Both diesel and petrol replacements can be produced from biomass and waste organic materials by gasification followed by FT synthesis. This process yields a mixture of

Fig. 8.26. The percentage of agricultural land required to produce 5, 20 and 100% of UK diesel using biodiesel, FT diesel and microalgae. The UK’s agricultural land is 18,016,981 ha (18 Mha) and the diesel required is 23,989,000 t. Yield of biodiesel is 1t/ha, FT diesel 1.35 t/ha and microalgal biodiesel 22.93 t/ha.

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Fig. 8.28. The percentage of agricultural land required to produce 5, 20 and 100% ethanol replacement for petrol from sugar and FT petrol. The petrol use in the UK is 26,916,216,000 l (19,918,000 t) and the agricultural land is 18,016,981 ha. Ethanol yields of sugarbeet are 2500-3000 l/ha; wheat 500-2750 l/ha; lignocellulose 1840-3580 l/ha. FT petrol and FT kerosene yields are 1.35 t/ha.

hydrocarbon which includes diesel and petrol (naphtha) fractions. It has been estimated that FT diesel production is about 45% efficient and therefore the 7,854,422 t of woody biomass available in the UK would yield 3,534,489 t FT diesel if all the products from the FT process were diesel. Other authors give different efficiencies for the pro­duction of liquid fuels from biomass. Van der Drift and Boerrigter (2006) quote that the biomass to syngas efficiency was 80% and syngas to liquid fuels 71%, giving an overall efficiency of 57%, and suggest a value of 63% for the production of liquid fuels if all the products are regarded as diesel.

However, the FT process produces a mixture of HC and the yield of diesel is generally around 30% of the total HC produced. This would represent 1,060,346 t of diesel from 7,854,422 t of biomass. The demand for diesel is 23,989,000 t and therefore FT diesel could provide 5.6% of this demand. Clearly any improvement in efficiency would improve the yield, but at present to increase the amount of FT diesel more biomass crops will need to be planted. In some cases with the FT processes, if the naphtha fraction is recycled it gives an overall yield of diesel of above 50%. The FT production of petrol has the same problems as diesel and petrol constitutes 30-40% of the total product.

The land required to produce FT diesel is shown in Fig. 8.26. The supply of FT diesel assumes a yield of biomass of 10 t/ha, 45% conversion, and 30% of the products are diesel giving a yield of 1.35 t/ha of FT diesel. A 5% addition would require

888,0 ha, 4.9% of the agricultural land, and if 100% replacement was attempted it would use 98% of the present agricultural land. Even if 100% of the FT products were diesel, this would still represent 23-29% agricultural land, a figure which is too high as it would affect food production. The land required providing similar quantities of FT petrol and FT kerosene is given in Fig. 8.28. A 5% addition of FT petrol would need 4.1% of the agricultural land and 100% would require 82% of the land. The amount of kerosene required is less than diesel and petrol and therefore less land is needed with 5% requiring 2.2% of the agricultural land and 100% requiring 44.3% of the land. If all the biomass available in the UK was converted to FT petrol, it would yield 3.5 million t if solely used for petrol production, which is 13.0% of the total use. Thus, FT synthesis of petrol and diesel uses non-food crops but it still requires large land areas to give 100% replacement, which may compromise food crops. Another feature is that the woody biomass available in the UK is widely distributed so that not all of it will be available and it may need transporting some distance.

In addition, another problem with the conversion of biomass to fuels is that to be efficient FT production plants have to be large and it is envisaged that the UK would only need three to four of these units. These large units would involve the transportation of large quantities of biomass and subsequent use of fuel. The develop­ment of small, efficient, regional FT plants would be a considerable advance in the provision of FT diesel and petrol.

An alternative to the FT synthesis has been developed where the synthesis gas is converted into ethanol by a bacterial culture (section ‘Commercial Lignocellulose Processes’, Chapter 6). A company, Bioengineering Resources Inc. (BRI), has devel­oped this system and one unit uses 100,000 t of waste, generates 5-6 MW of power and yields 6-8 million gallons of ethanol (22.68-30.24 million l).

If the UK woody biomass of 7,854,422 t was available, it would supply 78 BRI units yielding 1769-2358 million l representing between 6.6 and 8.7% of the ethanol required for 100% replacement of petrol.