Natural Gas (NG)
Natural gas vehicles (NGVs) are methane-powered vehicles. The methane is derived from either fossil sources or biomethane (a raw biogas upgraded for vehicle use). According to the European Natural Gas Vehicle Association (ENGVA, 2007), in 2007 there were 6.7 million NGVs worldwide, including 820,400 in Europe (10.5 per cent of which were trucks). NGVs produce slightly lower emissions of CO2 than traditional petrol-engined cars and about the same levels of CO2 as the equivalent diesel-powered trucks. The benefit of NG is that it produces no nitrous oxides or particulate matter. Some manufacturers produce heavy goods vehicles that run purely on NG. Vehicles can be refuelled overnight at depots, which have purchased the required compressors. Dual-fuel NG/diesel or NG/petrol vehicles are also available.
Natural gas is still, however, fossil fuel-dependent, so does not represent a clean fuel. Methane is classed as a greenhouse gas (although the ENGVA argue that it should not be) and has a global warming potential that is 21 times that of CO2 (for an explanation of this, see Chapter 2). However, NG production from biomass actually reduces emissions of methane as it harnesses the methane normally emitted in the waste disposal process and transforms it into energy. Anaerobic digestion plants extract and process the methane from municipal waste and sewage. At present, although Sweden has plans for 80,000 biogas-powered vehicles by 2010, the UK is still in its infancy stages and has no biogas refuelling stations (Anon, 2008).
Liquid Petroleum Gas (LPG) and Compressed Natural Gas (CNG)
LPG comes mostly in the form of propane and butane. LPG is a heavy gas derived from the process of petroleum refining and natural gas extraction and is stored in liquid form. In the EU, 66 per cent of LPG comes from gas field extraction and 34 per cent from crude oil refining (AEGPL, 2007). CNG is derived from similar sources but remains a gas when compressed. LPG is now a reasonably common fuel for cars and buses. As an example, 99.8 per cent of Hong Kong's taxis and most new minibuses run on LPG. However, it does require some vehicle modifications, and although there has been an international push towards providing LPG refuelling stations, it is not yet as widely available as conventional fuels, and indeed is not yet available at all in some countries.
Environmentally, the benefits are sizeable reductions in nitrous oxides and particulate matter at the tailpipe. However, both LPG and CNG still originate from fossil fuels, so the well-to-wheel emissions of greenhouse gases are still high. Eyre, Fergusson and Mills (2002: 53) state that LPG does not offer lifecycle carbon benefits compared with diesel and conclude that 'LPG will have a rather marginal impact on either total road fuel energy demand or CO2 emissions and does not provide a pathway to non-fossil-fuel transport.'