The WTW values for gaseous fuels have been compared with CNG and petrol in Fig. 8.39. CNG requires more energy than petrol and diesel but produces about the same quantity of GHGs. When compressed biogas is used, the amount of energy used is greater than the fossil fuels, but biogas produced from liquid, solid manure and municipal solid waste saved considerable amounts of GHGs. Clearly improvements in energy use for biogas are required to make the process sustainable.

Fig. 8.38. The well-to-wheel (WTW) energy and emissions for the production of syn-diesel using the Fischer-Tropsch process with different starting materials. NG, natural gas. (From JRC, 2007.)

Fig. 8.40. The well-to-wheel (WTW) energy and emissions for gaseous fuels DME and hydrogen. DME produced from wood, coal and natural gas. Hydrogen compressed (C-H2) and liquid (L-H2) in a port injection internal combustion engine and compressed in a fuel cell (C-H2 FC). NG, natural gas; PISI, port injection spark ignition. (From JRC, 2007.)

DME requires more energy for its production than petrol and CNG due to gasifi­cation and gas cleaning steps, but if wood is used for its production the emissions are reduced to a very low level (Fig. 8.40). In contrast, DME produced from coal and natural gas has high energy and GHG values. Hydrogen is normally produced from natural gas when used in an internal combustion engine and requires more energy than petrol and produces more emissions, but these are considerably reduced if the hydro­gen is used in a fuel cell. Hydrogen stored as a liquid had higher energy and GHG values than hydrogen stored as a compressed gas. The hydrogen values are similar to CNG. The hydrogen figures are very dependent on the method used to generate

hydrogen and would be greatly improved if hydrogen was produced either biologically or from sustainable electricity. This can be seen in Fig. 8.41 where the methods of producing hydrogen have been compared. Electrolysis requires more energy than hydrogen production from wood, coal and natural gas, but electricity generated from wind, nuclear and wood reduces the GHG emissions considerably. The use of nuclear­generated electricity does, however, require the most energy. Hydrogen generated from wood also has a greatly reduced emission level. Natural gas reforming to produce hydrogen tends to be more efficient than gasification in terms of energy.

All the data on the WTW have been combined in a scatter figure plotting GHG against energy use (Fig. 8.42). The figure shows considerable variation in the WTW values for the biofuels. The low values for ethanol are from sugarcane production and the low GHG values for hydrogen, biogas and DME are due to using sustainable feedstocks such as wood. The higher GHG emissions are due to the use of fossil fuels such as coal. The ideal fuel would have low values for both GHGs and energy and this can perhaps be achieved by process and feedstock changes. The WTW study can be used to indicate which fuel and feedstock needs improving. Those fuels showing low GHG and energy are ethanol from sugarcane, lignocellulose and biodiesel from plant oils. All those with low GHG emissions have been produced from sustainable materials such as lignocellulose, wastes and plants.

Infrastructure

Biodiesel is a liquid fuel with a density close to that of diesel, non-toxic, biodegrad­able, with a high flash point which means that it can be used by the diesel supply infrastructure without any significant changes. In fact it is somewhat safer than diesel. Bioethanol has many of the characteristics of petrol except that it is hydro­scopic. The ability to accumulate water means that bioethanol cannot be transferred by pipeline because the water content causes corrosion (rust). This means that it has to be transferred by tanker and this is proving to be a problem in the USA where the transport provisions have not kept pace with ethanol production.

The gaseous fuels methane and hydrogen pose different problems from the liquid fuels. Methane is essentially the same as natural gas and could be transported through the natural gas infrastructure. Hydrogen once compressed can be run through pipe­lines with a loss of 0.77% per 100 km but with a low density a leak will disperse rapidly (Hammerschlag and Mazza, 2005).

If hydrogen is produced in situ by the electrolysis of water the efficiency is 74% and compression is 88% efficient, which means that only 65% of the energy will be delivered as hydrogen. Hammerschlag and Mazza (2005) quote: ‘In virtually any conceivable

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arrangement for supplying renewable or carbon-neutral energy to electric customers, delivering the electricity directly is more efficient than manufacturing hydrogen.’ Table 8.8 gives the fuel stations capable of supplying a range of alternative transport fuels including biodiesel and bioethanol for the years 2005 and 2007. Worldwide there are 140 stations supplying hydrogen but none for private cars in the UK.

Conclusions

Biofuels should not be considered in isolation as alternatives to fossil fuels but as a part of a drive towards the production of sustainable products normally produced from crude oil. Biorefineries which can produce a range of products including biofu­els from renewable resources should be developed.

Liquid fuels of all types will continue to be used for some time because of the dif­ficulty of supplying alternative biofuels, and the existing extensive infrastructure will continue to be used. The use of gaseous fuels such as hydrogen, CNG and LNG require extensive and costly modification to vehicles and in the case of hydrogen a completely new infrastructure. Thus, these are long-term solutions. Fuel cells may replace the internal combustion engine but these are still under development and will require modification according to the supply infrastructure, depending on the fuel used in the fuel cells. Bioethanol and biodiesel are fuels that can be used now in present vehicles and infrastructure. The main restrictions on these fuels are insufficient supply and their cost, which could be improved by the tax structure. FT diesel, FT petrol and DME are in a developmental stage, as technical advances are required to make their production economic. In the longer term, hybrid or electric cars may be the best option for short-distance travel and city use provided the cars are charged using renewable­generated electricity. Diesel and biodiesel will probably be retained for heavy trans­port; trains and cars will also use more diesel as this has better fuel consumption. It is air transport that has not been addressed, probably because fuel is cheap. Biodiesel can replace kerosene and successful tests have been carried out with turboprop engines.