Dimethyl ether is a simple ether formula CH3OCH3 which has properties similar to propane, butane and LPG (Table 5.6). Dimethyl ether is volatile, non-toxic, non­mutagenic, non-carcinogenic, has a sweet ether odour and has been regarded as being

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environmentally benign (Semelsberger et al., 2006). As DME is non-toxic and non­corrosive it is used mainly as a hairspray propellant, in cosmetics and in agricultural chemicals. DME has a high cetane value, no sulfur, little particulate matter (PM) emissions and can be competitive with LPG. Owing to these characteristics DME has been considered as a fuel for diesel engines, gas turbines and fuel cells. As its proper­ties are similar to those of propane and butane (Table 5.6), DME could be used to replace or supplement LPG for distributed power generation including gas turbines (Cocco et al., 2006). DME can also be reformed to produce hydrogen for fuel cells.

Dimethyl ether is produced in a two-step process where methanol is produced from syngas which is normally produced by the steam reformation of methane (natu­ral gas). The methanol is then dehydrated to form dimethyl ether. Syngas can be produced from waste and biomass so that DME could be produced from sustainable sources. To produce methanol the syngas needs a 1:1 ratio of carbon monoxide to hydrogen which can be adjusted by the water-shift reaction:

CO + 2H2 « CH3OH (5.12)

2CH3OH « CH3OCH3 + H2O (5.13)

DME has been shown to produce low noise, smoke-free combustion, and reduced NOx when used in an internal combustion engine (Huang et al., 2006). Because of its high cetane number and low boiling point, DME has been used at 100% or as an oxygenated addition to diesel. However, DME requires special fuel handling and storage as its properties are similar to LPG, and the lower energy means that a larger fuel tank will be required. The engine does not require modification but as the viscosity of DME is so low it can cause leakage in the pumps and injectors. Another consequence of the low fuel viscosity is a reduction in lubrication where lubricants need to be added to the fuel if used for long periods. The conclusions on DME were that it gave lower NOx and SOx, is soot-less and produces the highest well-to-wheel value compared with FT-biodiesel, biodiesel, methanol and ethanol (Semelsberger et al., 2006).

DME has also been shown to be suitable for gas turbines where performance and carbon dioxide emissions were improved when it was used in a chemically recuperated gas turbine (CRGT) (Cocco et al., 2006). One of the options to increase the efficiency of gas turbines is to recover the exhaust heat chemically. Most CRGT systems use methane

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steam reforming where carbon dioxide and hydrogen are formed and used as a fuel in the turbine. However, a high reforming temperature up to 600-800°C is required, which is higher than the exhaust temperature of commercial gas turbines. On the other hand methanol, DME and ethanol have lower reforming temperatures of 250-300°C, 300- 350°C and 400-500°C respectively which makes them more suitable. The overall reform­ing process is described below and a CRGT system is shown in Fig. 5.16:

The reformation is carried out at 290°C using Cu/SiO2 and HPA/Al2O3 catalysts. The CRGT system achieves an efficiency of 54%, 44% higher than a standard plant.

DME can also be used as a fuel for fuel cells as it can be reformed to produce hydrogen. The reformation is in two steps where the first step is an acid catalysis which converts dimethyl ether into methanol and the second step is the reformation of methanol over a Cu or Cu/Zn catalyst:

CH3OCH3 + H2O « 2CH3OH (5.17)

2CH3OH + 2H2O « 6H2 + 2CO2 (5.18)

Other similar compounds have been tested in diesel engines which include dimethyl carbonate and dimethoxy methane (Huang et al., 2006).

Fig. 5.16. A chemically recuperated gas turbine (CRGT) system using reforming dimethyl ether (DME) to utilize the exhaust heat. (From Cocco et al., 2006.)

Conclusions

Gaseous biofuels are a mixture of existing technologies and the promise of technology in the future. Methane is produced in landfill and anaerobic digesters and used for heating and electricity generation. Methane as a transport fuel has been proposed and the technology for the compression and liquefaction of the gas exists but may not be implemented. Methane is similar to LPG. LPG has been around for some years but the take-up of cars using this fuel has been very slow, probably due to the required expensive modifications to the car and the uneven distribution of LPG filling stations.

Much has been written about hydrogen and the ‘hydrogen economy’ but consider­able advances in technology will be required to make hydrogen a transport fuel that is sustainable. At present all hydrogen is made from natural gas and to be sustainable it needs to be produced from renewable resources such as biomass. The problem with hydrogen as a fuel is it needs to be stored so that sufficient fuel can be carried in a vehicle. At present, hydrogen can either be compressed gas or liquefied but both require considerable amounts of energy and special tanks. The production and distri­bution of hydrogen will also be required if it is to be used as a transport fuel. This will need the installation of a completely new infrastructure at a considerable cost. Despite the problems there is probably a future for hydrogen as a fuel for fuel-cell-powered vehicles; the question is whether hydrogen will be produced on-board or stored as hydrogen.