The gaseous fuels DME and hydrogen can be produced using a number of routes including those from biological materials. The carbon dioxide produced per unit of energy (g CO2/MJ) is shown in Fig. 8.21 for hydrogen and DME. The production of DME from syngas, hydrogen from electrolysis, natural gas and coal is compared with petrol, CNG, LPG, coal and gas in terms of carbon dioxide produced per unit of energy. The amount of carbon dioxide produced by DME is similar to petrol and LPG. Hydrogen production by all three routes produces more carbon dioxide than petrol especially when coal is used. This indicates one of the problems of producing hydrogen from fossil fuels.

In a study by the Joint Research Centre EU, the cost of reducing carbon dioxide emissions for a number of biofuels was calculated and some of the data is shown in Fig. 8.22. The GHGs avoided in a life-cycle analysis, well-to-wheel (WTW), are related to the cost (€/t) of carbon dioxide equivalents avoided. A life-cycle analysis systemati­cally identifies and evaluates opportunities for minimizing the overall environmental consequences of using resources and releases into the environment. In terms of GHGs avoided, biodiesel is slightly less expensive than ethanol whether produced from either sugarbeet or wheat. The fuels DME, ethanol and FT diesel produced from wood bio­mass avoid the most GHGs because of using a sustainable source and are inexpensive at around €100-500/t CO2 avoided. Unfortunately the production of the biofuels from wood has yet to be commercialized. Electrolysis of water to produce hydrogen using sustainable electricity from nuclear and wind power avoids large quantities of GHGs.

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Fig. 8.20. (a) The energy input (MJ/t) for the processes used to produce biodiesel from rapeseed under normal and low nitrogen cultivation. (b) The carbon dioxide (kg/t) produced from the various stages of biodiesel production from rapeseed under normal and low nitrogen conditions. (c) Greenhouse gas (kg/t) for biodiesel production from rapeseed using a conventional and low nitrogen process. (Redrawn from Mortimer et al, 2003.)

CNG, hydrogen from natural gas and ethanol from wheat only avoid moderate amounts of GHGs. Hydrogen for use in fuel cells produced by on-board reforming of petrol is the most expensive option.