Liquid Biofuels to Replace Diesel


Both transport and industry rely heavily on the diesel engine that is widely used to power lorries, trains, tractors, ships, pumps and generators. The USA uses 50 billion gallons (1 gallon = 3.8 l) annually (Louwrier, 1998) and the consumption in the UK was 23.9 million t (106) in 2006 (IEA, 2008). The engine designed by Diesel ran for the first time on 10 August 1893, and the patent when filed proposed that the fuel could be powdered coal, groundnut oil, castor oil or a petroleum-based fuel (Shay, 1993; Machacon et al., 2001). At this time, the growing petrochemical industry pro­vided the best fuel, a crude oil fraction, now called diesel, which has been the fuel of choice for diesel engines ever since this time. Conventional diesel is produced by the distillation of crude oil and collecting middle distillate fractions in the range of 175-370°C. The fuel contains hydrocarbons such as paraffins, naphthenes, olefins and aromatics containing from 15 to 20 carbon molecules. To replace diesel without modifying the engine, any substitute will have to be similar to diesel in the following properties:

• A calorific value of 38-40 MJ/kg is a measure of the energy available in the fuel.

• A cetane number of around 50 is a measure of the ignition quality of the fuel.

• The viscosity of the fuel is important as it affects the flow of the fuel through pipelines and injector nozzles where a high viscosity can cause poor atomization in the engine cylinder.

• The flash point is a measure of the volatile content of the fuel and gives a measure of the safety of the fuel. The flash point for diesel is 64-80°C.

• It must be obtained from renewable resources such as biomass, oil crops and waste.

• It must be available in large quantities. For example the current use of diesel in the UK is 23,989,000 t where a 5% addition (on an energy basis) requires an addition of 5.75% by volume, which is equal to 1,199,450 t (1499 million l).

There are a number of possible sources of diesel replacements produced from agricul­tural products or microbial cultures, which are first-, second — and third-generation biofuels (Fig. 7.1). Some of the sources are as follows:

• Long-chain hydrocarbons (C 30) extracted from herbaceous plants, which can be cracked to form diesel, is a first-generation biofuel.

• Long-chain hydrocarbons (C 30) accumulated by some microalgae, which can also be cracked to form diesel, is a first-generation biofuel.

• Pyrolysis of biomass or waste to form bio-oil, which can be converted to diesel, is a second-generation biofuel.

• Gasification of biomass followed by Fischer-Tropsch synthesis of diesel (FT diesel) is a second-generation biofuel.

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Fig. 7.1. The routes to the production of alternative diesels capable of replacing fossil fuel diesel.

• Transesterification of plant, animal and waste oils and fats to methyl esters (biodiesel) is a first-generation biofuel.

• Oil accumulated by some microalgae, extracted and transesterified into biodiesel, is a third-generation biofuel.

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