Glycerol is a by-product of the production of biodiesel by transesterification, and constitutes 10% of the quantities of the oil used. Rather than discard the glycerol formed, some market needs to be found, as any value obtained from glycerol will go some way to reduce the cost of biodiesel. Initially glycerol was sold to the chemical and soap industries and was used to reduce the overall cost of biodiesel. The effect of glycerol prices on the cost of biodiesel is shown in Fig. 7.20. The rapid growth in biodiesel production has produced a glycerol surplus which has resulted in a drastic decrease in glycerol prices and the closing of glycerol-producing facilities by compa- nies such as Dow Chemicals and Procter and Gamble (Yazdani and Gonzalez, 2007).
The price has dropped in the USA from US$0.25/lb in 2004 to US$0.025/lb in 2006.
Therefore, alternative uses for glycerol need to be considered.
There are a number of potential uses for the excess glycerol and these are shown in Fig. 7.21. The simplest option is combustion as glycerol has a calorific value (16 MJ/kg). Glycerol can also act as a substrate for anaerobic digestion where it has been shown to stimulate biogas production. It can also be metabolized by some microalgae which can be used as a source of oil or used in anaerobic digestion.
Several microbial species such as Klebsiella pneumoniae and Citrobacter freundii can ferment glycerol to produce 1,3-propanediol (Mu et al., 2006; Yazdani and Gonzalez, 2007). 1,3-propanediol is used to manufacture polymers (polyesters), cosmet- ics, foods and lubricants. Another option is to convert the glycerol by ether ification (Fig.
7.21) (Karinen and Krause, 2006) which is butoxy-1,2-propanediol which is another fuel. Etherification with isobutene in the presence of an acidic ion exchange resin pro- duces butoxy-1,2-propanediol which has a high octane number (122–128). Thus, this can be an alternative to methyl-tert-butyl ether (MTBE) which is used as an oxygenate.
Glycerol has been shown to act as a substrate for citric acid synthesis with Yarrowia lipolytica (Papanikolaou et al., 2008). Glycerol has been blended with petrol using a third liquid, ethanol or propanol, to make the two miscible (Fernando et al., 2007).
Finally, glycerol can be used as a substrate for the microbial production of plastics poly(3-hydroxybutyrate) PHB and poly(hydroxyalkanoates) PHA (Ashby et al., 2004).
0.505 0.51 0.515 0.52 0.525 0.53 0.535 0.54
0.22 0.33 0.44
Glycerol ($/kg)
Biodieselcosts($/l)
Fig. 7.20. The effect of glycerol prices on the cost of biodiesel production. (Redrawn from Hass, 2005.)
Conclusions
Biodiesel produced from plant oils, animal fats and waste cooking oils has been shown to be suitable for use in diesel engines and is currently being added to diesel as a 5% addition. The amount of biodiesel produced worldwide and in the EU is increasing but there are insufficient plant oils available to increase this addition to much more than 10%. If there is any more than a 10% addition, oil crops will begin to compromise food crops (Chapter 8, this volume). Therefore, the other sources of diesel replacements will have to be commercialized such as FT diesel, pyrolysis bio-oil and microalgae.
FT diesel is being produced from the fossil fuels coal and natural gas in two large plants. The plants have to be large to be economic, and even at the large scale, the fuel produced by the FT process is 2–4 times the cost of diesel. To be sustainable, the FT process needs to be run using biomass or waste materials and not fossil fuels. The large scale of the operation would introduce problems of transporting large quantities of biomass to these plants which would burn fuel. What is needed is an improvement in the costing of the FT process which is probable in gas cleaning and in the development of smaller economic units which could treat biomass locally rather than transport it many miles. Pyrolysis is somewhat simpler than the FT process but it would compete for biomass, and bio-oil needs processing before it can be used.
The last option is oil extracted from microalgae, which is at the development stage and so it is difficult to give costing accurately. However, microalgae as a source of biodiesel have a number of advantages over other plant-derived oils. Microalgae can be grown on non-agricultural land, can be grown in sea water, are more produc- tive than land plants and can be part of a CO2 sequestration system. Thus, they would appear to hold great promise for the future and there is considerable interest worldwide in microalgae.
Glycerol Blending
with petrol
Combustion
Anaerobic digeston
Butoxy-1,2-
propanediol 1,3-propanediol Citric acid Plastics PHB/PHA
Fig. 7.21. Possible uses for glycerol produced from the transesterification of oil.
8 The Benefits and Deficiencies of Biofuels
Introduction
Biofuels are energy sources derived from biological materials and are therefore renewable and sustainable, and can go some distance in replacing fossil fuels and reducing carbon dioxide emissions. Their biological nature separates them from other renewable energy sources such as wind, wave and solar power. Biofuels can be solid, liquid and gaseous and can be used to generate electricity and as transport fuels. No matter how biofuels are used, they have both benefits and shortcomings, and in this chapter these are explored.
The benefits of biofuels whether globally or to a single country are as follows:
1. Reduction in crude oil use. Liquid biofuels can supplement or replace petrol and diesel, and at low levels of blending, little engine modification is required. Biodiesel can be used up to 100% in a conventional diesel engine but higher blends of ethanol (85%) require either modifications or a flexible fuel engine. Biomass and biogas can reduce fossil fuel use for electricity generation.
2. Improvements in engine performance. Ethanol has a very high octane number and has been used to improve the octane levels of petrol. It is also a possible replacement for methyl tertiary butyl ether (MTBE) which is being phased out as an octane enhancer. Biodiesel addition will enhance diesel lubricity and raise the cetane number.
3. Air quality. Biofuels can improve air quality by reducing the emission of carbon monoxide (CO) from engines, sulfur dioxide and particulates (PM) when used pure or in blends.
4. Reduction in the emission of the greenhouse gases (GHGs) carbon dioxide and methane. The replacement of fossil fuels with biofuels can reduce significantly the production of carbon dioxide, and the use of biogas reduces methane emissions.
5. Toxicity. Biofuels are less toxic than conventional fuels, sulfur-free, and are easily biodegradable.
6. Production from waste. Some biofuels can also be made from wastes, for example, used cooking oil can be used to make biodiesel.
7. Agricultural benefits. Biofuel crops of all types will provide the rural economy with an alternative non-food crop and product market.
8. Reduction of fuel imports. By producing fuels in the country, imports will be reduced and the security of energy supply will be increased.
9. Infrastructure. No new infrastructure is required for the first- and second-generation liquid biofuels and some of the solid and gaseous biofuels.
10. Sustainability and renewability. Biofuels are sustainable and renewable, as they are produced from plants and animals.
However, there are shortcomings to the use of biological materials to replace fossil fuels which are as follows:
1. Biological material may not be able to produce enough fuel to replace fossil fuels completely, and extensive cultivation of biofuel crops will compete with food crops, perhaps driving up prices.
2. Large amounts of energy are required to produce some biofuels, giving them a low net energy gain.
3. Some of the second- and third-generation biofuels will require the introduction of a completely new infrastructure, for example, hydrogen.