In terms of pyrolysis being used to generate a suitable alternative to petroleum products, pyrolysis is seen as one of a family of more environmentally sound products compared to fossil fuel use. The main alternatives to fossil fuels are bioethanol, bio-diesel and bio-(pyrolysis oil). As briefly outlined in the introduction there are advantages and disadvantages of all of these. In economic terms, it is clear that bioethanol currently has a clear advantage over any second generation biofuels with bioethanol from Brazilian sugar cane being within a price range of 0.20-0.30 €/l76 compared to ethanol from lignocellulose or other biofuels which are around 0.80-1.00 €/l.76 Because of the cost of first generation bio-diesels (i. e. from vegetable or other plant oils) where production costs are 0.35-0.65 €/l, bioethanol production has greatly outstripped biodiesel production.77 The rate of growth of both of these is expected to increase as petroleum prices continue to increase but biodiesel has and will continue to rely on legislation and governmental

support.78

One advantage that second generation fuels that are recovered from low cost sources such as wood or waste materials have is that raw material costs are significantly lower than vegetable oils or animal fats.78 The cost of, e. g., rice husks is around 15-20 €/ton.79 The drawback in the use of pyrolysis to obtain usable oil products from cheap feedstocks is the high capital, maintenance and labour costs associated with the technology.79 However, as shown by Islam and Ani, provided large scale plants can be built, pyrolysis can be economic for as-produced pyrolysis oil and catalytically upgraded pyrolysis oil.79 Another advantage of pyrolysis lies in the use of materials that can be grown on relatively poor land. The IEA report data that suggest that by 2030, biofuels will contribute around 7% of transport fuel usage.80 This target can be achieved through conventional ethanol production but will significantly affect land usage with loss of pasture land to, e. g., sugar cane crops.81 This fuel driven use of land must be balanced by growing fears of food security82 and the availability of low value materials grown in non-arable areas can alleviate some of the fears associated with growth of fermentable crops. Pyrolysis also offers a considerable technical advantage as large-scale production of ethanol from lignocellulose is not generally thought possible within the next few years.83 There are now a number of demonstration plants built in the EU and USA, one of the largest being at Bastardo (Umbria) Italy which has a maximum throughput of 650 kg/h and is funded by the EU for research purposes.

There are other technologies for producing energy and bio-oils from waste materials and biomass. These can be grouped into prospective and established technology areas. Amongst the prospective methodologies are microalgal, supercritical fluid techniques and liquefaction. Microalgal production of bio-oils has been known since the early 1950s84 and is still an active area of research.85 Here, algae (microrganisms that convert sunlight, water and carbon dioxide to lipids or triaceylglycerol) can be used to effectively trap CO2 from emissions or organic degradation and can be harvested to yield up to 80% of their weight as oil. However, scale-up remains a problem and prices are not currently competitive with ethanol or plant oils. Supercritical fluids are being explored as potential technologies. They can be used to affect hydrolysis of biomass to a mixture of methane, carbon dioxide, carbon monoxide and hydrogen.86 Supercritical-CO2 is becoming an important industrial solvent (e. g. dry-cleaning, coffee extraction) because its ‘solvency’ can be controlled precisely through pressure. It has been used to extract useful products from biomass directly.87 Liquefaction of biomass is another area of research that has shown some promise but the products are normally quite high in viscosity and usually needs reducing gases such as CO and H2 to be present and this can further increase costs.88

Conventional technologies include (as mentioned above) various established catalyst methods. Combustion remains a proven technique and the combination of catalytic combustion with biomass gasification may afford opportunities to develop both sustainable and environmental friendly energy production.89 The use of fluidised bed gassifiers has been shown to be commercially viable for biomass use as demonstrated by Hamelinck and Faaij.90 The product of the gasification is syn gas (together with char and some methane) which is then used to generate methanol which is a useful fuel with a higher octane rating than petrol. For economy, char and hydrocarbons must also be used and a boiler to capture heat in the combined process alleviates some of the cost burden.90 Leduc et al. have shown that choice of site is all important in operating such plants economically.91 Syn gas can also be used to create petroleum-like fuels via the Fischer-Tropsch process.92,93 The product of the reaction is a distribution of largely aliphatic hydrocarbons. The Fischer-Tropsch process is an engineering challenge taking place at temperatures up to 300°C in pressures of up to 40 atmospheres. The catalysts, either cobalt or iron based, can have limited life and strongly effect the product distribution. Many of the challenges associated with catalysts for use in the high temperature, high hydrocarbon and high pressure environments needed for synthesis of oils via the Fischer-Tropsch process are the same when developing catalysts for catalytic pyrolysis.