Alternative Fuels: a Chemical Perspective

Alternative fuel technologies are being researched by many different companies, however many people do not understand what alternative fuels are available, and what their respective advantages and disadvantages are. This report gives information on two more common alternative fuels – hydrogen and biodiesel – from a chemistry perspective.

Alternative Transport Fuels

Introduction

The world is rapidly approaching a fuel crisis – crude oil reserves are running out, and hydrocarbons derived from crude oil are causing high levels of pollution. Alternative transport fuels need to be found to reduce pollution and save remaining crude oil reserves.

Two of the most promising fuels are hydrogen and biodiesel. Both are much less polluting than petrol, however there are other factors that need to be considered, including the energy used in production, their phase, energy density and safety.

Discussion

Hydrogen (H2) Hydrogen gas.png

Overview:

Hydrogen can be used to produce energy either by combustion ( H2 (g) + O2 (g) → H2O (g) ) or via a fuel cell, which does not involve combustion (Fuel Cell Energy 2013). There are vast stores of hydrogen on the earth, mostly in the form of water or hydrocarbons (U.S. Department of Energy (USDE) 2014).

Energy Density:

Hydrogen has a low molar heat of combustion (285 kJ/mol), due to its low molar mass. Its energy density is 141 MJ/kg (20 MJ/L) as a liquid, but only 0.013 MJ/L as a gas (Smith et al. 2012). As a liquid, hydrogen contains the most energy per kilogram out of all the fuels that have been tested. Unfortunately, this means that hydrogen needs to be in a liquid form (by compression and/or cooling) for it to be useful as a fuel. A better solution would be to store the hydrogen in a compound, and release it as needed; however this is not currently feasible (Smith et al. 2012).

Physical Data:

Hydrogen’s boiling point is -259°C, so it is a gas at standard temperature and pressure (Smith et al. 2012).

Energy Costs of Production:

It is very difficult to efficiently extract hydrogen from hydrogen containing compounds (USDE 2014). The 2 techniques currently used – steam reforming and electrolysis – both require a lot of energy. The huge amount of energy required is the main argument against using hydrogen as a fuel (Parliament of Australia (PA) 2010).

A considerable amount of energy is also required to cool, compress and store hydrogen as a liquid (Smith et al. 2012).

Portability, Safety and Storage:

Hydrogen’s low density means that it is hard to store enough hydrogen onboard vehicles to achieve the same driving range as normal cars (USDE 2014). Moreover, it takes a lot of power to keep it stored as a liquid.

Hydrogen is explosive; however when if a tank was to leak, the gas would rapidly disperse into the atmosphere, instead of remaining at the leak site like liquid hydrocarbon fuels. In many cases, hydrogen is safer to use than petrol (How Stuff Works 2015).

Sustainable vs. Non-sustainable:

Hydrogen is found virtually everywhere, either as water or hydrocarbons. Using renewable energy to extract hydrogen from these compounds would allow hydrogen to be used as a sustainable fuel; however using fossil fuel offsets any environmental benefits of using hydrogen in the place of petroleum.

Uses/Applications:

Hydrogen fuel cells can be used to create electricity in any application, including vehicles and large power plants. Cars powered by fuel cells are two to three times more efficient than conventional cars (USDE 2014).

Environmental Effects and Costs:

Hydrogen is the only fuel that does not emit pollutants in its combustion. The only product of its combustion is water, as is shown in its combustion equation: H2 (g) + O2 (g) → H2O (g) (Smith et al. 2012). However, engines that burn hydrogen do emit some pollutants, due to the burning of lubricants in the engine. Vehicles with fuel cells produce much less pollution (Smith et al. 2012). If unrenewable energy sources are used, then considerable pollution is also produced in creating the energy to extract the hydrogen.

Biodiesel biofuel-formula

Overview:

Biodiesel is a biofuel made from recycled fat and oils, or plant oils extracted from crops grown for this purpose. It can be used as a replacement for diesel (USDE 2014). The equation for the combustion of a typical biodiesel molecule is:  C19H36O2 (l) + 27 O2 (g) → 19 CO2 (g) + 18 H2O (g)   (Biofuels 2010).

Energy Density:

Biodiesel’s molar heat of combustion is approximately 11,000 kJ[1]. Its energy density is 37.8 MJ/kg (33.3 to 35.7 MJ/L) (Wikipedia 2015a). This is slightly less energy per kilogram than normal diesel.

Physical Data:

The boiling point of biodiesel is around 340°C (Science Direct 2002), so it is liquid at room temperature.

Energy Costs of Production:

Biodiesel can be made via transesterification, which uses methanol to convert oils into methyl esters (the biodiesel) and glycerol (Wikipedia 2015b). This process would use some energy.

Moreover, energy may be consumed in producing fertilisers, or tending to the crop.

Portability, Safety and Storage:

Overall, biodiesel has similar properties to diesel. However it is much safer to handle and store than diesel, as its flashpoint is considerably higher, i.e. it is less combustible (USDE 2014).

Sustainable vs. Non-sustainable:

As it is produced by plants, and thus obtains its energy from photosynthesis, biodiesel is fairly sustainable. Using fertilisers or heavy machinery to tend the crop reduces its sustainability. Biodiesel made from recycled oils is also relatively sustainable, as otherwise, those oils would have been thrown away.

Uses/Applications:

Biodiesel can be directly used in regular diesel engines; however it is usually used as a blend with regular diesel (USDE 2014). Even in small concentrations it increases fuel lubricity, and raises the cetane number (similar to the octane number) of fuel (USDE 2014).

At higher concentrations (over 20%) biodiesel has a solvency effect, which can clog filters and degrade seals (USDE 2014). However, the Biofuels Association of Australia (2015) claims that the solvency effect is beneficial, as it cleans out fuel lines. They claim only minimal changes to some rubber components are needed. The BAA further suggests that biodiesel works better than regular diesel with new engine technologies, so could extend the life of an engine.

Environmental Effects and Costs:

Pollutant emissions from diesel and biodiesel engines are comparable; however catalysts tend to work better with biodiesel than regular diesel (USDE 2014). Furthermore, carbon dioxide released from the combustion of biodiesel is offset by the carbon dioxide used by the plants grown for the feedstock (USDE 2014). According to the BAA (2015), one litre of biodiesel can reduce net CO2 emissions by 95%. Biodiesel does however produce more nitrous oxides when burned than regular diesel (USDE 2014).

Biodiesel is much safer for the environment than diesel if it is spilled or accidentally released (USDE 2014).

Conclusion

There are many, many alternative fuels that are currently being researched, and there are probably more that have not yet been discovered. However, all fuels have their disadvantages, as well as their advantages.

Due to Law of Conservation of Energy, it is impossible to create a perpetual energy cycle. This means that net energy needs to be put into a fuel cycle for it to continue. In the case of fossil fuels, this energy is provided by sunlight many years ago.

The best source of renewable energy input is sunlight – it is a continuous source of energy that is separate from the earth. Both plants and photovoltaic cells can be used to capture the energy in sunlight.

Below is a table summarising the main features of the 2 fuels considered.

Fuel Phase Energy Density (MJ/kg) Energy for Production Pollutants Emitted Safety
Hydrogen Gas 141 MJ/kg (liquid) Very high None Explosive
Biodiesel Liquid 37.8 MJ/kg From sunlight 95% less than petrol Flammable
Comparison
Petrol Liquid 47.9 MJ/kg Moderately low Relatively high Highly flammable
Diesel Liquid 41.0 MJ/kg Moderately low Relatively high Flammable

Hydrogen has the greatest energy density of any fuel – almost 3 times as much as petrol. Moreover, it is the only fuel that does not produce a polluting product when burned. In short, hydrogen has enormous potential as a fuel. However, the huge energy requirements for its extraction and compression to a liquid make it unviable at this point in time. Eventually, new research may discover a way to sustainably produce and distribute hydrogen, in which case hydrogen will become the fuel of choice.

Until then, biodiesel will probably be the next best fuel. Biodiesel obtains its energy from sunlight, produce up to 95% less emissions than diesel, and has an energy density similar to regular diesel.

Recommendations

In the short term, biodiesel appears to be the best alternative fuel. Although biodiesel yields much less energy per kilogram than hydrogen, it is much easier to produce, store and use. However, as technology develops, hydrogen holds the best prospects in terms of zero pollution and energy density.

 

Annotated Bibliography

Biofuels 2010, website, viewed 8 September 2015, http://biofuel.org.uk/. The biofuels website covers the details of many different types of biofuels, however it is not difficult to read.

Biofuels Association of Australia 2015, ‘Ethanol’, viewed 5 September 2015, http://biofuelsassociation.com.au/biofuels/biodiesel/biodiesel-and-my-vehicle/. The website of the BAA contains information on 2 major biofuels – ethanol and biodiesel. The information may be slightly biased, as the BAA has a vested interest in convincing people that biofuels are the best fuels for the future.

Fuel Cell Energy 2013, ‘How do Fuel Cells Work?’, viewed 5 September 2015, http://www.fuelcellenergy.com/why-fuelcell-energy/how-do-fuel-cells-work/. This website contains information on how fuel cells work.

How Stuff Works 2015, ‘Hydrogen Fuel Safety’, viewed 14 September 2015, http://auto.howstuffworks.com/fuel-efficiency/alternative-fuels/dangerous-hydrogen-fuel2.htm. Written for the general public, this webpage explains the dangers of using hydrogen fuel in cars.

PA – see Parliament of Australia 2010

Parliament of Australia 2010, ‘Alternative Fuels’, viewed 5 September 2015, http://www.aph.gov.au/About_Parliament/Parliamentary_Departments/Parliamentary_Library/Browse_by_Topic/ClimateChange/responses/mitigation/Emissions/Alternative. This page examines several options for fuels, in slightly more scientific language than other sources. The article should not show any bias, as it is supposed to aid in making a decision about alternative fuels that Australia could use in the future.

Science Direct 2002, ‘Volatility and boiling points of biodiesel from vegetable oils and tallow’, viewed 8 September 2015, http://www.sciencedirect.com/science/article/pii/S0961953401000745. This is an abstract of a scientific article. As such, it is in scientific language and should be very reliable.

Smith, D, Monteath, S, Gould, M & Smith, R 2012, Chemistry in Use Book 1, Cengage Learning, Melbourne. This source is a secondary textbook, so can be expected to be fairly reliable and unbiased.

USDE 2014 – see U.S. Department of Energy 2014

U.S. Department of Energy 2014, ‘Alternative Fuels Data Center’, viewed 5 September 2015, http://www.afdc.energy.gov/. The website of the USDE contains a large amount of information about alternative fuels and their use in vehicles. It should be fairly reliable and unbiased; however it is written from an American point of view.

Wikipedia 2015a, ‘Energy Content of Biofuels’, viewed 8 September 2015, https://en.wikipedia.org/wiki/Energy_content_of_biofuel. This article contains much information about biofuels and their energy content. However, the information may be unreliable and biased, as anyone can edit it.

Wikipedia 2015b, ‘Biodiesel’, viewed 8 September 2015, https://en.wikipedia.org/wiki/Biodiesel. As above, the reliability of the article is dubious; however there is a lot of information, which seems relatively reliable compared to other sources.

[1] Molar heat of combustion = energy density per kg / no. moles = (37,800 kJ/kg) / (1000g / 292g/mol) = 11,045 kJ/mol

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