Notes on Bioenergy, Biofuels
A review of issues with bioenergy and biofuels - v1.1, May 2016
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To meet our ever increasing energy demand the world has almost exclusively relied on fossil fuels in recent history. The industrial revolution in the 19th century was firmly grounded on the increasing exploitation of coal as a fuel for steel manufacture, new heavy machinery such as steam engines and railways. Biofuels have only briefly been in fashion in the early days of the internal combustion engine. Rudolf Diesel (http://en.wikipedia.org/wiki/Rudolf_Diesel), the inventor of the self-ignition combustion engine, believed in the use of peanut oil in 1911 to run his machines:
''The Diesel engine can be fed with vegetable oils and would help considerably in the development of agriculture of the countries which will use it. This may appear a futuristic dream but I can predict with great conviction that this use of the Diesel engine may in the future be of great importance.'' (Source: Hall (1981))
Even if the modern motor car of the future may be powered by electric motors, biofuels probably have to power airplanes, simply because of the superior energy density of liquid fuels. In the media and press the term biofuels and bioenergy are often used synonymously, but strictly speaking, the two terms are not identical:
The units may provide a way of distinguishing between fuel and energy. A good explanation of biofuels is at http://www.nrel.gov/learning/re_biofuels.html.
- Biofuels: Denote renewable, liquid, but also solid (e.g. wood) or gaseous fuels (e.g. biogas) made from recent, nonfossil biological plant matter (such as crops or organic residues), that can be used in the transport sector but also for domestic and industrial heating. Even though all modes of transport (air, rail, water and road) can run on biofuels, a primary target for biofuels is road transport with cars, buses, lorries and motor cycles. Currently, liquid biofuels are rarely used in concentrated form, but are mixed with fossil petrol or Diesel to obtain a blend. Biodiesel and bioethanol are widely known biofuels. Raw, concentrated vegetable oil (e.g. rape seed oil) is a pure biofuel, that can only be combusted in modified Diesel engines. Feedstock biofuels are oil and tree seeds, grain, beets and roots, leaves (plural of leaf), grasses, wood, but also waste materials such as manure, waste water, fermentation residues, slurries, abattoir and food waste. Units for biofuels are often mass in [kg] or volume in . The EU Directive 2009/28/EC (Renewable Energy Directive or RED for short) defines the term biofuels in its own way in Article 2i where it says:
''Article 2 (i): biofuels means liquid or gaseous fuel for transport produced from biomass''.
So the RED limits the term biofuel to transport fuel. Literature other than the RED do not adopt this narrow definition of biofuels.
- Bioenergy: A broader term that includes, but goes beyond the term biofuels to comprise more forms of renewable biomass energy, such as heat and electricity from biofuels. Bioenergy can be viewed as the useful product from biofuels. The units of energy are [J], [kJ] or [kWh].
Biomass is a term given to biological matter that either grew naturally without human interference, or in a cultured environment (e.g. monocultures), and made use of photosynthesis. This includes grown materials such as
(e.g. Hall and Moss (1983), page 5). Biomass has created interest because it represents a renewable source of energy.
- trees, shrubs, reeds, wood
- a multitude of crops and plants, containing oils and hydrocarbons
- aquatic plants such as algae, water plants and weeds
- organic wastes such as slurries, farm residues, food waste, forestry residues
Energy is measured in joules [J], an SI unit, and several forms of energy exist:
Power P on the other hand is energy per time and has units of Watts or Joules [J] per time in [s]:
- Chemical energy: stored in the bonds of chemical substances; when the bonds are broken the stored energy is unleashed and becomes kinetic energy
- Electrical energy: Movement of charged particles such as the flow of electrons along a wire.
- Mechanical energy: A physical object that is moving at speed possesses kinetic energy.
- (Electromagnetic) radiation energy: A radiator or an internal combustion engine emits thermal radiation; light or solar energy warms the earth and stimulates the retinas of your eyes; these phenomena are caused by electromagnetic radiation.
. Power is energy per time and the term energy flux indicates this time dependence better than then term power. Power stations are typically rated in megawatts [MW] of electrical power (output).
Plants and trees appear green due to the selective absorbtion of certain wavelengths by chlorophyll. Wavelengths that are reflected back from the leaf are seen by the human eye. In autumn chlorophyll gradually stops absorbing solar energy which makes leafs change from green to yellow. A similar process is the ripening of green bananas - green chlorophyll is broken down into sugars. The banana turns yellow.
The three main solar energy conversion pathways have different energy conversion efficiencies.
All biological matter contains water. Biomass can be analysed or split into the following compositional fractions:
Just like with the digestion of food in the gut, not all theoretical energy in the food (feedstock) can be extracted by the body's metabolism. A certain percentage of the energy content is not digested and remains in the residue (human faecies). With human food the metabolisable energy in the food is somewhat smaller than the energy per mass value [MJ/kg] measured in a bomb calorimeter. Similarly, not all biofuel conversion technologies are equally suited to convert the individual fractions of biomass. For example, woody biomass is not suited for anaerobic digestion or for bioethanol (yet), but is ideal for thermal conversion. Figure 3 illustrates mass and compositional concepts that are relevant in the conversion to
- Fibre or lignocellulose: Biopolymers that give the plant stiffness; fibre consists of the three subgroups
The fibre fraction of biomass is recalcitrant - it does not break down in anaerobic digestion, but can of course be combusted or pyrolysed. In X-ray diffraction analysis, cellulose occurs on both crystalline and amorphous form (e.g. Bos and Donald (1999))
- Cellulose: Polymer chains consisting of long strands of 1-4 glucose units (several thousand glucose units); also called fibrills;
- Hemicellulose: Polymers of hexose and pentose (sugars)
- Lignin: Complex polymer, consisting of the three types of lignols: p-couramyl (H), guaiacyl (G), and syringyl (S); different plant species have various types of lignin;
Composition of biological matter. Main component is water, which can be dried of. What is left after drying is called TS (total solids).
A simple but useful measure for biomass material is total solids TS and volatile solids VS.
TS is usually, but not always, expressed as a percentage of the original or natural biomass sample that typically includes a substantial amount of water. With unaltered biomass more than half of the volume can be water as shown in Figure 4. For example TS for a biomass material could be expressed as '27 % of total weight'. This means, that after drying a biomass sample with originally 1 gram of mass in an oven at 105C over night, 0.27 grams of solid material is left. The terms organic dry matter and TS are often used interchangeably, but some textbooks do differentiate between the two terms. Water concentration for wood is 8 to 40 % by mass. The emphasis with ODM (organic dry matter) is on 'organic' carbon, excluding inorganic carbon such as calcium carbonate .
There were a number of reasons why energy from biomas was and still is considered attractive:
- Renewable: Compared to fossil sources biofuels are renewable. They can not be depleted.
- Net carbon: When biofuels are combusted no 'new' carbon dioxide is emitted into the atmosphere. All or most of the carbon dioxide is short term cyclic.
- Diversification: Not relying on a single, fossil fuel source is a good thing. This increases energy security.
- Rural economies: Local, domestic rather than remote economies are strengthend.
Biomass generation primarily relies on renewable solar energy as shown in Figure 1. 1st generation biofuels employ proven technologies available today. 2nd generation biofuels are more advanced, but are less often produced in full scale operating plants, and offer more energy yield per ecological expense. Common 1st generation biofuels together with their feedstock include
BTL represents a technology rather than a biofuel and a graphical overview of 1st generation biofuels is shown in Figure 5. In Brazil, the dominant biofuel is ethanol (= ethyl alcohol) from sugar cane. Bioethanol from sugar cane also is a 1st generation biofuel.
- Biodiesel: From crops grown on quality agricultural land
- Bioethanol (ethyl alcolhol): Distilled from grains, sugarcane, potatoes, fruit; shorter starch molecules are converted to ethanol through fermentation.
- Biogas: From energy crops
- Biobutanol: An alcohol with higher energy density and lower volatility than ethanol; same feedstock as bioethanol i.e. can be made from sugar cane, sugar beet, corn, wheat, sorghum. Biobutanol has lower vapour pressure than petrol, i.e. exhibits lower VOCs.
- Wood and BTL (Biomass to Liquid): Biomass such as wood, straw or coppice is first gasified and liquid fuel is synthesised from gas. Derived from older technologies such as CTL (= Coal to Liquid) and GTL (= Gas to Liquid).
There is growing concern that 1st generation biofuels are not as sustainable as originally thought (Sanderson (2006), Odling-Smee (2007), Manuel (2007)). A variety of biofuel feedstocks and technology pathways exist and not all are beneficial. Biofuels with low sustainability are branded 1st generation biofuels. The most serious issues with (1st generation) biofuels are:
So what is the answer to these biofuel Achilles' heels? In response to these drawbacks a more advanced class of biofuels were developed, known as 2nd generation biofuels (https://en.wikipedia.org/wiki/Second-generation_biofuels). As an example, food waste can be converted to biogas and such a feedstock from residue material does not suffer from
Clearly, 2nd generation biofuels provide more environmental benefits compared to 1st generation biofuels.
- Land use and land use change: Diversion of large portions of land to biofuels.
- Potentially increased carbon emissions from land use change: If grass land is ploughed under to grow biofuels, up to 300 tonnes of per hectare can be released. Forests can release 600 to 1000 tonnes per hectare if converted to energy crops (Source: www.biograce.net).
- Potentially increased nitrogen oxide emissions from industrial fertilisers by a factor of 3 to 5 (Crutzen et al. (2007))
- Indirect fossil fuel use through artificial fertilisers and soil acidification
- Crop residues stabilise soil organic carbon (Reijnders (2008)) and should not be taken off the field.
- Water use from irrigation
- Soil erosion
- Monocultures: Threaten biodiversity of plants and animals.
- Herbicide and pesticide use: Leak into the food chain.
A list of the four most common 1st generation biofuels. The feedstock typically comes from edible biomass which is undesirable.
Harvesting usually leaves behind a crop residue, that is left on the fields and replenishes the soil with nutrients and carbon. Taking away these crop residues for biofuels or for incineration can have negative effects and long term effects are not well known (Lemke et al. (2010), Lal (2005)). Removing crop residue from agricultural fields after the harvest can have the following disadvantages (Lal (2009)):
So removing the crop residue to make biofuels may also file under 'unsustainable practice'. This view is not shared by all researchers. It has been claimed that removal of straw for biofuels is acceptable (Gabrielle and Gagnaire (2008)).
- Natural nutrient cycling: Plant nutrients including N,P,K, and Ca are not recycled any more and this may induce more use of industrial fertilisers.
- Soil moisture: It is more difficult to conserve moisture and water infiltration into the ground.
- Rain and wind erosion: Crop residues protect the soil.
- Soil carbon: Is not replenished.
Despite the fact that they have only recently been re-discovered, biofuels have already been classified into 1st and 2nd generation fuels. Two factors determine which generation the biofuel belongs to - the feedstock and the conversion technology. Ethanol, propanol and butanol are (bio)alcohols.
|Type of biofuel
||edible crops, e.g.
||soybean, sunflower, palm oil
||waste cooking oil
||edible crops, e.g.
||nonedible biomass (cellulosic ethanol)
||sugar cane, maize, wheat
||wood, straw, wastes
||edible crops, e.g.
||sugar cane, maize, wheat
||edible crops, e.g.
||maize, grass, sugar beet
||manure, slurry, food waste, sewage
||municipal waste, waste wood,
||dry crop residues
2nd generation biofuels are produced using more advanced technologies and better feedstock compared to traditional 1st generation biofuels. Table 1 lists a feedstock matrix for current 1st and 2nd generation biofuels. Some of the characteristics and benefits from 2nd generation biofuels are:
In a reply to Searchinger et al. (2009), Bent Sorensen Sorensen (2010) points out that 2nd generation biofuels are of two types:
- Food-fuel debate: Do not eat into our food supply because the feedstock consists of inedible biomass such as
biological waste materials (e.g. used cooking oil, animal fats, food waste)
- Greenhouse gases: Carbon and other GHG emissions are reduced compared to 1st generation biofuels.
- Infrastructure: The currently existing petrol, Diesel or gas networks can be used for distribution of energy to end users.
- Advanced conversion to biofuel: Ability to break up lignocellulosic feedstock and convert it to fuel. If cellulose is broken down into fermentable feedstock, this indicates a 2nd generation biofuel - Figure 6.
Novel methods are researched to exploit the previously recalcitrant lignocellulosic fraction of the plant matter. The challenge is to keep parasitic energy demand low and minimise toxic chemicals used to break down recalcitrant lignocellulose (e.g. Ragauskas et al. (2006)). Examples of 2nd generation biofuels include
- Biofuels derived from cellulosic and lignocellulosic plant biomass grown on marginal land (i.e. land not used for food crops).
- Biofuels derived from organic residues or wastes, where the nutrients are taken back to land, implementing nutrient recycling. There is no or little primary energy input into producing these wastes; the energy inputs have already been expended anyway.
The Fischer-Tropsch thermo-chemical conversion pathway is not based on a single but a family of chemical reactions. In a first step syngas is generated:
- Lignocellulosic ethanol: The production of ethanol from lignin and cellulose makes it possible to use nonedible trees or grasses as feedstocks. With advanced conversion methods long chains of cellulose in woody materials are broken down and converted to fuel.
- Biodiesel from waste vegetable oil: An example is the Brighton Yellow Lemon bus that runs on waste biodiesel - http://youtu.be/vdJuQ36y3_k.
- BTL (biomass-to-liquids): BTL-fuels (BTL = biomass-to-liquid) can be produced from various types of biomass by pyrolysis/gasification and subsequent synthesis of the liquid fuel from the gas mixture. A subsection of BTL is known as GTL (Gas-to-Liquid), a thermo-chemical (rather than a biological) conversion pathway. A key process in GTL (Gas-to-Liquid) pathway is the Fischer-Tropsch conversion.
- Lipids (http://www.chem.qmul.ac.uk/iupac/lipid) from algae are usually branded as a 3rd generation biofuel but the potential of this pathway is highly debatable. Algal biofuels is still in their infancy (e.g. http://www.oilgae.com.)
A more general version of the above syngas reaction is
possibly assisted by a catalyst to instigate the reaction. The actual Fischer-Tropsch reaction can be written as
with a catalyst such as Fe or Co initiating the reaction. The term 'hydrocarbonchain' consists of molecules in a chain. The final liquid fuel is obtained in a refining reaction (e.g. http://www.fischer-tropsch.org). Fischer-Tropsch synthesis works with many feedstocks such as coal, natural gas, or biomass. An example of larger scale Fischer-Tropsch BTL (biomass-to-liquid) is
Also, part of South Africa's fossil Diesel is produced from coal employing a type of Fischer-Tropsch synthesis, run by the petroleum company Sasol (http://www.sasol.com). This is of course not a biofuel.
Volkswagen have marketed their own brand of 2nd generation BTL (biomass-to-liquid) biofuels under the name sunfuel 0.1. Audi supports the US biotech company http://www.jouleunlimited.com who research into 3rd generation liquid biofuels from algae, grown with waste carbon dioxide. Critics brand these as 'caviar fuels' - they are far too expensive to be produced on a large scale.
The differentiation between 1st and 2nd generation biofuels is often based on the performance in the three key areas (1) food fuel dilemma, (2) biomass-to-fuel conversion efficiency and (3) carbon emissivity per unit of energy delivered.
Organic waste materials can be classified into wet and dry which determines the energy conversion technology.
Waste can be described as material and goods that have been discarded by society, are now unwanted, and need to be disposed of in some way. Waste can be classified as dry and wet as shown in Figure 7, and it
is a clear indicator of a non-cyclic society. In a fully cyclic, recycling society, there should be no or only little waste, as all material is going back into the system for re-use. Societies are currently squandering resources on a large scale by intercepting the cycle, and the term 'waste' is actually inappropriate (Drackner (2005)). In Europe every citizen generates about 3.5 tonnes of waste each year (http://scp.eionet.europa.eu/themes/waste, accessed Feb 2013), and a substantial fraction of municipal tax goes into disposing of this enormous mountain of waste. The incineration of household waste is widely classed as an energy-from-waste technology (http://en.wikipedia.org/wiki/Waste-to-energy). Further energy-from-waste options are:
An example of the latter is http://www.edmontonbiofuels.ca and a video about the project is http://youtu.be/4AnMzntAde4. It is clear that energy from waste ticks a lot of boxes, so why are we not doing more of this? The answer is that one box can't be ticked - being able to satisfy the excessive energy demand of Western societies. Figure 8 compares electric output power of energy-from-waste power plants versus conventional fossil and nuclear plants in the UK. Energy-from-waste is comparable to a drop in the ocean and remains a marginal energy supply technology. Further examples of energy from waste are:
- Anaerobic digestion of organic waste: Organic household waste causes about 5 % of global greenhouse gas (GHG) emissions, mainly from methane emissions in landfills (Bogner et al. (2008); 4th assessment report http://www.ipcc.ch).
- Landfill gas.
- Palm oil waste in Malaysia can be used to generate energy (Keong (2005)).
- Pyrolysis and gasification: 'waste-to-biofuels' to generate syngas.
- Ely Power Station in Cambridgeshire is the largest straw burning power station in the world and generates over 270GWh of electricity per year - http://www.eprl.co.uk or http://www.renewables-map.co.uk and search for 'Ely power station'. The plant fires straw bales and other field residues, generates steam at 540C and 92 bar, which in turn generates electricity. A logistics network has been setup. The company behind Ely is 'Energy Power Resources' and it generates some 10 % of the UK's renewable electricity (in 2010) through five large biomass-fuelled generators. An overview of the Ely straw burning power plant is http://www.face-online.org.uk/non-food-crops/from-grass-to-grid.
- Thetford power plant, Norfolk: In the East of England there are two poultry litter power plants. The 38.5MW plant in Thetford, Norfolk, is Europe's largest biomass chicken litter fuelled electricity generator. The 12.7MW power station in Eye, Suffolk was the world's first poultry litter fuelled generating plant. The litter is sourced from a large number of farms in the region, and the combustion ashes produced by the power stations are marketed as high quality agricultural fertiliser.
- Enerkem plant in Edmonton, Alberta, Canada: In August 2010, Enerkem Inc http://www.enerkem.com started building a waste-to-biofuels plant in Edmonton, Alberta in Canada. The plant converts municipal solid wastes (MSW) into liquid fuels. The plant converts carbon-rich wastes into synthetic gas (syngas) and then methanol and ethanol.
Waste is generated in large quantities in our society. This material needs to be disposed of anyway, and if we can produce energy, then we have a double benefit ('free lunch' effect). Advantages of waste-to-energy technologies are:
Potential disadvantages of energy from waste are:
- Double benefits: Waste such as MSW (municipal solid waste) needs to be disposed of anyway; producing energy helps offsetting the environmental cost of the waste stream and replaces fossil electricity;
- Low or zero cost feedstock: No or very little ecological/financial cost for providing feedstock to generate energy;
- Carbon intensity: If electricity is generated the carbon emissivity per kWh is a lot lower compared to fossil fuel feedstock. The technology is carbon neutral or even carbon negative (if electricity replaces fossil electricity); with organic-waste-to-biogas technology (AD) there are no anthropogenic emissions, only biogenic , which (arguably) does not contribute to global warming;
- No resource depletion: Waste provides a renewable source of energy (rather than a depleting source with fossil fuels).
- Nutrients: In the case of anaerobic digestion of organic wastes, nutrient cycling provides ecological benefits, as mineral fertiliser can be replaced with digestate;
- With wet waste the high moisture content and low energy density means transport is costly in economic and in energy terms. Parasitic Diesel fuel is required to collect the waste.
- Compared to the huge energy demand with our current life style, energy from waste is a drop in the ocean.
- National (rather than European) legislation may not reward the true benefits of energy from waste.
In many coutries in Europe food waste from domestic households is not collected separately but is 'disposed' off together with the residual waste. This is completely unacceptable and food waste needs to be separated out and either composted or digested to produce biogas, as much of organic waste is water - Figure 4. Advantages of the food-waste-to-biogas process compared to a 1st generation biofuel, are as follows:
Figure 9 depicts the food waste to biogas process chain. One way to assess the energy performance of renewables in simple terms, is to establish the net energy balance ratio (NEB ratio). It is important to realise that two types of energy balances can be made, one relative to the calorific value of the generated biomethane and one relative to the electric energy produced by the CHP engines in the anaerobic digestion plant:
- no direct energy input is required to produce the fuel food waste;
- no mineral fertiliser is required to produce the biofuel (subsequently no heavy metal contamination of the soil, no run-off into rivers and aquifers);
- no direct use of insecticides or herbicides;
- no land use change and no diversion of food farmland to agrifuels;
- recycling of digestate residue back into the nitrogen cycle;
- biological conversion of carbon into methane not limited by the Carnot (in) efficiency
(the conversion of biomethane to electricity is however, subject to the Carnot efficiency law);
Attention should therefore be paid to what type of end energy (methane or electricity) is going into the energy balance. The following conclusions can be drawn from conduction an energy balance ratio for an anaerobic digestion plant fed on food waste, that produces biogas:
- Net energy balance ratio based on the calorific value of the generated methane: The energy balance ratio of the process chain shown in Figure 9 is comparatively high at around 4 to 5. As the AD plant requires some parasitic energy, a certain percentage of the energy contained in the harvested biomethane (e.g. 20 % or 25 %) is subtracted from the methane yield before computing the energy balance ratio.
- Net energy balance ratio based on the calorific value of the generated electricity: As electricity is generated using a reciprocating combustion engine, only about 1/3rd of the energy in the biomethane can be converted to electricity. In most real life cases, low grade heat from the CHP engine is flared off, and the net energy balance ratio will therefore be substantially lower compared to the above methane energy balance.
- The energy balance is based on an operational analysis, i.e. the energy balance did not include embodied energy of infrastructure (food waste bins and caddies, collection lorry, AD plant, digestate tanker).
- The analysis did not include upgrading to compressed gas for transport.
- Waste heat from the CHP plant is used to heat the digester, but this thermal energy has been ignored in the analysis.
- In the case of electricity generation from biogas, the efficiency of the AD plant improves dramatically if all waste heat of the CHP can be made use of.
- The production of food and food waste is assumed to have zero energy input; food production is outside the system boundaries.
Illustration of process steps and system boundary in a food waste-to-biogas chain. The energy analysis includes operational energies but not infrastructural energies.
According to DEFRA statistics http://www.defra.gov.uk/statistics/environment/waste/wrfg23-wrmsannual/ for the period from April 2010 to March 2011 a total of
The amount of waste is going down from year to year - Figure 10.
- 449 kg of household waste was generated per person in the UK.
- 189 kg of this was recycled and
- 264 kg was not recycled, i.e. landfilled or incinerated.
The Renewable Energy Directive 2009/28/EC, or short RED, is the legislative pillar that carries the energy policy of the EU into the next decade - http://eurlex.europa.eu/LexUriServ/LexUriServ.do?uri=OJ:L:2009:140:0016:0062:en:PDF. The 'rapporteur' (= appointed person) responsible for dealing with the RED in the European Parliament was Claude Turmes, a Green MEP from Luxemburg - http://www.claudeturmes.lu. Key points in the RED are as follows:
According to Article 17 of the RED, carbon emissions are a key sustainability criterion:
- Came into force in June 2009 and revolves around two targets - a renewable share of the total energy mix of 20 % in the EU and a renewable transport energy share of 10 % by 2020. These targets are minimum targets and are set in units of energy rather than units of volume (e.g. Charles and Wooders (2012), page 10).
- The 20 % energy target is the average target for all of the EU, but countries are assigned individual, mandatory renewable energy targets according to their capabilities. Some countries will therefore need to exceed the 20 % target whereas weaker countries are allowed lower renewable energy targets, as shown in Figure 11.
- Individual renewable energy shares per member state are listed on page 31 of the RED. For example, Malta gets by with only 10 % by 2020, whereas Sweden is expected to achieve a renewable share of 49 % by 2020. The UK is required to reach 15 % renewable energy in 2020.
- Energy-from-waste receives particular attention and member states are allowed to count their energy-from-waste share twice towards the renewable energy target values (Article 21 of the RED). For example, if a member state generates 2 % of its national transport energy consumption from wastes, then this counts as 4%:
''... for the use of energy from renewable sources in .. transport .., the contribution made by biofuels produced from wastes, residues, non-food cellulosic material, and ligno-cellulosic material shall be considered to be twice that made by other biofuels.'' (Renewable Energy Directive, Article 21 paragraph 2, page 26 of 47 page pdf file)
In the Directive's Article 21 wastes are priviledged if and only if they serve as feedstock for transport fuel - they are not generally priviledged for electricity generation.
- The RED is only moderately specific what type of wastes or residues classify for energy-from-waste.
- The RED requires a number of sustainability criteria to be met by biofuels. The EU commission responded to severe criticisme in the food-fuel debate and more than two pages of sustainability criteria are listed in Article 17 of the RED:
''...energy from biofuels ... shall be taken into account ... only if they fulfil the sustainability criteria set out in paragraphs 2 to 6'' (of Article 17).
- Each member state needs to produce a national renewable energy action plan NREAP.
''The greenhouse gas emission saving from the use of biofuels ... shall be at least 35 %.''
Article 17 gradually raises the bar in 2017 and again in 2018 as follows:
''From 1 January 2017, the greenhouse gas emission saving from ... biofuels ... shall be at least 50 %.
From 1 January 2018 that greenhouse gas emission saving shall be at least 60 % for biofuels and
bioliquids produced in installations in which production started on or after 1 January 2017.''
To conclude, the RED introduces the following sustainability criteria in Article 17:
The RED clarifies the EU Biofuels Directive 2003/30/EC from 8 May 2003, which aims to blend 5.75 % of biofuel into transport fuels in all EU member states by 2010 and 10% by 2020. Biofuel blending mandates vary from country to country as shown in Figure 12.
- GHG savings: Biofuels must achieve 35 %, rising to 60 % carbon savings after 2017, when compared to fossil based fuels.
- Biodiversity: High diversity crop and forest land is excluded from biofuel production.
- Soil carbon: Land with high carbon stock is excluded from biofuel production.
In order to support and organise the implementatin of the renewable targets, a scheme called 'Concerted Action on the Renewable Energy Sources Directive (CA-RES) started in July 2010 - http://www.ca-res.eu. The RED must be transposed into national law in each member state by 5 December 2010 and on 21 June 2012 the EU Commission pointed its finger at four member states, Cyprus, Ireland, Malta and Slovenia. These four states have not transposed the Directive into national law in time - http://europa.eu/rapid/press-release_IP-12-640_en.htm. Obviously these countries had to tackle other issues that took priority over becoming a more sustainable society.
Biofuel blending mandates in the EU. Summaries of the renewable status per memeber state are at the CA-RES website (Concerted Action on the Renewable Energy (Sources) Directive) at http://www.ca-res.eu/index.php?id=307 (accessed Feb 2013).
Not long ago biofuels were regarded as the silver bullet for an eternal, future energy source that is able to stamp out our depedency from fossil fuels for once and for all. This phase has certainly waned and a more sobering view has taken over following the food fuel debate among scientists and policy makers. The food fuel dilemma and previously neglected land-use change effects led to the sobering view that biofuels are not as beneficial as originally thought. Land use change and the application of industrial fertilisers in conventional farming can completely wipe out an otherwise beneficial greenhouse gas balance of a biofuel. Fossil energy sources are hard to replace in the manufacture of mineral fertilisers, yet they guarantee high crop yields. Sustainability of biofuels clearly needs to be assessed in some way and this receives increasing attention from scientists and policy makers (e.g. Lovett et al. (2011)). The degree of sustainability of a biofuel is often reduced to looking at the following
three environmental impacts:
However, limiting assessment of a biofuel to the above criteria is inadequate - adoption of a biofuel on a larger scale requires a wider scope of potential impacts including social and labour effects, water consumption, long term soil stability, biodiversity, and numerous forms of pollution into the compartments air, soil and water (e.g. Gasparatos et al. (2013)). Up to about 35 environmental impacts are known (Buchholz et al. (2009)) and a convention needs to be established and adopted that specifies performance targets.
- Food security
- GHG emissions over the life cycle or compared to fossil fuel equivalents: Biofuels can be rated according to their global warming emissions, and this indicator is often reduced to the term 'carbon emissions', which distinguishes between three levels of achievement for biofuels as shown in Figure 13:
is only one greenhouse gas among several and it is necessary to take other powerful GHGases such as and into account. Biofuels can completely defeat their purpose and cause more climate change compared to conventional fossil fuels if forests or grass land are converted to high intensity crop land. In one study, out of 26 biofuels only 21 resulted in a reduction of GHG emissions, the remaining 5 had greater environmental impact than fossil fuel equivalents (Zah et al. (2007)). It is expected that 2nd generation biofuels are less carbon intensive than 1st generation biofuels, which in turn are meant to be less carbon intensive than fossil fuels. Carbon emissivity of biofuels needs to be evaluated thoroughly for each type of fuel and technology pathway. Carbon neutral hydrocarbons have been proposed (Zeman and Keith (2008)), where carbon dioxide is either captured from biomass or from ambient air.
- carbon positive: The biofuel cycle acts as a fossil carbon dioxide emitter.
- carbon neutral: The biofuel cycle exhibits an overall carbon balance where emissions equal absorbtion of carbon dioxide over the life cycle of the biofuel.
- carbon negative: The biofuel cycle acts as a carbon sink over the life cycle. More carbon is sequestered into plant matter than is released in biofuel combustion - this could for example be achieved with so-called LIHD (low input high diversity) plant species, grown on non-food agricultural land with lower than average fertility (Tilman et al. (2006)).
- Energy return per energy invested.
An important performance indicator for biofuels is GHGas emissions from the complete production chain. The term 'carbon' in 'carbon positive' stands for fossil, anthropogenic carbon dioxide. See also Figure 14.
Both scientists and policy makers recognise that metrics need to be introduced to curb mushrooming of unsustainable biofuels and foster promising pathways more selectively. So-called certification schemes are on the agenda and these have proven successful in various areas other than biofuels:
Simplistic illustration of the direction of carbon flows with biomass cultivation. Biogenic carbon emissions (section in magenta on the left) represents short-term cyclic carbon and the majority of biofuel studies do not take short term cyclic, biogenic into their balances.
In terms of biofuels several national and international entities have gone some way towards assessment and certification of biofuel pathways. The UN Food and Agriculture Organization FAO runs the Bioenergy and Food Security Criteria and Indicators (BEFSCI) project at http://www.fao.org/bioenergy/foodsecurity/befsci/compilation/en/ and schemes are:
- FSC Forest Stewardship Council https://ic.fsc.org: Provide methods for assessing forest management.
- RSPO Round table on Sustainable Palm Oil http://www.rspo.org: Develop principles and criteria for sustainable palm oil production.
- GOTS Global Organic Textile Standard http://www.global-standard.org: The website contains a database of producers of organic textiles; certification for the GOTS standard is based on visits to textile factories by accredited, independent bodies;
- Soil Association Certification http://www.sacert.org: The largest organic food and farming certification body in the UK.
Biofuels have a distinct advantage over fossil fuels - they only emit biogenic rather than fossil . If fossil is emitted into the atmosphere the net concentration of in the atmosphere rises.
Scientists consider biofuel combustion as carbon-neutral technology because the from combustion is cyclic - it has previously been captured by growing plants driven by photosynthesis. The identical amount of captured during the plant growth phase will later be released in the combustion of the plant matter. Plants rely on water, sun, and soil nutrients to grow. The net biogenic balance, however, is only cyclic, if no fossil fertilisers are used in farming.
To make a biofuel pathway sustainable a number of boxes need to be ticked (e.g. Shilton and Guieysse (2010)):
- Carbon emissivity
- Long term soil quality
- Biodiversity: Monocultures must be avoided.
- Nutrient cycling and eutrophication: Nutrient runoff must be minimised.
- LUC (land use change) and iLUC (indirect land use change): An unprecedented portion of farm land is now devoted to biofuels throughout the world and two types of land use change can be associated with the recent rise of biofuels:
Indirect land use change effects are hard to quantify and it is the indirect land use change that the EU wants to incorporate into environmental performance indicators for biofuels. It is not surprising that the policy is highly controversial with farmers and the agricultural lobby.
- direct LUC (land use change): Farmland previously used to produce food is now used to produce energy crops.
- indirect LUC: Farmland in Europe is increasingly used for energy crops. Cooking oil that was previously made from sunflower and rape seed in Europe, may now be displaced by palm oil. The effect is that more forest is cleared in Malaysia to plant palm oil trees. This is an indirect effect where local farming practices 'at home' are causing remote deforestation, which may make things worse than they were with fossil fuels. On top of loss of biodiversity, this type of land use change may cause a carbon debt which may take as long as 423 years to pay back with the new biofuel that replaced the rain forest (Fargione et al. (2008)).
- Water use: Maize (American English: corn) is a C4 plant grown for biofuels in the US for ethanol and in Germany for biogas, requires large amounts of water and nitrogen, which in many cases means artificial irrigation and intensive mineral fertiliser use. Crop rotation is essential to avoid soil nutrient depletion from maize cultivation. Industrial fertilisers (e.g. ammonium nitrate) release nitrogen oxides, an extremely potent GHG with a emission factor of 298 compared to .
Carbon emissions for cars bought in 2010 across Europe. Portugal and Denmark are leading. Surprisingly countries such as Estonia and Latvia are trailing. Data source: http://www.eea.europa.eu.
In the EU all countries are required to report carbon emissions for new vehicle sales to the European Commission. Figure
16 shows that there is a large gradient of new vehicle emissions across the 27 EU countries that does not seem to correlate with their economic income. In Denmark and Portugal new cars sold in 2010 exhibit record low emissions of only 127 grams per km whereas many other EU member countries exceed 160 grams of emitted per km driven. Passenger transport planes emit even more at 100-250 g/km per passenger, a bus 40-80 g/km, and a train 40-160g/km per passenger (Source: http://www.eea.europa.eu/green-tips/going-on-a-longer-trip-choose-wisely). The emissions plotted in Figure 16 are vehicle emissions. The system boundary is virtually 'around the vehicle' and excludes the fossil oil well and the oil refinery.
It has been pointed out that well-to-wheel studies adopt a much more holistic view (Geerken et al. (2005)) to assess the whole fuel and vehicle transport chain and therefore take on life cycle analysis concepts. However, there are important differences between well-to-wheel studies and life cycle analysis studies:
For example, a 2001 early well-to-wheel study in the US (Wallace et al. (2001)) only considers two environmental impacts - (1) total energy use and (2) the three greenhouse gases , and . The study claims to conduct a LCA of 27 different fossil and renewable fuel pathways. A CONCAWE well-to-wheel study claims that liquid biofuels such as biodiesel and bioethanol reduce greenhouse gas emissions compared to their fossil counterparts by 53 %. If previously ignored emissions from mineral fertilisers are taken into account in the study, GHG emissions with biodiesel are only a mere 7% better compared to fossil Diesel (Armstrong et al. (2002), page V). CONCAWE is a European oil industry association founded in 1963 and is short for CONservation of Clean Air and Water in Europe (http://www.concawe.eu). It is clear that environmental impacts other than pure GHG emissions also play a role in the environmental performance
of a fuel and must be considered as stated in a Swedish study from 2008:
- Infrastructure: Well-to-wheel studies typically exclude embodied energy of infrastructur and the vehicle.
- Environmental impacts: Well-to-wheel studies employ far fewer environmental impacts compared to LCA. In many cases well-to-wheel studies only measure GHG emissions for the various transport fuel process chains.
''Thus, the digesting of organic waste produces ... pollutants contributing to eutrophication (ammonia), and these indirect effects might even exceed the direct environmental benefits of replacing fossil vehicle fuels with biogas.'' (Börjesson and Mattiasson (2008), page 10).
Carbon dioxide equivalent emissions for various fossil and biofuels are plotted in Figure 17. Biogas as transport fuel comes out extremely well at less than minus 64 g of per MJ of fuel. The biofuel may be much less beneficial if impacts other than GHG are considered. Figure 17 does not take biogenic carbon emissions from the combustion of biogas into account.
Attempt to attach a number to greenhouse gas emissions to various types of transport fuels. Bar graphs depict grams of GHGas emissions as per fuel type. Drawn after Börjesson and Mattiasson (2008), Figure 3, page 10.
To assess and rate the environmental performance for transport fuels from biofuels, two types of analyses exist, that differ in their system boundary:
See http://www.mpoweruk.com/fuel_cells.htm for examples of well-to-wheel efficiencies for various propulsion systems.
In the literature the fuel well, the fuel refining, and the delivery to the petrol station is sometimes known as the upstream section and the part from the petrol station onwards, i.e. the vehicle, is called downstream section.
Well-to-wheel efficiencies can be comparatively low with biofuels, for example between 9 and 11 % (Ahlvik and Brandberg (2001) Figure 9, page 78), but a number of ways of producing well-to-wheel analyses exist (Ma et al. (2011)).
- Well-to-tank analysis: identical to well-to-wheel but the system boundary does not include the car, i.e. does not take into account efficiency of the vehicle technology. This can be regarded as a cradle-to-gate analysis.
- Well-to-wheel analysis: For a car running on liquid fuel, this would include mining of the oil well, processing/refining crude oil in the oil refinery, transport to the petrol station, conversion of fuel to turn the wheels and propel the car (i.e. conversion from chemical fuel energy into kinetic energy). A well-to-wheel analysis would correspond to a cradle-to-grave approach for both the vehicle fuel and the vehicle technology.
Let's look into a well-to-tank and a well-to-wheel analysis for biomethane. Say we are operating a large anaerobic digestion plant and produce biogas. The digester is fed with various organic materials such as (a) food waste, (b) agricultural waste and (c) garden waste. Let's see what a (1) cradle-to-gate analysis involves and (2) a cradle-to-wheel analysis, for the case where transport fuel is produced from upgraded biogas:
Both analyses are useful but have different emphasis. The well-to-tank analysis emphasises the production chain of the biofuel excluding the vehicle technology, whereas the well-to-wheel analysis takes a more holistic view by taking the vehicle technology and its efficiency into the system boundary.
- Well-to-tank: The analysis stops after the upgrading of biogas to compressed, pure methane. What we do with the generated biomethane energy is not specified. The analysis stops before the specific end user technology to move 80 kg of passenger from A to B. The system (analysis) boundary in this case is well-to-compressed-gas or cradle-to-gate where the gate is the compressed gas cylinder within the plant after upgrading of biogas. Vehicle technology is not taken into account.
- Well-to-wheel: The system (analysis) boundary goes further to the type of vehicle technology used to move an 80 kg passenger from A to B. There are two options how to convert the biogas into kinetic energy: (a) compressing biomethane and feeding an internal combustion engine or in the future a gas fuel cell. (b) Generating electricity from biomethane and charging an electric vehicle. Due to the extended system boundary beyond the factory gate, this scenario represents a well-to-wheel analysis.
There is evidence that well-to-wheel efficiency of electric vehicles is higher than that of vehicles powered by an internal combustion engine. The two transport technologies work with different types of energy sources or wells:
It is clear that a well-to-wheel analysis of a propulsion technology is complex right from the start. It is important to distinguish between (tank-to-wheel) efficiencies, and carbon intensity. With electric vehicles carbon intensity is measured in units of [
] and it is a somewhat nontransparent metric due to the dependency of carbon emissivity on the fuel mix burnt to generate electricity in the first place. The reason for the poorer conversion efficiency of the internal combustion engine is that much of the energy in the combustion engine is converted into low-grade (useless) heat rather than kinetic energy. Of course the electric vehicle motor system also is not 100 percent efficient due to thermal losses in battery charging, but overall, the losses with electric propulsion are lower compared to an internal combustion engine. Figure 19 plots well-to-wheel emissions per MJ generated for various biofuels, excluding the vehicle
fuel exhibits very low carbon emissions. Combustion of biogas fuel releases only (short term cyclic) biogenic carbon which does not count towards the (fossil fuel) greenhouse effect.
- Internal combustion engine powered vehicle: A fuel source, e.g. fossil, crude oil from an oil well, or renewable biomass, is taken to an oil refinery for upgrading to petrol and Diesel.
- Electrically powered vehicle: A fuel source, e.g. fossil or possibly renewable biomass is combusted (oxidised) and an electric generator produces electricity. Alternatively wind and solar radiation act as a renewable well to produce electricity with wind turbines or photovoltaic cells.
Example of a well-to-tank analysis. The vehicle technology is not included - it is situated outside the system boundary.
The three most common greenhouse gases and their origin in the atmosphere are:
Global warming is caused by a number of individual gases, but these are often aggregated into a single carbon dioxide equivalent GHG effect with units of kg of .
- Carbon dioxide : Burning of fossil fuels (gas, oil, coal), or burning of biogenic fuels (wood, straw, biogas)
solid waste, trees and wood products generates ; also emitted in chemical reactions such as the manufacture of cement and during the production of steel. A certain amount of carbon dioxide is removed from the atmosphere (or 'sequestered') when it is absorbed by plants in the biological carbon cycle.
- Methane : Emitted from livestock such as cattle and pigs; mining of coal, oil and natural gas; anaerobic decay of organic matter in landfills;
- Nitrous oxide : Emitted in modern agriculture, as well as in the combustion of fossil fuels in (Diesel) engines and the incineration of solid waste.
- Hydrofluorocarbons (HFCs)
- Perfuorocarbons (PFCs)
- Sulphur hexafluoride (SF6)
A gaseous or liquid molecule vibrates in a complex way when it is excited. This complex vibration, however, can be broken down into a superposition of simpler vibrations, called the normal modes of vibration. The normal modes of vibration consist of simple harmonic motion.
Molecular vibrations can be classified into two types:
Both types of vibrations can be symmetric and antisymmetric. For linear molecules such as , the atoms are in line and then the number of normal modes of vibration for such a linear molecule is given by
- Stretching (mode) vibration: change of bond length
- Deformation (mode) vibration: change of bond angle
where N is the number of atoms in the molecule. Since contains N = 3 atoms (one carbon and two oxygen atoms) this linear molecule is able to vibrate in 3 3 - 5 = 4 modes of vibrations. An animation of the modes of vibration of is in this video about IR spectroscopy at https://www.youtube.com/watch?v=TMLnUmbLwUI. For nonlinear molecules such as the water molecule the two hydrogen atoms are not aligned with the oxygen atom - they are not arranged in a straight line. In such a nonlinear molecule the number of normal modes of vibration is
The water molecule consists of N = 3 atoms and then the number of modes of vibration for the water molecule is
The water molecule oscillates in three normal modes of vibration.
The global warming potential GWP of a greenhouse gas indicates to what degree (or to what factor) the gas causes global warming compared to the chosen standard . What is the GWP of ? The answer is 1. acts as the reference gas and it has been given a GWP of 1. The calculation of the GWP of greenhouse gases is complicated by the fact that the various greenhouse gases don't reside in the atmosphere for good. Instead, greenhouse gases gradually decay in the atmosphere over time. Because the residence time of global warming gases are dynamic, rather than static with time, they are subject to debates among researchers. Decay times or residence times are not identical for the various green house gases - they range from a few years to thousands of years.
The GWP for methane and nitrogen (di)oxide are shown in Table 2. The physics behind the global warming effect is that the gas is absorbing solar, thermal radiation, that would otherwise be reflected back into outer space. Currently adopted values for GWP of methane (Table 2) may be underestimates due to gas-aerosol interactions (
urlhttp://www.giss.nasa.gov/research/news/20091029 and http://methanenet.org/content/methane-has-larger-gwp). The 100-year GWPs for methane may be as large as 33 rather than 21 (Figure 2 in Shindell et al. (2009)).
Example: According to the 4'th assessment report IPCC 2007 Nations (2007), the greenhouse gas dinitrogen monoxide , also known as nitrous oxide or laughing gas (http://en.wikipedia.org/wiki/Nitrous_oxide; do not confuse with nitrogen dioxide ) has a GWP of 298. Explain!
Answer: A GWP of 298 means that the gas is 298 times more potent in global warming than . Or in units, one kg of is as potent as 298 kg of . One kg of causes as much global warming as 298 kg of .
Global warming potentials GWPs of the three most common greenhouse gases relative to which has been allocated a GWP of 1. The currently accepted values are from the 4'th assessment report IPCC 2007 Nations (2007).
||in IPCC 1996
||in IPCC 2007
|nitrous oxide (dinitrogen monoxide)
No rule exists to make the IPCC 2007 Nations (2007) GWPs mandatory in carbon footprinting. According to http://www.biograce.net the global warming potentials in carbon dioxide equivalents for the three major greenhouse gases , and are:
The reason to use the above values rather than the IPCC 2007 values is explained at http://www.biograce.net:
- Carbon dioxide : 1
- Methane : 23
- Dinitrogen monoxide : 296
''This list of standard values contains the conversion factors that were used for calculating the default values in the Renewable Energy Directive (2009/28/EC) Annex V but for one exception: The Commission calculated its default values using global warming potentials of 25 for (methane) and 298 for (nitrous oxide), whereas in Annex V part C the Commission prescribes that global warming potentials of 23 for and 296 for should be used for calculations. As to ensure that calculations are in line with the directive's rules, our list of standard values follows the methodology laid down in Annex V part C and uses values of 23 and 296.'' (Source: http://www.biograce.net, viewed April 2016)
Video resources on biofuels:
Video resources on greenhouse gases and the GHG effect:
Climate change sceptics:
- Bellamy and Barrett (2007) Climate stability: an inconvenient proof.
- http://youtu.be/2ROw_cDKwc0: Professor Murray Salby of the Department of Environment and Geography at Macquarie Universiry in Sydney, gave a lecture in Hamburg in 2013. Salby claims the rise in carbon dioxide is natural, and not man made.
- http://www.co2science.org: Run by Dr Sherwood Idso; dubious sponsors for his website; it says '' All donations are kept confidential. Please consider supporting the Center.''
Greenhouse gases and global warming:
A simple quiz on biofuels is at https://www.isurvey.soton.ac.uk/7043.
Selected literature about biofuels:
LCAs and sustainability criteria for biofuels
Despite the progress in LCA models, the stability and accuracy of LCA has been debated, and biofuel process chains are often difficult to model.
Waste to energy:
- Lynd (1996) Overview and evaluation of fuel ethanol from cellulosic biomass: technology, economics, the environment, and policy.
- Wyman (1999) Biomass ethanol: technical progress, opportunities, and commercial challenges.
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- Dias de Oliveira et al. (2005) Ethanol as fuel: Energy, carbon dioxide balances, and ecological footprint.
- Hahn-Hägerdal et al. (2006) Bioethanol - the fuel of tomorrow from the residues of today.
- Farrell et al. (2006) Ethanol can contribute to energy and environmental goals.
- Kim and Dale (2008) Life cycle assessment of fuel ethanol derived from corn grain via dry milling.
- Schmer et al. (2008) Net energy of cellulosic ethanol from switchgrass.
Plant nutrients and nitrogen fertilisers:
Global statistical energy data:
- http://faostat.fao.org: World food and agriculture statistics provided by the UN; provides agricultural maps from numerous countries in the world with interactive GIS web page;
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