in a time-bound manner. Impact of such conversions will be seen in the form of reduction of WTW energy consumption and harmful emissions.